Abstract:
The invention relates to a device for coupling an inner shaft and an outer shaft sliding in the direction of a common axis thereof, with balls provided between the two shafts. The balls are arranged in axial grooves of the inner shaft and in axial grooves of the outer shaft. Each row of balls is maintained by a double elastic member that bears in the groove of the inner shaft and urges the balls along two rolling tracks so that they come into contact with the groove of the outer shaft, wherein each of the two rolling tracks is hinged through a pivot surface and a bearing surface of the corresponding shaft, and the difference between the radii defines two bearing areas defining a hinge axis parallel to the common axis.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This is a divisional application of Ser. No. 12/997,533, filed Dec. 10, 2010, which is a national phase application under 35 U.S.C. §371 of PCT Application No. PCT/FR2008/000874, filed Jun. 20, 2008, the contents of each of which are expressly incorporated herein by reference. 
     
    
     FIELD OF ART 
       [0002]    The invention relates to a device for rotatably coupling two sliding shafts along the common axis thereof. The coupling device of the invention particularly applies to an automotive vehicle steering column, by adapting it to the intermediate column portion connected to the steering gear case or to the upper column portion connected to the steering wheel. 
       BACKGROUND 
       [0003]    In the more particular case of the intermediate axis, the latter has a Cardan joint at each end: one Cardan joint is connected to the motion input of the steering gear case, and the other Cardan joint is connected to the upper column portion. In order to have correct steering of current automotive vehicles, it is required that the length of the intermediate axis be able to vary and adapt to the oscillations of the front gear of the vehicle, due to the profile and the surface condition of the road carpet. This characteristic is also required in order to make the assembling to the rack pinion easier, and to absorb motions upon a frontal impact of the vehicle. 
         [0004]    The intermediate axis should therefore have first a variable length, that is it should have a sliding function of two shafts with respect to each other along the column axis thereof, which is the intermediate portion axis. Moreover, a transmission function for the rotation movement between the two shafts and for the torque necessary for handling the steering is required. 
         [0005]    There are numerous sliding shaft coupling devices, which enable to combine the passing of the torque between the two shafts through splines which are provided on each of the two shafts with conjugated profiles. However, this type of device has a clearance after an endurance cycle equivalent to the service life of a vehicle, which is the one required for current cars. In order to delay the onset of such clearance, adjusting the sliding of both shafts upon manufacture is relatively tight, which requires upon the line assembly a quite high axial stress, hence a longer assembly time and more laboriousness upon such mounting. 
         [0006]    Such adjustment should be accurate in order to enable operational axial movements to be correctly absorbed. Except upon the passing of the torque, the axial stress is a function of the torque being transmitted, the sliding stress related to the torque being transmitted and the friction coefficient increases, and then there is a sudden axial release, causing jerks which prevent a good sliding adjustment with reduced clearance to be maintained; and being harmful for a good driving feeling. 
         [0007]    There are also coupling devices which use plastic injection on splined portions which are part of a male metal shaft and a female metal tube. This solution raises issues for absorbing axial movements under high torques; the sliding stresses increase proportionally to the friction forces between the two portions. Besides, the plastic injection wear leaves clearances. As a last solution, rolling elements and strain springs can be introduced between the shaft and the tube. This satisfying solution for a smooth sliding raises issues of angular rigidity because it is directly proportional to the stiffness and preloading of the springs. 
       SUMMARY 
       [0008]    The object of the present invention is to provide a coupling device for sliding shafts which avoids the previous drawbacks, that is a device which requires an axial stress not related to the torque to be transmitted. The axial stress should therefore experience a very small increase when the torque to be transmitted increases while having a strong angular rigidity of the transmission. Further, there should not be any clearance after the endurance cycle and the coupling device of both shafts should be able to be easily mounted in the possible existing overcrowdings on automotive vehicles and this with a reduced axial stress upon assembling. 
         [0009]    More precisely, the object of the invention is: 
         [0010]    reducing contact pressures due to the transmitted torque, in order to improve the service life of the product and maintain the initial characteristics; 
         [0011]    making the assembly easier and absorbing the size and geometry tolerances of the different components; 
         [0012]    not biasing the elastic member too much: and 
         [0013]    minimising the sliding stress variation and improving the characteristics of a prior application on the name of the Applicant NACAM. 
         [0014]    The invention relates to a coupling device for two shafts: an inner shaft and an outer shaft which slide into each other in the direction of the common axis thereof. Said coupling device for both shafts comprises balls, which are provided between the inner shaft and the outer shaft. 
         [0015]    In the coupling device, each of said balls is arranged, in the one hand, in a concave portion of the inner shaft and, on the other hand, in a concave portion of the outer shaft. 
         [0016]    Each of said balls moves along two rolling tracks arranged on either side of a median plan going through the common axis and the centre of said balls. Each of the two rolling tracks cooperates with the concave portion of one of the two shafts, which are all parallel to the common axis. Each of the two rolling tracks is pushed by an elastic member bearing in said concave portion, each of said balls moving directly against the concave portion of the other shaft. 
         [0017]    Each of the two rolling tracks has a bearing portion with a bearing side of each of said balls, and a pivot portion with a rounded pivot side applied against a rounded support side of the corresponding concave portion of the shaft. According to an essential characteristic of the invention, the radius of the support side of the shaft is different from the radius of the pivot side of the rolling track, in order to have two contact areas which define the position of a joint axis of the pivot being parallel to the common axis. 
         [0018]    The mounting is carried out such that at rest, when there is no torque transmission, and in operation, when there is a torque transmission, each of said balls is always in contact on either side of the median plan, through a bearing area with the corresponding rolling track arranged in one of the two shafts, and through a bearing area with the concave portion of the other shaft. 
         [0019]    According to some embodiments of the invention, the radius of the concave support side of the shaft is smaller than the radius of the convex pivot side of the rolling track; and in other embodiments, the radius of the support side of the shaft is convex and larger than the radius of the concave pivot side of the rolling track. In all these embodiments and for each of the two rolling tracks, the radii of the support side of the shaft and the pivot side of the rolling track can be constant or progressive. Further, the bearing portion has a bearing side of the balls, which can be plane, convex, concave or with double concavity. 
         [0020]    In order to increase the operating safety of the coupling device of the invention, male grooves and female grooves, having conjugated profiles with some clearance, are arranged on the inner shaft and on the outer shaft, such that in case the balls are lost, the torque can still be transmitted between the inner shaft and the outer shaft. 
         [0021]    Several structures of the coupling device can be designed. In one structure, the elastic member(s) is or are provided in the inner shaft. In another structure, the elastic member(s) is or are provided in the outer shaft. 
         [0022]    In another structure, some of the elastic members are provided in the inner shaft and the others in the outer shaft. 
         [0023]    In a particular arrangement of the invention, the coupling device has the balls being located in several axial rows. 
         [0024]    For each row of balls, there is provided in the outer shaft a concave portion having the shape of an axial groove, the section of which comprises two concave sides being tilted with respect to each other, which come into contact with the balls. 
         [0025]    For each row of balls, there is provided in the inner shaft a concave portion having the shape of an axial groove, the section of which comprises a bottom and two flanks, the bottom being substantially perpendicular to the median plan going through the common axis and the axis of the centres of the balls in said row. 
         [0026]    For each row of balls, two rolling tracks are arranged on either side of the median plan and cooperate with the concave portion having the shape of an axial groove of the inner shaft. Each rolling track has the shape of an axial bar, the section of which has a bearing portion and a pivot portion. 
         [0027]    The bearing portion has a bearing side determined to come into contact with the balls. The pivot portion has a rounded shape with a convex pivot side, which cooperates with a concave rounded support side, joining the bottom and the corresponding flank of the axial groove, in order to have two contact areas determining the position of the joint axis of the pivot being parallel to the common axis. Each of the two rolling tracks is pushed by a spring which bears on the bottom of the axial groove. 
         [0028]    In another particular arrangement of the invention, the coupling device has the balls being arranged in several axial rows. 
         [0029]    For each row of balls, there is provided in the inner shaft a concave portion having the shape of an axial groove, the section of which comprises two concave sides being tilted with respect to each other, which come into contact with the balls. 
         [0030]    For each row of balls, there is provided in the outer shaft a concave portion having the shape of an axial groove, the section of which comprises a bottom and two flanks, the bottom being substantially perpendicular to the median plan going through the common axis and the axis of the centres of the balls in said row. 
         [0031]    For each row of balls, two rolling tracks are arranged on either side of the median plan and cooperate with the concave portion having the shape of an axial groove of the outer shaft. Each rolling track has the shape of an axial bar, the section of which has a bearing portion and a pivot portion. 
         [0032]    The bearing portion has a bearing side determined to come into contact with the balls. The pivot portion has a rounded shape with a convex pivot side, which cooperates with a concave rounded support side joining the bottom and the corresponding flank of the axial groove, in order to have two contact areas determining the position of the joint axis of the pivot being parallel to the common axis. Each of the two rolling tracks is pushed by a spring which bears on the bottom of the axial groove. 
         [0033]    In another particular arrangement of the invention, the coupling device has the balls being arranged in several axial rows. 
         [0034]    For each row of balls, there is provided in the outer shaft a concave portion having the shape of an axial groove, the section of which comprises two concave sides being tilted with respect to each other, which come into contact with the balls. 
         [0035]    For each row of balls, there is provided in the inner shaft a concave portion having the shape of an axial groove, the section of which comprises a bottom and two flanks, the bottom being substantially perpendicular to the median plan going through the common axis and the axis of the centres of the balls in said row. 
         [0036]    For each row of balls, two rolling tracks are arranged on either side of the median plan and cooperate with the concave portion having the shape of an axial groove of the inner shaft. Each rolling track has the shape of an axial bar, the section of which has a bearing portion and a pivot portion. 
         [0037]    The bearing portion has a bearing side determined to come into contact with the balls. The pivot portion has a rounded shape with a concave pivot side, which cooperates with a convex rounded support side joining the bottom and the corresponding flank of the axial groove, in order to have two contact areas determining the position of the joint axis of the pivot being parallel to the common axis. Each of the two rolling tracks is pushed by a spring which bears on the bottom of the axial groove. 
         [0038]    In another particular arrangement of the invention, the coupling device has the balls being arranged in several axial rows. 
         [0039]    For each row of balls, there is provided in the inner shaft a concave portion having the shape of an axial groove, the section of which comprises two concave sides being tilted with respect to each other, which come into contact with the balls. 
         [0040]    For each row of balls, there is provided in the outer shaft a concave portion having the shape of an axial groove, the section of which comprises a bottom and two flanks, the bottom being substantially perpendicular to the median plan going through the common axis and the axis of the centres of the balls in said row. 
         [0041]    For each row of balls, two rolling tracks are arranged on either side of the median plan and cooperate with the concave portion having the shape of an axial groove of the outer shaft. Each rolling track has the shape of an axial bar, the section of which has a bearing portion and a pivot portion. 
         [0042]    The bearing portion has a bearing side determined to come into contact with the balls. The pivot portion has a rounded shape with a concave pivot side which cooperates with a convex rounded support side joining the bottom and the corresponding flank of the axial groove, in order to have two contact areas determining the position of the joint axis of the pivot being parallel to the common axis. 
         [0043]    Each of the two rolling tracks is pushed by a spring which bears on the bottom of the axial groove. 
         [0044]    According to different embodiments of the invention, each of the two rolling tracks is pushed by an elastic member which is a spring bearing on the bottom of the axial groove. 
         [0045]    The spring is made of a body provided with elastic members, and comprising an axial edge on either side and a staple at each axial end; said body being mounted in the bottom of the corresponding concave portion of the shaft and having a shape conjugated with said bottom, that is bulged or convex, having at each transverse end a concave shape receiving the corresponding axial edge of said spring, of which both staples hold on the corresponding shaft. The elastic members are transverse tabs arranged directly by cutting the body, said tabs being held alternatively on one axial edge, then on the other edge. 
         [0046]    In a particularly well-balanced assembly of the coupling device, the balls are arranged in three axial rows. The axial rows are transversely located at 120° from one another. The inner shaft is provided with three axial grooves, the axial grooves being transversely located at 120° from one another. The outer shaft is provided with three axial grooves, the axial grooves being transversely located at 120° from one another. 
         [0047]    In another particularly well-balanced assembly of the coupling device, the balls are arranged in two diametrically opposed axial rows. The inner shaft is provided with two diametrically opposed axial grooves. The outer shaft is provided with two diametrically opposed axial grooves. 
         [0048]    In a complete architecture of the coupling device according to the invention, the inner shaft is provided with three axial grooves, the axial grooves being transversely located at 120° from one another. In each axial groove, an elastic member and two rolling tracks having the shape of an axial bar are mounted. 
         [0049]    The balls are arranged in three axial rows, the axial rows being transversely located at 120° from one another. 
         [0050]    The whole ball sleeve with the rolling tracks and the elastic axial flanges is closed at each axial end by a shoulder and a retaining ring. The retaining ring engages into each of the axial grooves of the inner shaft. 
         [0051]    The outer shaft is provided with three axial grooves transversely located at 120° from one another, which slide on the rows of balls. The axial grooves have the desired length for enabling the required axial sliding of the outer shaft and the inner shaft. 
         [0052]    The coupling device according to the invention is very well applied either to the intermediate portion of an automotive vehicle steering column, or to the upper portion of an automotive vehicle steering column. 
         [0053]    The coupling device of two shafts along the common axis thereof according to the invention has therefore the advantage of always having two bearing areas for each of the two inner and outer shafts and for each row of balls, which are always in contact, even if there is no torque transmission. 
         [0054]    Further, there is a reduced axial stress which is necessary upon the line assembly. The accurate adjustment of the coupling device of the invention enables the axial jerks upon driving the vehicle to be avoided, and ensures a good endurance by avoiding the onset of a coupling clearance. Moreover, upon passing of the torque, the axial stress is largely reduced due to the rolling and sliding contact. Finally, the coupling device can be mounted easily in the existing overcrowding of automotive vehicles steering columns. 
         [0055]    Other characteristics and advantages of the present invention will appear clearly with the following description of several preferred embodiments of the invention, provided as non-limiting examples, referring to the corresponding appended drawings wherein: 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0056]      FIG. 1  is a schematic perspective view of an automotive vehicle steering system, wherein the coupling device of the invention is applied to the intermediate column portion; 
           [0057]      FIG. 2  is an axial view of the intermediate portion of the automotive vehicle steering system of  FIG. 1 , wherein an embodiment of the invention is depicted in a partial axial section view; 
           [0058]      FIG. 3  is an exploded perspective view of the whole coupling device of  FIG. 2 ; 
           [0059]      FIG. 4  is a cross-section along IV-IV of  FIG. 2 ; 
           [0060]      FIG. 5  is a cross-section along the plan of  FIG. 4  of another embodiment of the invention; 
           [0061]      FIG. 6  is a cross-section along the plan of  FIG. 4  of another embodiment of the invention; 
           [0062]      FIG. 7  is an enlarged partial view of  FIG. 4  with a rolling track; 
           [0063]      FIG. 8  is a view similar to  FIG. 7  of another embodiment of the rolling track; 
           [0064]      FIG. 9  is an enlarged partial view of the mounting of a row of balls of  FIG. 4 ; 
           [0065]      FIG. 10  is a view similar to  FIG. 9  with another rolling track according to the invention; 
           [0066]      FIG. 11  is a view similar to  FIG. 9  with another rolling track according to the invention; 
           [0067]      FIG. 12  is a view similar to  FIG. 9  with another rolling track according to the invention; 
           [0068]      FIG. 13  is a view similar to  FIGS. 9 to 12  with an elastic member in a free position; 
           [0069]      FIG. 14  is a view similar to  FIG. 13  with an elastic member in compressed position; 
           [0070]      FIG. 15  is a perspective view of the inner shaft and the spring corresponding to  FIG. 14 ; 
           [0071]      FIG. 16  is an enlarged partial view of the mounting of a row of balls along the plan of  FIG. 4 , of another embodiment of the invention; and 
           [0072]      FIG. 17  is a view similar to  FIG. 16  of another embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0073]    The invention relates to a rotatably coupling device of two shafts which slide into each other along the common axis thereof. 
         [0074]    This coupling device can be particularly well applied to an automotive vehicle steering system, as the one schematically depicted in  FIG. 1 . 
         [0075]    The depicted steering system comprises a steering column with an upper column portion  6  also known as column top, and an intermediate column portion  7  also known as intermediate axis. 
         [0076]    The upper column portion  6  is connected through the upper end thereof to the steering wheel  5 , and through the lower end thereof to the intermediate column portion  7 . 
         [0077]    The intermediate column portion  7  is connected through the upper end thereof to the upper column portion  6 , and through the lower end thereof to the steering gear case  8  of the steering rod  9 . 
         [0078]    The intermediate column portion  7  is connected et each of the ends thereof through a Cardan joint referenced as  10  for the upper column portion  6 , and also referenced as  10  for the steering gear case  8 . 
         [0079]    In the following part, the description relates to a coupling device which is arranged in the intermediate column portion  7 . The coupling device of the invention can also be arranged in the upper column portion  6 . 
         [0080]    An inner shaft  1  and an outer shaft  2  slide into each other along the direction of the common axis  4  thereof, which is depicted in  FIGS. 2 and 3 . The coupling device of both shafts comprises balls  3  in different embodiments described below. The balls  3  are arranged between the inner shaft  1  and the outer shaft  2 . In the coupling device of the invention, each of said balls  3  is arranged, on the one hand, in a concave portion of the inner shaft  1 , and, on the other hand, in a concave portion of the outer shaft  2 . 
         [0081]    Each of said balls  3  moves along two rolling tracks, which cooperate with the concave portion of one of the two shafts and which pivot around a joint axis. Each rolling track and the corresponding joint axis are parallel to the common axis  4 . Each of the two rolling tracks is pushed by an elastic member, which is arranged and bears in said concave portion. Each of said balls  3  moves directly against the concave portion of the other shaft. 
         [0082]    The coupling device according to the invention is arranged such that at rest, when there is no torque transmission, and in operation, when there is a torque transmission, the balls  3  are always in contact. On either side of a median plan going through the centre of the balls  3  and the common axis  4 , each of the balls  3  is always in contact through a bearing area with the corresponding rolling track arranged in one of the two shafts, and through a bearing area with the concave portion of the other shaft. 
         [0083]    The whole intermediate column portion  7  in an axial view is depicted in  FIG. 2 . The inner shaft  1  and the outer shaft  2  slide with the balls  3  along the common axis  4  thereof. The intermediate column portion  7  is connected to the steering gear case  8  through the Cardan joint  10 , and it is connected to the upper column portion  6  through the Cardan joint  10 . 
         [0084]    The coupling device, represented in an exploded view in  FIG. 3  and in cross-section in  FIG. 4 , comprises balls  3  which are arranged in three axial rows  20 . The axial rows  20  are transversely located at 120° from one another. The inner shaft  1  is provided with three axial grooves  131 , which are transversely located at 120° from one another. The outer shaft  2  is provided with three axial grooves  132 , which are transversely located at 120° from one another. 
         [0085]    For each row  20  of balls  3 , there is provided in the outer shaft  2  a concave portion having the shape of an axial groove  132 , the section of which comprises two concave sides  133 .  134  being tilted with respect to each other, which come into contact with the balls  3 . 
         [0086]    For each row  20  of balls  3 , there is provided in the inner shaft  1  a concave portion having the shape of an axial groove  131 , the section of which comprises a bottom  135  and two flanks  136 ,  137 . The bottom  135  is substantially perpendicular to the median plan  30  going through the common axis  4  and the axis of the centres of the balls  3  in said row  20 . 
         [0087]    For each row  20  of balls  3 , there are two rolling tracks  140  which are arranged on either side of the median plan  30 . The rolling tracks  140  cooperate with the concave portion having the shape of an axial groove  131  of the inner shaft  1 . 
         [0088]    Each rolling track  140  has the shape of an axial bar, the section of which has a bearing portion  141  and a pivot portion  142 . 
         [0089]    The bearing portion  141  has a bearing side  143 , which is determined to come into contact with the balls  3 . The pivot portion  142  having a rounded shape has a convex pivot side  144 , which cooperates with a concave rounded support side  138  or  139  of the axial groove  131 . The concave rounded support side  138  joins the bottom  135  and the flank  136  for one of the rolling tracks  140 , and the concave rounded side  139  joins the bottom  135  and the flank  137  for the other rolling track  140 . 
         [0090]    According to an essential characteristic of the invention shown in detail in  FIGS. 7 and 8 , the radius of the concave rounded support side  138  and the radius of the concave rounded support side  139  are smaller than the radius of the convex pivot side  144  of the corresponding rolling track  140 . This characteristic enables to have two contact areas between the shaft  1  and the corresponding rolling track  140 , the action of which determines the position of an axis which is the joint axis  145  of the pivot being parallel to the common axis  4 . 
         [0091]    Each of the two rolling tracks  140  is pushed by an elastic member  210  which bears on the bottom  135  of the axial groove  131 . 
         [0092]    The coupling device shown in a cross-section according to  FIG. 5  comprises balls  3  which are arranged in three axial rows  20 . 
         [0093]    The axial rows  20  are transversely located at 120° from one another. The inner shaft  1  is provided with three axial grooves  151 , which are transversely located at 120° from one another. The outer shaft  2  is provided with three axial grooves  152 , which are transversely located at 120° from one another. 
         [0094]    For each row  20  of balls  3 , there is provided in the inner shaft  1  a concave portion having the shape of an axial groove  151 , the section of which comprises two concave sides  153 .  154  being tilted with respect to each other, which come into contact with the balls  3 . 
         [0095]    For each row  20  of balls  3 , there is provided in the outer shaft  2  a concave portion having the shape of an axial groove  152 , the section of which comprises a bottom  155  and two flanks  156 ,  157 . The bottom  155  is substantially perpendicular to the median plan  30  going through the common axis  4  and the axis of the centres of the balls  3  in said row  20 . 
         [0096]    For each row  20  of balls  3 , two rolling tracks  160  are arranged on either side of the median plan  30 . The rolling tracks  160  cooperate with the concave portion having the shape of an axial groove  152  of the outer shaft  2 . Each rolling track  160  has the shape of an axial bar, the section of which has a bearing portion  161  and a pivot portion  162 . 
         [0097]    The bearing portion  161  has a bearing side  163 , which is determined to come into contact with the balls  3 . The pivot portion  162  having a rounded shape has a convex pivot side  164 , which cooperates with a concave rounded support side  158  or  159  of the axial groove  151 . The concave rounded support side  158  joins the bottom  155  and the flank  156  for one of the rolling tracks  160 , and the concave rounded side  159  joins the bottom  155  and the flank  157  for the other rolling track  160 . 
         [0098]    According to an essential characteristic of the invention shown in detail in  FIGS. 7 and 8 , the radius of the concave rounded support side  158  and the radius of the concave rounded support side  159  is smaller than the radius of the convex pivot side  164  of the corresponding rolling track  160 . This characteristic enables to have two contact areas between the shaft  2  and the corresponding rolling track  160 , the action of which determines the position of an axis which is the joint axis  165  of the pivot being parallel to the common axis  4 . 
         [0099]    Each of the two rolling tracks  160  is pushed by an elastic member  210  which bears on the bottom  155  of the axial groove  152 . 
         [0100]    The coupling device shown in cross-section in  FIG. 16  comprises balls  3  which are arranged in three axial rows  20 . 
         [0101]    The axial rows  20  are transversely located at 120° from one another. The inner shaft  1  is provided with three axial grooves  171 , which are transversely located at 120° from one another. The outer shaft  2  is provided with three axial grooves  172 , which are transversely located at 120° from one another. 
         [0102]    For each row  20  of balls  3 , there is provided in the outer shaft  2  a concave portion having the shape of an axial groove  172 , the section of which comprises two concave sides  173 ,  174  being tilted with respect to each other, which come into contact with the balls  3 . 
         [0103]    For each row  20  of balls  3 , there is provided in the inner shaft  1  a concave portion having the shape of an axial groove  171 , the section of which comprises a bottom  175  and two flanks  176 ,  177 . The bottom  175  is substantially perpendicular to the median plan  30  going through the common axis  4  and the axis of the centres of the balls  3  in said row  20 . 
         [0104]    For each row  20  of balls  3 , two rolling tracks  180  are arranged on either side of the median plan  30 . The rolling tracks  180  cooperate with the concave portion having the shape of an axial groove  171  of the inner shaft  1 . Each rolling track  180  has the shape of an axial bar, the section of which has a bearing portion  181  and a pivot portion  182 . 
         [0105]    The bearing portion  181  has a bearing side  183 , which is determined to come into contact with the balls  3 . The pivot portion  182  having a rounded shape has a concave pivot side  184 , which cooperates with a convex rounded support side  178  or  179  of the axial groove  171 . The convex rounded support side  178  joins the bottom  175  and the flank  176  for one of the rolling tracks  180 , and the convex rounded side  179  joins the bottom  175  and the flank  177  for the other rolling track  180 . 
         [0106]    According to an essential characteristic of the invention, the radius of the convex rounded support side  178  and the radius of the convex rounded support side  179  is larger than the radius of the concave pivot side  184  of the corresponding rolling track  180 . This characteristic enables to have two contact areas between the shaft  1  and the corresponding rolling track  180 , the action of which determines the position of an axis which is the joint axis  185  of the pivot being parallel to the common axis  4 . 
         [0107]    Each of the two rolling tracks  180  is pushed by an elastic member  210  which bears on the bottom  175  of the axial groove  171 . 
         [0108]    The coupling device shown in cross-section in  FIG. 17  comprises balls  3  which are arranged in several axial rows  20 . 
         [0109]    The axial rows  20  are transversely located at 120° from one another. The inner shaft  1  is provided with three axial grooves  191 , which are transversely located at 120° from one another. The outer shaft  2  is provided with three axial grooves  192 , which are transversely located at 120° from one another. 
         [0110]    For each row  20  of balls  3 , there is provided in the inner shaft  1  a concave portion having the shape of an axial groove  191 , the section of which comprises two concave sides  193 .  194  being tilted with respect to each other, which come into contact with the balls  3 . 
         [0111]    For each row  20  of balls  3 , there is provided in the outer shaft  2  a concave portion having the shape of an axial groove  192 , the section of which comprises a bottom  195  and two flanks  196 ,  197 . The bottom  195  is substantially perpendicular to the median plan  30  going through the common axis  4  and the axis of the centres of the balls  3  in said row  20 . 
         [0112]    For each row  20  of balls  3 , two rolling tracks  200  are arranged on either side of the median plan  30 . The rolling tracks  200  cooperate with the concave portion having the shape of an axial groove  192  of the outer shaft  2 . Each rolling track  200  has the shape of an axial bar, the section of which has a bearing portion  201  and a pivot portion  202 . 
         [0113]    The bearing portion  201  has a bearing side  203 , which is determined to come into contact with the balls  3 . The pivot portion  202  having a rounded shape has a concave pivot side  204 , which cooperates with a convex rounded support side  198  or  199  of the axial groove  191 . The convex rounded support side  198  joins the bottom  195  and the flank  196  for one of the rolling tracks  200 , and the convex rounded side  199  joins the bottom  195  and the flank  197  for the other rolling track  200 . 
         [0114]    According to an essential characteristic of the invention, the radius of the convex rounded support side  198  and the radius of the convex rounded support side  199  is larger than the radius of the concave pivot side  204  of the corresponding rolling track  200 . This characteristic enables to have two contact areas between the shaft  2  and the corresponding rolling track  200 , the action of which determines the position of an axis which is the joint axis  205  of the pivot being parallel to the common axis  4 . 
         [0115]    Each of the two rolling tracks  200  is pushed by an elastic member  210  which bears on the bottom  195  of the axial groove  192 . 
         [0116]    Whatever the embodiment depicted in  FIGS. 2 to 17 , each of the two rolling tracks  140 ,  160 ,  180 ,  200  is pushed by an elastic member which is a metal spring  210 . The spring  210  is made of an axial body  211  which is provided with elastic members  216 . The body  211  comprises on either transverse side an axial edge  212  and  213 , as shown in details in  FIGS. 13 ,  14  and  15 . The body  211  further comprises at each axial end a staple  214  and  215 . The elastic members are transverse tabs  216 , which are arranged directly by cutting the body  211 . The transverse tabs  216  are held alternately on an axial edge  212 , and then on the other axial edge  213 . The body  211  is mounted in the bottom  135 ,  155 ,  175 ,  195  of the corresponding concave portion of the relevant shaft, having a shape being conjugated with that of said bottom, that is bulged, in other words convex. Said bottom comprises, at each transverse end, a concave shape receiving the corresponding axial edge  212 ,  213  of said spring  210 , of which both staples  214 ,  215  hold on the corresponding shaft. 
         [0117]    In the embodiments shown in the FIGS., the radius of the support side of the shaft  1  or  2  and the radius of the pivot side of the relevant rolling track have constant values. In other embodiments shown in  FIG. 8 , the radius of the pivot side is progressive. In other not shown embodiments of the invention, it is the radius of the support side of the shaft  1  or  2  which is progressive. Finally, in other not shown embodiments of the invention, it is the radius of the pivot side and the radius of the support side which are progressive. As particularly shown in  FIGS. 6 ,  7 ,  8  and  9 , each of the two rolling tracks  140 ,  160 ,  180 ,  200  has the bearing portion  141 .  161 ,  181 ,  201  provided with a bearing side  143 ,  163 ,  183 ,  203  of the balls  3  which is flat. As shown in  FIG. 10 , the bearing side of the balls is convex, while according to  FIG. 11 , the bearing side is concave, and according to  FIG. 12 , the bearing side of the balls has a double concavity. 
         [0118]    As shown in  FIGS. 3 to 5 , the balls  3  can be arranged in three axial rows  20 , the axial rows  20  being transversely located at 120° from one another. The inner shaft  1  is provided with three axial grooves  131 ,  151 ,  171 ,  191 , the axial grooves  131 ,  151 ,  171 ,  191  being transversely located at 120° from one another. The outer shaft  2  is provided with three axial grooves  132 ,  152 ,  172 ,  192 , the axial grooves  132 ,  152 ,  172 ,  192  being transversely located at 120° from one another. 
         [0119]    As is the case for  FIG. 6 , the balls  3  can be arranged in two diametrically opposed axial rows  20 . The inner shaft  1  is provided with two diametrically opposed axial grooves  131 ,  151 ,  171 ,  191 ; and the outer shaft  2  is provided with two diametrically opposed axial grooves  132 ,  152 ,  172 ,  192 . 
         [0120]    As particularly shown in  FIG. 9 , male grooves  23  and female grooves  24  having conjugated profiles with some clearance, are provided on the sliding side  21  of the inner shaft  1  and on the sliding side  22  of the outer shaft  2 ; the male grooves having an apex  25  and two flanks  27 , the female grooves  24  having a bottom  26  and two flanks  28 . Thus, in case the balls  3  are lost, the torque can still be transmitted between the inner shaft  1  and the outer shaft  2 . 
         [0121]    As it is particularly the case of  FIGS. 4 ,  6 , and  16 , the elastic members or springs  210  can be arranged in the inner shaft  1  and push both rolling tracks  140 ,  180  which cooperate with the inner shaft. 
         [0122]    As particularly for  FIGS. 5 and 17 , the elastic members or springs  210  can be arranged in the outer shaft  2  and push both rolling tracks  160 ,  200  which cooperate with the outer shaft. 
         [0123]    While remaining under the scope of the invention, some of the elastic members or springs  210  can also be arranged in the inner shaft  1 , and the others in the outer shaft  2 , said elastic members pushing both corresponding rolling tracks, which cooperate with the relevant shaft. 
         [0124]    In the embodiment of  FIGS. 3 ,  4  and  16 , the inner shaft  1  is provided with three axial grooves  131 ,  171 . The axial grooves  131 ,  171  are transversely located at 120° from one another. In each axial groove  131 ,  171 , an axial elastic member or spring  210  and two rolling tracks  140 ,  180  having the shape of an axial bar are mounted. 
         [0125]    The balls  3  are arranged in three axial rows  20 , which are transversely located at 120° from one another. 
         [0126]    The whole assembly of balls  3  with the rolling tracks  140 ,  180  and the elastic members or springs  210  is closed at each axial end by a shoulder  15  and by a retaining ring  12 . The retaining ring  12  engages into each of the axial grooves  131 ,  171  of the inner shaft  1 . 
         [0127]    The outer shaft  2  is provided with three axial grooves  132 ,  172  transversely located at 120° from one another, which slide on the rows  20  of balls  3 . The axial grooves  132 ,  172  should have the desired length in order to enable the required axial sliding of the outer shaft  2  and the inner shaft  1 . 
         [0128]    The essential characteristics of the invention are reminded thereafter. 
         [0129]    The balls are in contact, on the one hand, with one of the shafts and, on the other hand, with both rolling tracks. The rolling tracks are in contact with the other shaft and can pivot in the shaft. The rolling tracks are in contact with elastic members which are intended to maintain the contact between the rolling tracks and the balls at one point. 
         [0130]    Upon assembling the system, the elastic members enable the size variations of the different components to be compensated. It can be called tolerance absorption through elastic member deformation. 
         [0131]    Upon applying a torque, the load is transmitted from the tube on the shaft through the balls and the rolling tracks. The load application line goes through the centre of the ball and the pivot of the rolling track. Thus, the load is directly taken by the shaft without biasing the elastic member. The angular rigidity is very important and independent of the stiffness of the elastic members. 
         [0132]    Upon an axial movement of the tube with respect to the shaft, the sliding stress of the system is a function of the stresses on the balls inserted by the elastic members. 
         [0133]    Upon applying a torque, the sliding stress of the system is then a function of stresses on the balls inserted by the elastic members and the torque applied to the system.