Crank (mechanism)
From Wikipedia, the free encyclopedia
A crank is an arm attached at right angles to a rotating shaft by which reciprocating motion is imparted to or received from the shaft. It is 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 attached to it. Attached to the end of the crank by a pivot is a rod, usually called a connecting rod. The end of the rod attached to the crank moves in a circular motion, while the other end is usually constrained to move in a linear sliding motion, in and out.
The term often refers to a human-powered crank which is used to manually turn an axle, as in a bicycle crankset or a brace and bit drill. In this case a person's arm or leg serves as the connecting rod, applying reciprocating force to the crank. 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.
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[edit] Examples
Familiar examples include:
[edit] Using a hand
- mechanical pencil sharpener
- fishing reel and other reels for cables, wires, ropes, etc.
- manually operated car window
- the crank set that drives a trikke through its handles.
[edit] Using the feet
- the crankset that drives a bicycle via the pedals.
- treadle sewing machine
[edit] 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.
[edit] Kinematics
The displacement of the end of the connecting rod is approximately proportional to the cosine of the angle of rotation of the crank, when it is measured from top dead center. So the reciprocating motion created by a steadily rotating crank and connecting rod is approximately simple harmonic motion:
where x is the distance of the end of the connecting rod from the crank axle, l is the length of the connecting rod, r is the length of the crank, and α is the angle of the crank measured from top dead center.
Technically, the reciprocating motion of the end of the connecting rod departs slightly from sinusoidal motion due to the changing angle of the connecting rod during the cycle.
[edit] History
The eccentrically mounted handle of the rotary handmill which appeared in 5th century BC Celtiberian Spain and ultimately spread across the Roman Empire constitutes a crank.[1][2] In China, hand-operated cranks appeared during the Han Dynasty (202 BC-220 AD), as Han era glazed-earthenware tomb models portray, and was used thereafter in China for silk-reeling and hemp-spinning, for the agricultural winnowing fan, in the water-powered flour-sifter, for hydraulic-powered metallurgic bellows, and in the well windlass.[3]
A Roman iron crank handle was excavated in Augusta Raurica, Switzerland. The 82.5 cm long piece with a 15 cm long handle is of yet unknown purpose and dates to no later than ca. 250 AD.[4] The earliest evidence for the crank as part of a machine, that is in combination with a connecting rod, appears in late Roman water-powered saw mills dating from the late 3rd (at Hierapolis) to 6th century AD (at Ephesus and Gerasa).[5]
Simple crank mechanism are the basis of many pre industrial foot operated machines.
A device shown in the early 9th century Carolingian manuscript Utrecht Psalter is a crank handle used with a rotary grindstone.[6] The crank also appears in the mid-9th century in several of the hydraulic devices described by the Banū Mūsā brothers in their Book of Ingenious Devices. Scholars point to the use of crank handles in trepanation drills in a 10th century work by the Spanish Muslim surgeon Abu al-Qasim al-Zahrawi (936–1013).[6] The Benedictine monk Theophilus Presbyter (c. 1070–c.1125) described crank handles "used in the turning of casting cores" according to Needham.[7]
Al-Jazari (1136–1206) described a crank and connecting rod system in a rotating machine in two of his water-raising machines.[8] His twin-cylinder pump incorporated a crankshaft which, like the modern crankshaft, consisted of a wheel setting several crank pins into motion.[9]
The Italian physician and inventor Guido da Vigevano (c. 1280–1349) made illustrations for a paddle boat and a war carriages that were propelled by manually turned crankshafts and gear wheels.[10] The crank became common in Europe by the early 15th century, seen in the works of those such as the military engineer Konrad Kyeser (1366–after 1405).[10]
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.
[edit] See also
[edit] References
- ^ a b Ritti, Grewe & Kessener 2007, p. 159
- ^ a b Lucas 2005, p. 5, fn. 9
- ^ Needham 1986, pp. 118–119.
- ^ Laur-Belart 1988, p. 51–52, 56, fig. 42
- ^ Ritti, Grewe & Kessener 2007, p. 161
- ^ a b Needham 1986, p. 112.
- ^ Needham 1986, pp. 112–113.
- ^ Ahmad Y Hassan. The Crank-Connecting Rod System in a Continuously Rotating Machine.
- ^ Sally Ganchy, Sarah Gancher (2009), Islam and Science, Medicine, and Technology, The Rosen Publishing Group, p. 41, ISBN 1435850661
- ^ a b Needham 1986, p. 113.
[edit] Bibliography
- Lucas, Adam Robert (2005), "Industrial Milling in the Ancient and Medieval Worlds. A Survey of the Evidence for an Industrial Revolution in Medieval Europe", Technology and Culture 46: 1-30
- Laur-Belart, Rudolf (1988), Führer durch Augusta Raurica (5th ed.), Augst
- Needham, Joseph (1991), Science and Civilisation in China: Volume 4, Physics and Physical Technology: Part 2, Mechanical Engineering, Cambridge University Press, ISBN 0521058031.
- Ritti, Tullia; Grewe, Klaus; Kessener, Paul (2007), "A Relief of a Water-powered Stone Saw Mill on a Sarcophagus at Hierapolis and its Implications", Journal of Roman Archaeology 20: 138–163
[edit] External links
- Crank highlight: Hypervideo of construction and operation of a four cylinder internal combustion engine courtesy of Ford Motor Company
- Kinematic Models for Design Digital Library (KMODDL) - Movies and photos of hundreds of working mechanical-systems models at Cornell University. Also includes an e-book library of classic texts on mechanical design and engineering.
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