Hydraulic pump control

Abstract

 A fluid pressure supply system incorporates two pumping units P1 and P2 for supplying fluid under pressure to a load 13. Pump P1 supplies the load continuously. Pump P2 is also connected to the load 13 through a load valve 15 when pressure in the load is low but is connected to drain through the load valve when load pressure is high. A pressure relief valve 14 controls the load valve 15 by a connection from port 22 to port 28 which applies system pressure to the load valve at low pressure and drain pressure via port 23 at higher pressures in response to relief valve spool movement. The high control pressure moves load valve spool 25 to connect its inlet port 29 to load 13 through port 33. When system pressure is high and control pressure is low, inlet 29 is connected to drain port 31 so that pump P2 is off-load in that it is pumping direct to drain.

Description

[0001] This invention relates to fluid pressure supply systems for supplying hydraulic fluid under pressure to loads such as for example power steering or automatic transmission systems for motor vehicles.

[0002] Hitherto it has been common practice to provide a single positive displacement pump in conjunction with a pressure relief valve. The rating of the pump is determined by the maximum flow rate and pressure demand of the load. Such pumps typically operate at much less than maximum rating for the majority of operating time and much energy is wasted in pumping fluid up to relief valve pressure and discharging it through the relief valve.

[0003] Dual output pumps or twin pumps have been proposed in which the first pump operates continuously and the second is brought on load as required by flow sensitive control means. Such.control means include a flow restrictor upstream of the load to sense the higher flow rate and thus the requirement for the second pump. A disadvantage with this arrangement is that the restrictor inevitably has a pressure drop across it, which places an additional energy consuming load on the pump.

[0004] An object of the invention is to provide a fluid pressure supply system which provides a high output when required but has a low overall energy consumption.

[0005] According to one aspect of the invention there is provided a fluid pressure supply system comprising main and auxiliary positive displacement fluid pressure sources, a connection to a load from the main source, a pressure relief valve arranged to limit pressure at the load connection to a normal working pressure and a drain connection from the auxiliary source whereby it can pump to drain at low pressure, wherein flow cut off means in the drain connection is controlled by the pressure at the load connection to close the drain connection when this pressure falls below normal working pressure and there is a controlled connection from the auxiliary source to the load connection to permit flow from the auxiliary source to the load connection when the drain connection is closed.

[0006] Preferably the system incorporates a non-return valve in the controlled connection from the auxiliary source to the load connection whereby fluid flows from the auxiliary source to the load connection when the auxiliary source pressure exceeds the load connection pressure but reverse flow is prevented.

[0007] Preferably the flow cut off means and the controlled connection from the auxiliary source to the load connection are incorporated in a common valve which closes the flow cut off means and opens the controlled connection from the auxiliary source to the load connection in a single action.

[0008] Preferably the pressure relief valve is a spool valve with inlet and drain connections and a spool controlled by spring loading and by pressure in the load connection to open the drain connection when normal working pressure is reached, the spool valve also having an inlet from the auxiliary source connected to drain through the spool valve when load pressure is below normal working pressure but disconnected from drain by movement of the spool as normal working pressure is reached. The valve spool may also incorporate the controlled connection from the auxiliary source to the load connection, control being by movement of the spool in response to load pressure.

[0009] According to a second aspect of the present invention there is provided control valve means of a dual output pump for supplying fluid under pressure to a load and including relief valve means to limit the maximum supply pressure of fluid to said load, a first pump outlet for direct connection to said load and a second pump outlet for connection to said load through load valve means, said relief valve means comprising a spool valve having an inlet port for connection to the load supply, a drain pore and a valve spool movable against return spring means in response to increasing pressure at said inlet port to interconnect the inlet and drain ports above a predetermined supply pressure, said load valve means having an inlet port for connection to the second pump outlet, an outlet port for connection to the load and a drain port, the load valve means being responsive to the position of the relief valve spool to interconnect its inlet and outlet ports below a predetermined supply pressure and to interconnect its inlet and reservoir ports above that pressure.

[0010] Preferably said spool valve includes a signal port and a second drain port, the valve spool being movable to interconnect the inlet port and signal port below the predetermined supply pressure and to interconnect the signal and second drain ports above that pressure, the load valve means being responsive to fluid pressure at said signal port.

[0011] In one embodiment said relief valve means and said load valve means comprise a spool valve having a common valve spool, said supply port and outlet port being common and the valve spool having an internal passage from the inlet to the outlet ports, a non-return valve in the passage preventing fluid flow from the outlet to the inlet ports only.

[0012] Other features of the invention are included in the following description of three preferred embodiments shown, by way of example only, in the accompanying drawings in which:-

Figure 1 is a schematic hydraulic circuit diagram showing the valve arrangement according to a first embodiment of the invention;

Figure 2 is a diagram similart to figure 1 and showing the valve arrangement of a second embodiment of the invention;
and

Figure 3 is a further diagram showing a simplified valve according to a third embodiment of the invention.

[0013] With reference to figure 1 there is shown a fluid reservoir 11 from which a pump assembly 12 supplies fluid under pressure to a fluid load 13.

[0014] The pump assembly 12 comprises a main positive displacement pump or pressure source P1 and auxiliary positive displacement pump or pressure source P2 intended to be brought on-load as required. The pump assembly may be a single pump with two separate working chambers or may comprise two independent pumps driven together from a common drive. A wide variety of conventional types of pump are suitable.

[0015] The output from main pump P1 is supplied through load connection 10 to a load 13. The load may be an automatic transmission which has a high demand for fluid while starting up or shifting gear but a low demand at most other times. Another load which can be supplied is a power steering pump which has high fluid pressure demand for manoeuvring but lower demand at other times. A pressure relief valve 14 limits maximum system pressure and a load valve 15 diverts the output of auxiliary pump P2 between drain and the load as will be described.

[0016] The relief valve 14 comprises a spool 16 returned to one end of its bore 17 by a spring 18. An inlet port 19 is connected to the load connection 10 and a drain port 21 is provided to the reservoir 11. In all positions of the valve spool, inlet 19 is connected through bore 9 to the end of the spool remote from the spring so that inlet pressure acts on the spool in opposition to the spring load. Increasing fluid pressure in the relief valve urges the spool 16 to move against the force of spring 18 and thus causes the inlet and drain ports to become interconnected as a land 8 clears drain port 21. This drain port opens when normal working pressure is reached.

[0017] A signal port 22 of the relief valve is connected to the inlet port 19. Up to a predetermined pressure somewhat less than relief pressure this signal port is connected to inlet port 19 through the valve, but land 7 closes off this port 22 at this predetermined pressure. As working pressure is reached, land 22 again clears signal port 22 but then connects it to a further drain port 23.

[0018] The load valve 15 comprises a spool 25 urged to one end of its bore 26 by a return spring 27. A signal port 28 connected to the port 22 opens onto the end of the spool 25 remote from spring 27.

[0019] In the rest positon, with spool 25 resting against the inner end of its valve body adjacent port 28, i.e. slightly above the position shown, an inlet port 29 from auxiliary pump P2 is connected to a drain port 31 through a space between valve lands 5 and 6. A passage through the spool 26 from land 5 includes a non-return valve 32 and provides communication between the inlet port 29 and an outlet port 33 connected to the load 13 in the rest position and in the position shown.

[0020] Operation of the load valve is as follows:-

[0021] Both pumps P1 and P2 run continuously and provide a continuous flow of fluid. Pump P2 is off-load in that it is pumping fluid against very little resistance through inlet port 29 and drain port 31. Direct communication from inlet port 29 to outlet port 33 is closed by the land 5 and reverse flow through the passage in the spool 25 is prevented by the non-return valve 32.

[0022] Pump P1 supplies fluid direct to the load, and on starting up the system pressure rises in the load, the load connection and the relief valve. Spool 16 of relief valve begins to move against its return spring as pressure builds up. This pressure also acts on the load valve spool 25 via inlet port 19 and signal ports 22 and 28.

[0023] At a relatively low fluid pressure well below normal working pressure the spool 25 moves against the force of spring 27 so that land 5 closes the drain port 31 from inlet port 29. In the intermediate position shown the inlet port 29 and outlet port 33 are interconnected through the non-return valve 32 so that fluid from the auxiliary pump is supplied through this valve to the load connection 10. Further movement of the spool 25 connects the ports 29 and 33 directly bringing the pump P2 fully on-load to supplement the output of pump P1 to fill the load at relatively low pressure. The non-return valve connection ensures that during the transition between connection of pump P2 to drain and bringing it on-load, a flow path is always available for fluid from the pump P2.

[0024] As fluid pressure progressively rises, further movement of the relief valve spool 16 moves land 7 across the signal port 22 and so closes communication between the inlet port 19 and signal port 22 and connects the port 22 to the drain port 23.

[0025] The load valve spool 25 then returns under the influence of spring 27 to its rest position to reconnect the inlet port 29 to drain port 31 so that the pump P2 comes off-load. Flow from port 33 to port 29 is prevented, as before, by the non-return valve 32 and the land 5 of the spool 25. This off-loading of pump P2 occurs at a pressure very slightly below normal working pressure. When normal working pressure is reached in the load connection 11 by means of further fluid from pump P1, land 8 opens port 21 to prevent normal working pressure from being exceeded.

[0026] Should fluid demand from the load 13 increase beyond the capacity of main pump P1, system pressure drops until the relief valve spool reconnects the inlet port 19 to signal port 22 by partial retraction of spool 16. The load valve spool 25 then moves to bring the auxiliary pump P2 on-load again until load demand is satisfied and system pressure risesalmost to normal working pressure.

[0027] With this system a main pump of low capacity can be used so that a relatively small volume of fluid is pumped up to working pressure and discharged through the relief valve. The auxiliary pump is available to come on load for periods of high demand but is normally pumping at very low pressure and so consuming very little energy. In this way considerable energy savings are effected.

[0028] The pressure at which the auxiliary pump is brought on-load and the capacities of the two pumps can be determined by the particular duty required. Equal pump capacities has the advantage of simplifying pump design.

[0029] Figure 2 shows an alternative embodiment, using the same relief valve as in figure 1, but a modified load valve.

[0030] The load valve 35 has an inlet port 36, outlet port 37, signal port 38 and drain port 39 connected as previously described. A light spring 41 biases a spool 42 to one end of its bore 43 (as shown) in which position the inlet port 36 and outlet port 37 are interconnected, the drain port 39 being closed by a land 47 of the spool. A further land 48 controls communication with the outlet port 37. A non-return valve 44 is again provided in a passage within the spool.

[0031] Fluid pressure at the outlet port 37 acts through a damping restriction 45 on one end of the spool 35 in opposition to the light spring 41 and the signal port 38 is connected to the other end of the spool.

[0032] Operation of the load valve is as follows:-Initially, in the rest condition shown, the drain port 39 of the load valve is closed by land 47 and both pumps P1 and P2 contribute flow of fluid to the load 13. Steadily increasing fluid pressure at the signal port 38 acting on one end of the spool balances pressure at the outlet port 37 acting on the other end of the spool, the light spring 41 holding the spool 42 in the rest position shown. This arrangement gives a quick fill and fast initial pressure rise to the load 13.

[0033] When system pressure builds up to a level at which the relief valve spool 16 interconnects the signal port 22 and drain port 23 as previously described port 38 is connected to drain through the relief valve so that system pressure on the opposite end of spool 42 moves the spool to the opposite end of its stroke. Damping orifice 45 controls the rate at which the spool moves. This spool movement brings land 47 clear of the drain port 39 and so connects inlet port 36 to drain. Direct communication between the outlet port 37 and the inlet port 36 is prevented by land 48 and the non-return valve 44 prevents such a connection through the passage in the spool.

[0034] The auxiliary pump P2 is consequently brought off-load and idles. Further pressure increase from main pump P1 is then limited by the relief valve 14.

[0035] The passage in the spool is provided for the same reason as the corresponding passage in figure 1. As spool 42 is moving there is a transitional stage at which land 48 is just closing off connection from inlet 36 to outlet 37 and the drain connection to port 39 is only just opening. At this time the passage in the spool allows an alternative path for fluid from pump P2. As soon as the drain connection is open, the non-return valve 44 prevents reverse flow through the spool to drain.

[0036] Should fluid demand from the load 13 increase beyond the capacity of the main pump, fluid pressure drops until the relief valve spool reconnects the signal port 22 with system pressure from the inlet port 19. fluid pressure at the outlet port 37 again becomes balanced by that at the signal port 38 so spool 42 moves under the influence of spring 14 to reconnect the inlet port 36 and outlet port 37. Connection is via the non-return valve 44 until the outlet port 37 is uncovered by the land 48 of spool 42 to establish direct communication.

[0037] The auxiliary pump P2 is thus brought on-load until the load demand is satisfied, at which stage the pump is again connected to drain as described above.

[0038] Figure 3 shows a simplified system with a spool valve 51 in which the functions of relief valve and load valve are combined. The valve 47 is similar in many respects to the load valve of figure 2. A spool 52 slides within the bore of a valve body and is urged to one end of the bore by a control spring 53. In this situation, there is direct connection between two valve lands 54 and 55 between an inlet 56 from pump P2 and an outlet 57 leading to the load. In this condition, the common pressure at inlet 56 and outlet 57 is connected through a non-return valve 58 and a passage 59 in the spool to the end of the spool opposite relief valve spring 53 to provide a pressure force in opposition to the spring force. The valve also incorporates drain ports 61 and 62 which in the rest position of the valve are closed off by lands 54 and 63.

[0039] The operation of the system is as follows. Initially, fluid from both pumps Pl and P2 is applied to the load to give a quick fill of the load. As pressure builds up in the system, this pressure acting on one end of the spool 52 tends to move the spool against the load of relief valve spring 53. After some movement, edge B on land 55 begins to close,off communication with the outlet port 57 from the inlet port 56, thus momentarily diverting flow from P2 through the non-return valve 58 and a space between lands 55 and 63 to the outlet. At about this stage of operation, drain port 61 begins to be uncovered under the control of edge C on land 54. This escape of fluid to drain reduces the pressure of pump P2 and the non-return valve 58 closes to prevent reverse flow from outlet 57, at the pressure of pump Pl, towards inlet 56 which is becoming connected to drain. Further pumping from the pump P1 increases system pressure towards normal working pressure and at the same time this pressure is communicated through the gap between lands 55 and 63 and through passage 59 to continue to act as a control pressure against the relief valve spring 53. Further movement of the spool 52 fully opens the connection from pump P2 through inlet 56 and drain port 61, thereby bringing pump P2 fully off-load. Eventually, the stage is reached where edge A of land 63 begins to open outlet port 57 to drain port 62. This edge A in conjunction with port 57 acts as a pressure relief valve in that as pressure from pump P1 tends to increase and open up the edge, the pressure is relieved through the edge and to the drain port 62.

[0040] When pressure in the load drops due to a demand for fluid greater than the capacity of pump P1, the relief valve edge A closes as the spool 52 is moved by spring 53 to provide the whole of output of pump P1.to the load. Further movement of the spool under the influence of the reduced system pressure and the load of spring 53 causes edge C to close off the drain connection from pump P2 and also causes edge B to open the connection from pump P2 to the outlet 57 and thus to the load. The degree of opening depends on the extent to which the load pressure has dropped.

[0041] When load pressure increases again, the system presssure moves the spool against the force of relief valve spring 53 until communication from pump P2 to the outlet 57 is prevented by edge A and the drain connection is opened by edge C. As before, a slight further increase in system pressure to normal working pressure from pump P1 brings the pressure regulating function into operation.

Claims

1. A fluid pressure supply system comprising main (Pl) and auxiliary (P2) positive displacement fluid pressure sources, a connection to a load (13) from the main source, a pressure relief valve (14, 14 or 51) arranged to limit pressure at the load connection (10) to a normal working pressure and a drain connection (31, 39 or 61) from the auxiliary source whereby it can pump to drain at low pressure, characterised by flow cut off means (5, 47 or 54) in the drain connection controlled by the pressure at the load connection to close the drain connection when this pressure falls below normal working pressure and a controlled connection from the auxiliary source to the load connection (29 to 33 controlled by 5 or 36 to 37 controlled by 48 and 44 or 56 to 57 controlled by 55 and 58) to permit flow from the auxiliary source to the load connection(10)when the drain connection is closed.

2. A fluid pressure supply system according to claim 1, characterised by a non-return valve (32, 44 or 58) in the controlled connection from the auxiliary source (P2) to the load connection (10) whereby fluid flows from the auxiliary source to the load connection when the auxiliary source pressure exceeds the load connection pressure but reverse flow is prevented.

3. A fluid pressure supply system according to claim 1 or claim 2 characterised in that the flow cut off means (5, 47 or 54) and the controlled connection from the auxiliary source to the load connection (29 to 33 controlled by 5 or 36 to 37 controlled by 48 and 44 or 56 to 57 controlled by 55 and 58) are incorporated in a common valve (15, 35 or 51) which closes the flow cut off means and opens the controlled connection from the auxiliary source to the load connection in a single action.

4. A fluid pressure supply system according to any preceding claim characterised in that the pressure relief valve (51 in figure 3) is a spool valve with inlet and drain connections (57 and 62) and a spool (52) controlled by spring loading (spring 53) and by pressure in the load connection to open the drain connection (62) when normal working pressure is reached, the spool valve also having an inlet (56) from the auxiliary source connected to drain (61) through the spool valve when load pressure is below normal working pressure but disconnected from drain by movement of the spool (56 closing 61) as normal working pressure is reached.

5. A fluid pressure supply system according to claim 4 characterised in that the valve spool also incorporates the controlled connection from the auxiliary source to the load connection (56 to 57 controlled by 55 and 58), control being by movement of the spool in response to load pressure.

6. Control valve means of a dual output pump (P1, P2) for supplying fluid under pressure to a load (13) and including relief valve means (14, 14 or 51) to limit the maximum supply pressure of fluid to said load, characterised by a first pump outlet (from P1) for direct connection to said load and a second pump outlet (from P2) for connection to said load through load valve means (15, 35 or 51) said relieve valve means comprising a spool valve having an inlet port (19, 19 or 57) for connection to the load supply, a drain port (23, 23, or 62) and a valve spool (16, 16 or 52) movable against return spring means (18, 18 or 53) in response to increasing pressure at said inlet port to interconnect the inlet (19) and drain (23) ports above a predetermined supply pressure, said load valve means having an inlet port (29, 36 or 56) for connection to the second pump outlet, an outlet port (33, 37 or 57) for connection to the load and a drain port (31, 39 or 63), the load valve means being responsive to the position of the relief valve spool to interconnect the inlet and outlet ports (29 to 33 or 36 to 37) of the load valve below a predetermined supply pressure and to interconnect its inlet and drain ports (29 to 31 or 36 to 39) above that pressure.

7. Control valve means according to claim 6, characterised in that said spool valve (14) includes a signal port (22) and a second drain port (23), the valve spool being movable to interconnect the supply port (19) and signal port (22) below the predetermined inlet pressure and to interconnect the signal and second drain ports (22 to 23) above that pressure, the load valve means (15 or 35) being responsive to fluid pressure at said signal port.

8. Control valve means according to claim 7, characterised in that the load valve means (15 or 35) comprise a spool valve having an end port (28 or 38) for connection to the signal port and a vlave spool (25 or 42) movable in response to fluid pressure at the end port from a first position in which the inlet and outlet ports are interconnected (29 to 33 or 36 to 37) to a second position in which the inlet and drain ports are interconnected (29 to 31 or 29 to 39).

9. Control valve means according to claim 8, characterised in that the valve spool (25 or 42) of the load valve means includes an internal passage from the inlet to the outlet ports, a non-return valve (32 or 44) in the passage preventing fluid flow from the outlet to the inlet ports only.

10. Control valve means according to claim 9, characterised in that supply pressure acts on one end of the valve spool (42 figure 2) of the load valve means in opposition to a light spring (41) and signal port pressure at said end port.

11. Control valve means according to claim 6, characterised in that said relief valve means and said load valve means comprise a spool valve(51)having a common valve spool, the relief valve inlet port and load valve outlet port (57) being common and the valve spool having an internal passage from the load valve inlet port to the common port, a non-return valve in the passage preventing fluid flow from the common port to the load valve inlet port.

Drawing