Crankshaft

This is about the mechanical part. For the comic strip about an old, curmudgeonly bus driver, see Crankshaft (comic strip). For the ice hockey player nicknamed "Crankshaft", see Douglas Murray (ice hockey).

File:Cshaft.gif

Crankshaft (red), pistons (gray) in their cylinders (blue), and flywheel (black)

The crankshaft, sometimes casually abbreviated to crank, is the part of an engine which translates reciprocating linear piston motion into rotation. To convert the reciprocating motion into rotation, the crankshaft has "crank throws" or "crankpins", 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.

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.

History

The earliest use of the crank in a machine occurs in the crank-driven winnowing machine from China during the Han Dynasty (202 BC - 220 AD). A crank mechanism was first used in a water mill of late antiquity. Evidence appears on a late 3rd century relief of a saw mill from Hierapolis, Asia Minor. Two 6th century saw mills excavated at Ephesus, Asia Minor, and Gerasa, Jordan, worked with a similar mechanism. In China, a crank and connecting rod machine appeared in the 5th century, followed by a crank and connecting rod machine with a piston rod in the 6th century. In the 9th century, the non-manual crank appears in several of the hydraulic machines described by the Banu Musa brothers in their Book of Ingenious Devices. These automatically-operated cranks appear in several devices described in the book, two of which contain an action which approximates to that of a crankshaft. The Banu Musa brothers' automatic crank would not have allowed a full rotation, but only a small modification was required to convert it to a crankshaft.

A device shown in the early 9th century Carolingian manuscript Utrecht Psalter is a crank handle used with a rotary grindstone. Scholars point to the use of crank handles in trepanation drills in a 10th century work by the Spanish Muslim surgeon Abu al-Qasim al-Zahrawi (936–1013).

Crankshaft

In 1206, the Arabic inventor, Al-Jazari, described the earliest known crankshaft, which he incorporated with a crank and connecting rod mechanism in his twin-cylinder pump. Al-Jazari's mechanism consisted of a wheel setting several crank pins into motion. His water pump also employed the first known crank-slider mechanism.

Crankshafts were later described by Konrad Kyeser (d. 1405), Francesco di Giorgio (1439–1502), Leonardo da Vinci (1452–1519), and Taqi al-Din who incorporated it in a six-cylinder pump in 1551. A Dutch "farmer" Cornelis Corneliszoon van Uitgeest also described a crankshaft in 1592. His wind-powered sawmill used a crankshaft to convert a windmill's circular motion into a back-and-forward motion powering the saw. Corneliszoon was granted a patent for the crankshaft in 1597.

Design

File:Four stroke engine diagram.jpg

Components of a typical, four stroke cycle, DOHC piston engine. (E) Exhaust camshaft, (I) Intake camshaft, (S) Spark plug, (V) Valves, (P) Piston, (R) Connecting rod, (C) Crankshaft, (W) Water jacket for coolant flow.

Large engines are usually multicylinder to reduce pulsations from individual firing strokes, with more than one piston attached to a complex crankshaft. Many small engines, such as those found in mopeds or garden machinery, are single cylinder and use only a single piston, simplifying crankshaft design. This engine can also be built with no riveted seam.

Bearings

The crankshaft has a linear axis about which it rotates, typically with several bearing journals riding on replaceable bearings (the main bearings) held in the engine block. 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 a factor in the rise of V8 engines, with their shorter crankshafts, in preference to straight-8 engines. The long crankshafts of the latter suffered from an unacceptable amount of flex when engine designers began using higher compression ratios and higher rotational speeds. High performance engines often have more main bearings than their lower performance cousins for this reason.

Piston stroke

The distance 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-speed torque of an engine is to increase the stroke. This also increases the reciprocating vibration, however, limiting the high speed 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 "under square" or long-stroke, in that the stroke is longer than the diameter of the cylinder bore.

Engine configuration

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, in older days sometimes 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. 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 120 degrees apart, as in the GM 3800 engine. While production V8 engines use four crank throws spaced 90 degrees apart, high-performance V8 engines often use a "flat" crankshaft with throws spaced 180 degrees apart. The difference can be heard as the flat-plane crankshafts result in the engine having a smoother, higher-pitched sound than cross-plane (for example, IRL IndyCar Series compared to NASCAR Nextel Cup, or a Ferrari 355 compared to a Chevrolet Corvette). See the main article on crossplane crankshafts.

Engine balance

For some engines it is necessary to provide counterweights for the reciprocating mass of each piston and connecting rod to improve engine balance. These are typically cast as part of the crankshaft but, occasionally, are bolt-on pieces. While counter weights add a considerable amount of weight to the crankshaft it provides a smoother running engine and allows higher RPMs to be reached.

Rotary engines

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 could be considered the equivalent of the crankshaft in a piston engine.

Construction

File:Marine Crankshafts 8b03602r.jpg

Continental engine marine crankshafts, 1942

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.

Forging and casting

Crankshafts can be forged from a steel bar usually through roll forging or cast in ductile steel. 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 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. Iron crankshafts are today mostly found in cheaper production engines (such as those found in the Ford Focus diesel 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.

Machining

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 inhomogeneities of the material's chemical composition generated during casting) doesn’t follow the shape of the crankshaft (which is undesirable), this is usually not a problem since higher quality steels which normally are difficult to forge can be used. These crankshafts tend to be very expensive due to the large amount of material removal which needs to be done 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 production runs of crankshafts to be made without high costs.

Fatigue strength

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 radii in most cases are rolled, this also leaves some compressive residual stress in the surface which prevents cracks from forming.

Hardening

Most production crankshafts use induction hardened bearing surfaces since that method gives good results 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 nitridization instead. Nitridization is slower and thereby more costly, and in addition it puts certain demands on the alloying metals in the steel, in order to be able to create stable nitrides. The advantage with nitridization 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 any negative effects on the steel, such as annealing. With crankshafts that operate on roller bearings, the use of carburization tends to be favored due to the high Hertzian contact stresses in such an application. Like nitriding, carburization also leaves some compressive residual stresses in the surface.

Counterweights

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.

Stress on crankshafts

The shaft is subjected to various forces but generally needs to be analysed in two positions. Firstly, failure may occur at the position of maximum bending; this may be at the centre of the crank or at either end. 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 conrod needs to be checked for shear at the position of maximal twisting. The pressure at this position is the maximal pressure, but only a fraction of maximal pressure. ==

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
• Camshaft
• Cam

External links

Template:External links

• Animated representations of the vibrations characteristic of various two cylinder engine and crankshaft configurations
• Balancing engines
• Crankshaft highlight: Construction and operation of four cylinder internal combustion engine courtesy of Ford Motor Company
• The FOUR-STROKE CYCLE / OTTO CYCLE
• Kinematic Models for Design Digital Library (KMODDL) - Movies and photos of hundreds of working mechanical-systems models at Cornell University. Also includes an e-book library of classic texts on mechanical design and engineering.