Power Transmission Glossary#


Center to center (C2C) refers to the distance between the centers of a pair of sprockets, pulleys or gears. This will affect chain/belt tension and gear meshing, so calculating this correctly is essential.

Clearance Diameter#

The clearance diameter is the diameter of the imaginary circle that contains the entire pulley, sprocket, or gear. For pulleys and sprockets, this includes the belt or chain on the component. Clearance diameter is used to check for interferences with other mechanisms. It is usually larger then Pitch Diameter (PD) and Outside Diameter (OD)

GT2 Belt#

GT2 belt is a type of synchronous timing belt commonly used on drivetrains and other mechanisms. Its available in different widths and pitches, although the most common is GT2 3mm (3mm Pitch), compatible with REV Robotics COTS Pulleys. See Timing Belt

HTD Belt#

HTD belt is a type of synchronous timing belt commonly used on drivetrains. It is available in different widths to accommodate different sized pulleys. The most common is HTD3 (3mm Pitch) and HTD5 (5mm Pitch) belts, as these are compatible with goBILDA COTS pulleys. See Timing Belt


An idler gear, sprocket, or pulley is one that is purposely not used for driving anything else on the shaft. The purpose of this idler is, in the case of gears, to transfer power to another direction.

For chain and belt, idlers are more common, and are usually adjustable to maintain tension.


Loctite is thread locking fluid used so that bolts do not come loose under use and vibration. Loctite should be applied to the threads of the bolts. There are two types of Loctite: blue, which is removable, and red, which is permanent (and we mean it).


It is highly recommended that teams use Loctite on all motor and servo mounts, as well as any mechanism prone to vibration.


THE BOTTLE COLOR AND THE FLUID COLOR ARE REVERSED. When we refer to the “color”, we mean the fluid color. Blue loctite usually comes in a red bottle.

A red bottle containing blue (removable) loctite, and a blue bottle containing red (permament) loctite

Blue Loctite (removable, in red tube), Red Loctite (permament, in blue tube)#

Outside Diameter (OD)#

The diameter of the imaginary circle traced by the outermost face of the teeth on a gear, sprocket, or pulley. This will be larger then the Pitch Diameter (PD) for a gear, but smaller then the Pitch Diameter (PD) for a pulley or sprocket.


Pitch refers to the distance between the center of one tooth of a gear, sprocket, or pulley to another. In chains it refers to the distance from one pin to another, and in belts it refers to the distance betewen one groove to another.

Pitch Diameter (PD)#

An imaginary circle around a component used for various calculations. For gears, it is the imaginary circle that mates with any other gear’s pitch diameter when the gears are properly spaced. For chain and belt, it is the circle which is traced by the middle of the belt or chain as the pulley or sprocket rotates. For gears, the pitch diameter will be smaller then the Outside Diameter (OD), but with chain and belts, the pitch diameter will be larger. \(PD = (tooth * pitch)/\pi\)


A sprocket is a mechanical part that transfers power through its cogs, which fit into chain. It is similar to a gear, except that instead of meshing with another gear, the sprocket meshes with chain. See Chain

A REV Robotics Delrin 20 Tooth #25 sprocket

Delrin 20 Tooth #25 sprocket#