ABSTRACT

High performance plastic gears are increasingly replacing metal gears in several applications due to many advantages. The main ones being lower weight, no need for lubrication, cheaper mass production, significantly better noise, vibration and harshness (NVH) behavior and chemical/corrosion resistance. Naturally, they exhibit also some drawbacks against metal gears, e.g. lower load bearing capacity, thermal sensitivity and inferior manufacturing tolerances. Plastic gears are mainly produced by injection molding, which enables great design flexibility, e.g. joining several machine elements into one molded part. Thermal overload, fatigue and wear are the most common failure modes for plastic spur gears. To ensure a reliable operation of the gearbox each gear needs to be appropriately designed in order to avoid failure within the required lifespan and operating conditions. There is currently no international standard available for the mechanical design of plastic gears. The most up-to-date and comprehensive tools are the German guideline VDI 2736 and Japanese standard JIS B 1759, which provide the design procedures and control models for all common failure modes. While the proposed procedures are feasible, the problem arises as each control model requires some gear-specific and experimentally determined material data, to which the calculated values are compared. Fatigue rating is conducted by comparing the calculated stress to a material’s fatigue limit, for wear prediction the wear factors are required and for calculating the operating temperature a good assessment on the coefficient of friction is required. A significant lack of required material data is the major drawback which is preventing a larger adoption of plastic gears. The study provides a comprehensive overview of the state-of-the art models employed for rating the thermal, fatigue and wear failure modes, outlining their shortcomings and proposing several improvements to the models. Furthermore experimental methods are presented, which can be employed to characterize the gear-specific material data required during the rating procedures on individual failure modes.  

Author(s): Damijan Zorko, Rok Kalister, Borut Cerne

ISBN: 978-1-64353-170-0