Bolted Joint Preload Distribution from Torque Tightening

The purpose of this paper is to develop an understanding of how bolt preloads are distributed within a joint as each bolt is tightened in turn by the use of a calibrated torque wrench. It discusses how the order that the joints nuts/bolts are tightened can affect the final bolt preload. It also investigates the effect on incrementally increasing the bolt preload through a series of applications of the controlled torque tightening sequence.

Classical analysis methods are used to develop a method of analysis that can be applied to most preloaded bolted joints. It is assumed that the static friction coefficient is approximately 15% less than the dynamic friction.

It is found that the bolt preload distribution across the joint can range from slightly above the target preload to significantly less than the target preload. The bolts with a preload greater than the target preload are found to be those tightened towards the end of the tightening sequence, usually located close to the outer edges of the joint’s bolt array. The bolts with a preload less than the target preload are those tightened early in the tightening sequence, located centrally within the joints bolt array.

The methods presented can be used to optimise bolted joint design and assembly procedures. Optimising the design of preloaded bolted joints leads to more efficient use of the joints.