AGMA 920-B15 pdf download.AGMA Information Sheet Materials for Plastic Gears.
Although plastic materials are successfully used in 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 significant effect on plastic gear performance.
2.1 Elastic and viscoelastic behavior
Most structural metals behave as essentially elastic materials. Plastics, on the other hand, behave as a combination of elastic and viscous materials, with the balance varying considerably with the type of plastic, its molecular structure, and the type, quantity, and 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 data and their relationship to the specific application.
2.2 Response 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 load is removed. When the material has a viscous component combined with the elastic, the initial deformation will increase with time under load (creep deformation) and 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 family of curves resulting from varying the constant load (stress) and recording the increasing creep strain is shown in Figure 1. As the polymer is held under constant stress (load) over time, the creep strain initially 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 2). However, for ductile polymers, the material will experience another increase 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 transition from secondary to tertiary creep.
Creep deformation appears not to be a factor in gears under continuous operation because the load is applied to gear teeth only for a short time duration. However, for gears run into stalled conditions creep deformation and creep rupture of polymers needs to be considered.
2.3 Effect of rate of load application
Because of the time dependent nature of viscoelastic plastic materials, the strength properties and elasticity modulus are typically greater when the load is applied and removed more rapidly. See Figure 3.
This characteristic is especially important in gear applications.
2.4 Effect of temperature
2.4.1 Strength and deformation
Because a higher temperature reduces the resistance to movement of the polymer chains, the material at high temperatures can be viewed as less viscous (decrease of the viscous component). This decrease in the viscous component of polymers at higher temperatures causes the strength and stiffness properties to decrease with increasing temperatures (see Figure 3). T emperature increases of the polymer at critical locations in gears could result from friction, hysteresis, or both in combination. This temperature rise of the gear material at critical locations could, therefore, reduce the load resisting capability of the gear. This condition is a significant factor to consider in gear performance.AGMA 920 pdf download.