Calculation Methods and Measurement of the Heating of Small Plastic Gears

The heating is always a more important problem for drive units with plastic gears, even in simpler applications than with steel gears. The strength of plastics can change dramatically even at a temperature that occurs in the everyday environment, such as in places exposed to direct sunlight. In gearboxes, this can have a significant effect on the behaviour without rapid failure of the components, while higher temperatures lead to greater deformation of the tooth surface and greater wear and thus, it can reduce the contact ratio. At the same time, plastic gears are often used without lubrication, and in such circumstances, temperature also has a significant effect on the tribological properties of plastics. There are several methods for calculating the heating of plastic gears, which can give conflicting results. Some of these have become obsolete over the years but are still used today, for example, the VDI 2545 or the modified VDI 2545 calculation method. In the past, we have been able to demonstrate that reactive loads, especially in the drive units with small plastic gears performing a supporting function can lead to significant local temperature rises in the contacting teeth. Pulsating or continuous but variable reactive loads can cause heating similar to or greater than in the case of the normal operation. This heating can significantly affect the operating clearance and the strength characteristics of the gears. The temperature rising effects of reactive loads are measured with the help of special drive units, which proved to be a very useful method. In this article, we present some of the most commonly applied computing methods of heating. This is followed by a description of the specific drive units that we use to investigate the heating induced by the reactive loads and the principles of the measurement that can be performed with them.