How Tolerances Affect the Performance of Plastic Gears
Plastic gear are taking over many of the applications that were once reserved for metal alloys. They have the ability to run lubrication free, are much lighter than their metal counterparts, and perform well at high power transmission tiers. They also have the added benefit of being water resistant which means they won’t corrode or overheat in humid environments.
They are also less expensive to produce and replace. There are a number of different types of plastics that can be used to make gears. Some of the more popular include thermoplastic polymers such as nylon and acetal copolymer which have a good reputation for fatigue and strength over a wide temperature range. Other plastics, such as linear polyphenylene sulfides and long fiber plastics, have even better chemical resistance and can endure higher temperatures without deforming or becoming brittle.
Choosing the best plastic to use for your application is important because there are a number of variables that will impact the overall performance of your gear. For example, you’ll want to consider how rigid the gear needs to be and if it can be made to run at very low speeds. You’ll also need to take into account the size of the shaft that it is going to be attached to. If the shaft is not attached correctly, it can cause issues with transferring torque or causing wear and tear on the gears themselves.
To help prevent these problems, it’s essential to take into account the tolerances that are required when designing a plastic gear. Tolerances are the amount of variation that is allowed between the dimensions of the molded part and the desired specification. The closer the tolerances are, the more accurate the final product will be.
The type of injection molding process is another factor that can impact the tolerances of a plastic gear. For instance, hot-runner and injection molds require careful selection to minimize the effects of variations in temperature and pressure on the mold cavity and the molded parts. Cold-runner and continuous injection systems are not subject to these same fluctuations.
Plastic gears should be designed with a wide range of radii to spread stress in inner corners and improve the flow of the material over outer ones. This will help to keep the parts from concentrating and damaging the nominal wall.
It is also critical to carefully consider how the gear will attach to its hub. Press-fit knurled and splined shafts require extra mold precision and attention to processing for a secure mount without over-stressing the plastic. Insert-molded hubs grip the shaft better, but can induce residual stresses in the plastic when they shrink onto the shaft. Ultrasonic insertion of a knurled hub is ideal and will reduce these stresses significantly.