Page 185 - Root Cause Failure Analysis

GearboxedReducers 173

Table 161 Gear Characteristics Overview (continued)
Gear Type Characteristics
Attributes/Positives Negatives
~~~~~~~~~ ~ ~ ~
Bevel, miter Same number of teeth in both gears, operate on
shafts at 90"
Bevel, hypoid Connects nonintersecting shafts, high pinion Lower efficiency, dif-
strength, allows the use of compact straddle ficult to lubricate due
mounting on the gear and pinion, recommended to high tooth-contact
when maximum smoothness required, compact pressures, materials of
system even with large reduction ratios, speed- construction (steel)
reducing and speed-increasing drive require use of
extreme-pressure
lubricants

Planetary or Compact transmission with driving and driven
epicyclic shafts in line, large speed reduction when
required

Worm. Provide high-ratio speed reduction over wide Lower efficiency; heat
cylindrical range of speed ratios (60: 1 and higher from a removal difficult,
single reduction, can go as high as 500: l), quiet which restricts use to
transmission of power between shafts at 90". low-speed applica-
reversible unit available. low wear, can be self- tions
locking

Worm, double- Increased load capacity Lower efficiencies
enveloping

Source: Integrated Systems. Inc.

There are three main classes of spur gears: external tooth, internal tooth, and rack-
and-pinion. The external tooth variety shown in Figure 14-1 is the most common.
Figure 14-2 illustrates an internal gear. and Figure 14-3 shows a rack or straight-line
spur gear.

The spur gear is cylindrical and has straight teeth cut parallel to its rotational axis. The
tooth size of spur gears is established by the diametrical pitch. Spur-gear design
accommodates mostly rolling, rather than sliding, contact of the tooth surfaces and
tooth contact occurs along a line parallel to the axis. Such rolling contact produces
less heat and yields high mechanical efficiency, often up to 99 percent.

An internal spur gear, in combination with a standard spur-gear pinion. provides a
compact drive mechanism for transmitting motion between parallel shafts that rotate