Gear transmissions normally transfer power by means of two or more gears that directly mesh with each other.
However, a gear can also be used in combination with a fixed part, such as a gear rack.
These kinds of drives allow the gear rack or gear to move longitudinally. This is often done to change the speed, the clutch and/or the direction of the energy source, in the case of a reciprocating motion.
Gear racks are usually made of steel or stainless steel, but they are also available in other materials such as plastic, for example. Gear racks can also be supplied in hardened variants and/or with a surface treatment.
The size of a drive consisting of gears and gear racks is determined by its "module" and number of teeth. The module is the ratio between the pitch circle diameter and number of teeth. The underlying formula is: m=d/z. Here, m = module, d = pitch circle diameter and z = number of teeth.
Gears and gear racks with differing module values cannot mesh, so the module must be the same for gears and gear racks.
A gear drive with a large module can transmit greater forces than one with a small module.
Common standards for transmission ratios are DIN 867 and DIN 868.
