Hydraulic lifting devices of tractors

Claim 1

IN A TRACTOR HAVING A HITCH FOR CONNECTING AN IMPLEMENT THERETO AND A HYDRAULIC CONTROL SYSTEM HAVING A MANUAL CONTROL LEVER FOR CONTROLLING SAID SYSTEM FOR RAISING AND LOWERING THE HITCH, SAID SYSTEM INCLUDING A FLUID RAM CONNECTED TO THE HITCH, A FLUID CIRCUIT COMPRISING A PUMP, A RESERVOIR, AND A DISTRIBUTOR COMPRISING A PILOT VALVE HAVING A NEUTRAL POSITION AND OPERABLE FROM SAID NEUTRAL POSITION IN OPPOSITE DIRECTIONS TO EXTREME OPERATIVE POSITIONS FOR CONTROLLING THE ACTION OF SAID FLUID ON SAID FLUID RAM, MEANS DEFINING TWO PARALLEL FLOW PATHS IN SAID FLUID CIRCUIT CONNECTING SAID PUMP WITH SAID FLUID RAM, ONE OF SAID FLOW PATHS BEING CONNECTED BETWEEN SAID PUMP AND FLUID RAM AND INCLUDING A FIRST NON-RETURN VALVE, A SECOND FLOW-PATH OF SAID TWO FLOW-PATHS CONNECTED TO SAID ONE FLOW PATH UPSTREAM OF SAID FIRST NON-RETURN VALVE AND DOWNSTREAM THEREOF COMPRISING A THROTTLE VALVE AND A SECOND NON-RETURN VALVE BIASED TO A CLOSED POSITION AND SERIALLY CONNECTED WITH SAID THROTTLE VALVE, A BY-PASS VALVE BIASED TO A CLOSED POSITION AND IN COMMUNICATION WITH SAID SECOND FLOW PATH UPSTREAM OF SAID SECOND NON-RETURN VALVE TO BYPASS THE FLUID TO SAID RESERVOIR, FOLLOW-UP LINKAGE CONNECTING SAID PILOT VALVE TO THE HITCH AND ACTIVATED BY SAID RAM FOR POSITIONING SAID PILOT VALVE IN SAID NEUTRAL POSITION AND CONNECTED TO SAID MANUAL CONTROL LEVER FOR POSITIONING SAID PILOT VALVE IN SAID EXTREME POSITIONS, SAID BYPASS VALVE BEING CONTROLLED BY SAID PILOT VALVE AND BEING OPEN AT THE POSITIONING OF SAID PILOT VALVE IN SAID NEUTRAL POSITION, SAID CONTROL LEVER BEING OPERABLE TO TWO OPERATIVE POSITIONS FOR CONTROLLING THE MOVEMENT OF SAID PILOT VALVE, IN ONE DIRECTION TO AN EXTREME POSITION FOR CONTROLLING THE CLOSING OF SAID BY-PASS VALVE THEREBY TO SUPPLY FLUID UNDER PRESSURE TO THE FLUID RAM AND IN ANOTHER DIRECTION TO ANOTHER EXTREME POSITION FOR OPENING SAID SECOND NON-RETURN VALVE AND SAID BY-PASS VALVE TO ALLOW DISCHARGE OF SAID FLUID FROM SAID RAM, AND SAID THROTTLE VALVE HAVING MEANS OPERABLE SELECTIVELY TO RESTRICT THE FLOW THROUGH SAID SECOND FLOW PATH FOR INCREASING THE PRESSURE UPSTREAM OF SAID BY-PASS VALVE TO APPLY TO THE FLUID RAM AN INCREASED PRESSURE TENDING TO ELEVATE SAID HITCH TO EFFECT A TRANSFER OF A FRACTION OF THE LOAD ON THE IMPLEMENT TO SAID HITCH.