Accelerated lifetime testing of reinforced polymer gears

The main advantages of polymer gears compared to metal gears are low manufacturing costs for mass production, vibration damping, and there is no need for a lubricant. In the literature and guidelines, the allowable gear endurance limits for bending and contact stresses are mainly given for polyamides (PA) and polyacetals (POM). A large number of suitable polymer gear materials is available, but the standards offer little support for the lifetime calculations of polymer gears from other materials. Therefore, the testing of gear geometry and materials combinations cannot be avoided in the design of an optimal gear drive. However, gear testing is very time-consuming and expensive, especially when testing several different material combinations in different testing conditions. By applying the upgraded accelerated testing procedure, gear test time and costs can decrease significantly. Determination of the gear temperature during meshing is needed for the precise calculation of plastic gears. The presented temperature calculation model is corrected and improved with input parameters, which were determined from the test results. Accelerated tests were conducted on different combinations of reinforced and unreinforced commercially available materials: PA6, PA66, POM and PPS. Glass and carbon fiber were used for reinforcement. The research goal was characterization of different material pairs with the coefficient of friction, time strength, wear, and the failure mechanism in relation to load cycles and load level. The paper's contribution are some general guidelines for selecting polymer material for gears, such as fiber reinforcement improves the allowable stress level at up to a few million load cycles; unreinforced polymers are better for a higher number of load cycles. Also, PTFE - the internal lubricant significantly reduces a coefficient of friction if added to PA polymers, and is less efficient in combination with POM.