GB2232208A - A variable displacement vane pump - Google Patents

Abstract

A variable displacement vane pump (1), e.g. for providing power assistance to a vehicle steering system (47), comprises a cam ring (3) which is movable between minimum and maximum operating positions, and a pressure relief and flow control valve arrangement (29) having a flow control orifice and a hydraulic liquid feedback circuit. When power assistance is not required the cam ring (3) is in a minimum position and the pump (1) operates in a standby output mode to provide a pilot hydraulic liquid flow via orifices 44 and 24, line 45 and steering control valve 46 to keep the hydraulic system served by the pump (1) in a charged condition. When power assistance is required the cam ring (3) is moved with the use of a back pressure to the maximum position and the pump (1) operates in a maximum output mode to provide a maximum hydraulic liquid flow. The flow control valve (30) is moved against spring (43) by the pressure differential created across orifice (24) when the flow reaches a predetermined level to release excess flow to spill port (42). Excess pressure is relieved to spill port (42) by the automatic opening of relief valve (34). <IMAGE>

Description

A VARIABLE DISPLACEMENT VANE PUMP This invention relates to a variable displacement vane pump.

Variable displacement vane pumps are well known.

Some known variable displacement vane pumps can be wasteful of energy in that they tend to operate at maximum output at low pump speeds, even although this maximum output is often not required for long periods of time. More specifically, in conventional hydraulic power assisted steering systems, the maximum power assistance and the predetermined steering angle velocity are required during parking and low vehicle speeds, when tyre scrubbing loads are highest. Because of this, hydraulic pumps are employed which are sized to give sufficient flow at engine idle speed to achieve the required steering angle velocity. In the conventional hydraulically operated power assisted steering systems, the pump is a fixed displacement pump and this means that at high engine speeds such as in motorway driving, the flow from the fixed displacement pump far exceeds the system requirements.In order to prevent all the hydraulic flow from the pump passing through relatively small clearances in the vehicle steering control valve, which would cause excessive back pressure and excessively high temperatures in the system, a flow control valve is fitted. This flow control valve diverts excess liquid flow away from the steering control valve and back to a fluid reservoir, or back to the pump inlet. This reduces the back pressure but a flow of oil sufficient to operate the steering cylinder at the full steering angle velocity still has to pass through small clearances in the steering control valve, and some back pressure is still present in the system.Furthermore, because the flow control valve is caused to operate by a pressure drop across a flow control orifice situated in the pump discharge, the pressure within the pump is higher than in the remainder of the system by the amount of the pressure drop across the flow control orifice. The pressures being generated at high pump speed by a pump which is large enough to supply sufficient flow for the system requirements at engine idle speed, together with the pumping losses caused by moving large amount of-hydraulic liquid such as oil through the flow control valve, result in the waste of large amounts of energy, together with the generation of excessive heat within the power steering system.

The waste of large amounts of energy is undesirable. More and more cars are going to front wheel drive and power assisted steering is often required or preferred. However, with smaller cars, relatively small engines are employed and these engines are not designed to have so much excess power that large amounts of energy can be wasted. Furthermore, it is desirable to avoid excessive heat generation. This is especially so in hot climates or where vehicles are running at high speed on motorways. If too much heat is generated, this can cause the hydraulic liquid to become too thin so that the hydraulic liquid may provide less lubrication than is required and the hydraulic liquid may tend to leak. Decreases in lubrication may in turn cause seais to wear and excessive heat may cause the seal material to harden, so that the seals then do not seal properly.

In an attempt to meet the above mentioned problems, the previously used fixed displacement pumps have been replaced by variable displacement pumps. More specifically, it is known to employ a flow sensitive single sided vane pump in a power assisted steering system, the pump displacement being reduced as pump speed is increased.

The variable displacement pump includes a novable cam ring which is held at a position of rest and in a maximum output position by a spring. As the pump speed increases, the cam ring is moved from its maximum output position by pressurised oil from each side of a discharge orifice which is connected to sealed chambers which pressure equal but opposing areas of the outside periphery of the cam ring. As flow through the orifice increases, a pressure difference between the upstream and downstream sides of the orifice is created.This pressure difference acts on the opposing areas by the pressure from the upstream side of the orifice (the greatest pressure),being directed to the side of the cam ring where it will act to overcome the spring when a predetermined pressure difference is reached, and move the cam ring from its maximum output position by an amount which increases as the pump speed increases, and thus a nearly constant pump displacement is maintained. Such a variable displacement pump is known as a flow sensitive pump but a disadvantage of this type of pump is that the normal pump size has to be large enough to supply sufficient flow at engine idle speed to operate the steering system at the specified maximum steering angle velocity, which limits the amount of energy saving available at the lower pump speeds.Useful energy savings only occur at high vehicle speeds, which in many cases may only be a small percentage of the total vehicle running time.

It is an aim of the present invention to reduce the above mentioned problems.

Accordingly, this invention provides a variable displacement vane pump comprising a cam ring which is movable between minimum and maximum operating positions, and a pressure relief and flow control valve arrangement, the pressure relief and flow control valve arrangement having a flow control orifice and a hydraulic liquid feedback circuit, and the pump being such that during operation when power assistance is not required the cam ring is in a minimum position and the pump operates in a standby output mode to provide a pilot hydraulic liquid flow which keeps a hydraulic system served by the pump in a charged condition, and with the pump also being such that when power assistance is required the cam ring is moved with the use of a back pressure to the maximum position and the pump operates in a maximum output mode to provide a maximum hydraulic liquid flow.

Energy savings can thus be made as the pump only operates in a maximum output mode when maximum hydraulic liquid flow is required. This avoids providing maximum hydraulic liquid flow at times when the flow is not required, thus saving on energy and also minimising on heat generated. If the variable displacement pump is used in a power steering system or similar hydraulic circuit, then it may use less power than a conventional system and it may run on lower system temperatures. This is due to the use of the variable displacement pump to provide the hydraulic system with a flow of oil which is less than is required to achieve a specified flow control, for example to achieve a specified steering angle velocity, when the system is not providing power assistance, but which reacts to back pressure generated in the hydraulic circuit when full power assistance is required to provide the full flow required by the hydraulic system. The inclusion of the pressure relief and flow control valve arrangement enables control of the flow and pressure to the specified levels throughout the operating cycles of the variable displacement pump.

The flow control may operate as in a normal system with the same flow control setting, but under low pressure conditions the flow control in the present invention will operate at higher pump speed due to the cam ring remaining in its minimum operating position the pressure relief valve may operate exactly as in a normal system to limit the maximum pressure to a predetermined level.

The pressure relief and flow control valve arrangement may comprise a single unit having a combined pressure relief valve and a flow control valve.

Alternatively, the pressure relief and flow control valve arrangement may comprise a separate pressure relief valve and a seperate flow control valve.

Preferably, the variable displacement vane pump is a variable capacity rotary vane pump. The vane pump is preferably a flat vane pump but it could be a roller vane pump or a slipper vane pump.

The cam ring may be held against a stop member in the minimum position by biasing means. Preferably, the biasing means is a spring.

The variable displacement pump may be one in which the pilot hydraulic liquid flow is directed to a system control valve, the pilot hydraulic liquid flow being one which is less than is needed for providing the power assistance at a maximum predetermined requirement but one which creates back pressure when the system control valve is operated, and the variable displacement pump being one in which the back pressure acts on the cam ring to move it to the maximum position via pressure-responsive means.

The pressure-responsive means may be a piston which pushes on the cam ring. Alternatively, the pressure-responsive means may be a part of the cam ring which is constructed and adapted to be pressurised.

The present invention also provides a hydraulic system when provided with the variable displacement vane pump. The hydraulic system is preferably a hydraulic power assisted steering system. The hydraulic system may however be another type of hydraulic system if desired.

The present invention further provides a vehicle when provided with the hydraulic power assisted steering system.

Embodiments of the invention will now be described solely by way of example and with reference to the accompanying drawings in which: Figure 1 shows a first variable displacement vane pump in a hydraulic system in the form of a vehicle power assisted steering system; Figure 2 shows a first modified pump similar to that shown in Figure 1; Figure 3 shows a second modified pump similar to that shown in Figure 1; Figure 4 shows a third modified pump similar to that shown in Figure 1; Figure 5 shows a different position of the pump shown in Figure 4; Figure 6 shows a fourth modified pump similar to that shown in Figure 1; Figure 7 shows the pump shown in Figure 6 but in a different position; Figure 8 shows another variable displacement power assisted steering vane pump; and Figure 9 shows a typical flow graph.

Referring to Figure 1, there is shown a pump 1 having a recess 2 which contains a movable cam ring 3.

The movable cam ring 3 moves in an arc around a pivot 4.

The preferred position of the pivot 4 is on a line at 900 to a line of eccentricity of the cam ring 3, and passing through a point of minimum cam eccentricity 5.

This is the eccentricity of the cam ring 3 at rest on stop 15. The advantage of this position is that when the pump is running with the cam ring 3 in the minimum output position, the internal pressures will be almost equally balanced about the pivot 4, and any increase in pressure within the pump due to flow control occuring at high speed, will not tend to move the cam ring 3 to the maximum output position.

The pivot 4 is arranged to form a seal with the pump housing 1.

The outer periphery of the cam ring 3 has a protrusion 6. One side of the protrusion 6 is an arc 7.

The arc 7 is struck from the centre of the pivot 4. The protrusion 6 is located in a similarly shaped recess in the pump housing, one side also being an arc 8 which is struck from the centre of the pivot 4 and which provides a minimum running clearance with the arc 7.

The protrusion 6 has a slot 9 which contains a sealing member 10. The sealing member 10 runs on the arc 8 during cam movement. The sealing member 10 and the pivot 4 provide a sealed cavity 11 between the outside periphery of the cam ring 3 and the wall of the recess 2, and the pump end covers. A pressuring hole 12 connects the bottom of the slot 9 to the sealed cavity 11 so that oil pressure keeps the sealing member 10 in contact with the arc surface 8.

The radius of the cam housing cavity 2A at the opposite side of the pivot 4 from the sealed cavity 11 is struck from the centre of maximum cam eccentricity 13 and is the same radius as the outside radius of the cam ring 3A. This radius of the cam housing cavity 2A acts as the maximum output stop for the cam ring 3.

Alternatively, an adjustable stop 54 may be provided in a suitable position on the arc 2A if adjustable maximum flow output is required. The advantage of using the cam housing radius 2A as the maximum output stop is that on full cam ring displacement, the cam ring 3 will sit snugly against the radius 2A and will reduce leakage of oil or other hydraulic liquid around the outside of the cam ring 3 from a discharge port 20 to an inlet port 19.

A spring 14 pushes the cam ring 3 against a minimum output stop 15. A vane carrier 16 is driven from an engine via a shaft 48 passing through its centre. The vane carrier 16 contains a number of vanes 17 which slide in slots 50 so that the vanes 17 move outwards, initially by centrifugal force and, when the pump is pumping, by oil pressure which is fed from the discharge port 20 to the underside of the vanes 17 in slots 50 and contact the inner surface 18 of the cam ring 3. This inner surface 18 is offset eccentric to the shaft centre and is shown in Figure 1 as having a circular form but in practice it will be a cam form developed to produce quiet operation.

As the vane carrier 16 rotates, the vanes 17 form a series of expanding and contracting pumping chambers which are arranged to coincide with the inlet ports 19 during expansion, and with the discharge ports 20 during contraction. The inlet ports 19 are formed in the pump end covers and they are supplied with oil from a reservoir 51 via a conduit 52. The discharge ports 20 are also formed in the pump end covers and they are connected to a cavity 21 in a valve bore 31 by a passageway 22 in the pump housing 1.

A discharge pipe connector 23 is fitted to the valve bore 31 adjacent the cavity 21. The discharge pipe connector 23 contains a primary orifice 24 and an annulus 25, linked by a drilling 26 from the downstream side of the primary orifice 24. The annulus 25 is connected through a conduit 53 which connects to a valve spring chamber 27, via a secondary orifice 28, and which also connects with the sealed cavity or chamber 11.

The pump housing 1 also contains a combined flow control and pressure release valve 29 which is of a conventional type used in power assisted steering systems.

A valve body 30 is slidably housed in a valve bore 31. The valve body 30 has an inner stepped bore 32 within which is accommodated a relief valve assembly comprising a large fixed ball or plug 39 in the largest diameter of the bore 32, and a smaller movable ball 34 which is pressed by a spring 35 on to a angled seating 33 to form a pressure tight seal. A hole 36 connects a ball pressurising cavity 37 with a spring chamber 27. The ball or plug 39 is an interference fit to the bore 32 and is adapted to be pressed in to a depth necessary to compress the spring 35 against the ball 34, in order to provide the required preloading.

The valve body 30 is a relief valve body 30. This valve body 30 has a groove 40 around its outer diameter.

The groove 40 is connected to the inner bore 32 by holes 41. The groove 40 is connected to a relief valve spill hole 42 at all times. A flow control spring 43 keeps the valve body 30 seated on the discharge pipe connector 23. A slot across the narrow end of the valve body 30 allows unrestricted passage of oil from the cavity 21, out through the primary orifice 24 along a high pressure hose 45 to a conventional steering control valve 46.

The control valve 46 is incorporated in a conventional power steering box or a power steering rack and pinion gear 47. The control valve 46 is operated by the rotary movement of a vehicle steering wheel, via a flexible torque measuring element such for example as a torsion bar, a spiral spring or a leaf spring. The flexible torque measuring element converts the torque precisely and with no free play into as small a control travel as possible. The control edges which are in the form of chamfers or bevels, move as a result of the control travel to divert the flow of hydrualic liquid to the appropriate steering cylinder.

In its central 'open centres position, the control valve allows hydraulic liquid delivered by the pump to flow back to the fluid reservoir 51 through the small cross-section opening created by the control edges. With the control valve in this position, which in the vehicle corresponds to straight ahead driving, the movable cam ring 3 will remain pressed against the minimum output stop by the spring 14. The rotation of the carrier 16 and the vanes 17 by the driving member from the engine, cause the expanding and contracting pumping chambers to deliver a flow of hydraulic liquid from the discharge port 20 through the passageway 22 into the cavity 21, from where the hydraulic liquid passes through the slot 30 and out through the primary orifice 24 into the hydraulic hose 45 to the steering control valve 46.The hydraulic liquid also flows from the cavity 21 along the conduit 53 into the spring chamber 27, via the secondary orifice 28. This hydraulic liquid then flows into the pressure chamber 11 so that any pressure downstream of the orifice 24 is also present in these areas.

In the minimum output condition, the flow from the pump will be similar to the minimum output curve shown in the graph in Figure 9 which will be described in more detail hereinbelow. From this, it will be seen that a point in the speed range is reached where the flow corresponds to the specified flow control setting. In the case of the graph, the flow control setting is 2.5 gallons per minute and the minimum output setting of the cam ring 3 is 50 zó of the maximum setting, which is set to give 2.5 gallons per minute 1000 r.p.m.

Flow control therefore occurs at 2000 r.p.m. caused by a pressure drop across the primary orifice 24 which causes the pressure acting on the valve body 29 in the chamber 21 to be higher than the pressure in the spring chamber 27.

The valve will therefore overcome the flow control spring 43 and move to a position where a sealing land 49 uncovers the spill hole 42 (which may be connected to either a reservoir or the pump inlet) and spills hydraulic liquid sufficient to balance the valve and maintain the flow nearly constant at 2.5 gallons per minute.

A slightrise inflow is due to the increase of load on the spring 43 as the valve moves further open as the speed increases.

Despite the fact that the steering system is receiving the full specified flow, the power consumed and the running temperature of the system will be lower than in a conventional power assisted steering system because the cam ring 3 in the variable displacement pump is still in its minimum output position and has in effect reduced the size of the pump.

During engine idle and low speed running, the cam ring 3 remains in the minimum output position until the steering wheel turns and moves the control valve 46 and diverts oil to the steering cylinder which, being a closed volume, causes the pressure in the system to rise.

The pressure rise also occurs in the sealed chamber 11 and acts on the outer surface of the cam ring 3 until, with rising pressure, the force is sufficient to overcome the load exerted by the spring 14 and the cam ring 3 moves on to the maximum output stop 24 and the pump produces its full flow. This full flow corresponds to the maximum output curve shown in the graph in Figure 9.

If the pump speed rises with the pressure still present in the system, the flow control valve will open to bypass hydraulic liquid in a normal way.

If the pressure rises above the release valve setting, this is usually caused by the wheels of the vehicle coming up against the full lock stops, or by contacting some obstruction such as a kerb edge. This means that the flow out of the discharge stops so there is no pressure drop across the orifice 24. However, the pressure rises until it reaches the blow off setting of the relief valve pilot ball 34 which then lifts from its seat and releases a flow of hydraulic liquid along the inner bore 32 and out through the holes 41 around the annulus 40, and out of the spill hole 42. This flow passing through the secondary orifice 28 causes a pressure drop in the spring cavity 27, and the valve 30 overcomes the spring 43 and uncovers the spill hole 42 to spill oil back to the reservoir until the pressure is stabilised at the relief valve setting.

The pump is shown with the cam ring 3 on the minimum output stop 15, and the combined flow control/ pressure relief valve 29 is shown held closed by the flow control spring 43, as would apply with the pump running without providing steering assistance and with the output flow below the specified flow control setting.

Adjustments to the steering characteristics and the sensitivity of the cam ring 3 can be made by changing the shape and size of the control edges in the steering control valve 46 and the characteristics of the spring 14, or alternatively changing the cross sectional area of the pressure responsive means for moving the cam ring 3.

Referring now to Figure 2, there is shown a pump similar to that shown in Figure 1. In Figure 2, the cam ring 3 is on the minimum output stop 15, and the combined flow control valve/pressure relief valve 29 is opened to the spill hole 42 to provide flow control. This flow control will be, for example, when the pump is running at high speed with no power assistance being provided.

Figure 3 shows a pump similar to that shown in Figures 1 and 2 but with the cam ring 3 held on the maximum output stop 2A by pressure in the cavity 11; and the combined flow control/pressure relief valve 29 open to spill excess oil from the spill hole 42. This may occur in, for example, running conditions where the pressure in the system has risen above the relief valve blow off setting.

Figure 4 shows a variable capacity power assisted steering pump similar to that shown in Figure 1. In Figure 4, the pump is one where the force to move the movable cam ring from the minimum output stop to the maximum stop 2A is effected by means of a piston 50.

As shown in Figure 4, the piston 50 also acts as the minimum output stop and is pressurised from the downstream side of the orifice 24. During times of demand for steering assistance, the piston 50 pushes on the movable cam ring 3, causing the movable cam ring 3 to rotate about the pivot 4. The cam ring 3 is shown in the minimum output position, and the relief valve/flow control valve 29 is closed as occurs in low speed/low pressure running conditions.

Figure 5 shows the same pump as shown in Figure 4 but with the cam ring 3 in the maximum output position, and the relief valve/flow control valve 29 open as in high pressure running conditions.

Figure 6 shows a variable capacity power assisted steering pump similar to that shown in Figure 1 except that there is no means for moving the cam ring 3 except by internal pressure within the cam ring. In Figure 6, the pivot 4 is positioned on a line at 900 to the line of eccentricity of the cam form and it passes through the shaft centre. Thus internal pressurised surfaces of the cam ring 3 are unevenly distributed relative to the pivot 4. Thus, when the pump pressures reach a predetermined level, they cause the cam ring 3 to rotate around the pivot 4 by means of internal pressure on the cam ring 3 only, to overcome the spring 14 and to move the cam ring 3 from the minimum output stop 15 to the maximum output stop 2A.

In Figure 6, the cam ring 3 is shown in the minimum output position and the pressure relief/flow control valve is shown closed.

Figure 7 shows the same pump as in Figure 6 but with the cam ring 3 in the maximum output position, and the pressure relief valve 29 open as in high pressure conditions.

Figure 8 shows a variable displacement power assisted steering pump where the movable cam ring 3 moves linearly in an elongated pocket 2 from a minimum output stop 15 to a maximum output stop. The force to move the cam ring 3 may be applied by pressurising the end of the elongated pocket 2, adjacent to the minimum output stop. The pressure also acts on the outside profile of the cam ring 3A to move the cam ring 3A.

The force to move the cam ring 3A may also be by means of a separate pressurised piston 50 similar to that shown in Figure 4 and which pushes the cam ring 3 to cause it to move.

The cam ring 3 shown pressed against an adjustable maximum output stop 54 by pressure in the cavity 55 acting on the piston 50. The combined flow control/pressure relief valve 29 is shown open to the spill hole 42, as would occur with the pump running with pressures above the relief valve blow off setting, or with the pump pressure lower than the relief valve blow off pressure, but with the pump output higher than the flow control setting.

Figure 9 shows a typical flow graph for the maximum output and minimum output cam positions with the flow control set at 2.5 gallons per minute, and maximum output set at 2.5 gallons per 1000 revolutions, and minimum pump output set at 50% of the maximum output, i.e. 2.5 gallons per 2000 revolutions.

It is to be appreciated that the embodiments of the invention described above with reference to the accompanying drawings have been given by way of example only and that modifications may be effected. Thus, for example, the drawings show variable displacement pumps in the form of a vane pump having flat vanes. Vane pumps having roller vanes or slipper vanes may however be employed. Furthermore, it has been mentioned above that the pressure to move the cam ring 3 is taken from the downstream side of the orifice when the wheel is turned. Whilst this is preferred, the pressure to move the cam ring may also be taken from the upstream side of the orifice. The variable displacement capacity pump may be installed in otherwise conventional power assisted steering systems to achieve energy savings and lower running temperatures, and they may also be installed in other hydraulic systems.

Claims (14)

1. A variable displacement vane pump comprising a cam ring which is movable between minimum and maximum operating positions, and a pressure relief and flow control valve arrangement, the pressure relief and flow control valve arrangement having a flow control orifice and a hydraulic liquid feedback circuit, and the pump being such that during operation when power assistance is not required the cam ring is in a minimum position and the pump operates in a standby output mode to provide a pilot hydraulic liquid flow which keeps a hydraulic system served by the pump in a charged condition, and with the pump also being such that when power assistance is required the cam ring is moved with the use of a back pressure to the maximum position and the pump operates in a maximum output mode to provide a maximum hydraulic liquid flow.

2. A variable displacement vane pump according to claim 1 in which the pressure relief and flow control valve arrangement comprises a single unit having a combined pressure relief valve and a flow control valve.

3. A variable displacement vane pump according to claim 1 in which the pressure relief and flow control valve arrangement comprise a separate pressure relief valve and a separate flow control valve.

4. A variable displacement vane pump according to any one of the preceding claims and which is a variable capacity rotary vane pump.

5. A variable displacement vane pump according to claim 4 and which is a flat vane pump.

6. A variable displacement vane pump according to any one of the preceding claims in which the cam ring is held against a stop member in the minimum position by biasing means.

7. A variable displacement vane pump according to claim 6 in which the biasing means is a spring.

8. A variable displacement vane pump according to any one of the preceding claims in which the pilot hydraulic liquid flow is directed to a system control valve, the pilot hydraulic liquid flow being one which is less than is needed for providing the power assistance at a maximum predetermined requirement but one which creates back pressure when the system control valve is operated, and the variable displacement pump being one in which the back pressure acts on the cam ring to move it to the maximum position via pressure-responsive means.

9. A variable displacement vane pump according to claim 8 in which the pressure-responsive means is a piston which pushes on the cam ring.

10. A variable displacement vane pump according to claim 8 in which the pressure-responsive means is a part of the cam ring which is constructed and adapted to be pressurized.

11. A variable displacement vane pump substantially as herein described with reference to the accompanying drawings.

12. A hydraulic system when provided with a variable displacement vane pump according to any one of the preceding claims.

13. A hydraulic system according to claim 12 and which is a hydraulic power assisted steering system.

14. A vehicle when provided with the hydraulic power assisted steering system as claimed in claim 13.