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A common application of bolted joints is for detachable connections which contain pressurised fluid. These may be rendered leakproof either by O-rings or by gaskets. An O-ring is an endless ring of circular cross- section, which is relatively compliant and usually elastomeric. It is lightly nipped when assembled, subsequent application of fluid pressure extrudes the ring to plug all potential leakage paths. Such joints are called self- energising as they do not rely on preload for their effectiveness.
This example demonstrates the static analysis of a self-energising joint.
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Static indeterminacy example |
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An eyebolt 1 and tubular sleeve 2 are supported by the plate 3 as in (a), (b) below.
Initially the nut is just finger tight ( 'snubbed' - there are no gaps in, and no appreciable loads on the assembly).
Subsequently the nut is tightened by screwing it a known distance Δ along the thread, then the temperature of the sleeve only is increased by t, and finally the load P is suspended from the eye.
Given the axial stiffnesses k1 and k2, resolve the indeterminacy in the final loaded state.
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The washer and support plate 3 are thin - ie. very short axially compared to the bolt and sleeve - and therefore relatively rigid since k is proportional to 1/L. It follows that the only deformations of significance are those of the bolt 1 and sleeve 2.
The sequence of load application (heat, tightening, external load) is irrelevant in this elastic analysis - what matters is the final loaded state after all three have been applied. The significant deformations (δ1, δ2) in this final loaded state are assumed to be as shown in (b) in which deformations are grossly exaggerated in order to clarify compatibility constraints. Other deformation assumptions may be equally valid - the result should not be affected by the assumption provided any subsequent analysis is consistent with the assumption. More of this anon.
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Loads in an elastic bolted assembly |
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The safety of a threaded fastener depends upon the actual load on it and the maximum load it can withstand. The latter has been considered above. Since the fastener is part of an indeterminate assembly it is now necessary to analyse such an assembly to find out just what is the actual load on the fastener itself.
The analysis is very similar to that carried out in an earlier chapter in the context of indeterminate assemblies.
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