GB896859A - Improvements in or relating to hydraulic control mechanism - Google Patents

Abstract

896,859. Fluid-pressure servomotorcontrol systems. MASSEY - FERGUSON (UNITED KINGDOM) Ltd. Sept. 3, 1958 [Sept. 3, 1957], No. 27746/57. Class 135. [Also in Group XXXI] A tractor with an hyraulic powerlift mechanism has a piston 13 and cylinder 12 for raising, lowering or holding stationary a hitch-linkage connected thereto, a control valve 23 which is between the cylinder and a pump 17, which is subject to joint manual and automatic control and which is movable into an equilibrium position to supply hydraulic fluid to the cylinder and piston at a restricted rate sufficient to maintain a pressure to support the hitch-linkage when loaded; it also has a valve 21 to relieve the pump when it is supporting the load. A gear-wheel pump 17 draws oil through a filter 20 from the tractor sump 18 and feeds it through the valve 21 and a non-return valve 105 to a valve 23. The valve 23 is operated either by lever 38 or lever 39 according as the hand lever 36, to which they are both pivoted, is in its range A-B or its range B-C. In the range A-B the levers set the working depth subject to draft-control through the linkage 45, 46 as described in the Specification 896,858, in this Group, by causing the piston-rod 14 to rotate an arm 15 on a cross-shaft 16 which carries lift arms connected to the hitch linkage (not shown). In the range B-C the shaft 16 is moved similarly but, control is exercised by a cam 81 and stud 82 which serve to lift the lever 39 from the valve 23 when the shaft 16 reaches a position corresponding to the setting of lever 36. When the spool 31 of valve 23 is fully raised oil at pressure is fed through a non-return valve 24 to the cylinder 12 excess oil is discharged through the valve 21. At " draft control " the lever 38 depresses the spool so that most of the oil is returned to the sump 18 through the orifice 26 and the load on the hitch-linkage forces oil from the cylinder under the valve 24, past a check valve 125 which closes if the flow is too rapid and out through an adjustable valve 121 to the orifice 26. If the load drops excessively the draft-control gear operates to cause lever 38 to release the spool 31, so that after alternate rising and falling a position of equilibrium is reached with the outlet 26 almost closed and the oil being delivered to the cylinder at a restricted rate. Operation at " position control " is similar, but the spool 31 is brought to its equilibrium position by action of the cam 81 on the lever 39. In neither instance does the equilibrium position correspond to any fixed position of the hand lever. The relief valve 21 relies for its operation on the pressure differences between the top 94 and bottom internal surfaces of its piston due to the hole 101. When the difference is sufficient to overcome the spring 95 the valve rises and snaps open due to additional pressure of the oil which reaches the underside of the piston. If desired, the valve 21 can be reset by opening the valve 121 by hand. Specification 896,860 also is referred to.