AGMA 920-A01 pdf free.American Gear Manufacturers Association – Materials for Plastic Gears.
Afthough plastic materials are successfuPy used wi place of metals in load carrying applications such as gears, there are important differences between the two types of materials, These differences generally appear in combination and can have a sigrwflcant effect on plastic gear performance.
2.1 ElastIc and vlsco.lastlc behavior
Most structural metals behave as essentially elastc materials. Plastics, on the other hand, behave as a combination of elastic end viscous materials, with the balance varying considerably with the type of plastic, its molecular structfxe, and the type, quantity, end orientation of any additives. This special nature of plastic materials does not interfere with their use in a very wide range of applications which benefit from their many other special properties. It does, however, require a thorough understanding of reported material properties date end their relationship to the specific application.
2.2 Respons. to load
When load is applied to elastic materials such as steel, the resulting deformation is essentially immediate, constant over time, independent of a wide range of temperature, and fully recoverable when the bad is removed. When the material has a viscous component combined with the elastic, the Initial deformation will increase with time under bed (creep deformation) end will depend to a considerable degree on temperature. When the load is removed, there will be some delayed recovery and, possibly some permanent deformation,
The time dependent deformation of ductile polymers under constant load is quantified in creep testing. A fem)y of curves resulting from varying the constant load (stress) and recording the increasing creep strain is shown in figure Ia. As the polymer Is hek under constant stress (bad) over time, the creep strain intally increases at a rapid rate (primary creep) and then plateaus to a significantly lower creep strain rate (secondary creep). For nonductile polymers the material will experience creep rupture while deforming under secondary creep (see figure 1b). However, for ductile polymers, the material will experience another increese in creep strain rate (tertiary creep) and will creep rupture in tertiary creep. For non-ductile polymers the locus of creep rupture points forms the creep rupture envelope.However, the creep rupture envelope for ductile polymers is the locus of points resulting from the trensition from secondary to tertiary creep.deformation and creep rupture of polymers needs to be considered.
2.3 Effect of rate of load application
Because of the time dependant nature of viscoelas- tic plastic materials, the strength properties and elasticity modulus are typicallygreater whenthe load is applied and removed more rapidly. See figure 2.This cheracteristic is especially important in gear applications.AGMA 920 pdf download.