In simple terms, a crankshaft is a shaft driven by a crank mechanism, consisting of a series of cranks and crankpins to which the connecting rods of an engine are attached. It is a mechanical part able to perform a conversion between reciprocating motion and rotational motion.
In a reciprocating engine, it translates reciprocationg motion of the piston into rotational motion. In order to do the conversion between two motions, 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.
A crankshaft is typically connected 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 farthest from the output end acting on the torsional elasticity of the metal.
The crankshaft is supported by the engine block, with the engine's main bearings allowing the crankshaft to rotate within the block. The up-down motion of each piston is transferred to the crankshaft via connecting rods. A flywheel is often attached to the front end of the crankshaft, in order to smooth out the power delivery and reduce torsional vibration.
Cross-Plane vs Flat-Plane Configuration
Most production V8 engines use crank throws spaced 90° apart, which is called a "cross-plane" configuration (such as the Ford modular engine and the GM LS engine). Flat-plane crankshafts were used on several early V8 engines. Now, they are used on several high-performance V8 engines (such as the Ferrari 355) with throws spaced 180° apart, essentially resulting in two inline-four engines running in a common crankcase. These engines are usually able to rev higher, however they have more vibration
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 are subjected to enormous stresses, in some cases more than 19,000 pounds per cylinder. In determining strength, there are five manufacturing options, with the strongest being forged steel:
1. Cast Iron
2. Nodular Cast Iron
3. Cast Steel
4. Forged Steel
5. Billet Steel
The difference between cast iron and nodular cast iron is the shape of the graphite particles in the iron. Cast iron is not just iron, pure iron is too soft to be used as a crankshaft. In regular cast iron, the graphite particles are flakes. In nodular cast iron, they are spherical nodules. This gives the cast iron more strength and flexibility.
Cast steel is stronger due to the fact that steel is stronger than iron. To make a cast crankshaft, they heat up iron or steel until it is melted then pour it into a mold, the same as with cast pistons. Like cast pistons, there is less finish machining to do so they cost less.
With forged steel cranks, a metal bar is heated until it is soft then pounded into the rough shape of the crankshaft. From there it is machined into the final shape. There is more machine work involved, along with the cost of the forging equipment it makes for a higher cost of the final piece. There are different alloys of steel used, that give a crank different levels of strength.
There is one more option for manufacturing a crankshaft, and that is a billet steel crankshaft. To make them, a much larger bar of steel is forged into a cylinder shape as large as the total diameter of the finished crank, then it is machined to the final shape. As you can imagine that is a lot more machining and the final cost reflects this.
A cast iron or nodular cast iron crank is probably enough for most mild street motors. Higher horsepower and higher sustained rpm cause the stress on a crank to increase. Most aftermarket crankshaft manufacturers will have an approximate horsepower level for their cranks.
One thing to think about is that depending on the alloy used, a cast steel crank can be almost as strong as a forged crank.
As far as forged versus billet, it depends on who you believe on which one is stronger, but given the price of a billet crank, there should be some advantage to it. There is a theory that while forging aligns the crystals of the steel, all the bending that is needed to make the final shape breaks some of those. Versus the billet, that while it doesn’t use as much pressure as forging and doesn’t produce as tight of a grain, it isn’t disrupted by bending. Instead it is machined to the final shape.
The Challenger’s high-output 6.1 SRT and 6.4 SRT/Scat Pack engines have a forged crankshaft. The 5.7 Hemi and the 3.6 Pentastar engines have a nodular cast iron crankshaft.
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 damping. With forged crankshafts, vanadium micro-alloyed 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.
Hemi Forged Crankshaft
Cast 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.
For racing, crankshafts can also be machined out of a billet- often a bar of high quality vacuum re-melted steel. These crankshafts tend to be very expensive due to the large amount of material that must be removed with lathes and milling machines, the high material cost, and the additional heat treatment required. However, since no expensive tooling is needed, this production method allows small production runs without high costs.
In an effort to reduce costs, used crankshafts may also be machined. A good core may often be easily reconditioned by a crankshaft grinding process. Severely damaged crankshafts may also be repaired with a welding operation, prior to grinding, that utilizes a submerged arc welding machine. To accommodate the smaller journal diameters a ground crankshaft has, and possibly an oversized thrust dimension, undersized engine bearings are used to allow for precise clearances during operation.
Machining or re-manufacturing crankshafts are precision machined to exact tolerances with no odd size crankshaft bearings or journals. Thrust surfaces are micro-polished to provide precise surface finishes for smooth engine operation and reduced thrust bearing wear. Shot-peening can add an additional layer of hardness to the re-manufactured crankshaft. Every journal is inspected and measured with critical accuracy. After machining, oil holes are chamfered to improve lubrication and every journal polished to a smooth finish for long bearing life. Re-manufactured crankshafts are thoroughly cleaned with special emphasis to flushing and brushing out oil passages to remove any contaminants.
Here is a further in-depth technical discussion of crankshafts:
The crankshaft is able to rotate in the engine block due to the main bearings. Since the crankshaft is subject to large sideways forces from each cylinder, bearings are located at various points along the crankshaft, not just one at each end. This was a factor in V8 engines replacing straight-8 engines in the 1950s. The long crankshafts of the latter suffered from an unacceptable amount of flex when engine designers began using higher compression ratios and higher engine speeds (rpm). High performance engines often have more main bearings than lower performance engines.
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.
Crankshaft with flying arms (the boomerang-shaped link between the crank pins)
In some engine configurations, the crankshaft contains direct links between adjacent crank pins, without the usual intermediate main bearing. These links are called flying arms. This arrangement is sometimes used in V6 and V* engines, as it enables the engine to be designed with different V angles than what would otherwise be required to create an even firing interval, while still using fewer main bearings than would normally be required with a single piston per crank throw. This arrangement reduces weight and engine length at the expense of reduced crankshaft rigidity.
The distance the axis of the crank throws from the axis of the crankshaft determines the stroke length of the engine. Most modern car engines are classified as "over square" or short-stroke, wherein the stroke is less than the diameter of the cylinder bore. A common way to increase the low-RPM torque of an engine is to increase the stroke, sometimes known as "stroking" the engine. Traditionally, the trade-off for a long-stroke engine was reduced power and increased vibration at high rpm.