Q. Washer – necessary for bolts?

'Fastener' is a general term which is used to describe something that is used as a restraint for holding things together or attaching them to other things. 

The major physical distinction between bolts and screws is that screws are completely full of threads whereas bolts comprises shanks without threads. Though a better interpretation of differences between the two is that bolts are always fitted with nuts. Quite the reverse, screws are generally used with tapped holes. 

High friction grip bolts are generally used in structural steelwork. They usually comprises high tensile strength bolts and nuts with washers. Bolts are tightened to a shank tension so that transverse load across the joint is resisted by friction between the plates instead of the bolt shank's shear strength. 

Purpose of installing washers in a typical bolting system is to distribute the loads under bolt heads and nuts by providing a larger area under stress. Or else the bearing stress of bolts can exceed the bearing strength of connecting materials and this results in the loss of preload of bolts and creeping of materials. Otherwise flanged fasteners in place of using washers could be adopted to attain the same purpose.

Fig: Washer