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.
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A pinion is usually the smallest gear in a gear drive train. In many cases, such as remote controlled toys, the pinion is also the drive gear. In the case of John Blenkinsop's The Salamanca the pinion was rather large.
2. in the case of radio controlled cars with an engine (i.e. nitro) this pinion gear can be referred to as a "Clutch bell"