Patent Publication Number: US-2021189687-A1

Title: Rotary drive system and hydraulic excavator

Description:
TECHNICAL FIELD 
     The present invention relates to a rotary drive system and a hydraulic excavator. This application claims priority based on Japanese Patent Application No. 2018-015909, filed on Jan. 31, 2018, in Japan, the contents of which are incorporated herein by reference. 
     BACKGROUND TECHNOLOGY 
     Patent Document 1 describes a rotary drive system in which an electric motor and a speed reducer for reducing the speed of the rotation of the electric motor are integrally provided. The speed reducer includes a plurality of stages of planetary gear mechanisms as transmission portions accommodated in the speed reducer casing. 
     Lubricating oil is stored in the space in the speed reducer casing, and each planetary gear mechanism is immersed in the lubricating oil. 
     On the other hand, in order to remove heat generated from the rotor and stator during operation of the electric motor, cooling oil is supplied to the inside of the electric motor casing. A lower portion of a space in the electric motor casing is used as a tank in which the cooling oil is stored. The cooling oil discharged from the electric motor is cooled at the outside thereof and then is again supplied into the electric motor casing. 
     PRIOR ART DOCUMENT 
     Patent Document 
     [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2009-79627. 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     In the rotary drive system described above, some of the cooling oil is stored in part of the electric motor casing while storing the lubricating oil in the speed reducer casing. That is, since there is a tank for storing lubricating oil or cooling oil in each of the speed reducer and the motor, the size of the apparatus may be increased. 
     Further, it is necessary to individually manage the lubricating oil of the speed reducer and the cooling oil of the electric motor, and for example, it is necessary to provide an oil inspection pipe for inspecting the oil quantity and a supply port for supplying the lubricating oil separately. Therefore, there is an increase in cost. 
     The present invention has been made in view of such problems, and it is an object to provide a rotary drive system and a hydraulic excavator using the same which are capable of reducing cost while achieving compactness. 
     Means for Solving the Problem 
     An aspect of the present invention provides a rotary drive system, including: an electric motor including a rotary shaft provided so as to be rotatable about an axis extending in a vertical direction, a rotor core fixed to an outer peripheral surface of the rotary shaft, a stator surrounding the rotor core from the outer peripheral side of the rotor core, and an electric motor casing forming a first accommodating space that accommodates the rotary shaft, the rotor core and the stator so that a lower portion of the rotary shaft projects downward, and forming a communication hole communicating downward, an output shaft provided so as to be rotatable about the axis below the rotary shaft; a transmission portion in which a rotation of the rotary shaft is reduced in speed and is transmitted to the output shaft; a speed reducer having a speed reducer casing that accommodates the output shaft and the transmission portion so as to project downward a lower portion of the output shaft and forms a second accommodating space communicating with the first accommodating space through the communication hole; and a lubricating oil-circulating unit including a lubricating oil flow path that connects the first accommodating space and the second accommodating space, and a lubricating oil pump that is provided in the lubricating oil flow path and is configured to pump a lubricating oil from the second accommodating space side to the first accommodating space side. 
     According to the rotary drive system having the above structure, lubricating oil supplied into the electric motor casing is introduced into the speed reducer casing through the communication hole. Therefore, it is possible to supply again the lubricating oil in the speed reducer casing to the electric motor by the lubricating oil-circulating unit. 
     Accordingly, it is possible to consistently carry out cooling of the electric motor and lubricating of the speed reducer through a single lubricating oil-circulating unit. Therefore, a tank for storing lubricating oil in the electric motor is not necessary. 
     Further, it is not necessary to separately manage the amount of oil of the speed reducer and the electric motor. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a side view of a hydraulic excavator provided with a rotary drive system related to an embodiment of the present invention. 
         FIG. 2  is a plan view of the hydraulic excavator provided with the rotary drive system according to the embodiment of the present invention. 
         FIG. 3  is a schematic diagram showing an outline of the rotary drive system relating to the embodiment of the present invention. 
         FIG. 4  is a longitudinal sectional view of the rotary drive device in the rotary drive system according to the embodiment of the present invention. 
         FIG. 5  is an enlarged view of the electric motor in  FIG. 4 . 
         FIG. 6  is a vertical sectional view of the electric motor of the rotary drive system according to the embodiment of the present invention at a position different from that shown in  FIG. 4 . 
         FIG. 7  is an enlarged view of the speed reducer in  FIG. 4 . 
         FIG. 8  is a partially enlarged view of the reducer in  FIG. 7 , showing the liquid level of lubricating oil at the time of stopping the operation. 
         FIG. 9  is a partially enlarged view of a speed reducer in  FIG. 7 , showing a liquid level of lubricating oil during operation. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     The present invention will be described in detail below with reference to  FIG. 1  to  FIG. 9 . 
     &lt;Work Machine&gt; 
     As shown in  FIGS. 1 and 2 , a hydraulic excavator  200  as a work machine includes an undercarriage  210 , a swing circle  220 , and an upper swing body  230 . In the following description, the direction in which gravity is applied in a state in which the work machine is placed on the horizontal surface is referred to as a “vertical direction”. Further, the front of an operator&#39;s seat in a cab  231  described later is simply referred to as “forward”, and the rear of the operator&#39;s seat is simply referred to as “rearward”. 
     The undercarriage  210  includes a pair of right and left crawler belts  211 ,  211  and the crawler belts  211 , 211  are driven by a travel-use hydraulic motor (not shown) thereby travelling the hydraulic excavator  200 . 
     The swing circle  220  is a member for connecting the undercarriage  210  and the upper swing body  230 , and includes an outer race  221 , an inner race  222 , and a swing pinion  223 . The outer race  221  is supported by the undercarriage  210  and has an annular shape centered on a swing axis L extending in correspondence with the vertical direction. The inner race  222  is an annular member that is coaxial with the outer race  221 , and is disposed inside the outer race  221 . The inner race  222  is supported so as to be relatively rotatable about the swing axis L with respect to the outer race  221 . The swing pinion  223  meshes with inner teeth of the inner race  222 , and the inner race  222  rotates relative to the outer race  221  by a rotation of the swing pinion  223 . 
     The upper swing body  230  is disposed so as to be capable of swinging about the swing axis L with respect to the undercarriage  210  by being supported by the inner race  222 . The upper swing body  230  includes a cab  231 , a work equipment  232 , an engine  236  provided rearward of the cab and the work equipment, a generator motor  237 , a hydraulic pump  238 , an inverter  239 , a capacitor  240 , and a rotary drive system  1 . 
     The cab  231  is disposed forward and on the left side of the upper swing body  230 , and is provided with an operator&#39;s seat. The work equipment  232  is provided so as to extend forward the upper swing body  230 , and has a boom  233 , an arm  234 , and a bucket  235 . 
     The work equipment  232  performs various operations such as excavation by driving the boom  233 , the arm  234 , and the bucket  235  by respective hydraulic cylinders (not shown). 
     Shafts of the engine  236  and the generator motor  237  are connected to each other. The generator motor  237  is driven by the engine  236  to generate electric power. Rotary shafts of the generator motor  237  and the hydraulic pump  238  are connected to each other. The hydraulic pump  238  is driven by the engine  236 . Hydraulic pressure generated by driving the hydraulic pump  238  drives the aforementioned travel-use hydraulic motor and each of the hydraulic cylinders. In addition, the connection method for the engine  236 , the generator motor  237 , and the hydraulic pump  238  is not limited to the present embodiment and can be in an arbitrary known method. 
     The generator motor  237 , the capacitor  240  and the rotary drive system  1  are electrically connected to each other via the inverter  239 . In addition, another storage device such as a lithium ion battery, or the like, may be used instead of the capacitor  240 . 
     The rotary drive system  1  is arranged in a vertical placed state such that an axis O as the center of rotation coincides with the vertical direction. The output of the rotary drive system  1  is transmitted to a swing pinion  223  that meshes with the inner teeth of the inner race  222 . 
     The hydraulic excavator  200  drives the rotary drive system  1  by electric power generated by the generator motor  237  or by electric power from the capacitor  240 . A driving force of the rotary drive system  1  is transmitted to the inner race  222  via the swing pinion  223 . As a result, the inner race  222  rotates relative to the outer race  221 , thereby swinging the upper swing body  230 . 
     When the swing of the upper swing body  230  is decelerated, the rotary drive system  1  functions as a generator to generate electric power as regenerative energy. This electric power is stored in the capacitor  240  via the inverter  239 . The electric power stored in the capacitor  240  is supplied to the generator motor  237  at the time of accelerating the engine  236 . By the generator motor  237  being driven by the electric power of the capacitor, the generator motor  237  assists an output of the engine  236 . 
     &lt;Rotary Drive System&gt; 
     As shown in  FIG. 3 , the rotary drive system  1  includes a rotary drive device  10 , an oil inspection unit  160 , and a lubricating oil-circulating unit  150 . 
     &lt;Rotary Drive Device&gt; 
     The rotary drive device  10  includes an electric motor  20  and a speed reducer  60  provided integrally with the electric motor  20 . The speed reducer  60  is disposed below the electric motor  20 . 
     &lt;Electric Motor&gt; 
     As shown in  FIGS. 3 to 6 , the electric motor  20  includes an electric motor casing  21 , a stator  30 , and a rotor  38 . 
     &lt;Electric Motor Casing&gt; 
     As shown in  FIG. 5 , the electric motor casing  21  is a member that forms an outer shape of the electric motor  20 . The motor casing  21  has an upper casing  22  and a lower casing  25 . 
     The upper casing  22  has a bottomed cylindrical shape having an upper cylindrical portion  23  that has a cylindrical shape extending in the vertical direction (axial O direction) and an upper bottom portion  24  that closes an upper portion of the upper cylindrical portion  23 . An inner peripheral surface  23   a  of the upper cylindrical portion  23  has a circular shape in a sectional shape orthogonal to the axis O. An upper through-hole  24   a  passing through the upper bottom portion  24  so as to be centered on the axis O is formed in the upper bottom portion  24 . An annular convex portion  24   b  that projects from a surface of the upper bottom portion  24  facing downward so as to have an annular shape centered on the axis O is formed around the upper through-hole  24   a . An upper flange  23   b  is provided in a lower end of the upper cylindrical portion  23  so as to protrude from an outer peripheral surface of the upper cylindrical portion  23  toward the outer peripheral side thereof. 
     The lower casing  25  has a bottomed cylindrical shape having a cylindrical lower portion  26  that has a cylindrical shape extending in the vertical direction and a lower bottom portion  27  that closes a lower portion of the lower cylindrical portion  26 . An outer peripheral surface  26   a  and an inner peripheral surface  26   b  of the lower cylindrical portion  26  have a circular shape in a sectional shape orthogonal to the axis O. A lower flange  27   f  is provided in a lower end of the lower cylindrical portion  26  so as to protrude from the lower cylindrical portion  26  toward the outer peripheral side thereof. As shown in  FIG. 6 , a lower fitting portion  26   d  is formed in a radially inner portion and at a corner portion of an upper end of the lower cylindrical portion  26 . A plurality of lower fitting portions  26   d  are formed at intervals in the peripheral direction. A surface of the lower fitting portion  26   d  facing inward in the radial direction has a shape in which a sectional shape orthogonal to the axis O is a circle centered on the axis O. A surface facing upward of the lower fitting portion  26   d  has a flat shape orthogonal to the axis O. 
     A lower through-hole  27   a  passing through the lower bottom portion  27   a  so as to be centered on the axis O is formed in the lower bottom portion  27   a . A portion around the lower through-hole  27   a  in the surface facing upward of the lower bottom portion  27  is a first bottom surface  27   b  having an annular shape and having a flat shape orthogonal to the axis O. A second bottom surface  27   c  (see  FIG. 6 ) and a third bottom surface  27   d  (see  FIG. 5 ) are formed around the first bottom surface  27   b  of the lower bottom portion  27 . 
     As shown in  FIG. 6 , the second bottom surface  27   c  is a portion adjacent to the outer peripheral side of the first bottom surface  27   b , and is formed to be one step higher than the first bottom surface  27   b . The second bottom surface  27   c  forms a flat shape orthogonal to the axis O. A plurality of second bottom surfaces  27   c  are formed at intervals in the peripheral direction of the axis O. 
     As shown in  FIG. 5 , the third bottom surface  27   d  is provided adjacent to the outer peripheral side of the first bottom surface  27   b  similarly to the second bottom surface  27   c  and adjacent to the second bottom surface  27   c  in the peripheral direction. The third bottom surface  27   d  is formed to be one step higher than the second bottom surface  27   c . A plurality of third bottom surfaces  27   d  arc formed at intervals in the peripheral direction of the axis O. In the present embodiment, the plurality of second bottom surfaces  27   c  and the plurality of third bottom surfaces  27   d  are alternately provided in the peripheral direction. An inner peripheral surface  26   b  of the lower cylindrical portion  26  is connected to an outer peripheral side of the second bottom surface  27   c  and the third bottom surface  27   d.    
     As shown in  FIG. 5 , an electric motor-side accommodating recess  27  is formed in a portion of a surface facing downward of the lower bottom portion  27  in a peripheral-direction position corresponding to the third bottom surface  27   d  so as to recess upward from the lower surface of the lower bottom portion  27 . A plurality of the electric motor-side accommodating recesses  27   e  are formed at intervals in the peripheral direction so as to correspond to the third bottom surface  27   d.    
     The lower cylindrical portion  26  is fitted to the upper cylindrical portion  23  so as to be inserted from below. The outer peripheral surface  26   a  of the lower cylindrical portion  26  is fitted onto the inner peripheral surface  23   a  of the upper cylindrical portion  23 . The upper flange  23   b  and the lower flange  27   f  are in contact with each other over the peripheral direction. As a result, the lower cylindrical portion  26  and the upper cylindrical portion  23  are integrally fixed to each other. A space inside the electric motor casing  21  formed by the lower cylindrical portion  26  and the upper cylindrical portion  23  is a first accommodating space R 1 . 
     &lt;Stator&gt; 
     The stator  30  is provided with a stator core  31  and a coil  32 . 
     The stator core  31  is constituted by stacking a plurality of electromagnetic steel plates in the vertical direction, and includes a core main body  31   a  and a core convex portion  31   b.    
     The core main body  31   a  is constituted by a yoke having a cylindrical shape centered on the axis O and teeth formed at intervals with each other in the peripheral direction of the yoke so as to project from an inner peripheral surface of the yoke. 
     The core convex portion  31   b  is formed so as to project from the outer peripheral surface of the core main body  31   a . A plurality of core convex portions  31   b  are provided at intervals in the peripheral direction. The core convex portion  31   b  extends over the entire vertical direction of the core main body  31   a.    
     A plurality of coils  32  are provided to correspond to each of the teeth, and are wound around each of the teeth. As a result, the plurality of coils  32  are provided at intervals in the peripheral direction. 
     A portion of each coil  32  projecting upward from the stator core  31  is an upper coil end  32   a . A portion of each coil  32  projecting downward from the stator core  31  is a lower coil end  32   b . As the winding constituting the coil  32 , for example, a rectangular winding, a sectional shape of which has a quadrangular shape, is used. 
     In the present embodiment, the stator core  31  in the stator  30  is fitted on the upper casing  22  and the lower casing  25  of the electric motor casing  21 . That is, as shown in  FIG. 5 , the end portion on the outer peripheral side of the core convex portion  31   b  in the stator core  31  is fitted onto the inner peripheral surface  23   a  of the upper cylindrical portion  23  in the upper casing  23 . On the other hand, as shown in  FIG. 6 , an end portion on the outer peripheral side in the lower end of the core main body  31   a  in the stator core  31  is fitted onto the lower fitting portion  26   d  of the lower cylindrical portion  26  in the lower casing  25 . 
     Further, in the present embodiment, as shown in  FIG. 5 , a bolt insertion hole (not shown) penetrating in the vertical direction is formed in the core convex portion  31   b  of the stator core  31 . A bolt  33  is inserted into the bolt insertion hole from above. A lower end of the bolt  33  is fixed to a bolt fixing hole  26   e  formed in an upper-end surface  26   c  in the lower cylindrical portion  26  of the lower casing  25 . As a result, the stator core  31  is fixed and integrated to the lower casing  25 . 
     &lt;Rotor&gt; 
     As shown in  FIGS. 5 and 6 , the rotor  38  includes a rotary shaft  40 , a rotor core  42 , a lower end plate  45 , and an upper end plate  46 . 
     &lt;Rotary Shaft&gt; 
     The rotary shaft  40  is a rod-shaped member extending along the axis O. The rotary shaft  40  is disposed so as to penetrate an inside of the stator  30  in the vertical direction inside the casing. An upper end of the rotary shaft  40  projects upward the electric motor casing  21  through the upper through-hole  24   a  in the upper bottom portion  24  in the upper casing  22 . The upper end of the rotary shaft  40  may be accommodated in the electric motor casing  21 . 
     An upper seal  35  is provided between an inner peripheral surface of the upper through-hole  24   a  of the upper bottom portion  24  and an outer peripheral surface of the rotary shaft  40 . As a result, liquid tightness in the upper end inside the electric motor casing  21  is secured. 
     An upper bearing  36  having an annular shape centered on the axis O is provided on an inner peripheral surface of the annular convex portion  24   b  in the upper bottom portion  24 . The rotary shaft  40  is vertically inserted into the upper bearing  36 , and an upper portion of the rotary shaft  40  is supported by the upper bearing  36  so as to be rotatable about the axis O. 
     A lower bearing  37  having an annular shape around the axis O is provided on an inner peripheral surface of the lower through-hole  27   a  in the lower bottom portion  27 . The rotary shaft  40  is vertically inserted into the lower bearing  37 , and the lower portion of the rotary shaft  40  is supported by the lower bearing  37  so as to be rotatable about the axis O. 
     A center hole  40   a  extending downward from the upper end of the rotary shaft  40  and a radial hole  40   b  extending from the center hole  40   a  to the outer peripheral surface of the rotary shaft  40  are formed in the rotary shaft  40 . 
     The center hole  40   a  does not extend over the entire vertical direction of the rotary shaft  40 , but extends from the upper end of the rotary shaft  40  to a middle way toward the lower end of the rotary shaft. As a result, the rotary shaft  40  has a hollow structure in a portion where the center hole  40   a  is formed from the upper end to the lower end, and the remaining portion on the lower side is a solid structure. 
     The radial hole  40   b  extends in the radial direction so that an extending direction thereof is aligned with the direction orthogonal to the axis O. A radially inner end portion of the radial hole  40   b  communicates with the lower portion of the center hole  40   a . A radially outer end portion of the radial hole  40   b  opens into the outer peripheral surface of the rotary shaft  40 . A plurality of radial holes  40   b  are formed at intervals in the peripheral direction. 
     &lt;Rotor Core&gt; 
     The rotor core  42  has a cylindrical shape centered on the axis O, and an inner peripheral surface  42   a  of the rotor core is fitted on the outer peripheral surface of the rotary shaft  40  from an outside thereof. An upper end of the rotor core  42  fitted on the rotary shaft  40  from the outside thereof is a position in the vertical direction corresponding to the lower end of the center hole  40   a . An outer peripheral surface of the rotor core  42  has a cylindrical surface shape centered on the axis O and faces the inner peripheral surface of the stator  30 . The rotor core  42  is constituted by stacking a plurality of electromagnetic steel plates in the vertical direction. 
     On the inner peripheral surface  42   a  of the rotor core  42 , a plurality of inner axial-direction flow paths  42   b  having a groove shape extending over the entire axial O direction are formed at intervals in the peripheral direction. In a portion on an outer peripheral side of the inner axial-direction flow path  42   b  in an inside of the rotor core  42 , an outer axial-direction flow path  42   c  extending over the entire axial O direction is formed. 
     A plurality of permanent magnets (not shown) are embedded in the rotor core  42  at intervals in the peripheral direction. 
     &lt;Lower End Plate&gt; 
     The lower end plate  45  is a disc-like member extending in a direction orthogonal to the axis O and having a circular outer shape centered on the axis O. The lower end plate  45  is fixed to the rotor core  42  so as to be stacked from below the rotor core  42 . 
     A connection flow path  45   a  extending in the radial direction is formed on an upper surface of the lower end plate  45 . A plurality of connection flow paths  45   a  are formed at intervals in the peripheral direction. The connection flow path  45   a  connects the inner axial-direction flow path  42   b  and the outer axial-direction flow path  42   c  of the rotor core  42  in the radial direction. 
     &lt;Upper End Plate&gt; 
     The upper end plate  46  is a disk-shaped member extending in a direction orthogonal to the axis O and having a circular outer shape centered on the axis O similarly to the lower end plate  45 . The upper portion end plate  46  is fixed to the rotor core  42  so as to be stacked from above the rotor core  42 . The upper end plate  46  closes the inner axial-direction flow path  42   b  in the rotor core  42  from above. A plurality of discharge holes  46   a  penetrating in the vertical direction are formed in the upper end plate  46  at intervals in the peripheral direction. Each of the discharge holes  46   a  communicates with the outer axial-direction flow path  42   c  in the rotor core  42 . 
     As a result, a cooling flow path in which the lubricating oil flows in the order of the central hole  40   a , the radial hole  40   b , the inner axial-direction flow path  42   b , the connection flow path  45   a , the outer axial-direction flow path  42   c , and the discharge hole  46   a  is formed in the rotor  38 . 
     &lt;Communication Hole&gt; 
     As shown in  FIG. 6 , the electric motor casing  21  has a communication hole  50  that communicates the first accommodating space R 1  in the electric motor casing  21  to a lower side thereof. 
     In the present embodiment, the communication hole  50  is formed as a main oil drain hole  50   a , an auxiliary oil drain hole  50   b , an outer peripheral-side oil drain hole  50   c , and a bearing oil drain hole  50   d.    
     The main oil drain hole  50   a  is formed so as to open into the second bottom surface  27   c  in the lower bottom portion  27  of the lower casing  25 , and vertically passes through the lower bottom portion  27 . A plurality of main oil drain holes  50   a  are formed at intervals in the peripheral direction so as to correspond to each of the second bottom surfaces  27   c.    
     The auxiliary oil drain hole  50   b  is formed so as to open into the first bottom surface  27   d  in the lower bottom portion  27  of the lower casing  25 , and vertically passes through the lower bottom portion  27 . A plurality of auxiliary oil drain holes  50   b  are formed at intervals in the peripheral direction. The flow path sectional area of the auxiliary oil drain hole  50   b , which is a cross-sectional area orthogonal to the axis O, is smaller than the flow path sectional area of the main oil drain hole  50   a.    
     An upper end of the outer peripheral-side oil drain hole  50   c  is opened into the upper end face  26   c  of the lower cylindrical portion  26  and the outer peripheral-side oil drain hole  50   c  vertically passes through the lower cylindrical portion  26 . A plurality of outer peripheral-side oil drain holes  50   c  are formed at intervals in the peripheral direction. The plurality of outer peripheral-side oil drain holes  50   c  are formed at intervals in the peripheral direction so as to avoid the bolt fixing holes  26   e  for fixing the stator core  31 . The outer peripheral-side oil drain hole  50   c  may have a slit shape in which the peripheral direction is a longitudinal direction. 
     The bearing oil drain hole  50   d  is formed in the lower bearings  37 . As shown in  FIG. 6 , the lower bearing  37  includes an inner ring  37   a , an outer ring  37   b , a rolling body  37   c , and a bearing shield  37   d.    
     The inner ring  37   a  is an annular member, and an inner peripheral surface thereof is fixed to the outer peripheral surface of the rotary shaft  40 . The outer ring  37   b  is an annular member provided on the outer peripheral side of the inner ring  37   a  so as to be spaced apart therefrom, and the outer peripheral surface of the outer ring  37   b  is fixed to the inner peripheral surface of the lower through-hole  27   a  of the lower bottom portion  27 . The rolling body  37   c  has a spherical shape, and a plurality of rolling bodies  37   c  is arranged in the peripheral direction so as to be interposed between the inner ring  37   a  and the outer ring  37   b . The bearing shield  37   d  is an annular member fixed to a lower end of the outer peripheral surface of the inner ring  37   a . The bearing shield  37   d  has a plate shape having a plate thickness in the vertical direction. A clearance is formed over the peripheral direction between an outer peripheral end of the bearing shield  37   d  and the inner peripheral surface of the outer ring  37   b . The clearance is a bearing oil drain hole  50   d . An opening area of the bearing oil drain hole  50   d  is smaller than the flow path sectional area of the auxiliary oil drain hole  50   b.    
     The heights of upper ends of the inner ring  37   a  and the outer ring  37   b  of the lower bearing  37  are flush with the first bottom surface  27   b . Therefore, the height of the opening at the upper end between the inner ring  37   a  and the outer ring  37   b  in the lower bearing  37  is the same as the height in the upper end of the auxiliary oil drain hole  50   b . In addition, the height of the upper end of the auxiliary oil drain hole  50   b  may be lower than the upper end of the lower bearing  37 . That is, the auxiliary oil drain hole  50   b  may be opened at a portion of the upper end of the lower bearing  37  on a bottom surface of the electric motor casing  21  or lower thereof 
     &lt;Speed Reducer&gt; 
     Next, the speed reducer  60  will be described with reference to  FIG. 7 . The speed reducer  60  includes a speed reducer casing  61 , an output shaft  70 , a transmission portion  80 , and a brake mechanism  120 . 
     &lt;Speed Reducer Casing&gt; 
     The speed reducer casing  61  has a cylindrical shape extending along the axis O and opening upward and downward. The upper end of the speed reducer casing  61  abuts the lower flange  27   f  of the lower casing  25  in the electric motor casing  21  over the peripheral direction. The speed reducer casing  61  is integrally fixed to the lower flange  27   f  via bolts (not shown) or the like. An upper opening of the speed reducer casing  61  is closed by the lower casing  25  of the electric motor casing  21 . 
     &lt;Output Shaft&gt; 
     The output shaft  70  has a rod shape extending along the axis O. A rotation of the output shaft  70  becomes an output of the rotary drive system  1 . An upper portion of the output shaft  70  is disposed in the speed reducer casing  61  and a lower portion of the output shaft  70  is disposed so as to project downward from the speed reducer casing  61 . An output shaft bearing  71  for rotatably supporting the output shaft  70  about the axis O is provided on a lower portion of an inner peripheral surface of the speed reducer casing  61 . As the output shaft bearing  71 , for example, a self-aligning roller bearing is used. A lower portion projecting downward from the speed reducer casing  61  in the output shaft  70  is connected to the swing pinion  223 . 
     On the inner peripheral surface of the speed reducer casing  61 , further below the output shaft bearing  71 , a lower seal  72  for scaling an annular space between the inner peripheral surface of the speed reducer casing  61  and an outer peripheral surface of the output shaft  70  is provided. A space in the speed reducer casing  61  closed from below by the lower seal  72  is defined as a second accommodating space R 2 . The lower portion of the rotary shaft  40  projecting downward from the electric motor casing  21  is positioned at an upper portion of the second accommodating space R 2 . 
     &lt;Transmission Portion&gt; 
     The transmission portion  80  is provided in the second accommodating space R 2  in the speed reducer casing  61 . The transmission unit  80  has a function in which the rotational power of the rotary shaft  40  is input, the rotational speed thereof is reduced and is transmitted to the output shaft  70 . 
     The transmission portion  80  is constituted by a plurality of stages of planetary gear mechanisms that sequentially decelerate the number of revolutions from the rotary shaft  40  to the output shaft  70 . As the plurality of planetary gear mechanisms, in the present embodiment, three planetary gear mechanisms that are the first stage planetary gear mechanism  90 , the second stage planetary gear mechanism  100 , and the third stage planetary gear mechanism  110  are provided. 
     &lt;First Stage Planetary Gear Mechanism&gt; 
     The first stage planetary gear mechanism  90  is a planetary gear mechanism of the first stage. The first stage planetary gear mechanism  90  includes a first stage transmission shaft  91 , a first stage planetary gear  92 , and a first stage carrier  93 . 
     The first stage transmission shaft  91  includes a fitting cylindrical portion  91   a  and a disk portion  91   b . The fitting cylindrical portion  91   a  has a cylindrical shape centered on the axis O, and the lower end thereof is closed. The fitting cylindrical portion  91   a  is fitted from an outside thereof to the lower portion of the rotary shaft  40  from the lower end thereof. As a result, the fitting cylindrical portion  91   a  is rotatable about the axis O integrally with the rotary shaft  40 . The disk portion  91   b  protrudes from the lower portion of an outer peripheral surface of the fitting cylindrical portion  91   a  to the outer peripheral side. The disk portion  91   b  has a disc shape centered on the axis O. First stage sun gear teeth  91   c , which are outer gear teeth centered on the axis O, are formed on an outer peripheral surface of the disk portion  91   b.    
     The first stage planetary gear  92  is a gear that has a disc shape, and the first stage planetary gear teeth  92   a  are formed on an outer peripheral surface thereof. A plurality of first stage planetary gears  92  are provided at intervals in the peripheral direction around the first stage transmission shaft  91 . The first stage planetary gear teeth  92   a  of each first stage planetary gear  92  are engaged with the first stage sun gear teeth  91   c  of the corresponding first stage transmission shaft  91 . Vertical positions of the first stage planetary gears  92  are the same as each other. 
     Here, at a portion corresponding to an arrangement portion of the first planetary gear teeth  92  in an inner peripheral surface of the speed reducer casing  61 , first stage inner gear teeth  62   a  in which a first stage inner gear tooth  62   a  is formed over the entire peripheral direction of the inner peripheral surface of the speed reducer casing  21  is formed on the first inner peripheral surface  63   a  of the speed reducer casing  61 . The first inner peripheral surface  63   a  has a circular shape in a sectional shape orthogonal to the axis O. The first stage planetary gear teeth  92   a  of the first stage planetary gears  92  mesh with the first stage sun gear teeth  91   c  and also mesh with the first stage inner gear teeth  62   a.    
     The first stage carrier  93  is a member that supports the first stage planetary gear  92  so as to be rotatable and be capable of revolving around the axis O of the first stage transmission shaft  91 . The first stage carrier  93  includes a first stage carrier shaft  94 , a first stage upper plate portion  95 , and a first stage lower plate portion  96 . 
     A plurality of first stage carrier shafts  94  are provided so as to correspond to the respective first stage planetary gears  92 . The first stage carrier shaft  94  passes through the center of each first stage planetary gear  92  in the vertical direction and supports the first stage planetary gear  92  so as to be rotatable. 
     The first stage upper plate portion  95  has a disc shape centered on the axis O. The first stage upper plate portion  95  is disposed above each of the first stage planetary gears  92  so as to face the first stage planetary gears  92  from above. A first stage insertion hole  95   a  through which the rotary shaft  40  and the first stage transmission shaft  91  are inserted in the vertical direction is formed in the center of the first stage upper plate portion  95 . 
     The first stage lower plate portion  96  has a disc shape centered on the axis O. The first stage upper plate portion  95  is disposed below each of the first stage planetary gears  92  so as to face the first stage planetary gears  92 . A first stage connecting hole  96   a  passing through in the vertical direction is formed in the center of the first stage lower plate portion  96 . 
     An upper end of each first stage carrier shaft  94  is fixed to the first stage upper plate portion  95  and a lower end thereof is fixed to the first stage lower plate portion  96 . Accordingly, each first stage planetary gear  92  is supported by the first stage carrier  93  so as to be sandwiched between the first stage upper plate portion  95  and the first stage lower plate portion  96  from the vertical direction. 
     &lt;Second Stage Planetary Gear Mechanism&gt; 
     The second stage planetary gear mechanism  100  is a planetary gear mechanism in the middle stage. The second stage planetary gear mechanism  100  includes a second stage transmission shaft  101 , a second stage planetary gear  102 , and a second stage carrier  103 . The second stage transmission shaft  101  is a shaft extending with the axis O of the rotary shaft  40  as the center below the first stage transmission shaft  91 . An upper end of the second stage transmission shaft  101  is spaced apart from the lower end of the first stage transmission shaft  91 . As a result, the second stage transmission shaft  101  is relatively rotatable about the axis O with respect to the first stage transmission shaft  91 . In addition, the upper end of the second stage transmission shaft  101  and the lower end of the first stage transmission shaft  91  may be configured to be in sliding contact with each other, or a low-friction sliding contact member may be interposed therebetween. 
     An upper portion of an outer peripheral surface of the second stage transmission shaft  101  is connected to the first stage connection hole  96   a  of the first stage lower plate portion  96  of the first stage carrier  93  in the first stage planetary gear mechanism  90 . Thus, the second stage transmission shaft  101  rotates about the axis O integrally with the first stage carrier  93 . The second stage transmission shaft  101  may be, for example, spline-fitted, to the first stage connection hole  96   a  of the first stage lower plate portion  96  of the first stage carrier  93 . 
     The second stage sun gear teeth  101   a  as outer gear teeth with the axis O as the center are formed on a lower portion of the outer peripheral surface of the second stage transmission shaft  101 . 
     The second stage planetary gear  102  is a gear having a disc shape, and the second stage planetary gear teeth  102   a  are formed on the outer peripheral surface thereof. A plurality of second stage planetary gears  102  are provided at intervals in the peripheral direction around the second stage transmission shaft  101 . The second stage planetary gear teeth  102   a  of each of the second stage planetary gears  102  mesh with the second stage sun gear teeth  101   a  of the corresponding second stage transmission shaft  101 . Positions in the vertical direction of the second stage planetary gears  102  are the same as each other. 
     The second stage inner gear teeth  62   b  are formed over the entire peripheral area of the inner peripheral surface of the speed reducer casing  61 , in portions corresponding to the positions where the second stage planetary gears  102  are disposed on the inner peripheral surface of the speed reducer casing  61 . The second inner gear teeth  62   b  are formed on the second inner peripheral surface  63   b  of the speed reducer casing  61 . The second inner peripheral surface  63   b  has a circular shape in a sectional shape orthogonal to the axis O, and has an inner diameter larger than that of the first inner peripheral surface  63   a.    
     The second stage planetary gear teeth  102   a  of the second stage planetary gear  102  mesh with the second stage sun gear teeth  101   a  and also mesh with the second stage inner gear teeth  62   b.    
     The second stage carrier  103  is a member that supports the second stage planetary gear  102  so as to be rotatable and be capable of revolving around the axis O of the second stage transmission shaft  101 . The second stage carrier  103  includes a second stage carrier shaft  104 , a second stage upper plate portion  105 , and a second stage lower plate portion  106 . 
     A plurality of second stage carrier shafts  104  are provided so as to correspond to the respective second stage planetary gears  102 . The second stage carrier shaft  104  passes through the center of each second stage planetary gear  102  in the vertical direction and supports the second stage planetary gear  102  so as to be rotatable. 
     The second stage upper plate portion  105  has a disc shape centered on the axis O. The second stage upper plate portion  105  is disposed above each second stage planetary gear  102  so as to face the second stage planetary gears  102  from above. A second stage insertion hole  105   a  through which the second stage transmission shaft  101  is inserted in the vertical direction is formed in the center of the second stage upper plate portion  105 . In the present embodiment, part of the first stage lower plate portion  96  of the first stage carrier  93  is disposed in the second stage insertion hole  105   a.    
     The second stage lower plate portion  106  has a disc shape centered on the axis O. The second stage lower plate portion  106  is disposed below each of the second stage planetary gears  102  so as to face the second stage planetary gears  102  from below. A second stage connecting hole  106   a  passing through in the vertical direction is formed in the center of the second stage lower plate portion  106 . 
     An upper end of each second stage carrier shaft  104  is fixed to the second stage upper plate portion  105  and a lower end thereof fixed to the second stage lower plate portion  106 . Accordingly, each second stage planetary gear  102  is supported by the second stage carrier  103  so as to be sandwiched by the second stage upper plate portion  105  and the second stage lower plate portion  106  from the vertical direction. 
     &lt;Third Stage Planetary Gear Mechanism&gt; 
     The third stage planetary gear mechanism  110  is a planetary gear mechanism of the final stage. The third stage planetary gear mechanism  110  includes a third stage transmission shaft  111 , a third stage planetary gear  112 , and a third stage carrier  113 . 
     The third stage transmission shaft  111  is an axis extending with the axis O of the rotary shaft  40  as the center below the second stage transmission shaft  101 . The upper end of the third stage transmission shaft  111  is spaced apart from the lower end of the second stage transmission shaft  101 . As a result, the third stage transmission shaft  111  is relatively rotatable about the axis O with respect to the second stage transmission shaft  101 . The upper end of the third stage transmission shaft  111  and the lower end of the second stage transmission shaft  101  may be configured to be in sliding contact with each other, or a low-friction sliding contact member may be interposed therebetween. 
     The lower end of the third stage transmission shaft  111  is opposed to the upper end of the output shaft  70  so as to be spaced apart from the upper end of the output shaft  62 . The third stage transmission shaft  111  and the output shaft  70  are relatively rotatable to each other about the axis O. The lower end of the third stage transmission shaft  111  and the upper end of the output shaft  70  may be configured to be in sliding contact with each other, or a low-friction sliding member having low friction may be interposed therebetween. 
     The upper portion of an outer peripheral surface of the third stage transmission shaft  111  is connected to the second stage connection hole  106   a  of a lower plate portion of the second stage carrier  103  in the second stage planetary gear mechanism  100 . As a result, the third stage transmission shaft  111  rotates about the axis O integrally with the second stage carrier  103 . The third stage transmission shaft  111  may be, for example, spline-fitted to the second stage connection hole  106   a  of the lower plate portion of the second stage carrier  103 . 
     The third stage sun gear teeth  111   a  as outer gear teeth centered on the axis O are formed on a lower portion in the outer peripheral surface of the third stage transmission shaft  111 . 
     The third stage planetary gear  112  is a gear that has a disc shape, and the third stage planetary gear teeth  112   a  are formed on the outer peripheral surface thereof. A plurality of third stage planetary gears  112  are provided at intervals in the peripheral direction around the third stage transmission shaft  111 . The third stage planetary gear teeth  112   a  of each of the third stage planetary gears  112  mesh with the third stage sun gear teeth  111   a  of the corresponding third stage transmission shaft  111 . Positions in the vertical direction of the third stage planetary gears  112  are the same as each other. 
     The second stage inner gear teeth  62   c  are formed over the entire peripheral area of the inner peripheral surface of the speed reducer casing  61 , in portions corresponding to the positions where the third stage planetary gears  112  are disposed on the inner peripheral surface of the speed reducer casing  61 . The third inner gear teeth  62   c  are formed on the second inner surface  63   b  of the speed reducer casing  61  as similar to the second inner gear teeth  62   b . The third stage planetary gear teeth  112   a  of the third stage planetary gear  112  mesh with the third stage sun gear teeth  111   a  and also mesh with the third stage inner gear teeth  62   c.    
     The third stage carrier  113  is a member that supports the third stage planetary gear  112  so as to be rotatable and capable of revolving around the axis O of the third stage transmission shaft  111 . The third stage carrier  113  includes a third stage carrier shaft  114 , a third stage upper plate portion  115 , and a third stage lower plate portion  116 . 
     A plurality of third stage carrier shafts  114  are provided so as to correspond to the respective third stage planetary gears  112 . The third stage carrier shaft  114  passes through the center of each third stage planetary gear  112  in the vertical direction and supports the third stage planetary gear  112  so as to be rotatable. 
     The third stage upper plate portion  115  has a disc shape centered on the axis O. The third stage upper plate portion  115  is disposed above each of the third stage planetary gears  112  so as to face the third stage planetary gears  112  from above. A third stage insertion hole  115   a  through which the third stage transmission shaft  111  is inserted in the vertical direction is formed in the center of the third stage upper plate portion  115 . In the present embodiment, part of the second stage lower plate portion  106  of the second stage carrier  103  is disposed in the third stage insertion hole  115   a.    
     The third stage lower plate portion  116  has a disc shape centered on the axis O. The third stage upper plate portion  115  is disposed below each of the third stage planetary gears  112  so as to face the third stage planetary gears  112  from below. A third stage connecting hole  116   a  passing through in the vertical direction is formed in the center of the third stage lower plate portion  116 . The third stage connection hole  116   a  is connected to an upper portion of the outer peripheral surface of the output shaft  70 . The third stage connection hole  116   a  and the outer peripheral surface of the output shaft  70  may be spline-fitted. As a result, the third stage carrier  113  and the output shaft  70  are integrally rotated about the axis O. 
     An upper end of each third stage carrier shaft  114  is fixed to the third stage upper plate portion  115  and a lower end portion thereof is fixed to the third stage lower plate portion  116 . Accordingly, each third stage planetary gear  112  is supported by the third stage carrier  113  so as to be sandwiched by the third stage upper plate portion  115  and the third stage lower plate portion  116  from the vertical direction. 
     &lt;Brake Mechanism&gt; 
     As shown in  FIGS. 3 and 7 , the brake mechanism  120  is disposed above the first stage planetary gear mechanism  90  in the speed reducer casing  61 . 
     As shown in  FIG. 7 , the brake mechanism  120  includes a disk support portion  121 , a brake disk  122 , a brake plate  123 , a brake piston  130 , and a brake spring  140 . 
     The disk support portion  121  is a member having a cylindrical shape centered on the axis O. The lower end of the disk support portion  121  is integrally fixed to the first stage upper plate portion  95  of the first stage carrier  93  in the first stage planetary gear mechanism  90  over the peripheral direction. On the inner peripheral side of the disk support portion  121 , a lower portion of the rotary shaft  40  and a portion of the first stage transmission shaft  91  are positioned. 
     The brake disk  122  is an annular member, and a plurality of brake disks  122  are arranged at intervals in the vertical direction so as to protrude from an outer peripheral surface of the disk support portion  121 . The brake disc  122  has a plate shape in which the vertical direction is the thickness direction. 
     The brake plate  123  is an annular member, and a plurality of brake plates  123  are arranged at intervals in the vertical direction so as to protrude from the inner peripheral surface of the speed reducer casing  61 . In the present embodiment, the brake plate  123  is provided so as to protrude from the first sliding contact-inner peripheral surface  64   a  on the inner peripheral surface of the speed reducer casing  61 . The plurality of brake plates  123  and the plurality of brake disks  122  are alternately arranged in the order of the brake plate  123  and the brake disk  122  from the upper side to the lower side. The brake plate  123  and the brake disk  122  are capable of being brought into contact with each other. 
     The brake piston  130  is a member having an annular shape centered on the axis O, and is disposed so as to be movable in the vertical direction above the brake plate  123 . The brake piston  130  is disposed so as to face from below the lower bottom portion  27  of the lower casing  25  in the electric motor casing  21 . A lower portion of an outer peripheral surface of the brake piston  130  is a first sliding contact-outer peripheral surface  131 . 
     The first sliding contact-outer peripheral surface  131  of the brake piston  130  is slidable in the vertical direction with respect to the first sliding contact-inner peripheral surface  64   a  of the speed reducer casing  61 . 
     An upper portion on the outer peripheral surface of the brake piston  130  is a second sliding contact-outer peripheral surface  132 . An outer diameter of the second sliding contact-outer peripheral surface  132  is larger than that of the first sliding contact-outer peripheral surface  131 . The second sliding contact-outer peripheral surface  132  of the brake piston  130  is slidable in the axial O direction with respect to the second sliding contact-inner peripheral surface  64   b  of the speed reducer casing  61 . An inner diameter of the second sliding contact-inner peripheral surface  64   b  of the speed reducer casing  61  is larger than that of the first sliding contact-inner peripheral surface  64   a.    
     A step portion between the first sliding contact-outer peripheral surface  131  and the second sliding contact-outer peripheral surface  132  in the brake piston  130  has a flat shape orthogonal to the axis O and faces downward, and is a piston-side step surface  133  having an annular shape. 
     The step portion between the first sliding contact-inner peripheral surface  64   a  and the second sliding contact-inner peripheral surface  64   b  in the speed reducer casing  61  has a flat shape orthogonal to the axis O faces upward, and is a casing-side step surface  64   c  having an annular shape. 
     The piston-side step surface  133  and the casing-side step surface  64   c  face in the vertical direction, and are changed between a state of being in contact with each other and a state of being separated from each other in accordance with a movement in the vertical direction of the brake piston  130 . An annular space defined by separating the piston-side step surface  133  and the casing-side step surface  64   c  from each other is a hydraulic pressure supply space R 4 . 
     A hydraulic pressure supply hole  61   a  that is capable of supplying hydraulic pressure to the hydraulic pressure supply space R 4  from the outside is formed in the speed reducer casing  61 . Hydraulic pressure generated by a hydraulic pump is supplied to the hydraulic pressure supply hole  61   a.    
     An annular lower surface in the brake piston  130  is a plate contact surface  134 . The plate abutting surface  134  comes into contact with the brake plate  123  from above over the entire peripheral direction. 
     A plurality of piston-side accommodating recesses  135  which are recessed from above and formed at intervals from each other in the peripheral direction are formed on an annular upper surface in the brake piston  130 . Positions in the peripheral direction of the piston-side accommodating recesses  135  corresponds to the respective positions in the peripheral direction of the motor-side accommodating recesses  27   e  formed in the lower casing  25  of the electric motor casing  21 . 
     The brake spring  140  is accommodated in each spring accommodating portion R 3  defined by the piston-side accommodating recess  135  and the motor-side accommodating recess  27   e  which are opposed to each other in the vertical direction. The brake spring  140  is a coil spring which extends in a direction parallel to the axis O, and is accommodated in the spring accommodating portion R 3  in a compressed state. 
     &lt;Liquid Level of Lubricating Oil&gt; 
     As shown in  FIG. 8 , lubricating oil is stored in the second accommodating space R 2  in the speed reducer casing  61 . That is, the second accommodating space R 2  is used as a tank for storing lubricating oil. The liquid level S of the stored tank is set to a predetermined height in a state where the axis O is oriented in the vertical direction and the rotary drive system  1  is stopped (a state in which the liquid level S is stable). In the present embodiment, the height of the liquid level S of the lubricating oil is set to be lower than the first stage planetary gear  92  of the first stage planetary gear mechanism  90  which is the first stage and higher than the second stage planetary gear  102  of the second stage planetary gear  100  which is the middle stage. 
     Even in a state where the lubricating oil is circulated by the lubricating oil-circulating unit  150  described later, the height of the liquid level S of the lubricating oil in the second accommodating space R 2  maintains the above relationship. 
     &lt;Oil Inspection Unit&gt; 
     As shown in  FIG. 7 , the oil inspection unit  160  is used to detect a liquid level S of lubricating oil stored in the second accommodating space R 2  in the speed reducer casing  61  as a tank. In the present embodiment, the oil inspection unit  160  is provided only in the speed reducer  60  among the electric motor  20  and the speed reducer  60 . 
     The oil inspection unit  160  includes an oil inspection pipe  161  and an oil inspection rod  162 . 
     The oil inspection pipe  161  includes a horizontal pipe  161   a  having a tubular shape and extending radially outward from the outer peripheral surface of the speed reducer casing  61 , and a vertical pipe  161   b  having a tubular shape and extending upward from the horizontal pipe  161   a  and communicating with the horizontal pipe  161   a.    
     As shown in  FIGS. 4 and 7 , an oil inspection hole  65  passing through the speed reducer  60  in the horizontal direction (the direction orthogonal to the axis O) is formed at a predetermined height position in the speed reducer casing  61 . In the present embodiment, the oil inspection hole  65  is open into the second inner peripheral surface  63   b  of the speed reducer casing  61 . The horizontal pipe  161   a  in the oil inspection pipe  161  is provided so as to communicate with the oil inspection hole  65 . That is, a space inside the oil inspection hole  65  is continuous so as to maintain the height of a lower end of the oil inspection hole  65  in the space inside the horizontal pipe  161   a.    
     The oil inspection rod  162  is a rod-shaped member inserted from above the vertical pipe  161   b . In the state in which the oil inspection rod  162  is accommodated in the vertical pipe  161   b , the lower end of the oil inspection rod  162  is in contact with a bottom surface of the space inside the horizontal pipe  161   a , or is opposed to the bottom surface thereof so as to have a clearance. 
     When lubricating oil is stored in the reducer casing  61  by an appropriate amount, the lower end of the oil inspection rod  162  comes into contact with the lubricating oil. On the other hand, when the amount of lubricating oil is insufficient, the lower end of the oil inspection rod  162  is dried without coming into contact with the lubricating oil. The operation of the oil inspection is performed by extracting the oil inspection rod  162  from the vertical pipe  161   b  and visually observing an adhering state of the lubricating oil on the lower end of the oil inspection rod  162 . The liquid level S of the lubricating oil stored in the second accommodating space R 2  as a tank is set to a height at which lubricating oil can enter into the inside of the oil inspection pipe. Therefore, the height of the liquid level S is set to be substantially the same as or slightly higher than the lower end of an opening of the oil inspection hole  65 . 
     &lt;Height of Oil Inspection Hole&gt; 
     As shown in  FIG. 7 , the height of the oil inspection hole  65  of the speed reducer casing  61  is lower than the first stage planetary gear  92  of the first stage planetary gear mechanism  90 , which is the first stage, and is higher than the second stage planetary gear  102  of the second stage planetary gear mechanism  100 , which is the middle stage. 
     More specifically, the height of the lower end of the opening of the oil inspection hole  65  is lower than the first stage planetary gear  92  of the first stage planetary gear mechanism  90 , which is the first stage, and is higher than the second stage planetary gear  102  of the second stage planetary gear mechanism  100 , which is the middle stage. In the present embodiment, the height of an upper end of the opening of the oil inspection hole  65  is also positioned below the first stage planetary gear  92  of the first stage planetary gear mechanism  90 . The upper end of the oil inspection hole  65  may be positioned above the lower end of the first stage planetary gear  92 . 
     &lt;Throttle Portion&gt; 
     As shown in  FIG. 7 , the second stage upper plate portion  105  and the second stage lower plate portion  106  in the second stage carrier  103  of the second stage planetary gear mechanism  100  are respectively opposed to the second inner peripheral surface  63   b  in the speed reducer casing  61  over the peripheral direction. An outer diameter of an outer peripheral surface of the second stage upper plate portion  105  is larger than an outer diameter of an outer peripheral surface of the second stage lower plate portion  106 . Thus, a clearance between the outer peripheral surface of the second stage upper plate portion  105  and the second inner peripheral surface  63   b  of the speed reducer casing  61  is smaller than a clearance between an inner peripheral surface of the second stage lower plate portion  106  and the second inner peripheral surface  63   b  of the speed reducer casing  61 . Moreover, the outer peripheral surface of the second upper plate portion  105  is opposed to the oil inspection hole  65  in the horizontal direction. In the present embodiment, a lower surface of the second stage upper plate portion  105  is positioned below the lower end of the opening of the oil inspection hole  65 , and an upper surface of the second stage upper plate portion  105  is positioned between the lower end and the upper end of the oil inspection hole  65 . The second upper plate portion  105  functions as a throttle portion  170  that reduces the amount of lubricating oil that flows into the oil inspection pipe  161 . 
     &lt;Lubricating Oil-Circulating Unit&gt; 
     As shown in  FIG. 3 , the lubricating oil-circulating unit  150  supplies lubricating oil into the first accommodating space R 1  in the electric motor casing  21 , and supplies again the lubricating oil recovered from an inside of the second accommodating space R 2  in the speed reducer casing  61  to the first accommodating space R 1 . 
     The lubricating oil-circulating unit  150  includes a lubricating oil flow path  151 , a lubricating oil pump  152 , a cooling portion  153 , and a strainer  154 . 
     The lubricating oil flow path  151  is a flow path formed by a flow path forming member such as a pipe provided outside the rotary drive device  10 . A first end of the lubricating oil flow path  151 , which is an end portion on the upstream side of the lubricating oil flow path, is connected to the second accommodating space R 2  in the speed reducer casing  61 . In the present embodiment, the first end of the lubricating oil flow path  151  is connected to a portion between the output shaft bearing  71  and the lower seal  72  in the second accommodating space R 2 . 
     A second end of the lubricating oil flow path  151 , which is an end portion at the downstream side of the lubricating oil flow path, is connected to an opening of the center hole  40   a  in the upper end of the rotary shaft  40 . The second end of the lubricating oil flow path  151  is connected to the first accommodating space R 1  in the electric motor casing  21  via a cooling flow path in the rotor  38 . 
     The lubricating oil pump  152  is provided in the middle of the lubricating oil flow path  151 , and pumps lubricating oil from the first end toward the second end of the lubricating oil flow path  151 , that is, from the second accommodating space R 2  side toward the first accommodating space R 1  side. 
     The cooling portion  153  is provided at a portion on the downstream side of the lubricating oil pump  152  in the lubricating oil flow path  151 . The cooling portion  153  cools the lubricating oil flowing through the lubricating oil flow path  151  by exchanging heat with the external atmosphere. 
     The strainer  154  is provided at a portion on the upstream side of the lubricating oil pump  152  in the lubricating oil flow path  151 . The strainer  154  has a filter for removing dirt and dust from lubricating oil passing through the lubricating oil flow path  151 . The strainer  154  is preferably provided with a magnetic filter for removing iron powder generated from, for example, the gear teeth of the speed reducer  60 . 
     Operation and Effects 
     When the rotary drive system  1  is stopped, that is, the stoppage of the hydraulic excavator  200 , hydraulic pressure is not generated by the hydraulic pump  238 , and the hydraulic pressure is not supplied to a hydraulic pressure supply space R 4  in the brake mechanism  120 . Therefore, the brake piston  130  of the brake mechanism  120  is in a state of being pressed downward by the brake spring  140 . As a result, the brake piston  130  presses the brake plate  123 , whereby the brake plate  60  and the brake disk  20  are in a state of being braked by the frictional force between the brake plate  123  and the brake disk  122 . Lubricating oil is stored up to the liquid level S in the second accommodating space R 2  of the speed reducer casing  61 . 
     On the other hand, when the engine  236  of the hydraulic excavator  200  is operated and hydraulic pressure is generated by the hydraulic pump  238 , some of the hydraulic pressure is reduced to a pilot pressure by means of a pressure-reducing means such as a hydraulic throttle. When an operator of the hydraulic excavator  200  performs an unlocking operation of the lock lever, the lock switch, or the like, which enables the operation of the work equipment  232  and the upper swing body  230 , the hydraulic pressure reduced in pressure is supplied to the hydraulic pressure supply space R 4  based on the operation. The hydraulic pressure causes the brake piston  130  to move upward against the pressure force of the brake spring  140 . As a result, the brake is released and the speed reducer  60  and the electric motor  20  are in a rotatable state. 
     AC power is supplied to each coil  32  of the stator  30  of the electric motor  20  via the inverter  239 , and the permanent magnets follow the rotating magnetic field generated by the coils  32 , so that the rotor  38  rotates with respect to the stator  30 . The rotation of the rotary shaft  40  of the rotor  38  is reduced in speed through the transmission portion  80  in the speed reducer  60 , and is transmitted to the output shaft  70 . In the present embodiment, reducing speed is sequentially performed via three stages of the planetary gear mechanisms. Swing motion of the upper swing body  230  is carried out by the rotation of the output shaft  70 . 
     When the upper swing body  230  swings, the electric motor  20  is driven with high torque. Therefore, the rotor core  42  and the permanent magnets reach a high temperature due to iron loss in the rotor core  42  and eddy current loss in the permanent magnets. At the same time, the stator  30  reaches a high temperature due to copper loss at the coil  32  and iron loss at the stator core  31 . When the stator  30  reaches a high temperature, the rotor core  42  reaches a higher temperature by the radiant heat of the stator  30 . Therefore, the cooling oil is supplied into the electric motor  20  by the lubricating oil-circulating unit  150 . 
     When the lubricating oil pump  152  of the lubricating oil-circulating unit  150  is operated, some of the lubricating oil stored in the second accommodating space R 2  is supplied from the upper end into the center hole  40   a  of the rotary shaft  40  in the rotor  38  of the electric motor  20  through the lubricating oil flow path  151 . The lubricating oil supplied to the center hole  40   a  of the rotary shaft  40  cools the rotor core  42  and permanent magnets in the process of flowing through the radial hole  40   b , the inner axial-direction flow path  42   b , the connection flow path  45   a , and the outer axial-direction flow path  42   c . The lubricating oil discharged through the discharge hole  46   a  is spread outward in the radial direction by the centrifugal force caused by the rotation of the rotor  38 . As a result, lubricating oil is supplied to the stator core  31  and the coil  32  of the stator  30  and cooling of the stator  30  is achieved. Thereafter, the lubricating oil which has fallen from the stator  30  is discharged from an inside of the electric motor  20  through the communication hole  50  formed in the electric motor casing  21 . When the rotary drive system  1  is operated, the lubricating oil is discharged to the lower side of the electric motor  20  mainly through the main oil drain hole  50   a  and the outer peripheral side oil drain hole  50   c.    
     The lubricating oil is discharged to the lower side of the electric motor  20  through the communication hole  50 , so that the lubricating oil is supplied to the second accommodating space R 2  in the speed reducer casing  61 . The lubricating oil supplied to the second accommodating space R 2  so as to fall down from the communication hole  50  lubricates each of the gear teeth of the first stage planetary gear mechanism  90 , and then returned to the lubricating oil stored in the second accommodating space R 2 . The second stage planetary gear mechanism  100  and the third stage planetary gear mechanism  110  are immersed in the lubricating oil stored in the second accommodating space R 2 , and thus the lubricity of each of the gear teeth is secured. 
     As described above, according to the rotary drive system  1  of the present embodiment, the lubricating oil supplied into the electric motor casing  21  is introduced into the speed reducer casing  61  through the communication hole  50 . The lubricating oil merges with the lubricating oil stored in the speed reducer casing  61  as a tank. It is possible to supply some of the stored lubricating oil to the electric motor  20  again. As a result, it is possible to consistently carry out the cooling of the rotor  38  and the stator  30  of the electric motor  20  and the lubricating of the transmission portion  80  in the speed reducer  60  via the lubricating oil-circulating unit  150 . 
     Therefore, it is not necessary to form a tank for storing lubricating oil in the electric motor  20 . Therefore, it is possible to prevent the electric motor  20  from becoming larger in size, and the entire rotary drive system  1  can be made compact. 
     Further, it is not necessary to separately manage the amount of oil of the speed reducer  60  and the electric motor  20 . When the electric motor  20  and the speed reducer  60  are each provided with a tank for storing lubricating oil or cooling oil, it is necessary to provide separate oil inspection units  160  for managing the respective liquid levels S. Further, it is necessary to separately manage the properties of the lubricating oil and the properties of the cooling oil, thus making maintenance cumbersome. 
     In the present embodiment, since the lubricating oil is stored only in the speed reducer  60  side, it is possible to manage the liquid level S of the lubricating oil only by providing one oil inspection unit  160 . Therefore, it is possible to reduce the production cost as compared with the case where the oil inspection unit  160  is provided in each of the speed reducer  60  and the electric motor  20 . Further, since only the properties of one lubricating oil need to be managed, it is possible to improve maintenance performance. 
     Here, the first stage transmission shaft  91  and the first stage planetary gear  92  directly connected to the rotary shaft  40  of the speed reducer  60  are rotated at high speed in accordance with the rotational speed of the rotary shaft  40 . Therefore, in the case where the first stage transmission shaft  91  and the first stage planetary gear  92  are immersed in lubricating oil, stirring loss increases and efficiency is lowered. Further, the variation in the liquid level S of the lubricating oil also increases. 
     In contrast, in the present embodiment, the liquid level S of the lubricating oil stored in the second accommodating space R 2  in the speed reducer casing  61  is positioned below the first stage planetary gear  92  which rotates at the highest speed and the first stage sun gear teeth  91   c  of the first stage transmission shaft  91 . Therefore, it is possible to suppress an increase in stirring loss. 
     On the other hand, the second stage planetary gear  102  and the second stage sun gear  101   a , which have been reduced in rotational speed by one stage, are positioned below the liquid level S of the stored lubricating oil. Therefore, since each rotational speed of the second stage planetary gear  102  and the second stage sun gear  101   a  is smaller than that of the first stage planetary gear mechanism  90 , there is no significant increase in stirring loss even when the lubricating oil is immersed in the lubricating oil. Accordingly, it is possible to appropriately perform lubrication of the second stage planetary gear mechanism  100  and the third stage planetary gear mechanism  110  while suppressing stirring loss. 
     In addition, since lubrication in the first stage planetary gear  92  is carried out by the lubricating oil flowing down through the communication hole  50  of the electric motor casing  21 , inadvertent deterioration in lubricity at the first stage does not occur. 
     The height position of the oil inspection hole  65  in the oil inspection unit  160  corresponds to the height of the liquid level S of the stored lubricating oil to be managed. 
     In the present embodiment, since the height position of the oil inspection hole  65  is lower than the first stage planetary gear  92  and above the second stage planetary gear  102 , it is possible to appropriately lubricate the planetary gear mechanism while reducing stirring loss. 
     Here, when the hydraulic excavator  200  is positioned at an inclined surface, the liquid level S of the lubricating oil stored in the second accommodating space R 2  may fluctuate. Also, when the rotary drive system  1  is rotated, the stored lubricating oil is affected by centrifugal force, and as a result, the liquid level S may fluctuate. 
     In the present embodiment, a throttle portion  170  that suppresses an inflow of lubricating oil to be introduced into the oil inspection hole  65  is formed at a height position corresponding to the oil inspection hole  65 . Therefore, it is possible to prevent the lubricating oil from excessively flowing into the oil inspection pipe  161  inadvertently. That is, the throttle portion  170  causes pressure loss to the lubricating oil that is going to flow into the oil inspection pipe  161 , and thus, it is possible to suppress an increase of an amount of the inflow. Accordingly, it is possible to stabilize the liquid level S in the oil inspection pipe  161 . As a result, for example, it is possible to suppress leakage of the lubricating oil from the oil inspection pipe  161 . 
     In particular, in the present embodiment, the second stage upper plate portion  105  of the second stage carrier  103  in the second stage planetary gear mechanism  100  is the throttle portion  170 . Thus, for example, as shown in  FIG. 9 , even when the lubricating oil is displaced radially outward due to centrifugal force, it is possible to stabilize the liquid level S in the oil inspection pipe  161  by the throttle portion  170  causing pressure loss to the oil inspection pipe  161 , and it is possible to prevent the lubricating oil from inadvertently flowing into the oil inspection pipe  161 . Further, since the throttle portion  170  can be configured without providing separate parts, cost reduction can be achieved. 
     In the present embodiment, as shown in  FIG. 6 , since the main oil drain hole  50   a  is opened above the lower bearing  37 , it is possible to be in a state of always supplying the lubricating oil introduced into the first accommodating space R 1  of the electric motor casing  21  to the lower bearing  37  during operation of the rotary drive system  1 . As a result, it is possible to rotate and support stably the rotary shaft  40 . 
     On the other hand, when the operation of the rotary drive system  1  is stopped, it is possible to discharge the lubricating oil remaining in the first accommodating space R 1  to the speed reducer  60  side from the bearing oil drain hole  50   d  formed in the lower bearing  37 , and at the same time, it is possible to discharge the lubricating oil to the speed reducer  60  side via the auxiliary oil drain hole  50   b . Thus, the lubricating oil is smoothly collected on the speed reducer  60  side without retaining the lubricating oil in the electric motor  20  side at the time of stopping, and it is possible to merge the lubricating oil with the lubricating oil stored in the second accommodating space R 2 . 
     Other Embodiments 
     Although the embodiment of the present invention has been described above, the present invention is not limited thereto and can be appropriately changed without departing from the technical idea of the present invention. 
     In the present embodiment, an example has been described in which the transmission portion  80  has a total of three stages, that is, a first stage, a middle stage, and a final stage, of the planetary gear mechanisms, but the present invention is not limited thereto, and may include, for example, only one stage, two stages, four stages or more, of the planetary gear mechanisms. The planetary gear mechanism in the middle stage may be divided into a plurality of stages. 
     In the embodiment, the liquid level S of the lubricating oil in the second accommodating space R 2  is positioned below the first stage planetary gear  92  and above the second stage planetary gear  102 , but may be positioned, for example, below the second stage planetary gear  102  and above the third stage planetary gear  112 . That is, it is sufficient that the liquid level S is positioned below the first stage planetary gear  92  and above any one of the second and subsequent planetary gears. Accordingly, it is possible to appropriately lubricate the planetary gear having a relatively low rotational speed while reducing stirring loss due to a high rotational speed of the planetary gear. 
     Similarly, in the embodiment, the height position of the oil inspection pipe  161  is positioned below the first stage planetary gear  92  and above the second stage planetary gear  102 ; however, the height position may be positioned, for example, below the second stage planetary gear and above the third stage planetary gear  112 . 
     The structure of the rotor  38  is not limited to the present embodiment, and may have other cooling structures. 
     The throttle portion  170  of the embodiment need not necessarily be provided. In addition, another structure different from the second stage carrier  103  may be provided as the throttle portion. 
     In the embodiment, although an example in which the present invention is applied to the rotary drive system  1  of the hydraulic excavator  200  as a work machine has been described, the above-described rotary drive system  1  may be applied to a mechanism that swings or rotates part of a different work machine. 
     INDUSTRIAL APPLICABILITY 
     The present invention is applicable to a rotary drive system and a hydraulic excavator using the rotary drive system. According to the present invention, it is possible to achieve reduction in cost while achieving compactness. 
     EXPLANATION OF REFERENCE SIGNS 
     
         
           1 : Rotary Drive System; 
           10 : Rotary Drive Device; 
           20 : Electric Motor; 
           21 : Electric Motor Casing; 
           22 : Upper Casing; 
           23 : Upper Cylindrical Portion; 
           23   a : Inner Peripheral Surface; 
           23   b : Upper Flange; 
           24 : Upper Bottom Portion; 
           24   a : Upper Through-Hole; 
           24   b : Annular Convex Portion; 
           25 : Lower Casing; 
           26 : Lower Cylindrical Part 
           26   a : Outer Peripheral Surface; 
           26   b : Inner Peripheral Surface; 
           26   c : Upper-end Surface. 
           26   d : Lower Fitting Portion; 
           26   e : Bolt Fixing Hole; 
           27 : Lower Bottom Portion; 
           27   a : Lower Through-Hole; 
           27   b : First Bottom Surface (bottom surface of the electric motor casing); 
           27   c : Second Bottom Surface (bottom surface of the electric motor casing); 
           27   d : Third Bottom Surface (bottom surface of the electric motor casing); 
           27   e : Electric Motor-side Accommodating Recess; 
           27   f : Lower Flange; 
           30 : Stator; 
           31 : Stator Core; 
           31   a : Core Main Body; 
           31   b : Core Convex Portion; 
           32 : Coil; 
           32   a : Upper Coil End; 
           32   b : Lower Coil End; 
           33 : Bolt; 
           35 : Upper Seal; 
           36 : Upper Bearing; 
           37 : Lower Bearing; 
           37   a : Inner Ring; 
           37   b  Outer Ring; 
           37   c : Rolling Body; 
           37   d : Bearing Shield; 
           38 : Rotor; 
           40 : Rotary Shaft; 
           40   a : Center Hole; 
           40   b : Radial Hole; 
           42 : Rotor Core; 
           42   a : Inner Peripheral Surface; 
           42   b : Inner Axial-direction Flow Path; 
           42   c : Outer Axial-direction Flow Path; 
           45 : Lower End Plate; 
           45   a : Connection Flow Path; 
           46 : Upper End Plate; 
           46   a : Discharge Hole; 
           50 : Communication Hole; 
           50   a : Main Oil Drain Hole; 
           50   b : Auxiliary Oil Drain Hole; 
           50   c : Outer Peripheral-side Oil Drain Hole; 
           50   d : Bearing Oil Drain Hole; 
           60 : Speed Reducer; 
           61 : Speed Reducer Casing; 
           61   a : Hydraulic Pressure Supply Hole; 
           62   a : First Stage Inner Gear Teeth; 
           62   b : Second Stage Inner Gear Teeth; 
           62   c : Third Stage Inner Gear Teeth; 
           63   a : First Inner Peripheral Surface; 
           63   b : Second Inner Peripheral Surface; 
           64   a : First Sliding Contact-Inner Peripheral Surface; 
           64   b : Second Sliding Contact-Inner Peripheral Surface; 
           64   c : Casing-side Step Surface; 
           65 : Oil Inspection Hole; 
           70 : Output Shaft; 
           71 : Output Shaft Bearing; 
           72 : Lower Seal; 
           80 : Transmission Portion; 
           90 : First Stage Planetary Gear Mechanism; 
           91 : First Stage Transmission Shaft; 
           91   a : Fitting Cylindrical Portion; 
           91   b : Disk Portion; 
           91   c : First Stage Sun Gear Teeth; 
           92 : First Stage Planetary Gear; 
           92   a : First Stage Planetary Gear Teeth; 
           93 : First Stage Carrier; 
           94 : First Stage Carrier Shaft; 
           95 : First Upper Plate Portion; 
           95   a : First Stage Insertion Hole; 
           96 : First Stage Lower Plate Portion; 
           96   a : First Stage Connection Hole; 
           100 : Second Stage Planetary Gear Mechanism; 
           101 : Second Stage Transmission Shaft; 
           101   a : Second Stage Sun Gear Teeth; 
           102 : Second Stage Planetary Gear; 
           102   a : Second Stage Planetary Gear Teeth; 
           103 : Second Stage Carrier (Carrier). 
           104 : Second Stage Carrier Shaft; 
           105 : Second Upper Plate Portion (Upper Plate Portion). 
           105   a : Second Stage Insertion Hole; 
           106 : Second Stage Lower Plate Portion (Lower Plate Portion). 
           106   a : Second Stage Connection Hole; 
           110 : Third Stage Planetary Gear Mechanism; 
           111 : Third Stage Transmission Shaft; 
           111   a : Third Stage Sun Gear Teeth; 
           112 : Third Stage Planetary Gear; 
           112   a : Third Stage Planetary Gear Teeth; 
           113 : Third Stage Carrier; 
           114 : Third Stage Carrier Shaft; 
           115 : Third Stage Upper Plate Portion; 
           115   a : Third Stage Insertion Hole; 
           116 : Third Stage Lower Plate Portion; 
           116   a : Third Stage Connection Hole; 
           120 : Brake Mechanism; 
           121 : Disk Support Portion; 
           122 : Brake Disc; 
           123 : Brake Plate; 
           130 : Brake Piston; 
           131 : First Sliding Contact-Outer Peripheral Surface; 
           132 : Second Sliding Contact-Outer Peripheral Surface; 
           133 : Piston-side Step Surface; 
           134 : Plate Contact Surface; 
           135 : Piston-side Accommodating Recess; 
           140 : Brake Spring; 
           150 : Lubricating Oil-Circulating Unit; 
           151 : Lubricating Oil Flow Path; 
           152 : Lubricating Oil Pump; 
           153 : Cooling Portion; 
           154 : Strainer; 
           160 : Oil Inspection Unit; 
           161 : Oil Inspection Pipe; 
           161   a : Horizontal Pipe; 
           161   b : Vertical Pipe; 
           162 : Oil Inspection Rod; 
           170 : Throttle Portion; 
           200 : Hydraulic Excavator; 
           211 : Crawler Belt; 
           210 : Undercarriage; 
           220 : Swing Circle; 
           221 : Outer Race; 
           222 : Inner Race; 
           223 : Swing Pinion; 
           230 : Upper Swing Body; 
           231 : Cab; 
           232 : Work Equipment; 
           233 : Boom; 
           234 : Arm; 
           235 : Bucket; 
           236 : Engine; 
           237 : Generator Motor; 
           238 : Hydraulic Pump; 
           239 : Inverter; 
           240 : Capacitor; 
         L: Swing Axis; 
         O: Axis; 
         S: Liquid Level; 
         R 1 : First Accommodating Space; 
         R 2 : Second Accommodating Space; 
         R 3 : Spring Accommodating Portion; 
         R 4 : Hydraulic Pressure Supply Space