What is the purpose of a washer used with a nut and bolt?

A flat washer spreads the clamping force of a bolt or nut over a larger area, which distributes the load and protects the surface from damage and from the fastener digging in. A lock washer (split or toothed) adds tension or bite that helps keep the nut or bolt from vibrating loose. Washers are simple but important fasteners-system components. Choosing and placing washers correctly improves the strength and durability of a bolted joint. Understanding the role of washers — load distribution and, for lock washers, resisting loosening — is part of the fasteners topic.