What Causes a Stud Bolt to Loosen

by

Charles BahandiStud bolts are highly beneficial when dealing with larger steel like stainless steel pipes and steel flanges, due to its strength, wider flexibility of length, and ability to more or less permanently fasten the joint. Aside from the obvious strength issue, the problem that may occur with stud bolts and nuts is the possibility of it to self-loosen as the time passes, and eventually put the integrity of joints at risk, most especially when exposed to vibrations and movements.

How Will a Stud Bolt Fail?

Often fatigue failure is a result of the stud bolt self-loosening which reduces the clamp force acting on the joint. Joint slip then occurs which leads the the bolt being subjected to bending loads and subsequently failing by fatigue.

Stud bolts are manufactured in such a way that the bolt shank will fail first before the threads strip, this just means that the full strength of the thread can ride out forces greater than what would be necessary to fracture the actual bolt shank. But for it to actually be fractured, there must be a minimum amount of bolt thread engaging the nut, known as minimum thread engagement.

It is believed that vibration causes bolt loosening. By far the most frequent cause of loosening is side sliding of the nut or stud bolt head relative to the joint, resulting in relative motion occurring in the threads. If this does not occur, then the bolts will not loosen, even if the joint is subjected to severe vibration. By a detailed analysis of the joint, it is possible to determine the clamp force required to be provided by the bolts to prevent joint slip.

What is the Preload of a Stud Bolt?

For the most part stud bolts don’t face the above problem due to the ability to produce what is called a clamping load, or a preload, which if large enough, will ensure joint integrity. The preload is the force that is created when torque is applied to a bolt to fasten two or more objects together. The bolt is pulled into tension as torque is applied, while the fastened parts experience compression.

This tension, as long as it’s within the elastic limit of the bolt, exerts an equal and opposite force called the tensile stress which works to keep the bolt from loosening. Above all secondary safety measures such as lock nuts and washers, the principle of tightening the bolt down sufficiently to begin with, is the most crucial factor in preventing premature loosening.

THERE ARE THREE COMMON CAUSES OF THE RELATIVE MOTION OCCURRING IN THE THREADS:

Bending of parts which results in forces being induced at the friction surface. If slip occurs, the head and threads will slip which can lead to loosening.

Differential thermal effects caused as a result of either differences in temperature or differences in clamped materials.Applied forces on the joint can lead to shifting of the joint surfaces leading to bolt loosening.

Article Source:

eArticlesOnline.com}