Can any body shed some light on the existence of a standard or otherwise that governs the use of washers on studbolts? ie. When can you get away with not using them?
One important use is to reduce the surface pressure to acceptable levels for the material being clamped.
Another is to reduce the portion of the external load carried by the bolt, which is especially critical (because of fatigue) in the case of dynamic loads. It does this by increasing the relative stiffness of the clamped materials (k = AE/L, where A is the effective pressure area, which can be increased by using a hardened washer). In a bolted joint, the preload compresses the clamped members and stretches the bolt. An external tensile load will tend to separate the members, which would tend to stretch the bolt further (increasing the bolt load), but at the same time will also tend to relieve the compression of the clamped members (decreasing the bolt load). In reality, the joint will find some happy medium of compression/extension. The stiffer the members, the smaller the fraction of the external load the bolt will “see”.
Shigley and Norton both cover this topic. I prefer Norton, especially for his coverage of calculating the joint stiffness factor. And, www.boltscience.com has a great wealth of information on bolted joints.
Above is a snippet.