In its primitive form, a wheel is a circular block of a hard and durable material at whose center has been bored a hole through which is placed an axle bearing about which the wheel rotates when torque is applied to the wheel about its axis. The wheel and axle assembly can be considered one of the six simple machines. When placed vertically under a load-bearing platform or case, the wheel turning on the horizontal axle makes it possible to transport heavy loads. This arrangement is the main topic of this article, but there are many other applications of a wheel addressed in the corresponding articles: when placed horizontally, the wheel turning on its vertical axle provides the spinning motion used to shape materials; when mounted on a column connected to a rudder or to the steering mechanism of a wheeled vehicle, it can be used to control the direction of a vessel or vehicle; when connected to a crank or engine, a wheel can store, release, or transmit energy. A wheel and axle with force applied to create torque at one radius can translate this to a different force at a different radius, also with a different linear velocity.
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