Abstract:
A tripod joints includes a joint outer part and a joint inner part. The joint inner part has a tripod star with ball pins, which transmits a drive torque from the joint inner part to the joint outer part via a pressure element and via rolling bodies which are arranged between the joint outer part and a running surface of the pressure element. The running surface of the pressure element is curved in the rolling direction of the rolling bodies. The force may be distributed more uniformly between the rolling bodies when large drive torques are intended to be transmitted. The tripod joint may be suitable for the displaceable and pivotable driving connection of two shaft ends, e.g., in conjunction with drive trains or half shafts of motor vehicles.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to Application No. 101 41 439.0, filed in the Federal Republic of Germany on Aug. 23, 2001, which is expressly incorporated herein in its entirety by reference thereto. 
     FIELD OF THE INVENTION 
     The present invention relates to a tripod joint for two ends of a drive shaft. 
     BACKGROUND INFORMATION 
     Tripod joints are used, for example, as half shafts of motor vehicles. In this case, the tripod joints are used for transmitting drive torques between two drive elements of a drive train. The tripod joints permit relative displacement and relative pivoting of the drive elements to be compensated for. For the use in the case of half shafts of a motor vehicle, relative movements of this type are caused by spring deflections of the vehicle wheels. 
     U.S. Pat. No. 4,619,628 describes a tripod joint having a joint outer part and a joint inner part held in the joint outer part. The joint inner part has a tripod star having pins with a ball body. The ball bodies are accommodated pivotably in a partially spherical universal ball joint of a pressure element and are therefore mounted pivotably with respect to the pressure element. The pressure element has a running surface on the side facing away from the ball body. Rolling bodies are arranged between the running surface and a mating surface of the joint outer part, in order to transmit the drive torque. 
     It is an object of the present invention to provide a tripod joint which is optimized with regard to the forces which occur in the region of the running surfaces, of the mating surfaces and of the rolling bodies. 
     SUMMARY 
     The above and other beneficial objects of the present invention are achieved by the tripoid joint as described herein. 
     Investigations have shown that, particularly when large drive torques need to be transmitted, elastic deformation occurs in the components which form the mating surfaces and the running surfaces. 
     By way of example, if a force is introduced centrally through the ball body, that subregion of the running surface which projects in the running direction bends as a consequence of the reaction forces exerted by the rolling bodies. As a consequence of this, the distance between the running surface and the associated mating surface is increased in this subregion (in the micrometer range). As a consequence of this, the force to be transmitted decreases in the outer subregions of the running surface. This leads to the force being distributed inhomogeneously over the running surface. With a predetermined maximum surface pressure in the region of maximum forces, the maximum force which can be transmitted is thus not utilized in the outer subregions, while an optimum force distribution is achieved only for small drive torques. 
     According to the present invention, the running surface of the pressure element is curved in the running direction of the rolling bodies. The curvature is such that the distance between the running surface and the planar mating surface decreases in the direction of the outer subregions. The curvature is configured such that, when large drive torques need to be transmitted, an approximately planar running surface is produced, so that the forces to be transmitted are approximately equal on all the rolling bodies. 
     In consequence, all the rolling bodies may be stressed to a uniform extent, thus resulting in improved running characteristics and reduced wear. The surface pressures on the running surface and on the mating surface are likewise optimized, so that the wear on these operating surfaces may also be reduced. The drive torques which may be transmitted may be increased for the same component dimensions. According to the present invention, a non-uniform force distribution is therefore accepted for small drive torques, while an optimum force distribution may be achieved for large drive torques. The pressure element may be designed to be thinner, and it is possible to deliberately accept elastic deformation of the pressure element, which may be compensated for by the curvatures for large loads. This may result in a more compact tripod joint. 
     The running surface of the pressure element may include entry inclines or radii in the entry region of the rolling bodies. This makes it possible to improve the threading of the rolling bodies into the running surface, and hence into the force flow. In addition to reducing the mechanical stress on the components involved, this may result in a reduction in sudden force changes, which occur as a result of the threading-in process, during movement or pivoting of the tripod joint. 
     Exemplary embodiments of the tripod joint according to the present invention will be explained in greater detail below with reference to the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a longitudinal cross-sectional view of a tripod joint according to an example embodiment of the present invention. 
     FIG. 2 is a cross-sectional view of a tripod joint according to an example embodiment of the present invention. 
     FIG. 3 illustrates a pressure element of a tripod joint according to an example embodiment of the present invention. 
     FIG. 4 illustrates the transmission forces that occur on a conventional pressure element of a tripod joint from rolling bodies during operation. 
     FIG. 5 illustrates the transmission forces that occur on a pressure element of a tripod joint according to an example embodiment of the present invention from the rolling bodies during operation when drive torques are small. 
     FIG. 6 illustrates the transmission forces that occur on a pressure element of a tripod joint according to an example embodiment of the present invention from rolling bodies during operation when drive torques are large. 
    
    
     DETAILED DESCRIPTION 
     A tripod joint  10  has a joint inner part  11  and a joint outer part  12  holding the joint inner part  11 . The joint inner part  11  and the joint outer part  12  are in each case connected, at least in a rotationally fixed manner, to a drive element of a drive train of a motor vehicle, for example to a drive shaft and a vehicle wheel. The tripod joint  10  is used for transmitting a drive torque between the joint inner part  11  and the joint outer part  12  while ensuring a relative displacement along the longitudinal axis  13 — 13  of the joint inner part  11  and along the longitudinal axis  14 — 14  of the joint outer part  12 , a relative pivoting of the joint inner part  11  with respect to the joint outer part  12 , which pivoting is associated with a change in the angle  15  between the longitudinal axes  13 — 13  and  14 — 14 , and a three-dimensional movement which arises from a combination of the abovementioned forms of movement. 
     The joint inner part  11  has, at the end arranged on the inside, three pins  16  which are formed as a single piece or a number of pieces together. The three pins  16  are orientated radially and are distributed in each case at 120° in the circumferential direction and form a tripod star. The pins  16  have in each case a partially spherical ball body  17 . In order to transmit forces in both circumferential directions, the ball body  17  bears, in each case in the region of the spherical lateral surface, against a correspondingly configured recess  18  of a pressure element  19 . On the opposite side of the pressure element  19 , which side faces a flat mating surface  20  of the joint outer part  12 , the pressure element is of flat configuration with a running surface  21 . 
     Cylindrical rolling bodies  23 , in particular rollers or needles, are held between the running surface  21  and the mating surface  20 , forming a linear contact. A plurality of rolling bodies  23  are guided in a cage  24 . In order to transmit circumferential forces in the opposite direction, each pin  16  is configured with two associated pressure elements  19 , the rolling bodies  23  and the surfaces  20  and  21 , all symmetrical to a pin central plane accommodating the longitudinal axis  13 — 13 . 
     The running surface  21  of a pressure element  19  may have a rectangular form. Consequently, as many rolling bodies  23  as possible may be used thereby forming a load-bearing contact with a reduced surface pressure. The present invention is not limited to pressure elements  19  with rectangular forms. Circular or oval pressure elements  19  are also possible. 
     The joint outer part  12  has a recess  25  orientated in the direction of the longitudinal axis  14 — 14  with an essentially circular, central hole  26  and three holding spaces  27  which are orientated radially and are distributed in each case at 120° in the circumferential direction and are used in each case for holding and supporting a pin  16 , two pressure elements  19  and rolling bodies  23 . In the section illustrated in FIG. 2, the holding spaces  27  have an essentially U-shaped contour open in the direction of the hole  26 , the side limbs of the U-shaped contour being formed with the mating surfaces  20 . In the example embodiment illustrated in FIG. 2, the side limbs are of rectilinear configuration without a transitional region to the mating surfaces  20 . In the direction of the hole  26 , the side limbs do not, in particular, have any projections or depressions, but rather merge into the hole  26  with an enlargement of the spacing. In the position of the tripod joint illustrated in FIG. 2, the rolling bodies  23  together with the cage  24  are arranged spaced apart radially from the main limb of the U-shaped contour. 
     As illustrated in FIG. 2, the rolling bodies  23  are guided in a cage  24 . The rolling bodies  23  are guided in the cages  24  with the relative position of the longitudinal axes  31  of the rolling bodies  23  with respect to each other being ensured. The cages  24  are guided in the radial direction with respect to the pressure element  19  over shoulders  32  engaging around and enclosing the pressure element  19 . The cages  24  may be “clipped” via the shoulders  32  onto the pressure element  19 , as illustrated. The cages  24  may furthermore be centered in the running direction of the rolling bodies  23  via centering or spring elements  33 . Two cages  24  of a pin  16  may be guided and centred via a common spring element  33 . 
     Two pressure elements  19 , which are each associated with one ball body  17  for both circumferential directions, may be connected via two connecting webs to form an integral pressure body. A bayonet connection may be configured so that the ball body  17  may be inserted into the integral pressure body. 
     According to the example embodiment illustrated in FIG. 2, two spring elements  33  are connected to the pressure elements  19  or the ball body  17  via a respective fastening arrangement  36 . The spring elements  33  in each case have two elastic fingers  37  which bear against the opposite cages  24  or are connected thereto, for the purpose of supporting them. 
     In comparison with the ball body  17 , the cage  24  having the rolling bodies has, in particular, just two degrees of freedom: a suitably selected connection of the ball body  17  to the pressure elements  19  may ensure pivotability about an axis perpendicular with respect to the plane defined by the longitudinal axis  13 — 13  of the joint inner part  11  and the longitudinal axis of the pins  16 . The second degree of freedom is the connection between the cage  24  and pressure elements  19 , which connection may be displaced in a translatory manner. In order to ensure that the pressure element  19  may pivot with respect to the ball body  17 , the pressure element  19  may hold the ball body  17  in a universal ball joint. 
     As illustrated in FIG. 4, the pressure element  19  has transitional regions  38  between the end surfaces  39  and the running surface  21 . In the simplest case, the transitional regions are in the form of a phase. Alternatively, they may be curved, e.g., with a smooth transition to the running surface  21 . The transitional regions  38  may improve the threading-in process for the rolling bodies  23  between the running surface  21  and the mating surface  20  during operation of the tripod joint  10 . 
     FIG. 4 illustrates a conventional pressure element  19  with a planar running surface  21  in the unloaded state. The load transmitted when the drive torques are high lead to elastic deformation of the outer subregions  40  of the pressure element  19  in direction  41 , i.e., away from the associated mating surface  20 . The force transmitted in the outer subregions  40  decreases as a result of bending and as a result of an increased distance between the mating surface  20  and the running surface  21  in the outer subregions  40 . 
     FIG. 5 illustrates shows the force distribution on a pressure element  19  for small drive torques for an example embodiment of the present invention. The resulting curvature of the running surface  21 , which is oriented in the direction opposite the direction  40  from the centre in the running direction, results in the distance between the running surface  21  and the associated mating surface  20  being less in the outer subregions  40  than that from the inner subregions  42 . Accordingly, the forces which may be transmitted in the outer subregions  40  may be greater than in the inner subregions  42 . 
     FIG. 6 illustrates the force distribution on pressure element  19  for high drive torques for an example embodiment of the present invention. The curvature of the running surface  21  is compensated for by the elastic deformation of the pressure element  19 . All the subregions  40 ,  42  of the running surface  21  are thus at approximately the same distance from the mating surfaces  20 . The forces which occur in the subregions  40 ,  42  are approximately uniformly distributed. 
     The necessary curvature of the running surface  21  may be determined by a finite element calculation of the running surface/rolling body/mating surface system for the drive torques to be expected. The running surfaces  21  may be curved in the form of a circular arc, with a radius R as illustrated in FIG.  3 . In the example embodiment illustrated in FIG. 3, the difference in the distances between the central subregions  42  of the running surface  21  and the outer subregions  40  from the mating surface  20  is in the micrometer range. 
     The present invention is not limited to running surfaces  21  with circular arc curvatures. Other curvatures are possible and are included herein. In addition, a tripod joint according to the present invention may include a mating surface  20  with a curved configuration as an alternative to or in addition to the curved running surface  21 . 
     The configuration according to the present invention is suitable for any arrangement of a tripod joint for which a pressure element is connected in the force flow. Reference is made, for example, to U.S. Pat. No. 4,619,628, German Published Patent Application No. 28 16 646 or U.S. Pat. No. 4,708,693. 
     According to the present invention, the rolling bodies  23  may be guided in the cage  24  with parallel longitudinal axes  31  of the rolling bodies  23 , or with longitudinal axes  31  inclined at an acute angle to one another. 
     If the mating surfaces  20  are planar, the rolling bodies  23  may be in the form of cylindrical rollers each with the same radii so that the contact surfaces of the rolling bodies  23  that face the pressure element  19  are on one plane. 
     The example embodiments above include arrangements only given by way of example. A combination of the described features for different example embodiments is possible. Further features, in particular features which have not been described, of the device parts belonging to the invention are to be taken from the device-part geometries illustrated in the Figures.