Page 191 - Root Cause Failure Analysis

GearboxesJReducers 179

pair, respectively. Special bevel gears can be manufactured to operate at any desired
shaft angle, as shown in Figure 14-1 1.

As with spur gears, the tooth size of bevel gears is established by the diametrical
pitch. Because the tooth size varies along its length, measurements must be taken at a
specific point. Note that, because each gear in a bevel-gear set must have the same
pressure angle, tooth length, and diametrical pitch, they are manufactured and distrib-
uted only as mated pairs. Like spur gears, bevel gears are available in pressure angles
of 14.5" and 20".

Because there generally is no room to support bevel gears at both ends due to the
intersecting shafts, one or both gears overhang their supporting shafts. This, referred
to as an overhung load, may result in shaft deflection and gear misalignment, causing
poor tooth contact and accelerated wear.

Straight or Plain
Straight-bevel gears, also known as plain beipels, are the most commonly used and
simplest type of bevel gear (Figure 14-12). They have teeth cut straight across the
face of the gear. These gears are recommended for peripheral speeds up to 1 ,OOO ft per
minute in cases where quietness and maximum smoothness are not crucial. This gear
type produces thrust loads in a direction that tends to cause the pair to separate.

Zero1
Zerol-bevel gears are similar to straight-bevel gears, carry the same ratings, and can
be used in the same mountings. These gears, which should be considered spiral-bevel
gears having a spiral angle of zero, have curved teeth that lie in the same general

I

Figure 1611 ' angle, which can be at any degree (Neale 1993).