Abstract:
A constant velocity joint has outer and inner joint members formed with aligned ball grooves. A torque-transmitting ball is contained in each set of grooves and is captured within windows of a cage disposed between the joint members. The radial groove profile provides a large operating joint angle but with a relatively low R x  ratio. At zero joint angle the balls are positioned in a ball center plane and define a ball center radius (BCR) in the ball center plane. As the joint angulates, the balls roll on the grooves and, while maintained in a ball plane, are displaced radially relative to the ball center radius (BCR). R x =BCR/x, which is the ball displacement. The groove profile includes a straight section coupled with a double arc or polynomial curved section which attains high joint angle at a relatively low R x  ratio.

Description:
[0001]    The disclosure incorporates the constant velocity joint disclosed in U.S. provisional patent application No. 60/353,672, filed Jan. 31, 2002, whose priority is claimed for this application. 
     
    
     
       TECHNICAL FIELD  
         [0002]    The technical field of this invention is constant velocity joints.  
         BACKGROUND OF THE INVENTION  
         [0003]    A well known constant velocity joint, used in the drive axles of front wheel drive motor vehicles, is the “Rzeppa” joint, described in U.S. Pat. No. 2,046,584, in which a driving member and a driven member transmit torque through a plurality (generally 6) of balls captured in longitudinal grooves in the driving and driven member and a cage. The geometry of the arrangement ensures that the balls are always aligned in a plane which bisects the angle between a pair of planes normal to the driving and driven axes of rotation; and constant velocity rotation of the driven member is thus assured, regardless of the joint angle between driving and driven members. As the joint angle increases, however, some portion of the driven member, usually the output shaft, eventually abuts some portion of the driving member to define a maximum joint angle. A variety of design factors and constraints work together to limit this maximum joint angle, and considerable effort has been expended in attempting to increase it. Such increases in maximum joint angle have generally come at the cost of an increase in joint package size, which is not desirable in the crowded engine compartments of front drive vehicles.  
           [0004]    A modification of the original “Rzeppa” design is shown in U.S. Pat. No. 3,879,960 to Welschof et al. The constant velocity joint of this design is undercut-free: that is, the grooves of the outer joint member have a radial profile at the open end which is parallel to the joint axis so that they are not undercut in the longitudinal direction. The undercut-free design of this reference provides a reasonably high maximum joint angle; but an even higher joint angle is desired.  
           [0005]    Commonly owned U.S. Pat. No. 6,186,899 shows various constant velocity joint groove profiles which increase joint angle through a corresponding increase in the ratio (R x ) of the ball center radius (BCR) to the inward axial displacement (x) of the ball center relative to BCR, such that R x =BCR/x=45 and greater at a joint angle of 46°. It is not always desirable to increase R x  to the required range of 45 and above in order to achieve the higher joint angles.  
         SUMMARY OF THE INVENTION  
         [0006]    The constant velocity joint of this invention provides an increase in maximum joint angle, relative to similarly constructed joints of the prior art, without a corresponding increase in joint package size. Thus, for any desired maximum joint angle, the constant velocity joint of this invention allows a smaller joint package size than a conventional “Rzeppa” joint of the prior art.  
           [0007]    The constant velocity joint of this invention has a modified radial groove profile which significantly decreases ball movement radially inward of the ball center radius so that the ball cage may be increased in diameter without a corresponding increase in ball center radius. The radial groove profile provides a required predetermined funnel angle for ball control at the ball centered point of contact but minimal change in distance from the joint center over most of the axially inner side of the grooves in the outer joint member. One embodiment provides an undercut-free dual arc configuration of the ball grooves in which inner and middle arc segments are formed on the ball grooves of the outer joint member having centers of curvature which are axially offset to one side of the joint center and radially offset from one another and which transition into an undercut-free straight section at the open end of the joint. Another embodiment provides an undercut-free groove configuration in which a straight section of the outer joint ball grooves extends from the open end and transitions into a polynomial arc commencing at a transition point axially outward of the ball center plane at zero joint angle and extending inwardly toward an opposite closed end, and wherein the slope of curvature tangent to transition point is between 6 and 10°.  
           [0008]    Both embodiments provide high angulation to the constant velocity joint without requiring an increase in package size of the outer joint member. In each case, the ratio R x =BCR/x=30 to &lt;45, at 46° joint angle, thus achieving high joint angle with a relatively low R x  ratio as compared to the joints described in aforementioned U.S. Pat. No. 6,186,899. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    These and other features and advantages of the present invention will become more readily appreciated when considered in connection with the following detailed description and appended drawings, wherein:  
         [0010]    [0010]FIG. 1 is an axial section of a preferred embodiment of a constant velocity joint according to this invention, the section taken through a pair of opposing balls;  
         [0011]    [0011]FIG. 2 is an axial section of the constant velocity joint of FIG. 1 which is rotated by 30 degrees relative to the section shown in FIG. 1;  
         [0012]    [0012]FIG. 3 is a diagram illustrating a geometric constraint on joint angle in a constant velocity joint of the prior art;  
         [0013]    [0013]FIG. 4 is a diagram illustrating, in comparison with the diagram of  
         [0014]    [0014]FIG. 3, the larger geometrically constrained joint angle of an embodiment of a constant velocity joint according to this invention;  
         [0015]    [0015]FIG. 5 illustrates radial movement of a ball within the ball cage of a constant velocity joint of the prior art;  
         [0016]    [0016]FIG. 6 illustrates radial movement of a ball within the ball cage of an embodiment of a constant velocity joint according to this invention;  
         [0017]    [0017]FIG. 7 illustrates the radial profile of ball grooves in the outer and inner joint members;  
         [0018]    [0018]FIG. 8 is a trace of the outer joint ball groove profile;  
         [0019]    [0019]FIG. 9 is an axial section of an alternative embodiment of the constant velocity joint of this invention; and  
         [0020]    [0020]FIG. 10 is a trace of the outer joint ball groove profile of the alternative embodiment.  
     
    
     DETAILED DESCRIPTION  
       [0021]    The constant velocity joint of FIGS. 1 and 2 has an outer joint member generally indicated at  10  presenting an outer race  12  having an inner spherical surface  14  and an integral shaft  16  axially aligned with outer race  12 . The joint further has an inner joint member generally indicated at  20  presenting an inner race  22  having an outer spherical surface  24  and adapted by opening  23  to receive an axially aligned shaft, not shown in FIGS. 1 and 2 but partially shown in FIG. 4, which projects in a direction generally opposite to that of shaft  16 . In this embodiment, as in most such constant velocity joints, outer joint member  10  is the driven member and inner joint member  20  is the driving member; but the invention is not so limited. Each joint member  10 ,  20  has an axis, which are common along axis A when the joint members  10 ,  20  are at zero degree joint angle, as illustrated in FIGS. 1 and 2.  
         [0022]    Inner spherical surface  14  of outer joint member  10  is formed with a plurality of circumferentially spaced longitudinal grooves  15 , which extend axially from the open end  13  of outer race  12  in the direction of shaft  16 . Grooves  15  are arranged in opposed pairs and equally spaced circumferentially around inner spherical surface  14 . Likewise, outer spherical surface  24  of inner race  20  is broken by a plurality of longitudinal grooves  25 , which extend axially thereacross. Grooves  25  are arranged in opposed pairs and equally spaced circumferentially around outer spherical surface  24 . Grooves  15  and  25  will be further described below.  
         [0023]    A cage  30  is provided between outer joint member  10  and inner joint member  20 . Cage  30  has an outer spherical surface  32  which abuts inner spherical surface  14  of outer race  12  and further has an inner spherical surface  34  which abuts outer spherical surface  24  of inner race  22 . The spherical surfaces of outer race  12 , inner race  22  and cage  30  are all formed from centers which coincide in the assembled joint at a single point called the joint center C, shown in the drawings. Thus, outer race  12 , inner race  22  and cage  30  therebetween are each free to rotate independently of the others, within limits, except as constrained by the balls to be described.  
         [0024]    A plurality of cage windows  35 , equal in number to the number of grooves  15  and grooves  25 , are equally spaced circumferentially around cage  30 . Cage windows  35  are preferably rectangular openings through cage  30  from outer spherical surface  32  to inner spherical surface  34  having opposing axially extending sides aligned in two parallel axial planes in the normal manner. Each cage window  35  retains a ball  40 , which is also retained in one of grooves  15  of outer race  12  and in one of grooves  25  of inner race  22 . The number of balls  40  is thus equal to the number of cage windows  35 , grooves  15  and grooves  25 : namely 6 in this embodiment.  
         [0025]    The arrangement described to this point is similar to a standard “Rzeppa” constant velocity joint and generally operates in the manner normal for such joints. In operation, balls  40  transmit torque between outer race  12  and inner race  22  to turn output shaft  16 . When the joint angle is zero, with inner joint member  20  and outer joint member  10  (and thus input and output shafts) coaxial, cage  35  maintains balls  40  with the ball centers in a single plane P normal to the axis. When the joint angle becomes non-zero, grooves  15  become non-parallel with grooves  25  except in the plane of the joint angle. The position of each ball  40  is then determined by the crossing point of the particular one of grooves  15  and the particular one of grooves  25  which contains that ball; and the balls thus move back and forth in the grooves in a cycle synchronized with rotation of the joint. As each ball momentarily passes through the plane of the joint angle, the grooves momentarily become parallel, but the cage maintains the ball in the proper position. Provided that the grooves are correctly designed, the balls are thus automatically maintained at all times with their centers in a plane that bisects the angle between planes normal to the outer member and the inner member. The constant velocity joint of this invention differs from that of other “Rzeppa” joints in providing a new profile of the grooves  15  and  25  which allows cage  35  to be made radially larger without increasing the ball center radius (BCR). The result is a greater maximum joint angle with substantially no increase in the package size of the joint.  
         [0026]    The main limitation on maximum joint angle can be seen in FIG. 3, which represents a standard “Rzeppa” joint of the prior art. Only one half outer race  112  of outer joint member  110  is shown; and only an attached shaft  126  of a corresponding inner race is shown. All other parts are removed for simplicity, but they are understood to be present and operable as described above to determine the relative positions of the parts shown. In FIG. 3, the joint center  150  is shown as the intersection of four lines: the joint axis  151  (which is also the axis of outer joint member  10 ), normal line  152  (which is perpendicular to joint axis  151  in the plane of the joint angle: the plane of FIG. 3), the axis  153  of shaft  126 , which is also the axis of the attached inner race and thus defines the joint angle with joint axis  151 , and a line  154  from joint center  151  through point  155 . Point  155  is the first point of contact between shaft  126  being rotated counter-clockwise in FIG. 3 and outer race  112  and thus defines the maximum joint angle. Point  155  is located at the outer (right) edge of inner spherical surface  114 .  
         [0027]    The maximum joint angle is shown in FIG. 3 as angle C′. In order for this angle to increase, the sum of angles A′ and B′ must decrease. But angle A′ represents the amount of spherical “wrap-around” which is required to retain the cage within outer race  112 . Assuming the joint is optimally designed, this angle cannot be decreased without decreasing this retention capability below its design value. Thus, in order for angle C′ to increase, angle B must decrease. But, again assuming optimal joint design, angle B′ cannot be decreased by reducing radius of shaft  126  without reducing the strength of shaft  126  below its design value.  
         [0028]    Angle B′ can be reduced, however by increasing radius “S” of inner spherical surface  114 . FIG. 4 shows a joint corresponding to that of FIGS. 1 and 2, with a new, larger radius, which results in a new inner spherical surface  14 . The amount of increase is also exaggerated in FIG. 4 for demonstration purposes, and is not to be considered in scale. The new point of contact  55  between shaft  26  and outer race  12  is still on the original line  154  (angle A′ relative to normal line  52  has not changed). But because the radius of shaft  26  subtends a smaller arc at the larger distance from joint center  50 , the former axis  153  of shaft  126  has now moved counter-clockwise to become the new axis  53  of shaft  26 . Former angle B′ has decreased B′(−), and joint angle C′(+), has increased. An increase on the order of about 3 degrees is attainable over standard Rzeppa joints: for example, from 47 to 50 degrees in an undercut-free joint.  
         [0029]    With spherical surface  14  at a greater distance from the joint axis, cage  30  and spherical surface  24  are also enlarged radially by essentially similar distances. But the ball center radius (BCR), which is the distance from the joint center to the centers of the balls at zero joint angle, is not significantly changed. Thus, it is possible to avoid increasing the joint package size. It is necessary, however, to provide new profiles for grooves  15  and  25  to create a different ball movement within cage windows  35  consistent with constant velocity rotation of the joint.  
         [0030]    When the joint angle is non-zero, balls  40  move in and out radially relative to the joint center as they move back and forth along the grooves. This is due to the groove geometry: in particular, the fact that the grooves are constructed relative to a center of rotation offset from the joint center. The balls pass through the ball center radius twice during each rotation of the joint as they move outward beyond the ball center radius and inward within the ball center radius. Since cage  30  moves with a constant radius centered on the joint center C, the balls move radially inward and outward relative to cage  30 . The movement can be pictured in a prior art joint in FIG. 5 as the movement of a spot contact of a ball on an inner surface  136  of a window  135  in cage  130 . With the extreme point of ball movement in the groove toward the open end of outer race  112  labeled O, the centered position labeled “BCR” and the extreme point of ball movement in the groove toward the closed end of outer race  112  labeled I, the movement describes generally a figure eight pattern on the cage window surface. This pattern at least partially determines the dimensions of the cage, since cage  130  must retain the balls at all times. It can be seen that, if cage  130  is moved significantly outward with an increase in its radius without changing the ball center radius, the ball movement of the prior art grooves can move the contact point completely off the cage at the inner end of ball travel along the groove at large joint angles; and this is an undesirable result. As previously mentioned, if the ball center radius is increased along with the cage radius, this causes the entire joint package to increase in size; and this is not a preferred result.  
         [0031]    The groove geometry of the joint of this invention allows an increase in cage radius without a significant corresponding increase in the ball center radius by changing the groove geometry to reduce the radial movement of the balls below the ball center radius while increasing it above the ball center radius. This is demonstrated in FIG. 6, which shows cage  30  moved upward (radially outward) relative to the ball center radius. The distance between the ball center radius and the radially innermost point I is seen to be reduced from X 0  to X 1  so that the contact point remains on cage window inner surface  36  through an entire rotation of the joint. This enables the cage to be radially enlarged without correspondingly enlarging the ball center radius, and thus joint package size; and the larger cage permits a greater joint angle. This change can be expressed as an increase in the ratio R x =BCR/X, where BCR is the ball center radius of the joint at a joint angle of zero and X is the maximum travel of the ball centers below the ball center radius at a joint angle of 46 degrees. Typical values of R x  for joints of the prior art are in a range around 18. Values achieved in this invention exceed 30 but are less than 45.  
         [0032]    The groove geometry that produces the beneficial ball movement shown in FIG. 6 is described with reference to FIGS. 7 and 8. FIG. 7 shows the ball  40  in the centered position in an undercut joint constructed according to a first embodiment of the invention, with the outer and inner joint members  10 ,  20  at a zero degree angle. FIG. 7 illustrates the radial groove profiles of the outer and inner joint members  10 ,  20  that cooperate to generate the ball movement of FIG. 6, producing high joint angle with relatively low R x  ratio. The groove profiles of the outer and inner joint members  10 ,  20  are essentially the same, but oppositely arranged to provide the desired funnel angle of the grooves. FIG. 8 shows further details of the groove profile of the outer joint  10 , with it being understood that the details are equally applicable to the grooves of the inner joint  20 , but oppositely arranged as shown in FIG. 7.  
         [0033]    The profile of the grooves  15  of the outer joint  10  includes an undercut-free straight section  60 , which is parallel to the longitudinal axis A of the outer joint member  10  and extending from the open end  13  inwardly to a juncture point  62  spaced axially outwardly of the center plane P as shown in FIGS. 7 and 8. The groove profile further includes two arcuate sections R 1  and R 2  which have different centers of radii, as will be described below with reference to FIG. 8, and which are tangent at a point t lying in the plane P. The intersection of the plane P and axis A defines a center point C of the joint which remains the same at all angles of joint members  10 ,  20 .  
         [0034]    Turning now in more detail to FIG. 8, the groove profile of the outer joint  10  is shown in relation to the center plane P and axis A, including the straight section  60  and arcuate sections R 1  and R 2 . The first arcuate section R 1  is centered at point C 1  lying on the longitudinal axis A and is offset axially outwardly from the center point C of the joint toward the open end  13 . The first arcuate section R 1  is tangent with the straight section  60  at point  62  at the same distance outwardly from the center plane P as the offset between C 1  and C.  
         [0035]    The second arcuate section R 2  extends inwardly from the first arcuate section R 1  at tangent point t and is larger in radius than that of R 1 . R 2  is centered at point C 2  at a location spaced a predetermined distance y below the axis A of the outer joint  10  and offset axially toward the open end  13  by a distance greater than that between C 1  and C. The vertical distance y can be adjusted by changing the radius of R 2  in order to change the X 1  value of FIG. 6 and thus the R x  ratio to achieve, at a joint angle of 46°, an R x  value of between 30 and &lt;45.  
         [0036]    A similar result is obtained according to the alternative groove profile illustrated in FIGS. 9 and 10 pursuant to a second alternative embodiment of the invention. The same reference numerals are used where appropriate to indicate corresponding features with that of the first invention. The groove profile may include a straight section  60  that is parallel to the axis A and extends between the open end  13  and a transition point  62  spaced axially outwardly from the center plane P of the joint when the joint members  10 ,  20  are at zero angle. The groove profile transitions at point  62  into an arcuate section R 3  which is defined by a polynomial such that the following constraints are met: first, the horizontal section  60  must intersect the curve of R 3  at point  62 , the slope of tangency of the curve R 3  at the point g lying in the plane P must be set at an angle α of between 6 to 10° relative to the horizontal axis A. Such a profile of the ball grooves (the profile of the inner balls grooves being equal but opposite) yields a R x  ratio of greater than 30 but less than 45 measured at a joint angle of 46°. The slope of the polynomial at the open end of the joint (outer race) is about 0-25° relative to horizontal axis A. Accordingly, the second embodiment groove profile, like the first embodiment groove profile, yields a relatively high joint angle of up to 50° but with a correspondingly low R x  ratio, and may also be applicable to lower joint angles. The polynomial groove profile for high angle joint applications (i.e., those where the joint angle is about 50°) may be represented by the polynomial equation, High angle Y=0.0002x 3 −0.0092x 2 +0.1053x+27. The groove profile for lower angle joint (about 47°) may be represented by the polynomial equation Low angle Y=−0.0002x 3 +0.0132x 2 +0.1092x +27.  
         [0037]    Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. The invention is defined by the claims.