Estimation of lifetime for plastic gears

The importance of plastic gears for modern industry is growing every year. The engineer sizing plastic gears has a very difficult task. There is no international standard available for the strength analysis. The only method publicly accepted is the German guideline VDI2545. In addition to a lack of calculation methods there is a need for measuring material properties. This paper shall give an overview over the current situation and will provide some guidelines how to rate plastic gears, how to handle the lack of material data available and how to conduct measurements of material properties to make them suitable for the available calculation methods. Introduction The number of gears produced out of plastic is getting dramatically larger. This is primarily due to the improvement of plastic materials strength. The properties of plastic can be varied in a large range, especially when compared to steel. It is now possible to select an optimal material for a specific task based on the following properties: strength, wear, stiffness, damping and noise production. In spite of the growing use of plastic gears the scientific research is astonishingly low, especially compared to the resources used for research on metal gears. A mirror of this situation is the availability of standards. The standard used for the strength analysis of cylindrical plastic gears is the German guideline VDI 2545 and is poor compared to the respective standards AGMA 2001 and ISO 6336 for metal gears. AGMA 909-A06, ANSI/AGMA 1006-A97 and ANSI/AGMA 1106-A97 address plastic gears, but only the geometry. ANSI/AGMA 920-A01 offers much general information about the applicability of plastic material for gears and presents the typical test procedures. The VDI 2545 is currently invalid and fits into the overall lack of research into plastic gearing. One of the major restrictions of the VDI 2545 is the availability of data for only three different materials (PA12, PA66, POM). Currently several groups are attempting to produce reliable data for modern materials. These experiments have proven to be expensive and are very time consuming. A design engineer therefore needs to know how he can design new plastic gears successfully without the use of a valid standard and without scientific materials research data. The engineer must rely on knowledge gained from past experience. Usually metal gears are produced in a generating process. Plastic gears most often are injection molded. If the insert for the mold is manufactured with EDM (e.g. wire erosion) the tooth form can be optimized without additional costs. For generated gears this is only possible with special tools, which increases the costs. On the other hand, typical injection molded gears have a relatively low quality (ISO 9-10), a problem, however, which can be handled with special arrangements. Optimized plastic gear tooth forms are also designated “hybrid toothing” in literature. Es ti m at io n of li fe ti m e fo r p la st ic g ea rs