Hydraulic pumping units

Claim 1

A HYDRAULIC PUMPING UNIT COMPRISING, IN COMBINATION, A LIQUID TANK, A RECIPROCATING PUMP HAVING A CRANKCASE, A CYLINDER FIXEDLY SECURED ON SAID CRANK-CASE, A PLUNGER PISTON SLIDABLY MOUNTED IN SAID CYLINDER, DRIVING MEANS IN SAID CRANK-CASE OPERATIVELY CONNECTED WITH THE END OF SAID PISTON, AN INLET COMMUNICATING WITH SAID TANK THROUGH A FEEDING DUCT UNDER THE CONTROL OF AN INTAKE VALVE, AN OUTLET PROVIDED WITH A DELIVERY VALVE AND A WORKING CHAMBER FORMED BETWEEN SAID PISTON AND SAID TOW VALVES; A FIRST ANNULAR SPACE BETWEEN THE WALL OF SAID CYLINDER AND AN AXIAL ZONE OF SAID PISTON SPACED FROM ITS BOTH ENDS, MEANS TO CONDUCT LIQUID FROM SAID FIRST ANNULAR SPACE INTO SAID LIQUID TANK, A SECOND ANNULAR SPACE BETWEEN SAID CYLINDER AND AN AXIAL LENGTH OF SAID PISTON SPACED FROM SAID FIRST ANNULAR SPACE AND FROM THE DRIVEN END OF SAID PISTON, A RESERVOIR OF A LUBRICATING LIQUID, MEANS TO ESTABLISH A PERMANENT AND FREE COMMUNICATION BETWEEN SAID SECOND ANNULAR SPACE AND SAID RESERVOIR SUCH THAT SAID SECOND SPACE REMAINS FILLED WITH LUBRICATING LIQUID AND MEANS TO MAINTAIN THE LIQUID IN SAID SECOND SPACE UNDER A PRESSURE SLIGHTLY HIGHER THAN THE PRESSURE IN SAID CRANK-CASE, THE FREE END PORTION OF SAID PISTON WHICH IS COMPRISED BETWEEN SAID SECOND ANNULAR SPACE AND THE WORKING END OF SAID PISTON BEING FITTED IN SAID CYLINDER WITH A CLEARANCE SUFFICIENTLY LARGER THAN THAT OF THE REMAINING, GUIDED, PORTION OF SAID PISTON TO PREVENT ANY METAL-TO-METAL CONTACT BETWEEN SAID FREE END PORTION OF THE PISTON AND SAID CYLINDER.