Abstract:
A multifunction hydraulic flow metering valve and a circuit therefore comprising a main valve, a bleed flow orifice, and a servo valve acting with the main valve provides for proportional speed control for lowering loads and as an anti-cavitation check valve. A pilot valve in the circuit acting with the main valve provides the circuit with a pressure limiting relief valve.

Description:
This application discloses and claims matter disclosed in copending U.S. patent application Ser. No. 024,058, filed Mar. 26, 1979 now U.S. Pat. No. 4,201,052, having a common assignee with the present patent application. 
     BACKGROUND AND SUMMARY 
     This invention relates to a hydraulic flow metering circuit and a metering valve therefor and particularly to such circuits and valves having multiple functions. 
     In the control of return fluid flow from loads applied to hydraulic actuators such as cylinders on earthmoving and construction vehicles, it is customary to provide spool-type valve elements with each valve element serving various individual functions such as speed control of lowering loads, limiting excessive pressure, preventing or minimizing cavitation in the hydraulic cylinders, and holding a load stationary in a preselected position. 
     However, use of spool-type valve elements having a movable spool member lead to certain disadvantages, such as instability of the spool member while throttling fluid flow to decelerate a lowering load resulting in erratic or jerking movements of the load, and drifting of a stationary load from a preselected position due to leakage of fluid past clearances necessary to proper operation of the spool member. 
     The present invention is directed to a circuit comprising a single hydraulic flow metering valve which will function as a proportional speed control valve for lowering loads, a pressure limiting relief valve, and an anti-cavitation check valve. 
     In accordance with the present invention, a poppet type main valve meters return fluid flow from loaded cylinders in response to a variable orifice formed in the main valve, which is controlled by an integral servo valve, and to fluid flow through a bleed flow orifice which is controlled by a pilot valve. The metering valve is designed so that the following three different valve functions can be accomplished: 
     When the main valve is controlled by fluid flow through the variable orifice, the servo valve provides a machine operator with a proportional speed control for lowering loads. 
     If cylinder fluid pressure drops below return fluid pressure in the system, the main valve will open allowing fluid from the return to flow to the cylinder thereby operating as an anti-cavitation check valve. 
     The pilot valve controls bleed flow from the bleed flow orifice through the main valve to provide operation as a pressure limiting relief valve. 
     A fuller understanding of the invention may be had from consideration of the following description and claims taken together with the accompanying drawings. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic of the hydraulic circuit embodying the invention; 
     FIG. 2 is a diagramatic sectional view of a preferred embodiment of the invention; 
     FIG. 3 is another embodiment of the hydraulic circuit of FIG. 1. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, the hydraulic control valve circuit embodying the invention includes a load pressure line 10 adapted to be connected to a source of load pressure and a return flow line 12 adapted to be connected to the return flow of a pump or other source of fluid. A main valve 14 is spring loaded to yieldingly shut off fluid flow between lines 10 and 12. Main valve 14 is controlled by a bleed flow through a bleed flow orifice 18 positioned between pressure line 10 and bleed flow end 16 of valve 14, and a variable orifice, more fully described below, formed between bleed flow orifice 18 and return flow line 12. 
     A pilot valve 20 controls the bleed flow and thereby the opening of main valve 14. Pilot valve 20 includes a pilot piston 22 mounted for movement in a pilot bore 24. Pilot bore 24 includes a pressure end 26 connected to load pressure through line 10, a bleed flow chamber 28 connected to bleed flow end 16 of valve 14, a drain chamber 30 connected to return flow, and a spring end 32 also connected to return flow. Pilot piston 22 is yieldingly urged toward pressure end 26 to shut off flow between bleed flow chamber 28 and drain chamber 30 by a spring member 34 positioned in spring end 32 of pilot bore 24. 
     Main valve 14 is mounted for movement in a main bore 36. Main bore 36 includes a return chamber 38 formed in one end thereof, a load pressure chamber 40 adjacent return chamber 38, a bleed chamber 42 spaced from return chamber 38 by pressure chamber 40, and a control pressure chamber 44 formed at the opposite end of main bore 36. 
     Return chamber 38 is connected to the return flow through line 12 with pressure chamber 40 connected to the load pressure source through line 10, bleed chamber 42 is connected to bleed flow chamber 28 of pilot bore 24 and through bleed flow orifice 18 through bleed flow line 46, and control pressure chamber 44 is connected to a source of control pressure through a control line 48. 
     Main valve 14 includes a main piston 50 mounted for movement in main bore 36. Main piston 50 is formed with a head end 51 having a tapered portion 52 extending into return chamber 38. Tapered portion 52 is adapted for engagement with a main valve seat 54 formed at the juncture of load pressure chamber 40 and main bore 36. A spring member 56 yieldingly urges main piston 50 in the direction of return chamber 38 to seat tapered portion 52 in engagement with main valve seat 54 to shut off fluid flow between load pressure chamber 40 and return chamber 38. 
     Main piston 50 includes a large area or rear end 58 and head end 51 contacts valve seat 54 at a diameter less than that of rear end 58 thereby forming a first differential area which is acted on by fluid pressure in load pressure chamber 40. The portion of head end 51 extending into return chamber 38 forms a second differential area which is acted on by fluid pressure in return chamber 38. 
     Main piston 50 further includes a counterbore 60 formed in rear end 58 and which together with the walls of main bore 36 define bleed chamber 42. A metering passage 62 formed through head end 51 provides a path for fluid flow between bleed chamber 42 and return chamber 38. 
     A poppet type servo valve 64 having a cone shaped end 65 extends from control chamber 44 in the direction of main piston 50 and is yieldingly urged by spring member 56 into seated engagement with metering passage 62 to shut off fluid flow therethrough. Movement of cone shaped end 65 into or out of engagement with metering passage 62 generates a variable orifice therebetween for metering fluid flow between bleed chamber 42 and return chamber 38. 
     A servo piston 66 associated with servo valve 64 is slidably mounted for movement relative thereto in control chamber 44. Servo piston 66 is yieldingly urged by a spring member 68 to resist movement thereof by control pressure in control chamber 44. Under the urging of control pressure in chamber 44 servo piston 66 acts on servo valve 64 against the force of spring members 56 and 68 to unseat the engagement of the servo valve 64 with metering passage 62. 
     METERING OPERATION AS A FLOW CONTROL VALVE 
     Metered flow, from load pressure to return, will be proportional to the applied control pressure, with this proportionality achieved in the following manner. Control pressure, acting on the exposed area of the servo piston 66, generates a force that moves the servo piston 66 and poppet valve 64 in a direction away from main piston 50 against the force of springs 56 and 68. Motion stops when the spring force becomes equal to the control pressure-generated force. Because the springs have a linear rate, displacement of the servo valve 64 will be proportional to the applied control pressure. 
     As servo valve 64 moves, it separates from main piston 50 allowing fluid flow through the variable orifice so formed and through metering passage 62 into return chamber 38. The resulting drop in pressure across metering passage 62 due to the variable orifice reduces the closing force on large area end 58 of main piston 50 to a lesser value than the opening force developed on the first differential area of piston 50 by the load pressure. The main piston 50, therefore, will move following the motion of the servo valve 64. As a steady-state condition, the flow area between the servo valve 64 and the variable orifice must result in a bleed flow rate through the variable orifice that develops a closing pressure on large area end 58 of main piston 50 that exactly balances the opening force on the first differential area of main piston 50. Since the gain ratio between the servo valve motion and bleed chamber 42 pressure is very high, the positional difference between the servo valve 64 and main piston 50 is negligible, and main piston 50 displacement may be considered as being directly proportional to applied control pressure. If the variable metering orifice in the main piston 50 is of constant width, metered flow will be proportional to piston displacement and to applied control pressure, assuming a constant pressure differential between load and return. 
     OPERATION AS A RELIEF VALVE 
     If the load pressure exceeds a predetermined &#34;cracking&#34; pressure, the pilot valve 20 will be displaced to open a flow path from the load pressure through bleed flow orifice 18, to return. This bleed flow reduces the pressure and closing force on large area end 58 of the main piston 50, and in the same manner as described above for flow control, the main piston will open, allowing flow from load to return. If load pressure tends to drop below the cracking pressure, pilot valve 20 will close, causing an increase in the pressure and closing force on the main piston 50. 
     OPERATION AS AN ANTI-CAVITATION VALVE 
     In the type of system for which this valve is intended, load pressure may drop below return line pressure as the result of an overhauling load. To prevent cavitation, the valve is designed to open in response to such a pressure differential, allowing fluid flow from the return line into the load circuit. 
     If return line pressure in return chamber 38, acting on the second differential area of main piston 50, generates a force exceeding the sum of the forces developed by load pressure acting on an equal area and spring 56, main piston 50 will move away from valve seat 54, opening a flow path from return to load. A free sliding fit between servo piston 66, and servo valve 64 and the use of two springs 56 and 68 allows the closing force exerted by servo valve 64 on main piston 50 to be held to a minimum and, therefore, requiring a relatively small pressure differential to open the main valve 14. Note that if servo valve 64 and servo piston 66 were one piece and only one spring having a force equal to the sum of forces of springs 56 and 68 were used, the other two functions of the valve, as described above, would not be significantly affected, but the higher spring force of the single spring would require an unacceptably high pressure differential to overcome it and open the valve. 
     FIG. 2 shows a preferred embodiment of the hydraulic control circuit of the instant invention as a unitary multiple function control valve wherein corresponding elements shown in FIG. 1 are provided with a suffix a. 
     The control valve circuit of FIG. 1 is shown housed in a body 70 which includes a load pressure passage 10a  spaced from a return passage 12a. A main bore 36a  formed in body 70 extends from return chamber or passage 12a  through pressure passage 10a and terminates at a bearing member 72 held in position in main bore 36a by an end cap 74 portion of body 70. End cap portion 74 of body 70 includes a servo bore 76 spaced from main bore 36a by bearing member 72 and terminates at a distal end 78. A pilot bore 24a is formed in body 70 spaced from main bore 36a and includes a pressure end 26a in communication with and extending transverse of pressure passage 10a, a drain chamber 30a, and a spring end 32a spaced from pressure end 26a by drain flow chamber 30a. 
     Body 70 further includes a bleed flow passage 46a  extending between main bore 36a adjacent bearing member 72 and in communication with pressure end 26a of pilot bore 24a through bleed flow orifice 18a adjacent drain chamber 30a, a drain passage 80 interconnecting distal end 78 of servo bore 76 and drain chamber 30a of pilot bore 24a with return passage 12a, and a control passage 48a in communication with servo bore 76 adjacent bearing member 72. 
     A pilot valve 20a is mounted for movement in pilot bore 24a and is yieldingly urged to shut off fluid flow between bleed flow passage 46a and metered flow passage 80. Pilot valve 20a includes a pilot piston 22a extending through pilot bore 24a between spring end 32a and pressure end 26a. Pilot piston 22a is of two-piece construction and includes a metering section 82 and spring end section 83. Section 82 is formed with a tapered portion 84 positioned in drain chamber 30a and seats in bore 24a to form a leakproof seal when system pressure is below the cracking pressure level of the pilot valve. Metering section 82 also includes a shoulder portion 86 extending into pressure end 26a of bore 24a in which one or more metering slots 87 are formed. A reduced diameter portion 88 of metering section 82 spaces shoulder portion 86 from a head portion 90 of metering section 82 defining therebetween a bleed flow chamber 28a and having slots 87 in communication therewith and in which one end of bleed flow passage 46a  terminates. Bleed flow chamber 28a is in communication with pressure passage 10a through a bleed flow orifice 18a formed in head portion 90. 
     Spring end section 83 is held in abutting relationship with metering section 82 by a spring member 34a and is slightly smaller in diameter than shoulder portion 86 and head portion 90 of metering section 82. The differential area formed by the slight differences in diameters is acted on by load pressure in pressure end 22a generating a force tending to open the pilot valve against the force exerted by spring member 34a. 
     Spring member 34a is positioned in pilot bore 24a  between spring end section 83 and an adjustment member 94 in threaded engagement with body 70. Adjustment member 94 provides a means for varying the amount of compression of spring 34a thereby providing for adjusting the cracking pressure level of the pilot valve. 
     A main piston 50a is movably mounted in main bore 36a  to yieldingly shut off fluid flow between pressure passage 10a and return passage 12a. Main piston 50a includes a large area or rear end 58a and a spool section 96 extending from a tapered portion of piston 50a into return passage 12a and terminates at a head end 51a. Tapered portion 52a is adapted for seated engagement with a valve seat 54a formed on body 70 at the juncture of pressure passage 10a and main bore 36a. Spool section 96 is formed with a plurality of radial notches 98 terminating adjacent tapered portion 52a. Tapered portion 52a is proportioned for low leakage when in seated engagement with valve seat 54a. When fully closed, poppet action at tapered portion 52a on the valve seat 54a provides a virtually leak proof seal. As tapered portion 52a moves away from valve seat 54a, main piston 50a behaves as a sliding or spool type valve and fluid flow is metered through radial notches 98. By selective dimensioning of the width of radial notches 98, it is possible to control flow gain through the valve as contrasted to a very high flow gain that would result if the main piston was a pure poppet type valve with only poppet action between the main piston and the valve seat. 
     Fluid pressure in pressure passage 10a acts on main piston 50a on a first differential area formed by tapered portion 52a contacting valve seat 54a at a diameter less than that of rear end 58a. Fluid pressure in return passage 12a acts on a second differential area of main piston 50a formed by the radial surfaces of spool section 96 exposed to the fluid in the return passage. 
     Main piston 50a further includes a counterbore 60a formed in rear end 58a and a metering passage 62a extending from the bottom of counterbore 60a through spool section 96 into return passage 12a. Counterbore 60a, main bore 36a,  and bearing member 72 define therebetween a bleed chamber 42a in main bore 36a which is connected to return passage 12a through metering passage 62a and to bleed flow chamber 28a in pilot bore 24a through bleed flow passage 46a. 
     A servo valve 64a having a stem portion 100 supported for sliding movement in bearing member 72 extends from servo bore 76 through bearing member 72 and terminates in cone shaped end 65a. Cone shaped end 65a is yieldingly urged into engagement with metering passage 62a to shut off fluid flow therethrough by a spring member 77 arranged in bleed chamber 42a between bearing member 72 and a spring seat 104 positioned on stem 100 adjacent cone end 65a.  Movement of cone end 65a into or out of engagement with metering passage 12a generates a variable orifice therebetween for metering fluid flow between bleed chamber 42a and return passage 12a. 
     A servo piston 66a associated with poppet valve 64a is mounted for movement relative thereto in servo bore 76. Servo piston 66a includes a counterbore 106 which together with bearing member 72 and the walls of servo bore 76 define a control pressure chamber 44a in communication with control pressure passage 48a. A relatively light spring member 107 positioned between bearing member 72 and the bottom of counterbore 106 keeps servo piston 66a in contact with a shoulder portion 108 of servo valve 64a and prevents servo piston 66a from blocking passage 48a. Under the urging of control pressure in chamber 44a , servo piston 66a  acts to unseat cone end 65a through shoulder portion 108 for controlling the variable orifice metering action of the servo valve 64a. 
     Note that in this embodiment, spring member 77 combines the functions of springs 56 and 68 shown and described above in relation to FIG. 1. As previously mentioned, the resulting higher spring force limits the use of this embodiment as an anti-cavitation check valve. 
     One of the features of the control valve is low leakage in the shut off position. The seats between main piston 50a and body 70 and poppet valve 64a and metering orifice 62a can be considered as positive seals with zero leakage. With reasonable tolerances very low leakage rates can be maintained. 
     It will be apparent to those skilled in the art that many changes may be made to the above described invention without departing from the spirit of the invention and the scope of the appended claims. 
     One such change, by way of example, is shown in FIG. 3 wherein like elements have the same reference numerals as in FIG. 1 with the suffix b added. 
     FIG. 3 shows a spool type servo valve 108 in place of the servo piston 66 and poppet type servo valve 64 of FIG. 1. Servo valve 108 includes a piston end 110 positioned in control chamber 44b, a spool member 112 extending from piston end 110 into and through a bore 114 formed in main piston 50b, and having a metering passage 62b formed therein in communication with return chamber 38b through the end of spool member 112 positioned in bore 114. Servo valve 108 is mounted for movement in bore 114 and is yieldingly urged by spring member 68b to shut off fluid flow between bleed chamber 42b and return chamber 38b through metering passage 62b. Servo valve 108 is operable by control pressure applied to piston end 110 for generating a variable orifice as passage 62b is exposed to bleed chamber 42b thereby metering fluid flow through metering passage 62b between bleed chamber 42b and return chamber 38b. 
     Servo valve 108 has the advantages that system pressure forces have less effect on the spool type valve force balance than on the poppet type valve force balance and movement of servo valve 108 is not required when the circuit is functioning as a relief valve. A disadvantage of the spool type valve, as previously mentioned, is that it is susceptible to leakage through the clearances between bore 114 and spool member 112.