"This Self-Locking Pulley from Portable Winch is designed for use in rescue operations and any other situation where your load may fall or roll back. Features a brake cam system that allows the rope to move in only one direction. Works With 1/2 in. and 3/8 in. ropes, Material Aluminum, Application Pulley. Spring-loaded cam release-cord disengaged Two pulleys used together give you a 41 mechanical advantage Mounted on self-lubricating bushings and stainless steel axles Aluminum swing sides and main plate For 1/2in. and 3/8in. ropes"