A machine washer is a thin plate typically disk-shaped, but sometimes square with a hole (typically in the middle) that is normally used to distribute the load of a threaded fastener, such as a bolt or nut. Other uses are as a spacer,, wear pad, preload indicating device, locking device, and to reduce vibration (rubber washer). Machine Washers often have an outer diameter (OD) about twice their inner diameter (ID), but this can vary quite widely.
Machine Washers are usually metal or plastic. High-quality bolted joints require hardened steel washers to prevent the loss of pre-load due to brinelling after the torque is applied. Machine Washers are also important for preventing galvanic corrosion, particularly by insulating steel screws from aluminium surfaces. Machine Washers may also be used in rotating applications, as a bearing.
A thrust washer is used when a rolling element bearing is not needed either from a cost-performance perspective or due to space restraints. Coatings can be used in attempt to reduce wear and friction either by hardening their surface, or providing a solid lubricant.