is error in motion that occurs when gears change
direction. It exists because there is always some gap between the tailing
face of the driving tooth and the leading face of the tooth behind it on the
driven gear, and that gap must be closed before force can be transferred in
the new direction. The term "backlash" can also be used to refer to the size
of the gap, not just the phenomenon it causes; thus, one could speak of a
pair of gears as having, for example, "0.1 mm of backlash." A pair of gears
could be designed to have zero backlash, but this would presuppose
perfection in manufacturing, uniform thermal expansion characteristics
throughout the system, and no lubricant. Therefore, gear pairs are designed
to have some backlash. It is usually provided by reducing the tooth
thickness of each gear by half the desired gap distance. In the case of a
large gear and a small pinion, however, the backlash is usually taken
entirely off the gear and the pinion is given full sized teeth. Backlash can
also be provided by moving the gears farther apart. For situations, such as
instrumentation and control, where precision is important, backlash can be
minimised through one of several techniques. For instance, the gear can be
split along a plane perpendicular to the axis, one half fixed to the shaft
in the usual manner, the other half placed alongside it, free to rotate
about the shaft, but with springs between the two halves providing relative
torque between them, so that one achieves, in effect, a single gear with
expanding teeth. Another method involves tapering the teeth in the axial
direction and providing for the gear to be slid in the axial direction to
take up slack.