washer

Commonly used to cushion parts between the connector and the nut, the purpose is to act as a spacer to protect the connector surface from being scratched by the nut, dispersing the nut's pressure connector and reducing vibration caused by the separation. The gasket is usually metal or plastic, usually a flat metal ring. Due to the use of indentation torque to prevent the loss of preload, so special occasions need hardened steel washers. Especially by insulating the steel screws from the aluminum surface.