CA1264138A - Differential pressure biased spool type valve controller - Google Patents

Abstract

DIFFERENTIAL PRESSURE BIASED SPOOL TYPE VALVE
CONTROLLER FOR A PNEUMATIC DUAL DIAPHRAGM
CONTROL SYSTEM

ABSTRACT OF DISCLOSURE

The invention relates to a direct acting pneumatic control system for controlling a dual diaphragm, or double acting cylinder type automatic guiding system. The control device and system is used to selectively adjust the position of one end of a guide roll relative to the opposite end, in order to adjust the position of an endless web traveling over a series of rolls.
In order to adjust and control the guide roll position, it is necessary to alternately exhaust one diaphragm, and or cylinder end, while maintaining the flow of air to the opposite diaphragm or cylinder end. This in effect shifts the end of the roll in a lateral direction, effectively steering the web in the proper direction to effect the necessary web correction.
To activate and control the valve position, the control valve is equipped with a palm which follows the web and con-tinually monitors the web position and activates the control valve spool to modulate the "flow-exhaust" rates to bring the web to the corrected position with no over-run or "hunting" for posi-tion. The palm and rod must follow the web when the web is moving away from the palm. It is necessary to have thrust within the valve to accomplish this function. In this design this thrust is accomplished through the use of differential pressure across the spool ends with the necessary porting within the valve to accomplish the spool displacement.

Description

~2~

FIELD OF INVENTION

The present invention relates to a control valve to con-tinually monitor and control a guide roll position and to effec-tively cause this guide roll to be moved in the necessary direc-tion to maintain an endless web traveling in a straight run around a series of rolls.

SUMMARY AND OBJECTS OF INVENTION

The present invention presents a spool type pneumatic controller valve which operates within a 35 to 75 psi input range and feeds this output pressure directly to a ~uide mechanism to accomplish necessary control and corrections. The present control valve is unique in that it is not a pilot device but handles full flow directly to diaphragms or a cylinder end without the benefit of additional linkage, mechanisms or pneuma-tic boosters.
The controller valve of the present invention is designed to control a guide roll very precisely and this is accomplished through the particular design of the porting within the valve. The "dead zone" or area of ~no correction" at the palm's neutral position is approximately one-eighth inch.
This porting design and arrangement was designed and developed to

(2) ~Z~ik!L~3~3 give ~ast and acc-lrate response even at the low range o~ lnlet pressure operation o~ 30-35 psi, and to maintain any correction to the guide roll position until an additional correction is called for as a result of a change in the position of the web or material being guided and controlled. This unique design feature of the valve controller eliminates the "hunting" characteristic usùally associated with a guide valve of this type. This design reature within the controller valve is possible because of the relatively small movement oE the spool within the valve body com-pared to web movement. ~he present spool design and the large ratio of palm movement to spool movement makes the valve sen-sitive and responsive to small changes in web position.
It is, therefore, an object of the present invention to provide a spool valve for a fluidic guiding system of the type that controls the running angle of material passing over a guide roll that overcomes many of the disadvantages and drawbacks of conventional control systems.
A further object of the present invention is to provide a control valve for a fluidic guiding system of the character referred to above which is simple in construction, includes rela-tively few moving parts, is reliable, and which requires little or no maintenance.
It is also an object of the present invention to provide a spool type control valve for a web guide roll guiding system that accurately and precisely responds to the movement of the web or other material passing over the guide roll.

(3) 3~

Another object of the ?resent invention resides in the provision of a spool t~pe control valve ~oc such a guiding system which includes a pivot arm directly operating the sliding spool which is responsive to lateral movement of material passing over the guide roll for actuating the spool accordingly so as to vary and control the running angle of the material passing over the same guide roll.
A further object of the present invention resides in the provision of a spool type control valve for a guiding system that is relatively inexpensive, but which is rugged in construction.
It is also an object of the present invention to provide a spool type control valve for controlling a web guiding system of the character referred to above which re~uires no adjustments and whose operation is not subject to temperature changes.
Still a further object of the present invention resides in the provision of a spool type control valve for a web guiding system of the character referred to above which is applicable and can be used with all web guiding applications.
A further object of the present invention resides in the provision of a spool type control valve for a web guiding system that requires no air supply treatment due to the relatively large orifices utilized.
Another object of the present invention resides in the provision of a web guiding system control valve which includes a universal mounting that enables the same to be mounted either

(4) ~z~
r.i~ht h~ncled or Le-ft harlded and whic~l can be converlientLy located remotely ~`rom the guiding .system.
Another ob.jec-t ol! the present invention resides in -the provision of a web g~liding system spool -t-ype control valve th..lt can be u.secl on single dia-ph:r~gm spring re-tllrn -type guiding sys-tems by simply plugging one outle-t prot.
S-ti:Ll a f~lrther object Oe the present inven-tion resides :in the provi.sion of a web guiding system control valve that includes a following arm and palm tha-t is biased to continuously monitor the edge of the material passing over a guide roll, wherein the pressure e~erted against the guide arm, in order -to follow the -~eb in a contact relationship, is of a relatively low pressure.
It is also an object of the present invention to provide a spool type control valve of the charac-ter referred to above wherein the biasing action eluded to is achieved through the accomplishment of a differential pressure across the ends of the valve's spool.
Another object of the present invention resides in the provision of a spool type control valve for a web guiding system that is completely enclosed so as to prevent contamination.
Finally it is an object of the present inven-tion to provide a web guiding sys-tem con.trol valve that is very compact.

lZ~ 3B

:In keep:ing wi-th the fore~o-ing there is prov-ide~l in accordance with the presen-t invention a spoo:l type controller assesnbly -tor con-tro:L:Iing and direct:ing ail rrom an air source to either side of a dual diaphragm pne~Ima~ic gIlid:ing system that is operatively corInected -to a guide roll ~'or moving and adjusting the guide ro:ll so as to appropriately guide material such as wire OI' felt web over -the guide roll, said spool type controller comprising:
A. a valve having a body, a spool movably moun-ted therein, an air inlet port, a pair of outlet ports, an exhaust port means formed therein wi-th each outle-t port being adapted to be communicatively connected to a respective diaphragm of said pneumat:ic guiding system;
B. biasing means operatively associated with said spool for biasing -the same toward a selected position within said valve;
C. spool actuating means for engaging an edge of said material passing over said guide roll and moving said spool back and forth within said valve in response to the lateral shifting of said material as the same passes over said guide roll wherein the movemen-t of said spool, as a result of the lateral movement o~ said material, results in -the guiding system adjusting the position of said guide roll so as to adjust the running angle of the material passing over said guide roll;
- 5a -~Z~

~ . wherein said spool act~l~ting rneans includes pivot arm means clirectlv cou-p:Led to said spool for moving saicl spool ~ack and forth within said valve as said pivot arm means is moved back and forth, said p:ivot arm means .inclllding

5 an inner end movably molln-ted -to said spool and extending thererom and fur-ther ;nclucl-ing a remo-te end having a follower secured thereto for engaging the edge of saicl ma-terial passing over said guide roll, such that said pivo-t arm means actuates and moves said spool bac,k and f'orth within said valve in response -to the lateral shifting of -the material passing over said guide roll;
E. said spool and pivot arm adapted to assume a first neutral position and wherein from said neutral position said pivot arm may pivo-t -to second and third extreme positions on each side of said neutra:L position; and F. wherein said valve and spool inc,lude means for opening said outlet ports and exposing the same to air pressure while closing said exhallst port means in response to said spool and pivot arm assuming said neutral posi-tion, thereby preven-ting the valve from experiencing "hunting".
BR _F DESCRIPTION OF DRAWINGS
The invention is illus-trated by way of example in the accompanying drawing wherein:

- 5b -3~

Fio,llre l is a sicle view ill~lstr~tion of an end.less web p,lss:irlg arollnd a series o~ ro:Lle~s incl~lclin~ a ~ui.de ~ol.ler.
Fi~gllres 2a, 2b and 2c are -top pl.an views of three separ~-te web ~rrangements similar -to tha-t shown in Figure 1 wherein in each o-f -the th:ree cases the g~ide roller is shown in a partic-llar position to illustrate the natural direction that the web tends to follow as a resu:Lt of -the par-ticular positioning of one end of the guicle roller.
Figure 3 is a schematic illustration of the guide roll, control valve, and associated plumbing for directing air to and -through the control valve and into the guide roll control system.
Figure 4 is a sectional view illustrating the spool type controller of the present invention with the spool being shown in a neutral posi-tion.
Figure 5 is a fragmentary enlarged sectional view showino a selected por-tion of the valve and its spool with the spool being in the neutral position.
Figure 5A is a sectional view of the valYe shown in Figure 4 particularly ilLustrating the side by-pass air channe:L formed in the body of the valve that enables air to be channeled to both sides of the spool.

126~3B

Figure 6 is a schematic illustration o the control valve o the present invention illustrating the flow of air therethrough when the palm of the control valve as shown in Figure 4 assumes the leftmost extreme position (position No. 2).
Figure 7 is a schematic illustration of the control valve of the present invention illustrating the flow of air therethrough when the palm of the control valve as shown in Figure 4 assumes the neutral position (position No. 1).
Figure 8 is a schematic illustration of the control valve of the present invention illustrating the flow of air therethrough when the palm of the control valve as shown in Figure 4 assumes the rightmost extreme position (position No. 3).
Figure 9 is a graph illustration showing pressure measurements for various palm movements o~ the control valve of the present invention.

DESCRIPTION OF INVENTION

With further reference to the drawings, reference is made to Figures 1, 2a, 2b and 2c in order to illustrate the manner of maintaining proper running alignment of an endless web.
In Figure 1 an endless web 10 is shown passing around a series of carrier rolls 12. A guide roll, indicated by the numeral 16, is provided and as seen in Figure 1 web 10 travels thereover. In addition a palm or guide arm 18 that contacts the edge of web 10 is illustrated in Figure 1. Palm or follower 18, as will be ~7) ~Z~ 38 ~nderstood from subseq~ent portions of this disclosure, is uti-li~ed in conjunctio~ ~ith a control system that is responsive to lateral or side-to-side movement of the web 10 for appropriately correcting and controlling the running ali~nment of the web 10.
In Figure 2a, the position of guide roll 16 results in the web 10 moving in the direction of the arrow shown therein.
By moving one end of guide roll 16 to the position shown in Figure 2b causes web 10 to move generally in the direction of the arrow shown therein, By moving guide roll 16 to the position shown in Figure 2c results in the web lQ moving in the direction of the arrow as shown therein.
Therefore, in the case of an endless web 10, it is desirable to continuously sense the position of the edge of the ~eb 10 with a palm or follower 18 (as illustrated in Figure 1) and in response ~o that position to adjust the end of guide roll 16 so as to appropriately control and correct the alignment of the web 10.
Now turning to Figure 3, an actuator or guide system for moving guide roll 16 back and forth is shown therein and indi-cated generally by the numeral 20. Actuator 20 is of a conven-tional design and includes a saddle bearing holder 22 that is designed to receive an end bearing assembly of guide roll 160 ~isposed on each side of saddle bearing holder 22 is a pair of diaphragms 24 and 26. It is appreciated that by inflating and deflating ~he respective diaphragms 24 and 26, that saddle bearing holder 22 and accordingly guide roll 16, can be moved back and (8~

~Z~38 forth. I~ is this back-and-forth control mo~Jement that results in the continuo~s control o~ the alignment of web 10.
The present invention particularly relates to a control valve, 1ndicated generally by the numeral 15, for controlling actuator 20 and the respecti~e diaphragms 24 and 26 thereof.
Details of the control valve will be specifically dealt with sub-sequently herein. Peior to viewing details of sontroller valve 15, a general discussion of the air flow to the control valve and on to the actuator 20 will be dealt with.
In this regard, reference is made to Figure 3. Therein an air supply indicated by the numeral 28 is provided. Air from air supply 28 is directed through an inlet line 13 to a par-ticular inlet port of the control valve 15. Air passing through line 13 wlll pass through an on-off valve 30, filter 32 and a pressure regulator 34.
Control valve 15 is mounted adjacent the traveling web 10 by a support structure 210 ~5 will be discussed in more detail subsequently herein, control valve 15 includes a palm or follower 18 that continuously monitors the edge of web 10 and responds to the lateral movement thereof so as to continuously vary the output flow from control valve 15. Note in Figure 3 that control valve 15 further includes two additional ports, that again will be discussed in more detail subsequently hereinO
These two ports are operatively connected to diaphragms 24 and 26 via lines 17 and 19.

(9) ~Z~4~3~3 Turning to Figure 4, there is shown therein a spool type valve controller, indicated generall~ by the numeral 15. Control valve 15 comprises a housing structure 36 which includes a sliding spool 37. Secured to each end of the spool by hardware 43 and 44 are rolling diaphragms 38 and 39. These diaphragms are also secured between housing 36 and respective end covers 36a and 36b thereby forming separate chambers 41 and 42 at each end of spool 37.
Valve 15 includes an inlet port 40 formed in end cover 36a that directs inlet pressure to both chambers 41 and 42 through a by-pass port 36c formed in housing 36 and end caps 36a and 36b~ This by-pass port 36c is shown particularly in Figure 5A.
By specifically designing a suitable difference in an effective area b,etween diaphragms 38 and 39, the spool is biased to assure that palm 18 and connecting rod 45 will follow the web 10 being guided.
Pressure supply to the valve proper is accomplished through a port 37a formed through the center of spool 37. Flow gains entrance to port 37a through an opening located in retaining screw 43. Port 37a terminates short of the palm actuating mechanism area but extends sufficiently to feed pressure ports 37b and 37c.
Continuing to refer to Figure 4, a rod 45 extends from pivot arm 47 which is pivotably mounted to valve body 36 through a pivot pin 46. This imparts thrust to spool 37 in either a ~10~

le~t or right direction, the particular direction beirlg deter-mined by the movement of pal~ 18. A ball cap 48 is designed to receive pivot arm 47. Ball cap 48 includes a circular section and is fitted in a mating circular bore in spool 37. This allows the rotary motion of pivot arm 47 to convert to slidinq motion between ball cap 48 and spool 37. Expressed in another way, as rod 45 is swung back and forth, pivot arm 47 can rotate or oscillate in ball cap 48 and ball cap ~8 can move transversely or back and forth within the spool 37 and within the confines of the valve housing 36. This design essentially permits the rod 45 and extending palm 54 to effectively move spool 37 back and forth.
A bellows 49 is provided to protect the interior of the valve from exterior contaminants and as shown in Figure 4 extends from the valve body 36 to cover the pivot arm 47 extending from the valve.~
Provided in valve housing 36 are two outlet ports 50 and 51. In addition there is an exhaust port 52. A vent 53 is pro-vided to prevent the build up of back pressure against diaphragm 38 by any pressure leakage past spool 37.
Formed on spool 37 is a raised annular exhaust control shoulder 35 that is flanked on each side by raised annular control collars 31 and 33. Exhaust shoulder 35 includes a transverse channel or cross over 37d which allows air entering the valve through either ports 50 or 51 to cross over and be ~11) exhausted through exhaus~ port 52. It is appreciated that the shifting of spool 37 in either direction will allow exhaust port 52 to be opened to one of the ports 50 or 51.
As seen in Figures 4 and 5, in a neutral position, outlet ports 50 and 51 are aligned with respective raised annular collars 31 and 33 so that the port5 50 and 51 assume a generally closed position. However, the annular collars 31 and 33 are each slightly notched so as to continuously supply a pressure to each of the diaphragms 24 and 26 of the actuator or guiding system 20 This compensates for any pressure loss due to "blow by" when the valve is in a neutral position. Note in Figure 5 that the notched area of annular collar 31 is specifically shown and re~erred to by 31a.
In Figure 4 valve 15 is shown in the neutral posi-tion. In the neutral position, inlet air enters port 40 and spool ports 37a, 37b and 37c. This maintains inlet air pressure on outlet ports 50 and 51. Exhaust port 52 is closed.
In Figure 4 construction line 60 represents palm posi-tion No. 2 which moves a maximum of twenty degrees from the neutral position. In this position port 50 is fully open to inlet pressure via ports 37a and 37b. Port 51 is opened to exhaust port 52 via cross over 37d.
Also in Figure 4, construction line 62 represents a third palm position. In this third position, the palm 54 moves a maximum of twenty degrees to the right of the neutral position, (12) ~ZÇ~L3~

as shown in Figure 4. In this third palrn position, port 51 is fully open to inlet pressure via ports 37a and 37c. Port 50 is opened to exhaust port 52 via cross over 37d.
Palm position No. 2 and palm position No. 3 occur at the full extremes of the palrn's movementO When palm 54 moves slightly toward construction line 60, outlet port 50 becomes increasingly exposed to inlet pressure and outlet port 51 becomes increasingly exposed to exhaust port 520 For minor corrections to the web 10 position, the flow rates are low and the corresponding speed of correction is slow. This design elimina-tes over-reaction and consequent "hunting" of the guide.
Essentially the flow to the respective diaphragms 24 and 26 (or to a cylinder end) is proportional to the displacement of palm 18 relative to the neutral position.
The opposite conditions prevail when palm 18 moves slightly towards the No. 3 position tCOnStrUCtiOn line 62). That ls, outlet port 51 becomes increasingly exposed to inlet pressure while outlet port 50 becomes increasingly exposed to exhaust port 52. Again the flows are proportional to spool displacement.
In Figures 6, 7 and 8, the flow of fluid or air through valve 15 is illustrated in each of the three positions just described. In Figure 6, palm position No. 2 (construction line 60) is illustrated. In Figure 7~ the neutral position is illustrated. Figure 8 illustrates the third position or the position occupied by the palm when the same assumes that position represented by construction line 62 in Figure 4.

(13) 3~

It should be reiterated that valve 15 and spool 37 are particularly designed such that the effective areas on the opposite side of the spool are not equal. Because of this there exists a differential pressure across the ends of the spool 37 and in the present case, this results in the spool tending to be displaced toward the left (see Figure 4). Therefore, it is appreciated that palm 18 follows the edge of the web or material lO passing over guide roll 16. As the palm 18 follows the edge of web lO, the rod or swing arm 45 is operative to control actuator or guiding system 20. As the palm 18 oscillates between the two extreme positions indicated in Figure 4, it is appreciated that the respective diaphragms 24 and 26 are altern~tely inflated and exhausted and this results in the end of guide roll 16 being moved laterally back and forth so as to adjust the running angle of the material or web ]0 passing over the various carrier rolls 12.
Figure 9 displays a graph Oe actual diaphragm pressures plotted from the ~ero or neutral position of the actuator or guide roll carrier. These pressures are given for each 0.25~ of palm movement, and illustrate the gradual and relatively straight line increase in pressure on the high pressure side and the corresponding decrease in pressure on the exhaust or low pressure side.
The gradual increase in force on the ~igh pressure side results in a very uniform speed of movement, and provides precise control of the guide roll through the entire range of movement.
Reversal of high pressure and low pressure sides would result in a mirror image of the graph.
~14) ~6~3~3 The present invention may, of course, be carried out in other specific ways than those herein set forth without departing from the spirit and essential characteristics of the invention.
The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, and all changes comi.ny within the meaning and equivalency range of the appended Claims are intended to be embraced therein.

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Claims (13)

CLAIMED INVENTION

What Is Claimed Is:

1. A spool type controller assembly for controlling and directing air from an air source to either side of a dual diaphragm pneumatic guiding system that is operatively connected to a guide roll for moving and adjusting the guide roll so as to appropriately guide material such as wire or felt web over the guide roll, said spool type controller comprising:

A. a valve having a body, a spool movably mounted therein, an air inlet port, a pair of outlet ports, an exhaust port means formed therein with each outlet port being adapted to be communicatively connected to a respective diaphragm of said pneumatic guiding system;
B. biasing means operatively associated with said spool for biasing the same toward a selected position within said valve;
C. spool actuating means for engaging an edge of said material passing over said guide roll and moving said spool back and forth within said valve in response to the lateral shifting of said material as the same passes over said guide roll wherein the movement of said spool, as a result of the lateral movement of said material, results in the guiding system adjusting the position of said guide roll so as to adjust the running angle of the material passing over said guide roll;
D. wherein said spool actuating means includes pivot arm means directly coupled to said spool for moving said spool back and forth within said valve as said pviot arm means is moved back and forth, said pivot arm means including an inner end movably mounted to said spool and extending therefrom and further including a remote end having a follower secured thereto for engaging the edge of said material passing over said guide roll, such that said pivot arm means actuates and moves said spool back and forth within said valve in response to the lateral shifting of the material passing over said guide roll;
E. said spool and pivot arm adapted to assume a first neutral position and wherein from said neutral position said pivot arm may pivot to second and third extreme positions on each side of said neutral position; and F. wherein said valve and spool include means for opening said outlet ports and exposing the same to air pressure while closing said exhaust port means in response to said spool and pivot arm assuming said neutral position, thereby preventing the valve from experiencing "hunting".

2. The spool type controller of Claim 1 wherein said spool includes opposite ends and wherein said biasing means includes means for generating a differential pressure across said opposite ends of said spool such that because of the presence of the differential pressure said spool is biased toward a selected position.

3. The spool type controller of Claim 2 wherein said inlet port is connected to a source of compressed air and wherein said means for generating said differential pressure includes the provision of a pair of chambers disposed adjacent opposite ends of said spool and communicatively connected to said inlet port for receiving and holding compressed air therein and wherein said compressed air within said chambers acts against adjacent ends of said spool, and wherein one end of said spool includes a greater effective surface area exposed to the compressed air of the adja-cent chamber than the other end of said spool thereby giving rise to said differential pressure across the opposite ends of said spool and wherein said spool is biased by said differential pressure toward the end of the spool opposite the end having the greater surface area exposed to the adjacent chamber.

4. The spool type controller of Claim 1 including a joint assembly operatively interconnected between said spool and said pivot arm means.

5. The spool type controller of Claim 4 wherein said joint assembly includes a cavity actually formed in said spool and an opening providing access thereto, and a movable member contained within said cavity and operatively connected to said pivot arm means such that as said pivot arm is moved back and forth said spool is likewise moved via said joint assembly.

6. The spool type controller of Claim 5 further including a pivot pin connecting said pivot arm means with said valve adjacent said joint assembly converting rotary to sliding motion.

7. The spool type controller of Claim 6 including a pivot arm opening formed in said valve through which said pivot arm means projects, and wherein there is provided a flexible boot secured to said valve adjacent said pivot arm opening and which encompasses a portion of said pivot arm extending from said valve.

8. The spool type controller of Claim 2 including a pair of diaphragms secured across opposite ends of said spool so as to form opposed chambers adjacent each end of said spool and to generally seal the formed opposed chambers from the area surrounding said spool.

9. The spool type controller or Claim 8 wherein there is provided securing means for securing each diaphragm directly to the end of said spool such that said diaphragm rolls back and forth with said spool as the same is actuated; and wherein there is further provided means for securing said diaphragm to said valve body such that the respective diaphragms can roll back and forth with said spool.

10. The spool type controller of Claim 4 including means for moving said joint assembly back and forth within said valve in a direction transverse to the longitudinal axis of said spool as said pivot arm is swung back and forth.

11. A method of controlling a dual diaphragm fluid type control system that in turn controls the running angle of materials such as a web passing over a guide roll of the control system, said method comprising:
A. providing a spool type valve with two outlet ports and an exhaust port and operatively interconnecting the same between said dual diaphragm fluid type control system and a source of fluid pressure;

B. moving said spool back and forth within said valve to alternately direct fluid to each of the dual diaphragms of said fluid type control system for moving said guide roll back and forth and accordingly varying the running angle of material passing over said guide roll;
C. communicatively connecting said outlet ports with the dual diaphragms of the fluid control system;
D. moving said spool back and forth through a neutral position within said valve to alternately direct fluid to each of the dual diaphragms of said fluid type control system for moving said guide roll back and forth and accordingly varying the running angle of material passing over said guide roll;
E. opening said two outlet ports and exposing the same to pressurized air and closing said exhaust port in response to said spool assuming said neutral position, thereby preventing the spool type valve from "hunting" during the control process;
F. said steps of moving said spools back and forth within said valve including engaging the edge of the material passing over said guide roll with a follower;
G. moving a pivot arm back and forth in accordance with the movement of said follower; and H. connecting said pivot arm with said spool and moving said spool back and forth in response to said pivot arm being moved back and forth so as to control the flow of said fluid from said valve to said respective diaphragms of said dual diaphragm control system for moving and adjusting the position of the guide roll and varying the running angle of material passing over said guide roll.

12. The method of Claim 11 including the step of biasing the spool to a first position by generating a differential pressure across the ends of said spools.

13. The method of Claim 12 including the step of directing fluid into an inlet within said valve and into opposed chambers disposed on opposite ends of said spool, and providing a greater spool exposed surface area to the adjacent chamber about one end of said spool than the other so as to generate a dif-ferential pressure across the spool ends so as to naturally bias the spool toward the spool end opposite the end with the greater surface area exposed to the adjacent chamber.