A rack may be considered to be a gear of an infinite Pitch circle diameter. In consequence, remarks with are applicable to gears are equally applicable for racks.

In order to be able to mesh, the rack must be of an identical module to the gear. Also, the helical angles of the gears must be the same. A spur rack should be able to mesh with a spur gear of the same module. A parallel helical gear should be able to mesh with a helical gear, if the module is identical and the helical angle is complementary...

Note: Racks with an H.A. of 45?are not manufactured. They are not at all interesting because the angle is too large and the ensuing loss of couple is to high.

The gear has a rotating movement, whereas the rack has a linear translation.

For a rack, it is important to know the pitch and the module.

Pitch: p = m

When a very long rack is required, separate racks should be laid end to end. These racks are said to be joinable, i.e. they are cut at the lowest point of the teeth so that they can be mounted end to end.