Accelerated lifetime testing of reinforced polymer gears

Abstract

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.

Agile Development of Polymer Power Transmission Systems for e-Mobility—A Novel Methodology Based on an e-Bike Drive Case Study

“…The step test is a fast prediction of tribological properties and polymer strength and helps to identify the most promising material pairs, which are then followed up in life tests (durability tests). Coefficient of friction can be determined from operating parameters and measured temperature as presented in [ 69 ]. Lifetime tests on a test bench with standard size gears (at selected speed and torque).…”

“…Gear testing is very time-consuming, especially when several different material combinations are tested under different test conditions. By using the accelerated testing method, the time and cost of gear testing could be significantly reduced [ 69 ]. Poorer mechanical properties of polymer gears at higher temperatures are an important limitation [ 67 , 70 ].…”

A review on materials and performance characteristics of polymer gears

“…Use of glass fiber and carbon fiber reinforcements are commonly found in PA gears with varying filling percentage between a range of 20% and 35%, 38,39 15%–30%, 39,40 respectively. Glass fiber significantly increases the durability of polyamide at higher load levels 19 but has no influence of speed on gear life. 41 Comparatively high amount of glass reinforcement improves thermal resistance and thereby decreases the specific wear rate.…”

Fatigue and Wear Performance of Autoclave-Processed and Vacuum-Infused Carbon Fibre Reinforced Polymer Gears

“…Normally, the static tensile strength and modulus of elasticity can be increased by adding short reinforcing fibres. However, for dynamically loaded components, polymer gears without short fibre reinforcements can perform better than the composite gears reinforced with short fibres [ 14 ]. While the fibres increase the modulus of elasticity and the quasi-static strength of the composite, the interfaces between the hard fibres and the soft matrix can become areas of microscopic stress concentration and, thus, act as crack initiators during fatigue loading [ 15 , 16 ].…”