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# Gear Glossary

TermDefinitionFormulae
Active Profile The part a gear tooth which actually comes into contact with its mating tooth along the line of action.
Addendum (A) The difference between pitch circle diameter and external diameter of a gear. Distance to the highest point of a tooth, measured from the pitch circle diameter. A=M

Modification made to gears with low numbers of teeth to prevent undercutting and improve tooth strength.

Backlash (B)

The clearance between mating teeth which causes lost motion during operation. Also called ‘play’.

Base Circle Diameter (BCD)

The diameter of the base cylinder from which the involute is generated.

BCD = N·Mcos PA

Base Pitch (BP)

The circular pitch in the plane of rotation at the base circle.

BP = π Mcos PA
Basic Rack

A straight-sided rack shape featuring teeth along a flat surface.

Bore Length

The total length through a gear bore.

Bottom Land

The lowest point along the surface of the gear between two adjacent tooth flanks.

Centre Distance (CD)

The distance between pitch circle diameters of two mating gears.

CD = PCD (pinion) / 2 + PCD (gear) / 2

Circular Pitch (CP)

The distance along the pitch circle between corresponding points of adjacent teeth.

CP = π M

Circular Tooth Thickness (CTT)

The distance along the pitch circle between opposite sides of one gear tooth.

CTT = π M / 2

Clearance Operating (CO)

The distance by which the dedendum of a given gear extends beyond the addendum of a mating gear.

CO = D/A

Dedendum (D)

The difference between pitch circle diameter and root diameter of a gear. Distance to the bottom land, measured from the pitch circle diameter.

Diametric Pitch (DP)

The number of teeth per inches of pitch diameter. Generally used as an imperial equivalent to module.

Double Flank Gear Test

A basic gear rolling inspection which tests for variations in the input and output motion when a gear is used alongside a ‘perfect’ master gear.

Face Width

The axial width of each gear tooth.

The radius of the curve which forms the fillet at the base of each tooth.

Gear Ratio

The ratio of the number of teeth on two mating gears.

Hub

An enlarged central cylinder around the bore of a gear which improves overall gear strength and provides a greater surface area for fastening. Can also be fitted with a threaded hole for securing by threaded fastener. Gears can be manufactured with or without this feature.

Line of Action

The hypothetical straight line between base circle diameters of two mating gears. Also known as the involute.

Indicator Limits

The nominal diameter of a gear measured from the pitch circle diameter.

Involute

The hypothetical straight line between base circle diameters of two mating gears. Also known as the line of action.

Module (M)

The size of a tooth measured as a ratio of mm to pitch diameter.

M=A

Number of Teeth (N)

The number of teeth on a given gear.

N = Pd·D

Outside Diameter (OD)

The external diameter of a gear, measured around the highest point of each tooth.

OD = PCD + (2M)

Pitch (P)

The general distance between each tooth. Can be measured in multiple ways from different points, including base pitch, circular pitch and diametric pitch.

PCD = (N+2K)·M

Pitch Circle Diameter (PCD)

The hypothetical circle which runs directly through the middle of a gear’s teeth. In theory, the pitch circle diameter represents the point at which motion is transferred through a gear system.

CD = PCD (pinion) / 2 + PCD (gear) / 2

Pressure Angle (PA)

The angle between the tooth face and the tangent of the pitch circle diameter. This is the angle at which pressure is transferred. This angle is generally 20º unless otherwise stated.

Root Diameter (RD)

The diameter of the gear, measured along the bottom land of each tooth.

RD= OD – (2H)

Total Composite Error (TCE)

The overall error in a gear over one complete rotation of the double flank gear test.

Tooth to Tooth Composite Error (TTCE)

The individual error of each tooth on the gear in the double flank gear test. Particularly useful for larger gears or worn teeth.

Undercut

The point at which the base of a gear tooth is cut inwards to create space for rolling teeth and promote smooth operation. Can weaken teeth, and can be increased, reduced, or removed entirely.

N (min ) – 2/Sin² PA

Whole Depth (H)

The total depth of a tooth measured from the addendum to the dedendum.

H = A + D

Working Depth (WD)

The total depth of engagement between two mating teeth. Measure by combining the addendum of both meshed gears.

WD = 2A