Abstract:
The power transmission device includes an internal gear and an external gear that is inscribed in the internal gear and engages with the internal gear, and can transmit an input power to an attachment. The power transmission device further includes: an inner pin for bringing out a relative rotation component between the internal gear and the external gear; and an output flange connected to the inner pin. In this configuration, the inner pin and the output flange are integrally formed as one member, and a mounting hole for connecting the output flange to the attachment is formed in a surface of the output flange that is opposite to the inner pin.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a power transmission device. More particularly, the present invention relates to a power transmission device capable of transmitting an input power to a next-stage member (including an attachment), which can be suitably used for driving a robot wrist.  
         [0003]     2. Description of the Related Art  
         [0004]     A power transmission device disclosed in Japanese Patent Laid-Open Publication No. 2002-61720 is conventionally known. This power transmission device has an output shaft in the shape of a flange and can be directly attached to a next-stage member (e.g., an attachment of an industrial robot).  
         [0005]      FIGS. 4A, 4B , and  4 C show a power transmission device  290  having approximately the same structure as the aforementioned conventional power transmission device disclosed in Japanese Patent Laid-Open Publication No. 2002-61720.  FIG. 4A  is a front view of the power transmission device  290 ,  FIG. 4B  is a cross-sectional view thereof, taken along the line IVB-IVB in  FIG. 4A , and  FIG. 4C  is a rear view thereof.  
         [0006]     The power transmission device  290  includes an input shaft  260 , an eccentric body  240  that is rotated in an eccentric manner by rotation of the input shaft  260 , a bearing  230  for the eccentric body that transmits the eccentric rotation of the eccentric body  240 , an external gear  238  that is fitted with the bearing  230 , and an internal gear  234  in which the external gear  238  is inscribed. The internal gear  234  and the external gear  238  engage with each other. There is a small difference between the number of teeth of the internal gear  234  and that of the external gear  238 . The internal gear  234  also serves as a casing  250 .  
         [0007]     The external gear  238  has a plurality of inner pin holes  238   a . An inner pin  236  and an inner roller  232  are freely inserted into each of the inner pin holes  238   a.    
         [0008]     The inner pin  236  is fitted into a first output flange  200  and a second output flange  202 . The first output flange  200  and the second output flange  202  are connected to each other via a carrier bolt  228 .  
         [0009]     The reference numeral  270  in  FIG. 4A  denotes a mounting hole used for mounting an attachment (not shown) of a robot onto the power transmission device  290 .  
         [0010]     When the input shaft  260  rotates around a shaft center O 4 , the eccentric body  240  provided on an outer circumference of the input shaft  260  also rotates. The rotation of the eccentric body  240  tries to cause oscillating rotation of the external gear  238  around the input shaft  260 . However, the rotation of the external gear  238  is constrained by the internal gear  234 . Therefore, the external gear  238  makes an oscillating movement almost only, while being in contact with the internal gear  234 .  
         [0011]     The oscillating movement component of the oscillating rotation of the external gear  238  is absorbed by the inner pin hole  238   a  and the inner pin  236  (and the inner roller  232 ). Only the rotation component generated by the difference between the number of teeth of the external gear  238  and that of the internal gear  234  is transmitted to the attachment via the first output flange  200 .  
         [0012]     When the above-described power transmission device is used especially in an industrial robot, a next-stage member such as an attachment to be mounted onto the power transmission device (hereinafter, simply referred to as an attachment or the like) is inevitably different depending on the purpose of the industrial robot, e.g., welding, transport, or assembly. Thus, the power transmission device should be able to transmit a power to various types of attachment or the like.  
         [0013]     Therefore, when a mounting hole (e.g., a tap) is formed in the output flange of the power transmission device and the attachment or the like is mounted by means of a mounting screw or the like, if another member (e.g., the inner pin or the carrier bolt) is fitted into the output flange, it is inevitably necessary to form the mounting hole at a position other than a position of the other member. Thus, the position of the mounting hole and the number of mounting holes that can be formed are limited (see  FIG. 4A ).  
         [0014]     In other words, for some types of attachment or the like to be mounted, the mounting hole cannot be formed while sufficient mounting strength is ensured. Thus, in some cases, it is necessary to use a separate joint flange for connection, or a problem is caused that the power transmission device or a mounting portion of the attachment or the like to be mounted has to be redesigned.  
       SUMMARY OF THE INVENTION  
       [0015]     In view of the foregoing problems, various exemplary embodiments of this invention provide a power transmission device including an output flange that has a high degree of freedom of design for a mounting hole used for attaching an attachment or the like to the power transmission device, thereby allowing a wider variety of attachment or the like to be directly attached to the power transmission device without using a separate joint flange or redesigning the power transmission device and the like.  
         [0016]     Various exemplary embodiments of the present invention provide a power transmission device for driving a robot wrist. The power transmission device includes an internal gear and an external gear that is inscribed in the internal gear and engages with the internal gear, and can transmit an input power to an attachment. The power transmission device further includes: an inner pin for bringing out a relative rotation component between the internal gear and the external gear; and an output flange connected to the inner pin. In this configuration, the inner pin and the output flange are integrally formed as one member, and a mounting hole for connecting the output flange to the attachment is formed in a surface of the output flange that is opposite to the inner pin.  
         [0017]     According to various exemplary embodiments of the invention, the inner pin and the output flange are integrally formed. Thus, it is possible to avoid a problem in which the mounting hole for attaching the attachment or the like has to be designed at a position other than a position of the inner pin. Therefore, the mounting hole for attaching the attachment or the like can be designed more freely.  
         [0018]     The present invention can be applied to a reducer in which the inner pin is supported at both ends. Moreover, the present invention can be applied to a reducer in which the inner pin projects from the output flange while being supported at one end, as in the aforementioned example, as well as an exemplary embodiment described later. Incidentally, in the reducer in which the inner pin is supported at both ends, the use of a carrier bolt is not denied. However, the inner pin according to various exemplary embodiments of the present invention is integrated with the output flange and has strength that is sufficient to serve as the carrier bolt. Thus, an operation of the present invention can be more significantly achieved by completely eliminating the carrier bolt and making all the pins serve as inner pins.  
         [0019]     In this description, a side closer to a working portion (attachment) in an industrial robot is referred to as a “next stage.” 
         [0020]     According to various exemplary embodiments of the present invention, it is unnecessary to use a separate joint flange in accordance with the attachment or the like or change design of the power transmission device and the like. Furthermore, the power transmission device that is compact, has a good balance of rotation, and has high strength can be designed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]      FIG. 1  is a partial cross-sectional view of a whole power transmission device  190  attached to a robot wrist according to an exemplary embodiment of the present invention;  
         [0022]      FIGS. 2A and 2B  show the whole power transmission device according to the exemplary embodiment of the present invention,  
         [0023]      FIG. 2A  being a front view thereof, and  FIG. 2B  being a cross-sectional view thereof, taken along the line IIB-IIB in  FIG. 2A ;  
         [0024]      FIGS. 3A and 3B  show the power transmission device shown in  FIGS. 2A and 2B  in which a mounting hole for a next-stage member is provided,  FIG. 3A  being a front view thereof, and  FIG. 3B  being a cross-sectional view thereof, taken along the line IIIB-IIIB in  FIG. 3A ; and  
         [0025]      FIGS. 4A, 4B , and  4 C show a whole power transmission device of a conventional example,  FIG. 4A  being a front view thereof,  FIG. 4B  being a cross-sectional view thereof, taken along the line IVB-IVB in  FIG. 4A , and  FIG. 4C  being a rear view thereof. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]     An exemplary embodiment of the present invention is now described with reference to the accompanying drawings. In the description and the drawings, components that are the same or similar as/to those in the aforementioned conventional example are labeled with reference numerals in which last two digits are the same as those in the conventional example, and the description of those components is omitted in an appropriate manner. That is, only a difference between the exemplary embodiments of the present invention and the conventional example is described.  
         [0027]      FIG. 1  is a partial cross-sectional view showing a whole power transmission device according to an exemplary embodiment of the present invention. The power transmission device is attached to a robot wrist. In the following description, the robot wrist means a portion including the fourth one of a plurality of axes included in a robot and all the following portions. More specifically, the robot wrist means a portion including an arm portion of the robot formed by basic three axes, i.e., a pivot axis, a back-and-forth axis, and a vertical axis and the following portion (a portion arranged more closely to the attachment).  
         [0028]     A wrist including three joints J 4 , J 5 , and J 6  is provided in a robot arm  154  extended from the arm portion. An attachment  176  is attached to an end of the robot arm  154 .  FIG. 1  only shows a part of the attachment  176 . Although three joints are provided in the wrist in the present exemplary embodiment, the number of the joints is not limited thereto. Four or more, or two or less, joints may form the wrist. Each of those joints J 4 , J 5 , and J 6  includes a power transmission device. More specifically, the joint J 4  includes a power transmission device  190 , the joint J 5  includes a power transmission device  490  ( FIG. 1  only shows an appearance thereof), and the joint J 6  includes a power transmission device  390 . The joint J 4  is arranged to be rotatable in an X-direction around a shaft center O 1 , the joint J 5  is arranged to be rotatable in a Y-direction around a shaft center O 2 , and the joint J 6  is arranged to be rotatable in a Z-direction around a shaft center O 3 . According to this structure, cooperating rotation of those joints J 4 , J 5 , and J 6  enables the attachment  176  to be freely manipulated three-dimensionally.  
         [0029]     The power transmission devices  190 ,  390 , and  490  respectively included in the joints J 4 , J 5 , and J 6  have the same structure basically, although they are different in detail.  
         [0030]     Next, the power transmission device  190  will be described as a representative of the power transmission devices  190 ,  390 , and  490  with reference to  FIGS. 2A and 2B . The other power transmission devices  390  and  490  have approximately the same structure as the power transmission device  190 . Therefore, the same or similar components in the power transmission devices  390  and  490  as/to those in the power transmission device  190  are labeled with reference numerals that are the same in last two digits as those in the power transmission device  190 , and redundant description is omitted.  FIGS. 2A and 2B  show the whole power transmission device  190 .  FIG. 2A  is a front view thereof, and  FIG. 2B  is a cross-sectional view thereof, taken along the line IIB-IIB.  
         [0031]     In the present exemplary embodiment, an inner pin  136  and a first output flange (output flange)  100  are integrally formed as one member. Herein, the phrase “be integrally formed as one member” does not mean that a plurality of parts are integrated by fixing them by press fitting, adhesion, or the like but means that they are originally formed with integrity by forging or the like.  
         [0032]     The first output flange  100  integrally formed with the inner pin  136  is connected and fixed to a second output flange  102  (second output flange) via the inner pin  136  by means of a bolt  128  screwed from a side of the second output flange  102  that is opposite to the inner pin  136 . No carrier bolt is used in this arrangement. A first output flange surface  100   a  is secured on the first output flange  100  on a side opposite to the inner pin  136 , as shown with hatching in  FIG. 2A . The first output flange surface  100   a  has nothing formed thereon and is flat. Therefore, a mounting hole can be freely formed in the first output flange surface  100   a  in advance or in accordance with a hole position in a next-stage member  131  later.  FIGS. 3A and 3B  show an example of formation of an exemplary mounting hole  170 .  
         [0033]     An operation of the present exemplary embodiment will now be described. In the following, redundant description is omitted and only a difference between the present exemplary embodiment and the conventional example is described.  
         [0034]     Since the inner pin  136  for transmitting a rotation component of an external gear  138  is formed integrally with the first output flange  100 , the first output flange surface  100   a  is secured on the side of the first output flange  100  that is close to the next-stage member  131  (i.e., on the side opposite to the inner pin  136 ) as shown with hatching (see  FIG. 2A ). In the exemplary embodiment shown in  FIGS. 3A and 3B , screw holes  170  are formed in the first output flange surface  100   a  at similar positions to those in  FIG. 4A  for the sake of convenience. However, the position at which the screw hole  170  is formed is not limited thereto, as is apparent from comparison between  FIG. 3A  and  FIG. 4A . This is because an end face of the inner pin  236  or the carrier bolt  228  that was conventionally located on the first output flange surface  100   a  is not located on the first output flange surface  100   a  in the present exemplary embodiment.  
         [0035]     Therefore, even if the next-stage member is changed, there are few needs of using a separate joint flange or changing design of the power transmission device in accordance with the next-stage member.  
         [0036]     Especially, in the case of a power transmission device attached and used in a wrist of an industrial robot, it is preferable to make the power transmission device as light and small as possible in order to precisely control the robot (position the robot), secure a wide work range, and save an electric power.  
         [0037]     Therefore, it is highly significant that the degree of freedom of determining the position at which the mounting hole  170  is processed is improved without increasing the weight or the like, as in the present exemplary embodiment.  
         [0038]     Moreover, in the present exemplary embodiment, the first output flange  100  and the second output flange  102  are connected to each other by means of the inner pin  136  only without using a carrier bolt conventionally used. However, the first output flange is integrated with the inner pin  136  and the second output flange is tightly connected to the first output flange by means of the bolt  128 . Therefore, each of the inner pins  136  can sufficiently fulfill a connecting function of the carrier bolt conventionally used.  
         [0039]     In addition, all the inner pins  136  can contribute to power transmission. Therefore, a load applied to each inner pin  136  is reduced because the inner pin  136  is also arranged at a position at which the carrier bolt is conventionally arranged.  
         [0040]     Furthermore, the inner pin  136  and the bolt  128  are arranged evenly in a radial direction. Therefore, the power transmission device that has a good balance of rotation during an operation can be achieved.  
         [0041]     In the present exemplary embodiment, a top end of the inner pin  136  is supported by the second output flange  102 . Alternatively, another arrangement may be employed in which the second output flange is omitted and the inner pin  136  projects from the first output flange  100  while being supported at one end. That is, the arrangement around the top end of the inner pin  136  (the side close to the second output flange in the above exemplary embodiment) is not specifically limited.  
         [0042]     Moreover, the external gear is formed of three pieces in the present exemplary embodiment. However, the structure of the external gear is not necessarily limited thereto. The number of pieces forming the external gear may be selected in accordance with a transmission capacity (e.g., one or two).  
         [0043]     The most significant effect of the present invention can be achieved when the present invention is applied to a power transmission device attached to a wrist of an industrial robot, as described in the exemplary embodiment. A servomotor may be connected with the power transmission device. However, it is apparent that the present invention can be also applied to another type of power transmission device for transmitting a power to another machine.  
         [0044]     The disclosure of Japanese Patent Application No. 2005-86971 filed Mar. 24, 2005 including specification, drawing and claim are incorporated herein by reference in its entirety.