Abstract:
A power transmission device is provided, the power transmission device comprising: a motor including a rotor having a permanent magnet embedded therein and a stator, and configured to generate a rotational force; a reducer configured to transmit the rotational force of the motor; and a wet brake mechanism configured to put a brake the rotation of the motor. Spaces accommodating the motor, the reducer, and the brake mechanism communicate with each other so that lubricant oil can flow through the spaces. A leakage magnetic flux at an axial end face of the rotor away from the brake mechanism is greater than that of the end face toward the brake mechanism.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a power transmission device. 
         [0003]    2. Description of the Related Art 
         [0004]    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. In an oil bath motor such as this, metal powder created as a result of sliding motion between components may be circulated along with the lubricant oil and supplied to the slide portions with the result that the slide portions may be unusually abraded. 
         [0005]    One prior art discloses a compressor comprising: a power device composed of a stator and a rotor having a permanent magnet embedded therein; an airtight container for storing lubricant oil; and an oil feeding mechanism for pumping the lubricant oil stored in the airtight container so as to circulate the lubricant, wherein a through hole in which the lubricant flows is formed by drilling the rotor so as to be integrated with an air gap portion in which the permanent magnet is embedded. This allows iron powder mixed with the lubricant oil and circulated to be captured by the permanent magnet. 
       SUMMARY OF THE INVENTION 
       [0006]    The power transmission device according to one aspect of the present invention comprises: a motor including a rotor having a permanent magnet embedded therein and a stator, and configured to generate a rotational force; a reducer configured to transmit the rotational force of the motor; and a wet brake mechanism configured to put a brake on the rotation of the motor. 
         [0007]    Spaces accommodating the motor, the reducer, and the brake mechanism communicate with each other so that lubricant oil can flow through the spaces, and a leakage magnetic flux at an axial end face of the rotor away from the brake mechanism is greater than that of the end face toward the brake mechanism. 
         [0008]    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 
         [0009]    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: 
           [0010]      FIG. 1  shows the structure of a power transmission device according to one embodiment of the present invention; 
           [0011]      FIG. 2  is a top view of an end plate of  FIG. 1 ; and 
           [0012]      FIG. 3  shows the structure of a power transmission device of a structure in which a brake mechanism is provided between the IPM motor and the reducer. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    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. 
         [0014]    Some power transmission devices that use an oil bath motor as an engine are provided with a wet brake mechanism configured to produce a braking force by causing friction plates to be in contact with each other. By applying the technology of the prior art to the power transmission device as described above, iron powder attracted by the permanent magnet will be supplied to the neighborhood of the friction plates with the result that the friction plates may be unusually abraded. 
         [0015]    In this background, there is a need to provide a technology capable of capturing metal powder in the lubricant oil and reducing abrasion of friction plates of a brake mechanism in a power transmission device of a structure in which the motor, the reducer, and the brake mechanism are soaked in an oil bath. 
         [0016]      FIG. 1  shows the structure of a power transmission device  100  according to one embodiment of the present invention embedded 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 . 
         [0017]    The power transmission device  100  comprises a reducer  10 , an interior permanent magnet (IPM) motor  12 , and a brake mechanism  14 , and is used to drive the wheels of a utility vehicle independently. 
         [0018]    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. 
         [0019]    Two externally-toothed gears  24  and  26  are oscillatably fitted to the outer periphery 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 . 
         [0020]    The internally-toothed gear  28  primarily comprises 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  28 B, and an internally-toothed gear body  28 D rotatably retaining the retention pins  28 C and integrated with a casing  30 . 
         [0021]    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 . 
         [0022]    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°. 
         [0023]    The casing  30  of the reducer  10  is rotatably supported by the first carrier body  34  and the second carrier body  38  secured to the vehicle frame, via a pair of main bearings  46  and  47 . A wheel member  48  is jointed via bolts  49  to the lateral face 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 dashed-two dotted lines of  FIG. 1 ). 
         [0024]    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 bearings  52  and  54  in DF (face to face) arrangement. 
         [0025]    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 are 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 an output shaft  70  via an engagement part (not shown). The side of the output shaft  70  toward the vehicle is rotatably supported via a bearing  82  by an extension  60 A extending inward from a motor casing  60 . The side of the output shaft  70  away from the vehicle is jointed by the input shaft  16  of the reducer  10  via a spline  70   a.    
         [0026]    A stator  64  is fixed to the motor casing  60 . A coil for generating a magnetic field is wound around the stator  64 . The parts of the 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. 
         [0027]    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  comprises 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. 
         [0028]    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. 
         [0029]    Meanwhile, the rotatable friction plates  783  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  70 B. This allows the rotational friction plates  78 B 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 each of the rotatable friction plates  78 B. 
         [0030]    The brake piston  84  is located to slide 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. 
         [0031]    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. 
         [0032]    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 lubricant is sealed in this space and can flow through the space. 
         [0033]    A description will now be given of the operation of the power transmission  100  performed when the IPM motor  12  is driven. 
         [0034]    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. 
         [0035]    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  280  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 . 
         [0036]    A description will now be given of the braking operation of the power transmission device  100  performed by the brake mechanism  14 . 
         [0037]    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  78 A 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  78 B, the brake action of the output shaft  70  is exerted. 
         [0038]    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  78 B 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. 
         [0039]    A description will now be given of how metal powder in the lubricant oil is captured using magnetic flux generated by the permanent magnet in the rotor, which is one of characteristic features of the embodiment. 
         [0040]    As shown in  FIG. 1 , end plates  72  and  74  for preventing the permanent magnets embedded in the rotor from being dislocated while in rotation are fitted to the respective axial end faces of the rotor  66 . The end plates are made of stainless steel or aluminum. 
         [0041]      FIG. 2  is a top view of the end plate  72  away from the brake mechanism  14 . A through hole  72 A is formed in each of positions corresponding to permanent magnets  76 B (air gaps  66 A) formed in the rotor. By forming the rotor end face with through holes that allow direct contact between the permanent magnet and the lubricant oil, the magnetic flux from the permanent magnet can exude through the rotor end face and capture metal powder created mainly as a result of a sliding motion between components in the reducer. This can mitigate abnormal abrasion between components caused as a result of metal powder in the lubricant oil being caught in the sliding portion and can therefore extend the life of the reducer and the brake mechanism. 
         [0042]    The diameter of the through holes  72 A is illustrated as being substantially identical to the width of the air gaps  66 A but may be slightly smaller than the width of the gaps. Two or more through holes may be formed per a single air gap. 
         [0043]    According to this embodiment, the through holes  72 A are formed in the end plate  72  at one of the end faces of the rotor away from the brake mechanism  14 , but the end plate  74  at the end face toward the brake mechanism  14  is not provided with such through holes. This ensures that the leakage magnetic flux from the rotor end face away from the brake mechanism is larger than that of the end face toward the brake mechanism. Therefore, the metal powder in the lubricant is attracted more toward the end face away from the brake mechanism. This prevents a large amount of metal powder from flowing around the friction plates of the brake mechanism and prevents abnormal abrasion of the friction plates. 
         [0044]    Described above is an explanation based on an exemplary embodiment. The embodiment is intended to be illustrative only and it will be obvious to those skilled in the art that various modifications to constituting elements and processes could be developed and that such modifications are also within the scope of the present invention. 
         [0045]    It has been described that only one of the end plates is formed with through holes in the embodiment described above. However, other structures may also be used to ensure that the leakage magnetic flux from the rotor end face away from the brake mechanism is larger than that of the end face toward the brake mechanism. A description will now be given of such examples. 
         [0046]      FIG. 3  shows the structure of a power transmission device  200  according to one example. The basic structure and the operation are the same as those of the power transmission device  100  shown in  FIG. 1  except that a brake mechanism  114  is provided between the reducer  110  and the IPM motor  112 , so that a detailed description will be omitted. 
         [0047]    As in  FIG. 1 , air gaps  166 A extending in the axial direction are formed in a rotor  166  of the IPM motor  112 . Permanent magnets  176 A and  176 B are embedded in the air gaps. In the illustrated example, only one of the axial end faces of the rotor  166  toward the brake mechanism  114  is provided with an end plate  172 . The end face away from the brake mechanism is not provided with an end plate. By providing only one of the rotor end faces with an end plate, it is ensured that the leakage magnetic flux from the rotor end face away from the brake mechanism is larger than that of the end face toward the brake mechanism. 
         [0048]    In an alternative example, the thickness of one end plate provided at one rotor end face may be different from that of the other end face so as to create a difference in the amount of leakage magnetic flux from the rotor end faces. In other words, the end plate at the end face toward the brake mechanism may be thicker and the end plate away from the brake mechanism may be thinner. 
         [0049]    In a still alternative example, the material of the end plate provided at one rotor end face may be different from that of the other end face so as to create a difference in the amount of leakage magnetic flux from the rotor end faces. For example, the end plate toward the brake mechanism may be formed of a nonmagnetic material and the end plate away from the brake mechanism may be formed of a magnetic material. 
         [0050]    In order to increase the chance that metal powder is captured more at the end plate away from the brake mechanism, means may be provided to guide the lubricant toward the neighborhood of the end plate. Generally, when a rotor is rotated in the lubricant, a flow that draws the lubricant toward the surface of the rotor is generated due to the viscocity of the lubricant. The phenomenon can be exploited such that the outer surface of the rotor or the inner surface of the stator may be provided with skews to create a flow toward a space away from the brake mechanism when the rotor is rotated in a particular direction. 
         [0051]    Preferably, the particular direction in which the rotor is rotated is a direction more frequently used than the other direction because this will increase the efficiency of capturing metal powder. In the case of a forklift, the particular direction of rotation of the rotor is a direction of rotation corresponding to the forward movement of the forklift. 
         [0052]    Instead of or in addition to rotor or stator skews, means may be provided that positively guides the lubricant. For example, the output shaft  70  may be formed to be hollow and through holes that allow the lubricant to flow in the output shaft may be formed in the neighborhood of the end faces of the rotor. When the rotor with this structure is rotated, the lubricant can be drawn to the hollow portion of the output shaft  70  from the through hole located toward the brake mechanism and discharged from the through hole located away from the brake mechanism. Alternatively, the output shaft  70  may be provided with a fin that generates a flow from the side toward the brake mechanism to the opposite side. 
         [0053]    It has been described that a reducer mechanism of oscillating and internally meshing type is used in the embodiment described above. However, the reducer mechanism according to the invention is not limited to the oscillating and internally meshing type. For example, the reducer may have other 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 comprise multiple shafts or multiple stages. 
         [0054]    It should be understood that the invention is not limited to the above-described embodiment, but may be modified into various forms on the basis of the spirit of the invention. Additionally, the modifications are included in the scope of the invention. 
         [0055]    Priority is claimed to Japanese Patent Application No. 2012-037240, filed Feb. 23, 2012, the entire content of which is incorporated herein by reference.