**Gear Tooth Nomenclature**

**Pitch circle diameter** For two meshing gears these are the diameters of the discs
that would transmit the same
velocity ratio by friction (if there was no slipping) as the gears.

**Module, ***m* this indicates the tooth size and is the number of mm of pitch circle diameter (p.c.d.) per
tooth. For gears to mesh, their modules must be equal.

Gear ISO standards and design methods are now normally based on the module.

EG a gear of module 3 has 16 teeth, its pitch circle diameter is: 3 x 16 = 48 mm.

**Diametral pitch, ***P* is the number of teeth per m or mm of gear pitch circle diameter.

*P* = 1/*m*

**Circular pitch , ***p* is the distance from one point on the pitch circle of one tooth to
the corresponding point on the adjacent tooth, measured around the pitch circle. *pP*=PI

The first diagram shows the tooth geometry for the ISO basic rack in terms of module
1 size teeth; PI means 3.14159

The diagram below defines some of the terms used for helical gearing.

The angle *psi* is the *helix angle*.

The transverse circular pitch, in the plane of rotation, is: *p*_{t}
(usually called the circular pitch).

The normal circular pitch, perpendicular to the tooth axis, is: *p*_{n}

The normal pressure angle, *phi*_{n} is different from the
transverse pressure angle, *phi*_{t} and these are related by the
equation:

cos*(helix angle)* = tan *(normal pressure angle)*/tan*(transverse
pressure angle)*.

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to Module Introduction

David Grieve, 15th October 2001.