stationary disc 3 and to determine respectively the non operating condition u201c r u201d and operating condition u201c w u201d of the sealing ring 50 the drive means 7 include at least a slider 70 projecting transversely from the control rod 6 and provided with two faces 71 72 parallel to each other but sloping with respect to the longitudinal axis of the rod 6 the slider 70 is aimed at engaging in a complementary groove 44 made in the shank 40 so that the axial thrust exerted on the rod 6 in one or the other direction generates a component of a force parallel to the axis of discs 3 4 by means of the contact surfaces 71 72 of the slider 70 and the corresponding surfaces of the groove 44 thus according to the direction of this component axial movements of the movable disc 4 away from or close to the stationary disc 3 are carried out in a preferred embodiment of the drive means 7 two parallel flat surfaces 60 are made in the lower portion 6 b of the rod 6 from which two respective symmetrical sliders 70 protrude the shank 40 of the movable disc 4 features a longitudinal slit 42 arranged on a median plane of the shank 40 which is thus divided in two portions the facing faces of the slit 42 feature two of the above mentioned mirror like grooves 44 within which the lower portion 6 b of the rod 6 can be slidably introduced with its flat surfaces 60 and the two sliders 70 can engage with the corresponding grooves 44 in order.