In an advantageous embodiment of a BUTTERFLY VALVES the seat of the valve has a seating surface defining a section of an oblique cone having its apex lying in the wall of the cylindrical housing, a section of the cone perpendicular to the axis of the housing being in the shape of a circle and the rotating axis of the valve member lying excentrically in relation to the axis of the cylindrical housing and lying displaced in axial direction in relation to the plane of the sealing edge of the valve member.

In such an embodiment of a Butterfly Valves it is under normal circumstances possible by increasing the rotating force on the spindle of the valve member to increase the pressure of the valve member on the sealing edge of the seat so that one has a better sealing. Then however, it is necessary that the edge of the valve member is of elastic material. This again has the disadvantage that the embodiment can only be used when the working conditions are such that the elastic material maintains its elasticity.

When non-elastic material for the sealing edges were used, such an embodiment would be only possible when the parts of the valve would be machined in ideal way and there would be an exact fit which would be maintained in difficult working conditions such as big temperature differences.

According to the invention it is possible to have an embodiment of such a valve with sealing edges of the valve member and of the seat of a stiff material, more especially metal or plastic. Butterfly ValvesThen the sealing effect needs not to be obtained by increasing the force on the spindle of the valve member but by displacing the spindle of the valve member in the closed position in the direction of the seat.

In an advantageous embodiment according to the invention the actuating member of the bearing sleeves can be biassed by a spring for rotating the bearing sleeves in the direction in which the valve member is removed from its seat. Then the valve member automatically is released from its seat when the actuating device is actuated in opposite direction.

According to the invention the actuating device for the valve member and for the bearing sleeves can consist of a cylinder and a piston, one of which is connected with the actuating member of the valve member and the other of which is connected with the actuating member of the bearing sleeves, said cylinder having connections for supplying a pressure fluid on either side of the piston. By supplying pressure fluid on one side of the piston the valve member can be closed and be pressed onto the seat and by supplying the pressure fluid on the other side of the piston the valve member can be released from its seat.

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