US3685537A

US3685537A - Fluid flow control valve

Info

Publication number: US3685537A
Authority: US
Inventors: Yasuo Kita; Tadao Hayashi
Original assignee: Shimadzu Corp
Current assignee: Shimadzu Corp; Shimadzu Seisakusho Ltd
Priority date: 1970-04-14
Filing date: 1970-04-14
Publication date: 1972-08-22
Anticipated expiration: 1989-08-22

Abstract

A fluid flow control valve in which a spool is slidably disposed in the working hollow of the valve housing formed with first and second ports and is provided with an orifice portion for communicating between said second port and hollow; a rod is slidably mounted through said spool so as to move at least in the urging direction of a resilient member together with said spool and its one end is exposed to pressure of a value lower than that in said two chambers, whereby the balancing movement of the spool may determine the opening of the orifice portion in such a manner as to reduce the discharge of fluid under pressure as a value of fluid pressure in the side of the first port increases.

Description

United States Patent Kita et al.

[ 1 Aug. 22, 1972 [54] FLUID FLOW CONTROL VALVE [72] Inventors: Yasuo Kita; Tadao Hayashi, both of 211 Appl. No.: 28,391

[52] US. Cl. ..137/493.6, 137/493.9, 188/349, 303/6 [51] Int. Cl ..F16k 17/26 [58] Field of Search ..l37/493.6, 493.9, 505.13; 188/349; 303/6 C [56] References Cited UNITED STATES PATENTS 2,386,051 10/1945 Kempton ..137/505.l3 3,508,792 4/1970 Bueler ..137/493.9 2,451,707 10/ 1948 Armstrong ..137/493.6 X 3,418,024 12/1968 Stelzer ..188/349 X 722,047 3/1903 Robison ..137/505.l3 1,029,097 6/1912 Atkinson 137/505. 1 3 1,240,682 9/1917 Crompton l 37/493.6

Primary Examiner-Harold W. Weakley Att0rneyCraig & Antonelli ABSTRACT A fluid flow control valve in which a spool is slidably disposed in the working hollow of the valve housing formed with first and second ports and is provided with an orifice portion for communicating between said second port and hollow; a rod is slidably mounted through said spool so as to move at least in the urging direction of a resilient member together with said spool and its one end is exposed to pressure of a value lower than that in said two chambers, whereby the balancing movement of the spool may determine the opening ofthe orifice portion in such a manner as to reduce the discharge of fluid under pressure as a value of fluid pressure in the side of the first port increases.

12 Claims, 2 Drawing Figures FLUID FLOW CONTROL VALVE The present invention relates to a fluid flow control valve and, more particularly, to a fluid flow control valve for use in a hydraulic load controlled cylinder employed in, for example, a fork lift truck and effectively operable so as to automatically regulate the discharge of fluid under loaded pressure in accordance with change of a value of load imposed on the lift cylinder in the case where the present invention is applied in the fork lift truck for operating its fork carrier.

It has been well known that the recent demand in connection with the workability of, for example, a fork lift truck is to improve the lifting speed of the fork carrier of said fork lift truck.

In order to meet this demand, it has been generally proposed to reduce the diameter of the lift cylinder, rather than to improve a pumping means for supplying fluid under pressure to the lift cylinder. According to such a proposal, a value of fluid pressure in the hydraulic circuit connecting the reservoir and the lift cylinder will increase in accordance with increase of a value of load imposed on the lift cylinder sufficient enough to make the manual operation of the fluid flow control valve difficult. Accordingly, the fork carrier descending speed will become relatively fast unless the fluid flow control valve is operated carefully. Furthermore, in such a fluid flow control valve, the fork lift truck will suffer from sudden impact due to inertia of a load on the fork carrier upon excessive discharge of fluid under pressure from the lift cylinder, thus accompanying an operational danger. Therefore, an experienced operator of the conventional fork lift truck is required.

It has been contemplated to provide a throttle valve with the only object to improve the fork carrier descending speed at the time when a load imposed on the fork carrier has a relatively higher value. In this instance, the fork carrier lifting speed will, on the other hand, be lowered at the time when the fork carrier is imposed with a relatively lower value of load or is nonloaded.

Accordingly, the present invention has been made with one of its objects being the elimination of the above defects inherent to the conventional fluid flow control valve and has for an object the provision of an improved fluid flow control valve effective to automatically control the discharge of fluid under pressure from the lift cylinder in accordance with change of a value of the load imposed on the lift cylinder through the fork carrier of the fork lift truck.

The fluid control valve of the present invention comprises, in its principle construction, a valve housing formed with first and second ports and a working hollow communicating between said two ports, a spool dividing said working hollow into two working chambers and formed with an orifice portion for communicating between said second port and said working hollow, means for communicating between said two chambers, a rod slidably mounted through said spool and having its one end being exposed to pressure of a value lower than that in said two chambers, and a resilient member interposed between said valve housing and said spool for urging the latter in one direction, said rod being capable of moving at least in the urging direction 'of said resilient member together with said spool upon movement of the latter in said direction,

whereby the balancing movement of the spool may determine the opening of the orifice portion in such a manner as to reduce the discharge of fluid under pressure as a valve of fluid pressure in the side of the first port increases. I

Alternatively, the fluid flow control valve thus constructed in the above manner may be provided with a check valve between said first port and one of the two working chambers, in other words, at a position over the orifice portion so that the sliding movement of said check valve over the orifice portion may determine the opening of said orifice portion so as to permit the discharge of fluid under pressure from the first port to the second port, but restrict the flow of fluid under pressure from the second port to the first port.

The present invention will be hereinafter fully described with reference to the attached drawing. However it should be noted that, for the convenience of this description, the description will proceed in connection with the fluid flow control valve connected with the lift cylinder of a fork lift truck.

Referring now to the attached drawings;

FIG. 1 is a side sectional view of a fluid flow control valve in one embodiment of the present invention, and

FIG. 2 is a plan view of an essential portion of a spool incorporated in the fluid flow control valve shown in FIG. 1.

Referring now to FIG. I, it will be understood that a valve housing 10 having a stepped blind hollow 11 therein is formed at its upper portion with first port 12 communicating with a loaded unit such as a lift cylinder (not shown) at its lower portion with a second port 13 communicating with a fluid reservoir through a pumping means (both of the latter being not shown), and its right-hand portion with a threaded opening 14 for receiving a closing member or plug 15 fluid-tightly attached thereto through an O-shaped ring 16 of resilient material, said closing member of plug 15 being of any desired shape provided that it acts to seal said hollow 11 in the fluid-tight condition when threaded to said opening 14. The hollow 11 within said valve housing 10 communicates between said first and

second ports

12 and 13 and is divided into two working chambers 11a and 11b, respectively positioned at the left and right, with an annular chamber 11c therebetween by a spool 17 disposed in said hollow l 1.

The spool 17 is provided with a variable orifice portion 18 of which the opening forms such a V shaped cutout portion and is slidably disposed in said hollow of the valve housing 10 in such a manner as to extend from right chamber 11b to left chamber 11a through annular chamber 11c. This spool 17 is also slidably provided with a rod 19, which will be later described, in the coaxial relation thereto and both chambers 11a and 1 1b communicated with each other by means of an annular gap 20 formed between the inner peripheral surface of an annular collar 17a integrally radially projecting from the inner surface of the spool 17 and the outer peripheral surface of said rod 19. The annular gap 20 may be substituted with a narrow passage or a perforation (not shown) extending through the rod 19 so as to communicate both chambers 11a and 11b so long as it serves to equalize values of fluid under pressure within said chambers 1 la and 1 lb.

Slidably mounted on the outer periphery of the spool 17 over the orifice portion 18 is an annular check valve 21, the sliding movement of said check valve 21 over the orifice portion 18 being capable of determining the opening of the latter so as to permit the one-way flow of fluid under pressure from first port 12 to second port 13, but restrict the reverse flow. thereof, This check valve 21 is formed with a stepped portion on its outer periphery in such a manner that a working area thereof on which fluid under pressure to be supplied from first port 12 to second port 13 acts is larger than another working area thereof on which fluid under pressure to be reversely 'suppliedfrom second port 13 to first port 12 acts. a

,In this arrangement, the rod 19 is, as hereinbefore described, extended through the spool 17 in the axial direction thereof and in coaxial relation thereto, one end of which is a free end residing in left chamber 11a while the other end is slidably supported by a central bore 22 formed at the center of the closing member or plug 15. A resilient sealing member 23 is rigidly engaged around the inner peripheral surface of said central bore 22 so that an intermediate portion of said rod adjacent to the right-hand end of the rod tightly contacts thereto while said right-hand end is exposed to the atmosphere 24 or pressure of a value lower than that in the two chambers 11a and 11b through a filter member 25 covering said central bore 22 for the prevention of foreign matter penetrating thereinto.

Disposed between the annular collar 17a on the inner surface of the spool 17 and the interior side of the plug 15 is a compressing spring 26 for urging the spool 17 to the left. Also disposed between the stepped portion of the check valve 21 and the interior side of the plug, 15 is a compressing spring 27 for urging the check valve to the left. The rod 19 is formed at a position corresponding to the annular collar 17a with a detent ring 28 on the left side thereof so that the rod 19 will move 1 to the right together with the spool 17 when the movement of the spool 17 to the right takes place.

The operation of the fluid flow control valve of the present invention will be hereinafter described.

When a lift cylinder (not shown) for lifting or tilting a load carrier means is imposed with a load of high value and subsequently fluid pressure within said lift cylinder is to be discharged from the port 13 through the port 12 connected with said lift cylinder, fluid pressure of a high value P is applied to the port 12 and concurrently the chamber 11b. At the same time, this fluid pressure P0 is applied to the chamber 11a through the spool 17 and the annular gap 20 communicating between the both chambers 1 1a and 11b.

Now, assuming that f is a value of force acting on the spool 17 from the chamber 11b, f is a value of force acting on the spool 17 from the chamber 11a, A is a value of the total cross-sectional area of the spool 17, a is a value of the cross-sectional-area of the rod 19 and Pa is a value of atmospheric pressure, then a value of force'F exerted by the fluid pressure Poon the spool 17 wherein;

, f= (3) The force in the left direction is positive in the above formula. In this instance, since the value of atmospheric pressure Pa is smaller than the value of the fluid pressure P0 or a(Po -Pa) 0, the spool- 17 is moved to the response to change of a value of load imposed on the lift cylinder, the compression spring 26 must be pro? vided for urging the spool 17 to the' left, a value of resiliency Fs of said compressionspring 26 being such that, when it becomes equal tothe value of said force F the following relationship is established I the spool 17 will cease its movement to the left thereby to determine the opening of the orifice portion 18.

However, when the product of thevalue of the crosssectional area of the rod 19 vandthe value of the atmosph'en'c pressure 24 is fixed, the following relationship may beestablished;

Fs Po'a C.

Fs==P0 (6) wherein C designates a value of said product which is constant, Accordingly, from the aboveformula (6); it is apparent that the value of the resiliency F s of the spring- 26 will vary in proportion to the value of the fluid pressure P0. In other words, as the value of the fluid pressure Po increases, the resiliency Fs of the compression spring 26 is accumulated by the spool 17 moving to the right because of no fluid pressure on one side of the rod 19 exposed to the atmosphere 24.

In this arrangement, if the orifice portion 18 on the spool 17 is designed in the form of such a V-shaped cutout portion as illustrated in FIG. 2, the opening of the orifice portion 18 may be reduced as the spool 17 moves to the right whereby fluid under pressure to be discharged from the lift cylinder across the fluid flow control valve of the present invention is substantially regulated by said orifice portion 18. Y

The operational principle of the fluid flow control valve of the present invention as hereinbefore described is substantially the same even when a value of the load imposed on the lift cylinder is relatively small, that is a low load. However, it should be noted that, at this time, the spool 1 7 moves to the left so as to increase the opening of the orifice portion 18 and thereby to permit the discharge of a relatively great amount of the fluid under pressure across the control valve assembly of the present invention.

On the other hand, even when fluid under pressure is to be supplied to first port 12 and then to the lift cylinder from second port 13 communicating with the fluid reservoir, said fluid under pressure will be controlled by the variable orifice portion 18 which is automatically operated in the same manner as hereinbefore described.

Hereinafter, the operation of the check valve 21' incorporated in the fluid flow control valve of the present invention will be described particularly with reference to FIG. 2.

It should be noted that the annular check valve 21 is constantly urged to the left by the spring 27. Accordingly, discharge of fluid pressure from first port 12 to second port 13 may take place in the same manner as hereinbefore described with reference to FIG. 1. However, in the case where fluid pressure is tobe supplied from first port 13 to second port 12, and then if a loaded pressure of the lift cylinder of a relatively higher value is applied to the chamber 11, fluid under pressure will flow from the port 12 to the port 13 little by little through the orifice portion 18 because the total pressure receiving area in the side of the port 13 is smaller than that in the side of the chamber 11.

However, when a value of fluid pressure applied to second port 13 increases over the value of said load pressure as fluid under pressure is supplied by a pumping means (not shown), the following relationship will be established;

P-a Pa" 7 wherein P designates a value of fluid pressure supplied in the side of the port 13, a designates a value of the total area of the check valve 21 to which fluid pressure in the side of the port 13 is to be applied and a" designates a value of the total area of the check valve 21 to which fluid pressure in the side of the chamber 1 1 is to be applied.

Therefore, it is apparent that the check valve 21 will move to the right under the influence of the fluid pressure in the side of the port 13 until it is located at a position where a value of the force which is represented by the formula, Pa' P-a", acting on the check valve 21 is substantially equalized to a value of the resiliency of the compression spring 27. As a result thereof, the opening of the orifice portion 18 on the spool 17 increases so as to permit the flow of a great amount of fluid from the port 13 to the port 12 and then to the lift cylinder.

So long as the pumping means (not shown) is operated to supply fluid under pressure to the port 13, and even if the value of the fluid pressure in the side of the chamber 11 is subsequently reduced to generate a force of a value (P-a), the check valve 21 will move to the right under the influence of fluid pressure in the side of the port 13, thereby increasing the opening of the orifice portion 18 on the spool 17 so as to permit the flow of a great amount of fluid from the port 13 to the port 12 and then to the lift cylinder.

Although one form of the present invention has been fully disclosed by way of example with reference to FIGS. 1 and 2 of the attached drawings, alternative embodiments of these examples may be employed. For example, the shape of the cut-out portion of the orifice portion 18 on the spool 17 may be modified to suit to a desired fluid flow characteristic. Furthermore, although in the embodiment of FIGS. 1 and 2, the rod 19 is extended through the spool 17 with its left-hand portion being engaged with the annular collar 17a on the spool 17 by means of the detent ring 28 so as to move to the right upon movement of the spool 17 in the same direction, all that is necessary for this arrangement is an integral movement of the rod 19 with the spool 17 taking place upon movement of the spool 17 in the urging direction of the spring 26, which may be a resilient member, while the right-hand extremity of the rod 19 is in position opposite to said urging direction of the spring 26. Alternatively, this rod 19 may be integrally formed with the spool. In connection thereto, it will be apparently understood that the positioning of the spring 26 with respect to the spool modified.

Although the check valve 21 shown in FIG. 1 is mounted on the spool 17 slidably over the orifice portion 18, means for communicating between the port 13 to the chamber 11 may be provided together with a suitable non-return valve operable in such a manner that said non-retum valve will permit the flow of fluid under pressure from the port to the chamber 11 but restrict the reverse flow of said fluid under pressure from the chamber 1 1 to the port 13.

The fluid flow control valve of the present invention having the construction as hereinbefore fully described is effective to reduce the descending speed of, for example, a fork or bucket carrier of the fork lift truck by controlling the rate of flow of fluid under pressure from the lift cylinder to the fluid reservoir. Therefore, the descending movement of the form or bucket carrier at the time when said carrier is imposed with a load of relatively higher value can be smoothly performed, which is heretofore considered a problem to be solved. Of course, with the fluid flow control valve of the present invention, the working efficiency is substantially improved with reduction of dangers that may arise upon erroneous operation.

According to the present invention, since the spool in the fluid flow control valve is operated not by the pressure differential but by the loaded pressure of the lift cylinder, steady operation of the carrier can be appreciated without flow surge. In addition thereto, since the check valve is provided in said fluid flow control valve, discharge of fluid under pressure from the lift cylinder to the fluid resevoir can be restricted whereas supply thereof from the reservoir to the lift cylinder can be facilitated.

Therefore, in the case where the fluid flow control valve of the present invention is adapted in a fork lift truck for effectively operating its fork camenthe lifting speed of said fork carrier can be improved while the descending speed thereof is smoothly controlled with improved performance. However, the fluid flow control valve of the present invention has a wide range of application in addition to the fork lift truck.

What is claimed is:

1. A fluid flow control valve which comprises a valve housing having a working hollow therein and formed with first and second ports, a spool slidably disposed in said working hollow of said valve housing so as to divide said working hollow into two chambers and formed with an orifice portion communicating between said first and second ports via one of said two chambers, means for communicating between said two chambers, a rod slidably disposed within said valve housing and having one end exposed to pressure of the value lower than in said two chambers, a first resilient member accommodated in said valve housing for urg-- ing said spool in one direction, said rod having the other end coaxially connected with and parallelly extending along said spool so that, only when said rod is moved in the direction of the first mentioned end of said rod under the influence of fluid medium supplied from the first portto the other chamber through said communicating means, said spool can be moved against the action of! said first resilient member together with said rod, an annular check valve slidably mounted on said spool in said valve housing so as to cover part of said orifice portion of said spool, and a second resilient member accommodated in said valve housing for urging-said check valve in one direction, to abut a portion of said housing surrounding theorifice I I portion of said spool the opening of said orifice portion I being-determined by said annular check. 'valve as said spool moves in the oppositeidirection against the action of said first resilient member, whereby the fluid'medium can be permitted to flowfromthe first port to the second port through the orifice portion in a small amount, when the. pressure at the first port is higher I than that at the second port, said annular 'check valve being adapted to move in the opposite direction against the action of said second resilientmember only when first port. I

2. A fluid flow control .valve comprising:

, the pressure at the second port is higher than that at the a valve housing including a working hollow -ther ein,'

to'gether with a first and second port communicating with said hollow; I i i II means for dividing said working hollow into first and second working chambers, comprising a spool,

slidably disposed within said working hollow and/ being provided with a means for efiecting control-.

.lable fluid communication between said first and second ports through one of said working chambers of said hollow, said'fluid communication effecting means including an orifice formed in said means'for permitting fluid communication between said first and second working chambers of said hollow; 1 I a rod slidably disposed within said valve housing, and havingone end thereof exposed to pressure lower than the fluid pressure in said first and second chambers;

I means, contactingsaid spool and said housing for urging said spool in a first I predetermined direction, said urging means comprising a first resilient member; and I means, coupled to saidrod and contactable with said spool, for effecting the movement of said spool in a second direction opposite to said first direction, against the urging provided by said first resilient member, in response to the application of fluid pressure supplied from said first port through said fluid communication effecting means to said first chamber of said hollow, whereby said spool can be moved against the action of said first resilientmember together with said rod to effect the control of fluid communication between said first and second ports through said orifice, wherein I said means for effecting fluid communication between said first and. second port's further. in-

I cludes means, responsive *to the pressure differential between said first and second ports, for controlling the degree of opening of said orifice, assai ants: megawatt: ing and surrounding a' portion of said orifice, and a second resilient means, disposed within said valve housing, 'forurging said annular check valve in said first redetermined direction, said check valve 'i 1 being so dimensioned as .to' permit said check valve to slide in said opposite-direction over said orifice only in response to the fluid pressure atxsaid .direction. j

I 3.A valve according to claim 2, wherein said means' for permitting fluid-communication between said first. and second working chambers comprises an" annular gap disposed withina portion of said pool surrounding said rod.

said spool andsaid annular checkvalve.

6. A valve according toclaim '5, whereins'aid hollow. i- A is ,a stepped blind hollow, the first divided workingl chamber of which has a cross section less the cross section of said second divided working chamber. I

7. A valve accordingtoclairn 6, wherein each of said first and second-ports disposed on respectivefopiv posite sides of .the interior of said housing hollow I against which said annular check valve. is urged by said second spring.

8. A valve according to claim 5,-wherein said closing plug includes a slot into which a first end of-said rod slides and a filter member disposed thereiri- 9. A valve according to claim 2, wherein said first and second resilient members comprise first and I 7 second concentrically disposed springs, respectively 1 contacting a closing plug-portion of said housing and said spool and said annular check valve. I

10. A .valve according to claim 2, whereinsaid orifice comprises a first tapered portion extending from" one I end of said spool and a second tapered portion extending from said first tapered portion. a

l l. A valve according toclaim 10, wherein the areaof the orifice defined by said first taperedportion is greater than the area of the orifice defined by said second tapered portion.

12. A valve according to claim 11, wherein the I length of said orifice in an axial direction is greater than the corresponding dimension of said annular check valve.

=0: a: a: a

second port being greater than the fluidpresstireat said first port, so as to ove'rcomeEthe urging-force of said. second resilient means in said first;

Claims

1. A fluid flow control valve which comprises a valve housing having a working hollow therein and formed with first and second ports, a spool slidably disposed in said working hollow of said valve housing so as to divide said working hollow into two chambers and formed with an orifice portion communicating between said first and second ports via one of said two chambers, means for communicating between said two chambers, a rod slidably disposed within said valve housing and having one end exposed to pressure of the value lower than in said two chambers, a first resilient member accommodated in said valve housing for urging said spool in one direction, said rod having the other end coaxially connected with and parallelly extending along said spool so that, only when said rod is moved in the direction of the first mentioned end of said rod under the influence of fluid medium supplied from the first port to the other chamber through said communicating means, said spool can be moved against the action of said first resilient member together with said rod, an annular check valve slidably mounted on said spool in said valve housing so as to cover part of said orifice portion of said spool, and a second resilient member accommodated in said valve housing for urging said check valve in one direction, to abut a portion of said housing surrounding the orifice portion of said spool the opening of said orifice portion being determined by said annular check valve as said spool moves in the opposite direction against the action of said first resilient member, whereby the fluid medium can be permitted to flow from the first port to the second port through the orifice portion in a small amount when the pressure at the first port is higher than that at the second port, said annular check valve being adapted to move in the opposite direction against the action of said second resilient member only when the pressure at the second port is higher than that at the first port.

2. A fluid flow control valve comprising: a valve housing including a working hollow therein, together with a first and second port communicating with said hollow; means for dividing said working hollow into first and second working chambers, comprising a spool, slidably disposed within said working hollow and being provided with a means for effecting controllable fluid communication between said first and second ports through one of said working chambers of said hollow, said fluid communication effecting means including an orifice formed in said spool; means for permitting fluid communication between said first and second working chambers of said hollow; a rod slidably disposed within said valve housing, and having one end thereof exposed to pressure lower than the fluid pressure in said first and second chambers; means, contacting said spool and said housing for urging said spool in a first predetermined direction, said urging means comprising a first resilient member; and means, coupled to said rod and contactable with said spool, for effecting the movement of said spool in a second direction opposite to said first direction, against the urging provided bY said first resilient member, in response to the application of fluid pressure supplied from said first port through said fluid communication effecting means to said first chamber of said hollow, whereby said spool can be moved against the action of said first resilient member together with said rod to effect the control of fluid communication between said first and second ports through said orifice, wherein said means for effecting fluid communication between said first and second ports further includes means, responsive to the pressure differential between said first and second ports, for controlling the degree of opening of said orifice, including an annular check valve slidably mounted on said spool and abutting a portion of said housing and surrounding a portion of said orifice, and a second resilient means, disposed within said valve housing, for urging said annular check valve in said first predetermined direction, said check valve being so dimensioned as to permit said check valve to slide in said opposite direction over said orifice only in response to the fluid pressure at said second port being greater than the fluid pressure at said first port, so as to overcome the urging force of said second resilient means in said first direction.

3. A valve according to claim 2, wherein said means for permitting fluid communication between said first and second working chambers comprises an annular gap disposed within a portion of said spool surrounding said rod.

4. A valve according to claim 3, wherein said means for urging said spool in said first predetermined direction comprises an annular collar located on the interior portion of said spool.

5. A valve according to claim 4, wherein said first and second resilient members comprise first and second concentrically disposed springs, respectively contacting a closing plug portion of said housing and said spool and said annular check valve.

6. A valve according to claim 5, wherein said hollow is a stepped blind hollow, the first divided working chamber of which has a cross section less than the cross section of said second divided working chamber.

7. A valve according to claim 6, wherein each of said first and second ports are disposed on respective opposite sides of the interior of said housing hollow against which said annular check valve is urged by said second spring.

8. A valve according to claim 5, wherein said closing plug includes a slot into which a first end of said rod slides and a filter member disposed therein.

9. A valve according to claim 2, wherein said first and second resilient members comprise first and second concentrically disposed springs, respectively contacting a closing plug portion of said housing and said spool and said annular check valve.

10. A valve according to claim 2, wherein said orifice comprises a first tapered portion extending from one end of said spool and a second tapered portion extending from said first tapered portion.

11. A valve according to claim 10, wherein the area of the orifice defined by said first tapered portion is greater than the area of the orifice defined by said second tapered portion.

12. A valve according to claim 11, wherein the length of said orifice in an axial direction is greater than the corresponding dimension of said annular check valve.