One of the fundamental differences
between plastic and metal gears is their
differing rates of thermal expansion. An
unfilled engineering plastic such as
nylon or acetal will have four to five
times the thermal expansion coefficient
of steel. If the gear mesh is expected
to operate at elevated temperatures, the
designer must account for this expansion
or risk interference at high
temperatures or low contact ratio at low
temperatures. Historically, this is
achieved by altering backlash and root
clearance of the mating pair to
accommodate expansion.
Such an approach is perfectly
acceptable for gears with similar
expansion rates. However, if a plastic
gear with a relatively high thermal
expansion is in mesh with a steel gear
at an elevated temperature, the method
will cause improper meshing action. The
higher thermal expansion rate of the
plastic gear will cause its basic gear
geometry to change much more
dramatically than the steel gear. This
change in geometry due to thermal
expansion is very similar to thermal
shrinkage during the cooling cycle in
the mold. And the result will be gears
operating with dissimilar base pitches.
The effect of thermal shrink and
expansion on plastic gear geometry must
be thoroughly understood before such
gears can be properly designed or
inspected. This paper will examine the
behavior and governing equations for
this type of application. In summary,
the reader will be able to calculate the
actual gear geometry of the mold that
produces the finished part as well as
determine the change that will occur in
the finished part due to operating at
elevated temperatures.