To ensure reliable operation of the gearbox, each gear needs to be appropriately designed to avoid failure within the required lifespan and operating conditions. Plastic gears can fail due to different failure modes, i.e., fatigue fracture, wear or plastic deformation, which is usually thermally induced. Examples of the possible failure modes are shown in Figure 1. The fatigue failure mode can result in root fracture (Figure 1a), flank fracture, or, in some cases also pitting. Out of the three, the most common fatigue failure mode is root fracture, while flank fracture is often correlated with unfavorable contact characteristics of the gear pair, and pitting was only observed in some oil-lubricated cases. Wear, shown in Figure 1b, is another common failure mode for plastic gears. The degree of wear the gear exhibits depends on a variety of factors, e.g., operating temperature, lubrication, load, material of the mating gear, etc. Notable wear of the flank profile, deviating from the involute shape, leads to an elevated level of transmission error and worse NVH performance. As the wear progresses significantly, it also results in the breaking of teeth, with cracks originating from the worn tooth profile. The acceptable extent of wear varies depending on the specific use case. In applications demanding high precision (such as robotics and sensors), minimal wear is permissible, whereas in applications with lower precision requirements (like household appliances, power tools, and e-bike drives), a relatively substantial degree of wear is acceptable, involving a reduction in tooth thickness within the range of 20–30 percent of the gear module.

There is currently still no international standard available for the mechanical design of plastic gears, which would provide all the required tools and rating procedures to conduct design control against all possible failure modes. The most up-to-date and comprehensive is the German guideline VDI 2736: Part 2 (Ref. 9), where the design rating procedures for each failure mode are proposed. A flowchart representing the entire failure mode control process is shown in Figure 2. While the proposed procedures are feasible, the real problem arises as each control model requires some gear-specific material data, which is very limited. To patch this problem, VDI 2736: Part 4 (Ref. 22) provides testing procedures on how to generate the required material data.

Step 1. Calculate the operating temperature for the gear pair under design To ensure the reliable operation of a plastic gear, its operating temperature needs to be lower than the permissible temperature for a continuous load. The coefficient of friction for the selected material pair is needed in this step to be able to calculate the heat generated by friction. 

 Step 2. Root strength control The actual stress in the tooth’s root needs to be lower than the material’s fatigue limit at the desired number of load cycles (1 rotation of the gear is 1 load cycle on each tooth). Knowledge of the material’s fatigue strength is required to complete this step. 

 Step 3. Flank strength control against pitting This step is usually performed only for plastic gear pairs running in oil, as this is the only operating condition where pitting on plastic gears is sometimes observed. For dry-running or grease-lubricated plastic gears, root fatigue or wear are the most frequent failure modes. Step 4. Wear control 

Needs to be conducted for dry running plastic gears. It is recommended to do this control step also for grease-lubricated plastic gears if the plastic material is reinforced with fibers. Pulled out and cracked fiber particles tend to mix with the grease, forming an abrasive medium. The wear factor for the material pair of choice is required to conduct this step. 

 Step 5. Teeth deflection control Excessive deflection of teeth should be avoided to prevent teeth jamming and irregular meshing. Elastic modulus is required in this step to be able to calculate the tooth’s deflection. Step 6. Control of the static load In some applications, the gears are loaded with a high static load, e.g., holding some weight in a defined position. In that case, the gears need to be rated against a static load, and knowledge of the material’s tensile strength is required. It is not necessary to always conduct all the design rating steps. Step 1, Step 2 and Step 4 are advised to be always conducted, while others are case dependent.