Computer Aided Design 3 Assignment
Design and Development TUTOR: Tony Roberts
STUDENT NAME: Arvydas Gordejevas
STUDENT NO: 2602021
SUBMISSION DATE: 11th March 2011
Contents
1. Introduction ...................................................................................................................................... 3 2. Gears ................................................................................................................................................. 3 2.1 Gear Design ................................................................................................................................. 3 2.2 Spur Gears ................................................................................................................................... 3 2.3 Helical Gears ................................................................................................................................ 4 3. Belt and Chain Drive.......................................................................................................................... 4 3.1 Belt Drives ................................................................................................................................... 5 3.2 Chain Drives................................................................................................................................. 6 4. Advantages ....................................................................................................................................... 6 4.1 Advantages of Gear Drives .......................................................................................................... 6 4.2 Advantages of Belt Drives ............................................................................................................ 7 4.3 Advantages of Chain Drives ......................................................................................................... 7 5. Concept Design ................................................................................................................................. 7 6. Supporting Material Calculations ...................................................................................................... 8 6.1 Spur Gears Calculation ................................................................................................................ 8 6.2 Bevel Gears Calculation (differential) .......................................................................................... 9 7. Relative position within the overall vehicle .................................................................................... 10 8. Collaboration .................................................................................................................................. 10
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Central Differential 1. Introduction Our team have been assigned to design and develop a scale radio controlled vehicle based on the popular off-road buggy format. Every single member has chosen their subsystem from the overall vehicle. As my part of responsibility I have chosen a ‘central differential’ which also includes gears. Power transmission between shafts can be achieved by a variety of means including belt, chain and gear drives and their use should be compared for suitability and optimization for any given application. And by looking into the last two years design of the central differential we can see that the teams have chosen to transfer power using belt drive. But when you look in a first 2 years work, team chosen spur gears and a belt drive. So before the decision can be made of what is the best for our project, the research needed to be done and to understand how. 2. Gears1 The purpose of the gears in a manual transmission or transaxle is to transmit rotating motion. Gears are normally mounted on a shaft, and they transmit rotating motion from one parallel shaft to another. Gears and shafts can interact in one of three ways: the shaft can drive the gear; the gear can drive the shaft; or the gear can be free to turn on the shaft. In this last case, the gear acts as an idler gear. Sets of gears can be used to multiply torque and decrease speed, increase speed and decrease torque, or transfer torque and leave speed unchanged. 2.1 Gear Design1 Gear pitch is very important factor in gear design and operation. Gear pitch refers to the number of teeth per given unit of pitch diameter. A simple way of determining gear pitch is to divide the number of teeth by the pitch diameter of the gear. For example, if a gear has thirty-six teeth and a 6-inch pitch diameter, it has a gear pitch of six (Figure 1). The important fact to remember is that gears must have the same pitch to operate together. A five-pitch gear meshes only with another five-pitch gear; a six-pitch only with a six-pitch, and so on.
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Figure 1 Determining gear pitch.
2.2 Spur Gears1 The spur gear is the simplest gear design used in manual transmissions and transaxles. As shown in Figure 2, spur gear teeth are cut straight across the edge parallel to the gear’s shaft. During operation, meshed spur gears have only one tooth in full contact at a time. Its straight tooth design is the spur gear’s main advantage. It minimizes the chances of popping out of gear, an important 1 consideration during acceleration/deceleration and reverse Figure 2 Spur gears have teeth cut straight across the gear operation. For this reason, spur gears are often used for the reverse edge parallel to the shaft. gear. 1
Erjavec, J. (2004) Automotive Technology: a Systems Approach. 4th ed. USA: Delmar Cengage Learning.
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The spur gear’s major drawback is the clicking noise that occurs as teeth contact one another. At higher speeds, this clicking becomes a constant whine. Quieter gears, such as the helical design, are often used to eliminate this gear whine problem. 2.3 Helical Gears2 Helical gears have their teeth cut at an angle that allows more than one tooth to be in contact (Figure 3). Helical gears carry more load than equivalent-size spur gears and operate more quietly and smoothly. The disadvantage of helical gears is that they develop end thrust. End thrust is a lateral force exerted on the end of the gear shaft. 2 Thrust bearings are required to reduce Figure 32 Helical gear. Figure 4 Herringbone gear. the effect of this end thrust. Double helical gears are designed to eliminate the end thrust and provide long life under heavy loads. However, they are more difficult and costly to manufacture. The herringbone gear shown in Figure 4 is a double helical gear without space between the two opposing sets of teeth. Helical gears can be either right-handed or left-handed, depending on the direction the spiral appears to go when the gear is viewed face-on. When mounted on parallel shafts, one helical gear must be right-handed and the other left-handed. Two gears with the same direction spiral do not mesh in a parallel mounted arrangement.1
3. Belt and Chain Drive3 Belt and chain drives are used to transmit power from one rotational drive to another. A belt is a flexible power transmission element that runs tightly on a set of pulleys. A chain drive consists of a series of pin-connected links that run on a set of sprockets (Figure 5). The speed ratio between the driving and driven shaft is dependent on the ratio of the pulley or sprocket diameters as is given by:
Where: Vpitchline – pitchline velocity (m/s) – angular velocity of driving pulley or sprocket (rad/s) – angular velocity of driven pulley or sprocket (rad/s) R1– radius of driving pulley or sprocket (m) R2– radius of driven pulley or sprocket (m) 2 3
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Figure 5 Belt drive and chain drive.
Madsen, D. (2001) Engineering Drawing and Design. 3rd ed. USA: Delmar Cengage Learning. Childs, P. (2003) Mechanical Design. 2nd ed. Oxford: A Butterworth-Heinemann Title.
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Both belt and chain drives can transmit power between shafts that are widely separated giving designers greater scope for control over machine layout. In comparison to gears they do not require such precision in the location of centre distances. Both belt and chain drives are often cheaper than the equivalent gear drive. Belts and chains are generally complementary covering a range of operational requirements. In general, belt drives are used when rotational speeds are of the order of 10 to 60 m/s. At lower speeds the tension in the belt becomes too high for typical belt sections. At higher speeds centrifugal forces throw the belts off the pulleys reducing the torque capacity and dynamic phenomena reduce the effectiveness and life of the belt drive. Chain drives are typically used at lower speeds and consequently higher torques than belts. Recall that for a rotating machine torque is proportional to power/ . As the angular velocity reduces, for a given power, the torque increases. Belt drives have numerous advantages over gear and chain drives including easy installation, low maintenance, high reliability, adaptability to non-parallel drive and high transmission speeds. The principle disadvantages of belt drives are their limited power transmission capacity and limited speed ratio capability. Belt drives are less compact than either gear or chain drives and are susceptible to changes in environmental conditions, such as contamination with lubricants. In addition, vibration and shock loading can damage belts. Chains are usually more compact than belt drives for a given speed ratio and power capacity. Chain drives are generally more economical than the equivalent gear drive and are usually competitive with belt drives. Chains are inherently stronger than belt drives due to the use of steels in their manufacture and can therefore support higher tension and transmit greater power. The disadvantages of chain drives are limited speed ratios and power transmission capability and also safety issues. Chain drives can break and get thrown off the sprockets with large forces and high speeds. Guards should be provided for chain drives and some belt drives to prevent damage caused by a broken chain or belt and also to prevent careless access to the chain or belt drive. 3.1 Belt Drives3 There are various types of belt drive configurations including flat, round, V, wedge and synchronous belt drives, each with their individual merits. The cross-sections of various belts are illustrated in Figure 6. Most belts are manufactured from rubber or polymer-based materials. Because of their good ‘twistability’, belt drives are well suited to applications where the rotating shafts are in different planes. Some of the standard layouts are shown in Figure 7.
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Figure 6 Various belt cross-sections.
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Figure 7 Pulley configurations.
Flat belts have high strength, can be used for large speed ratios (>8:1), have a low pulley cost, give low noise levels and are good at absorbing torsional vibration. The driving force is limited by the friction between the belt and the pulley. The most widely used type of belt in industrial and automotive applications is the V belt or wedge belt. The V- or wedge-shape causes the belt to wedge into the corresponding groove in the pulley 5|P age
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increasing friction and torque capacity. Note that long centre distances are not recommended for V or wedge belts. Synchronous belts, also called timing belts, have teeth which mesh with corresponding teeth on the pulleys. It combines the advantages of normal friction belt drives with the capability of synchronous drive. A disadvantage of synchronous belts can be the noise generated by compression of air between the teeth especially at high speeds. Table 1 list the comparative merits of various belt drives.
Table 1 Comparison of belt performance.
3.2 Chain Drives3 Chain drives are usually manufactured using high strength steel and for this reason are capable of transmitting high torque. Chain drives are complementary and competitive with belt drives serving the function of transmitting a wide range of powers for shaft speeds up to about 6000 rpm. At higher speeds the cyclic impact between the chain links and the sprocket teeth, high noise and difficulties in providing lubrication, limit the application of chain drives. Table 2 shows a comparison of chain, belt and gear attributes.
Table 2 Comparison of chain, belt and gear performance. A, excellent; B, good; C, poor.
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4. Advantages It is important to evaluate the advantages and disadvantages of each type of mechanism for the proper design and implementation of the final product. Therefore now we are able to differentiate whether to use a chain, belt or gear drive for a given application. A quick view of the advantages list in gear, belt and chain drives. 4.1 Advantages of Gear Drives2 Gear drives are commonly used in some applications rather than belt or chain drives for the following reasons:
They are more rugged and durable than most belt and chain drives. They are more efficient than belt or chain drives. They are more practical when space limitations required the shortest distance between centres. They have great maximum speed ratios. They work better when high horsepower and load capabilities are important. They may be used where timing and synchronization are required. Chain drives may work effectively in these applications, but belt drives are not recommended because of slippage.
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4.2 Advantages of Belt Drives2 Belt drives are often the choice of the designer when some of the following characteristics are important:
They are up to 95% efficient, especially at high speeds. They are designed to slip when an overload occurs. They resist abrasion. They require no lubrication, because there is no metal-to-metal contact, except at shaft bearings. They are normally smooth running and less noisy. Belt drives, especially those using flat belts, may be used more effectively when very long shaft centre distances are required. They can operate effectively at high speed ranges. They have flexible shaft centre distances, where gear drives are restricted. They are less expensive than gear or chain drives. They are easy to assemble and install and have flexible tolerances. They absorb shock well. They are easy and inexpensive to maintain.
4.3 Advantages of Chain Drives2 Chain drive mechanisms have certain design advantages over both gear and belt drives:
They have flexible shaft centre distances, where gear drives are restricted. They are less expensive than gear drives, in most cases. They have simpler installation and assembly tolerances than gear drives. They provide better shock-absorbing qualities than gears. They have no slippage as compared to belt drives, resulting in more efficient operation. They have lower loads on shaft bearings because tension is not required as with belt drives. They are easy to install. They are not affected by sun, heat, or oil and do not deteriorate with age as do belts. They are more effective at lower speeds than belts. They require little adjustment, while belt drives require frequent adjustment.
5. Concept Design Therefore a question arises: why do all this 4 years a team where using a belt drive? As from the research we can see that for this project a gear drive would be the best solution. Gears are the most rugged and durable of the mechanisms, transmitting motion with little or no slippage. They can satisfy high power demands at efficiencies of up to 98%.2 I do agree that belt drive is easier and cheaper to maintain; but do we want to put the belt on every single time its slips away? Do we want to change a belt every week because it’s broke, or rather change a gear every month? Even if the engine were too far to use a gear drive, I believe it could be repositioned before agreed the positions to all components. Remote control cars are made to be enjoyed more than fixing them. Therefore my research is saying to go with a stronger solution – gear drive. As I’ve continued examining what previous year team have done on central differential, I noticed that a frame they have built is not strong enough, where’s a support missing as this entire frame is 7|P age
unstable. As you can see in a Figure 8, taken from the last years work and year before, it is built from two walls and another wall is built on a top, and all the fixings that are holding this part into the chassis is the screws underneath the walls (possibility to tip over). Plus maintaining central differential will be hard with this design as you don’t have an easy way to get it apart and take the differential out. As a frame will need to fit a central differential with a different drive than previous year, it would be best to create a new one which will also help to get an easy access to the central differential.
Figure 8 Central differential casing.
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6. Supporting material calculations Successively engaging teeth give a constant velocity ratio and different types are available to suit different relative of the axes of the shafts. Most teeth are of the ‘involute’ type. The nomenclature for spur gears is given in the Figure 9.
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Figure 9 Nomenclature for spur gears.
6.1 Spur Gears Calculation4 Symbols used for Figure 11: F = tooth force Ft = tangential component of tooth force Fs = separating component of tooth force = pressure angle of teeth D1 = pitch circle diameter of driver gear D2 = pitch circle diameter of driven gear N1 = speed of driver gear N2 = speed of driven gear n1 = number of teeth in driver gear n2 = number of teeth in driven gear P = power T = torque = efficiency 4
Figure 11
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Carvill, J. (1994) Mechanical Engineer’s Data Handbook. Oxford: Butterworth Heinemann.
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Formulas: Tangential force on gears Separating force on gears Torque on driver gear Torque on driven gear Speed ratio Input power Output power Efficiency 6.2 Bevel Gears Calculation (differential)4 Straight bevel gears Let: pressure angle of teeth pinion pitch cone angle Tangential force on gears = Separating force Pinion thrust Gear thrust
Spiral bevel gear Let: spiral angle of pinion normal pressure angle Force on pinion Force on gear For the diagram shown the signs are ‘+’ for Fp and ‘-‘ for Fg. The signs are reversed if the hand of the helix is reversed or the speed is reversed; they remain the same if both are reversed. Therefore as all formulas known for the calculations, further decision can be made in deciding what gear size to use. 9|P age
7. Relative position within the overall vehicle
The relative position of my part within the overall vehicle will be in a center. Keeping the dogbones same length for the front and rear differentials. In this case both dogbones will have only one share force calculation to be done. And the height of the differential will be decided after gear diameter is chosen. 8. Collaboration The key pieces of information that will be needed for my collaboration with a team are:
Where the engine going to be mounted? What distance do I have for my drive? The dimension of the engine height is going to be mounted? What drive we going to use for transmitting motion from one shaft to another? What break system going to be used? What is the height of the front and rear differential casings?
As a team we have made a block diagram from where we all could see who is collaborating with whom:
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From the above diagram the following collaborative relationships have been drawn. SILVIO
MONDAY
ARVYDAS Monday, Silvio and Arvydas
IVAN
ROMANA
TAYO
ARVYDAS Romana - Ivan - Tayo –Arvydas
SILVIO ARVYDAS
TAYO
ABDEL Arvydas -Silvio – Abdel - Tayo RAPHAEL
ABDEL
BRUNO
Abdel - Bruno -Raphael
References: [1] Erjavec, J. (2004) Automotive Technology: a Systems Approach. 4th ed. USA: Delmar Cengage Learning. rd [2] Madsen, D. (2001) Engineering Drawing and Design. 3 ed. USA: Delmar Cengage Learning. nd [3] Childs, P. (2003) Mechanical Design. 2 ed. Oxford: A Butterworth-Heinemann Title. [4] Carvill, J. (1994) Mechanical Engineer’s Data Handbook. Oxford: Butterworth Heinemann.
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