What is Gear Terminology

 

Addendum: Height of tooth above Pitch circle or the radial distance between the Pitch circle and Top of the Tooth.

 

Pitch diameter: The diameter of the pitch circle.

 

Module:  Ratio of pitch diameter to the number of teeth.

 

Pressure angle   : The angle which means a tooth profile and a radial line at its pitch point. In Involute teeth pressure angle is often described as the angle between the line of action and the line tangent to the pitch circle.

 

Approach ratio :The ratio of the arc of the approach to the arc of the action.

 

Arc of action : Arc of the Pitch circle through which a tooth travels from the first point of contact with the mating tooth to the point where contact ceases.

 

Arc of approach : Arc of the Pitch circle through which a tooth travels from the first (initial) point of contact with the mating tooth to the pitch point .

 

Arc of recess : Arc of the Pitch circle through which a tooth travels from its contact with the mating tooth at the pitch point to  the point where its contact ceases.

 

Axial plane : In a pair of gears it is the plane that contain the two axes. In a single gear it may be any plane containing the axes and  a given point.

 

Backlash : The amount by which the width of a tooth space exceeds the thickness of the engaging  tooth on the pitch circle

 

Base circle : The circle from which an involute  tooth curve is generated or developed.

 

Circular pitch : Length of the arc of the pitch circle between the centers or other corresponding points of adjacent teeth.

Normal circular pitch is the circular pitch in the normal plane.

 

Circular thickness : The length of the arc between the two sides of a gear tooth on the pitch circle unless otherwise specified. Normal Circular thickness is the circular thickness in the normal plane.

 

Clearance: The amount by which the dedendum in given gear exceeds the addendum of its mating gear.

It is also the radial distance between the top of a tooth and the bottom of the mating tooth space.

 

Contact diameter: The smallest diameter on a gear tooth with which the mating gear makes contact..

 

Contact ratio: The ratio of the arc of the action to the circular pitch. It is sometimes thought of as the average number of teeth in contact.For involute gears the contact ratio is obtained most directly as the ratio of the length of action to the base pitch.

 

Base Helix angle : The angle at the base cylinder of an involute gear  that the  tooth makes with the gear axis.

 

Base Pitch : In an involute gear it is the pitch on the base circle or along the line of action .

The normal base pitch is the base pitch in the normal plane and the axial base pitch  is the base pitch in the axial plane.

 

Center distance : The distance between the parallel axis of spur gears  and parallel helical axis  or between the crossed axes of crossed helical gears  and worm gears .

Also it is the distance between the centers of the pitch circles.

 

Dedendum : The depth of tooth space below the pitch circle or the radial dimension between the pitch circle and the bottom of the tooth space.

 

Diametral Pitch : The ratio of the number of teeth  to the number of inches of pitch diameter- equals number of gear teeth to each inch of pitch diameter.