Different teeth profile shapes of polymer gears and comparison of their performance

Abstract

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...

Durability Investigation of plastic gears

“…Previous research showed a clear distinction between Sand E-gear geometry in favour of former, which can be attributed to already mentioned attributes of S-gears [2][3][4][5]. Two types of experiments were conducted: a) durability tests where gears made of POM (driving) and PA66 (driven) run up to failure and tests where both gears were made of POM; b) long term tests up to about 20 million cycles.…”

“…Tests are summarized in Fig. 7 [4] are presented, with a far better durability. Gears are with the same geometry, experiments conducted on the basically the same machine.…”

“…Some tests with small plastic gears conducted to assess Wöhler curves for gears with various material combinations and long-term tests with smaller loads to run up to 10 or 20 million cycles in the controlled environment have been conducted recently [2][3][4][5]. Results comparing S-and E-gear shape indicate better performance of S-gears.…”

Comparison of Selected Parameters of a Planetary Gearbox with Involute and Convex–Concave Teeth Flank Profiles

“…Therefore, the contact load is lower [31].Tests were performed, which led to the confirmation of the assumption that 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 [32].The S-gears were also used by the design of the planocentric gear boxes and devices for large transmission ratios, primarily applied in automation and mechatronic industry, which decrease rotational movement and increase torque to the largest possible extent in the smallest available space [33].In all applications where the reduction gear must be as small as possible, a planetary gearset is preferred as it has the additional advantage of the input and output shafts being coaxial. The Wolfrom gear is suitable for high speed ratios, and has an efficiency that is not optimal but still acceptable.…”

“…Tests were performed, which led to the confirmation of the assumption that 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 [32].…”