Tightening Sequences

The Appropriate Sequence to Tighten a Joint

Because in the vast majority of bolted all bolts are not tightened simultaneously, the effect of tightening one bolt in the group as an effect on the preload in other previously tightened bolts in the group. Such effects are called elastic interactions or sometimes bolt crosstalk. The mechanism that causes this is illustrated in the diagram below.

Compression under a tightened bolt
The outer two bolts have been tightened compressing the joint under the bolts. The middle bolt is subsequently tightened compressing the joint directly under the bolt but also compressing the joint slightly under the two other bolts leading to a loss of preload in these bolts. Presented below are two examples of tightening sequences that have been shown to result in minimising bolt preload variations due to elastic interactions, that will minimise the preload scatter within a joint. If the joint is critical it would be wise to consider specifying a multiple pass tightening sequence. With such a sequence, each bolt is tightened more than once so as to reduce the preload reduction caused by the tightening of the other bolts in the joint.

Tightening sequence for a rectangular joint
Tests have shown that the elastic interaction between bolts in a joint can have a significant effect on the preload (a reduction of 35%). If a gasket is present between the joint surfaces the effect can be even more pronounced.