Plastic gearwheels show several advantages compared to metallic ones, such as lower weight, dry run capability and the potential for cost-effective mass production. However, disadvantages, such as inferior mechanical and thermal properties as well as less geometrical precision need to be compensated by design measures. Though, present design methods have a very limited applicability, since several plastic specific properties such as viscoelasticity, fiber orientation or damage mechanisms are not taken into account to an appropriate degree. Therefore, in this project cooperating with the Institute of Machine Elements, Gears, and Transmissions (Technical University Kaiserslautern) a multi physical modelling approach which should be used as a basis for applicable design methods is supposed to be worked out and validated experimentally.
A careful consideration of fiber orientation is an essential aspect of designing parts from fiber reinforced materials. Computer tomographic scans of short fiber reinforced plastic gearwheels showed a main fiber orientation along the flow direction of the injection molding process (see figure 1). Furthermore, it has to be considered that tensile specimens which are used for mechanical testing also do not show a homogeneous fiber distribution. Different main fiber directions can be detected inside the cross section of the specimen. The outer layers show a very intense main direction, while the main direction of the middle layer is rotated and much less pronounced (see figure 2).
Since polyamide is one of the preferred materials for plastic gearwheels, several external influences such as temperature, humidity content and strain rate also have to be taken ino account in mechanical testing to enable designing gearwheels for real operating conditions. Additionally, an appropriate design depends on a proper understanding of failure mechanisms. These mechanisms are a complex interaction of e. g. (plastic) deformation, fatigue and wear. Therefore, a comprehensive tribological characterization is supposed to give information on friction and wear depending on parameters such as temperature or sliding velocity.
The project “Design of Injection Molded Plastic Gearwheels” is funded by the Federal Ministry of Economic Affairs and Energy, on the basis of a decision by the German Bundestag (funding reference 20379N).
Stefan Schmidt, M. Eng.
Institut für Verbundwerkstoffe GmbH
Phone: +49 (0) 631 2017 274