An undercut crankshaft is a form of

crankshaft A crankshaft is a mechanical component used in a piston engine to convert the reciprocating motion into rotational motion. The crankshaft is a rotating shaft containing one or more crankpins, that are driven by the pistons via the connecting ...

for

piston engine A reciprocating engine, also often known as a piston engine, is typically a heat engine that uses one or more reciprocating pistons to convert high temperature and high pressure into a rotating motion. This article describes the common featu ...

s, where the overall length of the crankshaft is shortened by overlapping the

main bearing Main may refer to: Geography * Main River (disambiguation) **Most commonly the Main (river) in Germany * Main, Iran, a village in Fars Province *"Spanish Main", the Caribbean coasts of mainland Spanish territories in the 16th and 17th centuries ...

s of the crankshaft with the big end bearings of the

connecting rod A connecting rod, also called a 'con rod', is the part of a piston engine which connects the piston to the crankshaft. Together with the crank, the connecting rod converts the reciprocating motion of the piston into the rotation of the cranksh ...

Although this seems impossible at first glance, the geometry of the engine does permit this overlap. The big-end bearings of connecting rods are wider (along the crank axis) than the thickness of the rod itself. By making the crank webs with an offset to them, it is possible to place the main bearings closer together, closely sandwiching the rod passing between them, where this is now a narrower gap than the bearing size. The main bearings are similarly confined and each main bearing can be supported by only a thin wall across the

crankcase In a piston engine, the crankcase is the housing that surrounds the crankshaft. In most modern engines, the crankcase is integrated into the engine block. Two-stroke engines typically use a crankcase-compression design, resulting in the fuel/a ...

. The difficulty with these engines is in their manufacture. The overall shape of the crankshaft and its webs is not a problem, but finishing the bearing surfaces by

grinding Grind is the cross-sectional shape of a blade. Grind, grinds, or grinding may also refer to: Grinding action * Grinding (abrasive cutting), a method of crafting * Grinding (dance), suggestive club dancing * Grinding (video gaming), repetitive and ...

is. There is no longer access for the large abrasive discs of a crankshaft grinder to reach each bearing, the offset crankshaft webs now being in the way. As a result, each bearing must be made and finish-ground individually; then assembled into a built-up crankshaft, usually by

shrink-fitting Shrink-fitting is a technique in which an interference fit is achieved by a relative size change after assembly. This is usually achieved by heating or cooling one component before assembly and allowing it to return to the ambient temperature after ...

. One of the few engines to use undercut crankshafts was the

Bugatti U-16 The Bugatti U-16 was a 16-cylinder water-cooled double-8 vertical in-line "U engine", designed by Ettore Bugatti in 1915-1916 and built in France in small numbers. The US Bolling Commission bought a license to build the engine in the US, and sm ...

aircraft engine, a typical

Bugatti Automobiles Ettore Bugatti was a German then French manufacturer of high-performance automobiles. The company was founded in 1909 in the then-German city of Molsheim, Alsace, by the Italian-born industrial designer Ettore Bugatti. The cars w ...

approach, where only their unlimited budget and attention to detail could afford such complexities. The U-16 engine also had each 8-cylinder crankshaft built as two 4-cylinder crankshafts, assembled by a bolted taper joint. This joint may be seen at the left-hand side of the close-up drawing. The French Chaise 100 hp engine of 1930 used a similar built up crankshaft with a '

quincunx A quincunx () is a geometric pattern consisting of five points arranged in a cross, with four of them forming a square or rectangle and a fifth at its center. The same pattern has other names, including "in saltire" or "in cross" in heraldry (dep ...

' (staggered) cylinder layout to give a short, compact engine.

References

Crankshafts {{mech-engineering-stub

See also

References