What is the primary purpose of using a washer when installing bolts?

The primary purpose of using a washer when installing bolts is to distribute the load. When a bolt is tightened, it exerts a significant force on the surface it contacts. Without a washer, this force can concentrate on a small area, which might lead to deformation of the material or create stress concentrations that can weaken the connection. A washer helps to spread this force over a larger area, thus reducing the risk of damage to the materials being fastened. This load distribution is crucial in maintaining the integrity and strength of the assembly, particularly in applications where vibrations or thermal expansion and contraction can affect the joint.

Using a washer also aids in preventing the bolt or nut from loosening due to vibrations by providing a stable interface. It does not primarily serve to increase weight, act merely as a spacer, or secure the nut directly, although these effects can be secondary benefits in specific applications. However, the main function remains centered on effective load distribution, preserving the structural integrity of the assembly.