Position hold and release mechanisms for flow control valves

Claim 1

THE COMBINATION WITH LOAD HANDLING MECHANISM HAVING A LOAD HANDLING IMPLEMENT AND A FLUID MOTOR OPERATIVELY CONNECTED TO SAID IMPLEMENT TO MOVE THE LATTER TO LOAD LIFTING, LOAD DUMPING, AND LOAD ENGAGING THRUST POSITIONS, OF A PUMP, A RESERVOIR, AND A FLOW CONTROL VALVE ALL OPERATIVELY INTERCONNECTED TO CONDUIT FLUID UNDER PRESSURE DELIVERED BY SAID PUMP FROM SAID RESERVIOR TO SAID MOTOR VIA SAID VALVE AND TO CONDUCT FLUID DISPLACED BY SAID MOTOR TO SAID RESERVOIR VIA SAID VALVE; SAID VALVE COMPRISING A HOUSING HAVING A VALVE BORE INTERSECTED AXIALLY THEREALONG BY AN INLET PORT HAVING FLUID COMMUNICATION WITH THE DELIVERY PORT OF SAID PUMP, BY A SERVICE PORT HAVING FLUID COMMUNICATION WITH A PORT OF SAID MOTOR, AND BY A RETURN PORT HAVING FLUID COMMUNICATION WITH SAID RESERVIOR; A SPOOL AXIALLY RECIPROCABLE IN SAID BORE FROM A FIRST SPRING-HELD POSITION WHEREAT FLUID COMMUNICATION IS BLOCKED THEREBY BETWEEN SAID INLET PORT AND SAID SERVICE PORT TO EITHER OF TWO OTHER POSITIONS WHEREAT SAID INLET PORT IS IN FLUID COMMUNICATION WITH SAID SERVICE PORT AND WHEREAT SAID SERVICE PORT IS IN FLUID COMMUNICATION WITH SAID RETURN PORT RESPECTIVELY; RESTRICTOR MEANS EFFECTIVE TO IMPEDE RETURN FLOW OF FLUID FROM SAID SERVICE PORT TO SAID RETURN PORT WHEN SAID SPOOL IS IN THE LAST-MENTIONED ONE OF SAID TWO OTHER POSITIONS THUS TO BUILD UP BACK PRESSURE IN SUCH RETURNING FLUID; MEANS DEFINING A CHAMBER UPSTREAM OF SAID RESTRICTOR MEANS TO WHICH A PORTION OF SAID SPOOL IS EXPOSED THUS TO HOLD SAID SPOOL IN THAT POSITION AS LONG AS THE BACK PRESSURE IN SAID CHAMBER ACTING ON THE EXPOSED PORTION THEREOF EXCEEDS THE SPRING PRESSURE TENDING TO MOVE SAID SPOOL TO SAID FIRST POSITION; A NORMALLY OPEN VALVE DISPOSED ADJACENT SAID IMPLEMENT ARRANGED, WHEN OPEN, TO CONDUCT RETURNING FLUID FROM SAID SERVICE PORT INTO SAID CHAMBER; AND MEANS RESPONSIVE TO MOVEMENT OF SAID IMPLEMENT TO THRUST POSITION OPERATIVE TO CLOSE SAID NORMALLY OPEN VALVE WHEREBY BACK PRESSURE IN SAID CHAMBER IS RELEASED THROUGH SAID RESTRICTOR MEANS TO SAID RETURN PORT THUS TO PERMIT SPRING PRESSURE RETURN OF SAID SPOOL TO SAID FIRST POSITION.