Multi-ply bellows is widely used because its flexibility and pressure tightness. However, the analytical treatment of its behaviours is difficult compared with that of single-ply bellows, since the uncertainty of friction between plies exists. In this paper the strength evaluation equations of multi-ply bellows are examined and the friction effect between plies is investigated through simplified models and detailed FEM analyses of two-ply bellows for example including the effect of material plasticity and internal pressure. Mainly following results are obtained. - The behaviours of two-ply bellows without friction can be analysed approximately by using single-ply bellows model even in the elastic-plastic region. The effort of numerical analysis can be reduced if two-ply bellows analyses can be replaced by the analyses of single-ply bellows model. - The effect of friction is relatively small up to about 0.4 by friction coefficient. Therefore, the behaviours of two-ply bellows can be evaluated practically ignoring friction effect. - The strength of multi-ply bellows against pressure, i.e. buckling, can be evaluated conservatively by assuming no friction, even if considered the internal pressure with friction. Therefore, the evaluation method without friction is recommended for design.
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