Although some crankshafts seem to stand up to very high rev/min indefinitely, breakage is by no means unknown on a tuned engine. Examples of damage from prolonged over-revving being the destruction of timing wheels and chains. Torsional vibration can also be very destructive and cause fatigue to the crankshaft itself. With this in mind, undoubtedly the most valuable modification that can be carried out on engines that are perpetually driven at high rev/min is careful dynamic balancing of the crankshaft and flywheel, preferably as an assembly however this is not always possible.
The shaft is either cast or forged, usually in one piece, however on certain engines including many pre-war Bugattis, the crank is built up in sections. The shaft consists of the journals, which rotate in and are supported by the main bearings, and the crankpins which rotate in the big end bearings of the connecting rods. These in turn connect the crankpins to the pistons.
The crankpins and journals are joined together by the webs, which on many engines also act as counterweights creating a counterbalance mass to that of the crankpin. The excessive loads would otherwise overload the main bearings at high rev/min. This goes a long way in maintaining smoother engine running.
Crankshafts are of utmost importance when raising the rev limit of an engine, although in race applications, it is usually the last component to receive attention. With this in mind, cast iron crankshafts are normally substituted for a forged steel alternative as these tend to be stronger and less prone to wear and breakage. A more expensive option is the use of a billet crankshaft.
A fillet or radius is formed around the area where the webs and journals join in order to eliminate sharp corners, which would be a point of weakness that could lead to fatigue, or even breakage of the crankshaft. This is of particular importance in crankshafts that are subject to extremely heavy loads or significant stresses i.e race engines. The distance between the centre of the crankpin and that of the main bearing journal is referred to as either the "crank radius" or occasionally the "throw".
Pictured below is a diagram showing the basic construction of a crankshaft from an in-line four-cylinder engine.
Oil travels under pressure from a pump and/or by means of a "splash system" (which will be described later in a separate chapter on engine lubrication). Oil flows through drillings in the crankshaft running from the centres of the main bearing journals to those of the crankpins, in order to provide continual lubrication to the bearings and relevant surfaces. Some crankshafts feature hollow journals and crankpins to save weight, and these must obviously use seals in the form of caps or plugs bolted to their ends to prevent oil leakage.
Oil is retained within the crankcase by means of seals or/and retainers at either end of the crankshaft housing. Two commonly used methods are lip-type seals and scroll-type retainers. The former is used on most modern engines and has been in use for many years. It consists of a circular steel strip encased in a synthetic rubber which fits into a recess between the shaft and crankcase (rear) or timing case (front) and is held into place using a "garter" spring and sometimes retro-fitted to older engines in place of, or in addition to, the traditional scroll and flinger ring oil retainer. Scroll-type oil retainers use a thin metal ring, formed around the shaft, and therefore rotating with it. Any oil that reaches it is flung off by its rotational motion and drained back to the sump. Behind the flinger ring is the scroll, a helical groove machined into the crankshaft and housed inside the stationary rear housing, with a corresponding helix. This works very much like the groove of a thread or screw, bringing back into the crankcase any oil that should reach it as it turns in the stationary housing.
Pulleys and/or vibration dampers, along with gears or sprockets, are usually fitted to an extended section of the front end/nose of the crankshaft in order to drive the camshaft and components such as cooling fans, alternators/generators, power steering pumps and air conditioning compressors via belts, or in the case of the camshaft this can also be achieved by means of chains or gears. Pulleys and gears are generally located onto the end of the shaft by means of a tapered seat or a woodruff key and held in place by a large nut, or bolted onto the shaft, depending on the application.
A vibration damper typically consists of a metal disc with a ring of rubber bonded onto it. This helps to control torsional vibration, which is a slight twisting and untwisting of the crankshaft caused by the downward thrusts of the pistons during power strokes, which in turn causes sudden thrusts on the crankshaft.
Crankshafts with a longer throw/increased radius can be used to increase engine capacity. However, the connecting rod lengths must be reduced in order to make this work. This method is known as "stroking" and increases torque outputs considerably but can sometimes slightly reduce the safe rev limit of an engine. Many years ago, engines featuring very long strokes were the norm and excessive piston speeds were known to exert heavy loads onto both main and big-end bearings, with the final result being their imminent failure. However, the big-end and main bearings on today's high-revving modern engines tend to normally only fail due to lack of lubrication caused by poor maintenance, rather than the excessive loading previously mentioned.
The flywheel is a large and heavy carefully balanced disc, bolted onto the rear of the crankshaft to act as a reservoir for mechanical energy created by the power strokes of the engine. This is achieved by maintaining the crankshaft turning at a steady rate in between power strokes in order to assist smooth running of the engine. Provision is given so that the flywheel may only be fitted in one position and therefore will not run out of true with the crankshaft, which would set up a heavy vibration placing components under much undue stress. However, a very small amount of imbalance is tolerable.
The clutch shaft spigot bearing or bush is also included in the rear end of the crankshaft and in simple terms, its job is to locate the gearbox input shaft, carrying the clutch driven plate.
A more modern method of driving ancillary components is to do so from the rear end of the crankshaft, by means of a READ (rear end accessory drive) system, using gears from the crankshaft to drive components such as the alternator, from a common power take-off. One such manufacturer to employ this technology, among others, is Volvo and its primary focus is to save space by creating a more compact unit.
Popular materials used in the construction of crankshafts include steel and iron and they are manufactured by means or either forging or casting. Forging is a process where the material is formed and shaped by means of heating and then compressing or beating into shape, making for a stronger construction. Casting involves pouring heated liquid metal into moulds and has the added bonuses of stiffness and a more lightweight construction that requires considerably less machining than the forged counterparts and produces a stiffer end product. This is of great importance when we consider the short throws and large journals found on modern crankshafts. Upon completion, the shaft is then ground in places to a very smooth and accurate surface (sometimes an accuracy of less than one thousandth of an inch/0.025mm) with high-performance crankshafts being machined all over, further reducing oil drag.
Nitriding and induction hardening are methods used to prevent both torsional oscillation and shaft flexing due to constant load. Nitriding is where steel is mixed with nitrogenous metals and heated to around 500° for several hours in an ammonia filled atmosphere, which helps to prevent corrosion and increase the hardness of the shaft. This is achieved as the nitrogen from the ammonia is absorbed by the steel shaft, forming a hard iron nitride surface.
The electrical process of induction hardening involves the hardening of a steel surface by means of exposing the material to a magnetic field, causing it to heat, and then cooling it rapidly by quenching, usually done via a water spray.
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