Wheel and Axle
The wheel has undoubtedly been among the most significant of human inventions. Prior to its invention, population growth was limited, because people could not move easily to find more food and water once their resources had been diminished. The population in any area had to remain small enough for the land to support that group indefinitely. Only small quantities of goods could be transported, so trade was not common. Many other tasks which are basic to our way of life today could not be accomplished without the wheel.
The wheel has had many uses throughout human history. One of the earliest was the potter's wheel, which first appeared in Egypt around 3000 B.C. Wheels have also been used as winches to draw water up from wells or anchors up onto a ship, as waterwheels and windmills to generate power, as grinding mills, as pulleys, and as spinning wheels for textile workers.
Perhaps the most common use of the wheel, however, has been in transportation. Sometime before 3500 B.C., people in Mesopotamia began to use sleighs-like vehicles to increase the load their beasts of burden could carry. By 3500 B.C., the runners of the sleighs had been replaced with disk wheels fixed to an axle to which a cart body was attached. The earliest examples of these inventions are two- and four-wheeled Mesopotamian chariots. At first, wheels were made of wood planks fastened together, then cut into circles and often bound with cross-struts. Natural knot-holes were frequently placed in the center to serve as holes for the axle.
Around 3000 B.C., rims, made of wood studded with nails or metal strips, were added to the wheel. Between 2000 and 1000 B.C., spoked wheels came into use in Mesopotamia, Egypt, China, and Scandinavia. Spoked wheels were much lighter and could be made with a larger diameter, allowing chariots to be driven at high speeds over rough ground--which made them well-suited for use in battle. The axles and rawhide bearings of these vehicles were lubricated with animal fat, vegetable fat, or mineral oil.
Vehicle wheel design experienced only slight changes until the sixteenth century A.D., when dished wheels were invented. The dish wheel spokes were arranged in a flattened cone, so that the tops of the wheels angled out to accommodate wider, swaying loads on narrow tracks. Wheels continued to be made from wood with metal rims until the 1870s, when wire wheels were developed for the bicycle.
The pneumatic tire, a rubber casing inflated with compressed air, became more popular beginning in the late 1880s. The wheels on early automobiles were wooden and spoked or made of wire, although some had artillery wheels (one-piece, cast iron copies of wooden wheels). During the 1930s, light, strong, and inexpensive pressed steel wheels replaced other varieties for use on cars. Cast wheels, similar to the old artillery wheels but made of light, strong magnesium or aluminum alloy are now found on some sports cars.
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