A capstan is a rotating machine used to control or apply force to another element, usually linear. The term was first used for the capstan on a ship, which is used to control ropes that are wound around them. The rope is wound around one or more times, but is not stored on a capstan - that arrangement is usually called a winch. Ships' capstans were traditionally manually operated - consisting of a shaped wooden drum with handles inserted into the rim, at which men could push or pull. Modern capstans are powered electrically.
Capstans are also found in the mechanisms of tape recorders, where they have a similar function - they apply force to the tape causing it to be drawn off the spool, past the heads, and onto the take-up spool. Such capstans are precision-machined spindles, with a very accurate surface profile - any out-of-roundness or imperfections cause an audible effect called "flutter". The tape is held against the capstan by a rubber wheel called the pinch wheel or pinch roller.
The capstan is analogous to a rack and pinion arrangement found in gear systems.
