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This article presents a lifespan testing analysis of polymer gears manufactured by cutting. Compared to injection molding, machine cutting provides higher accuracy of gear geometry. Two different tooth flank geometries were tested; i.e. involute and S-gears. In theory, S-gears have several advantages over involute gears due to the convex/concave contact between the matching flanks. The theoretical tooth flank geometry of S-gears provides more rolling and less sliding between the matching flanks, compared to involute gears. The convex/concave contact leads to lower contact stress, which in combination with less sliding means lower losses due to sliding friction and consequently less heat generated. The goal of our research was to prove that tooth flank geometry affects the lifetime of polymer gears, and to find the mechanisms and quantitative differences in the performance of both analyzed geometries. The gears were tested on specially designed testing equipment, which allows exact adjustment of the central axis distance. Two different material pairs (POM/POM and POM/PA66) of the drive and driven gears were tested. Each test was done at a constant moment load and a constant rotational speed. Several tests were conducted using the same conditions due to repeatability analysis. All the tests were performed till the failure of the gear pair and without lubrication. In lifespan testing, the polymer S-gears showed better performance and longer lifespan than involute polymer gears.Keywords : Polymer gears, Lifespan testing, Temperature, S-gears, Involute IntroductionThe involute shape of the tooth flank was suggested by Leonard Euler (1775). Due to its unique properties, the involute shape of the tooth flank prevailed and most of the gears nowadays are involute. The condition that a tooth flank shape should fulfil in order to provide a constant gear ratio is defined by the law of gearing. Many curves, not just the involute one, fulfil it, which has yielded many other tooth flank shapes that are rarely used in practice. The purpose of developing new gear shapes is to improve the disadvantages of involute gears, such as: the convex/convex contact, teeth undercutting, and gears with a small number of teeth have a shorter dedendum, which leads to increased sliding and losses due to friction. Besides involute gearing, cycloidal gearing is the next best known type, mainly used in wrist watch mechanisms. Kapelevich (2013) dedicated much of his research work to gears with asymmetric tooth flanks, where the active tooth flank is still of involute shape. Mohan and Senthilvelan (2014) studied the lifespan of asymmetric composite gears. Kim (2006) and Düzcükoglu (2009) also worked on improving the performance of polymer gears. They used the involute tooth profile and attempted to extend the lifespan of gears by virtue of geometric changes in teeth width. Koide et al. (2017) investigated the impact of the sine-curve gear on the tooth surface temperature and gear power transmission efficiency. Hlebanja et al. (2008) suggeste...

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