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Bolt Tightening Sequences
Tests have shown that elastic interactions 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.
The effect that bolt crosstalk can have in reducing bolt preload
Because in the vast majority of bolted joints, all bolts are not tightened simultaneously, the effect of tightening one bolt in the group has 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 diagrams 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.
Ideal bolt tightening sequences
Presented below are two examples of tightening sequences that have been shown to result in minimising bolt preload variations due to elastic interactions. This 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 elastic interactions 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.
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