The importance of proper pre-loading of coupling bolts cannot be too strongly stated. Most coupling bolt problems relate to inadequate pre-loading of the bolts during installation, or relaxation of the bolt load in service due to rough or inaccurately machined coupling faces. This will eventually result in working of the coupling joint under the applied loads leading to fretting and seizing of the bolts and coupling faces, which will eventually lead to fatigue failures of the coupling bolts.

This paper looks into the effect of the dynamic response of the system on the bolt design with two examples from our own experience in this industry. One case experienced vibratory torque exceeding the nominal torque of the system and the other case experienced vibratory torque equal to the nominal torque of the system. Since nominal torque is used as design criteria for bolt design according to the classification society's requirement, one of the cases had high vibratory torque exceeding the design criteria of the coupling bolts. Compounding the problem is Poisson's effect on the bolt flange and the surrounding flange material, which severely reduces the interference fit during bolt preloading. This, combined with excessive vibratory torque, generates relative micro motion between the bolt shanks and flange holes as observed in actual failures.

The strength of a bolted assembly for resisting dynamic loads depends greatly upon the relative flexibilities of the bolts and of the reacting members. If the bolts could be made very flexible and the flange assembly very rigid, then, only a very small proportion of the dynamic load would be felt by the coupling bolts until actual separation of the coupling faces occur. Consequently, it is very important in future design of coupling bolts and coupling flanges to provide very rigid flange designs and to give the coupling bolts as much flexibility as possible by under cutting the bolt shanks as much as other considerations will permit. The amount of under cutting that can be permitted will depend on the length of the fitted portion of the bolt, and the magnitude of the interference fit required. It must be understood that if the interference fit is too excessive it can become a problem to achieve proper pre-loading of the bolts during assembly because the stretch of the bolt will tend to be restricted to the portion between the nut and the fitted part of the shank. This may lead to the strain not being distributed along the length of the bolt, but will tend to be confined to the undercut shank and threads between the fitted portion and the abutment face of the nut.

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