Abstract:
A known device for increasing the functional reliability for a hydraulic circuit of a hydraulic clutch having a pressure reservoir ( 25 ) and a release cylinder ( 5 ) which comprises a primary piston ( 14 ) and a secondary piston ( 16 ). A compensation volume is developed in the sense that when the vehicle is at a standstill and the parking brake released, an unintentional release of the clutch is prevented. A pressure-monitoring/pressure-supply device ( 28 ) is used for determining a reference operating pressure and establishes a static pressure which, in case of any system error, only allows displacement of the secondary piston ( 16 ) such that the contact pressure of the clutch discs, against one another, is still sufficient to maintain the vehicle at a standstill.

Description:
This application is a national stage completion of PCT/EP2003/013271 filed Nov. 26, 2003 which claims priority from German Application Serial No. 102 55 713.6 filed Nov. 29, 2002. 
   FIELD OF THE INVENTION 
   The invention concerns a device for increasing the functional reliability for a hydraulic circuit of a hydraulically operable clutch. 
   BACKGROUND OF THE INVENTION 
   An increase of the functional reliability of the hydraulic circuit of a hydraulically operable clutch generally aims at maintaining the existing shifting state of the clutch and obtaining between the shifting states a defined transition characteristic which can be reproduced and thus effectively controlled by the driver of the vehicle concerned. 
   To meet the requirements, pressure intensifiers are known from the prior art from which a sufficient feedback goes to the clutch pedal so that the engagement and disengagement ratios sought by the driver can be adapted to the existing situation. It is very important here to implement play and wear compensation in order to ensure a coincident response characteristic during the service life of the clutch. 
   Release cylinders thus are known from the prior art in which automatic wear compensation is implemented. In that case, the release cylinder can have two pistons; the axial spacing between which is determined by the volume of a liquid located between the pistons; there resulting an adaptation of the volume depending on the wear state of the clutch. 
   However, when the vehicle is stopped, there is the danger that in case the working valve is not sealed, hydraulic liquid flows into the intermediate space between primary and secondary pistons, via a bleeding hole and, due to the volume increase, moves the secondary piston so that a slow opening of the clutch cannot be ruled out with certainty whereby the vehicle, when the parking brake is open, could unintentionally set itself in motion. 
   In DE 197 17 486 C2 is described a pneumatic pressure intensifier for a hydraulic clutch operation for motor vehicles. In the pressure intensifier, a pneumatic working space is provided which can be loaded with pressurized air for pressure intensification of a hydraulic working space. One control piston, coordinated with the pneumatic working space and serving for engaging and disengaging the pressure intensification source, is activated by a working piston located in the hydraulic circuit so that a leakage, due to pressure increase in the release cylinder, does not occur. 
   However, the described pneumatic intensifier of the device has a complicated construction and, to that extent, is susceptible to noise and comparatively costly to produce. 
   Therefore, the invention is based on the problem of increasing the functional reliability of an operation in the hydraulic circuit of hydraulic clutch by comparatively simple means. 
   SUMMARY OF THE INVENTION 
   One pressure-monitoring/pressure-supply device is coordinated with the pressure reservoir which determines a reference operating pressure and a differential pressure so as to reduce the operating pressure when the vehicle is stationary and the engine is disconnected for obtaining a residual static pressure. 
   The invention is associated with the advantage that when the vehicle is started again after stoppage with disconnected motor, as a result of the steadily provided static pressure the hydraulic clutch, is quickly available and, at the same time, the stress of the pressure reservoir is reduced since it is not exposed to the full operating pressure. The permanently abutting full operating pressure can lead to an intensified diffusion of the operating medium, particularly when the pressure reservoir is designed as a diaphragm reservoir. The consequence would be increased operating and maintenance costs. It is further ensured that when static pressure abuts, the clutch does not open or can open only to the extent that a sufficiently high drag torque is retained on the clutch so that the vehicle does not roll away. 
   In a preferred development of the invention, the inventive device has one pressure-limiting unit which makes an adjustment and limitation of the static pressure possible based on a maximum value which, when action is exerted upon the liquid volume located between primary and secondary pistons, allows only a displacement of the secondary piston of the release cylinder such that the contact pressure of the clutch pressure plate on the clutch disc still suffices to keep the vehicle at a standstill. It is thus ensured that when a gear is introduced and the parking brake released, even on an inclined road, the vehicle is held in the position concerned. 
   To determine the maximum admissible static pressure, it is possible to use the relative displacement of the piston rod that actuates the release lever of the clutch. The piston rod can thus remain in operative connection with an incremental travel measuring system from which the information, concerning the covered path of the piston rod, can be relayed as standard to the pressure-monitoring/pressure-supply device for establishing the functional dependence of the static pressure on the axial displacement of the piston rod. 
   In addition, it is basically possible to detect the distortion characteristics of the piston spring and set it in relation to the momentarily prevalent pressure. 
   To prevent the selected static pressure from lowering, due to inner leakages in the hydraulic circuit which would result in delaying the availability of the vehicle after a period of time without operation, in a convenient development of the invention, one trigger element is coordinated with the pressure-monitoring/pressure-supply device which, preferably timed, causes a revision of the static pressure. The static pressure is conveniently corrected when a reference pressure, deemed admissible, is fallen below. For the triggering, the appertaining electronic system always has to be activated only briefly, e.g., once per hour which as a consequence, will have only an unimportant consumption of energy. Should the pressure fall below the preset value determined, then the pump, or any other such device for producing the operating pressure, has to be operated until the required operating pressure is again produced. For the driver, corresponding information can be provided in case the leakage rate in the hydraulic system shows a significant error in the system which can be detected when, in the course of time, the pressure drop exceeds a specific value. 
   Should the possibility be ruled out that under action of the static pressure upon the release cylinder there generates a volume increase between primary and secondary pistons and thus a force action upon the release lever of the clutch which leads to a reduction of the transmissible torque and to a partial opening of the clutch then, in development of the invention, by way of control of a working valve, the primary piston can be displaced to the extent of shutting a bleeding hole communicating with the compensation volume. Due to the volume increase in the intermediate space, there also exists the danger that when a clutch is actuated, the diaphragm spring between the two pistons could be over-pressed and destroyed and that the clutch no longer closes again or closes with great delay. 
   When the clutch is closed, the primary piston of the release cylinder is positioned so that the bleeding hole of the primary piston is shut in order that no change of volume affects the liquid volume located between primary and secondary pistons. 
   To allow a wear compensation of the piston, the pressure control unit is briefly deactivated by a trigger switch in a preferred development of the invention so that the primary piston of the release cylinder, for a volume compensation, releases the bleeding hole to the space between the primary and the secondary pistons. The triggering ensures that an opening of the bleeding hole has not to occur upon each actuation of the clutch, but only on and off, and yet a sufficient wear compensation results and the above described danger is prevented. A triggered travel recording of the piston can ensure that the vehicle does not roll away. An adjustment of the clutch would be detected and the static pressure adapted. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described, by way of example, with reference to the accompanying drawings in which: 
       FIG. 1  is the basic construction of a hydraulic clutch actuation deice with a pneumatic intensifier; 
       FIG. 2  is a release cylinder of a clutch actuation according to  FIG. 1  with primary and secondary pistons; and 
       FIG. 3  is a graph of the release cylinder with symbolic tying of a pressure-monitoring/pressure-supply device and one pressure control unit. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A hydraulic clutch actuation device with pneumatic intensifier essentially consists, according to  FIG. 1 , of a master cylinder  3  provided with a compensation tank  1  and actuated by a clutch pedal  2 , one pressure intensifier  4  rear-mounted on the master cylinder, one release cylinder  5  acting upon the release mechanism of the clutch and hydraulic pipes  6  and  7  which create an impermeable connection between the master cylinder  3  and the pressure intensifier  4 , the same as between the pressure intensifier  4  and the release cylinder  5 . 
   The pressure intensifier  4  has one pneumatic area  9  which acts upon hydraulic area  8  and the adjoining hydraulic pipes  6 ,  7  and in an active state is loaded with a compressed gas provided by a compressor  10  via a pneumatic line  11 . 
   In  FIG. 2 , the release cylinder  5  is not shown in a controlled state. A housing  12 , basically designed of cylindrical shape, is provided with a stepped hole  13  one primary piston  14  with a seal ring  15  and one secondary piston  16  with a seal ring  17  are axially movably disposed. A spacing between the primary piston  14  and the secondary piston  16  and thus the practical possibility of creating a free space for an automatic wear compensation of parts of the clutch is achieved by a spiral spring  18  which limits the approximation of the secondary piston  16  to the primary piston  14  in case of pressure relief of the primary piston  14 . The secondary piston  16  acts upon a piston rod  19  which is passed into a recess of a front plate  20  of a release cylinder  5  and extends to the release lever  21  of the clutch actuation. For protection, the piston rod  19  is surrounded by bellows which by their terminal area sit upon the front plate  20  of the housing  12 . 
   On the end of the housing  12  remote from the piston rod  19 , the hydraulic pipe  7  is impermeably connected with a connecting area  22  of the housing  12 . One bleeding hole  23  provided in the connecting area  22  produces in the terminal side stop of the primary piston  14  a connection of the hydraulic pipe  7  with a compensation space  26  between the primary piston  14  and the secondary piston  16  so that the liquid in the compensation space  26  between the primary piston  14  and the secondary piston  16  communicates with the liquid provided via the hydraulic pipe  7 . The space between the piston  14  and  16  can be altered by altering the liquid volume enclosed between the primary piston  14  and the secondary piston  16 , thus achieving a wear compensation in the area of the clutch. 
   As shown in  FIG. 3 , the release cylinder  5  of the hydraulic clutch actuation is controlled by a working valve  24  designed as 2/2 directional seat valve which, in one of its shifting states, relays the hydraulic system pressure originating from a pressure reservoir  25  to the connecting area  22  of the release cylinder  5  after the primary piston  14  is axially displaced from its end position and shuts the bleeding hole  23 . By way of the liquid of constant volume in the compensation space  26  between primary piston  14  and second piston  16  there result the movement of the primary piston  14 , the same as of the secondary piston  16  and also the piston rod  19  whereby the release lever  21  is actuated and the clutch opened. 
   One pump  34  driven by a motor  29  conveys hydraulic liquid from a tank  27 , via a recoil valve  35 , in direction toward the pressure reservoir  25 . In order that the pressure is monitored in the system, a pressure-limiting valve  36  is provided which allows hydraulic liquid to flow back to the tank  27  should it be necessary. 
   When the clutch closes, the spring-loaded release lever  21  moves back to its original position in  FIG. 3  and, at the same time, produces an axial movement to the left of the secondary piston  16  of the enclosed hydraulic liquid and primary piston  14  until the latter has reached its final position in the housing  12  of the release cylinder  5 . A working valve  33 , designed as  2 / 2  directional seat valve, is in a shifting position here in which a volume return flow can result in the tank  27 . Adapted shifting positions of the working valves  24  and  33  additionally allow an oil volume compensation in an end position of the primary piston  14 , via the bleeding hole  23 , released by the primary piston  14  and thus an adaptation of the clutch actuation to the wear characteristics of the of the clutch. 
   If the vehicle is stopped and the motor is shut off, a pressure-monitoring/pressure-supply device  28  receives a corresponding information on the basis of which the monitoring/pressure-supply device  28  determining the system pressure and reducing it by a differential pressure so that when gear has been introduced and parking brake is open, the vehicle is prevented from rolling away even on inclined roads and a quick availability of the clutch actuation is nevertheless ensured from then on. 
   For this the pressure-monitoring/pressure-supply device  28  ensures that the pressure detected in the pressure-monitoring/pressure-supply device  28  from system and differential pressure is limited to a static pressure which, during action upon the secondary piston  16  and thus upon piston rod  19  and release lever  21 , cannot lead to a partial release of the clutch. Such can always be the case when, due to error in the system, for example, when the working valve  24  does not exactly close, a slow pressure buildup occurs on the release cylinder  5  which, when the bleeding hole  23  is open, would result in a volume increase of the liquid-filled compensation space  26  between primary piston  14  and secondary piston  16 . 
   Therefore, by adequate minimal control of a shifting element, such as the working valve  24 , by way of a pressure control unit  30  provided in the pressure-monitoring/pressure-supply device  28 , due to the pressure generating on the release cylinder  5 , the primary piston  14  is displaced in a direction of the “clutch open” state and thereby the bleeding hole  23  is kept closed while the clutch remains closed. Alternatively, the primary piston  14  in the “clutch closed” state could always be moved forward to the extent that the bleeding hole  23  is shut. Then, only after a clutch actuation, the primary piston  14  is briefly moved back up to the stop and the bleeding hole  23  opened in order to be subsequently brought again to the position in which the bleeding hole  23  is closed. Since the clutch wear per clutch operation is small, this method suffices to make a reliable wear compensation possible and shut the bleeding hole  23  when its operation is not needed. Thus an uncontrolled oil flow in the intermediate space between primary and secondary pistons  14 ,  16  is reliably prevented. 
   As one other alternative, the volume change between the primary piston  14  and the secondary piston  16  and therewith a wear compensation of the clutch can also result by a trigger switch  31  which, likewise, can also be situated in the pressure-monitoring/pressure-supply device  28  and, at certain intervals, briefly deactivates the shutting of the bleeding hole  23  by the primary piston  14  to enable a volume exchange to take place. 
   Besides, the pressure-monitoring/pressure-supply device  28  communicates with an incremental path sensor  32  by which the information relative to the covered path of the piston rod  19  is relayed as standard to the pressure-monitoring/pressure-supply device  28  for establishing the functional dependence of the static pressure on the axial displacement of the piston rod  19 . 
   The pressure control unit  30  can be designed so as to work independently of the pressure-monitoring/pressure-supply device  28 , thus being active when the clutch is closed under different operation conditions and also when the motor is active. 
   REFERENCE NUMERALS 
   
       
         1  compensation tank 
         2  clutch pedal 
         3  master cylinder 
         4  pressure intensifier 
         5  release cylinder 
         6  hydraulic pipe 
         7  hydraulic pipe 
         8  hydraulic area 
         9  pneumatic area 
         10  compressor 
         11  pneumatic pipe 
         12  housing 
         13  stepped hole 
         14  primary piston 
         15  seal ring 
         16  secondary piston 
         17  seal ring 
         18  spiral spring 
         19  piston rod 
         20  front plate 
         21  release lever 
         22  connecting area 
         23  bleeding hole 
         24  working valve 
         25  pressure reservoir 
         26  compensation space 
         27  tank 
         28  pressure-monitoring/pressure-supply device 
         29  motor 
         30  pressure control unit 
         31  trigger switch 
         32  path sensor 
         33  working valve 
         34  pump 
         35  recoil valve 
         36  pressure-limiting valve