Retaining rings install into an assembly to restrict the movement of rotating or moving components. They are removable with specialized tools and come in two forms, depending on the application. External retaining rings fit over a shaft and lock into a milled groove. This retains any components on the shaft and helps prevent free-floating shafts from slipping out of the assembly. Internal retaining rings install into grooves on the inner surface of a hole and help retain components inside the assembly. They are used for applications where maintenance or disassembly is rare, such as hydraulic cylinders.