CONVEYORS
Evaluating the indentation rolling resistance performance of conveyor belt covers As conveying distances and belt speeds continue to increase, greater emphasis is being placed on energy efficiency in belt conveyor design. As a result, TUNRA Bulk Solids has seen an increasing number of inquiries which focus on the testing and analysis of higher energy-efficiency conveyor belts and idlers. RESEARCH BY HAGER AND Hintz [1] has concluded that up to approximately 60 per cent of energy consumption in long distance conveying systems is due to the indentation of the belt cover passing over the idlers. Considerable research efforts have been placed in reducing such energy losses, including the development of low rolling resistance compounds by conveyor manufacturers [2]. Indentation rolling resistance is affected by several factors, including the idler roll diameter, belt loading, belt cover thickness and the viscoelastic properties of the bottom cover. Such properties are also dependent on operating conditions including temperature, belt speed and belt load, which imposes challenges when trying to predict the indentation losses of a conveying system.
Predicting indentation rolling resistance
There are two general approaches for determining the indentation rolling resistance losses of a conveying system, these are the large sample and small sample methods. The large sample method uses a full sample of belt in a
Figure 3: Small sample preparation from complete belt section.
Figure 1: Large Sample Indentation Rolling Resistance rig at TUNRA Bulk Solids, the University of Newcastle
closed loop, including the carcass, carry and pulley covers, whereas the small sample method uses a small sample of the pulley (bottom) cover of the belt only. Large sample indentation loss method There are different test setups associated with the large sample method, one such test method is described in German standard DIN 22123 [3] and another is included in Australian standard, AS 1334.13:2017 [4]. For these test methods, the indentation rolling resistance is measured directly by an instrumented idler roll, as a function of normal load, for a specific pulley cover compound, idler roller diameter, pulley cover thickness, temperature and belt speed [5]. The indentation rolling resistance is typically reported per unit belt width to enable comparison between compound types and to allow designers to scale results for the actual belt width to be installed on a system. The Conveyor Equipment Manufacturers Association (CEMA) [6] includes a method for utilising the large sample measurements to calculate the indentation losses when the required idler diameters and belt cover thicknesses of a system are different from what was tested.
28 І Australian Bulk Handling Review: September/October 2021
The large sample testing facility located at TUNRA Bulk Solids, shown in Figure 1, can carry out indentation rolling resistance testing according to both DIN 22123 and AS 1334.13:2017, as well as having the ability to measure conveyor belt flexure resistance [5, 7]. It accommodates samples between 400 millimetres to 600 millimetres wide and 29,000 millimetres long (full spliced length). Small Sample Indentation Loss Method CEMA 7th Edition [6] has included a small sample indentation loss model with constants for several example rubber cover compounds that can be applied to the CEMA model for initial design feasibility studies. For the small sample method, the dynamic physical properties of a rubber sample piece, with a thickness of up to three millimetres, are measured over a range of temperatures, strain amplitudes and frequencies. The dynamic physical properties, being the viscoelastic properties of the belt conveyor cover rubber, are obtained by applying an oscillating mechanical deformation to a sample and measuring the resultant stress. The stress response of a viscoelastic
