Abstract:
The invention concerns a constant velocity tripod joint for motor vehicle transmission systems comprising a male element ( 2 ) including several branches ( 4 ), a female element ( 8 ) defining for each branch a pair of tracks ( 9 A,  9 B), symmetrical with respect to a plane (P), and, mounted on each branch, a mechanical transmission member ( 11 ) comprising an outer roller ( 12 ) designed to run on one of the corresponding tracks. Each track comprises a central zone ( 14 A,  14 B) having a rectilinear transverse profile and inclined at an angle a relative to the plan (P) and a bearing ( 15 A,  15 B) for maintaining the roller radially. Each roller has a peripheral surface whereof the central zone ( 31 ) is conical and matching the central zones of the tracks, and a front surface ( 32 ) adapted to be urged into contact more or less perpendicularly with said bearing surfaces of the tracks under the effect of a driving torque. The invention is applicable to constant velocity tripod joints for transmission systems in motor vehicles.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a constant-velocity universal joint. 
     The invention applies in particular to tripot constant-velocity joints for motor vehicle transmissions. 
     A tripot constant-velocity joint of this kind generally comprises a male element with ternary symmetry, or tripod, secured to a first rotating shaft, and a female element with ternary symmetry, or bell housing, secured to a second rotating shaft. 
     In an embodiment known from document FR-A-2 701 741, each mechanical transmission member comprises an internal ring, arranged inside the external roller, and which swivels and slides about a spherical bearing surface of the corresponding arm. 
     Each mechanical transmission member also comprises means of coupling the internal ring and the external roller to allow their relative pivoting about a common axis of revolution. The coupling means allow only a limited relative translational movement between the internal ring and the external roller along the common axis of revolution. 
     In general, the external roller comprises a peripheral surface of torus-shaped cross section. The, transverse profile of each track is in the form of a broken arc which is symmetric with respect to a mid-plane orthogonal to the corresponding longitudinal and radial plane. 
     Such a structure, through the rolling of the external rollers along one of the tracks of the corresponding pair, allows the constant-velocity joint to operate at an angle of misalignment between the two rotating shafts when a driving torque is applied. 
     During such operation under torque, each peripheral surface of an external roller bears against a track of the corresponding pair of tracks, and there is a small clearance between this peripheral surface and the other track of said pair. In addition, each arm is given a back and forth translational movement with respect to the corresponding pair of tracks parallel to the corresponding longitudinal and radial plane. This back and forth movement in the longitudinal and radial plane is due, on the one hand, to the inclination of the arm and, on the other hand, to the orbital movement, known as the offset, of the tripod at a frequency which is three times the rotational speed, as is well known in the art. 
     Such a back and forth movement of the arms gives rise, in the case of each external roller, to a back and forth rocking movement of the roller about the part of its peripheral surface bearing against one of the tracks. The rocking movement is brought about on the one hand by the friction between the spherical bearing surface of the arm and the corresponding internal ring and, on the other hand, by the moving of the point of contact between the bearing surface of the arm and the corresponding internal ring. 
     Thus, for each arm, the part of the peripheral surface of the roller which is diametrically opposite the part which is bearing oscillates between the two half-arcs forming the broken-arc profile of the track on which the peripheral surface is not bearing. 
     Such an oscillatory movement gives rise to friction phenomena and may even cause jamming between the roller and the track on which it is not bearing, particularly in the half-arc of the track profile located farthest toward the inside of the bell housing. 
     The purpose of the invention is to overcome these problems by providing a mechanical joint which can operate with an angle of misalignment with less friction and at the same time limiting the risk of jamming. 
     SUMMARY OF THE INVENTION 
     Another subject of the invention is a mechanical transmission member for a universal joint as defined hereinabove. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood from reading the description which will follow, given merely by way of example and with reference to the appended drawings, in which: 
     FIG. 1 is a part view in cross section of a tripot-type constant-velocity universal joint according to a first embodiment of the invention, and 
     FIG. 2 is a view similar to FIG. 1, illustrating another embodiment of a tripot-type constant-velocity universal joint according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 partially illustrates a tripot-type constant-velocity joint  1  intended for a motor vehicle transmission, and essentially comprising the following components: 
     (1) a male element or tripod  2 , with ternary symmetry with respect to a central axis X-X (orthogonal to the plane of FIG.  1 ), and which comprises a hub  3  and three radial arms  4  spaced angularly at 120° and just one of which is depicted. The end part of each arm  4  forms a spherical bearing surface  5  formed integrally with it and centered on the axis Y—Y of the corresponding arm  4 . This male element  2  is secured to a first rotating shaft  6 . 
     (2) a female element or bell housing  8  with ternary symmetry with respect to a central axis X′-X′, the latter axis being coincident with the axis X-X when the joint is in the aligned position depicted. On each side of each arm  4 , this bell housing has two tracks  9 A and  9 B facing each other. This female element  8  is secured to a second rotating shaft (not depicted). 
     (3) for each arm  4 , a mechanical transmission member  11  which comprises an external roller  12  of axis of revolution Z-Z coincident with the axis Y—Y of the corresponding arm  4  in the position depicted in FIG.  1 . The external roller  12  is intended to roll along one or other of the corresponding tracks  9 A and  9 B. 
     As the three mechanical transmission members  11  are identical, and because of the ternary symmetry of the male element  2  and of the female element  8 , only the part of the joint  1  which is depicted in FIG. 1 will be described. 
     The directrix of the tracks  9 A and  9 B is, for example, a substantially straight line parallel to the axis X′-X′. These tracks are symmetric with respect to one another about a longitudinal and radial plane P (orthogonal to the plane of FIG. 1) of the female element  8 . 
     Each track  9 A,  9 B has a central region  14 A,  14 B along which the roller  12  rolls and a bearing surface  15 A,  15 B for radially retaining the roller  12 . 
     The central regions  14 A and  14 B each stretch on each side of the mid-plane Q (orthogonal to the plane P) of the tracks  9 A and  9 B. 
     The transverse profiles, that is to say the profiles as seen in a plane transversal to the axis X′-X′, such as the plane of FIG. 1, of the central regions  14 A,  14 B of the tracks  9 A,  9 B are substantially straight and inclined by an angle α with respect to the longitudinal and radial plane P. The transverse profiles of the central regions  14 A and  14 B therefore converge radially towards the inside of the bell housing  8  at an angle substantially equal to 2α. 
     The transverse profiles of the bearing surfaces  15 A and  15 B are substantially straight and orthogonal to the longitudinal and radial plane P. 
     The bearing surfaces  15 A and  15 B are situated radially on the outside of the bell housing  8 , that is to say radially on the diverging side of the transverse profiles of the central region  14 A and  14 B. 
     For each track  9 A,  9 B, the transverse profiles of the central regions  14 A,  14 B and of the bearing surfaces  15 A,  15 B therefore make an acute angle. 
     The bearing surfaces  15 A,  15 B and the central regions  14 A,  14 B of each track  9 A,  9 B are connected by parts which have curved transverse profiles. 
     The mechanical transmission member  11  comprises, on the one hand, an internal ring  18 , of cylindrical overall shape with axis of revolution Z-Z, and which is arranged inside the external roller  12  and, on the other hand, means  19  of coupling the internal ring  18  and the external roller  12 . These coupling means  19  comprise a needle ring  21  arranged between a cylindrical surface  22  of the ring  18  which is radially external with respect to the axis Z-Z, and a cylindrical surface  23  of the external roller  12  which is radially internal with respect to the axis Z-Z. These coupling means  19  further comprise two flat thrust washers  24  and  25  arranged on each side of the ring  18  and of the needle ring  21 . 
     The periphery of each thrust washer  24 ,  25  is housed in an annular groove formed in the surface  23 . The washers  24  and  25  hold the needle ring  21  and the internal ring  18  therebetween, with a small clearance C along the axis Z-Z. 
     The coupling means  19  therefore allow relative pivoting between the roller  12  and the ring  18  about the axis Z-Z and allow their limited relative translational movement along the axis Z-Z. 
     The internal ring  18  comprises a surface  27 , radially internal with respect to the axis Z-Z, which is substantially cylindrical, which delimits an opening to accommodate the arm  4 . The spherical bearing surface  5  of the arm  4  and the surface  27  of the ring  18  allow a swivelling and sliding movement about the axis Y—Y between the ring  18  and the arm  4 . 
     The external roller  12  has a peripheral surface  30 , radially external with respect to the axis Z-Z, which has a central region  31 . This roller  12  also has a frontal surface  32  and a rear surface  33 . 
     The central region  31 , extends, in the direction of the axis Z-Z, on each side of the mid-plane Q′ of the roller  12 . This plane Q′, orthogonal to the axis Z-Z, is substantially coincident with the plane Q in the position depicted in FIG.  1 . 
     The tracks  9 A and  9 B keep the mid-plane Q′ of the roller  12  substantially orthogonal to the plane P. 
     The central region  31  is substantially frustoconical and converges, at an angle substantially equal to 2α, toward the hub  3  of the tripod  2 , that is to say in the same radial direction as the transverse profiles of the central regions  14 A and  14 B of the tracks  9 A and  9 B. The central region  31  of the roller is therefore, in meridian cross section, substantially the conjugate of the central regions  14 A and  14 B of the tracks  9 A and  9 B. 
     The frontal surface  32  is substantially a flat annulus of axis Z-Z. This surface  32  is situated radially on the divergent side of the region  31  of the external roller  12 . The frontal surface  32  of the roller  12  is therefore, in meridian cross section, substantially the conjugate of the bearing surfaces  15 A and  15 B of the tracks  9 A and  9 B. The rear surface  33  is substantially a flat annulus of axis Z-Z and is located radially on the inside of the joint  1 . 
     The region  31  of the external roller  12  is connected to the frontal surface  32  and rear surface  33  by parts, the profiles of which are curved in meridian cross section, of the roller  12 . 
     The way in which the joint  1  works is as follows. 
     When, for example, the male element  2  is driven in the counter-clockwise direction in FIG. 1, the roller  12  comes to bear against the track  9 A in order to transmit the torque to the female element  8 . 
     The arm  4  therefore transmits to the corresponding mechanical transmission member  11  a force F parallel to the plane Q′. The point M of application of this force F is the point of contact between the bearing surface  5  and the surface  27  of the ring  18 . 
     For each mechanical transmission member  11 , the central region  14 A of the track  9 A and the central region  31  of the roller  12  bear against each other. The transverse profiles of these central regions means that they press together substantially flat and implies the generation of a reaction force which, because of the inclination of the transverse profiles of these central parts, has a component R directed radially toward the outside of the joint  1 . 
     This radial component R tends to move the roller  12  radially toward the outside of the bell housing  8  until the frontal surface  32  of the roller  12  comes to bear against the bearing surfaces  15 A and  15 B of the tracks  9 A and  9 B. The surface  32  of the roller  12  comes into contact almost at right angles with the bearing surfaces  15 A and  15 B, which presents no risk of jamming. 
     When the joint  1  is operating at an angle of misalignment between the shafts of axes X-X and X′-X′, the arm  4  is given a back and forth movement in radial translation with respect to the tracks  9 A and  9 B, in the plane P, because, on the one hand, of the inclination of the arm and, on the other hand, of the orbital movement, known as the offset, of the tripod at a frequency which is three times the rotational speed, as is well known in the art. The amplitude of this movement is greater in the direction directed radially toward the inside of the joint  1  than in the direction directed radially toward the outside of the joint  1 . 
     When, during this back and forth movement, the arm  4  moves radially toward the inside of the joint  1  (downward in FIG.  1 ), the point M at which the force F is applied moves radially toward the inside of the joint  1 , and so would tend to cause the roller  12  to rock in the clockwise direction in FIG.  1 . In addition, during this movement, the arm  4  through friction exerts a force F 1  on the internal ring  1  along the axis Z-Z, which force F 1  is oriented radially toward the inside of the bell housing  8 . This force F 1  would also tend to cause the roller  12  to rock in the clockwise direction in FIG.  1 . 
     The flat pressing between the peripheral surface  30  of the roller and the track  9 A at their central regions  14 A and  31 , and the inclination of the transverse profiles of these central parts, make it possible, through the component R, to compensate for the effect of this force F 1  and the movement of the point M, so that the roller  12  remains bearing against the bearing surfaces  15 A and  15 f of the tracks  9 A and  9 B. 
     When the arm  4  moves radially toward the outside of the joint  1  (upward in FIG.  1 ), the point M of application of the force F moves in the same direction. At the same time, the arm  4 , through friction, exerts a force F 2  on the internal ring  18  along the axis Z-Z and directed toward the outside of the bell housing  8 . The movement of the point M and the force F 2  tend to keep the external roller  12  bearing against the bearing surfaces  15 A and  15 B of the tracks  9 A and  9 B. 
     Thus, the external roller  12  remains stable, without its part facing the central region  14 B of the track  9 B, against which it is not bearing, oscillating. 
     The angle α is determined so that it just compensates for the effects of the force F 1  and of the movement of the point M of application of the force F radially toward the inside of the joint  1 . In this way, the reaction forces F 3  exerted by the bearing surfaces  15 A and  15 B of the tracks  9 A and  9 B on the external roller  12  are limited, regardless of the direction in which the arm  3  moves. 
     The oscillations of the roller  12  and therefore the risks of the latter jamming when the joint  1  is operating at an angle of misalignment, are therefore limited. 
     Finally, the curved profiles in the parts connecting the central regions  14 A and  14 B to the bearing surfaces  15 A and  15 B of the tracks  9 A and  9 B make it possible to avoid high pressures at the ends of the bearing between the central region of the external roller  12  and the tracks  9 A and  9 B. 
     It should be noted that these curved profiles are obtained partly by relief-machining the ends of the central region  31  of the bearing surface  30 . 
     In general, the angle α may be between about 1 and 10° and preferably between about 3 and 5°. 
     According to variants, the transverse profiles of the central regions  14 A and  14 B of the tracks  9 A and  9 B converge radially toward the outside of the bell housing  8 . In the latter instances, the central part  31  of the external roller  12  also converges radially toward the outside and the marginal retaining parts of the tracks  9 A and  9 B are situated radially on the inside of the bell housing  8 . 
     FIG. 2 illustrates a second embodiment of a constant-velocity joint  1  according to the invention, which can be distinguished from the one illustrated in FIG. 1 as follows. 
     The internal ring  18  has an annular shoulder  40  of axis Z-Z on the radially internal surface  27  of this ring  18 . This shoulder  40  is situated radially on the outside of the joint  1 . 
     The length, along the axis Z-Z, of the surface  22  of the ring  18  is greater than the length of the surface  22  of the joint  1  in FIG.  1 . 
     The surface  23  of the external roller  12  has two annular shoulders  41  of axis Z-Z, arranged on each side of the needle ring  21  along this axis. 
     The coupling means  19  also do not have thrust washers  24  or  25 . 
     The coupling means  19  thus allow a pivoting movement about the axis Z-Z and a sliding movement along this axis between the external roller  12  and the internal ring  18 . 
     In addition, the internal ring  18  swivels and slides along the axis Z-Z about the spherical bearing surface  5  of the arm  4 . 
     The latter sliding movement is, however, limited radially toward the inside of the joint  1  by the shoulder  40  and, toward the outside, by the end  42  of the bell housing, between the bearing surfaces  15 A and  15 B. 
     The shoulders  41  of the external roller  12  allow the needle ring  21  to be held in place with respect to the roller  12 . 
     This second embodiment makes it possible to reduce the forces transmitted by the internal ring  18  to the roller  12  along the axis Z-Z because the relative movement along this axis between the internal ring  18  and the external roller  12  is provided via the needle ring  21 .