Abstract:
A continuously variable transmission including a drive element rotatable about a drive axis, an output element rotatable about an output axis parallel to the drive axis, and an adjusting mechanism with which the drive axis and the output axis can be laterally offset from each other. Coupling elements operate between the drive element and the output element and are spaced from each other in the peripheral direction of the drive element and output element for transmitting torque between the elements. The coupling elements change the movement of a peripheral region of the drive element faster than that of the output element so that forces acting on the coupling elements from the drive element and the output element lie substantially in a common plane that extends perpendicularly to the axes of rotation. The continuously variable transmission can be incorporated into a transfer case for an all-wheel drive system.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a continuously variable transmission as well as to a transfer case that includes a continuously variable transmission. 
   2. Description of the Related Art 
     FIG. 5  shows a known drive train of an all-wheel-drive motor vehicle. An internal combustion engine  10  is connected through a transmission  12 , for example a gearbox, to a transfer case  14  from which an output shaft  16  leads to a drive shaft  18 , which is connected to a rear axle differential  20 . A further output shaft  22  extends from transfer case  14  and is connected through a further shaft  24  to a front axle differential  26 . Such drive trains are known in the most varied configurations. 
   For various reasons, particularly vehicle movement dynamics reasons, it is advisable to divide the output torque of the transmission  12  to the wheels of the rear axle and the wheels of the front axle according to the driving condition in various ways. That can be accomplished by means of clutches, planetary gear sets, and differentials interconnected in various ways. One possibility consists also in integrating a continuously variable transmission (a transmission having a continuously variable transmission ratio) into the transfer case  14 , whose transmission ratio can be changed within a limited range in such a way that according to the operating conditions the wheels of the front axle rotate slightly faster or slightly slower than those of the rear axle. 
   A continuously variable transmission that can be utilized in such a transfer case is described, for example, in German Patent Publication No. DE 200 11 649 U1. The transmission includes a disk converter and a planetary gear set, which are coupled to each other in such a way that one of three main parts of the planetary gear set is controlled in a continuously variable way by the disk converter, a second part is coupled to the driven shaft of the transmission, and a third part is coupled to an output shaft of the transmission. The disk converter includes a drum as a driving component and a crowned disk as a driven component. The crowned disk has pins that each engage a groove of an associated intermediate disk. Each of the axially adjacent intermediate disks is connected to a radially inner surface of the drum through a one-way clutch. By an eccentric displacement between the drum and the crowned disk, the rotational speed ratio between the drum and the crowned disk can be modified. The known disk converter is relatively complicated in its configuration and has relatively large space requirements. 
   A continuously variable transmission that has a disk-shaped input element and an output element is known from U.S. Pat. No. 6,327,926. The input element has a radially-inwardly-open annular groove within which coupling elements move, which are each designed with a projecting pin whose enlarged end is received in a corresponding radially-extending groove of the output element. The rotational speed ratio between the input element and the output element changes by an eccentric displacement between the two. The elements have a relatively complex design. Furthermore, the axially-extending pins of the coupling elements must have a high rigidity because they are subjected to bending stresses. 
   It is an object of the invention to produce a continuously variable transmission that can particularly be used in transfer cases and can be economically manufactured with a compact design. 
   SUMMARY OF THE INVENTION 
   The object of the invention is achieved with a continuously variable transmission that includes a drive element that is rotatable about a drive axis, an output element that is rotatable about an output axis A that is parallel to the drive axis, an adjustment mechanism with which the distance between the drive axis and the output axis can be changed, and spaced coupling elements operating between the drive element and the output element in the peripheral direction of the drive element and the output element for transmitting torque. The coupling elements block a displacement of the corresponding peripheral region of the drive element faster than that of the output element, whereby the forces engaging the coupling elements of the drive element and the output element lie approximately in a common plane that is perpendicular to the axes of rotation. 
   The drive element is advantageously formed with mutually circumferentially-spaced drive projections arranged concentrically relative to the drive axis, which project into an annular groove that is arranged on the drive element concentrically relative to the output axis and which interact with the coupling elements arranged in the annular groove. 
   In a preferred embodiment of the continuously variable transmission, the coupling elements are composed of two radially adjacent coupling bodies that form an included angle somewhat smaller than 180° in the direction of rotation of the drive element and the output element, that have radially opposite surfaces that lie on the radially inner wall and the radially outer wall of the annular groove, and that together support each other through their mutually facing sides. 
   The mutually facing sides of the coupling bodies advantageously form rolling surfaces that roll on each other when there is a change in the angle between the coupling bodies. 
   The coupling bodies of the coupling elements are biased in the direction of an extended position. 
   In a preferred embodiment of a continuously variable transmission in accordance with the invention, the coupling bodies are formed with noses on the side opposite to the direction of rotation of the drive element and of the output element in the region of their mutually facing ends, against which rest the drive projections of the drive element. 
   The drive projections are preferably constructed having a thickness that diminishes radially inwardly when viewed in section perpendicular to the axes of rotation, and on their side opposite to the direction of rotation lie abut a leaf spring that biases the coupling bodies in an extended position direction. 
   The lateral surfaces of the drive projections that face in the direction of rotation preferably run approximately radially relative to the drive axis. 
   A transfer case that, as described above, includes a continuously variable transmission in accordance with the invention, advantageously has an input shaft supported in a transmission housing, within which the output element is also supported, in which a bracket can be pivoted about the bearing axis of the input shaft, on which the drive element of the continuously variable transmission is supported. 
   A drive gear of the transfer case is advantageously arranged on the same axis as the drive shaft and is non-rotatably connected therewith, an output gear is arranged on the same axis as the drive element and is non-rotatably connected therewith, and the drive gear and the output gear are connected to each other by means of an endless torque-transmitting means. 
   The continuously variable transmission in accordance with the invention can be installed with advantage wherever a compactly-built transmission with continuously variable transmission ratio and small spread is needed. 
   The transfer case in accordance with the invention has a very compact structure, if needed for the endless torque-transmitting means, a simply-designed tension regulating unit, since the spacing between the drive gear and the output gear does not change with a transmission ratio adjustment of the continuously variable transmission, and operates smoothly and uniformly, since numerous drive projections with coupling elements arranged between them can be provided without difficulty. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be explained below in exemplary fashion with reference to the schematic drawings and with further details, wherein: 
       FIG. 1  is a longitudinal section through a transfer case with an integrated continuously variable transmission, 
       FIG. 2  is an end elevational view of a drive element as viewed from the right in  FIG. 1  with the drive projections shown in section, 
       FIG. 3  is an enlarged representation of  FIG. 2 , 
       FIG. 4  is a schematic for explaining the functional principle of the continuously variable transmission in accordance with the invention, and 
       FIG. 5  is a view of a known drive train of an all-wheel-drive motor vehicle. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In accordance with  FIG. 1 , a transfer case identified overall by reference numeral  30  has a housing  32 , in which an input shaft  34  passing through the housing is supported, which in accordance with  FIG. 1  is connected on the left side to the transmission  12  ( FIG. 5 ), and on the right side forms the output shaft  16  in accordance with  FIG. 1 . The input shaft  34  is configured as one piece with a drive gear  36 . 
   An output shaft  37  is rotatably supported in the housing  32  about an output axis A spaced radially from the drive shaft  34 . 
   A bracket  38  having a bearing pin  40  on which is supported a drive element  42  of a continuously variable transmission, identified overall by reference numeral  44 , is pivotably supported on the same axis as the input shaft  34 . In the view of  FIG. 1 , the axis of the bearing pin  40  and the axis A of the output shaft  37  lie on the same straight line. The drive element  42  has a tubular body that is supported on the bearing pin  40  and forms an output gear  46  relative to the drive gear  36 , whereby an endless torque-transmitting means  48 , for example a toothed belt, passes around the drive gear  36  and the output gear  46 . 
   By mounting the bracket  38  on the same axis as the input shaft  34  or the drive gear  36 , the result is that if the bracket  38  is pivoted, the distance between the output gear  46  and the drive gear  36  does not change, so that the length of the endless torque-transmitting means  48  also does not change. An electric motor  50  that drives a spindle  52 , for example, which engages with a corresponding thread formed at an end of the bracket  38 , is provided for pivoting the bracket  38 . 
   The body of the drive element  42  that forms the output gear  46  is rigidly connected to, or is formed as one piece with, an annular disk  56 , and is coaxial with the axis of rotation B of the drive element  42  from which circularly-arranged, peripherally-spaced drive projections  58  extend. Drive projections  58  extend into an annular groove  60  that is formed on an annular disk  62  that is rigidly connected to the output shaft  37 , or is formed as one piece therewith. The annular disk  62  together with the output shaft  37  forms an output element of the continuously variable transmission  44  identified overall by reference numeral  64 . 
   Coupling elements  66  are arranged in the annular groove  60  between each two peripherally adjacent drive projections  58 . 
   As best seen in  FIGS. 2 and 3 , each coupling element  66  is composed of a radially inner coupling portion  68  and a radially outer coupling portion  70 , which are oppositely arranged through crowned rolling surfaces  72  and  74 , and with reference to the counterclockwise direction relative to  FIG. 3 , which is the direction of rotation of the components of the continuously variable transmission, form an angle of somewhat less than 180° with each other and are thus therefore not completely in an extended direction. The lateral surfaces of the coupling portions  68  and  70  that face in the direction of rotation are respectively radially outwardly or radially inwardly recessed in which a leaf spring  76  is supported that biases the coupling portions  68  and  70  in the direction of an extended position. The lateral surfaces of the coupling portions  68 ,  70  that face away from the direction of rotation are provided with noses  78  and  80  on their mutually facing end regions, against which lie the drive projections  58 . 
   The drive projections  58  have a wedge shape in a section perpendicular to the axis of rotation B, wherein their thickness diminishes radially inwardly. The lateral surfaces of the drive projections  58  that face the noses  78  and  80  run in an approximately radial direction relative to the axis of rotation of the drive element  42 . As shown, the arrangement is such that each drive projection  58  lies against the noses  78  and  80  of the coupling portions of a coupling element and the leaf spring  76  of an adjacent coupling element, and a radially inner contact surface  82  of the radially inner coupling portion  68  rests on the radially inner wall of the annular groove  60 , and a radially outer contact surface  84  of the radially outer coupling portion  70  rests on the outer annular wall of the annular groove  60 . 
   With the described arrangement, a transmission of torque from the drive projections  58  of the drive element  42  is achieved through the coupling elements  66  onto the walls of the annular groove  60  of the output element  64  in a plane II—II (see  FIG. 1 ), that is perpendicular to the axes of rotation A and B. 
   The function of the described continuously variable transmission formed substantially by the components  42 ,  64 , and  66  is explained first in principle with reference to  FIG. 4 , in which crossed arms  86  are rigidly connected with each other and form a drive element  87  rotatable about the axis of rotation B, a wheel  88  provided with external teeth is supported for rotation about an axis A, and coupling elements  90  are pivotally supported at the outer ends of the arms  86  and are respectively pulled in the clockwise direction by means of springs  92 , that is, in the direction toward the arms  86 . The coupling elements  90  are bent at their ends and engage the tooth system  94 , wherein the tooth system is formed in such a way that a coupling element  90  cannot move in a clockwise direction relative to the tooth system, but the tooth system can move clockwise relative to a coupling element. 
   It is initially assumed that the axes of rotation A and B coincide. If the drive element  87  composed of the arms  86  is driven in a clockwise direction, all the arms  90  that are arranged symmetrically with reference to the wheel  88  take along the tooth system  92 , so that the wheel  88  rotates at the same rotational speed as the component formed by the arms  86 . If the axis B of the drive component  87  is displaced relative to the axis A by the dimension a (shown in  FIG. 4 ), the effective radius for the engagement of the upper clutch elements  90  shown in  FIG. 4 , whose distance from the axis of rotation B is a maximum, is equal to R+a, wherein R is the radius of the wheel  88 . The effective radius for the engagement between the tooth system  94  and the lower clutch elements  90  relative to its peripheral speed, amounts to R−a. Therewith, the peripheral speed of the upper engagement (P) in accordance with  FIG. 4  is ω(a+R) when the rotational speed of the component composed of the arms  92  is ω, whereas the peripheral speed of the lower clutch elements  90  is merely ω(R−a) when they engage tooth system  94 . The tooth system  94 , and therewith the wheel  88 , are consequently driven by the respective upper coupling elements  90  at a peripheral speed that is greater than the peripheral speed of the other coupling elements, so that those lag behind relative to the tooth system  94 . The rotational speed of the wheel  88  is increased by the factor R/R−a relative to the position at which axes B and A coincide. 
   The foregoing considerations can be transferred directly to the continuously variable transmission  44  if the drive element  42  is thought of as equivalent to drive element  87 , coupling elements  90  are equivalent to the coupling elements  66 , and the wheel  88  is equivalent to the output element  64 . If the axis A of the output element  64  coincides with the axis B of the drive element  42 , the drive projections  58  of the drive element  42 , which are wedge-shaped in cross section, are in a radial position in such a way that they are accommodated approximately in the region of their half thickness between noses  78  and  80  and the crowns of the leaf springs  76 , wherein the individual components are in mutual contact over the entire periphery. If the drive element  42 , and therewith the drive projections  58  in accordance with  FIG. 3 , are rotated counterclockwise (arrow F), the coupling portions  68  and  70  are additionally pushed in the direction of their extended position by the force applied to their noses, so that they are additionally pushed against the peripheral walls of the annular groove  60  and carry along the output element  64 , so that when axes A and B coincide the output element  64  rotates at the same rotational speed as the drive element. 
   If the output element  64  lags behind relative to the drive element  42 , that is, if the annular groove moves with a slower peripheral speed than the drive element  42 , then the coupling portions  68  and  70  are pressed into increasing contact, whereby the engagement between the coupling portions and the walls of the annular groove is reinforced, so that a very high torque can be transmitted in the counterclockwise direction from the drive element  42  to the output element  64 . The torque transfer capacity in the opposite direction is considerably smaller, because the coupling portions  68  and  70  are pressed, by the friction force against the walls of the annular groove  60 , against the force of the leaf springs in the direction against the extended position when they overtake the annular groove  60  in a clockwise direction, whereby the friction force is reduced. 
   If the drive element  42  is displaced into an eccentric position relative to the output element  64  by pivoting of the bracket  38 , outwardly from the plane of the drawing in accordance with  FIG. 1 , which corresponds to a horizontal movement in accordance with the view of  FIGS. 2 and 3 , which for clarity is shown as a perpendicular movement, the upper drive projections  58  move downward, relative to  FIG. 3 , so that an additional force is exerted on the noses  78  and  80  and the leaf springs  75  as a result of the increasing thickness of the drive projections, which increasingly prestresses the coupling portions  68  and  70  into the extended direction, whereas the lower drive projections  58 , relative to  FIG. 3 , press the coupling portions  68  and  70  with a reduced force into an extended position. Consequently, the engagement between the upper coupling portions, relative to  FIG. 3 , and the output element  64  is reinforced, so that the output element  64  is driven in its upper region at an effective radius r+e, when r is the effective radius of the drive element in a concentric position between the output element and the drive element. The rotational speed of output element  64  consequently increases relative to the concentric position in the ratio (r+e)/r. In its lower region, or outside the uppermost region, the output element  64 , or its annular groove, can overtake the coupling portions  68  and  70 , as explained above. 
   Consequently, the invention provides a compactly-designed transfer case with integrated continuously variable transmission, wherein the rotational speed of the output element  64  relative to the drive element  42  can be transformed in accordance with the effective radius r and the maximum possible eccentricity e into, for example, up to a 15% higher speed. By appropriately selecting the diameter of the drive gear  36  and the output gear  46 , or also by coordinating the front axle differential therewith, the overall transmission ratio of the front axle relative to the rear axle can be transformed into a slower speed. If, for example, one selects the fixed transmission ratio of the front axle as 0.95, then one can achieve an overall transmission ratio of 0.95–1.05 between the front axle and the rear axle with a transmission ratio of the continuously variable transmission of 1.0–1.1, so that the torque delivered to the front axle can amount to between 0 and 100% of the overall torque. 
   The described stepless transmission, in a modified design of the overall transmission, can be arranged parallel to a fixed planetary distributor between the front and rear axles. Further, a simple clutch can be arranged in series with the continuously variable transmission, which can be disengaged if the ABS system is activated, to improve the control of the rotational speeds of the wheels by the ABS control unit. That clutch can also be mechanically operated if the continuously stepless transmission is in its 1:1 position. 
   The invention can be modified in many ways. For example, the torque can be transmitted between the drive projections  58  of the drive element, and the annular groove  60  of the output element by other coupling elements than those described as two coupling portions that roll on each other, and which are known in numerous embodiments as unilaterally acting freewheeling elements. 
   The following claims are illustrative and are without prejudice to acquiring wider patent protection. The applicants reserve the right to claim additional combinations of features disclosed in the specification and/or drawings. 
   The references contained in the dependent claims point to further developments of the object of the main claim by means of the features of the particular claim; they are not to be construed as a renunciation of independent, objective protection for the combinations of features of the related dependent claims. 
   Although the subject matter of the dependent claims can constitute separate and independent inventions in the light of the state of the art on the priority date, the applicants reserve the right to make them the subject of independent claims or separate statements. They can, moreover, also embody independent inventions that can be produced from the independent developments of the subject matter of the included dependent claims. 
   The exemplary embodiments are not to be considered to be limitations in accordance with the invention. On the contrary, many changes and variations are possible within the scope of the invention in the existing disclosure, in particular such variants, elements, and combinations and/or materials which, for example, are inventive by combining or modifying single features that are in combination and are described individually in relation to the general specification and embodiments as well as the claims and shown in the drawings, as well as elements or method steps that can be derived by a person skilled in the art in the light of the disclosed solutions of the problem, and which by means of combined features lead to a new object or new method steps or sequences of method steps.