A crankpin or crank pin, also known as a rod bearing journal, is a mechanical device in an

engine An engine or motor is a machine designed to convert one or more forms of energy into mechanical energy. Available energy sources include potential energy (e.g. energy of the Earth's gravitational field as exploited in hydroelectric power ...

which connects the crankshaft to the connecting rod for each cylinder. It has a cylindrical surface, to allow the crankpin to rotate relative to the "big end" of the connecting rod. The most common configuration is for a crankpin to serve one cylinder. However, many V engines have each crankpin shared by each pair of cylinders.

Design

The crankpin connects to the larger end of the connecting rod for each cylinder. This end of the connecting rod is called the "big end", as opposed to the "small end" or "little end" (which connects to the wrist/gudgeon pin in the piston). The bearing which allows the crankpin to rotate around its shaft is called the "rod bearing". In automotive engines, the most common type of rod bearing is the

plain bearing A plain bearing, or more commonly sliding contact bearing and slide bearing (in railroading sometimes called a solid bearing, journal bearing, or friction bearing), is the simplest type of bearing, comprising just a bearing surface and no roll ...

, however bushings or

roller bearing In mechanical engineering, a rolling-element bearing, also known as a rolling bearing, is a bearing which carries a load by placing rolling elements (such as balls or rollers) between two concentric, grooved rings called races. The relative m ...

s are also used in some engines.

Configurations

In a

single-cylinder engine A single-cylinder engine, sometimes called a thumper, is a piston engine with one cylinder. This engine is often used for motorcycles, motor scooters, go-karts, all-terrain vehicles, radio-controlled vehicles, portable tools and garden machinery ...

, straight engine or flat engine, each crankpin normally serves just one cylinder. This results for a relatively simple design which is the cheapest to produce. Some

V-twin engine A V-twin engine, also called a V2 engine, is a two-cylinder piston engine where the cylinders share a common crankshaft and are arranged in a V configuration. Although widely associated with motorcycles (installed either transversely or longi ...

s use a single cylinder per crankpin. Most

V engine A V engine, sometimes called a Vee engine, is a common configuration for internal combustion engines. It consists of two cylinder banks—usually with the same number of cylinders in each bank—connected to a common crankshaft. These cylinder ...

s have each pair of cylinders sharing a crankpin. This usually requires an offset between the cylinders in each bank, resulting in a simple connecting rod design. If a cylinder offset is not used, then the connecting rods must be articulated or forked at the big end. Forked connecting rods are mainly used in V-twin motorcycle engines, but in the past were found on a number of automobile and aero engines, such as the

Rolls-Royce Merlin The Rolls-Royce Merlin is a British liquid-cooled V-12 piston aero engine of 27-litres (1,650 cu in) capacity. Rolls-Royce designed the engine and first ran it in 1933 as a private venture. Initially known as the PV-12, it was late ...

aero engine of the WWII era. Articulated connecting rods consist of a "master" rod attached to the crank pin, with a "slave" rod connected to the big end of master rod. This design was used in older or exotic V engines.

Radial engine The radial engine is a reciprocating type internal combustion engine configuration in which the cylinders "radiate" outward from a central crankcase like the spokes of a wheel. It resembles a stylized star when viewed from the front, and is ...

s use a more complicated version of articulated connecting rods, where a single "master" connecting rod attached to the single crankpin (one for each row in multi-row designs), and smaller bearings for each of the corresponding cylinders machined into the big end of the master rod. Trier Bahnhof Triebachse Baureihe 44

Cylindrical crank pins were fitted onto the driving wheels of steam locomotives. They were connected to the driving rods that transmitted power from the

cylinder A cylinder (from ) has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base. A cylinder may also be defined as an infin ...

to the wheel. The crank pin was usually made of high-quality steel because it had to withstand high forces. The crank pin of a locomotive corresponds to the offset of a crankshaft in other

crank drive A crank is an arm attached at a right angle to a rotating shaft by which circular motion is imparted to or received from the shaft. When combined with a connecting rod, it can be used to convert circular motion into reciprocating motion, or vice ...

s. The distance from the centre of the crank pin to the centre of the wheel is also called offset and is exactly half the stroke of the pistons.

References

{{Automotive engine, state=expanded Crankshafts Engine technology

Design

Configurations

See also

References