Plastic worm gear drives are commonly designed by employing a metal worm in pair with a plastic worm wheel. Plastic worms, although viable, are used less often due to the complexities involved in injection molding and much lower achievable load carrying capacity and service life compared to metal variants. In this regard, available design guidelines and standards, like the VDI 2736: Part 3 (Ref. 6), further discussed in the following sections, also focus primarily on the evaluation of plastic worm wheels. The design of this type of worm wheels involves a durability control against the most commonly experienced failure modes during gear running (Fig. 2). In general, there are various failure modes that can, depending on the materials, loads, and lubrication conditions, occur on plastic worm wheels (Ref. 7), including: 

 Thermal failure 

 Root fatigue failure 

 Flank fatigue failure 

 Pitting 

 Wear 

 Viscoplastic tooth deformation

These are, in essence, the same types of failure modes exhibited by cylindrical (parallel axis) plastic gears. However, there are certain differences in the way these failure modes manifest and their probability of occurrence. In the following sections, the main failure modes will be presented in more detail, with methods of durability evaluation and experimental characterization further discussed.