Plastic Gearing

The material used to manufacture individual components of a system can affect things like cost of manufacture, precision of the finished system and the load that can be put on the individual component. Gear materials have basic requirements such as strength to handle the gear loading, good wear and friction characteristics against the material of the mating gear. Plastic gears are serious alternatives to traditional metal gears in a wide variety of applications. Plastic gears are great options for certain applications for their desirable characteristics such as quietness of operation and their capability to absorb shock and vibration, but also have less load-capacity and lower accuracy than metal gears.

The design opportunities that plastic gears afford are a major advantage. They can be molded in shapes difficult to machine in metal. Other functional elements such as springs and pawls can be molded into them, thereby consolidating parts and trimming the cost, weight, and complexity of geared speed reducers. Plastic gears can also be more cost effective than their metal counterpart due to faster cycle times and less tool wear or the use of injection-molding process. Plastic gears are manufactured with the same machining process as are metal gears, usually milling or hobbing, plastic gears can also be manufactured in far less operations than metal gears. The very low cutting forces permit high infeed rates. This reduces manufacturing cost. They are also usually less dense than their metal counterparts, which means that they are lightweight and have lower inertia. Plastic gears can generally be operated with little to no lubrication, but temperature rise due to meshing may be controlled by the cooling effect of a lubricant as well as to reduce friction or other mechanical system failures. Corrosion resistance may also have to be taken under consideration when using lubricants. Often, in the case of high-speed rotational speeds, lubrication is critical. The greater resilience that is inherent to plastic gears causes the tolerances to be less critical than metal gears.

Engineers and designers can’t view plastic gears as just metal gears cast in thermoplastic. They must pay attention to special issues and considerations unique to plastic gears. In designing plastic gears, the effects of heat and moisture must be given careful consideration. Plastic gears have larger coefficients of thermal expansion. Also, they have an affinity to absorb moisture and swell. Due to the thermal expansion of plastic gears and tendency to absorb moisture, one should make sure that meshing gears do not bind in the course of service. To some extent, the flexibility of the bearings and clearances can compensate for thermal expansion.

Gear teeth have a lower elastic modulus and mesh stiffness, so they deflect more under load. Consequently the designer generally needs to increase backlash and tip relief to prevent interference between mating teeth. Several means are available for introducing backlash into the system such as the enlargement of the center distance. Care must be taken, however, to ensure that the contact ratio remains adequate. It is possible also to thin out the tooth profile during manufacturing, but this requires careful consideration of the tooth geometry.  If a small change in the center distance is necessary and feasible, it probably represents the best and least expensive compromise.

The most important part of a gear is the gear teeth. Without the teeth, the gear is simply a wheel and is of little use in transmitting motion or power. That being said, a major consideration for using plastic gears are the bending and contact stresses put on the gear teeth when the gears are in mesh. Bending stresses can try to bend the gear teeth and shear the teeth from the bulk material of the gear. These forces lead to failures by tooth breakage due to static loading and fatigue action. Contact stresses can lead to failure in the surface of the gear teeth, or wear.  In other words, plastic teeth deflect more under load and spread the load over more teeth. In most applications, load-sharing increases the load-bearing capacity of plastic gears. To assure a satisfactory life, the gears must be designed so that the dynamic surface stresses are within the surface endurance limit of the material. In order to minimize stress concentration and maximize the life of a plastic gear, the root fillet radius should be as large as possible, consistent with conjugate gear action. Sudden changes in cross section and sharp corners should be avoided, especially in view of the possibility of additional residual stresses which may have occurred in the course of the molding operation. Plastic gears are great options for certain applications for their desirable characteristics such as quietness of operation and their capability to absorb shock and vibration.

What material the mating gear is manufactured from is another important consideration when designing plastic gears for specific applications. For plastic on metal gear pairs, composites may wear differently against a relatively hard metal than against soft metal with some formulations wearing better than others. If one of the gears of a mated pair is metal, there will be a heat sink that combats a high temperature rise. The effectiveness depends upon the particular metal, amount of metal mass, and rotational speed. For plastic on plastic gear pairs, composite selection becomes increasingly complex than metal on plastic pairing. These wear combinations are extremely difficult to predict and can only be confirmed by testing as there is not extensive knowledge of the effects in gear applications.

Types of Failure/Wear:

Remember when selecting plastic gear materials, look for those with sufficient strength and stiffness to handle the expected loads. Then make sure that any changes in dimensions and frictional characteristics due to environmental conditions are acceptable for the application. Material suppliers can provide a lot of useful information on these properties for various plastics. But you may still need to test prototype gears under realistic operating and environmental conditions to verify that they’ll perform as intended.

There are several design factors need to be taken under account since the plastic gear teeth would be attached to some plastic part. One of which is the nominal wall of the component. The thickness of the nominal wall will influence the strength, cost, weight and precision of the part, so the thickness of the nominal wall should be substantial enough to have the necessary strength for the part. The biggest problem associated with large changes in wall thickness is that the thicker sections will not cool as quickly as the thin sections and will therefore shrink more. This can result in part warp and out of tolerance parts. Another consideration would be the potential for stress concentrations and a reduction in flow where two walls meet in any plastic part. Therefore corners should have radii to spread out the stress in inner corners and improve the material flow path on outer corners. The simplest gear a flat gear with no rim or hub gated in the center. This gear will not have any differential shrinkage, since it has a single nominal wall with no changes in thickness. When designing a plastic gear that has a hub or a rim, careful consideration must be given to the thickness of the various parts. Tooth thickness and height have already been determined by the requirements of tooth strength. The difficulty lies in deciding which part of the gear is the nominal wall, and what is the relationship between the feature and the other parts. Each part of the gear should be designed to perform the desired function without forgetting the basic plastic design guidelines. As with any design guidelines, compromise will undoubtedly have to be made.