GB2047434A

GB2047434A - Pressure relief valve

Info

Publication number: GB2047434A
Application number: GB8009961A
Authority: GB
Original assignee: Sperry Corp
Current assignee: Sperry Corp
Priority date: 1979-03-26
Filing date: 1980-03-25
Publication date: 1980-11-26
Also published as: SE8002300L; AU5682580A; DE3011233C2; DE3011233A1; CA1130167A; GB2047434B; SE436784B; AU534468B2; IN154880B

Abstract

A hydraulic pressure relief circuit includes a main valve (14), a bleed flow orifice (18) to provide stability to the main valve, a pilot valve (20) having a differential area piston (30,32) and a damping orifice (46) in conjunction with an accumulator (44). System pressure is limited by opening the main valve and allowing fluid flow from the system back to tank. The main valve is controlled as a bleed flow servo wherein the pilot valve (20) meters the bleed flow from the bleed flow orifice (18) to tank, thereby controlling the opening of the main valve. Fluid flow through the damping orifice (46) in conjunction with the accumulator (44) acts on area (A1) of the pilot valve to provide damping forces acting on the pilot valve with both rising and falling system pressures, thereby limiting the maximum rate of pressure increase in the circuit (10). <IMAGE>

Description

SPECIFICATION Pressure relief valve This invention relates to hydraulic pressure relief valves and circuits and, more particularly, to rate damped relief valves and circuits.

A relief valve is used in hydraulic circuits to limit the maximum pressure that can develop.

However in hydraulic circuits utilizing relief valves, undesirable momentary high pressures above the set limit or overshoot can develop in response to transient conditions. Such a transient condition, for example, can occur when a lowering load is brought to an abrupt halt hydraulically, generating momentary pressure rate increases in the system ranging between approximately 0.83 to 20.7 GPa per second (GPa = Gigapascal = 109 Pascals).

Such repeated overshoots tend to shorten the service life of affected system components.

The present invention is directed to a novel hydraulic pressure relief valve and circuit that will eliminate the overshoot under most conditions and limit the maximum rate of pressure increase in the circuit.

The present invention provides a hydraulic pressure relief valve circuit comprising main valve means operable to shut off fluid flow between a source of system pressure and a return passage; bleed flow means connected to said source of system pressure and to a bleed pressure chamber associated with said main valve means for restricting fluid flow to said bleed pressure chamber; pilot valve means connected to said source of system pressure and to a pilot chamber of said main valve means for metering fluid flow from said bleed pressure chamber to said return passage; accumulator means connected to said source of system pressure and to said pilot valve means, for containing a volume of fluid; and damping means connected to said source of system pressure, to said pilot valve means, and to said accumulator means for restricting fluid flow from said source of system pressure to said pilot valve means and said accumulator means.

The main valve can be either a slide or poppet type of conventional design. The pilot valve preferably has a differential area piston.

System pressure is limited by opening of the main valve and allowing flow from the system line back to tank. The main valve is controlled as a bleed servo wherein the pilot valve meters the bleed flow from the bleed flow orifice to tank, thereby controlling the opening of the main valve. Fluid flow through the damping orifice, in conjunction with the accumulator, limits the rate of pressure rise which acts on the pilot valve, thereby limiting the maximum rate of pressure increase in the main system circuit.

The invention is further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of the hydraulic pressure relief valve circuit; Figure 2 is a partial sectional view of a preferred embodiment thereof; Figure 2a is a partial sectional view of Fig.

2 showing an alternative location of the bleed flow orifice; Figure 3 is a graph showing an undesirable transient pressure rise in terms of pressure and time in a non-damped relief valve circuit; and Figure 4 is a graph showing a desirable relationship of a transient pressure rise in terms of pressure and time in a damped relief valve circuit embodying the invention.

Referring to Fig. 1, a hydraulic pressure relief valve circuit embodying the invention includes a system pressure line 10 adapted to be connected to a source of system pressue extending to a main stage valve 14 and a tank return line 1 2 adapted to be connected to a tank or reservoir. Main stage valve 14, which may be either a poppet or slide type valve of conventional design is spring-loaded at its rear or large area end 1 6 to yieldingly shut off flow between lines 10 and 1 2. System pressure is limited by opening main stage valve 14 and allowing flow from pressure line 10 to tank return line 1 2 back to the tank.

Main stage valve 14 is controlled by a pilot circuit which includes a bleed flow orifice 1 8 positioned between line 10 and a bleed pressure chamber at rear end 1 6 of valve 1 4. A pilot valve 20 controls the bleed flow and thereby the opening of main valve 1 4.

Pilot valve 20 includes a pilot bore 22, which has a spring end 24, a drain chamber 26, a bleed flow chamber 27, and a pressure end 28 and in which a differential area piston is mounted for movement. The piston includes a reduced piston section 30 in spring end 24 and a metering piston sectin 32 extending from drain chamber 26 in pressure end 28.

Piston sections 30 and 32 are formed with a reduced area A, and pressure area A2, respectively, with metering piston section 32 having one or more metering slots 36 formed therein in open communication with bleed flow chamber 27.

A bleed flow line 38 extends between the bleed pressure chamber at the rear end 1 6 of main valve 14 and bleed flow chamber 27.

Slot 36 meters the bleed flow from rear end 1 6 of main valve 1 4 through line 38 into drain chamber 26 and on to the tank through a metered flow line 40. A spring member 42 bearing against reduced area A1 yieldingly urges the differential area piston of valve 20 toward pressure end 28 to shut off metered flow between chambers 26 and 27. Pilot valve 20 has spring end 24 of bore 22 and a fluid compression means, such as a contained volume or accumulator 44, connected to system pressure downstream of a damping orifice 46 through a damping flow line 48.

The differential area piston of valve 20 in conjunction with damping orifice 46 and ac cumulator 44 provides for a damping action on the pressure rise of system pressure in the circuit. Pressure exerted on area A2 of piston section 32 tends to open fluid flow through metering slot 36 and pressure exerted on reduced area A, of piston section 30 tends to close off flow through metering slot 36. This permits damping forces to be developed in the following manner.

If system pressure rises rapidly, area A2 being in direct communication with system pressure, will sense the pressure rise immedi ately. The pressure sensed by area A, will rise more slowly because sufficient flow of fluid must be developed either through damping orifice 46 or by displacement of area A, to compress fluid in accumulator 44, as presently described. The rate of pressure increase sensed at area A, is controlled by the sizes selected for damping orifice 46 and accumulator 44. If the rate of pressure rise in the system pressure is sufficiently high, the differ ence in pressures acting on areas A, and A2 will generate a force sufficient ta overcome the spring force exerted by spring member 42 and open the pilot valve 20 through metering slot 36 and thereby the opening of main valve 14, dumping system fluid flow to tank.A rapid drop in system pressure, while the main valve is relieving, will have an opposite effect.

The damping orifice 46 will restrict fluid flow that develops as the result of decompression of the fluid volume in accumulator 44, caus ing pressure sensed at area A1 to remain higher than the pressure sensed at area A2 thereby acting to close pilot valve 20. Damping forces are therefore developed with both rising and falling system pressures.

The cracking pressure Cp of pilot valve 20 is determined by the difference in piston areas A, and A2 and a force Fs exerted by the pilot valve spring member 42. Thus, the cracking pressure has the following relationship to the cross-sectional areas A, and A2: Fs Cp= A2 - A1 If system pressure rises above this value, the net force exerted on the two piston areas A, and A2 will exceed the force of spring member 42 and the piston will move to further compress the spring 42, uncovering metering slot 36 resulting in metering the bleed flow to tank. The metered bleed flow reduces the pressure at the large end 1 6 of main stage valve 14 and valve 14 opens allowing system pressure to return to tank, resulting in a drop in the system pressure.

The circuit will operate as a closed-loop servo and will maintain system pressure at, or slightly above, the cracking pressure of pilot valve 20.

Speed of response of the circuit is governed by the rate of spring member 42 and the width of metering slot 36. With a reasonably fast response, damping orifice 46 in conjunction with accumulator 44 will prevent instability and add damping to the extent that the speed of response is overdamped thereby limiting the maximum rate of increase of the system pressure. The value of this limit is a function of the volume of accumulator 44 and the size of damping orifice 46. The maximum rate of increase can be established at a desired level by selecting the proper size of each.

Fig. 2 shows a preferred embodiment of the hydraulic circuit as a unitary body relief valve wherein corresponding elements are provided with a suffix a.

The relief valve circuit of Fig. 1 as shown is housed in a body 11, Fig. 2, having a system pressure passage 1 0a and a return flow passage 1 2a spaced therefrom by a main bore 1 3 having a rear end portion 15 which terminates at an end wall 1 7 spaced from pressure passage 1 0a. A pilot bore 22a formed in body 11 in spaced relation to main bore 1 3 includes a system pressure end 28a, an enlarged intermediate portion 21 forming a drain chamber 26a, and a spring end 24a.

Pressure end 28a extends from and transversely to pressure passage 1 Oa and is in communication with rear end portion 1 5 of main bore 1 3 through a bleed flow passage 38a. Intermediate portion 21 of pilot bore 22a is in communication with return flow passage 1 2a through metered flow passage 40a. Body 11 also includes a contained volume 44a having a first end 27 in communication with spring end 24a of pilot bore 22a through an accumulator passage 48a and a second end 29 in communication with pressure passage 1 0a through a damping orifice 46a.

A main valve piston 14a is movably mounted in main bore 1 3 to yieldingly shut off flow between pressure passage 1 Oa and return passage 12a. Piston 1 4a includes a large area end 1 6a having a counterbore 31 formed therein which together with rear end portion 1 5 and end wall 1 7 of main bore 1 3 define a main chamber 35 in which a main valve spring 37 functions to urge main valve piston 1 4a in seated engagement with valve seat 39 formed in main bore 1 3.

A pilot valve 20a is mounted for movement in pilot bore 22a. Pilot valve 20a comprises a two-piece differential area piston having a reduced area piston 30a extending from metering chamber 26a into spring end 24a of pilot bore 22a and a metering piston 32a extending from drain chamber 26a into pressure end 28a of pilot bore 22a.

Metering piston 32a is formed with a tapered portion 41 positioned in drain chamber 26a in abutting relation to reduced area piston 30a. A shoulder portion 45 of metering piston 32a extends into pressure end 28a of bore 22a and a reduced diameter portion 43 of metering piston 32a spaces shoulder portion 45 from a head portion 47 of metering piston 32a. Shoulder portion 45 and head portion 47 define therebetween a bleed flow chamber 27a in pressure end 28a of bore 22a in which one end of bleed flow passage 38a terminates. Bleed flow chamber 27a is in communication with pressure passage 1 0a through a bleed fow orifice 1 8a formed in head poriton 47. Metering piston 32a is further provided with one or more metering slots 36a formed on shoulder portion 45 which are in communication with bleed flow chamber 27a.A spring member 42a is positioned in pilot bore 22a between spring piston 30 and an adjustment member 51 in threaded engagement with body 11 and yieldingly urges metering piston 32a into seated engagement with a valve seat 53 formed in bore 22a adjacent drain chamber 26a. Inward or outward movement of adjustment member 51 relative to body 11 varies the amount of compression of spring 42a thereby providing a means for adjusting the cracking pressure level at which the relief valve will relieve.

Figs. 3 and 4 show in graphical form a comparison of the rate of pressure rise in a non-damped relief valve circuit, Fig. 3, and a damped rate limiting relief valve, Fig. 4, constructed in accord with the instant invention.

Both circuits have the same predetermined steady-state relief valve setting. However, for a typical system with a conventional relief valve, as shown in Fig. 3, several oscillations occur where the pressures exceed the relief valve setting before the system stabilizes. In such a system, the relief valve setting is first reached much more rapidly than in the rate limited circuit. The momentary overshoots exceed the relief valve setting and the rate of pressure rise may range approximately betweeen 20.7 to 0.83 GPa per second resulting in shocks in the system that tend to shorten the service life of affected system components.

The graph of Fig. 4 shows that, for one embodiment of circuit according to the invention, the initial pressure rise is the same in both cases but with rate limiting the rise is more gradual and is relatively peak free with only a moderate pressure rise peak occurring well below the steady-state pressure setting.

The moderate peak occurs just prior to the opening of the main valve with a pressure rise of approximately 0.28 to 0.55 GPa per second.

Many changes may be made to the above described circuit without departing from the invention.

An example of such changes is shown in Fig. 2a, wherein reference numeral 55 shows the bleed flow orifice 1 8a of Fig. 2 formed through the wall of counterbore 31 communicating pressure passage 1 0a with bleed flow chamber 27a (Fig. 2) through main valve chamber 35 and bleed flow passage 38a.

The above-described hydraulic pressure relief circuit is useful in the hydraulic system described in co-pending application No. (agent's reference A4991 /B).

Claims

1. A hydraulic pressure relief valve circuit comprising: main valve means operable to shut off fluid flow between a source of system pressure and a return passage; bleed flow means connected to said source of system pressure and to a bleed pressure chamber associated with said main valve means for restricting fluid flow to said bleed pressure chamber; pilot valve means connected to said source of system pressure and to a pilot chamber of said main valve means for metering fluid flow from said pressure chamber to said return passage; accumulator means connected to said source of system pressure and to said pilot valve means, for containing a volume of fluid; and damping means connected to said source of system pressure, to said pilot valve means, and to said accumulator means for restricting fluid flow from said source of system pressure to said pilot valve means and said accumulator means.

2. A hydraulic circuit as claimed in claim 1, wherein said main valve means includes a large area end and said bleed flow means is connected between said source of system pressure and said large area end.

3. A hydraulic circuit as claimed in claim 2, wherein said pilot valve means is connected to said large area end.

4. A hydraulic circuit as claimed in claim 3, wherein said pilot valve means includes a bleed flow chamber connected to said bleed flow means and to said bleed pressure chamber which is at said large area end.

5. A hydraulic circuit as claimed in claim 4, wherein said pilot valve means includes a drain chamber and is operable to shut off fluid flow between said drain chamber and said bleed flow chamber.

6. A hydraulic circuit as claimed in claim 5, wherein said pilot valve means includes one or more metering slots in communication with said bleed flow chamber, and is operable to open said bleed flow chamber to said drain chamber through said metering slots.

7. A hydraulic circuit as claimed in claim 5 or 6, wherein said drain chamber is connected to said return passage.

8. A hydraulic circuit as claimed in any of claims 1 to 7, wherein said pilot valve means includes a differential area piston and a spring member, said piston being operable to yieldingly shut off metered fluid flow to said return passage, said piston having a large area in communicaion with said source of system pressure and a reduced area in communication with a said accumulator means and said damping means.

9. A hydraulic circuit as claimed in claim 8, wherein said differential area piston is operable to meter fluid flow to said return tank upon sensing a predetermined cracking pressure, said cracking pressure (cup) being determined by the difference between the areas of said reduced area (A,) and said large area (A2) of said piston and by the force (Fs) exerted by said spring member in accordance with the following relationship: Cp = Fs/A2A,.

10. A hydraulic circuit as claimed in claim 8 or 9, wherein said differential area piston is of a two-piece construction which includes a reduced area piston in abutting relation to a metering piston, the reduced area piston having said reduced area formed thereon and the metering piston having said large area formed thereon.

11. A hydraulic pressure relief valve circuit as claimed in claim 1, which also comprises a body having therein said return passage and a system pressure passage forming said source of system pressure, said passages being in spaced relationship, said body also having therein a main bore extending between said pressure passage and said return passage, said main valve being mounted for movement in said main bore and being yieldingly urged to shut off fluid flow between said system pressure and said return passages, said body further having therein a pilot bore spaced from said main bore and in communication with said system pressure passage, a bleed flow passage which forms said bleed flow means and which extends from said main bore to said pilot bore, and a drain passage extending from said pilot bore to said return passage, said pilot valve means being mounted for movement in said pilot bore and being yieldingly urged to shut off fluid flow between said bleed flow and said drain passages, a bleed flow orifice being located n said body to restrict fluid flow from said system pressure passage to said bleed flow passage, a metering slot being formed in said pilot valve and being adapted upon movement of said pilot valve to meter fluid flow from said bleed flow passage to said drain passage, said body also having therein a contained volume which forms said accumulator means and which is spaced from said pilot bore and is in communication with said system pressure passage and said pilot bore, and a damping orifice which forms said damping means and which is in communitation with said system pressure passage, said contained volume, and said pilot bore, said damping orifice restricting fluid flow from said system pressure passage to said pilot bore and said contained volume.

1 2. A hydraulic circuit as claimed in claim 11, wherein said pilot valve includes a differential area piston and a spring member positioned in said pilot bore and acting on said piston, said piston having a large area in communication with said system pressure passage and a reduced area in communication with said contained volume and said damping orifice, said differential area piston being oper able to meter fluid flow to said drain passage upon sensing a predetermined cracking pressure, (Cp) being determined by the difference between the reduced area (A,) and the large area (A2) of said piston and a force (Fs) exerted by said spring member having the following relationship:: Cp = Fs/A2A" said differential area piston being of a two piece construction which includes a reduced area piston in abutting relation to a metering piston, the reduced area piston having said reduced area formed thereon and the meter ing piston having said large area formed thereon, and in which said main valve means includes a large area end in communication with said bleed flow passage and said bleed flow orifice restricting fluid flow from said system pressure passage to said large area end, and said pilot valve means includes a bleed flow chamber in communication with said bleed flow passage and a drain chamber in communication with said return passage, and said pilot valve means being operable to shut off fluid flow between said chambers and including' one of more metering slots in communication with said bleed flow chamber, said pilot valve means being operable to open said bleed flow chamber to said drain chamber through said metering slot, said bleed flow orifice being formed through said pilot valve means.

1 3. A hydraulic pressure relief valve cir cuit constructed and adapted to operate sub stantially as herein described with reference to and as illustrated in the accompanying draw ings.