US3195559A

US3195559A - Spool valve assembly

Info

Publication number: US3195559A
Authority: US
Inventors: Hugh J Stacey
Original assignee: Parker Hannifin Corp
Current assignee: Parker Hannifin Corp
Priority date: 1959-05-12
Filing date: 1962-12-07
Publication date: 1965-07-20
Anticipated expiration: 1982-07-20

Description

July 20, 1965 H. J. STACEY SPOOL VALVE ASSEMBLY 2 Sheets-Sheet 1 Original Filed May 12, 1959 m m m m i J l n m 5 2;; 7. n1 r O l l 2 a 7 F I. W YB E 4 m m & & H M A u w m a a 1 9 a 5 2 7 1 1 9 ,7 e fl 2 l o ll 5 5 w a 5 5 5/ 5 2 A M 6 5 H, mm M- i w 4 w 9 55% 3 9 r a w 5 .5 2 a a A J 2 Y 1. 7 6 A 3 7 r 3 3 3 7 2 i M i INVENTOR.

HUGH J. STACEY TORNEYS y 0, 1965 H. J. STACEY 3,195,559

SPOOL VALVE ASSEMBLY Original Filed May 12, 1959 2 Sheets-Sheet 2 INVENTOR. HUGH J. STACEY BY 9%, 77/0,? 9 19M ATTORNEYS United States Patent 3,195,559 819681, VALVE ASSEMBLY Hugh J. Stacey, Chesterland, Ghio, assignor to Parker- Hannifin Corporation, @ieveiand, Ghio, a corporation of Ohio Continuation of application Ser. No. 812,687, May 12, 1959. This application Dec. '7, 1962, Ser. No. 243,714

13 Claims. (Cl. 137-27ii) Lower positions in which the pressure inlet port is selectively communicated with one of the service ports to either raise or lower the bucket, blade, or other implement to which the piston in the cylinder assembly is operatively connected, and in which the other service port is communicated with the tank port; and a Float position in which the pressure inlet port is communicated with each other to permit descent of the implement, and thus of the piston in the cylinder, by gravity.

In a broad sense, spool valves of this type are known in the art, it being one principal aim of the present invention to provide a novel restriction arrangement between one service port and the tank port which is effective to build up back pressure in the return circuit so that cavitation in the pressure circuit is prevented, as when the spool is in Lower position. Thus, when the bucket or blade engages the ground full line pressure will be immediately available for penetration or digging action.

It is another object of this invention to provide a spool valve assembly of the character indicated in which the valve spool is hollow and is provided with a novel restriction therein which, in the Float position of the spool, communicates the head end of the cylinder with the tank port to accommodate the greater displacement as compared with that of the rod end, such restriction also being effective to build up back pressure during the float or down stroke of the piston under the influence of the load thereon (the bucket or blade and its supporting arm) so that the rod end of the cylinder will be kept filled with oil preparatory to power lowering at a greater speed.

It is another object of this invention to provide a spool valve assembly of the character indicated in which the spool assembly may be turned end for end in the valve housing so that its actuating portion is disposed at either end of the valve housing.

It is yet another object of this invention to provide a spool valve assembly of the character indicated in which the relief valve in the valve housing may, likewise, be turned end for end so as to place the adjusting mechanism therefor at either end of the valve housing.

It is still another object of this invention to provide a novel sleeve extension arrangement which extends part way into the passage leading to the tank port to serve as a cut-off for fluid flow through the hollow spool when the latter is shifted to Raise position, whereby pressure builds up in the hoist cylinder to effectively raise the implement actuated by the piston therein.

Other objects and advantages of the present invention will become apparent as the following description proclaims, the following description and the annexed drawing setting forth in detail a certain illustrative embodiment of the invention, this being indicative, however, of but one of the various ways in which the principle of the invention may be employed.

In said annexed drawing:

FIG. 1 is a cross-section view through the valve assembly in a plane containing the axes of the relief valve and of the valve spool;

FIG. 2 is a fragmentary cross-section view taken substantially along the line 22, FIG. 1; and

FIG. 3 is a cross-section view taken substantially along the line 33, FIG. 1.

Referring now more particularly to the drawing, the valve assembly 1 herein shown comprises a valve housing 2 formed with mounting holes by which it may be secured in place and further formed with a pressure inlet port 4 which intersects the relief valve bore 5, the check valve bore 6, and the valve spool bore 7, the latter at two

places

8 and 9. The housing 2 is formed with a tank port 10 which has a top branch 11 and a bottom branch 12 intersecting relief valve bore 5 and spool bore 7 as shown and a middle branch 14 which intersects only spool bore 7.

The housing 2 is additionally formed with two service or

cylinder ports

15 and 16 which intersect the spool bore '7 at regions adjacent the respective tank port branches 11 and 12, and as indicated by the

lines

17 and 18, one service port 15 is connected to the head end of a double acting cylinder assembly 19 and the other service port 16 is connected to the rod end of said double acting cylinder assembly 19.

The check valve bore 6 has a pair of spring-biased check valves 20 therein. The inlet side of each check valve 26 communicates by way of bore 6 with the pres sure inlet port 4 and when the check valves are unseated, fluid flows therepast into the respective

secondary inlet ports

21 and 23, one of which is disposed between the top service port 15 and the top branch 8 of the inlet port 4 and the other of which is disposed between the bottom service port 16 and the bottom inlet branch 9.

With reference to the relief valve bore 5 which is intersected by the tank port branches 11 and 12, there is disposed in one portion of such bore a threaded plug 25 which serves as an abutment for the tubular relief valve seat member 26, the plug being sealed as by means of the O-ring 27, and the tubular end of the seat member being sealed in bore 5 as by means of an O-ring 28.

Screwed into the other and identical end of the relief valve bore 5 is a relief valve bonnet 29 which serves to retain the valve guide sleeve 30 in place with its inner end coaxial with, but axially spaced from, the seat 31. Slidably guided in said sleeve 30 is the tubular relief valve member 32 which is pressed by the spring 34 to cause its lower end to engage seat 31 in fluid-tight manner, the reilef valve member 32 being sealed in the sleeve 30 as by means of the O-ring 35. The upper end of the spring 34 bears on the spring guide 36 which, in turn, is engaged by an adjusting screw 37 which has threaded engagement in the bonnet 29 and which is locked in adjusted position by means of the lock nut 38 and cap 39, there being intervening washers it? between the bonnet and lock nut and between the lock nut and the cap respectively.

As is evident, whenever the pressure in the inlet port 4 acting on the annular area A is less than the force exerted by the spring 34, the relief valve member 32 will remain in seated position. However, should the fluid pressure in port 4 increase to an extent such that the force on the annular area A exceeds the force exerted by the spring 34-, the relief valve member 32 will be forced upwardly away from the seat 31, whereupon, fluid may flow through Patented July 20, 1965,

the tubular seat member '2 into the bottom branch 12 of the tank port 10. a

It is to be noted that the relief valvebore is-the same at both ends so that the entirerelief valve assembly may be reversed end for end to bring the adjusting mechanism to the bottom portion of the bore, as viewed in FIG. 1. I

Thus, in the event of encountering close quarters at: the r top of the valve housing 2'with insuificient room for removal of the cap 39 .and turningof the adjusting screw .37 aswith a screwdriver or the like, the relief valve assembly may be reversed to'place the screw 37at the bottom. V

Referring now in detail to the. valve spool 45 andits cap 39 and adjusting spring-centering and float-position detent mechanism, it

can be seen that the'spool 45 is of' hollow-construction with one end of the hole 46 closedby a plug 47 which is threadedly engaged therewith andwhich is formed with a crossbore 48 for an actuator (not shown).

45 is a close sliding fit inthe spool'bore 7.

Starting at. the actuating end of the. spool: there are provided alternatelands and grooves with the lands 50,

The

spool

51, 52, 53, and 54, separ'ated'by intervening .gro'ovesSd, V

57, 58, and 5?, respectively. The lands 5t), 51., 53, and 54, are each, formed with metering slots, as shown, and,

in addition, thelands 50, and 53, are formed with open-' ings or slots 60 and ,61 which intersect the spool hole id for reasons whichiwill be presently explained' Finally, the land 51 just beneath the metering slotjthereof is formed with. an'orifice 62, the purpose for'which likewise will be explained in detail. p

Here again, as in connection with the relief valve bore 5, the spool bore 7 "issymmetrical withrespect tothe centerline'of the inlet and

tank ports

4 and 10, such bore and within the part 1%, by means of internal and external 'O-rings as shown and, if desired, a wiping type seal 197 may be provided between parts 164 and 10510 wipe foreignmattter fromthe surface of the spool '45. as the spool is moved down or up to'its several different positions. The packing assembly may be held in place as by screws ldrwthat, extend. through parts 103 and 105 and I t that have threaded engagement with housing 2.

From the foregoing description it is clear that by reason of the symmetrical formation of thespool bore 7 with its enlargements 53 and counterbores 64, the sleeve 100 and associated packing assembly and the restrictor sleeve 70 and associated spring-centering assembly, may be reversed end for end "with the packing assembly at the top and the spring-centering assembly at the bottom, as viewed in FIG. 1. Of course, when such sleeves and their associated assemblies are reversed, the valve spool 45 must also be and 50-51, which prevent flow of fluid between the respec- 7 being formed with an enlargement 63 and counterbore 64 at each end. p a t Fitted in the enlargement 63 in one end of the spool bore 7 is a restrictor'sleeve 70 which, adjacent its iower end, is formed with one or more restricted passages 71 tive service ports and 6 and the top and bottom branches 11 andlz of the tank port 10 and which prevent flow of fluid between the service ports and the re spective

secondary inlet branches

21 and 23.

Now, in order to raisethe piston 115 in the cylinder 116 and thus lift or hoist the load, such as a bucket and arm, or a blade and" arm, etc., the spool 45 is moved to the raise positio'nFR, whereby fluid under pressure in the inlet port 4- unseats the'upper check valve so communicating the annular space 72 therewithin with the 7 spool valve bore enlargement 63 by means of internal and 7 external O-rings 73 and 74. The restrictor sleeve 'is held in place as by means of a plate 75 which is seated in the counterbore 64. I

The upper end of the spool 45 has engaged with a shoulder thereof a spring retainer and axially spaced therefrom is another spring retainer 81' which engages a snap ring 82 fitted into. a groove in'the spool, the spring 83 being compressed between said retainers 8i) and 81. The upper end of the spool is formedwith a head 84 which is engaged in acounterbore in the detent member 85 which, adjacent its lower end, is formed with a pcripheral groove 86 and which, adjacent its upper'end, is formed -with a beveled shoulder 87, the split resilient that fluid flows into the upper secondary inlet 21, through the groove 59 of'th'e spool 45', through thefservice port 15 and line 17, into the head end of the cylinder 116. The fluid displaced from the rod end of the cylinder 116 flows through the line 18 to the other service port 15, through the groove. 56,,an-d through the bottom branch 1215f the tank port 10 via the space between the end of lower sleeve 1th) and surface 101. It is to be noted that as the spool is, shifted from N to R, the slots 60 are progressively restricted by sleeve while the orifice 62 is progressivley communicated with the bottom secondaryinlet 23,. and, .thereafter the metering slots of land 53 gradually cut off the flow from top inlet branch 8' to middle-tank branch 14 so that pressure builds up in service port 15 for hoisting of the load on piston 115.

' As the spool 45 is so moved to R position the top spring ring 89 or the like, being normallyengaged with the beveled shoulder 87 whereby spring 83 'maintains the spool 45 in fNeutral position, as shown in FIG. 1. The split resilient ring 89 is contained'in a groove formed between the

parts

90, and 91 which are threaded together and sealed as by means of the O-ring 92. The functions of the spring-centering and detent mechanism will pres ently be described in connection with the four different positions to which'the spool 45 may be shifted. The,

retainer 81is moved downwardly therewith to further compress the spring 83, whereby whenvspool actuating force isreleased, the expansive force of the spring 83 will automaticallyreturn the spool 45 to neutral position Now, let us assume that it is desired to lower thepiston 115 and load underpower control at a rapid rate. To do this, the spool 45 is. moved to lower position L,

whereby the bottom spring retainer 80 moves upwardly with the spool to furthercompr ess the spring 83 against the top retainer 81, the :detent member tls being held in place by the spring ring 89 which engages the beveled shoulder 37. During the initial portion of the movement i ofthe spool from neutral position N to lower position L, the slots 66 are progressively-cutoff,- the metering slots in land 51 come into play to gradually out Oh? the a shoulder or flange 192 which is disposed in clampedposition in a groove defined .by the parts 103,104, and,

of. a packing assembly, the part 103 being'seated in the counterbore and sealed by means of the O-ring 106,

and the intermediate part being sealed to thespool land bottom inlet branch 9 from the, middle tank branch 14, and the spool orifice 62 issb'roughtinto communication with the bottom inlet branch 9. In the lower position LTof the spo0l-45 fluid under, pressure flows from the inlet port 4 through the, bottom check valve 24) into thesecondary inlet 23 and past the groove 56 of the spool into the service port 16, and thence through the line 18 to the rod end of the cylinder 116, thereby aiding gravity in moving the piston 115 and load downwardly. The fluid displaced from the head end of the cylinder 116 flows through the line 1'7 into the service port 15 and thence past the groove 59 of the spool into the top branch 11 of the tank port by way of the restricted passages 71 in the restrictor sleeve 7d and thus, a back pressure will build up in the head end of the cylinder 115 to prevent cavitation in the rod end and to keep the rod end filled with fluid. The slots 61) and orifice 62 are at this time respectively in communication with the bottom service port 16 and with the bottom branch 9 of the inlet port 4. When the spool actuating force is released, the spring 83 acting on the bottom retainer 8t) and spool will automatically return the spool from lower position 1" to neutral position N.

When it is desired to connect together the head and rod ends of the cylinder 116 which is at the so-called float position, the spool 45 is moved up all the way to the float position F, whereby the detent member 85 will be shoved up and the beveled shoulder 87 thereof will expand the spring detent ring 89. The expansive force of the spring 83 will now move the detent member 85 with respect to the spool 45 until the detent 39 springs into the groove 86 thereof. In such float position F, the top branch 8 of the inlet port 4 is communicated with the center branch 14 of the tank port 10 by way of the spool groove 57. In this position also, the

service ports

15 and 16 are communicated with each other through the slots 61 and 69 and the spool hole 46. Thus, as the piston 115 with its load floats downwardly by gravity, the fluid displaced from the head end of the cylinder 116 flows through line 17 into the service port 15 and through the slots 61 and spool hole 46 to the slots 60 and through the slots 61 and spool hole 46 to the slots 60 and other service port 16 and thence by way of the line 13 into the rod end of the cylinder 116. Because the displacement of the head end is greater than that of the rod end, the excess fluid will be returned to the tank by way of the spool orifice 62 which now is in communication with the center branch 14 of the tank port 19. The orifice 62 is of such size that a slight back pressure is built up to prevent too rapid descent of the piston 116 and its load, whereby the rod end of the cylinder will always remain filled with fluid ready for immediate power lowering by shifting the spool 45 I from the float position F, to the lower position L.

It will be seen that the spring detent 89 engaged in the groove 86 will retain the spool 45 in the float position'F until it is manually shifted downwardly to disengage the detent 89 from the groove 86, whereupon the head 84 will move the detent member 85 to set the detent ring 89 against beveled shoulder 87, the latter action occurring when the spool has moved downwardly to the neutral position. Such float position allows the piston 115 to move in the cylinder 11d and it may be desired to follow up by power lowering to effect penetration or digging action of the bucket or blade.

As aforesaid, the formation of the valvespool bore 7 in symmetrical fashion from its middle to its opposite ends enables mounting of the restrictor sleeve 71) and centering spring and detent mechanism at either end thereof with the sleeve 1% and packing assembly at the other end.

With further reference to the sleeve 11th as shown in FIG. 1, it is to be noted that it extends into the tank port branch 12 and terminates short of the surface 191, whereby when the valve spool 45 is shifted from position N for free flow of fluid as aforesaid. Moreover, when the relief valve member 32 is forced away from the seat 31 the relieved fluid will flow from the inlet port 4 into the tank port branch 12 and will freely flow around the annular space between the sleeve ltltl and the tank port branch 12 to the tank port 10.

The sleeve 1% extending into the tank port branch 12, as shown in FIG. 1, affords prompt cutoff of the slots 6%) from the bottom tank port branch 12 when the spool 45 is shifted from position N to position R and, similarly, the slots 6i) are rather promptly communicated with the service port 16 as the spool 45 is shifted, for example, from position N to position F. Accordingly, the sleeve 1% makes possible the use of shorter strokes of the spool 45 thereby minimizing the length of the housing 2 in a direction axially of the spool bore 7. The sleeve 1% also makes possible the use of a housing 2 of minimum thickness since the tank port branches 11 and 12 are of substantial axial extent, whereby they need be of smaller diameter than otherwise to afford free flow of fluid as aforesaid through the annular space defined be tween the sleeve 1431) and the surrounding tank port branch 11 or 12, as the case may be.

Other modes of applying the principle of the invention may be employed, change being made as regards the details described, provided the features stated in any of the following claims, or the equivalent of such, be employed.

I therefore particularly point out and distinctly claim as my invention:

1. A spool valve assembly comprising a housing formed with a bore intersected axially therealong by a pressure inlet port, a tank port, and a pair of service ports adapted for connection with a hydraulic cylinder assembly to raise and lower a load on the piston thereof; a hollow valve spool axially reciprocable in such bore and formed with alternate lands and grooves and with openings through certain lands thereof to provide a neutral positon whereat said inlet and tank ports are in fluid communication and said service ports are blocked by lands of said spool, two operating positions whereat said inlet and tank ports are communicated selectively with said service ports to actuate such cylinder assembly in either di rection; and a float position whereat said service ports are communicated with each other and said inlet and tank ports are communicated with each other; and restrictor means effective, in one operating position only, to build up a back pressure in one service port when that service port is communicated with said tank port; said restrictor means comprising a perforate sleeve mounted in said housing to constitute an extension of such bore and through which fluid flows from said one service port to said tank port.

2. A spool valve assembly comprising a housing formed with a bore intersected axially therealong by a pressure inlet port, a tank port, and a pair of service ports adapted for connection with a hydraulic cylinder assembly to raise and lower a load on the piston thereof; a hollow valve spool axially reciprocable in such bore and formed with alternate lands and grooves and with openings through certain lands thereof to provide a neutral position whereat said inlet and tank ports are in fluid communication and said service ports are blocked by lands of said spool, two operating positions whereat said inlet and tank ports are communicated selectively with said service ports to actuate said cylinder assembly in either direction, and a float position whereat said service ports are communicated with each other and said inlet and tank ports are communicated with each other; said service ports, in the float position of said spool, being communicated with each other through the interior of said spool, and said spool being formed with an orifice which leads from the interior thereof to said tank port whereby, in the case of a differential displacement cylinder assembly, excess fluid from I one end flows through such orifice to the tank port andv back pressure builds up in the interior of said spool to prevent cavitation in the other end of such cylinder assembly, such orifice and one of the openings in said spool being arranged so that in one operating position of said spool they communicate'the inlet portwith the same service port that is at that time communicated with said inlet port, and a sleeve in said housing extending into said tank;

port and constituting an extension of such 'hore efiectiveto block such opening as said spool is moved from neutral position to the other operating position.

3. A spool valve assembly comprising a housing formed j with abore intersected axially therealong by a pressure inlet port, a tank port, and a pair of service ports adapted for connection with a hydraulic cylinder assembly to raise; and lower a load on the piston'thereof; a'hollow valve spool axially reciprocable in such bore and formed with alternate lands and grooves and with openings through certain lands thereof to provide a neutral position whereat said inlet and tank ports are in fluid communication and said service ports are blocked by lands of said spoolftwo operating positions whereat said inlet and tankports are alternatelands and grooves to. provide a neutral position whereatsaid inlet and tank ports are in fluid communication and said service ports are blocked by lands 'ofsa id spool, and two operating positions wherea-t said inlet and tank ports are communicated selectively with said service ports to'actuate such cylinder assembly in either direction; a pair of longitudinally spaced openings in said valve [spool in fluid communication with each other through the interior thereof; and a sleeve means mounted in said housing extending into said tank port and constituting an extension of such bore from the ad- 'jacent service port, said sleeve 'means being operative tobloick one of said openings as said valve spool is moved to one of said operating positions, saidsleeve means also being of smaller diameter than said tank port to. thus definean annular passage through which fluid is adapted to flow. I v 10. The spool valve assembly of claim 9 wherein said housing is formed with another bore, and a relief valve communicated selectively with said service ports to ac: tuate such cylinder assembly in eit'ner'direction, and a I float position whereat said service ports are 1 communicated with each other and said 'inlet and tank ports '{are' communicated with each other; and first and second restrictor means respectively eflective,in one operating position only of said spool, to build up a back pressure in one service port when that service port is communicated with said tank port and, in the float position of said spool, 1

bleed fluid from between said intercommunicating',service ports: to said tank port andyto build up a back pressure which, in the case of adifferential displacement cylinder.

in such another bore to communicatesaid inlet port with said tank port'when' the fluidpressure in said inlet port exceeds a predetermined maximum, such fluid flowing through said tank port and around said sleeve means.

, 11'. Thespool valve assembly-of c1aim 9 wherein the inner end of said sleeve means is spaced axially outwardly from the respective end of said bore and the adjacent service port.

12. A'spool valve assemblycomprising a housing formed with a bore intersected, axially therealong by a pressure inlet port, a tank port, and aservice port adapted for connection with a hydraulic .cylinder'assembly to actuate the piston thereof; a valve spool axially recipassembly, accommodates excess fiuid from one end and prevents cavitation in the other end; said first restrictor. comprising a perforate sleeve mounted in said housing to constitute an'extension of such bore and through which fluid flows from said one service port to said tank port.

4. The spool valve assembly of claim 3 wherein said second restrictor comprises an orifice through the wall.

of said spool, said spool having openings leading to the interior thereof which'communicate with the respective A service ports when said spool is in the float position.

5. The spool valve assembly of claim4 wherein said spool is formed with openings through the wall thereof which register with the respective service ports in the float position.

6. The spool valve assembly bore is enlarged at both ends to receive said sleeves inter changeably therein.

7. The spool valve assembly of claim 6 wherein'sai'd spool has spring means at one end effective to automatical- I Ely return it to neutral position from both operating positions and an actuating portion at'the other end by which it may be manually shifted from neutral to said operating and, float positions, said spool, together with said sleeves, being.

reversible end for end in such bore to'thus reverse the with respect to said housing. 1 V

8. The spool, valve assembly of claim 7 wherein said housing is formed with another bore which is parallel'to position of said spring means and'said actuating portion such valve spool bore, and a relief valve in such another bore to communicate said inlet port with said tank' port of claim 5 wherein such,

rocable in such bore and formed with alternate lands and grooves to provide a neutral positionwhereat said inlet port and tank port are in fluid communication and said service port is blocked by a land of said spool, two operating positions whereat said "inlet portand tank port are communicated selectively with said service port; and restrictor means effective to build u'pa back pressure in said service-portwhen communicated with said tank port; said restrictor means comprising a perforate sleeve mounted in said housing to constitute an extension of such bore and through which fluid flows from said service port to said tank port.

13. A' spool valve assembly comprising a housing "formed with 'a bore intersected axially therealongiby a pressure inlet port, a tank port, and; a pair of service ports adapted for connection with a hydraulic cylinder assembly to raise and lower a load on the piston thereof;

a valve spool'axially reciproca ble in such bore and formed with alternate lands and grooves and with openings through certain lands thereof to provide a neutral position whereat said inlet port-andtank port are in fluid communication and said service ports are blocked by lands of said spool, and two operating positions whereat said I inlet port and tank port are communicatedselectively when the fluid pressure in saidinlet port exceeds a predetermined maximum, said relief valve-having at one end thereof, adjusting means by which the opening pres-' sure is changed, such another bore and relief 'valve' being formed so that said adjusting means may be-disposed adjacent either end of such another bore 9. A spool valve assembly comprising a housing formed with a bore intersected axially therealong by a pressure inlet port, a tank port, and a pair of service ports adapted for connection with a h ydrauliccylinder assembly-tdraise and lower a load on thepiston thereof; a 'hollow valve spool axially reciprocable in such bore and formed with .vvithsaidservice ports to actuate such cylinder assembly in either direction; and restrictor means eflective in. at;

least one operating position to build up a back pressure in one service port when that service'por-t is communicated with said tank port; said restrictor means comprising a perforate sleeve mounted in said housing to constitute an extensionof such'bore and through which fluid flows from said one service port to said tank port.

Claims

1. A SPOOL VALVE ASSEMBLY COMPRISING A HOUSING FORMED WITH A BORE INTERSECTED AXIALLY THEREALONG BY A PRESSURE INLET PORT, A TANK PORT, AND A PAIR OF SERVICE PORTS ADAPTED FOR CONNECTION WITH A HYDRAULIC CYLINDER ASSEMBLY TO RAISE AND LOWER A LOAD ON THE PISTON THEREOF; A HOLLOW VALVE SPOOL AXIALLY RECIPROCABLE IN SUCH BORE AND FORMED WITH ALTERNATE LANDS AND GROOVES AND WITH OPENINGS THROUGH CERTAIN LANDS THEREOF TO PROVIDE A NEUTRAL POSITION WHEREAT SAID INLET AND TANK PORTS ARE IN FLUID COMMUNICATION AND SAID SERVICE PORTS ARE BLOCKED BY LANDS OF SAID SPOOL, TWO OPERATING POSITIONS WHEREAT SAID INLET AND TANK PORTS ARE COMMUNICATED SELECTIVELY WITH SAID SERVICE PORTS TO ACTUATE SUCH CYLINDER ASSEMBLY IN EITHER DIRECTION; AND A FLOAT POSITION WHEREAT SAID SERVICE PORTS ARE COMMUNICATED WITH EACH OTHER AND SAID INLET AND TANK PORTS ARE COMMUNICATED WITH EACH OTHER; AND RESTRICTOR MEANS EFFECTIVE, IN ONE OPERATING POSITION ONLY, TO BUILD UP A BACK PRESSURE IN ONE SERVICE PORT WHEN THAT SERVICE PORT IS COMMUNICATED WITH SAID TANK PORT; SAID RESTRICTOR MEANS COMPRISING A PERFORATE SLEEVE MOUNTED IN SAID HOUSING TO CONSTITUTE AN EXTENSION OF SUCH BORE AND THROUGH WHICH FLUID FLOWS FROM SAID ONE SERVICE PORT TO SAID TANK PORT.