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Have you ever looked at a gear drawing and been overwhelmed by all the different specifications, wondering what it all means?
Gear terminology means all definitions used to describe a gear or the terms used to design a gear.Let's know the terminology used in Gears .Gears are widely used in mechanical industries for power transmission. It is used where small distance between driver and driven shaft. It is the only positive drive which can transfer exact velocity ratio to the driven shaft, that’s why it is widely used in various machines and other mechanical devices.

The following terms, which are mostly used to describe a gear, are as follow.

Number of Teeth :
The quantity of teeth on the gear. This term becomes slightly more complicated if sector gears are specified, in which case the manufactured quantity of teeth will be called out along with the total number of teeth of the full gear.

Diametral Pitch :
The quantity of teeth per inch.

Pitch Circle:
It is an imaginary circle which is in pure rolling action. The motion of the gear is describe by the pitch circle motion.

Pitch Circle diameter:
The diameter of the pitch circle from the center of the gear is known as pitch circle diameter. The gear diameter is described by its pitch circle diameter.

Pitch point:
When the two gears are in contact, the common point of both of pitch circle of meshing gears is known as pitch point.

Pressure angle or angle of obliquity:
Pressure angle is the angle between common normal to the pitch circle to the common tangent to the pitch point.

Distance between the pitch circle to the top of the tooth in radial direction is known as addendum.
Dedendum:
Distance between the pitch circle to the bottom of the tooth in radial direction, is known as dedendum of the gear.

The circle passes from the top of the tooth is known as addendum circle. This circle is concentric with pitch circle.

Dedendum circle:
The circle passes from the bottom of the tooth is known as dedendum circle. This circle is also concentric with pitch circle and addendum circle.

Module:
The ratio of the pitch circle diameter in millimeters to the total number of teeth is known as module. It is reciprocal of the diametrical pitch.

Clearance:
When two gears are in meshing condition, the radial distance from top of a tooth of one gear to the bottom of the tooth of another gear is known as clearance. The circle passes from the top of the tooth in meshing condition is known as clearance angle.

Total depth:
The sum of the addendum and dedendum of a gear is known as total depth. It is the distance between addendum circle to the dedendum circle measure along radial direction.

Working depth:
The distance between addendum circle to the clearance circle measured along radial direction is known as working depth of the gear.

Full Depth :
Full depth is sum of the addendum and the dedendum.
Face Width :
Face width is length of tooth parallel to axes.

The axial distance required for a helical gear tooth to make one complete revolution around the gear.

Whole Depth :
The gear tooth depth from the tip to the root.

Diametral Pitch :
Diametral pitch (p) is the number of teeth per unit volume.
p = (Number of Teeth) / (Diameter of Pitch circle)
Gear Ratio :
Gear ratio is numbers of teeth of larger gear to smaller gear.
Pressure Line :
Pressure line is the common normal at the point of contact of mating gears along which the driving tooth exerts force on the driven tooth.

Pressure Angle :
Pressure angle is the angle between the pressure line and common tangent to pitch circles. It is also called angle of obliquity. high pressure angle requires wider base and stronger teeth.

Helix Angle:

The tooth angle relative to the central axis. Helix angles typically range from 10° to 45° depending on the design requirements. Gears with 0° helix angles are called spur gears. Helical gears (over 0°) increase contact ratio and improve gear noise, but also produce an axial load that has to be accounted for in the bearing design.

Helix Hand:
The direction of the helix angle expressed as either right or left. Looking down the central axis of the gear, a right hand helix angles from left to right down the tooth face away from you. The opposite is true for a left hand helix.
Tooth thickness:
Distance of the tooth measured along the circumference of the pitch circle is known as tooth thickness.

Normal Tooth Thickness :
The tooth thickness measured normal or perpendicular to the central axis of the gear—also at the pitch diameter.

Transverse Tooth Thickness :
The tooth thickness measured normal or perpendicular to the helix angle—also at the pitch diameter.

Effective Tooth Thickness :
The theoretical tooth thickness that accounts for manufacturing process errors.

Measured or Actual Tooth Thickness :
The tooth thickness specified that does not account for manufacturing process errors and is used for measurement and inspection of the gear.Tooth space:
Distance between the two adjacent tooth measured along the circumference of the pitch circle is known as the tooth space.

Backlash:
It is the difference between the tooth thickness and the tooth space. It prevents jamming of the gears in meshing condition.

Profile:
It is the curved formed by the face and flank is known as profile of the tooth. Gear tooth are generally have cycloidal or involute profile.

Path of contact:
The curved traced by the point of contact of two teeth form beginning to the end of engagement is known as path of contact.

Arc of contact:
It is the curve traced by the pitch point form the beginning to the end of engagement is known as arc of contact.

Arc of approach:
The portion of the path of contact from beginning of engagement to the pitch point is known as arc of approach.

Arc of recess:
The portion of the path of contact form pitch point to the end of the engagement is known as arc of recess.