Abstract:
An automatic transmission having at the input a starting element ( 4 ), a reversing set ( 46 ), a first electronic control unit ( 65 ) and a cone pulley ( 29 ) fixedly supported on one primary shaft ( 18 ) and axially thereto a hydraulically adjustable cone pulley ( 30 ), both pulleys forming a primary variator part ( 19 ). In addition there is a cone pulley ( 31 ) fixedly supported on a secondary shaft ( 21 ) and axially thereto a hydraulically adjustable cone pulley ( 32 ) together forming a secondary variator part ( 20 ) wherein a belt-type organ ( 37 ) between the cone pulleys ( 29, 30, 31,32 ) drivingly connects the primary variator part ( 19 ) with the secondary variator part ( 20 ). The starting element ( 4 ) is a dry running disc clutch whose disc is non-rotatably connected with the primary shaft ( 18 ) and interacts in a clutch housing ( 14 ) with a non-rotatably but axially movably disposed pressure plate ( 7 ) actuated via axial bearings ( 9 ) by a non-rotating actuator ( 10, 11 ), the clutch housing ( 14 ) being non-rotatably connected with an input shaft ( 2 ) of a prime mover ( 1 ). The purpose is to reduce the rotating masses and to improve the adjustment of the variator ( 19, 20 ).

Description:
BACKGROUND OF THE INVENTION 
     The invention concerns an automatic transmission. 
     The automatically controlled transmission has a continuously variable ratio step with a primary variator pulley and a secondary variator pulley, which are drivingly interconnected by a belt-type organ in the form of a chain or belt. Such so-called CVT transmissions, continuously variable transmissions, are known already, e.g. from  ATZ Automobiltechnische Zeitschrift  96 (1994), pp. 578 to 587. Aside from the variator, a disc clutch in the form of a hydrodynamic torque converter with lock-up clutch is provided on the input side of the transmission. A pump wheel of the hydrodynamic torque converter is driven by an internal combustion piston engine as the prime mover and is non-rotatably connected with a power part of a hydraulic pump, such as a radial piston pump. 
     Situated between the variator and the hydrodynamic torque converter is a reversing set in planetary design which has one clutch for the forward drive and one brake for the reverse drive. For adapting the variator thrust to a convenient total ratio of the transmission, an intermediate step is rear-mounted on the variator. The intermediate step can also serve as adaptation of direction of rotation and also takes care of an output coaxial with the input shaft. 
     Each one of the primary and secondary variator pulleys has a cone pulley fixedly connected, respectively, with the primary shaft and secondary shaft and an axially displaceable cone rotatable therewith. The displaceable cones have a piston chamber for the hydraulic axial adjustment. To change the reduction ratio, an electronic regulating device, of the transmission, controls the piston chambers of the movable cones, the pressure medium reaching the rotating piston chambers via ducts in the shafts. 
     Depending on the rotational speed, the pressure medium is subject to different centrifugal forces which override the control and must be accordingly taken into account. In addition, the pressure medium in the rotating piston chambers increases the masses of the rotating parts and therewith the drag torque which acts negatively, the required contact pressures and thus upon the efficiency of the transmission when adjusting speed. The torque converter exerts a similarly negative influence in relation to the drag torque of the transmission even when the lock-up clutch is closed in order to keep the hydrodynamic losses in the converter low in some operating ranges. CVT transmissions are basically known for small cars, which have a simple starting element in the form of a magnetic powder or multi-disc wet clutch, instead of the hydrodynamic torque converter. 
     The problem, on which the invention is based, is to reduce the drag torque of the rotating parts and the flow rate need of the hydraulic actuators, the same as to improve the operating behavior. 
     SUMMARY OF THE INVENTION 
     According to the invention, the starting element is a dry-disc clutch whose disc is non-rotatably connected with the primary shaft and interacts in a clutch housing with a non-rotatably, but axially movably disposed pressure plate. A non-rotating actuator conveniently situated on a clutch bell housing actuates via an axial bearing the pressure plate, which presses the disc against a friction surface in the clutch housing. The clutch housing itself, which can form part of an inertia mass of a prime mover, is non-rotatably connected with the input shaft of the prime mover, such as a crankshaft or a flywheel connected therewith. 
     In open state, the dry disc clutch clearly separates the prime mover from the transmission without the tow torques being transmitted to the primary shaft by the fluid friction. Further creep of the vehicle is prevented when parked. The rotating masses of the disc clutch, connected with the transmission, are very small. They consist only of the disc connected with the primary shaft while the actuator is situated in the clutch bell housing and does not rotate. 
     The actuator conveniently consists of a hydraulic piston which, designed as an annular piston, in an annular cylinder of the clutch bell housing and is loaded with hydraulic oil by a transmission oil pump, the control being effected via an electronic pressure-control unit of a control unit of the transmission. The hydraulic piston closes the clutch while a recoil spring opens the disc clutch. It is thus ensured that, in case of failure of the hydraulic system, the clutch is brought to the safe open state. The power part of the transmission oil pump is non-rotatably connected with the pressure plate. Thus, it is driven at the rotational speed of the prime mover and thereby consumes only a small amount of power at idling speed and in a low rotational speed range, whereby the efficiency of the transmission is improved. 
     According to a development of the invention, two redundant annular pistons act upon the pressure plate, via the axial bearing, so as to ensure the driveability as long as possible. In normal operation, a first annular piston is controlled by means of the transmission oil pump, while a second control unit controls the second annular piston by means of a pressure supply pump, independent of the transmission oil pump, when a first electronic control unit and/or the hydraulic system of the transmission fails and/or a electronic control unit detects an ABS operation. By ABS operation is understood an operation state in which an anti-lock system reacts. 
     An independent pressure supply can serve the ABS hydraulic system and the oil supply of an auxiliary steering pump, the second electronic control unit can be integrated in an engine control unit or in an ABS control unit. If one of the electronic control units detects an ABS operation, drive line slip losses are reduced when the anti-lock system is active. 
     The hydraulic pistons are conveniently designed as annular pistons and consecutively shifted in series. They can be shifted into each other so that one piston serves wholly or partly as a cylinder for the other hydraulic piston. The annular pistons surround the extension of the primary shaft and the drag between the pressure plate and the transmission oil pump. The drag is designed as a hollow shaft and supported on the extension of the primary shaft. Thereby a small construction volume results and, in addition, a good reaction behavior of the clutch is obtained by the short hydraulic connections. 
     According to a development of the invention, in order further to reduce the rotating masses, cylinder-piston units, which adjust the movable cones of the variator, are situated fixed to the housing. Thereby is obtained that the hydraulic medium is not exposed to any centrifugal forces which must be taken into account in the control. The forces, which are exerted upon the variator pulleys by the cylinder-piston units, support themselves in the transmission housing via the primary shaft or the secondary shaft and the appertaining fixed bearing. Between the cylinder-piston units and the adjustable cones, axial bearings are provided which reduce the friction. In this case, relatively short hydraulic ducts fixed to the housing can also be designed with a large cross-section whereby the response behavior of the variator is improved. Finally, it is possible to omit the otherwise usual dynamic pressure-compensating chamber and expensive seals between parts having different rotational speeds. 
     According to one other development of the invention, a reversing set in planetary design is situated on the secondary shaft. In addition to a sun gear and an internal geared wheel, it has a double planet so that the reduction ratio of the reversing set can be equally designed in both directions. The total ratio of the transmission and the adaptation, specific to the vehicle of the direction of rotation during forward drive, can be determined by a rear-mounted intermediate step. The sun gear of the reversing set is non-rotatably connected with the secondary shaft and in forward drive range can be coupled via a clutch with a planet carrier. For reverse drive, the clutch is opened and the internal geared wheel slowed down by a brake. For the output, an output gear wheel is non-rotatably connected with the planet carrier. 
     To be able to transmit sufficient torque in the forward drive, it is convenient that the internal disc carrier of the clutch designed as disc clutch be connected with the planet carrier and the external disc carrier with the secondary shaft and the sun gear. For the friction surfaces thus results a larger, more effective diameter based on the secondary shaft. 
     The brake is conveniently designed centered in position as one-disc brake, wherein the brake disc fastened to the internal geared wheel is situated in a brake caliper between two pistons which are controlled via a brake valve. Together with the small rotating mass of the brake disc, the advantage results that the two brake pistons in the double-piston arrangement, such as has proved satisfactory for vehicle service brakes, quickly release the brake disc during pressure break down by slightly recoiling. Thereby are clearly reduced drag losses in the forward drive operation in which the reversing gear set rotates as block. 
     The annular piston for actuating the starting clutch of the transmission hydraulic system is conveniently controlled by an electronic pressure-control unit via a pressure-control valve. The characteristic of the pressure-control unit is laid out so that the appertaining cylinder of the first annular piston is ventilated when the first electronic control unit of the transmission passes to emergency operation. The disc clutch opens and thus changes to the safety state. 
     When the first electronic control unit, which is capable of regulating the transmission, passes to the interference mode bit, the second annular piston is conveniently controlled directly by an electronic pressure-control unit placed in the transmission. At the same time, the second piston is supplied with energy by an independent pressure supply, such as an auxiliary steering pump or an anti-lock system. The control of the second annular piston can be integrated in an electronic engine control or can be effected by a pressure-control valve integrated in an ABS block. 
     The first electronic control unit of the transmission, the electronic control unit of the anti-lock system and the electronic control unit of the prime mover conveniently communicate, via a CAN bus (controller area network), and thus exchange data relevant to the control. The characteristic of the control of the second annular piston is designed so that, in case of power failure, it ventilates the appertaining cylinder and thus the disc clutch is opened. Thus, in case of failure of the transmission control, the engine control takes over the control logic for the disc clutch. Besides, the ABS control unit can for its part improve during ABS operation, the regulation excellence and the driving stability by its opening during ABS operation, via the second annular piston, the disc clutch and thus clearly reducing the drive line drag torque by uncoupling the prime mover. 
     The movable cones of the variator can be conveniently controlled, according to the “master-slave principle”, i.e. the pressure in the cylinder-piston unit for the secondary variator pulley corresponds to the secondary torque to be transmitted, and the pressure of the cylinder-piston unit for the primary variator pulley is determined by a ratio regulator according to the selected cylinder surface ratio. The surfaces of the cylinder are ideally selected so as not to fall below the contact pressure need of the secondary variator pulley, taking into account the smallest possible system pressure for maintaining the pre-control pressure supply of about  6  bar. The theoretical minimum pressure for constant drive in the total load and rotational speed range must likewise be laid out on the value. 
     The movable cones of the variator can also be controlled according to the “partner principle”, i.e. equal surfaces are used for the cylinder-piston units of the primary side and secondary side of the variator and the ratio is controlled via a square slide. 
     In both cases, since the cylinder-piston units are stationary in the housing and not exposed to any centrifugal forces, an oil supply for a dynamic pressure-compensation chamber and the latter, itself, can be omitted. 
     In one other development of the invention, it is proposed that the transmitting capacity of the clutch and brake of the reversing gear set be selected so that the pressure on the secondary variator pulley, including the required safety factor, is directly used as operation pressure for the clutch and brake. Thereby it is possible with an electronic pressure-control unit to make the contact pressure for the cones, the clutch pressure and the brake pressure available. 
     It is also advantageous that the reversing gear set be shifted only when the disc clutch is open and the shifting process be controlled by pressure ramps on the pressure on the secondary variator pulley. It is possible to implement the shifting process with a good shifting comfort without volume damper for the clutch and/or the brake. 
     The brake of the reversing gear set is actuated by a precontrolled brake valve via a closed system. In reverse drive, the working piston of the brake valve is connected directly, via a shift valve, with the pressure of the secondary variator pulley by the position of the shift valve and the working volume of the working piston joins the pistons to the brake disc against the force of a spring. In the neutral position of the selector lever, the spring moves the brake piston and the working piston back to the initial position. Thereby the brake disc can freely move causing no drag losses either in the forward drive or in the neutral position. 
     In this system, since the variator cannot be uncoupled in the sense of a parking adjustment, in order to assist a return to “low”, it is further proposed that for pressure control in the clutch or in the brake, a pressure-regulating valve be front-amounted on the shift valve. 
     Finally, it is advantageous for control of the operation cycle, for regulation of the variator ratio and regulation of the disc clutch to measure by Hall sensors the rotational speed of the prime mover on the clutch housing, the rotational speed of the primary variator pulley on the primary shaft, the rotational speed of the secondary variator pulley on the secondary shaft and the initial rotational speed of the output shaft on a differential. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other advantages result from the description he drawing that follows. In the drawing is shown an embodiment of the invention. The description and the claims contain numerous features in combination. The expert will conveniently regard the features separately and make with them logical added combinations. In the drawing: 
     FIG. 1 is a diagrammatic representation of an inventive transmission in longitudinal section; 
     FIG. 2 is a detail according to the line II in FIG. 1; 
     FIG. 3 is a detail according to the line III in FIG. 1; 
     FIG. 4 is a brake valve; and 
     FIG. 5 is a circuit diagram of an electrohydraulic control. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A prime mover  1 , as a rule an internal combustion piston engine, drives via an input shaft  2 , its crankshaft, a flywheel designed clutch housing  3  of a disc clutch  4 , in the instant case a dry starting clutch. The latter has one disc  5  pressed by a pressure plate  7  on to a friction surface of the clutch housing  3 . The input shaft  2  and the housing  3  firmly connected therewith are rotatably supported by a bearing  26  in a housing (not shown in detail) of the prime mover  1 . 
     A disc carrier  6  of the disc  5  is non-rotatably connected with a primary shaft  18  of a primary variator pulley  19  so that when the disc clutch  4  is engaged, a torque is transmitted from the prime mover  1  to the primary shaft  18  which is the input shaft of the transmission. The pressure plate  7 , which rotates with the clutch housing  3 , is actuated, via an axial bearing  9 , by a first annular piston  10  which is an annular piston guided in an annular cylinder formed by a second hydraulic piston  11  which is axially displaceably in a clutch bell housing  14  firmly connected with a transmission housing  16 . The annular pistons  10  and  11  are shifted in parallel and act redundantly upon the disc clutch  4 . 
     The pressure disc  7  has radially inwardly oriented arms  8  to which attaches a hollow-shaft designed drag  13  for a transmission oil pump  12 . The drag  13  extends radially within the annular pistons  10 ,  11  and is supported by bearings  17  upon the primary shaft  18 . The power part of the transmission oil pump  12  is thus driven by the prime mover  1  at the speed thereof so that at low rotational speeds of the prime mover  1  the transmission oil pump  12  consumes only a small amount of power. Thereby is improved the total efficiency of the transmission. 
     The primary variator pulley  19  and secondary variator pulley  20  are drivingly interconnected via a belt-type organ  37  in the form of a thrust articulated belt and provide a continuously variable ratio step. The primary variator pulley  19  has one cone  29  firmly connected with the primary shaft  18  and one cone  30  axially displaceable upon balls  34  in a longitudinal guide  33 . The secondary variator pulley  20  has one cone  31  firmly connected with a secondary shaft  21  and one cone  32  non-rotatably connected with the secondary shaft  21  but axially movable on balls  36  in a longitudinal guide  35 . The belt-type organ  37  runs respectively between the cones  29 ,  30  and  31 ,  32  on an effective contact diameter, the ratio of the contact diameter of the primary variator pulley  19  to the contact diameter of the secondary variator pulley  20  determining the reduction ratio of the continuously variable reduction step. 
     The reduction ratio can be changed by axial adjustment of the cone  30  and  32 . An annular piston  39  upon the primary side and an annular piston  41  on the secondary side serve for this purpose. The pistons  39  and  41  are in corresponding annular cylinders  38  and  40  firmly attached to the housing and can be controlled by the hydraulic system of the transmission via an electronic control unit  65  of the transmission. To reduce friction, the pistons  39  and  41  act via axial bearings  42 ,  43  upon the cones  30 ,  32  coordinated with them. Since the cylinders  38  and  40 , the same as the pistons  39  and  41 , are arranged firmly attached to the housing, their masses do not increase the rotating masses of the transmission. In addition, no centrifugal force acts upon the hydraulic medium in the cylinders  38  and  40  so that no displacement forces subject to centrifugal force have to be taken into account or compensated. 
     The primary shaft  18  is supported by a movable bearing  23  in the transmission housing  16 , by a fixed bearing  22  in an intermediate plate  15  fastened to the transmission housing  16 . The axial displacement forces of the piston  39  are reinforced within the cone  29  via the axial bearing  42 , the primary variator pulley  19 , the primary shaft  18  and via the fixed bearing  22 . FIG. 2 shows an enlarged section corresponding to the line  11  in FIG. 1 from which it results that an inner bearing ring of the fixed bearing  22  is axially fixed between a collar of the primary shaft  18  and a shaft nut  27  while the outer bearing ring of the fixed bearing  22  is axially fixed between the cylinder  38  and the intermediate plate  15 . A sealing ring  44  serves to seal the piston  39  in the cylinder  38 . 
     The secondary shaft  21  is correspondingly supported in the housing  16 . On the side of the cone  31  firmly connected with the secondary shaft  21 , the secondary shaft  21  is supported in a movable bearing  25  while, on the other end, it is supported in a fixed bearing  24 . The inner bearing ring of the fixed bearing  24  is axially fixed between a shaft collar of the secondary shaft  21  and a shaft nut  28 . The outer bearing ring of the fixed bearing  24  is axially retained between the cylinder  40  and the housing  16 . A sealing ring  45  seals the working space between the piston  41  and the cylinder  40 . FIG. 3 shows an enlarged section corresponding to the line III in FIG.  1 . The axial displacement forces of the piston  41  are reinforced within the secondary shaft  21  via the axial bearing  43 , via the secondary variator pulley  20 , via the secondary shaft  21  and the fixed bearing  24 . 
     A reversing gear set  46  in planetary design is situated on the extension of the secondary shaft  21  between the movable bearing  25  and the cone  31  firmly connected with the secondary shaft  21 . The reversing gear set  46  has a sun gear  47 , a double planet set with two planetary gears  48  and  49  and an internal geared wheel  50 . The sun gear  47  is non-rotatably connected with the secondary shaft  21 . The planetary gears  48 , 49  are supported on a planet carrier  51  which, in turn, is supported on the extension of the secondary shaft  21  and non-rotatably connected with an output gear  57 . 
     In order to shift from a neutral position  81  of the reversing gear set  46 , where the internal geared wheel  50  and the planet carrier  51  turn freely, to a forward drive position  79 , the planet carrier  51  is connected, via a clutch  52 , with the secondary shaft  21  or the sun gear  47  so that the output gear  57  is driven at the rotational speed of the secondary shaft  21 , the reversing gear set  46  rotating in block. To shift to a reverse drive position  80 , the clutch  22  is opened and the internal geared wheel  50  locked in the housing  16  by a brake  53 . Thereby the output gear  57  is driven in a reverse direction by the sun gear  47  via the planetary gears  48 ,  49  and the planet carrier  51 . In a reduction ratio one of the reversing gear set  46 , the output gear  57  also turns at the rotational speed of the secondary shaft  21 , but in a reverse direction of rotation. The reversing gear set  46  is conveniently shifted only when the disc clutch  4  is open. 
     The brake  53  is a fixed caliper brake which moves between two brake pistons  54 . The latter are situated in a brake caliper  56  fixed to the housing and are actuated by a precontrolled brake valve  83  (FIG.  4 ). Such brakes have proved satisfactory as vehicle service brakes and offer the advantage that the pistons  54  slightly recoil during pressure break down and fully release the brake disc  55  so that in forward drive operation the drag losses of the reversing gear set  46  rotating as block are clearly reduced. Besides, the rotating masses are scarcely enlarged by the light brake disc  55 . 
     The clutch  52  is designed as disc clutch, wherein its outer disc carrier is connected with the secondary shaft  21  and its inner disc carrier with the planet carrier  51 . 
     The output gear  57  operates via an intermediate step  58  and a differential  59  upon output shafts  60 . By virtue of the intermediate step  58  the general direction of rotation of the transmission and the total ratio can be adapted to the requirements of the vehicle. 
     To control the operation cycle, to regulate the variator ratio and to regulate the disc clutch  4 , rotational speeds on different parts of the transmission are detected by means of rotational speed sensors  61 ,  62 ,  63 ,  64 , which are conveniently Hall sensors. The rotational speed sensor  61  is placed on the periphery of the clutch housing  3  or of the flywheel and detects the rotational speed of the prime mover  1 . The rotational speed sensor  62  is situated on the periphery of the cone  29  that is firmly connected with the primary shaft  18  and detects the rotational speed of the primary variator pulley  19  while the rotational speed sensor  63  is located on the periphery of the cone  32  of the secondary variator pulley  20  and thus detects the rotational speed of the secondary variator pulley  20 . The reduction ratio of the variator pulleys  19 ,  20  can be calculated from the detected rotational speeds of the rotational speed sensors  62 ,  63 . The output rotational speed of the transmission is detected by the rotational speed sensor  64  which is provided on the periphery of the differential  59 . The rotational speed signals with added vehicle parameters, input parameters and drive parameters are processed in the first electronic control unit  65  to adjust signals for the disc clutch  4 , for the primary and secondary variator pulleys  19 ,  20  and for the clutch  52  and the brake  53  of the reversing gear set  46 . 
     The first electronic control unit  65  comprises electronic pressure-control units  66 ,  67 ,  68 ,  69 ,  70  which, via pressure-control valves  73 ,  76 ,  77  and pressure-regulating valves  72 ,  75 , modify according to the respective use the pressure produced by the transmission oil pump  12  (FIG.  5 ). The transmission oil pump  12  sucks oil from an oil sump  82  and delivers it into the hydraulic system wherein the pressure level in the secondary side piston chamber  40  is determined via a pressure-limiting valve  71 . Thus, the first annular piston  10  is directly controlled via the pressure-control valve  73  by the electronic pressure-control unit  68  of the first electronic control unit  65 . The characteristic of the pressure-control unit  68  is laid out so that the first annular piston  10  is ventilated and the disc clutch  4  is opened when the first electronic control unit  65 , associated with the transmission, passes to emergency operation. For this case is provided the second hydraulic piston  11  which, via a pressure supply pump  93  on the vehicle side, such as an auxiliary steering pump or an ABS pressurized-oil supply is supplied with energy. The second hydraulic piston  11  is directly controlled by an electronic pressure-control unit  70  supported in the transmission, e.g. for the case that the pressure supply pump  93  takes over the oil supply. But it can also be controlled directly from a pressure-control valve  76  integrated in the ABS block  96 . In this case, the characteristic of the pressure-control unit  76  is also laid out so that in case of power failure the second hydraulic piston  11  is ventilated, i.e. the disc clutch  4  is opened. 
     The operation logic of the control of the second hydraulic piston  11  is integrated in a second electronic control unit  95 , e.g. the control unit of the prime mover  1  or of the anti-block system. The second electronic control unit  95  communicates with the first electronic control unit  65  via a CAN bus. In case of failure of the first electronic control unit  65 , it takes over the control logic for the disc clutch  4 . 
     The cone  30  on the primary side or the cone  32  on the secondary side can be controlled according to the master-slave principle, i.e. the pressure produced by the transmission oil pump  12  corresponds to the torque to be transmitted to the secondary variator pulley  20  and the pressure drawn therefrom by the pressure-regulating valve  75  for the cone  30  on the primary side corresponding to the selected cylinder surface ratio of the cylinders  38 ,  40  is determined by the ratio regulator. According to the partner principle, in a control equal surfaces of the cylinders  38 ,  40  are provided on the primary side and secondary side and the ratio is controlled by using a square slide. Since the cylinders  38  and  40  are disposed firmly attached to the housing, the displacement forces of the pistons  39 ,  40  are independent of the centrifugal forces. 
     The surfaces of the cylinders  38 ,  40  are ideally selected so that the contact force need be ensured on the secondary side without falling below the lowest possible system pressure for maintaining the precontrol pressure supply of about six bar. The theoretical minimum pressure for a constant forward drive in the whole load and rotational speed range must likewise be laid out at the value. 
     A selector lever  78  serves to introduce the forward drive position  79  or the reverse drive position  80  out of the neutral position  81  and via a shift valve  74  pressurizes in the forward drive position  79  the clutch  52  while in the reverse drive position  80  the brake  53  is controlled via the precontrolled brake valve  83  (FIG.  4 ). The shifting operation between the neutral position  81 , the forward drive position  79  and the reverse drive position  80  always is carried out when the disc clutch  4  is open. 
     The shifting cycle and the shifting comfort are produced by pressure ramps of the pressure generated by the transmission oil pump  12 . Thereby volume dampers for the clutch  52  and the brake  53  become unnecessary. The brake  53  is actuated, via a closed system, by the precontrolled brake valve  83 . The latter has a working piston  85  which is axially movably passed into a brake cylinder  84  and on its periphery is sealed in relation thereto by a seal  86 . The working piston  85  forms in the brake cylinder  84  a working chamber  87  where a spring  88  is situated which presses the working piston  85  into an initial position. In this position, the working piston  85  abuts with a stop  94  on a front wall of the brake cylinder  84  and the brake  53  is opened. 
     In the proximity of the front wall is located a pressure connection  89  through which, in the reverse drive position  80  of the selector lever  78 , pressure is fed via the shift valve  74 , the pressure moving the working piston  85  against the force of the spring  88 . At the same time, a control collar  92  traverses a pressure connection  90  for a precontrol pressure and closes the pressure connection  90  to the working chamber  87 . The hydraulic medium, enclosed in the working chamber  87 , is fed to the brake piston  54  of the brake  53  via an outlet  9 . When the pressure drops on the pressure connection  89 , the spring  88  moves the working piston  85  back to the initial position, the brake  53  becomes aerated. Thereby is obtained that the brake disc  55  in neutral position  81  and in forward drive operation  79  moves completely free and that no drag losses generate on the brake  53 . 
     When the shift valve  74  is directly loaded with the pressure produced by the transmission oil pump  12 , and the clutch  52  and the brake  53  are designed so as to be able, at this pressure and with adequate safety values, to transmit a torque corresponding to the secondary torque, even though it is possible to uncouple the variator in order to assist a parking adjustment. Therefore, it is convenient to front-mount a pressure-regulating valve  72  on the shift valve  74  which is controlled by an electronic pressure-control unit  67 . Thereby the secondary variator pulley  20  can be uncoupled, via the reversing gear set  46 , by the output  60  for parking adjustment. With the pressure-regulating valve  72 , the clutch  52  and the brake  53  can be controlled separately from the pressure level of the transmission oil pump  12 . The control by the electronic pressure-control unit  67  makes possible, via the first electronic control unit  65 , a higher prioritization than the pressure distribution via the shift valve  74 . 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 Reference numerals 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                  1 
                 prime mover 
               
               
                   
                  2 
                 input shaft 
               
               
                   
                  3 
                 clutch housing 
               
               
                   
                  4 
                 disc clutch 
               
               
                   
                  5 
                 disc 
               
               
                   
                  6 
                 disc carrier 
               
               
                   
                  7 
                 pressure plate 
               
               
                   
                  8 
                 arms 
               
               
                   
                  9 
                 axial bearing 
               
               
                   
                 10 
                 annular piston 
               
               
                   
                 11 
                 annular piston 
               
               
                   
                 12 
                 transmission oil pump 
               
               
                   
                 13 
                 drag 
               
               
                   
                 14 
                 clutch bell housing 
               
               
                   
                 15 
                 intermediate plate 
               
               
                   
                 16 
                 transmission housing 
               
               
                   
                 17 
                 bearing 
               
               
                   
                 18 
                 primary shaft 
               
               
                   
                 19 
                 variator pulley (primary) 
               
               
                   
                 20 
                 variator pulley (secondary) 
               
               
                   
                 21 
                 secondary shaft 
               
               
                   
                 22 
                 fixed bearing 
               
               
                   
                 23 
                 movable bearing 
               
               
                   
                 24 
                 fixed bearing 
               
               
                   
                 25 
                 movable bearing 
               
               
                   
                 26 
                 bearing 
               
               
                   
                 27 
                 shaft nut 
               
               
                   
                 28 
                 shaft nut 
               
               
                   
                 29 
                 cone 
               
               
                   
                 30 
                 cone 
               
               
                   
                 31 
                 cone 
               
               
                   
                 32 
                 cone 
               
               
                   
                 33 
                 longitudinal guide 
               
               
                   
                 34 
                 ball 
               
               
                   
                 35 
                 longitudinal guide 
               
               
                   
                 36 
                 ball 
               
               
                   
                 37 
                 belt-type organ 
               
               
                   
                 38 
                 cylinder 
               
               
                   
                 39 
                 piston 
               
               
                   
                 40 
                 cylinder 
               
               
                   
                 41 
                 piston 
               
               
                   
                 42 
                 axial bearing 
               
               
                   
                 43 
                 axial bearing 
               
               
                   
                 44 
                 sealing ring 
               
               
                   
                 45 
                 sealing ring 
               
               
                   
                 46 
                 reversing gear set 
               
               
                   
                 47 
                 sun gear 
               
               
                   
                 48 
                 planetary gear 
               
               
                   
                 49 
                 planetary gear 
               
               
                   
                 50 
                 internal geared wheel 
               
               
                   
                 51 
                 planet carrier 
               
               
                   
                 52 
                 clutch 
               
               
                   
                 53 
                 brake 
               
               
                   
                 54 
                 brake piston 
               
               
                   
                 55 
                 brake disc 
               
               
                   
                 56 
                 brake caliper 
               
               
                   
                 57 
                 output gear 
               
               
                   
                 58 
                 intermediate step 
               
               
                   
                 59 
                 differential 
               
               
                   
                 60 
                 output shaft 
               
               
                   
                 61 
                 rotational speed sensor 
               
               
                   
                 62 
                 rotational speed sensor 
               
               
                   
                 63 
                 rotational speed sensor 
               
               
                   
                 64 
                 rotational speed sensor 
               
               
                   
                 65 
                 first electronic control unit 
               
               
                   
                 66 
                 electronic pressure-control unit 
               
               
                   
                 67 
                 electronic pressure-control unit 
               
               
                   
                 68 
                 electronic pressure-control unit 
               
               
                   
                 69 
                 electronic pressure-control unit 
               
               
                   
                 70 
                 electronic pressure-control unit 
               
               
                   
                 71 
                 pressure-limiting valve 
               
               
                   
                 72 
                 pressure-regulating valve 
               
               
                   
                 73 
                 pressure-control valve 
               
               
                   
                 74 
                 shift valve 
               
               
                   
                 75 
                 pressure-regulating valve 
               
               
                   
                 76 
                 pressure-control valve 
               
               
                   
                 77 
                 pressure-control valve 
               
               
                   
                 78 
                 selector lever 
               
               
                   
                 79 
                 forward drive 
               
               
                   
                 80 
                 reverse drive 
               
               
                   
                 81 
                 neutral 
               
               
                   
                 82 
                 oil sump 
               
               
                   
                 83 
                 precontrolled brake valve 
               
               
                   
                 84 
                 brake cylinder 
               
               
                   
                 85 
                 working piston 
               
               
                   
                 86 
                 seal 
               
               
                   
                 87 
                 working chamber 
               
               
                   
                 88 
                 spring 
               
               
                   
                 89 
                 pressure connection 
               
               
                   
                 90 
                 pressure connection precontrol 
               
               
                   
                 91 
                 outlet 
               
               
                   
                 92 
                 control collar 
               
               
                   
                 93 
                 pressure supply pump 
               
               
                   
                 94 
                 stop 
               
               
                   
                 95 
                 second electronic control unit 
               
               
                   
                 96 
                 ABS block