Crank (mechanism)

• 1. Introduction

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  The crank is one of the most quietly consequential mechanisms in human history. An arm fixed at a right angle to a rotating shaft, it does something deceptively simple: it converts circular motion into back-and-forth motion, or the reverse. Every bicycle pedal, every fishing reel, every old car starter uses this principle. But the crank's story stretches back thousands of years, from Han dynasty winnowing fans to the stone sawmills of Roman Syria, from medieval grindstones to the explicit six-step instructions printed in a 1918 Reo automobile owner's manual for how not to break your wrist starting an engine by hand. What forces shaped its slow diffusion across civilizations? Why did it take so long to grasp something that now seems obvious? And what does the crank reveal about how mechanical ideas actually spread?

• 2. The Mechanics of Conversion

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  A connecting rod is the crank's essential partner. Without it, a crank can only impart or receive rotation. With it, the pair can translate the push-and-pull of a piston into the spin of a wheel, or reverse that transaction entirely. The connecting rod, often called a conrod, attaches to the crank's outer end by a pivot, creating a linkage that makes engines possible.

  Scholars F. Lisheng and T. Qingjun drew a careful distinction between the hand-crank of an ancient rotary quern and a true crank. A true crank, in their formulation, requires the combination of a hand-crank with a push-and-pull connecting rod joined by a hinge. That might sound like splitting hairs, but the distinction mattered enormously for engineering history. A rotating handle on a millstone is a useful tool. A crank-and-conrod system is an engine waiting to happen.

  Almost all reciprocating engines, from the earliest steam machines to modern internal combustion designs, incorporate this arrangement. The pistons move back and forth; the cranks, gathered into a crankshaft, translate that motion into rotation. Every revolution of a car's engine is a testament to a mechanism whose core geometry was worked out long before anyone thought to burn fuel inside a cylinder.

• 3. Ancient Roots

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  One of the oldest confirmed appearances of the crank dates to the Western Han dynasty, which ran from 202 BC to 9 AD. Pottery models of crank-operated agricultural winnowing fans were unearthed from this period, and Chinese craftsmen used the crank-and-connecting rod in blasting apparatus, textile machinery, and grain processing equipment across the same era. The device began in the manually operated quern and a grain decortication item called a long, then spread to flour-sifting, treadle spinning wheels, water-powered furnace bellows, and silk-reeling machines.

  On the other side of the ancient world, the Antikythera mechanism, dated to around 200 BC, incorporated a crank to manually set its astronomical starting date. Rotary querns first appeared in Spain between 600 BC and 500 BC before spreading eastward. Ancient Egyptian drills from the Old Kingdom period, spanning 2686 to 2181 BCE, resembled a crank in form, and the Egyptians even had a hieroglyph for the tool, though these drills did not operate as true cranks in the mechanical sense.

  Roman evidence is more concrete. A Roman iron crank of unknown purpose, dated to the 2nd century AD, was excavated at Augusta Raurica in Switzerland. It measured 82.5 cm in length with a 15 cm bronze handle at one end, the other handle lost to time. Near Munich, in Aschheim, a 40 cm iron crank was found alongside shattered millstones, and the crank-operated mill it belonged to was dated to the late 2nd century AD as well.

• 4. Roman Sawmills and What They Imply

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  The Hierapolis sawmill in Roman Asia, dated to the 3rd century AD, is one of the most discussed machines from antiquity. A carved relief on its pediment shows a waterwheel fed by a mill race driving, through a gear train, two frame saws that cut rectangular stone blocks. The mechanism required connecting rods and, by mechanical necessity, cranks. The inscription on the relief is written in Greek.

  Two further sawmills from the 6th century AD, one at Gerasa in Roman Syria and one at Ephesus in Greek Ionia, also used crank-and-connecting-rod arrangements, though without gear trains. These three finds, taken together, push back the confirmed date of the crank-and-connecting-rod by what scholars described as a full millennium relative to earlier estimates.

  The implications are significant. Late 4th century poet Ausonius mentioned water-powered marble saws near Trier, in what is now Germany. Around the same period, the Greek Christian saint Gregory of Nyssa described similar installations in Anatolia. Together these references point to widespread use of water-powered machinery across many parts of the Roman Empire. As one analysis noted, by Roman times all the elements needed to construct a steam engine, which was not invented until 1712, were already present: Hero's aeolipile for steam power, cylinder and piston mechanisms in metal force pumps, non-return valves in water pumps, and gearing in mills and clocks.

• 5. Medieval Encounters

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  Around 830 AD, a pen drawing in the Carolingian manuscript known as the Utrecht Psalter showed a rotary grindstone operated by a crank handle. Scholars believe this drawing traced back to a late antique original, making it the earliest known representation of a rotary grindstone. A musical tract attributed to the abbot Odo of Cluny, who lived from around 878 to 942, described a stringed instrument sounded by a resined wheel turned with a crank.

  From the Middle East, the Banu Musa brothers described crank-like devices in their Book of Ingenious Devices in the mid-9th century, though those devices only made partial rotations and transmitted limited power. Al-Jazari, who lived from 1136 to 1206, went further: he documented a crank-and-connecting-rod system in two of his water-raising machines, and his twin-cylinder pump incorporated a full crankshaft.

  The Benedictine monk Theophilus Presbyter, active around 1070 to 1125, described crank handles used in the turning of casting cores. The Italian physician Guido da Vigevano, who lived from around 1280 to 1349, drew illustrations of paddle boats and war carriages driven by manually turned compound cranks and gear wheels. The Luttrell Psalter, dating to around 1340, described a grindstone rotated by two cranks, one at each end of its axle. By the 15th century, geared hand-mills operated with one or two cranks had also appeared.

• 6. Renaissance Spread

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  By the early 15th century, the crank had become common across Europe, and the German military engineer Konrad Kyeser was among those documenting its uses most thoroughly. His illustrated work Bellifortis depicted cranked windlasses for spanning siege crossbows, cranked chains of buckets for lifting water, and cranks fitted to wheels of bells. Kyeser also applied crank handles to Archimedes screws, replacing the ancient practice of working the pipe by treading underfoot.

  The compound crank, a more complex variant, made its earliest documented appearance in depictions of the carpenter's brace between 1420 and 1430, drawn in various northern European artworks. A German writer known only as the Anonymous of the Hussite Wars traced its rapid development in three stages: first the connecting rod reappeared applied to cranks; then double compound cranks were equipped with connecting rods; then the flywheel was added to carry the mechanism through the dead spot in its rotation where it would otherwise stall.

  Roberto Valturio improved on paddle-wheel concepts in 1463 with a boat using five sets of cranks, all joined to a single power source by one connecting rod. In Italian sketchbooks, Taccola provided the earliest evidence of a compound crank with a connecting rod, though he did not fully understand the mechanics. Pisanello demonstrated a sound grasp of crank motion somewhat later, painting a piston pump driven by a water wheel and operated by two simple cranks and two connecting rods.

  Agostino Ramelli's 1588 work The Diverse and Artifactitious Machines depicted eighteen examples of cranks and connecting rods integrated into machine design. Georg Andreas Bockler's Theatrum Machinarum Novum raised that count to 45 different machines using the mechanism, representing one third of the total machines in the book.

• 7. The Age of Hand-Starting

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  Cranks were standard on phonographs before the 1930s, where they wound clockwork motors to power playback. On early 20th century automobiles, hand cranks served as the primary method for starting internal combustion engines before electric starters arrived. In the United Kingdom these were called starting handles.

  The 1918 Reo owner's manual preserved a six-step procedure for hand-cranking that reveals just how demanding and hazardous the task was. Step six carried a specific warning: grasp the starting crank, push it in to engage the ratchet with the crankshaft pin, and turn the motor by giving a quick upward pull. Never push down, the manual stated, because if the motor kicked back, it would endanger the operator. Backfire from an improperly started engine could and did break wrists.

  The Citroen 2CV, produced from 1948 to 1990, was the last car model to incorporate a hand crank as standard equipment. The crank axle, a crankshaft that also functions as an axle, survived in a different context: it remains in use on steam locomotives with inside cylinders, where the geometry of the mechanism serves both rotational and structural roles at once.