AN EXPERIMENTAL STUDY ON THE RELAXATION OF BOLTS

Loss of pre-load with time, commonly known as ‘relaxation’ is an established phenomena. Behaviour of a bolted joint depends upon the pre-load in the bolts in use, not the pre-load introduced by the mechanic. Loss of pre-load is expected due to many factors such as embedment relaxation, gasket creep, elastic interactions, and vibration loosening or stress relaxation. In a gasketed joint, due to the gasket flexibility, relaxation in almost all bolts is always substantial during preliminary bolt tightening passes, as 80 to 100% loss is common hence resulting in a dynamic behaviour. It is observed that pre-load in a gasketed joint is controlled to a certain extent only in the final bolt-tightening passes. Experimental study presented in this paper highlights the factors affecting the amount of bolt preload relaxation with time. Important considerations are recommended to reduce bolt relaxation. Both the short and long term relaxations are recorded and a ‘best fit’ model for relaxation behaviour is derived.