WHEEL

a circular frame of hard material that may be solid, partly solid, or spoked and that is capable of turning on an axle. A Sumerian (Erech) pictograph, dated about 3500 BC, shows a sledge equipped with wheels. The idea of wheeled transportation may have come from the use of logs for rollers, but the oldest known wheels were wooden disks consisting of three carved planks clamped together by transverse struts. Spoked wheels appeared about 2000 BC, when they were in use on chariots in Asia Minor. Later developments included iron hubs (centerpieces) turning on greased axles, and the introduction of a tire in the form of an iron ring that was expanded by heat and dropped over the rim and that on cooling shrank and drew the members tightly together. The use of a wheel (turntable) for pottery had also developed in Mesopotamia by 3500 BC. The early waterwheels, used for lifting water from a lower to a higher level for irrigation, consisted of a number of pots tied to the rim of a wheel that was caused to rotate about a horizontal axis by running water or by a treadmill. The lower pots were submerged and filled in the running stream; when they reached their highest position, they poured their contents into a trough that carried the water to the fields. The three power sources used in the Middle Agesanimal, water, and windwere all exploited by means of wheels. One method of driving millstones for grinding grain was to fit a long horizontal arm to the vertical shaft connected to the stone and pull or push it with a horse or other beast of burden. Waterwheels and windmills were also used to drive millstones. Because the wheel made controlled rotary motion possible, it was of decisive importance in machine design. Rotating machines for performing repetitive operations driven by steam engines were important elements in the Industrial Revolution. Rotary motion permits a continuity in magnitude and direction that is impossible with linear motion, which in a machine always involves reversals and changes in magnitude.