Flank load carrying capacity of high performance plastic gears

(29.07.2019)
Current market developments show an increasing trend towards the use of plastic gears for higher power transmission applications. Low density and high damping characteristics on the one hand are just two of several plastic specific advantages. Temperature dependent material parameters and gear strength numbers on the other hand, however, restrict the scope of application. Therefore, the knowledge of the tooth temperature and the corresponding load carrying capacity is of essential importance for plastic gear development.

Current calculation methods such as VDI 2736 are derived from steel standards and therefore neglect some important plastic specific material characteristics. In the scope of a research project conducted successfully at FZG, several thermoplastic materials have been investigated regarding tooth root load carrying capacity. Modified calculation approaches to calculate tooth root stresses have been developed.

In a current research project, adequate thermoplastic materials such as Polyetheretherketone (PEEK) are now investigated regarding tooth flank load carrying capacity under oil-lubricated conditions. Experimental investigations for the determination of the tooth temperatures as well as test runs in back-to-back gear test rigs are performed. The objective is to extend the current state of knowledge regarding flank load carrying capacity for high performance plastic gears and to develop a more comprehensive and reliable calculation method.

VDI 2736 is the basis for the design of plastic drives. However, the required strength values are only available for a very limited selection of plastic materials. PEEK is a high performance plastic that is particularly suitable for applications in high temperature environments. Compared to other thermoplastics, PEEK retains its Young's modulus and strength even at material temperatures above 100 °C since the glass transition temperature is around 150 °C. However, VDI 2736 does not contain any strength values for this material, which complicates the targeted design of PEEK plastic gears.

Extensive experimental investigations in back-to-back test rigs with injection molded and oil lubricated test gears provide initial results on the tooth flank load carrying capacity and damage characteristics of PEEK materials. The back-to-back tests show that pitting damages, which resemble those occurring on steel gears, are developing. Flank damages such as wear, which can usually be observed in dry-running plastic gears, play a subordinate role in operation under oil lubricated conditions. Thermal damage such as melting of the tooth flanks can also be prevented by the effective heat dissipation capabilities of the lubricant.

The so far determined results prove the typical damage characteristics on the flanks of plastic gears and provide initial information on the strength values of PEEK materials.

A comparison with alternative materials such as PA66 at the same temperature level confirms the suitability of PEEK as a high-temperature material since PEEK reaches noticeably higher pitting load carrying capacity in high temperature environments.

Detailed results of the comprehensive investigations will be presented at the VDI International Conference on High Performance Plastic Gears in September 2019.


Authors

From left to right: Christopher Illenberger, Dr.-Ing. Thomas Tobie, Prof. Dr.-Ing. Karsten Stahl
Gear Research Centre (FZG), Technische Universitat Munchen (TUM), Garching, Germany