Mechanical device consisting of a bar of rectangular cross section (the rack), having teeth on one side that mesh with teeth on a small gear (the pinion).
If the pinion rotates about a fixed axis, the rack will move in a straight path. Some automobile steering mechanisms have rack-and-pinion drives that use this principle. If the rack is fixed and the pinion is carried in bearings on a table guided on tracks parallel to the rack, rotation of the pinion shaft will move the table parallel to the rack. On machine tools , this principle is used to obtain rapid movements of worktables.