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 but operates on a different principle, being that of the
pole lathe
A pole lathe, also known as a springpole lathe, is a wood-turning lathe that uses the elasticity within a long pole as a return spring for a treadle. Pressing the treadle pulls on a cord that is usually wrapped around the piece of wood or billet be ...
. It consists of a
capstan which is a reel or
pulley 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
Tension may refer to:
Science
* Psychological stress
* Tension (physics), a force related to the stretching of an object (the opposite of compression)
* Tension (geology), a stress which stretches rocks in two opposite directions
* Voltage or el ...
.
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
A universal joint (also called a universal coupling or U-joint) is a joint or coupling connecting rigid shafts whose axes are inclined to each other. It is commonly used in shafts that transmit rotary motion. It consists of a pair of hinges ...
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
In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of th ...
-limiting
clutch. 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 is a system of components, linkages, and other parts that allows a driver to control the direction of the vehicle.
Introduction
The most conventional steering arrangement allows a driver to turn the front wheels of a vehicle using ...
mechanism of vehicles, etc.
The slip torque of the capstan can be set to any desired torque by adjusting the tension on the cable according to the
capstan equation.
External links
{{DEFAULTSORT:Capstan And Bowstring
Automotive steering technologies