US3882678A

US3882678A - Automatic connecting valves for reserve pumps

Info

Publication number: US3882678A
Application number: US476432A
Authority: US
Inventors: Rolf Fassbender
Original assignee: ZF Friedrichshafen AG
Current assignee: ZF Friedrichshafen AG
Priority date: 1973-06-08
Filing date: 1974-06-05
Publication date: 1975-05-13
Anticipated expiration: 1992-05-13
Also published as: DE2329328A1; YU145874A; SE398623B; JPS5818521B2; DE2329328C2; YU39071B; SE7407132L; GB1469458A; FR2232715A1; BR7404655D0; FR2232715B1; JPS5032374A; IT1013262B; ES426731A1

Abstract

In systems using a main pump and a reserve pump where it is essential that pressure feed to a consumer unit, e.g., a vehicle booster steering device, be available in the event of failure of the main pump, the invention provides quick shifting to a reserve pump. The effect is achieved in two embodiments of the invention by means of a valve which is spring biased to a closed flow condition of the main stream from a main pump, but opening the main flow responsive to consumer unit demand, while, however, permitting continuous reserve pump flow to reach the consumer unit prior to such opening. A pilot stream from the main pump establishes a pressure differential on the valve to control response to main pump failure for cutting in the reserve pump. The pilot flow is through a pressure chamber means in which a drop in pressure due either to unit demand or main pump failure is effective to cause valve actuation for main flow opening, or valve opening for reserve pump flow. An important feature of the invention resides in the fact that the pressure drop is achieved either by loss of main pump pressure or reduction in discharge rate, e.g., due to a slipping drive belt of the main pump.

Description

United States Patent [1 1 Fassbender 51 May 13, 1975v [75] Inventor: Rolf Fassbender, Mutlangen,

Germany [73] Assignee: Zahnradfabrik Friedrichshafen Aktiengesellschaft, Postfach, Germany [22] Filed: June 5, 1974 [21] Appl. No.: 476,432

[56] References Cited UNITED STATES PATENTS 2,440,371 4/1948 Holley 417/286 FOREIGN PATENTS OR APPLICATIONS Germany 60/405 Primary Examiner-Edgar W. Geoghegan [57] ABSTRACT In systems using a main pump and a reserve pump where it is essential that pressure feed to a consumer unit, e.g., a vehicle booster steering device, be avail able in the event of failure of the main pump, the invention provides quick shifting to a reserve pump. The effect is achieved in two embodiments of the invention by means of a valve which is spring biased to a closed flow condition of the main stream from a main pump, but opening the main flow responsive to consumer unit demand, while, however, permitting continuous reserve pump flow to reach the consumer unit prior to such opening. A pilot stream from the main pump establishes a pressure differential on the valve to control response to main pump failure for cutting in the reserve .pump. The pilot flow is through a pressure chamber means in which a drop in pressure due either to unit demand or main pump failure is effective to cause valve actuation for main flow opening, or valve opening for reserve pump flow. An important feature of the invention resides in the fact that the pressure drop is achieved either by loss of main pump pressure or reduction in discharge rate, e.g., due to a slipping drive belt of the main pump.

12 Claims, 2 Drawing Figures PATENTEI] MAY 1 3197s D2 122 C2 B2 FIG.2

AUTOMATIC CONNECTING VALVES FOR RESERVE PUMPS Briefly, the invention comprises a valve construction, which, for simplicity, is of the slidable piston valve type having annular grooves which coact for flow control of main and reserve pumps with annular grooves in a valve housing and connecting with inlet means from the pump and outlet means for exhaust of the reserve pump and connection to a consumer unit. Each end of the piston valve is exposed to a pressure chamber across which is a pressure differential when the main pump is operative. Such pressure differential is effected by main pump pressure causing a pilot flow between the chambers through a choke passage. A spring normally biases the piston valve to main flow shut-off except for the pilot flow in one embodiment, although in a second embodiment the pilot flow is likewise shut-off until there is a drop in pressure of one of the pressure chambers increasing the pressure differential sufficiently to overcome the spring bias for opening main flow to a consumer unit. Thus, the upstream pressure chamber is always connected to the main pump while the downstream pressure chamber is always connected to the consumer unit. Accordingly, when pressure is dropped in the downstream chamber upon demand of the consumer unit by virtue of the valving therein being opened, the differential pressure increase effects the actuation of the piston valve essentially due to output pressure of an operative main pump. Thus, main flow is connected and reserve flow shut-off. However, should main pump failure occur, the differential pressure is no longer sufficient to hold the piston valve in actuated position against the spring and accordingly the spring moves the piston valve back to initial position. In the initial position the reserve pump provides continuous flow to the consumer unit available for immediate use, but such flow is cut off upon actuation of the piston valve to open main flow to the consumer unit. Upon reestablishing the initial position of the piston valve, due either to completion of need by the consumer unit or failure of the main pump, reserve flow again is established to the consumer unit. Various check valves are provided in the system in order to prevent short circuiting of flow from either pump under certain conditions as will be described.

The invention has an advantage over previous known arrangements for the reason that the valving is responsive not only to pressure drop but also to some malfunction effecting flow rate reduction through the choke passage which translates flow rate into pressure differential. This causes the piston valve to be responsive to flow rate whereby a reduction causes a decrease in differential pressure and permits the spring to shift the piston valve to the initial position in order to make certain of connection to the reserve pump.

Prior art devices are responsive only to pressure drop and it would be possible for a main pump to maintain a sufficient pressure to keep the reserve pump shut off through the valve and yet not provide a sufficient discharge rate for the consumer device. This, of course, makes it virtually impossible, for example, to steer a vehicle. In other words, the diminishing of main pump discharge rate provides insufficient pilot stream to maintain proper pressure differential for holding the piston valve in main flow open condition.

A detailed description of the invention now follows in which:

FIG. 1 is a longitudinal cross section of the valve construction showing connections to pumps, a tank, and a consumer unit, wherein there is continuous pilot flow from the main pump through the valve feeding to the consumer unit at all times that the main pump is operative.

FIG. 2 is a longitudinal cross section of an embodiment wherein pilot flow is normally cut off until differential pressure on the piston valve effects movement for establishment of the pilot flow and wherein grooves of the valve construction supplant one of the check valves of the previous embodiment for preventing flow short circuit.

The invention comprises a construction having a housing 1 with a throughbore 2 in which is slidably disposed a spool or sleeve-like piston valve 3. Suitable conduit connection fittings, e.g. 20, close bore 2, as shown, at one end. At each end of the housing is a pressure chamber such as the pressure chamber 4 at the upstream end of bore 2 and the pressure chamber 5 at the downstream end. Pressure chamber 4 connects via conduit or line 6 to the main pump 7 for ultimate feed to a pressureoperated unit 8, such as a booster steering device, via line 9 connecting to pressure chamber 5. Flow takes place at a relatively small rate from chamber 5 and is continuous while pump 7 is operative.

Within the pressure chamber 5 is a compression spring 10 socketed in the end of piston valve 3 and bearing against fitting 2a to bias valve 3 to an initial and limiting neutral position against a locking ring 3a as shown. Housing 1 has a port 11 for intake from a reserve pump 12, further on the housing has a port 13 connecting to unit 8, and the housing also has an outlet port 14 connecting to the oil reservoir or tank 15. Port 13 communicates with a housing groove 16 and port 14 connects with a housing groove 17.

Piston valve 3 is provided with radial bore 18 and axial throughbore 19 for communication to annular groove 20 which is thus connected with pressure in chamber 4. The piston valve also has an annular groove 21 which in all positions of the piston valve communicates with port 11 for inflow of pressure feed from reserve pump 12 either to feed unit 8 or exhaust to tank 15.

Bore l9 terminates in a choke passage 22 leading to pressure chamber 5 and pressure chamber 5 communicates with a check valve 23 in fitting 2a closing in the direction of the pressure chamber 5 by spring bias as shown. Accordingly, it will be understood that fluid flow from main pump 7 can effect passageway by virtue of pressure opening check valve 23 to reach unit 8 via line 9. However, such flow consists of a relatively small control or pilot stream causing a differential pressure in

chambers

4 and 5 due to the choke passage 22 to effect automatic shifting of piston valve 3. Thus, the upstream chamber 4 would have a greater pressure to cause shifting of the piston valve to the left to send flow from pump 7 to unit 8 via

passages

19, 18, 20, l6, l3, 9 and a check valve 24. This occurs when main pump 7 is fully operative as hereinafter explained, otherwise reserve pump 12 provides booster pressure to unit 8. The coacting control edges of the grooves in the housing and in the piston valve are in effect gaps A1, B1, C1, opening and/or closing to effect passageways in the usual manner of piston valve operation. However, it

should be noted that when piston valve 3 is shifted to the left from the position in FIG. 1, the gap C1 will first open, exhausting pump 12 to tank 15 followed by closing gap B1 shutting off pump 12 to the unit 8 and finally by opening of gap Al effecting full main pump flow to unit 8.-Thus', the pressure differential means acting on the piston valve is responsive to an increaseor decrease in pressure in chamber 5 to effect actuation of the piston valve either by high differential or, on low differential by spring bias. Operation:

As seen in-FIG. 1 the piston valve 3 is in neutral posi' tion and there is communication of reserve pump flow to unit 8 from tank 15 via pump 12, groove 21, port 13 to line 9 via check valve 24. Regardless of pressure from main pump.7 in line 9 check valve 24 can open to feed reserve pump flow to unit 8 when required. Thus, if main pump 7 is inoperative, reserve pump 12 provides steering boostpress'ure'.

However, assuming normal operation of main pump I 7, a small pilotflo'w passes from tank 15 via pump 7,

bores '19, 18, (filling groove 20), choke passage 22, the check valve 23, line 9, to unit 8. This flow is available for immediate steering boost although upon demand of unit 8, such as actuation of valving in the boost mechanism (not shown), a greater flow throughchoke passage 22 occurs. This flow increase causes a differential pressure in cham bers 4 and 5, dependent not only on pressure but also rate of flow. Thus, a rate is controlled by choke passage 22 large enough so that the

chambers

4 and 5 communicate to provide the continuous pilot stream from chamber 5 to unit 8. Thus, a differential pressure drop exists wherein pressure is greater in chamber. 4 upstream of the choke passage 22 to effect ashift to the left ofvalve spool 3 against spring 10, clue to the area of valve 3 exposed to higher pressure in chamber 4-than'in chamber 5. The differential pressure necessary to overcome spring occurs upon demand of unit 8',when pressure drop due to.a rate increase of flow from chamber 5 takes place. The shift first opens gap'Cl and then closes gap Bl, reserve pump flow then I exhausting to tank via port 14, and subsequently gap A1 opens for main pump heavyflow via grooves and 16. Some pilot flow past valve 23 still continues. However, should main pump 7 fail, check valve 23 will close to prevent short circuit or bypass of reserve pump 12 flow from line 9 back'through choke 22, bore 19 and pump 7 (by effecting hydraulic motor rotation) to tank 15. Thus, in'event of such failure of. the mainpump 7, the pressure drop from chamber 4 to chamber 5 would not be'sufficient to keep .spring 10 compressed, and

spring 10 would return the valve 3 to the position of .FIG..1'. This connects the reserve pump 12 to line 9 for .feed to unit 8.

In normal operation of main pump 7 should gaps B1 1 and C1 be open in some valve 3 position, flow from pumpf7fvia choke passage 22 to line 9 could short cir- The modification of FIG. 2 differs essentiallyifrom that shown in FIG. 1 in that there is no check valve of a gap D2 is relied on to block a short-circuiting of the reserve pump 1 12 in the event of failure of the main' pump 107 should piston valve 103 be in open position I at the time of such failure.

Thus, the components of FIG. 2 comprise the housi f ing 101 having the piston valve 103 slidable therein, with an upstream pressure chamber 104 fedvia line a 106 from main pump 107. Differential pressure can w occur between upstream chamber 104 and downstream? chamber 105 only after leftward shift of the piston." valve acting against spring 110 which in neutral position maintains the piston valve 103 in its limiting position toward the right. Flow from reserve pump 112] passes through open gap B2 and then to the line than feeds unit 108, all as hereinabove described. Inthisin- 1 stance, the choke passage 122 is a radial bore in the pis-f 1 ton valve, which will be seen to connect to the axial I chamber 105 upon shift. Feed to line 109 is viaa radial bore 103a in the end skirt formation of the piston valve.

Thus, upon demand of'unit 108, thesudden drop in pressure in chamber 105 will effect shifting ofthe piston valve 103 to the left due to pressure in chamber 104. Flow through chokepassage 122 then occurs upon opening of gap D2 effecting the pilot streamwhich by differential pressure in the pressure chambers will control the action of the piston valve as hereinabove explained. l-Iowever, in the event of failure of the main pump 107, spring 110 will'move the piston. valve to the extreme right limiting position thus closing gap D2 to prevent short-circuiting of outlet flow from re V serve pump 112 which is at that time fe'edingunit'108 via open gap B2 and the check valve shown, 124.

The effect and operation of the gaps C2 and A2 are precisely as described for the gaps C1 and A1, respectively, ofFlG. 1.

It will be apparent that the design of the components of the invention are fairly simple, due regard being had 11 I for. the strength of the compressionspring and also for into a main flow and a pilot flow which governs the pressure differential and thus the actuation of the pis-" ton valve. The choke passage 22 is suitably designed so i as not to cause any significant loss of main pump pressure available to the consumer unit upon shift 'of the piston valve. Thus, it is only necessary to have a pilot. stream sufficient to hold the piston valve in anactuated position against the spring. Further, in addition to a minimum pilot stream being desirable for the above reasons it is preferable to have it of such a valve that the gap C1 has neither too much overlap, nor too little, so that upon actuation of the piston valve there willbe a quick change for full flow from the reserve pump.

ln instances where the main pump has relatively low operational pressure, the system is faced with frequent actuation of the piston valve in the embodiment of FIG,:

1. However, this is somewhat overcome by the continuous flow of the pilot stream to the consumer unit at all times, Further, even under conditions of frequent valve actuation there will be no'pressure failures occuring in feed to the consumer device.

The rapidity of valve actuation may be provided for a'relatively large choke passage 22 in the case of FIG. 1 or 122 in the case of HO. 2. t

Essentially, the invention provides pressure feed to a consumer unit at all times with a minimum of leakage.

I claim: 1. A valve for controlling main flow from a main pump and reserve flow from a reserve pump, to a consumer'unit; t

comprising a housing having a main pump inlet and a reserve pump inlet; said valve having passage means for a pilot stream and main flow from said main pump; outlet means for main flow from said main pump and reserve flow'from said reserve pump to a consumer unit; actuable valve means in said housing; said valve means and'said housing having coacting elements relatively positionable to open main flow to said outlet means and close reserve flow or to close main flowand open reserve flow to said outlet means; A

bias means normally biasing said valve means to close main flow and open reserve flow to said outlet means; i

pressure differential means comprising upstream and downstream pressurechambermeans operative to actuate said valve means to overcome said bias means to open main'flow and close reserve flow upon'increase ofpressure differential; further comprising choke passage means operable to pass said pilot' stream from said upstream to said downstream pressure chambenmeans; and a connection from said downstream pressure chamber means to said outlet'means whereby a reduction in pressure differential to a predetermined value permits said bias means to actuate said valve means to effect reserve flow to said consumer unit.

2. A valve for controlling main flow from a main.

pump and reserve flow from a reserve pump, to a consumer unit;

comprising a housing having a main pump inlet and a reserve pump inlet;

main flow outlet means for main flow from said main pump and for reserve flow from said reserve pump. to a consumer unit;

actuable valve means in said housing;

said valve means and said housing having coacting elements relatively positionable to open main flow to said outlet means and close reserve flow, or, to close main flow and open reserve flow to said outlet means;

hias means normally biasing said valve means to close main flow and open reserve flow to said outlet means;

pressure differential means comprising communicable upstream and downstream pressure chamber means effecting pressures operative on said valve means to actuate said valve means against said bias means to open main flow and close reserve flow upon relative increase of pressure in said upstream chamber means; said pressure differential means comprising pilot stream means providing pilot flow from said main pump to said upstream pressure chamber means to effect a normally higher pressure therein for maintaining said valve means in an actuated position; I

and a connection from said downstream pressure chamber means to said outlet means whereby relative increase of pressure therein reduces the differential pressure between said chambers a predetermined value in event of main pump failure to permit said bias means to actuate said valve means to effect reserve flow to said consumer unit. 3. A valve for controlling main flow from a main pump and reserve flow from a reserve pump, to a consumer unit; i comprising a housing having a main pump a reserve pu'mp inlet; main flow outlet means for main flow from said main pump and for reserve flow from said reserve pump, to a consumer unit; l actuable valve means in said housing; said valve means and said housing having coacting elements relatively positionable to open main flow to said outlet means and close reserve flow, or, to close main flow and open reserve flow to said outlet means; bias means normally biasing said valve means to close inlet and main flow and open reserve flow to said outlet means;

pressure differential means comprising upstream and downstream pressure chamber means having pressures acting on said valve means operative to overcome said bias means andactuate said valve means to open-main flow and close reserve flow; said differential pressure means comprising pilot stream means providing a pilot stream flowing from said upstream to said downstream pressure chamber-means in an actuated position of said valve means when said main pump is operative to effect a relative reduction of pressure in said downstream pressure chamber means whereby a predetermined differential pressure maintains main flow to said outlet means;

and a connection from said downstream pressure chamber means to said outlet means whereby a decrease in pressure in said upstream pressure chamber means to a'predetermined value in event of main pump failure permits said bias means to actuate said valve means to effect reserve flow to said consumer unit.

4. A valve for controlling main flow from a main pump and reserve flow from a reserve pump, to a consumer unit;

comprising a housing having a main pump inlet and a reserve pump inlet;

said valve having passage means for a pilot stream and main flow from said main pump; outlet means for main flow from said main pump and reserve flow from said reserve pump to a consumer unit;

actuable valve means in said housing;

v bias means normally'biasing said valve means to close main flow and openreserve flow to said outlet means- .1

. pressuredifferential means comprising upstream and downstream pressure chamber meansoperative to actuate said valve means to overcome said .bias meansto open main flow and close reserve flow upon increase of pressure differential;

said pressure differential means being operable to permit flow ofsaid pilot stream from said upstream to said downstream pressureehamber means and effecting a pressure drop in the course of said flow;

and a connectionfrom said downstream pressure 1 I chamber means tosaid outletmeans whereby a reduction in pressure differential to a predetermined I value pcrmits s'e id bias meansto actuate said valve meansto effect reserve flowto said consumer unit. 5. A valve as setforth in claim 4, said valve means co'm'prising'a slidablejpiston valve having a throughbore connecting with said upstream pressure chamber means;

a choke passage "jsaid throughbore and with said downstream pressure chamber means in at least one position of said piston valve; l v a housing for said valve means in which said piston valve is slidable; said housing and piston valve having coacting grooves to effect or cut-off flow from said pumps to said consumer unit;

a spring acting against said'piston-valve to bias it to a position wherein flow from said reserve pump is effected and flow from-said main pump-is cut-off.

6..A valve as set forth in claim 5, includinga check valve in said outlet means to preclude-'flow from said pilot stream from short circuiting through said valve means from said outlet means when saidwpiston valve is being actuated.

7. A valve as set forth in claim 5, including a check valve in said connection from said downstream chamber means to prevent flow from said reserve pump from short circuiting through said valve means and main pump when said piston valve is being actuated, in event of failure of said main pump.

8. A valve as set forth in claim 5, said housing having a bore; v I said piston valve being slidable in said bore; said pressure chamber, means being portions of said bore at opposite respective ends of said piston valve;

in's'aid piston valve connecting with a wall across the throughbore of said piston valve;

in pilot flow cut off position by bias of said spring and respective grooves in said piston valve and housing, If

said piston valve.

-10. A valve as set forth in claim 5, including a check valve in said connection from said downstream chamr ber means to prevent flow from said. reserve pump from I I short circuiting through said valve means when said pis- I ton valve is being actuated;

said housing having a bore; said piston valve being slidable in said bores said pressure chamber means being portions'of said bore at opposite respective ends of saidp'isto'n,

valve; a wall across the throughbore of said piston valve; said choke passage being through said wall for effecting an exhaust outlet connectable with the reserve pump inlet when saidvalve means is in actuated position effecting main flowto said consumer unit,

in pilot flow cut-off-position in coaction with said housflows only upon actuation of said prstonvalve;

wherein said piston valve and housing have'respec tive coacting grooves for control of flow from said pumps to said consumer unit;

and wherein a groove of said piston valve and a said piston valve is being actuated, in event of'failure of said main pump. =l

whereby said pilot stream flows only upon actuation of ing a continuous pilot stream to said consumer unit from said main pump through said check valve. 11. A valve as set forth in claim 5, said housing hav- 12. A valve as set forth in claim 5, said: piston valve having a radial bore effecting said choke passage and connecting with said throughbore and being normally ing by bias of said spring and whereby saidpilotstream

Claims

1. A valve for controlling main flow from a main pump and reserve flow from a reserve pump, to a consumer unit; comprising a housing having a main pump inlet and a reserve pump inlet; said valve having passage means for a pilot stream and main flow from said main pump; outlet means for main flow from said main pump and reserve flow from said reserve pump to a consumer unit; actuable valve means in said housing; said valve means and said housing having coacting elements relatively positionable to open main flow to said outlet means and close reserve flow or to close main flow and open reserve flow to said outlet means; bias means normally biasing said valve means to close main flow and open reserve flow to said outlet means; pressure differential means comprising upstream and downstream pressure chamber means operative to actuate said valve means to overcome said bias means to open main flow and close reserve flow upon increase of pressure differential; further comprising choke passage means operable to pass said pilot stream from said upstream to said downstream pressure chamber means; and a connection from said downstream pressure chamber means to said outlet means whereby a reduction in pressure differential to a predetermined value permits said bias means to actuate said valve means to effect reserve flow to said consumer unit.

2. A valve for controlling main flow from a main pump and reserve flow from a reserve pump, to a consumer unit; comprising a housing having a main pump inlet and a reserve pump inlet; main flow outlet means for main flow from said main pump and for reserve flow from said reserve pump, to a consumer unit; actuable valve means in said housing; said valve means and said housing having coacting elements relatively positionable to open main flow to said outlet means and close reserve flow, or, to close main flow and open reserve flow to said outlet means; bias means normally biasing said valve means to close main flow and open reserve flow to said outlet means; pressure differential means comprising communicable upstream and downstream pressure chamber means effecting pressures operative on said valve means to actuate said valve means against said bias means to open main flow and close reserve flow upon relative increase of pressure in said upstream chamber means; said pressure differential means comprising pilot stream means providing pilot flow from said main pump to said upstream pressure chamber means to effect a normally higher pressure therein for maintaining said valve means in an actuated position; and a connection from said downstream pressure chamber means to said outlet means whereby relative increase of pressure therein reduces the differential pressure between said chambers a predetermined value in event of main pump failure to permit said bias means to actuate said valve means to effect reserve flow to said consumer unit.

3. A valve for controlling main flow from a main pump and reserve flow from a reserve pump, to a consumer unit; comprising a housing having a main pump inlet and a reserve pump inlet; main flow outlet means for main flow from said main pump and for reserve flow from said reserve pump, to a consumer unit; actuable valve means in said housing; said valve means and said housing having coacting elements relatively positionable to open main flow to said outlet means and close reserve flow, or, to close main flow and open reserve flow to said outlet means; bias means normally biasing said valve means to close main flow and open reserve flow to said outlet means; pressure differential means comprising upstream and downstream pressure chamber means having pressures acting on said valve means operative to overcome said bias means and actuate said valve means to open main flow and close reserve flow; said differential pressure means comprisIng pilot stream means providing a pilot stream flowing from said upstream to said downstream pressure chamber means in an actuated position of said valve means when said main pump is operative to effect a relative reduction of pressure in said downstream pressure chamber means whereby a predetermined differential pressure maintains main flow to said outlet means; and a connection from said downstream pressure chamber means to said outlet means whereby a decrease in pressure in said upstream pressure chamber means to a predetermined value in event of main pump failure permits said bias means to actuate said valve means to effect reserve flow to said consumer unit.

4. A valve for controlling main flow from a main pump and reserve flow from a reserve pump, to a consumer unit; comprising a housing having a main pump inlet and a reserve pump inlet; said valve having passage means for a pilot stream and main flow from said main pump; outlet means for main flow from said main pump and reserve flow from said reserve pump to a consumer unit; actuable valve means in said housing; said valve means and said housing having coacting elements relatively positionable to open main flow to said outlet means and close reserve flow, or, to close main flow and open reserve flow to said outlet means; bias means normally biasing said valve means to close main flow and open reserve flow to said outlet means; pressure differential means comprising upstream and downstream pressure chamber means operative to actuate said valve means to overcome said bias means to open main flow and close reserve flow upon increase of pressure differential; said pressure differential means being operable to permit flow of said pilot stream from said upstream to said downstream pressure chamber means and effecting a pressure drop in the course of said flow; and a connection from said downstream pressure chamber means to said outlet means whereby a reduction in pressure differential to a predetermined value permits said bias means to actuate said valve means to effect reserve flow to said consumer unit.

5. A valve as set forth in claim 4, said valve means comprising a slidable piston valve having a throughbore connecting with said upstream pressure chamber means; a choke passage in said piston valve connecting with said throughbore and with said downstream pressure chamber means in at least one position of said piston valve; a housing for said valve means in which said piston valve is slidable; said housing and piston valve having coacting grooves to effect or cut-off flow from said pumps to said consumer unit; a spring acting against said piston valve to bias it to a position wherein flow from said reserve pump is effected and flow from said main pump is cut-off.

6. A valve as set forth in claim 5, including a check valve in said outlet means to preclude flow from said pilot stream from short circuiting through said valve means from said outlet means when said piston valve is being actuated.

8. A valve as set forth in claim 5, said housing having a bore; said piston valve being slidable in said bore; said pressure chamber means being portions of said bore at opposite respective ends of said piston valve; a wall across the throughbore of said piston valve; said choke passage being through said wall for effecting a continuous pilot stream to said consumer unit from said main pump through said check valve.

9. A valve as set forth in claim 5, said piston valve having a radial bore effecting said choke passage and connecting with said throughbore and being normally in pilot flow cut off position by bias of said spring and rEspective grooves in said piston valve and housing, whereby said pilot stream flows only upon actuation of said piston valve.

10. A valve as set forth in claim 5, including a check valve in said connection from said downstream chamber means to prevent flow from said reserve pump from short circuiting through said valve means when said piston valve is being actuated; said housing having a bore; said piston valve being slidable in said bore; said pressure chamber means being portions of said bore at opposite respective ends of said piston valve; a wall across the throughbore of said piston valve; said choke passage being through said wall for effecting a continuous pilot stream to said consumer unit from said main pump through said check valve.

11. A valve as set forth in claim 5, said housing having an exhaust outlet connectable with the reserve pump inlet when said valve means is in actuated position effecting main flow to said consumer unit.

12. A valve as set forth in claim 5, said piston valve having a radial bore effecting said choke passage and connecting with said throughbore and being normally in pilot flow cut-off position in coaction with said housing by bias of said spring and whereby said pilot stream flows only upon actuation of said piston valve; wherein said piston valve and housing have respective coacting grooves for control of flow from said pumps to said consumer unit; and wherein a groove of said piston valve and a groove of said housing are intermediate the connection from said downstream pressure chamber means and said radial bore and coact to prevent pilot stream flow when said piston valve is biased to said main flow cut-off, whereby to prevent flow from said reserve pump from short circuiting through said valve means and main pump when said piston valve is being actuated, in event of failure of said main pump.