A review on failure characteristics of polymeric gears

The importance of polymer gears is increasing continuously since its advent in 1950s and developing at a very fast pace due to some of its advantage over metallic gears like light weight, self-lubricating, low noise, etc. However, due to its low mechanical property and poor heat conduction, its failure mechanism is observed entirely different comparative to metallic gears. This article presents different behavior of failures which takes place in polymer gears viz. wear, thermal and fatigue failure. Review of different polymeric materials like Nylon, Acetal, etc. are reviewed here. It is observed that acetal gear fail due to wear while nylon gear fail due to fracture below the pitch line, nearer to the root. It is also observed that mechanical property and resistance to thermal deformation of composite nylon gear improves to a significant level. Influence of rotational speed up to 1200 rpm on life cycles of gear is found negligible up to the limit of bending stress of 15MPa. Whereas influence of rotational speed is found considerable above the bending stress of 20MPa and hence the mode of failure becomes plastic deformation. Based on the literature review it is observed that an asymmetric polymer gear is being preferred in specific operating conditions where there is a requirement of high load carrying capacity and in case of unidirectional transmission loading. Bending fatigue performance of polymer gear is found improved when its pressure angle on drive side is increased from 20° to 34° as in case of an asymmetric gear.