Abstract:
A pressure-medium actuated clutch that includes a plurality of clutch disks and a piston slidably positioned for movement toward and away from the disk to selectively engage and to disengage the clutch. A pressure chamber is provided adjacent to the piston and communicates with a source of pressurized fluid. A pilot-pressure-operated control valve is provided to control the pressure of the pressurized fluid, and a flow constriction is provided in a conduit extending from the control valve to the pressure chamber. A method for actuating the clutch is also disclosed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates a pressure-medium-operated clutch, especially for the drive train of a motor vehicle. Additionally, the invention also relates to a method for operating such a clutch. 
     2. Description of the Related Art 
     Pressure-medium-operated clutches can be used, for example, as drive components for an automatic-transmission-equipped motor vehicle, such as is described in German Patent DE 198 00 490. The automatic transmission can be a multiple-stage automatic transmission or it can be a continuously variable transmission (CVT), such as a belt-driven, conical pulley transmission. Such a clutch can also be utilized in a manually-operated transmission. 
     Pressure-medium-operated clutches have the disadvantage that, beginning at an unpressurized state, a comparatively long time elapses before torque is transmitted, because the clutch-activation cylinder must first be filled to cause engagement. During that time period critical vehicle or safety conditions can arise. 
     A purpose of the present invention is to provide a clutch and a method to activate the clutch in order to provide improved safety. 
     The present invention solves the problems by means having the advantageous features hereinafter described. 
     SUMMARY OF THE INVENTION 
     Briefly stated, in accordance with one aspect of the present invention, a pressure-medium-actuated clutch is provided that includes an annular pressure plate slidably supported for movement toward and away from a plurality of clutch disks. The pressure plate includes a piston surface that is spring biased toward a disengaged condition of the clutch. A pressure chamber is formed adjacent to and facing the piston surface, wherein the pressure chamber is adapted to receive pressurized fluid to cause the piston surface and pressure plate to move toward an engaged condition of the clutch. A pressure-medium control system includes at least one pilot-pressure-controlled valve, wherein at the beginning of actuation with a substantially unpressurized clutch pressure chamber the chamber is filled more quickly at higher pressure. A control means is provided that is operable upon reaching one of a desired pressure in the pressure chamber and a predetermined time interval to fill the pressure chamber with fluid more slowly. 
     In accordance with another aspect of the present invention a method is provided for operating a pressure-medium-operated clutch, the method including providing a pressure-reducing valve between a clutch pressure chamber and a source of pressurized fluid. A pilot pressure applied to a valve member carried within the pressure-reducing valve is controlled to provide a desired clutch-operating pressure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is further described herein in relation to the following drawings, in which: 
     FIG. 1 is a cross-sectional view of a clutch in accordance with the present invention; 
     FIG. 2 is an enlarged view of detail “B” of the multiple-disk clutch shown in FIG. 1; and 
     FIG. 3 shows an hydraulic system for operating the clutch. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a cross-sectional view of an embodiment of a multiple-disk clutch  1 . At the input side of multiple-disk clutch  1  a shaft  2  (only partially shown in FIG. 1) is provided that is also connected with a set of planetary gears (not shown) or with a set of conical drive elements (not shown). Shaft  2  is rotatably supported in a housing by means of a first roller bearing  3  and by a second roller bearing (not shown). FIG. 1 shows only cover  4  of the housing 
     The-multiple-disk clutch can be provided with a further, reverse clutch that in conformity with German patent publication DE 198 00 490 can be radially positioned outside clutch  1 . The invention further relates to a clutch as disclosed in the earlier German patent publication DE 19 800 490, the entire contents of which is hereby expressly incorporated herein by reference to the same extent as if fully rewritten. 
     Supported on shaft  2  and axially adjacent to roller bearing  3  is an annular flange  5  that carries at its outer periphery a sheet metal component  6  that has a U-shaped cross section. Reference numeral  7  designates a ball that is carried in an axial slot to serve as a positive drive and to allow an inner disk carrier  8  to move axially, but not radially, relative to a pressure plate  9 . Pressure plate  9  can also be referred to as a piston. 
     The outer region of the multiple-disk clutch is surrounded by a clutch casing  10  that encloses inner disks  11  and outer disks  12 . Casing  10  also forms an outer disk carrier  13  that is positioned radially outwardly of outer disks  12  and that includes axially-extending teeth  14 . In a similar manner, inner disks  11  are carried on axially-extending external teeth  15  of inner disk carrier  8  for axial movement therealong. 
     An annular hub  16  carries an engagement surface  17  in such a manner that inner disks  11  and outer disks  12 , which are arranged between a pair of end disks  18 , can be driven between engagement surface  17  and a corresponding engagement surface  19  on pressure plate  9  for purposes of torque transmission upon the application of a perpendicular force. 
     Illustrated in FIG. 1 by reference numeral  20  is an oil partition, which at its radially outermost edge overlaps the outer edge of a plate spring  21 . A thrust ring  22  carried on the outer circumference of shaft  2  serves as an axial support for an adjacent membrane  23  that is shown more clearly in FIG.  2 . 
     Pressurized hydraulic fluid can be supplied from a pump (not shown) through a conduit  24  that is either positioned within or that is formed in shaft  2 . After passing through conduit  24 , the hydraulic fluid reaches an end area  25  in shaft  2 , from which a radial conduit  26  branches. The pressurized hydraulic fluid flows through conduit  26  and into a first pressure chamber  27  bounded by annular flange  5  and membrane  23 , and when within chamber  27  the fluid exerts pressure against membrane  23 . The surface of membrane  23  that faces in the opposite direction from first pressure chamber  27  rests against a piston  28 , which faces the active face of plate spring  21 , against which piston lies an annular seal  29  which, in turn, presses against the active face of plate spring  21 . 
     First pressure chamber  27  together with pressure plate  9  forms a piston/cylinder unit to actuate the clutch. A pressure increase in first pressure chamber  27 , caused by the pressurized fluid introduced into chamber  27 , causes membrane  23  and piston  28  to move to the right, as viewed in the plane of the drawing, against the biasing force of plate spring  21 . Piston  28  is integral with pressure plate  9  so that when first pressure chamber  27  is initially pressurized, there is also a displacement toward the right, when viewed in the plane of the drawing, of engagement surface  19  of pressure plate  9 , and consequently disks  11 ,  12 , and  18  come into contacting engagement. Because the structure of membrane  23  includes two circular troughs  30 , similar to rolled creases, and because membrane  30  is a thin element, pressurizing first pressure chamber  27  results in a substantially friction-free and consequently a hysteresis-free actuation of piston  28 . 
     As a result, even a slight change in the pressure in first pressure chamber  27 , acting on membrane  23 , provides a substantially friction-free, axial displacement of piston  28 , and thereby of engagement surface  19  of pressure plate  9  and of disks  11 ,  12 , and  18 . It should be noted in this respect that multiple-disk clutch  1  is shown in FIG. 2 as open, and therefore engagement surface  19  has not yet contacted end disk  18 . It is evident that upon a further increase in the pressure within first pressure chamber  27  there will be a corresponding displacement of piston  28 , and hence of engagement surface  19 , causing a corresponding perpendicular reaction force between inner disks  11  and outer disks  12 . A slight pressure change within first pressure chamber  27  for the purpose of regulating the torque transmitted by multiple-disk clutch  1 , within the range of regulation of about 1/10 the possible path of displacement of disks  11 ,  12 , and  18 , results in a commensurately small axial displacement of the disks along axially-extending teeth  14 ,  15  of disk carriers  8 ,  13 . 
     When a slight change in the torque transmitted by multiple-disk clutch  1  is desired, it can be implemented by means of a control for slightly changing the pressure of the pressurized fluid fed into first pressure chamber  27 , whereby a corresponding slight axial displacement of inner disks  11  and outer disks  12  is carried out by slight movement of membrane  23 , piston  28 , pressure plate  9  and engagement surface  19 . As a result, inner disks  11  and outer disks  12 , by virtue of complementary spring tongues  31 ,  32  that engage with respective axially-extending teeth  14 ,  15 , non-rotatably engage inner disk carrier  8  and outer disk carrier  13 . The slight pressure change in first pressure chamber  27  and the resulting slight axial displacement of the disks results in a change in the normal force between the disks, without spring tongues  31 ,  32  that engage axially-extending teeth  14 ,  15  being displaced in the direction of the teeth, so that a desired, slight change in torque no longer causes relative displacement between the side flanks  33  of spring tongues  31 ,  32  and axially-extending teeth  14 ,  15 . 
     In operation, multiple-disk clutch  1  is subjected to rotary motion, so that pressure fluid that is introduced into pressure chamber  27  through conduit  24  is likewise subjected to rotary motion. As a result of the circulation of the pressure fluid, from which circulation first pressure chamber  27  is supplied, fluid can similarly be supplied to fluid chamber  37 , through a conduit  35  within the output shaft and that is coaxial with conduit  24 , and through a conduit  36  connected with conduit  35 . Fluid is introduced through an opening  38  in the radially-inwardly-lying portion of oil divider  20  as well as through an opening  39  in hub  16 , so that the fluid can arrive in the area of disks  11 ,  12 ,  18  and can be introduced to provide cooling because of the heat resulting from the frictional engagement between the disks. 
     The oil flow takes place in such a way that the oil that flows through bore  36  as a result of clutch engagement can be guided to multiple-disk clutch  1 , or upon disengagement of the clutch it can be guided to a radially-outwardly-lying reverse-movement clutch or diverted. Correspondingly, based upon the position of pressure plate  9  the fluid is led in the direction of arrow  51  through bores in portions  20  and  16  and  13 . Portion  20  thereby carries a relatively small bore. 
     With pressure plate  9 ,  28  in engagement with the clutch disks, a radially inward passageway is opened between pressure plate  28  and thrust ring  22 , as a result of which the passageway is further enlarged radially outwardly and is larger than bore  38  in portion  20 . Thus the fluid flows in the direction of arrow  50  into clutch  1 . 
     In FIG. 3 the clutch is identified by reference numeral  120 . Conventionally, clutch  120  includes a set of disks  121  and a piston  122  that is movable inwardly in a chamber  123  within a cylinder  124  by means of hydraulic pressure, to engage the clutch in such a way that piston  122 , upon enlargement of chamber  123 , presses the disks of the set of disks  121  against a corresponding friction lining. The higher the pressure within chamber  123 , the larger the coupling torque transferred by driving clutch  120 . 
     Chamber  123  is connected through a hydraulic conduit  125  with a manually-movable valve  130  which, for safety, abruptly empties chamber  123  when switching into to the P and N positions. In the D position conduit  125  communicates with conduit  131  which, in turn, communicates with a pressure-reducing valve  140  positioned after manually-movable valve  130 . The pressure-reducing valve includes an inlet E 1  to receive pilot pressure and an inlet E 2  to receive the full system pressure provided by a pump  160  through a conduit  162 , as well as an inlet E 3  described further below, and an outlet A connected to conduit  131  to provide the clutch operating pressure. 
     Inlet E 1  communicates through a conduit  151  with an electromagnetic valve  150 , such as a control valve or a proportional valve, which is controlled by a control unit (not shown) for regulating the level of current flowing in the valve&#39;s magnet in order to set the pilot pressure at inlet E 1 . The pressure produced by pump  160  is applied through conduit  161  to a pressure-reducing valve  170  that provides a constant pressure of, for example, 5 bar to conduit  163 , which pressure is utilized by control valve  150  to provide, according to its setting, the pilot pressure at inlet E 1 . 
     Control valve  150  produces a downstream flow that is dependent upon the pressure provided by pressure reducing valve  170  (for example 5 bar) to control the pilot pressure (for example 0 to 5 bar) at inlet E 1 . The flow is used as a set point for the clutch pressure, preferably using software and by means of parameters such as, for example, throttle-valve position, etc. Depending on the plunger position of control valve  150 , the pressure from pressure reducing valve  170  present at inlet E 1  is maintained constant by fluid discharge through the outlet to sump  152  so that the pilot pressure corresponds with the set point. 
     Pressure-reducing valve  140  includes a valve spool  141 . In accordance with the invention, spool  141  is biased toward the right, as viewed in FIG. 3, by a spring  142 , preferably in the form of a coil spring. Spring  142  presses against an end face S 1  of spool  141 . As a result, and before valve spool  141  reaches its end position, control edge  144  of valve spool  141  is shifted toward inlet E 2  so that the system pressure is communicated from inlet E 2  to outlet A and passes through a feedback branch  145  to inlet E 3 , where it exerts on control surface S 2  a pressure opposite to the force of spring  142 , whereby valve spool  141  is shifted so far to the left until communication between control edge  144  and inlet E 2  again is interrupted. The pressure acting on clutch  120  is therefore related to the combination of the pressure from spring  142  plus the pilot pressure at inlet E 1 . 
     The method of operation of the described control arrangement will now be explained in further detail. The clutch pressure need not be controlled very precisely during the time chamber  123  of drive clutch  120  is being pre-filled or emptied. No torque is being transmitted at those times. Hence pressure-reducing valve  140  is only biased by spring  142  so that the minimal clutch pressure in chamber  123  corresponds with the pressure exerted by plate spring  21 . In that case the pilot pressure set by control valve  150  is zero. 
     During normal vehicle operation, by appropriate operation of control valve  150  to engage drive clutch  120 , the pilot pressure at inlet E 1  is raised from zero in such a way that spool  141  in pressure-reducing valve  140  is shifted in such a manner that the full system pressure applied to inlet E 2  consistent with the pressure exerted by spring  142  plus the pilot pressure, is transmitted to clutch  120 . As soon as the desired clutch pressure has been attained, the force applied to control surface S 2  through feedback branch  145  interrupts the connection between inlet E 2  and outlet A. In that way the pilot pressure can immediately be used to transmit a torque. When engaging the clutch or accelerating, no time will be wasted, as would be needed to fill space  123  using the state-of-the-art apparatus. 
     Also during normal vehicle operation, because of the effect of spring  142  the clutch pressure is decreased to the pressure needed to transfer a creeping torque (creeping pressure) due to the effect of spring  142 , which is in the range of the biasing force of the plate spring. 
     In order to adjust the torque to be transmitted by the clutch, an estimated torque is selected. The pressure through proportional valve  150  is adjusted according to a characteristic curve stored in a computer unit. For that purpose the proportional valve is provided with a current i to operate the valve to set the nominal pressure. An objective of the invention is to set, for a first time interval, a current that is substantially higher than that necessary to provide the input torque. As a result, the area of conduit  125  upstream of constriction  199  is subjected to a higher pressure and thus chamber  123  fills more quickly. When the nominal pressure in chamber  123  is at least nearly reached, the current is switched back to the value i corresponding with the nominal pressure to control proportional valve  150 . This switchover to the lower current also can take place after a given time interval has elapsed. 
     The current setting to rapidly fill the cylinder can be determined by various parameters such as temperature, angular speeds of motor and gears, vehicle speed, or throttle-valve position. 
     The clutch pressure can be detected by a pressure sensor. 
     The claims included in the application are illustrative and are without prejudice to acquiring wider patent protection. The applicant reserves the right to claim additional 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 renunciation to independent, objective protection for the features of the related dependent claims. 
     The objects of the dependent claims also constitute independent inventions comprising a structure independent of the object of the previous dependent claims. 
     The invention is also not restricted to the description of the illustrative embodiments. On the contrary, many changes and variations are possible within the scope of the invention, in particular such variants, elements, and combinations and/or materials which, for example, are inventive by combining or modifying features, or elements, or method steps described individually in relation to the general specification and embodiments and claims and shown in the drawings, and which by means of combined features lead to a new object or new method steps or sequences of method steps, as well as manufacturing, testing and operational procedures.