Gearing Up With Plastic

This article explains the engineering design concepts behind injection-molded gears that make drives lighter, quieter, and less costly. As the experience base with plastic gears has grown, computer-aided-design (CAD) tools have advanced. The most powerful advantages of plastic gears may be the design opportunities they afford. Gear geometries overlooked by designers used to metal are often easy to mold in plastic, and they can reduce drive size, weight, and cost. The operating speed of plastic gears obviously impacts operating temperature. However, rapid-loading rates can also affect material properties. For some materials, the faster a tooth is loaded, the higher the effective modulus and strength. It is highlighted that designers of plastic gears should also pay special attention to shaft attachment. Bore tolerances naturally impact true center distances, sometimes resulting in loss of proper gear action. Problems with prototypes can also tempt gear designers to change resins—a costly mistake given the different shrinkage characteristics of various plastics. It is better to rework the tooth profile than switch the material, unless it is clear that the wrong material was chosen.