GB851340A - Improvements in hydraulic control systems for agricultural tractors - Google Patents

Abstract

851,340. Tractor power-lift mechanisms. SOC. INDUSTRIELLE GENERALE DE MECANIQUE APPLIQUEE S.I.G.M.A. Apr il 10, 1959 [April 14, 1958], No. 12266/59. Class 79(1). In a hydraulic power lift system of a tractor, valve means are provided for selecting automatic load transfer to the tractor wheels from the plough &c. by a system sensitive to increase of draft force and including a second valve means operable by the driver to obtain load transfer when the wheels slip under conditions in which the automatic device does not operate as when travelling over slippery ground &c. The system may be applied to a tractor having a double acting lift cylinder 14 Fig. 3 which is manually positioncontrolled by a lever 32 through a servo valve 24 with a follow-up linkage 34, 38 coupled to the lift piston 18 In this case the supply lines 22, 23 to opposite ends of the cylinder 14 are closed by non-return valves 25, 26 which are operable by pistons 30 for return flow. The non-return valves 25, 26 may however be replaced by a shuttle valve or the system may be applied to a single acting ram. The additional valve means for load transfer comprises a valve 39 operable manually by a lever 65 and a second valve 49 having a draft responsive control rod 50. The valve 39, Fig. 4, normally permits pressure fluid to flow from the pump 12 through line 21 to the positioncontrol servo valve 24 but includes a plunger 40 which closes the line 21 on initial movement of the lever 65 to the position (shown) for selecting load transfer conditions. In this position the supply line 21 is coupled through lines 22a, 23a to the lift cylinder 14 but pressure build-up to operate the cylinder by unbalance is prevented due to the escape of fluid to a return line 43 through a central bore 44 over restriction slots 64. Further movement of the valve by the lever 65 against a spring 66 gradually closes the restriction between the slots 64 and a bore portion 62 to cause a build-up of pressure with consequent partial operation of the lift linkage. The rise in pressure is limited, to a value below that which causes lifting of the plough, by a calibrated orifice 70 which permits escape of pressure fluid to a central bore 68 and return line 69. The automatic load transfer control valve 49 is coupled to the return line 43 through a short-circuiting valve 67 and comprises restriction slots 58 cooperating with a bore 57. Movement of the rod 50 against an adjustable spring 53 results in build up of pressure to operate the lift cylinder which is again limited by the calibrated orifice 70.