Abstract:
A pressure control valve under the control of a pilot pressure generates a working pressure for a pressure-actuated apparatus. The valve has a control piston with two piston elements of different diameter. A working-pressure chamber is confined between the two piston elements, and a pilot-pressure chamber is confined between one of the control-piston elements and an additional piston. The additional piston takes a position dependent on a momentarily existing difference between the working pressure and the pilot pressure and, through an elastic force-coupler device, applies a position-dependent force to the control piston.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The invention relates to a pressure control valve that serves to generate a variable working pressure for actuating a device such as the start-up clutch of a motor vehicle, where the variation of the working pressure is effected by a controlling pressure, e.g., a pilot pressure, while the supply pressure delivered to the pressure control valve by a pressure source, for example a pump, remains constant.  
           [0002]    The functional dependency between the pilot pressure and the working pressure is subject to a diversity of requirements. For example, in the engagement phase of a start-up clutch, a change in the pilot pressure should at first cause only a small change in the working pressure in order to allow a precise control of the working pressure. However, with a continued increase in the pilot pressure, the working pressure should increase at a steeper rate to ensure that the clutch is securely engaged and works in a slip-free or at least substantially slip-free manner.  
           [0003]    This kind of a progressive pressure control characteristic can be realized, e.g., with the known pressure control valve according to DE 198 20 389 A1, which has a control piston coaxially surrounded by an outer piston with at least three radial openings. The pressure control valve has connector conduits, one of which introduces the working pressure to the control piston in such a manner that the control piston moves in opposition to the combined forces of a spring and the pilot pressure. A further conduit performs the function of applying the pilot pressure to the outer piston. When a certain threshold value of the pilot pressure is exceeded, the outer piston will move in opposition to the force of another spring, whereby the aforementioned spring force that opposes the movement of the control piston is increased. As a result, the gradient slope of the working pressure in function of the pilot pressure is increased, so that the pressure control valve has a correspondingly non-linear characteristic.  
           [0004]    The pressure control valve of the foregoing description suffers from the drawback of a relatively complex design and requires a comparatively complicated manufacturing process.  
         OBJECT OF THE INVENTION  
         [0005]    It is therefore the object of the present invention to provide a pressure control valve of a less complicated design, in which a desired functional relationship between pilot pressure and working pressure can be achieved through an appropriated choice of dimensions, in particular for providing the pressure control valve with a progressive characteristic where the working pressure as a function of the pilot pressure rises at a steeper gradient in that part of the characteristic where the pilot pressure is high than in the part where the pilot pressure is low.  
         SUMMARY OF THE INVENTION  
         [0006]    According to the invention, the foregoing objective is met by a pressure control valve with a working-pressure chamber and a pilot-pressure chamber. The working-pressure chamber receives a supply pressure from a pressure source by way of a supply-pressure port and a throttle passage and delivers a working pressure to a pressure-actuated apparatus by way of a working-pressure port. The pilot-pressure chamber receives a pilot pressure from a pilot-pressure control device. The working-pressure chamber is axially confined between two piston elements of a control piston, and the pilot-pressure chamber is axially confined between one of the control-piston elements and an additional piston. In the presence of a constant supply pressure, the working pressure generated at the working-pressure port varies as a function of the pilot pressure. The additional piston takes a position dependent on a momentarily existing difference between the working pressure and the pilot pressure and, through an elastic force-coupler device, applies a position-dependent force to the control piston. In addition, the valve has one or more drain ports.  
           [0007]    Preferably, the aforementioned force-coupler device is designed as a coupler spring arranged in such a manner that one end of the spring acts against the additional piston while the other end of the spring acts against the control piston.  
           [0008]    In a further developed embodiment of the invention, the additional piston is elastically biased into a rest position and connected to the working pressure in such a way that the additional piston will leave the rest position only when the working pressure exceeds the pilot pressure by a predetermined amount.  
           [0009]    In an embodiment that includes the aforementioned coupler spring as well as the elastic biasing of the additional piston, it is advantageous if the coupler spring is in a force-free state when the additional piston is in the rest position.  
           [0010]    The elastic biasing of the additional piston can be advantageously performed by a biasing spring.  
           [0011]    In preferred embodiments of the invention, the housing of the pressure control valve has a cylindrical interior space divided into three cylinder-bore sections of different respective diameters arranged in order of diameter size from smallest to largest. The first piston element of the control piston runs inside the first and narrowest cylinder-bore section, while the second piston element runs in the second, i.e. intermediate, cylinder-bore section and the additional piston runs in the third, i.e. widest, cylinder-bore section. As described above, the working-pressure chamber is confined between the first and second piston elements, and the pilot-pressure chamber is confined between the second piston element and the additional piston.  
           [0012]    With the arrangement of three cylinder-bore sections as just described, it is advantageous if the drain port is located in the intermediate cylinder-bore section.  
           [0013]    In design arrangements where the pressure control valve has three cylinder-bore sections, it is further advantageous if the supply-pressure port is arranged to enter the valve in the area of the first cylinder-bore section at a place occupied by the first piston element. The aforementioned throttle passage from the supply-pressure port to the working-pressure chamber can be designed as a narrow gap between the circumference of the first piston element and the inside cylinder wall of the first cylinder-bore section.  
           [0014]    In a further preferred embodiment, the pressure control valve according to the invention has a dead-end chamber on the side of the first piston element that faces away from the working-pressure chamber. The dead-end chamber is open to an additional drain port. The aforementioned narrow gap between the first piston element and the housing now throttles the fluid flow from the supply-pressure port in both directions, i.e., into the working-pressure chamber as well as into the dead-end chamber.  
           [0015]    The pressure control valve according to the present invention is distinguished by a comparatively simple design and by its adaptability to diverse requirements. By appropriate dimensioning of the two springs and of the effective pressure-bearing surfaces of the pistons, it is possible to achieve a desired valve characteristic, i.e., a desired functional relationship between the pilot pressure and the working pressure.  
           [0016]    The novel features that are considered as characteristic of the invention are set forth in particular in the appended claims. The improved apparatus itself, however, both as to its construction and its mode of operation, together with additional features and advantages thereof, will be best understood upon perusal of the following detailed description of certain presently preferred specific embodiments with reference to the accompanying drawing.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    In the following detailed description, examples of embodiments of the invention are discussed on the basis of the attached drawing in which  
         [0018]    [0018]FIG. 1 represents a schematic cross-sectional view of a pressure control valve according to the invention; and  
         [0019]    [0019]FIG. 2 represents an example of the functional dependency between the pilot pressure and the working pressure.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0020]    In accordance with FIG. 1, the pressure control valve has five essential components: a housing  2 , a control piston  4 , an additional piston  6 , a coupler spring  8 , and a biasing spring  10 .  
         [0021]    The housing overall encloses a cylindrical space with three coaxial cylinder-bore sections  12 ,  14 ,  16  lying adjacent to each other. The section  12  on the right of FIG. 1 has a smaller diameter, and the section  16  on the left has a larger diameter than the section  14  in the middle.  
         [0022]    The control piston  4  has a second piston element  18  guided in the bore section  14  with essentially fluid-tight wall contact. The second piston element  18  is connected by way of a shaft  20  to a first piston element  22  that moves with a small amount of wall clearance inside the bore section  12 . The additional piston  6 , whose overall shape resembles a hat, is guided in the bore section  16  with essentially fluid-tight wall contact.  
         [0023]    In the travel range of the first piston element  22 , the cylinder-bore section  12  has a supply-pressure port  26  receiving pressure medium, preferably hydraulic fluid, that is pressurized with a supply pressure p V . The pressure may be generated, e.g., by a pump delivering fluid at a high and preferably constant pressure level.  
         [0024]    The portion of the cylinder-bore section  16  between the additional piston  6  and the control piston  4  forms a pilot-pressure chamber  27  with a pilot-pressure port  28  receiving pressure medium at a pilot pressure p S . The pilot pressure p S  can be considerably smaller than the supply pressure.  
         [0025]    The portion of the bore section  14  between the piston elements  18  and  22  forms a working-pressure chamber  29 , where a working pressure p A  is generated and delivered by way of the working-pressure port  30  to a pressure-actuated apparatus such as a vehicle clutch. A connector conduit  32  leads from the working-pressure port  30  to the far side of the cylinder-bore section  16 , whereby the working pressure p A  is applied to the side of the additional piston  6  that faces away from the control piston  4 . The working-pressure chamber  29  is connected to the supply-pressure port  26  by way of a throttle passage  31  that is formed by the narrow gap between the first piston element  22  and the interior housing wall.  
         [0026]    A first drain port  34  exits from the bore section  14 . The aperture of the first drain port  34  is controlled by the position of the shutter edge  36  of the piston element  18 , so that the width of the passage from the working-pressure chamber  29  through the first drain port  34  is controlled by the position of the control piston  4 .  
         [0027]    To the right of the first piston element  22  in the representation of FIG. 1, the valve housing has a dead-end chamber  38  with a second drain port  40 . The dead-end chamber is connected to the supply port  26  by way of a throttle passage  42 .  
         [0028]    The biasing spring  10 , which is seated against the step in the interior housing wall between the bore sections  14  and  16 , is designed and dimensioned to urge the additional piston  6  with a biasing force to its home position on the left side of FIG. 1, where the additional piston  6  will rest against the end wall of the cylinder section  16 , although FIG. 1 does not show the additional piston  6  in its home position.  
         [0029]    The coupler spring  8 , which is seated between the control piston  4  and the additional piston  6  is designed and dimensioned to be essentially force-neutral when the additional piston  6  is in the home position described above.  
         [0030]    The pressure control valve works as follows:  
         [0031]    The relationship between the working pressure p A  and the pilot pressure p S  is expressed through the equation  
         p A (A 2 −A 1 )=p S  A 2   (1)  
         [0032]    where A 2  represents the effective aspect area of the second piston element  18  (equal to the cross-sectional area of the bore section  14 ) and A 1  represents the effective aspect area of the first piston element  22  (equal to the cross-sectional area of the bore section  12 ), with the assumption that the cross-sectional area of the shaft  20  has a negligible influence in comparison to the cross-sectional areas of the piston elements  18  and  22 . The equilibrium represented by equation (1) is maintained by the cooperation between the shutter edge  36  and the first drain port  34  with very small movements of the control piston  4 . Thus, the relationship between working pressure and pilot pressure is represented by the equation  
         p A =p S  k 1   (2)  
         [0033]    where k 1 =A 2 /(A 2 −A 1 )  
         [0034]    The foregoing equations (1) and (2) remain valid as long as the additional piston  6  is in its home position, i.e., as long as  
             p   s     &lt;     p   s   *       =     F       A   3          (       k   1     -   1     )           ,                         
 
         [0035]    wherein F represents the biasing force of the biasing spring, A 3  represents the cross-sectional area of the additional piston  6 , and p S * represents the threshold value of the pilot pressure p S  at which the additional piston will overcome the biasing spring force F.  
         [0036]    When the pressure difference p A −p S  produces a resultant force on the additional piston  6  that is greater than the biasing force of the biasing spring  10 , the additional piston  6  is pushed more and more to the right, i.e., to a position as illustrated in FIG. 1. This causes the coupler spring  8  to apply an additional force to the control piston  4 , which will have the same effect as if an additional amount of pilot pressure were acting on the control piston  4 . As a result, the slope of p A  as a function of p S  becomes steeper.  
         [0037]    When the additional piston is free from its rest position, the equilibrium of forces on the additional piston  6  and the control piston  4 , respectively, is represented by the equations  
         (p A −p S )A 3=F+XC   1 +XC 2 ;  
         [0038]    and  
         p A (A 2 −A 1 )=p S A 2 +XC 2 ,  
         [0039]    wherein C 1  represents the spring constant of the biasing spring  10 , C 2  represents the spring constant of the coupler spring  8 , and X represents the decrease in the distance between the additional piston  6  and the control piston  4  (which is approximately equal to the displacement of the additional piston  6 ).  
         [0040]    The preceding equations can be solved for p A  in terms of p S  and p S *, with the result  
         p A =p S *k 1 +(p S −p S *)k 2 ,  
         [0041]    where k 2  is a constant. The last equation is illustrated by the graph of FIG. 2.  
         [0042]    When the pilot pressure p S  is smaller than p S *, the relationship between p A  and p S  is represented by the linear equation (2). When the pilot pressure p S  reaches the threshold level p S *, i.e., at the point where the biasing force of the biasing spring  10  is matched by the resultant of the pressure forces on the additional piston, the coupler spring  8  begins to have an increasing effect and as a consequence, p A  begins to increase at a steeper rate.  
         [0043]    By selecting appropriate dimensions and force-displacement characteristics for the springs  8  and  10 , it is possible to achieve a functional relationship between p A  and p S  where, after a transitional range around p S * (in FIG. 2), the graph continues to the right at a linear rate of increase. Thus, the pressure control valve can be designed to essentially have a characteristic with two linear parts of different slope angle.  
         [0044]    The dead-end chamber  38  allows the control piston  4  to move with minimal friction.  
         [0045]    As is self-evident, the pressure control valve of the foregoing description can be modified in any number of ways. As an example, the connector conduit  32  could be integrated in the housing. Depending on different requirements, the diameter sizes of the cylinder-bore sections could be arranged in different order. For example, the requirements could call for the diameter sizes to increase from left to right in FIG. 1, or for the bore-sections to have equal diameters.  
         [0046]    The throttle passages  31  and  42  can be designed with a small flow-passage cross-section, so that the pressure control valve works with a low level of leakage losses.  
         [0047]    Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic and specific aspects of the aforedescribed contribution to the art and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the appended claims.