Patent GB-681817-A (GB)
Title: Hydraulic control mechanism
IPC: F15B13/04
Filed: 1949-06-29
Abstract: 681,817. Hydraulic transmission of power ; fluidactuated valves. CATERPILLAR TRACTOR CO. June 29, 1949 [March 28, 1949], No. 17172/49. Classes 69 (ii) and 135. In a hydraulic circuit comprising a liquid supply tank 10, Fig. 2, a double-acting hydraulicallyactuated jack 15, a control valve 12 for controlling the flow of liquid to and from the jack through a pair of power lines 13, 14, each of which communicates with a different end of the jack, and means for preventing the return of liquid from either end of the jack to the tank at a greater rate than that at which the liquid is directed to the opposite end of the jack comprising a passage 61, Fig. 1, through which all the liquid displaced from either end of the jack must pass in returning to the tank 10, there is provided a counterbalance valve 62, disposed within and normally closing the passage 61 and means, such as a piston face 63 on the end of the counterbalance valve 62, actuated by liquid pressure from the pump for moving the valve 62 to its open position permitting liquid flow through the passage when liquid pressure is in the power line directing liquid from the pump to the jack. The hydraulic jack 15, Fig. 2, is pivotally attached to a tractor or the like whilst its piston rod 16 is similarly connected to the bulldozer blade 17 or other implement. The control valve member 21 which slides in bore 20 is operated through linkage from a manually-operated rocking shaft 40 extending through and rotatable in a wall of the tank 10. The member 21 is normally urged towards its neutral position by a spring 42, Fig. 1, that is enclosed for a part of its length by a sleeve 49 which serves to limit the axial movement of the member 21 ; the latter has four positions namely "neutral", "float", "raise" and "lower" each of which provides for a different position of implement. When the member 21 is moved to the "float" position it is held against the spring 42 by roller 50, Fig. 2, engaging a detent 52 in a bell-crank lever 53, loaded by a spring 54 which opposes further leftward movement of the member 21 and so enables the operator to identify the "float" position by feel. Liquid returning to the tank 10 is discharged from manifold 81 of large cross-section with two outlets 82 and 83 in order to reduce the discharge velocity and minimise turbulence in the tank. The counterbalance valve comprises a member 62, Fig. 1, sliding in a bore 60, the member 62 being urged to the closed position shown by a spring 65 and opened by the pressure liquid from the pump, flowing to the jack 15, acting on the member 62 after traversing a pipe 66 so that exhaust liquid from the jack flows to the tank through metering notches 67 in member 62 only when there is sufficient pressure in one of the lines 13 or 14 to compress spring 65. The other end of pipe 66 may be connected to either of lines 13 and 14 by a shuttle valve, the sliding valve member 72, Fig. 2, of which is loaded by a spring 77, the valve being opened by liquid from passage 69 under the control of member 21. When the implement is lowered a smaller volume of liquid is displaced, due to the piston rod, than is required to fill the other side of the jack and a lower pressure results in bore 20 than in the tank whereupon extra liquid is taken from the tank through a pipe 78, Fig. 1, which communicates with bore 60, the pipe 78 being normally closed by a check valve 79 which is so loaded that it opens when a pressure differential occurs between bore 20 and the tank. The maximum pressure in the lines 13 and 14 is limited by a relief valve 85 in a passage 86 providing communication between bore 60 and recess 30 which opens into the tank, the passage having a restricted orifice 80 to reduce the pressure upon actuation of the relief valve 85. Passage 86 communicates with a bore 87 that joins two recesses 30 and 31 with which the member 21 co-operates ; the bore 87 is, however, normally closed by a dump valve 88 so that the pump output is directed to the tank after the pressure has been reduced by the orifice 90 and the dump valve thereby opened. The dump valve, which is urged to the closed position by a spring and liquid from one of the power lines and is moved to the open position by liquid from the pump, also serves to relieve excessive pressure when during movement of member 21 the spacing of collars 23 and 27 therein may temporarily prevent the power lines and return passages from being in communication with each other or the pump. In a modification, the member 21 has a counterbore at each end which houses a spring-loaded check valve, the two valves controlling the liquid displaced from the jack and being operated by the pressure liquid. Two of the collars of the member 21 are provided with metering notches and the pump incorporates a relief valve. The shuttle and counterbalance valves are dispensed with.