GB720461A - Improvements in or relating to a hydraulic control system - Google Patents

Abstract

720,461. Hydraulic presses; fluid-pressure servomotor control systems. UNITED ENGINEERING & FOUNDRY CO. March 11, 1949 [March 18, 1948], No. 6713/49. Classes 69 (2) and 135. A control system for a hydraulic press having a main cylinder 4 and balancing cylinders 5 for the movable head 3 of the press, the main cylinder having a greater area than the balancing cylinders, comprises a pump 9 for supplying a constant volume of fluid at varying pressures to the main cylinder 4, a constant source of fluid 12 for the balancing cylinders and a non-return valve 27 interconnecting the balance cylinders and the main cylinder to effect rapid lowering of the movable press head. The cylinder 4 is connected to the supply pump 9 through a valve 22 and the cylinders 5 are under constant pressure from the source 12 which comprises an accumulator fed by an auxiliary pump 14, each pump drawing from a common sump 15; a relief valve 21 is connected between the outlet of the pump 9 and the sump 15. A chain 58 secured at one end to the platen 3 passes round a sprocket 57 and has a weight 62 secured at the other end. The valve 22 comprises a main pressure duct 8a and a relief duct 18a controlled by a valve 25 connected to a servomotor 41. A pipe 29 communicating with the cylinders 5 is connected to the duct 8a through the non-return valve 27 which is normally held on its seat by a spring-pressed rod 36 which is connected to a piston 34 in a cylinder 33. The servomotor 41 is connected to a pump 44 by pressure line 45 and exhaust line 46 and comprises a casing 63, Fig. 2, formed with chambers 91, 92 and 87 communicating with outlet pipe 46 and inlet pipe 45 respectively, and a bore forming a cylinder 66 for a piston 64, the stem 64a of which slides in a bore in the casing 63. An internal valve 65 having a central bore 65a is formed with lands 83, 84, 85, and slides in a bore 71 in the stem 64a which bore is formed with chambers 93, 89, 94 which communicate by radial bores with the chambers 91, 87, 92, respectively. The stem 64a has ducts 102, 101 communicating with the bore 71 and with the lower and upper surfaces of the piston 64 respectively. A pipe 39 connects the upper cylinder space 66 to the cylinder 33 controlling the valve 27. A connection 105 to the lower surface of the piston 64 is plugged. An extension 47a of a control element 47 passes through clearance holes 74 in the stem 64a and slides in a bore 73 in valve 65. Lateral movement of the element 47 causes vertical movement of the valve 65. The stem 64a is connected by an extension 43 to the valve 25. The element 47 is connected to a link 49, Fig. 1, carrying a roller 52 which bears, under the influence of a spring 54, on a cam 53 secured to the sprocket 57. A link 51 is pivoted to the link 49 and to a hand lever 48. In the neutral position of the valve 65 the liquid in pipe 45 and ducts 102, 101 is by-passed to the outlet pipe 46. Movement of the element 47 to the right causes the valve 65 to move down and thereby progressively connect the duct 101 to the supply line 45 and duct 102 to exhaust line 46, causing the piston 64 to move down tending to close the valve 25 and the press decends opposed by the pressure in the cylinders 5. Downward movement of piston 64 returns the valve 65 to a relative neutral position and movement of piston 64 ceases. When the valve 65 is at the top of its stroke full pressure is applied to the piston 64 and the valve 25 is fully closed. This full pressure is sufficient to actuate the piston 34 to remove the spring pressure from the valve 27 which permits the cylinders 5 to vent the line 8a. The press is raised by moving the element 47 to the left which supplies liquid to the opposite end of the piston 64 to open the valve 25, the spring 37 closing the valve 27. Movement of the element 47 by the lever 48 is counteracted by the cam 53; the lever 48 must therefore be kept in motion to complete an operation. Adjustable throttle 20 may be included in the line 29 to equalize the speed of raising and lowering. In a modification a second larger valve is fitted in the sump return line in addition to the valve 25, in order to allow rapid raising of the press. This valve is operated by the pressure from the connection 105. The servomotor may be designed to give the operator a better feel of the assistance provided by the servomotor. Specifications 591,589, 656,487 and 720,476 are referred to.