Crankshafts & Parts

The crankshaft, sometimes casually abbreviated to crank, is that part of an engine which translates reciprocating linear piston motion into rotation. more...

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 furthest from the output end acting on the torsional elasticity of the metal.

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 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.

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