Abstract:
A motor includes: a rotor including an output shaft; a stator provided with a coil wound around the stator; and a casing configured to accommodate the rotor and the stator and to contain a coolant liquid therein, wherein skews are formed in at least one of the rotor and the stator. A hollow portion extending an axial direction is formed in the output shaft. The hollow portion includes an opening that opens in the casing at an end surface of the output shaft where a liquid surface of the coolant liquid is relatively higher due to rotation of the rotor and that communicates with the casing near an end opposite to the end surface.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to motors in which a coolant liquid is used. 
     2. Description of the Related Art 
     Oil bath motors are structured such that a motor and a reducer are encapsulated in spaces communicating with each other and lubricant oil is circulated in the spaces. For example, one prior art discloses rotary electric machine configured such that a rotation shaft of the machine is hollow, and oil-laden coolant is blasted from an external cooling fluid supplying unit to a space located inside a housing and accommodating a rotor, via holes in the rotation shaft. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention relates to a motor including: a rotor including an output shaft; a stator provided with a coil wound around the stator; and a casing configured to accommodate the rotor and the stator and to contain a coolant liquid therein, wherein skews are formed in at least one of the rotor and the stator, wherein a hollow portion extending an axial direction is formed in the output shaft. The hollow portion includes an opening that opens in the casing at an end surface of the output shaft where a liquid surface of the coolant liquid is relatively higher due to rotation of the rotor and that communicates with the casing near an end opposite to the end surface. 
     Optional combinations of the aforementioned constituting elements, and implementations of the invention in the form of methods, apparatuses, and systems may also be practiced as additional modes of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments will now be described, by way of example only, with reference to the accompanying drawings, which are meant to be exemplary, not limiting, and wherein like elements are numbered alike in several figures, in which: 
         FIG. 1  shows the structure of a power transmission device using a motor according to one embodiment of the present invention built in a wheel of a forklift; 
         FIG. 2  shows the liquid surface of the coolant liquid occurring when the motor is not in operation; 
         FIG. 3  shows how the liquid surface varies when the prior art motor in which a through hole is not formed in the motor shaft is operated; 
         FIG. 4  shows how the liquid surface varies when the motor according to the embodiment is operated. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention will now be described by reference to the preferred embodiments. This does not intend to limit the scope of the present invention, but to exemplify the invention. 
     With the technology described in the prior art, it is necessary to circulate the coolant using the cooling fluid supplying unit. 
     In this background, there is a need to provide a technology capable of circulating cooling liquid in a closed space accommodating a motor, without using an external device such as a pump. 
       FIG. 1  shows the structure of a power transmission device  100  using a motor according to one embodiment of the present invention built in a wheel of a forklift.  FIG. 1  is a cross section that results when the power transmission device  100  is severed by a vertical plane that includes the central axis of the device  100 . 
     The power transmission device  100  includes a reducer  10 , an interior permanent magnet (IPM) motor  12 , and a brake mechanism  14 , and is used to drive the wheels  50  of a utility vehicle independently. 
     The reducer  10  is a kind of planetary gear reducer of eccentric oscillation and meshing type. An input shaft  16  is located at the radial center of externally-toothed gears  24  and  26  described later. Two eccentric bodies  18  and  20  eccentric relative to the input shaft  16  are formed so as to be integrated with the input shaft  16 . The two eccentric bodies  18  and  20  are eccentric relative to each other by a phase difference of 180°. The eccentric bodies  18  and  20  may be configured as components independent of the input shaft  16  and fixed to the input shaft  16  using a key, etc. 
     Two externally-toothed gears  24  and  26  are oscillatably fitted to the outer circumference of the eccentric bodies  18  and  20 , respectively, via roller bearings  21  and  23 . The externally-toothed gears  24  and  26  internally mesh with an internally-toothed gear  28 . 
     The internally-toothed gear  28  primarily includes cylindrical internal gear pins  28 A and  28 B forming internally-toothed gears, retention pins  28 C extending through the internal gear pins  28 A and  28 B and rotatably retaining the pins  28 A and  288 , and an internally-toothed gear body  28 D rotatably retaining the retention pins  28 C and integrated with a casing  30 . 
     A first carrier body  34  fixed to a vehicle frame (not shown) is located at the axial end of the externally-toothed gears  24  and  26  toward the vehicle. At the axial end of the externally-toothed gears  24  and  26  away from the vehicle is located a second carrier body  38  integrated with the first carrier body  24  via carrier bolts  36  and carrier pins  42 . Internal pins  40  are formed to be integrated with the second carrier body  38 . 
     Twelve through holes having the equal diameter are formed at positions offset from the shaft center of the externally-toothed gear  24  so as to be equidistant from each other. The carrier pins  42  are inserted through three of these through holes equidistant from each other by 120°, and internal pins  40  are inserted through the remaining nine pins, Gear teeth of waveform are formed at the outer circumference of the externally-toothed gear  24 . As the gear teeth move on the internal gear pins  28 A of the internally-toothed gear  28 , maintaining contact with the internal gear pins  28 A, the externally-toothed gear  24  is capable of oscillating within a plane defined about a central axis normal to the plane. The externally-toothed gear  26  is similarly structured as the externally-toothed gear  24  except that there is a phase difference of 180°. 
     The casing  30  of the reducer  10  is rotatably supported via a pair of main bearings  46  and  47  by the first carrier body  34  and the second carrier body  38  secured to the vehicle frame. A wheel member  48  is jointed via bolts  49  to the end surface of the casing  30  away from the vehicle. A tire  50  of a forklift (not shown) is mounted to the wheel member  48 . The reducer  10  is accommodated within an axial range of the tire  50  (within the range denoted by a dashed two dotted line of  FIG. 1 ). 
     The input shaft  16  of the reducer  10  is rotatably supported by the first carrier body  34  and the second carrier body  38  via a pair of angular contact ball bearings  52  and  54  in DF (face-to-face) arrangement. 
     The main bearings  46 ,  47  and the angular contact ball bearings  52 ,  54  in the reducer  10  are open-end bearings and lubricated by a coolant liquid sealed in the casing, as described later. 
     The IPM motor  12  is provided with a stator  64  and a rotor  66  each configured with magnetic steel sheets. A plurality of air gaps  66 A extending in the axial direction is formed in the magnetic steel sheets composing the rotor  66 . 
     Permanent magnets  76 A and  7 B are embedded in the gaps. IPM motors, in which permanent magnets are embedded in the rotor, have higher efficiency than SPM motors, in which permanent magnets are attached to the surface of the rotor, and are suitable as a motor to drive a forklift. The magnetic steel sheets composing the rotor  66  are integrated with each other by bolts  67  and are integrated with a motor output shaft  70  via an engagement part (not shown). The side of the motor output shaft  70  toward the vehicle is rotatably supported via a bearing  82  by an extension  60 A extending inward from a motor casing GO. The side of the motor output shaft  70  away from the vehicle is jointed by the input shaft  16  of the reducer  10  via a spline  70   a.    
     The stator  64  is fixed to the motor casing  60 . A coil for forming a magnetic field is wound around the stator  64 . The parts of coil that are folded back to provide a winding extend axially from the ends of the stator  64  as coil ends  68 A and  68 B. 
     The inner circumferential surface of the stator  64  facing the rotor  66  is formed with skews (not shown) for improving voltage waveform and reducing cogging torque. 
     Instead of forming skews in the stator  64 , skews may be formed on the outer circumferential surface of the rotor  66  facing the stator  64 . Alternatively, skews may be formed on both the inner circumferential surface of the stator  64  and the outer circumferential surface of the rotor  66 . In the latter case, the direction of twist of the skews in the stator is identical to that of the rotor. 
     End plates  72  and  74  for preventing the permanent magnet embedded in the rotor from being dislocated while in rotation are fitted to the respective axial end surfaces of the rotor  66 . For example, the end plates are made of stainless steel or aluminum. 
     A hollow portion  90  extending in the axial direction is formed inside the motor output shaft  70 . The end of the hollow portion  90  toward the vehicle communicates with a space  801 , at an opening  96 . The input shaft  16  of the reducer  10  is inserted at the end of the hollow portion  90  away from the vehicle, as indicated above. The motor output shaft  70  is formed with through holes  92  and  94  that extend in the radial direction and open to the side surface of the output shaft  70 , so as to be adjacent to the end plates  74  and  72  at the respective ends of the rotor  66 . The through holes  92  and  94  provide communication between the hollow portion  90  and spaces  80 L and  80 R. At least two through holes  92  and  94  are provided in the circumferential direction of the motor output shaft  70 . Preferably three or more through holes are provided in rotation symmetry. However, only one through hole may be provided. In the axial direction, the through holes  92  and  94  are provided such that the openings thereof in the side surfaces of the output shaft  70  are respectively located radially inward from the coil ends  68 A and  68 B extending at the respective ends of the stator  64 . 
     A helical channel (not shown) with a direction of twist opposite to that of the skew on the inner circumferential surface of the stator or the outer circumferential surface of the rotor is formed on the inner circumferential surface of the output shaft  70 . The helical channel functions as a means to guide the coolant liquid, which enters the hollow portion  90  via the opening  96 , away from the vehicle, i.e., from the side of the space  801 , toward the side of the space  80 R, which will be described later in detail. 
     The brake mechanism  14  puts a brake on the rotation of the output shaft  70 . The brake mechanism  14  is accommodated interior to the coil end  68 A of the coil wound around the stator  64  in the radial direction. The brake mechanism is provided with a multi-plate brake  78  having a plurality of friction plates. The friction plates of the multi-plate brake  78  includes a plurality of (four, in the illustrated case) fixed friction plates  78 A and a plurality of (three, in the illustrated case) rotatable friction plates  78 B. 
     The fixed friction plates  78 A are fixed in the circumferential direction between a brake piston  84  located to block the rear end of the motor casing  60  of the IPM motor  12  and the extension  60 A of the casing  60  by thorough pins (not shown). The fixed friction plates  78 A are movable in the axial direction along the thorough pins. 
     Meanwhile, the rotatable friction plates  78 B are built in the output shaft  70 , which is rotated as one piece with the rotor  66 , and is rotatable as one piece with the output shaft  70 . A spline  70 B is formed in the axial direction at the outer circumference of the output shaft  70 . The inner circumferential ends of the rotatable friction plates  783  are engaged with the spline  703 . This allows the rotational friction plates  783  to be integrated with each other in the circumferential direction via the output shaft  70  and the spline  70 B and to be movable in the axial direction of the output shaft  70 . A friction sheet (not shown) is adhesively attached to the surface of the rotatable friction plates  78 B. 
     The brake piston  84  is located to oscillate in a cylinder that communicates with a hydraulic mechanism (not shown) via an oil passage  86 . When the operator of the forklift performs a braking maneuver, pressurized oil is supplied from the hydraulic mechanism to the cylinder via the oil passage  86 , and the brake piston  84  pressurizes the fixed friction plate  78 A closest to the vehicle in the axial direction. 
     The rotor  66  of the IPM motor  12 , the output shaft  70 , the friction plates  78 A,  78 B of the brake mechanism  14 , the input shaft  16  of the reducer  10 , the casing  30  (output shaft of the reducer  10 ), and the wheel member  48  are located coaxially. 
     The IPM motor  12  and the brake mechanism  14  are formed as wet mechanisms, and the interior spaces of the reducer  10 , the IPM motor  12 , and the brake mechanism  14  communicate with each other to form a single, closed space. The coolant liquid is sealed in this space and can flow through the space. The coolant liquid not only functions to cool the rotor  66  and the stator  64  of the IPM motor but also functions as a lubricant for the bearings and slide portions inside the reducer and the motor. 
     A description will now be given of the operation of the power transmission  100  performed when the IPM motor  12  is driven. 
     When the operator of the forklift maneuvers the forklift to move forward or backward, the rotor  66  and the output shaft  70  are rotated relative to the stator  64  of the IPM motor  12 . The rotation of the output shaft  70  is transmitted to the input shaft  16  of the reducer  10  via the spline  70 A. When the input shaft  16  is rotated, the outer circumferences of the eccentric bodies  18  and  20  move eccentrically, causing the externally-toothed gears  24  and  26  to oscillate via the roller bearings  21  and  23 . The oscillation causes the positions of meshing between the outer teeth of the externally-toothed gears  24 ,  26  and the internal gear pins  28 A,  28 B of the internally-toothed gear  28 , respectively, to be shifted successively. 
     The difference in the number of teeth between the externally-toothed gears  24 ,  26  and the internally-toothed gear  28  is defined to be “one”. The rotation of the externally-toothed gears  24  and  26  is restrained by the internal pins  40  fixed to the first carrier body  34 , which is fixed to the vehicle frame. Therefore, each time the input shaft  16  is rotated 360°, the internally-toothed gear  28  is rotated relative to the externally-toothed gears  24  and  26 , the rotation of which is restrained, by an angle defined by the difference in the number of teeth. As a result, the rotation of the input shaft  16  causes the casing  30  integrated with the internally-toothed gear body  28 D at a rotational speed reduced by 1/(the number of teeth of the internally-toothed gear). The rotation of the casing  30  causes the tire  50  of the forklift to be rotated via the wheel member  48  fixed to the casing  30  by the bolts  49 . 
     A description will now be given of the braking operation of the power transmission device  100  performed by the brake mechanism  14 . 
     When the operator of the forklift performs a braking maneuver, pressurized oil is supplied from the hydraulic mechanism to the cylinder via the oil passage  86 , causing the brake piston  84  to move away from the vehicle (toward right in the figure) within the cylinder. As a result, the fixed friction plate  78 A closest to the vehicle is pressurized by the brake piston  84  to move away from the vehicle. Then, the plurality of fixed friction plates  78 A and the rotatable friction plates  78 B come into contact with each other successively with a strong force. As described above, the fixed friction plates  7 BA are fixed in the circumferential direction via the through pins, and the rotatable friction plates  78 B are integrated with the output shaft  70  in the circumferential direction via the spline  70 B built in the output shaft  70 . Therefore, as a result of the friction plates  78 A and the rotatable plates  78 B being in strong contact with each other via the friction sheets adhesively attached to the rotatable friction plates  783 , the brake action of the output shaft  70  is exerted. 
     When the operator stops the braking maneuver, the supply of the pressurized oil in the cylinder is stopped. Consequently, the restoring force of a spring  84 A interposed between the extension  60 A and the brake piston  84  returns the brake piston  84  toward the vehicle, causing the fixed friction plates  78 A to return to the initial axial positions. In association with this, the rotatable friction plates  783  also return to the initial axial positions, causing the fixed friction plates  78 A to lose contact with the rotatable friction plates  78 B and causing the brake action to disappear. 
       FIG. 2  shows the liquid surface of the coolant liquid (lubricant) occurring when the motor is not in operation. The shaded portion in the figure indicates the coolant liquid. As shown in the figure, according to the embodiment, the coolant liquid of an amount sufficient to immerse parts of the bearing  82  of the IPM motor  12 , the roller bearings  21  and  23  of the reducer  10 , and the angular contact ball bearings  52  and  54  in the liquid is sealed in the casings  30  and  60  with the central axis being horizontal. 
       FIG. 3  shows how the liquid surface varies when the prior art motor in which a through hole is not formed in the motor shaft is operated. When the rotor is rotated, the coolant liquid is drawn to the surface of the rotor due to its viscocity and begins to flow in a direction identical to the direction of rotation. In particular, the coolant liquid located in the gap between the rotor and the stator is pushed by the skews formed on the inner circumferential surface of the stator or the outer circumferential surface of the rotor in an axial direction defined by the skews. If the direction of skews is defined to create a flow in an axial direction toward the vehicle, the height of the liquid surface differs in the space SOL toward the vehicle from that of the opposite space  80 L, as shown in  FIG. 3 . If this phenomenon occurs, the roller bearings  21  and  23  and the angular contact ball bearings  52  and  54  in the reducer  10  may not be immersed in the coolant liquid as shown in  FIG. 3  so that sufficient lubrication performance may not be provided. 
     If the volume of coolant liquid is small, only the lower half of the coil, the primary heat source of the motor, will be immersed in the coolant liquid. Therefore, the upper half of the coil cannot be cooled and the motor would lack heat radiation performance. 
     In order to increase the cooling performance of the motor, the amount of contact of the coolant liquid with the upper half of the coil need be increased. If the method of increasing the amount of coolant liquid sealed in the casing is employed for this purpose, the rotational load of the rotor due to the viscocity resistance of the coolant liquid will be increased so that the motor efficiency will be lowered. 
     In this embodiment, the aforementioned problem is solved by forming the motor output shaft having a hollow portion with a through hole that extends in the radial direction of the output shaft and that opens at the side surface of the shaft. 
       FIG. 4  shows how the liquid surface varies when the motor according to this embodiment is operated. The arrows in the figure indicate the directions of flow of the coolant liquid and the direction of rotation of the output shaft. As the rotor  66  is rotated due to the electromagnetic action between the stator and the rotor, the coolant liquid is drawn to the surface of the rotor due to its viscocity and begins to flow in a direction identical to the direction of rotation of the rotor, same as in  FIG. 3 . The coolant liquid located in the gap between the rotor and the stator is pushed by the skews formed on the inner circumferential surface of the stator or the outer circumferential surface of the rotor in an axial direction defined by the skews. 
     As the rotational speed of the rotor is increased and the liquid surface of the coolant liquid in the space  80 L toward the vehicle becomes higher than the lower end of the opening  96  of the motor output shaft  70 , the coolant liquid flows into the hollow portion  90 . Due to the rotation of the helical channel formed on the inner circumferential surface of the output shaft, a flow of the coolant liquid that causes the coolant liquid to move on the inner circumferential surface from the opening  96  toward the vehicle to the shaft end opposite to the vehicle is created. 
     The centrifugal force exerted on the output shaft  70  causes a portion of the coolant liquid moving on the inner circumferential surface of the output shaft  70  to pass through the through holes  92  and  94  and to spread in droplets toward the coil ends  68 A and  68 B located above the through holes  92  and  94 . Droplets of the coolant liquid spread via the through holes  92  and  94  are attached to the coil ends  68 A and  68 B and deprive the coil of heat before falling on the liquid surface by gravitation. Droplets of the coolant liquid spread via the through holes  94  are supplied to the reducer, bypassing the output shaft  70 . The remainder of the coolant liquid is supplied to the reducer  10  via the gap of the spline  70 . 
     The action of the hollow portion  90  to transport the coolant liquid prevents the liquid surface in the space  80 L toward the vehicle from becoming excessively higher than the liquid surface in the opposite space  80 R, ensuring that the liquid surface on the left side approaches that of the right side. As a result, the roller bearings  21  and  23  and the angular contact ball bearings  52  and  54  in the reducer  10  will also be immersed in the coolant liquid to provide sufficient lubrication performance. 
     While  FIG. 4  shows that the inner diameter of the hollow portion is constant, the inner diameter may not be constant. For example, the diameter may be smaller toward the end surface away from the vehicle than at the end surface toward the vehicle. 
     The figure also depicts the inner diameter of the through holes  92  and  94  extending in the radial direction as being constant. Alternatively, the through holes may be formed such that the opening at the inner circumferential surface is larger than the opening at the side surface. This enhances the speed of spreading of the coolant liquid via the through holes. 
     Further, the through holes  92  and  94  may not extend in a direction perpendicular to the central axis but may extend at an angle. This allows droplets of the coolant liquid to be spread to coil end portions other than the portions immediately above the opening at the side surface. 
     Where a plurality of through holes  92  and a plurality of through holes  94  are provided, the angle of inclination may be different from one through hole to another. This allows droplets of the coolant liquid to be spread in an extensive range over the coil ends. 
     Which of the liquid surface in the space  80 L at one end of the rotor or that of the space  80 R at the other end is higher depends on the direction of rotation of the rotor, and the direction of skews on the inner circumferential surface of the stator or the outer circumferential surface of the rotor. Since the coolant liquid is introduced in the hollow portion  90  via the opening  96  according to this embodiment, the liquid surface in the space  80 L needs to be higher than that of the space  80 R. It is therefore preferable to define the direction of skews on the inner circumferential surface of the stator or the outer circumferential surface of the rotor so that a flow of the coolant liquid toward the space  80 L is created in association with the direction of rotation of the rotor that is frequently used. In the case that a forklift is operated by the power transmission device  100 , the direction of rotation of the rotor that is frequently used is the direction of rotation corresponding to the forward movement of the forklift. 
     It is preferable to measure a difference in liquid surface level between the spaces  80 L and  80 R at the respective ends of the rotor occurring when the rotor is driven at a predetermined rotational speed (preferably, when the rotor is driven at a high, frequently-used speed) and to optimally design the amount of coolant liquid sealed in the casings  30  and  60  such that a proper amount of coolant liquid is found in the reducer side even when the motor is operated at the predetermined rotational speed. 
       FIG. 4  shows that the end of the motor output shaft  70  away from the vehicle is coupled to the input shaft  16  of the reducer  10  via the spline  70 A. Alternatively, a larger opening may be provided at the end of the motor output shaft  70  away from the vehicle by coupling the input shaft  16  of the reducer  10  to the motor output shaft  70  by using other means (e.g. by using a coupling). Still alternatively, the motor output shaft  70  may be coupled to the input shaft  16  so as to completely close the end away from the vehicle. Even in the latter case, since droplets of the coolant liquid spread via the through hole  94  are supplied to the reducer  10  by bypassing the output shaft  70 , there will not be shortage of the coolant liquid in the reducer  10 . Even if the motor output shaft  70  is coupled to the input shaft  16  of the reducer  10  via a spline, the end of the motor output shaft away from the vehicle can be completely closed by using a spline with little gap or by with interference fit. 
     Still alternatively, the inner circumferential surface of the motor output shaft  70  may be configured as a flat surface instead of being formed with a helical channel. This is because, if a difference in liquid surface level of the coolant liquid is created between the ends of the rotor, the associated difference in potential energy of the liquid surface between the space  80 L toward the vehicle and the space  80 R away from the vehicle causes the coolant liquid to be transferred from the vehicle side to the side away from the vehicle via the hollow portion  90  even if a helical channel is not provided. Instead of or in addition to forming a helical channel, an impeller may be provided in the hollow portion  90  to create a flow of the coolant liquid from the vehicle side to the side away from the vehicle. 
     As described above, the difference in liquid surface level of the coolant liquid between the ends of the rotor, created by the rotation of the rotor and the skews on the inner circumferential surface of the stator or the outer circumferential surface of the rotor, could be taken advantage of to guide the coolant liquid into the hollow portion via the opening at the end surface of the output shaft and to transport the coolant liquid from the side with a higher liquid surface level to the side with a lower liquid surface level via the hollow portion. Therefore, the difference in liquid surface level between the ends of the rotor can be reduced to immerse the slide portions of the motor and the reducer in the coolant without using an external device such as a pump. 
     Since the centrifugal force exerted on the output shaft causes the coolant liquid to be spread toward the coil ends via the through holes formed in the hollow portion, the coil ends can be cooled efficiently even if the amount of coolant liquid sealed in the casings is small (e.g. even if the amount is just sufficient to immerse a part of the lower half of the rotor when the output shaft is positioned horizontally). This prevents the resistance of stirring met when the rotor is rotated from being increased due to an increase in the amount of coolant liquid. 
     It has been described in the foregoing about some embodiments of the invention. It will be obvious to those skilled in the art that these embodiments are intended to be illustrative only and various modifications to constituting elements and processes could be developed and that such modifications are also within the scope of the present invention. 
     The structure where the brake mechanism is provided toward the vehicle side of IPM motor is described by way of example. Alternatively, the present invention can be equally applied to the structure where the brake mechanism is provided between the reducer and the IPM motor. The present invention can also be applied to any type of oil bath motors having other structures where the coolant liquid of the motor also serves as the lubricant in the slide portions such as bearings. 
     A reducer mechanism of oscillating and internally meshing type is used in the embodiments above. However, the reducer mechanism of the reducer used in combination with the motor according to the embodiments is not limited to the one described. For example, the reducer may have other reducer mechanisms such as a simple planetary gear reducer mechanism. The reducer may not necessarily have a single-stage reducer mechanism in which the input shaft and the output shaft are coaxial. Alternatively, the reducer mechanism may include multiple shafts or multiple stages. 
     In the embodiments above, it has been described that the motor according to the invention is used in combination with a planetary gear reducer of eccentric oscillation and meshing type in which the input shaft (eccentric body shaft)  16  is provided at the center of the internally-toothed gear  28 . Alternatively, the motor according to the present invention can be used in combination with a planetary gear reducer of a type in which several eccentric body shafts are provided at positions offset from the center of the internally-toothed gear. 
     In the embodiments above, it has been described that the planetary gear reducer of eccentric oscillation and meshing type is configured to fix the first carrier body  34  and the second carrier body  38  and to output rotation from the casing  30 . Alternatively, the planetary gear reducer may be configured to fix the casing  30  and to output rotation from the first carrier body  34  and the second carrier body  38 . In this case, the spin component of the externally-toothed gears  24  and  26  is transmitted to the first carrier body  34  and the second carrier body  38  via the internal pins  40 . 
     The motor according to the present invention is not limited to be used to drive the wheels of a utility vehicle such as a forklift, but may be applied to any application. 
     Priority is claimed to Japanese Patent Application No. 2012-077931, filed Mar. 29, 2012, the entire content of which is incorporated herein by reference.