Patent GB-1396926-A (GB)
Title: Control device for load-independent regulation of hydraulic  consumers
IPC: F15B13/02
Filed: 1972-06-14
Abstract: 1396926 Fluid-pressure servosystems KOPPEN & LETHEM TRADING CO AG and APPINGEDAMMER BRONSMOTORENFABRIEK NV 14 June 1972 [29 June 1971] 27853/72 Heading G3P [Also in Divisions Fl and F2] Each of a bank of spool valves 2a-2c mounted between a valve block 1 and an end-plate 3 comprises a manually-operable spool 10 adapted to connect either of work ports 6, 7 to supply passage 48 whilst simultaneously connecting the other work port to a respective exhaust passage 50a or 50c, the latter passages being inter-connected by passage 50b in end-plate 3, Fig. 6. Spool 16 in block 1 is adapted to maintain the pressure in supply passage 48 a constant amount greater than the highest pressure prevailing in the work ports currently connected to the supply passage, and in the embodiment shown this is achieved by subjecting the lower end of the spool to supply pressure via passages 17, Fig. 6, and the upper end to a pilot pressure in chamber 52 representing the highest work port pressure, the latter pressure being assisted by spring 19. Spool 16 thus bypasses fluid from inlet chamber 46 to exhaust chamber 47 to maintain a constant differential between supply and pilot pressures. The pilot pressure is established by pressure reducing valves 8, 9 associated one with each work port. Upon initial displacement of a spool 10, work port pressure is applied via 61, 63 or 62, 64 to a respective valve 8 or 9 where it acts to depress a spool 28, Fig. 5A, against spring 34 to allow flow via 68, 69, to passage 70 (in the case of each valve 8) or 71 (in the case of each valve 9). The pressure downstream of each spool 28 acts via 69a to tend to reclose the spool so that each spool seeks to establish a pilot pressure in passage 70 or 71 which is less than the respective work port pressure by an amount determined by spring 34. Passages 70, 71 communicate with each other and transmit the highest pilot pressure to chamber 52 via an adjustable restrictor (44). In order to allow the pilot pressure in chamber 52 and passages 70, 71 to dissipate when the work port pressure falls, a constant leakage flow from passages 70, 71 is maintained by spool 37, Fig. 7, through which fluid leaks to chamber 79b and thence to exhaust passage under the control of land 40. Spool 16 is also under the control of a pilot valve 22, Fig. 6,  which relieves excess pressure in chamber 52. In a modification each valve 8, 9 uses a lift-type closure member instead of a spool, and spool 16 may alternatively control flow from chamber 47 to chamber 46, connection 5 then serving as an inlet while connection 4 is blanked-off. In this case, exhaust passages 50a-c discharge via an opening in end-plate 3 instead of via chamber 47. In a further modification, Fig. 17 (not shown) passages 50a-c again discharge via an opening in the end-plate but spool 16 is arranged, as in Fig. 6, to control by-pass to chamber 47 which is disconnected from passage 50c and from which bypass fluid returns to drain via a restrictor. The pressure upstream of the restrictor acts on the displacement-control piston of a pump to adjust the pump delivery in the sense to minimize bypass flow. A development of this system, Fig. 18 (not shown) also incorporates in intermediate plate (108) a further pilot valve (28a) which is connected to passages 70, 71 and transmits the pilot pressure to chamber 52 modified in accordance with the pump delivery pressure which is sensed by a spring-loaded piston controlling a potentiometer which adjusts an electrical load applied by a solenoid armature to the pilot valve 28a.