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 versa. The arm may be a bent portion of the shaft, or a separate arm or disk attached to it. Attached to the end of the crank by a pivot is a rod, usually called a connecting rod (conrod).
The term often refers to a human-powered crank which is used to manually turn an axle, as in a bicycle crankset or a brace and bit drill. In this case a person's arm or leg serves as the connecting rod, applying reciprocating force to the crank. There is usually a bar perpendicular to the other end of the arm, often with a freely rotatable handle or pedal attached.
Contents
Examples
Familiar examples include:
Foot-powered cranks
The crankset that drives a bicycle via the pedals.
Treadle sewing machine
Engines
Almost all reciprocating engines use cranks (with connecting rods) to transform the back-and-forth motion of the pistons into rotary motion. The cranks are incorporated into a crankshaft.
History
Asia
It was thought that evidence of the earliest true crank handle was found in a Han era glazed-earthenware tomb model of an agricultural winnowing fan dated no later than 200 AD, but since then a series of similar pottery models with crank operated winnowing fans were unearthed, with one of them dating back to the Western Han dynasty (202 BC – 9 AD). The Chinese used the crank-and-connecting rod in ancient blasting apparatus, textile machinery and agricultural machinery no later than the Western Han dynasty (202 BC – 9 AD). It was first used in the manually operated quern and long (grain decortication item) before evolving into other devices. According to F. Lisheng and T. Qingjun, the hand-crank of the rotary quern was different from a crank, which was the combination of a hand-crank and a push-and-pull connecting rod by a hinge. Eventually crank-and-connecting rods were used in the inter-conversion or rotary and reciprocating motion for other applications such as flour-sifting, treadle spinning wheels, water-powered furnace bellows, and silk-reeling machines.
Middle East
Ancient Egyptians had manual drills resembling a crank at the time of the Old Kingdom (2686–2181 BCE) and even a hieroglyph for the tool. However the Ancient Egyptian drill didn't operate as a true crank.
Later evidence for the crank, combined with a connecting rod in a machine, appears in the Ancient Greek Hierapolis sawmill in Roman Asia from the 3rd century AD and two stone sawmills at Gerasa, Roman Syria, and Ephesus, Greek Ionia under Rome, (both 6th century AD). 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. The crank and connecting rod mechanisms of the other two archaeologically attested sawmills worked without a gear train.
The crank appears in the mid-9th century in several of the hydraulic devices described by the Banū Mūsā brothers in their Book of Ingenious Devices. These devices, however, made only partial rotations and could not transmit much power, although only a small modification would have been required to convert it to a crankshaft.
Al-Jazari (1136–1206) described a crank and connecting rod system in a rotating machine in two of his water-raising machines. His twin-cylinder pump incorporated a crankshaft. A crank is later also described in an early 15th-century Arabic manuscript of Hero of Alexandria's Mechanics.
Europe
The first rotary hand mills, or rotary querns, appeared in Spain (600 BC – 500 BC), before they spread to the East. The handle near the outer edge of the rotary part makes the crank, a human arm powering the rotation would be the connecting rod. According to F. Lisheng and T. Qingjun, the hand-crank of the rotary quern was different from a crank, which was the combination of a hand-crank and a push-and-pull connecting rod by a hinge.
The Antikythera mechanism, dated to around 200 BC, used a crank as a part of its mechanism. The crank was used to manually setup the starting date for a prediction.Later evidence for the crank, combined with a connecting rod in a machine, appears in the Ancient Greek Hierapolis sawmill in Roman Asia from the 3rd century AD and two stone sawmills at Gerasa, Roman Syria, and Ephesus, Greek Ionia under Rome, (both 6th century AD). 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. The crank and connecting rod mechanisms of the other two archaeologically attested sawmills worked without a gear train.
A Roman iron crank of yet unknown purpose dating to the 2nd century AD was excavated in Augusta Raurica, Switzerland. The 82.5 cm (32 inches) long piece has fitted to one end a 15 cm (6 inches) long bronze handle, the other handle being lost.
An true iron crank about 40 cm (16 inches) long was excavated, along with a pair of shattered mill-stones of 50 to 65 cm (20 to 26 inches) diameter and diverse iron items, in Aschheim, close to Munich. The crank-operated Roman mill is dated to the late 2nd century AD. However an often cited modern reconstruction of a bucket-chain pump driven by hand-cranked flywheels from the Nemi ships has been dismissed as "archaeological fantasy".
In ancient literature, there is a reference to the workings of water-powered marble saws close to Trier, now Germany, by the late 4th century poet Ausonius; about the same time, these mill types seem also to be indicated by the Greek Christian saint Gregory of Nyssa from Anatolia, demonstrating a diversified use of water-power in many parts of the Roman Empire The three finds push back the date of the invention of the crank and connecting rod by a full millennium:
20th century
Cranks were formerly common on some machines in the early 20th century; for example almost all phonographs before the 1930s were powered by clockwork motors wound with cranks. Reciprocating piston engines use cranks to convert the linear piston motion into rotational motion. Internal combustion engines of early 20th century automobiles were usually started with hand cranks (known as starting handles in the UK), before electric starters came into general use. The last car model which incorporated a crank was the Citroën 2CV 1948-1990
The 1918 Reo owner's manual describes how to hand crank the automobile:
First: Make sure the gear shifting lever is in neutral position.
Second: The clutch pedal is unlatched and the clutch engaged. The brake pedal is pushed forward as far as possible setting brakes on the rear wheel.
Third: See that spark control lever, which is the short lever located on top of the steering wheel on the right side, is back as far as possible toward the driver and the long lever, on top of the steering column controlling the carburetor, is pushed forward about one inch from its retarded position.
Fourth: Turn ignition switch to point marked "B" or "M"
Fifth: Set the carburetor control on the steering column to the point marked "START." Be sure there is gasoline in the carburetor. Test for this by pressing down on the small pin projecting from the front of the bowl until the carburetor floods. If it fails to flood it shows that the fuel is not being delivered to the carburetor properly and the motor cannot be expected to start. See instructions on page 56 for filling the vacuum tank.
Sixth: When it is certain the carburetor has a supply of fuel, grasp the handle of starting crank, push in endwise to engage ratchet with crank shaft pin and turn over the motor by giving a quick upward pull. Never push down, because if for any reason the motor should kick back, it would endanger the operator.
Crank axle
A crank axle is a crankshaft which also serves the purpose of an axle. It is used on steam locomotives with inside cylinders.
