# Gear Terminology: Basic definitions with PDF | The Mechanical post

This is The Mechanical post and today you would be reading about What is gear? and terminology of gears.

## What is gear?

A gear is a cylindrical or conical-shaped machine element having teeth cut on its periphery which meshes with another toothed part to transmit torque and rotary motion.

Gears are used widely for power transmission as they have a positive drive and the capability of transferring an exact velocity ratio.

They are also a highly efficient mode of power transmission when compared to belt, chain, or rope drive.

Do you know?

Gears were invented by the Greeks in the third century.

## Terminology of gear

The following are the important terms used for gears:

### Pitch circle

A Pitch circle is an imaginary circle that will contact the pitch circle of another gear when in a mesh.

### Pitch circle diameter

The pitch circle diameter is the diameter of the pitch circle. Usually, the size of the gear is defined by its pitch circle diameter. It is generally denoted by the letter "D".

 See source., CC BY-SA 3.0, via Wikimedia Commons

### Pitch point

Pich point is the point of contact of the pitch circles of gears in mesh.

### Pressure angle

The angle made by the common normal at the pitch point and the common tangent at the pitch point is called the pressure angle.

### Face of tooth

The part of the tooth above the pitch circle is called the face of the tooth.

### Flank

The part of the tooth below the pitch circle is called the flank.

### Profile

The profile is the curve formed by the face and the flank of the tooth. In short, it is the shape of the tooth. Cycloidal and involute are the two main types of gear profile.

### Face width

The Face width is the width of the tooth when measured parallel to the axis of the gear.

### Top land

The topmost part of the tooth is known as the top land.

### Tooth thickness

The distance of the tooth from one flank to another flank of the same tooth or the distance of the tooth measured along the pitch circle is the tooth thickness.

### Tooth space

The distance or space between two adjacent gear teeth when measured along the pitch circle is known as the tooth space.

### Circular pitch

The distance from one point on one tooth to the same point on the adjacent tooth, when measured along the pitch circle, is known as circular pitch.

### Diametral pitch

Diametral pitch is the ratio of the number of teeth (N) of gear to its pitch circle diameter (D).

Mathematically,  Diametral pitch = N / D

The circle drawn such that it passes through the top of the teeth and is concentric with the pitch circle is called the addendum circle.

### Dedendum circle

The circle drawn such that it passes through the bottom of the teeth of the gear and is concentric with the pitch circle is known as the Dedenddum circle.

### Dedendum

The radial distance between the dedendum and the pitch circle is known as Dedendum.

### Total depth

The radial distance between the addendum and the dedendum is the total depth.
OR

The total depth is the sum of the addendum and the dedendum of the gear.

### Module

Module is defined as the ratio of pitch circle diameter (D) to the number of teeth (N).

Mathematically,    m = D / N

### Clearance

When 2 gears are in mesh, then the radial distance between the top of the tooth of one gear to the bottom of the tooth of another gear is called clearance.

### Gear Backlash

Backlash is the difference between tooth space and tooth thickness when the distance is measured along the pitch circle.

Backlash in a gear helps prevent jamming of the gear. It also acts as clearance for thermal expansion and allows a thin film of lubricating oil. Manufacturing errors in gear also lead to the formation of backlash between the gear.

 source- en:User: GearHeads, User: Slashme, CC BY-SA 3.0, via Wikimedia Commons

## FAQ on gear terminology

#### What is a module in gear?

A module in gear is the ratio of the pitch circle diameter of gear to the number of teeth on the same gear.

#### What is gear terminology?

Gear terminology are the terms related to a gear that helps in the design of it and describes the shape and size of gear.

#### What is an addendum and dedendum?

An addendum is the radial distance between the pitch circle and the addendum circle whereas the distance from the pitch circle to the dedendum circle is the dedendum.

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