JPS5843596B2 - vehicle power steering pump - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a pump, and more particularly to a power steering pump for use in and controlling a vehicle steering system.

Power steering pumps for use in vehicle steering systems are well known and there are several power steering pump designs.

Such pumps incorporate means for controlling fluid flow to the steering system in response to the pressure required of the steering fluid.

Additionally, means are typically provided to control the output from the pump to avoid directing excessive amounts of fluid into the system at high vehicle speeds.

Typically, this function is performed by a bypass valve, as shown in U.S. Pat. No. 3,200,752, which bypasses output pump flow to the pump inlet in response to pressure acting thereon.

By controlling the output flow to the system with the bypass valve, pump output is controlled and maintained to provide adequate fluid to the system for steering and to prevent excess flow to the system at high vehicle speeds.

In the type of system disclosed in U.S. Pat. No. 3,200,752, the bypass valve must bypass a significant volume of fluid to provide proper operation of the pump to the system.

For example, at very high speeds the amount of fluid bypassed is on the order of 25 gallons per minute, which of course requires substantially larger valves to bypass such fluid.

Recently, power steering pumps have been substantially improved, as disclosed in U.S. Pat. No. 3,822,965.

This patent omits a bypass valve such as is commonly used in power steering pumps.

Rather than using a bypass valve to bypass excessive fluid flow from the pump, the pump structure allows the pump to be opened by movement of a cheek plate that delimits a portion of the pump chamber that operates the pump discharge structure. and controls were developed.

a pump chamber connected to the inlet when the cheek plate moves;
A pump chamber connected with the outlet is in direct communication to allow fluid to flow from the outlet of the pump across the face of the pump discharge mechanism and cheek plate to the inlet of the pump.

No. 3,822,965, the cheek plate is moved by fluid pressure acting thereon, and a fluid pressure cavity is provided on the opposite side of the cheek plate from the pumping mechanism.

The control valve controlling the pressure within the cavity is a relatively small valve compared to the bypass valves described above and, of course, does not function as a primary bypass for fluid across the face of the cheek plate.

The present invention relates to a system of the type disclosed in U.S. Pat. controlling the flow rate of fluid into the system supplied by the pump to adjust and maintain the desired flow rate of fluid;

The cheek plate moves in response to changes in pressure acting thereon, which pressure changes are controlled by appropriate control valves responsive to a variety of different conditions, including excessive flow or high pressure in the system.

More particularly, the present invention is directed to the problem of stabilizing a control valve that controls the pressure within the cavity to control the position of the cheek plate.

The travel distance of the control valve to move the cheek plate to the fully open position is relatively small.

For example, the total travel of the control valve is 0.06 inches.
5 mm).

Therefore, any instability that even slightly affects the position of the control valve will affect the pressure in the cheek plate cavity, which in turn will affect the position of the cheek plate and thus affect performance.

Therefore, although not essential to uniformly and accurately controlling the pressure within the cheek plate cavity, it is necessary to maintain the control valve in the proper position and to avoid unnecessary forces on the control valve that would tend to displace the control valve. This is desirable.

Applicants have recognized that a variety of different structures may be used to stabilize the control valve that controls the pressure acting on the cheek plate.

However, in the preferred embodiment described herein, a small steady flow is provided that is directed through the control valve and stabilizes the control valve.

In the present invention, when the control valve is moved to a position to relieve the pressure in the cavity to control the movement of the cheek plate, the valve first moves to open and first communicates with the cavity to relieve the pressure in the cavity. , then a small steady flow of fluid from the pump outlet is provided across the control valve, and this steady flow serves to stabilize the control valve.

This steady flow has the nature of a slight leakage flow and a large volume bypass flow from the system occurs across the cheek plate.

What is essentially understood is that the constant valve flow in this case is simply a leakage flow and not a bypass flow as found in the prior art to maintain flow into the system at an appropriate level.

Further features and advantages of the invention will be apparent to those skilled in the art from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings.

The present invention is preferably implemented in a power steering pump 10.

This power steering pump 10 includes a member 12 and an outer shell 1 screwed into this member at a position indicated by reference numeral 14.
It has an assembly 11 consisting of 3.

The assembly 11 has a pump chamber 15 formed within the outer shell 13 in which is positioned a pump element of a pumping mechanism 16 for pumping fluid.

The pumping mechanism may be of any conventional construction, in this embodiment comprising a cam ring 20 suitably radially positioned relative to member 12 of assembly 11 by dowels 21.

The cam ring 20 has an internal bore into which a series of slippers 22 are rotatably mounted.

This slipper is attached to a slot located in the rotor 23.

The rotor 23 is rotationally driven by an input shaft 24 having a drive spline connected to the inner diameter of the rotor, as shown at 25.

The slipper 22 is attached to the cam ring 20 by a series of springs 26.
biased outwardly into engagement with the inner periphery of.

Adjacent slippers form pump pockets that expand and contract due to the cam ring shape as the rotor rotates.

This particular type of pump is known as a slipper pump, and its construction is well known and will not be described in detail here.

Of course, it is clear that when the input shaft 24 rotates, the rotor 23 is rotated to move the slipper 22 around the inner circumference of the cam ring 20.

As the slippers move around the inner circumference of the cam ring 20, they cooperate with inlets and outlets (not shown) formed in the port plate 29.

As the slipper passes through the inlet and outlet, the pump pocket formed between the two slippers expands and contracts.

Fluid is introduced into the expanding pump pocket and fluid is forced out of the contracting pocket.

The shapes of the inlets and outlets do not particularly form part of the invention and will not be discussed in detail here.

Preferably, the c4 pump has a double protrusion structure and has two
It has one inlet and two outlets.

Additionally, this embodiment does not show the particular passageway connecting the inlet to the fluid supply, nor does it show the entire outlet passageway communicating with the outlet.

Pump 10, such as that of US Pat. No. 3,822,965, has a cheek plate opening feature.

More specifically, the pump 10 has a cheek plate (side plate) 30 that delimits a part of the pump chamber 15 where the pumping action occurs.

The cheek plate 30 consists of a plurality of stamped metal plates, the details of which will not be described here.

The cheek plate is biased by a spring 31 into engagement with the pump discharge mechanism.

The axial surfaces 32 of the cheek plates engage adjacent axial surfaces of the cam ring and rotor and function to seal or block fluid flow from the pump pocket communicating with the inlet to the pump pocket communicating with the outlet.

Thus, when the cheek plate 30 is in the position shown in FIG. 1, the full power of the pump is transmitted to the steering gear since there is no fluid bypass between the inlet and outlet pump pockets.

However, if the cheek plate 30 is moved to the right from the position shown in FIG. It is clear that they will be in direct communication.

This significantly reduces any flow rate of fluid into the system supplied by the pump.

Of course, when the amount of movement of the cheek plate 30 is large, the amount of fluid bypassed is also large.

It is therefore clear that by precisely controlling the position of the cheek plate 30, precise control of fluid into the system can be achieved.

In order to accurately position the cheek plate, the pump 10 is located on the right side (looking at the drawing) of the cheek plate 30 and is generally designated by the reference numeral 3.
It is equipped with a pressure cavity indicated by 5.

It is apparent that any pressure in cavity 35 will bias cheek plate 30 into engagement with pump discharge mechanism 16, causing the cheek plate to move to the left and increase flow from the pump.

The fluid pressure in the cavity 35 is communicated to the cavity by suitable passage means 40 provided in the cheek plate.

This passage 40 communicates with the outlet of the pump.

Accordingly, fluid pressure is communicated to the cavity to drive the cheek plate to the position shown in FIG.

The cheek plate is provided with a seal in the form of an 0-IJ song 8 surrounding the cheek plate.

This O-ring 48 is configured to maintain a sealing relationship between the outer circumference of the cheek plate and the inner circumference of member 13 of assembly 11.

Therefore, there is no fluid leakage between the member 13 and the outer periphery of the cheek plate 30.

Therefore, the only fluid that enters cavity 35 is through passage 40, and the dimensions of cavity 35 must be accurately determined as will be apparent from the following description.

Of course, it is clear from the above that the force acting on the cheek plate 30 includes the output pressure of the pump acting on the face 32 of the cheek plate 30 to move the cheek plate to the right, and also to move the cheek plate to the left. The forces acting on include the pressure within the spring 31 and the cavity 35.

Furthermore, it is clear that by controlling the pressure within the cavity the position of the cheek plate can be controlled.

More specifically, pump 10 includes a control valve mechanism 50 that controls the pressure within cavity 35 .

This valve mechanism is positioned in a hole 51 provided in pump housing member 12.

Hole 51 and valve mechanism 50 communicate with cavity 35 through a passage 52 in member 12 and a hollow dowel pin 53.

Dowel pin 53 is connected to housing member 12 and extends through port plate 29 and cam ring 20 into cheek plate 30.

It is clear that this hollow dowel pin 53 communicates with the passage 52 at one end and with the cavity 35 at the other end.

What is further clear is that the dowel pins guide the axial movement of the cheek plate 30.

Furthermore, it is apparent that the cheek plate cannot rotate and the dowel pins help prevent that rotation.

Valve mechanism 50 reduces flow to the system in response to a momentary increase in fluid pressure in some cases.

In this case, the valve mechanism causes the pressure in cavity 35 to increase, so that the cheek plate moves to the left and increases the output of the pump to match the pressure and flow rate required by the system.

Additionally, the valve mechanism 50 reduces the pressure within the cavity 35 when excessive fluid flows into the system, such as at high pump speeds, thereby allowing the cheek plate 30 to move under the forces acting on its surface 32. Serves to bypass fluid flow into the system.

Accordingly, the pump 10 including the valve mechanism 50 is disclosed in U.S. Pat.
, 822,965 to regulate fluid flow to the power steering system.

Valve mechanism 50 includes a spool valve member 60 (see FIG. 2).
.

The spool valve member 60 is mounted within a sleeve member 61 that is positioned within the bore 51.

This sleeve member 61 has a plurality of lands 62, 63 and 63a.

The lands are axially spaced along sleeve member 61 and define a pair of circumferentially extending grooves or fluid passageways 64, 65 between them and bore 51.

Sleeve member 61 abuts shoulder 70 of housing member 12 at its right end.

At the left end of the sleeve member 61, as shown in FIG. 2, the sleeve member engages a screen 71 (shown schematically) which connects the left end of the sleeve member 61 to the open lower lower portion 4 which allows fluid to flow into the system. It is inserted between the press-fit plug member 73 and the press-fit plug member 73.

A high pressure IJ valve assembly 80 is suitably carried on the right end of sleeve member 61.

This IJ IJ-F valve assembly is described in U.S. Patent No. 3,822,
No. 965 is preferred and will not be described here.

A spring member 81 acts between the IJ valve assembly 80 and the valve spool 60 to bias the valve spool to the left in the drawing.

This valve spool 60 has an annular land member 8 that engages with a snap ring 83 supported on the inner periphery of the sleeve member 61.
2, and this land member is urged by a spring 81 and comes into pressure contact with a snap ring 83 as shown in FIG.

Passage or chamber 64 communicates with the inlet of the pump via passage 95, and chamber 65 communicates with passage 52 and thus with cavity 35.

The valve spool 60 has a projection 10 that projects through the passageway 74.
has 0.

An orifice 91 is formed between the outer surface of projection 100 and member 73.

The pump outlet is shown schematically at 90 in FIG. 2, and fluid from the pump outlet flows through screen 71 and then through orifice 91 and passage 74 to the power steering gear.

Furthermore, the valve spool 60 has a passageway 101 extending therethrough which communicates at the outer end of the projection 100 with the outlet conduit or system and at its inner end with the chamber 81a in which the spring 81 is located.

Therefore, in the position shown in FIG.
acts on the outer surface 110 of the land 82 and further on the surface 111 of the land 82,
It will be apparent that the pressure tends to act to move the valve spool 60 to the right as viewed in FIG.

It is further apparent that the spring 81 acting on the end face 112 of the spool and the pressure within the chamber 81a cause the valve spool to move to the left as viewed in FIG.

Of course, before starting the pump, the cheek plate 30 is biased by the spring 31 to the position shown in FIG. is pumped through the system.

If the system is a conventional central open system, the fluid is returned to the reservoir and from there to the pump in a known manner.

Of course, before starting, the valve spool 60 is in the position shown in FIG. 2 (which corresponds to the position shown in FIG. The opening 121 of 61 is covered.

This opening 121 communicates the groove 65 with the inner bore of the sleeve member 61.

When land 120 blocks communication between groove 65 and the inner bore of sleeve 61, cheek plate cavity 35 is blocked by the valve spool and fluid flows into the cavity through passageway 40, increasing the pressure within the cavity. .

As the pump speed increases when the parts are in this state, ie, in the position of FIGS. 1 and 3, the output flow rate increases proportionately.

This proportional flow rate occurs during the first or relatively bottom pump speed range.

However, as the pump speed increases and the flow rate to the system increases, the pressure acting on the face of the valve spool 60 acts to move the valve spool to the right.

Of course, the pressure acting on the surfaces 111 is due to the pressure drop caused by the flow through the orifice 91, so that these surfaces
The second speed range is higher than the first speed range where the pump speed is relatively low.
The valve spool is dimensioned to move the valve spool to the right from the position shown in FIG. 3 to the position shown in FIG. 4 when the range is reached.

When the valve spool is moved to the right to the position shown in FIG. be able to.

Additionally, regulated flow control is provided due to the tapered shape of the valve spool section 120.

Of course, fluid flows through passage 121 into the interior of valve sleeve 61 and through passage 131, groove 64 and passage 95 to the pump inlet.

This, of course, relieves the pressure in the cavity 35 of the cheek plate, and the pressure acting on the face 32 of the cheek plate 30 causes the cheek plate to move to the right in FIG. This results in a direct bypass from the pump outlet to the inlet.

This results in controlled flow into the system.

When valve spool 60 moves to the position shown in FIG. 4, valve spool land 82 moves to the position shown in FIG. It leaks from land 82 to flow through groove 64.

This fluid flow is referred to as a stable flow and is a very small fluid flow whose function is to stabilize the valve spool 60.

This flow alone slightly reduces fluid flow into the system.

The stabilizing flow 135 takes place after the pressure in the cheek plate cavity 35 is relieved by the passage formed by the land 120 passing through the passage 121.

Regarding the point, line 140 on the valve spool in the drawing
and line 141 is the distance between passage 136 and passage 121.
is greater than the distance between the leftmost edges of

As a result, the land 120 opens the passage 121, but the land 82 still blocks the passage 136, and this passage 136 begins to release the pressure in the cavity of the cheek plate (the line 140 is the passage 1
21) is not released until a stable flow is formed.

It is apparent that when valve spool 60 is in the position of FIG. 4, there is flow through passage 121 and passage 131 as indicated by arrow 150.

Additionally, there is flow to the exit of the system as indicated by arrow 151.

As is well known, these flows create forces acting on the valve spool 60 in the nature of pressure as well as fluid force.

Additionally, the valve spool 60 is subject to vibrational forces and the like that can destabilize the valve spool 60.

It is clear that the steady flow indicated by arrow 135 provides a stabilizing effect on the valve spool 60 and precisely controls the position of the cheek plate, thus providing a stable valve spool for precisely controlling the flow into the system. This is extremely important.

After the valve spool has been moved to the position shown in FIG. 4, the valve spool can be moved or adjusted in that position to control fluid flow to the stem during relatively high pump speeds.

For example, if the fluid pressure in the system increases for some reason, the flow through orifice 91 will be momentarily reduced.

As a result, the pressure acting on the surface 111 and the surface 110.112
The difference in pressure acting on the valve spool is reduced, thus causing the valve spool to move to the left as viewed in FIG. 2, reducing the escape of pressure in the cheek plate cavity.

This increases the pressure in the cheek plate cavity 35 such that the cheek plate 30 moves closer to the pumping mechanism 16, instantaneously increasing fluid flow into the system.

Furthermore, if the speed of the vehicle increases and the pump speed increases accordingly, the fluid through the orifice 91 will momentarily increase and the pressure drop across the orifice 91 will increase.

As a result of this, the difference between the pressure acting on surface 111 on the one hand and the pressure acting on surfaces 110° and 112 on the other hand increases, the pressure relief in cavity 35 increases and the valve spool moves to the right, Therefore, the pressure acting on the face 32 of the cheek plate causes the cheek plate 30 to move to the right.

Any movement of valve spool 60 to the left from the position shown in FIG. 4 is in a direction that interferes with fluid flow 135.

Furthermore, any movement of the valve spool 60 towards the right is caused by the spring 8
1, an arbitrary fluid pressure acts on the chamber 81a.

As a result, it is clear that there is some resistance when the valve spool is moved in either direction from the position shown in FIG. Furthermore, it functions to stabilize the valve spool in any position.

The characteristics of the operation of the system of the invention are clear from the above.

However, in order to fully understand the present invention and to show how it compares with the prior art known to the applicant, FIGS.
Attached is a graph of output flow rate versus pump speed shown in the figure.

For example, with reference to FIG. 5, it is apparent that as the speed of the pump increases, the output flow rate to the system supplied by the pump increases proportionally to the increase in pump speed.

Without flow control on the pump, the total area defined by the triangle shown in FIG. 5 is the area that represents the flow into the system based on increasing pump speed.

As mentioned above, a continuous increase in flow to the system as the pump speed increases is unsuitable for power steering pumps, and flow control should be performed to minimize the flow of fluid into the system at high speeds. It is extremely important to recognize that this must be done.

The diadarum shown in Figure 5 is similar to Dudley's US Pat.
23,244 is an outline of the structure and operation of the system.

Without going into all the details of the Dudley patent, the Dudley patent discloses a system in which a rotatable cylinder block is moved to bypass fluid.

The cylinder block is loaded by pressure in a chamber between them, and the flow into this chamber is caused by leakage around the cylinder block and between the pistons within the cylinder block.

As shown in FIG. 5, Area A represents the volume of fluid that does not flow into the system due to unloaded (open) or movement of the cylinder block relative to the port plate.

Region B represents the volume of fluid that does not flow into the system due to leakage around the cylinder block, and Region C represents the volume of fluid that does not flow into the system due to leakage around the piston. Shows the flow rate into the system at speed.

Based on a detailed examination of the Dudley patent, it is clear that
The inability to precisely control region C is due to the fact that uncontrolled leakage creates pressure during the leakage between the cylinder blocks.

It is further evident that all of the flows described above that reduce flow into the system occur simultaneously; in fact, bypass flow and piston leakage flow occur immediately upon operation of the pump.

FIG. 6 is a graph showing the operating characteristics of Slap, US Pat. No. 2,839,003.

Although this patent operates somewhat similar to the Dudley principle, the Slap patent includes a bypass valve to provide bypass flow that occurs before the cheek plate opens.

This bypass flow is the main flow that controls power output, and leaving the cylinder block unloaded (open) is a safety feature.

Region B of the Slap patent indicates the substantial region of fluid bypassed by the bypass valve.

Area A represents the portion of fluid flow bypassed due to opening of the cylinder block due to cylinder block movement, and area C represents the reduced portion of flow into the system due to piston leakage and leakage around the cylinder block.

It is clear that there is a timing difference in the Slap patent, and that the bypass valve opens to bypass fluid from the outlet before the cylinder block opens, and that this opening of the cylinder block is due to control at very high speeds. It is about being alone.

The graph in FIG. 7 illustrates the operation of US Pat. No. 3,822,965.

This patent precisely controls fluid pressure within the cavity of the cheek plate.

Region B is the portion of reduced flow into the system, which is a slightly metered flow from the cheek plate cavity.

Region A has more excess flow than region B provided by the opening of the cheek plate.

The lines defining the area indicate the regulated flow of fluid into the system.

The area is bounded by a sharp 6" knee of the curve as indicated by the symbol X in contrast to the smooth curve shown in FIGS. 5 and 6.

This, of course, provides optimally sensitive control and precise opening of the cheek plate.

FIG. 8 is a graph of the operating characteristics of the present invention.

The area indicated by C is the part of the flow from the cheek plate cavity 35 to □.

Region B is a stable flow. Region A is the portion of the flow that is bypassed due to opening of the cheek plate and movement of the cheek plate.

It is clear that stable flow region B begins at a very low flow rate and some time after the cheek plate begins to open.

In addition, a sharply curved 6 knee"

Further in accordance with the present invention, testing of fluid flow rates used for stabilization was conducted.

At different rotational speeds and different types of pump constructions, the stable flow rate or percentage of steady flow rate will vary.

For some pump configurations, at 7000 rpm the percentage of steady flow to total pump displacement is approximately 9.8% and 6.
．． It was 6qb.

These percentages are higher than needed for stability due to machine dimensions that give the machine room.

However, it is clear that the steady flow rate alone, which is a very small percentage of the total pump output, will give effective results as will be apparent to those skilled in the art.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of a power steering pump implementing the present invention, FIG. 2 is an enlarged sectional view of a part of the pump shown in FIG. 1,
3 and 4 are partial sectional views of the pump of FIG. 2 showing parts in different positions, FIGS. 5 to 7 are graphs showing the operating characteristics of conventional pumps, and FIG. 8 is a diagram showing the pump of the present invention. It is a graph showing the operating characteristics of the pump. 12... Member, 15... Pump chamber, 2
2...Slippers, 23...Rotor, 3
0... Teak board, 35... Blank space, 50
... Valve mechanism, 60 ... Valve spool 62
．． 63,63a...Land, 64.65...
... Fluid passage, 91 ... Orifice, 100
...Protrusion.

Claims (1)

[Scope of Claims] 1. A power steering pump for a vehicle that supplies fluid to a power steering system, the housing having an inlet and an outlet and forming a pump chamber, and capable of pumping fluid from the inlet to the outlet. pumping means having a pumping element forming a series of pumping pockets disposed in said pumping chamber and expanding and contracting to pump fluid; and during a first range of pumping speeds, pumping means having pumping elements forming a series of pumping pockets for pumping fluid; an apparatus for increasing fluid flow to the system and providing a substantially constant flow of fluid to the system during a second range of pump speed directly following the first range, the apparatus comprising: a cheek plate whose one axial side in the first speed range is located in a sealing position that blocks fluid communication between the pump pockets and acts to be driven away from the sealing position by the pressure of the outlet; means for forming a cavity on the other axial side of the cheek plate; means for forming a passageway for directing fluid from the outlet to the cavity having an internal pressure driving the cheek plate toward a sealed position; is movable to release the pressure in the cavity when reaches the second speed range, and the pressure applied to the cheek plate during the second speed range moves the cheek plate from the outlet to the inlet. valve means having a valve member operable to control pressure within the cavity to form a fluid bypass across one side of the cheek plate; and a pump during pressure relief of the cavity by the valve member. means for stabilizing the valve member by creating fluid flow through the valve member with fluid flow from the outlet into the system. 2. The valve means comprises a valve spool having at least two axially spaced lands, a first land cooperating with a first port communicating with the cavity, and a first land communicating with the pump inlet. a second port in communication and a second land cooperating with said first land;
forming a fluid flow from the cavity through the first port and through the first land;
the second land cuts it to form a stable fluid flow from the outlet to the inlet, and the first and second lands and the first and second ports are arranged in a cavity in the cheek plate. 2. The pump of claim 1, wherein the pump is positioned to form said steady flow after first communicating with said inlet. 3. The pump of claim 2, wherein said steady flow is on the order of less than 10 percent of the pump's output at high speed. 4. The stable flow and the relief flow in the cavity are arranged in the first direction so that these flows are at least generally in opposite axial directions.
3. A pump according to claim 2, wherein the pump returns to said inlet through port means disposed between a land and a second land. 5 further comprising means for forming an orifice disposed at the pump outlet, said valve means comprising a valve spool having lands and faces acted upon by pressure on opposite sides of said orifice, said pressure displacing the valve spool; The pump according to claim 1, which presses the pump so as to press the pump. 6. The valve spool has at least two axially spaced lands, a first land cooperating with a first port in communication with the cavity, and a first land in communication with the pump inlet. a second land cooperating with a second port, said first land passing from said cavity through said first port as it passes through said first port;
the second land forming a stable fluid flow across it from the outlet to the inlet, the first and second lands and the first
6. The pump of claim 5, wherein: and a second port is positioned to provide said steady flow after said cheek plate cavity first communicates with said population. 7 The valve spool is axially aligned with the orifice.
7. A pump according to claim 6, wherein the pump is arranged in parallel. 8. The valve spool has a protrusion extending therefrom, the orifice being partially defined by an outer surface of the protrusion, and the protrusion extending therethrough for communicating pressure acting on the lands of the valve spool to the system. 6. A pump according to claim 5, having a passageway such that the outlet pressure acts on a face of the valve spool. 9 A pump for supplying fluid to a system, the housing having an inlet and an outlet and forming a pump chamber, and a housing configured to pump fluid from the inlet to the outlet, the housing being configured to pump fluid from the inlet to the outlet. pump means having a pump element forming a series of pump pockets that expand and contract to operate, and a seal having a side opposite the pump means and supported by the housing to isolate fluid communication between the pockets; a cheek plate located in a position and moved axially from the sealed position to allow fluid to flow directly between the pockets and bypassed to the system, and a cavity on the other side of the cheek plate bounded by the housing; a fluid passageway disposed between the cheek plate and the housing for directing fluid flow from the outlet into the cavity for driving the cheek plate to the sealed position; and a fluid passageway disposed between the cheek plate and the housing from the pump chamber to the cavity. a seal for blocking leakage of entering pressure fluid; an orifice at said outlet through which fluid flows into the system; and a valve member movable within said cavity for displacing said cheek plate away from said pumping means. and means cooperating with said valve member to stabilize said valve member by forming a fluid flow from said outlet to said inlet during relief of said cavity by said valve means. supplying fluid to the system, the valve member having surfaces actuated by fluid pressure on opposite sides of the orifice and movable to control the pressure within the cavity to control the position of the cheek plate. pump.