Patent Publication Number: US-2010113206-A1

Title: Eccentric oscillating type speed reducer and apparatus for rotating stabilizer shaft using the eccentric oscillating type speed reducer

Description:
TECHNICAL FIELD 
     The present invention relates to an eccentric oscillating type speed reducer that reduces a speed of an input rotation, and to a rotation apparatus for a stabilizer shaft, which uses the eccentric oscillating type speed reducer. 
     BACKGROUND ART 
     As a conventional eccentric oscillating type speed reducer, for example, the following speed reducer described in JP-B-5-86506 is known. 
     This speed reducer has a case provided with internal teeth by impacting substantially half of each of a large number of cylindrical pins into the inner periphery thereof, a carrier inserted into the case so as to be able to perform relative rotation with respect to the case, a pinion supported by the carrier so as to have external teeth formed on the outer periphery thereof to mesh with the internal teeth, and a crank shaft configured to have an eccentric portion inserted into a central portion of the pinion and to rotate to cause the pinion to perform eccentric rotation. 
     In recent years, such a speed reducer has been required to reduce the manufacturing cost thereof without reducing a transmitted torque. However, in a case where the internal teeth are constructed by impacting the cylindrical pins into the case, as described above, the number of components is increased. In addition, a process of impacting the pins into pin grooves formed in the case is required. Consequently, the manufacturing cost is high. Thus, it is considered to reduce the manufacturing cost by forming the internal teeth integrally with the case, by cutting work, precision casting, or the like, and by using internal and external teeth with low machining accuracy. Also, a heat treatment distortion of tooth portions (the internal teeth, and the external teeth) is eliminated, and the manner of meshing between the internal teeth and the external teeth is improved to an ideal manner by polishing processing on the tooth portions after the tooth portions are hardened by performing heat treatment processing thereon. Also, it is considered to reduce the manufacturing cost of the speed reducer by omitting the polishing processing for eliminating the distortion. 
     DISCLOSURE OF THE INVENTION 
     Problems that the Invention is to Solve 
     However, in the case of using the aforementioned internal and external teeth with low machining accuracy or the internal and external teeth, which are formed by omitting the polishing processing for eliminating the heat treatment distortion, in the eccentric oscillating type speed reducer, the concentration of a large meshing load on the vicinity of a part, at which the manner of meshing between the internal teeth and the external teeth largely differs from an ideal manner, is caused in a meshing region in which the inner teeth mesh with the external teeth while a torque is transmitted. Consequently, the conventional eccentric oscillating type speed reducer has a problem that the torque transmitting capability of the eccentric oscillating type speed reducer is largely reduced. 
     An object of the present invention is to provide an eccentric oscillating type speed reducer that can be manufactured at low cost while preventing torque transmitting capability thereof from being degraded, and to provide a rotation apparatus for a stabilizer shaft, which uses this eccentric oscillating type speed reducer. 
     Means for Solving the Problems 
     First, such an object can be achieved by an eccentric oscillating type speed reducer which comprises a case with the inner periphery of which a large number of internal teeth are formed integrally, a carrier capable of performing relative rotation with respect to the case, a pinion supported by the carrier and configured so that external teeth meshing with the internal teeth are formed on an outer periphery thereof, and a crank shaft which has an eccentric portion inserted into the pinion and which rotates to cause the pinion to perform eccentric rotation. In this eccentric oscillating type speed reducer, a part of the case, which is in a meshing region in which the internal teeth mesh with the external teeth, can bend in a direction in which a radius of curvature thereof decreases. 
     Second, such an object can be achieved by a rotation apparatus for a stabilizer shaft using an eccentric oscillating type speed reducer, which comprises the aforementioned eccentric oscillating type speed reducer, and a drive motor configured to give a torque to a crank shaft of the eccentric oscillating type speed reducer. In this rotation apparatus, a first stabilizer element constituting one side of a stabilizer shaft is fixed to the case of the speed reducer, while a second stabilizer element constituting the remaining one side of the stabilizer shaft is fixed to the carrier of the speed reducer. 
     ADVANTAGES OF THE INVENTION 
     According to the present invention, a large number of the internal teeth are formed integrally with the inner periphery of the case. A part of the case, which is in a meshing region in which the internal teeth and the external teeth of the pinions mesh with one another, is enabled to bend in a direction which the radius of curvature decreases. Thus, when the concentration of the meshing load on the part, in which the manner of meshing between the internal teeth and the external teeth largely differs from an ideal manner, is caused by the eccentric rotation of the pinion, a portion of the case, which is in the vicinity of this part, bends (consequently, the portion of the case swells radially outwardly) in a direction in which the radius of curvature decreases. Accordingly, the meshing load is uniformized by being dispersed in the circumferential direction of the case. Thus, the manufacturing cost of the eccentric oscillating type speed reducer can be reduced while the torque transmitting capability is restrained from being reduced. 
     Further, in a case where the eccentric oscillating type speed reducer is used in a limited narrow space, particularly, in a case where the eccentric oscillating type speed reducer is used in the rotation apparatus for the stabilizer shaft, the outside diameter of the eccentric oscillating type speed reducer is a fairly small diameter, the polishing processing of the internal teeth and the external teeth are difficult to perform. However, according to the present invention, a part of the case, which is in the meshing region, can bend in a direction in which the radius of curvature decreases. Thus, upon completion of hardening processing, such as heat treatment processing, of the tooth portions, the meshing load is uniformized by being dispersed in the circumferential direction of the case, even without performing polishing processing on the external teeth. Consequently, the present invention can preferably be used especially, in such a case. 
     Furthermore, the aforementioned internal teeth are constituted by teeth having a circular arc tooth profile. In addition, the external teeth are constituted by teeth having a trochoidal tooth profile. Thus, the concentration of the meshing load can be mitigated. Also, the eccentric oscillating type speed reducer can be manufactured at low cost. Further, a seal member is interposed between the inner periphery of the case at an axial one side of each pinion and the outer periphery of the carrier. On the other hand, in a case where the bearing is interposed only between a part of the inner periphery of the case, which is provided at the axial other side of the pinion, and the outer periphery of the carrier. The case and the carrier are caused by the bearing to perform relative rotation. Thus, the carrier and the case are cantilevered. Consequently, as compared with a case where the carrier and the case are supported in a center impeller manner, and where the bearings are interposed between the inner peripheries of the case, which are provided at both axially outer sides of the pinion, and the outer periphery of the case, the case is more likely to bend in a direction in which the radius of curvature thereof decreases. Consequently, the concentration of a meshing load can be more effectively constrained. 
     Further, the outside diameter of the bearing is set to be less than the diameter of the addendum circle of the internal teeth. Consequently, the thickness (radial thickness) of the case at the axial other side of the two pinions, more specifically, that of the axial other side end portion of the case can be increased. Accordingly, the support stiffness of this part is increased. Thus, the strength of the eccentric oscillating type speed reducer is increased. Furthermore, the aforementioned carrier is configured to have a pair of end plates disposed at both axially outer sides of the pinion and to have also bolts that fasten the endplates by being inserted into both end plates from axial one side to the axial other side. Thus, the assembly of the carrier can be facilitated. Consequently, an eccentric oscillating type speed reducer, which can easily be assembled, is implemented. Moreover, an attaching flange, to which the drive motor is attached, is attached to the case. In addition, a first stabilizer element is fixed to the attaching flange. Thus, the first stabilizer element is fixed to the case via the attaching flange. Consequently, a drive motor can easily be attached to the attaching flange. Accordingly, the drive motor can easily be incorporated into the rotation apparatus for the stabilizer shaft. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front cross-sectional view illustrating Embodiment 1 according to the present invention. 
         FIG. 2  is a cross-sectional view taken in the direction of arrows I-I shown in  FIG. 1 . 
         FIG. 3  is an enlarged cross-sectional view illustrating the vicinities of internally toothed gears and externally toothed gears. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Embodiment 1 
     Hereinafter, Embodiment 1 if the present invention is described with reference to the accompanying drawings. 
     In  FIGS. 1 ,  2 , and  3 , reference numeral  11  designates an eccentric oscillating type speed reducer that reduces an input rotation and outputs the reduced rotation. The eccentric oscillating type speed reducer  11  has a cylindrical case  12 . A large number of internal teeth  13  whose tooth traces axially extend are formed integrally by, e.g., cutting using a hob or a shaping cutter or by, e.g., precision casting. 
     The internal teeth  13  are constituted by a large number of convex circular arc portions  14  which radially inwardly protrude and which have outer contours of a predetermined radius of curvature, and a large number of concave circular arc portions  15  each of which smoothly connects the adjacent concave circular arc portions  14  and which are radially outwardly dented. Consequently, the internal teeth  13  are constituted by teeth having a circular arc tooth profile. Incidentally, the expression “smoothly connect” means that two circular arc curves are connected to each other so that the adjacent circular arc curves have contact with each other, that is, the circular arc of the convex circular arc portion  14  and the circular arc of the concave circular arc portion  15  are connected to each other so as to have one common point and as to share one tangential line in common at this common point. 
     In the case  12 , a plurality of (two in the present embodiment) ring-like pinions  20  are axially arranged and accommodated. Outer teeth  21  constituted by a large number of teeth of a trochoidal tooth profile, more specifically, an epitrochoidal tooth profile are formed on the outer peripheries of these pinions  20 . Incidentally, the number of the external teeth  21  of the pinions  20  is less than that of the internal teeth  13  by one or two, more specifically, only by one in the present embodiment. Further, the external teeth  21  mesh with the internal teeth  13  in a state in which these pinions  20  are inscribed in the case  12 . However, the maximum meshing portions (parts at which the depth of meshing is the largest) of the two pinions  20  are shifted in phase by 180 degrees with each other. Incidentally, in a case where the internal teeth  13  are formed integrally with the case  12  as described above, the internal teeth  13  and the external teeth  21  come into slide-contact with each other. Thus, sometimes, the internal teeth  13  and the external teeth  21  are early worn away. Accordingly, the hardness of surfaces of the internal teeth  13  and the external teeth  21  are made to be high by applying ion-nitriding thereon. Further, each component of the eccentric oscillating type speed reducer  11  is made of metal. More specifically, the case  12  and the pinions  20  use structural alloys, such as carbon steels for machine structural use and chrome molybdenum high-strength steels. 
     Incidentally, the range of meshing in the circumferential direction between the external teeth  21  and the internal teeth  13  (range where the external teeth  21  and the internal teeth  13  mesh and transmit torque) varies from 45 degrees to 180 degrees, preferably, from 80 degrees to 100 degrees. Further, preferably, the radius of curvature of the circular arc tooth file of the internal teeth  13  (the convex circular arc portions  14 ) ranges from 0.2 mm to 0.5 mm. The reasons are as follows. The machining and the finishing of the internal teeth  13  are very difficult in a case where the radius of curvature of the internal teeth  13  is less than 0.2 mm. On the other hand, in a case where the radius of curvature thereof exceeds 0.5 mm, the Hertzian stress in the surfaces of the teeth increases, so that the peeling of the surfaces of the internal teeth  13  and the external teeth  21  are liable to occur. Also, it becomes difficult to set the number of the internal teeth  13  to be equal to or more than 120 without increasing the pitch diameter of each of the external teeth  21  and the diameter of a circle connecting the centers of the circular arcs of the internal teeth  13 . Consequently, a large speed reduction ratio cannot easily be obtained. 
     In each of the aforementioned pinions  20 , a plurality of (four in the present embodiment) through holes  22 , which axially penetrate therethrough, are formed. Theses through holes  22  are arranged at uniform intervals in the circumferential direction of each of the pinions  20 . Reference numeral  23  designates a carrier inserted into the case  12 . This carrier  23  has a pair of end plates  24  and  25  disposed at both axially outer sides of the two pinions  20 , in the present embodiment, one side disk-like end plate  24  placed on one axial side of the two pinions  20 , and the other side end plate  25  placed on the other axial side thereof, and a plurality of bolts  26  (the number of which is equal to that of the through holes  22 ) that fasten both the end plates  24  and  25  to each other by inserting the plurality of bolts  26  into both the end plates  24  and  25  from one axial side to the other axial side and by screwing the other axial side portions thereof into the other side end plate  25 . 
     Further, the carrier  23  has a plurality of, more specifically, in the present embodiment, four cylindrical column members  27  extending axially. These column members  27  are loosely fit into the through holes  22 , respectively. The bolts  26  pass through in the column members  27 , respectively. Each of the column members  27  is sandwiched by the one side end plate  24  and the other side end plate  25  from both axial sides by screwing the bolts  26  into the other side end plate  25 . The axial length of each of the column members  27  is slightly longer than a total thickness of the two pinions  20 . Incidentally, although the column members  27  are formed separately from both the end plates  24  and  25 , the column members can be formed integrally with the one side end plate or with the other side end plate according to the present invention. 
     Reference numeral  31  designates an oil seal serving as a seal member interposed between the inner periphery of the case  12  on axial one side of the two pinions  20  and the outer periphery of the carrier  23 , more particularly, between the inner periphery of the axial one side end portion  16  of the case  12  and the outer periphery of the one side end plate  24 . This oil seal  31  seals between the case  12  and the carrier  23  and prevents a lubricant agent or the like from leaking out from therebetween and prevents also dust, dirt or the like from entering the case  12 . Reference numeral  32  denotes a couple of bearings interposed only between the inner periphery of the case  12  on the axial other side of the two pinions  20  and the outer periphery of the carrier  23 , more particularly, only between the inner periphery of the axial other side end portion  17  of the case  12  and the outer periphery of the other side end plate  25 . The couple of bearings are disposed by being slightly spaced from each other. These bearings  23  enable the case  12  and the carrier  23  to perform relative rotation. 
     Both the end plates  24  and  25 , the bolts  26 , and the column members  27 , which have been described above, in their entirety are inserted into the case  12 , and constitute the aforementioned carrier  23  capable of performing relative rotation with respect to the case  12 . Further, in a case where the bearings  32  are interposed only between a part of the inner periphery of the case  12 , which is provided at the axial other side of the two pinions  20 , and the outer periphery of the carrier  23  in a portion between the inner periphery of the case  12  and the outer periphery of the carrier  23  as described above, the carrier  23  and the case  12  are cantilevered. Thus, as compared with a case where the carrier  23  and the case  12  are supported in a center impeller manner, and where the bearings  32  are interposed between the inner peripheries of the case  12 , which are provided at both axially outer sides of the two pinion  20 , and the outer periphery of the case  12 , the case  12  is more likely to bend in a direction in which the radius of curvature thereof decreases. Consequently, the concentration of a meshing load can be more effectively constrained. 
     Further, the outside diameter of the bearings  32  is less than the diameter of the addendum circle of the internal teeth  13 . Consequently, the thickness (radial thickness) of the case  12  at the axial other side of the two pinions  20 , more specifically, that of the axial other side end portion  17  of the case  12  can be increased. Accordingly, a thick wall part, whose inside diameter is less than the diameter of the addendum circle of the internal teeth  13 , is formed in the axial other side end portion  17  of the case  12 . The support stiffness of this part (part abutting against the bearings  32  of the case  12 ) is increased. Thus, the strength of the eccentric oscillating type speed reducer  11  is increased. 
     Then, in a case where the bolts  26  are inserted into both the end plates  24  and  25  from axially one side to the axially other side, as described above, the bolts  26  are less subjected to interference from the bearings  32  which are relatively small in diameter. Thus, as compared with a case where the bolts  26  are inserted into both the end plates  24  and  25  from the axial other side to the axial one side, the assembly of the carrier  23  is easily achieved. Consequently, the manufacturing cost of the eccentric oscillating type speed reducer  11  can be reduced. 
     Reference numeral  33  designates a plurality of (eight in the present embodiment, which is a number that is twice the number of the through holes  22 ) pin holes formed in each pinion  20 , which extend axially. These pin holes  33  are arranged pair by pair (two by two) between each adjacent pair of the through holes  22  arranged in the circumferential direction of each pinion  20 . Reference numeral  34  denotes pins the number of which is equal to that (eight) of the pin holes  33 . An axially central portion of each of such a plurality of pins  34  is loosely fit into an associated one of the pin holes  33 . On the other hand, both axial end portions of each of the pins  34  are supported by the carrier  23 . More particularly, both axial end portions of each of the pins  34  are supported by and fixed to the carrier  23  by being press-fit into both the end plates  24  and  25 , respectively. 
     Reference numeral  35  designates a ring rotatably fit onto the axially central portion of each pin  34  so that the number of the rings is equal to the number (two) of the pinions  20 . The inside diameter of these rings  35  is substantially equal to the outside diameter of each pin  34 . Thus, the inner periphery of the ring  35  comes into slide-contact with the outer periphery of the pin  34 . Further, the outside diameter of the ring  35  is less than the inside diameter of the pin hole  33  by an amount which is twice the eccentricity amount of an eccentric portion  43  of a crank shaft  40 , which will be described below. The outer periphery of the ring  35  is in rolling contact with the inner periphery of the pin hole  33 . Consequently, the pinions  20  are eccentrically rotatably supported by the carrier  23  via the pins  34  and the rings  35 . 
     Incidentally, each set of the aforementioned pin  34  and the aforementioned ring  35  in their entirety is configured so as to be inserted into an associated one of the pin holes  33  by engaging the central part thereof with the associated one of the pin holes  33 , as to have both end portions thereof, which are supported by the carrier  23 , and as to constitute an engaging pin  36  which performs relative rotation with respect to each pinion  20 . Incidentally, according to the present invention, in addition to the constituting of each engaging pin by integrating each pin and the associated ring with each other, both axial end portions of each engaging pin can be configured to be rotatably supported by both the end plates, respectively. Alternatively, each engaging pin can be constituted a pin with an eccentric portion by omitting the rings. In addition, both axial end portions of each engaging pin can be configured to be rotatably supported by both the endplates so that the outer periphery of the eccentric portion is brought into slide or rolling contact with the pinions (pin holes). 
     Reference numeral  40  designates one hollow crank shaft, which is loosely fit into a crank shaft hole  39  formed on the central axis of the carrier  23  and extends axially. This crank shaft  40  is such that both end portions thereof are supported by the end plates. More particularly, one end portion of the crank shaft  40  is rotatably supported by the one side end plate  24  via a bearing  41 , while the other end portion thereof is rotatably supported by the other side end plate  25  via a bearing  42 . Further, this crank shaft  40  has two decentered eccentric portions  43  at the central portion thereof. These eccentric portions  43  are arranged at axial positions so as to overlap with the pinion  20 . Moreover, these eccentric portions  43  are shifted in phase only by 180 degrees. Additionally, the eccentric portions  43  are inserted into a crank hole  44  formed in central portions of the pinions  20  so as to penetrate axially therethrough in a state in which a cylindrical roller bearing  45  is interposed therebetween. Further, when the crank shaft  40  rotates, the pinions  20  eccentrically rotate. 
     Incidentally, preferably, the outside diameter of the cylindrical roller bearing  45  is set to be within a range of 30% to 65% of the diameter D of a circle connecting the centers of the circular arcs of the internal teeth  13  of the case  12 , more particularly, the diameter D of a circle connecting the centers of curvature (the centers of circular arcs) P of the convex circular arc portions  14 . The reasons are as follows. In a case where the outside diameter of the cylindrical roller bearing  45  is less than 30% of the diameter D, the load capability of the cylindrical roller bearing  45  is reduced, so that transmitted torque is reduced. On the other hand, in a case where the outside diameter of the cylindrical roller bearing  45  exceeds 65% of the diameter D, the diameter of the pins  34  is reduced. Alternatively, the radially thickness of a part of each of the pinions  20 , in which the pins  34  are inserted, is reduced. Thus, similarly, there is a fear of reduction in the transmitted torque. However, in a case where the outside diameter of the cylindrical roller bearing  45  is set to be within the aforementioned range, such effects are balanced. Consequently, the transmitted torque can have a large value. Furthermore, it is more preferable that the outside diameter of the cylindrical roller bearing  45  is set to be within a range of 40% to 60% of the diameter D. This is because the aforementioned advantages can be surely obtained. 
     Further, when drive rotation is input to the crank shaft  40 , the crank shaft  40  rotates around the axis of rotation thereof. Consequently, the eccentric portion  43  of the crank shaft  40  rotates in the crank hole  44  of each of the pinions  20 . The pinions  20  perform eccentric oscillating rotations. At that time, the number of the external teeth  21  of the pinion  20  is less than that of the internal teeth  13  of the case  12  only by one. Thus, the speed of the relative rotation between the case  12  and the carrier  23  is considerably reduced, so that the case  12  and the carrier  23  perform relative rotation at low speed. The case  12 , the pinions  20 , the carrier  23 , and the crank shaft  40  in their entirety, which have been described, constitute the eccentric oscillating type speed reducer  11  capable of reducing input rotation at a high ratio. 
     Incidentally, in a case where the machining accuracy of at least one of a set of the internal teeth  13  of the case  12  and a set of the external teeth  21  of the pinions  20  is low in the aforementioned eccentric oscillating type speed reducer  11 , and where polishing processing for eliminating a heat treatment distortion is omitted, a large meshing load is concentrated on the vicinity of a part, at which the manner of meshing between the internal teeth  13  and the external teeth  21  differs largely from an ideal manner, in a meshing region  50  in which the internal teeth  13  mesh with the external teeth  21 , while torque is transmitted therebetween. 
     Thus, in the present embodiment, a part of the case  12 , which is in the meshing region  50  in which the internal teeth  13  and the external teeth  21  of the pinion  20 , is made to be thin, as compared with the thickness of the conventional case, and is thus enabled to bend in a direction in which the radius of curvature thereof is reduced. Consequently, during a rated torque is transmitted, when the concentration of a large meshing load on the vicinity of a part, at which the manner of meshing between the internal teeth  13  and the external teeth  21  differs largely from an ideal manner, is caused by the eccentric rotation of each of the pinions  20  in the meshing region  50 , a portion of the case  12 , which is in the vicinity of this part, bends in a direction in which the radius of curvature thereof is reduced (consequently, this portion swells radially outwardly). Accordingly, the meshing load is uniformized by being dispersed in the circumferential direction of the case  12 . 
     As a result, the manufacturing cost of the eccentric oscillating type speed reducer  11  can be reduced while torque transmitting capability is restrained from being reduced. Further, in the present embodiment, the internal teeth  13  are constituted by teeth having a circular arc tooth profile, as described above. In addition, the external teeth  21  are constituted by teeth having a trochoidal tooth profile. Thus, the concentration of the meshing load can be further mitigated. Also, the eccentric oscillating type speed reducer  11  can be manufactured at low cost. Incidentally, because the case  12  is configured to be able to flex (bend), as described above, it is preferable that the case  12  is made of a material whose hardness and toughness are higher than those of the pinions  20 . 
     Further, in the present embodiment, in a case where a thickness from the bottom of each of the internal teeth  13  to the outer periphery  51  of the case  12  in the aforementioned meshing region  50  is set at the minimum thickness T in the aforementioned meshing region  50 , the meshing region  50  of the aforementioned case  12  is connected to the axial other side end portion  17  by a thin wall portion  18  whose thickness is less than the minimum thickness T. Consequently, the flexing (bending) of the case in the aforementioned meshing region  50  is further facilitated. Furthermore, in order to mitigate the concentration of stress, a circular arc portion (R) R 1  having a predetermined radius of curvature R 1  is formed on the border between the meshing region  50  and the thin wall portion  18 , while a circular arc portion R 2  having a predetermined radius of curvature R 2  is formed on the border between the thin wall portion  18  and the axial other side end portion  17 . Incidentally, the difference in thickness between both sides of the latter circular arc portion R 2  is larger than that in thickness between both sides of the latter circular arc portion R 1 . Thus, the radius R 2  of curvature of the latter circular arc portion is larger than that of curvature of the former circular arc portion. 
     Further, the former circular arc portion R 1  is formed of the top portion of the inner tooth  13 . Therefore, burrs are prevented from being produced when the internal teeth  13  are machined. Incidentally, the aforementioned circular arc portions R 1  and R 2  can be formed by combining a plurality of circular arcs that differ in radius of curvature from one another. Furthermore, in order to flex (bend) the case  12 , as described above, in a case where the diameter D of a circle connecting the centers of curvature (the centers of circular arcs) P of the convex circular arc portions  41  exceeds 60 mm and is equal to or less than 100 mm, it is sufficient to set the minimum thickness T to be within the range of 3% to 8% of the diameter D, preferably, 3% to 5% of the diameter D. 
     Moreover, in a case where the aforementioned diameter D exceeds 100 mm and is equal to or less than 200 mm, it is sufficient to set the aforementioned minimum thickness T to be within the range of 3% to 7% of the aforementioned diameter D, preferably, 4% to 6% of the diameter D. Additionally, in a case where the aforementioned diameter D exceeds 200 mm and is equal to or less than 300 mm, it is sufficient to set the aforementioned minimum thickness T to be within the range of 2% to 6% of the aforementioned diameter D, preferably, 3% to 5% of the diameter D. Further, in a case where the aforementioned diameter D exceeds 300 mm and is equal to or less than 400 mm, it is sufficient to set the aforementioned minimum thickness T to be within the range of 1% to 5% of the aforementioned diameter D, preferably, 2% to 4% of the diameter D. Thus, as described above, the case  12  can be bent, and the meshing can be uniformized. 
     An example of the aforementioned eccentric oscillating type speed reducer  11  is described hereinbelow. The outside diameter of the case  12  was 102.00 mm. Both of the diameter D of a circle connecting the centers P of the circular arcs of the internal teeth  13  and the diameter of a pitch circle of the external teeth  21  of each pinion  20  were 95.0 mm. The diameter of the addendum circle of the internal teeth  13  was 94.10 mm. The diameter of the root circle of the internal teeth  13  was 95.151 mm. The diameter of the addendum circle and that of the root circle of each pinion  20  were 94.48 mm and 93.72 mm, respectively. The radius of curvature of the circular arc tooth profile of the internal teeth  13  was 0.45 mm. The pitch of the internal teeth  13  was 1.50 mm. The minimum thickness T of the case  12  in the meshing region  50  is 3.425 mm (3.60% of the diameter D). The number of the internal teeth  13  was 200. The number of the external teeth  14  was 199. Further, in a case where a rated torque was given to such an eccentric oscillating type speed reducer  11 , an amount of swelling radially outwardly was 20 μm when a part of the case  12 , which was in the meshing region  50 , bent. 
     Reference numeral  53  is a substantially disk-like attaching flange, which is attached to the other side surface (the other side surface of the thick wall portion of the case  12 ) by being positioned with high precision by smooth fitting. A drive motor  45  is attached to the radially central portion of the other side surface of this attaching flange  53  with bolts  55 . Consequently, this drive motor  54  is coaxially configured with the crank shaft  40 . An end portion of the rotating shaft  56  of this drive motor  54  is inserted into and spline-connected to a hollow hole  57  of the crank shaft  40 . Thus, the rotating shaft  56  of this drive motor  54  is connected (directly connected in the present embodiment) to the other end portion of the crank shaft  40 . Consequently, when the drive motor  54  operates, so that a torque is given to the crank shaft  40  from the rotating shaft  56 , the crank shaft  40  rotates around the axis of rotation thereof. 
     Incidentally, the aforementioned eccentric oscillating type speed reducer  11  and the drive motor  54  in their entirety constitute a rotation apparatus  58  for a stabilizer shaft  61 , which positively gives a torque to the stabilizer shaft  61  against twist caused in the stabilizer shaft  61  (to be described below) based on roll. Reference numeral  60  designates a first stabilizer element which constitutes one side portion of the stabilizer shaft  61  provided in a vehicle in order to remain balance of a vehicle (an automobile, a railway vehicle, or the like), and which is coaxial with the central axis of the aforementioned eccentric oscillating type speed reducer  11 . The other end of this first stabilizer element  60  is connected to an arm (not shown) attached to a wheel (right wheel in the present embodiment). 
     On the other hand, a bottomed cylindrical cover portion  62 , which moves in concert with the attaching flange  53  and surrounds the drive motor  54 , is formed at one end of the first stabilizer element  60  integrally therewith. This cover portion  62  is positioned at a radially outer end portion of the other side surface of the attaching flange  53  by smooth fitting. The cover portion  62  is fixed by being fastened together with the attaching flange  53  and the case  12  by a bolt  63 , one end of which extends into the thick wall portion of the case  12 . Consequently, the first stabilizer element  60  is fixed to the case  12  via the attaching flange  53 . Thus, the drive motor  54  can easily be attached to the attaching flange  53 . Consequently, the drive motor  54  can easily be incorporated into the rotation apparatus  58  for the stabilizer shaft. 
     Reference numeral  66  designates a second stabilizer element, which constitutes the remaining one side of the aforementioned stabilizer shaft  61  and is coaxial with the central axis of the aforementioned eccentric oscillating type speed reducer  11 . One end of the second stabilizer element  66  is fixed to an arm (not shown) attached to a wheel (left wheel in the present embodiment). On the other hand, a disk-like portion  67  for closing one end opening of a crank shaft hole  39  is formed at the other end of the second stabilizer element  66  integrally therewith. The disk-like portion  67  is fixed to one side surface of the carrier  23  with a plurality of bolts  68 . The positioning of these components is performed by faucet connecting, by which the outer peripheral surface of a faucet portion formed on the central portion of the other side surface of the aforementioned disk-like portion  67  is brought into surface contact with the crank shaft hole  39 . Incidentally, the axially central portion of each of the first stabilizer element  60  and the second stabilizer element  66  is rotatably attached to a vehicle via a bearing (not shown). 
     Incidentally, in a case where the eccentric oscillating type speed reducer  11  is used in a limited space, such as the rotation apparatus  58  for the stabilizer shaft, the outside diameter of the eccentric oscillating type speed reducer  11  is a fairly small diameter. Thus, the polishing processing of the internal teeth  13  and the external teeth  21  is difficult to perform. However, in the present embodiment, a part of the case  12 , which is in the meshing region  50 , can be bent in a direction in which the radius of curvature thereof is decreased. Accordingly, upon completion of performing hardening treatment, such as heat treatment processing, on the teeth portions, the meshing load is dispersed in the circumferential direction of the case  12  and is uniformized when the pinions rotate, even without performing polishing processing thereon. Thus, the aforementioned eccentric oscillating type speed reducer  11  can be particularly suitably used in the rotation apparatus  58  for the stabilizer shaft. 
     Incidentally, the rotation apparatus  58  and the stabilizer shaft  61  in their entirety are attached to a vehicle (not shown) and constitute an active stabilizer apparatus  70  capable of positively giving a counterbalancing force against roll, which is generated in the vehicle mainly at the time of turning driving thereof, to the stabilizer shaft to restrain the left and right wheels from moving in vertically opposite phases. Additionally, such an active stabilizer apparatus  70  can be mounted in one or both of front or rear portions of a vehicle. 
     Next, an operation of the aforementioned Embodiment 1 is described below. 
     When a vehicle, to which the active stabilizer apparatus  70  is attached, is caused to perform turning-driving, roll is generated in a vehicle. At that time, the drive motor  54  is operated on the basis of, for example, lateral G detected by a sensor (not shown), or the like. When a torque is given to the crank shaft  40  from the rotating shaft  56  by operating the drive motor  54 , the crank shaft  40  rotates around the axis of rotation thereof. Consequently, the eccentric portion  43  of the crank shaft  40  rotates in the crank hole  44  of each pinion  20  to cause the pinions  20  to perform eccentrically oscillating rotations. However, the number of the external teeth  21  of the pinion  20  is less than that of the internal teeth  13  of the case  12  only by one. Thus, the speed of the relative rotation between the case  12  and the carrier  23  is considerably reduced, so that a rotation of the crank shaft  40  is transmitted to at least one (both in the present embodiment) of the case  12  and the carrier  23 . Consequently, the case  12  and the carrier  23  are caused to perform relative rotation (reverse rotation) at low speed. 
     Accordingly, the first stabilizer element  60  fixed to the case  12 , and the second stabilizer element  66  fixed to the carrier  23  relatively reverse rotation at low speed. As a result, a counterbalancing torque against twist generated in the stabilizer shaft  61  based on roll is given to the stabilizer shaft  61 . Thus, the generation of roll is effectively prevented. A vehicle, which is turning-driving, is held in a balanced condition. 
     Incidentally, as described above, a large number of internal teeth  13  are integrally formed on the inner periphery of the case  12 . In addition, a part of the case  12 , which is in the meshing region  50  in which the aforementioned internal teeth  13  and the external teeth  21  of the pinions  20  mesh, is enabled to bend in a direction in which the radius of curvature decreases. Thus, even when the concentration of a large meshing load on the vicinity of a part, at which the manner of meshing between the internal teeth  13  and the external teeth  21  differs largely from an ideal manner, is caused by the eccentric rotation of each of the pinions  20  in the meshing region  50 , a portion of the case  12 , which is in the vicinity of this part, bends in a direction in which the radius of curvature thereof is reduced (consequently, this portion swells radially outwardly). Accordingly, the aforementioned meshing load is uniformized by being dispersed in the circumferential direction of the case  12 . Consequently, the manufacturing cost of the eccentric oscillating type speed reducer  11  can be reduced while torque transmitting capability is restrained from being reduced. 
     Further, in a case where the eccentric oscillating type speed reducer  11  is used in a limited space, particularly, in the aforementioned rotation apparatus  58  for the stabilizer shaft, the finish polishing processing of the internal teeth  13  and the external teeth  21  is difficult to perform. However, the eccentric oscillating type speed reducer  11  is such that a part of the case  12 , which is in the meshing region  50 , can bend in a direction in which the radius of curvature decreases. Thus, the meshing load is uniformized by being dispersed in the circumferential direction of the case  12 , even without performing polishing processing on the internal teeth  13  and the external teeth  21 . 
     Incidentally, in the aforementioned embodiment, the single crank shaft  40  is disposed on the central axis of the eccentric oscillating type speed reducer  11 . However, according to the present invention, a plurality of crank shafts can be disposed at a uniform distance from the central axis of the eccentric oscillating type speed reducer  11  and at uniform angular intervals, instead of the crank shaft  40 . At that time, the pinions are supported by the carrier via the plurality of crank shafts. 
     Further, in the aforementioned embodiment, both the case  12  and the carrier  23  are rotated. However, according to the present invention, the case  12  and the carrier  23  can be configured so that one of the case  12  and the carrier  23  is fixed, while the remaining one of the case  12  and the carrier  23  is rotated. Furthermore, in the aforementioned embodiment, the internal teeth  13  and the external teeth  21  with low machining accuracy are used. However, according to the present invention, teeth with low machining accuracy can be used as one of a set of the internal teeth  13  and a set of the external teeth  21 . Alternatively, teeth with high machining accuracy can be used as both the set of the internal teeth  13  and the set of the external teeth  21 . 
     INDUSTRIAL APPLICABILITY 
     The present invention can be applied to an industrial field of an eccentric oscillating type speed reducer configured to reduce a speed by causing pinions to perform eccentric rotations using a crank shaft.