Designing Plastic Gears To Last

Plastic gears, when properly designed, offer many advantages compared to metal gears. We have found out the hard way, however, that the Lewis formula most engineers use to design metal gears will cause problems when applied to plastic gears. Differences in the characteristics of metal and plastic can lead to skewed results.

Having tested alternative plastic materials, Intech engineers identified Power-CoreTM material, a gravity cast nylon 12 composite cast around a metal hub, as a material that exhibits superior stability over time and carries high loads under varying conditions.

Usually, prototype parts must be fabricated and tested to define the point of failure. At Intech, however, formulae empirically derived during 10 years of experimentation with a complete range of gear sizes give our engineers a unique ability to predict gear life for INTECH Power-Core. The formulae apply at a wide range of rotational loads and speeds, with and without lubrication.

Gear calculation for Power-Core gears does not require including backlash to compensate for swelling due to moisture absorption. Also, the metal hub used in our gears reduces thermal expansion. In short, the high accuracy of gear machining, and the fact that the properties input to the gear calculation do not change significantly during the gear's operation, make our software a reliable tool.

cast and machined plastic gear on metal hub

Empirically derived formulae enable design engineers to maximize the useful life of plastic gears.

Printed in Design News / 5-22-95