Crankshaft

2006-06-26T12:51:14Z

68.124.65.79:

[[Image:Cshaft.gif|300px|thumb|Crankshaft, pistons, and flywheel]]
[[Image:Marine Crankshafts 8b03602r.jpg|right|thumb|250px|[[Continental engine]] marine crankshafts, 1942]]
:''For the comic strip about an old, curmudgeonly bus driver, see [[Crankshaft (comic strip)]].''

The '''crankshaft''', sometimes casually abbreviated to ''[[crank (mechanism)|crank]]'', is that part of an [[engine]] which translates [[reciprocate|reciprocating]] [[linear]] [[piston]] motion into rotation. It typically connects to a [[flywheel]], to reduce the pulsation characteristic of the [[four stroke cycle]], and sometimes a torsional or vibrational damper at the opposite end, to reduce the [[torsion (mechanics)|torsion]] vibrations often caused along the length of the crankshaft by the cylinders furthest from the output end acting on the torsional elasticity of the metal. The crankshaft was invented by the Turkish inventor [[Al-Jazari]] in the 12th century.

==Design==
Large engines are usually [[engine configuration|multicylinder]] to reduce pulsations from individual firing [[stroke (engines)|strokes]], with more than one piston attached to a more complex crankshaft; but many small engines, such as those found in [[moped]]s or garden machinery, are single cylinder and use only a single piston, simplifying crankshaft design. The crankshaft has a linear [[Axis of rotation|axis]] about which it rotates, typically with several bearing journals riding on replaceable [[plain bearing|bearings]] held in the engine block, the [[main bearing]]s. As the crankshaft undergoes a great deal of sideways load from each cylinder in a multicylinder engine, it must be supported by several such bearings, not just one at each end; this was also a factor in the rise of [[V8]] engines with their shorter crankshafts, in preference to [[straight-8]] engines. High performance engines will often have more main bearings than their lower performance cousins, for this reason. In addition, to convert the reciprocating motion into rotation, the crankshaft has "crank throws" or "crank pins", additional bearing surfaces whose axis is offset from that of the crank, to which the "big ends" of the [[connecting rod]]s from each cylinder attach. The distance of the axis of the crank throws from the axis of the crankshaft determines the piston [[stroke (engines)|stroke]] measurement, and thus [[engine displacement]]; a common way to increase the power of an engine is to increase the stroke. This also increases the reciprocating vibration, however, limiting the high [[RPM]] capability of the engine; in compensation, it improves the low speed operation of the engine, as the longer intake stroke through smaller valve(s) results in greater turbulence and mixing of the intake charge. For this reason, even such high speed production engines as current [[Honda]] engines are classified as long-stroke, in that the stroke is larger than the diameter of the [[cylinder bore]]. In production [[V engine|V]] or [[flat engine|flat]] engines, neighboring connecting rods attach side by side to the same crank throw, simplifying crank design.

The configuration and number of pistons in relation to each other and the crank leads to [[straight engine|straight]], [[V engine|V]] or [[flat engine|flat]] engines. The same basic [[engine block]] can be used with different crankshafts, however, to alter the [[firing order]]; for instance, the 90 degree [[V6]] engine configuration, usually derived by using [[V6|six cylinders]] of a [[V8]] engine with what is basically a shortened version of the V8 crankshaft, produces an engine with an [[V6#Odd and even firing|inherent pulsation in the power flow]] due to the "missing" two cylinders, often reduced by use of [[balance shaft]]s. The same engine, however, can be made to provide evenly spaced power pulses by using a crankshaft with an individual crank throw for each cylinder, spaced so that the pistons are actually phased 60 degrees apart, as in the [[GM 3800 engine]]. Similarly, while production V8 engines use 4 crank throws spaced 90 degrees apart, racing engines often use a "flat" crankshaft with throws spaced 180 degrees apart, accounting for the higher pitched, smoother sound of [[Indy Racing League|IRL]] engines compared to [[NASCAR]] engines, for example. In engines other than the flat configuration, it is necessary to provide [[counterweight]]s for the reciprocating mass of each piston and connecting rod; these are typically cast as part of the crankshaft, but occasionally are bolt-on pieces. This adds considerably to the weight of the crankshaft; crankshafts from [[Volkswagen]], [[Porsche]], and [[Corvair]] flat engines, lacking counterweights, are easily carried around by hand, compared to crankshafts for inline or V engines, which need to be handled and transported as heavy chunks of metal.

Many early aircraft engines (and a few in other applications) had the crankshaft fixed to the [[airframe]] and instead the cylinders rotated, known as a [[rotary engine]] design.

In the [[Wankel engine]], the rotors drive the eccentric shaft, which can be considered the equivalent of the crankshaft in a piston engine.

==Construction==
Crankshafts can be [[forge]]d or [[cast]] from either [[plain-carbon steel|mild steel]] or [[high strength steel]], or machined out of a single [[Billet (manufacturing)|billet]] of forged steel. Mild steel is only used for engines in models or other such applications, where the engine runs but does not supply power. Cast crankshafts are usually found in production engines, with forged and billet crankshafts being more expensive but reliable for higher performance. The rough casting or forging is machined to size and shape, the holes are drilled, the main and connecting rod bearing journals are precision ground and [[case hardening|case hardened]], and the appropriate holes are threaded.

ThyssenKrupp and Bharat Forge Ltd are largest manufacturers of Crankshafts. They employ forging for the making of Crankshafts, Axle Beams, Steering Knuckles and other Automobile Components.

==Stress analysis of crankshaft==
The crankshaft is subjected to various forces but it needs to be checked in two positions.
First, failure may occur at the position of maximum bending. In such a condition the failure is due to bending and the pressure in the cylinder is maximal. Second, the crank may fail due to twisting, so the crankpin needs to be checked for shear at the position of maximal twisting. The pressure at this position is not the maximal pressure, but a fraction of maximal pressure.

== See also ==
* [[Crankset|bicycle crankset]]
* [[Crank (mechanism)]]
* [[Brace (tool)]]
* [[Controlled Combustion Engine]]

==External links==
*[http://www.mustangandfords.com/howto/29178/ Nicely detailed discussion of crankshaft features, from ''Mustang & Fords'' magazine, with many photographs]
*[http://pdmec4.mecc.unipd.it/~cos/DINAMOTO/twin%20motors/twin.html Animated representations of the vibrations characteristic of various two cylinder engine and crankshaft configurations]
*[http://www.babcox.com/editorial/ar/eb10330.htm Balancing engines]

[[Category:engine technology]]
[[Category:Auto parts]]

[[af:Krukas]]
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[[he:גל ארכובה]]
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Crankshaft