Abstract:
In a speed reducer, because the idler gears  62  that mesh with the drive gear  58  and the external gears  55  fixed to the crankshafts  45  are provided between those gears  55  and  58,  the reduction of the diameters of the external gears  55  and the drive gear  58  can be easily accomplished. Consequently, the inertia moment, and hence the size of the servomotor for driving, become smaller, and vibration and the noise caused by engagement can be reduced as well. The radial distance L between the crankshaft  45  and the center axis Z of the speed reducer  64  can be easily increased to improve the rigidity of the speed reducer  64  in the circumferential direction.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to an eccentric orbiting reducer where the speed reduction is achieved by eccentrically revolving the pinions. 
     In general, the eccentric orbiting reducer has been used in various applications, especially as a reducer for robotics, because it is compact and yet provides a high reduction ratio. 
     However, the eccentric orbiting reducer  11  available on the market has a shortcoming in that its rigidity in the rotational direction, which is an important property of a reducer for robotics, is low because the radial distance L between the crankshaft  12  and the center axis Z of the reducer  11  is relatively short as shown in FIG.  3 . 
     One possible idea for solving such a problem would be to use larger diameter gears for both the external gear  14  fixed on the crankshaft  12  and a drive gear  15  that meshes with the external gear  14  in order to increase the above-mentioned radial distance L as shown in FIG. 4, thus improving the rigidity of the reducer  11  in the rotational direction. 
     However, such a reducer  11  with a large diameter external gear  14  and drive gear  15  has problems such that it has a large inertia moment because of large gears, so that it tends to generate substantial vibration during acceleration and deceleration and it also requires a larger servomotor to drive the drive gear  15 . Moreover, since the external gears  14  and the drive gear  15  are large, it can create interference with the external gear  14  in the circumferential direction and noise due to a high circumferential speed. 
     SUMMARY OF THE INVENTION 
     The present invention intends to provide an eccentric orbiting type speed reducer that produces little vibration and noise, while it is capable of easily improving its circumferential rigidity although it is compact and economical and is also capable of having its drive gear concentric with the center axis of the reducer while increasing the radial distance L. 
     An eccentric orbiting type speed reducer, constructed as a preferred embodiment of the present invention, comprises: a cylindrical member having internal gear teeth on its inner circumference; pinions having external gear teeth that mesh with the internal gear teeth each having a number of teeth slightly less than the internal gear teeth; a carrier having pillars that penetrate through the pinions in an axial direction; multiple crankshafts that are arranged equiangularly in a circumferential direction, are rotatably supported by the carrier at both axial ends, penetrate through the pinions in the middle, and cause the pinions to eccentrically revolve when they rotate; external gears each fixed on one end of a respective one of the crankshafts; a drive gear located in a position surrounded by the external gears; and multiple idler gears, each of which is rotatably supported by the carrier to be located between a respective one of the external gears and the drive gears to mesh with those gears, wherein the rotation of the drive gear is transmitted through the idler gears and the external gears to the crankshafts to cause the pinions to revolve eccentrically as well as to reduce the eccentric rotation of the pinions by means of the external and internal gear teeth, so that the cylindrical member or the carrier can rotate at a slow speed. 
     By having idler gears that mesh with the external gears fixed to the crankshafts and the drive gear between them, the reduction of the diameters of the external gears and the drive gear can be easily achieved. As a consequence, the inertia moment of the drive gear as well as vibration during acceleration and deceleration become smaller, hence reducing the size of the servomotor as well. The reduction of the diameters of the external gear and the drive gear also reduces the circumferential speed of the external gears and the drive gear, which in turn reduces noise to improve the working environment. Having idler gears increases the distance between the crankshaft and the reducer center axis of the reducer in the radial direction, which contributes to an easy improvement of the rigidity of the speed reducer in the circumferential direction. 
     In order to rotate the pinion smoothly, it is necessary to have the rotating phases of the three crankshafts match completely, but the above-mentioned requirements can be satisfied as long as the assembly condition described in claim 2 is satisfied. 
     The present disclosure relates to the subject matter contained in Japanese patent application No. Hei. 2000-29478 (filed on Feb. 7, 2000), which is expressly incorporated herein by reference in its entirety. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross sectional front view showing an embodiment of the invention 
     FIG. 2 is a Side view of the front stage gear speed reducing unit 
     FIG. 3 is a front view of a related eccentric orbiting type speed reducer 
     FIG. 4 is a font view of a conceivable example of the related eccentric orbiting type speed reducer 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of the invention is described referring to the accompanying drawings. 
     FIGS. 1 and 2 show a flange  21  of the servomotor, which will be described later, and a cylindrical member  22 , which is attached to a robot arm (not illustrated) and also fixed on one end of the flange  21 . Multiple internal tooth pins  23  inserted halfway and fixed thereby to the inner circumference of the cylindrical member  22  serve as the internal teeth and these internal tooth pins  23  extend axially and are also equally spaced in the circumferential direction. The cylindrical member  22  contains two ring-shaped pinions  24  and  25  placed axially apart from each other, and external gear teeth  26  and  27  formed on the outer circumference of pinions  24  and  25  respectively, where each of them has a number of teeth slightly less than the number of the internal tooth pins  23 . While these external teeth  26  and  27  of both of these pinions  24  and  25  mesh with the internal tooth pins  23  of the cylindrical member  22 , and the largest meshing points of these pinions  24  and  25  (the deepest points of the engagements of the teeth) are 180 degrees apart in phase angle. 
     The pinions  24  and  25  have multiple idle holes  31  and through holes  32  reciprocally formed in the circumferential direction. A carrier  35  is attached on a stationary robot member (not shown) enclosed in a cylindrical member  22 , and the carrier  35  consists of a pair of circular disk-shaped end plates  36  and  37  that are placed on both external sides of the pinions  24  and  25  in the axial direction and pillars  39 , one end of which is integrally connected to the end plate  36  and the other end connected to the end plate  37  by means of multiple bolts  38 . The pillars  39  that connect the end plates  36  and  37  extend axially and loosely fit with idle holes  31  of the pinions  24  and  25 . Bearings  40  and  41  are provided between the outer circumference of the end plates  36  and  37  and the inner circumference of the cylindrical member  22  and the cylindrical member  22  is supported rotatably by the carrier  35  by means of these bearings  40  and  41 . 
     Multiple (3 in this case to match with the number of through holes  32 ) crankshafts  45  are spaced equiangularly in the circumferential direction, and one end of the crankshaft  45  in the axial direction is rotatably supported by the end plate  36  via a bearing  46 , while the other end in the axial direction is rotatably supported by the end plate  37  via a bearing  47 . Each crankshaft  45  has two eccentric cams  48  and  49 , which are offset at an equal distance from the center axis of the crankshaft  45 , in the middle of the shaft in the axial direction, and these eccentric cams  48  and  49  are 180 degrees apart in phase angle. The middle of the crankshaft  45 , i.e., the eccentric cams  48  and  49 , are inserted into the through hole  32  of the pinions  24  and  25  provided in roller bearings  51  and  52  respectively. When these crankshafts  45  rotate with a constant speed in the same direction, the pinions  24  and  25  revolve eccentrically with phases 180 degrees apart. 
     An external gear  55  is fixed to an end of each crankshaft  45  protruding from the end plate  37  to the other side. These external gears  55  are spaced equiangularly on a circle. A drive shaft  56  is supported rotatably by the flange  21  via a bearing  57  and this drive shaft  56  rotates driven by a servomotor (not illustrated). A large number of external teeth are formed on one end of the drive shaft  56  on the outer circumference and these external teeth constitute a drive gear  58  which drives and rotates. The drive gear  58  is surrounded by the external gears  55  and is concentric with the cylindrical member  22 . 
     Multiple (as many as the number of the external gears  55 ) support shafts  60  are attached to the end plate  37  and the other end of each support shaft  60  supports an idler gear  62  rotatably via a roller bearing  61 . Consequently, these multiple (as many as the number of external gears  55 ) idler gears  62  are supported rotatably by the carrier  35  via the supporting shafts  60 . These idler gears  62  are provided between the external gears  55  and the drive gear  58  in such a way that their rotating shafts are on the straight lines connecting the rotating shafts of the external gears  55  and the drive gear  58 ., and so that they mesh with both the external gears  55  and the drive gear  58 . These external gears  55 , the drive gear  58  and the idler gears  62  constitute a front stage gear speed reducing unit  63 , while the cylindrical member  22 , the pinions  24  and  25 , the carrier  35 , the crankshafts  45 , and the front gear reducing unit  63  constitute an eccentric orbiting type speed reducer  64 . 
     In order for the pinions  24  and  25  to rotate smoothly, the rotating phases of all (three spaced equiangularly, in this case) the crankshafts  45  must match completely. The assembly constituting condition for the front stage gear speed reducing unit  63  is as follows assuming A is the number of teeth of the drive gear  58 , B is the number of teeth of the external gear  55 , and C is the number of teeth of the idler gear  62 . The condition is: when the drive gear  58  is rotating clockwise between any pair of external gears  55  of the three external gears  55 , the value obtained from either of the following equations (1) and (2) is a positive integer and further when the drive gear  58  is rotating counterclockwise between the same pair of external gears  55 , the value of the other equation is also a positive integer. 
     
       
           N =(2 A+B )/3 + C   (1) 
       
     
     
       
           P =( A+ 2 B )/3+ C   (2) 
       
     
     If the equation (1) is rewritten using a positive integer M=N−C, the following equation (3) is obtained. 
     
       
         2 A+B= 3 M   (3) 
       
     
     Similarly, if the equation (2) is rewritten using a positive integer Q=P−C, the following equation (4) is obtained. 
     
       
           A+ 2 B= 3 Q   (4) 
       
     
     If the number of teeth of the drive gear  58  and the number of teeth of the external gear  55  satisfy both of the above equations (3) and (4) when the rotating direction of the drive gear  58  is reversed, the assembly conditions will be met and the rotating phase of the three crankshafts  45  match completely and the pinions  24  and  25  rotate smoothly. 
     Next, the operation of an embodiment of the invention is described. 
     In reducing speed using the eccentric orbiting type speed reducer  64 , the drive shaft  56  and the drive gear  58  are driven by a servomotor (not illustrated) and the rotation of this drive gear  58  is transmitted to the crankshafts  45  via the idler gears  62  and the external gears  55 . Consequently, all the crankshafts  45  rotate around the center axis in the same direction at the same speed. The eccentric cams  48  and  49  of the crankshafts  45  rotate eccentrically inside the through holes  32  of the pinions  24  and  25  and cause the pinions  24  and  25  to rotate eccentrically (revolution). Since the number of external gear teeth  26  and  27  are slightly smaller than the number of internal tooth pins  23 , the eccentric revolution of the pinions  24  and  25  is reduced in speed by means of the external gear teeth  26  and  27  and the internal tooth pins  23  at a high reduction ratio, and is transmitted to the cylindrical member  22  to cause the cylindrical member  22  to rotate at a low speed. 
     Since the idler gears  62  are provided between the external gears  55  fixed to the crankshafts  45  and the drive gear  58  to mesh with both gears  55  and  58 , it is easy to make the diameter of the external gears  55  and the drive gear  58  smaller. Consequently, it is possible to reduce the inertia moment of the drive gear  58  and vibration during acceleration and deceleration, and so the compact size of the servomotor used to drive the drive gear  58  become sufficient. Moreover, reducing the diameter of the external gears  55  and the drive gears  58  reduce circumferential speeds and noise, which results in a better working environment. Furthermore, the use of the idler gears  62  makes it possible to increase the radial distance L between the crankshafts  45  and the center axis Z of the speed reducer  64 , which in turn makes it easy to improve the circumferential rigidity of the speed reducer  64 . 
     Although the cylindrical member  22  is rotatable, the carrier  35  is stationary, and a rotation inputted into the crankshafts  45  is speed-reduced by the pinions  24  and  25  to be outputted to the cylindrical member  22  in the above embodiment, and it is also possible to make the cylinder stationary and the carrier rotatable so that a rotation inputted into the crankshafts can be speed-reduced and outputted to the carrier. 
     As described above, the invention provides a compact and inexpensive speed reducer with less vibration and noise and high rigidity in the circumferential direction.