Washers: Are They as Good as We Think?

Abstract

Many bolting professionals in the heavy industrial industry rely on the belief that “Washers play an essential role in generating and sustaining integrity in bolted assemblies.” [1] It is believed that washers improve the reliability of bolted joints by providing a consistent K-Factor by introducing a flat even plane on which the nut can rotate. During practical training we noticed that using washers resulted in greater difference in bolt load between studs than experienced without washers. That experience led the authors to hypothesize that washers do not improve the accuracy and repeatability of bolted flange joints. This paper reports the findings of a study conducted to test this hypothesis. The study recorded the variance in measured K-Factor when tightening nuts against various flange surfaces with and without washers and determined that, when properly lubricated, the performance of studs against various flange surfaces did not vary greatly from the performance of studs against washers. Additionally, in some cases washers introduced a source of variance in bolt load. In addition to their perceived benefit in generating and maintaining tension in the bolted flange joint, washers are understood to provide benefits including preventing damage to the flange surface, preventing galling of the flange surface, and preventing nut embedment. This study focused on the use of washers in generating the expected bolt load and did not assess those additional benefits of washer usage.