For the comic strip about an old, curmudgeonly bus driver, see Crankshaft (comic strip).

The 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.

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Design

Tiny 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, whose long crankshafts suffered from an unacceptable amount of flex when engine designs began using a higher compression ratio and improved-breathing over head valves allowed higher RPMs. 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 low-RPM torque 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.

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 four 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 IndyCar Series engines compared to NASCAR Nextel Cup 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. Rotary engines such as the wankel engine are referred to as pistonless rotary engines.

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

Construction

Crankshafts can be monolithic (made in a single piece) or assembled from several pieces. Monolithic crankshafts are most common, but some smaller and larger engines use assembled crankshafts.

Crankshafts can be forged from a steel bar or cast in ductile iron. Today more and more manufacturers tend to favor the use of forged crankshafts due to their lighter weight, more compact dimensions and better inherent dampening. With forged crankshafts, vanadium microalloyed steels are mostly used as these steels can be air cooled after forging reaching high strengths without additional heat treatment, with exception to the surface hardening of the bearing surfaces. The low alloy content also makes the material cheaper than high alloy steels. Carbon steels are also used but these require additional heat treatment to reach the desired properties. Cast iron crankshafts are today mostly found in cheaper production engines where the loads are lower. Some engines also use cast iron crankshafts for low output versions while the more expensive high output version use forged steel.

Crankshafts can also be machined out of a billet, often using a bar of high quality vacuum remelted steel. Even though the fiber flow (local inhomogeneity of the materials chemical composition generated during casting) isn’t following the shape of the crankshaft, which is undesirable, this is usually not a problem since higher quality steels can be used (steels that normally are difficult to forge). These crankshafts tend to be very expensive due to the large amount of material removal by using lathes and milling machines, the high material cost and the additional heat treatment required. However, since no expensive tooling is required this production method allows small series of crankshafts to be made without high costs.

The fatigue strength of crankshafts is usually increased by using a radius at the ends of each main and crankpin bearing. The radius itself reduces the stress in these critical areas, but since the radiuses in most cases are rolled, this also leaves some compressive residual stress in the surface which prevents cracks to form.

Most production crankshafts use induction hardened bearing surfaces since that method gives good result with low costs. It also allows the crankshaft to be reground without having to redo the hardening. But high performance crankshafts, billet crankshafts in particular, tend to use nitriding instead. Nitriding is slower and thereby more costly, and in addition it put certain demands on the alloying metals in the steel, this in order to be able to create stable nitrides. The advantage with nitriding is that it can be done at low temperatures, it produces a very hard surface and the process will leave some compressive residual stress in the surface which is good for the fatigue properties of the crankshaft. The low temperature during treatment is advantageous in that it doesn’t have a negative effect on the steel. With crankshafts that operate on roller bearings, the use of carburizing tends to be favored due to the high hertzian contact stresses in such an application. Like nitriding, carburizing also leaves some compressive residual stresses in the surface.

Some expensive high performance crankshafts also use heavy metal counterweights to make the crankshaft more compact. The heavy metal used is most often a tungsten alloy but depleted uranium has also been used. A cheaper option is to use lead, but compared with tungsten its density is much lower.

Germany's ThyssenKrupp, Turkey's Yapı-Tek Çelik Sanayi and India's Bharat Forge Ltd are the largest manufacturers of crankshafts. They use forgings to make crankshafts, axle beams, steering knuckles and other automobile components.

Stress analysis of crankshaft

The shaft 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.

References

See also

• Crankcase, the housing that surrounds the crankshaft
• Bicycle crankset
• Crank (mechanism)
• Brace (tool)
• Controlled Combustion Engine
• Piston motion equations
• Hudson Motor Car Company, balanced crankshaft in 1916 allowed higher RPM & more power

Heat engine types/configurations using thermodynamic cycles (Power cycles)
Stroke number and stroke parting
Crower six stroke Four-stroke cycle Scuderi Split Cycle Engine One-stroke cycle Six stroke engine Two-stroke cycle
Different work volume types (incl. Pistonless rotary engine)
Britalus Rotary Engine Combustion chamber Controlled Combustion Engine Jet engine Orbital engine Piston engine Quasiturbine Rocket engine Swing-piston engine Toroidal engine Trochilic engine Tschudi engine Twingle engine Wankel engine
Different work volume ports and main forms of
Cylinder head porting D slide valve Four-stroke cycle engine valves Manifold Multi-valve Piston valve Poppet valve Rocket engine nozzles Sleeve valve
Different pistons layouts
Bourke engine Delta engine Double acting/differential cylinder Opposed piston engine Radial engine Rotary engine Single cylinder engine Stelzer engine Straight engine
Different main rotational motion mechanisms or arc to (or even almost) pistons back-and-forth.
Sector straight-line linkage [1] Connecting rod Coomber Rotary Engine [2] Crank Substitute Engine [3] Crankshaft Evans linkage [4] Cam Parallel motion Peaucellier-Lipkin linkage Piston rod QRMC Stirling/HydraLink [5] Revolving Cylinder Engine [6] Rhombic drive Scotch yoke Swashplate Swashplate engine Watt's linkage
One-way stop-and-go like rotational motion mechanisms to rotation.
Toroidal engine Trochilic engine

External links

• Nicely detailed discussion of crankshaft features, from Mustang & Fords magazine, with many photographs
• Animated representations of the vibrations characteristic of various two cylinder engine and crankshaft configurations
• Balancing engines
• An interesting tool used to electrically emulate crankshaft position sensors
• Crankshaft highlight: Construction and operation of four cylinder internal combustion engine courtesy of Ford Motor Company