Bolt loads reduce significantly after tightening of bolted flanged connections in which expanded PTFE (Polytetrafluoroethylene) gaskets are used. Bolts are often retightened after a while to compensate for the reduction of bolt loads in practice. The visco-elastic characteristic of PTFE gaskets is the main cause for the phenomenon. The flexibility of the bolt-flange system also affects the phenomenon. However, effects of the retightening on the residual bolt loads and the tightness of flanged connections have not been clarified yet.
In this paper, an analytical model of a bolted flanged connection, in which the flanges, the bolts and the gasket are represented by spring constants, is proposed. The residual bolt forces and the deflection of the gasket were estimated based on the analytical model of the bolted flanged connection and a creep characteristic of the gasket. Thus, the relationship between gasket deflection and the tightness of the bolted flanged connection are clarified. Based on the analytical method, the effectiveness of retightening was discussed from the view points of the residual bolt load and the tightness.