Abstract:
Disclosed is a speed reducer capable of precisely and reliably maintaining a rotational force of the output shaft and of maximizing its efficiency by allowing the rotational force of the driving shaft to transmit and to slow down via a plurality of bevel gears and by preventing the fluctuation of the output shaft so as to prevent uneven wear due to the long working and to prevent the damage due to the fluctuation. The speed reducer comprises a pair of driving gears  11,12  installed at a one end of a driving shaft  10  driven by a driving means, a pair of driven gears  21,22  meshed with the driving gear  11,21  and a rotational member  200  for outputting the rotational force to slow down, in which the driven gears  21,22  comprise a bevel gear, respectively and the rotational member  200  is coupled to the driven gears  21,22.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates generally to a speed reducer having a simple configuration, which is capable of precisely and reliably delivering the power to slow down. 
         [0003]    2. Description of the Prior Art 
         [0004]    Generally, the speed reducer which has been conventionally widely used in related industries includes a pinion shaft that receives a power of the motor, a reduction gear that is integrally coupled to a one side of the pinion shaft, a ring gear meshing and rotating with the reduction gear, and a ring gear shaft for inducing the transmitted power to a drive shaft, the ring gear shaft having a pin hole in which a pin is inserted so as to fix the ring gear shaft into the ring gear. 
         [0005]    However, in the conventional speed reducer as described above, the output shaft of the speed reducer is coupled with the ring gear shaft by means of the pin so that the degree of coupling is not strong. Accordingly, it does not guarantee a smooth power transmission. Furthermore, when the speed reducer is operated for a long time, then the pin, the ring gear shaft and the output shaft, etc. cannot stand the rotational force and thereby resulting in frequent damage of these machine parts. As a result, it causes a failure of the reduction gear. 
         [0006]    In general, the ring gear is fixed on the ring gear shaft by means of a key. Likewise, these ring gear and the ring gear shaft may be easily and frequently damaged due to the rotation force while the speed reducer is operated for a long time. This leads to occurrence of excessive maintenance costs for the speed reducer. 
         [0007]    Meanwhile, since a rotational force may be transmitted through the output shaft of a one end of the driving shaft in the conventional speed reducer, the efficiency of power transfer is limited. 
         [0008]    Furthermore, in the conventional speed reducer, since the output shaft is not rigidly fixed, the fluctuations and the uneven wear of the output shaft was terribly occurred, and thereby resulting in the limitation of the transfer of minute rotational force. Since most of the mechanical constitution is very complicate and the overall volume is large, it is hard to use in a system that requires a precise control such as joints of the robot and this leads to occurrence of excessive manufacturing costs. 
       SUMMARY OF THE INVENTION 
       [0009]    In consideration of the above-mentioned disadvantages or inconveniences of the conventional techniques, an object of the present invention is to provide a speed reducer having a simple configuration, which is capable of precisely and reliably delivering the power to slow down, and which can be miniaturized and can operate stably without fluctuation. 
         [0010]    Other object of the present invention is to provide a speed reducer can be widely applied because it can employ various gears such as a planet gear, a spur gear and a harmonic gear, etc. as the reduction gear for decelerating a rotational speed. 
         [0011]    In order to achieve the objects, the present invention provides a speed reducer comprising: 
         [0012]    a driving gear being installed at a one end of a driving shaft driven by a driving means; 
         [0013]    a driven gear for changing the direction of the rotational force of the driving gear, the driven gear being meshed with the driving gear; and 
         [0014]    a rotational member for outputting the rotational force to slow down, the rotational member being coupled to the driven gear. 
         [0015]    Preferably, the driving gear comprises a first driving gear and a second driving gear, which are positioned facing each other with making a pair and are different from each other in size, and wherein the driven gear comprises a first driven gear and a second driven gear, which are positioned facing each other with making a pair, in which the first driven gear and the second driven gear are meshed with the first driving gear and the second driving gear, respectively. 
         [0016]    The first driving gear and the second driving gear are installed at a driving shaft with facing each other. Likewise, the first driven gear and the second driven gear are installed at a driven shaft with facing each other. The driven gears are meshed with the driving gear, respectively so that they output a driving force to the outside. 
         [0017]    Especially, since the first driving gear and the first driven gear have a different size with the same gear ratio, and similarly the second driving gear and the second driven gear have a different size with the same gear ratio, the first driven gear and the second driven gear rotate by the same revolutions. 
         [0018]    If the drive gears and the driven gears are configured to have different sizes as described above, they can be installed at one driving shaft so that they can be coupled to the driven shaft, respectively without interference with each other. As a result, it is possible to minimize the total size of the reduction gear and thereby resulting in manufacturing of the precision reduction gear. Since the driving force can be outputted from both sides of the driving shaft and it is transferred to one rotational member, the speed reducer can precisely and reliably deliver the power in a state that it may operate stably without fluctuation. 
         [0019]    Preferably, the driving gears and the driven gears comprise a bevel gear, respectively. Alternatively, the driving gears and the driven gears comprise a worm screw and a worm wheel, respectively. Any gear system which is capable of changing the direction of the rotational force of the driving gear into a direction perpendicular to the original direction may be employed. 
         [0020]    The driven gear is supported by a driven shaft which comprises a non-rotating fixed shaft, and the driving shaft being installed at the driven shaft in a manner that it passes through a middle portion of the driven shaft. Since the driving gear rotates with the driving shaft as a unit and the driven shaft supports the driven gear, the driven gear may be idling-rotated on the driven shaft. Thus, although the driving shaft and the driven shaft intersect with each other, they do not give any hindrance in each of the power transmission. 
         [0021]    Furthermore, in order to achieve the objects, the present invention provides a speed reducer comprising: 
         [0022]    a driving shaft driven by a driving means; 
         [0023]    a pair of driving gears comprising a first driving gear and a second driving gear, in which the first driving gear and the second driving gear are installed at a one end of the driving shaft and are positioned facing each other, and the first driving gear and the second driving gear have a different size with the same gear ratio and comprise a bevel gear, respectively; 
         [0024]    driven gears comprising a first driven gear and a second driven gear, which are positioned facing each other with making a pair, in which the first driven gear and the second driven gear are meshed with the first driving gear and the second driving gear, respectively and they comprise a bevel gear, respectively; 
         [0025]    a driven shaft for supporting the first driven gear and the second driven gear, in which the driving shaft is installed at the driven shaft in a manner that it passes through a middle portion of the driven shaft; and 
         [0026]    a rotational member for outputting the rotational force to slow down, the rotational member being coupled to the driven gear. 
         [0027]    An idle gear for supporting the other end of the driven gear is installed at the driving shaft and is positioned facing to the driving gear, in which the idle gear and the driving gear have a same size with the same gear ratio. 
         [0028]    Preferably, the speed reducer further comprises an auxiliary speed reducing means installed at the outside of the driven gear. 
         [0029]    The auxiliary speed reducing means comprises a sun gear installed at the driven gear, a planetary gear meshed with the sun gear, and a ring gear meshed with the planetary gear. Alternatively, the auxiliary speed reducing means comprises a combination of spur gears. Alternatively, the auxiliary speed reducing means comprises a harmonic gear. Any rotational force transmitted from the driving shaft may be outputted to slow down due to the operation of the auxiliary speed reducing means. The rotational member for outputting the driving force to slow down can be designed various types in accordance with applications. Preferably, the rotational member may be installed to the driven gear in a manner that it can rotate against the driven gear at 360 degrees. Alternatively, the rotational member is eccentrically mounted to the driven shaft in a manner that it can eccentrically rotate against the driven shaft in an eccentric state. 
         [0030]    As described above, the speed reducer according to the present invention can precisely and reliably deliver the power to slow down and can operate stably without fluctuation by employing the gear system which is capable of changing the direction of the rotational force into a direction perpendicular to the original direction. 
         [0031]    Since the speed reducer according to the present invention can employ various gears such as a planet gear, a spur gear and a harmonic gear, etc. as the reduction gear for decelerating a rotational speed, it can be widely applied. 
         [0032]    Furthermore, the speed reducer according to the present invention can minimize the total size of the reduction gear, thereby resulting in the reduction of the manufacturing cost. Furthermore, the speed reducer according to the present invention can be applied in various fields that require miniaturization and refinement. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]    The above objects and other characteristics and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings, in which: 
           [0034]      FIG. 1  is an exterior perspective view of a speed reducer according to a preferred first embodiment of the present invention; 
           [0035]      FIG. 2  is an exploded perspective view of the speed reducer as illustrated in  FIG. 1 ; 
           [0036]      FIG. 3  shows an external appearance of the speed reducer as illustrated in  FIG. 1 ; 
           [0037]      FIG. 4  shows an application state that the speed reducer as illustrated in  FIG. 1  is installed at a robot joint; 
           [0038]      FIG. 5  is a partial side sectional view of the speed reducer and the robot joint as illustrated in  FIG. 4 ; 
           [0039]      FIG. 6  is an exterior perspective view of the speed reducer according to the preferred first embodiment of the present invention, for showing a state that a fluctuation prevention means is installed at the speed reducer; 
           [0040]      FIG. 7  is a sectional view of the robot joint employing the speed reducer as illustrated in  FIG. 6 ; 
           [0041]      FIG. 8  is a partial side sectional view of the speed reducer and the robot joint as illustrated in  FIG. 7 ; 
           [0042]      FIG. 9  is an exploded perspective view of a speed reducer according to another exemplarily embodiment of the present invention, 
           [0043]      FIG. 10  is an exterior perspective view of a speed reducer according to a preferred second embodiment of the present invention; 
           [0044]      FIG. 11  is an exploded perspective view of the speed reducer as illustrated in  FIG. 10 ; 
           [0045]      FIG. 12  is a partial side sectional view of the speed reducer and the robot joint, for showing an application state that the speed reducer as illustrated in  FIG. 10  is installed at the robot joint; 
           [0046]      FIG. 13  is an exterior perspective view of the speed reducer according to the preferred second embodiment of the present invention, for showing a state that a fluctuation prevention means is installed at the speed reducer; 
           [0047]      FIG. 14  is an exploded perspective view of the speed reducer and the robot joint, for showing the application state that the speed reducer as illustrated in  FIG. 12  is installed at the robot joint; 
           [0048]      FIG. 15  is an exterior perspective view of a speed reducer according to a preferred third embodiment of the present invention; 
           [0049]      FIG. 16  is an exterior perspective view of the speed reducer according to the preferred third embodiment of the present invention, for showing a state that a fluctuation prevention means is installed at the speed reducer; 
           [0050]      FIG. 17  shows an external appearance of the speed reducer according to the preferred third embodiment of the present invention; 
           [0051]      FIG. 18  is an exterior perspective view of the speed reducer and the robot joint, for showing the application state that the speed reducer according to the preferred third embodiment of the present invention is installed at the robot joint; 
           [0052]      FIG. 19  is an exterior perspective view of a speed reducer according to a preferred fourth embodiment of the present invention; 
           [0053]      FIG. 20  is an exploded perspective view of the speed reducer as illustrated in  FIG. 19 ; 
           [0054]      FIG. 21  is an exterior perspective view of the speed reducer according to the preferred fourth embodiment of the present invention, for showing a state that a fluctuation prevention means is installed at the speed reducer; 
           [0055]      FIG. 22  shows an application state that the speed reducer as illustrated in  FIG. 21  is installed at a robot joint; and 
           [0056]      FIG. 23  is an exterior perspective view of the speed reducer and the robot joint, for showing the application state that the speed reducer according to the preferred fourth embodiment of the present invention is installed at the robot joint. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0057]    Hereinafter, the constitution of the abalone habitat reef according to the present invention will be explained in more detail with reference to the accompanying drawings. 
         [0058]    Prior to proceeding to the more detailed description of the preferred embodiments according to the present invention, it should be noted that, for the sake of clarity and understanding of the invention identical components which have identical functions have been identified with identical reference numerals throughout the different views which are illustrated in each of the attached drawing Figures. 
         [0059]      FIG. 1  is an exterior perspective view of a speed reducer according to a preferred first embodiment of the present invention,  FIG. 2  is an exploded perspective view of the speed reducer as illustrated in  FIG. 1 ,  FIG. 3  shows an external appearance of the speed reducer as illustrated in  FIG. 1 , and  FIG. 4  shows an application state that the speed reducer as illustrated in  FIG. 1  is installed at a robot joint. 
         [0060]    Referring to  FIGS. 1 to 4 , the speed reducer comprises a driving gear installed at a one end of a driving shaft  10  which rotates by receiving a driving force from a driving means such as a motor, an engine, etc. The speed reducer further comprises a driven gear for changing the direction of the rotational force of the driving gear into a direction perpendicular to the original direction. The driven gear is installed to the driving gear. As shown in the attached drawings  FIG. 1  to  FIG. 4 , the driving gear and the driven gear comprises a bevel gear. 
         [0061]    The driving gear includes a first driving gear  11  and the second driving gear  12 , which are positioned on the driving shaft  10  facing each other with making a pair and are different from each other in size, and they have the same gear ratio. 
         [0062]    The driven gear comprises a first driven gear  21  and a second driven gear  22 , which are positioned on a driven shaft  20  facing each other with making a pair. The first driven gear  21  and the second driven gear  22  are meshed with the first driving gear  10  and the second driving gear  12 , respectively. The first driven gear  21  and the second driven gear  22  are installed on the driven shaft  20  in a manner that they are arranged in a direction perpendicular to the driving shaft  10 . Accordingly, the direction of the driven shaft  20  is perpendicular to the direction of the driving shaft  10 . 
         [0063]    The driven shaft  20  comprises a non-rotating fixed shaft. The first driven gear  21  and the second driven gear  22  are installed on the driven shaft  20  in a manner that they can rotate about the driven shaft  20 . Since the driving shaft  10  passes through a middle portion of the driven shaft  20 , it is possible to reduce the total volume of the speed reducer. 
         [0064]    Like the structure of the first driving gear  11  and the second driving gear  12 , the first driven gear  21  and the second driven gear  22  have a different size and a same gear ratio. The first driven gear  21  is meshed with the first driving gear  11  and the second driven gear  22  is meshed with the second driving gear  12 , respectively. Due to this gear system, the rotational force of the driving shaft  10  can be transmitted in the direction perpendicular to the direction of the driving shaft  10 . 
         [0065]    The speed reducer as described above can be designed as a speed reducer  100  which may be embedded in a case  101  as best shown in  FIG. 3 . The driving shaft  10  outwardly extends from a one surface of the speed reducer  100  so that it may be connected with a driving means. Side surfaces of the first driven gear  21  and the second driven gear  22  or a member for transmitting the rotational force of the driving shaft  10  may be mounted to both sides of the speed reducer  100 . As shown in  FIG. 4 , a rotational member  200  to be rotated can be mounted to the connecting member or the surfaces of the first driven gear  21  and the second driven gear  22 . 
         [0066]      FIG. 5  is a partial side sectional view of the speed reducer and the robot joint as illustrated in  FIG. 4 . As shown in  FIG. 5 , a pair of connecting members  201  extending from an end of the rotational member  200  can be connected with outer portions of the driven gear. 
         [0067]    Hereinafter, the operation of the speed reducer according to the first embodiment of the present invention will be simply explained. 
         [0068]    If the driving shaft  10  rotates by receiving a driving power from a driving means, the first driving gear  11  and the second driving gear  12  installed on the same driving shaft also rotate. Continuously, the first driven gear  21  meshed with the first driving gear  11  and the second driven gear  22  meshed with the second driving gear  12  rotate, respectively. Since the driving gears  11 , 12  rotate in a state that they contact with an outer circumferential surface of the driven gears  21 , 22 , respectively, the driven gears  21 , 22  have the same rotational direction with each other. 
         [0069]    Since the connecting member  201  connected to the driven gears may output the same movement, the rotational member  200  can rotate by receiving a stable rotational force outputted from both side surfaces of the speed reducer  100 . 
         [0070]    Meanwhile,  FIG. 6  is an exterior perspective view of the speed reducer according to the preferred first embodiment of the present invention, for showing a state that a fluctuation prevention means is installed at the speed reducer,  FIG. 7  is a sectional view of the robot joint employing the speed reducer as illustrated in  FIG. 6 , and  FIG. 8  is a partial side sectional view of the speed reducer and the robot joint as illustrated in  FIG. 7 . 
         [0071]    In the first embodiment according to the present invention, since the driving gears  11 , 12  rotate in a state that they contact with an outer circumferential surface of the driven gears  21 , 22 , respectively, a stress to be biased to one side may be generated. This leads the generation of fine tremor or vibration in the speed reducer. Accordingly, it is required to employ to a means for preventing the generation of fine tremor or vibration. 
         [0072]    In order to solve this problem, an idle gear that is a fluctuation prevention means is further installed at the driving shaft  10  so as to transmit a driving power more reliable. 
         [0073]    The idle gear comprises a first idle gear  31  and a second idle gear  32 . The first idle gear  31  has the same size as the first driving gear  11  and it has the same gear ratio as the first driving gear  11 . The first idle gear  31  is meshed with an outer circumferential surface of the first driven gear  21  so as to support the first driven gear  21  in a manner that it is opposite to the first driving gear  11  on the driving shaft  10 . Likewise, the second idle gear  32  has the same size as the second driving gear  12  and it has the same gear ratio as the second driving gear  12 . The second idle gear  32  is meshed with an outer circumferential surface of the second driven gear  22  so as to support the second driven gear  22  in a manner that it is opposite to the second driving gear  12  on the driving shaft  10 . 
         [0074]    The first idle gear  31  and the second idle gear  32  idling on the driving shaft  10  so that they support the driven gears  21 , 22 , respectively. As a result, the driven gears  21 , 22  can be supported at both sides of the driven shaft  20  and can transmit the driving power at the same gear ratio. Consequently, this gear system can transfer more precise rotation. 
         [0075]    In the first embodiment according to the present invention, as described above, the driving gears are installed on the driving shaft in a state that they are opposite with each other and driven gears being meshed with these driving gears are installed on the driven shaft in a state that they are opposite with each other. Due to this structure, there is no interference between the first driving gear  11  and the second driven gear  22 , or between the second driving gear  12  and the first driven gear  21 . This leads to the smooth power transmission of the gear system. Also, it is possible to reduce the total volume of the speed reducer and it can make more precise speed reducer. 
         [0076]    Although the driving gears and the driven gears comprise a bevel gear so as to output the rotational force in a direction perpendicular to the original direction, most of the gear which is capable of changing the direction of power transmission may be employed. For example,  FIG. 9  is an exploded perspective view of a speed reducer according to another exemplarily embodiment of the present invention, for showing the driving gear and the driven gear comprise a worm screw and a worm wheel, respectively. 
         [0077]      FIGS. 10 to 14  show the speed reducer according to a preferred second embodiment of the present invention.  FIG. 10  is an exterior perspective view of the speed reducer,  FIG. 11  is an exploded perspective view of the speed reducer as illustrated in  FIG. 10 , and  FIG. 12  is a partial side sectional view of the speed reducer and the robot joint. 
         [0078]    Referring to  FIGS. 10 to 12 , the speed reducer according to the preferred second embodiment of the present invention has the same configuration as that of the first embodiment. In other words, the speed reducer according to the preferred second embodiment of the present invention comprises the first driving gear  11  and the second driving gear  12  which comprise a bevel gear, respectively and are installed on the driving shaft  10  in a state that they are opposite to each other; the first driven gear  21  and the driven gear  22  which are installed on the driven shaft in a state that they are opposite to each other and they can be meshed with the first driving gear  11  and the second driving gear  12 ; and the driven shaft  20  for supporting the first driven gear  21  and the driven gear  22 . The speed reducer according to the preferred second embodiment of the present invention further comprises an auxiliary speed reducing means for transmitting the slowed rotational force to further slow down. 
         [0079]    The auxiliary speed reducing means comprises a sun gear  51  installed at the driven gear, a planetary gear  52  meshed with the sun gear  51 , and a ring gear  53  meshed with the planetary gear  52 . The sun gear  51  is installed on the driven shaft  20  in a state that it can rotate at outer side surface of the driven gears  21 , 22  as a unit. 
         [0080]    The ring gear  53  may be directly installed in the case  101  of the speed reducer  100  as shown in the drawings. A gear shaft of the planetary gear  52  is assembled and fixed to the connecting member  201  of the rotational member  200  to be coupled to the speed reducer  100 . 
         [0081]    Due to this structure, the rotational force of the first driving gear  11  and the second driving gear  12  provided by the driving shaft  10  can be transmitted to the first driven gear  21  and the second driven gear  22  to slow down, in turns, after decelerating it again by means of the auxiliary speed reducing means, it can be transmitted to the rotational member  200  coupled to the gear shaft  54  of the planetary gear  52 . 
         [0082]      FIGS. 13 and 14  show a state that a fluctuation prevention means is installed at the speed reducer as in the first embodiment according to the present invention. As shown in  FIGS. 13 and 14 , the first idle gear  31  and the second idle gear  32  for supporting the first driven gear  21  and the second driven gear  22  are installed on the driving shaft  10 . 
         [0083]      FIGS. 15 to 18  show the speed reducer according to a preferred third embodiment of the present invention.  FIG. 15  is an exterior perspective view of the speed reducer,  FIG. 16  is an exterior perspective view of the speed reducer, for showing a state that a fluctuation prevention means is installed at the speed reducer,  FIG. 17  shows an external appearance of the speed reducer installed within a case, and  FIG. 18  is an exterior perspective view of the speed reducer and the robot joint, for showing the application state that the speed reducer is installed at the robot joint. 
         [0084]    Referring to  FIGS. 15 to 18 , the speed reducer according to the preferred third embodiment of the present invention has the same configuration as that of the first embodiment. 
         [0085]    In other words, the speed reducer according to the preferred third embodiment of the present invention comprises the first driving gear  11  and the second driving gear  12  which comprise a bevel gear, respectively and are installed on the driving shaft  10  in a state that they are opposite to each other; the first driven gear  21  and the driven gear  22  which are installed on the driven shaft in a state that they are opposite to each other and they can be meshed with the first driving gear  11  and the second driving gear  12 ; and the driven shaft  20  for supporting the first driven gear  21  and the driven gear  22 . The speed reducer according to the preferred third embodiment of the present invention further comprises an auxiliary speed reducing means for transmitting the slowed rotational force to further slow down. 
         [0086]    The auxiliary speed reducing means comprises an output gear  61  installed on the driven shaft  20  in a manner that it may integrally formed or fixed with an outer side surface of the driven gears  21 , 22  as a unit, and a spur gear  62  meshed with the output gear  61 . 
         [0087]    Due to this structure, the rotational force of the first driving gear  11  and the second driving gear  12  provided by the driving shaft  10  can be transmitted to the first driven gear  21  and the second driven gear  22  to slow down, in turns, after decelerating it again by means of the spur gear  62 , it can be transmitted to the rotational member  200 . Appropriate number of spur gears can be added between the output gear  61  and the spur gear  62  according to the need. Furthermore, as shown in  FIG. 16 , it is possible to additionally install the first idle gear  31  and the second idle gear  32  can be installed on the driving shaft  10  so as to prevent fluctuation during the power transmission. 
         [0088]      FIG. 19  is an exterior perspective view of a speed reducer according to a preferred fourth embodiment of the present invention,  FIG. 20  is an exploded perspective view of the speed reducer as illustrated in  FIG. 19 ,  FIG. 21  is an exterior perspective view of the speed reducer, for showing a state that a fluctuation prevention means is installed at the speed reducer,  FIG. 22  shows an application state that the speed reducer as illustrated in  FIG. 21  is installed at a robot joint, and  FIG. 23  is an exterior perspective view of the speed reducer and the robot joint, for showing the application state that the speed reducer according to the preferred fourth embodiment of the present invention is installed at the robot joint. 
         [0089]    Referring to  FIGS. 19 to 23 , the speed reducer according to the preferred fourth embodiment of the present invention has the same configuration as that of the first embodiment. 
         [0090]    In other words, the speed reducer according to the preferred fourth embodiment of the present invention comprises the first driving gear  11  and the second driving gear  12  which comprise a bevel gear, respectively and are installed on the driving shaft  10  in a state that they are opposite to each other; the first driven gear  21  and the driven gear  22  which are installed on the driven shaft in a state that they are opposite to each other and they can be meshed with the first driving gear  11  and the second driving gear  12 ; and the driven shaft  20  for supporting the first driven gear  21  and the driven gear  22 . The speed reducer according to the preferred third embodiment of the present invention further comprises an auxiliary speed reducing means for transmitting the slowed rotational force to further slow down. 
         [0091]    The auxiliary speed reducing means comprises a harmonic gear installed on the driven shaft  20  in a manner that it may integrally formed or fixed with an outer side surface of the driven gears  21 , 22  as a unit. An internal gear  71  that is a part of the harmonic gear  70  may be directly formed at an inner side of the case  101 . 
         [0092]    Due to this structure, the rotational force of the first driving gear  11  and the second driving gear  12  provided by the driving shaft  10  can be transmitted to the first driven gear  21  and the second driven gear  22  to slow down, in turns, after decelerating it again by means of the harmonic gear  70 , it can be transmitted to the rotational member  200 . 
         [0093]    Furthermore, as shown in  FIGS. 21 and 22 , it is possible to additionally install the first idle gear  31  and the second idle gear  32  can be installed on the driving shaft  10  so as to prevent fluctuation during the power transmission. 
         [0094]    As described above, the speed reducer according to the present invention can be applied to various instruments. For example, if the reduction gear and the rotational member are used in the joint of robot, the rotational member functions as the robot arm. 
         [0095]    When the driving gear and the driven gear are installed on the driving shaft and the driven shaft with making a pair, they can have a different gear ratio so as to allow them to rotate at a different rotational speed. 
         [0096]    When the driven gears are installed on the driven shaft with making a pair, one rotational member can be connected to outer side surfaces of the driven gears. Alternatively, different rotational members can be connected to the individual driven gear. Alternatively, different rotational members can be installed in a manner that they can rotate at a different rotational speed. The rotational member is installed to the driven gear in a manner that it can rotate against the driven gear at 360 degrees. Alternatively, the rotational member is eccentrically mounted to the driven shaft in a manner that it can eccentrically rotate against the driven shaft in an eccentric state. 
         [0097]    It is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the above description or illustrated in the drawings. 
         [0098]    The invention is capable of other embodiments and of being practiced and carried out in various ways by modifying the structure of artificial reef as needs of manufacturer of the artificial reef on the basis of its application. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting. 
         [0099]    While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention.