Page 194 - Root Cause Failure Analysis

182 Root Cause Failure Analysis
increasing the demands on the lubricant is the material of construction, as both the
driven and driving gears are made of steel. This requires the use of special extreme-
pressure lubricants that have both oiliness and antiweld properties that can withstand
the high contact pressures and rubbing speeds.

Despite its demand for special lubrication, this gear type is in widespread use in
industrial and automotive applications. It is used extensively in rear axles of automo-
biles having rear-wheel drives and increasingly is being used in industrial machinery.

Worm
The worm and gear, which are illustrated in Figure 14-16, are used to transmit motion
and power when a high-ratio speed reduction is required. They accommodate a wide
range of speed ratios (60: 1 and higher can be obtained from a single reduction and can
go as high as 500:l). In most worm-gear sets, the worm is the driver and the gear the
driven member. They provide a steady, quiet transmission of power between shafts at
right angles and can be self-locking. Thus, torque on the gear will not cause the worm
to rotate.

The contact surface of the screw on the worm slides along the gear teeth. Because of
the high level of rubbing between the worm and wheel teeth, however, slightly less
efficiency is obtained than with precision spur gears. Note that large helix angles on
the gear teeth produce higher efficiencies. Another problem with this gear type is heat
removal, a limitation that restricts their use to low-speed applications.

Figure 14-16 Worm gear (Nelson 1986).