Abstract:
The present invention provides a lightweight, compact, and low-cost tripod type constant velocity universal joint while maintaining strength of overall tripod type constant velocity universal joint. The dimensional ratio of each component of the tripod type constant velocity universal joint is set as follows: 0.80&lt;Dor&lt;0.90, 0.105&lt;Tor&lt;0.115, 0.025&lt;fir&lt;0.045, 0.025&lt;t 2   r &lt;0.075, 0.53&lt;Dir&lt;0.63, 0.065&lt;Tir&lt;0.085, 0.32&lt;Dj 1   r &lt;0.42, and 0.32&lt;Dj 2   r &lt;0.35. In this event, the subscript “r” means that each dimension is divided by the trunnion journal pitch circle diameter D (=PCD) of the joint.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a tripod type constant velocity universal joint, for example, for use in a drivetrain of an automobile drive system and for transmission of a rotating force at a constant angular velocity between two rotary shafts on a nonlinear line of the automobile drive system. 
   2. Description of the Background Art 
   Demand of weight reduction and size reduction of tripod type constant velocity universal joints is increasing to improve automobile mileage. Responding to the demand, a tripod type constant velocity universal joint of, for example, Japanese unexamined patent publication 2004-257418 is proposed. The tripod type constant velocity universal joint set forth in this Japanese unexamined patent publication 2004-257418 is of a double roller type where each outer roller rolls in parallel to a track groove. The journal root portion has a non-circular cross-section in which a diameter in the joint circumferential direction is larger than that in the joint axial direction. This feature brings about lightweight, compactness and reduced cost of the joints while keeping the strength at the root portion of the trunnion journal that tends to be a weakest portion of the trunnion. 
   However, in the event that an ultimate design is applied to the tripod type constant velocity universal joint of Japanese unexamined patent publication 2004-257418 in order to promote a lightweight compact conversion, it has been found out that it is insufficient to solely make the cross section of the journal root portion noncircular to maintain the strength at the root portion of the trunnion journal. That is, considering the strength of the whole tripod type constant velocity universal joint, it has been found it is difficult to promote a lightweight compact conversion of the whole tripod type constant velocity universal joint unless dimensions of each component, particularly, the shape and the dimensions of the roller cassette and trunnion journal root portion are optimally designed. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to solve the above-mentioned problems and to provide a lightweight, compact, and low-cost tripod type constant velocity universal joint while maintaining the strength of the whole tripod type constant velocity universal joint. 
   In order to solve the above-mentioned problems, the first aspect of the present invention is a tripod type constant velocity universal joint, which comprises an outer joint member being coupled to and adapted to transmit torque to a first rotary shaft and having track grooves which extend in the axial direction at trisected positions in a circumferential direction on the inner circumferential surface thereof, an inner joint member being coupled to and adapted to transmit torque to a second rotary shaft and comprising a boss directly connected to the second rotary shaft and a journal protruding from the circumferential trisected positions of the boss in the radial direction, and a roller cassette that stands between the outer joint member and the inner joint member to transmit torque and comprising an outer roller that can roll along the track grooves of the outer joint member, an inner roller fitted to the journal of the inner joint member and a plurality of rolling elements disposed between the outer roller and the inner roller, wherein Do denotes the outside diameter of the outer roller, To the radial wall thickness of the outer roller, t  1  the collar thickness on the outside of the joint radial side of the outer roller, t  2  the collar thickness on the inside of the joint radial side of the outer roller, Di the outside diameter of the inner roller, Ti the wall thickness of the inner roller, Dj  1  the diameter in the joint peripheral direction of the cross-sectional shape at the root portion of trunnion journal, Dj 2  the diameter in the joint axial direction of the cross-sectional shape at the joint of trunnion journal, and D the trunnion journal pitch circle diameter (PCD) of the joint; then, in the event that it is defined that the outside diameter ratio of the outer roller Dor=Do/D, wall thickness ratio Tor=To/D, outer collar thickness ratio t 1   r =t 1 /D, inner collar thickness ratio t 2   r =t 2 /D, outside diameter ratio of inner roller Dir=Di/D, wall thickness ratio Tir=Ti/D, joint circumferential diameter ratio at the joint of trunnion journal Dj  1   r =Dj  1 /D, and joint axial diameter ratio at the joint of trunnion journal Dj 2   r =Dj 2 /D, each of the above-mentioned ratios is set to the range of 0.80&lt;Dor&lt;0.90, 0.105&lt;Tor&lt;0.115, 0.025&lt;th&lt;0.045, 0.025&lt;t 2   r &lt;0.075, 0.53&lt;Dir&lt;0.63, 0.065&lt;Tir&lt;0.085, 0.32&lt;Dj 1   r&lt; 0.42, and 0.32&lt;Dj 2   r &lt;0.35. In this event, Dor, Tor, tir, t 2   r  denote the ratio of the outer roller dimensions, and Dir and Tir are ratios of the inner roller dimensions. Setting them to the above-mentioned proportions provides a good balance of strength and rigidity of the relevant portions, lightweight compact conversion and low cost conversion, while maintaining the required strength. 
   Dj 1   r  and Dj 2   r  are ratios of dimensions of the trunnion journal portion. Setting the ratio to the above-mentioned proportions provides the required operating angle of the constant velocity universal joint, a good balance between the strength and the rigidity of the relevant portions, lightweight compact conversion and low cost conversion, while maintaining the required strength. 
   As mentioned above, setting the dimensional ratio of each component of the constant velocity universal joint to a specified range realizes lightweight, compact, and low-cost constant velocity joints with keeping the strength of overall tripod type constant velocity universal joints. 
   The second aspect of the present invention lies in that undercuts for the inner roller assembly are provided in the joint circumferential direction on the nearly spherical trunnion journal. 
   This enables smooth assembly by greatly tilting a roller cassette when the roller cassette is assembled to a trunnion journal. 
   According to the first and second aspects of the invention, the third aspect of the present invention lies in that the journal root portion has a noncircular cross section whose diameter in the joint circumferential direction is larger than that in the joint axial direction. 
   This can improve strength in the circumferential direction of the journal neck bottom that is the position most susceptible to the maximum stress when a torque load is applied and can make the joint still more compact. 
   According to the first through third aspects of the invention, the fourth aspect of the present invention is that the radius of curvature R 1  of the journal root portion in the joint circumferential direction is constant over the specified angle range from the top of the joint circumferential direction. 
   Even in the event that the radius of curvature R 1  is made constant throughout a specified angle range, it is possible to achieve small size, light weight, and low cost without sacrificing the maximum operating angle of a joint while maintaining strength at the journal root portion, which is most likely to become the weakest portion of the tripod. 
   According to the first to fourth aspects of the invention, the fifth aspect of the present invention lies in that the ratio of radius of curvature R 1  in the joint circumferential direction of the journal root portion to the radius of curvature R 2  in the joint axial direction is set to 2.0&lt;a&lt;6.5 where a denotes the value of the ratio of R 1  to R 2 . In the case of a&lt;2.0, R 2  is too large, causing the joint to interfere with a roller cassette  20 , and the maximum operating angle of the joint decreases. Consequently, the joint PCD must be increased to avoid interference, which possibly results in an increased outside diameter of the joint. In the case of a&gt;6.5, R 2  is too small and the forging formability may be degraded. 
   According to the first to fifth aspects of the invention; the sixth aspect of the present invention lies in that the ratio of radius of curvature R 1  in the joint circumferential direction of the journal root portion to the female spline PCD of the boss is set to 0.13&lt;b&lt;0.25 where b denotes the value of the ratio of R 1  to PCD. 
   In the case of b&lt;0.13, R 1  is too small and the strength of the tripod  16  may be possibly decreased. In the case of b&gt;0.25, R 1  is too large, causing the joint to interfere with the roller cassette  20  and the maximum operating angle of the joint decreases. Consequently, the joint PCD must be increased to avoid interference, and an increased joint outside diameter may result. 
   As described above, setting the dimensional ratio of each component of the tripod type constant velocity universal joint to the proper value leads to a tripod type constant velocity universal joint that can satisfy all the requirements of light weight, compactness, and low cost while maintaining the strength of the whole joint. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is a front view of a tripod used for a tripod type constant velocity universal joint according to the present invention and  FIG. 1B  is a cross-sectional view taken in the direction of the arrows along the line b-b of  FIG. 1A ; 
       FIG. 2A  is a front view of a trunnion journal and  FIG. 2B  is a side view of the trunnion journal; 
       FIG. 3  is a transverse cross sectional view of a tripod type constant velocity universal joint; 
       FIG. 4  is a longitudinal cross sectional view of a tripod type constant velocity universal joint; 
       FIG. 5  is an enlarged view of the transverse cross section of the tripod type constant velocity universal joint; 
       FIG. 6  is a cross-sectional view of an outer roller; 
       FIG. 7  is a cross-sectional view of an inner roller; and 
       FIG. 8  is a cross-sectional view that explains an assembly procedure of a tripod kit. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Hereinafter, embodiments of the present invention will now be described in detail with reference to the drawings. 
   (Basic Configuration of a Tripod Type Constant Velocity Universal Joint) 
     FIG. 3  through  FIG. 5  show a basic configuration of a tripod type constant velocity universal joint  11 . This constant velocity universal joint  11  is composed of a hollow cylindrical outer joint member  14  which is fixed to the end portion of a first rotary shaft  12  such as a drive shaft, etc. and a tripod (inner joint member)  16  which is fixed to the end portion of a second rotary shaft  13  such as rotary shaft, etc. on the wheel side. 
   The outer joint member  14  is formed integral with the first rotary shaft  12  by a forging process and has a recessed groove  14   a  extending in the axial direction at the trisected position in the circumferential direction of the inner circumferential surface thereof. Each recessed groove  14   a  is recessed from the inner circumferential surface of the outer joint member  14  radially outward and is composed of a pair of guide surfaces  14   b  facing each other in the circumferential direction and a bottom surface which is located in the radial outside of the outer joint member and connects both guide surfaces  14   b . The pair of guide surfaces  14   b  provide a track on which the outer roller  26 , later discussed, is guided in the outer joint member axial direction and roll and transmit torque to the outer roller  26 . In addition, to part of the bottom surface of the recessed groove  14   a , a guiding shoulder surface  14   c  that guides rolling of the outer roller  26  is formed. This guiding shoulder surface  14   c  allows the outer roller  26  to maintain a posture parallel to the outer joint member axial direction when the outer roller  26  travels in the recessed groove  14   a  and plays a role to smoothly roll the outer roller  26 . 
   The tripod  16  is composed of a boss  16   a  and a trunnion journal  16   b  formed integral by forging processing. The boss  16   a  is fixed to the end portion of the second rotary shaft  13 . For example, a spline shaft  25  formed on the second rotary shaft  13  and a spline hole formed on the boss  16   a  are fitted to each other and positioned with a retaining ring  13   a . The trunnion journal  16   b  protrudes in the radial direction from the circumferential trisected position of the boss  16   a . The end of each trunnion journal  16   b  assumes a spherical shape. 
   Each trunnion journal  16   b  supports a roller cassette  20 . The roller cassette  20  is of a double roller type composed of an inner roller  22  and an outer roller  26  adapted to rotate relatively via a needle roller  24 . In order to prevent slip-out of the needle roller  24 , an annular collar  38  is provided for the outer roller  26  on both end sides of the needle roller  24 . The inner circumferential surface of the inner roller  22  is a sphere having a radius of curvature nearly the same as that of the spherical outer circumferential surface of the trunnion journal  16   b , and the inner roller  22  and the trunnion journal  16   b  are spherically fitted with each other. By this, the circumference of the spherical outer circumferential surface of the trunnion journal  16   b  rockably supports the spherical inner circumferential surface of the inner roller  22 . 
   Between the cylindrical outer circumferential surface of the inner roller  22  and the cylindrical inner circumferential surface of the outer roller  26 , multiple needle rollers  24  stand. This configuration enables relative rotation and axial displacement of the inner roller  22  and the outer roller  26 . 
   The outer roller  26  is housed in the recessed groove  14   a  of the outer joint member  14 . The pair of guide surfaces  14   b  that compose each recessed groove  14   a  is arcuate, and is nearly same as that of the generator of the outer circumferential surface of the outer roller  26  in the transverse cross section ( FIG. 3 ) of the outer joint member  14 . Consequently, the outer roller  26  is rotatably supported between a pair of these guide surfaces  14   b.    
   In the event that, for example, the first rotary shaft  12  rotates while the tripod type constant velocity universal joint configured as above is in use, the rotating force is transmitted from the outer joint member  14  to the boss  16   a  of the tripod  16  via the roller cassette  20  and the trunnion journal  16   b  and rotates the second rotary shaft  13 . Furthermore, in the event that the center shaft of the first rotary shaft  12  and the center shaft of the second rotary shaft  13  are not aligned as shown in  FIG. 4 , in other words, in the event that the tripod type constant velocity journal joint takes an operating angle, each trunnion journal  16   b  displaces in the rocking direction with respect to the guide surface  14   b  of the recessed groove  14   a  as both rotary shafts rotate while setting the tripod  16  at the center of the displacement. In such event, the outer roller  26  supported by the trunnion journal  16   b  is displaced in the axial direction of the trunnion journal  16   b  while it rolls on the guide surface  14   b  of the recessed groove  14   a . These movements, as known, secure the constant velocity between the first and the second rotary shafts  12 ,  13 . 
   (Shape of the Trunnion Journal) 
   As shown in  FIGS. 1A and 1B , the root portion of the trunnion journal  16   b  is of a noncircular cross section whose diameter in the joint circumferential direction is greater than the diameter in the joint axial direction. This noncircular cross section typically has a shape of an ellipse with the minor axis directed in the joint axial direction as shown in  FIG. 1B . Needless to say, it is, possible to apply noncircular cross sections other than the ellipse to the root portion of the trunnion journal  16   b . In such event, it is recommendable to make the contour line of the noncircular cross section a smooth continuous curve with care to prevent stress concentration. 
   By designing the root portion of the trunnion journal  16   b  to have a noncircular cross section whose diameter in the joint circumferential direction is greater than the diameter in the joint axial direction, the following advantages can be obtained. That is, because the interference undercuts for assembling the journal neck bottom portion with the roller need to exist only on the joint axial direction of the journal neck bottom portion in the event that the roller cassette is tilted in the joint axial direction on the plane surface and assembled to the trunnion journal, no interference undercut for assembly is required for the joint circumferential direction side. At the position in the neck bottom circumferential direction that is likely to be susceptible to the maximum stress when a torque load is received, there is no interference undercut and strength improvement is enabled, and also a more compact joint can be achieved. 
   As shown in  FIGS. 2A and 2B , assume that the journal root portion  16   e  which connects the boss  16   a  with the journal  16   b  of the tripod  16  is of a recessed arcuate profile, the radius of curvature in the joint circumferential direction (torque load direction) be R 1  and the radius of curvature in the joint axial direction be R 2 , we have R 1 &gt;R 2  and the radius of curvature gradually decreases from R 1  to R 2 . 
   By this, it becomes possible to provide a lightweight, compact, and low-cost tripod type constant velocity universal joint without sacrificing the maximum operating angle of the joint with the strength of the journal root portion maintained, where the strength of the tripod  16  becomes the weakest. 
   Let “a” denote the value of the ratio of the radius of curvature R 1  in the joint circumferential direction (torque load direction) to the radius of curvature R 2  in the joint axial direction. It is desirable to set a to 2.0&lt;a&lt;6.5. In the case of a&lt;2.0, R 2  is excessively large and causes the joint to interfere with the roller cassette  20 , and the maximum operating angle of the joint decreases; this requires the joint PCD to be increased to avoid interference, possibly resulting in an increase in the joint outside diameter. In the case of a&gt;6.5, R 2  is excessively small and forging formability may be degraded. 
   In addition, let b denote the value of the ratio of the radius of curvature R 1  in the joint circumferential direction (torque load direction) to the female spline  25  of the boss  16   a  PCD; then, it is desirable to set b to 0.13&lt;b&lt;0.25. In the case of b&lt;0.13, R 1  is too small and the strength of the tripod  16  may be possibly decreased. In the case of b&gt;0.25, R 1  is too large, causing the joint to interfere with the roller cassette  20  and the maximum operating angle of the joint decreases. Consequently, the joint PCD must be increased to avoid interference, and an increased joint outside diameter may result. 
   In the first embodiment, it is also possible to make the radius of curvature R 1  in the joint circumferential direction (torque load direction) of the journal root portion  16   e  that connects the boss  16   a  of the tripod  16  with the journal  16   b  constant throughout the angle range shown with reference character A ( FIG. 1B ). In such event, the extension of the relevant angle range is gradually reduced to the radius of curvature R 2  in the joint axial direction. 
   Even in the event that the radius of curvature R 1  is made constant throughout a specified angle range, it is possible to achieve small size, light weight, and low cost without sacrificing the maximum operating angle of a joint while maintaining strength at the journal root portion  16   e , which is most likely to become the weakest portion of the tripod  16 . The range of angle A is preferably made larger to the extent which does not exert effects on the maximum operating angle of the joint and is preferably. 
   By the way, what is indicated by reference character  16   d  in  FIG. 2B  is the undercut for assembling the inner roller  22  formed by cutting part of the nearly spherical journal  16   b  in the joint circumferential direction. 
   (Optimal Dimensional Ratio of Outer Roller, Inner Roller and Journal Root Portion) 
   The range of the optimal dimensional ratio of the outer roller  26 , inner roller  22  and journal root portion of the tripod type constant velocity universal joint is discussed herein. The “dimensional ratio” is the ratio of the joint to the trunnion journal pitch circle diameter D (=PCD) (see  FIG. 5 ). As any one of the dimensional ratios of components exceeds the lower limit or the upper limit, the strength or rigidity lowers at the portion departing from the dimensional ratio of the tripod type constant velocity universal journal or the joint outside diameter increases, and high strength and compactness are difficult to achieve even if other dimensional ratios are included in the upper and the lower limit ranges of optimum dimensional ratios. Reference characters of dimensions of each portion are shown in  FIG. 1B ,  FIG. 5 ,  FIG. 6 , and  FIG. 7 . 
   (Assembly of Roller Cassette) 
     FIG. 8  shows the procedure when the roller cassette  20  is assembled to the trunnion journal  16   b . As shown in  FIG. 8 , a large chamfer  16   c  is provided to the end side (right side of  FIG. 4  and left side in  FIG. 8 ) of the second rotary shaft  13  of the boss  16   a  of the tripod  16 . This chamfer  16   c  and the undercut  16   d  in the joint circumferential direction discussed before enable the roller cassette  20  to be greatly tilted and assembled as shown by an imaginary line in  FIG. 8  when the roller cassette  20  is assembled to the trunnion journal  16   b . And because the roller cassette  20  (of the inner roller  22 ) interferes with the trunnion journal  16   b  only at opposite two places in the direction to which the torque load is exerted (direction perpendicular to the paper surface of  FIG. 8 ), assembly is enabled by pressing in the inner roller  22  while it is being elasto-deformed. 
   By the way, the interference undercuts between the trunnion journal  16   b  and the roller cassette  20  only need to exist at the trunnion journal neck bottom portion in the joint axial direction, and no interference undercuts are required for the trunnion journal neck bottom portion in the joint circumferential direction. Because in the case of this specification, no interference undercuts are required at the neck-bottom circumferential direction that is susceptible to the maximum stress when a torque load is exerted, the strength can be increased and also a more compact tripod type constant velocity universal joint can be obtained. Furthermore, the interference may be avoided by providing a plane surface to two positions (outside the load range), the position to which the torque load of the trunnion journal  16   b  is exerted and the position in the perpendicular direction. 
   Because according to the above construction, in the—high-performance type tripod type constant velocity universal joint, the torque is transmitted between the spherical inner circumferential surface of the inner roller  22  and the spherical trunnion journal  16   b , the contact surface pressure is suppressed to a low level, which is advantageous from the viewpoint of strength and durability, and at the same time, the neck bottom strength of the trunnion journal  16   b  is improved; it, therefore, becomes possible to provide a tripod type constant velocity universal joint, which can satisfy all the requirements of high performance, high strength, high durability, and compactness. 
   As described above, the preferred embodiments of the present invention have been described and illustrated in detail, but it is clearly understood that the present invention shall not be limited to the above-mentioned embodiments but various modifications may be made possible based on the technical ideas expressed in the appended claims.