Journal of Mechanical Engineering 2011 11

57 (2011) 1 11

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11 year 2011 volume 57 no.

Platnica SV-JME 57(2011)11_03.pdf 2 7.11.2011 12:05:47

Strojniški vestnik – Journal of Mechanical Engineering (SV-JME) Aim and Scope The international journal publishes original and (mini)review articles covering the concepts of materials science, mechanics, kinematics, thermodynamics, energy and environment, mechatronics and robotics, fluid mechanics, tribology, cybernetics, industrial engineering and structural analysis.

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Editor in Chief Vincenc Butala University of Ljubljana Faculty of Mechanical Engineering, Slovenia Co-Editor Borut Buchmeister University of Maribor Faculty of Mechanical Engineering, Slovenia Technical Editor Pika Škraba University of Ljubljana Faculty of Mechanical Engineering, Slovenia

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Editorial Office University of Ljubljana (UL) Faculty of Mechanical Engineering SV-JME Aškerčeva 6, SI-1000 Ljubljana, Slovenia Phone: 386-(0)1-4771 137 Fax: 386-(0)1-2518 567 E-mail: info@sv-jme.eu http://www.sv-jme.eu Founders and Publishers University of Ljubljana (UL) Faculty of Mechanical Engineering, Slovenia University of Maribor (UM) Faculty of Mechanical Engineering, Slovenia Association of Mechanical Engineers of Slovenia Chamber of Commerce and Industry of Slovenia Metal Processing Industry Association Cover: Development of algorithms, simulation models and grippers for robot cooperation to achieve an increase in efficiency and flexibility of assembly systems (main picture).

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Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11 Contents

Contents Strojniški vestnik - Journal of Mechanical Engineering volume 57, (2011), number 11 Ljubljana, November 2011 ISSN 0039-2480 Published monthly Papers Vid Novak, Rok Petkovšek, Boštjan Podobnik, Janez Možina: CW Fiber Laser for Second Harmonic Generation Dario Croccolo, Massimiliano De Agostinis, Nicolò Vincenzi: Structural Analysis of an Articulated Urban Bus Chassis via FEM: a Methodology Applied to a Case Study Timo Kiekbusch, Daniel Sappok, Bernd Sauer, Ian Howard: Calculation of the Combined Torsional Mesh Stiffness of Spur Gears with Two- and Three-Dimensional Parametrical FE Models Gordana Ostojic, Stevan Stankovski, Djordje Vukelic, Milovan Lazarevic, Janko Hodolic, Branko Tadic, Stevan Odri: Implementation of Automatic Identification Technology in a Process of Fixture Assembly/Disassembly Tomaz Brajlih, Tadej Tasic, Igor Drstvensek, Bogdan Valentan, Miodrag Hadzistevic, Vojko Pogacar, Joze Balic, Bojan Acko: Possibilities of Using Three-Dimensional Optical Scanning in Complex Geometrical Inspection Andrej Ljubenko, Alojz Poredoš, Miran Zager: Effects of Hot-Water-Pipeline Renovation in a District Heating System Dragan V. Petrović, Časlav B. Mitrović, Nataša R. Trišovic, Zorana Z. Golubović: On the Particles Size Distributions of Diatomaceous Earth and Perlite Granulations Vladan Radlovački, Ivan Beker, Vidosav Majstorović, Mladen Pečujlija, Dragutin Stanivuković, Bato Kamberović: Quality Managers’ Estimates of Quality Management Principles Application in Certified Organisations in Transitional Conditions - Is Serbia Close to TQM? Instructions for Authors

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Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, 789-798 DOI:10.5545/sv-jme.2011.109

Paper received: 20.05.2011 Paper accepted: 05.09.2011

CW Fiber Laser for Second Harmonic Generation Novak, V. – Petkovšek, R. – Podobnik, B. – Možina, J. Vid Novak1 – Rok Petkovšek2,* – Boštjan Podobnik1 – Janez Možina2 1 LPKF Laser & Elektronika d.o.o, Slovenia 2 University of Ljubljana, Faculty of Mechanical Engineering, Slovenia

We report on a reduced-complexity laser-diode-seeded master-oscillator-power-amplifier setup of a continuous wave fiber laser, with a single-stage ytterbium doped photonic crystal fiber amplifier. The laser is capable of generating up to 7.5 W single-transverse-mode, narrow-linewidth, polarized output suitable for second harmonic generation. The approach used possesses a further power scaling potential. ©2011 Journal of Mechanical Engineering. All rights reserved.

Keywords: Yb fiber amplifier, MOPA, narrow-linewidth, single mode, polarized output, second harmonic generation 0 INTRODUCTION Laser material processing has become a widespread technology in industrial manufacturing [1] to [3]. In various applications, such as e.g. in micro-processing, a high-quality (i.e. singletransverse-mode) output at high power levels is desired [4] to [7]. In conventional laser sources, such as e.g. in bulk solid state lasers, thermally induced optical mode distortions restrict their power scaling potential when a good beam quality needs to be maintained. Therefore, several geometries of the gain medium have been introduced (thin-disc, slab, fiber) in order to overcome these issues.

Fig. 1. Basic fiber laser setup; PUMP: pump source, FBG: fiber Bragg grating, SPL: passive/ active fiber splice, YDF: ytterbium doped fiber, CL: collimator lens The fiber laser architecture (Fig. 1) turns out to be a particularly attractive alternative, not only for its capacity to generate raw optical power – up to 10 kW in a single transverse mode has been achieved [8] – but also for a number of other features that distinguish it from other laser setups, and that lead to a rapid penetration of fiber laser systems into applications formerly dominated by other lasers.

An active optical fiber has a rare-earthdoped silica core that serves as a gain medium as well as an optical waveguide that supports only a single transverse mode of propagation, providing long interaction lengths between the gain medium and the guided laser and pump light. A high single-pass-gain gives a low laser threshold and simple amplifier setups are possible even with quasi-three-level dopants such as ytterbium, which offers a high optical-to-optical efficiency [9] due to its low quantum defect.

A low quantum defect, a large surfaceto-volume ratio of the core as well as a high damage threshold and low loss of the silica host enable an efficient thermal management and a high-power output. Ytterbium also offers a broad gain bandwidth (975 to 1180 nm) as well as a broad absorption band, covering wavelengths (900 to 980 nm) at which high power semiconductor pump diodes are at their best.

Fully integrated structure provides compact, robust, alignment-free laser setup that is compatible with highly efficient fiberpigtailed high power pump diodes and various fiber-integrated devices such as e.g. fiber Bragg gratings, fiber couplers, etc.

Fiber lasers also offer the possibility of operating in the pulsed regime. Several different techniques have been employed to achieve a broad pulse parameter space. Active Q-switching in free space coupled [10] to [14] and all-fiber design [14] and [15], passive Q-switching [16] and [17], as well as direct [18] to [20] and external modulation [21] and [22] of a seed laser can be used to generate ns and sub-ns pulse durations

*Corr. Author’s Address: University of Ljubljana, Faculty of Mechanical Engineering, Aškerčeva 6, 1000 Ljubljana, Slovenia, rok.petkovsek@fs.uni-lj.si

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with PRF (pulse repetition frequency) in the kHz region. In the ps region, semiconductor seed diode gain switching [15] and active modelocking setups [16] can provide pulses with PRF up to the GHz range. Passive modelocking techniques [17] and [18] offer sub 100 fs pulse lengths at PRF of several 100 MHz. In either pulsed or CW regime, laser (resonator) configurations (Fig. 1) [8] and [27] as well as master oscillator power amplifier (MOPA) configurations (Fig. 2) [7], [28] and [29] can be used for power scaling of fiber lasers.

Whereas the laser configuration offers simplicity and compactness, the MOPA approach allows for a more refined control of both temporal and spectral characteristics at high powers. In a MOPA architecture, the output from a highly controlled, low power seed laser can be amplified to high power levels while preserving the desired seed characteristics. Narrow-linewidth operation in CW [19] or pulsed [20] regime and highly-controlled pulsed operation [21] to [24] at high power levels are typically realized using a MOPA design.

A narrow-linewidth output of a MOPA laser can be coherently combined into a synthetic aperture laser with a power scaling potential that can far exceed that of a single-fiber laser. A linearly polarized narrow-linewidth beam can also be efficiently transformed into a higher-order harmonic radiation, e.g. by using a critical phase matching technique in a nonlinear optical crystal.

Second harmonic generation (SHG) produces a radiation in the green part of the visible spectrum which can be employed in various industrial and medical applications, such as e.g. thin-filmtransistor (TFT) annealing [25], post deposition annealing of ferro-electric thin film for ferroelectric random access memory (FeRAM) and piezoelectric devices [26] or photochemical tooth bleaching [27], where it can significantly outperform existing solutions.

A significant progress has been made in the field of research on narrow-linewidth fiber lasers with reported powers of up to 500 W [19].

MOPA is a well established approach in the field of generating a narrow-linewidth polarized output [19] and [20], that is suitable for SHG. Various seed sources have been used - solid state bulk and fiber laser as well as semiconductor laser diode.

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However, multiple-stage amplifiers have had to be used to achieve even a medium power level output (~10W), with each stage requiring a separate pump and thermal management system and a high-power inter-stage optical isolator, rendering such an approach to be rather complex.

Fig. 2. MOPA (Master oscillator power amplifier) fiber laser architecture with a two stage fiber amplifier; FI: Faraday isolator, CMB: fiber combiner, SPL: passive/active fiber splice, PUMP: pump source, SEED: seed source, YDF: ytterbium doped fiber, CL: collimator lens Recent advances in semiconductor laser diode technology [28] and [29] and the introduction of advanced photonic crystal fiber (PCF) design with an increased pump absorption have opened up a possibility to reduce the complexity of the MOPA architecture.

For the purpose of achieving a single frequency output in the 10 W region, we have focused our work on a diode-seeded single stage amplifier MOPA configuration using an air-clad PCF with ytterbium-doped silica core. Since high gain as well as high efficiency cannot be achieved in a single stage amplifier at the same time, the design had to be optimized to achieve a high enough gain to amplify the seed to the required output power without the need to use excessive pump powers. A single semiconductor singleemitter-based pump module has been proved to be sufficient.

Novak, V. – Petkovšek, R. – Podobnik, B. – Možina, J.

Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, 789-798

In this paper, a single stage amplifier MOPA arrangement, providing up to 7.5 W of linearly polarized narrow-linewidth output at 1064 nm, suitable for SHG is presented.

This simplified arrangement is well-suited for integration into various industrial applications.

1 EXPERIMENTAL SETUP The experimental setup of our MOPA system is shown in Fig. 3. A 1064 nm continuous wave (CW) fiber-pigtailed semiconductor laser diode master oscillator has been used as a seed source for a single stage ytterbium doped fiber amplifier.

Fig. 3. Experimental setup of a MOPA system with a semiconductor laser diode master oscillator (i.e. the seed source) and a single stage photonic crystal fiber amplifier; LDS: seed laser diode, LDP: pump laser diode, TEC: thermo-electric cooler, FI: Faraday isolator, DM: dichroic mirror, YDF: ytterbium doped fiber The seed diode delivers a narrow-linewidth (300 MHz), single-transverse-mode, linearly

polarized beam with an electronically adjustable output power of up to 150 mW. It has been temperature stabilized at 25 °C using a diodeintegrated thermo-electric cooler.

An ytterbium-doped photonic-crystal fiber has been used as the amplifier medium. Active fiber’s cross section is shown in Fig. 4. Strictly single-transverse mode guidance is achieved, using a step index profile between the 15 μm core with a numerical aperture (NA) of 0.055 and the 135 μm inner (pump) cladding (NA = 0.6). A microstructured region of air channels, inserted into a silica substrate, that run along the entire fiber length, separates the inner and outer cladding, providing a high-NA waveguide for low-brightness pump light. An all-glass pure silica cladding structure (i.e. the absence of a polymer outer cladding) alleviates the risk of fiber damage at high levels of launched pump power and offers a significant power scaling potential.

Fig. 4. Active fiber cross section. Strictly singletransverse-mode guiding is achieved, using a step index profile between the core and the inner cladding; polarization maintaining fiber has two orthogonal axes of propagation, a slow and a fast one, which are generated by the stress rods on each side of the core; air cladding region separates the inner and the outer cladding, providing a high NA waveguide for lowbrightness pump light The active fiber is polarization maintained (PM) with two orthogonal axes of propagation.

A strong birefringence is created by inducing a

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constant stress-field within the fiber core by using stress-applying members on each side of the core.

Birefringence breaks the circular symmetry, thus creating two principal transmission axes within the core, providing a strong isolation against cross-coupling of guided light between the two orthogonal eigenmodes, while the fiber is being subjected to external stress and temperature perturbations along its length.

PM fiber’s birefringence axis has been aligned with the seed polarization plane in order to excite a single polarization mode and thus maintain the polarization state along the amplification path in the PM fiber.

A fiber pigtailed 915 nm semiconductor laser diode delivering up to 25 W in the CW regime has been used as the pump source for the fiber amplifier in a counter-propagating pumping scheme. An electronically controllable current source has been used as the pump diode driver while an external thermo-electric cooler provides the means for a temperature-stabilized diode operation at peak absorption wavelength.

Both the seed and the pump beam have been free-space coupled into the fiber amplifier using bulk optics. The coupling efficiency has been determined, using a short fiber piece, to be 75 and 80% for the seed and pump beam respectively. The absorption of the 915 nm pump light in the core has also been measured, obtaining a value of 90%.

The fiber ends have been precisely cleaved at an angle of approximately 8° to the fiber axis to prevent parasitic lasing in the amplifier due to the Fresnel back-reflection on the fiber facets, effectively coupling the back-reflected light into radiation modes and thus preventing their amplification in the backward direction.

A dichroic mirror has been used to separate the amplified laser beam and the input pump beam on the output side of the amplifier.

The high amplifier gain necessitates the use of an optical isolator to protect the seed source against the amplified back-reflected beam. A bulk optic Faraday isolator with 42 dB isolation has therefore been inserted between the seed source and the fiber amplifier input.

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2 EXPERIMENTAL RESULTS AND DISCUSSION Experimental results have been grouped into the following subsections: Output power and amplifier gain, Amplifier efficiency, Polarization contrast, Output power stability and Emission spectrum.

2.1 Output Power and Amplifier Gain Electronically adjustable seed current that drives the semiconductor master oscillator allows for a refined control over the optical seed power coupled into the fiber amplifier. Data were acquired by varying the seed current and by simultaneously recording the resulting output power. Fig. 5 shows the 1064 nm laser output power PL from the ytterbium PCF amplifier as a function of the input seed power Ps at several different levels of launched pump power Pp.

Fig. 5. Average laser output power PL versus launched seed power Ps; the legend shows the pump power Pp used, for each graph respectively With increasing seed power the gain saturates and the output power only becomes a function of the launched pump power in the saturated regime. The output power scales linearly with pump power up to approx. 15 W of pump power, whereas a transition into a sub-linear

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Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, 789-798

relationship is observed above this value due to pump saturation.

A maximum output power of 7.5 W at 1064 nm was achieved in the saturated regime for a launched pump power of 18.3 W, corresponding to a single pass gain of approximately 24 dB. The maximum achievable output power was only limited by the available pump power.

Fig. 6 shows the amplifier gain characteristics. Amplifier gain PL / Ps as a function of launched seed power Ps has been plotted for various levels of pump power Pp. The gain increases monotonically with pump power and decreases monotonically with seed power. A maximum gain of 29.5 dB has been achieved at the seed power of 2 mW and pump power of 18.3 W.

stage that operates in the highly saturated regime throughout the whole fiber length.

Here, a single-stage single-pass amplifier has been used in order to reduce the complexity of the laser setup, rendering the high levels of efficiency out of reach since the input segment of the amplifier cannot be saturated and acts therefore, as an amplified stimulated emission (ASE) source.

Fig. 7. Fiber amplifier efficiency PL / Pp versus the seed power Ps for three different levels of pump power Pp (as shown in the legend); efficiency is defined here as a quotient of total output power at 1064 nm PL versus the pump power launched into the active fiber Pp Fig. 6. Amplifier gain PL / Ps as a function of launched seed power Ps; the legend shows the pump power Pp used, for each individual graph Notice that by reducing the seed power the small signal gain becomes increasingly saturated with respect to launched pump power.

2.2 Amplifier Efficiency A single-stage single-pass amplifier inherently suffers from a lower optical-to-optical efficiency than a multiple stage amplifier where the overall efficiency is predominantly determined by the highly efficient final power amplifier

Therefore, a careful amplifier design optimization has been undertaken in order to achieve a high enough gain to amplify the seed to the required output power, as well as to get a reduced ASE output and the highest possible efficiency, so that only one single-emitter based pumping module would be sufficient to pump the amplifier, leading to a simpler thermal management and pump driver system.

An air-clad PCF with a strong overlap between the pump modes and the active core that is a direct consequence of a high-NA (0.6), small diameter (135 mm) pump cladding has been used to ensure a high pump light absorption, leading to a short unsaturated length of the amplifier,

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effectively reducing the ASE output. A plot of amplifier efficiency PL / Pp as a function of the launched seed power Ps is shown in Fig. 7.

Efficiencies up to 50% have been achieved as well as a laser-to-ASE contrast up to 10 dB as shown in Fig. 8.

Fig. 9. Output beam polarization contrast Pmax / Pmin at 1064 nm versus the launched pump power Pp at 10 mW of seed power Ps 2.4 Output Power Stability

Fig. 8. Percentage of laser power PL (i.e. power at 1064 nm) in the amplifier output Po as a function of launched seed power Ps; the legend shows the pump power Pp used, for each graph respectively

A long term (1 hour) as well as short term (2 s) output power stability has been measured using a thermopile sensor and a photodiode respectively.

The output power stability on the 1 h scale was measured to be 1.3% RMS and 2.4% on the 2 s scale.

2.3 Polarization Contrast The 1064 nm output of the amplifier is linearly polarized. Fig. 9 shows the dependence of the polarization contrast on the launched pump power Pp at 10 mW of input seed power Ps. The polarization contrast has been measured using a rotating beam-splitter cube and is defined here as a ratio of maximum vs. minimum transmitted power Pmax / Pmin at 1064 nm.

Whereas a high contrast of approx.

100 to 120 could be maintained up to approx.

10 W of pump power, a reduction of the achievable contrast is observed when approaching higher levels. Nevertheless, a value of over 30 could be achieved over the entire range.

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Fig. 10. Output power (PL) stability over an interval of 1 hour, measured using a thermopile sensor; photodiode measured short term stability on the seconds scale is shown on the inset

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Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, 789-798

2.5 Emission Spectrum The emission spectrum of the amplifier output has been investigated using an optical spectrum analyzer (OSA). In addition to the 1064 nm signal, the spectrum also exhibits an ASE band, as shown in Figs. 11 and 12.

The overall amplifier spectral fidelity could not be determined using an OSA due to its limited resolution bandwidth (70 pm) that is orders of magnitude wider than that required to sample a 300 MHz laser line at 1064 nm. However, it

still enabled us to examine various ASE spectral features that were present in the amplifier output.

A 5-by-2 matrix of optical spectra is given in Fig. 11, providing a comparison of the unsaturated versus the saturated operation of the amplifier at various pump and seed powers used.

A row-vise comparison shows that at low pump powers, no significant ASE is present. However, a significant build-up of ASE can be observed when the pump is increased. An even further increase results in the onset of spurious lasing as confirmed by the presence of sharp peaks in the

Fig. 11. Output power spectrum comparison between the unsaturated and saturated regime; a 5-by2 matrix of optical spectra is given, where the bright line in the left and right columns represents the spectrum at launched seed powers Ps of 10 and 30 mW respectively, whereas the first, second, third, fourth and fifth row refer to launched pump powers Pp of 0, 1.01, 3.09, 5.17 and 7.29 W respectively; unseeded amplifier response is given as a comparison for rows 2 to 5 (dark line) CW Fiber Laser for Second Harmonic Generation

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ASE spectrum. Introducing a 10 mW seed signal into the amplifier causes a significant reduction of ASE only at lower pump powers but it does not suppress the lasing modes at higher pumping levels.

A column-wise comparison on the other hand shows that by increasing the seed power the gain can be saturated and consequently, the ASE modes effectively suppressed even at the highest pumping levels. The seed diode optical output has been sufficient to maintain a spurious-lasing free operation throughout the entire range of pump powers. Fig. 12 shows the output power spectrum in the highly saturated regime at a seed power of 57 mW and at four different pump levels.

linewidth output suitable for second harmonic generation. Amplifier gain of 24 dB at the highest optical output power has been achieved and we have managed to maintain the polarization contrast of over 30 throughout the entire output power range. Amplifier efficiencies up to 50% and laser-to-ASE contrasts of up to 10 dB have been obtained.

The presented setup has the potential to be fully fiber integrated and scaled well above the 10 W level since its output has only been limited by the currently available pump power.

4 ACKNOWLEDGMENT The operation has been partly financed by the European Union, European Social Fund.

5 REFERENCES

Fig. 12. Output power spectrum in the highly saturated regime at a seed power Ps of 57 mW and at four different levels of launched pump power PP (as shown on the graph); a rise of approx. 10 dB of ASE spectrum peak is observed, as well as a slight shift of the ASE peak towards lower wavelengths The ASE spectral features lie more than 26 dB below the signal level without any evidence of the lasing modes being present.

3 CONCLUSION We have demonstrated a reduced complexity MOPA system with a single-stage ytterbium PCF amplifier, generating up to 7.5 W single-transverse-mode linearly-polarized narrow796

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[17] Röser, F., Schimpf, D., Schmidt, O., Ortaç, B., Rademaker, K., Limpert, J., Tünnermann, A. (2007). 90 W average power 100 uJ energy femtosecond fiber chirped-pulse amplification system. Optics Letters, vol.

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[19] Jeong, Y., Nilsson, J., Sahu, J.K., Payne, D.N., Horley, R., Hickey, L.M.B., Turner, P.W. (2007). Power scaling of singlefrequency ytterbium-doped fiber masteroscillator power-amplifier sources up to 500 W. IEEE Journal of Selected Topics in Quantum Electronics, vol. 13, no. 3, p. 546551.

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[21] Vu, K.T., Malinowski, A., Richardson, D.J., Ghiringhelli, F., Hickey, L.M.B., Zervas, M.N. (2006). Adaptive pulse shape control in a diode-seeded nanosecond fiber MOPA system. Optics Express, vol. 14, no. 23, p.

10996-11001.

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[23] Malinowski, A., Vu, K.T., Chen, K.K., Nilsson, J., Jeong, Y., Alam, S., Lin, D., Richardson, D.J. (2009). High power pulsed fiber MOPA system incorporating electrooptic modulator based adaptive pulse shaping. Optics Express, vol. 17, no. 23, p.

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Novak, V. – Petkovšek, R. – Podobnik, B. – Možina, J.

Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, 799-809 DOI:10.554.5/sv-jme.2011.077

Paper received: 05.04.2011 Paper accepted: 04.10.2011

Structural Analysis of an Articulated Urban Bus Chassis via FEM: a Methodology Applied to a Case Study Croccolo, D. ‒ De Agostinis, M. ‒ Vincenzi, N. Dario Croccolo* ‒ Massimiliano De Agostinis ‒ Nicolò Vincenzi DIEM, Department of Mechanical Engineering, University of Bologna, Italy

This paper deals with the static structural analysis of an articulated urban bus chassis, carried out with the Finite Elements Method. The purpose of this work is to simulate and forecast the structural response of the chassis, in terms of stress, strain and displacement, under several loading and constraining conditions, which aim at reflecting the actual duty cycle of the bus. A thorough interaction with the customer company allowed the authors to adequately define the loading scheme and to constrain the structure properly. Sensitivity analyses about FEM parameters have been run, in order to achieve an adequate trade off between computational time and results accuracy. Obtained results have been double checked by employing both solid (3D) and shell (2D) elements for each simulation. Eventually, the customer has been notified of critical issues and the related suggested improvements. ©2011 Journal of Mechanical Engineering. All rights reserved.

Keywords: bus, structure, frame, chassis, case study 0 INTRODUCTION The vehicle under investigation is an articulated bus characterized by a length of 18 m, realised by the joining of two chassis (Fig. 1), capable of carrying up to 160 passengers and with a mass at full load of about 30,000 kg. Urban buses, as most part of passenger vehicles, are built around a tubular chassis that bears both the weight of the vehicle itself and the weight of passengers and luggage.

Fig.1. Urban bus and respective chassis A good chassis shall also meets precise stiffness requirements in order to allow a safe drive under the most diverse traffic conditions.

The chassis is realized by means of rectangular section tubular beams (Fig. 1), having external dimensions within the range of 30 to 150 mm and *Corr. Author’s Address: DIEM - Facoltà di Ingegneria, Viale Risorgimento 2, 40136 Bologna, Italia, dario.croccolo@unibo.it

wall thicknesses within the range of 2 to 8 mm, joined each other by full welding; several added bent sheets-ribs increase the overall assembly stiffness. A structural steel with a Young’s modulus E = 200 GPa, a Poisson’s ratio ν = 0.3, a yield stress Sy = 400 MPa and a mass density ρ = 7,850 kg/m3 has been employed. Modern FEM analysis capabilities, mainly in terms of computational resources, allow the vehicle manufacturer to fix structural issues before performing field tests, hence shortening the overall design and engineering phase, as demonstrated in [1] to [4].

Despites of the noticeable size of the chassis, the single beam used to realize the chassis is generally no longer than 1.5 m. Moreover, the displacements object of investigation are several orders of magnitude smaller than the overall dimensions of the chassis. Therefore, an accurate choice of both the finite elements size and the contact elements definition is necessary. The analysis is limited to the sprung part of the vehicle: it is fundamental to remember that when the bus is performing a cornering manoeuvre, the centripetal accelerationforce given by the contact between asphalt and tyres together with the inertia forces acting on the body, make the suspension springs outside the curve compress, while those on the opposite side stretch. This results in a rolling movement of the vehicle body, which must be taken into account even when performing a static analysis, because the gravitational and inertia forces generate 799

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different stress/strain distributions within the structure depending on its position with respect to a system of coordinates which is fixed to the ground, as suggested in [5]. Such behaviour could be well simulated by a bus model, which included the whole suspensions group [6].

1 PRELIMINARY REMARKS The top level assembly of the chassis consists of about 1,500 parts, which belong to sub–structures representing, for instance, the floor, the roof and the body sides of the chassis. Due to hardware limitations, it would be impossible to analyze the whole structure (Fig. 1) at one time, hence the overall assembly has been divided into two sub assemblies, one being the front half of the chassis and one the rear part of it: from now on, they will be respectively referred to as A-chassis and B-chassis. Such a large structures would be generally analyzed by introducing rough approximations mainly concerning the loading and constraining hypotheses (uniform loads distributions [7]) or by means of beam (1D) elements [8].

The A-chassis, whose two front wheels are steering while the rear ones are fixed, comprehends the pilot’s station: two passengers doors open on the right side of this chassis. A half part of the articulation system (the device that joins the two halves of the bus allowing them to rotate respectively around the vertical axis), is installed by the rear side of the A-chassis.

The B-chassis has only two wheels on the rear and hangs on the A-chassis by means of the other part of the articulation system. This chassis has no steering devices but it carries the engine group on the rear left side: two passengers doors are on the right side. Each chassis has been analyzed under six different loading and constraining schemes (cases), which aimed at simulating the chassis behaviour under the following conditions: (a) the action of gravitational acceleration; (b) the braking at the upper deceleration limit of the vehicle; (c) the both sides cornering manoeuvres at the capsizing limit; (d) the both sides torsion due to uneven road surface.

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A Cartesian Coordinate System has been chosen [5] with its origin into centre of the articulation system: X-axis is oriented as the driving gear, Z-axis is pointing upwards and Y-axis follows the right hand rule.

2 FEA SETUP The main structural elements of steel framed structures (e.g. the bus chassis, Fig. 1) have to be studied as the assembly of three different components, namely, columns, beam and their joints (Fig. 2).

Fig. 2. Example of steel framed structure: main structural components (column, beam and joint) The capacity of steel frames to resist loads is determined more by the strength and, in particular, the stiffness of the joints than by the properties of the members themselves [9]. In practice, beam-to-column joints in conventional analysis and design of steel frameworks are usually assumed to behave either ideally pinned or fully rigid. Conversely, experimental investigations [10] show that the true behaviour of joints lies in between that of ideally compliant and fully rigid: such joints are referred to as semi-rigid joints.

Overestimating the joint stiffness may result in underestimating the forces developed in beam and column and the overall displacement of the global frame structure. Neglecting the real behaviour of the joint may lead to unrealistic predictions of the response and reliability of steel frames [11]: both of these extreme assumptions are inaccurate and uneconomic. When approaching the problem by a numerical (FEM) standpoint the latter issues occur in formulating the contact parameters between two or more structural members. When dealing with contacts there is a lot more to control

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than the mesh size type [12] and in particular the contact formulation as well as the contact stiffness parameters. Finite Elements Analyses have been performed by means of the Ansys Code, Workbench Release 12. In order to manage the contact formulation, the Augmented Lagrangian Method has been chosen as it allows, by manually setting the contact stiffness parameters, a better approximation of the interactions occurring within the contact areas of welded structures. As a matter of fact the Augmented Lagrangian is an iterative method working by two consequent steps: firstly, like a simple Penalty Method, locally modifying the bodies’ normal stiffness in the contact region until the equilibrium is satisfied. Then, if any interference (penetration) occurred, it proceeds adding a convenient pressure to the mating surfaces, until the interference is overridden. The contact stiffness parameter (normal or tangential) can be input as an absolute value (KN or KT) or as a factor (FKN or FKT) to the default Hertz contact stiffness KH, which depends on the component geometry and material. For surface-to-surface contact elements, Ref. [13] recommends a FKN value in the range from 0.001 to 100 (default value 1.0): changes in such range strongly affect the solution in terms of stresses, strains and displacements. FKN value shall be tuned by means of experimental analyses. Experimental results on a full welded T-joint, comparable in dimensions and welding method to that used on the bus chassis (Fig. 3), have been obtained by Yang and Kim [10].

Fig. 3. Full welded T-joint specimen tested in [9] In particular, they established that under the maximum force (for the linear elastic field) of 42.1 kN applied to the upper hinge a deflection

of 12 mm occurs at the same point. Numerical results in terms of upper beam displacements as a function of the FKN parameter are reported in Table 1 and shown in Fig. 4: a FKN value between 0.01 and 0.005 offers a reliable prediction of experimental results.

Table 1. FEA displacements as a function of the FKN parameter Experimental displacement ≈ 12 mm [9] FEA displacement FKN - Normal Stiffness [mm] Factor Default (1.0) 8.13 0.1 8.51 0.05 8.80 0.01 10.68 0.005 12.70 0.001 26.82

Fig. 4. Deformed shape (14× magnification) of the T-joint specimen tested in [9] as a function of the contact normal stiffness factor FKN in Table 1 The evaluation of the tensile state in the vicinity of the weld toe (e.g. hot spot stress method

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or the peak stress method recently proposed by Meneghetti et al. [14]) is not the object of this work. No welds are, accordingly, modelled in the chassis geometry; the connection between beam and column is managed by the contact stiffness parameters, as shown in [8]. In order to check the numerical structural stress state of the beam and column (far from the weld toe), a comparison with the data evaluated via strain gages in a square-tosquare hollow section T-joint [14] and [15], has been performed. Both solid (3D) mesh and shell (2D) mesh have been compared to the results obtained in the reference geometry reported in Fig. 5 [14] and [15].

The applied loads are reported in Fig. 5, while the stress results (in terms of maximum principal stress as suggested by the peak stress method [14]) in Fig. 6. An attentive examination of the results shows that at a distance almost equal to the cross section dimensions (dashed lines) the stresses evaluated both via 3D mesh and via 2D mesh (without modeling the weld) converged to the reference one [14] and [15].

As suggested in [16], tubular joints have been meshed by using shell (2D) elements that represent the mid-surfaces of the joint member walls. As shown previously and as accurately demonstrated in [17], the results are perfectly comparable with the ones obtained by means of a solid (3D) mesh: the second technique is still ten times more demanding than the first in terms of disk space.

3 FEA LOADING CASES Each load applied to the A-chassis as well as to the B-chassis has been introduced as a lumped mass. These masses undergo acceleration components imposed by the conditions described in Section 1, and are attached to one or more surfaces belonging to one or more chassis components. Using lumped masses rather than remote forces results in a speed up of the workflow when the boundary conditions have to be changed.

For example, the gravitational acceleration can

Fig. 5. The square-to-square hollow section T-joint specimen tested in [13] and [14] a)

b)

c)

Fig. 6. Comparison in stress distributions (deformed shape 50x magnification) far from the joint between beam and column (dashed lines); a) reference geometry [13] and [14] with the welded joint modelled, b) solid mesh without the weld, c) shell mesh without the weld; mesh size: 1 mm 802

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be quickly changed into the forward or lateral accelerations acting on the system.

For remote boundaries conditions, such as the lumped masses defined, Ansys allows the control of the specific geometry behaviour, which can be defined as either rigid or deformable. A deformable behaviour has been chosen here by the authors, as it represents the actual response of the structure well: in fact, on the other hand, the masses application areas would result in being unreasonably undeformable.

Since the B-chassis is attached to the A-chassis by means of an articulation device, the analyses on the B-chassis were carried out first, assuming the constraints between the B-chassis and the A-chassis to be of the hinge type, and to be applied on the edges of the articulation device.

Accordingly, when performing the corresponding analysis the reaction forces of the same magnitude evaluated on the articulation device were applied, but opposite in direction to the A-chassis.

The results in terms of total displacement and Von Mises equivalent stress distribution have been computed and analyzed for each condition.

Constraint reactions in magnitude and direction have been checked to be equal to the imposed loads in magnitude and direction as an overall verification of the simulation process.

line formed by a single edge, can be considered equivalent to an ideal hinge that locks rotational motions around X-axis and Z-axis and translation along each axis. Then, the standard gravitational acceleration g (9.81 m/s2) has been applied to the whole mass system.

Fig. 7. B-chassis: example of distributed masses related to the main body structure

3.1 B-Chassis At first, lumped masses (represented as spheres) have been applied to the chassis. In Fig. 7, for instance, 2,950 kg of distributed masses belonging to coatings and body panels are shown: each sphere has the same color of its target parts.

Masses related to onboard systems, windows and doors (1,046 kg) and those belonging to passengers’ mass (4,931 kg) have been, instead, represented in Fig. 8. The 1,500 kg engine mass has been subdivided into three lumped masses: each of them has been applied to the relevant engine mount on the chassis. The B-chassis self weight is 1,658 kg.

3.1.1 B-Chassis, Gravitational Acceleration Fixed supports have been applied both to the rear axle edges and to the articulation device edges: this type of constraints, since applied to a

Fig. 8. B-chassis loads and constraints – loading case 3.1.2 3.1.2 B-Chassis, Gravitational Acceleration and Braking Deceleration In order to simulate the effects of a severe brake, a 0.75·g acceleration [5] and [18] has been added along the positive X direction. The gravitational acceleration still acts on the system.

Moreover, the rear axle edges constraints have been redefined according to Fig. 8, allowing them to translate only along X-axis (Z-axis and Y-axis displacements are still equal to zero). At the same time, braking forces Fμ (Eq. (1)) have been applied to the lower edges of the rear axle (see Fig. 8, flag D and E), by imposing the Coulomb friction law [18] to [20]:

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Fμ = μ·FZr,B . (1) This hypothesis considers the rear wheels subjected to the braking force Fμ, while the front articulation edges hold up the remaining X-axis force Rx given by Eq. (2) (Fig. 8a): Rx = mB-chassis·0.75·g ‒ Fμ .

(2)

Such a constraint scheme produces the highest stresses and displacements on the structure because the whole B-chassis is subjected to both the bending moment and the compression force generated by the inertial loads. Conversely, in the two remaining schemes (Figs. 8b and c) loads and moments are partially supported by the rear constraint and, therefore, they are not affecting the central section of the frame.

Since FZr,B changes during the brake depending on the load transfer from the rear axle to the front support [21] and [22], the correct FZr,B value has been determined by some iterative analyses: firstly, the vertical constraint reaction on the rear axle are determined from the static equilibrium (FZr,B_1) and the simulation has been run using Fμ = μ·FZr,B_1, then the actual vertical constraint reactions on the rear axle (FZr,B_2) has been calculated. A new simulation has been run again assuming Fμ = μ·FZr,B_2 and calculating the actual vertical constraint reactions on the rear axle (FZr,B_3). The same procedure of assuming Fμ = μ·FZr,B_i and calculating the actual vertical constraint reactions (FZr,B_i+1) lasted until no significant discrepancy has been found between two subsequent values of FZr,B (FZr,B_i ≈ FZr,B_i+1).

3.1.3 B-Chassis, Gravitational Acceleration and Cornering Since the chassis is not symmetric about XZ-plane (see Fig. 7), two simulations have been run in order to evaluate the effects of both right and left cornering manoeuvres performed on a plain ground. Since the standard gravitational acceleration is always present, an acceleration vector having a magnitude of 0.75·g and directed along Y-axis, (positive or negative depending on the turning direction) has been added to the system. The constraints remain the same used for the brake simulation. It is important to verify that the rear axle reactions provided by the analysis exclude any capsizing tendency of the vehicle 804

when subjected to this loading case: both of the rear constraints must have positive reactions along Z-axis.

3.1.4 B-Chassis, Torsion The chassis could be subjected to torsion when, for example, the bus should run on an uneven asphalt mat. In order to recreate such a condition, two analyses have been performed, suppressing two of the total four constraints at a time and applying the sole standard gravitational acceleration. The supports to be suppressed have been chosen as follows: (i) left articulation constraint and right rear axle constraint; (ii) right articulation constraint and left rear axle constraint.

In this way, the chassis could twist around the axis that joints the remaining supports.

3.2 A-Chassis Lumped masses were applied to this chassis and for the B-chassis: 2901 kg of distributed masses belonging to coatings and body panels, 494 kg related to windows and doors, 5822 kg belonging to passengers and the driver and 1540 kg related to relevant systems have beenw applied to the chassis. The A-chassis self weight is 2252 kg.

Fig. 9. A-chassis loads and constraints – loading case 3.2.1

Fig. 10. A-chassis loads and constraints – loading case 3.2.2 As mentioned before, the reactions on the articulation device edges have beencarried over

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from the analyses performed on the B-chassis (opposite in direction), as shown in Figs. 9 and 10.

3.2.1 A-Chassis, Gravitational Acceleration Fixed supports have been applied both to the rear and to the front axle edges: this type of constraints is equivalent to an ideal hinge locking rotational motions around X-axis and Z-axis and translation along each axis. The standard gravitational acceleration g (9.81 m/s2) has been applied to the system.

3.2.2 A-Chassis, Gravitational Acceleration and Braking Deceleration In order to simulate the effects of a severe brake, a 0.75路g acceleration has been added along the positive X direction. The gravitational acceleration still acts on the system. Moreover, braking forces have been applied to the lower edges of the rear axle, which have been allowed to translate along X-axis, while the front axle edges have been locked towards the three components of translation. As for the B-chassis, such constraining hypotheses make the chassis working under the worst condition of free deflection length. Braking forces intensity defined by iteration, as was formerly done for the B-chassis.

3.2.3 A-Chassis, Gravitational Acceleration and Cornering Since the chassis is not symmetric about XZ-plane (the passenger doors are located on the right side), two simulations have been run in order to evaluate the effects of both right and left cornering manoeuvres performed on a plain ground. The standard gravitational acceleration still acts on the system, together with an acceleration vector directed along Y-axis with a 0.75路g magnitude (positive or negative depending on the turning direction).

The constraints are still the same used for the brake simulation. The rear axle reactions provided by the analysis exclude any capsizing tendency of the vehicle when subjected to this loading case: in fact both of the rear constraints have positive reactions along Z-axis.

3.2.4 A-Chassis, Torsion Two torsion analyses have been performed also on the A-chassis by suppressing two of the four supports at a time and applying the standard gravitational acceleration. The supports to be suppressed have been chosen as follows: (i) Left front axle constraint and right rear axle constraint; (ii) Right front axle constraint and left rear axle constraint.

In this way, the chassis can twist around the axis that joints the remaining supports.

4 FEA RESULTS AND DISCUSSION The sum of the external loads applied to the structure and the sum of the constraints reactions have been compared for each analysis in order to evaluate the overall equilibrium of the chassis and, therefore, exclude macroscopic errors affecting the results. The resume of such verification is reported in the Appendix at the end of the manuscript: the results are subdivided into four kinds of loading cases (vertical, braking, cornering, torsion) respectively, for the A-chassis and B-chassis in terms of constraints or external loads direction and magnitude. As shown in the Appendix, discrepancies are always lower than 0.3%. A stress limit equal to Sl = 150 MPa (safety factor of 2.6 with respect to the yield limit), has been chosen in accordance with the customer: the multiaxial stress states have been compared with the uniaxial material properties by means of the Von-Mises yield criterion. Even if static analyses have been performed, Meznar and Lazovic [23], Lan et al. [24] and Kim et al. [25] have demonstrated the importance of these preliminary FEA results for further experimental analyses (e.g. strain gauges as deep demonstrated in [23]) performed on typical duty cycles.

4.1 A-Chassis The A-chassis has a good overall response to every imposed loading condition, since no significant area of it exceeded the established equivalent stress limit Sl. Braking and cornering conditions, according to [23], are the most severe because stresses show up close to Sl, interesting

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a)

b) Fig. 11. Stress distributions in A (a) and B (b) chassis in most severe (cornering) loading condition

different areas depending on the loading case.

Some chassis portions underneath the floor and close to the articulation device are impacted by the effects of braking with Von-Mises stress mean values of about 100 MPa (Fig. 11a), essentially due to forces transmitted by the B-chassis via the articulation device itself. The right side pillars and the J-shaped tubular beams, which connect the roof to the right body side, have stress values of about 130 MPa when the left-cornering loading case is applied. It is worth mentioning that stresses recorded in the right-cornering loading case are much lower than those of the left-cornering loading case only due to the left-side being stiffer than the right-side, as a consequence of a lack of door holes on the right.

The rooftop area has demonstrated to always have the greatest displacement values Δ, differentiated as a function of the loading conditions: a magnitude of about 6 and 12 mm is reached when gravity and braking loading cases are applied, respectively. The peak values of about 24 and 18 mm are reached for cornering and for torsion respectively. The deformed shapes of the structure, due to the applied loads, are reported in Figs. 12 to 15, with an appropriate scale factor (20× magnification). As the performances in terms of stresses and displacements of the A-chassis have been judged to be compliant with the specifications, no structural improvement has been suggested to the customer.

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4.2 B-Chassis Left-cornering loading case (Fig. 11b) is the most severe condition for the B-chassis as well, causing wide areas of the tubular beam shown in Fig. 16a, which appreciably exceed the equivalent stress limit Sl, as Von-Mises stresses on such component assume values close to 190 MPa. Indeed, such a beam had been noticed to be a critical component for all the loading cases since it has the highest stress values in the whole structure.

Therefore, the original 3 mm thick beam has been replaced with a 5 mm thick one, and a new leftcornering simulation has been performed in order to validate the change. During the cornering manoeuvres the stress value on the body-side pillars results of about 140 MPa, as reported in Fig. 16b, which is now an adequate value: hence, the stress decrease for the proposed solution is equal to 26%. Elsewhere, the B-chassis shows a fair behaviour, since stress and displacement remain beneath the established limits.

The maximum displacement values are located on the front left engine support (Δ = 9 mm) when braking loads are applied (Fig. 13) and on the rooftop (Δ = 11 mm) when gravity loads are applied (Fig. 12). Eventually, during the cornering to the left (Fig. 14) a peak value of about 27 mm is reached on the rooftop, which becomes about 33 mm when torsion occurs (Fig. 15). As mentioned before some images of the deformed structure due to the different loading cases, are reported in Figs.

12 to15.

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Fig. 12. Total displacements – gravity loading case

Fig. 13. Total displacements – braking loading case

Fig. 14. Total displacements – left cornering loading case 5 CONCLUSIONS

Fig. 15. Total displacements – Torsion loading case (scale factor 20×) a)

The static structural analysis of an articulated urban bus chassis, with a total length of 18 m, has been performed via Finite Elements Method. The frame behaviour towards four different loading conditions, representative of its typical duty cycle, has been analysed: the action of gravitational acceleration, the braking at the upper deceleration limit of the vehicle, the cornering manoeuvres and the torsion due to uneven road b)

Fig. 16. Von-Mises equivalent stress values on the critical beam; a) original beam, b) modified beam Structural Analysis of an Articulated Urban Bus Chassis via FEM: a Methodology Applied to a Case Study

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surface. Sensitivity analyses in order to evaluate the welded joint performances have been carried out in order to obtain reliable results in terms of stiffness and displacements of the chassis (steel framed structure). Braking and cornering conditions have been demonstrated to be the most severe, especially on the B-Chassis (the rear one).

Needful improvements have been suggested to the manufacturer in order to help achieve the target strength/stiffness characteristics on the whole structure.

6 REFERENCES [1] Croccolo, D., Cuppini, R., Vincenzi, N. (2007). The design and optimization of forkpin compression coupling in front motorbike suspensions. Finite Elements in Analysis and Design, vol. 43, p. 977-988, DOI:10.1016/j. finel.2007.06.016.

[2] Croccolo, D., Cuppini, R., Vincenzi, N. (2009). Design improvement of clamped joints in front motorbike suspension based on FEM analysis. Finite Elements in Analysis and Design, vol. 45, p. 406-414, DOI:10.1016/j.finel.2008.11.007.

[3] Croccolo, D., De Agostinis, M., Vincenzi, N. (2010). Recent improvements and design formulae applied to front motorbike suspensions. Engineering Failure Analysis, vol. 17, p. 1173-1187, DOI:10.1016/j. engfailanal.2010.02.002.

[4] Croccolo, D., De Agostinis, M., Vincenzi, N. (2011). Failure analysis of bolted joints: effect of friction coefficients in torque preloading relationship. Engineering Failure Analysis, vol. 18, p. 364-373, DOI:10.1016/j. engfailanal.2010.09.015.

[5] ISO8855 (1991). Road vehicles Vehicle dynamics and road-holding ability. International Organization for Standardization, Geneva.

[6] Reimperl, J., Stoll, H., Betzler, J.W. (2001).

The Automotive Chassis: Engineering Principles (2001), II Edition, SAE International, Warrendale.

[7] Larrodé, E., Miravete, A., Fernàndez, F.J. (1995). A new concept of a bus structure made of composite materials by using continuous transversal frames.

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Composite Structures, vol. 32, p. 345-356, DOI:10.1016/0263-8223(95)00060-7.

[8] Gauchia, A., Diaz, V., Boada, M.J.L., Boada, B.L. (2010). Torsional stiffness and weight optimization of a real bus structure.

International Journal of Automotive Technology, vol. 11, p. 41-47, DOI:10.1007/ s12239-010-0006-4.

[9] Elnashai, A.S., Elghazouli, A.Y. (2003).

Seismic behaviour of semi-rigid steel frames.

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[10] Yang, C.M., Kim, Y.M. (2007). Cyclic behaviour of bolted and welded beam-tocolumn joints. International Journal of Mechanical Science, vol. 49, p. 635-649, DOI:10.1016/j.ijmecsci.2006.09.022.

[11] Hadianfard, M.A., Razani, R. (2003). Effects of semi-rigid behaviour of connection in the reliability of steel frames. Structural Safety, vol. 25, p. 123-138, DOI:10.1016/S01674730(02)00046-2.

[12] Lanoue, F., Vadean, A., Sanschagrin, B. (2009). Finite element analysis and contact modelling considerations of interference fits for fretting fatigue strength calculations.

Simulation Modelling Practice and Theory, vol. 17, p. 1587-1602, DOI:10.1016/j. simpat.2009.06.017.

[13] Ansys Theory Manual 12.0.

[14] Meneghetti, G., Atzori, B., Manara, G. (2010). The peak stress method applied to fatigue assessments of steel tubular welded joints subjected to mode I loading.

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[15] Chiew, S.P., Lee, C.K., Lie, S.T., Ji, H.L. (2007). Fatigue behaviors of square-tosquare hollow section T-joint with corner crack: experimental studies. Engineering Fracture Mechanics, vol. 74, p. 703-720, DOI:10.1016/j.engfracmech.2006.06.022.

[16] Lee, M.M.K. (1999). Strength, stress and fracture analyses of offshore tubular joints using finite elements. Journal of Constructional Steel Research, vol.

51, p. 265-286, DOI:10.1016/S0143974X(99)00025-5.

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[17] Pey, L.P., Soh, A.K., Soh, C.K. (1995).

Partial implementation of compatibility conditions in modeling tubular joints using brick and shell elements. Finite Elements in Analysis and Design, vol. 20, p. 127-138.

DOI:10.1016/0168-874X(95)00020-T. [18] ISO611 (2003). Road vehicles ‒ Braking of automotive vehicles and their trailersVocabulary. International Organization for Standardization, Geneva.

[19] Pacejka, H. (2005). Tyre and Vehicle Dinamycs, Elsevier, Amsterdam.

[20] Watanabe, K., Yamakawa, J., Tanaka, M., Sasaki, T. (2007). Turning characteristics of multi-axle vehicles. Journal of Terramechanics, vol. 44, p. 81-87, DOI:10.1016/j.jterra.2006.01.007.

[21] Corno, M., Savaresi, S.M., Tanelli, M., Fabbri, L. (2008). On optimal motorcycle braking. Control Engineering Practice, vol.

16, p.

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[22] Genta, G., Morello, L. (2002). The Automotive Chassis Vol. 1: Components design. Springer.

[23] Meznar, D., Lazovic, M. (2010). The strength of the bus structure with the determination of critical points. Strojniski vestnik – Journal of Mechanical Engineering, vol. 56, p. 544550.

[24] Lan, F., Chen, J., Lin, J. (2004).

Comparative analysis for bus side structures and lightweight optimization.

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[25] Kim, M.H., Suh, M.W., Bae, D.H. (2001).

Development of an optimum design technique for the bus window pillar member. Proceedings of the Institution of Mechanical Engineers, Part D – Journal of Automobile Engineering, vol. 215, p. 11-20, DOI:10.1243/0954407011525421.

APPENDIX Here below the comparison between constraints reactions and applied external loads is reported: results are subdivided into the four kinds of loading cases, respectively for the A-chassis and B-chassis.

A-chassis 3.2.1 GRAVITY ONLY CONSTRAINT NAME

LEFT FRONT RIGHT FRONT LEFT REAR RIGHT REAR TOTAL REACTION EXTERNAL LOAD ERROR 3.2.2 BRAKE CONSTRAINT NAME

LEFT FRONT RIGHT FRONT LEFT REAR RIGHT REAR TOTAL REACTION EXTERNAL LOAD ERROR 3.2.3 CORNERING CONSTRAINT NAME

LEFT FRONT RIGHT FRONT LEFT REAR RIGHT REAR TOTAL REACTION EXTERNAL LOAD ERROR 3.2.4 TORSION CONSTRAINT NAME

LEFT FRONT RIGHT REAR TOTAL REACTION EXTERNAL LOAD ERROR

UNIT N N N N N N % UNIT N N N N N N % UNIT N N N N N N % UNIT N N N N %

X 1,554 1,860 11,363 11,018 25,795 25,796 0.004

DIRECTION Y -6,554 6,541 -18,517 18,530 0 0 0.000

Z 31,199 28,781 42,098 40,907 142,985 142,987 0.001

X -32,659 -32,098 0 0 -64,757 -64,764 0.011

DIRECTION Y -12,976 12,976 0 0 0 0 0.000

Z 41,133 38,219 41,543 40,850 161,745 161,767 0.014

X 0 0 0 0 0 0 0.000

DIRECTION Y 16,010 30,689 17,343 52,678 116,720 116,723 0.000

Z 11,504 49,680 8123 73,627 142,934 142,937 0.002

X 1,394 24,424 25,818 25,796 0.085

DIRECTION Y -3,575 2,387 -1,188 -1,185 0.253

Z 59,909 83,076 142,985 142,987 0.001

B-chassis 3.1.1 GRAVITY ONLY CONSTRAINT NAME

UNIT

REAR AXLE ARTICULATION

N N

X -25,796 25,796

TOTAL REACTION EXTERNAL LOAD ERROR 3.1.2 BRAKE CONSTRAINT NAME

N N %

0 0 0.000

REAR AXLE ARTICULATION

N N

X 0 -31,726

TOTAL REACTION EXTERNAL LOAD ERROR 3.1.3 CORNERING CONSTRAINT NAME

N N %

-31,726 -31,735 0.028

DIRECTION Y Z 103 84,308 -103 34,193

0 118,501 0 118,513 0.000 0.010

RHS REAR AXLE LHS REAR AXLE ARTICULATION

TOTAL REACTION EXTERNAL LOAD ERROR 3.1.4 TORSION CONSTRAINT NAME

N N N

N N %

X 0 0 0

0 0 0.000

DIRECTION Y Z 65,934 81,846 5,542 21,292 19,155 15,363

90,631 118,501 90,638 118,513 0.008 0.010

REAR AXLE ARTICULATION TOTAL REACTION EXTERNAL LOAD ERROR

N N N N %

X -14,424 14,424 0 0 0.000

DIRECTION Y Z 5,516 102,050 -5,516 16,451 0 118,501 0 118,513 0.000 0.010

UNIT

UNIT

UNIT

DIRECTION Y Z 1,185 103,090 -1,185 15,413

0 118,503 0 118,513 0.000 0.008

Structural Analysis of an Articulated Urban Bus Chassis via FEM: a Methodology Applied to a Case Study

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Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, 810-818 DOI: 10.5545/sv-jme.2010.248

Paper received: 07.12.2010 Paper accepted: 02.08.2011

Calculation of the Combined Torsional Mesh Stiffness of Spur Gears with Two- and Three-Dimensional Parametrical FE Models Kiekbusch, T. ‒ Sappok, D. ‒ Sauer, B. ‒ Howard, I. Timo Kiekbusch1,* ‒ Daniel Sappok1 ‒ Bernd Sauer1 ‒ Ian Howard2 1 University of Kaiserslautern, Institute for Machine Elements, Gears and Transmissions, Germany 2 Curtin University Perth, Department of Mechanical Engineering, Australia

The torsional mesh stiffness is one of the most important characteristics of spur gears. This paper presents the development of detailed two- and three-dimensional finite element models which can be used to calculate the torsional mesh stiffness. Using the parametrical design language of the FE software ANSYS the models offer the possibility to generate various different pairs of spur gears and include an adaptive meshing algorithm for the contact zones. Due to the short computation times the 2D model is well suited to simulate a variety of different gear pairs in a short time period. The more complex 3D model features more options in terms of investigating tooth face modifications for further studies. The resulting values of the torsional stiffness can be used – for example – in multi body simulations of gearboxes.

The results from the 2D FEA are used to derive a simple formula for the combined torsional stiffness of spur gears in mesh. The results presented are based on the individual stiffness of the three main components – body, teeth and contact. Hence, the introduced formula uses these three parts to determine the overall stiffness for a wide range of gears and gear ratio combinations.

Finally, the results from both the two- and three-dimensional finite element model and the derived formula are compared and the results from the 3D model are checked against results obtained by analytical equations. ©2011 Journal of Mechanical Engineering. All rights reserved.

Keywords: gear dynamic modelling, spur gear, finite element modelling, torsional mesh stiffness, contact stiffness 0 INTRODUCTION

1 THE FINITE ELEMENT GEAR MODELS

Gears are the main component of many different kinds of rotating machinery and they are often a critical part to the function of the machinery. There have been many attempts in recent years to understand and describe the process of the meshing of spur gears. As the process of meshing is very complex and difficult to describe, the finite element analysis is the method of choice to investigate the underlying relationships.

The approach used in this paper to determine the stiffness of spurs gears in mesh is the development of a FE model of the complete gear arrangement. Recent studies on this topic [1] to [4] have shown that these models produce the best results in comparison with single-tooth models or partial teeth models. As computer hardware and FE software advances, working with these rather complex models is now feasible.

In the following sections the development of the two- and three-dimensional finite element models is described. This involves the fully parametrical creation of the gear shape, the meshing and the simulation process including the adaptive meshing.

810

1.1 2D FE Model Fully parametrical APDL-scripts (ANSYS Parametric Design Language) are used to describe the geometry of the gears and to control the simulation process. The first step is the generation of the geometry consisting of lines and areas, which in the next step are meshed with a relatively coarse FE-mesh. The result is a FE model of pinion and gear which is shown in Fig. 1.

The process of creating various different pairs of gears is automated with the powerful

*Corr. Author’s Address: Institute for Machine Elements, Gears and Transmissons, Gottlieb-Daimler-Str., 67663 Kaiserslautern, Germany, timo.kiekbusch@mv.uni-kl.de

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design language of ANSYS. Without changing the model itself many parameters like modulus, number of teeth, shaft radius etc. can easily be varied as all parameters are provided in a data input file. This file is loaded during the preprocessing.

Fig. 1. Result from the first APDL-script; premeshed geometry As the contact between pinion and gear is the most important and critical part of the gear simulation the description of the tooth involutes and feet require the highest possible precision during the modelling process. Therefore, both geometric details are generated by using a very high number of keypoints. The location of these points is calculated on the basis of the data input file. The keypoints are connected with splines to represent the outline of the tooth geometry.

After adding the gear body the areas created are meshed with a coarse mesh. This model (see Fig. 1) is the basis for the next steps of the modelling process namely the mesh refinement at the contact point(s). This refinement is realised using another APDL-Script. The result is shown in Fig. 2. This script also adds the constraints, contact elements and different torque loads to the model, solves and post processes the job. The constraints used in the model are as follows: the hub of the gear is completely constrained from motion; the nodes at the hub of the pinion can only rotate around the centre of the pinion. The torque is applied using a force on every node at the driving gear’s hub, adding up to the specified torque.

A quasi-static simulation method is used for the determination of the mesh stiffness. This means that the stiffness is calculated at several

different angular positions of the gears. Therefore, the gears have to be rotated to the corresponding position before the model is solved. At every roll angle the contact point(s) change so that the mesh refinement at the contact point(s) require an adaptive remesh algorithm which takes into account the actual contact situation.

a) b) Fig. 2. Adaptive refining of the mesh at the contact point(s); a) remeshed contact for single contact, b) remeshed contact for double contact During the automated postprocessing the following results are extracted from the model: combined torsional mesh stiffness, deformation of gear body, teeth and contact zone for both pinion and gear. This data is written to a text file for further processing.

When analysing gears with tooth shape errors or profile correction, an initial gap between the teeth can occur. In order to create a very flexible model which can be used in further studies, the model was built to be able to solve problems including an initial gap between meshing teeth. Typically a static FE model cannot be solved if some parts of the model do not have enough constraints as the stiffness matrix becomes singular [5]. This can happen, for example, if a contact is not initially closed and in this case the pinion can rotate a small angle without any resistance or stiffness.

To avoid rigid body motion, a weak spring is attached to the pinion. This small stiffness prevents the unintentional rotational motion. This spring is only used for the initial simulation step with a very small torque just to get the teeth in contact. When the actual load torque is applied, the spring is disabled using the birth/death of elements command [6]. This approach can handle much bigger gaps (rigid body motion) than the

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automatic adjustment which tends to be used by default.

1.2 3D FE Model There are some restrictions when using a two dimensional FE model for the analysis of spur gears in mesh. For example, simulating helical gears or applying tooth face modifications in the direction of the rotation axis of the gear like crowning or face angle correction is not possible. By using a three dimensional FE model these restrictions can be evaded. Furthermore, the influence of misalignment between the rotation axes of the gears as well as shaft centre distance changes on the mesh stiffness can be investigated.

The disadvantages of a three dimensional versus a two dimensional model are higher complexity and hardware requirements as well as computation times.

In order to overcome the restrictions that are given by the 2D model a three dimensional FE model built on the latter was developed. Again a fully parametrical approach was used to create the shape of the gears and an initial coarse mesh. In addition the model was designed to support the modelling of helical gears and the application of tooth face modifications for further studies.

To create the gear model, each gear wheel is sliced into multiple layers along the rotation axis. For each layer, the two dimensional shape of the gear is modelled by keypoints which are connected with straights, arcs and splines. The tooth involutes and roots are described with a high number of keypoints to provide a very high accuracy like in the 2D model. As each 2D shape of the gear is built separately, the tooth shape can be changed along the rotation axis. This procedure is a prerequisite to model helical gears and to apply the above mentioned tooth face modifications.

Only the teeth which are involved in one mesh cycle are created for each gear. The surrounding teeth have only a subsidiary influence on the mesh stiffness but demand a higher amount of elements which leads to higher computation times and hardware requirements. The gear bodies are completely modelled.

During the modelling process the gears are completely meshed using a mapped meshing with hexahedral elements. In this stage, the mesh 812

in the area of contact, that is on the tooth faces on the loaded side of the teeth, is comparatively coarse. Mesh transitions between the body areas adjacent to modelled teeth and the rest of the gear body are used to reduce the amount of elements.

A completely meshed gear pair is shown in Fig. 3.

Fig. 3. Meshed gear pair created during the model build process A relatively fine mesh is needed to accurately simulate the non-linear contact deformation between the tooth faces. There are two options to refine the mesh in the area of contact during the simulation process. The first one is to adaptively refine the mesh at the position(s) were the tooth face contact(s) occurs.

The second one is to refine the complete tooth faces on the loaded side of the teeth. Fig. 4 shows the mesh refinement for two meshing angles with the first option.

The modelling process generates a model database file and a parameter file which comprises all necessary parameters. These files are used by an APDL-script in which the boundary conditions and the contact elements are created, the solving process is started and finally a fully automated postprocessing is conducted.

In order to close an initial gap between the tooth faces and avoid rigid body motion, the usage of a small stiffness spring attached to the driving gear’s hub is adopted from the 2D model.

This procedure is extended with a method to compute the spring stiffness and the initial torque load according to the actual gap size. Thereby the amount of simulation steps to close the initial gap can be reduced to a minimum which consequently reduces the simulation time.

During the postprocessing the same results as within the 2D model are extracted.

Kiekbusch, T. ‒ Sappok, D. ‒ Sauer, B. ‒ Howard, I.

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a) b) Fig. 4. Adaptive refining of the mesh at the contact point(s) for the 3D model; a) remeshed contact for single contact, b) remeshed contact for double contact

That is the combined torsional mesh stiffness, the deformation of gear body, teeth and contact zone for pinion and gear.

2 THE COMBINED TORSIONAL MESH STIFFNESS The definition of the combined torsional mesh stiffness Km used in this paper is the quotient of input load T [Nm] and driving gear hub rotation under load T.E. [rad] [6] to [8]:

Km =

T . (1) T .E .

This definition can be used for the dynamic simulation of spur gear systems as it directly describes the relation between load torque and relative motion of the gears.

In 2001, Jia [2] introduced a common formula describing the combined torsional mesh stiffness by body and tooth bending stiffness. This simplification neglects the effect of the applied torque on the gearing's stiffness. The results from the FE model, however, show that there is an influence of the torque on the resulting combined torsional mesh stiffness as shown in Fig. 5.

The studies of the deformational behaviour of the gear wheels show that the stiffness of gear body and teeth are almost load-independent while the contact deformation is non-linear, as a Hertzian contact occurs between the teeth of the gears. The influence of the applied torque on the component’s stiffness is shown in Fig. 6. The position and width of the handover region between single and double contact zone are also torque-dependent which has also been shown previously [1].

Fig. 5. Torsional mesh stiffness for a complete mesh cycle with different torque loads (model with 1:1 gear ratio, 23 teeth, modulus 6 mm, steel) Calculation of the Combined Torsional Mesh Stiffness of Spur Gears with Two- and Three-Dimensional Parametrical FE Models

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Fig. 6. Influence of the applied torque on body, teeth and contact stiffness (model with 1:1 gear ratio, 23 teeth, modulus 6 mm, steel) 3 TORSIONAL STIFFNESS OF A SINGLE GEAR The description of the torsional mesh stiffness is based on the assumption that the stiffness of body, teeth and contact zone can be considered to act like three springs in a row, which means that the combined stiffness Ki [Nm/rad] for each pinion and gear can be calculated as:

(

Ki = K B ,i −1 + KT ,i −1 + KC ,i −1

)

−1

, (2)

where KB,i is the gear body stiffness, KT,i the tooth stiffness and KC,i the contact stiffness with i being P or G for pinion resp. gear.

Fig. 7. Nodes selected to determine the component’s deformation It has to be mentioned that these values are not the actual stiffness values of the particular component, but items which help to develop the common formula for the mesh stiffness. In the following pages the combined gear stiffness Ki is used to calculate the stiffness for the two gears in mesh. The values of KB,i, KT,i and KC,i can be obtained from the FE model. For this purpose 814

the deformations of body, teeth and contact zone have to be separated. In the FE model different nodes are used to read out their displacements.

These displacements are put into relation with the applied torque which results in the component’s stiffness. The nodes which are chosen to receive the deformation data are placed at the shaft radius, the dedendum radius and at the radius of contact; in each case in the middle of the tooth in contact and in addition one node at the contact point. Fig. 7 shows the placement of the selected nodes for a single tooth pair in contact.

3.1 Stiffness for the Driving Gear with a Single Pair of Teeth in Contact The description of the torsional stiffness of the different components requires an analysis which gearing parameters affect the particular stiffness. The relevant parameters for each component are pointed out in the corresponding sections.

The range of the gear model parameters which have been used for this research is shown in Table 1.

Table 1. Range of parameters for the models used in this research Parameter Number of teeth z Modulus m Torque load T Gear ratio u

Kiekbusch, T. ‒ Sappok, D. ‒ Sauer, B. ‒ Howard, I.

Unit [-] [mm] [Nm] [-]

Min 7 3 0.1 0.34

Max 50 15 1000 2.94

Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, 810-818

3.1.1 Stiffness of the Gear Body The stiffness of the gear body is assumed to only depend on the following parameters: shaft radius rs dedendum radius rd, face width w and Young’s modulus E. A simplified model (see Fig. 8) is used to analyse the influence of the above mentioned parameters on the body’s stiffness. Different combinations of parameters were used to come to the following Eq.:

1.6

K B , P = cB ⋅ E ⋅ w ⋅ ln ( rd − rs )

⋅ rs1.6 , (3)

where cB is a coefficient which was found out to be 9.555e-4.

The results from this equation reproduce the results from the FE model within an accuracy of about 5% for gear bodies with a outer radius between 10 and 200 mm and various inner radii.

where cT is a coefficient which was found out to be 3.2e-5. The results from this Eq. are within 7% of the results from the FE model for the parameter range analysed.

3.1.3 Stiffness of the Teeth Contact In contrast to the stiffness mentioned above, the stiffness of the contact between the meshing teeth is highly non-linear with the load as it is a Hertzian contact between two curved surfaces. Therefore, the load torque T needs to be considered in the equation describing the contact stiffness. In addition, the following parameters have been found out to have a significant influence: modulus m and number of teeth z, contact radius, Young’s modulus E and face width w. The contact stiffness is approximated by:

KC , P = cC ⋅ E ⋅ w ⋅ m1.85 ⋅ z 2 ⋅ T 0.105 , (5)

where the coefficient cC is 7.937e-5. The results for the contact stiffness were found to be within 10% of the results from the FE model.

3.2 Stiffness for the Driven Gear with a Single Pair of Teeth in Contact

Fig. 8. Simplified model of the gear body with applied constraints and forces 3.1.2 Bending Stiffness of the Teeth As the teeth basically bend under load, the assumption is made that the parameters that influence the stiffness of the teeth are the same as those of a bending beam. These parameters are: height and width w of the teeth and Young’s modulus E. The influence of the radius at which the tooth is located and the tooth height was taken into account with the parameters modulus m and number of teeth z. This results in the stiffness of the tooth KT,P:

KT , P = cT ⋅ E ⋅ w ⋅ m 2 ⋅ z 2.2 , (4)

The stiffness values determined above are related to the driving gear (index P). With the given gear ratio u, the driven gear’s stiffness (index G) can be directly deduced taking into account that both load torque and radius of contact are u-times the respective torque and contact radius of the driving gear. So the body, teeth and contact stiffnesses are calculated by: 2

z  Ki ,G = Ki , P ⋅ u 2 = Ki , P ⋅  P  , (6)  zG  with i being the indices B, T and C for body, teeth and contact. It has to be mentioned that the parameters of the driven gear have to be used for the calculation of its stiffness (e.g. zG, EG).

3.3 Stiffness in the Double Contact Zone The body’s stiffness value in the double contact zone is obtained by adding the factor fB. The only difference between single and double contact for the body stiffness is the width of the

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area which is affected by the load. This influence is considered to be rather small which was confirmed by the FE model. Hence, the factor fB was found to be equal to 1.1: , Ki , B ,double = f B ⋅ Ki , B ,single = 1.1 ⋅ Ki , B ,single. (7) The description of the double contact zone teeth stiffness assumes that both teeth pairs share the load equally. This means that each tooth’s deformation is half the deformation as in the single contact zone. This results in a stiffness value twice as high as for the single contact:

Ki ,T ,double = 2 ⋅ Ki ,T ,single . (8)

The evaluation of the contact stiffness shows that the difference between single and double contact cannot only be described by a simple factor but by an additional change of the exponent for the applied load (cf. Eq. (5)):

Ki ,C ,double = cC ⋅ E ⋅ w ⋅ m1.85 ⋅ z 2 ⋅ T 0.068 , (9)

where cC is 1.1905e-4.

3.4 The Formula for the Combined Torsional Mesh Stiffness The stiffness for the single gear (KP, KG) is calculated by assuming all three springs in a row as mentioned above. The combined torsional mesh stiffness Km can be derived by considering the two gears as two springs in series (see Fig. 9):

(

K m = K P −1 + KG −1

)

−1

. (10)

The results from the formula are within 10% from the 2D FE model’s results for most input parameter sets. This accuracy seems to be good enough for most of the cases where the formula can be used as there are many other different factors which influence the value of the mesh stiffness, like the shaft to collar connection and the lubrication of the gears.

Fig. 9. Simplified model of the combined torsional mesh stiffness of two gears in mesh A simple modification during the calculation can be made to take into account the small variation of the stiffness inside the single or double contact zone. Adding a quadratic correction term will result in a lower difference between the simulated and the calculated values over the whole mesh cycle (see Fig. 10). The modified stiffness Km* against the distance from the actual relative roll angle to the centre of the single resp. double contact zone ΔΩ is:

K m * = K m ⋅ (1 − c ⋅ ∆©2 ), (11)

with the factor c which has to be adapted on basis of the FE model.

The determination of the position and width of the hand-over region, however, still needs further investigation which can be done using the FE model.

Fig. 10. Comparison of the FE results with quadratic correction term (Eq. (11)) 816

Kiekbusch, T. ‒ Sappok, D. ‒ Sauer, B. ‒ Howard, I.

Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, 810-818

4.1 2D – 3D – Mesh Stiffness Formula

4 RESULTS Due to the fact that the FEA of a spur gear pair is complex and therefore prone to errors the results have to be checked for plausibility.

Hence the results from the 2D and 3D model as well as the mesh stiffness formula are compared with each other. Additionally, the results from the 3D model are checked against analytical results according to DIN 3990 [9]. A variety of gear pairs has been used for this comparison. Table 2 shows an excerpt from the compared gear pairs.

Table 2. Investigated gear pairs (excerpt) Variant-No. z1 z2 m [mm] α [°] w1 [mm] w2 [mm] Rs1 [mm] Rs2 [mm] E-Modulus [GPa]

1 23 23 6 20 16 15 15 15 210

2 25 25 2 20 28 24 8 8 210

3 36 43 11 20 58 50 80 100 210

4 13 31 4 20 16 15 10 20 210

The torsional mesh stiffness has been calculated for the single contact zone (SCZ) and the double contact zone (DCZ) to check if the two FE models and the mesh stiffness formula produce consistent results.

Table 3 shows the stiffness results and the proportion between FE and mesh stiffness formula for each FE model.

The results show a maximum deviation of 10% that implies that the FEA and mesh stiffness formula produce consistent results for the torsional mesh stiffness.

4.2 3D – DIN 3990 DIN 3990 provides methods to calculate the load capacity of cylindrical gears which includes the determination of the tooth spring stiffness. That is the normal tooth load which is needed to deform a meshing tooth pair with 1 mm tooth width perpendicular to the tooth involute for 1 mm. This deformation corresponds to the base circle arc length and thus a rotation angle which can be converted into the before defined torsional mesh stiffness. These results are compared to

Table 3. Comparison of the torsional mesh stiffness (Km_MSF) between 2D (Km_2D), 3D FE simulation (Km_3D) and the mesh stiffness formula (italic: proportion values) Variant-No. Torque Load [Nm] Km_MSF [106 Nm/rad] SCZ Km_2D [106 Nm/rad] Km_3D [106 Nm/rad] Km_MSF [106 Nm/rad] DCZ Km_2D [106 Nm/rad] Km_3D [106 Nm/rad]

1 1000 0.562 0.573 0.594 0.722 0.751 0.781

1.00 1.02 1.06 1.00 1.04 1.08

2 75 0.152 0.146 0.154 0.206 0.207 0.215

1.00 0.96 1.01 1.00 1.00 1.04

3 1000 22.8 20.2 22.1 31.4 29.7 30.9

1.00 0.89 0.97 1.00 0.95 0.98

0.100 0.098 0.102 0.138 0.141 0.145

4 100

1.00 0.98 1.02 1.00 1.02 1.05

Table 4. Comparison of the torsional mesh stiffness between 3D FE simulation (Km_3D) and DIN 3990 (Km_DIN) Variant-No. Torque Load [Nm] Tooth Spring Stiffness [N/(mm∙μm)] Linear Distributed Load [N/mm] Km_3D [106 Nm/rad] SCZ Km_DIN [106 Nm/rad] Km_3D / Km_DIN

1 1000 15.1 966 0.594 0.652 0.91

2 75 15.0 125 0.154 0.159 0.97

3 1000 17.4 101 22.1 24.0 0.92

Calculation of the Combined Torsional Mesh Stiffness of Spur Gears with Two- and Three-Dimensional Parametrical FE Models

4 100 14.4 256 0.102 0.103 0.99 817

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results obtained by the 3D FE model in the single contact zone (SCZ) which is shown in Table 4.

The results from the 3D FE model reproduce the results from DIN 3990 within an accuracy of about 10%. This shows that the results from the FEA are good. In particular if the real contact situation is taken into account which includes effects like friction, lubrication and tolerances a deviation of 10% has to be regarded little.

5 CONCLUSION This paper presents detailed two- and threedimensional FE models to create a set of gears and simulate the torsional mesh stiffness for one mesh cycle. By using the ANSYS parametric design language the models are fully parametric and both models feature an adaptive meshing algorithm for the contact zones.

The 2D FE model is used to derive a simple formula for the combined torsional mesh stiffness of spur gears in mesh. This formula uses the three main parts of a gear – body, teeth and contact – to calculate the overall stiffness of the gear pair. The 3D FE model is introduced as the basis for further studies. With a 3D model it will be possible to simulate helical gears or apply tooth face modifications like crowning or face angle corrections. Furthermore, the influence of meshing interferences like misalignment between the gears axes due to shaft, bearing or housing deformation and tolerances can be investigated.

The resulting values from the FE models and the mesh stiffness formula can be used in dynamic simulations such as multi body simulation of gearboxes. A benefit of the developed formula is the fact that only the basic gearing parameters are needed to derive the torsional mesh stiffness.

Still the FE models feature the option to analyse stresses in critical areas of the gears – for example the tooth root ‒ as well as the contact pressure on the tooth faces.

A comparison of the results with each other and with analytical equations shows the plausibility of both FE models and the mesh stiffness formula. When compared to the real

818

contact situation the conducted simulations and calculations involve some simplifications – that is for example lubrication, friction and tolerances.

Due to this the obtained results and the determined deviations are completely satisfying.

6 REFERENCES [1] Wang, J., Howard, I. (2004). The torsional stiffness of involute spur gears. Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science, vol. 218, no. 1, p. 131-142.

[2] Jia, S., Howard, I., Wang, J. (2001). A common formula for spur gear mesh stiffness. Proceedings of the JSME International Conference on Motion and Power Transmissions, p. 1-4.

[3] Wang, J., Howard, I. (2005). Finite element analysis of high contact ratio spur gears in mesh. Journal of Tribology, vol. 127, no. 3, p. 469-483.

[4] Jia, S., Howard, I. (2006). Comparison of localised spalling and crack damage from dynamic modelling of spur gear vibrations.

Mechanical Systems and Signal Processing, vol. 20, no. 2, p. 332-349.

[5] Madenci, E., Guven, I. (2006). The finite element method and applications in engineering using ANSYS. Springer-Verlag, New York.

[6] Wang, J., Howard, I. (2006). Comprehensive analysis of spur gears in mesh with various types of profile modifications. Proceedings International Conference on Mechanical Transmissions, p. 42-47.

[7] Sirichai, S. (1999). Torsional properties of spur gears in mesh using nonlinear finite element analysis. PhD Thesis. Curtin University, Perth.

[8] Wang, J. (2003).

Numerical and experimental analysis of spur gears in mesh.

Curtin University, Perth.

[9] DIN 3990 T1 (1987). Tragfähigkeitsberechnung von Stirnrädern - Calculation of load capacity of cylindrical gears.

Deutsches Institut für Normung. Berlin.

Kiekbusch, T. ‒ Sappok, D. ‒ Sauer, B. ‒ Howard, I.

Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, 819-825 DOI:10.5545/sv-jme.2010.131

Paper received: 10.6.2010 Paper accepted: 24.6.2011

Implementation of Automatic Identification Technology in a Process of Fixture Assembly/Disassembly Ostojic, G. ‒ Stankovski, S. ‒ Vukelic, D. ‒ Lazarevic, M. ‒ Hodolic, J. ‒ Tadic, B. ‒ Odri, S. Gordana Ostojic1,* ‒ Stevan Stankovski1 ‒ Djordje Vukelic1 ‒ Milovan Lazarevic1 ‒ Janko Hodolic1 ‒ Branko Tadic2 ‒ Stevan Odri1 1 University of Novi Sad, Faculty of Technical Science, Serbia 2 University of Kragujevac, Faculty of Mechanical Engineering, Serbia

Radio Frequency Identification (RFID) technology presents automatic identification technology that can be used in product life cycle various phases, especially in the manufacturing phase. The analysis of possible application of RFID technology in machining and inspection operations for fixture manufacturing assembly/disassembly process is presented in this paper. Furthermore, assemble/disassemble fixture manufacturing system structure and conception is presented. An analysis involves hardware and software components that the designed system for assembly/disassembly needs to have. Suggested system verification was done in laboratory conditions. Verification uses ninety-six parts and adequate fixtures. The paper concludes with final remarks, discussing advantages and disadvantages of the developed system. ©2011 Journal of Mechanical Engineering. All rights reserved.

Keywords: fixture, automatic identification, RFID technology

0 INTRODUCTION Modern manufacturing is characterized by large product ranges, frequent changes of production programmes, constant demands for higher product quality, etc. Two important factors which allow the integration of the mentioned characteristics are scheduling and organisation.

Scheduling represents the allocation of scarce sources to tasks in a definite period of time [1].

One of the basic requirements of every production system is production optimization, which is the minimization of production time and costs, on the one hand, and maximization of profit, on the other [2]. The efficiency of the machining process is influenced by numerous parameters, one of which is fixtures. Fixtures are used to reliably locate and clamp the workpiece during a number of operations (machining, welding, inspection, etc.), which are all part of manufacturing. Fixtures significantly influence the output effects of the production process, bearing in mind that costs of fixture manufacture can contribute to the total manufacturing costs [3] by more than 20%. Such percentage ratio confirms the fact that those are expensive and high-quality devices, while their design and manufacture deserve special attention.

Considering the costs and time consumption, the manufacture of novel fixture design solutions can be broken down into two key processes: • fixture design process, • fixture assembly/disassembly process.

All other processes needed to manufacture a fixture (e.g. inspection of assembly precision) require less time while incurring lower costs.

The majority of investigations have so far focused on the specific area of development of methodologies for fixture design. So far, many different approaches have been attempted in the research of fixture design. Specifically, it can be discerned between several fields of research in this area. Most of them refer to the development of fixture design systems and the development of methodologies for fixture design optimization.

Numerous techniques have been employed to optimize fixture design including artificial neural networks (ANN) [4], finite element analysis (FEA) [5], genetic algorithms (GA) [6], as well as combinations of some methods such as FEA and GA [7], ACA and FEA [8], and GA and ant colony algorithm (ACA) [9]. Those techniques of optimization were generally focused on defining optimal location for certain fixture elements, predominantly the locating and/or clamping

*Corr. Author’s Address: University of Novi Sad, Faculty of Technical Science, Trg Dositeja Obradovica 6, 21000 Novi Sad, Serbia, goca@uns.ac.rs

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elements. Minimization of workpiece deformation under the influence of forces was the most often used goal function. This allowed a definition of all the suitable locations for particular fixture elements. Nevertheless, the main drawback of this approach stems from the fact that it fails to provide a selection of specific fixture elements as well as the final fixture solution. These questions were addressed by the research focused on the development of fixture design systems.

With respect to the development of fixture design systems, there exist a number of viable approaches: expert systems [10] and [11], systems based on ANN [12], systems based on case-based reasoning (CBR) methodology [13] and [14], and other knowledge-based systems [15] and [16]. Each of them can be attributed particular advantages and disadvantages.

Much less attention has been paid to the problem of fixture assembly/disassembly. The most frequently applied methodologies have been: feature-based methodology [17] and [18], knowledge-based methodology [19], geometrybased methodology [20] and [21], and virtual reality-based methodology [22]. To verify their methodologies, researchers have used various CAD systems. However, the main issue with these research works is that they pertain to fixture assembly planning while disregarding the process of physical fixture assembly/disassembly. These methodologies are based on theoretical knowledge and software simulations of fixture assembly which have not been tested in practice, either in laboratory or in industrial environment.

Assembly and disassembly are parts of the production process and are crucial factors in the competitiveness of the industry in general [23].

These processes have a growing impact on fixture manufacture considering that complex workpieces require complex fixtures. Some fixture designs contain more than 150 elements. A large number of fixture elements also prolongs the time required for fixture assembly/disassembly. According to the published research, the time and cost share of the assembly and disassembly ranges from 15 to 25%, depending on the number of fixture elements and its complexity. This percentage indicates that special attention must be paid to the very process of efficient assembly/disassembly of fixtures in 820

order to increase productivity and lower the total costs of manufacture.

Bearing in mind their importance for the production process, the automation of fixture design, assembly/disassembly and identification should be realized to advance the complete process. The automation should primarily involve automatic identification of fixtures during the assembly/disassembly. In this paper, the emphasis is placed on the improvement of the process through shortening of their duration, through automated identification of fixture elements and fixture assemblies.

1 RFID TECHNOLOGY Radio Frequency Identification (RFID) is a system for automated data identification and acquisition [24], which allows collection and wireless (radio wave) transfer of productionand business-related data. Since the moment, a product/fixture is manufactured, to the beginning of its exploitation or disassembly, RFID technology allows real-time identification, during delivery, storage, or any other process taking place within an enterprise. By means of radio waves, the data are acquisitioned and transferred in a wireless mode between production and business processes in real time. This unique way of identification is adjusted in such a way that it allows the information on product/fixture to correspond to the information on the side of the company or the host system. Using RFID technology, it is possible to track products and equipment, including fixtures, with minimum human intervention.

This can potentially cut back operating costs and increase real-time visibility during the complete product/fixture life cycle.

The RFID system consists of: a computer (or PLC), RFID reader, antenna (which can be integrated in a RFID reader) and transponder – tag. The antenna is used to amplify the signal, which is emitted by the reader to the tag, as well as the signal, which is returned to the reader by the tag, which increases the tag-reading range.

The RFID reader can be a stationary or a portable device, which can activate and pick up the signals emitted by the tags. It consists of the power unit, antenna and a PC board, and its primary role is to receive and send RF signals to the tags by

Ostojic, G. ‒ Stankovski, S. ‒ Vukelic, D. ‒ Lazarevic, M. ‒ Hodolic, J. ‒ Tadic, B. ‒ Odri, S.

Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, 819-825

means of an antenna. From a computer or a PLC, the reader receives instructions generated by the dedicated software. The control unit inside the reader executes the received instructions [24].

The readers differ by range and operating frequency. Similar to the tags, readers can have small range (up to several centimetres), medium range (up to 1 meter), and long range (tens of meters, with an additional antenna). In addition, there are readers equipped with potentiometer for range regulation.

The tags consist of a microchip (which stores alpha-numerical code for product/fixture labelling), an antenna (copper wire ‒ coil) and an optional power source (e.g. battery). They exist in a variety of forms: various pendants, circular or square plates, magnetic cards, or some other form, depending on the area of application (Fig. 1). Smart labels are a special type of tags which can be placed on, or built into a palette or any sort of product/fixture.

• •

is prohibitive in an industrial environment where there are oil stains and other impurities.

Bar code labels are often hard to place on palettes or products/fixtures.

Finally, the greatest deficiency of a bar code is that, in case ID needs to be changed on a palette or product/fixture, a new label must be used, which increases material consumption and requires additional time to finish the operation.

Fig. 2. Illustration showing the broad range of frequencies within the electromagnetic spectrum that RFID system can utilise Fig. 1. Various forms of RFID tags (Summit Automation Co., Ltd.) The components of RFID systems are selected depending on the area of application. Of primary consequence is the operating frequency of the components (Fig. 2). In most of the countries, the operating frequencies for RFID systems are strictly defined.

Bar code reading requires an operator to manipulate product/fixture in order to be detected by the reader, or to manipulate the reader itself.

This requires a number of workers: • Bar code must not have any impurities, or otherwise the reading is erroneous. This

A significant advantage of RFID systems is that they do not require contact for proper functioning. Tags can be read in any industrial environment, which can involve snow, fog, ice, colour stains, dirt and similarly. RFID tags also read fast - in most cases the response is faster than 100 milliseconds. A new generation of readers have the ability to simultaneously read several tags. Thus, the whole storage area can be read at once instead of scanning each article individually.

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2 EXPERIMENTAL RESEARCH STRUCTURE Considering all the advantages of RFID, especially the possibility of non-contact identification and application in an industrial (unclean) environment, this technology was chosen for fixture assembly/disassembly case study.

In production systems, where various variants of products and frequent changes of production programme are common, there is also the need for multiple changes of fixtures. Due to intensive fixture changes and the inability to store an already used fixture for further usage, it is more feasible to disassembly such a fixture and re-use its elements in a new fixture which is made for the current workpiece. With this in mind, a system was designed which should allow assembly/disassembly and inspection of fixture assembly precision in all available work stations. The system is designed bearing in mind that the assembly/disassembly and inspection operators are familiar neither with the planned tasks of fixture assembly/disassembly (fixture assembly/disassembly planning is stochastic ‒ to accommodate production requirements) nor with all the fixture variants. Instead, they use instructions especially defined for assembly/ disassembly of each particular fixture.

The system proposed in this paper for fixture assembly/disassembly and inspection consists of: four work stations which are identically equipped with tools, measurement devices, a computer which is connected to the database and an RFID reader (Fig. 3). All the work stations are interconnected by a conveyor belt which transports palette carriers, and are supplied by fixture elements from a central storage. In addition, the system also features two work stations equipped with a computer and an RFID tag. These work stations are manned by operators who take out and return fixture elements back to the storage, while also performing selection of fixture elements according to their present state.

2.1 Fixture Assembly Procedure According to the scheduled plan of production, work order is sent to the storage of fixture elements which defines the required fixture 822

with the BoM (Bill of Material). Warehouse operator takes the kitting box and selects required fixture elements from the storage. He then takes a card-shaped RFID tag and writes in the code of the work order. In this way, a direct relationship between the collected fixture elements and their assembly instructions is established. All the data are stored into a unified data base. In addition, the storage inventory state is automatically reduced by the number of issued elements, which provides efficient updating of inventory state.

Subsequently, the operator inserts the RFID tag into a kitting box together with fixture elements and places the box onto palette carrier on the conveyor belt. The kitting box is then taken by the first available operator and the RFID tag is taken out. Once the RFID tag is read, the computer displays (according to work order code) assembly instructions which must be carried out.

Upon the completion of assembly procedure, the operator inspects assembly precision and RFID tag is updated to indicate assembly completion.

After that, the fixture is returned to the storage of finished fixtures together with its RFID tag, wherefrom the fixture is directed to the appropriate machine tool, according to the process plan. This procedure is the same for all fixtures except those whose base plate dimensions preclude them from being fitted onto conveyor belt. In such cases, the operator takes the base plate from the buffer and assemblies the rest of the fixture elements according to the work order (Fig. 3).

2.2 Fixture Disassembly Procedure The used fixtures and their corresponding RFID tags arrive at the storage for fixtures that are to be disassembled. Here, they are taken by available operators and transported to their work stations. At the workstation, the operator takes the RFID tag and enters the data at the beginning of the fixture disassembly procedure. Based on the code read from the RFID tag, a sequence of disassembly instructions is drawn from the data base for the particular fixture. In the next step, the operator disassembles the fixture. Upon completion of disassembly, he takes the kitting box and places all the disassembled fixture elements into it. Then, the RFID is updated with the data on disassembly completion and the tag is inserted into the kitting

Ostojic, G. ‒ Stankovski, S. ‒ Vukelic, D. ‒ Lazarevic, M. ‒ Hodolic, J. ‒ Tadic, B. ‒ Odri, S.

Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, 819-825

Fig. 3. Assembly/disassembly process with the use of RFID technology box. The kitting box is placed on a palette on a conveyor belt. It is then taken by the operator in charge of the storage inventory. The operator reads the fixture BoM from the RFID tag which he took from the kitting box. He then checks the number of available elements and performs their selection.

The elements which conform to the requirements (are not deformed, broken, or of the changed structure, etc.) are returned to the storage, while the rest are selected according to their state. The present state of each BoM element is established by the operator (to be returned, not to be returned to the warehouse), after which the warehouse inventory state is automatically updated. In this way, procurement of fixture elements is always up to date which directly influences production continuity (Fig. 3).

It should be emphasized that throughout its life cycle, a fixture is constantly monitored

by RFID technology ‒ from the moment of taking fixture elements from the storage and their assembly, through exploitation, up to the disassembly and return of elements to the storage.

3 RESULTS AND DISCUSSION The proposed system was verified in vitro.

In order to obtain relevant data, complete research was conducted under laboratory conditions. Prior to verification, a detailed study of the system was conducted. Fixtures were identified with all the necessary operations for their assembly/ disassembly. In addition, all possible strategies for the selection of fixtures and their elements were analyzed. Verification was performed on a total of 96 fixtures, which, according to applicability in certain machining operations and the similarity of assembly/disassembly sequences, could be classified into following groups: • fixture group 1 - comprises 44 fixture design solutions for machining operations; • fixture group 2 - comprises 52 fixture design solutions for inspection operations; The conducted research yielded results shown in Table 1. Based on the results, the following conclusions can be drawn: • For fixtures of Group 1, the difference in average assembly times with manual and

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•

RFID identification amounts to 22% in favour of RFID technology. With disassembly, the situation is similar, while the average disassembly time is 17% shorter when using RFID. For fixtures of Group 2, the difference in average assembly times with manual and RFID identification amounts to 21% in favour of RFID technology. With disassembly, the situation is similar, while the average disassembly time is 15% shorter when using RFID.

Table 1. System verification results Fixture groups Process 1 2 Average time of fixture assembly (min/fixture) with 98 83 manual identification Average time of fixture assembly (min/fixture) with 76 66 RFID identification Average time of fixture disassembly (min/fixture) with 82 69 manual identification Average time of fixture disassembly (min/fixture) with 68 59 RFID identification 4 CONCLUSIONS The proposed concept of the system provides complete control of materials flow related not only to the manufacturing process itself, but also to its environment. Furthermore, this concept makes the data on the number of fixtures which are to be assembled, disassembled readily available, as well as the fixture elements which can be re-used (which are already storage or are transported to the storage), fixture elements which can be reconstructed, the quantities and types of materials for recycling (quantities and types of secondary materials).

Analyzing the results of system verification in vitro, it can be noted that the ratio of percentages varies in some cases by more than 20%, but is always in favour of the RFID identification, which justifies its application in the given examples. A feasibility study is required to support the idea of 824

introducing the RFID technology into systems for fixture assembly/disassembly.

In addition, the systems, which incorporate the RFID technology, show greater flexibility in terms of the ability to accept and machine various types of fixtures, as well as other products.

Finally, the following can be concluded: the system proposed and tested in this paper performed satisfactorily with various types of fixtures, which were grouped according to similarity of sequences within the process of assembly/disassembly. It allows the reduction of the main assembly/disassembly time, which directly steps up productivity.

5 REFERENCES [1] Palčič, I., Buchmeister, B., Polajnar, A. (2010). Analysis of innovation concepts in Slovenian manufacturing companies.

Strojniški vestnik - Journal of Mechanical Engineering, vol. 56, no. 12, p. 803-810.

[2] Kušar, J., Berlec, T., Žefran, F., Starbek, M. (2010). Reduction of machine setup time.

Strojniški vestnik - Journal of Mechanical Engineering, vol. 56, no. 12, p. 833-845.

[3] Bi, Z.M., Zhang, W.J. (2001). Flexible fixture design and automation: Review, issues and future directions. International Journal of Production Research, vol. 39, no. 13, p.

2867-2894.

[4] Župerl, U., Čuš, F. (2002). A model for analysing and optimazing fixtures. Strojniški vestnik - Journal of Mechanical Engineering, vol. 48, no. 2, p. 73-86.

[5] Wang, Y., Xie, J.F., Wang, Z.J., Gindy, N. (2008). A parametric FEA system for fixturing of thin-walled cylindrical components.

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[6] Kulankara, K., Satyanarayana, S., Melkote, S.N. (2002). Iterative fixture layout and clamping force optimization using the genetic algorithm. Journal of Manufacturing Science and Engineering, vol. 124, no. 1, p. 119-125.

[7] Chen, W., Ni, L., Xue, J. (2008).

Deformation control through fixture layout design and clamping force optimization.

The International Journal of Advanced

Ostojic, G. ‒ Stankovski, S. ‒ Vukelic, D. ‒ Lazarevic, M. ‒ Hodolic, J. ‒ Tadic, B. ‒ Odri, S.

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Manufacturing Technology, vol. 38, no. 9-10, p. 860-867.

[8] Padmanaban, K.P., Arulshri, K.P., Prabhakaran, G. (2009).

Machining fixture layout design using ant colony algorithm based continuous optimization method. International Journal of Advanced Manufacturing Technology, vol. 45, no. 9-10, p. 922-934.

[9] Padmanaban, K.P., Prabhaharan, G. (2008).

Dynamic analysis on optimal placement of fixturing elements using evolutionary techniques.

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4177-4214.

[10] Bugtai, N., Young, R.I.M. (1998). Information models in an integrated fixture decision support tool. Journal of Materials Processing Technology, vol. 76, no. 1-3, p. 29-35.

[11] Lin, Z.C., Yang, C.B. (1995). An expert system for fixturing design for face milling using modular fixture. The International Journal of Advanced Manufacturing Technology, vol. 10, no. 6, p. 379-388.

[12] Hu, C.Q., Lin, Z.Q., Lai, X.M. (2006).

Concept design of checking fixture for auto-body parts based on neural networks.

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[13] Liqing, F., Kumar, A.S. (2005). XMLbased representation in a CBR system for fixture design. Computer-Aided Design & Applications, vol. 2, no. 1-4, p. 339-348.

[14] Peng, G., Chen, G., Liu, X. (2010). Using CBR to develop a VR-based integrated system for machining fixture design.

Assembly Automation, vol. 30, no. 3, p. 228239.

[15] Ameri, F., Summers, J.D. (2008). An ontology for representation of fixture design knowledge. Computer-Aided Design & Applications, vol. 5, no. 5, p. 601-611.

[16] Vukelic, D, Župerl, U., Hodolic, J. (2009).

Complex system for fixture selection,

modification, and design. The International Journal of Advanced Manufacturing Technology, vol. 45, no. 7-8, p. 731-748.

[17] Liou, F.W., Suen, D. (1992). The development of a feature-based fixture process planning system for flexible assembly. Journal of Manufacturing Systems, vol. 11, no. 2, p. 102113.

[18] Ma, W., Lei, Z., Rong, Y. (1998). FIX-DES: A computer-aided modular fixture configuration design system. The International Journal of Advanced Manufacturing Technology, vol.

14, no. 1, p. 21-32.

[19] Kakish, J., Zhang, P.L., Zeid, I. (2000).

Towards the design and development of a knowledge-based universal modular jigs and fixtures system. Journal of Intelligent Manufacturing, vol. 11, no. 4, p. 381-401.

[20] Dai, J.R., Nee, A.Y.C., Fuh, J.Y.H., Kumar, A.S. (1997). An approach to automating modular fixture design and assembly.

Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture, vol. 211, no. 7, p. 509-521.

[21] Wu, Y., Rong, Y., Ma, W., LeClair, S.R. (1998). Automated modular fixture planning: Geometric analysis. Robotics & ComputerIntegrated Manufacturing, vol. 14, no. 1, p.

1-15.

[22] Peng, G., He, X., Yu, H., Hou, X., Khalil, A. (2008). Precise manipulation approach to facilitate interactive modular fixture assembly design in a virtual environment. Assembly Automation, vol. 28, no. 3, p. 216-224.

[23] Herakovič, N. (2007). Computer and machine vision in robot-based assembly. Strojniški vestnik - Journal of Mechanical Engineering, vol. 53, no. 12, p. 858-873.

[24] Ostojic, G., Lazarevic, M., Stankovski, S., Cosic, I., Radosavljevic, Z. (2008).

Radio frequency identification technology application in disassembly systems. Strojniški vestnik - Journal of Mechanical Engineering, vol. 54, no. 11, p. 759-767.

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Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, 826-833 DOI:10.5545/sv-jme.2010.152

Paper received: 07.07.2010 Paper accepted: 03.08.2011

Possibilities of Using Three-Dimensional Optical Scanning in Complex Geometrical Inspection

Brajlih, T. ‒ Tasic, T. ‒ Drstvensek, I. ‒ Valentan, B. ‒ Hadzistevic, M. ‒ Pogacar, V. ‒ Balic, J. ‒ Acko, B. Tomaz Brajlih1,* ‒ Tadej Tasic1 ‒ Igor Drstvensek1 ‒ Bogdan Valentan1 ‒ Miodrag Hadzistevic2 ‒ Vojko Pogacar1 ‒ Joze Balic1 ‒ Bojan Acko1 1University of Maribor, Faculty of Mechanical Engineering, Slovenia 2University of Novi Sad, Faculty of Technical Sciences, Serbia Non-contact optical three-dimensional measuring, scanning and digitising are increasingly present in quality assurance systems. Simple scanning procedures, high density of data acquired in a single scan, and the possibility of integrated reverse engineering and inspection, are all advantages of optical scanning compared to conventional measuring methods. Due to the three-dimensional acquisition of measuring data, an optical scanner is often considered to be an alternative possibility for coordinate measuring machines.

However, the accuracy of the measured data acquired by optical scanning (even with a high-end system) is still far below the level achieved by high-level coordinate measuring machines. This paper examines the possibilities of using a three-dimensional scanner for workpiece inspection. The first part presents a special field of workpiece inspection in which, even with currently achievable accuracy, optical scanning is a viable solution for the inspection of manufactured parts. In addition, the achievable dimensional accuracy of an optical scanner is tested by scanning several gauge blocks. In conclusion, a head to head comparison with a coordinate measuring machine is made by scanning and verifying a sphere. ©2011 Journal of Mechanical Engineering. All rights reserved.

Keywords: 3D optical scanning, uncertainty of measurement, geometry inspection, rapid manufacturing, reverse engineering, quality assurance 0 INTRODUCTION A three-dimensional optical scanner acquires geometry data from an existing physical object. This data is used to construct a virtual three-dimensional model of the scanned object that can be used for various applications, such as reverse engineering, inspection and quality management, rapid prototyping, cultural heritage documentation and restoration.

The research presented in this paper was performed using a GOM ATOS™ II threedimensional scanner, which is currently installed at the Faculty of Mechanical Engineering in Maribor (Fig. 1). The ATOS™ scanning system is based on the triangulation principle: The sensor unit projects different fringe patterns onto the scanned objects, which are then recorded by two cameras. Each single measurement generates up to 4 million data points. The scanner records only those points visible by both cameras in a single scan. In order to digitize a complete object, several individual measurements are required from different angles. Based on reference points (circular markers), which are attached directly on 826

to the object or on the measuring plate or a fixture, ATOS transforms these individual measurements automatically into a common global coordinate system.

Fig. 1. Atos II optical scanner schematics The ATOS™ II three-dimensional scanner is equipped with several different projector and camera lenses setups (Table 1) that enable scanning to be performed inside different

*Corr. Author’s Address: University of Maribor, Faculty of Mechanical Engineering, Smetanova 17, SI-2000 Maribor, Slovenia, tomaz.brajlih@uni-mb.si

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measuring volumes. The selection of larger measuring volume quickens the scanning of large parts by reducing the number of required consequent scans. However, the accuracy of scans decreases as volume envelope size increases.

Table 1. Measuring volume setups Measuring volume (L×W×H) [mm]

Mea- Projector Camera suring lens lens point [mm] [mm] distance [mm] 1700×1360×1360 1.33 6 8 1200×960×960 0.94 6 8 800×640×640 0.62 8 12 550×440×400 0.43 8 12 350×280×280 0.27 12 17 250×200×200 0.2 17 23 175×140×135 0.14 23 35 135×108×95 0.11 35 50 1 APPLICATIONS FOR 3D SCANNERS There are already several fields where three-dimensional scanning is an established method of data acquisition. In mechanical engineering, 3D scanners are often used for workpiece inspection, deformation analysis, reverse engineering and reengineering of moulds and dies and general quality control procedures [1]. Civil engineering also uses three-dimensional scanning during building inspection, custom fit furniture design, and cultural heritage protection and renovations. Forensics use optical scanning during crime scene investigation and data preservation. The textile industry uses scanning for digitalization of the human figure in custom fit product design. The movie industry also widely uses three-dimensional scanning for various CGI effects creation. Its use has also spread to marketing and advertisement industry [2]. This has already caused some degree of specialization by scanner manufactures regarding the indented field of use.

Our 3D scanner GOM ATOS™ II has already been used for many industrial measurement tasks, but recently we have strengthened co–operation with the University

Clinical Centre of Maribor, and one of our joint projects included measurement of cranial implants. This measurement will be presented in the next chapter.

2 INSPECTION OF CRANIAL IMPLANTS The recent development of various rapid manufacturing technologies has opened new possibilities of customized product manufacturing.

Lower accuracy, rough surfaces and relatively high production costs are still factors limiting the implementation of these technologies in the production processes. However, there are some special fields where rapid manufacturing is currently replacing conventional machining [3].

One of these fields is the manufacturing of customized cranial implants. Low accuracy demands, desired rougher surfaces and individual part production make rapid manufacturing technologies a better choice when manufacturing cranial implants (Fig. 2) [4]. Because these parts are essentially medical products, some form of inspection must be performed prior to implantation, in order to establish any manufacturing inaccuracies and possible postprocessing deformities.

Fig. 2. Cranial implant and skull model There are several factors that make three-dimensional optical scanning (Fig. 3) a favourable inspection method compared to a coordinate measuring machine. The CAD data of a cranial implant is usually (due to the established modelling method) a polygon mesh in the STL

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file format. This fact can make the accurate importation of CAD data (essential for inspection) into CMM software rather difficult. On the other hand, an STL mesh can be considered as a native format for three-dimensional scanner software, making inspection using STL CAD data much easier. In addition, accuracy demands regarding cranial implants are within the limits of ±0.5 mm, making the higher accuracy of a CMM (compared to a high-end optical scanner) unnecessary. There is also a possibility of measuring probe damage, especially in the case of continuous scanning due to implants usual (desired) surface roughness.

was based on implants STL file incorporated during manufacturing. Two independent scans of each side of the implant were taken (Fig. 4). Each side was polygonized into an independent mesh.

An essential step during the part inspection is the mutual registration of scanned and CAD data. In ATOS software, registration is usually performed in two steps. Firstly, the meshes are manually registered by marking four (or more) common points on each mesh [6]. Due to two independent meshes being the results, both were registered separately (Fig. 5).

Fig. 3. Scanning of the implant

Fig. 5. Manual pre-registration of parts

Fig. 4. The complete side of the implant was digitized in a single scan The next section presents an established inspection method for a specific implant. This particular implant was manufactured using a selective laser melting method (on EOSINT M270™ rapid manufacturing machine) from titanium alloy [5]. A physical model of the patient’s skull was also manufactured (from polyamide) for rough inspection and engineersurgeon communication. However, an additional inspection prior to the operation was carried out by means of ATOS II optical scanner. This inspection 828

Fig. 6. Global vs. local registration Semi-automatic best–fit registration is performed over the next step. Inspection of the implant was performed by best-fit registration using two different strategies. One strategy was by taking into account both meshes globally and the other by just regarding the local fixture structure (Fig. 6). This resulted in slightly different colourcoded deviation meshes. Both results are useful for the final verification of the manufactured implant,

Brajlih, T. ‒ Tasic, T. ‒ Drstvensek, I. ‒ Valentan, B. ‒ Hadzistevic, M. ‒ Pogacar, V. ‒ Balic, J. ‒ Acko, B.

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which is basically an engineer and surgeon mutual go/no-go decision prior to operational implantation.

By observing the inspection results (Fig. 7) we can note some problematic areas regarding the part (especially areas where a support structure had to be removed), where deviation is greater than desired ±0.5 mm limits. However, due to these areas not being critically located, the implant was approved, and later successfully implanted.

Fig. 7. Colour coded deviation mesh (global vs. local registration) 3 EXPERIMENTAL UNCERTAINTY EVALUATION Due to complex calculations regarding 3D geometry from a very large number of measured points, it is almost impossible to calculate universal task-independent uncertainty in accordance with [7] and [8] for three-coordinate measuring machines. Calibration procedures for establishing the measuring uncertainty of CMMs are usually task-dependent and are used in special cases for very precise measuring tasks. However, the industry seeks universal CMM checks, which are fast and give sufficient information on the expected accuracy over a wide range of measuring tasks. Acceptance and reverification tests were standardized for this reason [9]. Different artefacts such as ball plates, step gauges, gauge blocks, rings and balls are used to verify uncertainty, as specified by the CMM producer. However, no such procedures have been generally accepted and standardized for 3D scanners and other optical devices. The producers of such devices have developed their own standards and perform periodical checks for their customers.

We have been attempting to transform our rich experiences in CMM verification in the field of 3D scanning. The first verifications were performed by means of scanning gauge blocks and balls. Further work is presently focused on developing special 3D artefacts and corresponding

verification procedures. The work is being executed within the Euramet joint research project TP 3 JRP 2.2.

Our first verifications of the 3D scanner GOM ATOS II were limited to the smallest measuring volume in which the best accuracy is expected.

3.1 Gauge Blocks Scanning Several gauge blocks were scanned and digitised in order to test the achievable accuracy of the optical scanner (Fig. 8). Scanning was performed using the smallest measuring volume in order to acquire as accurate scans as possible.

Also, the number of subsequent scans was limited to two subsequent scans in order to minimize an error during scan assembly. Two scans were necessary to acquire enough data about both gauge block reference surfaces.

Fig. 8. Gauge block scanning The digitizing and post-processing of the scanned data was performed using ATOS™ firmware. The first step was to take two subsequent scans of the gauge block (Fig. 9).

Fig. 9. Scan assembly The initial result of scanning is a point cloud file of acquired points. This form is unsuitable for further work so the next step was to polygonize the point cloud into three-dimensional polygonal mesh. The polygonization process (Fig. 10) can be controlled using several parameters.

For this test the reference point areas were cut-out,

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the polygonization detail was high, and we did not initially perform post processing.

Next, 9 points were selected on the opposite side of the gauge block in the following pattern. One approximately at the centre of the surface, four in the surrounding area, and four close to the corners (Fig. 13). Then, nine projected normal distances for these points to the plane 1 were measured.

Fig. 10. Polygonization of point cloud In the next step the undesirable data (unintentionally scanned parts of the measuring table) were removed. Also the scan noise was removed by smoothing the mesh using an allowed surface deviation of 0.005 mm (Fig. 11). Finally, the mesh was checked for possible triangulation errors (bad edges, reverse normals, etc.).

Fig. 11. Mesh postprocessing

Fig. 13. Projected point-distance definition An average for these nine distances was calculated as a result of the gauge block scanning.

Three different gauge blocks were taken (20, 30 and 70 mm nominal) for the purpose of this test.

Each gauge block was scanned ten times.

The results of the gauge block scanning (Figs. 14 to 16):

Data about gauge block length was acquired by the following procedure. Firstly, a few triangles were selected on one of the reference plains. Then, the selection of triangles was increased by curvature in order to select as many triangles as possible describing the desired plane.

A best-fit plane primitive was created based on this selection. All points inside the 3-sigma limits of the selected triangles were taken for plane creation (Fig. 12).

Fig. 14. Results of the 20 mm gauge block scanning

Fig. 12. Plane definition 830

The above diagrams show scattering around nominal value within certain limits not systematically dependent on the scanned nominal length [7]. In fact, the scattering at 70 mm is somewhat higher than the other two, but the mean

Brajlih, T. ‒ Tasic, T. ‒ Drstvensek, I. ‒ Valentan, B. ‒ Hadzistevic, M. ‒ Pogacar, V. ‒ Balic, J. ‒ Acko, B.

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deviation from the nominal length is the smallest one. Standard deviations for different nominal lengths are s20 = 7.1 µm, s30 = 5.9 µm, s70 = 11.2 µm.

µm over the whole measurement range. Expanded uncertainty at a confidence level of approx. 95% is then U = 25 µm.

3.2 Issue of Spray Coating Scanned Parts

Fig. 15. Results of the 30 mm gauge block scanning

Fig. 16. Results of the 70 mm gauge block scanning While the deviations of the arithmetic means are as follows: d 20 = 7.1 µm, d 30 = 5.9 µm, d 70 = 11.2 µm.

Since the mathematical model of this measurement is very complex, it is assumed that the uncertainty results in scattering and systematic deviations [7] and [10]. The propagation law gives us the following relations:

Since optical scanners are based on optical data acquisition, there are problems when scanning parts with reflective or dark coloured surfaces.

Often, it is necessary to coat the surface with a white spray in order to scan a certain part. In our case the gauge block surfaces were too reflective for scanning without applying a small amount of spray to the surface. The thickness of the spray coating inevitably contributes to error regarding scanned data. The manufacturer of the Ti-oxidebased spray usually used for these purpose claims that the thickness of the coating should be below 0.001 mm if the spray is used appropriately.

The definition of appropriate spray use is highly subjective and described only as applying as little spray as (and when) necessary. This very much depends on experience and skill of the person performing the spraying (and scanning). In order to roughly examine the possible influence of spray coating, the 20 mm and 30 mm gauge blocks were (after the initial scanning) sprayed again and this time the coating was really exaggerated far beyond the “appropriate use” level. Both blocks were then scanned again (each twice) (Fig. 17). a)

b)

2

u20 = d 20 + s20 2 = 9.0 µm ,

2

2

u30 = d 30 + s30 = 8.1 µm , 2

u70 = d 70 + s70 2 = 11.2 µm .

The above uncertainties differ a little, but no systematic dependence on scanned length can be observed. Therefore, it is assumed that standard uncertainty is within the limits of u = 12

Fig. 17. Results of thickly sprayed gauge block scanning; a) 20 mm gauge block, b) 30 mm gauge block Both results show scanning error well within the limits set by previous testing. It can be assumed, that among other parameters which also contribute to scanning inaccuracy (nominal resolution, scan assembly, polygonization and

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mesh smoothing, etc.); spray thickness does not have a predominant influence. Naturally, it cannot be disregarded, but at the same time the fact that some (a majority) of parts have to be sprayed does not automatically disqualify optical scanning as a future alternative to conventional inspection methods.

3.3 Sphere Scanning The next phase of testing was performed by scanning a metal sphere. The first reason for this phase was a step towards a more complex geometry of the scanned part, which makes a three-dimensional scanner a more suitable means.

Secondly, by scanning a sphere, necessary data for geometrical primitive creation can be acquired by a single scan, eliminating the error caused by subsequent scan assembly [11]. Additionally, verification of the scanned data was made by comparing the scanned results to the measured results gathered by the coordinate measuring machine.

Fig. 19. CMM sphere inspection

Fig. 20. Sphere primitive definition from scanned data The diameters of the generated best-fit primitives were taken as a result of scanning.

The following diagram compares the results of scanning with those acquired with the CMM (Fig. 21).

Fig. 18. Reference sphere measurement with a CMM ZEISS UMC 850 The sphere was measured using a Zeiss UMC-850™ coordinate measuring machine (Figs. 18 and 19) by using standard procedure for sphere measurements [12]. Five subsequent measurements were taken. Then, the same ball was scanned (five times) by means of the ATOS™ II optical scanner (Fig. 20). Each scanning consisted of a single scan and subsequent sphere primitive generation based on the collected data [13].

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Fig. 21. Sphere verification ATOS vs. CMM The results for the sphere are much closer to the CMM data than the results of gauge blocks compared to their nominal values. This increase

Brajlih, T. ‒ Tasic, T. ‒ Drstvensek, I. ‒ Valentan, B. ‒ Hadzistevic, M. ‒ Pogacar, V. ‒ Balic, J. ‒ Acko, B.

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in accuracy can be attributed to the elimination of subsequent scan assembly error, during the postprocessing of the scanned data. It has to be noted that it was also necessary for the sphere to spraycoat prior to scanning, due to a reflective surface.

If the uncertainty is estimated in the same simplified way as in the case of gauge block scanning, the expanded uncertainty at k = 2 is U = 5 µm.

4 CONCLUSION This paper is the result of research of the possibilities of customized medical implant inspection. Complex geometry, rough surface and polygon-only CAD data are all common properties of customized implants manufactured by additive technologies. This makes conventional coordinate measuring machines rather unsuitable for this type of inspection [14]. This resulted in a search for alternative methods, and optical scanning seemed a viable solution. Several tests on achievable accuracy were performed by means of scanning gauge blocks and other well-defined geometrical forms. These tests proved that by using a highend system, optical scanning can be successfully used in complex geometrical inspection and for currently defined accuracy demands for cranial implants. With the future development of optical scanning system and the increasing accuracy of scanned data, the use of this system will undoubtedly widen to other areas of inspection, as an alternative to conventional measurement methods.

5 REFERENCES [1] GOM. Quality control of injection moulded parts, from http://www.gom.com/EN/B0C. html, accessed on: 2009-05-31.

[2] Pogacar, V. (2007). Integrated renaissance of design. Proceedings of the 1st DAAAM International specialized conference on additive technologies, p. 9-12.

[3] Dolinsek, S. (2005). Wear characteristics of laser sintered molding tools. Wear ‒ An International Journal on the Science and Technology of Friction, Lubrication and Wear, vol. 259, no. 7-12, p. 1241-1247.

[4] Drstvensek, I., Strojnik, T., Brajlih, T., Valentan, B. (2007). Rapid Technologies

supporting surgical operation - case study.

Proceedings of the 1st DAAAM International specialized conference on additive technologies, p. 53-56.

[5] Drstvensek, I. (2004). Layered Technologies.

Faculty of Mechanical Engineering, Maribor.

[6] Wesche, L. (2006) GOM ATOS II users guidelines. GOM, Braunschweig.

[7] BIPM. Evaluation of measurement dataGuide to the Expression of Uncertainty in Measurement from http://www.bipm. o rg / u t i l s / c o m m o n / d o c u m e n t s / j c g m / JCGM_100_2008_E.pdf, accesed on: 201003-10.

[8] Ačko, B. (2003). A universal model for evaluation measuring uncertainty in calibration. International Journal of Simulation Modelling, vol. 2, no. 4, p. 121129.

[9] EN ISO 10360 (2000). Geometrical product specifications (GPS) – Acceptance and reverification tests for coordinate measuring machines (CMM), parts 1-5. International Organization for Standardization. Geneva.

[10] Ačko, B., Godina, A. (2005). Verification of the conventional measuring uncertainty evaluation model with Monte Carlo simulation.

International Journal of Simulation Modelling, vol. 4, no. 2, p. 76-84.

[11] Balič, J., Klančnik, S., Brezovnik, S. (2008).

Feature extraction from CAD model for milling strategy prediction. Strojniški vestnik ‒ Journal of Mechanical Engineering, vol.

54, no. 5, p. 301-307.

[12] Ačko, B. (2007). Calibration of measuring instruments on a coordinate measuring machine.

Advances in Production Engineering & Management, vol. 2, no. 3, p.

127-134.

[13] Yin, Z.W. (2004). Direct integration of reverse engineering and rapid prototyping based on the properties of NURBS or B-spline. Precision Engineering, vol. 28, no.

3, p. 293-301.

[14] Gusel, A., Ačko, B., Mudronja, V. (2009).

Measurement uncertainty in calibration of measurement surface plates flatness.

Strojniški vestnik - Journal of Mechanical Engineering, vol. 55, no. 5, p. 286-292.

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Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, 834-842 DOI:10.5545/sv-jme.2010.227

Paper received: 02.11.2010 Paper accepted: 27.07.2011

Effects of Hot-Water-Pipeline Renovation in a District Heating System Ljubenko, A. ‒ Poredoš, A. ‒ Zager, M. Andrej Ljubenko1,* ‒ Alojz Poredoš1 ‒ Miran Zager2 1 University of Ljubljana, Faculty of Mechanical Engineering, Slovenia 2 Municipal Utility Company Velenje, Slovenia

In this paper the attention is on the heat losses of a main, aboveground supply, hot-water pipeline, whose length is 5430 m. It is situated between the Šoštanj Thermal Power Plant (TEŠ) and the Central Energy Station (CES) Velenje as part of the district heating system in the Šaleška Valley. It was renovated because major heat losses were identified. A numerical analysis and a comparison between the temperature fields of the pipe’s insulation before and after the renovation was performed to discover the main causes for the high heat losses before renovation. An integral measuring method for determining the heat losses was adapted and used to determine the pipeline’s heat losses after the renovation. Taking into consideration the real operating parameters, the annual energy savings and the consequent operational cost reduction, due to the lower heat losses, resulting from the pipeline renovations, are presented. The calculation is made with measured heat-loss coefficients and is therefore based on the real state of the pipeline. The aim is to show the necessity to investigate the heat losses of a hot-water pipeline whose design or state of insulation is questionable. ©2011 Journal of Mechanical Engineering. All rights reserved.

Keywords: district heating, heat losses, pipeline renovation, thermal bridges 0 INTRODUCTION In district heating (DH) systems a substantial amount of energy losses is linked with the energy transport to consumers because heat losses in the distribution network (DN) occur.

Therefore, it is essential to focus a lot of attention on the system design, operation and management [1]. Hot-water pipeline insulation must be a key parameter in DN design to achieve its sufficient thermo-economic operation [2]. Heat losses from district heating pipes seem vastly underestimated

by DH companies because of insufficient data of actual heat losses, as highlighted in [3]. In [4] high heat losses of a district heating system were attributed to poor thermal insulation and overdimensioning of pipe sizes.

A review of available literature about steady-state heat losses in buried district heating pipes was carried out in [5]. Steady-state heat losses from buried pre-insulated pipes generally consider heat transfer coefficient between the pipe and the ground. In addition, heat transfer coefficients between supply and return pipes are also considered. However, from the engineering point of view acceptable results are obtained 834

by considering only one linear heat transfer coefficient for each pipe and one temperature difference, between water and the ground [6].

The construction of the pipeline’s insulation plays a very important role because possible thermal bridges can substantially decrease its insulation properties. When a detailed analysis of the causes for high heat losses in a pipeline with complex insulation geometry is desired, we resort to numerical program tools [6] and [7]. During the numerical analysis a lot of generalizations and presumptions are usually made. In addition, the actual state of the insulation is not considered.

Therefore, a heat-loss measurement gives better results. Thus, it is important to have available experimental methods to determine the state of the insulation on existing DH systems.

A distinction is made between the experimental methods that require the DH network to be taken out of normal operation and those which do not. Their basics were described

by Bøhm [8]. It is usually not feasible and very expensive to take a DH network out of a normal operation because it requires the disconnection of consumers and special equipment. Therefore, it is usually in the best interest of the DH company to

*Corr. Author’s Address: University of Ljubljana, Faculty of Mechanical Engineering, Aškerčeva 6, 1000 Ljubljana, Slovenia, andrej.ljubenko@fs.uni-lj.si

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determine the heat losses and the critical sections in the DH network during an undisturbed heat supply to all consumers. This is feasible with the following methods: measurement of the insulation or ground temperatures [9], infrared thermography (the TX model) [10], the integral method and heat meters balance. The only one that enables us to determine all of the heat losses on a chosen section is the integral method, which consists of a temperature decline (ΔT) and mass-flow ( m ) measurement. The heat losses are calculated as:

 p ∆T , (1) Q = mc

where cp is the specific heat of the water.

DH systems have a long lifetime, during which a substantial decrease in the quality of the insulation or the technical progress in its construction or materials can occur [11]. New state-of-the-art systems are usually constructed of pre insulated pipes with highly-efficient polyurethane (PUR) insulation. Such pipes can include insulation-embedded copper wires for the purpose of leak detection and diffusion barriers to avoid aging of the insulation material due to diffusion of gases [12] to [14]. Thus, renovation of individual sections in existing DH systems can lead to important energy and financial savings. This leads to a short payback period for the renovation investment [15]. In addition, energy losses greatly affect the advantages of DH systems, compared to individual heating systems [16]. In a report [11]

it is suggested that heating utilities should review their concepts for insulating the district-heating lines at regular intervals of about 5 years and to evaluate the heat losses of their DN with regard to possible savings potentials.

1 DISTRICT HEATING IN THE ŠALEŠKA VALLEY DH systems can be divided into three basic parts: energy production, distribution network and off take sites by the consumers.

For the needs of the DH system in Šaleška Valley the heat is produced in the Šoštanj Thermal Power Plant (TEŠ). There, heat and power are partially produced in combination (condensing turbine) and partially produced separately. The heating power is 192 MW. At the location of the TEŠ the heat is purchased by the Municipal Utility Company Velenje, which supplies heat through a branched DN to the consumers. Fig. 1 shows the whole DN, where the overall length of the supply pipelines is more than 140 km and is composed of several intermediate heat stations, reducing stations and dividing shafts. These diversify the network and/ or change the temperature and pressure regimes.

The biggest heat stations are the Central Dividing Heat Station (CDHS), which divides the pipeline from the TEŠ in the directions Šoštanj and Velenje, and the Central Energy Station

Fig. 1. Analyzed aboveground pipeline in the DH system of Šaleška Valley Effects of Hot-Water-Pipeline Renovation in a District Heating System

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Fig. 2. Individual pipeline: number of operating days in 2007 (CES). In the CES the pipeline is branched for the needs of the heat supply to Velenje. Between these two stations there are two, 5430-m-long, main aboveground pipelines, with different inner diameters (ID) of the steel pipe, (named as ID 450 and ID 350) which supply heat to all the consumers in the direction CDHS-Velenje.

It is inappropriate for both hot-water pipelines to always be in operation as the need for heat is not constant throughout the year. The purpose is operation optimization since one pipeline has lower heat losses than both of them. Therefore, only one should be in operation if it can supply sufficient quantities of heat. The results of the optimization in 2007 are shown in Fig. 2.

The supply-water temperature is mostly dependent on the time of the year and the location in the network. The temperature is changed due to system temperature optimization, which is the most accessible way to reduce the heat losses, because it often does not require renovations.

The DN in Šaleška Valley consists of underground pipes in concrete ducts, pre-insulated underground pipes and aboveground pipes. To a large extent the heat is distributed in a two-pipe system, where there is one supply and one return pipeline. However, some of the sections consist of a three-pipe system, where the supply of heat for space heating and domestic hot water is divided.

1.1 ID 350 Construction before Renovation Steel pipes inner diameter of the analyzed aboveground pipeline (ID 350) is 350 mm. It is 836

composed of an insulated pipe, fixed and sliding supports. The overall heat losses of the pipeline are the sum of the losses of its parts. The pipe is insulated with 120-mm-thick mineral wool. The outer aluminum cover is supported with six steel beams (Fig. 3).

Fig. 3. Pipe’s cross-section before and after the renovation at the location of the steel beams The fixed and sliding supports are the elements, which, if not properly designed, in addition to their structural capabilities, also represent distinct thermal bridges.

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1.2 ID 350 Construction after Renovation The construction of all the hot-water pipeline parts that contribute to the heat losses was renovated. This means that the pipe’s insulation, as well as the sliding and fixed supports, were renovated. There was no need to renovate the steel pipe and it was not replaced. A comparison of the pipe’s insulation before and after the renovation can be seen in Fig. 3. The reason for the two-layer insulation after the renovation is that the supply temperatures are too high for polyurethane, the investment savings and the better insulating properties.

The design of the fixed and sliding supports was improved with the implementation of Teflon washers between the steel support and the pipe.

With the washers the contact surface is partially reduced, but primarily, the thermal resistance is increased.

According to the information acquired from the Municipal Utility Company Velenje in the whole DH system in Šaleška Valley annual water leakage is 21,900 m3, which amounts to about 2.06 GWh of heat. Their assessment of the annual system heat losses is 83 GWh. Since the analyzed pipeline is aboveground (water leakages are easily spotted) and the heat losses of water leakage represent only about 2.5% of the total system heat losses they were neglected for the purpose of this study.

accuracy ±10%. As a result of the study the pipeline was later renovated.

A numerical analysis was made, where the temperature fields of the insulation construction at the location of the steel beams was calculated to better understand the causes of the high heat losses before the renovation. The results are compared to the renovated state.

To test the effects of the renovation, the heat losses after the renovation were measured.

2.1 Numerical Analysis of the Insulation Construction The numerical analysis was made using Ansys Fluent as the program tool. The accounted boundary conditions are as follows: a supplywater temperature of 140 °C and a surroundingair temperature of ‒10 °C. The temperature field before and after the renovation is displayed in Figs. 4 and 5. The vast thermal bridges can easily be seen in Fig. 4. However, they are removed in Fig. 5, which leads to extensive energy savings.

2 HOT-WATER PIPELINE’S HEAT LOSSES Before the renovation the high pipeline’s heat losses were identified in a study [17]. There the foundation for the heat-loss measurements was the integral measuring method, where the temperatures Tin and Tout were measured. At both locations, in addition to the temperatures, the volume flows were also measured. From the two different flow data (because of the undisturbed heat supply), the average mass flow was calculated.

The acquired data was used to calculate the heat losses, which are, for the purpose of determining the effects of the renovation, summarized in this paper. The determined heat losses were 456 W/m.

At the temperatures of the supply water (140 °C) and the surrounding air (‒10 °C) the heatloss coefficient was 3.04 W/(mK), its measuring

Fig. 4. Temperature field (K) of the pipe’s insulation construction before renovation Fig. 6 shows the apparent temperature rise in the outer surface temperatures before the renovation on the location of the circumference where the steel beams are.

Fluent makes it possible to calculate and report the heat flows on any edge. The results are 331 W/m before renovation and 70 W/m after renovation. The heat losses, due to the improvement in the insulation construction, are therefore reduced by 79%. This proves that the

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measured high heat losses before the renovation can be mainly attributed to an inappropriate insulation design and not to its worn state.

However, the numerically obtained results do not illustrate the overall hot-water pipeline’s heat losses, because all of the pipeline’s parts and their state are not included. Therefore, the effects of the renovation are based on the measured results.

It is also meaningless to calculate the heat loss coefficients based on numerical analyses because they would not show all hot-water pipeline heat loses, but just the critical part.

pipelines heat losses are higher than the ones calculated in Figs. 4 and 5 because of real construction and materials used (not ideal), ageing of the insulation and heat losses on fix and sliding supports.

2.2 Measured Heat Losses after Renovation While performing heat-loss measurements in the Šaleška Valley DN specific conditions occurred. Therefore, some presumptions and generalizations for the integral experimental method had to be made. The heat supply to all of the consumers had to be undisturbed during the heat-losses measurement. Thus, it was not possible to plug off in-between consumers. Therefore, the mass flow was not constant throughout the whole section. However, a relatively small amount (about 10%) of heat was being offtaken at the measured section. Most of it was being transferred to consumers in Velenje (Fig. 1).

Fig. 5. Temperature field (K) of the pipe’s insulation construction after renovation

Fig. 7. Searching for the maximum cross-correlation coefficient

Fig. 6. Pipe’s superficial temperature before renovation The purpose of numerical analyses, which was to assess the main deficiency of insulation construction before renovation, was met. Six steel beams which support the outer aluminum cover before renovation are the main construction features that lead to high heat losses. Measured 838

While performing the test measurements unperiodic, random fluctuations in the supply temperature of 1 to 2 °C were discovered, which made it possible to determine the time of the temperature wave’s travel through the section.

This can be determined by searching for the maximum in the cross-correlation coefficient [18] for the discrete data from two independent temperature sensors Tin and Tout (Fig. 7). Time measured since the beginning of temperature

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measurement is marked as t. Tin is the temperature measured at the beginning of the section and Tout at the end. The cross-correlation coefficient shows a linear relationship of the data from the two temperature sensors, so by discovering its maximum it can be determined when the water from temperature sensor Tin has reached Tout.

Consequently, an average segment water velocity can be determined. During the measurements, stationary flow conditions were required, which were monitored by an ultrasonic volume flow meter at the location of the temperature sensor Tout.

Fig. 7 shows the basic principle of the temperature wave’s travel time determination.

Considering the fluctuations of the temperature Tin, the time interval was determined, and with a chosen sampling rate the number of discrete data n, which are considered in correlation (the gray area in Fig. 7). This interval has to be large enough to cover the characteristic random temperature changes. In the next step the maximum of the cross-correlation coefficient was searched between n data Tin and the same amount of Tout.

The searching was designed in such a way as to make it possible to find the intervals where Tin and Tout are the most correlated. Time at the beginning of interval with Tin is marked as ta and time at the beginning of interval with Tout is marked as tb. Then the temperature wave’s travel time was calculated as:

τ = tb ‒ ta . (2) Based on the temperature wave’s travel time the average section water velocity can be calculated: L v= ,

(3) τ where L is the section length.

Next, the average mass calculated:

m = A ⋅ v ⋅ ρ ,

flow

was (4)

where A is pipe’s cross-section and ρ is the average water density (between Tin and Tout).

At this point enough information is known to calculate the section’s heat losses:

(

)

 p Tin − Tout . (5) Q = mc

More characteristic and comparable with each other is the information about the heat losses that each meter of hot-water pipeline has: Q

(6) q = , L and the heat-loss coefficient: q

, (7) H= Tw − Tsurr which shows the pipeline’s heat losses for a given temperature difference between the system’s circulating water Tw and that of the surroundings Tsurr.

In the considered data interval, acquired while performing the measurement, the average supply temperature was 140.7 °C and the air temperature was 2.9 °C. These temperatures led to heat losses of 100.4 W/m and a heat-loss coefficient of 0.73 W/(mK). Accuracy of the temperature change (Tin ‒ Tout) was calculated as ±5.8% and the accuracy of mass flow assessed as ±5% which results in calculated accuracy of heatloss coefficient ±8%.

3 EVALUATION OF THE HEAT LOSSES To determine the effects of the hotwater pipeline renovation, its annual heat losses before and after the renovation were calculated.

Knowing the price of heat for the DH company, econonomical evaluation was conducted.

The heat losses for an individual month of the year are calculated as:

Qm = H·(Tw,m ‒ Tsurr,m)·tm·L ,

(8)

where Qm are the pipeline’s monthly heat losses, Tw,m is the average monthly circulatingwater temperature, Tsurr,m is average monthly surrounding-air temperature, tm is the time of the evaluated month and L is the pipeline’s length.

If the price of the heat is c, the cost of the heat losses is calculated as:

C = Qm·c .

(9) 4 EFFECTS OF THE RENOVATION OF THE HOT-WATER SUPPLY PIPELINE CDHS – CES ID 350 When calculating the effects of the renovation the real supply and air temperatures as

Effects of Hot-Water-Pipeline Renovation in a District Heating System

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Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, 834-842

well as the operation time and the price of the heat have to be considered. The data used in this paper were for the year 2007. The price that the DH company, Public Utility Company Velenje, had to pay to TEŠ was 10.38 €/MWh. The remaining data are shown in Figs. 2 and 8.

4.1 Reduction of Heat Loss in the Supply Pipeline When observing Figs. 9 and 10 it is important to consider the hot-water pipeline

operation time. In the periods January–September and November–December it was always in operation. In October it only operated for two days. Therefore, the columns for October are proportionally lower.

In Fig. 9 the calculated monthly heat losses are shown for the pipeline before and after the renovation. The black column also points out the difference.

The direct consequences of the heatloss reduction are financial savings due to lower

Fig. 8. Average monthly temperatures in 2007

Fig. 9. Pipeline’s heat losses 840

Ljubenko, A. ‒ Poredoš, A. ‒ Zager, M.

Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, 834-842

amounts of heat, which the DH company has to purchase from TEŠ. These are shown in Fig. 10.

If we add up the results from Figs. 9 and 10 the following results are acquired. On an annual level calculated pipelines heat losses amounted to 15,635 MWh. To purchase this heat from TEŠ the Public Utility Company Velenje would have to pay 162,295 €. After renovation the calculated heat losses are 3,600 MWh. To purchase this heat 37,371 € are needed. Thus we can calculate the savings due to pipeline renovation. With renovation 12,035 MWh of heat is saved which is in financial terms 124,924 €.

conductivity, neglected influence of nonlinear heat losses due to radiation of pipes outer surface), linearly dependent on the temperature difference between the water temperature in the pipe and the surrounding air. Thus, it enables heat-loss calculations for arbitrary conditions.

The effects of the hot-water pipeline have revealed that its insulation before renovation was unapropriate. After renovation, the heat losses determined on measured results were reduced by 77%. This data is comparable with a study [4] where the efficiency of heat insulation for one district heating system is estimated as three times less than in a good example case.

Based on the price of heat and the temperature conditions data from 2007, the financial savings due to the heat-loss reduction after renovation were calculated. It was found out that the cost of the heat losses in 2007 for the unrenovated 5430-m-long pipeline would be 162,295 €. After the renovation they were 37,371 €.

6 REFERENCES

Fig. 10. Financial savings due to pipeline renovation 5 CONCLUSIONS A numerical analysis of the pipe’s insulation has led to a conclusion that the causes for the high heat losses before renovation are mainly due to the design of the insulation construction.

An adapted integral method for the experimental heat-loss determination has been suitable for an assessment of the pipeline’s heat losses. The cross-correlation water-velocity measurement enables an average mass-flow calculation on a measured section, which is the best possible actual state assessment while the heat supply is undisturbed.

The heat-loss coefficient demonstrates the fact that the pipeline’s heat losses are, under certain presumptions (temperature independent thermal

[1] Verda, V., Borchiellini, R., Calì, M. (2001). A thermoeconomic Approach for the Analysis of District Heating Systems. International Journal of Applied Thermodynamics, vol. 4, p. 183-190.

[2] Ӧztürk, İ.T., Karabay, H., Bilgen, E. (2006).

Thermo-economic optimization of hot water piping systems: A comparison study. Energy, vol. 31, p. 2094-2107.

[3] Ostergaard, T. (2005). Why should district heating companies spend money on carrying out hydraulic analyses. News from Danish Board of District heating, no. 1.

[4] Hlebnikov, A., Dementjeva, N., Siirde, A. (2009). Optimization of Narva district heating network and analysis of competitiveness of oil shale chp building in Narva. Oil Shale, vol. 26, p. 269-282.

[5] Bøhm, B. (2000). On transient heat losses from buried district heating pipes.

International Journal of Energy Research, vol. 24, p. 1311-1334.

[6] Dalla Rosa, A., Li, H., Svendsen, S. (2011).

Method for optimal design of pipes for low-

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energy district heating, with focus on heat losses. Energy, vol. 36, p. 2407-2418.

[7] Poredoš, A., Poredoš, A., Škerget, L. (2001).

Some possibilities for reduction of heat losses in district energy systems. SDDE – Slovene District Energy Association, Ljubljana. (in Slovene) [8] Bøhm, B. (1999). In – situ determination of heat losses from buried district heating pipes.

News from Danish Board of District heating, no. 4.

[9] Perpar, M., Žun, I., Gregorc, J., Bajrič, S., Zajšek, B. (2010). Setting up of measuring system for evaluation of heat losses on district heating network of Ljubljana. International Conference on District Energy Proceedings, p. 159-166.

[10] Zinko, H., Bjärklev, J., Bjurström, H., Bjurström, M., Bøhm, B., Koskelainen, L., Phetteplace, G. (1996). Quantitative heat loss determination by means of infrared thermography – The TX model. International Energy Agency, IEA District Heating – Annex 4 – Network Supervision.

[11] Schmitt, F., Hoffman, H.W., Göhler, T. (2005). Strategies to manage heat losses – technique and economy. International Energy Agency, Program of Research, Development and Demonstration on District Heating, Mannheim.

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[12] Olsen, P.K., Lambertsen H., Hummelshøj R., Bøhm B., Christansen C.H., Svendsen S., Larsen C.T., Worm J. (2008). A new low-temperature district heating system for low-energy buildings. The 11th International Symposium on District Heating and Cooling.

[13] LOGSTOR. District Heating, from http://www.logstor.com/showpage.php? pageid=5765476, accessed on 2011-06-07.

[14] Persson, C., Jarfelt, U., Ramnäs, O., Reidhav, C. (2006). Insulating performance of flexible district heating pipes. 10th International Symposium on District Heating and Cooling.

[15] Bejan, A., Tsatsaronis, G., Moran, M. (1996). Economic analysis. Thermal Design and Optimization. John Wiley and Sons, Hoboken, p. 333-462.

[16] Çomakli, K., Yüksel, B., Çomakli, Ӧ. (2004).

Evaluation of energy and exergy losses in district heating network. Applied Thermal Engineering, vol. 24, p. 1009-1017.

[17] Poredoš, A., Besednjak, D., Dimnik, M. (1994). Heat losses analysis of an aboveground pipeline in a district heating system of Šaleška valley. University of Ljubljana, Faculty of Mechanical Engineering, Ljubljana. (in Slovene) [18] Bendat, J.S., Piersol, A.G. (2010). Random data: analyses and measurement procedures.

John Wiley & Sons, Inc., New York.

Ljubenko, A. ‒ Poredoš, A. ‒ Zager, M.

Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, 843-850 DOI:10.5545/sv-jme.2010.050

Paper received: 02.03.2010 Paper accepted: 24.06.2011

On the Particles Size Distributions of Diatomaceous Earth and Perlite Granulations Petrović, D.V. ‒ Mitrović, Č.B. ‒ Trišovic, N.R. ‒ Golubović, Z.Z. Dragan V. Petrović1 ‒ Časlav B. Mitrović2,* ‒ Nataša R. Trišovic2 ‒ Zorana Z. Golubović2 1 University of Belgrade, Faculty of Agriculture, Serbia 2 University of Belgrade, Faculty of Mechanical Engineering, Serbia

When filtering products with a high level of non-soluble solids and containing filtration-inhibiting substances, a variety of different filter aids can be used. Among others, kieselguhr and perlite are possible materials for this purpose. The selection of the granulation that should be applied depends on the filtered liquid medium and the desired retention level. While perlite is most commonly used for rough filtration, kieselguhr is also suitable for more subtile filtration. In the paper, particle size distributions of three kieselguhr granulations and a perlite granulation are analysed. Particle sizes are measured by the morphometric method, commonly used in the microbiology. Basic statistic parameters are calculated for all considered samples and compared. The applicability of hyperbolic and log-hyperbolic functions, in describing the particles size distributions of these granulations is verified. Depending on the imposed filtration requirements, this approach enables simple modelling and composition of a wide variety of different granulations characterised by appropriate particle size distributions from a few available granulations. ©2011 Journal of Mechanical Engineering. All rights reserved.

Keywords: filtration, kieselguhr, perlite, granulation, particle size distribution 0 INTRODUCTION Filtration of fluids is an important step in a variety of technological processes. There are several types of filtration. In order to optimise the filtration process, they can be applied either independently, or combined. However, one of the most often used is mechanical filtration [1]. In the filtration of this kind, substances are separated from each other by the application of adequate filtration mediums and procedures. In this way, contaminants are restrained at the surface, or inside the volume of the filtration medium. Depending on the initial and final process conditions, the filtration procedure can include one or more succeeding filtration steps. In the latter case, fluid is gradually filtered – primary filter performs rough filtration, while the last filtration step extracts the finest contaminants from the fluid.

To date, a variety of filtering mediums and filter aids have been developed and reported in a rich literature data base. The filtering medium has been fabricated as ceramics [2] and [3], polymers [4] and [5], metals [6] and [7], zeolite [8] and [9], celulose [10] and [11], carbon [11] and [12], composite materials [13] and [14], their combinations [15],[16] and [25], etc.

The frequently used filter aids materials are: kieselguhr (also known as diatomaceous earth - DE) and perlite, etc. Distributions of particle sizes of these two filter aid materials significantly influence the filtration process.

DE is a chalky sedimentary material, comprised of the skeletal remains of prehistoric water microorganisms (single-celled algae) [17], called diatoms. They are characterized by size in the range from under 5 to over 100 mm and porous structure with openings as small as 0.1 mm in diameter. The diatom skeletons and housings are mined, ground up and thermally treated to render a powder composed of microscopic solid particles, which occupy only 15% of total filtration volume.

The combination of small pore sizes, large specific surface area, firmness of the particles and high porosity allow DE to efficiently remove small particles from liquid mediums at high filtration rates. The low solubility of DE in water and diluted acids (less than 1%), as well as the odorless, tasteless, and chemically inert characteristics, make it safe for filtering water or other liquids intended for human consumption.

DE is commonly applied in different areas: medicine and bioprocessing [15] and [18], the food and beverage industry [19] to [21], purification of

*Corr. Author’s Address: University of Belgrade, Faculty of Mechanical Engineering, Kr. Marije 16, 11000, Belgrade, Serbia, cmitrovic@mas.bg.ac.rs

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potable water, contaminated ground- and surface water [22] and [23], decontamination of sewage liquids and waste water [24], pharmacy [25], etc. However, there are some difficulties related to efficient recycling of DE and its expensive disposal due to increasing landfill taxes [22]. Fortunately, there are some promising attempts to further processing and using the DE sludge from deep filtration beds [19] and [24].

Perlite is an amorphous volcanic glass that has 2 to 5% of combined water. At temperatures of 760 to 1100 °C, it softens. Water trapped in the structure of the material vaporizes and escapes, causing the expansion of the material [26]. Due to its low density after being processed, and relatively low price, many commercial applications for perlite have been developed, including the removal of: hazardous metals [27] and [28], dye pollutions, particulates from exhaust gases, medicine and bioprocessing [15] and [18], as well as in the food and beverage industry [11].

A vast variety of DE and perlite granulations exists. The selection of the applied granulation depends on the filtered fluid properties and the desired retention level. While perlit is most commonly used for rough filtration, DE is also suitable for more subtile filtration. However, these two materials are frequently combined in a common filtering process [11].

The particle size distributions of DE and perlite granulations, together with size distributions and concentration of solid contaminants in the dirty inlet and outlet purified fluid, directly influence deep filter properties and govern the preparation of adequate filtering mediums and aids. Consequently, these problems have been extensively studied worldwide.

In this paper, particle size distributions of three DE and one perlit granulation are analysed.

Sizes of acquired particles are measured by the morphometric method. The applicability of hyperbolic and log-hyperbolic functions, in describing the particles size distributions of these granulations is verified. Depending on the filtration requirements, this approach facilitates modelling and composition of variety of different granulations characterised by appropriate particle size distributions from a few available granulations.

844

1 MATERIAL AND METHODS The paper presents and analyses particles size distributions of three DE granulations (Pura, Super and Ultra –made by Pall SeitzSchenk), and a perlite granulation (Harbolite, type 900 produced

by World Minerals). Distributions of their particle sizes are measured by the morpho-metric method.

It is developed primarily for microbiological analysis, but it is quite effective and accurate for estimating the size of various microscopic objects including solid particles. Each granulation sample was separately mixed with distilled water - the so-called »dilution ratio« of 10-3.

Concentration of the sample was performed with Pall filtration equipment, using 47 mm filter disc membranes whose absolute removal rating is 0.2 mm. Proximately before counting the particles, filter discs were flushed by redistilled water, and obtained suspension was centrifuged to provide an appropriate concentration for measuring.

Consequently, sizes of DE and perlite particles were measured. An analogue technique, also based on the application of optical microscope, can be used in the technical practice to research the flocs size distributions in the suspensions of biological water treatment plants.

Basic statistic parameters are calculated for each of the four samples following the common procedure. Particles of each sample are sorted according to their diameters in the intervals of Δd = 5 µm in width. This way, each of m particle size-classes is represented by the interval midpoint di and the absolute frequency (number of particles) Ni = N(di). Consequently, the sum of absolute frequencies equals the total sample particles number:

N=

m

∑

Ni =

i =1

m

∑ N ( d ). i

(1)

i =1

When the relative frequencies:

Q0i = Q0 ( di ) =

Ni , ( i = 1, 2,..., m ) , N

(2)

of the particle sizes are evaluated, basic statistical parameters are calculated. Arithmetic mean and standard deviation:

d=

m

∑Q

0i

i =1

Petrović, D.V. ‒ Mitrović, Č.B. ‒ Trišovic, N.R. ‒ Golubović, Z.Z.

⋅ di , (3)

Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, 843-850

σ=

m

∑Q ⋅(d − d ) 0i

2

, (4)

i

can be represented by hyperbolic function in the following form:

i =1

represent an expected value and dispersion of data sample, respectively. Skewness S and flatness F factor: m

∑Q ⋅(d − d )

3

0i

S=

i =1

i

m

∑Q ⋅(d − d ) 0i

, (5)

σ3 4

i

i =1

, (6) σ4 characterise a distribution shape: symmetry and peakedness, respectively. For the Gaussian function S = 0 and F = 3. A higher difference between the empirical and these theoretical values of S and F means larger discrepancy of the empirical distribution according to the normal.

Characterisation of some distribution by different statistical description parameters provides useful information. However, additional approaches also exist ‒ approximating a measured distribution with interpolation, spline and appropriate analytic fitting functions, [23] to [25].

The approach based on data fitting is used in the present paper. Its purpose is to characterize a large amount of information contained in the empirical distribution, with a small number of parameters ‒ constants in the probability distribution function (PDF), evaluated with the fitting procedure.

In general, normal PDF is the most commonly used statistical function. However, PDFs of real samples very often significantly deviate from this function. Among others, a possible approach for overcoming these situations is based on the hyperbolic function, introduced

by Barndorf-Nielsen (1977) to describe the asymmetric distributions of the wind-blown sand particle sizes [26] and has been extensively used up to date [27] and [28]. This method is used in the present paper for modelling the PDFs of filtering granulation particle sizes.

The probability density function:

F=

pdf ( x ) =

d  PDF ( x )  dx

,

(7)

pdf ( x ) = g ( x ) = A (α , β , δ ) ⋅

2   (8) ⋅ exp  −α ⋅ δ 2 + ( x − µ ) + β ⋅ ( x − µ )  .  

The four arbitrary parameters, α > 0, |β| > α, δ > 0 and µ ∈ ( −∞, ∞ ) that define its shape are evaluated numerically on the basis of adequate empirical data set. Parameter A is the normalization constant that ensures the hyperbolic function, Eq. (8), to satisfy the standard condition of the theory of probability, the so-called normalizing condition: ∞

∫

∞

g ( x ) ⋅ dx =

−∞

∫ pdf ( x ) ⋅ dx = 1. (9)

−∞

This means that the probability of all possible realizations of some event i.e. the probability of a sure event is 1 (100%). This constant is defined by formula:

A (α , β , δ ) =

α2 − β2 , 2 ⋅ α ⋅ δ ⋅ K1 (ξ ) (10)

ξ = δ ⋅ α2 − β2 , where K1(ξ) is the Bessel function of the third kind and the first order. If a natural logarithm ln(x) is applied in Eq. (8) instead of x, the log-hyperbolic function arise.

To analyse some properties of the hyperbolic function, Eq. (8) can be written as:

ln  g ( x )  = ln ( A ) − 2

−α ⋅ δ 2 + ( x − µ ) + β ⋅ ( x − µ ) .

(11)

This functional form shows that two asymptotes exist in this situation, having slopes (α+β) and ‒(α‒β). In addition, the parameter μ gives the abscissa of the point of intersection of the two asymptotes and is, therefore, regarded as a location parameter. The location μ, the scale parameter δ and the mode point ν of the hyperbolic distribution are interrelated:

ν =µ+

δ ⋅β α2 − β2

. (12)

The radius of curvature at the mode point, On the Particles Size Distributions of Diatomaceous Earth and Perlite Granulations

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ρν =

α 2 ⋅δ 2

(α

2

−β

2

)

3

, (13)

characterises the distribution peakedness. For the normal function ρν equals the variance σ2.

Basic parameters of hyperbolic function are sketched in Fig. 1.

In general, the hyperbolic function possesses thicker tails in comparison to the normal Gaussian. It may be skewed to the left (β < 0), symmetric (β = 0) or right-skewed (β > 0).

All these properties of the hyperbolic function are also illustrated in Fig. 1. Following [28], by giving parameters in the hyperbolic function certain suitable limits, it is possible to convert it to the normal (Gaussian) and Laplace (symmetrical or asymmetrical) distribution.

2

m

∧   SSE =  yi − y i  =  i =1 

∑

m

∑ε

2 i . (15)

i =1

It represents a sum of squares of regression errors εi, i.e. the differences between the true (measured) values yi with respect to fitted values y , where m is the number of fitted data »points«, i equal to the number of particle size-class intervals in this case. The root mean square error, or standard error of estimate: 2

m

RMSE =

SSE = m−k

m

∧   yi − y   i =1  = m−k

∑

∑ε i =1

2 i

m−k

, (16)

is the second popular characterization parameter of fitting accuracy. Deviations are referred to each predicted value. Thus, SSE has m − 4 degrees of freedom if a regression line is hyperbolic, since k = 4 degrees of freedom are lost: the four constants in the Eq. (8) are evaluated. The smaller values, defined by Eqs. (15) and (16), mean the higher fitting accuracy.

However, the most commonly used is the coefficient of determination or R-square: m

R2 =

SST − SSE SSE = 1− = 1− SST SST

∧    yi − y i   i =1 

∑ m

∑( y − y)

2

2

∈ [ 0,1 1] , (17)

i

Fig. 1. Geometric interpretation of relevant parameters of hyperbolic function For a known pdf(x) of a specified granulation, where x is a particle diameter d, the participation of the particles having diameters within limits dmin and dmax can be evaluated:

P ( d min < x < d max ) =

d min

∫

pdf ( x ) ⋅ dx. (14)

d max

A standard question that follows empirical data fitting procedures is related to their accuracy.

The most commonly, fitting accuracy can be estimated by the error sum of squares, also called the residual sum of squares:

846

i =1

which represents the ratio between the residual sum of squares SSE, Eq. (15), and the total sum of squares:

SST =

2

m

∑ ( y − y ) , (18) i

i =1

where y denotes the mean value. As closer is the value of R2 to 1 it means the better fitting.

2 RESULTS AND DISSCUSSION Basic statistical descriptive parameters of three DE granulations (Pura, Super and Ultra) and a perlite granulation, measured by the morpho­ metric method described above, are given in Table 1. The mean diameter value varies from 7.56 µm

Petrović, D.V. ‒ Mitrović, Č.B. ‒ Trišovic, N.R. ‒ Golubović, Z.Z.

Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, 843-850

Table 1. Basic statistical descriptive parameters of DE and perlite granulations Mean [mm] Max [mm] RMS [mm] Coef. of var. - Cv [%] Skewness [-] Flatness [-]

DE Pura 7.56 30.00 4.51 59.62 2.25 8.74

for DE Pura and being 18.72 µm for perlite. Corresponding maximum values are 30, 125 and 150 µm for DE granulations and 90 µm for perlite.

Coefficient of variation Cv, shows the largest size dispersion of DE Ultra medium around the mean value (Cv = 115.72%). In contrast, the DE Pura (Cv = 59.62%) and perlite (Cv = 67.87%) are more tightly concentrated around the mean values of their diameters, while DE Super, having value of Cv = 96.59% is in the middle with respect to other filter aid material according to the criteria of data dispersion.

Particle size distributions of all tested filter materials are skewed, showing deviations

DE Super 15.27 125.00 14.75 96.59 2.62 13.4

DE Ultra 16.64 150.00 19.25 115.72 3.08 15.28

Perlite 18.72 90.00 12.7 67.87 1.49 6.28

from the normal Gaussian distribution. Skewness factors are always positive, having values of 2.25, 2.62 and 3.08 for DE granulations Pura, Super and Ultra, respectively, and 1.49 for perlite. Thus, particle size distribution of the perlite granulation is the closest to the symmetrical behavior.

According to the flatness factor, with values of 8.74, 13.4, 15.28 and 6.28 for DE Pura, DE Super, DE Ultra and perlite, respectively, perlite granulation is the closest to the normal Gaussian distribution, while the DE Ultra granulation expresses the largest deviation from this function.

Fig. 2. Fitting results of the particle size distributions of three DE and one perlite granulation; a) hyperbolic function, b) log-hyperbolic function On the Particles Size Distributions of Diatomaceous Earth and Perlite Granulations

847

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Finally, data are fitted using the hyperbolic and log-hyperbolic function. The applied computer program follows algorithm [28]. The resulting data are presented in Fig. 2, where ordinate axis P [%] represents the empirical and estimated (fitted) values of the percent participation of the granulations particle size classes of diameter d [mm]. It is evident that both functions follow experimental data fairly well in all tested cases.

The parameters that characterize the accuracy of fitting procedures are given in Table 2. In the case of DE Pura and perlite, both models provide a nearly identical accuracy, i.e. SSE,

RMSE and R2 values are nearly identical. In the case of DE Super and DE Ultra, log-hyperbolic function gave slightly better results ‒ the R2 values are higher, while the SSE and RMSE are lower. However, in all tested cases, the R2 factors are very high, over 0.93, which confirms a capability of hyperbolic and log-hyperbolic model to describe particles size distributions of these filter materials. Furthermore, standard errors were up to 2.72% in all cases, which is acceptable from the practical engineering point of view.

Analytically describing the size distributions of different filtering granulations is a

Table 2. Determination factors of different DE and perlite filter medium particle sizes distributions fitting by hyperbolic and log-hyperbolic functions SSE [%] RMSE [%] R2 [-]

FUNCTION Hyperbolic Log-hyperbolic Hyperbolic Log-hyperbolic Hyperbolic Log-hyperbolic

DE Pura 14.65 14.84 2.71 2.72 0.9956 0.9956

DE Super 109.77 7.81 2.70 0.72 0.9389 0.9957

DE Ultra 96.74 23.51 2.26 1.11 0.9489 0.9876

Perlite 21.95 21.76 1.48 1.48 0.9707 0.9709

Fig. 3. The particle size distribution of composed granulation consisting of 80% DE Pura and 20% DE Ultra; a) particle size distributions of DE Pura, DE Ultra and their composition, predicted by loghyperbolic fit, b) comparison between the experimental and particle sizes distribution of composition predicted by log-hyperbolic fit 848

Petrović, D.V. ‒ Mitrović, Č.B. ‒ Trišovic, N.R. ‒ Golubović, Z.Z.

Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, 843-850

small but important step in developing the models of various filtration processes. Among others, starting from the known particle size distributions of just a few granulations, it is possible to compose a variety of filter mediums. Fig. 3 illustrates the prediction of the size distribution of a composed granulation consisting of 80% of DE Pura and 20% of DE Ultra medium, based on the fitting of the particle size distributions of these two materials by log-hyperbolic functions.

Fig. 3a illustrates log-hyperbolic fits of the particle size distributions of DE Pura, DE Ultra and their composition consisting of 80% DE Pura and 20% DE Ultra. The particle size distribution of this composition is estimated by combining the log-hyperbolic fitting functions of two parent DE components. Fig. 3b presents a comparison between the experimental and estimated particle size distribution of the composition. It is evident that estimated data agree well with the experimental points, verifying the applicability of the presented approach in predicting and modelling the particle size distributions of granulation compositions.

3 CONCLUSIONS The applicability of the presented experimental and numerical method, in analyzing and describing the particles size distributions of different filter granulation mediums, is verified in the paper. Both the hyperbolic and log-hyperbolic function gave similar fitting results. Still, the log-hyperbolic model has a slight advantage in resolving the problems of this kind. However, a more detailed analysis, based on larger samples of the tested but also of the additional filter granulations of various ranges and produced by various manufacturers, is needed in the future.

This will provide more detailed, reliable and accurate results. The other fitting functions should also be tested.

The presented approach facilitates a detailed analysis of granulation filter aid materials. This way, depending on the imposed filtration requirements, a wide variety of different granulations characterised by appropriate particle size distributions can be composed from a few available granulations. The analogue approach can be applied to modelling the particle size

distributions of the contaminants born by the fluid that is subjected to filtration, enabling establishing of interrelations between the inlet/outlet fluid parameters and filter granulations particle size distributions.

4 ACKNOWLEDGMENTS This research was supported by the Ministry of Science and Technological Development, Republic of Serbia – projects: OI 174011, TR35035, TR36001.

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[12] Wegmanna, M., Michen, B., Graule, T. (2008). Nanostructured surface modification of microporous ceramics for efficient virus filtration. Journal of the European Ceramic Society, vol. 28, p. 1603-1612.

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73-81.

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Experimental study on a new dual-layer granular bed filter for removing particulates.

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[18] Stevenson, G.D. (1997). Flow and filtration through granular media - the effect of grain 850

and particle size dispersion. Water Research, vol. 31, no. 2, p. 310-322.

[19] Wakeman, R. (2007). The influence of par­ ticle properties on filtration. Separation and Purification Technology, vol. 58, p. 234241.

[20] Tasić, J.S., Golubović, Z.Z., Petrović, V.D., Golubović, Đ.Z. (2009). On the applicability of morphometric method for evaluation of the waterborne particles size distribution.

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227-228.

[21] Žajdela, B., Hriberšek, M., Hribernik, A. (2008). Experimental investigations of porosity and permeability of flocs in the suspensions of biological water treatment plants. Strojniški vestnik - Journal of Mechanical Engineering, vol. 54, no. 7-8, p.

547-556.

[22] Maletić, R. (2005). Methods of statistical analysis. Faculty of Agriculture, Belgrade. (in Serbian) [23] Emri, I., Cvelbar, R. (2006). Using spline functions to smooth discrete data.

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[24] Grešovnik, I. (2007). The use of moving least squares for a smooth approximation of sampled data. Strojniški vestnik - Journal of Mechanical Engineering, vol. 53, no. 9, p.

582-598.

[25] Seber, F.A.G., Wild, J.C. (2003). Nonlinear regression, Wiley & Sons, New York.

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[28] Bhatia, J. C., Durst, F. (1988). LHPDF – A PC package for estimating parameters of the log-hyperbolic distribution, moments and mean parameters, Report LSTM 230/T/88, LSTM, Technische Fakultät, Universität Erlangen-Nürnberg, Erlagen.

Petrović, D.V. ‒ Mitrović, Č.B. ‒ Trišovic, N.R. ‒ Golubović, Z.Z.

Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, 851-861 DOI:10.5545/sv-jme.2010.204

Paper received: 29.09.2010 Paper accepted: 02.09.2011

Quality Managers’ Estimates of Quality Management Principles Application in Certified Organisations in Transitional Conditions - Is Serbia Close to TQM?

Radlovački, V. ‒ Beker, I. ‒ Majstorović, V. ‒ Pečujlija, M. ‒ Stanivuković, D. ‒ Kamberović, B. Vladan Radlovački1,* ‒ Ivan Beker1 ‒ Vidosav Majstorović2 ‒ Mladen Pečujlija1 ̶ Dragutin Stanivuković1 ‒ Bato Kamberović1 1 University of Novi Sad, Faculty of Technical Sciences, Department for Industrial Engineering and Management, Serbia 2 University of Belgrade, Faculty of Mechanical Engineering Belgrade, Department for Production Engineering, Serbia For this research, an ad hoc questionnaire was sent to a representative sample of quality ma­na­ gers in Serbian certified organisations to determine their estimate of the quality management principles ap­plication (QMPA) as an overall estimate of management commitment to quality management practice.

In a stable business environment, high impact of TQM elements application to the QMPA is likely to be ex­ pected. Objective parameters (number of employees, form of ownership of organisation, etc.) are not likely to form significant variations in managers’ perceptions of TQM elements.

It is determined that only a few of the selected TQM elements are predictors of a subjective estimate of the QMPA (estimates of QMS, changes after certification, human resource management and process management).

Observed objective parameters (number of employees and whether an organisation is employing a network administrator or not) showed statistically significant variations in estimates of TQM elements.

Research results point to the conclusion that Serbia is yet to take the long journey towards raising awareness about the management commitment to QM and TQM practice. It is possible that other economies in transition also have similar problems in this area. ©2011 Journal of Mechanical Engineering. All rights reserved.

Keywords: TQM, quality management, questionnaire study, subjective estimate of TQM elements, management commitment, transition 0 INTRODUCTION Approaches to quality improvement in­tro­ duced by the quality gurus E. Deming, J. Juran, A. Feigenbaum, K. Ishikawa, G. Taguchi and P. Crosby represent the basis for classical TQM phi­lo­sophy. However, the term »Total Quality Ma­nage­ment« still does not have a uniformly ac­ cepted definition.

One of the definitions of TQM provided in [1] is that »TQM is ... a collective, interlinked sys­ tem of quality practices that is associated with or­ ganisational performance«. According to the sa­me source, in the 1990s, TQM was mainly as­soci­ated with meeting the requirements set by the Malcolm Baldrige National Quality Award (MBNQA).

[2] defines TQM as “a holistic ma­ nage­ ment philosophy that strives for continual im­provement in all functions of an organisation”, whereas [3] *Corr. Author’s Address: Faculty of Technical Sciences, Trg Dositeja Obradovića 6, Novi Sad, Serbia, rule@uns.ac.rs

defines TQM as “a way of improving activities and performance in firms”.

[4] defines TQM as “a strategy to meet customers’ requirements” and “a management philosophy of seeking excellence in all aspects of business through organisation-wide continual improvement”.

Analysis of the relevant literature indicates that researchers in the field of TQM used the same or similar methods in different regions. [5] were among the first to describe in detail and apply the methodology of analysing subjective opinions collected through questionnaires. The foundations for their questionnaire were the theoretical grounds of 1990s.

[1] investigated relations between TQM practice, organisation performance and customer satisfaction in the USA. [2] conducted research on TQM practices and their effects on company 851

Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, 851-861

performance in USA, where she described a number of previous similar researches in detail.

[6] describes research of quality and innovation impact on the short-term performance of products in Spain, whereas [7] describes research in Spain about whether the organisations that implement TQM are better learning organisations. In Korea, [8] conducted research on the impact of MBNQA criteria on organisation quality per­ for­ man­ ce. [9] published research on the effects of the certification process on organisation per­for­man­ ce in Australia, [10] conducted similar em­pi­ri­cal research in Malaysia as did [11] in Greece.

Analysis of these sources shows that im­ ple­ mentation of TQM and a positive attitude towards TQM practice in a number of cases cor­ re­lates to an increase of business performance, although the uniformly accepted impact of TQM implementation and company performance is not determined. Mixed results, as reported by [2], could exist for three groups of reasons: the design of research, the way performance is measured and the use of different analytical frameworks. [12] reported no findings of substantial differences of critical factors across different countries. How­ ever, they stated, the set of determined TQM key fac­tors could not be considered as universal, primarily for the lack of information about a number of countries not covered by the research.

Therefore, differences between countries might be a fourth factor to add to Kaynak’s list. Expressed doubts related to universality do not inf­luence the research for it is analysing sub­jec­tive estimates on QMPA, not the key factors, or their impact to company performance.

0.1 Research Problem [2] provides a thorough review of studies related to TQM. It includes a number of views about how TQM can be operationalised and analysed.

The aim of this research is to observe quality managers’ subjective estimates of the qua­ lity management principles application (referred to as QMPA for the rest of the text) as an overall esti­mate of management commitment to QM and TQM. Focusing directly on estimates of QMPA is on­ly a part of the research. Investigating relations bet­ween estimates of QMPA and estimates of 852

other elements of TQM opeartionalisation is like­ ly to provide a wider picture of how quality ma­ nagers generally perceive TQM, and how or­ga­ nisations’ managements are generally com­mit­ted to TQM in the population observed. If ma­nage­ rs are committed to TQM, it is likely to ex­pect a correspondence between estimates of QM prin­cip­ les and TQM elements.

Quality management (QM) principles are defined in the ISO 9004 standard. The selection of TQM elements is treated later.

By thorough search of the available so­ur­ ces, it is determined that no systematic analyses re­lated to the QMPA in Serbian organisations have been carried out so far. This research may pro­vide some guidelines to managers willing to im­prove their business. Furthermore, it may provide so­me methodical innovations in the field.

0.2 Operationalisation of TQM Economic systems in transition generally do not operate in a favourable business en­vi­ron­ ment. The Serbian economy is generally cha­rac­te­ ri­sed by unstable business conditions, mainly low motivation of managers for achieving business success beyond the struggle for survival, high stress level of both managers and other employees on all levels, use of outdated technology, instability of supply chains and many other unfavourable attributes.

The lack of valid information in Serbia about financial and other performance indicators of organisations, the absence of a central registry of certified organisations, the lack of bench­mark­ ing data, the unwillingness to participate in the re­search and other factors did not disable the re­ search due to its focus on subjective estimates.

In order to carry out research to determine quality managers’ estimates of QMPA as an overall estimate of management commitment to QM practice in Serbia, the fact that consistent implementation of QM principles should assure the application of TQM and most of business excellence criteria was used.

These principles [namely: a) customer focus, b) leadership, c) involvement of people, d) pro­cess approach, e) system approach to ma­ na­ge­ment, f) continual improvement, g) factual ap­proach to decision-making and h) mutually be­

Radlovački, V. ‒ Beker, I. ‒ Majstorović, V. ‒ Pečujlija, M. ‒ Stanivuković, D. ‒ Kamberović, B.

Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, 851-861

ne­fi­cial supplier relationships] are described and analysed in [13].

The estimates of TQM operationalisation ele­ ments used in the research are (relations to cor­ res­ ponding ISO 9001 requirements are also enclosed): A Quality management principles application QMPA (proposed by the authors as a central element of the research), B QMS - quality management system, ISO 9001 chapter (ch.) 4 (proposed by the authors to see how managers perceive QMS in general), C key factors of QM success [12], ISO 9001 ch.

8.2, D approaches to improvement ([14], [15], [16] and [3]), ISO 9001 ch. 5.6, 8.2, 8.5, E changes after certification (proposed by the authors for information on how managers perceive benefits from the certification), F application of QM techniques [17], ISO 9001 ch. 8.1, G leadership ([16], [18], [19] and [3]), ISO 9001 ch. 5, 6, 7.1 and 7.5, H quality/strategy planning ([20] and [3]), ISO 9001 ch. 5.4 and 7.1, I human resource management ([21], [22], [20], [3] and [1]), ISO 9001 ch. 6.2, J purchase management [3], ISO 9001 ch. 7.4, K customer focus ([14], [15], [23], [16], [18], [19] and [3]), ISO 9001 ch. 5.2, 7.2, 7.5 and 8.2, L process management ([14], [16], [22], [20] and [3]), ISO 9001 ch. 4.1 and 7, M business results ([20] and [1]), ISO 9001 ch.

8.2, N learning ([14], [23], [16] and [7]) and O application of IT (proposed by the authors - information on the use level of IT in the population might provide relevant additional information).

While deciding on the construct, the idea was to focus to implementation of ISO 9001 requirements, not one-by-one, but as a whole.

This is, in short, why some requirements are not directly addressed by the construct.

0.3 Why Managers’ Subjective Estimates The major problem when investigating the relations between TQM elements and business

performance, as reported by [2], is obtaining the values of objective performance parameters of the companies. This problem is even more complex in Serbia today (problem is treated earlier).

Using the concept having the QMPA as the central point of the research enables exploring the interrelations of managers’ estimates of dif­fe­rent TQM elements and QMPA. Construct re­li­a­bi­lity/ validity test and findings of the research show that exploring exclusively perceptual data can provide valid and valuable results in the field.

This research is based on the data collected as a response from quality managers. [2] and [24] stated and confirmed that management leadership has a significant role in successful implementation of TQM practices and that management commitment support the motivation of employees to become more conscious of the need of a company to gain its goals. [2] reports of complaints from within the companies about the lack of management support and resistance to implement changes. The management com­mit­ ment is, she determined, often weak or missing.

1 RESEARCH HYPOTHESES Based on presented considerations, two general hypotheses as a basis for the model of the research were defined: H01: Subjective estimates of TQM element ap­ plication in an organisation may be statistically significant predictors of a subjective estimate of QMPA in an organisation and H02: Relevant objective parameters of or­ga­ni­sa­ti­ons may affect a subjective estimate of TQM ele­ments.

Now, it is possible to establish the following specific operating hypotheses resulting from the general H01 hypothesis: H1: Subjective estimate of QMS in organisations in general...

H2: Subjective estimate of QM success factors in organisations...

H3: Subjective estimate of current QM practice improvement approaches application in or­ga­ni­sa­ tions...

H4: Subjective estimate of changes in or­ga­ni­sa­ tions resulting from QMS implementation...

H5: Subjective estimate of the application of quality engineering techniques in organisations...

Quality Managers’ Estimates of Quality Management Principles Application in Certified Organisations in Transitional Conditions - Is Serbia Close to TQM?

853

Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, 851-861

H6: Subjective estimate of the application of lea­ dership in organisations...

H7: Subjective estimate of the application of qua­ li­ty and strategic planning in organisations...

H8: Subjective estimate of human resource ma­na­ ge­ment in organisations...

H9: Subjective estimate of purchase management in organisations...

H10: Subjective estimate of an organisation’s focus on customers...

H11: Subjective estimate of process management...

H12: Subjective estimate of business results and ana­lyses in organisations...

H13: Subjective estimate of learning in organisations...

H14: Subjective estimate of information technologies (IT) application as a support to the in­for­ma­tion system and QMS in organisations... ... may be a statistically significant pre­dic­ tor of a subjective estimate of QMPA. It is also possible to establish the following specific operating hypotheses resulting from the general hypothesis H02: H15: The number of employees in organisations...

H16: The form of organisation ownership...

H17: The nature of organisations (pro­ duc­ ti­ on/ service/combined)...

H18: The existence of a computer network, with an employed network administrator in an or­ga­ni­ sa­tion...

H19: The use of database and business ap­pli­ca­ tions in daily operation... ... may affect a subjective estimate of TQM elements.

A graphic presentation of the model re­sul­ ting from the general hypothesis H01 is shown in Fig. 1.

2 DESCRIPTION OF THE QUESTIONNAIRE For the purpose of the research, a questionnaire containing 147 questions divided into 15 groups (scales) was created.

Estimates are defined according to the discrete Likert-type scale.

The content validity of the questionnaire was carried out through subjective evaluation as in [3]. The items were considered as suitable because they were obtained from the literature reviews, in accordance with the goals of the research and characteristics of the population.

854

Fig. 1. Graphic presentation of the model resulting from H01 Values of Cronbach’s α coefficients ob­ tained by scale validity testing are given with each scale in the following text. All scales proved to be reliable (α > 0.7, [4]), as well as each of the items in the questionnaire.

Scale QMPA (A, α = 0.802) consists of 13 items which refer to a subjective estimate of QMPA in an organisation. This scale is a central scale of the research.

The scale "Estimate of QMS" (B, α = 0.738) consists of 8 items. An important factor of TQM application is the manner in which quality managers perceive be­ne­fits of the quality management system.

The scale "Key factors for QM success" (C, α = 0.776) consists of 9 items ‒ estimates whether process management, informing in QMS, employees interrelations, relations with users and other elements are key factors of QM success.

The scale "Approaches to improvement" (D, α = 0.754) consists of 5 items about standard approaches to management system improvement (internal audit, corrective and preventive actions and management review).

The scale “Changes after certification” (E, α = 0.938) consists of 18 items related to changes that were the result of the application of ISO 9001:2008 standard requirements (within first years after certification).

The scale "Quality engineering tech­ni­ques" (F, α = 0.942) consists of 12 items about quality

Radlovački, V. ‒ Beker, I. ‒ Majstorović, V. ‒ Pečujlija, M. ‒ Stanivuković, D. ‒ Kamberović, B.

Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, 851-861

engineering tech­niques application. Scales G ‒ N refer to a sub­jective estimate of organisation orientation to­wards gaining TQM. Those are: "Leadership" (G, 8 items, α = 0.937), "Qua­li­ ty/stra­te­gic planning” (H, 11 items, α = 0.927), "Human re­source ma­na­ge­ment” (I, 6 items, α = 0.872), "Pur­cha­se ma­na­ge­ment” (J, 2 items, α = 0.852), "Cus­to­mer focus” (K, 7 items, α = 0.795), "Process ma­na­ge­ment" (L, 7 items, α = 0.769), "Business results and analyses" (M, 6 items, α = 0.831 - practically, the trends of user satisfaction, profitability, employee satisfaction, business trends, productivity and performance measuring system effectiveness) and "Learning" (N, 3 items, α = 0.770).

The last one - “Application of IT” - refers to IT application as a support to the organisation information system (O, 32 items, α = 0.926).

3 SAMPLE AND DATA The questionnaire was sent for completion to ma­na­ge­ment representatives in 204 certified or­ ga­ni­sa­tions. 51 completed questionnaires (25%) were sent back.

Among organisations that submitted com­ ple­ted surveys, 12 are exclusively production or­ ga­nisations (23.5%), 14 organisations (27.5%) with combined core operations (both production and services) and 25 organisations (49%) exclusively providing services.

Seven organisations (13.73%) have from 1 to 9 employees, 17 organisations (33.33%) have from 10 to 49 employees, 23 organisations (45.10%) have from 50 to 249 employees and 4 organisations (7.84%) have 250 and more employees.

Among the respondents, 33 organisations (64.7%) are private property, 15 organisations (29.4%) are state property and 2 organisations (3.9%) are combined property.

The average age of QMS in the sample is 4.07 years, while σ is 2.78. Certification of the quality management system from the sample took place at least 1 year, or at the most 13 years before the survey.

4 RESULTS AND DISCUSSION Descriptive statistics are presented in Table 1. Respondents filled in 96.49% of the questionnaire items.

The highest mean is in scale K – Customer focus (4.46), and the lowest mean is in scale F – Application of quality engineering techniques (2.20). Table 1 presents all the scales of the questionnaire descriptors sorted descending by the mean.

4.1 Variations by Independent Variables The significant impact on estimates of TQM elements is present in the following cases: the number of employees (on three scales), and the existence of a computer network with an employed network administrator (as much as on 11 scales).

Table 1. Scales descriptive statistics No

Scales

1

K Customer focus J Purchase management G Leadership A QMPA N Learning C Key factors for QM success L Process management H Quality/strategy planning I Human resource management E Changes after certification O Application of IT B Estimate of QMS M Business results and analyses D Approaches to improvement F Quality engineering tech­ni­ques

2 3 4 5 6 7 8 9 10 11 12 13 14 15

Mean (sorted, desc.)

4.46 4.44 4.31 4.21 4.20

0.75 1.13 1.05 0.91 1.00

4.17

1.08

4.17

0.98

4.05

1.12

4.05

1.04

3.91

1.17

3.88 3.87

1.68 1.08

3.84

1.24

3.81

1.42

2.20

1.54

s

Other independent parameters (the nature of an organisation’s business, the use of databases

Quality Managers’ Estimates of Quality Management Principles Application in Certified Organisations in Transitional Conditions - Is Serbia Close to TQM?

855

Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, 851-861

Table 2. One-way ANOVA – Scales by independent: number of employees B Estimate of QMS

Between Groups Within Groups Total D Approaches Between Groups to Within Groups improvement Total M Business Between Groups results and Within Groups analyses Total O Applica­tion Between Groups of IT Within Groups Total

Sum of Squares 208.010 1000.813 1208.824 219.804 765.177 984.980 411.968 1106.777 1518.745 5475.404 30853.772 36329.176

and form of ownership) do not have a statistically significant impact on TQM elements.

Testing of the H02 showed that it could be accepted to a limited extent. Hypothesis H16, H17 and H19 are not confirmed.

4.1.1 Number of Employees (Hypothesis H15) Table 2 presents the results of the analysis.

A statistically significant impact on the scales B, D, M and O was determined. P-value for the scale O (slightly over 0.05) indicates that the impact of the number of employees on the scale O may conditionally be considered as significant.

Hypothesis H15 may be considered as confirmed to a limited extent.

4.1.2 Employed (Hypothesis H18)

Network

Administrator

The results of the analysis are presented in Table 3. A statistically significant impact on the scales A, B, C, D, E, I, K, L, M, N and O was determined.

The observed independent parameter shows a significant impact on subjective estimates of the TQM elements application. H18 is largely confirmed.

4.2 Predictors of a Subjective Estimate of the QM Principles Application (Scale A) The calculation (Table 4) shows that the regression model is fully statistically significant.

856

df 3 47 50 3 47 50 3 47 50 3 47 50

Mean Square 69.337 21.294

F 3.256

Sig. .030

73.268 16.280

4.500

.007

137.323 23.548

5.832

.002

1825.135 656.463

2.780

.051

The linear regression analysis (Table 5) showed that the scales B, C, E, I and L are statistically significant predictors of a subjective estimate of QMPA. 4.3 Interpretation of Results 4.3.1 Comments on H01 Validation The existence of high means might be attributed to the lack of proper information about the business excellent system, rather than to high QM performance of organisations from the sample.

Fig. 2 presents the results of regression model calculations obtained by testing general hypothesis H01.

Expected impacts of B, E, I and L scales on a subjective estimate of QMPA have been registered. Sub­jective estimates of QMS, changes after certi­fi­cation, human resource management and process management in an organisation are predictors of a subjective estimate of QMPA in Serbia.

Therefore, quality managers in Serbia estimate that, by certification, organisations have accomplished some of the main objectives – im­ proved process management, enhanced subjective es­timate of the system, gained substantial changes in an organisation and improved human resource ma­nagement.

However, the research shows that the ful­ fil­ment of customer requirements in Serbia seems not to be related to QMPA, i.e. not applied. It

Radlovački, V. ‒ Beker, I. ‒ Majstorović, V. ‒ Pečujlija, M. ‒ Stanivuković, D. ‒ Kamberović, B.

Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, 851-861

Table 3. One-way ANOVA – Scales by independent: existence of an employed network administrator

A QMPA B Estimate of QMS C Key factors for QM success D Approaches to improvement E Changes after certification I Human resource management K Customer focus L Process management M Business results and analyses N Learning O Application of IT

Between Groups Within Groups Total Between Groups Within Groups Total Between Groups Within Groups Total Between Groups Within Groups Total Between Groups Within Groups Total Between Groups Within Groups Total Between Groups Within Groups Total Between Groups Within Groups Total Between Groups Within Groups Total Between Groups Within Groups Total Between Groups Within Groups Total

Sum of Squares

df

Mean Square

F

Sig.

335.324 1513.265 1848.588 220.390

2 48 50 2

167.662 31.526

5.318

.008

.008

48

110.195 20.592

5.351

988.434 1208.824 193.105 1307.875 1500.980 225.570 759.410 984.980 2167.716 8252.441 10420.157

50 2 48 50 2 48 50 2 48 50 2 48 50

96.553 27.247

3.544

.037

112.785 15.821

7.129

.002

1083.858 171.926

6.304

.004

76.748 18.139

4.231

.020

5.728

.006

86.094 15.955

5.396

.008

133.793 26.066

5.133

.010

31.714 4.986

6.361

.004

9160.647 375.164

24.418

.000

153.497 870.660 1024.157

119.782

2

59.891

501.904 621.686 172.188 765.851 938.039 267.585 1251.160 1518.745 63.429 239.316 302.745 18321.294 18007.882 36329.176

48 50 2 48 50 2 48 50 2 48 50 2 48 50

10.456

Table 4. Regression model

1

Model Regression Residual Total

Sum of Squares 1610.103 238.485 1848.588

df 14 36 50

perfectly cor­ responds to the estimate of the application of qua­ lity engineering methods and techniques. The me­an of the scale F is by far the lowest in the ques­tion­naire (Table 1). [14] found no significant impact of continuous improvement and customer sa­tis­fac­tion (15 years ago).

Mean Square 115.007 6.625

F 17.361

Sig. .000a

The estimate of key factors for QM suc­ cess is a predictor of an estimate of QMPA, which indicates to the validity of hypothesis H2, but seems to be a paradox for β < 0. Managers in Serbia estimate that QM systems are more suc­ cessful if QM principles are less applied.

Quality Managers’ Estimates of Quality Management Principles Application in Certified Organisations in Transitional Conditions - Is Serbia Close to TQM?

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Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, 851-861

Table 5. Predictors of a subjective estimate of QMPA Model (Constant) O Application of IT N Learning M Business results and analyses L Process management K Customer focus J Purchase management 1 I Human resource management H Quality/strategy planning G Leadership F Quality engineering techniques E Changes after certification D Approaches to improvement C Key factors for QM success B Estimate of QMS

Unstandardized Coefficients Std. Error B 13.680 3.845 -.011 .023 .046 .290 -.044 .092 .338 .151 .130 .170 .410 .218 .492 .113 -.182 .101 .106 .117 -.010 .031 .178 .040 .026 .124 -.241 .099 .481 .115

Reasons for this finding are likely to be found in the aspirations of managers in Serbia and their familiarity with the purpose of QMPA. It is highly like­ly that mechanisms of internal audits, corrective and preventive actions and mana­ge­ment review are usually conducted for formal reasons (scale D is not a predictor of QMPA). Having in mind the fin­dings of [2] (“management’s lack of support, its resistance to change ... make it difficult to cul­ti­vate and take advantage of opportunities for TQM’s potential benefits”), findings of the re­ search so far give an impression that managers in Ser­bia are still not significantly committed to QM, TQM and continuous improvement. Some use­ful findings about the relationship between in­ter­nal and external audits and number of em­plo­yees of a company are given in [25].

Quality engineering techniques are im­ ple­men­ted poorly in Serbia. If va­li­da­ted methods are not used, then everything that is done is just a res­ponse to a situation, not the result of planning.

This also corresponds to the observed findings (scale H is not a predictor of QMPA). Achieving con­tinuous quality improvement implies the use of appropriate quality tools and techniques (as sta­ ted in [26]) and should be the obligatory aim in gaining leadership.

The subjective estimate of the leadership in organisations is also not a predictor of a sub­ 858

Standardized Coefficients

t

Sig.

-.050 .018 -.039 .241 .076 .142 .366 -.266 .122 -.023 .423 .019 -.217 .389

3.558 -.488 .157 -.474 2.235 .768 1.886 4.367 -1.797 .907 -.317 4.443 .209 -2.438 4.200

.001 .629 .876 .638 .032 .448 .067 .000 .081 .370 .753 .000 .836 .020 .000

β

jec­tive estimate of QMPA. The reasons are most likely very complex and detailed research thereof is beyond the scope of the research.

Fig. 2. Regression analysis results Quality managers should be aware of the need for the implementation of quality engine­ ering techniques.

A subjective estimate of quality and stra­te­ gic planning has not proved to be a predictor of QMPA either. Planning in unfavourable con­di­tions is much more complicated than planning in stable conditions.

The described findings imply the lack of awareness of these important relations bet­we­en business success and QMPA in Serbia.

Radlovački, V. ‒ Beker, I. ‒ Majstorović, V. ‒ Pečujlija, M. ‒ Stanivuković, D. ‒ Kamberović, B.

Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, 851-861

The part of the questionnaire related to hu­ man resource management, mostly treats basic re­qui­rements - training and employees’ attitudes.

These elements are predictors of QMPA. How­ ever, keeping in mind that these elements are obli­ gatory for the certification, the management is pres­sed to implement them and quality managers are involved.

As the price is usually the strongest (often the only) criterion for purchase in Serbia, it is pro­ bably the reason for not having purchase ma­na­ gement as a significant predictor of QMPA. Estimates of learning have also not proved to be a predictor of QMPA. However, it cannot be said that there is no on-the-job training – busi­ ness simply could not be continually conducted in such conditions. The cause may be in training ses­sions that are often conducted ad hoc, and are not recorded (the effectiveness thereof is eva­lu­ ated rarely in Serbian organisations). To im­ple­ ment learning appropriately in an organisation and aim it to gain better performance, managers should create constructive environment and motivate their employees ‒ this is also a part of ma­nagement commitment [2].

According to the authors’ experience, it is ra­re to see an example of effective IT application in Serbia today. The research has shown that the es­ti­mated level of IT application is also not a pre­ dic­tor of a subjective estimate of QMPA. 4.3.2 Comments on H02 Validation Two out of 6 independent parameters show statistically significant variations.

The number of employees influences esti­ ma­tes of QMS (B), approaches to improvement (D), business results (M) and the application of IT (O). Estimates significantly rise from micro to lar­ge organisations for all of the above. Managers of micro and small organisations may use this re­ sult to consider enhancing approaches to QMS im­ provement and IT efficiency. On the other hand, managers’ estimates of QMPA, quality plan­ ning, human resource management and other ele­ ments do not depend on organisational size. New research might offer an insight into the ca­u­ses and implications of these findings.

Taking into account the question of whet­ her an organisation has a network administrator employed or not proved to be very informative.

Here, the parameter is not blurred by subjectivity (as it probably is with the use of databases) ‒ an organisation simply employs administrator(s) or it does not. Estimates of almost every TQM element (even the one this research is focused on ‒ QMPA) show variations related to this parameter, and are higher if the organisation employs administrator. This is likely the consequence of the fact that decisionmaking based on facts derived from information systems is better when a network is administered from within the organisation. On the other hand, decision-making and QMS efficiency are closely related. Therefore, managers in Serbia can use these findings to consider employing a network administrator (or improving administering) for improving QMS efficiency. The efficiency of internal communications is a requirement of the ISO 9001 standard. Enhancing the information system is likely to provide better QMS. [27] report tight connections between TQM and IT in their detailed research.

5 CONCLUSIONS AND DIRECTIONS OF FURTHER RESEARCH This is, to the knowledge of authors, the first systematic review of TQM practice in Serbia.

The research has reached its aim and it has provided conclusions useful for both practitioners and researchers.

Estimates of the quality management principles application (QMPA) uncover the lack of awareness in an organisation’s management about the importance of applying QMPA, commitment to QM and, therefore, applying continuous improvements. It also contains observations on where to find some of the reasons for the treated standing of TQM practice (both subjective ‒ the lack of the managers’ awareness being one of the most important ‒ and objective ‒ the need for a more favourable organisational environment) in further research.

Findings have shown that it is possible to do valid research in the field based on subjective estimates of TQM elements only. The phase of construct validity using Cronbach alpha coefficients proved the items and scales are valid, as well as the regression model test. Furthermore, the determined importance of administering

Quality Managers’ Estimates of Quality Management Principles Application in Certified Organisations in Transitional Conditions - Is Serbia Close to TQM?

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Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, 851-861

computer networks and its impact to TQM elements empirically supports the validity of the model used.

Research results have also shown that the use of the QM principles application (QMPA) in the analysis of subjective estimates as a central point led to valid and valuable results.

It was not the intention of this research to explore the relationship between TQM practices and objective parameters of TQM performance.

This is the most significant drawback of the research. Keeping in mind the characteristics of the environment organisations operate in now, it is, at least, extremely difficult (if not impossible) to do more thorough research. Even in developed regions, managers are rarely willing to share these data with researchers [2].

It is not clear how “close” Serbia is to TQM, but it is obvious that the given attribute cannot illustrate the present standings. The “way” Serbia has to pass certainly goes through many points. Fostering the managers’ commitment to QM and TQM is one of them.

The study does not treat problems of integrating other management systems in QMS (such as environmental, OH&S) and their relations to QMPA. It does also not include any observations about the management of secondary materials and recycling (treated in detail in [28]), applying six-sigma, statistical and quality engineering techniques, and a number of other work process improvements, to increase performance and competitiveness. Some of these may be the subject of future research of TQM practice in Serbia on the way to improvement.

Managers are strongly advised to apply complex improvements using project strategies [29].

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Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11 Vsebina

Vsebina Strojniški vestnik - Journal of Mechanical Engineering letnik 57, (2011), številka 11 Ljubljana, november 2011 ISSN 0039-2480 Izhaja mesečno

Povzetki člankov Vid Novak, Rok Petkovšek, Boštjan Podobnik, Janez Možina: Kontinuirni vlakenski laser za frekvenčno podvojevanje Dario Croccolo, Massimiliano De Agostinis, Nicolò Vincenzi: Strukturna analiza šasije zgibnega mestnega avtobusa po MKE: uporaba metodologije na študiji primera Timo Kiekbusch, Daniel Sappok, Bernd Sauer, Ian Howard: Izračun kombinirane torzijske togosti ubiranja čelnih zobnikov z dvo- in tridimenzijskimi parametričnimi modeli po metodi končnih elementov Gordana Ostojic, Stevan Stankovski, Djordje Vukelic, Milovan Lazarevic, Janko Hodolic, Branko Tadic, Stevan Odri: Implementacija tehnologije samodejne identifikacije pri procesu montaže/demontaže vpenjal Tomaz Brajlih, Tadej Tasic, Igor Drstvensek, Bogdan Valentan, Miodrag Hadzistevic, Vojko Pogacar, Joze Balic, Bojan Acko: Možnosti uporabe trirazsežnega optičnega zajemanja pri preverjanju kompleksne geometrije Andrej Ljubenko, Alojz Poredoš, Miran Zager: Učinki sanacije vročevoda v sistemu daljinskega ogrevanja Dragan V. Petrović, Časlav B. Mitrović, Nataša R. Trišovic, Zorana Z. Golubović: O porazdelitvi velikosti delcev v diatomejski zemlji in perlitu različnih zrnavosti Vladan Radlovački, Ivan Beker, Vidosav Majstorović, Mladen Pečujlija, Dragutin Stanivuković, Bato Kamberović: Vodje kakovosti ocenjujejo uveljavljanje načel vodenja kakovosti v certificiranih organizacijah v pogojih tranzicije – ali je Srbija blizu celovitega obvladovanja kakovosti?

SI 163 SI 164 SI 165 SI 166 SI 167 SI 168 SI 169

SI 170

Navodila avtorjem

SI 171

Osebne vesti Doktorske disertacije in diplome

SI 173

Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, SI 163

Prejeto: 20.05.2011 Sprejeto: 05.09.2011

Kontinuirni vlakenski laser za frekvenčno podvojevanje Novak, V. – Petkovšek, R. – Podobnik, B. – Možina, J. Vid Novak1 – Rok Petkovšek2,* – Boštjan Podobnik1 – Janez Možina2 1 LPKF Laser & Elektronika d.o.o, Slovenija 2 Univerza v Ljubljani, Fakulteta za strojništvo, Ljubljana, Slovenija

Vlakenski laserji so zaradi številnih prednosti v primerjavi z doslej poznanimi in uporabljanimi laserskimi viri še posebej zanimivi za obdelavo materialov. Njihove glavne prednosti v primerjavi s klasičnimi vrstami laserjev so visok energijski izkoristek, kakovost žarka, kompaktnost in robustnost ter dolga življenjska doba.

Primernost laserskega izvora za obdelavo določa predvsem visoka svetilnost. Ta je pogojena s sposobnostjo doseganja visoke kakovosti izhodnega žarka pri visokih optičnih močeh, kar je osnovna značilnost vlakenskih laserjev. Če ima laser polariziran izhod in ustrezno spektralno širino, je mogoče z uporabo nelinearnega optičnega kristala doseči učinkovito frekvenčno pomnoževanje žarka in s tem še dodatno razširiti področje uporabe vlakenskih laserjev na obdelavo materialov, ki imajo nizko stopnjo absorpcije pri osnovni frekvenci, kot so npr. polimeri in kompozitni materiali.

V članku je predstavljen kontinuirni vlakenski laser tipa oscilator-ojačevalnik s polprevodniško vzbujevalno lasersko diodo in z enostopenjskim mikrostrukturiranim iterbijevim vlakenskim ojačevalnikom.

Uporabljena konfiguracija oscilator-ojačevalnik omogoča, da šibek signal enorodovne vzbujevalne laserske diode z močjo velikostnega reda nekaj 10 mW, spektralno širino 300 MHz in polarizacijskim kontrastom >30 dB ojačimo do izhodnih moči velikostnega reda nekaj W. Ojačeni laserski žarek z valovno dolžino 1064 nm ohrani kakovost, spektralno širino in polarizacijski kontrast, kar omogoča frekvenčno transformacijo žarka v vidni del spektra.

Postavljeni eksperimentalni vlakenski laser lahko pri stopnji ojačenja 24 dB doseže najvišjo izhodno moč 7,5 W. Izkoristek optično-optične pretvorbe znaša 50 %, polarizacijski kontrast ojačenega žarka pa presega vrednost 30 v celotnem razponu izhodne moči. Zasnova laserja omogoča tudi nadgradnjo z varjenimi spoji med posameznimi komponentami. S tem se lahko bistveno povečata kompaktnost in robustnost sistema. Prav tako pričakujemo možnost doseganja višje izhodne moči, saj je ta omejena le z razpoložljivo močjo črpalnega sistema ojačevalnika.

Z optimizacijo parametrov sistema in uporabo mikrostrukturiranega aktivnega vlakna s krajšo interakcijsko dolžino med aktivnim medijem ter laserskim in črpalnim žarkom je možno doseči večjo stopnjo ojačenja z eno samo ojačevalno stopnjo, kar predstavlja bistveno prednost predstavljenega sistema v primerjavi z obstoječimi večstopenjskimi izvedbami. ©2011 Strojniški vestnik. Vse pravice pridržane.

Keywords: iterbijev vlakenski ojačevalnik, laser tipa oscilator-ojačevalnik, laser z ozkopasovnim izhodom, laser s transverzalno-enorodovnim izhodom, laser s polariziranim izhodom, laser s frekvenčno podvojenim izhodom

*Naslov avtorja za dopisovanje: Univerza v Ljubljani, Fakulteta za strojništvo, Aškerčeva 6, 1000 Ljubljana, Slovenija, rok.petkovsek@fs.uni-lj.si

SI 163

Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, SI 164

Prejeto: 05.04.2011 Sprejeto: 04.10.2011

Strukturna analiza šasije zgibnega mestnega avtobusa po MKE: uporaba metodologije na študiji primera Croccolo, D. ‒ De Agostinis, M. ‒ Vincenzi, N. Dario Croccolo* ‒ Massimiliano De Agostinis ‒ Nicolò Vincenzi Univerza v Bologni, DIEM – Oddelek za strojništvo, Italija

Namen tega dela je simulacija in napovedovanje odzivov konstrukcije velikega mestnega avtobusa, ki je sestavljen iz dveh šasij in lahko prevaža do 160 potnikov. Avtobus je dolg 18 m in tehta pri polni obremenitvi približno 30.000 kg. Izračunane so napetosti, deformacije in premiki konstrukcije (jeklene cevne šasije) pri različnih obremenitvah in robnih pogojih za realne delovne obremenitve avtobusa.

Cilji raziskave so bili doseženi v tesnem sodelovanju s proizvajalcem avtobusa, ki je avtorjem posredoval informacije za ustrezno določitev obremenitev in robnih pogojev. Nato je bila opravljena analiza občutljivosti parametrov MKE, kot so vrste elementov in njihove karakteristične dimenzije, ki daje ustrezen kompromis med računskim časom in natančnostjo rezultatov.

Avtorji so pri reševanju strukturnega problema na velikih sestavih izkoristili zmogljivosti sodobnih komercialnih paketov za MKE. Analiziran je odziv okvirja na različne obremenitvene pogoje, ki se tipično pojavljajo pri delu: (i) delovanje gravitacijskega pospeška, (ii) zaviranje pri zgornji meji pojemka vozila, (iii) manevri zavijanja in (iv) torzijske obremenitve zaradi neravnin na cesti. Avtobusna šasija je izvedena kot varjen okvir, zato je bila še posebna pozornost posvečena stikom med elementi sestavov za pridobivanje zanesljivega odziva konstrukcije (togost in premiki). Komponente šasije so za časovno učinkovitejši model predstavljene kot 2D-elementi (lupinski) in kot 3D-elementi (polni kvadri ali tetraedri).

Avtobusna šasija in še zlasti njen zadnji del je izpostavljena največjim obremenitvam pri manevrih zaviranja in zavijanja. Potrebne izboljšave konstrukcije okvirja so bile določene na osnovi dovoljenega napetostnega stanja in ugotovljeno je bilo, da nekateri elementi okvirja potrebujejo ojačitve. Vse potrebne spremembe smo predlagali proizvajalcu.

Za verifikacijo rezultatov numeričnih simulacij je treba izvesti eksperimentalno analizo napetosti z merilnimi lističi. Članek ima praktično vrednost za vsakogar, ki se ukvarja s simulacijami odziva konstrukcije velikih varjenih sestavov. V njem je predstavljen tako značilen potek dela kot tudi kritični parametri, ki jih mora konstruktor imeti pod nadzorom, če želi zanesljive rezultate za napetosti in premike. ©2011 Strojniški vestnik. Vse pravice pridržane.

Keywords: avtobus, konstrukcija, okvir, šasija, študija primera, MKE

SI 164

*Naslov avtorja za dopisovanje: Univerza v Bologni, DIEM – Oddelek za strojništvo, Viale Risorgimento 2, 40136 Bologna, Italija, dario.croccolo@unibo.it

Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, SI 165

Prejeto: 07.12.2010 Sprejeto: 02.08.2011

Izračun kombinirane torzijske togosti ubiranja čelnih zobnikov z dvo- in tridimenzijskimi parametričnimi modeli po metodi končnih elementov Kiekbusch, T. ‒ Sappok, D. ‒ Sauer, B. ‒ Howard, I. Timo Kiekbusch1,* ‒ Daniel Sappok1 ‒ Bernd Sauer1 ‒ Ian Howard2 1 Univerza v Kaiserslauternu, Institut za strojne elemente, zobnike in prenosnike, Nemčija 2 Curtinova univerza v Perthu, Oddelek za strojništvo, Avstralija

Zobniki so ena glavnih komponent najrazličnejših rotacijskih strojev in imajo pogosto ključno vlogo pri delovanju strojev. Ena od najpomembnejših značilnosti čelnih zobnikov je torzijska togost ubiranja. Poznavanje torzijske togosti ubiranja zobniških parov je zato izjemno pomembno pri analizi in razvoju menjalnikov. V zadnjih letih je bilo več poskusov preučitve in popisovanja procesa ubiranja čelnih zobnikov. Proces ubiranja je zelo zahteven, zlasti ob upoštevanju sprememb bokov zob in realne situacije pri montaži zobnikov, ki vključuje tako napake pri poravnavi kot odstopanja razdalje med osema.

Prva izbira metode pri preučevanju medsebojnih razmerij je zato analiza po metodi končnih elementov.

Upoštevanje vseh vplivov geometrije na proces ubiranja zahteva podrobno 3D-analizo po metodi končnih elementov. 3D-simulacije s končnimi elementi pa zahtevajo veliko časa in virov in tako niso primerne npr. za simulacije več teles. Zato je poleg 3D-analize s končnimi elementi potreben še dodaten pristop. Za kompenzacijo slabosti 3D-modela po metodi končnih elementov je bil razvit 2D-model po metodi končnih elementov, iz katerega je bila izpeljana enostavna formula za izračun togosti ubiranja.

V članku je predstavljen razvoj podrobnih dvo- in tridimenzionalnih modelov po metodi končnih elementov, ki so uporabni za izračun torzijske togosti ubiranja. Simulacija se izvede samodejno s pomočjo parametričnega jezika programske opreme ANSYS za simulacije po MKE. Simulacija vključuje ustvarjanje zobniške dvojice, ubiranje zobnikov, uveljavitev robnih pogojev in obremenitev ter postopek reševanja in naknadne obdelave. Pri reševanju je bil za kontaktne površine uporabljen adaptivni algoritem mreženja, ki daje majhne elemente za izračun stika. Avtomatizirani postopek omogoča ustvarjanje in analizo različnih parov čelnih zobnikov. 2D-model je zaradi kratkih računskih časov primeren za hitro izvedbo simulacij različnih zobniških parov. Zahtevnejši 3D-model daje več možnosti za preučevanje sprememb bokov zob v nadaljnjih študijah. Dobljene vrednosti torzijske togosti je mogoče uporabiti npr. pri simulacijah menjalnikov z več telesi. Rezultati 2D-analize s končnimi elementi so bili uporabljeni za izpeljavo enostavne formule za kombinirano torzijsko togost čelnih zobnikov v ubiru. Predstavljeni so rezultati na osnovi posamičnih togosti treh glavnih komponent – telesa, zob in stika. Izpeljana formula določa s pomočjo teh treh delov celotno togost za različne kombinacije zobnikov in prestavnih razmerij.

Končno je predstavljena primerjava rezultatov dvo- in tridimenzionalnega modela po metodi končnih elementov ter rezultatov izpeljane formule, ter verifikacija rezultatov 3D-modela glede na rezultate analitičnih enačb. Primerjave kažejo verodostojnost tako obeh modelov MKE kakor tudi formule za togost ubiranja. Pri simulacijah in izračunih je bilo sicer nekaj poenostavitev z ozirom na realno kontaktno situacijo, upoštevano ni bilo npr. mazanje, trenje in tolerance. Dobljeni rezultati in ugotovljena odstopanja so v tem pogledu popolnoma zadovoljivi.

V članku sta predstavljena dva različna načina za analizo parov čelnih zobnikov. Enostavna formula omogoča hiter izračun torzijske togosti ubiranja samo iz glavnih parametrov zobnikov. Za razliko od drugih metod izračunavanja togosti ubiranja, npr. po DIN 3990, tukaj ni potrebno določati množice koeficientov, zato je uporaba formule zelo enostavna. Drugi način je uporaba 3D-modela s končnimi elementi. Model omogoča izračunavanje torzijske togosti ubiranja ob upoštevanju vseh geometrijskih vidikov zobnikov, zlasti sprememb bokov zob. Ena glavnih prednosti modela je tudi v tem, da se celotna simulacija izvaja v razširjenem komercialnem programskem paketu za analize po metodi končnih elementov. ©2011 Strojniški vestnik. Vse pravice pridržane.

Keywords: modeliranje dinamike zobnikov, čelni zobnik, modeliranje s končnimi elementi, torzijska togost ubiranja, kontaktna togost *Naslov avtorja za dopisovanje: Univerza v Kaiserslauternu, Institut za strojne elemente, zobnike in prenosnike, Gottlieb-Daimler-Str., 67663 Kaiserslautern, Nemčija, timo.kiekbusch@mv.uni-kl.de

SI 165

Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, SI 166

Prejeto: 10.6.2010 Sprejeto: 24.6.2011

Implementacija tehnologije samodejne identifikacije pri procesu montaže/demontaže vpenjal

Ostojic, G. ‒ Stankovski, S. ‒ Vukelic, D. ‒ Lazarevic, M. ‒ Hodolic, J. ‒ Tadic, B. ‒ Odri, S. Gordana Ostojic1,* ‒ Stevan Stankovski1 ‒ Djordje Vukelic1 ‒ Milovan Lazarevic1 ‒ Janko Hodolic1 ‒ Branko Tadic2 ‒ Stevan Odri1 1 Tehniška fakulteta, Univerza v Novem Sadu, Srbija 2 Fakulteta za strojništvo, Univerza v Kragujevcu, Srbija Namen tega članka je predstavitev novega pristopa k identifikaciji vpenjal pri montaži/demontaži vpenjal za operacije strojne obdelave in kontrole. Predlagani pristop omogoča popoln nadzor nad tokom materiala z ozirom na proizvodni proces in na njegovo okolje. Opisani pristop prav tako daje na razpolago podatke o številu vpenjal, ki jih je treba montirati ali demontirati, kakor tudi o elementih vpenjal, ki jih je mogoče ponovno uporabiti, o elementih vpenjal, ki jih je mogoče obnoviti, ter o količini in vrsti materialov za reciklažo.

Montaža/demontaža vpenjal je proces, ki ima vse večji vpliv na proizvodnjo vpenjal, saj kompleksni obdelovanci zahtevajo kompleksna vpenjala. Nekateri vpenjalni sistemi lahko vsebujejo tudi več kot 150 elementov. Veliko število vpenjalnih elementov pomeni tudi podaljšanje časa za montažo/demontažo vpenjal. Montaža/demontaža vpenjal mora biti učinkovita za povečanje produktivnosti in znižanje celotnih proizvodnih stroškov. Učinkovitost je mogoče doseči s samodejno identifikacijo elementov in sestavov vpenjal.

Tehnologija radiofrekvenčne identifikacije (RFID) je tehnologija samodejne identifikacije, ki je uporabna v različnih fazah življenjskega cikla izdelka, zlasti v fazi proizvodnje. V članku je predstavljena analiza možnosti uporabe tehnologije RFID pri strojni obdelavi in kontroli v procesu montaže/demontaže vpenjal. Predstavljena sta tudi koncept in struktura sistema montaže/demontaže vpenjal. V analizi so predstavljene komponente strojne in programske opreme, ki je vključena v sistem za montažo/demontažo vpenjal.

Verifikacija sistema je bila opravljena v laboratorijskih pogojih. Uporabljeno je bilo 96 delov in ustreznih vpenjal. Za vpenjala so bile določene vse operacije, ki so potrebne za njihovo montažo/ demontažo. Tehnologija RFID je spremljala vpenjala skozi celoten življenjski cikel - od izdaje elementov vpenjal iz skladišča, montaže, prek uporabe pa vse do demontaže in vračila elementov v skladišče.

Analiza rezultatov verifikacije laboratorijskega sistema kaže, da se odstotkovne vrednosti v nekaterih primerih spreminjajo tudi za več kot 20 %, vendar vedno v korist identifikacije RFID, s čimer je tudi upravičena njena uporaba.

Sistemi, ki vključujejo tehnologijo RFID, so bolj fleksibilni glede sposobnosti sprejema in obdelave različnih vrst vpenjal, pa tudi drugih izdelkov. Nadaljnje raziskave bi bilo smiselno usmeriti v možnosti sledenja spremembam statusa vpenjal v različnih fazah življenjskega cikla.

Sistem, ki je predlagan in preizkušen v tem članku, je zasnovan na tehnologiji samodejne identifikacije, kar je novost v primerjavi s konvencionalnimi sistemi. Verifikacija sistema v laboratorijskih pogojih je bila uspešno opravljena za različne vrste vpenjal, ki so bila razvrščena v skupine glede na podobnost postopkov pri procesu montaže/demontaže. Na ta način je omogočeno skrajšanje časa montaže/ demontaže ter neposredno izboljšanje produktivnosti. ©2011 Strojniški vestnik. Vse pravice pridržane.

Keywords: vpenjala, samodejna identifikacija, tehnologija RFID

SI 166

*Naslov avtorja za dopisovanje: Tehniška fakulteta, Univerza v Novem Sadu, Trg Dositeja Obradovica 6, 21000 Novi Sad, Srbija, goca@uns.ac.rs

Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, SI 167

Prejeto: 07.07.2010 Sprejeto: 03.08.2011

Možnosti uporabe trirazsežnega optičnega zajemanja pri preverjanju kompleksne geometrije

Brajlih, T. ‒ Tasic, T. ‒ Drstvensek, I. ‒ Valentan, B. ‒ Hadzistevic, M. ‒ Pogacar, V. ‒ Balic, J. ‒ Acko, B. Tomaz Brajlih1,* ‒ Tadej Tasic1 ‒ Igor Drstvensek1 ‒ Bogdan Valentan1 ‒ Miodrag Hadzistevic2 ‒ Vojko Pogacar1 ‒ Joze Balic1 ‒ Bojan Acko1 1Univerza v Mariboru, Fakulteta za strojništvo, Slovenija 2Univerza v Novem Sadu, Fakulteta tehničnih znanosti, Srbija Brezkontaktno optično zajemanje, merjenje in digitalizacija postajajo vedno bolj razširjeni postopki v sistemih zagotavljanja kakovosti. Prednosti optičnega skeniranja v primerjavi s konvencionalnimi kontaktnimi merilnimi postopki so preprosto zajemanje, visoka gostota pridobljenih podatkov ter povezava med povratnim inženirstvom in preverjanjem oblike. Optični skener se zaradi trirazsežnega zajemanja podatkov pogosto obravnava kot alternativa koordinatni merilni napravi. Prednost optičnega skeniranja je predvsem večja hitrost zajemanja podatkov, medtem ko natančnost še ne dosega ravni koordinatne merilne tehnike.

Ta prispevek obravnava možnosti uporabe optičnega skenerja pri preverjanju natančnosti izdelave.

Predstavljen je primer, pri katerem je za preverjanje natančnosti izdelka uporabljen optični skener GOM ATOS II. V prvem delu prispevka je predstavljeno specifično področje preverjanja natančnosti medicinskih vsadkov. Predstavljene so prednosti, zaradi katerih je optično skeniranje pri takšnih izdelkih primernejše od koordinatnega merjenja. V drugem delu prispevka so predstavljeni rezultati optičnega zajemanja geometrije merilnih kladic ter merilna negotovost postopka. V zadnjem delu je predstavljena neposredna primerjava rezultatov optičnega skeniranja in koordinatnega merjenja krogle.

Glede na rezultate merjenja merilnih kladic in primerjave rezultatov merjenja krogle smo dokazali, da je natančnost optičnega skenerja GOM ATOS II primerna za preverjanje oblike medinskih vsadkov z ozirom na zahtevano natančnost izdelka pred samim operacijskim posegom.

Nadaljnje raziskave na tem področju bodo namenjene predvsem ločevanju in vrednotenju merilnih pogreškov optičnega zajemanja, ki so posledica nenatančnosti same naprave ter pogreškov, ki nastajajo zaradi naknadne poobdelave (poligonizacije) zajetih podatkov.

Preizkus natančnosti optičnega sistema je bil omejen na merjenje merilnih kladic in na primerjavo meritve krogle s koordinatno merilno napravo.

Prispevek predstavlja izvirno področje preverjanja natančnosti izdelave kompleksnih geometrijskih oblik. Predstavljena sta postopek zajemanja in poobdelave podatkov ter metoda preizkusa natančnosti naprave. Prispevek je namenjen vsem, ki se ukvarjajo s preverjanjem kompleksnih geometrijskih oblik, saj se bo z razvojem novih sistemov za optično zajemanje oblik povečevala tudi njihova natančnost. Te naprave bodo zato postajale vedno pomembnejše na širšem področju zagotavljanja kakovosti in ne samo na specifičnem primeru, predstavljenem v tem članku. ©2011 Strojniški vestnik. Vse pravice pridržane.

Keywords: trirazsežno optično skeniranje, merilna negotovost, preverjanje geometrije, hitra izdelave, povratno inženirstvo, zagotavljanje kakovosti

*Naslov avtorja za dopisovanje: Univerza v Mariboru, Fakulteta za strojništvo, Smetanova 17, SI-2000 Maribor, Slovenija, tomaz.brajlih@uni-mb.si

SI 167

Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, SI 168

Prejeto: 02.11.2010 Sprejeto: 27.07.2011

Učinki sanacije vročevoda v sistemu daljinskega ogrevanja

Ljubenko, A. ‒ Poredoš, A. ‒ Zager, M. Andrej Ljubenko1,* ‒ Alojz Poredoš1 ‒ Miran Zager2 1 Univerza v Ljubljani, Fakulteta za strojništvo, Slovenija 2 Komunalno podjetje Velenje d.o.o., Slovenija Precejšen delež energijskih izgub pri sistemih daljinskega ogrevanja je povezan s transportom toplote do potrošnika. Izgube imajo pomemben vpliv na termoekonomsko ustreznost zagotavljanja energetske oskrbe odjemalcem, zato je nujno pazljivo načrtovanje, dimenzioniranje in upravljanje sistema. Zaradi pomanjkljivih podatkov o toplotnih izgubah distribucijske mreže jih distributerji pogosto podcenjujejo. Sistemi daljinskega ogrevanja imajo dolgo dobo trajanja, v kateri lahko pride do bistvenega poslabšanja izolacije cevi ali tehničnega napredka izvedbe in materialov izolacije. Zato lahko s sanacijo posameznih odsekov dosežemo pomembne energijske in finančne prihranke, ti pa pomenijo kratko vračilno dobo naložbe. Pri vročevodih vprašljive konstrukcije in stanja izolacije so nujne raziskave toplotnih izgub in možnosti za izboljšanje.

V prispevku je analiziran nadzemni magistralni vročevod ID 350 dolžine 5.430 m, ki povezuje Termoelektrarno Šoštanj in Centralno energetsko postajo kot del sistema daljinskega ogrevanja Šaleške doline. Zaradi ugotovljenih visokih toplotnih izgub je bil saniran.

Za določitev toplotnih izgub vročevoda obstaja vrsta eksperimentalnih metod. V nekaterih primerih jih lahko določimo ob nemoteni dobavi toplote do odjemalcev. Ob zahtevah po določitvi celotnih toplotnih izgub na izbranem odseku, kar zajema realno stanje vseh elementov vročevodnega omrežja, jih lahko določimo z integralno merilno metodo. Za primer nemotene oskrbe s toploto za odjemalce je bila razvita prilagojena integralna merilna metoda, katere osnova je korelacijsko merjenje hitrosti obtočne vode. Ob ugotovljenih visokih toplotnih izgubah vročevodov je potrebno ugotavljanje razlogov za njih.

Pred odločitvijo za sanacijo je nujna raziskava ali je obstoječa konstrukcija izolacije cevi primerna in so visoke toplotne izgube posledica dotrajanega stanja izolacije, ali pa je njena konstrukcija neustrezna in je potrebna obširnejša zamenjava tehnologije izolacije. V primeru kompleksne geometrije konstrukcije izolacije cevi določimo temperaturno polje, ki nastopa v njej, z numerično analizo.

Numerična analiza obravnavanega vročevoda je pokazala, da so glavni razlogi za visoke toplotne izgube vročevoda ID 350 pred sanacijo v neustrezni konstrukciji izolacije cevi. Prilagojena integralna merilna metoda se je izkazala kot primerna za določitev toplotnih izgub vročevoda. Določeni učinki sanacije so pokazali, da je bilo stanje pred sanacijo neustrezno. Po sanaciji so toplotne izgube manjše za 77%, kar bi po podatkih iz leta 2007 za distributerja pomenilo za 124.924 € manjše stroške za nabavo toplote.

Prispevek kaže na potrebo po analiziranju toplotnih izgub vročevodov z nepoznanim stanjem in toplotnimi izgubami, zaradi možnih velikih potencialov za izboljšanje energetske učinkovitosti in ekonomike obratovanja.

Na novo razvita je prilagojena integralna merilna metoda. Kombinacija numeričnih analiz in meritev kaže na glavne možnosti za izboljšanje energetske učinkovitosti in je koristna vsem, ki se ukvarjajo s tehnologijami izolacij cevi in distribucijo toplote po vročevodnem omrežju. ©2011 Strojniški vestnik. Vse pravice pridržane.

Ključne besede: daljinsko ogrevanje, sanacija vročevoda, toplotne izgube, toplotni mostovi

SI 168

*Naslov avtorja za dopisovanje: Univerza v Ljubljani, Fakulteta za strojništvo, Aškerčeva 6, 1000 Ljubljana, Slovenija, andrej.ljubenko@fs.uni-lj.si

Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, SI 169

Prejeto: 02.03.2010 Sprejeto: 24.06.2011

O porazdelitvi velikosti delcev v diatomejski zemlji in perlitu različnih zrnavosti

Petrović, D.V. ‒ Mitrović, Č.B. ‒ Trišovic, N.R. ‒ Golubović, Z.Z. Dragan V. Petrović1 ‒ Časlav B. Mitrović2,* ‒ Nataša R. Trišovic2 ‒ Zorana Z. Golubović2 1 Univerza v Beogradu, Fakulteta za kmetijstvo, Srbija 2 Univerza v Beogradu, Fakulteta za strojništvo, Srbija Filtriranje tekočin je pomemben del številnih tehnoloških procesov. Pri filtriranju se pogosto uporabljajo pomožni filtrirni materiali kot sta kieselgur (znan tudi kot diatomejska zemlja – DZ) in perlit.

Porazdelitev velikosti delcev v pomožnem filtrirnem materialu ima pomemben vpliv na proces filtracije.

Na voljo so različne zrnavosti DZ in perlita. Izbira zrnavosti je odvisna od lastnosti filtrirane tekočine in od želene stopnje filtracije. Perlit se običajno uporablja za grobo filtracijo, diatomejska zemlja pa je primerna tudi za finejšo filtracijo. Običajna je tudi kombinirana uporaba obeh materialov v skupnem filtrirnem procesu.

Za filtriranje medijev z visoko vsebnostjo netopnih trdnih delcev in z vsebnostjo snovi, ki inhibirajo filtracijo, je možno uporabiti različne pomožne filtrirne materiale. Med drugim se za ta namen uporabljata tudi kieselgur in perlit. Izbira uporabljene zrnavosti je odvisna od lastnosti filtrirane tekočine in od želene stopnje filtracije.

V članku je predstavljena in analizirana porazdelitev velikosti delcev za DZ treh zrnavosti in za eno zrnavost perlita. Porazdelitev velikosti delcev se meri z morfometrično metodo, ki je bila razvita predvsem za mikrobiološke analize, dokaj učinkovita in natančna pa je tudi pri ocenjevanju različnih mikroskopskih objektov, vključno s trdnimi delci.

V članku je predstavljen pristop na osnovi iskanja najboljšega prilega. Pri tem pristopu se velika količina podatkov popiše s pomočjo empirične porazdelitve z majhnim številom parametrov. Za vse obravnavane vzorce so bili izračunani in primerjani osnovni statistični parametri. Preverjena je bila uporabnost hiperbolične in log-hiperbolične funkcije za popis porazdelitve velikosti delcev določene zrnavosti. Porazdelitev velikosti delcev v vseh preizkušenih filtrirnih materialih je popačena in odstopa od normalne Gaussove porazdelitve.

V tem delu je preverjena uporabnost predstavljenih eksperimentalnih in numeričnih metod za analizo in popis porazdelitve velikosti delcev filtrirnih medijev različnih zrnavosti. Hiperbolična in log-hiperbolična funkcija dajeta podobne rezultate pri iskanju najboljšega prilega. Izkazalo se je, da ima pri tovrstnih nalogah rahlo prednost log-hiperbolični model.

Predstavljeni pristop omogoča podrobno analizo zrnavosti pomožnih filtrirnih materialov. Iz nekaj razpoložljivih zrnavosti je tako mogoče sestaviti vrsto različnih zrnavosti z ustrezno porazdelitvijo velikosti delcev, ki ustrezajo zahtevam glede filtracije.

Analogen pristop je uporaben tudi za modeliranje porazdelitve velikosti delcev onesnaževal v filtrirani tekočini ter omogoča ugotavljanje povezav med vstopnimi/izstopnimi parametri fluida in porazdelitvami velikosti delcev filtrirnega materiala. ©2011 Strojniški vestnik. Vse pravice pridržane.

Ključne besede: filtracija, kieselgur, perlit, zrnavost, porazdelitev velikosti delcev

*Naslov avtorja za dopisovanje: Univerza v Beogradu, Fakulteta za strojništvo, Kr. Marije 16, 11000, Beograd, Srbija, cmitrovic@mas.bg.ac.rs

SI 169

Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, SI 170

Prejeto: 29.09.2010 Sprejeto: 02.09.2011

Vodje kakovosti ocenjujejo uveljavljanje načel vodenja kakovosti v certificiranih organizacijah v pogojih tranzicije – ali je Srbija blizu celovitega obvladovanja kakovosti?

Radlovački, V. ‒ Beker, I. ‒ Majstorović, V. ‒ Pečujlija, M. ‒ Stanivuković, D. ‒ Kamberović, B. Vladan Radlovački1,* ‒ Ivan Beker1 ‒ Vidosav Majstorović2 ‒ Mladen Pečujlija1 ̶ Dragutin Stanivuković1 ‒ Bato Kamberović1 1 Tehniška fakulteta, Katedra za industrijski inženiring in management, Novi Sad, Srbija 2 Fakulteta za strojništvo, Katedra za proizvodni inženiring, Beograd, Srbija

Pridobivanje in analiza splošnih informacij o prediktorjih uveljavljanja načel vodenja kakovosti (QMPA) med elementi operacionalizacije celovitega obvladovanja kakovosti (TQM) je zanimivo tako za praktike kot za raziskovalce tega področja. Te informacije razkrivajo zelo pomembne in nezadostno raziskane odnose med praksami TQM in načini, kako te prakse dojemajo vodje kakovosti. Uporabne so lahko tudi kot smernice za splošne izboljšave sistemov obvladovanja kakovosti v organizacijah v regiji.

Splošni namen raziskave dopolnjujejo izsledki, ki se nanašajo na spremembe neodvisnih parametrov. Gre za izvirno raziskavo, saj avtorjem ni znano, da bi bile v regiji ali kje drugje opravljene študije z enakimi ali podobnimi cilji.

Med vodjami kakovosti v srbskih certificiranih organizacijah je bil izbran reprezentativni vzorec anketirancev, ki so prejeli ad hoc vprašalnik. Anketiranci so podali svoje ocene o naslednjih elementih: QMPA, splošna ocena sistema obvladovanja kakovosti (QMS), ključni dejavniki uspešnosti obvladovanja kakovosti (QM), pristop k izboljšavam, spremembe po certifikaciji, tehnike inženiringa kakovosti, vodenje, načrtovanje kakovosti/strategij, upravljanje s človeškimi viri, upravljanje nabav, usmerjenost h kupcem, vodenje procesov, poslovni rezultati in analize, učenje in uporaba informacijskih tehnologij (IT). Osrednji element študije so bile ocene elemente QMPA, ki so jih podali vodje kakovosti. Pri tem je bila uporabljena predpostavka, da višji oceni naštetih elementov ustrezajo višje ocene elementa QMPA. Anketa je zasnovana tako, da omogoča iskanje signifikantnih odvisnosti med osrednjim parametrom in več neodvisnimi spremenljivkami. Rezultati raziskave so bili validirani.

Za iskanje prediktorjev QMPA med elementi TQM je bila uporabljena multipla linearna regresija, za iskanje signifikantnih sprememb osrednjega parametra pa je bilo narejenih več enosmernih analiz variance (ANOVA).

Cilj raziskave je bil poiskati tiste ocene naštetih elementov TQM, ki so v korelaciji z ocenami vodij za QPMA. Te informacije omogočajo razkrivanje nekaterih slabosti današnjih sistemov obvladovanja kakovosti in vpeljavo nekaterih temeljnih izboljšav v sisteme obvladovanja kakovosti.

Med naštetimi elementi so samo štirje prediktorji QMPA: ocena QMS, vodenje človeških virov, vodenje procesov in spremembe po certifikaciji. Srbski vodje kakovosti nepričakovano ocenjujejo, da so sistemi obvladovanja kakovosti uspešnejši, če se manj uveljavljajo načela obvladovanja kakovosti (analize kažejo za ta element majhen, a negativen in signifikanten koeficient). Ugotovljeno je bilo tudi, da ima zaposlitev skrbnika omrežja signifikanten vpliv na ocene vodij za večino obravnavanih elementov TQM. Ti rezultati kažejo na tesne povezave med IT in QMS. Rezultati, predstavljeni v tem članku, so podlaga za sistemske izboljšave. Nadaljnje študije bi bile lahko usmerjene v snovanje praktičnih izboljšav in v preučitev posledic teh izboljšav.

Raziskava podaja vpogled v to, kako vodje kakovosti percipirajo uveljavljanje načel obvladovanja kakovosti. Kolikor je avtorjem znano, je ta zamisel izvirna. Rezultati študije dajejo tako praktikom kot raziskovalcem uporabne informacije o tem, kako postaviti smernice za snovanje učinkovitih izboljšav obvladovanja kakovosti. Raziskava daje tudi dragocene informacije o sistemih obvladovanja kakovosti v pogojih tranzicije. ©2011 Strojniški vestnik. Vse pravice pridržane.

Ključne besede: TQM, obvladovanje kakovosti, študija z vprašalnikom, subjektivna ocena elementov TQM, zavezanost managementa, tranzicija SI 170

*Naslov avtorja za dopisovanje: Tehniška fakulteta, Katedra za industrijski inženiring in management, Trg Dositeja Obradovića 6, Novi Sad, Srbija, rule@uns.ac.rs

Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, SI 171-172 Navodila avtojem

Navodila avtorjem Članke pošljite na naslov: Strojniški vestnik Journal of Mechanical Engineering Aškerčeva 6, 1000 Ljubljana, Slovenija Tel.: 00386 1 4771 137 Faks: 00386 1 2518 567 E-mail: info@sv-jme.eu strojniski.vestnik@fs.uni-lj.si Članki morajo biti napisani v angleškem jeziku. Strani morajo biti zaporedno označene.

Prispevki so lahko dolgi največ 10 strani. Daljši članki so lahko v objavo sprejeti iz posebnih razlogov, katere morate navesti v spremnem dopisu.

Kratki članki naj ne bodo daljši od štirih strani.

Navodila so v celoti na voljo v rubriki “Informacija za avtorje” na spletni strani revije: http://en.sv-jme.eu/ Prosimo vas, da članku priložite spremno pismo, ki naj vsebuje: 1. naslov članka, seznam avtorjev ter podatke avtorjev; 2. opredelitev članka v eno izmed tipologij; izvirni znanstveni (1.01), pregledni znanstveni (1.02) ali kratki znanstveni članek (1.03); 3. izjavo, da članek ni objavljen oziroma poslan v presojo za objavo drugam; 4. zaželeno je, da avtorji v spremnem pismu opredelijo ključni doprinos članka; 5. predlog dveh potencialnih recenzentov, ter kontaktne podatke recenzentov. Navedete lahko tudi razloge, zaradi katerih ne želite, da bi določen recenzent recenziral vaš članek.

OBLIKA ČLANKA Članek naj bo napisan v naslednji obliki: Naslov, ki primerno opisuje vsebino članka.

Povzetek, ki naj bo skrajšana oblika članka in naj ne presega 250 besed. Povzetek mora vsebovati osnove, jedro in cilje raziskave, uporabljeno metodologijo dela, povzetek rezultatov in osnovne sklepe.

- Uvod, v katerem naj bo pregled novejšega stanja in zadostne informacije za razumevanje ter pregled rezultatov dela, predstavljenih v članku.

- Teorija.

- -

-

Eksperimentalni del, ki naj vsebuje podatke o postavitvi preskusa in metode, uporabljene pri pridobitvi rezultatov.

- Rezultati, ki naj bodo jasno prikazani, po potrebi v obliki slik in preglednic.

- Razprava, v kateri naj bodo prikazane povezave in posplošitve, uporabljene za pridobitev rezultatov. Prikazana naj bo tudi pomembnost rezultatov in primerjava s poprej objavljenimi deli. (Zaradi narave posameznih raziskav so lahko rezultati in razprava, za jasnost in preprostejše bralčevo razumevanje, združeni v eno poglavje.)

- Sklepi, v katerih naj bo prikazan en ali več sklepov, ki izhajajo iz rezultatov in razprave.

- Literatura, ki mora biti v besedilu oštevilčena zaporedno in označena z oglatimi oklepaji [1] ter na koncu članka zbrana v seznamu literature.

Enote - uporabljajte standardne SI simbole in okrajšave. Simboli za fizične veličine naj bodo v ležečem tisku (npr. v, T, n itd.). Simboli za enote, ki vsebujejo črke, naj bodo v navadnem tisku (npr. ms1, K, min, mm itd.)

Okrajšave naj bodo, ko se prvič pojavijo v besedilu, izpisane v celoti, npr. časovno spremenljiva geometrija (ČSG).

Pomen simbolov in pripadajočih enot mora biti vedno razložen ali naveden v posebni tabeli na koncu članka pred referencami.

Slike morajo biti zaporedno oštevilčene in označene, v besedilu in podnaslovu, kot sl. 1, sl. 2 itn. Posnete naj bodo v ločljivosti, primerni za tisk, v kateremkoli od razširjenih formatov, npr. BMP, JPG, GIF. Diagrami in risbe morajo biti pripravljeni v vektorskem formatu, npr. CDR, AI. Vse slike morajo biti pripravljene v črnobeli tehniki, brez obrob okoli slik in na beli podlagi.

Ločeno pošljite vse slike v izvirni obliki Pri označevanju osi v diagramih, kadar je le mogoče, uporabite označbe veličin (npr. t, v, m itn.). V diagramih z več krivuljami, mora biti vsaka krivulja označena. Pomen oznake mora biti pojasnjen v podnapisu slike.

Tabele naj imajo svoj naslov in naj bodo zaporedno oštevilčene in tudi v besedilu poimenovane kot Tabela 1, Tabela 2 itd.. Poleg fizikalne veličine, npr t (v ležečem tisku), mora biti v oglatih oklepajih navedena tudi enota. V tabelah naj se ne podvajajo podatki, ki se nahajajo v besedilu.

SI 171

Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, SI 171-172

Potrditev sodelovanja ali pomoči pri pripravi članka je lahko navedena pred referencami. Navedite vir finančne podpore za raziskavo.

REFERENCE Seznam referenc MORA biti vključen v članek, oblikovan pa mora biti v skladu s sledečimi navodili. Navedene reference morajo biti citirane v besedilu. Vsaka navedena referenca je v besedilu oštevilčena s številko v oglatem oklepaju (npr. [3] ali [2] do [6] za več referenc). Sklicevanje na avtorja ni potrebno.

Reference morajo biti oštevilčene in razvrščene glede na to, kdaj se prvič pojavijo v članku in ne po abecednem vrstnem redu. Reference morajo biti popolne in točne. Vse neangleške oz. nenemške naslove je potrebno prevesti v angleški jezik z dodano opombo (in Slovene) na koncu Navajamo primere: Članki iz revij: Priimek 1, začetnica imena, priimek 2, začetnica imena (leto). Naslov. Ime revije, letnik, številka, strani, DOI oznaka.

[1] Hackenschmidt, R., Alber-Laukant, B., Rieg, F. (2010). Simulating nonlinear materials under centrifugal forces by using intelligent cross-linked simulations. Strojniški vestnik - Journal of Mechanical Engineering, vol.

57, no. 7-8, p. 531-538, DOI:10.5545/svjme.2011.013.

Ime revije ne sme biti okrajšano. Ime revije je zapisano v ležečem tisku.

Knjige: Priimek 1, začetnica imena, priimek 2, začetnica imena (leto). Naslov. Izdajatelj, kraj izdaje [2] Groover, M. P. (2007). Fundamentals of Modern Manufacturing. John Wiley & Sons, Hoboken.

Ime knjige je zapisano v ležečem tisku.

Poglavja iz knjig: Priimek 1, začetnica imena, priimek 2, začetnica imena (leto). Naslov poglavja. Urednik(i) knjige, naslov knjige. Izdajatelj, kraj izdaje, strani.

[3] Carbone, G., Ceccarelli, M. (2005). Legged robotic systems. Kordić, V., Lazinica, A., Merdan, M. (Eds.), Cutting Edge Robotics. Pro literatur Verlag, Mammendorf, p. 553-576. Članki s konferenc: Priimek 1, začetnica imena, priimek 2, začetnica imena (leto). Naslov. Naziv konference, strani.

[4] Štefanić, N., Martinčević-Mikić, S., Tošanović, N. (2009). Applied Lean System in Process

SI 172

Industry. MOTSP 2009 Conference Proceedings, p. 422-427.

Standardi: Standard (leto). Naslov. Ustanova. Kraj.

[5] ISO/DIS 16000-6.2:2002. Indoor Air – Part 6: Determination of Volatile Organic Compounds in Indoor and Chamber Air by Active Sampling on TENAX TA Sorbent, Thermal Desorption and Gas Chromatography using MSD/FID. International Organization for Standardization.

Geneva.

Spletne strani: Priimek, Začetnice imena podjetja. Naslov, z naslova http://naslov, datum dostopa.

[6] Rockwell Automation. Arena, from http://www. arenasimulation.com, accessed on 2009-09-27.

RAZŠIRJENI POVZETEK Ko je članek sprejet v objavo, avtorji pošljejo razširjeni povzetek na eni strani A4 (približno 3.500 - 4.000 znakov). Navodila za pripravo razširjenega povzetka so objavljeni na spletni strani http://sl.svjme.eu/informacije-za-avtorje/.

AVTORSKE PRAVICE Avtorji v uredništvo predložijo članek ob predpostavki, da članek prej ni bil nikjer objavljen, ni v postopku sprejema v objavo drugje in je bil prebran in potrjen s strani vseh avtorjev. Predložitev članka pomeni, da se avtorji avtomatično strinjajo s prenosom avtorskih pravic SV-JME, ko je članek sprejet v objavo. Vsem sprejetim člankom mora biti priloženo soglasje za prenos avtorskih pravic, katerega avtorji pošljejo uredniku. Članek mora biti izvirno delo avtorjev in brez pisnega dovoljenja izdajatelja ne sme biti v katerem koli jeziku objavljeno drugje.

Avtorju bo v potrditev poslana zadnja verzija članka. Morebitni popravki morajo biti minimalni in poslani v kratkem času. Zato je pomembno, da so članki že ob predložitvi napisani natančno.

Avtorji lahko stanje svojih sprejetih člankov spremljajo na http://en.sv-jme.eu/.

PLAČILO OBJAVE Domači avtorji vseh sprejetih prispevkov morajo za objavo plačati prispevek, le v primeru, da članek presega dovoljenih 10 strani oziroma za objavo barvnih strani v članku, in sicer za vsako dodatno stran 20 EUR ter dodatni strošek za barvni tisk, ki znaša 90,00 EUR na stran.

Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, SI 173-177 Osebne objave

Doktorske disertacije in diplome DOKTORSKE DISERTACIJE Na Fakulteti za strojništvo Univerze v Mariboru je z uspehom obranil svojo doktorsko disertacijo: dne 21. oktobra 2011 Aljaž ŠKERLAVAJ z naslovom: »Numerična analiza toka tekočine v vtočnem bazenu vertikalnih črpalk« (mentor: prof. dr. Leopold Škerget); Napovedovanje lokacije in intenzitete podvodnih in površinskih vrtincev je zelo pomembno v postopku načrtovanja vtočnih bazenov vertikalnih črpalk, kot na primer v nekaterih sistemih jedrskih elektrarn (v obratovalnih ali nezgodnih stanjih) ali v namakalnih sistemih. Močni podvodni ali površinski vrtinci so nezaželeni zaradi morebitne kavitacije, neenakomernega natoka vode na rotor in sesanja plavin ali zraka z vodne gladine, kar lahko poslabša delovanje črpalke ali celo povzroči njeno odpoved.

Lokacijo in intenziteto vrtincev v črpalnih bazenih se navadno ugotavlja eksperimentalno na pomanjšanih modelih vtočnih bazenov. Ker je izdelava modelov in eksperimentalno testiranje drago, lahko pričakujemo, da bodo v prihodnosti numerični izračuni pomagali pri izvedbi eksperimentalnih testov ali pa jih celo nadomestili.

V doktorski disertaciji smo testirali možnost napovedovanja toka tekočine v vtočnih bazenih z numeričnim izračunom.

Na podlagi primerjave enofaznih numeričnih izračunov in eksperimentalnih podatkov smo iskali primeren turbulentni model za takšen tok tekočine. Primerjava je bila izvedena v treh korakih. V prvem delu doktorske disertacije smo obravnavali numerični izračun talnega vrtinca v vtočnem bazenu. V drugem delu disertacije smo izvedli numerični izračun površinskega vrtinca. Odkrili smo, da je najprimernejši turbulentni model za simulacijo talnega in tudi površinskega vrtinca model prilagodljivih skal velikosti (SAS) s korekcijo ukrivljenosti (CC).

V drugem delu disertacije smo določali dolžino zračnega jedra površinskih vrtincev na osnovi predpostavke o Burgersovem tipu vrtinca. V tretjem delu disertacije smo uporabili model SAS-CC za numerični izračun laboratorijskega modela vtočnega bazena. Na podlagi primerjave z eksperimentalnimi rezultati glede oblike

površinskih vrtincev in napovedanega sesanja zraka z vodne gladine smo potrdili, da je model SAS-CC primerna izbira za numerične izračune vtočnih bazenov. * MAGISTRSKA DELA Na Fakulteti za strojništvo Univerze v Ljubljani je z uspehom zagovarjal svoje magistrsko delo: dne 12. oktobra 2011 Nikola VLAHOVIĆ z naslovom: »Mehanizmi nastajanja hrupa pri hladilnih stolpih« (mentor: prof. dr. Mirko Čudina). * Na Fakulteti za strojništvo Univerze v Mariboru je z uspehom zagovarjal svoje magistrsko delo: dne 7. oktobra 2011 Andrej FLOGIE z naslovom: »Načrtovanje in optimiranje procesov s posebnim poudarkom na vzdrževanju tehničnih sistemov« (mentor: prof. dr. Boris Aberšek); dne 13. oktobra 2011 Robert ROŠER z naslovom: »Visokohitrostni laserski razrez materiala« (mentor: prof. dr. Jože Balič). * SPECIALISTIČNA DELA Na Fakulteti za strojništvo Univerze v Ljubljani je z uspehom zagovarjal svoje specialistično delo delo: dne 26. oktobra 2011 Dejan TOMAŽINČIČ z naslovom: »Razvoj povezovalnih elementov nosilnih konstrukcij« (mentor: prof. dr. Jožef Duhovnik). * Na Fakulteti za strojništvo Univerze v Mariboru je z uspehom zagovarjal svoje specialistično delo delo: dne 13. oktobra 2011 Izidor ZUPANC z naslovom: »Razvoj ohišja vodnikov s pomočjo integracije sistemov« (mentor: prof. dr. Jože Balič).

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DIPLOMIRALI SO

*

Na Fakulteti za strojništvo Univerze v Ljubljani so pridobili naziv univerzitetni diplomirani inženir strojništva: dne 27. oktobra 2011: Blaž DEČMAN z naslovom: »Model kombiniranega aksialno radialnega toka odpiha pri razvlaknjenju kamene volne« (mentor: prof. dr.

Branko Širok); Tilen MRŽLJAK z naslovom: »Elektrolizer v napredni energijski oskrbi « (mentor: izr. prof. dr. Mihael Sekavčnik); Miha ŠKOF z naslovom: »Numerična analiza mehanskih lastnosti krila ultralahkega letala« (mentor: prof. dr. Boris Štok); dne 28. oktobra 2011: Rok DRAŠČIĆ z naslovom: »Zasnova in izdelava orodja za dvokomponentno brizganje termoplastičnih mas« (mentor: prof. dr. Janez Kopač, somentor: doc. dr. Davorin Kramar); Jaša JERAS z naslovom: »Neto ničenergijska stavba« (mentor: prof. dr. Vincenc Butala, somentor: doc. dr. Matjaž Prek); Blaž ŽUPAN z naslovom: »Razvoj prototipnih elementov pol-aktivnega vzglavnika za avtomobilski sedež« (mentor: doc. dr. Jernej Klemenc, somentor: prof. dr. Matija Fajdiga); Marko ŽURMAN z naslovom: »Zasnova in razvoj komore za fermentacijo testa na pladnjih« (mentor: doc. dr. Jernej Klemenc somentor: prof. dr. Matija Fajdiga).

Na Fakulteti za strojništvo Univerze v Mariboru so pridobili naziv univerzitetni diplomirani gospodarki inženir: dne 27. oktobra 2011: Tilen MARUŠIČ z naslovom: »Vpliv toplotne obdelave na lastnosti hitrostrnjenih trakov na osnovi zlitin Al-Mn« (mentor: izr. prof. dr. Franc Zupanič, mentor: prof. dr. Miroslav Rebernik, somentor: doc. dr. Gorazd Lojen); Gregor ŠIREC z naslovom: »Projekt razvoja novega izdelka mobilnega elektroencefalografa« (mentor: doc. dr. Iztok Palčič, mentor: prof. dr. Anton Hauc);

* Na Fakulteti za strojništvo Univerze v Mariboru so pridobili naziv univerzitetni diplomirani inženir strojništva: dne 12. oktobra 2011: Gašper FERME z naslovom: »Analiza in sinteza krmilnega sistema vodenja varnostnih vrat obdelovalnega stroja« (mentor: doc. dr. Uroš Župerl, somentor: prof. dr. Riko Šafarič); dne 27. oktobra 2011: Bogdan KOROŠEC z naslovom: »Zasnova in izdelava cepilnika lesa na gradbenem stroju« (mentor: izr. prof. dr. Ivan Pahole, mentor: doc. dr.

Mirko Ficko); Blaž PRISLAN z naslovom: »Načrtovanje proizvodnje voznih enot v podjetju HTZ I.P.« (mentor: doc. dr. Marjan Leber, mentor: izr. prof. dr. Borut Buchmeister).

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* Na Fakulteti za strojništvo Univerze v Ljubljani so pridobili naziv diplomirani inženir strojništva: dne 13. oktobra 2011: Aleš DEBENJAK z naslovom: »Izdelava strženske žice z laserskim varjenjem« (mentor: prof. dr. Janez Tušek); Grega POŽAR z naslovom: »Oblikovanje linije za izdelavo segmentov« (mentor: izr. prof. dr. Janez Kušar, somentor: prof. dr. Marko Starbek); Andrej ŠKAPIN z naslovom: »Izboljšanje vpetja ulitkov na vertikalni CNC-stružnici« (prof. dr. Mirko Soković, somentor: doc. dr. Davorin Kramar); dne 14. oktobra 2011: Denis BORKOVIĆ z naslovom: »Postavitev tehnologije frezanja miniaturnih grafitnih elektrod« (mentor: prof. dr. Janez Kopač, somentor: doc. dr. Franci Pušavec); Jure VESEL z naslovom: »Pospešeno testiranje lezenja materiala P24 z metodo majhnega bata« (mentor: izr. prof. dr. Roman Šturm); Nejc ZUPANČIČ z naslovom: »Eksperimentalna raziskava učinkovitosti dvostopenjskega modificiranega turbinskega mešala« (mentor: doc. dr. Andrej Bombač); dne 17. oktobra 2011: Jurček GORJUP z naslovom: »Merjenje in simuliranje aerodinamicnih sil na krilu« (mentor: izr. prof. dr. Tadej Kosel); Andreja STRAŽIŠČAR z naslovom: »Energijska analiza kompleksnega ogrevalnega sistema v različnih klimatskih pogojih« (mentor:

Strojniški vestnik - Journal of Mechanical Engineering 57(2011)11, SI 173-177

prof. dr. Vincenc Butala, somentor: doc. dr. Uroš Stritih); Miha TEVŽ z naslovom: »Naprava za lepljenje kapacitivnega senzorja na okrov ohišja kavnega avtomata« (mentor: prof. dr. Janez Diaci). * Na Fakulteti za strojništvo Univerze v Mariboru so pridobili naziv diplomirani inženir strojništva: dne 12. septembra 2011: Aleš DIMNIK z naslovom: »Določanje delovnih parametrov pri izvajanju stereolitografije

na praktičnih primerih« (mentor: prof. dr. Franci Čuš, somentor: prof. dr. Jože Balič); Matic POBERŽNIK z naslovom: »Razvoj in izdelava vijačne skobeljne glave za lesnoobdelovalne stroje« (mentor: izr. prof. dr.

Ivan Pahole, somentor: prof. dr. Franci Čuš); Jure RUDMAN z naslovom: »Statistično spremljanje proizvodnje gredi polževega gonila« (mentor: izr. prof. dr. Bojan Ačko, somentor: izr. prof. dr. Borut Buchmeister); Denis RUDOLF z naslovom: »Umerjanje koničnih navojev« (mentor: izr. prof. dr. Bojan Ačko).

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Janez Oman (1947-2011) V oktobru smo se poslovili od dolgoletnega kolega in prijatelja prof. dr. Janeza Omana, ki mu težka bolezen ni namenila milosti spokojnega uživanja sadov svojega dolgoletnega dela na Univerzi v Ljubljani, Fakulteti za strojništvo.

Rojen je bil je leta 1947 v Ljubljani, osnovno šolo in gimnazijo pa je obiskoval v Kranju. Študij je leta 1966 nadaljeval na Fakulteti za strojništvo Univerze v Ljubljani, kjer je leta 1973 diplomiral in se zatem tam tudi zaposlil.

Njegova delovna zavzetost in prirojen občutek do pedagoškega dela sta zaznamovala njegovo nadaljnjo uspešno pot visokošolskega pedagoga in znanstvenika. Leta 1978 je bil izvoljen v naziv asistenta in leta 1984 zaključil magistrski študij ter pet let za tem, leta 1989, obranil doktorsko disertacijo z naslovom ''Direktna absorpcija sončnega sevanja s trdnimi delci v plinu'' pri mentorju prof. dr. Petru Novaku. V naziv docent za področja Energetski sistemi, Toplotna postrojenja in Generatorji toplote je bil izvoljen 1991, decembra 1997 v naziv izredni profesor ter novembra 2007 v naziv redni profesor za področje energetski sistemi.

Znanstveno raziskovalno delo prof. dr. Janeza Omana se je od začetnih raziskav, ki so obsegale predvsem področje absorpcije koncentriranega sončnega sevanja, preusmerilo na področja smotrne rabe energije, preizkušanja in optimiziranja termoenergetskih sistemov, teorije zgorevanja in na nove tehnologije pri zgorevanju biomase.

Prof. dr. Janez Oman je ustanovil skupino za preizkušanje termoenergetskih sistemov in vpeljal to dejavnost v slovenski strokovni prostor.

Od leta 2004 dalje je bil vodja Laboratorija za termoenergetiko na Fakulteti za strojništvo. Bil je nosilec raziskovalnih programov s področja ekološkega in energetskega izboljšanja rabe domačih goriv. Rezultati in dosežki njegovega SI 176

znanstvenega in strokovnega dela v slovenski industriji so razvidni in merljivi. Odražajo se v obliki povišanih izkoristkov termoelektrarn in zniževanju škodljivih emisij iz kotlov in postrojenj v podjetjih, ki sodelujejo s Fakulteto za strojništvo Univerze v Ljubljani. Prof. dr. Janez Oman je zato v strokovnih krogih užival velik ugled. Rezultati poglobljenega sistematičnega dela, ki jih je skupina pod njegovim vodstvom objavljala v znanstveni periodiki, so bili opaženi tudi v tujini. Povabilo evropskega združenja proizvajalcev električne energije VGB s sedežem v Essnu, Nemčija, k sodelovanju pri pripravi posodobljenih standardov za preizkušanje velikih termoenergetskih postrojenj, je velik dosežek skupine, ki jo je vodil.

Prof. dr. Janez Oman je predaval na dodiplomskih in podiplomskih študijskih programih na več fakultetah Univerze v Ljubljani in bil gostujoči profesor na Univerzi v Padovi. Pri založbi Fakultete za strojništvo je leta 2004 prof. dr. Janez Oman skupaj z izr. prof. dr. Andrejem Senegačnikom objavil knjigo »Lastnosti zraka, goriv in dimnih plinov«, ki je zapolnila vrzel na tem področju slovenske tehniške literature. Samo leto zatem je pri isti založbi izšla tudi knjiga »Generatorji toplote«, namenjena tako študentom, kakor tudi strokovnjakom na področju energetike.

Prof. dr. Janez Oman je posebno pozornost namenjal doslednemu negovanju slovenskega strokovnega izrazoslovja, zato njegova knjižna dela predstavljajo pomembno oporno točko tudi v tem pogledu. Rezultate njegovega znanstvenega in strokovnega delovanja je strnil v več kot 100 znanstvenih in strokovnih objavah in več kot 150 strokovnih študij.

Pri študentih si je ustvaril prav poseben sloves: bil je zahteven in dosleden, predan stroki, a hkrati zavzet in pošten, vselej pripravljen na pogovor in pomoč. Mnogo študentov, ki so

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pod njegovim mentorstvom zaključili študij, so postopoma prevzemali najbolj odgovorne strokovne naloge v slovenski energetiki.

Teoretično še tako zapleteno materijo je znal približati študentom ter jih vzpodbujal k razumevanju praktične uporabe pridobljenega znanja. Njegova neizmerna volja do pedagoškega dela mu ni pošla tudi v obdobju, ki ga je zaznamovala težka bolezen; do zadnjega si je prizadeval ohraniti stik s študenti in pedagoškim delom.

Prof. dr. Janez Oman je bil klen Slovenec, trdnih in jasnih načel, ki jih je znal prepričljivo,

a brez nepotrebnih konfliktov zagovarjati. Bil je profesor širokih horizontov, vedno pripravljen poslušati druge in se seznanjati s temami izven njegovega ozkega znanstvenega področja. Svojo dušo si je spočil v objemu gora in brezpotjih narave, kamor je pogosto zahajal.

Na pokopališču v Kranju smo se 18. oktobra 2011 poslovili od spoštovanega univerzitetnega profesorja, strokovnjaka in dragega kolega, ki ga bomo ohranili v trajnem spominu. izr. prof. dr. Mihael Sekavčnik

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