A crankshaft—related to crank—is a mechanical part able to perform a conversion between reciprocating motion and rotational motion. In a reciprocating engine, it translates reciprocating motion of the piston into rotational motion; whereas in a reciprocating compressor, it converts the rotational motion into reciprocating 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.

It 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 earliest hand-operated cranks appeared in China during the Han Dynasty (202 BC-220 AD), as Han era glazed-earthenware tomb models portray, and was used thereafter in China for silk-reeling and hemp-spinning, for the agricultural winnowing fan, in the water-powered flour-sifter, for hydraulic-powered metallurgic bellows, and in the well windlass. In order to create a handle by means of a wheel to easily rotate their grain winnowers, the Chinese invented the crank handle and applied the centrifugal fan principle in the 2nd century BC. The crank handle was used in well-windlasses, querns, mills, and many silk making machines. The rotary winnowing fan greatly increased the efficiency of separating grain from husks and stalks. Harvesting grain by the use of rotary winnowing fan would not reach the Western world until the eighteenth century, where harvested grain was initially thrown up in the air by shovels or winnowing baskets. However, the potential of the crank of converting circular motion into reciprocal one never seems to have been fully realized in China, and the crank was typically absent from such machines until the turn of the 20th century.

The eccentrically mounted handle of the rotary handmill which appeared in 5th century BC Celtiberian Spain and ultimately spread across the Roman Empire constitutes a crank. A Roman iron crank of yet unknown purpose dating to the 2nd century AD was excavated in Augusta Raurica, Switzerland. The 82.5 cm long piece has fitted to one end a 15 cm long bronze handle, the other handle being lost.

A ca. 40 cm long true iron crank was excavated, along with a pair of shattered mill-stones of 50−65 cm diameter and diverse iron items, in Aschheim, close to Munich. The crank-operated Roman mill is dated to the late 2nd century. An often cited modern reconstruction of a bucket-chain pump driven by hand-cranked flywheels from the Nemi ships has been dismissed though as "archaeological fantasy".

Evidence for the crank combined with a connecting rod appears in the Hierapolis sawmill in Asia Minor from the 3rd century and two stone sawmills at Gerasa, Roman Syria, and Ephesus, Asia Minor (both 6th century). On the pediment of the Hierapolis mill, a waterwheel fed by a mill race is shown powering via a gear train two frame saws which cut rectangular blocks by the way of some kind of connecting rods and, through mechanical necessity, cranks. The accompanying inscription is in Greek.

This page was last edited on 13 June 2018, at 12:35 (UTC).
Reference: under CC BY-SA license.

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