Wear behaviour of acetal gear pairs

The wear behaviour of polymer gear pairs has been little studied. Most wear studies have used pin-on-disc or polymer against steel tests. The non-conformal contact and high transient temperatures in polymer gear pairs make it essential to study wear under actual running conditions. This paper describes the design and operation of a novel test rig for measuring the wear behaviour of polymer gears continuously. Preliminary results are presented and comparisons made between failures in acetal and nylon. Wear of acetal gears is studied in more detail. It is shown that wear of acetal gears is a complex process. Optical and scanning electron microscopy (SEM) show the driving and driven gears to have different features on the worn flanks, with a ridge forming at the pitch line on the driven gear and a valley on the driving gear. The wear surfaces over the addendum and dedendum are also strikingly different. This is related to the direction of rolling and friction forces on each gear face. The limits to the use of acetal as a gear material are considered. At low torques, life is limited by wear and at high loads the maximum permissible surface temperature is a limiting factor.