Edges, clearances, and wear: Little things that make big differences in bushing friction

Traditional pin-on-flat tribometers are necessary instruments for making direct measurements of tribological properties, but mechanical design of even simple systems often requires application-specific information that only component level testing can provide. This paper uses the design and operation of a bushing tribometer to elucidate geometric effects that plague bushing systems. For example, the nature of the bushing edge has a dramatic influence on the performance of the system in practice where the system is necessarily over-constrained. It is shown mathematically that the torque requirements increase as the wrap angle of the contact spreads for a constant friction coefficient; at steady state the equation for calculated torque requirement is found. This is demonstrated using ultra-high molecular weight polyethylene bushings that showed increasing torques at decreasing clearances. A polytetrafluoroethylene (PTFE) bushing with 150 μm interference initially required 50% more torque than a PTFE bushing with 350 μm clearance. As the two different bushings ran into steady state wear, the torque asymptotically approached the mathematically derived value. The results of these experiments provide designers insight into the design of successful bushing pairs and the ability to tune frictional torques without changing material through the selection of clearance.