Page 196 - Analysis and Design of Machine Elements

Analysis and Design of Machine Elements
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Table 8.1 Types of gear drives.
Gear drives Parallel-shaft gearings Spur gear drives
Helical gear drives
Herringbone gear drives
Pinion and rack drives
Internal gear drives
Intersecting-shaft gearings Straight bevel gear drives
Spiral bevel gear drives
Crossed gearings Crossed helical gear drives
Hypoid gear drives
Worm and wormgear drives

Spur gears are the most widely used type because of simplicity, low fabrication cost
and great precision. They have straight involute teeth parallel to the axis of the shaft that
carries them. Spur gears impose only radial loads on the supporting bearings. They are
usually limited to pitch line velocities around 20 m s −1 to avoidhighfrequencyvibration
and unacceptable noise [6].
Helical gears are superior to spur gears in terms of tooth strength, operation speed
and power transmission capacity because of the helix angle they possess. The gradual
engagement of helical gear due to angled teeth leads to a smoother and quieter operation
than spur gears. On the other hand, the angled teeth impose both radial and thrust loads
on the supporting bearings. To eliminate thrust loads on the supporting bearings when
transmitting high power, a herringbone gear can be used.
The rack and pinion drive is a special case of parallel-shaft gearing, as the rack can be
regarded as a cylindrical gear with an infinite pitch diameter. It may have either straight
spur gear teeth or helical gear teeth. Most gears are external gears. Internal gears are
used to achieve a short centre distance and are a necessity in epicyclic gear trains.
Intersecting-shaft gearings use straight bevel gears and spiral bevel gears to transmit
motion between intersecting shafts. Bevel gears have teeth formed on conical surfaces.
The straight bevel gear teeth appear similar to spur gear teeth, except they are tapped
in both tooth thickness and height. Bevel gears impose both radial and thrust loads on
the supporting bearings. They must be accurately mounted at an accurate axial distance
from the pitch cone apex for proper meshing. When bevel gears are made with teeth that
form a helix angle similar to that in helical gears, they are called spiral bevel gears. Spiral
bevel gears provide the advantage of gradual engagement along the tooth face. There-
fore, they operate more smoothly than straight bevel gears and can be made smaller for
a given power transmission capacity.
Shafts carrying a pair of helical gears are typically arranged parallel to each other.
However, when a pair of helical gears mesh with each other with nonintersecting shaft
axes, they are called crossed helical gear drives. In crossed helical gear drives, teeth
initially mesh at a point and, later, a line. Therefore, the load carrying capacity gradually
increases after a wear-in period. Hypoid gear drives resemble spiral bevel gear drives,
except that the shafts have a small offset and nonintersecting. For larger offsets, the
pinion begins to resemble a tapered worm and the set is then called a spiroid gearing.
Another example of crossed gearings whose shaft axes are neither parallel nor inter-
secting is the worm and wormgear drives. A worm and its mating wormgear operate on