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Wheel

Wheel, a mechanical device providing rotary motion by means of a disk or circular frame revolving on an axis. The rotary motion provided by the wheel is so basic to machinery that it is doubtful that mechanized civilization could have developed without it. Since the invention of the wheel in protohistoric times, its exploitation has gradually brought innumerable derivatives. It led first to a revolution in land transport; subsequently, by a series of mutations, machines were devised to lessen labor, increase its efficiency, and substitute power sources for the limited muscular capacities of humans and other animals. Enumeration of even a few of the derived forms suggests the magnitude of developments from the wheel; revolving shafts, pulleys, gears, and flywheels are involved in complex devices such as turbines, internal combustion engines, and electric motors. Some permit application of power directly at a working point, as in the winch and the circular saw; others transform natural sources of power into forms that can be transmitted, as in the windmill and the dynamo.

A wheel operates as an infinite series of levers. In a cartwheel, for example, each spoke is a lever, with the rim on the ground as its fulcrum. A connecting rod at an intermediate point on the radius of the wheel, as in a locomotive drive wheel, delivers power to the rim, the axle serving as the fulcrum. Reversing this, with a fixed axle, power applied to the rim provides reciprocating motion to a connecting rod. A set of gears of different sizes modifies power or speed in proportion to the lengths of their radii. Not only does a wheel, in effect, lever its load forward, but in contrast to a dragged object, it also minimizes friction. A person or horse is thus able to pull a load many times greater than what they could carry on their back.

Early History. Knowledge of the wheel's origin can be projected from its earliest known form, the two-wheeled cart of the Bronze Age in Sumer (3500 B.C.). Like all inventions it involved a combination of previously known devices. For 2,000 years rollers had been used to move heavy weights; loads had been transported on dryland sledges or shafts dragged by animals; and plows were drawn by pairs of oxen. With rollers under such sledges, held in place by guides, traction was greatly improved. The sequence may have been, first, to trim the excess at the middle of the roller to overcome difficulties with the guides, leaving the ends as rudimentary wheels, and then to mount free-turning wheels on fixed axles. The difficulty of cutting cross sections of sufficiently large logs with inadequate tools was surmounted by forming solid disks of three sections of plank crossed by two battens. The earliest vehicles had a single pole, indicating that pairs of plow oxen were put to the new task. The point of origin is uncertain; the archaeological record points to the Near East, although it may have been Inner Asia.

Wheeled vehicles were known in Sumer in 3500 B.C.; Assyria, 3000 B.C.; Indus Valley, 2500 B.C.; central and northern Europe, soon after 1000 B.C.; and Britain, about 500 B.C. This sequence indicates a single origin of the wheel and its subsequent slow adoption over the Old World. The only other independent invention approximating the wheel is from ancient Mexico, where toy clay animals resting on clay tubes acting as rollers have been found, but there is no indication that practical application followed.

Evolution and Applications. The earliest wheeled vehicles had limited use until inherent problems were solved. The four-wheeled wagon, almost as old as the cart, could be steered only by lifting it bodily until a swiveling front axle was devised. Draft by pairs of oxen or asses was slow. With the introduction of the swifter horse from the Asiatic steppes into Mesopotamia just after 2000 B.C., the cart was transformed into a lighter military chariot. The spoked wheel then widely adopted was first given a binding of leather, later of copper. To withstand wear, wheels were provided with studs, or the spokes were extended through the rims.

The first adaptation of the wheel for a machine was the waterwheel. A horizontal wheel set in a flowing stream, with its vertical shaft turning a millstone above, spread from the Near East in the 1st century B.C. Not long after, Roman engineers devised the more efficient vertical wheel to raise water for irrigation and, with simple gears, to operate flour mills. Extended use followed the development of the even more effective overshot wheel in the 3d century A.D. Greatly expanded employment came in the early Middle Ages, when the waterwheel operated mechanical hammers, ore stamp mills, and bellows by means of cams. Windmills harnessed another natural source of energy (see Windmill). The Eastern type, turning on a vertical axle, originated in Persia in the 5th to 7th centuries A.D.; probably independently, in western Europe, windmills with horizontal shafts first appeared in the 12th century. Pulleys were in use before 1500 B.C.; in the 1st century A.D. they were used in cranes where the windlass was powered by men inside a huge wheel built as a treadmill.

The adaptation of wheels as gears was a conceptual leap. Engaging wheel rims to transmit or modify motion is not obvious, and the invention may have owed in part to accidental juxtaposition of studded wheels. The first gear set was a pair of toothed wheels, axles at right angles, in vertical waterwheels. Cyclometers to be attached to vehicles, involving multiple gears, were invented around the 1st century B.C. and ultimately became the source of clock mechanisms. Far more sophisticated reduction gears and belted wheels did not make their appearance until the Middle Ages.

Other mechanisms providing rotary motion, as old or older than the wheel, entered into machine combinations with it. The weighted spindle was employed from Neolithic times for spinning thread; the spinning wheel, which coupled the spindle with a wheel, was an ancient Asiatic invention that reached Europe in the Middle Ages. The so-called potter's wheel, a flywheel, probably antedating the cart, was developed by pivoting the plate on which clay was hand molded. Equally ancient is the rotary hand mill (two superimposed stones for grinding flour), to which waterwheel power was added shortly before the Christian era. Grinding disks rotated by crank were known in China by about 1500 B.C.; the independently invented grindstone reached Europe after 500 A.D. Two devices, essentially rotating shafts, may have appeared independently of the wheel but became elaborated under its stimulus: the lathe, dating from the 2d millennium B.C. in Egypt and used widely by the Greeks after the 7th century B.C.; and the windlass, which came into use in about 1000 B.C. (See Wheel and Axle.)

Leslie Spier
University of New Mexico

Bibliography

Dennis, R. A., Making Wheels: A Technical Manual on Wheel Manufacture (Intermediate Technology Publs. 1994).

Peloubet, Don, ed., Wheelmaking: Wooden Wheel Design and Construction (Astragal Press 1996).

Piggott, Stuart, The Earliest Wheeled Transport: From the Atlantic Coast to the Caspian Sea (Cornell Univ. Press 1983).

 

Citar:

Spier, Leslie. "Wheel." Encyclopedia Americana. 2007. Grolier Online. 30 Mar. 2007 <http://ea.grolier.com/cgi-bin/article?assetid=0416450-00>