GEARS

Introduction
A gear or cogwheel is a rotating machine part having cut teeth, or cogs, which mesh with another
toothed part to transmit torque. Geared devices can change the speed, torque, and direction of
a power source. Gears almost always produce a change in torque, creating a mechanical advantage,
through their gear ratio, and thus may be considered a simple machine. The teeth on the two
meshing gears all have the same shape. Two or more meshing gears, working in a sequence, are
called a gear train or a transmission. A gear can mesh with a linear toothed part, called a rack,
producing translation instead of rotation.
The gears in a transmission are analogous to the wheels in a crossed, belt pulley system. An
advantage of gears is that the teeth of a gear prevent slippage.
When two gears mesh, if one gear is bigger than the other, a mechanical advantage is produced,
with the rotational speeds, and the torques, of the two gears differing in proportion to their
diameters.
Gear Terminologies
1.Module size
Using ISO (International Organization for Standardization) guidelines, Module Size is designated
as the unit representing gear tooth-sizes.

Module (m)
m = 1 (p = 3.1416)
m = 2 (p = 6.2832)
m = 4 (p = 12.566)

2. Pitch
If you multiply Module by Pi, you can obtain Pitch (p). Pitch is the distance between corresponding
points on adjacent teeth.

p = Pi x Module = πm

3. Pressure Angle ( α )
Pressure angle is the leaning angle of a gear tooth, an element determining the tooth profile.
Recently, the pressure angle (α) is usually set to 20°, however, 14.5° gears were prevalent.

4. Number of teeth
Number of teeth denotes the number of gear teeth.
5. Tooth Depth and Thickness
Tooth depth is determined from the size of the module (m). Introduced here are Tooth Profiles
(Full depth) specified by ISO and JIS (Japan Industrial Standards) standards.

6. Diameter of Gears (Size)

The size of gears is determined in accordance with the reference diameter (d) and determined by
these other factors; the base circle, Pitch, Tooth Thickness, Tooth Depth, Addendum and
Dedendum.

Gear functions
(i) The basic function of an gear is to transmit rotary motion as well as power from one shaft
to another.
(ii) Gears can also be used to amplify power.
(iii) Almost in every machine where speed control is needed gears are used.
(iv) Gears are used to transfer the mechanical advantage of motors and engines to drive or lift
objects.
(v) Gears are used to transfer the mechanical advantage of motors and engines to drive or lift
objects.

Gear Kinematics
Kinematics is the branch of classical mechanics which describes the motion of points, bodies
(objects) and systems of bodies (groups of objects) without consideration of the causes of
motion. Kinematics as a field of study is often referred to as the "geometry of motion".

For the gear train shown in the figure, calculate the rotational speed of the output gear if the
rotational speeds of the input gear and the arm are 500 rpm and 300 rpm respectively.
Gear measurement tests

At KHK, the following measuring instruments are used to test and inspect the gear accuracy.

1. Gear measuring machine……To measure the accuracy of tooth profile, tooth trace (flank
line), pitch and runout
2. 3-D coordinate measuring machine……To measure the pitch accuracy of racks
3. Composite gear tester……To test composite deviation

The measuring equipment used while working on gear production are:

 Vernier calipers, Micrometer calipers, Cylinder gauge……To measure inside and outside
diameters and tooth thickness
 Runout tester……To measure side face runout, and circumference (radial) runout
 Hardness testing machine……To measure hardness
 Micrometer calipers……To find span measurement
 Gear tooth vernier calipers……To measure tooth thickness of worm
 Worm gear tester……To measure tooth contact and backlash
 Bevel gear tester……To measure tooth contact and backlash
6. Spur gears
Spur gears or straight-cut gears are the simplest type of gear. They consist of a cylinder or disk
with teeth projecting radially. Though the teeth are not straight-sided (but usually of special
form to achieve a constant drive ratio, mainly involute but less commonly cycloidal), the edge
of each tooth is straight and aligned parallel to the axis of rotation. These gears mesh together
correctly only if fitted to parallel shafts. No axial thrust is created by the tooth loads. Spur gears
are excellent at moderate speeds but tend to be noisy at high speeds.

Spur gear measurement