US3864911A

US3864911A - Hydraulic System with Bi-Rotational Pump

Info

Publication number: US3864911A
Application number: US442483A
Authority: US
Inventors: Robert K Gellatly; Joel B Meredith; Richard A Green
Original assignee: General Cable Corp
Current assignee: General Cable Corp
Priority date: 1974-02-14
Filing date: 1974-02-14
Publication date: 1975-02-11
Anticipated expiration: 1992-02-11

Abstract

An hydraulic system employing bi-rotational pumps for directing fluid under pressure to an actuator. In one embodiment the return fluid from one port of the actuator is filtered by means of a pilot operated check valve in a by-pass circuit leading to a filter and fluid reservoir, with an additional check valve being provided to preclude flow from the by-pass circuit to the associated port of the pump. In another embodiment return flow from the head end of the actuator is directed to a filter and reservoir by a two way control valve which also functions to direct flow from the pump to pressurize the head end of the actuator. A pilot operated check valve is provided to direct flow from the head end of the actuator directly to the reservoir even should the two way control valve be inoperable. In another embodiment duplex bi-rotational pump systems provide high volume flow rate at a relatively low pressure, or a low volume flow rate at a relatively high pressure. In one embodiment of the duplex bi-rotational pump system a circuit is provided to filter the fluid returning from the actuator to the reservoir by means of a two-way flow control valve.

Description

[ 1 Feb. 11, 1975 HYDRAULIC SYSTEM WITH Bl-ROTATIONAL PUMP [75] Inventors: Robert K. Gellatly, Los Gatos; Joel B. Meredith, San Carlos; Richard A. Green, Mountain View, all of Calif.

[73] Assignee: General Cable Corporation, San

Carlos, Calif.

[22] Filed: Feb. 14, 1974 1211 Appl. No.: 442,483

Primary Examiner--Edgar W. Geoghegan Attorney, Agent, or Firm-Flehr, Hohbach, Test,

Albritton llerbert [57] ABSTRACT An hydraulic system employing bi-rotational pumps for directing fluid under pressure to an actuator. In one embodiment the return fluid from one port of the actuator is filtered by means of a pilot operated check valve in a by-pass circuit leading to a filter and fluid reservoir, with an additional check valve being provided to preclude flow from the by-pass circuit to the associated port of the pump. In another embodiment return flow from the head end of the actuator is directed to a filter and reservoir by a two way control valve which also functions to direct flow from the pump to pressurize the head end of the actuator. A pilot operated check valve is provided to direct flow from the head end of the actuator directly to the reservoir even should the two way control valve be inoperable. In another embodiment duplex bi-rotational pump systems provide high volume flow rate at a relatively low pressure, or a low volume flow rate at a relatively high pressure. in one embodiment of the duplex bi-rotational pump system a circuit is provided to filter the fluid returning from the actuator to the reservoir by means of a two-way flow control valve.

9 Claims, 6 Drawing Figures PATENIEDFEM 1 1915 v ties-4.911 SHEET 1 [IF 3 NE WE uw PATENIEB FEB 1' 1 I975 SHEET 2 BF 3 PATENTED H81 1 I975 SHEET 3 0F 3 mmw HYDRAULIC SYSTEM WITH'BI-ROTATIONAL PUMP BACKGROUND OF THE INVENTION This invention relates to hydraulic systems for delivering pressurized fluid to hydraulic actuators, and in particular relates to systems of the type described which employ bi-rotational pumps.

Various circuit designs have heretofore been developed for controlling hydraulic actuators, such as rotary or extensible hydraulic actuators or rams. Bi-rotational pumps have been developed for use in such hydraulic circuits to reduce design complexity and cost. Conventional bi-rotational pumps typically are of the intermeshing gear type which are reversibly actuated with the ports of the pump alternating between suction and pressure functions so that an extensible actuator, for example, can be either extended or retracted by driving the pump in either direction.

Heretofore it has been difficult to adequately filter the hydraulic fluid in systems employing bi-rotational pumps because the return fluid from the actuator is returned to the suction face of the pump, with the result that only the relatively small volume of fluid which is pumped from the reservoir is filtered. In the case of a linear cylinder type actuator, only the differential rod volume can bedumped to tank through a low micron pressure-line filter. Suction line filters must of necessity be limited to approximately 100 mesh screen to prevent pump cavitation due to inadequate intake flow.

ln many cases it has been found desirable to provide a dual stage pumping system in which one stage provides a high volume, low pressure flow to the actuator while another stage provides a low volume, high pressure flow. Hydraulic systems of the type used for elevating a boom, for example, advantageously employ such a dual stage system where in one stage the boom is lowered at a fast rate under a light load, and in another stage the boom is raised under a heavy load. Conventional dual stage systems employ unloading valves to achieve this purpose, but heretofore a dual stage system of the type described has not been proposed which makes it feasible to employ the advantages of a birotational pumping system.

OBJECTS AND SUMMARY OF THE INVENTION It is a general object of the invention to provide a new and improved hydraulic system employing bi-rotational pumps for supplying pressurized fluid to operate hydraulic actuators.

Another object is to provide a hydraulic system employing bi-rotational pumps in which there is provision for filtering of the flow of fluid returning from one port of the actuator to the reservoir.

Another object is to provide a bi-rotational pump system of the type described in which the return flow of fluid from one port of the actuator is blocked from the pump by a check valve and is directed to a filter through a valve in a by-pass circuit.

Another object is to provide a bi-rotational pump system of the type described which directs return flow from one port of the actuator through a two way control valve to a filter and reservoir.

Another object is to provide a bi-rotational pump system of the type described employing return line filtering by means ofa two way control valve in which provision is made for diverting the return fluid directly to the reservoir should the control valve become inoperable for any reason.-

Another object is to provide a duplex bi-rotational pump system of the type described which affords a first operating stage delivering low volume, high pressure flow to the actuator, and another stage delivering high volume, low pressure flow to the actuator.

Another object is to provide a duplex bi-rotational pump system of the type described in which provision is made for the filtering of the return flow from one end of the actuator through a two way flow control valve.

Another object is to provide a duplex bi-rotational pump system of the type described employing a two way flow control valve for return line filtering in which provision is made to divert the return fluid directly to the reservoir should the control valve become inoperable for any reason.

The invention provides a hydraulic system employing bi-rotational pumps for operating hydraulic actuators. The ports of the pump are fluidly connected with the actuator by circuit means which includes a fluid reservoir, a suction circuit directing fluid from the reservoir to theports, a first supply circuit directing fluid from one of the pump. ports to the actuator and a second supply circuit directing fluid from the other pump port to the actuator. In one embodiment return fluid from one port of the actuator is blocked from the pump by a check valve and is diverted to a filter and the fluid reservoir through a by-pass circuit under control of a pilot operated valve operating responsive to the pressure in the first supply circuit. In another embodiment the return flow from one end of the actuator is diverted through the second circuit to the filter and reservoir by means of a two-way control valve. Should the two-way control valve become inoperable for any reason the actuator can be operated in emergency situations through the provision ofa pilot operated valve which diverts the return flow in the second circuit directly to the reservoir. In another embodiment the hydraulic system employs duplex, i.e., dual stage, bi-rotational pumps with means to reversibly co-actuate the pumps for directing pressurized fluid through either of first and second supply circuits connected with separate ports of the actuator. For high volume, low pressure operation, flow from the two pumps is combined and directed to the actuator. For low volume, high pressure operation, an unloading valve diverts flow from a port of the first stage pump to the reservoir while a duplexing check valve precludes flow from the second stage pump to the reservoir. During high volume, low pressure operation means is provided to open the duplexing check valve responsive to pressure in the first supply circuit. In another embodiment of the duplex bi-rotational pump system return flow from one port of the actuator is blocked from the suction ports of the pumps and directed through a filter to the reservoir by means of a two-way flow control valve. A pilot operated check valve is provided to divert return fluid directly to the reservoir to permit operation of the actuator should the flow control valve become inoperable for any reason.

Additional objects and features of the invention will appear from the following description in which the preferred embodiments have been set forth in detail in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic diagram of one embodiment of the invention illustrating a bi-rotational pump system having provision for return line filtration by means of a check valve and by-pass circuit;

FIG. 2 is a schematic diagram of another embodiment providing a bi-rotational pump system with provision for return line filtration by means of a check valve and by-pass circuit;

FIG. 3 is a schematic diagram of another embodiment providing a bi-rotational pump system with provision for return line filtration by means of a two-way control valve;

FIG. 4 is a schematic diagram of another embodiment providing a bi-rotational pump system with provision for return line filtration by means of a two-way control valve;

FIG. 5 is a schematic diagram of another embodiment providing a duplex bi-rotational pump system; and

FIG. 6 is a schematic diagram of another embodiment providing a duplex bi-rotational pump system with provision for return line filtration by means of a two-way control valve.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the drawings FIG. 1 illustrates generally at 10 an hydraulic system adapted for operating a suitable hydraulic actuator 11 such as a rotary actuator or extensible cylinder or ram of the type used, for example, in the machine tool industry.

Hydraulic system 10 includes a suitable bi-rotational pump 12, preferably of a design which incorporates intermeshing gears rotatably mounted within a pump housing. The pump is reversibly actuated by means of a suitable motor 13, such as an electric motor, coupled with the pump through a drive shaft 14. Rotation of drive shaft 14 by the motor in one rotational direction or first operation mode draws fluid under suction from a first pump port 16 and directs the fluid under pressure through the other port 17. Rotation of the drive shaft in the opposite rotational sense or second mode reverses the fluid flow and draws fluid under suction from the second port 17 and directs it under pressure through the first port 16.

Circuit means is provided for fluidly interconnecting the pump ports with the actuator. The circuit means includes a fluid reservoir 18 connected with the pump ports through a bifurcated suction circuit 19. The fluid is drawn in this circuit through a suitable coarse filter 21, preferably of a size on the order of one hundred mesh, and is directed through a first branch 22 having a check valve 23 into the first port 16 during the first mode of operation, and through a branch conduit 24 through a check valve 25 into the second port 17 during the second mode of operation. The

respective check valves

23, 25 function to preclude return flow into the reservoir through whichever branch is connected with the pressure face of the pump, depending upon the mode of operation.

The circuit means further includes a first supply circuit comprising conduits 27, 28 which interconnect the first port of pump 12 with a first port 29 ofthe actuator. A second supply circuit is provided and includes

conduits

31, 32, 33 for interconnecting the second port 17 of the pump with the second port 34 of the actuator. One-way valve means comprising a check valve 36 is provided in conduit 31 of the second supply circuit to direct flow therethrough only in a direction from the pump to the actuator.

A bifurcated return circuit is provided in the circuit means and comprises first and

second branch conduits

37, 38 connected with respective first and second supply circuits and also connected with a common conduit 39 leading to a fluid reservoir 41 through a suitable fine filter 42, preferably of a mesh size on the order of 40 microns. While

separate reservoirs

18, 41 are illustrated schematically, the invention contemplates that the reservoirs could be interconnected, or a single reservoir provided, for the suction and pressure relief circuits. Suitable

pressure relief valves

45, 43 are provided in respective first and second branch conduits of the return circuit. Preferably the relief valves comprise normally closed valves which sense pressure in respective first and second supply circuits and open when the pressures in the respective circuits exceed a predetermined value that is selected to preclude overpressure damage to the system.

The circuit means further includes a by-pass circuit comprising a conduit 44 and pilot operated check valve 46 for directing fluid from conduit 32 of the second supply circuit to the return circuit and filter 42 during the second mode of pump operation in which the first supply circuit is pressurized. One end of conduit 44 is connected in the circuit on the outlet side of check valve 36, and the other end is connected with the return circuit on the outlet side of pressure relief valve 43. A pilot conduit 47 connects pilot operated check valve 46 with conduit 28 of the first supply circuit for communicating the pressure of the first supply circuit to valve 46 for opening the same after such pressure exceeds a predetermined value.

The use and operation of the hydraulic system of FIG. 1 will be described assuming, for example, that actuator is an extensible hydraulic ram having a rod end and a head end. The port 29 is at the rod end of the ram and the port 34 is at the head end. A valve having a setting to open at 800 psi is provided as the first pressure relief valve 45, and a valve having a setting to open at 1,500 psi is provided as the second pressure relief valve 43. For extending the ram motor 13 is energized to drive pump 12 in its first operational mode drawing fluid under suction through first port 16 and directing it under pressure through second port 17. Fluid is drawn into the first port from reservoir 18 and coarse filter 21 through the branch of suction circuit 22 and check valve 23. The pressurized fluid from second port 17 opens check valve 36 and flows through

conduits

32 and 33 of the second supply circuit into the head end of ram. The

check valves

25 and 46 and pressure relief valve 43 prevent pressurized fluid from returning to reservoir. Should an overload above 1,500 psi from the pressure face of the pump arise for any reason during this mode of operation relief valve 43 will open to dump the fluid to reservoir 41.

For retracting the actuator motor 13 is actuated to reversely drive pump 12 in its second mode in which fluid is drawn under suction through second port 17 and directed under pressure through first port 16. Fluid is supplied to the second port from reservoir 18 and filter 21 through branch conduit 24 and check valve 25. Pressurized fluid from the first port is directed through conduit 28 of the first supply circuit to the rod end of the ram. Should an overload above 800 psi from the pressure face of the pump arise in this mode, relief valve 45 will open to dump the fluid to reservoir through conduit 37 of the return circuit. As the ram retracts the pressure from the first supply circuit is communicated through pilot conduit 47 to open pilot operated check valve 46. This permits return fluid from the head end of ram to flow through conduit 33 of the second supply circuit into conduit 44 of the by-pass circuit and into reservoir 41 through fine filter 42. The check valve 36 in the second supply circuit precludes this return'flow from entering the second port of pump 12.

FIG. 2 illustrates an embodiment similar to that of FIG. 1 providing an hydraulic system 50 employing a bi-rotational pump 51 driven by motor 52 through drive shaft 53. The construction and operation of pump 51 and motor 52 are similar to that described for the embodiment of FIG. 1. Circuit means is provided to fluidly interconnect respective first and

second ports

54, 55 of the pump with the

ports

57, 58 of an actuator 59, such as a rotary actuator or extensible hydraulic ram. The circuit means include a bifurcated suction circuit having a conduit 61 drawing fluid from reservoir 62 through coarse filter 63 for supply through

branch conduits

64, 65 and

check valves

67, 68 to the pump ports.

A first supply circuit including conduits 70, 71 connects first port 54 of the pump with port 57 of the actuator and a second supply

circuit including conduits

72, 73 connects the second port of the pump with port 58 of the actuator.

A return circuitis' provided and includes a fine filter 74 leading to reservoir 76 from a first branch conduit 77 provided with a first pressure relief valve 78 and a second branch conduit 79 provided with a second pressure relief valve 81. Preferably the first and second relief valves comprise normally closed valves adapted to open responsive to upstream pressures in the respective supply circuit exceeding predetermined values.

A by-pass circuit is provided to direct return fluid from port 58 of the actuator to fine filter 74 and reservoir 76 without entering the second port 55 of the pump. The by-pass circuit includes a conduit 82 and pilot operated check valve 83 normally precluding flow from the second supply circuit to the return circuit. A pilot conduit 84 is connected to communicate pressure from the first supply circuit for opening check valve 83 when the first supply circuit pressure exceeds a predetermined value. A one-way flow valve such as the check valve 86 is provided in conduit 73 to permit flow only in a direction from the pump through the second supply circuit. Check valve 86 is connected in the circuit at a point between the pump and the connection with the conduit leading to check valve 83 of the bypass circuit.

The operation of the hydraulic system of the embodiment of FIG. 2 is similar to that described for the embodiment of H6. 1. Assuming that actuator 59 is an extensible hydraulic ram having its head end at port 58 and its rod end at port 57, it is extended by actuating motor 52 to turn drive shaft 53 in a rotational sense for operating pump 51 in a mode which draws fluid under suction through first port 54 and discharges it under pressure through second port 55 and through the second supply circuit into port 58. Return flow from the rod end of the ram is directed through conduit 71 where it combines with supply fluid from suction circuit conduit 64 for delivery into first port 54 of the pump. For retracting the ram motor 52'is actuated to reversely drive the pump to draw fluid from the reservoir 62 and second port 55 and deliver it under pressure into the first supply circuit and the rod end of the ram. Return fluid from the head end of the ram is directed through conduit 72 where it is blocked from reaching the pump by check valve 86. The pressure in the first supply circuit is simultaneously communicated through pilot conduit 84 to open pilot operated check valve 83 which directs the return flow through return circuit 82 and reservoir 76 through fine filter 74. This permits the relatively large volume of fluid from the head end of the ram to be filtered during its retraction stroke, and in addition the filtering action is positive in that the fluid is forced under pressure through the fine filter.

It will realized in the embodiment of FIG. 2 the second pressure relief valve 81 functions to unload pump overpressure condition occurring in a portion of the second supply circuit between pump 51 and check valve 83. This is in comparison to the system in the embodiment of FIG. 1 where the pressure relief valve-43 functionsto relieve an overpressure condition occurring in a portion of the second supply circuit between the ram and check valve 36 under positive pump pressure or back pressure from the ram.

FIG. 3 illustrates an embodiment providing an hydraulic system 90 employing a bi-rotational pump 91 actuated through drive shaft .92 by a suitable reversible motor 93. Preferably the pump and motor are of the type described above in connection with the embodiments of FIGS. 1 and 2. That is, motor 93 is adapted to be actuated to turn the drive shaft in a rotational sense for operating pump 91 in a first mode which draws fluid under suction through first port 94 and discharges it under pressure through second port 96, and in a reverse rotational sense to operate the pump in a second mode drawing fluid under suction through second port 96 and discharging it under pressure through first port 94.

Circuit means is provided for fluidly interconnecting the ports of the pump with a suitable actuator 97. The actuator is illustrated as an extensible hydraulic ram adapted to be mounted, as an example, for raising and lowering the boom of an aerial platform. The circuit means includes a fluid reservoir 98 and bifurcated suction circuit having a coarse filter 99 of a suitable size on the orderof one hundred mesh, a branch conduit 101 having a check valve 102 permitting flow only in a direction toward the first port 94 of the pump, and a branch conduit 103 having a check valve 104 directing flow only in a direction toward the second port 96.

A first supply

circuit including conduits

105, 106 directs flow from the first port of the pump to port 107 at the rod end of the ram, and a second supply

circuit including conduits

108, 109 directs flow from the sec ond port of the pump through a two-way control valve 111 to port 112 at the head end of the ram.

A return circuit is provided and includes first and

second branch conduits

113, 114 connected with respective first and second supply circuits and also connected with a common conduit 116 leading to a reservoir 117 through a fine filter 118 of a mesh size on the order of 40 microns. Suitable

pressure relief valves

119, 120 are provided in respective first and second branch conduits and preferably comprise normally closed valves. The valve 119 is adapted to open responsive to the pressure in first supply circuits exceeding a predetermined value, and the valve 120 is adapted to open responsive to the pressure in that portion of the second supply circuit between the pump and two-way valve 111 exceeding a predetermined value.

Two-way control valve 111 controls the flow to and from the head end of the actuator. Preferably control valve 111 comprises a suitable spool valve normally urged by the force of pilot assisted, adjustable spring servo unit 122 to the illustrated first position which interconnects 109 of the second supply circuit with a conduit 123 leading to the conduit 114 in return circuit and reservoir 117 through filter 118. The spool of control valve 111 is urged to its second position by the discharge pressure from pump port 96 communicated through pilot conduit 124 into servo unit 126. 1n the second position of the spool control valve 111 establishes communication between conduit 108 and conduit 109 in the second supply circuit to direct discharge fluid from the pump to port 112 in the head end of the ram.

Pilot conduits

127, 128 communicate pressure from conduit 106 of the first supply circuit into servo unit 122 to act against the end of the spool and assist the spring force in urging the spool to its first position for directing return fluid through the filter 118 into the reservoir as the ram is retracting. The spring force urging the spool to its first position may be adjustably varied to selectively vary the pressure limit at which the spool is shifted to its second position for extending the ram.

A pilot operated check valve 129 is provided in a branch conduit 131 connected between the suction circuit and conduit 108 of the second supply circuit. Check valve 129 normally precludes flow into the suction circuit through branch conduit 131, and is adapted to be opened responsive to the pressure in the first supply circuit exceeding a predetermined value. The pressure in the first supply circuit is communicated to check valve 129 by pilot conduit 132 connected with pilot conduit 127. Check valve 129 serves to permit the return fluid from the head end of the ram to flow directly to reservoir 98 should control valve 111 be inoperable and remain in its second position at the time pump 92 is actuated to pressurize the first supply circuit. This permits the boom to be lowered in an emergency situation where control valve 111 fails to function properly.

The operation of the embodiment of FIG. 3 will be described assuming that actuator 97 comprises an extensible ram mounted for elevating and lowering the boom of an aerial lift. First pressure relief valve 119 is selected to open responsive to a pressure in excess of 800 psi in the first supply circuit, and second relief valve 120 is selected to open responsive to a pressure in excess of 1,500 psi in the second supply circuit. For elevating the boom motor 93 is actuated to operate pump 91 in its first mode drawing fluid under suction through the suction circuit and discharging it under pressure into conduit 108 of the second supply circuit. The pressure in conduit 108 communicates through pilot conduit 124 to shift the spool of control valve 111 to its second position to establish communication from the pump into conduit 109 leading to the head end of ram. As the ram extends return fluid from its rod end is directed through the first supply circuit and into conduit where it combines with flow from the suction circuit to supply fluid into first port 94 of the pump.

For retracting the actuator to lower the boom the motor is reversely actuated to operate pump 91 in its second mode drawing fluid under pressure through second port 96 and discharging it under pressure through first port 94 into the first supply circuit and the rod end of the ram. The pressure in the first supply circuit is communicated through

pilot conduits

127 and 128 to assist the spring in servo 122 to shift the spool of control valve 111 to its illustrated first position. In this position the return flow from the head end of the ram is directed into conduit 123 of the return circuit and through fine filter 118 into reservoir 117. It will be realized that a relatively large volume of fluid is filtered in this manner on the retraction stroke of the ram. and in addition the fluid is forced through the filter under pressure from the ram.

FIG. 4 illustrates another embodiment providing an hydraulic system 136 similar to the embodiment of FIG. 3. This system incorporates a bi-rotational pump 137 operated through drive shaft 138 by reversible motor 139. The pump and motor are similar to that described in connection with the foregoing embodiments,

Circuit means is provided for fluidly interconnecting the first and

second ports

141, 142 of pump 137 with a suitable actuator 143, which preferably comprises an extensible ram. The circuit means includes a suction circuit having a reservoir 144 for supply fluid to the pump ports through a coarse filter 146. A check valve 147 in one branch 148 of the suction circuit permits flow only in a direction toward the first port of the pump, and a check valve 149 in the other branch 151 permits flow only in a direction toward the second port of the pump. A first supply

circuit having conduits

152, 153 directs flow from the first port of the pump to the rod end of the ram, and a second supply circuit having a conduit 154 directs flow from the second port through a two-way control valve 156 and conduit 157 to the head end of the ram.

A return circuit is provided to direct return fluid through a conduit 158, a fine filter 159 and into a reservoir 160. A first branch 161 of the return circuit is provided with a first pressure relief valve 162 adapted to open responsive to pressure in the first supply circuit exceeding a predetermined value, and a second branch 163 is provided with a second pressure relief valve 164 adapted to open responsive to the pressure in the second supply circuit exceeding another predetermined value.

Two-way control valve 156 preferably comprises a spool valve normally urged to its first position, as illustrated in FIG. 4, by the force of pilot assisted adjustable spring servo unit 166. The pressure of second supply conduit 154 is communicated to servo 166 through a pilot conduit 167. In its first position the spool directs discharge fluid from the pump into conduit 157 leading to the head end of ram. The spool is actuated to its second position by the pressure of first supply conduit 153 communicated through

pilot conduits

168, 169 into servo unit 171. In its second position the spool directs return fluid from the head end of ram into return conduit 158 and through the fine filter 159 to reservoir 160.

A branch conduit 172 and pilot operated check valve 173 are provided between the suction circuit and conduit 154 in the second supply conduit. Check valve 173 normally precludes flow from the second supply conduit into the suction circuit and is opened responsive to a predetermined pressure in the first supply conduit communicated through pilot conduits 168 and 174. With check valve 173 open in this manner return fluid from the head end of the ram is diverted to reservoir 144 should control valve 156 be inoperable with its spool valve lodged in the first position as the ram is retracted.

In operation of hydraulic system 136 it will be assumed that actuator 143 is an hydraulic ram mounted to raise and lower the boom of an aerial lift. First relief valve 162 is set to open responsive to the pressure in the first supply circuit exceeding 800 psi, and second relief valve 164 is set to open responsive to the pressure in the second supply circuit communicating with the pump discharge exceeding 1,500 psi. For extending the actuator is raise the boom motor 139 is actuated to operate pump 137 in its first mode drawing fluid under suction through first port 141 and discharging it under pressure into the second supply circuit. With the spool of the control valve 156 urged to its first position by the spring force of servo 166 assisted by the pressure in pilot conduit 167, pressurized fluid from the pump is directed into conduit 157 and the head end of the ram. Return fluid from the rod end is directed through the first supply circuit into conduit 152 where it combines with flow through branch conduit 148 of the suction circuit to supply the suction side of the pump.

For retracting the ram to lower the boom. motor 139 is revcrsely actuated to operate pump 137 in its second mode drawing fluid from the suction circuit and discharging it under pressure into conduit 153 of the first supply circuit and the rod end of the ram. The pressure in conduit 153 is communicated through

pilot conduits

168 and 169 to shift the spool of valve 156 to its second position. Return fluid from the head end of the ram then flows through conduit 157 and valve 156 into return conduit 158 and through fine filter 159 into the reservoir. Thus, a relatively large volume of fluid is filtered on the retraction stroke of the ram, and moreover the fluid is forced under pressure of the ram through the fine filter. FIG. illustrates another embodiment providing an hydraulic system 176 incorporating a pair of birotational first and second stage pumps 177, 178 connected to provide duplex, i.e., two-stage, operation. In one stage flow from the two pumps is combined for high volume, low pressure operation, and in another stage one pump is unloaded for low volume, high pressure operation. The pumps preferably are of the intermeshing gear pump type as described in the foregoing embodiments, and are mounted for reversible coactuation on a drive shaft 179 powered by reversible motor 181, such as an electric motor. Motor 181 is adapted to conjointly actuate the two pumps in a first mode for drawing fluid under suction through their respective

first ports

182, 183 and discharging it under pressure through their

respective seconds ports

184, 185, and in a second mode drawing fluid under suction through their second ports and discharging it under pressure through their first ports.

Circuit means is provided for fluidly interconnecting the ports of the pumps with a suitable actuator 187, iilustrated as an extensible hydraulic ram. The circuit means include a suction circuit having a conduit 188 drawing fluid from a reservoir 189 through a coarse tilter 191 of a size on the order of one hundred mesh. A

branch 192 of the suction circuit directs flow through a check valve 193 and

conduits

194, 195 and 196 leading to the first ports of the pumps, and another branch 5 directs flow through a check valve 198 and

conduits

199, 200 and 401 leading to the second ports of the pumps. Conduit 403 provides a first supply circuit directing flow to the rod end of the actuator, and conduit 204 provides a second supply circuit directing flow to the head end of the actuator. A bifurcated return circuit including conduit 205 is provided to direct return flow to reservoir 189 through a fine filter 201 ofa mesh size on the order of 40 microns. A first branch 202 of the return circuit includes a pressure relief valve 203 and is connected with the first supply circuit. A second branch 204 of the return circuit is provided with a second pressure relief valve 206 and is connected through a conduit 207 with conduit 204 of the second supply circuit. Preferably the pressure relief valves are normally closed valves adapted to open responsive to the upstream pressures in the circuits exceeding predetermined values.

Valve means for unloading the first stage pump 177 when the pressure in the second supply circuit exceeds a predetermined value it is provided and comprises an unloading valve 208 in a branch conduit 209 connected with the conduit 401 leading from the second port of the first stage pump, with the outlet of the unloading valve dumping fluid into reservoir 189. Preferably unloading valve 208 comprises a normally closed valve sensing pressure in the second supply circuit through conduit 207 and a conduit 211, with the valve being set to open when such pressure exceeds a predetermined value.

Duplexing valve means is provided to preclude flow from the second stage pump to the reservoir through the open unloading valve during the low volume, high pressure stage, and to permit suction flow to the first stage pump during the high volume. low pressure stage. The duplexing valve means comprises a pilot operated check valve 212 in a conduit 213 interconnecting the conduits leading to the second ports of the two pumps.

The check valve normally permits flow from the first stage pump to combine with that from the second stage pump for high volume, low pressure supplied to the head end of the actuator. A higher pressure in conduit 213 of the second supply circuit when unloading valve 208 is open functions to close the duplexing check valve so that a low volume, high pressure from second stage pump 178 is delivered to the head end of the actuator. During the low pressure stage duplexing check valve 212 opens responsive to a given pressure in the first supply circuit sensed through a pilot conduit 214 to permit suction fluid to be supplied to pump 177 from check valve 198,

conduits

197, 188 and reservoir 189.

A differential volume valve comprising a pilot operated check valve 216 is provided in a conduit 217 connected between conduit 204 of the second supply circuit and conduit 197 of the suction circuit. The valve 216 functions to divert a portion of the greater volume of flow from the head end of the actuator during retraction into the suction circuit. This valve is opened responsive to the pressure in first supply circuit as sensed through a pilot conduit 218 exceeding a predetermined value.

In operation, it will be assumed that actuator 187 comprises an extensible ram mounted to raise and lower the boom of an aerial lift. The bi-rotational pumps 177, 178 are each sized to produce a 1.5 gpm flow rate with motor 181 operating at 3,450 rpm, thereby providing at 2:1 flow ratio between low and high pressure modes. As desired, the pumps could be sized to provide different flow ratios, for example a 3:1 ratio between the low and high presssure modes. First pressure relief valve 203 is set to open responsive to pressure in the first supply circuit exceeding 850 psi and second pressure relief valve 206 is set to open responsive to pressure in the second supply circuit exceeding 1,500 psi. The unloading valve 208 is set to open responsive to pressure in the second supply circuit exceeding 1,000 psi.

For extending the ram to raise the boom, motor 181 is energized to conjointly drive the first and second stage pumps in a mode drawing fluid under suction through their respective

first ports

182, 183 and discharging it under pressure through

second ports

184, 185. Fluid from first stage pump 177 is directed through duplexing check valve 212 and the second supply circuit where it combines with the discharge flow from second stage pump 178 for delivery to the ram through conduit 204. As the ram extends and the load on the boom increases, the dual stage pump delivery mode is maintained until a pressure of 1,000 psi is exceeded in the second supply circuit. This pressure is communicated through

conduits

207 and 211 to open unloading valve 208. The discharge from first stage pump 177 then dumps through the unloading valve to the reservoir. This releases the upstream pressure on duplexing check valve 212 so that the pressure from second stage pump 178 closes the duplexing valve preventing second stage flow from discharging to reservoir. The system thus is capable of operating the ram under high pressure, low flow conditions utilizing relatively lower horsepower requirements as compared to that which would be required by utilizing the flow from two pumps. As the ram extends return flow from its rod end is directed through conduit 403 of the first supply circuit where it combines with the intake flow from check valve 193 of the suction circuit for delivery to the first and second stage pumps.

For retracting the actuator to lower the boom, motor 181 is energized to reversely and conjointly drive the two pumps in a low pressure, high volume mode drawing fluid under suction through their second ports 184, I85 and discharging it under pressure into conduit 203 of the first supply circuit and the rod end of the actuator. Pressure in the first supply circuit is communicated through pilot conduit 214 to open duplexing check valve 212. This permits intake fluid from check valve 198 of the suction circuit to flow through the duplexing check valve and supply the second port 184 of the first stage pump. Fluid supply to both pumps in this mode comes from the head end of actuator 187 through

conduits

204, 199, 213, 200 and 401 to pump

intake ports

185 and 184.

During the low pressure, high volume mode for retracting the actuator, pressure from the first supply circuit is communicated through pilot conduit 218 to open pilot operated check valve 216. This permits the differential volume between the head and rod ends of actuator 187 to flow through this valve and discharge to reservoir 189 through

conduits

217, 197 and 188 and filter 191.

FIG. 6 illustrates an embodiment of the invention providing an hydraulic system 220 incorporating birotational pumps 221 and 222 arranged to provide duplex, i.e., two-stage, flow to an actuator 223 with provision for filtration of return fluid from one end of the actuator. The first stage pump 221 and second stage pump 222 are mounted for conjoint reversible operation through a drive shaft 224 from a suitable motor 226, such as an electric motor. The two pumps preferably are of the previously described gear pump type and are adapted for operation between one mode drawing fluid under suction through their respective

first ports

227, 228 and discharging it under pressure through their respective

second ports

229, 230 and a second mode drawing fluid under suction through their second ports and discharging it under pressure through their first ports.

Circuit means is provided for fluidly interconnecting the ports of the pump with actuator 223, shown as an extensible ram. A suction circuit directs fluid from reservoir 232 through a coarse filter 233, of a size on the order of one hundred mesh, through a first branch conduit 234 with check valve 236 and a second branch conduit 237 with check valve 238. Fluid from the first branch conduit is supplied through conduit 239 to the first port of second stage pump 222 and through

conduits

241, 242 to the first port of first stage pump 221. Fluid from the second branch conduit 237 is supplied to the second port of the second stage pump through conduit 243, and through duplexing check valve 244 into conduit 246 to the second port of the first stage pump. A first supply circuit including a conduit 247 is provided to direct the flow from the first ports of the pumps to the rod end of the actuator, and a second supply circuit including a conduit 248 is provided to direct flow from the second ports of the pumps through a twoway control valve 249 to the head end of the actuator.

A return

circuit including conduits

251 and 252 is provided. to direct return fluid to the reservoir through a fine filter 253 of a mesh size on the order of 40 microns into reservoir 254. A first branch conduit 256 having a pressure relief valve 257 and a second branch conduit 258 having a second pressure relief valve 259 are provided to relieve overpressure from the supply circuits. Preferably the pressure relief valves comprise normally closed valves adapted to open responsive to the upstream pressure in respective supply circuits exceeding a predetermined value.

Unloading valve means is provided and comprises a normally closed pilot operated valve 261 in a conduit 262 connected between the first stage pump oulet conduit 246 and the return circuit. A pilot conduit 263 is provided to communicate second stage pump discharge pressure from

conduits

243 and 264 for opening unloading valve 261 responsive to such pressure exceeding a predetermined value.

The duplexing check valve 244 comprises a pilot operated check valve which normally precludes flow from the second stage pump to reservoir 254 when unloading valve 261 is open during the high pressure operating mode. Pilot conduits 266 and 267 communicate pressure from the first supply circuit to open the duplexing check valve permitting fluid to be supplied to second port 229 of the first stage pump when the pumps are operating in their low pressure mode for retracting the actuator.

Two position flow control valve 249 preferably comprises a spool valve normally urged by pilot pressure assisted spring servo unit 301 to its illustated first position for establishing communication between conduit 248 connected with the head end of the actuator and return conduit 251, filter 253, and reservoir 254. A pilot conduit'302 communicates pressure from the first supply circuit to servo 301 to assist in shifting the spool to its first position when the rod end of the actuator is pressurized. The spool is shifted toits second position by servo unit 303 responsive to the pressure communicated through pilot condut 304 of discharge fluid from the second port of pump 222. In the second position of the spool the pump discharge is directed to the head end of the actuator.

A pilot operated check valve 306 is provided in a conduit 307 connected between conduit 243 and the suction circuit. This check valve normaly precludes flow from the discharge of the second stage pump to the suction circuit, and is opened responsive to a predetermined pressure communicated from the first supply circuit through

pilot conduits

266 and 308. The function of check valve 306 is to insure that the actuator can be retracted should control valve 249 become inoperable with its spool lodged in its second position. If the control valve does fail to move from its second position when a retraction stroke is desired, check valve 306 will open as the pumps pressurize the first supply circuit, and return fluid from the head end of the actuator is then directed through the control valve and diverted through check valve 306 to the suction circuit.

In operation it will be assumed that actuator 223 comprises an hydraulic ram mounted to raise and lower the boom of an aerial lift. The bi-rotational pumps 221, 222 are each sized to deliver 1.5 gpm flow when driven at 3,450 rpm by motor 226. First pressure relief valve 257 is set to open responsive to first supply circuit pressure exceeding 850 psi, and second relief valve 259 is set to open responsive to second supply circuit pressure exceeding 2,000 psi. Unloading valve 261 is set to open responsive to the second stage pump discharge pressure from conduit 243 exceeding 1,200 psi.

For extending the actuator to elevate the boom, motor 226 is energized to conjointly operate the first and second stage pumps in a mode drawing fluid under suction through the first ports and discharging it under pressure through their second ports. Fluid from first stage pump 221 flows through duplexing check valve 244 and combines with flow from the second stage pump to communicate pressure through pilot conduit 304 to shift the spool of control valve 249 to its second position. thereby directing combined flow from the two pumps through conduit 248 to the head end of the ac tuator. Suction fluid is supplied to the two pumps from reservoir 232, through filter 233, check valve 236 and

conduits

239, 241, 242. Return fluid from the rod end of the actuator is directed through conduit 247 to com bine with the flow into the suction ports of the two pumps. This operational mode is maintained until the pressure in the second supply circuit exceeds 1,200 psi, at which point unloading valve 261 is opened due to the pressure communicated through pilot conduit 263. The discharge from first stage pump 221 then flows through the unloading valve and filter 253 to reservoir 254. The unloading of the discharge from the first stage pump reduces pressure in the free direction on check valve 244 causing it to close, thereby preventing discharge .14 from the second stage pump from reaching the reservoir. The second stage pumpthen continues to operate to supply low volume, high'pressure flow to'the head end of the actuator.

For retracting the actuator to lower the boom, motor 226 is reversed to drive'thetwo pumps in a mode discharging fluid under pressure throughtheir-lfirst ports through conduit 247 tothe rod end of the actuator. Pressure in the first supplycircuit iscommunicated through pilot conduit 302 to servo unit 301 to assist the spring force in shifting the spool valve to its'first position, thereby directing return flow fromthe heade'nd of the actuator to return conduit-251', filter 253-and reservoir 254. Pressure in the first supply circuit is also communicated'through pilot conduits 266'and '267 to open duplexing check valve 244. Thiszpermits suction fluid to flow from conduit264 into second port229 of the first stage pump. First circuit pressurealsoopens check valve 306 through pilot conduit-266, thereby permitting the differential volume fluid which is not supplied to the suction ports of the pumps to be discharged through conduit 307 to reservoir 232 through filter 233 in the event that valve 249 sticks in its second position.

While the foregoing embodiments are at present considered to be preferred it will be understood that variations and modifications may be made therein by those skilled in the art and it is intended to cover all such variations and modifications as fall withinthe true spirit and scope of the invention as expressed in the appended claims.

We claim:

1. ln a system for operating an hydraulic actuator, the combination of a first stage reversible pump, a second stage reversible pump, each pump having first and second ports and being capable of operating in a first mode in which its first and second ports assumerespective suction and pressure functionsand in a second mode in which its ports assume respective pressure and suction functions, fluid circuit means for directing fluid between the ports of the pumps and said actuator, said circuit means including fluid reservoir means, a suction circuit to direct fluid from the reservoir means to said ports, a first supply circuit to direct fluid under pressure from the first ports of said pumps to the actuator, a second supply circuit to direct fluid under pressure from the second ports of said pumps to said actuator, unloading valve means adapted to open for directing fluid from the second port of the first pump to the reservoir means responsive to pressure in the portion of the second supply circuit between the second port of the second pump and the actuator exceeding a given value, and duplexing valve'means for precluding fluid flow from the second port of the second pump to the reservoir means when the loading valve means is open.

2. A system as in claim 1 in which the circuit means includes a return circuit to direct fluid from said actuator to the second ports and to the reservoir means when the pumps are operated in their second modes.

3. A system as in claim 1 in which said return circuit includes valve means operable to open for directing return fluid from said portionof the second supply circuit to the suction circuit responsive to the pressure in the first supply circuit exceeding an established value.

4. A system as in claim 1 in which said duplexing valve means includes pilot operated check valve means operable to direct flow from the suction circuit to the second port of the first pump responsive to the pressure in said first supply circuit exceeding another given value when said pumps are operated in their second modes.

5. A system as in claim 1 in which the circuit means includes a return circuit to direct return fluid from the actuator to the reservoir means, filter means for filtering fluid in the return circuit, and flow control valve means operable for directing flow from the first and second stage pumps to the actuator through the second supply circuit when the pressure in the latter exceeds a predetermined value, said control valve means being further operable for directing return flow from the actuator through the return circuit and filter means to the reservoir means responsive to the pressure in the first supply circuit exceeding another predetermined value.

6. A system as in claim 5 in which the duplexing check valve means includes pilot operated check valve means operable to direct flow from the suction circuit to the second port of the first pump responsive to the pressure in said first supply circuit exceeding another given value when said pumps are operated in their second modes.

7. A system as in claim 5 which includes pilot operated check valve means for permitting flow from the portion of the second supply circuit between the second port of the second pump and the control valve means to the reservoir means responsive to pressure in the first supply circuit exceeding another given value to permit release of return flow from said actuator through the control valve means should the latter be rendered inoperable.

8. A system as in claim 5 which includes first pressure relief valve means to direct fluid from the actuator to the reservoir means through the first supply circuit when the pressure therein exceeds a first predetermined value, and second pressure relief valve means for directing fluid from the actuator to the reservoir through the second supply circuit when the pressure therein exceeds a second predetermined value greater than said first value.

9. A system as in claim 1 which includes means for co-actuating said pumps in a common selected mode for pumping fluid under pressure into selected first or

Claims

1. In a system for operating an hydraulic actuator, the combination of a first stage reversible pump, a second stage reversible pump, each pump having first and second ports and being capable of operating in a first mode in which its first and second ports assume respective suction and pressure functions and in a second mode in which its ports assume respective pressure and suction functions, fluid circuit means for directing fluid between the ports of the pumps and said actuator, said circuit means including fluid reservoir means, a suction circuit to direct fluid from the reservoir means to said ports, a first supply circuit to direct fluid under pressure from the first ports of said pumps to the actuator, a second supply circuit to direct fluid under pressure from the second ports of said pumps to said actuator, unloading valve means adapted to open for directing fluid from the second port of the first pump to the reservoir means responsive to pressure in the portion of the second supply circuit between the second port of the second pump and the actuator exceeding a given value, and duplexing valve means for precluding fluid flow from the second port of the second pump to the reservoir means when the loading valve means is open.

3. A system as in claim 1 in which said return circuit includes valve means operable to open for directing return fluid from said portion of the second supply circuit to the suction circuit responsive to the pressure in the first supply circuit exceeding an established value.

9. A system as in claim 1 which includes means for co-actuating said pumps in a common selected mode for pumping fluid under pressure into selected first or second supply circuits.