GB1236797A - Improvements in or relating to hydraulic power lifting systems - Google Patents

Abstract

1,236,797. Tractor power lift mechanisms. ROBERT BOSCH G.m.b.H. 26 June, 1968 [1 July, 1967], No. 30371/68. Heading A1P. A hydraulic power lifting system for tractors towing implements via a threepoint hitch linkage includes a regulating control apparatus 23 for initiating raising and lowering of the towed implement and a pressure control device 26 for controlling the pressure in the ram 16, 17 to a predetermined nominal value. The driver operates pre-control slide 35 to block line 34 and connect control passage 37, leading to main control slide 38, to return flow line 36, the control slide 38 then being actuated by a spring to disconnect operating line 33 from return line 36. A first control line 41, provided in pressure control device 26 is connected to the ram 16, 17 via operating line 33 and throttle 40, the throttle 40 being opened fully so that the same pressure prevails in line 41 and ram 16, 17. Hydraulic fluid is then pumped from a reservoir 18 to a pressure line 31 provided in pressure control device 26 via passages 21, 24, 30, and from pressure line 31 to ram 16, 17 via operating line 33 and one-way valve 32, the implement then being lifted accordingly. If the pressure in the ram 16, 17 exceeds the predetermined nominal value, pressure fluid in first control line 41 actuates a sensor 45, in the form of a plunger 48, causing the plunger 48 to lift off its seat 44<SP>1</SP> and connect control 41 to return line 36 via a chamber 43 and passages 59, 58. At the same time, plunger 48 causes pre-control slide 50 to raise against the action of spring 53, the loading of which is adjustable by a screw 54 which sets the predetermined nominal value. Raising of slide 50 connects chambers 56 and 57 and blocks off the chamber 57 from the return line 36. Fluid pumped to pressure line 31 then passes from line 60 and chamber 56 to a chamber 64 of a spring-loaded control slide 66 via chamber 57 and a line 62, thus pushing control slide 66 down against the action of a spring 67 so that pressure line 31 is connected to return line 36 via transverse bores 70 in slide 66, bore 69 in slide 66 and passages 59, 58. Fluid supply to ram 16, 17 is then stopped until the pressure in the ram has reached predetermined value when plunger 48 again seats against its seat 44<SP>1</SP>. If the regulating control apparatus 23 is used to control tractive forces and position of the towed implement, a maximum pressure (for opening sensor 45) in excess of the predetermined nominal pressure is set by screw 54. The pressure control device 26 then acts as a safety valve for the regulating control apparatus 23.