Abstract:
Provided is a joint structure for a robot which can prevent leakage of a lubricant which is charged to the interior of the joint structure while improving a drip-proof property. The joint structure for the robot includes: a first arm which is hollow; a second arm which is rotatably mounted to the first arm; a power transmission mechanism which is provided adjacent to the outside of the first arm, the power transmission mechanism including a gear and an inner space which houses the gear and is charged with a lubricant; a booster section which increases a pressure in an interior of the first arm to be higher than an outside pressure; and a one-way communication section which allows the interior of the first arm and the inner space to communicate with each other and a gas in the interior of the first arm to flow into the inner space, while preventing the lubricant in the inner space from flowing out to the interior of the first arm.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a joint structure for a robot. 
         [0003]    2. Description of the Related Art 
         [0004]    In a joint structure in which a robot arm and a wrist are connected to each other, techniques for increasing a pressure in the interior of the robot arm and the wrist to improve a drip-proof property have been known (e.g. Japanese Unexamined Patent Publication (Kokai) No. H7-75992). 
         [0005]    In the joint structure as described above, a technique for preventing outflow of a lubricant which is charged to the interior of the joint structure while improving a drip-proof property has been demanded. 
       SUMMARY OF THE INVENTION 
       [0006]    A joint structure of a robot comprises a hollow first arm, a second arm rotatably attached to the first arm, and a power transmission mechanism provided outside of the first arm so as to be adjacent to the first arm. The power transmission mechanism includes an inner space in which a lubricant is filled. 
         [0007]    The joint structure of a robot comprises a booster section configured to increase a pressure in an interior of the first arm to be higher than an outside pressure, and a one-way communication section configured to fluidly connect the interior of the first arm to the inner space so as to be in fluid communication with each other, so that the one-way communication section allows a gas in the interior of the first arm to flow into the inner space, while the one-way communication section prevents the lubricant in the inner space from flowing out to the interior of the first arm. 
         [0008]    The second arm may be hollow, and the inner space may be defined by the second arm. The one-way communication section may include a passage having a first opening which opens to the inner space and a second opening opposite the first opening, which opens to the interior of the first arm, and a one-way valve provided inside of the passage. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The above-mentioned or other objects, features, and advantages of the invention will be clarified from the description of embodiments with reference to the accompanying drawings, in which: 
           [0010]      FIG. 1  is a cross-sectional view of a joint structure of a robot according to an embodiment of the invention; 
           [0011]      FIG. 2  is a cross-sectional view of a joint structure of a robot according to another embodiment of the invention; 
           [0012]      FIG. 3  is a cross-sectional view of a joint structure of a robot according to still another embodiment of the invention; 
           [0013]      FIG. 4  is a cross-sectional view of a joint structure of a robot according to still another embodiment of the invention; and 
           [0014]      FIG. 5  is a cross-sectional view of a joint structure of a robot according to still another embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    Hereinafter, embodiments of the invention will be described in detail with reference to the drawings. First, referring to  FIG. 1 , a joint structure  10  of a robot according to an embodiment of the invention will be described. Note that, in the various embodiments described below, similar elements are assigned the same reference numerals, and overlapping description will be omitted. In addition, a rightward, leftward, upward, and downward directions in the following description respectively correspond to the rightward, leftward, upward, and downward directions in the drawings. 
         [0016]    The joint structure  10  includes a first arm  12 , a second arm  14 , a servo motor  16 , a power transmission mechanism  18 , a booster section  20 , and a one-way communication section  22 . 
         [0017]    The first arm  12  includes a body  24  and a cover  26 . The body  24  is hollow, and extends along an axis A 1 . An opening  24   b  is formed at a left wall  24   a  of the body  24 . 
         [0018]    The opening  24   b  is a hole which enables the operator to access to the inside of the body  24  during assembly of the joint structure  10  or the like. The cover  26  air-tightly covers the opening  24   b . The body  24  and the cover  26  define an inner space S 1  which is air-tightly sealed from the outside. 
         [0019]    The second arm  14  is attached to the first arm  12  so as to be rotatable about an axis A 2  orthogonal to the axis A 1 . The second arm  14  is formed with a recess  30 . The recess  30  is formed so as to be recessed rightward from a left end surface  28  of the second arm  14 , which faces the first arm  12 . 
         [0020]    A right wall  14   a  of the second arm  14 , which is located opposite the end surface  28 , is formed with an opening  14   b . The opening  14   b  is a hole which enables the operator to access to an inner space S 2  during assembly of the joint structure  10  or the like. The inner space S 2  will be described later. A cover  32  is attached to the wall  14   a  of the second arm  14  so as to air-tightly cover the opening  14   b.    
         [0021]    The servo motor  16  is fixed to an inner surface of a right wall  24   c  of the body  24 . The servo motor  16  includes an output shaft  16   a . The output shaft  16   a  extends along the axis A 2  so as to project out from the inner space S 1  of the first arm  12 . The servo motor  16  functions as a drive section configured to generate rotational force for rotating the second arm  14  relative to the first arm  12  about the axis A 2 . 
         [0022]    The power transmission mechanism  18  is provided outside of the first arm  12  so as to be adjacent to the first arm  12 , and housed in the recess  30  formed at the second arm  14 . The power transmission mechanism  18  transmits the rotational force of the output shaft  16   a  rotated by the servo motor  16  to the second arm  14 . 
         [0023]    Specifically, the power transmission mechanism  18  includes a strain wave gearing device  34  and a cross roller ring  35 . The strain wave gearing device  34  is a device referred to as harmonic drive (registered trademark), and includes a wave generator  42 , a flex spline  43 , and a circular spline  39 . 
         [0024]    The wave generator  42  includes an elliptical inner ring  42   a  fixed on the output shaft  16   a , a flexible outer ring  42   b , and a ball  42   c  arranged between the inner ring  42   a  and the outer ring  42   b . The outer ring  42   b  is deformed by the ball  42   c  so as to conform to an elliptical outer shape of the inner ring  42   a.    
         [0025]    The flex spline  43  is made from an elastic material, and one end thereof is fixed to the wall  24   c  of the first arm  12  by bolts  46 . An inner circumferential surface of the flex spline  43  is in contact with the outer ring  42   b  of the wave generator  42 . Accordingly, the flex spline  43  is deformed so as to conform to an elliptical outer shape of the outer ring  42   b . An outer circumferential surface of the flex spline  43  is formed with a gear. 
         [0026]    The circular spline  39  has a ring shape, and is fixed to the wall  14   a  of the second arm  14  by bolts  44 . An inner circumferential surface of the circular spline  39  is formed with a gear. 
         [0027]    The gear formed on the outer circumferential surface of the flex spline  43  engages the gear formed on the inner circumferential surface of the circular spline  39  at a position corresponding to a major-axis portion of the inner ring  42   a  of the wave generator  42 , while separating away from the gear formed on the inner circumferential surface of the circular spline  39  at a position corresponding to a minor-axis portion of the inner ring  42   a.    
         [0028]    As the output shaft  16   a  rotates, the engagement position between the gear formed on the outer circumferential surface of the flex spline  43  and the gear formed on the inner circumferential surface of the circular spline  39  rotates about the axis A 2 . By such a rotational engagement, the circular spline  39  receives force from the flex spline  43  fixed to the first arm  12  so as to be rotated about the axis A 2 . 
         [0029]    The cross roller ring  35  includes an inner ring  36 , an outer ring  38 , and a roller  40  arranged between the inner ring  36  and the outer ring  38 . The inner ring  36  has a ring shape, and a right end of the inner ring  36  is coupled with a left end of the circular spline  39 , thereby the inner ring  36  rotates integrally with the circular spline  39 . 
         [0030]    The outer ring  38  has a ring shape, and is fixed to the wall  24   c  of the first arm  12  by the bolts  46 . The inner ring  36  is rotatably supported by the roller  40  at radially inside of the outer ring  38 . 
         [0031]    The power transmission mechanism  18  has the inner space S 2 . The inner space S 2  is defined by the wall  24   c  of the first arm  12 , the cross roller ring  35 , the circular spline  39 , and the cover  32  which covers the opening  14   b  of the second arm  14 . 
         [0032]    The inner space S 2  is air-tightly sealed from the outside by oil seals and O-rings  52 . The inner space S 2  is filled with a lubricant in order to lubricate the components of the power transmission mechanism  18  (i.e., the strain wave gearing device  34  and the cross roller ring  35 ). 
         [0033]    The one-way communication section  22  fluidly connects the inner space S 1  of the first arm  12  to the inner space S 2  of the power transmission mechanism  18  so as to be in fluid communication with each other, so that the one-way communication section  22  allows a fluid to flow from the inner space S 1  to the inner space S 2 , while preventing a fluid-flow from the inner space S 2  to the inner space S 1 . 
         [0034]    In this embodiment, the one-way communication section  22  includes a tubular member  48  and a one-way valve  50 . The tubular member  48  has a first opening  48   a  and a second opening  48   b  opposite the first opening  48   a , and is inserted into a through hole  24   d  formed at the wall  24   c  of the first arm  12 . 
         [0035]    The first opening  48   a  opens to the inner space S 1  of the first arm  12 , while the second opening  48   b  opens to the inner space S 2  of the power transmission mechanism  18 . The tubular member  48  defines a passage for a fluid, which extends between the first opening  48   a  and the second opening  48   b , inside thereof. 
         [0036]    The one-way valve  50  is arranged in the passage defined inside of the tubular member  48 , and configured to allow a fluid to flow through the passage from the inner space S i  to the inner space S 2 , while it prevents a fluid from flowing from the inner space S 2  to the inner space S 1 . 
         [0037]    The booster section  20  includes an air pump or the like, and supplies a gas to the inner space S 1  of the first arm  12  so as to increase a pressure in the inner space S 1  to be higher than an outside pressure. 
         [0038]    When the pressure in the inner space S 1  is increased by the booster section  20 , a gas in the inner space S 1  flows through the one-way communication section  22  from the inner space S 1  toward the inner space S 2  to flow into the inner space S 2 . 
         [0039]    Thereby, a pressure in the inner space S 2  of the power transmission mechanism  18  also increases higher than the outside pressure. By increasing the pressures in the inner spaces S 1  and S 2  so as to be higher than the outside pressure in this way, it is possible to prevent a foreign material, such as cutting fluid, from entering the inner spaces S 1  and S 2 , during operation of the joint structure  10 . 
         [0040]    On the other hand, since the one-way communication section  22  prevents a fluid-flow from the inner space S 2  to the inner space S 1 , it is possible to prevent the lubricant filled in the inner space S 2  from flowing out to the inner space S 1  through the one-way communication section  22 . Thus, according to this embodiment, both entrance of the foreign material and outflow of the lubricant can be prevented by a simple configuration. 
         [0041]    Further, in this embodiment, the one-way communication section  22  can be easily mounted to the first arm  12  by merely inserting the tubular member  48 , which houses the one-way valve  50  therein, into the through hole  24   d.  Therefore, a manufacturing process for the joint structure  10  can be simplified. 
         [0042]    Next, referring to  FIG. 2 , a joint structure  60  of a robot according to another embodiment will be described. The joint structure  60  includes a first arm  62 , a second arm  64 , a drive section  66 , a power transmission mechanism  68 , the booster section  20 , and a one-way communication section  70 . 
         [0043]    The first arm  62  is hollow, and extends along an axis A 3 . The first arm  62  includes a body  72  and a cover  76 . The body  72  includes a wall  72   a  which faces the second arm  64 , a cylindrical boss  72   b  projecting leftward from the wall  72   a , and a projection  72   c  arranged at a position separated downward from the boss  72   b  so as to project leftward from the wall  72   a . The projection  72   c  is formed with a through hole  72   d.    
         [0044]    The cover  76  is fixed to the body  72  by bolts  78 . The body  72  and the cover  76  define an inner space S 3  of the first arm  62 . 
         [0045]    The second arm  64  is attached to the first arm  62  via the power transmission mechanism  68  so as to be rotatable about an axis A 4 . 
         [0046]    The drive section  66  generates rotational force for rotating the second arm  64  relative to the first arm  62  about the axis A 4 . Specifically, the drive section  66  includes a servo motor (not shown), a belt  80 , and a pulley  82 . The servo motor includes an output shaft and a pulley fixed to the output shaft (both not shown), and is fixed in the inner space S 3  of the first arm  62 . 
         [0047]    The pulley  82  includes a flange  82   a  and a shaft  82   b  projecting rightward from the flange  82   a , and is rotatable about the axis A 4 . 
         [0048]    The belt  80  is a ring-shaped member, and is stretched at one side thereof on an outer circumference of the pulley fixed to the output shaft of the servo motor, while being stretched at the other side on an outer circumference of the flange  82   a . The belt  80  rotates together with the output shaft of the servo motor, and in turn, rotates the pulley  82  about the axis A 4  along with its rotation. The rotation speed of the output shaft can be increased or reduces by changing an outer diameter of the pulley  82 . 
         [0049]    The power transmission mechanism  68  is provided outside of the first arm  62  and the second arm  64  so as to be adjacent to the first arm  62 , and transmits the rotational force generated by the drive section  66  to the second arm  64 . 
         [0050]    Specifically, the power transmission mechanism  68  includes a strain wave gearing device  84  and a cross roller ring  85 . Similarly to the above-mentioned strain wave gearing device  34 , the strain wave gearing device  84  is a device referred to as harmonic drive (registered trademark), and includes a wave generator  92 , a flex spline  86 , and a circular spline  88 . 
         [0051]    The wave generator  92  includes an elliptical inner ring  92   a  fixed on the shaft  82   b  of the pulley  82 , a flexible outer ring  92   b , and a ball  92   c  arranged between the inner ring  92   a  and the outer ring  92   b.    
         [0052]    The flex spline  86  is made from an elastic material, and one end thereof is fixed to a wall  64   a  of the second arm  64  by bolts  100 . An inner circumferential surface of the flex spline  86  is in contact with the outer ring  92   b  of the wave generator  92 , while an outer circumferential surface of the flex spline  86  is formed with a gear. 
         [0053]    The circular spline  88  has a ring shape, and is fixed to the wall  72   a  of the first arm  62  by bolts  96 . An inner circumferential surface of the circular spline  88  is formed with a gear. 
         [0054]    The gear formed on the outer circumferential surface of the flex spline  86  engages the gear formed on the inner circumferential surface of the circular spline  88  at a position corresponding to a major-axis portion of the inner ring  92   a  of the wave generator  92 , while separating away from the gear formed on the inner circumferential surface of the circular spline  88  at a position corresponding to a minor-axis portion of the inner ring  92   a.    
         [0055]    As the pulley  82  rotates, the engagement position between the gear formed on the outer circumferential surface of the flex spline  86  and the gear formed on the inner circumferential surface of the circular spline  88  rotates about the axis A 4 . By such a rotational engagement, the flex spline  86  receives force from the circular spline  88  fixed to the first arm  62  so as to be rotated about the axis A 4 . 
         [0056]    The cross roller ring  85  includes an outer ring  89  and a roller  90  arranged between the outer ring  89  and the circular spline  88 . The outer ring  89  has a ring shape, and is fixed to the wall  64   a  of the second arm  64  by the bolts  100 . The circular spline  88  is supported radially inside of the outer ring  89  by the roller  90  so as to be rotatable. 
         [0057]    A bearing  94  is interposed between an inner circumferential surface of the boss  72   b  and the shaft  82   b  so as to rotatably support the shaft  82   b . An oil seal  98  is arranged to be adjacent to the left side of the bearing  94 . The oil seal  98  air-tightly seals a gap between the inner circumferential surface of the boss  72   b  and the shaft  82   b.    
         [0058]    The power transmission mechanism  68  has an inner space S 4  which is defined by the boss  72   b  of the first arm  62 , the circular spline  88 , the cross roller ring  85 , the wall  64   a  of the second arm  64 , and the oil seal  98 . 
         [0059]    The inner space S 4  is air-tightly sealed from the outside by the oil seal  98  and the O-ring  52 . In order to lubricate the components of the power transmission mechanism  68  (i.e. the strain wave gearing device  84  and the cross roller ring  85 ), the inner space S 4  is filled with a lubricant. 
         [0060]    The one-way communication section  70  includes a through hole  102 , a first joint  108 , a second joint  110 , a pipe  112 , and a one-way valve  106 . The through hole  102  is formed in the boss  72   b  of the second arm  64  so that one end thereof opens to the inner space S 4  of the power transmission mechanism  68  and the other end thereof opens to the inner space S 3  of the first arm  62 . 
         [0061]    The first joint  108  is hollow and inserted into the through hole  102 . The first joint  108  has a first opening  108   a  which opens in the through hole  102  and a second opening  108   b  opposite the first opening  108   a.    
         [0062]    The second joint  110  is hollow and inserted into the through hole  72   d  formed at the projection  72   c . The second joint  110  has a first opening  110   a  and a second opening  110   b  opposite the first opening  110   a . The second opening  110   b  opens to the inner space S 3 . 
         [0063]    The pipe  112  is an elongated pipe made from an elastic material, wherein one end thereof is connected to the second opening  108   b  of the first joint  108  and the other end thereof is connected to the first opening  110   a  of the second joint  110 . 
         [0064]    In this way, the through hole  102 , the first joint  108 , the second joint  110 , and the pipe  112  define a passage for a fluid, one end of which opens to the inner space S 4  of the second arm  64  and the other end of which opens to the inner space S 3  of the first arm  62 . 
         [0065]    The one-way valve  106  is arranged inside of the second joint  110 . The one-way valve  106  allows a fluid to flow through the passage, which is defined by the through hole  102 , the first joint  108 , the second joint  110  and the pipe  112 , from the inner space S 3  to the inner space S 4 , while preventing a fluid-flow from the inner space S 4  to the inner space S 3 . 
         [0066]    The booster section  20  supplies a gas to the inner space S 3  of the first arm  62  so as to increase a pressure in the inner space S 3  to be higher than an outside pressure. When the pressure in the inner space S 3  is increased by the booster section  20 , the gas in the inner space S 3  flows into the second joint  110  via the second opening  110   b , passes through the pipe  112 , the first joint  108 , and the through hole  102 , and flows into the inner space S 4 . 
         [0067]    In this way, the one-way communication section  70  allows the gas to flow from the inner space S 3  to the inner space S 4 . Due to this, a pressure in the inner space S 4  of the power transmission mechanism  68  also increases higher than the outside pressure. By increasing the pressures in the inner spaces S 3  and S 4  so as to be higher than the outside pressure in this way, it is possible to prevent a foreign material, such as cutting fluid, from entering the inner spaces S 3  and S 4 , during operation of the joint structure  60 . 
         [0068]    On the other hand, since the one-way communication section  70  prevents a fluid-flow from the inner space S 4  to the inner space S 3  by the one-way valve  106 , it is possible to prevent the lubricant filled in the inner space S 4  from flowing out to the inner space S 3  through the one-way communication section  70 . Thus, according to this embodiment, both entrance of a foreign material and outflow of the lubricant can be prevented by a simple configuration. 
         [0069]    Further, in this embodiment, the one-way valve  106  can be arranged at a position separated away from the through hole  102  formed in the wall  72   a  of the first arm  62  by the elongated pipe  112 . According to this configuration, even when the one-way valve  106  cannot be directly mounted in the through hole  102  due to dimensional constraint of the inner space S 3  of the first arm  62 , etc., it is possible to arrange the one-way valve  106  at a suitable position. 
         [0070]    Next, referring to  FIG. 3 , a joint structure  120  of a robot according to still another embodiment will be described. The joint structure  120  includes an arm  122 , an arm  124 , an arm  126 , the drive section  66 , a drive section  128 , the power transmission mechanism  68 , a power transmission mechanism  130 , a flange  156 , the booster section  20 , and a one-way communication section  133 . The arm  122  (first arm) has a configuration similar to that of the above-mentioned first arm  62 . 
         [0071]    The arm  124  (first arm) extends along an axis A 5 , and includes a body  132  and a cover  134 . The body  132  includes a wall  132   a  which faces the arm  126  and a ring-shaped boss  132   b  which projects leftward from the wall  132   a.    
         [0072]    The cover  134  is fixed to the body  132  by bolts  136 . The body  132 , the cover  134 , and oil seals  160  and  164  define an inner space S 5  of the arm  124 . 
         [0073]    Similarly to the embodiment shown in  FIG. 2 , the drive section  66  includes the servo motor (not shown), the belt  80 , and the pulley  82 , and generates rotational force for rotating the arm  126  relative to the arm  122  about the axis A 4 . 
         [0074]    The drive section  128  generates rotational force for rotating the flange  156 . Specifically, the drive section  128  includes a servo motor (not shown), a belt  138 , and a pulley  140 . The servo motor includes an output shaft and a pulley fixed to the output shaft (both not shown), and is fixed in the inner space S 5  of the arm  124 . 
         [0075]    The pulley  140  includes a flange  140   a  and a shaft  140   b  projecting leftward from the flange  140   a , and is rotatable about an axis A 6  orthogonal to the axis A 5 . 
         [0076]    The belt  138  has a ring shape, and is stretched at one side thereof on an outer circumference of the pulley fixed to the output shaft of the servo motor fixed in the inner space S 5 , while being stretched at the other side on an outer circumference of the flange  140   a . The belt  138  rotates together with the output shaft of the servo motor fixed in the inner space S 5 , and in turn, rotates the pulley  140  about the axis A 6  along with its rotation. The rotation speed of the output shaft can be increased or reduced by adjusting outer diameters of the pulley  140  and the pulley fixed to the output shaft. 
         [0077]    The arm  126  (second arm) is hollow, and attached to the arm  122  via the power transmission mechanism  68  so as to be rotatable about the axis A 4 . Further, the arm  126  is supported by the arm  124  at a side opposite to the arm  122  so as to be rotatable about the axis A 6 . 
         [0078]    More specifically, the arm  126  includes walls  142 ,  144 ,  146 ,  148 ,  150 , and  152 . The wall  142  has a configuration similar to that of the above-mentioned wall  64   a , and faces the arm  122 . The wall  144  is arranged opposite to the wall  142 , and faces the arm  124 . The wall  146  extends between lower ends of the walls  142  and  144 . 
         [0079]    The wall  148  is a cylindrical wall arranged so as to surround the axis A 6 , and extends leftward from the wall  144  so as to be parallel to the axis A 6 . A bearing  158  and the oil seal  160  are interposed between the boss  132   b  of the arm  124  and the wall  148  of the arm  126 . 
         [0080]    The wall  150  is a ring-shaped wall extending from a left end of the wall  148  toward the axis A 6 . The wall  152  is cylindrical, and projects rightward from an inner circumferential edge of the wall  150  so as to be parallel to the axis A 6 . 
         [0081]    The oil seal  164  and a bearing  162  arranged adjacent to a left side of the oil seal  164  are interposed between the shaft  140   b  of the pulley  140  and the wall  152  of the arm  126 . The oil seal  164  air-tightly seals a gap between the shaft  140   b  and the wall  152 . The pulley  140  is supported by the bearing  162  so as to be rotatable about the axis A 6 . 
         [0082]    Similarly to the embodiment shown in  FIG. 2 , the power transmission mechanism  68  includes the strain wave gearing device  84  and the cross roller ring  85 , and transmits the rotational force generated by the drive section  66  to the arm  126 . The power transmission mechanism  68  is provided outside of the arm  122  so as to be adjacent to the arm  122 . 
         [0083]    The power transmission mechanism  130  transmits rotational force generated by the drive section  128  to the flange  156 . Specifically, the power transmission mechanism  130  includes a first bevel gear  168 , a gear member  170 , a strain wave gearing device  171 , and a cross roller ring  173 . The first bevel gear  168  is integrally fixed to a distal end of the shaft  140   b  of the pulley  140 , and rotates integrally with the pulley  140  about the axis A 6 . 
         [0084]    The gear member  170  is supported by a bearing  174  so as to be rotatable about an axis A 7 , and includes a second bevel gear  178  and a shaft  180 . The second bevel gear  178  engages the first bevel gear  168  so as to be rotated about the axis A 7  along with the rotation of the first bevel gear  168 . The shaft  180  is integrally fixed to the second bevel gear  178 , and extends upward from the second bevel gear  178  along the axis A 7 . 
         [0085]    Similarly to the above-mentioned strain wave gearing devices  34  and  84 , the strain wave gearing device  171  is a device referred to as harmonic drive (registered trademark), and includes a wave generator  172 , a flex spline  175 , and a circular spline  177 . 
         [0086]    The wave generator  172  includes an elliptical inner ring  172   a  fixed on the shaft  180  of the gear member  170 , a flexible outer ring  172   b , and a ball  172   c  arranged between the inner ring  172   a  and the outer ring  172   b.    
         [0087]    The flex spline  175  is made from an elastic material, and an upper end thereof is fixed to the flange  156 . An inner circumferential surface of the flex spline  175  is in contact with the outer ring  172   b  of the wave generator  172 , while an outer circumferential surface of the flex spline  175  is formed with a gear. 
         [0088]    The circular spline  177  has a ring shape, and is fixed to the arm  126  by bolts  166 . An inner circumferential surface of the circular spline  177  is formed with a gear. 
         [0089]    The gear formed on the outer circumferential surface of the flex spline  175  engages the gear formed on the inner circumferential surface of the circular spline  177  at a position corresponding to a major-axis portion of the inner ring  172   a  of the wave generator  172 , while separating away from the gear formed on the inner circumferential surface of the circular spline  177  at a position corresponding to a minor-axis portion of the inner ring  172   a.    
         [0090]    As the gear member  170  rotates, an engagement position between the gear formed on the outer circumferential surface of the flex spline  175  and the gear formed on the inner circumferential surface of the circular spline  177  rotates about the axis A 7 . By such a rotational engagement, the flex spline  175  receives force from the circular spline  177  fixed to the arm  126  so as to be rotated about the axis A 7 . 
         [0091]    The cross roller ring  173  includes an inner ring  186 , an outer ring  188  and a roller  184  arranged between the inner ring  186  and the outer ring  188 . The inner ring  186  has a ring shape, and is fixed to the flange  156  at an upper end thereof. 
         [0092]    The outer ring  188  has a ring shape, and is fixed to the arm  126  by the bolts  166 . The inner ring  186  is supported by the roller  184  at radially inside of the outer ring  188  so as to be rotatable. 
         [0093]    The power transmission mechanism  130  has an inner space S 7  which is defined by the hollow arm  126 . Further specifically, the inner space S 7  is defined by the walls  142 ,  144 ,  146 ,  148 ,  150  and  152  of the arm  126 , the oil seal  164 , the circular spline  177 , the cross roller ring  173 , and the flange  156 . 
         [0094]    The inner space S 7  is air-tightly sealed from the outside by the oil seal  164  and an  0 -ring  182 . In order to lubricate the components of the power transmission mechanism  130  (i.e. the first bevel gear  168 , the gear member  170 , the strain wave gearing device  171 , and the cross roller ring  173 ), the inner space S 7  is filled with a lubricant. 
         [0095]    The flange  156  is supported so as to be rotatable about the axis A 7 , and is rotated integrally with the flex spline  175  of the strain wave gearing device  171 . An end effector, such as a robot hand or a welding gun, a tool, or the like is connected to the flange  156 . 
         [0096]    The one-way communication section  133  fluidly connects the inner space S 5  of the arm  124  to the inner space S 7  of the power transmission mechanism  130  so as to be in fluid communication with each other, so that the one-way communication section  133  allows a fluid to flow from the inner space S 5  to the inner space S 7 , while preventing a fluid-flow from the inner space S 7  to the inner space S 5 . 
         [0097]    The one-way communication section  133  includes a tubular member  190  and a one-way valve  192 . The tubular member  190  has a first opening  190   a  and a second opening  190   b  opposite the first opening  190   a , and is inserted into a through hole  150   a  formed at the wall  150  of the arm  126 . 
         [0098]    The first opening  190   a  opens to the inner space S 7  of the power transmission mechanism  130 , while the second opening  190   b  opens to the inner space S 5 . The tubular member  190  defines a passage for a fluid, which extends between the first opening  190   a  and the second opening  190   b , inside thereof. 
         [0099]    The one-way valve  192  is arranged in the passage defined inside of the tubular member  190 , and allows a fluid to flow through the passage from the inner space S 5  to the inner space S 7 , while preventing a fluid-flow through the passage from the inner space S 7  to the inner space S 5 . 
         [0100]    The booster section  20  supplies a gas to the inner space S 5  of the arm  124  so as to increase a pressure in the inner space S 5  to be higher than an outside pressure. When the pressure in the inner space S 5  is increased by the booster section  20 , a gas in the inner space S 5  flows into the tubular member  190 , passes through the one-way valve  192 , and flows into the inner space S 7 . 
         [0101]    Due to this, a pressure in the inner space S 7  of the arm  126  also increases higher than the outside pressure. By increasing the pressures in the inner spaces S 5  and S 7  to be higher than the outside pressure in this way, it is possible to prevent a foreign material, such as cutting fluid, from entering the inner spaces S 5  and S 7  during operation of the joint structure  120 . 
         [0102]    On the other hand, since the one-way communication section  133  prevents the fluid-flow from the inner space S 7  to the inner space S 5  by the one-way valve  192 , it is possible to prevent the lubricant filled in the inner space S 7  from flowing out to the inner space S 5  through the one-way communication section  133 . Thus, according to this embodiment, both entrance of a foreign material and outflow of a lubricant can be prevented by a simple configuration. 
         [0103]    Next, referring to  FIG. 4 , a joint structure  200  of a robot according to still another embodiment will be described. The joint structure  200  differs from the joint structure  120  shown in  FIG. 3  in the following configuration, wherein the joint structure  200  further includes a one-way communication section  202 . 
         [0104]    The one-way communication section  202  fluidly connects the inner space S 4  of the power transmission mechanism  68  to the inner space S 7  of the power transmission mechanism  130  so as to be in fluid communication with each other, so that the one-way communication section  202  allows a fluid to flow from the inner space S 7  to the inner space S 4 , while preventing a fluid-flow from the inner space S 4  to the inner space S 7 . 
         [0105]    The one-way communication section  202  includes a tubular member  204  and a one-way valve  206 . The tubular member  204  has a first opening  204   a  and a second opening  204   b  opposite the first opening  204   a , and is inserted into a through hole  142   a  formed at the wall  142  of the arm  126 . 
         [0106]    The first opening  204   a  opens to the inner space S 4  of the power transmission mechanism  68 , while the second opening  204   b  opens to the inner space S 7  of the power transmission mechanism  130 . The tubular member  204  defines a passage for a fluid, which extends between the first opening  204   a  and the second opening  204   b , inside thereof. 
         [0107]    The one-way valve  206  is arranged in the passage defined inside of the tubular member  204 , and allows a fluid to flow through the passage from the inner space S 7  to the inner space S 4 , while preventing a fluid-flow through the passage from the inner space S 4  to the inner space S 7 . 
         [0108]    When the pressures in the inner space S 5  of the arm  124  and the inner space S 7  of the arm  126  are increased by the booster section  20 , a gas in the inner space S 7  passes through the one-way communication section  202  and flows into the inner space S 4 . Thereby, a pressure in the inner space S 4  of the power transmission mechanism  68  also increases higher than an outside pressure. 
         [0109]    By increasing the pressures in the inner spaces S 4r  S 5  and S 7  to be higher than the outside pressure in this way, it is possible to prevent a foreign material, such as cutting fluid, from entering the inner spaces S 4 , S 5  and S 7  during operation of the joint structure  200 . 
         [0110]    On the other hand, since the one-way communication section  202  prevents a fluid-flow from the inner space S 4  to the inner space S 7  by the one-way valve  206 , it is possible to prevent the lubricant filled in the inner space S 4  from flowing out to the inner space S 7  through the one-way communication section  202 . 
         [0111]    Next, referring to  FIG. 5 , a joint structure  210  of a robot according to still another embodiment will be described. The joint structure  210  differs from the joint structure  120  shown in  FIG. 3  in the following configuration, wherein the joint structure  210  further includes the one-way communication section  70 , a first booster section  20   a , and a second booster section  20   b.    
         [0112]    Similarly to the embodiment shown in  FIG. 2 , the one-way communication section  70  includes the through hole  102 , the first joint  108 , the second joint  110 , the pipe  112 , and the one-way valve  106 . 
         [0113]    Similarly to the booster section  20  shown in  FIG. 3 , the first booster section  20   a  supplies a gas to the inner space S 5  of the arm  124  so as to increase the pressure in the inner space S 5  to be higher than an outside pressure. Further, similarly to the booster section  20  shown in  FIG. 2 , the second booster section  20   b  supplies a gas to the inner space S 3  of the arm  122  so as to increase the pressure in the inner space S 3  to be higher than the outside pressure. 
         [0114]    According to this embodiment, the pressures in the inner spaces S 5 , S 7 , S 3  and S 4  are increased by the first booster section  20   a  and the second booster section  20   b , thereby it is possible to prevent a foreign material, such as cutting fluid, from entering the inner spaces S 5 , S 7 , S 3  and S 4  during operation of the joint structure  210 . 
         [0115]    On the other hand, since the one-way communication sections  133  and  70  respectively prevent the fluid-flows from the inner space S 7  to the inner space S 5  and from the inner space S 4  to the inner space S 3 , it is possible to prevent the outflows of the lubricants filled in the inner spaces S 7  and S 4 . 
         [0116]    Note that, in the above-mentioned embodiments, the power transmission mechanisms  18 ,  68  and  130  include the strain wave gearing devices  34 ,  84  and  171 , respectively. However, the power transmission mechanism may include any type of reducer, such as a differential gear reducer, a spur gear reducer, a planetary gear reducer, a cycloid reducer or the like. Alternatively, the power transmission mechanism may be one configured to transmit force by means of a chain or a belt, without a gear. 
         [0117]    While the invention has been described in terms of embodiments of the invention, the embodiments as described above do not limit the invention as recited in the claims. In addition, although a mode in which the features described in the embodiments of the invention are combined can be also included in the technical scope of the invention, all the combinations of these features are not always essential for solution means of the invention. Further, it is also apparent for those skilled in the art that various modifications or improvements can be added to the above embodiments. 
         [0118]    It should be noted that with regard to an execution order of each processing such as an operation, a procedure, a step, a process, a stage, and the like in the device, the system, the program, and the method as recited in the claims, the specification, and the drawings, each processing can be realized in an optional order unless explicit description is made in particular such as “before” and “prior to” and an output of prior processing is used in subsequent processing. With regard to an operation flow in the claims, the specification, and the drawings, even when description has been made using “first”, “next”, “then”, “subsequent” and the like for convenience sake, it is not meant that execution in this order is essential.