Abstract:
The invention concerns a continuously variable belt-drive transmission, especially for motor vehicles. In the pressure less state of the transmission, the axially displaceable cone pulley of the input shaft is loaded with an axial force by a spring system in a manner such that the contact pressure increases on the belt-drive mechanisms and the ratio of the transmission is reduced.

Description:
FIELD OF THE INVENTION 
   The invention concerns a continuously variable belt-drive transmission, especially for motor vehicles. 
   BACKGROUND OF THE INVENTION 
   Such belt-drive transmissions are generally known. Continuously variable belt-drive transmissions usually comprise a variator for ratio adjustment which has a first cone pulley pair upon an input shaft and a second cone pulley pair upon an output shaft, and a belt-drive means running between the cone pulley pairs. Each cone pulley pair consists of one first pulley (a fixed pulley) stationary in an axial direction and one second cone pulley (an adjusting pulley) displaceable in an axial direction. The input shaft of the variator is usually designated as the primary shaft and the first cone pulley pair accordingly as the primary pulley pair. The output shaft of the variator is, as a rule, similarly designated as the secondary shaft and the second cone pulley pair as the secondary pulley pair. The primary pulleys and secondary pulleys and thus the ratio are axially adjusted by a pressure medium. The pressure medium is usually conveyed via ducts onto one or more pressure chambers of the primary or secondary pulley, and a pressure-medium pump making available the oil pressure needed. 
   The pressure supply of the whole transmission is mostly made possible via a pressure-medium pump directly coupled with the input rotational speed. The volume of oil conveyed depends therefore directly on the input rotational speed. When now the input rotational speed drops down to zero, the transmission is no longer supplied with oil and a so-called pressureless state appears. In this state, the maximum ratio is usually adjusted. At the same time, the primary pulleys stay insofar as possible, apart from each other and the belt-drive means rests on the primary side upon the smallest possible friction radius. The friction radius of the belt-drive means upon the output shaft, which radius is preset by the secondary pulleys, is thus at a maximum. When the motor vehicle now has to be towed in this condition, serious problems can arise: 
   The drive wheels of the vehicle guide torque into the transmission which is in the pressureless state. The torque is transmitted to the drive wheel via the output shaft and the belt-drive means. In the pressureless state, since a minimum friction radius adjusts on the input side and a maximum friction radius accordingly on the output side, the input shaft has to accelerate very intensively. In the pressureless state, since the compression force of the cone pulleys on the primary side is minimal, a skidding of the belt-drive means upon the primary pulleys can occur under the intensive acceleration whereby the primary pulleys are seriously damaged. 
   Due to the skidding, it can also occur that one clutch connected with the input shaft is intensively accelerated even when the vehicle motor is idle. When a certain limit rotational speed is exceeded, an undesired engaging of the clutch can occur in the clutch arrangement due to the rotational pressure then appearing. Due to the trailing torque then prevailing, an overheating of the clutch and the damage thereof can result. 
   SUMMARY OF THE INVENTION 
   According to the invention, the aforementioned problems are solved by the fact that the load piston of the adjusting pulley of the input shaft is loaded with spring tension via a spring device whereby, when the transmission is in the pressureless state, the adjusting pulley is axially displaced in a direction toward the fixed pulley. The applied pressure of the cone pulleys thereby increases upon the belt-drive means, preventing a skidding of the belt-drive means during towing. In addition, the primary pulleys move in the axial direction on top of each other whereby the friction radius of the belt-drive means upon the primary pulleys increases and the ratio diminishes. The spring tension acts here only during the pressureless state of the transmission and, therefore, does not affect regulation of the variator during normal driving operation. Besides, the number of operating strokes of the spring device minimizes and this acts positively upon the service life thereof. 
   A first advantageous embodiment is implemented by at least one plate spring mounted outside a first pressure chamber of a contact pressure device of the adjusting pulley of the primary side. The plate spring braces a first load piston with a shaft-tied cylinder and is advantageously centered on a cylindrical part of the first load piston. Thereby the load piston, in the pressureless state, is axially pressed away from the cylinder to the fixed pulley. The adjusting pulley thus moves and increases the contact pressure upon the belt-drive means and, likewise, the friction radius of the belt-drive means. 
   One other advantageous embodiment contains a second plate spring which, the same as the first plate spring, is centered on a cylindrical part of the load piston. The second plate spring braces itself between the first plate spring and the cylinder. The first plate spring braces itself between the second plate spring and the outer side of the load piston. The spring tension upon the load piston and also the spring travel are increased without great expense by the second plate spring. 
   In both embodiments, in further advantageous arrangements, the first, the same as the second plate springs, can be centered on their radially inner side. 
   A circlip can be installed, as needed, so as to make a stoke limitation of the plate spring possible by simple means. With the stroke limitation, the travel of the plate spring can be adapted to different designs of variators. Besides, the service life of the plate spring can be substantially improved by the limitation of the stroke. 
   In the pressurized state of the transmission, the plate spring is permanently kept at its maximum engaged state. Thus, it does not affect the regulation of the variator during normal driving operation. Besides, the number of operating strokes of the spring is thereby reduced, which has a positive effect on the service life thereof. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will now be described, by way of example, with reference to the accompanying drawings in which: 
       FIG. 1  show parts essential to the invention of a continuously variable belt-drive transmission; 
       FIG. 2  shows a first spring arrangement; and 
       FIG. 3  shows a second spring arrangement. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows a view of the parts essential to the invention of a continuous belt-drive transmission. One first input shaft  1  carries the primary pulleys  2 ,  3  which are designed with tapered friction surfaces  25 ,  26 . One of the pulleys is designed as fixed pulley  2  fixedly supported on the shaft and one as an axially movable adjusting pulley  3 . Between the primary pulleys  2 ,  3  lies a belt-drive means  4  which, during the driving operation, transmits torque from the input shaft  1  to the output shaft  5 . Upon the output shaft  5  are supported secondary pulleys  6 ,  7  which are also designed with tapered friction surfaces. A first clutch  8  is fixedly connected with the input shaft  1 . The contact pressure devices  9 ,  10 , next to the adjusting pulleys, are diagrammatically shown. 
     FIG. 2  shows a detailed view of an advantageous arrangement. Upon the input shaft  1  sits an axially movable cone pulley, the so-called adjusting pulley  3 . In addition, a shaft-tied cylinder  11  is fastened upon the input shaft  1 . On the side of the cylinder  12  facing the adjusting pulley  2 , a plate spring  13  is pressed against a first load piston  14 . The plate spring  13  is centered at its radially outer side  15  by a cylindrical part of a load piston  16  and, in addition, it presses with a radially outer edge  15  against the shaft-tied cylinder  11 . The plate spring  13  presses with its radially inner edge  17  against the load piston  14 . A circlip  18  is inserted in the cylindrical part of the load piston  16 . The circlip  18  serves as stroke limit for the plate spring  13  so that, in a pressureless state of the transmission, a ratio of the transmission remains greater than one (iV&gt;1). The spring stroke can thereby be adapted by simple means to the total ratio of the transmission. The first load piston  14  with the adjusting pulley  3  form a first pressure chamber  19 . The cylinder  11  with a second load piston  20  and the input shaft  1  form a second pressure chamber  21 . 
   In one other advantageous embodiment, the plate spring  13  can also be centered at its radially inner side  17 . 
   In another advantageous embodiment, the plate spring  13  can also be inversely built by pressing with its radially inner side  17  against the cylinder  11  and with the radially outer side against the load piston  14 . At the same time, the pressure surface of the shaft-tied cylinder  12  is advantageously elongated inwardly in a radial direction. 
     FIG. 3  shows another advantageous embodiment of the invention. A second plate spring  22 , the same as the first plate spring  13 , is centered in a cylindrical part of a load piston  16 . Said second plate spring  22  braces itself between the first plate spring  13  and the shaft-tied cylinder  11 , pressing against the first plate spring  13  with its radially outer edge  30  and against the shaft-tied cylinder  11  with its radially inner edge  29 . The first plate spring  13  braces itself between the second plate spring  22  and the load piston  14 . By its radially inner edge  17  it presses against the load piston  14  and by its radially outer edge  15  against the second plate spring  22 . By said second plate spring  22 , the spring tension upon the load piston  14  and also the spring stroke can be increased without great expense. 
   In this embodiment the first plate spring  13  can be inversely installed in which case its radially inner side  17  presses against a second plate spring  22  and its radially outer side  15  presses against the load piston  14 . For this purpose the second plate spring  22  is advantageously applied to the radially inner side  29  of the shaft-tied cylinder  11 . 
   In this embodiment, the plate spring  13  can also be centered, in another design, on its radially inner side. 
   REFERENCE NUMERALS 
   
     
       
             
             
           
         
             
                 
             
           
           
             
                1 input shaft 
               17 radially inner end of the plate 
             
             
                2 fixed pulley 
               spring 
             
             
                3 adjusting pulley 
               18 circlip 
             
             
                4 belt-drive means 
               19 pressure chamber 
             
             
                5 output shaft 
               20 load piston 
             
             
                6 fixed pulley 
               21 pressure chamber 
             
             
                7 adjusting pulley 
               22 plate spring 
             
             
                8 clutch 
               23 outer edge 
             
             
                9 contact pressure device 
               24 inner edge 
             
             
               10 contact pressure device 
               25 tapered face 
             
             
               11 cylinder 
               26 tapered face 
             
             
               12 pressure face of the cylinder 
               27 tapered face 
             
             
               13 plate spring 
               28 tapered face 
             
             
               14 load piston 
               29 inner edge 
             
             
               15 radially outer end of the plate spring 
               30 outer edge 
             
             
               16 cylindrical part of the load piston