It is ascertained that at assembly and trial of pneumahydraulic systems (PHS), detachable seam of connection after removal from them the efforts of a tightening do not lose the air-tightness even in conditions of nominal loads. Basing on the theory of metal links and the carried out researches the calculations of diameter of a single spot of contact and actual square of contact depended on the pressure and from sort of processed material are done. The mathematical model is developed and offered which allows to judge about common character of the process cold weld in detachable connections, and also to select his optimal parameters.