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# Gear Terminology:  Pitch circle, Pitch circle diameter,  Pitch point, Pitch surface, Pressure angle or angle of obliquity,  Addendum, Dedendum, Addendum circle, Dedendum circle

## Pitch circle

It is an imaginary circle which by pure
rolling action, would give the same motion as the actual gear.

## Pitch circle diameter

It
is the diameter of the pitch circle. The size of the gear is usually specified by the pitch
circle diameter. It is also called as pitch diameter.

## Pitch point

It is a common
point of contact between two pitch circles.

Pitch surface

It is the surface
of the rolling discs which the meshing
gears have replaced
at the pitch circle.

Pressure angle or angle of obliquity

It is the angle between the common normal to two gear
teeth at the point of contact and the common tangent at the pitch point. It is usually
denoted by
f. The standard pressure angles are 141/2° and 20°.

distance of a tooth from the pitch circle to the top of the tooth.

## Dedendum

It is the radial distance of a tooth from the pitch circle to the bottom of the tooth.

It is the circle
drawn through the top of the teeth and is concentric with the
pitch circle.

Dedendum circle.

It is the circle
drawn through the bottom of the teeth.
It is also called root circle.

Note : Root circle diameter = Pitch circle
diameter × cos
f, where f is the pressure angle.

## Circular pitch

It
is the distance measured on the circumference of the pitch circle from a point of one
tooth to the corresponding
point
on the next tooth. It is usually denoted by p
c. Mathematically,

Circular pitch,      pc = p D/T

where            D = Diameter of the pitch circle, and

T = Number of teeth on the wheel.

A
little consideration will show that the two gears will mesh together
correctly, if the two wheels
have the same circular pitch.

Note : If D1 and D2 are the diameters of the two meshing gears having the teeth T1 and
T
2 respectively;
then for them to mesh correctly,

D1/ D2 T1/T2

## Diametral pitch

It is the ratio
of number of teeth to the pitch circle diameter
in millimetres. It denoted by
p
d.

## Module

It is the ratio
of the pitch circle diameter
in millimetres to the number
of teeth. It is
usually denoted by m.

Note
:
The recommended series of modules in Indian Standard are 1,
1.25, 1.5, 2, 2.5, 3, 4, 5,

6, 8, 10, 12, 16, 20, 25, 32, 40 and 50.

The modules 1.125, 1.375, 1.75,
2.25, 2.75, 3.5, 4.5,5.5, 7, 9, 11, 14, 18, 22, 28, 36 and

45 are of second choice.

 Spur gear

## Clearance

It is the radial distance from the top of the tooth to the bottom of the tooth, in a
meshing gear. A circle passing
through the top of the meshing gear is known as clearance
circle.

## Total depth

It is the
radial distance between the addendum and the dedendum circle of a gear. It is equal to the sum of the

## Working depth

distance from the addendum circle to the clearance circle. It is equal to the sum of the addendum of the two meshing gears.

## Tooth thickness

It is the
width of the tooth measured along the pitch
circle.

## Tooth space

It is the
width of space between the two adjacent teeth measured along the pitch circle.

## Backlash

It is the difference between the tooth space and the tooth thickness, as measured
on the pitch circle.

## Face of the tooth

It is surface
of the tooth above the pitch surface.

## Top land

It is the
surface of the top of the tooth.

## Flank of the tooth

It
is the surface of the tooth below the pitch
surface.

## Face width

It is the width
of the gear tooth measured
parallel to its axis.

## Profile

It is the curve
formed by the face and flank of the tooth.

It is the
radius that connects the root circle to the profile of the tooth.

## Path of contact

It is the path traced by the point of contact
of two teeth from the beginning
to the end of engagement.

## Length of the path of contact

It is the length
of the common
normal cut-off
by the addendum circles of the wheel and pinion.

## Arc of contact

It is the path traced by a point
on the pitch circle from the beginning to the end of engagement of a given
pair of teeth.
The arc of contact consists
of two parts, i.e.

## (a)Arc of approach

It is the portion
of the path of contact
from the beginning of the engagement to the pitch point.

## (b)  Arc of recess

It is the portion of the path of contact from the pitch point to
the end of the
engagement of a pair of teeth.

Note : The ratio of the length
of arc of contact to the circular
pitch is known as contact ratio i.e. number of pairs
of teeth in contact.