Crankshaft, sometimes casually abbreviated to crank, is the part of an engine which
translates 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 vibrations often caused along the length
of the crankshaft by the cylinders farthest from the output end acting on the torsional elasticity of the metal.
Large engines are usually multicylinder to reduce pulsations from individual firing strokes, with more than one piston attached to a more complex
crankshaft; but many small engines, such as those found in mopeds or garden machinery, are single cylinder and use only a single piston, simplifying
crankshaft design. The crankshaft has a linear axis about which it rotates, typically with several bearing journals riding on replaceable bearings held
in the engine block, the main bearings. 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
rods from each cylinder attach.
The distance of the axis of the crank throws from the axis of the crankshaft determines the piston 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 or 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, V or 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
six cylinders of a V8 engine with what is basically a shortened version of the V8 crankshaft, produces an engine with an inherent pulsation in the
power flow due to the "missing" two cylinders, often reduced by use of balance shafts. 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 IRL engines compared to NASCAR engines, for
example. In engines other than the flat configuration, it is necessary to provide counterweights 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.
Crankshafts can be forged or cast from iron. They can be machined out of a single billet of forged steel. A disadvantage to billet crankshafts is that
the grain structure is uni-directional. The only real advantage to billet crankshafts is its capability to produce very low amounts of custom designed
crankshafts. Untreated mild steel is only used for engines in models or other such applications, where the engine runs but does not supply high
power. Cast crankshafts are usually found in low cost production engines, where as now more and more automotive manufacturers are using forged
crankshafts in need of its durability for today's high powered engines. 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 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