Patent GB-783219-A (GB)
Title: Improvements in hydraulic control systems, in particular for operating a tractor rock shaft
IPC: F15B9/12
Filed: 1955-08-09
Abstract: 783,219. Fluid-pressure servomotors. SOC. INDUSTRIELLE GENERALE DE MECANIQUE APPLIQUEE S.I.G.M.A. Aug. 9, 1955 [Aug. 28, 1954], No. 22890/55. Class 135. In an hydraulic control system including a jack 2 the liquid supply to which is controlled by a main valve 20 operated by liquid controlled by a pilot valve 10, and mechanical means 16, 17 operable by the jack 2 to return the pilot valve 10 into neutral position in response to a given displacement of the relatively moving parts of the jack resulting from a displacement of the pilot valve 10 from the neutral position, the pilot valve 10 is movable in an hydraulic circuit at a pressure which is constantly lower than the maximum pressure of the circuit controlled by the main valve 30. The jack cylinder is pivoted on a tractor whilst the piston rod 5 acts on a power lift device via a crank 8 on a shaft 9. The pilot valve 10 slidable in a casing 11 is pivotally connected to an intermediate point of a lever 16 the ends of which latter are connected to a link 17 pivoted on the rod 5 and a manual control lever 12 in a guide 13. The displacements of the lever 12 are defined by a fork-shaped member 19. With the apparatus in the position of rest shown a pump 1 draws in oil from the reservoir 43 through a filter 45 and a conduit 44 and discharges it through conduits 21, 22 to the valve 20 from where it traverses conduits 23 and 24 and a spring-loaded valve 42 before re-entering the reservoir 43, the valve 42 being adjusted to give rise to a low pressure which is transmitted to a conduit 32. Upon moving the lever 12 leftwards to obtain upward movement of an implement, not shown, carried by the shaft 9 the valve 10 is urged to the right so that low-pressure oil from conduit 32 traverses a conduit 34 and acts upon the right end of valve 20 thus moving the latter to the left and compressing a centralizing spring 41; the oil in the chamber containing the spring 41 escapes to the reservoir by way of a conduit 35, valve 10 and a conduit 37. The supply of oil from the pump to conduit 23 is thereby cut-off and directed instead across a groove 25 into a conduit 26 and through a one-way valve 46 to act on the larger effective diameter of piston 3; the resistance offered to movement of the piston 3 causes the pressure in the circuit to rise. The oil behind the piston 3 exhausts through a conduit 27, groove 28 conduits 29 and 24 and both enters the reservoir and maintains the pressure in the low-pressure conduit 32; as the piston 3 completes its stroke it returns the valve 10 and hence valve 20 to their neutral positions. Positioning the lever 12 to the right of the position shown brings about leftward motion of the valve 10 and rightward movement of valve 20 and piston 3 to lower the implement, the movement of which is retarded due to the oil from the jack having to pass through a calibrated orifice in valve 46; a portion of the oil returning to the reservoir 43 passes through a safety valve 30 since the speed of the implement is the same for both up and down movements thereof whilst for the latter the effective area of the piston is smaller than that used for upward travel. The valves 10 and 20 are again returned to their neutral position shown as the piston 3 completes its stroke. The lever 12 may occupy any position between the two extremes on either side of the neutral position shown and the apparatus may be used to control more than one jack.