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.
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.
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