Universal joints are fitting components used to transmit shaft rotation at a different angle. Hook joints are the most commonly used type of universal joint. Using a single universal joint at just one shaft end will result in non-uniform speeds, so there must always be a universal joint at each shaft end to make the speed uniform. A single constant-velocity joint can be used to always keep the speed constant even when changing the angle. Products in this category are available in several varieties to match different operating environments. For example, there are covered models that prevent contaminants from entering bearings, products that have been fully hardened to improve fatigue strength, and pre-greased needle bearings designed to withstand high-speed use. Oiling and other maintenance procedures will enable longer operation.