Classification of Gears

Classification of Gears
Gears may also be classified according to the position of the axis of shaft:

1. Parallel
Spur Gear
Helical Gear
Rack and Pinion
2. Intersecting
Bevel Gear
3. Non-intersecting and Non-parallel

Gear Terminology and Basic Definition

Gear Definition

A Gear is a rotating machine part having cut teeth, or cogs, that meshes with another toothed part in order to transmit torque.

Gears are the most common source used for power transmission.

Two or more gears working in tandem are called a transmission and can produce a mechanical advantage through a Gear ratio and thus may be considered a simple machine.

The gears in a transmission are analogous to the wheels in a pulley.

The advantage of using gears is that the teeth of gear do not slip or it prevents slipping.

When two gears of an unequal number of teeth are combined, a mechanical advantage is produced, with both the rotational speeds and the torques of the two gears differing in a simple relationship.

You can see small gears for devices like wrist watches etc., and large or huge gears in big ships, vehicles, etc. The gear is a very important element of any mechanism.

Toothed gears are also used to change the speed, power, and direction between an input and output shaft.

Gears are the most common source used for power transmission.

Gears can be applied to two shafts which are in the following arrangements (position of shaft axes)-



Perpendicular and Intersecting

Perpendicular and Non-Intersecting

Inclined at an Arbitrary Angle

Gear Terminologies

Pitch Surface

The surface of the imaginary rolling cylinder (cone, etc.) that the toothed gear may be considered to replace at the pitch circle.

Base Circle

An imaginary circle used in involute gearing to generate the involutes that form the tooth profiles.

Pitch Circle

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

Addendum circle

A circle bounding the ends of the teeth, in the right section of the gear.

Dedendum circle

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


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


The radial distance between the pitch circle and the root circle (bottom of the tooth).


It is the radial distance from the top of the tooth to the bottom of the tooth, in
a meshing gear.

Clearance Circle

A circle passing through the top of the meshing gear is known as a clearance circle.

Face of a tooth

That part of the tooth surface lying outside the pitch surface or above the pitch surface.

Top land

It is the surface of the top of the tooth.


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

Flank of a tooth

The part of the tooth surface lying inside the pitch surface or below the pitch surface.

Circular thickness (tooth thickness)

The thickness of the tooth measured on the pitch circle. It is the length of an arc and not the length of a straight line.

You cannot copy content of this page