Abstract:
A hub unit for use in electrically assisted bicycles has an improved effect to radiate heat from a stator. A hub ( 7 ) has inner teeth ( 66   a ) meshable with planetary gears ( 65 ) and a fixing support shaft ( 57 ) projecting from opposite ends of the hub and provided fixedly relative to a motor housing ( 5 ). The motor housing ( 5 ) has no cylindrical wall ( 8 ) covering the outer peripheral surface of the stator ( 3 ) but leaves the stator outer peripheral surface exposed to effectively release the heat generated by the windings of the stator.

Description:
FIELD OF THE INVENTION 
   The present invention relates to hub units for use in electrically movable wheels, and to electrically assisted bicycles, wheelchairs and like vehicles comprising the hub unit. 
   BACKGROUND OF THE INVENTION 
     FIG. 9  shows an electrically movable wheel hub unit  2  for an electrically assisted bicycle. 
   The electrically movable wheel hub unit (hereinafter referred to as the “hub unit”) has a motor  9  comprising a stator  3  and a rotor  4 , a hub  7  enclosing the motor  9  therein and rotatingly drivable at a reduced speed by the motor  9  and a planetary gear reduction mechanism  6 , and fixing support shafts  57 ,  57   a  disposed on the axis of rotation of the hub  7  fixedly relative to a motor housing  5  and projecting outward from the hub  7 . 
   The stator  3  comprises a stack  31  of metal plates and windings  32  provided on the metal plate stack  3 . 
   The motor housing  5  comprises a cylindrical wall  8  and a pair of end plates  51 ,  52  closing opposite end openings of the wall  8 . The end plates  51 ,  52  are fastened to the cylindrical wall  8  with bolts  53   b ,  53   b.    
   The metal plate stack  31  of the stator  3  is fastened to one of the end plates,  51 , with a plurality of bolts  53   d  extending through the outer peripheral portion of the stack. 
   The fixing support shafts  57 ,  57   a  extending through respective opposite ends of the hub  7  are fixed to the front or rear fork of the bicycle, and the spokes (not shown) of the wheel are fitted into spoke attaching holes  74   a  in circumferential walls  74 ,  74  provided around the outer periphery of the hub  7 . 
   When windings  32  of the stator  3  are energized, the rotor  4  rotates. The rotation of the rotor  4  is transmitted to the hub  7  upon a speed reduction by the planetary gear reduction mechanism  6  to rotate the wheel of the bicycle. 
   In the case where a load not smaller than a specified value acts on the pedals of the bicycle when they are stepped on, the windings  32  of the stator  3  are energized, permitting the rider to run with ease even on slopes. 
   See, for example, JP-A No. 2002-514550. 
   The energization of the windings results in heat generation. 
   If the windings  32  are overheated, the rotor  4  can not be rotated with high torque, hence a need for a contrivance for releasing the heat of the windings  32 . 
   However, since the stator  3  is enclosed in the motor housing  5  which is closed, the stator  3  is unable to radiate heat effectively. 
   For this reason, it is practice to enclose a cooling liquid within the hub  7  to cool the motor housing  5  from outside and suppress the heat generation of the windings  32 . 
   When the cooling liquid is enclosed in the hub  7 , means or measure is needed for preventing leakage of the liquid to result in a corresponding increase in cost. The cooling liquid used correspondingly increases the weight of the hub unit. 
   The present invention provides a hub unit of the type described wherein the outer peripheral surface of the stator  3  is left exposed to face the inner surface of the hub  7 , with the cylindrical wall  8 , part of the motor housing  5 , dispensed with so as to cause the stator  3  to release heat more effectively for the prevention of overheating. The invention also provides a vehicle comprising the hub unit. 
   SUMMARY OF THE INVENTION 
   The present invention provides a hub unit  2  comprising a motor  9  composed of a stator  3  and a rotor  4 , a hub  7  enclosing the motor therein and rotatingly drivable by the rotation of the rotor  4 , and a fixing support shaft  57  disposed on an axis of rotation of the hub  7  and projecting from the motor housing  5  fixedly relative thereto, the support shaft  57  projecting outward beyond the hub  7 . 
   The motor housing  5  comprises a pair of end plates  51 ,  52  opposed to each other, with a stack  31  of metal plates of the stator  3  held therebetween, and fastened to each other by tightening up bolts to cause the end plates  51 ,  52  to clamp the metal plate stack  31  therebetween. 
   The stator  3  has an outer peripheral surface left exposed to face an inner surface of the hub  7  between the end plates  51 ,  52 . 
   The motor housing  5  has no cylindrical wall  8  surrounding the outer peripheral surface of the metal plate stack  31  of the stator  3 , so that even in the event of the windings of the stator  3  generating heat, the heat can be effectively released through the stator  3  to the outside of the motor housing  5 , consequently preventing the windings  32  from overheating. 
   This obviates the need to suppress the overheating of the windings  32  by the conventional means, i.e., a cooling liquid enclosed within the hub  7 . 
   Because the cylindrical wall  8  of the motor housing  5  can be dispensed with and further because there is no need for time and labor for enclosing the cooling liquid in the hub, the hub unit can be manufactured at a reduced cost. The hub unit can be obtained also with a weight reduction corresponding to the cylindrical wall  8  and the cooling liquid omitted. 
   The stator  3  can be given an increased outside diameter corresponding to the absence of the cylindrical wall  8 , with the result that increased torque is available. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a front view of an electrically assisted bicycle; 
       FIG. 2  is a sectional view of a hub unit; 
       FIG. 3  is a perspective view of the hub unit with a hub main body removed; 
       FIG. 4  is a perspective view of the hub unit; 
       FIG. 5  is an exploded perspective view of a stack of metal plates; 
       FIG. 6  is a sectional view of a hub unit of second embodiment; 
       FIG. 7  is a perspective view of the hub unit with a hub main body removed; 
       FIG. 8  is a sectional view of a hub unit of third embodiment; 
       FIG. 9  is a sectional view of a conventional hub unit; and 
       FIG. 10  is a perspective view of the hub unit with a hub main body removed. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows an example of electrically assisted bicycle  1  having a hub unit  2  of the present invention. 
   A hub unit  2  is attached to the lower end of a front fork  12  of the bicycle  1 , and the hub unit  2  and the rim  13  of a front wheel  11  are interconnected by spokes  14 ,  14 . 
   A battery  17  is mounted on the bicycle between a seat post  15  and a rear wheel  16 . 
   A sensor (not shown) is provided on a suitable portion on which the pedaling force of the pedals  18  acts for energizing a motor  9  of the hub unit  2  by the battery  17  when the load on the pedals is not smaller than a predetermined value. 
   The hub unit  2 , which basically has the same construction as the conventional example of  FIG. 9 , has the above-mentioned motor  9  which comprises a stator  3  and a rotor  4 , a hub  7  enclosing the motor  9  therein and rotatingly driven by the rotation of the rotor  4 , and a fixing support shaft  57  secured to a motor housing  5 , projecting therefrom in alignment with the axis of rotation of the hub  7  and extending outward from the hub  7 . 
   The stator  3  has a metal plate stack  31  which comprises a specified number of metal plates  31   a  blanked out as shown in  FIG. 5  and as already known. Each of the metal plates  31   a  is in the form of a ring having a plurality of T-shaped projections  31   b  arranged on the inner periphery of the ring at equal intervals circumferentially thereof and projecting toward the center of the ring. The metal plate  31   a  has twelve projections  31   b  according to the present embodiment. 
   A winding  32  is provided on a stack of projections  31   b  in phase as a unit. 
   Such a metal plate stack  31  is made conventionally by blanking out metal plates  31   a  from a metal sheet material with a blanking die and stacking the plates as blanked out at the same time. 
   For this reason, burrs created on respective adjacent pairs of blanked-out metal plates  31   a ,  31   a  are located at the same position or in alignment. 
   If burrs are formed on each single blanked-out metal plate  31   a  uniformly circumferentially thereof, the metal plate stack  31  will have a uniform thickness circumferentially thereof. 
   However, this is almost unlikely in actuality; when a single blanked-out metal plate  31   a  is observed, burrs are not positioned uniformly with respect to the circumferential direction. When metal plates  31   a  which are blanked out in succession are merely stacked in superposed layers, the single metal plate stack  31  will have a difference of about 0.1 to 0.3 mm in thickness and has a portion of large thickness and a portion of small thickness. 
   For the reason to be given later, there is a need to reduce the likelihood that the metal plate stack  31  will have a portion of large thickness and a portion of small thickness. Accordingly, the stack  31  is divided into units  31   c  each comprising a number of blanked-out metal plates  31   a , and the units  31   c  are stacked as displaced from one another through a predetermined angle. 
   According to the present embodiment, the units  31   c  are stacked as displaced from one another through an angle obtained by dividing 360 deg by the number of units  31   c  to make the metal plate stack  31 . For example, in the case where the number of units  31   c  is three, the three units  31   c  are displaced from one another by 120 deg, and a wire is wound around each of the projection stacks  31   b  to form the winding  32 . 
   The metal plate stack  31  can be made easily by blanking out metal plates  31   a  from a metal sheet material, stacking a specified number of blanked-out metal plates  31   a  upon blanking, rotating the metal sheet material through a predetermined angle, e.g., through 120 deg according to the present embodiment, within a horizontal plane every time the specified number of metal plates are blanked out. 
   Incidentally, the number of inward projections  31   b  on the metal plate  31   b  blanked out is limited to a number which can be divided by the number of units  31   c.    
   The rotor  4  is rotatably disposed inside the stator  3  concentrically therewith. 
   The rotor  4  is made by inserting a rotating shaft  41  through the center of a stack  42  of circular metal plates so as to render the shaft  41  rotatable with the metal plate stack  42 . 
   A plurality of permanent magnets  43  are embedded in the outer peripheral portion of the metal plate stack  42  in parallel to the rotating shaft  41  at equal intervals with S poles and N poles arranged alternately circumferentially of the stack. 
   The housing  5  for the motor  9  is made by holding the metal plate stack  31  of the stator  3  between a pair of first and second end plates  51 ,  52  opposed to each other axially of the stator  3  and fastening the two end plates  51 ,  52  together with a plurality of bolts  53  inserted through the outer peripheral portion of the stack  31  and tightened up. 
   The metal plate stack  31  of the stator  3  serves the function of fixedly positioning the first end plate  51  and the second end plate  52  as spaced apart by a given distance. Thus, the metal plate stack  31  serves also the function of a cylindrical wall  8  of the motor housing  5  in the conventional hub unit shown in  FIG. 9 . 
   The first end plate  51  and the second end plate  52  are provided with respective bearings  58 ,  58  inside the center portions thereof for the bearings  58 ,  58  to support opposite ends of the rotating shaft  41  of the rotor  4 . 
   The first end plate  51  and the second end plate  52  are provided at their outer peripheral edges with short inward cylinder portions  54 ,  55 , respectively. The short cylinder portions  54 ,  55  have at their inner ends respective stepped portions  54   a ,  55   a  having fitted therein opposite-end outer peripheral edges of the metal plate stack  31 . 
   The first end plate  51  has a boss  56  on the outer side of the center portion thereof. Extending through the boss  56  is a fixing support shaft  57  projecting outward. The shaft  57  is externally threaded as at  57   b.    
   Provided externally of the second end plate  52  is a reduction mechanism for transmitting the rotation of the rotor  4  to the hub  7  upon a speed reduction. 
   The reduction mechanism of the present embodiment is a planetary gear reduction mechanism  6 . 
   The reduction mechanism  6  has a planetary gear support frame  61  provided with three tubular legs  62  projecting toward and bearing against the second end plate  52  and equidistantly spaced circumferentially of the frame. The frame  61  is fastened to the second end plate  52  with bolts  63  extending through the respective legs  62 . 
   Extending between and supported by the second end plate  52  and the planetary gear support frame  61  are three support shafts  64  equidistantly spaced circumferentially of the second end plate  52  and each adapted to rotatably support a planetary gear  65  thereon. 
   The planetary gear  65  comprises a large gear  65   a  positioned closer to the second end plate  52  and a small gear  65   b  positioned on the opposite side and coaxial with the gear  65   a.    
   The large gear  65   a  of the planetary gear  65  is in mesh with a sun gear  60  formed on an end portion of the rotating shaft  41  of the rotor  4 . The small gear  65   b  of the planetary gear  65  is in mesh with inner teeth  66   a  on the closure plate  72  of the hub  7  to be described later. 
   The planetary gear support frame  61  has a boss  67  on the inner side of the center portion thereof. Extending through the center of the boss  67  is a fixing support shaft  57   a  projecting outward. The shaft  57   a  is externally threaded as at  57   b.    
   The hub  7  comprises a hub main body  71  generally in the form of a cup which is open at the center of its bottom wall and open over the entire area of the other side, and the above-mentioned closure plate  72  covering the entire opening. 
   The hub main body  71  covers the motor housing  5  and the planetary gear reduction mechanism  6 . A bearing  73  is provided in the bottom opening  70  around the boss  56  on the first end plate  51  of the motor housing  5 . 
   The closure plate  72  covers the entire opening of the hub main body  71  so as to conceal the support frame  61  of the reduction mechanism  6  and is fastened to the hub main body  71  with bolts  76 . 
   Fastened with screws to the inner side of the closure plate  72  is an annular member  66  concentric with the fixing support shaft  57 . The inner teeth  66   a  meshable with the planetary gears  65  are formed on the inner periphery of the annular member  66 . 
   The fixing support shaft  57   a  on the support frame  61  extends through the closure plate  72  to the outside. A bearing  75  is provided in the closure plate  72  around the fixing support shaft  57   a.    
   The hub main body  71  has a circumferential wall  74  formed on its outer peripheral surface and extending therearound at each of opposite side portions. Spoke attaching holes  74   a  are formed in the circumferential wall  74  and arranged at regular intervals circumferentially of the wall. 
   With reference to  FIG. 2 , the rotating shaft  41  of the rotor  4 , the fixing support shaft  57  on the first end plate  51  of the motor housing  5 , and the fixing support shaft  57   a  extending through the closure plate  72  of the hub  7  are arranged in alignment. 
   When both the fixing support shafts  57 ,  57   a  are secured to the front fork  12  of the bicycle by screw-thread engagement, the portions marked with lines slanting leftwardly downward in  FIG. 2  are fixed portions, and the portions marked with lines slanting rightwardly downward are rotating portions. 
   When a load in excess of a specified value acts on the bicycle pedals  18  upon the pedals being stepped on, the battery  17  energizes the windings  32  of the stator  3 , rotating the rotor  4 . 
   The rotation of the rotor  4  rotates the sun gear  60  on the end of the rotating shaft  41  to rotate the planetary gears  65  in position. The rotation of the planetary gears  65  is transmitted through the inner teeth  66   a  to the hub  7  on a speed reduction, drivingly rotating the front wheel  11 . 
   By virtue of the rotation of the rotor  4 , the bicycle  1  can be run with a reduced stepping force on the pedals  11 . 
   The energization of the windings  32  causes the windings  32  to generate heat, which is given to the stator  3 . However, the motor housing  5  does not have a cylindrical wall which prevents radiation of heat, so that the heat given to the stator  3  is released directly through an opening between the pair of end plates  51 ,  52  holding the stator  3  therebetween, whereby the windings  32  can be prevented from overheating without necessitating the cooling liquid conventionally enclosed in the hub  7 . 
   Because the cylindrical wall  8  of the motor housing  5  can be dispensed with and further because there is no need for time and labor for enclosing the cooling liquid in the hub, the hub unit can be manufactured at a reduced cost. The hub unit can be obtained also with a weight reduction corresponding to the cylindrical wall  8  and the cooling liquid omitted. 
   The stator  3  can be given an increased outside diameter corresponding to the absence of the cylindrical wall  8 , with the result that increased torque is available. 
   If the metal plate stack  31  of the stator  3  has varying thicknesses circumferentially thereof, the parallelism between the end plates  51 ,  52  involves errors, which produce a bending force acting on the rotating shaft  41  of the rotor  4  supported by bearings  58 ,  58  on the opposed end plates  51 ,  52 , giving rise to the problem of causing trouble to the smooth rotation of the rotor  4 . For this reason, an inherent problem will arise when the metal plate stack  31  of the stator  3  is held between the pair of end plates  51 ,  52  and if the end plates  51 ,  52  are fastened together by tightening up the bolts  53  so as to cause the thrust of screws to act on the stack  31  in the direction of stacking as practiced in the present embodiment. 
   According to the embodiment, however, a plurality of units  31   c  each comprising a specified number of metal plates  31   a  blanked out are stacked as displaced from one another by a predetermined angle so as to render the metal plate stack  31  uniform in thickness circumferentially thereof to the greatest possible extent. Even if the end plates  51 ,  52  are fastened by tightening up the bolts  53  so as to causes the thrust of screws to act on the metal plate stack  31  in the stacking direction, the first and second end plates  51 ,  52  can therefore be given parallelism with high accuracy. 
   Consequently, the rotor  4  can be rotated smoothly by giving improved parallelism to the bearings  58 ,  58  for supporting the opposite ends of the rotating shaft  41  of the rotor  4 . 
     FIG. 6  shows a second embodiment of hub unit  2 , and  FIG. 7  is a perspective view of the same with a hub main body removed. 
   This embodiment has the same construction as the hub unit  2  shown in  FIG. 2  except the motor housing  5  and the means for attaching the metal plate stack  31  of the stator  3  to the motor housing. Accordingly, the same description will not be given repeatedly. 
   The motor housing  5  comprises a first end plate  51 , second end plate  52 , a plurality of spacer rods  50  interposed between the two end plates  51 ,  52  for determining the distance therebetween, and fastening bolts  53   b ,  53   b  inserted through the end plate  51  or  52  and screwed into the spacer rods  50 . 
   The spacer rods  50  have a length slightly larger than the thickness of the metal plate stack  31  of the stator  3 . 
   The metal plate stack  31  is provided in its outer peripheral portion with grooves  33  for the respective spacer rods to fit in. 
   The stack  31  is fastened to the first end plate  51  or the second end plate  52  with bolts  53   a  extending through the outer peripheral portion of the stack  31 . 
   The metal plate stack  31  does not serve also as a component of the motor housing  5  unlike the stack  31  of the hub unit  2  shown in  FIG. 2 . 
   In the case of the embodiment of  FIG. 6 , therefore, no bending force will act on the rotating shaft  41  of the rotor  4  even if the stack  31  has varying thicknesses circumferentially thereof. 
     FIG. 8  shows a third embodiment of hub unit  2 . This embodiment has the same construction as the hub unit  2  shown in  FIG. 2  except that a single fixing support shaft  57  extends through a hub  7  and that a rotor  4  is provided on a tubular shaft  44  rotatably fitting around the support shaft  57 . 
   When the fixing support rods  57 ,  57   a  extend through respective opposite ends of the hub  7  to project outward therefrom and are each independent of the rotating shaft  41  of the rotor  4  as in the hub unit  2  shown in  FIG. 2 , the hub unit has a complex construction. Furthermore, errors involved in machining or assembling the components are likely to cause the deflection of the axes of the support shafts  57 ,  57   a  at the opposite ends, whereas the third embodiment can be free of these problems. 
   The hub unit  2  of the present invention, which is useful for electrically assisted bicycles, can be embodied alternatively for hubs for vehicles having wheels, for example, for wheelchairs. 
   The individual fixing support shafts  57 ,  57   a  of the first and second embodiments described are caused to extend outward from respective opposite ends of the hub  7 , and the single fixing support rod  57  of the third embodiment has opposite ends projecting from respective opposite ends of the hub  7 , so that the hub unit  2  can be supported at opposite ends, whereas when the hub unit  2  is to be supported at only one end thereof as in wheelchairs, one end of the fixing support shafts is omitted. 
   Apparently, the present invention can be modified and altered by one skilled in the art without departing from the spirit of the invention, and such modification is included within the scope of the invention as set forth in the appended claims.