**Contents**show

# 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°.

** ****Addendum**

** **It is the radial

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.

**Addendum circle****.**

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 pc. 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

T2 respectively;

then for them to mesh correctly,

D1/ D2= T1/T2

Gear Terminology |

**Diametral pitch**

** **It is the ratio

of number of teeth to the pitch circle diameter

in millimetres. It denoted by

pd.

** ****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

addendum and dedendum.

**Working**

**depth**

** **It is radial

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

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.

**Fillet radius**

** **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

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.