Crank (mechanism)

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A crank is an arm at right angles to a shaft (an axle or spindle), by which motion is imparted to or received from the shaft; it is also used to change circular into reciprocating motion, or reciprocating into circular motion. The arm may be a bent portion of the shaft, or a separate arm keyed to it.

One application is human-powered turning of the axle. Often there is a bar perpendicular to the other end of the arm, often with a freely rotatable handle on it to hold in the hand, or in the case of operation by a foot (usually with a second arm for the other foot), with a freely rotatable pedal.

Examples

Familiar examples include:

Using a hand

manual pencil sharpener
fishing reel and other reels for cables, wires, ropes, etc.
train window
manually operated car window
the crank set that drives a trikke through its handles.

Using feet

the crankset that drives a bicycle via the pedals.
treadle sewing machine

Engines

Almost all reciprocating engines use cranks to transform the back-and-forth motion of the pistons into rotary motion. The cranks are incorporated into a crankshaft.

History

The earliest hand-operated cranks appeared in China during the Han Dynasty (202 BC-220 AD), as Han era glazed-earthenware tomb models portray. It was however through Al-Jazari that the use of crank became widely established, as he was the first to incorporate a crankshaft in a machine. The connecting rod was also invented by Al-Jazari, and was used in a crank and connecting rod system in a rotating machine he developed in 1206, in two of his water-raising machines. Cranks were formerly common on some machines in the early 20th century; for example almost all phonographs before the 1930s were powered by clockwork motors wound with cranks, and internal combustion engines of automobiles were usually started with cranks (known as starting handles in the UK), before electric starters came into general use.

References

External links and books

Crank highlight: Hypervideo of construction and operation of a four cylinder internal combustion engine courtesy of Ford Motor Company
Needham, Joseph (1986). Science and Civilization in China: Volume 4, Physics and Physical Technology, Part 2, Mechanical Engineering. Taipei: Caves Books, Ltd.

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Last updated on Tuesday February 19, 2008 at 16:14:10 PST (GMT -0800)
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