Crankshaft
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Crankshaft.png
Marine_Crankshafts_8b03602r.jpg
Crankshaft is also the name of a comic strip about an old, curmudgeonly bus driver.
The crankshaft, sometimes casually abbreviated to crank, is that 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 torsional 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.
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Design
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 replacable 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, the rotary piston 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 forged or cast from either mild steel or high strength steel, or machined out of a single 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 hardened, and the appropriate holes are threaded.
Stress Analysis of Crankshaft
The crankshaft is subjected to various forces but it needs to be checked in two positions. Firstly, 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. Secondly, 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
External links
- Nicely detailed discussion of crankshaft features, from Mustang & Fords magazine, with many photographs (http://www.mustangandfords.com/howto/29178/)
- Animated representations of the vibrations characteristic of various two cylinder engine and crankshaft configurations (http://pdmec4.mecc.unipd.it/~cos/DINAMOTO/twin%20motors/twin.html)
- Balancing engines (http://www.babcox.com/editorial/ar/eb10330.htm)
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