Engineering

Friction is not friction

Experts from Elastotec and the University of Newcastle explain the why it is vital to understand the coefficient of friction between the pulley lagging and the belt for optimising the design, performance, efficiency, and safety of conveyor systems. 

The coefficient of friction between the pulley lagging and the belt is a critical parameter in conveyor design that governs the belt’s grip on the pulley and its ability to transmit torque. 

A higher coefficient of friction provides better grip and allows the conveyor to operate with higher loads, less slipping and greater efficiency, whereas a lower coefficient of friction can result in slipping and loss of power.

Currently, conveyor designers use generic coefficients of friction, specified in design standards (CEMA, ISO5048, DIN22101). These values are comparatively lower than conventional values and are blanketly applied across laggings of different designs, and of different origins. This results in a conservative conveyor design that translates into higher belt tensions, oversized belt rating, bigger conveyor structures and less efficiency. 

With conveyors continually emerging that are bigger and more powerful, there’s a need for a better understanding of the behaviour of lagging friction under real-world conditions that will allow for more efficient designs.

It is important to understand the coefficient of friction between pulley lagging and belt in conveyor design for several reasons:

  1. Design optimisation: The coefficient of friction between the pulley lagging and the belt affects the torque and power requirements of the conveyor system, which is critical for optimizing the design of the conveyor. A better understanding of the coefficient of friction allows conveyor designers to select the right type of pulley lagging and belt material and to accurately calculate the power requirements of the conveyor system.
  2. Improved performance: The coefficient of friction between the pulley lagging and the belt also affects the performance and efficiency of the conveyor system. A better understanding of the coefficient of friction allows conveyor designers to select the right type of pulley lagging that minimises belt and infrastructure loads.
  3. Cost savings: By optimising the design and performance of the conveyor system, a better understanding of the coefficient of friction can result in significant cost savings by reducing the need for maintenance, reducing the size of the drive components, and improving the overall efficiency of the conveyor system. This corresponds to less downtime.
  4. Enhanced safety: A better understanding of the coefficient of friction can also enhance safety by reducing the risk of slippage or belt failure, which can result in damage to the conveyor system or injury to workers.

Contrary to classic friction behaviour, the viscoelastic nature of the rubber belt and/or lagging allows it to deform and recover under stress. This causes the friction behaviour to be dynamic, with the coefficient responding to changes in normal (face) pressure, sliding (slip) velocity, sliding time and temperature. The resulting friction behaviour is complex, eliminating the possibility of a ‘blanket’ or ‘one size fits all’ approach in design.

The Conveyor Equipment Manufacturers Association (CEMA), a primary conveyor design standard, specifies the friction coefficients for conveyor design based on the results of laboratory tests and field observations. The friction coefficients provided by CEMA are intended to be used as a general reference for conveyor designers but may not apply in all cases. For instance, a varying coefficient of friction is specified for different levels of contamination, however, no information is provided for changes in belt tension (face pressure), slip velocity or temperature. The standard values are shown in Figure 1.

Figure 1: Coefficients of Friction from CEMA.

Elastotec has partnered with the University of Newcastle to further understand friction behaviour between pulley lagging and belt cover and understand the dependency of slip velocity, face pressure and temperature.

Testing equipment and method

The University of Newcastle, and TUNRA Bulk Solids, have designed and built a lagging friction test rig (Figure 2). The system consists of two opposing U-shaped sections supported by linear rails with the lagging attached to each side. Installed in an Ultimate Testing Machine (Shimadzu Autograph 50kN) conveyor belt samples are connected to the upper cross-head, with a steel plate backing, such that the bottom cover faces out towards the lagging plate. The belt is clamped at the top of the pull plate to allow the belt to stretch and better simulate the belt-pulley interaction. The system can test pulley lagging samples up to 300mm x 300mm, and any combination of belt and lagging up to 80mm thick on each side.

Figure 2: Friction test facility.

To conduct a test, constant face pressure is applied to the lagging/belt contact via pneumatic bellows. The presence of bellows on either side of the sample allows higher face pressures to be tested, and facilitates a ‘self-centring’ design, should any compression occur of the belt or samples during testing. The upper cross-head is set to the slip velocity specified, and the resultant friction force is measured. This is subsequently repeated at different face pressures, velocities, and in the coming months, temperature

Initial testing friction dependency with velocity 

Preliminary testing has been conducted on several pulley lagging samples to understand how the coefficient of friction is impacted by changes in normal pressure and velocities. Tests were conducted on diamond natural rubber and direct bond ceramic (see Figure 3), at a range of velocities (0.05mm/s-10mm/s) and face pressures (300kPa-1200kPa) with the results displayed in the surface plots below. The test temperature was approximately 23C. These ranges span those expected in typical applications. The lagging samples represent actual materials and profiles used in industry, with each having a unique tread type design.

Figure 3: Lagging test samples: Diamond natural rubber.
Figure 3: Lagging test samples: Direct bond ceramic.

 

It is noted that the results presented below represent a coefficient of friction associated with the lagging profile, and would differ if the identical materials were tested with a flat surface.

Natural rubber lagging

The surface and contour plot shown in Figure 4 illustrates how the coefficient of friction varies over the range of face pressures and velocities. As can be seen, higher face pressures correspond to a lower coefficient of friction, as does a lower sliding velocity. The velocity and load dependency cause a changing coefficient of friction from less than 0.3 to over 1.0. 

Figure 4: Surface fit and contour plot of natural rubber lagging.

Direct bond ceramic lagging

The results of the direct bond ceramic testing showed similar behaviour, with slightly less dependency on the face pressure than the other lagging tested. This is likely due to the rigid nature of the lagging, as compared to the viscoelastic nature of rubber lagging. In these results, the friction coefficient changed from approximately 0.3 to almost 1.0.  

Figure 5: Surface fit and contour plot of ceramic lagging.

Conclusions

The friction between the pulley lagging and the conveyor belt bottom cover shows dependencies on both slip velocity and face pressure. The relationship between friction coefficient, face pressure and velocity will help to better define the contact between the belt and pulley and in turn better define the stresses witnessed by the lagging. A better understanding of the frictional mechanics at play between the pulley lagging and belt bottom cover will help improve design and efficiency and ensure that lagging selection is based on an engineered analysis, for a given application. The next phase of work in this research is to expand testing to other lagging types and investigate the influence of temperature on outcomes. 

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