A wheel is a rotating component that is intended to turn on an axle bearing. The wheel is one of the key components of the wheel and axle which is one of the six simple machines. Wheels, in conjunction with axles, allow heavy objects to be moved easily facilitating movement or transportation while supporting a load, or performing labor in machines. Wheels are also used for other purposes, such as a ship's wheel, steering wheel, potter's wheel, and flywheel.
This Ljubljana Marshes Wheel with axle is the oldest wooden wheel yet discovered dating to Copper Age (c. 3130 BCE)
A depiction of an onager-drawn cart on the Sumerian "War" panel of the Standard of Ur (c. 2500 BCE)
Solid wheels on a heavy temple car, contrasted with the lighter wire-spoked wheels of the black roadster bicycle in the foreground
A figurine featuring the New World's independently invented wheel. Among the places where wheeled toys were found, Mesoamerica is the only one where the wheel was never put to practical use before the 16th century.
A bearing is a machine element that constrains relative motion to only the desired motion and reduces friction between moving parts. The design of the bearing may, for example, provide for free linear movement of the moving part or for free rotation around a fixed axis; or, it may prevent a motion by controlling the vectors of normal forces that bear on the moving parts. Most bearings facilitate the desired motion by minimizing friction. Bearings are classified broadly according to the type of operation, the motions allowed, or the directions of the loads (forces) applied to the parts.
A ball bearing
Drawing of Leonardo da Vinci (1452–1519) Study of a ball bearing
Early Timken tapered roller bearing with notched rollers
