Gears are used to transmit power or motion from one shaft to another without slip and provide a constant velocity ratio. The key terminology used to describe gear teeth include: 1) Addendum - The radial distance from the pitch circle to the tip of the tooth. 2) Dedendum - The radial distance from the pitch circle to the bottom of the tooth space. 3) Pitch circle - The imaginary circle used to define the gear size and spacing of teeth. There are several methods used to measure the thickness of gear teeth.

1. Introduction
Gears are mainly used for transmission of power or motion
from one shaft to another, without slip. Thus they are
positive in action and provide constant velocity ratio.

2. Terminology of Gear tooth
 Addendum:- This is the radial distance from the pitch
circle to the tip of the tooth. Its value is equal to one
module.
 Clearance:- This is the radial distance from the tip of a
tooth to the bottom of a mating tooth space when the
teeth are symmetrically engaged. Its standard value is
0.157 m.
 Dedendum:- This is the radial distance from the pitch
circle to the bottom of the tooth space.
 Dedendum = Addendum + Clearance = m+0.157 m=1.153m.

4. Face of Tooth:- It is that part of the tooth surface which is above the pitch
surface.
Flank of Tooth:- It is that part of the tooth surface which is lying below the pitch
surface.
Tooth thickness:- The thickness of tooth, measured along the pitch circle.
Whole depth= Addendum + Dedendum
Working depth= Addendum + Dedendum - Clearance
Pitch circle:- In every pair of gears in mesh, the two plain wheels in contact are
assumed to exist; which are known as pitch circle.
Pitch point:- It is the point of contact between the pitch circle of two gears in
mesh.

5. Circular pitch:- It is the arc distance measured around the pitch circle from a point on
one tooth to a corresponding point on the adjacent tooth.
Circular pitch = πd/T = πm
d = pitch circle diameter
T = no. of teeth
m = module = d/T
Diametral pitch = Number of teeth/Pitch circle diameter = T/d
Module = (1/ Diametral pitch)
Back lash = The diameter through which a gear can be rotated to bring its non-working
flank in contact with the teeth of mating gear.

6. Pressure angle or angle of obliquity:-It is the angle which common normal to the
two teeth at the point of contact makes with common tangent to the two pitch circles
at the pitch point.

7. Tooth thickness measurement:-Since tooth thickness is defined as length of an arc,
it is not possible to measure it directly. There are various methods of measuring the
gear tooth thickness.
(i)Measurement of tooth thickness by gear tooth vernier calliper (Chordal thickness
method)
(ii)Constant chord method
(iii)Base tangent method
(iv)Measurement by dimension over pins.