Abstract:
The invention relates to a device for controlling the contact pressure of a transmission element of a continuously variable transmission, especially the traction element of a flexible drive for motor vehicles. The inventive device is provided with at least two levers as the transmission elements that provide the contact pressure depending on torque and ratio. Said levers are pivoted to the driving transmission elements and to the driven transmission elements and act upon an axially displaceable transmission element that indirectly or directly exerts the contact pressure via at least one rolling element each. The lever length effective to exert the contact pressure can be varied in accordance with the selected transmission ratio.

Description:
FIELD OF INVENTION 
   The invention relates to a device for controlling the contact pressure of the transmission mechanism of a continuously variable transmission. 
   BACKGROUND OF THE INVENTION 
   Such continuously variable transmissions are, for example, in the form of toroid drives or traction mechanism drives, the contact pressure of the transmission means, such as a link chain or thrust link conveyor, being controlled for the most part as a function of torque. The purpose of such transmissions is on the one hand to permit transmission of the drive torque as free of slippage as possible but on the other to counteract efficiency losses and wear resulting from application of too great a contact pressure force to the transmission means. 
   DE 42 01 692 A1 also discloses additional configuration of the contact pressure so that it is not dependent on the transmission ratio, for example, the contact pressure on the link chain in a traction mechanism drive being higher with a smaller effective belt contact radius on the drive side and lower with a greater belt contact radius on the drive side. 
   SUMMARY OF THE INVENTION 
   The object of the invention is to present a device for control of the contact pressure of the transmission means of a continuously variable transmission which, while being of simple and rugged design, effects continuous adjustment of the contact pressure as a function both of torque and of transmission ratio. 
   It is claimed for the invention that this object and other advantageous developments are attained by the characteristics specified in the claims. 
   While rolling elements or balls which move up corresponding frontal ramps on the driving transmission element and on the driven transmission element are used in the disclosed device for contact pressure control (see, for example, FIG. 2 of DE 42 01 692 A1 referred to above), it is proposed in accordance with the invention that use be made of levers, ones which, in accordance with the characteristics formulated in the claims, are of lengths which may be varied as a function of the moving pulley position assigned and which then act upon the transmission element bringing about the contact pressure by way of rolling elements, either additionally by way of ramps or directly without ramps. 
   The contact pressure control may be exerted either directly, by way of the levers and the rolling elements, or indirectly by throttle control of superimposed hydraulic contact pressure. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features and advantages as well as a description of the invention will be presented below in conjunction with the diagrams presented in the accompanying drawings, in which: 
       FIG. 1  presents a section of a continuously variable belt contact transmission for motor vehicles with, a pair of drive wheels shown partly in longitudinal section and a device for control of the contact pressure of the traction mechanism as a function of torque and transmission ratio; 
       FIG. 1A  presents a second section of the continuously variable belt contact transmission shown in  FIG. 1 . 
       FIG. 2  a top view of the device shown in  FIG. 1  with straight levers and rollers as rolling elements; 
       FIG. 3  also a top view, a development of another device with multiple-arm levers and balls as rolling elements; 
       FIG. 4  a section along line IV-IV of  FIG. 3  through the alternative device; and 
       FIG. 5  a top view in the direction of arrow X in  FIG. 4 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A continuously variable belt contact transmission  10  of conventional design not thus far described is shown in a sectional view in  FIG. 1 ; in this connection reference is to be made, for example, to FIG. 1 of DE 195 45 492 A1. 
   The belt contact transmission  10  has a conventional driving wheel pair  12  rotatably mounted with a drive shaft  14  in a transmission housing not shown and drives a driven wheel pair (not shown) by way of a link chain  16 . 
   The driving wheel pair  12  (and, of course, the driven wheel pair as well) has a fixed wheel  18  and a non-rotating gearing wheel  20  which nevertheless may be moved axially by way of gearing  19 ; the link chain  16  may be continuously adjusted, by hydraulic adjustment of the gearing wheel  20  relative to the fixed wheel  18 , within the two end positions illustrated for the purpose of assigning a desired transmission ratio. For this purpose pressurized hydraulic fluid from a hydraulic control unit not shown is delivered by way of channel  22  in the drive shaft  14  to a first hydraulic chamber  24  or drained from it. 
   The gearing wheel  20  has non-rotatably mounted on it a drive bell  26  which pushes axially oriented sliding guides  30  by way of carriers  28  projecting inward through straight-line levers  32  (also see  FIG. 2 ). 
   The carriers  28  also engage longitudinal grooves  34  of a ringshaped control sleeve  36  which is guided on the shaft  14  by a hub section (or control edge)  38   b  the annular edge of which forms a choke in conjunction with a drain channel  40 . 
   Hydraulic fluid may be fed to the hydraulic chamber  44  bounded by the control sleeve  36  and an annular partition  42  rigidly connected to the shaft  14  by way of a channel  46  in the shaft  14  also connected to the hydraulic control unit and another channel  48  in the partition  42 , it being possible to control the pressure (=contact pressure of the link chain  16 ) among other means by way of the choke  38   a.    
   Six levers  32  distributed over the circumference are provided, each of which is hinge connected by way of bolts  50  to a driving gear wheel  52  of the belt contact transmission  10 .  FIG. 1A  shows a view of the structure from a different angle, showing a second lever  32 ′. The other transmission elements on the drive side with a gear wheel  54  on another gear shaft are not shown. The driving gear wheel  52  is rotatably mounted on the drive shaft  14  by way of a rolling bearing  56 . 
   The levers  32  support, between the support point on the carriers  28  and the bolt connection  50 , a roller  58  which is rotatably mounted on a bolt  60  of the levers  32  and operate each in conjunction with a ramp  62 , the ramps  62  being formed by molding in the control sleeve  36 . It is also to be noted that the carriers  28  on the drive bell  26  are guided with zero backlash in the sliding guide  30  and the control sleeve  36  and keep the control sleeve  36  nonrotatable by way of longitudinal grooves  34 . 
   Contact pressure control of the link chain  16  by way of the gearing wheel  20  is as follows: 
   If driving torque is introduced by way of the driving gear wheel  52 , such torque is transmitted by the six levers  32  (see  FIG. 2 ) and by the carriers  28  to the drive bell  26  and the gearing wheel  20 . 
   The control sleeve  36  is pretensioned to the left by the pressure of the hydraulic fluid in the hydraulic chamber  44  as shown in  FIGS. 1 and 2 , the ramps  62  being pressed against the rollers  58  and accordingly, because of their configuration (chamfering angle), counteract excursion of the levers  32  in both directions of rotation (=traction or thrust). 
   If the torque increases, the levers  32  overcome the hydraulic prestressing force and are deflected, the rollers  58  displacing the control sleeve  36  in  FIG. 1  and to the right by way of the ramps  62  and guiding the control edge  38   a  toward the drain channel  40 , and accordingly throttling the latter. Because of the oil volume flow introduced, this results in increase in pressure in the hydraulic chamber  44  and so increase in the contact pressure of the link chain  16  by way of the gearing wheel  20 . 
   Since the gearing wheel  20  and thus the drive bell  26  are displaced axially over a distance s (see  FIG. 1 ) on change in the transmission ratio of the belt contact transmission  10 , the carriers  28  also shift correspondingly relative to the levers  32  inside the longitudinal openings  30 . This leads to a change in the leverage relationships from a (constant) to b (variable), lower contact pressure obviously being introduced with increase in the length of the lever b (=link chain  16  on the outside); hence the torque-dependent contact pressure decreases continuously in the event of a link chain moving outward. 
     FIGS. 3 and 4  illustrate another exemplary embodiment; in order to avoid repetition, it will be described only to the extent that it differs significantly from the embodiment shown in  FIGS. 1 and 2 . Identical components are identified by the same reference numbers. 
   As is to be seen in  FIGS. 3 and 4 , the three levers  70  are configured in the shape of a cross; one arm  72  is coupled by means of a bolt  74  to the driving gear wheel  52 ′, the opposite projecting arm  76  is introduced into recesses  78  in the form of pockets in the annular partition  42 ′, and the two cross-arms  80 ,  82  operating in conjunction with rolling elements or balls  84 ,  86 . 
   The balls  84 ,  86  of the three levers are guided in annular rolling bearing cages  88 ,  90 ; the bearing cages  88 ,  90  have radially projecting carriers  92 ,  94  which extend into obliquely oriented guide slots  96 ,  98  in an annular extension  100  on the drive bell  26 ′. 
   The guide slots  96 ,  98  per lever  70  are convergent, as is to be seen in  FIG. 5 , so that, on displacement of the gearing wheel  20  with drive bell  26  or  26 ′, the rolling bearing cages  88 ,  90  rotate in opposite directions relative to each other by way of the carriers  92 ,  94 , and accordingly the balls  84 ,  86 , as is shown in  FIG. 3  with reference to the balls  84 , are displaced toward or away from each other and thus form the variable lever length b. 
   The constant lever length a results from the distance between the bolt connection  74  of the arms  72  and the point of engagement of the arms  76  of the pocket-shaped recesses  78  in the partition  42 ′. 
   The balls  84 ,  86  in turn act on a radial, annular stopping face  102  on the control sleeve  36 ′. 
   When drive torque is transmitted from the driving gear wheel  52 ′ by way of the levers  70  to the partition  42 ′ rigidly connected to the shaft, the levers  70  are rotated around the bolts  74 , where the control sleeve  36 ′ is correspondingly displaced by way of the arms  80 ,  82  (on application of tractive or thrust force) and the balls  84 ,  86 . The function is as described previously in connection with  FIGS. 1 and 2 . 
   In the event of change in the transmission ratio of the belt contact transmission  10  the gearing wheel  20  is displaced with that of the drive bell  26 ′, the rolling bearing cages  88 ,  90  with the balls  84 ,  86  being rotated in opposite directions relative to each other and the balls  84 ,  86  being moved toward or away from each other by way of the oblique guide slots  96 ,  98  and the carriers  92 ,  94  for the purpose of changing the lever length b. 
   Contact pressure control is applied by way of the control sleeve  36 ′ in the drive direction (tractive load application) by the balls  84  and in the event of propulsive load application by the balls  86  (or conversely). 
   If in the event of axial displacement of the gearing wheel  20  and the drive bell  26 ′ the rolling bearing cages  88 ,  90  with the balls  84 ,  86  are rotated so that the ball (e.g.,  84 ) relevant in the case of drive torque is moved closer to the swivelling axis (bolt  72 ) of the levers  70 , the hydraulic pressure application of the link chain  16  as already described in the foregoing increases as a result of reduction of the length of lever arm b (see  FIG. 3 ). The same applies in the event of torque reversal (propulsive operation), the balls  86  operating accordingly.