Rolling bearings for a wave gear

Abstract

Rolling bearing (32) for a wave gear (5) with a circular spline (10) having an internal toothing (11), a flex spline (20) arranged within the circular spline (10) and having an external toothing (21) which engages with the internal toothing (11), and a rotating component (30) arranged within the flex spline (20) which deforms the flex spline (20) into a non-circular shape to cause the external toothing (21) to partially engage with the internal toothing (11), wherein the rolling bearing (32) comprises: an inner bearing ring (34) which is rotatable together with a non-circular cam (31) arranged in the rotating component (30) and which is elastically deformable; an outer bearing ring (33) which is rotatable together with the flex spline (20) and which is elastically deformable; and a plurality of balls. (35), which are arranged between the inner bearing ring (34) and the outer bearing ring (33);a cage (36) with a plurality of pockets (37) arranged circumferentially, in which the balls (35) are received, wherein the cage (36) has a ring section (42) and a plurality of cage walls (41) extending axially from the ring section (42); the pockets (37) are arranged between cage walls (41) adjacent to each other circumferentially; an inner surface of the pocket (37), which can be brought into contact with the ball (35), is formed from a surface that is straight in the radial direction;and characterized in that a circumferential distance (K5) formed between the ball (35) and the cage wall (41) is equal to or greater than a radial distance of an annular space (E1, E2) formed between the bearing ring (33, 34) deformed into a non-circular shape and the cage (36), that if a direction of a long principal axis of the bearing ring (33, 34) deformed into a non-circular shape coincides with a vertical direction, the circumferential distance (K4) in the pocket (37) at a position in the direction of a short principal axis is equal to or greater than a radial distance (K3) of the annular space (E2) at a position of the long principal axis, and that at the position of the long principal axis the radial distance (K3) between the cage (36) and the inner bearing ring (34) is equal to or greater than the radial distance (K1) between the cage (36) and the outer bearing ring (33).

Description

BACKGROUND OF THE INVENTION 1. Field of the invention The present invention relates to a rolling bearing for a wave gear. 2. Description of the state of the art A wave gear is generally known which includes an annular circular spline 80, an annular flex spline 99 and a rotary component 89, as shown in Fig. 9 (see Japanese patent application publication no. S60-143244 (JP S60 - 143 244 A )). The circular spline 80 has an internal tooth 81. The flex spline 99 is arranged inside the circular spline 80 and has an external tooth 86 that meshes with the internal tooth 81. The rotary component 89 is located inside the flex spline 99. In the wave gear, the number of teeth on the external tooth 86 is less than the number of teeth on the internal tooth 81. The rotary component 89 comprises a cam 91 and a rolling bearing 90. The rolling bearing 90 is mounted externally onto the cam 91, with the flex spline 99 positioned externally over the rolling bearing 90. The cam 91 has an elliptical shape.Thus, the rolling bearing 90 and the flex spline 99, which lie outside the cam 91, are deflected into an elliptical shape, allowing the external teeth 86 of the flex spline 99 to partially engage with the internal teeth 81 of the circular spline 80. In other words, the flex spline 99, deflected into an elliptical shape, engages with the circular spline 80 at the section of the long major axis of the flex spline 99 that has been deflected into an elliptical shape, and is separated from the circular spline 80 at the section of a short major axis of the flex spline 99. The cam 91 is rotated to allow the principal axis positions (the positions where the flex spline 99 engages with the internal teeth 81) of the elliptical flex spline 99 to move relative to the circular spline 80. In connection with this movement, the flex spline 99 can be rotated, with the teeth of the flex spline 99 partially engaging with the circular spline 80. The elliptical rolling bearing 90 outside the cam 91 has an outer ring 98, an inner ring 92, a plurality of balls 96, and a ring-shaped cage 97. The flex spline 99 is placed externally over the outer ring 98. The inner ring 92 is placed externally over the cam 91. The balls 96 are located in an annular space 95 formed between the outer ring 98 and the inner ring 92. The cage 97 holds the balls 96. Fig. 10A is a cross-sectional view showing the rolling bearing 90 and a circumferential section thereof. The cage 97 has a ring section 97a and a plurality of cage walls 97b. The cage walls 97b extend axially from the ring section 97a. The cage 97 is known as a snap-in cage. Pockets 94, which hold the respective balls 96, are located in the circumferential direction between adjacent cage walls 97b in the cage 97. The outer ring 98, the inner ring 92, and the cage 97 in the rolling bearing 90 each have a perfectly round shape before the rolling bearing 90 is mounted on the cam 91. When the rolling bearing 90 is mounted on the cam 91, the outer ring 98 and the inner ring 92 are elastically deformed into an elliptical shape. In contrast, the cage 97 acts to maintain the perfectly round shape. In such a wave gear, any potential backlash of the rolling bearing 90 must be suppressed to prevent the elliptical shape of the flex spline 99 from being affected. For this purpose, the rolling bearing 90 for a wave gear has more balls than standard ball bearings. Fig. 10B is a representation showing the ball 96 and part of the cage 97 viewed in a direction parallel to the axis of the rolling bearing 90. In the cage 97 for a conventional rolling bearing 90 of a wave gear, the pocket 94 is formed along a spherical surface. The spherical surface is chosen to be slightly larger than the radius of the ball 96. Thus, the clearance formed between the ball 96 and the pocket 94 is very small. This configuration allows the cage 97 to be positioned in both a radial and an axial direction when the balls 96 and the pockets 94 come into contact with each other. In the rolling bearing 90, which is used for the wave gear according to Fig. 9 and which is mounted externally over the elliptical cam 91, the balls 96 are arranged in an elliptical configuration corresponding to the deformation of the inner ring 92 into an elliptical shape. Therefore, in a conventional rolling bearing 90, the balls 96 are arranged in an elliptical configuration with respect to the cage 97, which is designed to maintain a perfectly round shape. Particularly in sections S1 (see Fig. 9) corresponding to the long major axis of the ellipse, the distance between the ball 96 and the pocket 94 (see Fig. 10B) is partially lost. Thus, the cage 97 can be deformed, generating a local load. Furthermore, the cam 91 rotates, repeatedly causing such a load. Additionally, the rotational speed of the ball 96 changes between two positions S3 and S4, with section S1 of the long major axis in between (see Fig. 9).Consequently, a possible pre-travel or delay of the ball 96 can cause each of the balls 96 to collide with a cage wall 97b (pocket 94), resulting in an excessive load on the cage 97. Thus, the distance between the ball 96 and the pocket 94 (see Fig. 10B) can be increased to reduce the load generated on the cage 97 due to the relationship between the balls 96 in the elliptical arrangement and the cage 97 (pockets 94). However, in this case, the cage 97 is unstable in both the radial and axial directions. In particular, if the principal axis direction of the rolling bearing 90, which is deformed into an elliptical shape, coincides with a vertical direction (see Fig. 9), the balls 96 located in positions along the short principal axis can press against the cage walls 97b of the cage 97, and the cage 97 can flutter in conjunction with rotation. As a result, the cage 97 can be damaged in a short time, or vibrations or noise may occur. DE 11 2011 100 426 T5 and DE 11 2011 104 783 T5 disclose gear devices with roller or needle bearings. DE 14 50 057 A, considered the closest patent application, discloses a ball bearing for a tension shaft drive. SUMMARY OF THE INVENTION One object of the present invention is to provide a rolling bearing for a wave gear which enables the reduction of a load acting on a cage, while allowing stable rotation of the cage. A rolling bearing according to the present invention for a wave gear comprises a circular spline with internal teeth, a flex spline arranged within the circular spline and having external teeth that mesh with the internal teeth, and a rotating element arranged within the flex spline that deforms the flex spline into a non-circular shape such that the external teeth partially mesh with the internal teeth. The rolling bearing has an inner bearing ring that is rotatable together with a non-circular cam arranged in the rotating element and is elastically deformable, an outer bearing ring that is rotatable together with the flex spline and is elastically deformable, a plurality of balls arranged between the inner and outer bearing rings, and a cage with a plurality of circumferentially arranged pockets in which the balls are received.The cage has a ring section and a plurality of cage walls extending axially from the ring section. The pockets are located between adjacent cage walls in the circumferential direction. An inner surface of the pocket, which can be brought into contact with the ball, is formed by a surface that is straight in the radial direction. The circumferential distance formed between the ball and the cage wall is equal to or greater than the radial distance of an annular space formed between the bearing ring, which is deformed into a non-circular shape, and the cage. BRIEF DESCRIPTION OF THE DRAWING The foregoing and further details and advantages of the invention will become more apparent from the following description of exemplary embodiments with reference to the accompanying drawings, where the same reference numerals are used to denote the same elements and in which: Fig. 1 is a schematic representation of an embodiment of a wave gear with a rolling bearing according to the present invention, viewed in the axial direction; Fig. 2 is a vertical sectional view of the wave gear from Fig. 1; Fig. 3 is a vertical sectional view of the rolling bearing; Figs. 4A, 4B and 4C are representations showing modes of operation of the wave gear; Fig. 5 is a representation showing one half of a cage of the rolling bearing; Figs. 6A and 6B are representations showing a pocket in the cage and a ball held in the pocket, with Fig. 6A showing the pocket and the ball viewed in the radial direction and Fig. 6B showing the lateral view of the bearing.Fig. 6B is a view along line XX, indicated by the arrows in Fig. 6A; Fig. 7 is a view showing the wave gear seen in the axial direction, which is deformed into an ellipse; Fig. 8 is a view showing the wave gear seen in the axial direction, which is deformed into an ellipse; Fig. 9 is a view showing a conventional wave gear; and Fig. 10A is a sectional view showing a wave gear and its surroundings; and Fig. 10B is a view showing a partial view of balls and a cage, seen in a direction parallel to the axis of the rolling bearing. DETAILED DESCRIPTION OF EXECUTION FORMS One embodiment of the present invention is described with reference to the drawing. Fig. 1 is a schematic representation of an embodiment of a wave gear 5 with a rolling bearing 32 according to the present invention, viewed in the axial direction. Fig. 2 is a vertical sectional view of the wave gear 5. The wave gear 5 comprises a circular spline 10, a flex spline 20, and a rotary component 30. The Circular Spline 10 is a ring-shaped rigid component (metal component) and has an internal toothing 11 on one of its inner circumferential surfaces. The inner circumferential surface is a perfectly round surface with its center on an axis C. The Circular Spline 10 is attached to a housing (not shown in the drawing) of the wave gear 5. The flex spline 20 is located radially inside the circular spline 10. The flex spline 20 has an external toothing 21 on an outer circumferential surface of the circular spline 10, which is partially engaged with the internal toothing 11. The flex spline 20 of the present embodiment (see Fig. 2) is a metallic elastic body shaped like a thin cup, with a cylindrical section 22 and a bottom section 23. The external toothing 21 is located on an outer circumferential surface of the cylindrical section 22. An output shaft, not shown in the drawing, is attached to the bottom section 23. The number of teeth of the external toothing 21 of the flex spline 20 is less than the number of teeth of the internal toothing 11 of the circular spline 10. In the present embodiment, the number of teeth of the external toothing 21 is two fewer than the number of teeth of the internal toothing 11. The difference in the number of teeth is arbitrary. As described above, the cylindrical section 22 of the flex spline 22 can be deflected into a non-circular shape (an ellipse in the present embodiment) by elastic deformation as shown in Fig. 1. The external teeth 21 and the internal teeth 11 are engaged with each other at sections S1 corresponding to a long major axis of the ellipse and separated from each other at sections S2 corresponding to a short major axis of the ellipse. The rotating component 30 lies radially within the cylindrical section 22 of the flex spline 20. The rotating component 30 comprises a cam 31 and a rolling bearing 32. The cam 31 is not circular and, in the present embodiment, is elliptical (see Fig. 1). The rolling bearing 32 is positioned externally over the cam 31, and the cylindrical section 22 of the flex spline 20 is positioned externally over the rolling bearing 32. As shown in Fig. 3, the rolling bearing 32 has a thin outer bearing ring 33 (hereinafter referred to as the outer ring), a thin inner bearing ring 34 (hereinafter referred to as the inner ring), a plurality of balls 35, and an annular cage 36. The balls 35 are located between the outer ring 32 and the inner ring 34. The cage 36 has a number of pockets 37, in which the balls 35 are received, spaced at intervals in the circumferential direction. All pockets 37 have the same shape. The outer ring 33 and the inner ring 34 are ring-shaped, circumferential components made of metal, for example bearing steel, and can be elastically deformed in the radial direction (are easily elastically deformable) because the outer ring 33 and the inner ring 34 are thin. The thickness (maximum thickness) of the outer ring 33 and the inner ring 34 is, for example, one-seventh or more and half or less than the diameter of the balls 35. The balls 35 are also made of metal, for example bearing steel. The outer ring 33 has a raceway groove 38 on an inner circumferential surface, which has a circular arc-shaped cross-section. The inner ring 34 has a raceway groove 39 on an outer circumferential surface, which also has a circular arc-shaped cross-section. The balls 35 can roll along the raceway grooves 38 and 39. The inner ring 34 is fixed to the cam 31 (placed externally over the cam 31) such that the inner ring 34 and the cam 31 can rotate together. The outer ring 33 is attached to the cylindrical section 22 of the flex spline 20 (the cylindrical section 22 is placed externally over the outer ring 33) such that the outer ring 33 and the flex spline 20 can rotate together. The cam 31 is shaped like an ellipse along one of its outer circumferences (see Fig. 1), thus elastically deforming the inner ring 34 into an elliptical shape that conforms to the shape of the cam 31. The outer ring 33 and the cylindrical section 22 are also deformed into ellipses by the spheres 35. The rotary component 30, which has the cam 31 and the rolling bearing 32 as shown in Figures 1 and 2, is also referred to as a "strain wave generator". An input shaft, not shown in the drawing, is attached to the cam 31. Thus, the rotary component 30 deflects the flex spline 20 such that the flex spline 20 has an elliptical shape, which allows the external teeth 21 of the flex spline 20 to partially engage with the internal teeth of the circular spline 10. In the present embodiment, the external teeth 21 and the internal teeth 11 engage with each other at two points that are spaced 180 degrees apart. As shown in Fig. 1, the flex spline 20 is deflected by the rotating component 30 to assume an elliptical shape. The external teeth 21 and the internal teeth 11 are engaged with each other at sections S1 of the long major axis within the ellipse and separated from each other at sections S2 on a short major axis of the ellipse. The circular spline 10 is fixed. When, in this state, the cam 31 rotates clockwise about an axis C in Fig. 1 (compare Fig. 4a), the positions of sections S1 of the long major axis and of the flex spline 20 move (change), and thus the engagement sections between the external teeth 21 and the internal teeth 11 move (change). As shown in Fig. 4b, when the cam 31 is rotated 180 degrees in the state of Fig. 1, the flex spline 20 moves in one direction (counterclockwise) opposite to the direction of rotation of the cam 31 by a distance corresponding to one tooth, which is equal to half the difference in the number of teeth between the external teeth 21 and the internal teeth 11. Then, when the cam 31 is rotated a further 180 degrees in the state of Fig. 4b, as shown in Fig. 4c, the flex spline 20 moves in the opposite direction (counterclockwise) to the direction of rotation of the cam 31 by a distance corresponding to two teeth, equal to the difference in the number of teeth between the external teeth 21 and the internal teeth 11. The cam 31 can rotate together with the input shaft (not shown in the drawing). The flex spline 20 can rotate together with the output shaft (not shown in the drawing).Thus, the wave gear 5 outputs the rotation of the flex spline 20 in response to an input via the cam 31. In Figs. 4a, 4b and 4c, the cage 36 of the rolling bearing 32 is omitted. Fig. 5 shows half of the cage 36 of the rolling bearing 32. The cage 36 is a snap-in cage. The cage 36 has a circumferential ring section 42 and a plurality of cage walls 41 extending axially from the ring section 42. The pockets 37 are located between adjacent cage walls 41 in the circumferential direction. A ball 35 is received in each of the pockets 37. Before the rolling bearing 32 is mounted on the cam 31, the cage 36 has a circular (perfectly round) shape similar to the outer ring 33 and the inner ring 34. In the present invention, the cage 36 is made of a resin. Figures 6a and 6b are illustrations showing the pocket 37 in the cage 36 and the ball 35 held in the pocket 37. Figure 6a is a radial view, and Figure 6b is a view along line XX according to the arrows in Figure 6a. In Figures 5, 6, and 6b, an inner surface 45a of the cage wall 41, which faces the ball 35, and an inner surface 45b of the ring section 42, which reaches the ball 35, can come into contact with the ball 35. The inner surfaces 45a and 45b form an inner surface (pocket surface) 45 of the pocket 37, which can be brought into contact with the ball 35. The inner surface 45 is formed by a surface that is straight in the radial direction (in the direction perpendicular to the plane of Figure 6a). More precisely, the pocket 37 (inner surface 45) is formed along a cylindrical surface whose centerline direction corresponds to the radial direction. A suitable gap is formed between the inner surface 45 and the ball 35. Before the rolling bearing 32 is mounted on the cam 31, there is a circumferential clearance between the ball 35 and the cage walls 41 (hereinafter referred to as the circumferential clearance), as shown in Figs. 6a and 6b, which is designated “K5” in Figs. 6a and 6b. The circumferential clearance K5 is defined by a distance formed when a pocket center coincides with a ball center and represents a dimension of the distance in the circumferential direction. The pocket center and the ball center, which coincide, are designated by point Q in Fig. 6a. The pocket center is the center of the pocket 37 and is an intersection between a centerline of the cylindrical surface and the pitch circle of the ball 35. The ball center is the center of the ball 35. As shown in Fig.As shown in Figure 3, an outer circumferential surface 47 (radial outer surface) of the cage 36 has a shape adapted to a cylindrical surface whose centerline corresponds to the axis of the cage 36. An inner circumferential surface 48 (radial inner surface) of the cage 36 has a shape adapted to the cylindrical surface, the centerline of which corresponds to the axis of the cage 36. Fig. 7 shows a representation of the rolling bearing 32, which is deformed into an elliptical shape, viewed in the axial direction. Fig. 7 shows the contour shapes of an inner circumferential surface 46 of the outer ring 33, the outer circumferential surface 47 and the inner circumferential surface 48 of the cage 36, and an outer circumferential surface 49 of the inner ring 34. Since the cam 31, over which the rolling bearing 32 is mounted from the outside (see Fig. 1), is elliptical according to the description above, the inner ring 34 of the rolling bearing 32 is also elastically deformed into an elliptical shape, with the ball 35 arranged along the elliptical inner ring 34. Furthermore, the outer ring 33 is also elastically deformed into an elliptical shape. In contrast, in cage 36 a distance of suitable size (the circumferential distance K5 described above) is defined between the inner surface 45 of the pocket 37 and the sphere 35, as shown in Fig. 6a and Fig. 6b.Thus, the cage 36 retains its original perfectly round shape without following the spheres 35, which are arranged in an elliptical configuration. Consequently, in each of the pockets 37, the circumferential distance K5 between the sphere 35 (compare Fig. 6a and Fig. 6b) and the cage walls 41, located on both sides of the sphere 35, is present on both sides in the principal axis direction of Fig. 7. In Fig. 7, a top-to-bottom direction represents a (long) principal axis direction, and a sideways direction represents the direction of the short principal axis. When the cam 31 rotates during operation (during rotation) of the wave gear 5, the direction of the long principal axis and the direction of the short principal axis change accordingly. An outer annular space E1 is formed between the outer circumferential surface 47 of the cage 36 and the inner circumferential surface 46 of the outer ring 33. An inner annular space E2 is formed between the inner circumferential surface 48 of the cage 36 and the outer circumferential surface 49 of the inner ring 34. In the outer annular space E1, the radial distance K1 at the principal axis positions differs from the radial distance K2 at the positions of the short principal axis. K1 is greater than K2 (K1 > K2). The radial distance K1 at the principal axis positions is greatest in the outer annular space E1. The radial distance K2 at the positions of the short principal axis is smallest in the outer annular space E1. In the inner annular space E2, the radial distance K3 at the principal axis positions differs from the radial distance K4 at the positions of the short principal axis. K3 is smaller than K4 (K3 < K4). The radial distance K3 at the principal axis positions is smallest in the inner annular space E2. The radial distance K4 at the positions of the short principal axis is greatest in the inner annular space E2. The radial distances K1, K2, K3, and K4 are each defined by the dimension (in the radial direction) of the outer or inner annular space E1 or E2 formed between the outer ring 33 or the inner ring 34, deformed into an elliptical shape, and the perfectly round cage 36 when the rolling bearing 32 is stopped (not rotating) and the centers of the outer ring 33 and inner ring 34 coincide with the center of the cage 36. In the following description, the outer ring 33 and the inner ring 34 are arranged concentrically, and their centers are referred to as the bearing ring center. Since the outer ring 33 and the inner ring 34 are each deformed into an elliptical shape, the bearing ring center corresponds to the intersection of the long major axis and the short major axis (the center of the ellipse). The center of the cage 36 is referred to as the cage center. The circumferential distance K5 (see Fig. 6a and Fig. 6b) formed between the ball 35 in the pocket 37 at each position on the short axis in Fig. 7 and each of the cage walls 41 on both sides of the ball 35 is equal to or greater than the radial distance K1 (K5 ≥ K1) of the outer annular space E1 at each principal axis position, formed between the elliptical outer ring 33 and the perfectly round cage 36, with the bearing ring center coinciding with the cage center (see Fig. 7). In the present embodiment, at the principal axis position, the radial distance K3 between the cage 36 and the inner ring 34 is equal to or greater than the radial distance K1 between the cage 36 and the outer ring 33 (K3 ≥ K1), as shown in Fig. 7. The circumferential distance K5, set as shown in Fig. 6a and Fig. 6b, is larger than a conventional circumferential distance K5. Therefore, even if the spheres 35 are arranged in an elliptical configuration, a suitable distance is maintained between the sphere 35 and the cage wall 41 (the inner surface 45 of the pocket 37). Consequently, the cage 36 is less likely to be decelerated and retains its original perfectly round shape, thus preventing potential localized stresses on the cage 36. This reduces the stress on the cage 36. In particular, when the spheres 35 are arranged in an elliptical configuration, the distance between the sphere 35 and each of the cage walls 41 (the inner surface 45a of the pocket 37) is reduced at the principal axis positions of the ellipse.Even with a reduced distance, a suitable distance between the sphere 35 and each of the cage walls 41 (the inner surface 45a of the pocket 37) is maintained, as described above. As a result, potential local loads on the cage 36 can be modified. While the top-to-bottom direction corresponds to the vertical direction, and the principal axis direction of the inner ring 34 and the outer ring 33, each deformed into an elliptical shape, corresponds to the vertical direction (direction of gravity) in Fig. 7, the cage 36 moves vertically due to the influence of gravity (falling downwards in Fig. 7). At this point, the distance by which the cage 36 falls corresponds to the radial distance K1. Therefore, the present embodiment establishes the dimensional relationship K5 > K1, as described above. Thus, when the cage 36 falls vertically downwards while the rolling bearing 32 rotates and the principal axis direction coincides with the vertical direction, the cage 36 can come into contact with the inner circumferential surface 46 of the outer ring 33 at the principal axis position P1 (lower end in Fig. 7).7) before the cage wall 41 (the inner surface 45a of the pocket 37) comes into contact with the ball 35 at position P2 of the short main axis. Consequently, the cage 36 is guided more by the outer ring 33 than by the balls 35 and can rotate stably. Even if K5 = K1 while the rolling bearing 32 is not rotating, the rolling bearing 32 still rotates, so that a centrifugal force acts on the cage 36, and thus the diameter of the cage 36 is slightly increased. Thus, when the rolling bearing 32 is rotating, K5 > K1 is ensured and the cage 36 is guided more by the outer ring 33 than by the balls 35 at the main axis position P1 (the lower end of Fig. 7). At the main axis positions, K3 ≥ K1, as described above. Consequently, the cage 36 can be reliably guided more by the outer ring 33 than by the inner ring 34. Even if K3 = K1, while the rolling bearing 32 is stationary, the rolling bearing 32 rotates to exert a centrifugal force on the cage 36, thus slightly increasing the diameter of the cage 36. When the rolling bearing 32 is rotating, K3 > K1 is ensured, and the cage 36 is guided more by the outer ring 33 than by the inner ring 34. When the cam 31 rotates 90 degrees from the state shown in Fig. 7, the direction of the short main axis coincides with the vertical direction, while the main axis direction (direction of the long main axis) coincides with the horizontal direction (see Fig. 8). In this state, the cage 36 also moves (falls) downwards in the vertical direction (downwards in Fig. 8). The distance by which the cage 36 falls corresponds to the radial distance K2. Again, the circumferential distance K5 (see Fig. 6), which is formed at each main axis position between the ball 35 in the pocket 37 and each of the cage walls 41 on both sides of the ball 35, is greater than the radial distance K2 of the outer annular space E1 between the outer ring 33, which is deformed into an elliptical shape, and the perfectly round cage 36, while the bearing ring center coincides with the cage center (K5 > K2).In particular, in the present embodiment, the circumferential distance K5 is greater than or equal to the radial distance K1 of the outer annular space E1 at the principal axis positions (K5 ≥ K1) as described above. In the outer annular space E1, the radial distance K2 is smallest at the positions of the short principal axes, while the radial distance K1 is largest at the principal axis positions (K1 > K2), so that the relationship K5 ≥ K1 > K2 holds. As described above, the dimensional relationship K5 > K2 applies. Therefore, if the cage 36 falls downwards in the vertical direction while the rolling bearing 32 rotates, with the direction of the short main axis coinciding with the vertical direction, the cage 36 can come into contact with the inner circumferential surface 46 of the outer ring 33 at position P1 of the short main axis (lower end in Fig. 8) before the cage wall 41 comes into contact with the inner surface 45a of the pocket 37 and the ball 35 at the main axis position P2. Consequently, the cage 36 is guided more by the outer ring 33 than by the balls 35 and can rotate stably. Since K4 > K2 applies at the position of the short main axis, the cage 36 can be reliably guided more by the outer ring 33 than by the inner ring 34. As a result, in the present embodiment, the rolling bearing 32 serves as an outer ring-guided bearing, which allows the cage 36 to be guided by the outer ring 33. In the embodiment described above, the setting K3 > K1 in Fig. 7 allows the cage 36 to be guided by the outer ring 33. However, K3 < K1 can also be used. Even in this case, however, the circumferential distance K5 (see Fig. 6a and Fig. 6b) in the pocket 37 at each position of the short main axis in Fig. 7 is equal to or greater than the radial distance K3 of the inner annular space E2 at each main axis position, which is formed between the inner ring 34, which is deformed into an elliptical shape, and the perfectly round cage 36 when the bearing ring center coincides with the cage center (K5 ≥ K3). This is described with reference to Fig. 7. While the direction from top to bottom corresponds to the vertical direction, and the principal axis direction of the inner ring 34 and the outer ring 33, which are each deformed into an elliptical shape, corresponds to the vertical direction (direction of gravity), the cage 36 moves (falls) downwards in a vertical direction (downwards in Fig. 7). The amount by which the cage 36 falls corresponds to the radial distance K3. As described above, the dimensional relationship K5 ≥ K3 applies. Even if, during the rotation of the rolling bearing 32, the cage 36 falls downwards in a vertical direction and the main axis direction coincides with the vertical direction, the cage 36 can come into contact with the outer circumferential surface 49 of the inner ring 34 at the main axis position P1 (lower end in Fig. 7) before the cage wall 41 (inner surface 45a of the pocket 37) comes into contact with the ball 35 at the position P2 of the short main axis.Consequently, the cage 36 is guided more by the inner ring 34 than by the balls 35 and can rotate stably. However, if the cam 31 rotates at high speed and the rolling bearing 32 also rotates at high speed, the diameter of the cage 36 is increased by a centrifugal force, as described above. Therefore, K3 ≥ K1 is preferable to K3 < K1 to allow the cage 36 to be guided by the outer ring 33. As described above, in the rolling bearing 32 of the present embodiment, the circumferential distance K5, which is formed between the ball 35 and the cage wall 41, wherein the pocket center coincides with the ball center in the pocket 37 at the positions of the short main axis (see Fig. 6a and Fig. 6b), is equal to or greater than the radial distance K1 (K2) of the outer annular space E1, which is formed between the elliptically deformed outer ring 33 and the perfectly round cage 36, wherein the bearing ring center coincides with the cage center, as shown in Fig. 6a, Fig. 6b and Fig. 7. Furthermore, the inner surface (pocket surface) 45 of the pocket 37 in the cage 36, which can be brought into contact with the ball 35, is formed from a surface that is straight in the radial direction (see Fig. 6a and Fig. 6b). Even if the cam 31, which has an elliptical shape, causes the rolling bearing 32 to be deformed into an elliptical shape in order to bring the balls 35 into an elliptical arrangement, which locally reduces the distance between the balls 35 and each cage wall 41 (especially at the main axis positions), there is less probability that the cage walls 41 will be braked by the ball 35, thus preventing significant loads on the cage 36. That is to say, in the wave gear 5, the rolling bearing 32 is deformed into an elliptical shape, which brings the balls 35 into the elliptical arrangement.In the present embodiment, however, the inner surface 45 of the pocket 37 in the cage 36 is a surface that is straight in the radial direction, so that it is less likely to be affected by the spheres 35 which are arranged in the elliptical configuration. Consequently, the cage 36 can act in such a way as to maintain the circular (perfectly round) shape, thus avoiding potentially significant stresses. In the cage 36 of the present embodiment, the cage wall 41 has a decreasing circumferential dimension. This increases the distance between the pocket 37 and the ball 35 (circumferential distance K5: see Fig. 6a and Fig. 6b) in order to increase the degrees of freedom of relative movement between the balls 35 and the cage 36. The cage wall 41 also has a decreasing radial dimension (thickness) e (see Fig. 6b). This increases the degrees of freedom of movement of the cage 36 with respect to the outer ring 33 and the inner ring 34. This, in turn, enables a configuration that can eliminate (reduce) the load on the cage 36 due to a braking effect. As described above, the cage 36 maintains the appropriate distance between the ball 35 and the cage wall 41 (the inner surface 45 of the pocket 37). As a result, the cage 36 is less likely to be slowed down by the balls 35 and retains its original perfectly round shape. This prevents potential local stresses on the cage 36 due to, for example, the balls 35 pre-running or lagging behind. In other words, stresses on the cage 36 can be reduced. Furthermore, the cage 36 is made of a resin, and specifically a resin without reinforcement, such as reinforcing fibers (e.g., nylon 66). Therefore, the cage 36 (especially the cage walls 41) deforms easily, and there is less chance of breakage, even upon collision with the balls 35. Since the distances remain fixed as described above, the cage 36 can come into contact with the inner circumferential surface 46 of the outer ring 33 at position P1 of the long main shaft (the lower end in Fig. 7) before the cage wall 41 (the inner surface 45a of the pocket 37) comes into contact with the ball 35 at position P2 of the short main shaft. Consequently, the cage 36 is guided more by the outer ring 33 than by the balls 35 and can rotate stably. This prevents the cage 36 from impacting the balls 35 and thus breaking. This improves the durability of the cage 36, making the rolling bearing 32 more reliable for the wave gear. As a result, a reliable wave gear 5 can be obtained. The wave gear 5 and the rolling bearing 32 of the present invention are not limited to the shape shown in the drawing and can have any other shape within the scope of the present invention. In the embodiment described above, the case in which the cam 31 is elliptical was described. However, the cam 31 can have a different, non-circular shape. In the embodiment described above, the case in which the flex spline 20 is placed directly over the outer circumference of the rolling bearing 32 (the outer ring 33) from the outside was described. However, the flex spline 20 can be placed over the rolling bearing 32 with an elastic layer in between. An intermediate component can also be placed between the cam 31 and the inner ring 34. The rolling bearing for the wave gear of the present invention enables a reduction of the load generated in the cage and also allows the cage to rotate stably.

Claims

Rolling bearing (32) for a wave gear (5) with a circular spline (10) having an internal toothing (11), a flex spline (20) arranged within the circular spline (10) and having an external toothing (21) which engages with the internal toothing (11), and a rotating component (30) arranged within the flex spline (20) which deforms the flex spline (20) into a non-circular shape to cause the external toothing (21) to partially engage with the internal toothing (11), wherein the rolling bearing (32) comprises: an inner bearing ring (34) which is rotatable together with a non-circular cam (31) arranged in the rotating component (30) and which is elastically deformable; an outer bearing ring (33) which is rotatable together with the flex spline (20) and which is elastically deformable; and a plurality of balls. (35), which are arranged between the inner bearing ring (34) and the outer bearing ring (33);a cage (36) with a plurality of pockets (37) arranged circumferentially, in which the balls (35) are received, wherein the cage (36) has a ring section (42) and a plurality of cage walls (41) extending axially from the ring section (42); the pockets (37) are arranged between cage walls (41) adjacent to each other circumferentially; an inner surface of the pocket (37), which can be brought into contact with the ball (35), is formed from a surface that is straight in the radial direction;and characterized in that a circumferential distance (K5) formed between the ball (35) and the cage wall (41) is equal to or greater than a radial distance of an annular space (E1, E2) formed between the bearing ring (33, 34) deformed into a non-circular shape and the cage (36), that if a direction of a long principal axis of the bearing ring (33, 34) deformed into a non-circular shape coincides with a vertical direction, the circumferential distance (K4) in the pocket (37) at a position in the direction of a short principal axis is equal to or greater than a radial distance (K3) of the annular space (E2) at a position of the long principal axis, and that at the position of the long principal axis the radial distance (K3) between the cage (36) and the inner bearing ring (34) is equal to or greater than the radial distance (K1) between the cage (36) and the outer bearing ring (33). Rolling bearing (32) for a wave gear (5) according to claim 1, wherein the circumferential distance (K5) is equal to or greater than a radial distance (K1) of an outer annular space (E1) formed between the outer bearing ring (33) and the cage (36). Rolling bearing (32) for a wave gear (5) according to claim 1 or 2, wherein the circumferential distance (K5) is equal to or greater than the radial distance (K1) of the outer annular space (E1) at the position of the long main axis, wherein the outer annular space (E1) is formed between the outer bearing ring (33) and the cage (36).

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