wipers, reinforced legs, steering tubes, swivel locks and brakes, all implemented in an effort to reduce operator injuries in the workplace. Wheel diameter, wheel width, and tandem wheels The diameter of a caster wheel affects how easily the caster moves over particulate, rough or irregular surfaces. Large diameter caster wheels are able to bridge gaps like that between the floor and an elevator car. However, the larger the diameter of a caster wheel, the higher the caster support arm must be. Either the base of a low-hanging object must be lifted higher above the wheels, or the casters must hang out to the sides straddling the low-hanging supported object. While rotating around the vertical shaft, swivel caster wheels sweep out a space. Larger wheels require more of this space. Load capacity may be increased by using wider wheels with more ground contact area. However, when rotating a wide swivel caster in-place, the center part of the wheel-to-ground contact patch rotates slower than the regions further out to the sides. This difference in rotation speed across the base of the wheel contact patch causes wide wheels to resist rotation around the swivel, and this resistance increases as weight loading increases. An alternative way to increase load capacity while limiting swivel-rotation resistance is to use multiple narrow wheels in tandem on the same wheel axis. Each wheel has a comparatively narrower ground contact patch than a single wide wheel, so there is less resistance to turning in place on the swivel. Other related wheels There are four main classifications of wheels: A standard wheel has a center rotating hub (or bearing) and a compliant material on its outer side. A caster is a wheel mounted to a fork, with an optional, additional offset steering joint. An omnidirectional wheel (Mecanum wheel, Omni wheel, or Swedish wheel) is made of a large central hub with many additional smaller wheels mounted along the perimeter such that their axes are perpendicular to the central wheel. The central wheel can rotate around its axis like traditional wheels, but the smaller wheels can also enable movement perpendicular to the central axis. A spherical wheel is omnidirectional and is generally a spherical ball mounted inside a restraining fixture. An example is a ball transfer unit. See also Caster angle Caster board Bogie References Wheels