A hydraulic control system

Abstract: A hydraulic control system (10) that comprises a hydraulic pressure line (20), a hydraulic fluid control mechanism (22-28) and a lag network coupling the pressure line to the control mechanism for restricting flow of hydraulic flow therethrough, and thereby delaying and damping response of the control mechanism (22-28) to fluid pressure fluctuations at the hydraulic line (20). The lag network comprises check valve (30,32,50) that includes a flow passages interconnecting the hydraulic line (20) and the control mechanism, a valve element (34), and a spring (36) resiliently urging the valve element (34,40) to close the passage, such that resistance to fluid flow increases as fluid flow decreases between the hydraulic pressure line (20) and the control mechanism (22-28). Preferably, a pair of such check valves (30,32,50) are connected in parallel between the hydraulic pressure line and the control mechanism for controlably restricting fluid flow in both directions. To offset decreasing resistance as a function of increasing fluid flow through the check valve or valves, and orifice that exhibits increasing resistance as function of fluid flow may be connected in series with the valves.

Description: The present invention relates to a hydraulic control system according to the preamble of claim 1. In a hydraulic control system of that kind (DE-B 12 67 011) a compensator as a hydraulic control means is connected through a control duct to pump displacement control means. The control duct inductes a pair of check valves in an antiparallel arrangement, and each a needle valve connected in series to a respective one of the check valves, so that the control duct can be termed a lag network means. According to the description of DE-B 12 67 011, the needle valve provide orifices which determine the behavior of the control duct. Furthermore, the check valve suppress small pressure fluctuations in the pressure signal from compensator to pump. There are many instances in hydraulic control systems in which a lag network is connected between a hydraulic pressure line and a fluid control mechanism for restricting flow of fluid to the mechanism and thereby delaying and damping response of the control mechanism to fluctuations in fluid pressure at the fluid line. One example where a lag network of this character might be employed is in load-sensing pressure compensation of a variable displacement hydraulic pump of the type disclosed in U.S. Patent No. 4,695,230. Delay and damping of the compensation control system helps eliminate pressure pulsations to the control mechanism, and thereby helps prevent oscillating movement of the pump displacement control under heavy pump load. Conventionally, a lag network of the subject character comprises an orifice positioned in the hydraulic line to cooperate with a volume downstream of the orifice, formed by the line itself or by a separate accumulator, to restrict fluid flow. Such orifice/volume combination exhibits the desirable characteristic of attenuating the oscillating pressure on the volume resulting from oscillating flows passing through the orifice resistance. However, the orifice resistance to fluid flow is highly non-linear, and approaches zero as total fluid flow approaches zero. (The term "orifice resistance" refers to incremental resistance - i.e., a change in pressure divided by a change in flow about some steady operating flow through the orifice.) Thus, the filtering or attentuating property of the orifice/volume network fails at low flows because the orifice resistance approaches zero. It is therefore a general object of the present invention to provide a hydraulic control system embodying a lag network between the primary fluid pressure line and the fluid control mechanism that exhibits or is characterized by a resistance to fluid flow that increases as fluid flow approaches zero. Another and related object of the invention is to provide a system of the described character that maintains resistance at low flow as described, while at the same time exhibiting either constant resistance or increasing resistance to flow as flow increases. A further object of the invention is to provide a variable displacement pump control system that includes a pressure compensation network embodying such a lag network for controlling pump displacement as a function of load pressure. Yet another object of the invention is to provide a bidirectional check valve for in-line connection in a hydraulic fluid system that achieves a more nearly constant resistance with changes in fluid flow, particularly at low flow. The problem mentioned above is solved by the teaching of claim 1. A hydraulic control system in accordance with a first important aspect of the present invention comprises a hydraulic pressure line, a hydraulic fluid control mechanism and a lag network coupling the pressure line to the control mechanism for restricting flow of hydraulic flow therethrough, and thereby delaying and damping response of the control mechanism to fluid pressure fluctuations at the hydraulic line. The lag network comprises a check valve that includes a flow passage interconnecting the hydraulic line and the control mechanism, a valve element, and a spring resiliently urging the valve element to close the passage, such that resistance to fluid flow increases as fluid flow decreases in the hydraulic line feeding the control mechanism. Preferably, a pair of such check valves are connected in parallel between the hydraulic pressure line and the control mechanism for controllably restricting fluid flow in both directions. To offset decreasing resistance as a function of increasing fluid flow through the check valve or valves, an orifice that exhibits increasing resistance as function of fluid flow may be connected in series with the valves. This technique tends to linearize further the pressure drop/flow characteristics of the combination. In accordance with a second important aspect of the present invention, a pressure compensated variable displacement hydraulic pump control system comprises a variable displacement hydraulic pump including a displacement control yoke and a fluid output. A hydraulic pressure line is connected through a control valve system to the pump output, and a compensation network is responsive to fluid pressure at the pressure line to control displacement of the pump. A check valve, preferably a pair of parallel reversed check valves as previously described, are connected between the fluid pressure line and the pressure compensation mechanism for restricting and damping fluid flow to the control mechanism. Thus, pressure fluctuations at higher frequencies are isolated from the compensation network, subsequent oscillations at the pump output and load are reduced, and a higher degree of pump stability is obtained. In accordance with a third important aspect of the present invention, the parallel oppositely-poled check valves previously described are provided in a single bidirectional hydraulic check valve assembly that includes a housing having an internal cavity with fluid openings at axially opposed ends. A first valve element comprises a cup-shaped sleeve having a base adjacent to one axial end of the cavity and a sidewall axially slidably embraced by the housing within the cavity. A first fluid passage extends through the base of the sleeve adjacent to one of the cavity openings. A second valve element comprises a spool telescopically slidably received within the sidewall of the sleeve. A second fluid passage extends through the spool end from adjacent the second end of the housing cavity to internally adjacent the sidewall of the sleeve. A fluid passage is formed between the radially opposing surfaces of the sleeve and the spool for passing fluid therethrough as a function of axial position of the sleeve and spool with respect to each other. A coil spring is capture in compression between the sleeve and spool so as to urge the valve elements toward respective ends of the housing cavity. In the preferred embodiment of the invention, the fluid passage between the radially opposing surfaces of the sleeve and spool comprises at least one channel, and preferably a pair of diametrically opposed channels, formed in the outer wall of the spool. The channel or channels have a cross section to fluid flow that varies as a function of axial position of the valve elements with respect to each other. To restrict fluid passage at high fluid flow rates, one or both of the housing fluid openings may comprises a damping orifice of preselected diameter. Brief Description of the Drawings The invention, together with additional objects, features and advantages thereof, will be best understood from the following description, the appended claims and the accompanying drawings in which: Fig. 1 is a hydraulic schematic diagram of a pressure-compensated variable displacement pump control system in accordance with the present invention, Fig. 2 is a sectioned side elevational view diametrically bisecting a bidirectional check valve in accordance with a presently preferred embodiment of the invention, and Fig. 3 is a fragmentary view on an enlarged scale of that portion of Fig. 2 by the line 3. Fig. 1 illustrates a pressure-compensated variable displacement hydraulic pump control system 10 in accordance with one presently preferred embodiment of the invention as comprising a variable displacement pump 12 having a swash plate 14 movable to vary the stroke of the pump pistons. Pump 12 feeds fluid under pressure from a sump 16 through a system 18 of control valves to a fluid pressure line 20 for direction to a hydraulic load (not shown). A piston 22 is urged by a coil spring 24 and by the pressure of fluid in line 20 to position swash plate 14 for maximum pump displacement. A larger yoke-positioning piston 26 acts on swash plate 14 in opposition to piston 22. A system of load-sensing and pressure-compensation spool valves 28 receives hydraulic pressure from the output 13 of pump 12 and from fluid pressure line 20, and forms a compensator output pressure as a function of the pressure in line 20. The compensator output pressure acts on larger piston 26 to control position thereof. US-A-3,554,093 discloses a typical pump 12 of the type illustrated in Fig. 1. To the extent thus far described the pump control system of Fig. 1 is as illustrated, and described in greater detail in US-A-4,695,230, the disclosure of which is incorporated herein by reference. In accordance with the present invention, a pair of oppositely-orientated or oppositely-polarized check valves 30, 32 are connected in parallel between fluid line 20 and line 29 to load-sensing/pressure-compensation system 28. In particular, valve 30 comprises chamber 33 and a poppet element 34 that is resiliently urged by a coil spring 36 against a seat 38 for blocking flow of fluid from line 20 to line 29, while valve 32 comprises a chamber 39 and a poppet element 40 resiliently urged by a coil spring 42 against a seat 44 for blocking flow of fluid from line 29 to line 20. However, each valve 30, 32 permits flow of fluid opposite to such check direction, with the resistance to fluid flow varying as an inverse function with magnitude of fluid flow. That is, resistance of valve 32 to fluid flow from left to right (in the orientation of Fig. 1) approaches infinitly at zero flow (and a negative flow), but decreases with fluid flow from left to right that urges element 40 away from seat 44 against the force of spring 42. Likewise, resistance of valve 30 to fluid flow from right to left approaches infinitly at zero (and negative) fluid flow, but decreases with increasing fluid flow against the force of spring 36. Figs. 2 and 3 illustrate a bidirectional check valve assembly 50 in accordance with a presently preferred embodiment of the invention that combines both check valves 30, 32 of Fig. 1 into a unitary assembly. Valve assembly 50 comprises a cylindrical housing 52 having a pair of end plugs 54, 56 threadably received therewithin. End plugs 54, 56 have respective diametric channels 55, 57 on the inbound faces thereof. Housing 52 and end plugs 54, 56 together define an axially oriented internal fluid cavity 58 that is open at opposed cavity ends through coaxial fluid openings 60, 62 in end plugs 54, 56. A pair of check valve elements 64, 66 are telescopically slidably disposed within cavity 58. Valve element 64 comprises an end cap 68 threaded into one end of a hollow cylindrical sleeve 70. Cap 68 has an end flange 72 that captures a sealing ring 74 against the opposing end of sleeve 70 to form the generally cup-shaped contour of valve element 64 having a left-hand annular end surface 98 and a right hand circular end surface 100. A fluid passage 76 extends through cap 68 coaxially with housing 52 and fluid openings 60, 62. Valve element 66 is a piston or spool also of cup-shaped form having a cross-sectional area equal to the sum of right hand end surfaces 102 and 104 or the hollow 71 in the sleeve 70. Spool 66 is telescopically slidably positioned within sleeve 70. A T-shaped fluid passage 77 includes a central passage 78 that opens adjacent to and coaxially with opening 60 in plug 54, and a pair of diametrically oppositely oriented passages 80, 82 that extend from central passage 78 to the sidewalls of spindle 66 adjacent to the opposing inner wall surface of sleeve 70. A pair of channels or slots 85, 86 extend axially from the ends of passages 80, 82 along the outer surface of spool 66 toward a cylindrical shoulder 88 on the opposing inner wall surface of sleeve 70. Channels 84, 86 taper narrowingly from passages 80, 82 toward shoulder 88. A coil spring 90 is captured in compression between an internal pocket 92 in end cap 68 and an opposing internal pocket 94 in spool 66. Coil spring 90 thus urges valve elements 64, 66 axially outwardly toward abutment with associated end caps 56, 54 to the zero-flow position illustrated in Figs. 2 and 3. In operation, and first assuming fluid pressure from left to right in the orientation of Figs. 2 and 3, fluid enters cavity 58 through end plug 54 and passage 60. The pressure of hydraulic fluid against the opposing end surface 96 of spool 66 urges spool 66 to the right in Fig. 2 against the force of coil spring 90. At the same time, pressure against annular end surface 98 of sleeve 70, which preferably is equal in area to end surface 96 of spool 66, cooperates with spring 90 to urge valve element 64 to the right against the opposing face of plug 56. As spool 66 moves to the right under force of fluid pressure, channels 84, 86 begin to overlap shoulder 88, so that fluid flows through the channels past shoulder 88 into the volume between spool 66 and end cap 68, and then through passage 76 and opening 62 out of the valve assembly. Increased fluid pressure from left to right increases motion of spool 66 to the right, permitting greater fluid flow through channels 84, 86. It will be appreciated that the tapering contours of channels 84, 86 illustrated in the drawings are merely exemplary, and that other channel configurations and geometries may be employed to obtain fluid flow of any desired characteristics. In like manner, and this time assuming fluid pressure from right to left in Fig. 2, pressure on the end surface 100 minus the cross-sectional area of spool 66 urges valve element 64 to the left in Fig. 2. Preferably, the cross-sectional area of spool 66 is equal to the annular surface 98 (= area 100 minus areas 102 + 104) or half of circular surface 100. As fluid pressure increases, shoulder 88 is brought into radial registry with channels 84, 86, so that fluid can flow through the channels, through passages 80, 82, 78, and thence through opening 60 of plug 54. Thus, valve assembly 50 effectively functions as parallel check valves of opposite polarity of the character schematically illustrated at 30, 32 in Fig. 1. The channels or slots 84 and 86 represent restrictors, and one of them (or the clearances) can be shaped as an orifice of predetermined dimension thus assuring a minimum opening that is steadily effective to transmit pressure from line 20 to line 29 with a predetermined time-lag also when the other channel is closed. It is also possible to add an orifice in series to the check valves so thath the hydraulic arrangement exhibits inclreasing resistance as function of fluid flow.

Claims: A hydraulic control system that comprises a hydraulic pressure line (20), hydraulic fluid control means (28) and lag network means including check valve means (50) for restricting flow of hydraulic fluid therethrough; characterized in that said check valve means (50) comprises a housing (52, 54, 56) that includes means defining an internal cavity (58) having an axial dimension and fluid openings (60, 62) at axially opposed ends of said cavity, a first cup-shaped spool (64) having a base (100) adjacent to one axial end of said cavity (58), a sidewall (70) defining a hollow (71) and axially slidably embraced by said housing (52) within said cavity (58) and first fluid passage means (76) extending through said base (100) into said hollow (71), a second spool (66) telescopically slidably embraced within said sidewall (70) of said first spool (64) and second fluid passage means (77) that extend through said second spool (66) from one end (78) to said hollow (71) of said sidewall (70), third fluid passage means (84, 86, 88) between said first and second spools (64, 66) variably connecting said second passage means (77) to said first passage means (76) as a function of axial positions of said spools (64, 66) with respect to each other, and spring means (90) captured between said first (64) and said second spool (66) and urging said spools (64, 66) toward respective ends of said cavity (58). The system set forth in claim 1 wherein said third fluid passage means (84, 86, 88) comprises an internal shoulder (88) in said sidewall (70) of said first spool (64) positioned to close said third passage means when said spools (64, 66) are positioned by said spring means (90) at said axially opposed ends of said cavity (58). The system set forth in claim 2 wherein said shoulder (88) is axially spaced from the inner end (80, 82) of said second passage means (77) when said spools (64, 66) are positioned at said opposed ends of said cavity (58), and wherein said third passage means (84, 86, 88) further comprises a channel (84, 86) in ther outer surface of said second spool (66) opposed to said sidewall (70) and extending longitudinally from said second end (80, 82) of said second passage means (77) toward said shoulder (88), said channel (84, 86) having a cross section to fluid flow that varies longitudinally thereof. The system set forth in claim 3 wherein said second passage means (77) comprises a T-shaped passage having diametrically opposed second ends (80, 82), and wherein said third passage means (84, 86, 88) comprises a diametrically opposed pair of said channels (84, 86) in said outer surface of said second spool (66). The system set forth in any of claims 1-4 wherein said check valve means (30, 32, 50) interconnects said pressure line (20) and said control means (28) through a flow passage, a valve element (34, 40; 66) of said check valve means being resiliently urged to close said passage so as to exhibit increasing resistance to fluid flow as fluid flow decreases between said pressure line (20) and said control means (28) and delaying and damping response of said control means (28) to fluid pressure variations at said pressure line (20). The system set forth in claim 5 wherein said check valve means comprises a pair of said valve elements (34, 40) positioned in said passage and coupled to said spring means (36, 42) for restricting flow in opposite parallel directions through said passage. The system set forth in any of claims 1 to 6 further comprising a variable displacement pump (12) that includes pump displacement control means (26) coupled to said hydraulic fluid control means (28) and a pump output (13) supplying said pressure line (20) through a valve system (18). The system set forth in claim 7 wherein said hydraulic fluid control means (28) comprises means for controlling said displacement-control means (26) as a function of pressure in said pressure line (20). The system set forth in any of claim 1 to 8 wherein said lag network couples said hydraulic pressure line (20) to said hydraulic control means (28) and comprises an orifice that exhibits a resistance to fluid flow that increases as a function of fluid flow. The system set forth in any of claims 7 to 9 wherein said control means (28) comprises a compensator responsive to fluid pressure to control said displacement control means, and wherein said hydraulic pressure line (20) is connected to a load.

Inventors: NANDA VINOD K, TAPLIN LAEL B, NANDA, VINOD K., TAPLIN, LAEL B.
Applicants: VICKERS INC, VICKERS INCORPORATED
Application year: 1990
Main classification: F04B49
IPC codes: F04B1, F04B49