Abstract
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 the 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 is always substantial during preliminary passes, as 80 to 100% loss is not uncommon in almost all the bolts, resulting in a dynamic behaviour. Pre-load in a gasketed joint is stabilized and retained to certain extent in the final passes only. In a non-gasketed joint, due to no gasket and no rotation its static behaviour is concluded. This paper highlights the factors affecting the amount of relaxation with time and presents important considerations that can reduce this. Both the short and long term relaxations are recorded and a ‘best fit’ model for relaxation behaviour is derived.
Highlights
From a number of analytical and experimental studies of bolted flanged pipe joints, great importance is attributed to the bolting and assembly of the bolted joints due to different bolt behaviour that can be observed [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17]
Good quality and high strength bolts with original surface treatment, i.e. dry lubricant, proper bolt tightening speed by properly trained fitters, and with initial higher preload to compensate for the pre-load loss is recommended
It is recommended that bolts are used after undergoingseveral passes of loosening and tightening
Summary
From a number of analytical and experimental studies of bolted flanged pipe joints, great importance is attributed to the bolting and assembly of the bolted joints due to different bolt behaviour that can be observed [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17]. The bolts were preloaded close to the ultimate tensile strength of the bolt material, to study its relaxation behaviour This experimental study highlights important factors that affect amount of relaxation in bolts with time and presents important considerations that may be effective in reducing this occurrence. A bolt calibration unit developed for the measurement of force felt by the bolt was used Both the short and long term relaxations are recorded and a ‘best fit’ model for relaxation behaviour is derived for different pre-loads. Definitions of relaxation terms are given here for clarity: Short term relaxation: Most of the relaxation occurs shortly, after the joint has been assembled or at least soon after it has been put into service, due to the number of reasons, such as bolt bending, soft parts (gasket), improper tooling and torquing, bolt quality, nonparallelism of flange joint surfaces, geometric variance and so on
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