Abstract:
A crankshaft has eccentric holes communicating with each other. The eccentric hole extends in the axial direction of the crankshaft from one end surface of the crankshaft and is disposed so as to be eccentric in the same direction as a cam section. The eccentric hole extends in the axial direction of the crankshaft from the other end surface of the crankshaft and is disposed so as to be eccentric in the direction of the eccentricity of a cam section. A centrifugal force due to the rotation of the crankshaft generates a force couple about an axis which is perpendicular to the axis of the crankshaft. The force couple is reduced by the eccentric holes. The cam sections are arranged around the axis of rotation with the phases shifted by 180 degrees from each other. As a result, a translational force acting in the direction perpendicular to the axis of the crankshaft is also reduced.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an eccentric rocking type reduction gear, and more specifically, to an improvement of a rotation balance of a crank shaft that performs eccentric rocking on an external gear of the eccentric rocking type reduction gear. 
     BACKGROUND OF THE INVENTION 
     Eccentric rocking type reduction gears include a crank shaft. The crank shaft includes an eccentric cylindrical cam in order to perform eccentric rocking on an external gear. In general, the crank shaft is utilized as an input shaft, and rotates at a fast speed. Hence, fluctuating load acts on a bearing that supports the crank shaft due to centrifugal force produced inherently to the unbalanced shape of the eccentric cylindrical cam. For example, according to a reduction gear disclosed in Patent Document 1, in order to reduce such fluctuating load, the eccentric cylindrical cam is formed with a balancer weight. This suppresses the unbalance originating from the weight of the eccentric cylindrical cam. 
     Moreover, there are reduction gears that include two external gears. According to the reduction gears of this type, a crank shaft is provided with two eccentric cylindrical cams to support the two external gears, respectively. The respective eccentric cylindrical cams are disposed around the axial line of the crank shaft with respective phases shifted by 180 degrees from each other. This structure cancels translational force. 
     As explained above, according to the reduction gear of Patent Document 1, the balancer weight is formed inwardly of the eccentric cylindrical cam. This balancer weight eliminates the unbalance around the axial line of the crank shaft. However, the unbalance around the axial line orthogonal to the axial line of the crank shaft still remains unaddressed. Accordingly, couple is produced around the axial line orthogonal to the axial line of the crank shaft due to centrifugal force. 
     According to the reduction gears having the two external gears, the two eccentric cylindrical cams are disposed around the axial line of the crank shaft with respective phases being shifted by 180 degrees from each other. According to such a structure, the unbalance around the axial line of the crank shaft can be also addressed. However, the unbalance around the axial line orthogonal to the axial line of the crank shaft still remains unaddressed. Hence, couple is still produced around the axial line orthogonal to the axial line of the crank shaft. 
     The couple around the axial line orthogonal to the axial line of the crank shaft also applies fluctuating load to the bearing supporting the crank shaft. This often results in the shortage of the lifetime of the bearing. Moreover, the eccentric rocking type reduction gear is likely to generate vibration. 
     PRIOR ART DOCUMENT 
     Patent Document 
     
         
         Patent Document 1: Japanese Laid-Open Patent Publication No. 2007-247684 
       
    
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an eccentric rocking type reduction gear which reduces fluctuating load acting on a bearing due to a rotation of a crank shaft to extend the lifetime of the bearing, and which also suppresses a generation of vibration. 
     To accomplish the above object, a first aspect of the present invention provides an eccentric rocking type reduction gear comprising an internal gear, two external gears meshed with the internal gear, a crank shaft supporting both of the external gears, and a carrier rotating together with a rotation of both of the external gears, the crank shaft comprising first and second cylindrical cams disposed around an axial line of the crank shaft with respective phases being shifted by 180 degrees and in a manner offset from a rotation center of the crank shaft, both of the external gears being supported by the first and second cylindrical cams, respectively, in a freely rotatable manner and in a manner revolvable around an axial line of the internal gear, the carrier comprising a plurality of output pins fastened around an axial line of the carrier at an equal interval, each of the output pins being engaged with a plurality of through-holes provided in both of the external gears to be linked with a rotation movement of both of the external gears, the crank shaft being rotated as an input shaft and either one of the internal gear and the carrier being rotated as an output shaft, first and second eccentric holes which run in an axial direction of the crank shaft, and which are in communication with each other being formed in the crank shaft, the first eccentric hole running from a first end face of the crank shaft to a center position of the crank shaft in the axial direction, and being disposed in a manner offset in a same direction as that of the first cylindrical cam, and the second eccentric hole running from a second end face of the crank shaft to the center position of the crank shaft in the axial direction, and being disposed in a manner offset in a same direction as that of the second cylindrical cam. 
     According to such a structure, the first and second cylindrical cams are disposed around the crank shaft with respective phases being shifted from each other by 180 degrees. Hence, translational force due to centrifugal force acting on the crank shaft can be reduced. Moreover, the first and second eccentric holes reduce the moment of couple due to the centrifugal force acting on the crank shaft. Since both translational force and moment of couple are reduced as explained above, fluctuating load acting on the bearing supporting the crank shaft can be reduced. Accordingly, the lifetime of the bearing can be extended. Moreover, vibration caused by the reduction gear can be reduced. 
     In the above-explained eccentric rocking type reduction gear, it is preferable that axial-end balance adjusting portions which adjust a weight balance are provided at both ends of the crank shaft. 
     According to such a structure, the axial-end balance adjusting portions are provided at both ends of the crank shaft. Accordingly, the arm of couple can have the maximum length. Hence, when couple is produced around an axial line orthogonal to the axial line of the crank shaft, the adjustment for accomplishing the balancing can be reduced as much as possible. 
     In the above-explained eccentric rocking type reduction gear, it is preferable that the axial-end balance adjusting portions are provided at both end faces of the crank shaft, and are chamfers provided at respective circumference edges of openings of the first and second eccentric holes. 
     According to such a structure, by increasing the chamfering level in a chamfering process, the rotation balance of the crank shaft can be adjusted finely without any special process. 
     In the above-explained eccentric rocking type reduction gear, it is preferable that the axial-end balance adjusting portions are balancer weights provided at both ends of the crank shaft, respectively. 
     According to such a structure, by increasing or decreasing the weight of the balancer weight, the rotation balance of the crank shaft can be adjusted finely even after the assembling of the crank shaft is completed. 
     To accomplish the above object, a second of the present invention provides An eccentric rocking type reduction gear comprising an internal gear, two external gears meshed with the internal gear, a hollow crank shaft supporting both of the external gears, and a rotating carrier rotating together with a rotation of both of the external gears, the crank shaft comprising first and second cylindrical cams disposed around an axial line of the crank shaft with respective phases being shifted by 180 degrees and in a manner offset from a rotation center of the crank shaft, both of the external gears being supported by the first and second cylindrical cams, respectively, in a freely rotatable manner and in a manner revolvable around an axial line of the internal gear, the carrier comprising a plurality of output pins fastened around an axial line of the carrier at an equal interval, each of the output pins being engaged with a plurality of through-holes provided in both of the external gears to be linked with a rotation movement of both of the external gears, the crank shaft being rotated as an input shaft and either one of the internal gear and the carrier being rotated as an output shaft, two recesses being provided in an inner periphery of the crank shaft, and the respective recesses being disposed at opposite sides along offset directions of the first and second cylindrical cams, and being disposed at different positions along an axial direction of the crank shaft. 
     According to such a structure, the first and second cylindrical cams are disposed around the crank shaft with respective phases being shifted from each other by 180 degrees. Hence, translational force due to centrifugal force acting on the crank shaft can be reduced. Moreover, the two recesses are disposed in the inner periphery of the crank shaft at opposite sides to each other along respective offset directions of the first and second cylindrical cams. The respective recesses are disposed at different positions from each other along the axial direction of the crank shaft. In this case, both recesses cancel couple acting on the crank shaft when no such recesses are provided. Accordingly, moment of couple due to the centrifugal force acting on the crank shaft can be reduced. Since both translational force and moment of couple are reduced as explained above, fluctuating load acting on the bearing supporting the crank shaft can be reduced. Hence, the lifetime of the bearing can be extended. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a vertical cross-sectional view illustrating an eccentric rocking type reduction gear according to an embodiment of the present invention (a cross-sectional view taken along a line  1 - 1  in  FIG. 4 ); 
         FIG. 2  is a vertical cross-sectional view illustrating a crank shaft (a cross-sectional view taken along a line  2 - 2  in  FIG. 3 ); 
         FIG. 3  is a cross-sectional view taken along a line  3 - 3  in  FIG. 2 ; 
         FIG. 4  is a cross-sectional view taken along a line  4 - 4  in  FIG. 1 ; 
         FIG. 5  is a cross-sectional view illustrating a condition in which a wiring is caused to pass through an eccentric hole of the eccentric rocking type reduction gear; 
         FIG. 6(   a ) is a vertical cross-sectional view illustrating the crank shaft in an axial condition, and  FIG. 6(   b ) is a right side view illustrating the crank shaft in the axial condition; 
         FIG. 7(   a ) is a front view illustrating a solid member, and  FIG. 7(   b ) is a right side view illustrating the solid member; 
         FIG. 8(   a ) is a vertical cross-sectional view of the crank shaft having a communication portion not chamfered, and  FIG. 8(   b ) is a model diagram for explaining coupling by chamfered communication portion; 
         FIG. 9(   a ) is a vertical cross-sectional view of the crank shaft having both ends thereof not chamfered, and FIG.  9 ( b ) is a model diagram for explaining coupling by chamfered both ends of the crank shaft; 
         FIG. 10(   a ) is a vertical cross-sectional view illustrating the left end of a crank shaft according to another embodiment, and  FIG. 10(   b ) is a vertical cross-sectional view illustrating the right end of the crank shaft; 
         FIG. 11(   a ) is a vertical cross-sectional view of a crank shaft according to the other embodiment,  FIG. 11(   b ) is a cross-sectional view taken along a line  11   b - 11   b  in  FIG. 11(   a ), and  FIG. 11(   c ) is a cross-sectional view taken along a line  11   c - 11   c  in  FIG. 11(   a ). 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the present invention in which an eccentric rocking type reduction gear thereof is applied to a joint of a robot arm will now be explained with reference to  FIG. 1  to  FIGS. 9(   a ) and  9 ( b ). 
     &lt;Structure of Reduction Gear&gt; 
     As illustrated in  FIG. 1 , an eccentric rocking type reduction gear  1  is provided between a first arm  41  and a second arm  42 . The reduction gear  1  includes a cylindrical housing  2  and a pair of side plates  4  and  8 . The housing  2  is fastened to the first arm  41 . The side plate  4  is supported through a bearing  10  in a freely rotatable manner to an end of the housing  2  facing the first arm  41 . The side plate  8  is supported through a bearing  14  in a freely rotatable manner to an end of the housing  2  facing the second arm  42 . A crank shaft  3  is supported at the center of the housing  2  through the two side plates  4  and  8 . The side plate  4  holds a bearing  11 . The side plate  8  holds a bearing  15 . The crank shaft  3  is supported through both bearings  11  and  15  in a freely rotatable manner to both side plates  4  and  8 . 
     Two cylindrical cams  31  and  32  are formed integrally at the center of the crank shaft  3 . As illustrated in  FIG. 2 , the respective cams  31  and  32  are disposed in an eccentric manner by an offset level e 1  from a rotation axis a 1  of the crank shaft  3 . The cam  31  is disposed in an eccentric manner to the rotation axis a 1  in the opposite direction (vertical direction in  FIG. 1 ) to the cam  32 . As illustrated in  FIG. 3 , the cam  31  is disposed around the rotation axis a 1  of the crank shaft  3  with a phase shifted by 180 degrees from the cam  32 . In  FIG. 3 , the cam  31  is disposed in a manner shifted upwardly of the rotation axis a 1 . The cam  32  is disposed in a manner shifted downwardly of the rotation axis a 1 . 
     As illustrated in  FIG. 1 , an external gear  5  is supported on the outer periphery of the cam  31  in a freely rotatable manner through a bearing  12 . An external gear  6  is supported on the outer periphery of the cam  32  in a freely rotatable manner through a bearing  13 . As illustrated in  FIG. 4 , a plurality of through-holes  51  are formed in the external gear  5 . A plurality of through-holes  61  are formed in the external gear  6 . The respective through-holes  51  are disposed at an equal interval around the rotation center of the external gear  5  offset by the offset level e 1  from the rotation axis a 1  of the crank shaft  3 . Moreover, the respective through-holes  61  are disposed at an equal interval around the rotation center of the external gear  6  offset by the offset level e 1  from the rotation axis a 1  of the crank shaft  3 . Respective axial lines of the through-holes  51  and  61  are parallel to the rotation axis a 1  of the crank shaft  3 . 
     As illustrated in  FIG. 1 , the side plate  4  holds a plurality of output pins  7 . The number of the output pins  7  is consistent with the number of the respective through-holes  51  and  61 . As illustrated in  FIG. 4 , the respective output pins  7  are disposed at an equal interval around the rotation axis a 1  of the crank shaft  3 . As illustrated in  FIG. 1 , respective axial lines of the outer pins  7  are also parallel to the rotation axis a 1  of the crank shaft  3 . The respective output pins  7  pass through the respective through-holes  51  and  61  of the external gears  5  and  6 . A tip of each output pin  7  abuts the internal face of the side plate  8 . Bolts  16  are inserted in the side plate  8  from the external side. Each output pin  7  is tightened by each bolt  16 , thereby being fastened to the side plate  8 . 
     As illustrated in  FIG. 4 , an internal gear  21  is formed on the inner periphery of the housing  2 . As is indicated by dashed lines in  FIG. 4 , the external gears  5  and  6  have a pitch circle diameter in such a way that those gears mesh with the internal gear  21  at one location. Together with the rotation of the crank shaft  3 , the external gears  5  and  6  revolve around the rotation axis a 1  of the crank shaft  3  while being meshed with the internal gear  21 . The revolution radius of the external gears  5  and  6  is consistent with the offset level e 1  from the rotation axis a 1  of the crank shaft  3 . The through-holes  51  and  61  have an internal diameter d expressed by the following formula (1). The external gear  5  rotates while always causing the inner periphery of the through-hole  51  to contact the outer periphery of the output pin  7 . The external gear  6  rotates while always causing the inner periphery of the through-hole  61  to contact the outer periphery of the output pin  7 .
 
 d= 2 ·e   1   +D   (1)
 
     where e 1  is the offset level of the cams  31  and  32 , and D is the external diameter of the output pin  7 . 
     As illustrated in  FIG. 1 , a motor  40  includes a rotor  401  and a stator  402 . The rotor  401  is fastened to an end of the crank shaft  3  facing the first arm  41 . The stator  402  is fastened to a coupling part (joint) with the housing  2  in the first arm  41 . The stator  402  is disposed coaxially with the rotation axis a 1  of the crank shaft  3 . The crank shaft  3  rotates by allowing a current to flow through the stator  402 . The side plate  8  has an external face fastened with the second arm  42 . The second arm  42  turns around the axial line of the second arm  42  together with the rotation of the side plate  8 . At this time, the second arm  42  turns relative to the first arm  41  at a turning speed obtained by the rotation motion of the motor  40  having undergone the speed reduction by the reduction gear  1 . 
     &lt;Crank Shaft&gt; 
     Next, an explanation will be given of a shape of the crank shaft  3  in detail. 
     As illustrated in  FIG. 2 , the crank shaft  3  includes, in addition to the above-explained two cams  31  and  32 , a cylindrical part  38 . The cylindrical part  38  has its center matching the rotation axis a 1  of the crank shaft  3 . The cam  31  is disposed in such a manner as to be distant from the cam  32  by an interval L 2  along the rotation axis a 1 . The cams  31  and  32  have the same width L 1  and the same external diameter D 1 . Two eccentric holes  36  and  37  in communication with each other are formed in the crank shaft  3 . The respective eccentric holes  36  and  37  run along the rotation axis a 1 . 
     As illustrated in the right part of  FIG. 2 , the eccentric hole  36  runs from an axial end f (first end face) of the crank shaft  3  facing the first arm  41  to a center  100  of the crank shaft  3  along the axial line of the crank shaft  3 . The eccentric hole  36  has a length L 3  with reference to the axial end f. The eccentric hole  36  is disposed in such a manner as to be offset by an offset level e 2  to the rotation axis a 1  in the same direction (upward direction in  FIG. 2 ) as that of the cam  31 . The eccentric hole  36  has an internal diameter which is an internal diameter d 1 . 
     As illustrated in the left part of  FIG. 2 , the eccentric hole  37  runs from an axial end g (second end face) of the crank shaft  3  facing the second arm  42  to the center  100  along the axial line of the crank shaft  3 . The eccentric hole  37  has a length L 3  with reference to the axial end g. That is, the length of the eccentric hole  37  is the same as that of the eccentric hole  36 . The eccentric hole  37  is disposed in such a manner as to be offset by the offset level e 2  to the rotation axis a 1  in the same direction (downward direction in  FIG. 2 ) as that of the cam  32 . The eccentric hole  37  has an internal diameter that is an internal diameter d 1 . The internal diameter of the eccentric hole  37  is the same as that of the eccentric hole  36 . 
     The axial end f of the crank shaft  3  has a chamfer  34  formed around the entire circumference of the open end of the eccentric hole  36 . Likewise, the axial end g of the crank shaft  3  has a chamfer  35  around the entire circumference of the open end of the eccentric hole  37 . A communicated-part chamfer  33  is formed at a communicated part between the eccentric hole  36  and the eccentric hole  37 . The communicated-part chamfer  33  is formed at, in the inner periphery of the crank shaft  3 , a part near the cam  31  (the upper part in  FIG. 2 ) and a part near the cam  32  (the lower part in  FIG. 2 ), respectively. The upper communicated-part chamfer  33  is an inclined surface, and is inclined from the center  100  in such a manner as to become close to the inner periphery of the eccentric hole  37  toward the axial end g. The lower communicated-part chamfer  33  is also an inclined surface, and is inclined from the center  100  in such a manner as to become close to the inner periphery of the eccentric hole  36  toward the axial end f. 
     The respective parts of the crank shaft  3  have dimensions that satisfy the following formula (2).
 
 e   1   ·D   1   2   ·L   1 ( L   1   +L   2 )= e   2   ·d   1   2   ·L   3   2   (2)
 
     where e 1  is the offset level of the cams  31  and  32 , D 1  is the external diameter of the cams  31  and  32 , L 1  is the width of the cams  31  and  32 , and L 2  is an interval between the cams  31  and  32 . e 2  is an offset level of the eccentric holes  36  and  37 , d 1  is the internal diameter of the eccentric holes  36  and  37 , and L 3  is the length of the eccentric holes  36  and  37 . 
     &lt;Operation of External Gear&gt; 
     Next, an explanation will be given of the two external gears  5  and  6 . 
     As illustrated in  FIG. 4 , when the crank shaft  3  rotates, the external gear  5  revolves around the rotation axis a 1  of the crank shaft  3  while being meshed with the internal gear  21  of the housing  2 . At this time, when the number of gear teeth of the external gear  5  is Z 1  and the number of gear teeth of the internal gear  21  is Z 2 , the external gear  5  rotates relative to the crank shaft  3  by what corresponds to the difference in the number of gear teeth represented by Z 2 -Z 1  every time the crank shaft  3  performs one turn. That is, the external gear  5  revolves by one turn along the orbit of a circle having a radius that is the offset level e 1  relative to the housing  2 , and also rotates by (Z 2 −Z 1 )/Z 1 . The rotation motion of the external gear  5  is transmitted to the side plates  4  and  8  that are output shafts through respective contacts between the through-holes  51  and the output pins  7 . Like the external gear  5 , the rotation motion of the external gear  6  is transmitted to the side plates  4  and  8  through respective contacts between the through-holes  61  and the output pins  7 . In the present embodiment, the side plates  4  and  8  construct a carrier that is linked with the rotation motion of the external gears  5  and  6 . 
     For example, the robot arm has a drive device attached to the joint. There is a demand for such a drive device that it should be lightweight and have a high torque output. In this case, the eccentric rocking type reduction gear  1  is effective which can allow a compact drive motor to rotate at a fast speed, and which can perform speed reduction on such a rotation at a large reduction ratio to output high torque. According to the reduction gear  1  of this type, the crank shaft  3  that is an input shaft rotates at a fast speed. Hence, when the crank shaft  3  has an unbalanced portion, a fluctuating load due to centrifugal force, acts on the bearings  11  and  15 . In order to reduce such fluctuating load, it is necessary to let the crank shaft  3  to be balanced highly precisely. 
     &lt;Balancing of Crank Shaft&gt; 
     Next, an explanation will be given of the balancing of the crank shaft  3  in detail. First, a case in which the crank shaft  3  is a solid shaft will be examined. 
     As illustrated in  FIG. 6(   a ), when the crank shaft  3  is a solid shaft (when there is no eccentric holes  36  and  37 ), centrifugal force F, acting on the crank shaft  3  can be expressed as the following formula (3).
 
 F   c   =M   c   ·e   1 ·ω 2   (3)
 
     where M c  is the mass of cam  31 ,  32 , e 1  is the offset level of the cam  31 ,  32 , and ω is the rotation speed of the cam  31 ,  32 . The two cams  31  and  32  have the same offset level e 1 , e 1  and mass M c , M c . 
     Moreover, the mass M c  can be expressed by the following formula (4).
 
 M   c   =ρ·πD   1   2   ·L   1 /4  (4)
 
     where ρ is the density of the crank shaft  3  when it is a solid shaft, D 1  is the external diameter of the cam  31 ,  32 , and L 1  is the width of the cam  31 ,  32 . 
     Hence, when the formula (4) is applied to the formula (3), the centrifugal force F c  can be expressed as the following formula (5).
 
 F   c =ρ·π·D 1   2   ·L   1   ·e   1 ·ω 2 /4
 
     (5) As explained above, the cams  31  and  32  are disposed around the rotation axis a 1  of the crank shaft  3  in such a way that the cam  31  has the phase shifted by 180 degrees from the cam  32  as illustrated in  FIG. 6(   b ). Accordingly, the centrifugal force F c  by the cam  31  acts around the rotation axis a 1  of the crank shaft  3  in such a way that the phase is shifted by 180 degrees from the centrifugal force F c  of the cam  32 . That is, the centrifugal force F c  by the cam  31  acts in the opposite direction to the centrifugal force F c  by the cam  32 . In the condition illustrated in  FIGS. 6(   a ) and  6 ( b ), the upward centrifugal force F c  by the cam  31  and the downward centrifugal force F c  by the cam  32  act on the crank shaft  3 , respectively. 
     As illustrated in  FIG. 6(   a ), respective working points of the centrifugal forces F c  and F c  by the cams  31  and  32  are disposed on the rotation axis a 1  of the crank shaft  3 . The working point of the centrifugal force F c  by the cam  31  is disposed in a manner corresponding to the center of the cam  31  in the axial direction. The working point of the centrifugal force F c  by the cam  32  is disposed in a manner corresponding to the center of the cam  32  in the axial direction. As explained above, the cams  31  and  32  are disposed in such a manner as to be distant from each other by the interval L 2  along the axial direction of the crank shaft  3 . Hence, the working points of the centrifugal forces F c  and F c  by the cams  31  and  32  are distant from each other by a distance L 1 +L 2  (=L 1 /2+L 2 +L 1 /2). That is, the centrifugal forces F c , F c  by the cams  31  and  32  are equal to the concentrated loads acting at the two working points distant from each other by the distance L 1 +L 2  in the opposite directions and having the same magnitude. 
     Hence, the translational forces acting on the crank shaft  3  are canceled from each other. The translational force means force that causes the crank shaft  3  to move linearly in the direction orthogonal to the rotation axis a 1 . For example,  FIGS. 6(   a ) and  6 ( b ) illustrate the condition in which the two cams  31  and  32  are disposed in the opposite sides along the vertical direction. In this condition, as illustrated in  FIG. 6(   b ), when the crank shaft  3  is viewed from the axial direction, the centrifugal forces F c , F c  by the cams  31  and  32  act on the weight center of the crank shaft  3 , i.e., the rotation axis a 1  illustrated in the figure. At this time, the centrifugal force F c  by the cam  31  acts as the translational force that causes the crank shaft  3  to move in the upward direction. The centrifugal force F c  by the cam  32  acts as the translational force that causes the crank shaft  3  to move in the downward direction. As explained above, the centrifugal forces F c , F c  by the cams  31  and  32  act in the opposite directions and with the same magnitude, and thus the translational forces acting on the crank shaft  3  are subtracted and become zero at total. In this case, the moment of forces around the rotation axis a 1  of the crank shaft  3  is balanced. Accordingly, the unbalance around the rotation axis a 1  can be addressed. 
     When, however, the crank shaft  3  is viewed from a direction orthogonal to the offset direction of the cams  31  and  32  (the vertical direction in  FIG. 6(   b )), i.e., when the crank shaft  3  is viewed from the horizontal direction in  FIG. 6(   b ), as illustrated in  FIG. 6(   a ), the centrifugal forces F c , F c  by the cams  31  and  32  act as the couple to the crank shaft  3 . Accordingly, the moment of the couple with the magnitude of F c ·(L 1 +L 2 ) acts on the crank shaft  3  in the left-turn direction in  FIG. 6(   a ). According to the present embodiment, in order to suppress the moment of the couple, the crank shaft  3  is formed with the eccentric holes  36  and  37 . 
     The effect of the eccentric holes  36  and  37  acting on the centrifugal force of the crank shaft  3  is equivalent to the subtraction of the effect of the centrifugal force acting on the solid member matching the shapes of the eccentric holes  36  and  37 . The effect of the centrifugal force acting on a solid member  71  is as follow. The eccentric holes  36  and  37  are disposed in an offset manner by the same offset level e 2  from the rotation axis a 1 . Hence, as illustrated in  FIG. 7(   a ), a first portion  72  corresponding to the eccentric hole  36  of the solid member  71  and a second portion  73  corresponding to the eccentric hole  37  are disposed in an offset manner by the same offset level e 2  relative to the rotation axis a 1 . Moreover, the first and second portions  72  and  73  have the same mass M h . Accordingly, centrifugal force F h  by the first and second portions  72  and  73  can be expressed as the following formula (6).
 
 F   h   =M   h   ·e   2 ·ω 2   (6)
 
     where M h  is the mass of the first and second portions  72 ,  73 , e 2  is the offset level of the first and second portions  72 ,  73  relative to the rotation axis a 1 , and ω is a rotation speed. 
     Moreover, the mass can be expressed by the following formula (7).
 
 M   h =ρ·π·d 1   2   ·L   3 /4  (7)
 
     where ρ is the density of the solid member, d 1  is the internal diameter of the eccentric hole  36  (in this example, the external diameter of the first and second portions  72 ,  73 ), and L 3  is the length of the eccentric holes  36 ,  37  along the rotation axis a 1  (in this example, the length of the first and second portions  72 ,  73 ). 
     Hence, when the formula (7) is applied to the formula (6), the centrifugal force F h  by the first and second portions  72 ,  73  can be expressed as the following formula (8).
 
 F   h =ρ·π·d 1   2   ·L   3   ·e   2 ·ω 2 /4  (8)
 
     As explained above, the eccentric holes  36  and  37  are disposed in such a way that respective phases are shifted by 180 degrees from each other around the rotation axis a 1 . Accordingly, as illustrated in  FIG. 7(   b ), the first and second portions  72  and  73  are disposed in such a way that respective phases are shifted by 180 degrees from each other around the rotation axis a 1 . Moreover, the eccentric holes  36  and  37  adjoin to each other and are in communication with each other in the axial direction. Hence, the first and second portions  72 ,  73  have the shape joined with each other along the rotation axis a 1 . 
     As illustrated in  FIG. 7(   a ), respective working points of the centrifugal forces F h , F h  by the first and second portions  72 ,  73  are disposed on the rotation axis a 1  of the crank shaft  3 . The working point of the centrifugal force F h  by the first portion  72  is disposed in a manner corresponding to the center of the first portion  72  in the axial direction. The working point of the centrifugal force F h  by the second portion  73  is disposed in a manner corresponding to the center of the second portion  73  in the axial direction. Accordingly, the working points of the centrifugal forces F h , F h  by the first and second portions  72 ,  73  are distant from each other by a distance L 3  (=L 3 /2+L 3 /2). Accordingly, the centrifugal forces F h , F h  by the first and second portions  72 ,  73  are equivalent to the concentrated loads acting on the two working points distant from each other by the length L 3  in the opposite directions and with the same magnitude. Hence, likewise the above-explained cams  31  and  32 , when the solid member  71  is viewed from the axial direction, the translational forces acting on the solid member  71  are canceled. That is, as illustrated in  FIG. 7(   b ), the centrifugal force F h  acting in the upward direction and the centrifugal force F h  acting in the downward direction are canceled from each other. 
     When, however, the solid member  71  is viewed from the direction orthogonal to the offset direction of the first and second portions  72  and  73 , the centrifugal forces F h , F h  acting on the first and second portions  72 ,  73 , respectively act as couple to the solid member  71  as illustrated in  FIG. 7(   a ). That is, a moment of the couple with the magnitude of F h ·L 3  acts on the solid member  71  in the left-turn direction in  FIG. 7(   a ). Hence, the effect of the eccentric holes  36 ,  37  given to the centrifugal force of the crank shaft  3  is the moment of couple that is −F h ·L 3  in the left-turn direction in  FIG. 2 . 
     Accordingly, the effect of the centrifugal force acting on the crank shaft  3  with the two eccentric holes  36 ,  37  is as follow. First, when the crank shaft  3  is viewed from the axial direction, the centrifugal force acting in the upward direction in  FIG. 3  and the centrifugal force acting in the downward direction are canceled from each other. Accordingly, no translational force acts on the crank shaft  3 . Next, as illustrated in  FIG. 2 , when the crank shaft  3  is viewed from the direction orthogonal to the offset direction of the cams  31 ,  32 , the moment of couple with a magnitude expressed by the following formula (9) acts on the crank shaft  3  in the left-turn direction in  FIG. 2 .
 
Moment of couple= F   c ·( L   1   +L   2 )− F   h   L   3  
 
     (9) When the formula (5) and the formula (8) are applied to the formula (9), the moment of couple acting on the crank shaft  3  can be expressed as the following formula (10).
 
Moment of couple={ D   1   2   ·L   1 ·( L   1   +L   2 )· e   1   −d   1   2   ·L   3   2   ·e   2 }·ρ·π·ω 2 /4  (10)
 
     Respective dimension of the portions of the crank shaft  3  are designed so as to satisfy the relational expression of the above-explained formula (2). When the formula (2) is applied to the formula (10), it becomes clear that the moment of couple becomes zero. 
     In practice, it is necessary to consider centrifugal force F′ of the external gears  5  and  6  expressed by the following formula (11). When this centrifugal force F′ is taken into consideration, the formula (9) becomes the following formula (12).
 
 F′=m′·e   1 ω 2   (11)
 
     where m′ is the mass of the external gear  5 ,  6 .
 
( F   c   +F ′)·( L   1   +L   2 )− F   h   ·L   3   (12)&lt;
 
     &lt;Couple by Chamfering&gt; 
     The couple in the left-turn direction in  FIG. 2  acts on the crank shaft  3  due to the communicated-part chamfer  33 . However, couple in the right-turn direction also acts on the crank shaft  3  by the chamfers  34  and  35  at the axial ends. Accordingly, couple by the communicated-part chamfer  33  in the left-turn direction and couple by the chamfers  34  and  35  in the right-turn direction can be canceled from each other. A detailed explanation will be below given of the effects of the communicated-part chamfer  33  and the chamfers  34  and  35  working on the moment of the couple of the crank shaft  3 . 
     First, the moment of couple by the communicated-part chamfer  33  will be explained. 
     As illustrated in  FIG. 8(   a ), a condition in which no communicated-part chamfer  33  is provided is presumed first. That is, it is presumed that two edges  81 ,  81  which are eliminated when the communicated-part chamfer  33  is formed are present. As illustrated in  FIG. 8(   b ), the two edges  81 ,  81  can be expressed as a model having two weights  82 ,  82  joined to an axis  83  matching the rotation axis a 1 . Using this model, the two weights  82 ,  82  are rotated around the axis  83  (rotation axis a 1 ). When the two weights  82 ,  82  are disposed at opposite sides in the vertical direction, a moment of couple in the right-turn direction is produced by centrifugal forces acting on both two weights  82 ,  82 . 
     Conversely, a condition in which the communicated-part chamfer  33  is present as illustrated in  FIG. 2  matches a condition in which the two weights  82 ,  82  are omitted from the model illustrated in  FIG. 8(   b ). In this condition, no centrifugal force, and thus no moment of couple works. Accordingly, by providing the communicated-part chamfers  33 ,  33 , the moment of couple in the right-turn direction acting on the crank shaft  3  can be eliminated. In other words, a moment of couple in the left-turn direction can be caused to act on the crank shaft  3 . 
     Next, a moment of couple by the chamfers  34 ,  35  at the axial ends will be explained. 
     As illustrated in  FIG. 9(   a ), it is first presumed that there is no chamfers  34 ,  35  at the axial ends. That is, it is presumed that two ring members  84 ,  84  eliminated when the chamfers  34 ,  35  are formed are present. As illustrated in  FIG. 9(   b ), the two members  84 ,  84  can be expressed as a model having two ring weights  85 ,  85  joined to both ends of an axis  86  matching the rotation axis a 1 . Using this model, the two weights  85 ,  85  are rotated around the axis  86  (rotation axis a 1 ). When the two weights  85 ,  85  are disposed in an offset manner in the vertical direction, respective centrifugal forces by the two weights  85 ,  85  act oppositely in the vertical direction. At this time, a moment of couple in the left-turn direction is produced by the centrifugal forces of the two weights  85 ,  85 . 
     Conversely, a condition in which the chamfers  34 ,  35  are present as illustrated in  FIG. 2  matches a condition in which the two weights  85 ,  85  are eliminated from the model illustrated in  FIG. 9(   b ). In this condition, no centrifugal force, and thus no moment of couple works. Accordingly, by providing the chamfers  34 ,  35 , the moment of couple in the left-turn direction acting on the crank shaft  3  can be eliminated. In other words, a moment of couple in the right-turn direction is caused to act on the crank shaft  3 . 
     According to the present embodiment, the communicated-part chamfer  33  and the chamfers  34 ,  35  are provided in such a way that the couple in the left-turn direction by the communicated-part chamfer  33  is balanced with the couple in the right-turn direction by the chamfers  34 ,  35 . Accordingly, the couple in the left-turn direction by the communicated-part chamfer  33  and the couple in the right-turn direction by the chamfers  34 ,  35  can be canceled from each other. Hence, the chamfers  34 ,  35  function as an axial-end balancing part for finely adjusting the weight balance of the crank shaft  3 . 
     &lt;Adjustment of Weight Balance of Crank Shaft&gt; 
     Next, an explanation will be given of the adjustment of the weight balance of the crank shaft. 
     As explained above, the unbalancing of the crank shaft  3  can be eliminated in principle by forming the eccentric holes  36 ,  37 , the communicated-part chamfer  33 , and the chamfers  34 ,  35  at the axial ends in the predetermined shape. However, unbalancing inherent to an error in shape of respective portions of the crank shaft  3  often remains. Accordingly, after this unbalancing level is measured, the chamfers  34  and  35  are finish turned by a cutting tool like a turning tool based on the measured unbalancing level. By setting depths L 4  and L 5  of the chamfers  34  and  35  illustrated in  FIG. 2  to be an appropriate value in this manner, the unbalancing level of the crank shaft  3  can be easily suppressed to be equal to or smaller than a desired value. 
     &lt;Insertion of Wiring&gt; 
       FIG. 5  illustrates an example case in which a wiring is caused to pass through the interior of the crank shaft  3  when the reduction gear  1  is applied to the joint of the robot arm. In order to cause a wiring to pass through the interior of the crank shaft  3 , first, the reduction gear  1  is fastened to the first arm  41 . Next, a cylindrical guide member  502  is inserted so as not to contact the crank shaft  3 . A wiring bracket  501  is attached to an end of the guide member  502  facing the first arm  41 . The wiring bracket  501  is fastened to the internal space of the first arm  41 . Subsequently, a wiring  500  is caused to pass through the interior of a guide member  502  fastened to the first arm  41  via the wiring bracket  501 . At this time, the communicated-part chamfer  33  is provided at the communicated part between the eccentric hole  36  and the eccentric hole  37 . Accordingly, the wiring  500  does not get stuck on the uneven surface due to the eccentric disposition of the eccentric holes  36  and  37 . Hence, the wiring  500  can be easily inserted in the guide member  502 , and thus the joint of the robot arm can be easily assembled. A current is supplied to a hand (not shown) provided at the tip of the second arm  42  through the wiring  500 . 
     Advantages of Embodiment 
     Hence, according to the present embodiment, the following advantages can be accomplished. 
     (1) The unbalancing of the crank shaft  3  when the crank shaft  3  rotates can be reduced by providing the two eccentric holes  36  and  37 . This results in a reduction of the fluctuating load acting on the bearings  11  and  15  supporting the crank shaft  3 . Accordingly, the lifetime of the bearing in the reduction gear  1  can be extended. Moreover, an occurrence of vibration of the reduction gear  1  originating from the unbalancing of the crank shaft  3  when it rotates can be also suppressed. 
     The value of the unbalancing level when the crank shaft  3  rotates, and thus the value of the fluctuating load acting on the bearings  11  and  15  supporting the crank shaft  3  can be easily suppressed to a value equal to or smaller than a desired value by simply providing the eccentric holes  36  and  37  in the crank shaft  3 . Moreover, the lifetime of the bearing in the reduction gear  1  can be extended over a desired value, and the vibration of the reduction gear  1  originating from the unbalancing when the crank shaft  3  rotates can be suppressed to a value smaller than a desired value. 
     (2) The chamfers  34  and  35  as axial-end balance adjusting portions are provided at both ends of the crank shaft  3 . In this case, the moment of couple acting on the crank shaft  3  can be adjusted by adjusting the chamfering depth, etc., of the chamfers  34  and  35 . Moreover, the length of the arm of the couple can be maximized by providing the chamfers  34  and  35  at both ends of the crank shaft  3 . Accordingly, when couple is produced around the axial line orthogonal to the axial line of the crank shaft  3 , the adjusting level for obtaining the balancing in this case can be suppressed to a small level. 
     (3) The balancing of the couple acting on the crank shaft  3  is adjusted through the chamfers  34  and  35  at both ends of the crank shaft  3 . Accordingly, the rotation balance of the crank shaft  3  can be adjusted finely by increasing or decreasing the chamfering level in a chamfering process without any additional special process. 
     (4) The lifetime of the bearings  11  and  15  of the reduction gear  1  can be extended. This also extends the lifetime of the robot, etc., using the reduction gear  1 . Moreover, since vibration is little, the second arm  42  or the hand can be positioned precisely. 
     (5) The wiring or the pipe fitting can be caused to pass through the two eccentric holes  36  and  37  formed in the reduction gear  1 . Accordingly, the wiring space for the reduction gear  1  or the robot arm can be reduced. Hence, the motion of the robot is not interfered from the exterior by the wiring, etc. 
     (6) The communicated-part chamfers  33 ,  33  are provided at the communicated part (uneven surface part) between the two eccentric holes  36  and  37 . Accordingly, the insertion work of the wiring  50  is facilitated. 
     Other Embodiments 
     The present embodiment can be modified as follows. 
     In the present embodiment, the unbalance level of the crank shaft  3  is adjusted finely by increasing or decreasing the depths L 4  and L 5  of the chamfers  34  and  35 , but the following modification can be applied. For example, as illustrated in  FIG. 10(   a ), an additional hole  87  may be formed in at least either one end of the crank shaft  3 , or as illustrated in  FIG. 10(   b ), a balancer weight  88  may be added. Moreover, the balancing of the crank shaft  3  may be adjusted using both hole  87  and balancer weight  88 . Furthermore, the balancing may be adjusted by partially cutting the circumference surface of, not the end of the crank shaft  3  but the cylindrical part  38 . In this case, the hole  87  and the balancer weight  88  serve as the axial-end balance adjusting portions. 
     When the hole  87  is used to adjust the balancing of the couple acting on the crank shaft  3 , the rotation balance of the crank shaft  3  can be adjusted finely by increasing or decreasing the number, depth, and diameter of the hole  87  even after the assembling of the crank shaft  3  completes. When the balancer weight  88  is used to adjust the balance of the couple of the crank shaft  3 , the rotation balance of the crank shaft  3  can be adjusted finely by increasing or decreasing the number or weight of the balancer weight  88  even after the assembling of the crank shaft  3  completes. 
     The two eccentric holes  36  and  37  are provided in the present embodiment, but the following modification can be applied. That is, as illustrated in  FIG. 11(   a ), a hole  91  that passes all the way through the crank shaft  3  is formed along the rotation axis a 1 . First and second recesses  92  and  93  are formed by eliminating a part of the crank shaft  3  from the inner periphery of the hole  91 . The first and second recesses  92  and  93  are disposed at opposite sides along the offset directions of the cams  31  and  32 , respectively. That is, the first and second recesses  92  and  93  are disposed around the rotation axis a 1  with respective phases being shifted by 180 degrees from each other. More specifically, as illustrated in  FIG. 11(   b ), the first recess  92  is disposed near the cam  31  (upper side in  FIG. 11(   b )) in the inner periphery of the crank shaft  3 . As illustrated in  FIG. 11(   c ), the second recess  93  is disposed near the cam  32  (lower side in  FIG. 11(   c )) in the inner periphery of the crank shaft  3 . Moreover, the first and second recesses  92  and  93  are formed across the half circumference (a range within 180 degrees) around the rotation axis a 1 . As illustrated in  FIG. 11(   b ), the first recess  92  is formed across the upper half circumference. As illustrated in  FIG. 11(   c ), the second recess  93  is formed across the lower half circumference. Furthermore, as illustrated in  FIG. 11(   a ), the first and second recesses  92  and  93  are formed at different positions in the axial direction of the crank shaft  3 . The first and second recesses  92  and  93  work so as to cancel the moment of couple acting on the crank shaft  3  when no such recess is provided. Accordingly, the moment of couple by the centrifugal force acting on the crank shaft can be reduced. Therefore, the unbalancing when the crank shaft  3  rotates can be suppressed. 
     In the present embodiment, it is not necessary to set the dimensions of respective portions of the crank shaft  3  so as to satisfy the formula (2). In this case, also, the reducing effect of the moment of couple by the eccentric holes  36  and  37  can be accomplished, and thus the moment of couple acting on the crank shaft  3  can be reduced. 
     In the present embodiment, the lengths L 3  of the eccentric holes  36  and  37  are equal to each other, but may be different from each other. In this case, also, the couple reducing effect by the eccentric holes  36  and  37  can be accomplished. 
     The communicated-part chamfers  33 ,  33  can be omitted from the crank shaft  3 . In this case, also, the couple reducing effect by the two eccentric holes  36  and  37  can be accomplished. 
     The chamfers  34  and  35  can be omitted from the crank shaft  3 . In this case, also, the couple reducing effect by the two eccentric holes  36  and  37  can be accomplished. 
     In the present embodiment, the carrier including the two side plates  4  and  8  is utilized as an output shaft, but the internal gear  21  (housing  2 ) may be utilized as the output shaft. In this case, the joining between the housing  2  and the first arm  41  and the joining between the side plate  8  and the second arm  42  are released. The internal gear  21  (housing  2 ) is joined with the second arm  42 . 
     In the present embodiment, the reduction gear  1  is applied to the joint of the robot arm, but the preset invention is not limited to this case.