The Capstan and Bowstring is a mechanism that converts rotary motion into linear motion and vice versa. It is very similar in function to a rack and pinion

Rack and pinion

A rack and pinion is a type of linear actuator that comprises a pair of gears which convert rotational motion into linear motion. A circular gear called "the pinion" engages teeth on a linear "gear" bar called "the rack"; rotational motion applied to the pinion causes the rack to move, thereby...

 but operates on a different principle, being that of the Polelathe

Polelathe

A pole lathe is a wood-turning lathe that uses a long pole as a return spring for a treadle. Pressing the treadle with your foot pulls on a cord that is wrapped around the piece of wood or billet being turned. The other end of the cord reaches up to the end of a long springy pole. As the action is...

. It consists of a capstan

Capstan

Capstan may refer to:*Capstan , a rotating machine used to control or apply force to another element*Capstan , rotating spindles used to move recording tape through the mechanism of a tape recorder...

 which is a reel or pulley

Pulley

A pulley, also called a sheave or a drum, is a mechanism composed of a wheel on an axle or shaft that may have a groove between two flanges around its circumference. A rope, cable, belt, or chain usually runs over the wheel and inside the groove, if present...

 around which the bowstring is wound several turns. As the capstan is rotated, the bowstring is pulled in on one side and reeled out on the other. The free ends of the bowstring are coupled to the linear part of the mechanism, the "bow", which holds the string in tension.
Such mechanisms are not often found since the rack and pinion is generally more robust for the same job. However the capstan and bowstring is easily fabricated by the home builder from scratch, where accurate rack and pinion mechanisms are not, and also require much looser tolerances between the working parts. The angle between the capstan shaft and the linear element can also be variable without any special couplings needed, such as universal joint

Universal joint

A universal joint, universal coupling, U joint, Cardan joint, Hardy-Spicer joint, or Hooke's joint is a joint or coupling in a rigid rod that allows the rod to 'bend' in any direction, and is commonly used in shafts that transmit rotary motion...

s. The cable is typically steel and can be tensioned at the points where it is connected to the bow. Another feature of the capstan and bowstring is that it can be designed to slip at the ends of its travel if desired by the simple expedient of having the cable slip around the capstan, forming a simple torque

Torque

Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....

-limiting clutch

Clutch

A clutch is a mechanical device which provides for the transmission of power from one component to another...

. Capstan and bowstring mechanisms can be used wherever a rack and pinion or other rotary-to-linear conversion is found, such as in the steering

Steering

Steering is the term applied to the collection of components, linkages, etc. which will allow a vessel or vehicle to follow the desired course...

mechanism of vehicles, etc.