Loss of Preload in Threaded Fasteners With or Without Rotation

It is well known that a loss of preload tension in a threaded fastener frequently leads to a fatigue failure of the fastener. Loss of preload in threaded fasteners is usually attributed to rotation of the fastener due to vibration; however under some conditions preload may also be lost without relative rotation between the male and female components in a joint. In these cases, preload is lost due to embedment and relaxation of the joint members. Given that loss of preload is normally associated with rotation, there are numerous methods of “safetying” threaded fasteners to prevent rotation. Many of these methods are in-fact ineffective as they only prevent gross rotation, but allow sufficient rotation to cause a loss of preload. This paper discusses the shortcomings of several traditional fastener “safetying” methods such as safety wire, lock washers, and locking tabs. It also presents the factors that can lead to a loss of preload due to embedment and relaxation. A case study is presented where it is believed that a recurrent joint failure occurred because the inclusion of a traditional anti-rotation safety device in the design contributed to a total loss of preload due to embedment and relaxation.