Page 195 - Root Cause Failure Analysis

GearboxesfReducers 183

A major advantage of the worm gear is low wear, due mostly to a full-fluid lubricant
film. In addition, friction can be further reduced through the use of metals having low
coefficients of friction. For example, the wheel typically is made of bronze and the
worm of a highly finished hardened steel.

Most worms are cylindrical in shape with a uniform pitch diameter. However, a vari-
able pitch diameter is used in the double-enveloping worm. This configuration is used
when increased load capacity is required.

PERFORMANCE
With few exceptions, gears are one-directional power transmission devices. Unless a
special, bidirectional gear set is specified, gears have a specific direction of rotation
and will not provide smooth, trouble-free power transmission when the direction is
reversed. The reason for this one-directional limitation is that gear manufacturers do
not finish the nonpower side of the tooth profile. This is primarily a cost-savings issue
and should not affect gear operation.

The primary performance criteria for gear sets include efficiency, brake horsepower.
speed transients, startup, backlash, and ratios.

Efficiency
Gear efficiency varies with the type of gear used and the specific application.
Table 14-2 provides a comparison of the approximate efficiency range of various gear
types. The table assumes normal operation, where torsional loads are within the gear
set’s designed horsepower range. It also assumes that startup and speed change
torques are acceptable.

Table 1&2 Gear Efiiencies

Gear Type Efficiency Range (%)
Bevel gear, hypoid 90-98
Bevel gear, miter Not available
Bevel gear, spiral 97-99
Bevel gear, straight 97-99
Bevel gear, zero1 Not available
Helical gear, external 97-99
Helical gear-double, external (herringbone) 97-99
Spur gear, external 97-99
Worm, cylindrical 50-99
Worm, double-enveloping 50-98
Source: Adapted by Integrated Systems, Inc., from “Gears and Gear Drives.” 1996 Power Trunsmissiow
Design (Penton Publishing Inc.. 1996), pp. A199-A211.