14-06-2012, 12:34 PM
SEMINAR ON GEARS AND GEAR DRIVES
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GEAR TYPES
Gears are compact,
positive-engagement,
power transmission
elements that determine
the speed, torque, and direction
of rotation of driven
machine elements. Gear types may be
grouped into five main categories:
Spur, Helical, Bevel, Hypoid, and
Worm. Typically, shaft orientation, efficiency,
and speed determine which of
these types should be used for a particular
application.
Spur gears
Spur gears have straight teeth cut
parallel to the rotational axis. The
tooth form is based on the involute
curve, Figure 1. Practice has shown
that this design accommodates
mostly rolling, rather than sliding,
contact of the tooth surfaces.
External-tooth gears — The
most common type of spur gear,
Figure 3, has teeth cut on the outside
perimeter of mating cylindrical
wheels, with the larger wheel
called the gear and the smaller
wheel the pinion.
The simplest arrangement of
spur gears is a single pair of gears
called a single reduction stage,
where output rotation is in a direction
opposite that of the input. In
other words, one is clockwise while
the other is counter-clockwise.
Helical gears
Helical gearing differs from spur in
that helical teeth are cut across the
gear face at an angle rather than
straight, Figure 6. Thus, the contact
line of the meshing teeth progresses
across the face from the tip at one end
to the root of the other, reducing the
noise and vibration characteristic of
spur gears. Also, several teeth are in
contact at any one time, producing a
more gradual loading of the teeth that
reduces wear substantially.
Bevel gears
Unlike spur and
helical gears with
teeth cut from a cylindrical
blank, bevel
gears have teeth cut
on an angular or conical surface.
Bevel gears are used when input and
output shaft centerlines intersect.
Teeth are usually cut at an angle so
that the shaft axes intersect at 90 deg,
but any other angle may be used. A
special class of bevels called miter
gears have gears of the same size with
their shafts at right angles.