Patent Publication Number: US-2023160400-A1

Title: Hydraulic System for an Industrial Truck

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to German Patent Application No. 10 2021 130 272.2 filed Nov. 19, 2021, the disclosure of which is hereby incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a hydraulic system for an industrial truck, in particular a battery-powered electric industrial truck, with a lift drive to raise and lower a load handling device, wherein the hydraulic system has a first hydraulic pump that is designed to supply the lift drive with hydraulic fluid and a second hydraulic pump that can be actuated by means of a valve device for the supply of the lift drive. 
     Description of Related Art 
     Hydraulic systems of this type are generally used in battery-powered electric industrial trucks such as counterbalanced fork lift trucks or reach trucks, for example. 
     On battery-powered electric industrial trucks such as counterbalanced fork lift trucks or reach trucks, for example, hydraulic systems are known in which a single hydraulic pump is provided which supplies hydraulic fluid to the lift drive, with which a load handling device can be raised and lowered, as well as to optional additional consumers of a hydraulic work system, such as a tilting drive and/or a sideshifter drive. 
     The maximum displacement volume of the hydraulic pump that is driven by an electric drive motor, for example, is determined by the maximum load pressure on the lift drive and the output limit of the electric drive motor that drives the hydraulic pump. 
     In the event that the load handling device is to be lifted/raised with no load, i.e. lifted at a low load pressure, a significantly higher displacement volume of the hydraulic pump can be used than when the load handling device is lifted with a nominal load, i.e. lifted at a high load pressure, given the same torque or power curve of the electric drive motor. 
     On hydraulic systems of industrial trucks with a single hydraulic pump which supplies the lift drive with hydraulic fluid, an additional increase of the lifting speed of the load handling device can be achieved only by increasing the driving speed of the hydraulic pump. However, the lifting speed during no-load lifting is limited on account of the maximum requirement during lifting with a nominal load (torque limit or power output limit of the drive motor of the hydraulic pump) in the form of the pump size, i.e. of the displacement volume of the hydraulic pump, and the maximum driving speed of the hydraulic pump. 
     Especially on reach trucks, on which the lift drive makes possible a lifting height of the load handling means of more than 8 m, for example, higher no-load lifting speeds are advantageous to increase the cargo handling capacity of the reach truck. On account of the power output limit of the electric drive motor of the hydraulic pump, however, the maximum displacement volume of the hydraulic pump is defined by the maximum load pressure at nominal load. 
     To achieve higher lifting speeds of the load handling means, hydraulic systems of industrial trucks are known in which the hydraulic system has two hydraulic pumps. In these cases, a first hydraulic pump supplies the lift drive with hydraulic fluid under all operating conditions. To achieve higher lifting speeds of the load handling means, a second hydraulic pump is provided, which can be actuated as necessary by means of a valve device for the supply of the lift drive. 
     A hydraulic system of this type for an industrial truck with two hydraulic pumps in which the first hydraulic pump supplies the lift drive with hydraulic fluid under all operating conditions and a second hydraulic pump is actuated for the supply of the lift drive to achieve higher lifting speeds of the load handling device is known from DE 198 31 828 B1. In DE 198 31 828 B1, the first hydraulic pump and the second hydraulic pump are two separate hydraulic pump units, which results in a more complex and expensive construction. The second hydraulic pump is further connected via its own connecting line with the lift drive, as a result of which the cost and complexity of the construction is further increased as a result of the higher piping and assembly cost and effort required. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to make available a hydraulic system for an industrial truck. 
     According to the invention, this object is accomplished in that the first hydraulic pump and the second hydraulic pump are in the form of a two-pump hydraulic unit and the valve device is integrated into the two-pump hydraulic unit. 
     According to the invention, the two hydraulic pumps therefore form a two-pump hydraulic unit into which the valve device, by means of which the second hydraulic pump can be switched on to supply the lift drive, is integrated and therefore incorporated. 
     The combination of the two hydraulic pumps into one two-pump hydraulic unit results in particular in a reduced construction cost. The integration and therefore the incorporation of the valve device into the two-pump hydraulic unit results in a further reduction of the construction cost because the second hydraulic pump does not require its own connecting line for the connection to the lift drive, so that the cost for the piping of the second hydraulic pump to the lift drive and the related assembly cost and effort can be eliminated. 
     According to one advantageous embodiment of the invention, the two-pump hydraulic unit preferably has a housing and the valve device is incorporated into the housing. Consequently, a simpler incorporation and integration of the valve device into the two-pump hydraulic unit can be achieved. 
     According to one advantageous embodiment of the invention, an intake channel is preferably formed in the housing which is connected with an intake port of the first hydraulic pump and with an intake port of the second hydraulic pump, a first delivery channel is formed which is connected with a delivery port of the first hydraulic pump, and a second delivery channel is formed which is connected with a delivery port of the second hydraulic pump and with the first delivery channel. With the intake channel in the housing, the intake ports of the two hydraulic pumps can be connected, preferably in a simple manner, with a reservoir for the intake of hydraulic fluid. At the first delivery channel, which is connected with the delivery port of the first hydraulic pump and which is connected with the second delivery channel which is connected with the delivery port of the second hydraulic pump, the lift drive can preferably be connected in a simple manner with both hydraulic pumps by means of a connecting line. 
     According to one advantageous embodiment of the invention, the valve device preferably has a switched on position in which the delivery port of the second hydraulic pump is connected with the first delivery channel, and a switched off position in which the delivery port of the second hydraulic pump is connected with the intake channel. In the switched off position, therefore, by means of a connection of the delivery port of the second hydraulic pump with the intake channel, the second hydraulic pump can be short-circuited in a simple manner to the intake channel or to a reservoir connected to it. In the switched on position, therefore, by means of a connection of the delivery port of the second hydraulic pump with the first delivery channel, the second hydraulic pump can be connected to supply the lift drive and to increase the lifting speed of the load handling device. 
     According to one advantageous configuration of the invention, the valve device preferably has a check valve, in particular a spring-loaded check valve, located in the second delivery channel and opening toward the first delivery channel, as well as a control valve device which is located in a connecting channel that connects the second delivery channel with the intake channel. The connection of the delivery port of the second hydraulic pump with the first delivery channel can be controlled in a particularly simple manner with the check valve. With the control valve device, which is located in the connecting channel connecting the second delivery channel with the intake channel, in particular the connection of the delivery port of the second hydraulic pump with the intake channel can be controlled in a simple manner. 
     According to one advantageous embodiment of the invention, the control valve device preferably has a closed position that forms the switched on position and an open position that forms the switched off position. In the closed position of the control valve device, the connection of the delivery port of the second hydraulic pump with the intake channel is preferably shut off. As a result, the delivery port of the second hydraulic pump can preferably be connected via the opening check valve with the first delivery channel. In the open position of the control valve device, the delivery port of the second hydraulic pump can preferably be connected with the intake channel. 
     According to an alternative and likewise advantageous embodiment of the invention, the valve device preferably has a control valve device which is located in the second delivery channel and is connected to a connecting channel that is connected with the intake channel. With a control valve device of the type described, the connection of the delivery port of the second hydraulic pump with the first delivery channel and the connection of the delivery port of the second hydraulic pump with the intake channel can preferably be controlled in a simple manner. 
     According to one advantageous embodiment of the invention, the control valve device preferably has a control position that forms the switched on position in which the second delivery channel is open and the connection of the second delivery channel with the connecting channel is shut off, and a second control position that forms the switched off position in which the second delivery channel is connected with the connecting channel and the connection of the second delivery channel with the first delivery channel is shut off. In the first control position, in particular the connection of the delivery port of the second hydraulic pump with the first delivery channel for the switched on position can be established in a simple manner. In the second switched position, in particular the connection of the delivery port of the second hydraulic pump with the intake channel for the shutoff position can be established 
     According to one advantageous embodiment of the invention, the control valve device can preferably be a multi-way valve or a proportional valve. 
     According to one advantageous embodiment of the invention, preferably the control valve device can be actuated electrically by means of an electric actuator device, in particular by means of a magnet. With an electric actuator device, the control valve device can be actuated in a simple manner between the switched on position and the switched off position. 
     According to one advantageous embodiment of the invention, an electronic control device is preferably provided which is in communication on the input side with an operating element and on the output side with the valve device for its actuation, wherein the electronic control device is configured so that the valve device is controlled as a function of the actuation of the operating element. As a result, the valve device can be actuated in a simple manner as a function of the actuation of the operating element, such as a lever or switch, for example, by an operator of the industrial truck into the switched on position or the switched off position. 
     According to one advantageous configuration of the invention, the electronic control device is preferably designed so that when the operating element is actuated, the valve device is actuated into the switched on position or into the switched off position, and when the operating element is not actuated, the valve device is actuated into the switched off position or into the switched on position. By actuating the operating element, for example, an operator of the industrial truck can activate the switched on position in a simple manner to request a higher lifting speed from the lift drive. 
     According to one advantageous embodiment of the invention, preferably an electronic control device is provided which is in communication on the input side with a sensor device that measures the load pressure on the lift drive and on the output side with the valve device for its actuation, wherein the electronic control device is configured so that the valve device is controlled as a function of the load pressure on the lift drive. As a result, in particular the valve device can be actuated in a simple manner as a function of the current load pressure on the lift drive into the switched on position or the switched off position. 
     According to one advantageous configuration of the invention, the electronic control device is preferably designed so that the valve device is actuated into the switched on position at a load pressure on the lift drive below a limit load pressure, and is actuated into the switched off position at a load pressure on the lift drive above the limit load pressure. As a result, in particular in a simple manner, the second hydraulic pump can be switched on during a no-load lift or during a lifting operation in the partial-load range to achieve a higher lifting speed during a no-load lifting or during the lifting of a partial load. 
     According to one advantageous embodiment of the invention, the first hydraulic pump and the second hydraulic pump are preferably driven by a common drive motor, in particular an electric motor. Because the second hydraulic pump can be switched on and switched off as necessary by the valve device, in particular both hydraulic pumps can be driven by a common drive motor, as a result of which the cost of construction of the hydraulic system can be further reduced. 
     The invention has a series of advantages. 
     As a result of the switching on of the second hydraulic pump, for example at load pressures during lifting that are below the limit load pressure, significantly higher lifting speeds of the load handling device can be achieved with partial loads or no loads with a significantly smaller size of the drive motor that drives the hydraulic pumps. 
     The invention further makes possible a downsizing of the drive motor which drives the hydraulic pumps, because during lifting at the nominal load and therefore a high load pressure on the lift drive, the pump volume can be reduced by switching off the second hydraulic pump. 
     The second hydraulic pump can be switched off when the drive motor is started to achieve high starting speeds of the first hydraulic pump. Consequently a long useful life of bearings, in particular sliding-contact bearings, of the hydraulic pumps can be achieved. 
     The result is a further cost optimization because the integration of the valve device into the hydraulic double pump unit makes it possible to eliminate the cost of piping for the second hydraulic pump with the lift drive and the related costs of installation. 
     In connection with an operating element that can be actuated by an operator of the industrial truck, for example a lever or switch, which can additionally be actuated by a lift-drive-operating element such as a joystick that controls the lifting operation, the operator can actively initiate a boost function with an increased lifting speed during lifting operation of the lift drive by an additional actuation of the operating element. 
     With the invention, it also becomes possible in a simple manner to create a variant model of a standard industrial truck with a single hydraulic pump and a high-performance industrial truck with a hydraulic system according to the invention with a hydraulic double pump unit, because this variant model can be produced in the industrial trucks in a simple manner by two pump variants, namely a single pump and a hydraulic double pump unit according to the invention, without additional modifications or adaptations to the hydraulic systems of the industrial trucks. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Additional advantages and details of the invention are described in greater detail below with reference to the exemplary embodiments illustrated in the accompanying schematic figures, in which 
         FIG.  1    is a circuit diagram of a first embodiment of the invention. 
         FIG.  2    is a variant of the first embodiment illustrated in  FIG.  1    and 
         FIG.  3    is a circuit diagram of a second embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     In  FIGS.  1  to  3   , identical components are identified by the same reference numbers. 
       FIGS.  1  to  3    each show a hydraulic system  1  of an industrial truck, for example of a battery-powered, electrically operated industrial truck. 
     The hydraulic system  1  has a lift drive  2  to raise and lower a load handling device which is not illustrated in any further detail, such as a load fork comprising forks. In the illustrated exemplary embodiment, the lift drive  2  has one or more lifting cylinders  3 . 
     The hydraulic system  1  has a first hydraulic pump  5  which supplies the lift drive  2  with hydraulic fluid and a second hydraulic pump  6  which can be switched on by means of a valve device  7  for the supply of the lift drive  2 . 
     A common drive motor  8  is provided for the drive of the first hydraulic pump  5  and the second hydraulic pump  6 . In the illustrated exemplary embodiment, the drive motor  8  is an electric motor. 
     The first hydraulic pump  5  and the second hydraulic pump  6  are in the form of a hydraulic double pump unit  10 . The valve device  7  is integrated and thereby incorporated into the hydraulic double pump unit  10 . 
     For this purpose the hydraulic double pump unit  10  has a housing  11  into which the valve device  7  is incorporated. 
     The first hydraulic pump  5  has an intake port S 1  and a delivery port P 1 . Correspondingly, the second hydraulic pump  5  has an intake port S 2  and a delivery port P 2 . 
     In the housing  11  of the hydraulic double pump unit  10  there is an intake channel  12  which is connected with the intake port S 1  of the first hydraulic pump  5  and with the intake port S 2  of the second hydraulic pump  6 . The intake channel  12  is also in communication with a reservoir  13 , for example by means of a reservoir line  14  connected to the intake channel  12 , from which the two hydraulic pumps  5 ,  6  take in hydraulic fluid. 
     In the housing  11  of the hydraulic double pump unit  10  there is a first delivery channel  20  which is connected with the delivery port P 1  of the first hydraulic pump  5 . A connecting line  21  is connected to the first delivery channel  20  and connects the first delivery channel  20  with a multi-way valve block  22  of the hydraulic system  1 . The lifting cylinder  3  is connected to the multi-way valve block  22  by means of an additional connecting line  23 . The multi-way valve block  22  is provided with a multi-way control valve device with which the lifting operation and the lowering operation of the lift drive  2  can be controlled. In the lifting operation of the lift drive  2 , the first delivery channel  20  is connected by means of the multi-way control valve device with the connecting line  23 . In the lowering operation of the lift drive  2 , the connecting line  23  is connected by means of the multi-way control valve device with the reservoir  13 . 
     It is noted that additional hydraulic consumers can be connected to the multi-way control valve block  22  and controlled by means of corresponding multi-way control valve devices, for example a tilting operation of the load handling device and/or a sideshifter of the load handling device. 
     In the housing  11  of the hydraulic double pump unit  10  there is also a second delivery channel  30  which is connected with the delivery port P 2  of the second hydraulic pump  6 . The second delivery channel  30  is also connected to the first delivery channel  20  inside the housing  11 . 
     The valve device  7  illustrated in  FIGS.  1  to  3    has a switched on position in which the delivery port P 2  of the second hydraulic pump  6  is connected with the first delivery channel  20 , and a switched off position in which the delivery port P 2  of the second hydraulic pump  6  is connected with the intake channel  12 . 
     In the exemplary embodiments illustrated in  FIGS.  1  and  2   , the valve device  7  has a check valve  40  located in the second delivery channel  30 , which opens toward the first delivery channel  20 . In the exemplary embodiments illustrated in  FIGS.  1  and  2   , the check valve  40  is a spring-loaded check valve. The valve device  7  in  FIGS.  1  and  2    also has a control valve device  45  which is located in a connecting channel  46  that connects the second delivery channel  30  with the intake channel  12 . The connecting channel  46  is connected to the second delivery channel  30  between the delivery port P 2  and the check valve  40  and leads to the intake channel  12 . 
     In the exemplary embodiments illustrated in  FIGS.  1  and  2   , the control valve device  45  is a two-port, two-position valve and has a closed position  45   a  that forms the switched on position and an open position  45   b  that forms the switched off position. 
     In the exemplary embodiment illustrated in  FIG.  1   , the control valve device  45  is a switch valve. 
     In the exemplary embodiment illustrated in  FIG.  2   , the control valve device  45  is a proportional valve that has a throttling action in intermediate positions. 
     In the exemplary embodiment illustrated in  FIG.  1   , the control valve device  45  can be actuated electrically by means of an electric actuator device  50 , such as by means of a magnet, in particular a switching magnet, for example. In the exemplary embodiment illustrated in  FIG.  1   , the control valve device can be actuated by means of a spring  51  toward the closed position  45   a  and by means of the electric actuator device  50  toward the open position  45   b.    
     In the exemplary embodiment illustrated in  FIG.  2   , the control valve device  45  can be actuated electrically by means of an electric actuator device  50 , such as by means of a magnet, in particular a proportional-action magnet, for example. In the exemplary embodiment illustrated in  FIG.  2   , the control valve device can be actuated toward the open position  45   a  by means of a spring  51  and toward the closed position  45   a  by means of the electric actuator device  50 . 
     In the exemplary embodiment illustrated in  FIG.  3   , the valve device  7  has a control valve device  60  which is located in the second delivery channel  30  and is connected to a connecting channel  61  connected with the intake channel  12 . 
     In the illustrated exemplary embodiment, the control valve device  60  is a three-port, two-position valve which is connected at a first port A 1  to the segment of the second delivery channel  30  in communication with the delivery port P 2  of the second hydraulic pump  6 , at a second port A 2  to the segment of the second delivery channel  30  in communication with the first delivery channel  20  and at a third port A 3  to the connecting channel  61 . 
     The control valve device  60  has a first control position  60   a  that forms the switched on position, in which the first port A 1  is connected with the second port A 2  and the third port A 3  is shut off. In the control position  60   a , therefore, the second delivery channel  30  is open and the connection of the second delivery channel  30  with the connecting channel  61  is closed. The control valve device  60  has a second control position  60   b  that forms the switched off position, in which the first port A 1  is connected with the third port A 3  and the second port A 2  is closed. In the control position  60   b , therefore, the second delivery channel  30  is connected with the connecting channel  61  and the connection of the second delivery channel  30  with the first delivery channel  20  is closed. 
     In the exemplary embodiment illustrated in  FIG.  3   , the control valve device  60  is a proportional valve that has a throttling action in intermediate positions. Alternatively, the control valve device  60  can be a switch valve. 
     In the exemplary embodiment illustrated in  FIG.  3   , the control valve device  60  can be actuated electrically by means of an electric actuator device  70 , such as by means of a magnet, in particular a proportional-action magnet, for example. In the illustrated exemplary embodiment, the control valve device  60  can be actuated toward the first control position  60   a  by means of a spring  71  and toward the second control position  60   b  by means of the electric actuator device  50 . Alternatively, the control valve device  60  can be actuated toward the second control position  60   b  by means of a spring  71  and toward the first control position  60   a  by means of the electric actuator device  70 . 
     For the actuation of the valve device  7  formed by the control valve device  45  in  FIGS.  1  and  2    or the control valve device  60  in  FIG.  3   , an electronic control device  80  is provided which is in communication on the input side with a sensor device  81  that measures the load pressure of the lift drive  2  present in the connecting line  23 , and is in communication on the output side with the actuator device  50  or  70 . The electronic control device  80  is designed so that the control valve device  45  or  60  of the control valve device  7  is controlled as a function of the load pressure on the lift drive  2 . 
     The electronic control device  80  is designed so that the valve device  7  is actuated into the switched on position at a load pressure on the lift drive  2  below a limit load pressure, and is actuated into the switched off position at a load pressure on the lift drive  2  above the limit load pressure. 
     In  FIGS.  1  and  2   , for this purpose the control valve device  45  is actuated by the control device  80  during lifting operation of the lift drive  2 , when a load pressure on the lift drive  2  measured by means of the sensor device  81  is below the limit load pressure, into the closed position  45   a  that forms the switched on position  45   a , and by the control device  80 , during lifting operation of the lift drive  2  at a load pressure on the lift drive  2  measured by means of the sensor device  81  above the limit load pressure into the open position  45   b  that forms the switched off position. 
     In  FIG.  3   , for this purpose the control valve device  60  is actuated by the control device  80  during lifting operation of the lift drive  2 , when a load pressure on the lift drive  2  measured by means of the sensor device  81  is below the limit load pressure, into the first control position  60   a  that forms the switched on position, and by the control device  80 , during lifting operation of the lift drive  2  at a load pressure on the lift drive  2  measured by means of the sensor device  81  above the limit load pressure into the second control position  60   b  that forms the switched off position. 
     In the switched on position of the valve device  7  in  FIGS.  1  to  3   , it therefore becomes possible to achieve an increased lifting speed of the load handling device. 
     Alternatively or additionally, the electronic control device  80  can be in communication on the input side with an operating element  85  such as a lever or switch, for example, which can be actuated by the operator of the industrial truck, and on the output side with the actuator device  50  or  70 , wherein the electronic control device  80  is designed so that the control valve device  45  or  60  of the valve device  7  is controlled as a function of the actuation of the operating element  85 . 
     The electronic control device  80  is configured, for example, so that when the operating element  85  is actuated, the valve device  7  is actuated into the switched on position, and when the operating element of not actuated, the valve device  7  is actuated into the switched off position. 
     In  FIGS.  1  and  2   , for this purpose the control valve device  45  is actuated by the control device  80  during lifting operation of the lift drive  2  when the operating element  85  is additionally actuated into the closed position  45   a  that forms the switched on position, and by the control device  80  during lifting operation of the lift drive  2 , when the operating element  85  is not actuated, into the open position  45   b  that forms the switched off position. 
     In  FIG.  3   , for this purpose the control valve device  60  is actuated by the control device  80  during lifting operation of the lift drive  2  when the operating element  85  is also actuated into the first control position  60   a  that forms the switched on position, and by the control device  80  during lifting operation of the lift drive  2  when the operating element  85  is not actuated into the second control position  60   b  that forms the switched off position. 
     When the lift drive  2  is in lifting operation, the operator, by additionally actuating the operating element  85 , can actively activate the switched on position of the valve device  7  in  FIGS.  1  to  3    and therefore order an increased lifting speed of the load handling device. 
     Alternatively, the electronic control device  80  can be configured, for example, so that when the operating element  85  is actuated, the valve device  7  is actuated into the switched off position, and when the operating element is not actuated, the valve device  7  is actuated into the switched on position. 
     In the hydraulic system  1  according to the invention, the valve device  7  is integrated into the hydraulic double pump unit  10 . The second hydraulic pump  6  is switched on and off as a function of the load pressure during lifting operation of the lift drive  2 , as measured by the sensor device  81  in the connecting line  23  that leads to the lift drive  2 . Alternatively or additionally, the second hydraulic pump  6  can be switched on or off as a function of the actuation of the operating element  85 . 
     In the hydraulic system  1  according to the invention, if the flow of the second hydraulic pump  6  is not required and the valve device  7  is in the switched off position, the volume flow of the second hydraulic pump  6  is short-circuited directly to the intake channel  12  of the two hydraulic pumps  5 ,  6  inside the housing  11 . In the hydraulic system  1  according to the invention, if the volume flow of the second hydraulic pump  6  is required to increase the lifting speed of the load handling device and the valve device  7  is in the switched on position, the volume flow of the second hydraulic pump  6  is transported directly inside the housing  11  into the first delivery channel  20 . The second hydraulic pump  6  therefore does not require any external hoses or piping. 
     If the load pressure on the lift drive  2  in lifting operation is below the defined limit load pressure, e.g.  100  bar, the second hydraulic pump  6  can be switched on in the closed position  45   a  of the control valve device  45  in  FIGS.  1  and  2    or in the first control position  60   a  of the control valve device  60  in  FIG.  3   . It is therefore possible to achieve very high lifting speeds of the load handling device without a load and with a partial load. If, during lifting operation, the load pressure on the lift drive  2  exceeds the defined limit load pressure and, therefore, the pump torque of the two hydraulic pumps  5 ,  6  exceeds the maximum torque or the maximum output of the drive motor  8 , the second hydraulic pump  6  is short-circuited to the intake channel  12  and, therefore, to the reservoir  13  by an actuation of the control valve device  45  in  FIGS.  1  and  2    into the open position  45   b  or the control valve device  60  in  FIG.  3    is actuated into the second control position  60   b . During lifting operation, at a load pressure on the lift drive  2  that is above the defined limit load pressure, only the first hydraulic pump  5  therefore acts to lift the load handling device. The check valve  40  in  FIGS.  1  and  2    thereby prevents a short-circuiting of the first hydraulic pump  5  to the reservoir  13 . 
     The invention is not restricted to the exemplary embodiments illustrated in  FIGS.  1  to  3   . 
     In  FIG.  1   , the control valve device  45  can alternatively be a proportional valve. 
     In  FIG.  2   , the control valve device  45  can alternatively be a switch valve. 
     In  FIG.  3   , the control valve device  60  can alternatively be a switch valve.