Patent Publication Number: US-11655857-B2

Title: Tripod constant-velocity joint

Description:
This application claims priority from Japanese Patent Application No. 2019-094069 filed on May 17, 2019, the disclosure of which is herein incorporated by reference in its entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to a tripod constant-velocity joint capable of reducing a compelling force without reducing a durability of the constant-velocity joint. 
     BACKGROUND OF THE INVENTION 
     There is known a tripod constant-velocity joint that includes: an inner ring attached to a leg shaft; three trunnions protruding from the inner ring toward an outer peripheral side of the inner ring and having respective convex outer-circumferential surfaces; three roller units rotatably supported by the respective three trunnions; and a outer ring accommodating the three roller units, wherein the inner ring is disposed in the outer ring such that the inner ring is unrotatable relative to the outer ring and is movable relative to the outer ring in a direction of a rotation axis about which the outer ring is to be rotated. An example of such a tripod constant-velocity joint is disclosed in JP-2011-163411A. 
     SUMMARY OF THE INVENTION 
     By the way, in the above-described tripod constant-velocity joint, a tangent point at which the convex outer-circumferential surface of each of the three trunnions is in contact with a corresponding one of the roller units, is displaced in a trunnion protruding direction (in which the trunnion protrudes from the inner ring), with change of a joint angle corresponding to an angle that is defined by a rotation axis of the leg shaft (inner ring) and the rotation axis of the outer ring. Each of the roller units is constituted by an inner roller, an outer roller and a plurality of needles that are interposed between the inner and outer rollers in a radial direction, wherein the plurality of needles serve as rolling elements. The outer roller includes a radially inward flange portion provided in its axial end portion that is on a side of a distal end of a corresponding one of the trunnions. A snap ring is provided in another axial end portion of the outer roller that is on a side of a proximal end of the corresponding one of the trunnions. The radially inward flange portion and the snap ring, which are provided in the respective axial end portions of the outer roller, cooperate with each other to limit movements of the needles and the inner roller relative to the outer roller. 
     However, depending on the change of the joint angle corresponding to the angle that is defined by the rotation axis of the leg shaft (inner ring) and the rotation axis of the outer ring, a clearance between the inner and outer rollers on the side of the distal end of the trunnion is eliminated whereby the inner roller is brought into contact with the radially inward flange portion of the outer roller, so that there is a possibility of reduction of a durability of the constant-velocity joint due to a compelling force generated as a result of the elimination of the clearance. 
     The present invention was made in view of the background art described above. It is therefore an object of the present invention to provide a tripod constant-velocity joint having a high durability by restraining generation of a compelling force due to change of a joint angle. 
     The above object is achieved according to the following feature of the present invention. 
     According to the feature of the invention, there is provided a tripod constant-velocity joint including (a) an inner ring attached to a leg shaft; (b) three trunnions protruding from the inner ring toward an outer peripheral side of the inner ring, each of the three trunnions having a convex outer-circumferential surface; (c) three roller units rotatably supported by the respective three trunnions; and (d) a outer ring storing the three roller units. The inner ring is disposed in the outer ring such that the inner ring is unrotatable relative to the outer ring and is movable relative to the outer ring in a direction of an outer ring axis about which the outer ring is to be rotated. Each of the three roller units includes: (c-1) an inner roller slidably fitted at an inner circumferential surface thereof on the convex outer-circumferential surface of a corresponding one of the three trunnions, such that the inner circumferential surface of the inner roller and the convex outer-circumferential surface of the corresponding one of the three trunnions are in contact with each other at a tangent point that is to be reciprocatively moved during rotation of the tripod constant-velocity joint; (c-2) a plurality of cylindrical rolling elements each including a crowned end portion in an axial end portion thereof; (c-3) an outer roller supported by the inner roller through the plurality of cylindrical rolling elements that are interposed between the inner roller and the outer roller; and (c-4) a limiting portion provided integrally in the outer roller and protruding from the outer roller toward an inner peripheral side of the outer roller, so as to limit movements of the plurality of cylindrical rolling elements and the inner roller. The inner roller and the limiting portion cooperate to define therebetween a clearance in a direction of a center line of a corresponding one of the three trunnions, such that the clearance is smaller than an axial length of the crowned end portion of each of the plurality of cylindrical rolling elements, and is larger than a stroke distance of reciprocating movement of the tangent point when a joint angle of the tripod constant-velocity joint is a predetermined value. The joint angle of the tripod constant-velocity joint is an angle between the outer ring axis and an inner ring axis about which the inner ring is to be rotated. Further, for example, each of the three roller units further includes, in addition to the limiting portion as a first limiting portion, a second limiting portion provided to protrude from the outer roller toward the inner peripheral side of the outer roller and spaced apart from the first limiting portion by a predetermined spacing distance in an axial direction of the outer roller, wherein the second limiting portion cooperates with the first limiting portion to limit the movements of the plurality of cylindrical rolling elements and the inner roller that are located between the first and second limiting portions in the axial direction of the outer roller, and wherein the clearance corresponds to a value obtained by subtracting an axial length of the inner roller from the predetermined spacing distance between the first and second limiting portions. 
     In the tripod constant-velocity joint constructed as described above, the clearance between the inner roller and the limiting portion in the direction of the center line of the corresponding one of the three trunnions, is smaller than the axial length of the crowned end portion of each of the plurality of cylindrical rolling elements, and is larger than the stroke distance of the reciprocating movement of the tangent point (at which the inner roller and the corresponding one of the three trunnions are in contact with each other) when the joint angle is the predetermined value. Owing to this feature, the inner roller is not brought into contact with the limiting portion and accordingly the inner roller is restrained from being brought into contact at its edge portion with rolling surfaces of the respective rolling elements. It is therefore possible to retrain a compelling force applied from the inner roller and to provide the constant velocity joint with a high durability. 
     Preferably, the clearance between the inner roller and the limiting portion of a corresponding one of the three trunnions in the direction of the center line of the corresponding one of the three trunnions, is smaller than the stroke distance of reciprocating movement of the tangent point when the joint angle is 10 degrees. Owing to this feature, the clearance can be set to a value within an appropriate range. 
     Preferably, the predetermined value of the joint angle is a normal angle value that is a value of the joint angle in the most frequent case in a practical use of the tripod constant-velocity joint. For example, the predetermined value of the joint angle is 6 degrees. Owing to this feature, the clearance between the inner roller and the limiting portion of a corresponding one of the three trunnions in the direction of the center line of the corresponding one of the three trunnions, is set to a value larger than the stroke distance of reciprocating movement of the tangent point when the joint angle is the normal angle value, so that the inner roller is not brought into contact at its axial end surface with columnar-shaped outer circumferential surfaces of the respective rolling elements at least when the joint angle is the normal angle value, thereby making it possible to avoid a stress concentration on the rolling surface of any of the rolling elements, which could be caused by contact of the edge of the inner roller with the rolling surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a front view showing a vehicle drive-force transmitting shaft to which a tripod constant-velocity joint is applied, wherein the tripod constant-velocity joint is constructed according to an embodiment of the present invention; 
         FIG.  2    is a partially cross sectional view showing a outer ring that constitutes a part of the tripod constant-velocity joint shown in  FIG.  1   ; 
         FIG.  3    is a view showing a cross section of the tripod constant-velocity joint shown in  FIG.  1   , wherein the view is as seen from a left side of  FIG.  1    and the cross section contains a center line of a trunnion that is included in the tripod constant-velocity joint; 
         FIG.  4    is a cross sectional view schematically showing a clearance between an inner roller and a limiting portion of an outer roller that are included in the tripod constant-velocity joint shown in  FIG.  1   , in a direction parallel to the center line of the trunnion; 
         FIG.  5    is a schematic view for explaining a distance by which a tangent point (at which the inner roller and the trunnion are in contact with each other) is to be moved relative to a center of the trunnion so as to reach an upper stroke end of a reciprocating movement of the tangent point, in the tripod constant-velocity joint shown in  FIG.  1   ; 
         FIG.  6    is a schematic view for explaining a distance by which the tangent point is to be moved relative to the center of the trunnion so as to reach a lower stroke end of the reciprocating movement of the tangent point, in the tripod constant-velocity joint shown in  FIG.  1   ; 
         FIG.  7    is a is a view corresponding to the view of  FIG.  3    and showing a cross section of a tripod constant-velocity joint of a comparative example; and 
         FIG.  8    is a view showing the clearance in the tripod constant-velocity joint shown in  FIG.  1    and the clearance in the tripod constant-velocity joint shown in  FIG.  7   , in a comparative manner. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
     Hereinafter, a preferred embodiment of the invention will be described in detail with reference to the accompanying drawings. The figures of the drawings are simplified or deformed as needed, and each portion is not necessarily precisely depicted in terms of dimension ratio, shape, etc. 
     Embodiment 
       FIG.  1    is a view schematically showing a vehicle drive shaft  10  to which a constant velocity joint is applied, wherein the tripod constant-velocity joint is constructed according to an embodiment of the present invention. The vehicle drive shaft  10  of  FIG.  1    is, for example, a front drive shaft that is to be provided in a front portion of a FF vehicle such that the front drive shaft extends in a width direction of the FF vehicle. The vehicle drive shaft  10  is constituted mainly by, for example, an intermediate shaft (leg shaft)  12  made of a carbon steel for machine structure and a pair of constant velocity joints  14 ,  20  that are fixed to respective axially opposite end portions of the intermediate shaft  12 . 
     The constant velocity joint  14 , which is one of the pair of constant velocity joints  14 ,  20 , is provided in one of the axially opposite end portions of the intermediate shaft  12 , wherein the one of the axially opposite end portions is located in an outer side portion of the vehicle. The constant velocity joint  14  consists of a Birfield constant-velocity joint, and is connected to a hub  22  to which a front wheel (not shown) as a drive wheel of the vehicle is fixed. On the other hand, the constant velocity joint  20 , which is another one of the pair of constant velocity joints  14 ,  20 , is provided in another one of the axially opposite end portions of the intermediate shaft  12 , wherein the other one of the axially opposite end portions is located in an inner side portion of the vehicle. The constant velocity joint  20  consists of a sliding tripod constant-velocity joint, and is connected to a side gear of a final speed-reduction device (not shown). Thus, the vehicle drive shaft  10  is configured to transmit a drive force that is outputted from the final speed-reduction device included in a transaxle (not shown), to the front wheel as the drive wheel of the vehicle, for example. 
     The constant velocity joint  14  consisting of the Birfield constant-velocity joint includes an inner ring  15  fitted on an end portion  12   a  of the intermediate shaft  12  and a outer ring  16  defining therein a storing room  16   a  in which the inner ring  15  is stored. The end portion  12   a  corresponds to the above-described one of axially opposite end portions of the intermediate shaft  12 , wherein the one of the axially opposite end portions is located in the outer side portion of the vehicle. The outer ring  16  is to be rotated about its rotation axis C 4 , and includes a connecting shaft  16   b  that extends in a direction of the rotation axis C 4  so as to be fitted in a fitting hole  22   a  provided in the hub  22  such that the connecting shaft  16   b  is unrotatable relative to the hub  22 . The connecting shaft  16   b  has spline teeth provided on an outer circumferential surface of its axial end portion, so that the connecting shaft  16   b  is splined to the fitting hole  22   a  in which spline teeth are provided on its inner circumferential surface. 
     Between the above-described inner and outer rings  15 ,  16 , there are provided a substantially cylindrical cage  14   a  and a plurality of balls  14   b , such that the balls  14   b  are held in respective ball holding holes that are provided in the cage  14   a . A plurality of guide grooves  14   c  are provided in each of an outer circumferential surface of the inner ring  15  and an inner circumferential surface of the outer ring  16  and extend in a direction of axes of the inner and outer rings  15 ,  16 , so that the balls  14   b  are fitted in the respective guide grooves  14   c  and guided by the respective guide grooves  14   c . Owing to this arrangement, each of the outer ring  16  and the hub  22  is allowed to be pivoted or have a circular motion within a certain range about the above-described the above-described one of the axially opposite end portions of the intermediate shaft  12  which is located in the outer side portion of the vehicle. 
     An opening between the end portion  12   a  of the intermediate shaft  12  and the outer ring  16  is covered by a bellows-like tapered boot  18  that is made of a synthetic rubber formed of a soft resin. The boot  18  is fitted at its large-diameter end portion on an outer circumferential surface of the outer ring  16 , and is fitted at its small-diameter end portion on an outer circumferential surface of the intermediate shaft  12 . The boot  18  has an inner portion that is filled with a lubricating grease. 
     The constant velocity joint  20  consisting of the sliding tripod constant-velocity joint includes an inner ring  24  fitted on another end portion  12   b  of the intermediate shaft  12  and a outer ring  26  defining therein a storing room  26   a  in which the inner ring  24  is stored. The other end portion  12   b  corresponds to the above-described other of the axially opposite end portions of the intermediate shaft  12 , wherein the above-described other of the axially opposite end portions is located in the inner side portion of the vehicle. The outer ring  26  is to be rotated about its rotation axis (outer ring axis) C 2 , and includes a connecting shaft  26   b  that extends in a direction of the rotation axis C 2  so as to be fitted in a splined fitting hole provided in a center of the above-described side gear of the final speed-reduction device such that the connecting shaft  26   b  is unrotatable relative to the side gear of the final speed-reduction device. 
       FIG.  2    is a partially cross sectional view showing, in enlargement, the outer ring  26  of the constant velocity joint  20 . As shown in  FIG.  2   , the connecting shaft  26   b , which extends from a main body of the outer ring  26  in the direction of the rotation axis C 2 , includes a splined shaft portion  26   d  provided with splined teeth, a fitting shaft portion  26   e  which is contiguous to the splined shaft portion  26   d  and which has a diameter larger than a diameter of the splined shaft portion  26   d , and a large-diameter shaft portion  26   f  which is contiguous to the fitting shaft portion  26   e  and which has a diameter larger than a diameter of the fitting shaft portion  26   e . The fitting shaft portion  26   e  is slidably fitted in a differential casing (not shown) of the above-described final speed-reduction device, so as to be supported by the differential casing. 
     The inner ring  24  is provided with three trunnions (protrusions)  24   a  which are equi-angularly arranged in a circumferential direction of the inner ring  24  and which protrude toward an outer peripheral side of the inner ring  24 , so as to support three roller units  30 . The three roller units  30  are received in respective three guide slots  26   c  that are provided in an inner circumferential surface of the outer ring  26 . Each of the three guide slots  26   c  is elongated in a direction parallel to the rotation axis C 2  so as to guide a corresponding one of the three roller units  30  in the direction parallel to the rotation axis C 2 . With the three roller units  30  being received in the respective three guide slots  26   c , the inner ring  24  is disposed in the outer ring  26  such that the inner ring  24  is unrotatable relative to the outer ring  26  about the rotation axis C 2  and is movable relative to the outer ring  26  in the direction of the rotation axis C 2 . 
     An opening between the end portion  12   b  of the intermediate shaft  12  and the outer ring  16  is covered by a bellows-like tapered boot  28  that is made of a synthetic rubber formed of a soft resin. The boot  28  is fitted at its large-diameter end portion on the outer circumferential surface of the outer ring  16 , and is fitted at its small-diameter end portion on the outer circumferential surface of the intermediate shaft  12 . The boot  28  has an inner portion that is filled with a lubricating grease. 
       FIG.  3    is a cross sectional view showing each of the three guide slots  26   c  which receives a corresponding one of the three roller units  30  therein so as to guide the corresponding one of the three roller units  30  in the direction parallel to the rotation axis C 2 . As shown in  FIG.  3   , each of the inner and outer rings  24 ,  26  is allowed to be rotated or pivoted relative to the other of the inner and outer rings  24 ,  26  in a plane containing a corresponding one of the rotation axes C 1 , C 2  of the respective inner and outer rings  24 ,  26 , within a certain range of a joint angle θ, about the above-described other end portion  12   b  of the intermediate shaft  12 , wherein the other end portion  12   b  of the intermediate shaft  12  is located in the inner side portion of the vehicle, and is splined to the inner ring  24  unrotatably relative to the inner ring  24 . 
     The three trunnions  24   a  are equi-angualrly arranged in the circumferential direction of the inner ring  24 , and protrude toward the outer peripheral side of the inner ring  24 . In other words, each of the trunnions  24   a  protrudes in a direction of its center line C 3  away from the rotation axis C 1  of the inner ring  24  toward the outer peripheral side of the inner ring  24 . Further, each of the trunnions  24   a  has a convex outer-circumferential surface  24   b  that is a part-spherical surface convexed in a radial direction perpendicular to the center line C 3 . Each of the roller units  30  is fitted on the convex outer-circumferential surface  24   b  of a corresponding one of the trunnions  24   a , such that the convex outer-circumferential surface  24   b  is inscribed in a cylindrical inner circumferential surface  32   a  of an inner roller  32  of the roller units  30 . Each of the roller units  30  includes the inner roller  32 , a plurality of columnar or cylindrical rolling elements (needles)  34 , an annular outer roller  36  and a snap ring  38 . The inner roller  32  has the cylindrical inner circumferential surface  32   a  that is slidably fitted on the convex outer-circumferential surface  24   b  of a corresponding one of the trunnions  24   a . The outer roller  36  is supported by the inner roller  32  through the plurality of cylindrical rolling elements  34  that are interposed between the inner and outer rollers  32 ,  36  in a radial direction of the roller units  30 . 
     The outer roller  36  includes a flange portion  36   a  serving as a limiting portion for limiting movements of the plurality of the cylindrical rolling elements  34  and the inner roller  32  relative to the outer roller  36  in an axial direction of the outer roller  36 . The outer roller  36  has an annular groove  36   b  provided in its inner circumferential surface, and the snap ring  38  is fitted in the annular groove  36   b . The flange portion  36   a  serving as the limiting portion is provided integrally in the outer roller  36  and protruding from an axial end portion of the outer roller  36  toward an inner peripheral side of the outer roller  36 , wherein the axial end portion of the outer roller  36  is located on a side of a distal end of a corresponding one of the trunnions  24   a . The annular groove  36   b  is provided in another axial end portion of the outer roller  36 , wherein the other axial end portion of the outer roller  36  is located on a side of a proximal end of the corresponding one of the trunnions  24   a . Each of the three guide slots  26   c  of the outer ring  26  is provided with a pair of recessed grooves  26   g  that are opposed to each other in a width direction of the guide slot  26   c  that is elongated in the direction parallel to the rotation axis C 2 . The outer roller  36  has a convex outer-circumferential surface  36   c  that is fitted between the pair of recessed grooves  26   g . It is noted that the snap ring  38 , which is fitted in the annular groove  36   b  of the outer roller  36 , can be interpreted to serve as a second limiting portion which is spaced apart from the flange portion  36   a  serving as a first limiting portion by a predetermined spacing distance in the axial direction of the outer roller  36 , so as to cooperate with the flange portion  36   a  as the first limiting portion to limit the movements of the cylindrical rolling elements  34  and the inner roller  32  that are located between the first and second limiting portions in the axial direction of the outer roller  36 . 
     The outer roller  36  of each of the three roller units  30  is linearly movable along the recessed grooves  26   g  of a corresponding one of the three guide slots  26   c . Further, each of the three trunnions  24   a  having the convex outer-circumferential surface  24   b  are pivoted or have a circular motion with increase of the joint angle θ. A contact or tangent point T, at which the cylindrical inner circumferential surface  32   a  of the trunnion  24   a  is in contact with the cylindrical inner circumferential surface  32   a  of the inner roller  32 , is reciprocatively moved during rotation of the constant velocity joint  20 , relative to a tangent line L which is tangent to an imaginary circle PC and which is perpendicular to the center line C 3 , wherein the imaginary circle PC has a center lying at the rotation axis (inner ring axis) C 1  and passes a center M of the above-described part-spherical surface defining the convex outer-circumferential surface  24   b . The inner roller  32  is moved in the direction of the center line C 3  (in the axial direction of the outer roller  36 ) owing to friction generated between its cylindrical inner circumferential surface  32   a  and the convex outer-circumferential surface  24   b  of the trunnion  24   a  which is moved and which is in contact with the cylindrical inner circumferential surface  32   a  of the inner roller  32 . 
       FIG.  4    is a view schematically showing a clearance CL between the inner roller  32  and the flange portion  36   a  of the outer roller  36  in the direction of the center line C 3  of the trunnion  24   a  (in the axial direction of the outer roller  36 ). The inner roller  32  and the rolling elements  34  are limited, by the flange portion  36   a  and the snap ring  38 , from being moved relative to the outer roller  36  in the direction of the center line C 3  (in the axial direction of the outer roller  36 ). That is, the inner roller  32  and the rolling elements  34 , which serve as the respective first and second limiting portions, cooperate with each other to inhibit the outer roller  36  from being moved by a predetermined distance or more. The clearance CL shown in  FIG.  4    is set to a value that allows the inner roller  32  to be moved relative to the outer roller  36  by a distance shorter than the predetermined distance. The clearance CL corresponds to a value obtained by subtracting an axial length of the inner roller  32  from the above-described predetermined spacing distance between the inner roller  32  and the rolling elements  34  in the direction of the center line C 3  (in the axial direction of the outer roller  36 ). 
       FIG.  5    is a view showing a distance Du between the tangent point T and the above-described tangent line L (which is tangent to the imaginary circle PC and which is perpendicular to the center line C 3 ) when the tangent point T is in an upper stroke end of the reciprocating movement. This distance Du corresponds to a distance by which the tangent point T (at which the trunnion  24   a  and the inner roller  32  are in contact with each other) is to be moved relative to the center M of the above-described part-spherical surface defining the convex outer-circumferential surface  24   b  so as to reach the upper stroke end of the reciprocating movement. As shown in  FIG.  5   , the tangent point T is located on an outer peripheral side of the imaginary circle PC, and the distance Du is expressed by expression (1) given below, where “Dp” represents a radius of the imaginary circle PC and “θ” represents the joint angle. It is noted that the joint angle θ is defined as an angle between the center line C 3  and a line connecting between the center of the imaginary circle PC and the center M of the convex outer-circumferential surface  24   b , namely, an angle between the rotation axis C 1  of the inner ring  24  and the rotation axis C 2  of the outer ring  26 .
 
 Du =[(1−cos θ)/4 cos θ]· Dp   (1)
 
       FIG.  6    is a view showing a distance Dd between the tangent point T and the above-described tangent line L (which is tangent to the imaginary circle PC and which is perpendicular to the center line C 3 ) when the tangent point T is in a lower stroke end of the reciprocating movement. This distance Dd corresponds to a distance by which the tangent point T (at which the trunnion  24   a  and the inner roller  32  are in contact with each other) is to be moved relative to the center M of the above-described part-spherical surface defining the convex outer-circumferential surface  24   b  so as to reach the lower stroke end of the reciprocating movement. As shown in  FIG.  6   , the tangent point T is located on an inner peripheral side of the imaginary circle PC, and the distance Dd is expressed by expression (2) given below, where “Dp” represents the radius of the imaginary circle PC and “θ” represents the joint angle.
 
 Dd =(¾)·(1−cos θ)· Dp   (2)
 
     A stroke distance D of the reciprocating movement of the tangent point T (at which the convex outer-circumferential surface  24   b  of the trunnion  24   a  and the cylindrical inner circumferential surface  32   a  of the inner roller  32  are in contact with each other) corresponds to a sum of the above-described distances Du, Dd (i.e., D=Du+Dd). In the constant velocity joint  20  according to the present embodiment, the clearance CL between the inner roller  32  and the flange portion  36   a  of the outer roller  36  is set to a value that satisfies the following expression: 
     D 6 &lt;CL&lt;Ls, where “D 6 ” represents the stroke distance D of the reciprocating movement of the tangent point T when the joint angle θ is 6 degrees that is a normal angle value in a practical use of the constant velocity joint  20 , and “Ls” represents an axial length of a crowned end portion (parabolic-curvature end portion) of each of the cylindrical rolling elements  34 . 
     A normal angle value θn is a value of the joint angle θ in the most frequent case in which an average weight load acts on the vehicle, for example, in a case in which one person rides on the vehicle or a predetermined weight load acts on the vehicle. That is, the clearance CL is set to the value that is larger than the stroke distance D 6  of the reciprocating movement of the tangent point T when the joint angle θ is 6 degrees, because, in a vehicle adaptation study that was made in view of a rolling fatigue life and a noise/vibration performance of the tripod constant-velocity joint, it was determined that the constant velocity joint  20  should be adapted for a vehicle in which the joint angle θ of the constant velocity joint  20  is 6 degrees or less in the case in which one person rides on the vehicle or a predetermined weight load acts on the vehicle. 
     It is noted that an upper threshold value of the clearance CL may be smaller than a smaller one of the axial length Ls (of the crowned end portion of each of the cylindrical rolling elements  34 ) and the stroke distance D 10  of the reciprocating movement of the tangent point T when the joint angle θ is 10 degrees. That is, the clearance CL may be set to a value that satisfies the following expression: 
     D 6 &lt;CL&lt;Min (D 10 , Ls), where “D 10 ” represents the stroke distance D of the reciprocating movement of the tangent point T when the joint angle θ is 10 degrees. 
     For example, where the axial length Ls (of the crowned end portion of each of the cylindrical rolling elements  34 ) is larger than the distance D 10 , the clearance CL may be a set to a value that satisfies the following expression:
 
 D 6&lt; CL&lt;D 10
 
       FIG.  7    is a view corresponding to the view of  FIG.  3    and showing a tripod constant-velocity joint of a comparative example. In the constant velocity joint (tripod constant-velocity joint)  20  according to the present embodiment, as shown in  FIG.  3   , the first limiting portion is constituted by the flange portion  36   a  that is provided integrally in the outer roller  36  and protruding from the outer roller  36  toward the inner peripheral side of the outer roller  36 . On the other hand, in the tripod constant-velocity joint of the comparative example shown in  FIG.  7   , the first limiting portion is constituted by a snap ring  136   a , so that an inner roller  132  and cylindrical rolling elements  134  are located between the snap ring  136   a  and a snap ring  138  and are inhibited from being moved relative to an outer roller  136  in the direction of the center line C 3  (in the axial direction of the outer roller  136 ) by a predetermined distance or more. A clearance is provided between the inner roller  132  and the snap ring  136   a  to allow the inner roller  132  to be moved relative to the outer roller  136  by a distance shorter than the predetermined distance. In the construction of the tripod constant-velocity joint shown in  FIG.  7   , a function of preventing removal of the inner roller  132  is established by the two snap rings, thereby causing increase of the number of required components. Consequently, the increase of the number of the required components is likely to cause the clearance to be made too small due to dimensional variation. 
       FIG.  8    is a view showing the clearance in comparative examples 1 and 2 that are constructed as shown in  FIG.  7    and also in the present embodiment constructed as shown in  FIG.  3   , in a comparative manner. In  FIG.  8   , “▾” indicates a lower limit value of the clearance, “▴” indicates an upper limit value of the clearance, and “◯” indicates the median of the clearance. As shown in  FIG.  8   , in the comparative examples 1 and 2, a range between the lower and upper limit values of the clearance is large and variation of the clearance is large. On the other hand, in the embodiment shown in a right end part of  FIG.  8   , a range between the lower and upper limit values of the clearance CL is small, and variation of the clearance CL is small although not being shown in the drawings. 
     As described above, the tripod constant velocity joint  20  according to the present embodiment includes: the inner ring  24  attached to the intermediate shaft (leg shaft)  12 ; the three trunnions  24   a  protruding from the inner ring  24  toward the outer peripheral side of the inner ring  24  and having the respective convex outer-circumferential surfaces  24   b ; the three roller units  30  rotatably supported by the respective three trunnions  24   a ; and the outer ring  26  storing the three roller units  30 , wherein the inner ring  24  is disposed in the outer ring  26  such that the inner ring  24  is unrotatable relative to the outer ring  26  and is movable relative to the outer ring  26  in a direction of an outer ring axis about which the outer ring  26  is to be rotated. Each of the three roller units  30  includes: the inner roller  32  slidably fitted at the inner circumferential surface  32   a  thereof on the convex outer-circumferential surface  24   b  of a corresponding one of the three trunnions  24   a , such that the inner circumferential surface  32   a  of the inner roller  32  and the convex outer-circumferential surface  24   b  of the corresponding one of the three trunnions  24   a  are in contact with each other at the tangent point T that is to be reciprocatively moved during rotation of the tripod constant-velocity joint  20 ; the plurality of cylindrical rolling elements  34  each including the crowned end portion in the axial end portion thereof; the outer roller  36  supported by the inner roller  32  through the plurality of cylindrical rolling elements  34  that are interposed between the inner roller  32  and the outer roller  36 ; and the flange portion  36   a  (limiting portion) provided integrally in the outer roller  36  and protruding from the outer roller  36  toward the inner peripheral side of the outer roller  36 , so as to limit movements of the plurality of cylindrical rolling elements  34  and the inner roller  32 . The inner roller  32  and the flange portion  36   a  cooperate to define therebetween the clearance CL in the direction of the center line C 3  of a corresponding one of the three trunnions  24   a , such that the clearance CL is smaller than the axial length of the crowned end portion of each of the plurality of cylindrical rolling elements  34 , and is larger than the stroke distance of reciprocating movement of the tangent point T (at which the inner roller  32  and the corresponding one of the three trunnions  24   a  are in contact with each other) when the joint angle θ of the tripod constant-velocity joint  20  is the predetermined value. Owing to this construction, the inner roller  32  is not brought into contact with the flange portion  36   a  and accordingly the inner roller  32  is restrained from being brought into contact at its edge portion with rolling surfaces (columnar-shaped outer circumferential surfaces) of the respective rolling elements  34 . It is therefore possible to retrain a compelling force applied from the inner roller  32  and to provide the tripod constant-velocity joint  20  with a high durability. 
     Further, in the tripod constant velocity joint  20  according to the present embodiment, the clearance CL between the inner roller  32  and the flange portion  36   a  of a corresponding one of the three trunnions  24   a  in the direction of the center line C 3  of the corresponding one of the three trunnions  24   a , is smaller than the stroke distance D 10  of reciprocating movement of the tangent point T when the joint angle θ is 10 degrees. Owing to this feature, the clearance CL can be set to a value within an appropriate range. 
     Further, in the tripod constant velocity joint  20  according to the present embodiment, the predetermined value of the joint angle θ is a normal angle value θn that is a value of the joint angle θ in the most frequent case in a practical use of the tripod constant-velocity joint  20 . For example, the normal angle value θn of the joint angle θ is 6 degrees. Owing to this feature, the clearance CL between the inner roller  32  and the flange portion  36   a  of a corresponding one of the three trunnions  24   a  in the direction of the center line C 3  of the corresponding one of the three trunnions  24   a , is set to a value larger than the stroke distance of reciprocating movement of the tangent point T when the joint angle θ is six degrees as the normal angle value θn, so that the inner roller  32  is restrained from being brought into contact at its axial end surface with the columnar-shaped outer circumferential surfaces of the respective rolling elements  34  at least when the joint angle θ is six degrees as the normal angle value θn, thereby making it possible to avoid a stress concentration on the columnar-shaped outer circumferential surface as the rolling surface of any of the rolling elements  34 , which could be caused by contact of the edge of the inner roller  32  with the rolling surface. 
     While the preferred embodiment of this invention has been described in detail by reference to the drawings, it is to be understood that the invention may be otherwise embodied. 
     For example, in the constant velocity joint  20  according to the above-described embodiment, the inner ring  24  is splined to the end portion of the intermediate shaft  12 . However, the inner ring  24  may be provided integrally with the intermediate shaft  12 . 
     It is to be understood that the embodiment described above is given for illustrative purpose only, and that the present invention may be embodied with various modifications and improvements which may occur to those skilled in the art. 
     NOMENCLATURE OF ELEMENTS 
       12 : intermediate shaft (leg shaft) 
       20 : constant velocity joint (tripod constant-velocity joint) 
       24 : inner ring 
       24   a : trunnion 
       24   b : convex outer-circumferential surface 
       26 : outer ring (outer ring) 
       30 : roller units 
       32 : inner roller 
       32   a : cylindrical inner circumferential surface 
       34 : cylindrical rolling elements (rolling elements) 
       36 : outer roller 
       36   a : flange portion 
     C 2 : rotation axis of outer ring 
     C 3 : center line of trunnion 
     CL: clearance 
     D: stroke distance of reciprocating movement 
     Ls: axial length of crowned end portion 
     θ: joint angle