GB2368883A

GB2368883A - Regulating fluid flow from tandem gear pump assemblies

Info

Publication number: GB2368883A
Application number: GB0022358A
Authority: GB
Inventors: Gavin Davis
Original assignee: Sauer Danfoss GB Ltd
Current assignee: Sauer Danfoss Swindon Ltd
Priority date: 2000-09-13
Filing date: 2000-09-13
Publication date: 2002-05-15
Anticipated expiration: 2020-09-13
Also published as: GB2368883B; GB0022358D0

Abstract

A tandem gear pump assembly 2 having a first integral gear pump 4 and a second integral gear pump 6, both rotatably driven by a common drive shaft 8, is used to supply fluid to an external hydraulic system through a common outlet port 12. A control device is provided in fluid communication with an output of each integral gear pump 4,6 and the common outlet port 12 and regulates the fluid flow from the common outlet port 12 to a pre-determined rate when the fluid flow derived from one or both of the integral gear pumps 4,6 is sufficient to establish the pre-determined flow rate. The control device incorporates an unload valve 22, and a preferential flow divider 24 with a pre-set orifice 25 on its outlet side.

Description

TITLE Regulating Fluid Flow From Tandem Gear Pump Assemblies DESCRIPTION Field of the Invention The present invention relates to tandem gear pump assemblies and, in particular, to a control device and method for use with tandem gear pump assemblies to produce a substantially constant fluid flow output over a relatively wide range of operating speeds and to reduce power wastage.

Background Art A typical variable speed prime mover tandem gear pump assembly comprises two gear pumps operating in tandem from a common drive shaft wherein the output from each individual gear pump is combined and then transferred for use through an outlet port of the assembly. In such an assembly, the maximum operating speed is generally in the region of approximately four times the assembly idling speed. Although useful torque is developed from roughly two to four times the idling speed, most conventional assemblies are designed to run at speeds within an optimum range of three to three and a half times the idling speed so as to ensure that a relatively constant combined fluid flow output is established throughout assembly operation.

At lower speeds the flow rate developed at the output is insufficient to meet the requirements of a hydraulic system which it supplies, while at higher speeds the flow rate is more than sufficient so as to constitute a power wastage. Hence, the optimum speed range for such assemblies is relatively narrow. Indeed, even when operating within this relatively narrow optimum speed range, since the flow rate in the prior art assembly is at all times proportional to the drive shaft speed, a constant flow rate is not achievable if the speed is varied even slightly and such changes in the speed, no matter how small, cause fluctuations in the flow rate through the outlet port of the assembly thereby producing noise, vibration and enhanced wear of the pump assembly and the hydraulic system it supplies.

Thus, the purpose of the present invention is to provide a control device and method for use with a tandem gear pump assembly to provide a substantially constant flow rate from the common outlet gallery of the assembly over a relatively wide range of operating speeds and to reduce the power wastage at relatively high speeds of operation.

Summary of the Invention Accordingly the present invention provides a control device for regulating to a pre-determined rate the fluid flow from an outlet port of a tandem gear pump assembly having two gear pumps, the control device being in fluid communication with an output of each gear pump, the outlet port of the pump assembly and a tank, wherein: when the combined fluid flow derived from both of the gear pumps is insufficient to establish the pre-determined flow rate, the control device directs the fluid flow derived from both gear pumps to the outlet port; when the fluid flow derived from each gear pump individually is insufficient to establish the pre-determined flow rate but the combined fluid flow derived from both gear pumps is more than sufficient to establish the pre-determined flow rate, the control device diverts any excess fluid flow derived from both gear pumps over that required to establish the pre-determined flow rate to the tank and transmits the remainder to the outlet port; and when the fluid flow derived from one of the gear pumps is at least sufficient in itself to establish the pre-determined flow rate, the control device completely off-loads the fluid flow derived from the other of the gear pumps to the tank and diverts any excess fluid flow derived from said one of the gear pumps to the tank.

Preferably the control device comprises an unload valve to completely off-load the fluid flow derived from said other of the gear pumps to the tank and a preferential flow divider to divert the excess fluid flow derived from both gear pumps or from said one of the gear pumps to the tank.

In the preferred embodiment, the unload valve is supplied with fluid flow derived from the output of said other of the gear pumps and can direct that fluid flow to either the preferential flow divider or the tank. While the preferential flow divider is supplied with fluid flow derived from said one of the gear pumps along with any fluid flow supplied to it from the unload valve through a common inlet.

Advantageously, the pre-determined flow rate is set by providing an orifice between an output of the preferential flow divider and the common outlet port of the tandem gear pump assembly. Additionally, by changing the size of the orifice, the pre-determined flow rate can be varied.

Furthermore, the orifice may be used to trigger the preferential flow divider to divert all or a portion of the fluid flow supplied to it to the outlet port and any remainder to the tank.

A monitoring device may be provided to monitor the flow of fluid from the preferential flow divider to the tank and to activate the unload valve when the fluid flow derived from said one of the gear pumps is sufficient to establish the pre-determined flow rate.

The control device can be disposed within or outside of the pump assembly.

The invention also provides a method for regulating to a pre-determined rate the fluid flow from an outlet port of a tandem gear pump assembly having two gear pumps, the method comprising the steps of: directing the fluid flow derived from both gear pumps to the outlet port when the combined fluid flow derived from both of the gear pumps is insufficient to establish the pre-determined flow rate; diverting any excess fluid flow derived from both gear pumps over that required to establish the pre-determined flow rate to a tank when the fluid flow derived from each gear pump individually is insufficient to establish the pre-determined flow rate but the combined fluid flow derived from both gear pumps is more than sufficient to establish the pre-determined flow rate; and when the fluid flow derived from one of the gear pumps is at least sufficient in itself to establish the pre-determined flow rate, off-loading the fluid flow derived from the other of the gear pumps to the tank an- diverting any excess fluid flow derived from said one c the gear pumps to the tank.

Description of the Drawings Fig. 1 is a cross-section taken along an axis of a drive shaft of a tandem gear pump assembly incorporating a control device according to the invention; Fig. 2 is a schematic of the hydraulic circuit of the assembly and control device of Fig. l ; and Fig. 3 is a graph illustrating an operational output of the assembly of Fig. 1 over a wide range of speeds.

Description of the Preferred Embodiment Fig. 1 illustrates a control device 20 according to the invention for use with a tandem gear pump assembly 2. The assembly 2 houses a first gear pump 4 and a second gear pump 6 both of which are rotatably driven by a common drive shaft 8. Both gear pumps 4, 6 are simultaneously supplied with hydraulic fluid from a common inlet port 10 disposed on the assembly 2. The output from each of the gear pumps 4,6 is fed to the control device 20 as indicated by the arrows in the figure. The control device 20 is capable of selectively transmitting the fluid flow which it receives to a tank 14 and a common outlet port 12 disposed on the assembly 2.

The control device 20 incorporates an unload valve 22 and a preferential flow divider 24. Fluid flow derived from the output of the first gear pump 4 is directly communicated to the preferential flow divider 24. Fluid flow derived from the output of the second gear pump 6 is communicated to the unload valve 22. The unload valve 22 transmits the fluid flow it receives to either the preferential flow divider 24 through a non-return valve 26 or to a tank 14.

The manner in which the control device 20 operates will now be described with reference to Fig. 2 and Fig. 3. The preferential flow divider 24 is used to supply a constant, pre-determined flow of fluid to an external hydraulic system (not shown) via the common outlet port 12. The divider 24 is provided with a pre-set orifice 25 on its outlet side which monitors the fluid flow from the preferential flow divider 24 to the common outlet port 12 and establishes the pre-determined flow rate. The pre-determined flow rate can be set or altered by fitting pre-set orifices 25 of varying size to the preferential flow divider 24.

On start-up of the pump assembly 2, illustrated by a line joining a point A to a point B on the graph of Fig. 3, the fluid flow derived from both gear pumps 4,6 is insufficient to meet the desired output from the assembly 2 to the external hydraulic system as the actual flow rate of fluid through the common outlet port 12 is less than the pre-determined flow rate. Under these conditions the unload valve 22 directs all the fluid flow received from the second gear pump 6 to the preferential flow divider 24. The preferential flow divider 24 in turn transmits all fluid flow it receives from the unload valve 22 and the first gear pump 4 directly to the common outlet port 12.

The positions assumed by the unload valve 22 and the preferential flow divider 24, under these conditions, are as shown in Fig. 2.

As the speed of the common drive shaft 8 gradually increases, the flow of fluid derived from the gear pumps 4,6 improves until at the speed indicated at point B, the fluid flow developed by both gear pumps 4,6 is sufficient to meet the pre-determined fluid flow rate required by the external hydraulic system. Hence, from point A to point B the fluid flow developed by both gear pumps 4,6 is delivered by the control device 20 to the common outlet port 12 of the pump assembly 2.

As the speed of the common drive shaft 8 increases from point B, the combined output from both gear pumps 4,6 produces more fluid flow than the external hydraulic system requires. At this stage the pre-set orifice 25 initiates the preferential flow divider 24 (which assumes a position to the right of that shown in Fig. 2) to divert the excess fluid flow back to the tank 14 thereby maintaining the pre-determined flow rate through the common outlet port 12.

As the operating speed of the assembly 2 further increases from point B to a point C, an increasing amount of the fluid flow supplied to the preferential flow divider 24 is diverted back to the tank 14.

At point C, the fluid flow derived from the first gear pump 4 is sufficient in itself to meet the needs of the external hydraulic system without the addition of the flow from the second gear pump 6. At this point, a monitoring device 23 located in a fluid channel between the preferential flow divider 24 and the tank 14 triggers the unload valve 22 (which moves to a position to the left of that shown in Fig. 2) to divert all of the fluid flow derived from the second gear pump 6 directly to the tank 14. Thus the second gear pump 6 will only be operating at tank 14 pressure and not at system pressure. The second gear pump 6 is thereby off-loaded and this reduces power wastage. Therefore, in this condition, all of the fluid flow supplied to the external hydraulic system is derived from the first gear pump 4 only.

The monitoring device 23 is activated to trigger the unload valve 22 when the fluid flow derived from the first gear pump 4 is sufficient to establish the pre-determined flow rate. In a particular embodiment of the invention, the monitoring device 23 activates the unload valve when it discerns that the excess flow from the preferential flow divider 24 equals the fluid flow derived from the second gear pump 6. As an alternative to using the monitoring device 23, it is possible to develop a signal to trigger the unload valve 22 from the pre-set orifice 25.

Initially at point C, just after the unload valve 22 has been triggered to off-load the second gear pump 6, all of the fluid flow required by the external hydraulic system is derived from the first gear pump 4. Thus the preferential flow divider 24 resumes the position as shown in Fig. 2.

As the operating speed increases from point C, the preferential flow divider 24 again assumes the position to the right of that shown in Fig. 2 and thereby diverts any excess fluid flow back to the tank 14 while maintaining the pre-determined fluid flow rate at the common outlet port 12.

To ensure stability throughout assembly operation, the unload valve 22, in responding to the signal from the monitoring device 23, should desirably have a slight amount of hysteresis in its activation.

In the preferred embodiment, the control device 20 is incorporated within the pump assembly 2 but it is feasible for the control device 20 to be located external to the pump assembly 2. Furthermore, fluid supplied to the gear pumps 4,6 may be derived from different sources through separate inlet ports.

Reference is made to Fig. 3 for describing how conventional tandem gear pump assemblies function. As discussed previously many of the prior art assemblies directly combine the fluid flow derived from the individual gear pumps for delivery through a common outlet port. The output from such assemblies for varying operating speedis illustrated by the line labelled"combined flow fr both pumps"in Fig. 3. As such, the desired c pre-determined flow rate can only be established anu. maintained at point B.

In the present invention, the provision of a control device enables a tandem gear pump assembly to establish and maintain the pre-determined flow rate over a relatively wide range of operating speeds.

Claims

CLAIMS 1. A control device for regulating to a pre-determined rate the fluid flow from an outlet port of a tandem gear pump assembly having two gear pumps, the control device being in fluid communication with an output of each gear pump, the outlet port of the pump assembly and a tank, wherein: when the combined fluid flow derived from both of the gear pumps is insufficient to establish the pre-determined flow rate, the control device directs the fluid flow derived from both gear pumps to the outlet port; when the fluid flow derived from each gear pump individually is insufficient to establish the pre-determined flow rate but the combined fluid flow derived from both gear pumps is more than sufficient to establish the pre-determined flow rate, the control device diverts any excess fluid flow derived from both gear pumps over that required to establish the pre-determined flow rate to the tank and transmits the remainder to the outlet port; when the fluid flow derived from one of the gear pumps is at least sufficient in itself to establish the pre-determined flow rate, the control device completely off-loads the fluid flow derived from the other of the gear pumps to the tank and diverts any excess fluid flow derived from said one of the gear pumps to the tank.
2. A control device according to claim 1 comprising an unload valve to completely off-load the fluid flow derived from said other of the gear pumps to the tank.
3. A control device according to claim 2 further comprising a preferential flow divider to divert the excess fluid flow derived from both gear pumps or from said one of the gear pumps to the tank.
4. A control device according to claim 3, wherein the unload valve is supplied with fluid flow derived from the output of said other of the gear pumps and can direct that fluid flow to either the preferential flow divider or the tank.
5. A control device according to claim 4, wherein the preferential flow divider is supplied with fluid flow derived from said one of the gear pumps along with any fluid flow supplied to it from the unload valve through a common inlet.
6. A control device according to claim 5, wherein the common inlet to the preferential flow divider is provided with an orifice which sets the pre-determined flow rate.
7. A control device according to claim 6, wherein the orifice triggers the preferential flow divider to divert all or a portion of the fluid flow supplied to it to the outlet port and any remainder to the tank.
8. A control device according to claim 7, wherein the pre-determined flow rate can be changed by varying the size of the orifice.
9. A control device according to claim 7 or claim 8, wherein a monitoring device is provided to monitor the flow of fluid from the preferential flow divider to the tank and to activate the unload valve when the fluid flow derived from said one of the gear pumps is sufficient to establish the pre-determined flow rate.
10. A control device according to any preceding claim, being disposed within the pump assembly.
11. A control device as hereinbefore described with reference to the drawings.
12. A method for regulating to a pre-determined rate the fluid flow from an outlet port of a tandem gear pump assembly having a two gear pumps, the method comprising the steps of: directing the fluid flow derived from both gear pumps to the outlet port when the combined fluid flow derived from both of the gear pumps is insufficient to establish the pre-determined flow rate; diverting any excess fluid flow derived from both gear pumps over that required to establish the pre-determined flow rate to a tank when the fluid flow derived from each gear pump individually is insufficient to establish the pre-determined flow rate but the combined fluid flow derived from both gear pumps is more than sufficient to establish the pre-determined flow rate; when the fluid flow derived from one of the gear pumps is at least sufficient in itself to establish the pre-determined flow rate, off-loading the fluid flow derived from the other of the gear pumps to the tank and diverting any excess fluid flow derived from said one of the gear pumps to the tank.
13. A method as hereinbefore described with reference to the drawings.