itself linked to a sliding ring 27 on drive shaft 3 and driven rotationally through sliding keying by said shaft 3 this ring 27 comprises a chamber 28 for receiving a hydraulic fluid and with the piston 29 itself linked to axle 3 forms an actuator jack at rest this actuator jack is made to move downwards by the spring 30 pressing on the piston 29 itself linked to the axle 3 and consequently the flange 25 linked to the ring 27 is therefore made to move downwards and the tube 9 is not compressed if a pressurized fluid is introduced into the chamber 28 through the aperture 31 itself fed by a revolving joint 32 the sliding ring 27 and as a result the flange 25 are moved in the opposite direction i e upwards by the actuator jack consequently if the fluid is pressurized the mobile flange 25 is moved and the two rollers 21 21 u2032 linked to this flange compress the tube 9 against the rollers 20 20 u2032 mounted on the fixed flange 24 the tube 9 is therefore compressed and said tube 39 s tightness is ensured the compression force of tube 9 will be proportional to the pressure of the fluid entering the chamber 28 this pressure of the fluid may be proportional to the pumping pressure of the product and thus ensure the required tightness corresponding to the pumping pressure the elastomer of the tube 9 will only be called upon as much as necessary thus improving its life the powering of the shaft 3 and therefore of the two flanges 24 25.