Self-advancing mine roof supports

Claim 1

AN ADVANCING MINE ROOF SUPPORT COMPRISING: AT LEAST ONE VERTICAL EXTENSIBLE AND RETRACTABLE SUPPORT MEMBER OF THE TYPE INCORPORATING A CYLINDER AND PISTON ASSEMBLY TO THE VERTICAL CYLINDER OF WHICH FLUID UNDER PRESSURE CAN BE ADMITTED FOR THE PURPOSE OF EXTENDING THE SAID SUPPORT MEMBER INTO A ROOF-ENGAGING POSITION, SAID VERTICAL CYLINDER HAVING CONNECTED THERETO AN OUTLET CONDUIT CAPABLE OF SERVING AS A PASSAGE FOR THE EXHAUSTING OF FLUID FROM SAID VERTICAL CYLINDER FOR THE PURPOSE OF RETRACTING SAID SUPPORT MEMBER; A CONTROL VALVE ASSOCIATED WITH SAID OUTLET CONDUIT, SAID CONTROL VALVE COMPRISING A VALVE HOUSING DEFINING VALVE INLET AND VALVE OUTLET PASSAGES, A VALVE CLOSURE MEMBER MOVABLY LOCATED WITHIN A CHAMBER DEFINED BY THE HOUSING, AND SPRING MEANS ASSOCIATED WITH THE SAID VALVE CLOSURE MEMBER AND ADAPTED TO EXERT A RESILIENT FORCE IN A DIRECTION SUCH THAT THE SAID VALVE CLOSURE MEMBER IS NORMALLY MAINTAINED IN A CLOSED POSITION IN WHICH FLUID FLOW BETWEEN THE SAID VALVE INLET PASSAGE AND THE SAID VALVE OUTLET PASSAGE IS PREVENTED, SAID VALVE INLET PASSAGE HAVING HYDRAULIC CONNECTION WITH THE SAID OUTLET CONDUIT; ADVANCING MEANS CONNECTED TO SAID SUPPORT MEMBER AND TO AN ABUTMENT SPACED FROM THE SAID SUPPORT MEMBER, SAID ADVANCING MEANS BEING FURTHER ADAPTED TO EXERT AN ADVANCING FORCE WHEREBY SAID SUPPORT MEMBER IS URGED TOWARDS THE SAID ABUTMENT; VALVE ACTUATING MEANS INTERCONNECTING SAID ADVANCING MEANS AND SAID CONTROL VALVE AND ADAPTED TO TRANSMIT TO THE SAID VALVE CLOSURE MEMBER A VALVE ACTUATING FORCE PROPORTIONATE TO THE SAID ADVANCING FORCE IN OPPOSITION TO THE SAID RESILIENT FORCE WHEREBY, WHEN THE SAID ADVANCING FORCE ATTAINS A PREDETERMINED VALUE, THE VALVE ACTUATING CLOSURE MEMBER INTO AN OPEN POSITION IN WHICH FLUID FLOW IS ESTABLISHED BETWEEN THE SAID VALVE INLET AND OUTLET PASSAGES TO EXHAUST SAID VERTICAL CYLINDER.