A fixed-end constant velocity universal joint

Abstract

The present invention relates to a fixed-end constant velocity universal joint, comprising an outer planet wheel, an inner planet wheel, a ball cage and steel balls. The outer planet wheel is provided with 2N outer ball tracks, where N is greater than or equal to 3. The inner planet wheel is provided with 2N inner ball tracks, and the inner ball tracks and the outer ball tracks correspond to each other one by one to form a ball track, and the inner ball tracks and the outer ball tracks are symmetrical about the central plane of the universal joint. One end of the outer planet wheel is an open end, and the other end is a mounting end. The outer ball track comprises a first arc segment, a second arc segment, and a transition segment located between the first arc segment and the second arc segment. The first arc segment and its center are located on the side of the central plane of the universal joint facing the open end, and the second arc segment and its center are located on the side of the central plane of the universal joint facing the mounting end. The first arc segment and the second arc segment are both curved inwardly toward the outer planet wheel, and the first arc segment and the second arc segment are both tangently connected to the transition segment. The 2N outer ball tracks have the same shape, or N of the outer ball tracks have the same shape and the other N outer ball tracks have the same shape.

Description

Technical Field

[0001] The present invention relates to the technical field of universal joint transmission, in particular to a fixed-end constant velocity universal joint. Background Art

[0002] The function of the constant velocity universal joint is to connect two rotating shafts with an angle between them or with different relative positions, and to make the two shafts transmit power at the same angular velocity. It can overcome the non-constant velocity problem of ordinary cross-axis universal joints.

[0003] See attached Figure 1 The fixed-end constant velocity universal joint generally includes an outer star wheel 1, an inner star wheel 2, a ball cage 3 and steel balls 4. The outer star wheel 1 has several outer ball channels 5, and the inner star wheel 2 has the same number of inner ball channels 6. The outer ball channels 5 and the inner ball channels 6 correspond one to one to form a complete ball channel, and the outer ball channels 5 and the inner ball channels 6 are symmetrical about the center plane 9 of the universal joint. The steel balls 4 are arranged in each ball channel. The ball uses a window to accommodate the steel balls 4 and make the centers of the steel balls 4 move in the same plane. The function of the fixed-end constant velocity universal joint is to change the direction of power transmission without changing the rotational speed. In the fixed-end constant velocity universal joint, the inner star wheel 2, the ball cage 3 and the outer star wheel 1 are connected by spherical surfaces that match each other. The motion relationship, interaction force of each component and the function and performance of the universal joint are affected by the fixed joint ball channel structure, layout design, etc.

[0004] To operate at various swing angles, the ball track in existing designs consists of a trajectory consisting of an arc segment, or a segment of an arc plus a straight line segment, or several tangent arc segments, with its center offset from the universal joint's center plane 9. When the universal joint operates at a fixed or variable swing angle, the steel balls reciprocate in the ball track. The friction generated by this motion creates significant forces between the ball track and the steel balls of the inner star wheel, the ball track and the steel balls of the outer star wheel, the cage window and the steel balls, and the mating spherical surfaces. To prevent jamming, the center of the ball track needs to be offset from the center of the universal joint, an offset design. This offset design further increases the forces acting within the universal joint. As the operating swing angle increases, the temperature within the joint rises, increasing transmission losses and reducing transmission efficiency. The structure of some fixed-end constant velocity universal joints is designed by staggering two sets of different ball track structures. The steel balls moving in the two sets of ball tracks are subjected to forces in opposite directions, thereby reducing the overall force inside the structure and achieving high efficiency. However, this fixed joint with two sets of different ball track structures is affected by the circumferential phase arrangement and cannot weaken the axial force at large swing angles. Summary of the Invention

[0005] In view of the above-mentioned shortcomings of the prior art, the technical problem to be solved by the present invention is to provide a fixed-end constant velocity universal joint that can effectively reduce the axial force.

[0006] To achieve the above-mentioned objectives, the present invention provides a fixed-end constant velocity universal joint, comprising an outer planet wheel, an inner planet wheel, a ball cage and steel balls, wherein the outer planet wheel is provided with 2N outer ball tracks, where N is greater than or equal to 3, and the inner planet wheel is provided with 2N inner ball tracks, wherein the inner ball tracks and the outer ball tracks correspond to each other one by one to form a ball track, and the inner ball tracks and the outer ball tracks are symmetrical about the central plane of the universal joint, one end of the outer planet wheel is an open end, and the other end is a mounting end, the outer ball track comprises a first arc segment, a second arc segment, and a transition segment between the first arc segment and the second arc segment, the first arc segment and its center are located on the side of the central plane of the universal joint facing the open end, the second arc segment and its center are located on the side of the central plane of the universal joint facing the mounting end, the first arc segment and the second arc segment are both curved inwardly toward the outer planet wheel, and the first arc segment and the second arc segment are both tangentially connected to the transition segment; the 2N outer ball tracks have the same shape, or N of the outer ball tracks have the same shape and the other N outer ball tracks have the same shape.

[0007] Furthermore, the distances from the center of the first arc segment and the center of the second arc segment to the central axis of the universal joint are the same.

[0008] Furthermore, the distances from the center of the first arc segment and the center of the second arc segment to the central axis of the universal joint are different.

[0009] Furthermore, the transition segment is composed of a straight line segment, or is composed of several arc segments.

[0010] Furthermore, the transition segment is composed of a plurality of straight line segments and a plurality of circular arc segments.

[0011] Furthermore, the N identical outer ball tracks on the outer planet wheel are type A outer ball tracks and the other N identical outer ball tracks are type B outer ball tracks. The transition section of the type A outer ball track gradually tilts toward the central axis of the outer planet wheel from the opening end to the mounting end, and the transition section of the type B outer ball track gradually tilts toward the central axis of the outer planet wheel from the mounting end to the opening end.

[0012] Furthermore, the distances from the centers of the first arc segment and the second arc segment of the type A outer ball track to the central axis of the universal joint are the same and they are located on the same side of the central axis of the universal joint, and the radius of the first arc segment is greater than the radius of the second arc segment; the distances from the centers of the first arc segment and the second arc segment of the type B outer ball track to the central axis of the universal joint are the same and they are located on the same side of the central axis of the universal joint, and the radius of the first arc segment is smaller than the radius of the second arc segment.

[0013] Furthermore, the centers of the first arc segment and the second arc segment of the outer ball track are both located on the central axis of the universal joint.

[0014] Furthermore, N A-type outer ball tracks and N B-type outer ball tracks are alternately arranged in the circumferential direction of the outer star wheel.

[0015] Furthermore, one or both of the B-type outer ball track and the A-type outer ball track also include a third arc segment, which is located on the side of the first arc segment facing the open end, and the two are tangently connected, and the bending direction of the third arc segment is opposite to that of the first arc segment.

[0016] As described above, the fixed-end constant velocity universal joint according to the present invention has the following beneficial effects:

[0017] By setting up a ball lane of a specific shape, in two opposite ball lanes, the first arc segment and the second arc segment are respectively used to contact the steel balls therein, and the component forces of the steel balls in the two ball lanes in the direction of the axis of the outer star wheel or the inner star wheel can be effectively offset, thereby reducing or even eliminating the force of the steel balls on the window of the ball cage, further reducing the spherical force in the fixed-end constant velocity universal joint, and reducing the internal energy loss caused by movement. Compared with the existing general constant velocity fixed-end universal joint, it has significantly higher efficiency, and its main benefit characteristics are in line with the main development directions of vehicles such as improving energy efficiency and reducing losses. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a structural diagram of an existing traditional fixed-end constant velocity universal joint.

[0019] Figure 2 It is a structural schematic diagram of the open end side of the fixed-end constant velocity universal joint of the present invention.

[0020] Figure 3 for Figure 2 Cross-sectional view at CC in .

[0021] Figure 4 for Figure 2 Cross-sectional view at DD in.

[0022] Figure 5 The figure is a schematic diagram of the working of the fixed-end constant velocity universal joint of the present invention at a large swing angle.

[0023] Figure 6 The figure is a schematic diagram of the operation of the fixed-end constant velocity universal joint of the present invention at a small swing angle.

[0024] Explanation of Figure Numbers

[0025] 1 Alien Wheel

[0026] 2 Inner Star Wheel

[0027] 3 CVJs

[0028] 4 steel balls

[0029] 5 Outer Lane

[0030] 5a A-type outside fairway

[0031] 5b B-type outer fairway

[0032] 51 First arc segment

[0033] 52 Second arc segment

[0034] 53 Transition

[0035] 54 The third arc segment

[0036] 6 Inner Fairway

[0037] 7 Mounting end

[0038] 8 open end

[0039] 9 Universal joint center plane

[0040] 10 Universal joint center axis DETAILED DESCRIPTION

[0041] The following describes the implementation of the present invention through specific embodiments. People skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification.

[0042] It should be noted that the structures, proportions, sizes, etc. depicted in the drawings of this specification are only used to match the contents disclosed in the specification for people familiar with this technology to understand and read, and are not used to limit the conditions for the implementation of the present invention. Therefore, they have no substantial technical significance. Any modification of the structure, change in the proportional relationship, or adjustment of the size should still fall within the scope of the technical content disclosed by the present invention without affecting the efficacy and purpose of the present invention. At the same time, the terms such as "upper", "lower", "left", "right", "middle", etc. quoted in this specification are only for the convenience of description and are not used to limit the scope of the implementation of the present invention. Changes or adjustments in their relative relationships should also be regarded as the scope of the implementation of the present invention without substantially changing the technical content.

[0043] See also Figures 2 to 6 The present invention provides a fixed-end constant velocity universal joint, comprising an outer star wheel 1, an inner star wheel 2, a ball cage 3, and steel balls 4. The outer star wheel 1 is provided with 2N outer ball tracks 5, and the inner star wheel 2 is provided with 2N inner ball tracks 6, where N ≥ 3. The inner ball tracks 6 and the outer ball tracks 5 correspond to each other one by one to form a ball track, and the inner ball tracks 6 and the outer ball tracks 5 are symmetrical about the central plane 9 of the universal joint. One end of the outer star wheel 1 is an inner cavity opening, referred to as the opening end 8, and the other end is used to connect the shaft, referred to as the mounting end 7. The above parts are the same as conventional designs and will not be described in detail. The characteristic of the present invention is that the outer ball track 5 includes a first arc segment 51, a second arc segment 52, and a transition segment 53 located between the first arc segment 51 and the second arc segment 52, see Figure 3 and Figure 4, is a cross-sectional view along the centerline of the ball track. The first arc segment 51 and its center are located on the side of the universal joint center plane 9 facing the open end 8. The second arc segment 52 and its center are located on the side of the universal joint center plane 9 facing the mounting end 7. The transition segment 53 crosses the universal joint center plane 9. The first arc segment 51 and the second arc segment 52 both curve inward toward the outer planet 1, that is, their centers are located on the side of the inner cavity of the outer planet 1. The first arc segment 51 and the second arc segment 52 are both tangentially connected to the transition segment 53. That is, from the cross-section passing through the ball track centerline, from the opening end 8 to the mounting end 7, the outer ball track 5 includes the first arc segment 51, the transition segment 53, and the second arc segment 52. The first arc segment 51 and the second arc segment 52 are arc-shaped along the centerline of the ball track. In the present invention, the 2N outer ball tracks 5 have the same shape, or N of the outer ball tracks 5 have the same shape and the other N outer ball tracks 5 have the same shape. The shape and structure of the inner ball track 6 correspond to the outer ball track 5 and will not be described in detail. The distances between the center of the first arc segment 51 and the center of the second arc segment 52 and the central axis 10 of the universal joint (also the central axis of the outer planetary gear 1) can be equal or unequal. The center of the first arc segment 51 and the center of the second arc segment 52 can be located on the same side of the central axis 10 of the universal joint or on opposite sides of the central axis 10.

[0044] The working principle of the fixed end constant velocity universal joint of the present invention is: when the universal joint is at a large swing angle β1, see Figure 5, where the swing angle β1 is the angle between the central axis 10 of the universal joint (also the central axis of the outer planet 1) and the central axis of the inner planet 2. The steel balls 4 in one ball lane are located in the second arc segment 52, while the steel balls 4 in the opposite ball lane are located in the first arc segment 51. Because the centers of the steel balls 4 always lie on the center plane of the cage 3, based on the general principles of constant velocity universal joints, the angles formed between the steel balls 4 in these two ball lanes and the center planes of the outer planet 1 or the inner planet 2 are equal. The two steel balls 4 located in the first arc segment 51 and the second arc segment 52 are subjected to forces directed toward their respective centers. Therefore, the axial force components acting on the outer planet 1 or inner planet 2 are in opposite directions, and the resulting force cancels out and is very small. The operating principles in the other two opposing ball lanes are the same. As the swing angle changes, the angle between the steel ball 4 and the center plane of the outer planet wheel 1 or the center plane of the inner planet wheel 2 changes. The steel balls 4 in the two opposite ball lanes always have the same angle with the center plane of the outer planet wheel 1 or the center plane of the inner planet wheel 2, and always have the effect of canceling out the axial force components on the outer planet wheel 1 or the inner planet wheel 2. And at the same time, when the steel ball 4 in any ball lane is subjected to force, there is always a steel ball 4 in the corresponding ball lane that is subjected to an opposite force in the axial direction of the outer planet wheel 1 or the inner planet wheel 2. Therefore, when the resultant force in the fixed-end constant velocity joint is transferred between the steel ball 4 and the ball cage 3, the axial forces are very small after canceling out each other, and the spherical forces inside the fixed-end constant velocity joint are also very small after canceling out each other. When the working swing angle is increased, the internal movement will not increase the spherical force and will not cause the temperature inside the joint to be excessive. Therefore, the friction loss between the spherical surfaces inside the fixed-end constant velocity joint is low, and it is a very efficient structure. When the fixed joint is at a small swing angle β0, see Figure 6 The steel ball 4 in the ball track is in the transition section 53. At this time, since the swing angle β0 is very small, according to the force characteristics of the fixed end constant velocity universal joint, the internal force of the ball track is also very small.

[0045] The fixed-end constant velocity universal joint of the present invention is designed with a specific ball track shape so that the component force of the steel ball 4 along the axis of the universal joint at a large swing angle can be effectively offset, reducing the axial force and the force between the spherical surfaces in the universal joint, thereby reducing the internal energy loss caused by movement.

[0046] In this embodiment, see Figure 3 and Figure 4 The transition section 53 can be composed of a straight line segment, or composed of several arc segments, or composed of several arc segments and several straight line segments. The transition section 53 is required to be a smooth transition to ensure that the steel ball 4 moves smoothly in the ball lane. Of course, the transition section 53 can also be other smooth curve shapes.

[0047] In this embodiment, see Figure 2 、 Figure 3 and Figure 4As a preferred design, N outer ball tracks 5 have the same shape and are recorded as type A outer ball tracks 5a. The transition section 53 of the type A outer ball track 5a gradually tilts toward the central axis of the outer star wheel 1 from the opening end 8 to the mounting end 7, that is, the distance between the transition section 53 and the central axis 10 of the universal joint gradually decreases from the opening end 8 to the mounting end 7. The other N outer ball tracks 5 have the same shape and are recorded as type B outer ball tracks 5b. The transition section 53 of the type B outer ball track 5b gradually tilts toward the central axis of the outer star wheel 1 from the mounting end 7 to the opening end 8, that is, the distance between the transition section 53 and the central axis 10 of the universal joint gradually increases from the opening end 8 to the mounting end 7. Specifically in this embodiment, see Figure 3 and Figure 4 The distances from the centers of the first arc segment 51 and the second arc segment 52 of the A-type outer ball track 5a to the central axis 10 of the universal joint are the same and they are located on the same side of the central axis 10 of the universal joint. The radii of the first arc segment 51 and the second arc segment 52 are respectively recorded as R1 and R2, R1>R2. The distances from the centers of the first arc segment 51 and the second arc segment 52 of the B-type outer ball track 5b to the central axis 10 of the universal joint are also the same and they are located on the same side of the central axis 10 of the universal joint. The radii of the first arc segment 51 and the second arc segment 52 are respectively recorded as R3 and R4, R3<R4. With this design, when the swing angle is relatively small, the steel ball 4 is located in the area of ​​the transition section 53. Since the transition section 53 of the A-type outer ball track 5a is located on a side of the opening end 8 that is higher than the side of the mounting end 7, the axial component of the resultant force received by the steel ball 4 when in the transition section 53 of the A-type outer ball track 5a is directed to the opening end 8; correspondingly, the transition section 53 of the B-type outer ball track 5b is located on a side of the opening end 8 that is lower than the side of the mounting end 7, and the axial component of the resultant force received by the steel ball 4 in the transition section 53 of the B-type outer ball track 5b is directed to the mounting end 7. In this way, the steel balls 4 in the A-type outer ball track 5a and the steel balls 4 in the B-type outer ball track 5b are subjected to opposite axial forces and can offset each other. Therefore, the axial force can be well reduced when the swing angle is relatively small.

[0048] In this embodiment, see Figure 3 and Figure 4 The center O1 of the first arc segment 51 and the center O2 of the second arc segment 52 are preferably both located on the central axis 10 of the universal joint, but they may not be located on the central axis 10 of the universal joint. The distances between the center O1 of the first arc segment 51 and the center O2 of the second arc segment 52 and the central plane 9 of the universal joint can be set according to actual needs.

[0049] In this embodiment, see Figure 2The total number of outer ball tracks 5 is 8, with 4 A-type outer ball tracks 5a and 4 B-type outer ball tracks 5b, i.e., N=4. Preferably, the 4 A-type outer ball tracks 5a and the 4 B-type outer ball tracks 5b are arranged alternately around the circumference of the outer star wheel 1. That is, the two A-type outer ball tracks 5a are symmetrical about the central axis 10 of the universal joint, and the two B-type outer ball tracks 5b are symmetrical about the central axis 10 of the universal joint. In this case, the ball track opposite the A-type outer ball track 5a is also an A-type outer ball track 5a, and the ball track opposite the B-type outer ball track 5b is also a B-type outer ball track 5b. With this arrangement, the steel balls 4 in the two opposing A-type outer ball tracks 5a and the steel balls 4 in the two opposing B-type outer ball tracks 5b are subjected to force in opposite directions along the axial direction of the outer star wheel 1 or the inner star wheel 2, which can better offset the force components in that direction, making the force distribution more uniform and stable. This can further improve the working efficiency of the fixed joint, especially at small swing angles. Of course, in other embodiments, the outer ball lanes 5 may also be of other numbers, such as 6. In addition, the A-type outer ball lanes 5a and the B-type outer ball lanes 5b may not be arranged alternately.

[0050] In this embodiment, see Figure 4 As a preferred design, either or both the B-type outer ball track 5b and the A-type outer ball track 5a further include a third arc segment 54. The third arc segment 54 is located on the side of the first arc segment 51 facing the open end 8, and the two are tangentially connected. From the cross section through the centerline of the ball track, from the open end 8 to the mounting end 7, the B-type outer ball track 5b includes the third arc segment 54, the first arc segment 51, the transition segment 53, and the second arc segment 52 in sequence. The third arc segment 54 and the first arc segment 51 have a curvature direction opposite to that of the first arc segment 51. The radius R5 of the third arc segment 54 can be set according to actual needs, and it is preferred that both the B-type outer ball track 5b and the A-type outer ball track 5a be provided with the third arc segment 54. By providing the third arc segment 54, the swing angle and working angle of the fixed-end constant velocity universal joint can be increased.

[0051] In the present invention, the outer and inner ball races 5 and 6 are formed by milling, and the steel balls 4 within the races can reciprocate normally within any swing angle of the fixed-end constant velocity joint. The design of the races requires that they do not interfere with the assembly of the fixed-end constant velocity joint. Therefore, the inner star wheel 2 can be placed into the opening of the cage 3, and then vertically inserted into the outer star wheel 1. The inner star wheel 2 can then be flipped over, and the steel balls 4 can be installed one by one into the races. Finally, the fixed-end constant velocity joint can be assembled by press-fitting the solid shaft.

[0052] As can be seen from the above, the fixed-end constant velocity universal joint of the present invention has the following technical effects:

[0053] By setting up ball lanes of a specific shape, in two opposite ball lanes, the first arc segment 51 and the second arc segment 52 are respectively used to contact the steel balls 4 therein, and the component forces of the steel balls 4 in the two ball lanes in the axial direction of the outer star wheel 1 or the inner star wheel 2 can be effectively offset, thereby reducing or even eliminating the force of the steel balls 4 on the window of the ball cage 3, further reducing the spherical force in the fixed-end constant velocity universal joint, and reducing the internal energy loss caused by movement. Compared with the existing general constant velocity fixed-end universal joints, it has significantly higher efficiency, and its main benefit characteristics can be in line with the main development direction of vehicles such as improving energy efficiency and reducing losses.

[0054] In summary, the present invention effectively overcomes various shortcomings in the prior art and has high industrial utilization value.

[0055] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the present invention. Anyone skilled in the art may modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by one of ordinary skill in the art without departing from the spirit and technical principles disclosed herein are intended to be covered by the claims of the present invention.

Claims

1. A fixed-end constant velocity universal joint, comprising an outer star wheel (1), an inner star wheel (2), a ball cage (3) and steel balls (4), wherein the outer star wheel (1) is provided with 2N outer ball tracks (5), N ≥ 3, and the inner star wheel (2) is provided with 2N inner ball tracks (6), wherein the inner ball tracks (6) and the outer ball tracks (5) correspond to each other one by one to form ball tracks, and the inner ball tracks (6) and the outer ball tracks (5) are symmetrical about the central plane (9) of the universal joint, and wherein one end of the outer star wheel (1) is an open end (8) and the other end is a mounting end (7), and wherein: The outer ball track (5) comprises a first arc segment (51), a second arc segment (52), and a transition segment (53) located between the first arc segment (51) and the second arc segment (52); the first arc segment (51) and its center are located on the side of the universal joint center plane (9) facing the opening end (8); the second arc segment (52) and its center are located on the side of the universal joint center plane (9) facing the mounting end (7); the first arc segment (51) and the second arc segment (52) are both bent inwardly of the outer star wheel (1); and the first arc segment (51) and the second arc segment (52) are both tangent to the transition segment (53). The outer ball paths (5) are connected; the shapes of the 2N outer ball paths (5) are all the same, or N of the outer ball paths (5) are the same and the shapes of the other N outer ball paths (5) are the same; the N identical outer ball paths (5) on the outer star wheel (1) are type A outer ball paths (5a) and the other N identical outer ball paths (5) are type B outer ball paths (5b); the transition section (53) of the type A outer ball path (5a) gradually tilts toward the central axis of the outer star wheel (1) from the opening end (8) to the mounting end (7), and the transition section (53) of the type B outer ball path (5b) gradually tilts toward the central axis of the outer star wheel (1) from the mounting end (7) to the opening end (8).

2. The fixed-end constant velocity universal joint according to claim 1, characterized in that: The transition section (53) is composed of a straight line section or a plurality of circular arc sections.

3. The fixed-end constant velocity universal joint according to claim 1, characterized in that: The transition section (53) is composed of a plurality of straight line sections and a plurality of circular arc sections.

4. The fixed-end constant velocity universal joint according to claim 1, characterized in that: The distances from the center of the first arc segment (51) and the center of the second arc segment (52) to the central axis (10) of the universal joint are the same.

5. The fixed-end constant velocity universal joint according to claim 1, characterized in that: The distances from the center of the first arc segment (51) and the center of the second arc segment (52) to the central axis (10) of the universal joint are different.

6. The fixed-end constant velocity universal joint according to claim 1, characterized in that: The centers of the first arc segment (51) and the second arc segment (52) of the A-type outer ball track (5a) are at the same distance from the central axis (10) of the universal joint and are located on the same side of the central axis (10) of the universal joint, and the radius of the first arc segment (51) is greater than the radius of the second arc segment (52); the centers of the first arc segment (51) and the second arc segment (52) of the B-type outer ball track (5b) are at the same distance from the central axis (10) of the universal joint and are located on the same side of the central axis (10) of the universal joint, and the radius of the first arc segment (51) is smaller than the radius of the second arc segment (52).

7. The fixed-end constant velocity universal joint according to claim 1 or 6, characterized in that: The centers of the first arc segment (51) and the second arc segment (52) of the outer ball track (5) are both located on the central axis (10) of the universal joint.

8. The fixed-end constant velocity universal joint according to claim 5, characterized in that: N A-type outer ball tracks (5a) and N B-type outer ball tracks (5b) are alternately arranged in the circumferential direction of the outer star wheel (1).

9. The fixed-end constant velocity universal joint according to claim 5, characterized in that: One or both of the B-type outer ball track (5b) and the A-type outer ball track (5a) further include a third arc segment (54), wherein the third arc segment (54) is located on the side of the first arc segment (51) facing the opening end (8), and the two are tangentially connected, and the curvature direction of the third arc segment (54) is opposite to that of the first arc segment (51).

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