Pilot operated hydraulic control system for scrapers

Claim 1

A HYDRAULIC CONTROL UNIT FOR REGULATING FLUID FLOW TO AND FROM A HYDRAULIC JACK, SAID CONTROL UNIT COMPRISING A HOUSING HAVING A VALVE SPOOL RECIPROCABLY DISPOSED THEREIN AND ADAPTED TO ASSUME AN OPERATING POSITION AND AN INTERMEDIATE HOLD POSITION AND A FLOAT POSITION, MEANS FOR NORMALLY LOCATING SAID VALVE IN SAID INTERMEDIATE HOLD POSITION, A SOURCE OF COMPRESSED AIR, FIRST SECOND AIR CHAMBERS RESPECTIVELY FORMED IN SAID HOUSING AT OPPOSITE ENDS OF SAID SPOOL, MEANS FOR SELECTIVELY CONNECTING SAID FIRST AND SECOND AIR CHAMBERS WITH SAID SOURCE OF COMPRESSED AIR FOR SELECTIVELY MOVING THE SPOOL VALVE TO SAID OPERATING AND FLOAT POSITIONS RESPECTIVELY, MEANS DISPOSED ADJACENT SAID SECOND AIR CHAMBER AND HAVING AN ACTUATOR ADAPTED TO EXTEND INTO THE SECOND CHAMBER MOVE THE SPOOL VALVE TO THE FLOAT POSITION, MEANS FOR HOLDING THE ACTUATOR IN A RETRACTED POSITION, AND MEANS INCLUDING MANUALLY OPERABLE MEANS FOR CAUSING THE ACTUATOR TO EXTEND INTO THE SECOND AIR CHAMBER TO MOVE THE SPOOL VALVE TO THE FLOAT POSITION.