Is it necessary to use a flat washer on all bolt and nut assemblies?
Yes. Flat washers should be used under the head of the bolt and under the nut. Flat washers prevent the head of the bolt or the nut from damaging the work surface. They also provide a larger load-bearing area, and provide a smooth surface for nut or bolt head rotation during installation. It is also important to use a flat washer of the same grade. A common mistake is to use low carbon flat washers with grade 5 or grade 8 bolts. The washer, which is soft in comparison to the bolt and nut, will compress as the assembly is tightened. This makes it difficult, at best, to determine the proper assembly torque. Therefore, there is no way for you to know if the assembly is tightened properly. In use, if the assembly is subject to vibration or impact, the flat washer may compress even more, causing the bolt to relax. This will eventually lead to fatigue failure. Always use flat washers of the proper grade!
