Couplings join two shafts for the purpose of power transmission and may be rigid or flexible, depending on the load and alignment requirements of the two shafts, and screw collars terminate the end of a shaft to prevent the shaft from sliding in one direction. Collars and couplings lock onto the end of a shaft using either one or more set screws or other clamping mechanism and may also employ the use of a keyway for additional rigidity. Universal joints, flexible couplings, rigid couplings, shaft collars, and shaft-hub locking devices are common forms of this mechanism. Misalignment between shafts generally requires the use of a flexible connection between the shafts to reduce vibration during rotation and to simplify installation. Joiners connect two pipes or tubes for the purpose of extending their lengths and are generally rigid, using clamps to secure the connection. When choosing collars and couplings, consider the environment in which they are used and the load and alignment requirements of the application.