Patent Publication Number: US-2015064030-A1

Title: Fluid pressure drive unit

Description:
TECHNICAL FIELD 
     The present invention relates to a fluid pressure drive unit for adapted to supply a working fluid to and driving a fluid pressure actuator. 
     BACKGROUND ART 
     Conventionally, in a construction machine such as a power shovel, a hybrid structure in which a power generator is rotated by an extra output of an engine and emission energy of an actuator, electric power generated by the power generator is stored, and actuation of the actuator is assisted by using the stored electric power is used. In such a hybrid structure, a fluid pressure drive unit including an electric motor to be rotated with the stored electric power, and an assist pump to be driven and rotated by the electric motor, the assist pump for discharging a working fluid and assisting the actuation of the actuator by a main pump is used. 
     JP2011-127569A discloses an assist regeneration device including a motor generator to be actuated and rotated with electric energy, a regeneration motor for driving and rotating the motor generator with energy of a working fluid, and an assist pump to be driven and rotated by the motor generator, the assist pump for discharging the working fluid. 
     SUMMARY OF INVENTION 
     However, in the assist regeneration device of JP2011-127569A, when being driven and rotated or when generating the regenerative electric power, the motor generator generates heat. Therefore, there is a need for a cooling system of circulating a refrigerant by using a pump and cooling the motor generator from an exterior. 
     The present invention is achieved in consideration with the above problem, and an object thereof is to simplify a cooling mechanism of an electric motor in a hydraulic pressure drive unit. 
     According to one aspect of the present invention, a fluid pressure drive unit adapted to supply a working fluid to and driving a fluid pressure actuator is provided. The fluid pressure drive unit includes a fluid pressure pump that is configured to suction and discharge the working fluid, an electric motor that is configured to drive and rotate the fluid pressure pump, a power transmission mechanism that is configured to transmit a power between a rotation shaft of the fluid pressure pump and a rotation shaft of the electric motor, and a circulation mechanism that is configured to be driven by the power transmitted by the power transmission mechanism, the circulation mechanism that is configured to guide a lubricating fluid in the power transmission mechanism and cool the electric motor. 
     The details as well as other features and advantages of the present invention are set forth in the remainder of the specification and are shown in the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a front view showing a part of a fluid pressure drive unit according to an embodiment of the present invention in a sectional view. 
         FIG. 2  is a sectional view by line II-II of a fluid pressure pump motor in  FIG. 1 . 
         FIG. 3  is a sectional view of a plate, a power transmission mechanism, and a circulation mechanism in  FIG. 1 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, referring the drawings, a hydraulic drive unit  100  serving as a fluid pressure drive unit according to an embodiment of the present invention will be described. In the hydraulic drive unit  100 , working oil is used as a working fluid. It should be noted that instead of the working oil, other fluids such as working water may be used as the working fluid. 
     Firstly, referring to  FIGS. 1 to 3 , a configuration of the hydraulic drive unit  100  will be described. 
     The hydraulic drive unit  100  is to supply the working oil to and drive a hydraulic actuator (not shown) serving as a fluid pressure actuator. The hydraulic drive unit  100  is applied to a hybrid construction machine such as a power shovel for driving the hydraulic actuator with the working oil discharged from a main hydraulic pump (not shown) which is driven by a prime mover. 
     As shown in  FIG. 1 , the hydraulic drive unit  100  is provided with a hydraulic pump motor  1  serving as a fluid pressure pump motor which includes a hydraulic pump  10  serving as a fluid pressure pump for suctioning and discharging the working oil, and a hydraulic motor  20  serving as a fluid pressure motor to be driven and rotated with the supplied working oil. 
     The hydraulic drive unit  100  is also provided with an electric motor  30  arranged in parallel to the hydraulic pump motor  1 , a plate  40  having an identical surface to which the hydraulic pump motor  1  and the electric motor  30  are attached, a power transmission mechanism  50  for transmitting a power between a rotation shaft  2  of the hydraulic pump motor  1  and a rotation shaft (not shown) of the electric motor  30 , and a circulation mechanism  60  for guiding lubricant oil serving as a lubricating fluid in the power transmission mechanism  50  and cooling the electric motor  30 . 
     The hydraulic pump  10  and the hydraulic motor  20  forming the hydraulic pump motor  1  are respectively swash-plate-type variable piston pump motors. The hydraulic motor  20  is a piston pump motor of a larger scale than the hydraulic pump  10 . 
     As shown in  FIG. 2 , the hydraulic pump motor  1  is provided with a casing  3  for accommodating the hydraulic pump  10  and the hydraulic motor  20 , and the single rotation shaft  2  rotatably and axially supported on the casing  3  and commonly used for the hydraulic pump  10  and the hydraulic motor  20 . 
     The casing  3  has a flange portion  3   a  fastened to the plate  40  by bolts. The casing  3  has a supply and emission passage  4  through which the working oil to be supplied to the hydraulic pump  10  flows and the working oil emitted from the hydraulic motor  20  flows, a discharge passage  5  through which the working oil discharged from the hydraulic pump  10  flows, and a return passage  6  through which the working oil returned from the hydraulic actuator, to be supplied to the hydraulic motor  20  flows. 
     The supply and emission passage  4  communicates with a tank (not shown) in which the working oil is stored. The discharge passage  5  and the return passage  6  communicate with the hydraulic actuator. The supply and emission passage  4  is provided to oppose the discharge passage  5  and the return passage  6 . 
     The hydraulic pump  10  and the hydraulic motor  20  are arranged to oppose each other in the axial direction of the rotation shaft  2  across the supply and emission passage  4 , the discharge passage  5 , and the return passage  6 . 
     The hydraulic pump  10  suctions the working oil of the supply and emission passage  4  and discharges to the discharge passage  5 . The hydraulic pump  10  assists drive of the hydraulic actuator by the main hydraulic pump with the discharged working oil. The hydraulic pump  10  is provided with a cylinder block  11  coupled to the rotation shaft  2 , a plurality of pistons  13  respectively accommodated in a plurality of cylinders  12  which is defined in the cylinder block  11 , a swash plate  14  for letting the pistons  13  in sliding contact reciprocate, and a port plate  15  to be brought into sliding contact with an end surface of the cylinder block  11 . 
     The cylinder block  11  is formed into a substantially columnar shape, and rotated integrally with the rotation shaft  2 . The cylinder block  11  is driven and rotated by the rotation shaft  2 . In the cylinder block  11 , the plurality of cylinders  12  is formed in parallel with the rotation shaft  2 . 
     The cylinders  12  are arranged on an identical circumference of the cylinder block  11  centering on the rotation shaft  2  in an annular manner at fixed intervals. The pistons  13  are inserted into the respective cylinders  12 , and volume chambers  12   a  are defined between the cylinders and the pistons  13 . The volume chambers  12   a  communicate with the port plate  15  through communication holes. 
     When the cylinder block  11  is rotated together with the rotation shaft  2 , the pistons  13  are brought into sliding contact with the swash plate  14 . Thereby, the pistons  13  reciprocate in the cylinders  12  in accordance with a tilting angle of the swash plate  14 , and hence extend and contract the volume chambers  12   a.    
     The swash plate  14  is provided in such a manner that the tilting angle is adjustable by a capacity switching actuator (not shown). The swash plate  14  is tiltable into a state shown in  FIG. 2  from a state where the swash plate is perpendicular to the rotation shaft  2  with the tilting angle of zero. The tilting angle of the swash plate  14  is steplessly adjusted by the capacity switching actuator. 
     The port plate  15  is formed into a disc shape, and has a through hole into which the rotation shaft  2  is inserted in center thereof. The port plate  15  has a supply port  15   a  formed into an arc shape centering on the rotation shaft  2 , the supply port providing communication between the supply and emission passage  4  and the volume chambers  12   a,  and a discharge port  15   b  similarly formed into an arc shape centering on the rotation shaft  2 , the discharge port providing communication between the discharge passage  5  and the volume chambers  12   a.    
     In the hydraulic pump  10 , a region where the pistons  13  are brought into sliding contact with the swash plate  14  and the volume chambers  12   a  are extended is a suctioning region, and a region where the pistons  13  are brought into sliding contact with the swash plate  14  and the volume chambers  12   a  are contracted is a discharging region. The supply port  15   a  is formed in correspondence with the suctioning region, and the discharge port  15   b  is formed in correspondence with the discharging region. Thereby, in accordance with rotation of the cylinder block  11 , the working oil is suctioned into the volume chambers  12   a  facing the supply port  15   a,  and the working oil is discharged from the volume chambers  12   a  facing the discharge port  15   b.    
     The hydraulic motor  20  is driven and rotated with the working oil emitted from the hydraulic actuator. The hydraulic motor  20  is provided with a cylinder block  21  coupled to the rotation shaft  2 , a plurality of pistons  23  respectively accommodated in a plurality of cylinders  22  which is defined in the cylinder block  21 , a swash plate  24  for letting the pistons  23  in sliding contact reciprocate, and a port plate  25  to be brought into sliding contact with an end surface of the cylinder block  21 . The cylinder block  21 , the cylinders  22 , the pistons  23 , and the swash plate  24  of the hydraulic motor  20  only have different size from the configurations of the above hydraulic pump  10  but have the same configurations. Thus, description thereof is omitted. 
     The port plate  25  is formed into a disc shape, and has a through hole into which the rotation shaft  2  is inserted in center thereof. The port plate  25  has a supply port  25   a  formed into an arc shape centering on the rotation shaft  2 , the supply port  25   a  providing communication between the return passage  6  and volume chambers  22   a,  and an emission port  25   b  similarly formed into an arc shape centering on the rotation shaft  2 , the emission port  25   b  providing communication between the supply and emission passage  4  and the volume chambers  22   a.    
     In the hydraulic motor  20 , a region where the pistons  23  are brought into sliding contact with the swash plate  24  and the volume chambers  22   a  are extended is a suctioning region, and a region where the pistons  23  are brought into sliding contact with the swash plate  24  and the volume chambers  22   a  are contacted is an emitting region. The supply port  25   a  is formed in correspondence with the suctioning region, and the emission port  25   b  is formed in correspondence with the emitting region. Thereby, in accordance with rotation of the cylinder block  21 , the working oil is suctioned into the volume chambers  22   a  facing the supply port  25   a,  and the working oil is emitted from the volume chambers  22   a  facing the emission port  25   b.    
     The electric motor  30  drives and rotates the hydraulic pump  10 , and is capable of generating regenerative electric power by the rotation of the hydraulic motor  20 . The electric power generated in the electric motor  30  is stored in an electric power storage device (not shown). The electric motor  30  drives and rotates the hydraulic pump  10  by using the regenerative electric power regenerated by the rotation of the hydraulic motor  20  and stored in the electric power storage device. 
     As shown in  FIG. 1 , the plate  40  is a plate shape member having one surface  40   a  to which the hydraulic pump motor  1  and the electric motor  30  are attached, and the other surface  40   b  to which a casing  51  of the power transmission mechanism  50  is attached. Thereby, the power transmission mechanism  50  is provided to oppose the hydraulic pump motor  1  and the electric motor  30  across the plate  40 . In the plate  40 , a through hole (not shown) through which the rotation shaft  2  of the hydraulic pump motor  1  passes, a through hole (not shown) through which a rotation shaft of the electric motor  30  passes, and a reflux port  42  (refer to  FIG. 3 ) through which the lubricant oil after cooling the electric motor  30  is refluxed are formed. 
     As described above, in the hydraulic drive unit  100 , the hydraulic pump motor  1  and the electric motor  30  are arranged in a U shape through the plate  40  and the power transmission mechanism  50 . Therefore, as the hydraulic pump motor  1  and the electric motor  30  are arranged in parallel, the entire length of the hydraulic drive unit  100  can be shortened. Thus, mountability of the hydraulic drive unit  100  to the hybrid construction machine can be improved. 
     It should be noted that instead of the U shape arrangement, the hydraulic pump motor  1  may be attached to the one surface  40   a  of the plate  40 , and the electric motor  30  may be attached to the other surface  40   b  of the plate  40 . The hydraulic pump motor  1  and the electric motor  30  may be arranged in series across the plate  40 . 
     As shown in  FIG. 3 , the power transmission mechanism  50  is provided with the casing  51  fixed to the plate  40 , a first gear  52  to be rotated integrally with the rotation shaft  2  of the hydraulic pump motor  1 , a second gear  53  to be rotated integrally with the rotation shaft of the electric motor  30 , and an idle gear  54  provided between the first gear  52  and the second gear  53 , the idle gear  54  for transmitting the power. 
     The casing  51  accommodates the first gear  52 , the second gear  53 , and the idle gear  54 . The casing  51  is fastened by bolts in a state where an opening end surface  51   a  is abutted with the other surface  40   b  of the plate  40 . The lubricant oil is charged inside the casing  51 . The casing  51  has a through hole  51   b  formed on an end surface on the opposite side of the opening end surface  51   a,  the through hole  51   b  into which a rotation shaft of the idle gear  54  is inserted. 
     The first gear  52  has a recessed portion  52   a  formed on a rotation shaft, the recessed portion into which the rotation shaft  2  of the hydraulic pump motor  1  is inserted and fitted. Thereby, the first gear  52  is rotated integrally with the rotation shaft  2  of the hydraulic pump motor  1 . In the first gear  52 , one end of the rotation shaft is rotatably and axially supported on the plate  40  by a first bearing  52   b,  and the other end of the rotation shaft is rotatably and axially supported on the casing  51  by a second bearing  52   c.    
     Similarly, the second gear  53  has a recessed portion  53   a  formed on a rotation shaft, the recessed portion into which the rotation shaft of the electric motor  30  is inserted and fitted. Thereby, the second gear  53  is rotated integrally with the rotation shaft of the electric motor  30 . In the second gear  53 , one end of the rotation shaft is rotatably and axially supported on the plate  40  by a first bearing  53   b,  and the other end of the rotation shaft is rotatably and axially supported on the casing  51  by a second bearing  53   c.    
     The idle gear  54  is respectively meshed with the first gear  52  and the second gear  53  and transmits the power between the gears. In the idle gear  54 , one end of the rotation shaft is rotatably and axially supported on the plate  40  by a first bearing  54   b,  and a substantially center part of the rotation shaft is rotatably and axially supported on the casing  51  by a second bearing  54   c . The other end of the rotation shaft of the idle gear  54  is inserted into the through hole  51   b  and extended in a casing  61  of the circulation mechanism  60 . 
     In such a way, by providing the idle gear  54  between the first gear  52  and the second gear  53 , even in a case where the hydraulic pump motor  1  and the electric motor  30  are relatively distant from each other, diameters of the first gear  52  and the second gear  53  are suppressed from being large. Therefore, the power transmission mechanism  50  can be downsized, and the entire hydraulic drive unit  100  can be downsized. 
     By adjusting a gear ratio between the first gear  52  and the second gear  53 , a reduction ratio between the hydraulic pump motor  1  and the electric motor  30  can be set to be a proper value. 
     The circulation mechanism  60  is provided with the casing  61  whose interior communicates with an interior of the casing  51  of the power transmission mechanism  50 , an impeller  62  serving as a rotation member to be rotated integrally with the idle gear  54  in the casing  61 , a supply flow passage  63  for guiding the lubricating fluid stirred up by the impeller  62  to the electric motor  30 , and a reflux flow passage  64  for returning the lubricating fluid guided to the electric motor  30  into the power transmission mechanism  50 . 
     The casing  61  is fixed in a state where an opening end surface  61   a  is abutted with the casing  51  of the power transmission mechanism  50 . The lubricant oil charged in the interior of the casing  51  of the power transmission mechanism  50  flows into the interior of the casing  61 . In the casing  61 , a third bearing  54   d  for rotatably and axially supporting the other end of the rotation shaft of the idle gear  54  is provided. 
     The impeller  62  is a rotating part provided coaxially with the idle gear  54 . The impeller  62  is attached to the rotation shaft of the idle gear  54 . The impeller  62  is provided between the second bearing  54   c  and the third bearing  54   d.  It should be noted that the impeller  62  may be provided anywhere between the first bearing  54   b  and the third bearing  54   d.    
     The impeller  62  is rotated when the power transmission mechanism  50  transmits the power between the hydraulic pump motor  1  and the electric motor  30 , and stirs up the lubricant oil in the casing  51  of the power transmission mechanism  50  guided into the casing  61  toward an outer circumference. In accordance with an increase in the rotation number of the electric motor  30 , the rotation number of the impeller  62  is increased. Therefore, in accordance with an increase in a heat generation amount of the electric motor  30 , an amount of the lubricant oil stirred up by the impeller  62  is increased. 
     Since the impeller  62  is rotated integrally with the idle gear  54 , rotational fluctuation of the idle gear  54  can be reduced by the flywheel effect. Therefore, noises due to the rotational fluctuation of the idle gear  54  can be reduced. 
     It should be noted that instead of providing the impeller  62  to be rotated integrally with the idle gear  54 , the impeller may be provided to be rotated integrally with the first gear  52  or the second gear  53 . A plurality of impellers  62  may be provided, for example, impellers  62  are respectively provided in the first gear  52  and the second gear  53 . That is, the impeller  62  is to be rotated integrally with at least any one of the first gear  52 , the second gear  53 , and the idle gear  54 . 
     Instead of the impeller  62 , another mechanism such as a cylinder to be driven by the rotation of the idle gear  54 , the cylinder for stirring up the lubricant oil may be provided. That is, as long as the mechanism is capable of converting rotation motion of the idle gear  54  and stirring up the lubricant oil, any mechanism may be provided. 
     As shown in  FIG. 1 , the supply flow passage  63  is a pipe pulled out to an exterior from the casing  61  and coupled to an exterior of the electric motor  30 . The supply flow passage  63  is pulled out from a surface of the casing  61  facing the outer circumference of the impeller  62 . The lubricant oil guided through the supply flow passage  63  is supplied to an oil jacket (not shown) formed inside the electric motor  30 , and cools the electric motor  30 . 
     The reflux flow passage  64  is a pipe pulled out to the exterior from the electric motor  30  and coupled to the reflux port  42  (refer to  FIG. 3 ) formed in the plate  40 . Through the reflux flow passage  64 , the lubricant oil emitted from the oil jacket of the electric motor  30  is refluxed into the casing  51  of the power transmission mechanism  50 . It should be noted that instead of the configuration in which the supply flow passage  63  and the reflux flow passage  64  are provided in the exterior of the electric motor  30 , the supply flow passage  63  and the reflux flow passage  64  may be formed inside a casing of the electric motor  30 . 
     Next, actions of the hydraulic drive unit  100  will be described. 
     In a case where the hydraulic drive unit  100  assists the drive of the hydraulic actuator by the main hydraulic pump, the electric motor  30  is rotated by using the electric power preliminarily stored in the electric power storage device. By the rotation of the electric motor  30 , the rotation shaft  2  of the hydraulic pump motor  1  is driven and rotated via the power transmission mechanism  50 . 
     Regarding the hydraulic pump  10 , the tilting angle of the swash plate  14  is switched to have a predetermined value which is more than zero by the capacity switching actuator. In the hydraulic pump  10 , in accordance with the rotation of the cylinder block  11 , the pistons  13  reciprocate in the cylinders  12 . By this reciprocation of the pistons  13 , the working oil from the tank is suctioned into the volume chambers  12   a  through the supply port  15   a  of the port plate  15 . The working oil discharged from the volume chambers  12   a  is guided to the discharge passage  5  through the discharge port  15   b  of the port plate  15 . 
     Thereby, the working oil discharged from the hydraulic drive unit  100  is supplied for the drive of the hydraulic actuator, and assists the drive of the hydraulic actuator by the main hydraulic pump. 
     When the electric motor  30  drives and rotates the hydraulic pump motor  1 , the rotation of the second gear  53  is transmitted to the idle gear  54 , and the rotation of the idle gear  54  is transmitted to the first gear  52 . By rotating the idle gear  54 , the impeller  62  of the circulation mechanism  60  is rotated. 
     When the impeller  62  is rotated, the lubricant oil in the casing  51  of the power transmission mechanism  50  guided into the casing  61  of the circulation mechanism  60  through the through hole  51   b  is stirred up and supplied to the oil jacket of the electric motor  30  through the supply flow passage  63 . Therefore, the electric motor  30  can be cooled by heat exchange between the lubricant oil and the electric motor  30 . The lubricant oil after cooling the electric motor  30  is refluxed from the oil jacket of the electric motor  30  into the casing  51  of the power transmission mechanism  50  through the reflux flow passage  64 . 
     As described above, when the electric motor  30  drives and rotates the hydraulic pump motor  1 , the impeller  62  is rotated in accordance with transmission of the power by the power transmission mechanism  50 , and the lubricant oil is guided to the electric motor  30 . Therefore, since there is no need for providing a cooling system of cooling the electric motor  30  from the exterior, a cooling mechanism of the electric motor  30  in the hydraulic drive unit  100  can be simplified. 
     Only when the power transmission mechanism  50  transmits the power, that is, when the electric motor  30  is rotated and generates heat, the lubricant oil can be supplied and cooling can be performed. Therefore, in comparison to a case where the cooling is always performed by using the cooling system of cooling the electric motor  30  from the exterior, cooling efficiency can be more enhanced. 
     Since the lubricant oil stirred up by the impeller  62  cools the electric motor  30  and is refluxed, the lubricant oil in the power transmission mechanism  50  is circulated. Therefore, the lubricant oil in the power transmission mechanism  50  flows and moves. Thus, the bearings for axially supporting the first gear  52 , the second gear  53 , and the idle gear  54  are prevented from being burnt out due to shortage of the lubricant oil. 
     At this time, the hydraulic motor  20  is retained in such a manner that a tilting angle of the swash plate  24  becomes zero by the capacity switching actuator. Therefore, since the pistons  23  do not reciprocate in the cylinders  22 , a displacement volume by the pistons  23  becomes zero. Thus, since the hydraulic motor  20  does not supply and emit the working oil but only runs idle, a drive loss of the hydraulic motor  20  is suppressed. 
     Meanwhile, in a case where the regenerative electric power is generated with the working oil emitted from the hydraulic actuator, regarding the hydraulic motor  20 , the tilting angle of the swash plate  24  is switched to be a predetermined value which is more than zero by the capacity switching actuator. In the hydraulic motor  20 , in accordance with the rotation of the cylinder block  21 , the pistons  23  reciprocate in the cylinders  22 . By this reciprocation of the pistons  23 , the pressurized working oil returned from the hydraulic actuator through the return passage  6  flows into the volume chambers  22   a  through the supply port  25   a  of the port plate  25 . The pistons  23  reciprocate in the cylinders  22 , and the cylinder block  21  is driven and rotated. The working oil flowing into the volume chambers  22   a  is emitted to the supply and emission passage  4  through the emission port  25   b  of the port plate  25 , and refluxed to the tank. 
     The rotation shaft  2  is rotated integrally with the cylinder block  21 . The rotation of the rotation shaft  2  is transmitted to the rotation shaft of the electric motor  30  via the power transmission mechanism  50 . Thereby, the electric motor  30  can generate and store the regenerative electric power in the electric power storage device. 
     When the rotation of the rotation shaft  2  of the hydraulic pump motor  1  is transmitted to the electric motor  30 , the rotation of the first gear  52  is transmitted to the idle gear  54 , and the rotation of the idle gear  54  is transmitted to the second gear  53 . By rotating the idle gear  54 , the impeller  62  of the circulation mechanism  60  is rotated. Therefore, as well as a case where the electric motor  30  drives and rotates the hydraulic pump motor  1 , the electric motor  30  can be cooled by the heat exchange between the lubricant oil and the electric motor  30 . 
     At this time, the hydraulic pump  10  is retained in such a manner that the tilting angle of the swash plate  14  becomes zero by the capacity switching actuator. Therefore, since the pistons  13  do not reciprocate in the cylinders  12 , a displacement volume by the pistons  13  becomes zero. Thus, since the hydraulic pump  10  does not supply and emit the working oil but only runs idle, a drive loss of the hydraulic pump  10  is suppressed. 
     It should be noted that in a case where the hydraulic drive unit  100  assists supply of the working oil to a plurality of hydraulic actuators by the main hydraulic pump, there is sometimes a case where drive of one hydraulic actuator is assisted and the working oil is refluxed from other hydraulic actuators. 
     According to the above embodiment, the following effects are obtained. 
     The circulation mechanism  60  for guiding the lubricant oil in the power transmission mechanism  50  by the rotation of the impeller  62  and cooling the electric motor  30  is provided. This impeller  62  is rotated integrally with the idle gear  54  for transmitting the power between the first gear  52  and the second gear  53 . Therefore, when the electric motor  30  drives and rotates the hydraulic pump motor  1 , the impeller  62  is rotated in accordance with the transmission of the power by the power transmission mechanism  50 , and the lubricant oil is guided to the electric motor  30 . Thus, since there is no need for providing the cooling system of cooling the electric motor  30  from the exterior, the cooling mechanism of the electric motor  30  in the hydraulic drive unit  100  can be simplified. 
     Embodiments of the present invention were described above, but the above embodiments are merely examples of applications of the present invention, and the technical scope of the present invention is not limited to the specific constitutions of the above embodiments. 
     For example, the hydraulic drive unit  100  is to assist the drive of the hydraulic actuator by the main hydraulic pump. However, instead of this, the hydraulic actuator may be driven by using only the hydraulic drive unit  100 . 
     Both the hydraulic pump  10  and the hydraulic motor  20  are swash-plate-type piston pump motors. However, as long as the motors are variable motors in which a suction and discharge capacity is adjustable to be zero, the hydraulic pump and the hydraulic motor may be other types. The circulation mechanism  60  may supply the lubricant oil to the hydraulic pump motor  1 . 
     This application claims priority based on Japanese Patent Application No. 2012-075565 filed with the Japan Patent Office on Mar. 29, 2012, the entire contents of which are incorporated into this specification.