Patent Publication Number: US-9903442-B2

Title: Driving device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. National Stage of PCT/JP2014/054750, filed Feb. 26, 2014, which in turn claims priority to Japanese Patent Application No. JP 2013-047081, filed Mar. 8, 2013. The contents of these applications are incorporated herein by reference in their entireties. 
     TECHNICAL FIELD 
     The present specification discloses a technique related to a driving device. In particular, the present specification discloses a technique related to a driving device which rotates a rotated member while doubly supporting the rotated member at its both ends. 
     BACKGROUND ART 
     A driving device is known in which rotates a rotated member while doubly supporting the rotated member at its both ends. An example of this type of driving device is disclosed in Japanese Utility Model Application Publication No. H01-175135 (called Patent Document 1 below). The driving device of Patent Document 1 is provided with a first frame extending in one direction along a rotation axis of the rotated member, and a second frame extending in another direction along the rotation axis. A supporting member for rotatably supporting the rotated member is fixed to each of the first frame and the second frame. Each supporting member includes a case, and a shaft supported rotatably by the case. The case is fixed to the frame (the first frame, the second frame), and the shaft is fixed to the rotated member. A pair of bearings is disposed between the case and the shaft. 
     SUMMARY OF INVENTION 
     In a case of supporting a shaft rotatably by a case, it is technical common sense to dispose a pair of bearings between the case and the shaft in order to support the shaft stably. Consequently, in Patent Document 1, the pair of bearings is disposed between the case and the shaft in each supporting member. However, in a case where the rotated member is doubly supported at its both ends, an assembly error may occur when fixing the supporting members to the rotated member. When such an assembly error occurs, a force displacing relative positions of the case and the shaft is applied between the case and the shaft. However, since the relative positions of the case and the shaft are fixed when the pair of bearings is disposed between the case and the shaft, an excessive force is exerted on the bearings. Consequently, when the supporting members are to be fixed to both sides of the rotated member, an assembling operation must be performed with high accuracy. The present specification provides a novel driving device which solves the above problem. 
     The driving device taught in the present specification rotates a rotated member while doubly supporting the rotated member at its both ends. The driving device comprises a fixing member, a first supporting member and a second supporting member. The fixing member comprises a first frame extending in one direction along a rotation axis of a rotated member, and a second frame extending in another direction along the rotation axis of the rotated member. The first supporting member is attached to the first frame. The first supporting member is disposed between the first frame and the rotated member. The first supporting member comprises a first case, a first supporting shaft, and a first bearing. The first case is provided with a first through hole, and is fixed to one of the first frame and the rotated member. The first supporting shaft passes through the first through hole, and is fixed to the other of the first frame and the rotated member. The first bearing is disposed between the first case and the first supporting shaft. The second supporting member is attached to the second frame. The second supporting member is disposed between the second frame and the rotated member. The second supporting member comprises a second case, a second supporting shaft, and a second bearing. The second case is provided with a second through hole, and is fixed to one of the second frame and the rotated member. The second supporting shaft passes through the second through hole, and is fixed to the other of the second frame and the rotated member. The second bearing is disposed between the second case and the second supporting shaft. In this driving device, only one first bearing is disposed between the first case and the first supporting shaft, and only one second bearing is disposed between the second case and the second supporting shaft. 
     In the case of the aforementioned driving device, the first case may be fixed to the first frame, and the first supporting shaft may be fixed to the rotated member. Alternatively, the first case may be fixed to the rotated member, and the first supporting shaft may be fixed to the first frame. Similarly, the second case may be fixed to the second frame, and the second supporting shaft may be fixed to the rotated member. Alternatively, the second case may be fixed to the rotated member, and the second supporting shaft may be fixed to the second frame. 
     According to the aforementioned driving device, only the one first bearing is disposed between the first case and the first supporting shaft, and therefore even if an assembly error occurred when fixing the first supporting member to the rotated member, the relative positions of the first supporting shaft and the first case can change, with the first bearing as a fulcrum. Similarly, even if an assembly error occurred when fixing the second supporting member to the rotated member, the relative positions of the second supporting shaft and the second case can change, with the second bearing as a fulcrum. Consequently, it is possible to suppress excessive force being exerted on the bearings (the first bearing and the second bearing). Moreover, when the first supporting member and the second supporting member are attached to the fixed member, a configuration is completed in which both sides of the fixed member are supported by the pair of bearings. Consequently, the rotated member is stably supported by the fixing member of the driving device by the first supporting member and the second supporting member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a cross-sectional view of a state in which a fixed member has been attached to a driving device of a first embodiment; 
         FIG. 2  shows an enlarged cross-sectional view of a portion surrounded by a broken line II of  FIG. 1 ; 
         FIG. 3  shows an enlarged cross-sectional view of a portion surrounded by a broken line III of  FIG. 2 ; 
         FIG. 4  shows a cross-sectional view of a state in which a fixed member has been attached to a driving device of a second embodiment; 
         FIG. 5  shows an enlarged cross-sectional view of a portion surrounded by a broken line V of  FIG. 4 ; 
         FIG. 6  shows an enlarged cross-sectional view of a portion surrounded by a broken line VI of  FIG. 5 ; 
         FIG. 7  shows a cross-sectional view of a state in which a fixed member has been attached to a driving device of a third embodiment; 
         FIG. 8  shows an enlarged cross-sectional view of a portion surrounded by a broken line VIII of  FIG. 7 ; and 
         FIG. 9  shows an enlarged cross-sectional view of a portion surrounded by a broken line IX of  FIG. 8 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Below, several technical features of the embodiments taught in the present specification will be noted. Moreover, each of the items noted below solely have technical usefulness. 
     The first supporting member may comprise a reduction gear structure of the type in which a first external gear rotates eccentrically relative to a first internal gear. An example of this type of reduction gear structure is a reduction gear comprising a crankshaft, an eccentric rotary gear, and a rotation gear. The crankshaft extends along a rotation axis of an output unit of the reduction gear, and may comprise an eccentric body. The eccentric rotary gear engages with the eccentric body, and may rotate eccentrically with the rotation of the crankshaft. The rotation gear meshes with the eccentric rotary gear, and may have a number of teeth different from a number of teeth of the eccentric rotary gear. The rotation gear may be disposed coaxially with the rotation axis of the output unit of the reduction gear. 
     In a case where the first supporting member comprises the reduction gear structure comprising the first external gear, the first internal gear, and a first crankshaft, the first crankshaft may be supported by the first supporting shaft. The first eccentric body may be provided on the first crankshaft. Further, the first internal gear may be provided at a wall surface of a first through hole of a first case. The first external gear may engage with the first eccentric body, and may mesh with the first internal gear. 
     A second supporting member, like the first supporting member, may comprise a reduction gear structure of the type in which a second external gear rotates eccentrically relative to a second internal gear. Further, the second supporting member may comprise a reduction gear structure comprising a second external gear, a second internal gear, and a second crankshaft. In this reduction gear structure, also, the second crankshaft may be supported by a second supporting shaft. The second eccentric body may be provided on the second crankshaft. Further, the second internal gear may be provided at a wall surface of a second through hole of a second case. The second external gear may engage with the second eccentric body, and may mesh with the second internal gear. 
     In a case where both the first supporting member and the second supporting member comprise a reduction gear structure of the type in which the external gear rotates eccentrically relative to the internal gear, each of the first crankshaft and the second crankshaft may comprise two or more eccentric bodies. As an example, a third eccentric body which is symmetrically offset from the first eccentric body with respect to a first rotation axis of the first crankshaft may be provided on a first frame side than the first eccentric body of the first crankshaft. The third external gear may engage with the third eccentric body. Further, a fourth eccentric body which is symmetrically offset from the second eccentric body with respect to a second rotation axis of the second crankshaft may be provided on a second frame side than the second eccentric body of the second crankshaft. The fourth external gear may engage with the fourth eccentric body. When viewed along a rotation axis direction of the rotated member, the first external gear and the second external gear may be offset in the same direction with respect to the rotation axis of the rotated member. Moreover, in a case of this driving device, the first rotation axis and the second rotation axis may be coaxial, and the first eccentric body and the second eccentric body may be eccentric in the same direction with respect to the first rotation axis. 
     In the case where both the first supporting member and the second supporting member comprise the reduction gear structure of the type in which the external gear rotates eccentrically relative to the internal gear, the first supporting member and the second supporting member may comprise only one external gear. That is, the first external gear may be the only external gear provided to the first supporting member, and the second external gear may be the only external gear provided to the second supporting member. When viewed along the rotation axis direction of the rotated member, the first external gear and the second external gear may be symmetrically offset with respect to the rotation axis of the rotated member. In the case of this driving device, the first rotation axis of the first crankshaft and the second rotation axis of the second crankshaft may be coaxial, and the first eccentric body and the second eccentric body may be symmetrically offset with respect to the first rotation axis. 
     The first supporting shaft may be fixed to the first frame, the first case and the second case may be fixed to the rotated member, and the second supporting shaft may be fixed to the second frame. 
     EMBODIMENTS 
     First Embodiment 
       FIG. 1  shows a state in which a machine tool  120  has been attached to a driving device  100 . The machine tool  120  is an example of a rotated member. The driving device  100  is fixed to an output member  110  which rotates relative to a base shaft  104 . An entirety of the driving device  100  rotates about a rotation axis  106  relative to the base shaft  104 . The driving device  100  rotates the machine tool  120  about a rotation axis  160 . Moreover, the machine tool  120  comprises a body  122 , and a head unit  124  which rotates relative to the body  122 . A tool (not shown) is attached to the head unit  124 . 
     The driving device  100  comprises a fixing member  102 , a first reduction gear  30   a , and a second reduction gear  30   b . The fixing member  102  performs a position fixing of the machine tool  120  in the rotation axis  106  direction. The fixing member  102  comprises a first frame  102   a , a second frame  102   b , and a coupling frame  102   c . The first frame  102   a  extends in one direction along the rotation axis  160  of the machine tool  120 . The second frame  102   b  extends in the other direction along the rotation axis  160  of the machine tool  120 . That is, the second frame  102   b  extends in the opposite direction to the first frame  102   a  relative to the machine tool  120 . The coupling frame  102   c  couples the first frame  102   a  and the second frame  102   b , and is fixed to the output member  110 . The machine tool  120  is disposed between the first frame  102   a  and the second frame  102   b . The first reduction gear  30   a  is fixed to the first frame  102   a , and the second reduction gear  30   b  is fixed to the second frame  102   b . The first reduction gear  30   a  is an example of a first supporting member, and the second reduction gear  30   b  is an example of a second supporting member. 
     The first reduction gear  30   a  is disposed between the first frame  102   a  and the machine tool  120 . The first reduction gear  30   a  comprises a first case  62   a  and a first supporting shaft  56   a . The first supporting shaft  56   a  is supported rotatably by the first case  62   a . The first case  62   a  is fixed to the body  122  of the machine tool  120 , and the first supporting shaft  56   a  is fixed to the first frame  102   a . The second reduction gear  30   b  is disposed between the second frame  102   b  and the machine tool  120 . The second reduction gear  30   b  comprises a second case  62   b  and a second supporting shaft  56   b . The second case  62   b  is fixed to the body  122  of the machine tool  120 , and the second supporting shaft  56   b  is fixed to the second frame  102   b . The second supporting shaft  56   b  is supported rotatably by the second case  62   b . The rotation axis  160  corresponds to a rotation axis of output units (the cases  62   a ,  62   b ) of the first reduction gear  30   a  and the second reduction gear  30   b . Consequently, the machine tool  120  rotates about the rotation axis  160  with respect to the fixing member  102 . 
     A first motor case  132   a  is fixed to the first frame  102   a . A motor (not shown) for driving the first reduction gear  30   a  is housed within the first motor case  132   a . A first cover  133   a  is fixed to the first frame  102   a . The first cover  133   a  prevents the first motor case  132   a  from being exposed to the exterior of the driving device  100 . A second motor case  132   b  is fixed to the second frame  102   b . A motor for driving the second reduction gear  30   b  is housed within the second motor case  132   b . A second cover  133   b  is fixed to the second frame  102   b . The second cover  133   b  prevents the second motor case  132   b  from being exposed to the exterior of the driving device  100 . 
     The driving device  100  will be described in detail with reference to  FIG. 2 . Moreover, the first reduction gear  30   a  and the second reduction gear  30   b  comprise substantially identical configurations. Below, the first reduction gear  30   a  will be described in detail. Components of the second reduction gear  30   b  will be denoted by same reference numbers as in the first reduction gear  30   a , or by reference numbers having the same lower two digits, and a description thereof may be omitted. 
     The first case  62   a  is fixed to the body  122  of the machine tool  120  via a first connecting member  140   a . The first case  62   a  comprises a first through hole  58   a . The first supporting shaft  56   a  passes through the first through hole  58   a . The first supporting shaft  56   a  is fixed to the first frame  102   a  (the fixing member  102 ) via the first motor case  132   a . A first bearing  64   a  is disposed between the first case  62   a  and the first supporting shaft  56   a . The first bearing  64   a  is an angular contact ball bearing. 
     The first reduction gear  30   a  comprises a first internal gear  6   a , the first supporting shaft  56   a , a first crankshaft  12   a , and two external gears (first external gear  8   a , third external gear  4   a ). The first internal gear  6   a  is provided at a wall surface of the first through hole  58   a  of the first case  62   a . The rotation axis  160  corresponds to rotation axes of the first internal gear  6   a  and the first supporting shaft  56   a . The first crankshaft  12   a  is supported by the first supporting shaft  56   a . A rotation axis  20   a  is a rotation axis of the first crankshaft  12   a  (may be called first rotation axis  20   a  below). The first rotation axis  20   a  is parallel to the rotation axis  160 . That is, the first crankshaft  12   a  extends parallel to the rotation axis  160  at a position offset from the rotation axis  160 . Moreover, the first reduction gear  30   a  comprises three first crankshafts  12   a . Each of the first crankshafts  12   a  is disposed to be equally spaced from each other around the rotation axis  160 . 
     Each first crankshaft  12   a  comprises two eccentric bodies (first eccentric body  10   a , third eccentric body  2   a ). In a first rotation axis  20   a  direction, the first eccentric body  10   a  is disposed on the machine tool  120  side than the third eccentric body  2   a . The first eccentric body  10   a  and the third eccentric body  2   a  are eccentric symmetrically with respect to the first rotation axis  20   a . The first external gear  8   a  engages with the first eccentric body  10   a , and the third external gear  4   a  engages with the third eccentric body  2   a . In the rotation axis  160  direction, the first external gear  8   a  is disposed on the machine tool  120  side than the third external gear  4   a . A number of teeth of the first external gear  8   a  and that of the third external gear  4   a  is identical. In the rotation axis  160  direction, the first bearing  64   a  is disposed on the first motor case  132   a  side (the first frame  102   a  side) than the first external gear  8   a  and the third external gear  4   a . Only one first bearing  64   a  is disposed between the first case  62   a  and the first supporting shaft  56   a.    
     The second case  62   b  is fixed to the body  122  of the machine tool  120  via a second connecting member  140   b . The second case  62   b  comprises a second through hole  58   b . The second supporting shaft  56   b  passes through the second through hole  58   b . The second supporting shaft  56   b  is fixed to the second frame  102   b  via the second motor case  132   b . A second bearing  64   b  is disposed between the second case  62   b  and the second supporting shaft  56   b . The second bearing  64   b  is an angular contact ball bearing. 
     The second reduction gear  30   b  comprises a second internal gear  6   b , the second supporting shaft  56   b , second crankshafts  12   b  and two external gears (second external gear  8   b , fourth external gear  4   b ). The rotation axis  160  also corresponds to rotation axes of the second internal gear  6   b  and the second supporting shaft  56   b . A rotation axis  20   b  is a rotation axis of each second crankshaft  12   b  (may be called second rotation axis  20   b  below). The second rotation axis  20   b  is coaxial with the first rotation axis  20   a . That is, each second crankshaft  12   b  is coaxial with the first crankshaft  12   a . Each second crankshaft  12   b  also extends parallel to the rotation axis  160 . 
     Each second crankshaft  12   b  comprises two eccentric bodies (second eccentric body  10   b , fourth eccentric body  2   b ). In the second rotation axis  20   b  direction, the second eccentric body  10   b  is disposed on the machine tool  120  side than the fourth eccentric body  2   b . The second eccentric body  10   b  and the fourth eccentric body  2   b  are offset symmetrically with respect to the second rotation axis  20   b . The second external gear  8   b  engages with the second eccentric body  10   b , and the fourth external gear  4   b  engages with the fourth eccentric body  2   b . In the rotation axis  160  direction, the second external gear  8   b  is disposed on the machine tool  120  side than the fourth external gear  4   b . In the rotation axis  160  direction, the second bearing  64   b  is disposed on the second motor case  132   a  side (the second frame  102   b  side) than the second external gear  8   b  and the fourth external gear  4   b . Only one second bearing  64   b  is disposed between the second case  62   b  and the second supporting shaft  56   b.    
     The configuration of the first reduction gear  30   a  will be described in more detail with reference to  FIG. 3 . As described above, the first reduction gear  30   a  and the second reduction gear  30   b  comprise substantially identical configurations. Consequently, only the configuration of the first reduction gear  30   a  will be described, and a description of the configuration of the second reduction gear  30   b  will be omitted. 
     The first internal gear  6   a  is structured by aligning inner teeth pins  5   a  along a circumferential direction on a wall surface of the first through hole  58   a . In the rotation axis  160  direction, the first internal gear  6   a  is provided at a center of the first case  62   a . The first supporting shaft  56   a  comprises a first plate  50   a  and a second plate  54   a . The second plate  54   a  comprises a columnar portion  52   a . The columnar portion  52   a  extends toward the first plate  50   a  from the second plate  54   a . The columnar portion  52   a  is fixed to the first plate  50   a . In the rotation axis  160  direction, the first plate  50   a  is positioned at the machine tool  120  side, and the second plate  54   a  is positioned at the motor case  132   a  side (the first frame  102   a  side). The first bearing  64   a  is disposed between the second plate  54   a  and the first case  62   a . A bearing is not disposed between the first plate  50   a  and the first case  62   a . That is, the first supporting shaft  56   a  is supported by the first case  62   a  via the one first bearing  64   a  at a position away from the machine tool  120 . 
     Each first crankshaft  12   a  is supported by the first supporting shaft  56   a  via a pair of bearings  14   a . The pair of bearings  14   a  is tapered roller bearings. Movement of the first crankshaft  12   a  in an axial direction and in a radial direction with respect to the first supporting shaft  56   a  is regulated by the pair of bearings  14   a . The first eccentric body  10   a  is positioned at the machine tool  120  side, and the third eccentric body  2   a  is positioned at the motor case  132   a  side (the first frame  102   a  side). 
     A first center through hole  34   a , a first supporting shaft through hole  36   a , and a first crankshaft through hole  24   a  are provided in the first external gear  8   a . A third center through hole  32   a , a third supporting shaft through hole  38   a , and a third crankshaft through hole  26   a  are provided in the third external gear  4   a . A cylindrical member  28   a  passes through the first center through hole  34   a  and the third center through hole  32   a . The cylindrical member  28   a  is fixed to the first plate  50   a  and the second plate  54   a . The columnar portion  52   a  passes through the first supporting shaft through hole  36   a  and the third supporting shaft through hole  38   a . A clearance is provided between the columnar portion  52   a  and the first supporting shaft through hole  36   a , and between the columnar portion  52   a  and the third supporting shaft through hole  38   a . The first eccentric body  10   a  engages with the first crankshaft through hole  24   a  via a cylindrical roller bearing  22   a . The third eccentric body  2   a  engages with the third crankshaft through hole  26   a  via a cylindrical roller bearing  18   a.    
     An oil seal  40   a  is disposed between the first case  62   a  and the first plate  50   a , an oil seal  7   a  is disposed between the first case  62   a  and the second plate  54   a , an oil seal  16   a  is disposed between the first plate  52   a  and the first crankshaft  12   a , and an oil seal  9   a  is disposed between the second plate  54   a  and the first crankshaft  12   a . Lubricant (oil) within the first reduction gear  30   a  is prevented from leaking to the exterior of the first reduction gear  30   a  by the oil seals  40   a ,  7   a ,  16   a  and  9   a.    
     A motor (not shown) for driving the first reduction gear  30   a  is housed within the first motor case  132   a . Torque of the motor is transmitted to the first crankshafts  12   a . When the first crankshafts  12   a  rotate, the first eccentric body  10   a  and the third eccentric body  2   a  rotate eccentrically around the first rotation axis  20   a . The first external gear  8   a  and the third external gear  4   a  rotate eccentrically with the eccentric rotation of the eccentric bodies  10   a ,  2   a  while meshing with the first internal gear  6   a . An offset directions of the first external gear  8   a  and the third external gear  4   a  are symmetrical relative to the rotation axis  160 . 
     A number of teeth of the first internal gear  6   a  and the numbers of teeth of the external gears  8   a ,  4   a  are different. Consequently, when the external gears  8   a ,  4   a  rotate eccentrically, the external gears  8   a ,  4   a  rotate relative to the first internal gear  6   a . The external gears  8   a ,  4   a  can also be called eccentric rotary gears of the first reduction gear  30   a . The first internal gear  6   a  can also be called a rotation gear of the first reduction gear  30   a . The external gears  8   a ,  4   a  are supported by the first supporting shaft  56   a  via the first crankshaft  12   a . Therefore, when the external gears  8   a ,  4   a  rotate eccentrically, the first supporting shaft  56   a  rotates relative to the first internal gear  6   a . The first internal gear  6   a  (the first case  62   a ) is fixed to the body  122  of the machine tool  120  via the connecting member  140   a . Further, the first supporting shaft  56   a  is fixed to the fixing member  102  (the first frame  102   a ) via the first motor case  132   a . Consequently, when the external gears  8   a ,  4   a  rotate eccentrically, the machine tool  120  rotates around the rotation axis  160  with respect to the fixing member  102 . The driving of the first reduction gear  30   a  and the driving of the second reduction gear  30   b  are performed simultaneously. Consequently, the second reduction gear  30   b  also performs the same movement as the first reduction gear  30   a . Moreover, the external gears  8   b ,  4   b  can also be called eccentric rotary gears of the second reduction gear  30   b . The second internal gear  6   b  can also be called a rotation gear of the second reduction gear  30   b . The driving device  100  supports the machine tool  120  at both ends, and can tilt the position of a tool (not shown) attached to the head unit  124 . 
     The characteristics of the driving device  100  will be described. As described above, the first case  62   a  and the second case  62   b  are fixed to the machine tool  120 . Further, the first supporting shaft  56   a  and the second supporting shaft  56   b  are fixed to the fixing member  102 . When the machine tool  120  is attached to the driving device  100 , the first case  62   a , the second case  62   b  and the machine tool  120  are integrated, and the first supporting shaft  56   a , the second supporting shaft  56   b  and the fixing member  102  are integrated. The integrated body, which is the first case  62   a , the second case  62   b  and the machine tool  120 , is supported by the pair of bearings (the first bearing  64   a  and the second bearing  64   b ) with respect to the integrated body which is the first supporting shaft  56   a , the second supporting shaft  56   b  and the fixing member  102 . The machine tool  120  can be supported at both ends with respect to the fixing member  102 . Consequently, it is possible to control the position of the machine tool  120  around the rotation axis  160  with high precision. 
     Moreover, in a conventional driving device, a pair of bearings is disposed between the case and the shaft for each reduction gear. Since the shaft is doubly supported at both ends by the case, the relative position of the shaft and the case in each reduction gear is stable. However, in the case where one rotated member (e.g., the machine tool) is doubly supported at its both ends by two reduction gears, a force which causes a relative positional shift to occur between the shaft and the case is applied when an assembly error occurs upon attaching the reduction gears to the rotated member. As a result, a force may be applied to the pair of bearings disposed between the shaft and the case, and a life of the bearings may be reduced. In order to avoid this type of inconvenience, in the conventional driving device, the two reduction gears had been required to be attached to the rotated member with high precision. 
     In the driving device taught in the present specification, only one bearing is disposed between the shaft and the case in each of the reduction gears. Consequently, even if the assembly error occurs upon attaching the two reduction gears to the machine tool, the positions of the shaft and the case can shift relatively. As a result, it is possible to suppress the excessive force being applied to the bearing. That is, in the technique disclosed in the present specification, the shaft is intentionally not supported firmly by the case in each of the reduction gears. Thereby, it is possible to realize a highly durable driving device (having a long life). 
     Other characteristics of the driving device  100  will be described. As described above, the first rotation axis  20   a  of each first crankshaft  12   a , and the second rotation axis  20   b  of each second crankshaft  12   b  are coaxial. When viewed along one direction of the first rotation axis  20   a , offset directions of the first eccentric body  10   a  and the second eccentric body  10   b  are the same. Consequently, the movement of the eccentric rotation of the first external gear  8   a , and the movement of the eccentric rotation of the second external gear  8   b  is equal at both sides of the machine tool  120 . As a result, when driving the driving device  100 , the direction of force applied to the first internal gear  6   a  from the first external gear  8   a  is equal to the direction of force applied to the second internal gear  6   b  from the second external gear  8   b . It is possible to suppress vibration of the machine tool  120 . Moreover, when viewed along the one direction of the first rotation axis  20   a , offset directions of the third eccentric body  2   a  and the fourth eccentric body  2   b  are the same. The movement of the eccentric rotation of the third external gear  4   a , and the movement of the eccentric rotation of the fourth external gear  4   b  are equal at both sides of the machine tool  120 . 
     Further, as described above, in the first reduction gear  30   a , the offset direction of the first eccentric body  10   a  with respect to the first rotation axis  20   a  is symmetrical to the offset direction of the third eccentric body  2   a . Consequently, when driving the first reduction gear  30   a , the positions of the first external gear  8   a  and the third external gear  4   a  are always symmetrical with respect to the rotation axis  160 . Consequently, it is possible to improve the rotational balance of the first crankshafts  12   a . This characteristic can also be obtained in the second reduction gear  30   b.    
     Moreover, in the present embodiment, a structure was described in which the first rotation axis  20   a  and the second rotation axis  20   b  are coaxial, and the first eccentric body  10   a  and the second eccentric body  10   b  are eccentric in the same direction with respect to the rotation axes  20   a ,  20   b . However, in a structure in which the movement of the eccentric rotation of the first external gear  8   a  and the movement of the eccentric rotation of the second external gear  8   b  is equal at both sides of the machine tool  120 , the first rotation axis  20   a  and the second rotation axis  20   b  may not be coaxial. That is, the first external gear  8   a  of the first reduction gear  30   a  and the second external gear  8   b  of the second reduction gear  30   b  may be offset in the same direction with respect to the rotation axis  160 . For example, a structure is possible in which the first crankshafts  12   a  and the second crankshafts  12   b  are disposed symmetrically with respect to the rotation axis  160  and, when viewed along one direction of the first rotation axis  20   a  (or the second rotation axis  20   b ), the offset direction of the first eccentric body  10   a  and the offset direction of the second eccentric body  10   b  are the same. 
     Further, in the present embodiment, an example was described using reduction gears as the first supporting member and the second supporting member. However, a supporting member not having a reduction gear structure may be used instead of the first reduction gear  30   a  and/or the second reduction gear  30   b  of the present embodiment. In this case, also, only one bearing is disposed between the case and the shaft. In the case of using a supporting member not having a reduction gear structure, the machine tool  120  can be caused to rotate about the rotation axis  160  by attaching a motor to the case or the shaft. Further, a reduction gear in which an internal gear rotates eccentrically can be used as the first supporting member and/or the second supporting member. The important thing is that each of the two supporting members (first supporting member and second supporting member) disposed at both sides of the machine tool  120  is provided with a case and a shaft, and that only one bearing is disposed between the case and the shaft. The same applies in the following embodiments. 
     Second Embodiment 
     A driving device  200  will be described. The driving device  200  is a modification of the driving device  100 . Specifically, in the driving device  200 , only the configuration of the supporting member (reduction gear) is different from that of the driving device  100 . Consequently, members which are the same as the members of driving device  100  will be denoted by the same reference numbers as in the driving device  100  or by reference numbers having the same lower two digits, and a description thereof may thereby be omitted. Further, the configuration of a first reduction gear  230   a  is described, and a description of the configuration of a second reduction gear  230   b  may be omitted. 
     As shown in  FIG. 4  to  FIG. 6 , only one first eccentric body  210   a  is provided on a first crankshaft  212   a  of the first reduction gear  230   a . Similarly, only one second eccentric body  210   b  is provided on a second crankshaft  212   b  of the second reduction gear  230   b . A first rotation axis  220   a  of the first crankshaft  212   a  and a second rotation axis  220   b  of the second crankshaft  212   b  are coaxial. Viewed along one direction along the first rotation axis  220   a , an offset direction of the first eccentric body  210   a , and an offset direction of the second eccentric body  210   b  are symmetrical with respect to the first rotation axis  220   a . That is, a center of the first eccentric body  210   a  and a center of the second eccentric body  210   b  are symmetrical with respect to the first rotation axis  220   a . A first external gear  208   a  is engaged with the first eccentric body  210   a , and a second external gear  208   b  is engaged with the second eccentric body  210   b . Consequently, when viewed along one direction of a rotation axis  260 , the first external gear  208   a  and the second external gear  208   b  are eccentric in opposite directions (are symmetrical with respect to the rotation axis  260 ). 
     As shown in  FIG. 5  and  FIG. 6 , in the first reduction gear  230   a , a first bearing  264   a  is disposed between a first plate  250   a  and a first case  262   a . A bearing is not disposed between a second plate  254   a  and the first case  262   a . A first supporting shaft  256   a  is supported by the first case  262   a  via the one first bearing  264   a  on the machine tool  120  side than the first external gear  208   a . In other words, in the rotation axis  260  direction, the first bearing  264   a  is disposed on the machine tool  120  side than the first external gear  208   a.    
     Similarly, in the second reduction gear  230   b , a second supporting shaft  256   b  is supported by a second case  262   b  via one second bearing  264   b  on the machine tool  120  side than the second external gear  208   b . That is, in the rotation axis  260  direction, the second bearing  264   b  is disposed on the machine tool  120  side than the second external gear  208   b.    
     The characteristics of the driving device  200  will be described. As described above, in the driving device  200 , each of the first reduction gear  230   a  and the second reduction gear  230   b  is provided with only one external gear (the first external gear  208   a , the second external gear  208   b ). Consequently, compared with a reduction gear (e.g., the reduction gears  30   a ,  30   b  of embodiment  1 ) provided with a plurality of external gears, it is possible to reduce the length in the axial direction of the reduction gears. That is, according to the technique disclosed in the present embodiment, it is possible to realize a compact driving device. 
     Further, the first external gear  208   a  and the second external gear  208   b  are eccentric symmetrically with respect to the rotation axis  260 . Consequently, the force is balanced which is applied from the two external gears (the first external gear  208   a , the second external gear  208   b ) to the integrated body, in which the first case  262   a , the machine tool  120 , and the second case  262   b  are combined. 
     The first reduction gear  230   a  and the second reduction gear  230   b  can also be considered as being a reduction gear provided with two external gears, of which part is divided and disposed at both sides of the machine tool  120 . For example, in the case of the driving device  100  of the first embodiment, the first reduction gear  30   a  comprises the two external gears  8   a ,  4   a  which are eccentric symmetrically with respect to the rotation axis  160 . The driving device  200  of the present embodiment can be considered as a structure in which the two external gears  8   a ,  4   a  of the driving device  100  of the first embodiment are divided, and one of the external gears has been disposed on the opposite side of the machine tool  120 . 
     In the driving device  200 , the first bearing  264   a  and the second bearing  264   b  are each disposed further to the machine tool  120  side than the first external gear  208   a  and the second external gear  208   b . That is, the first external gear  208   a  and the second external gear  208   b  are disposed outside a range where the pair of bearings (the first bearing  264   a , the second bearing  264   b ) support the integrated body which is the cases  262   a ,  262   b  and the machine tool  120 . Consequently, the machine tool  120  can suppress vibration caused by force being applied to the cases  262   a ,  262   b  from the external gears  208   a ,  208   b.    
     Moreover, in the present embodiment, a structure was described in which the first rotation axis  220   a  and the second rotation axis  220   b  are coaxial, and the first eccentric body  210   a  and the second eccentric body  210   b  are eccentric symmetrically with respect to the first rotation axis  220   a . However, the important thing is that the first external gear  208   a  and the second external gear  208   b  rotate eccentrically in a state of being eccentric in opposite directions relative to the rotation axis  260 . With this type of structure, the first eccentric body  210   a  and the second eccentric body  210   b  need not be symmetrically eccentric with respect to the first rotation axis  220   a  (the second rotation axis  220   b ). For example, the structure may be such that the first crankshaft  212   a  and the second crankshaft  212   b  are disposed symmetrically with respect to the rotation axis  260 , and the offset direction of the first eccentric body  210   a  and the offset direction of the second eccentric body  210   b  are opposite when viewed along one direction of the first rotation axis  220   a  (or the second rotation axis  220   b ). 
     Third Embodiment 
     A driving device  300  will be described. The driving device  300  is a modification of the driving device  200 . Specifically, the driving device  300  differs from the driving device  200  only in the structure of reduction gears  330   a  and  330   b . Consequently, members the same as those of the driving device  200  will be denoted by the same reference numbers as those of the driving device  200  or by reference numbers having the same lower two digits, and a description thereof is thereby omitted. Further, the configuration of the first reduction gear  330   a  is described, and a description of the configuration of the second reduction gear  330   b  may be omitted. 
     As shown in  FIG. 7  to  FIG. 9 , in the driving device  300 , a first crankshaft  312   a  and a second crankshaft  312   b  are disposed coaxially with a rotation axis  360 . That is, the crankshafts  312   a ,  312   b  are disposed on the rotation axis  360  that is an output unit of the first reduction gear  330   a  and the second reduction gear  330   b . The first crankshaft  312   a  is provided with only one first eccentric body  310   a , and the second crankshaft  312   b  is provided with only one second eccentric body  310   b . Offset directions of the first eccentric body  310   a  and of the second eccentric body  310   b  are symmetrical with respect to the rotation axis  360 . 
     As shown in  FIG. 8 , the first reduction gear  330   a  comprises first driven crankshafts  370   a . The first reduction gear  330   a  comprises three first driven crankshafts  370   a . The first driven crankshafts  370   a  are each disposed equally spaced from each other around the rotation axis  360 . Each of the first driven crankshafts  370   a  is provided with only one first driven eccentric body  372   a . The second reduction gear  330   b  comprises second driven crankshafts  370   b . The second reduction gear  330   b  comprises three second driven crankshafts  370   b . The second driven crankshafts  370   b  are each disposed equally spaced from each other around the rotation axis  360 . A rotation axis  380   a  of each first driven crankshaft  370   a  and a rotation axis  380   b  of each second driven crankshaft  370   b  are coaxial. Each of the second driven crankshafts  370   b  is provided with only one second driven eccentric body  372   b . When viewed along one direction of the rotation axis  380   a , an offset direction of the first driven eccentric body  372   a  and an offset direction of the second driven eccentric body  372   b  are opposite with respect to the rotation axis  380   a.    
     Torque of a motor (not shown) is not transmitted directly to the first driven crankshafts  370   a  and the second driven crankshafts  370   b . The first driven crankshafts  370   a  and the second driven crankshafts  370   b  rotate with the eccentric rotation of external gears  308   a ,  308   b . Since each of the first reduction gear  330   a  and the second reduction gear  330   b  comprises the first driven crankshafts  370   a  and the second driven crankshafts  370   b , the first external gear  308   a  and the second external gear  308   b  rotate smoothly. 
     As shown in  FIG. 9 , the first crankshaft  312   a  is supported by a first supporting shaft  356   a  via a pair of bearings  314   a . The pair of bearings  314   a  is deep groove ball bearings. Each first driven crankshaft  370   a  is supported by the first supporting shaft  356   a  via a pair of bearings  384   a . The pair of bearings  384   a  is tapered roller bearings. Movement of the first driven crankshafts  370   a  in the axial direction and radial direction is regulated by the tapered roller bearings  384   a.    
     The first external gear  308   a  comprises a crankshaft through hole  324   a , driven crankshaft through holes  386   a , and supporting shaft through holes  336   a . The crankshaft through hole  324   a  is provided in a center of the first external gear  308   a . The driven crankshaft through holes  386   a  and the supporting shaft through holes  336   a  are provided alternately in the circumferential direction of the first external gear  308   a . The first eccentric body  310   a  engages with the crankshaft through hole  324   a  via a cylindrical roller bearing  322   a . The first driven eccentric body  372   a  engages with the driven crankshaft through holes  386   a  via cylindrical roller bearings  374   a . A columnar portion  352   a  of the first supporting shaft  356   a  passes through the supporting shaft through hole  336   a . An oil seal  382   a  is disposed between the first driven crankshaft  370   a  and a first plate  350   a , and an oil seal  388   a  is disposed between the first driven crankshaft  370   a  and a second plate  354   a.    
     Specific examples of the present invention have been described in detail, however, these are mere exemplary indications and thus do not limit the scope of the claims. The art described in the claims includes modifications and variations of the specific examples presented above. Technical features described in the description and the drawings may technically be useful alone or in various combinations, and are not limited to the combinations as originally claimed. Further, the art described in the description and the drawings may concurrently achieve a plurality of aims, and technical significance thereof resides in achieving any one of such aims.