avoids damage during the positioning process and provides sufficient prestressing to ensure a tight seal to improve operation the clamping device additionally comprises a support ring on which the resilient torus rests the radial dimensions of the support ring are such that it can also follow any radial movements of the shaft another embodiment is the arrangement of the space in the neck close to the shutter around the collar the clamping device is located in the immediate vicinity of the first seal and should be mounted from inside the body preferably the clamping device comprises a metal spring with a helical winding in another embodiment the flexible cushion comprises a rigid washer which limits the axial movement in yet another embodiment the sleeve comprises a ring which ensures radial compression of a resilient collar of the cushion or the collar of an elastomer sleeve according to another embodiment the clamping device consists of a cage enveloping the collar against the space the neck and partly the shaft side the cage may extend radially towards flanges between the sleeve and the body this cage radially follows the movement imposed by the deformation of the shaft under the effect of the differential pressure exerted on the butterfly this results in a significant improvement in performance with the clamping device consisting of a swivel bearing guiding the shaft and at the same time allowing a rocking movement consisting of a radial and axial movement it may easily be imagined that the deformation of the butterfly valve and the shaft under the effect of the differential pressure applied to the butterfly valve.