The wear mechanisms of acetal in unlubricated rolling-sliding contact

The wear mechanisms of acetal (polyoxymethylene or POM) running against acetal in non-conformal, unlubricated rolling-sliding contacts have been investigated over a range of loads, rolling speeds and slip ratios. A wear and friction testing machine designed and built at Birmingham allows acceptable contact stresses for polymers and composites to be combined with a wide range of rolling speeds and slip ratios under well-controlled conditions. Based on the observed drastic changes in the wear rate with running conditions, wear has been divided into two regions: mild and severe. In the mild wear region, the wear rates are below 10^-6 μm cycle^-1 and are independent of rolling speed and slip ratio and vary approximately with the peak hertzian contact stress. Scanning electron microscopy examination shows that the wear appears to be at least partly due to mechanical fatigue of the surface. In the severe wear region, wear rates may exceed 10^-4 μm cycle^-1 and are very sensitive to operating conditions. The surface temperature is shown to be above 110°C and wear is caused by lateral cracking of the surface and subsequent tearing. These results are compared with the wear behaviour of POM gear pairs and close correlation is shown to exist. It is shown that, provided the temperature limitation of the material is observed, POM gears and other non-conformal components should have a life of around 10⁹ cycles.