Unlike a wheel on a car or truck, a caster is an undriven wheel that is mounted to objects and enables them to be moved. The little wheels on an office chair or a shopping cart are good examples for the use of casters in everyday life. Industrial applications also utilize casters, for example on carts and dollies or on assembly turntables in manufacturing plants. Some of these vehicles move extremely heavy loads, in some instances up to 50,000 pounds. Custom-build heavy duty casters are usually made of cast iron and have polyurethane tires, which do not track dirt. Often, several wheels are used on the same axis to increase load capacity and to distribute weight.