Abstract:
A piston for the variable delimitation of a pressure chamber in a housing of a hydraulic cylinder, in particular a clutch master cylinder for motor vehicles, has a main section on which a running surface is provided for a sealing element on the housing side. The sealing element seals a pressure chamber in an operating position of the piston. The piston also has an after-running device, which, in a normal position of the piston, connects the pressure chamber to an after-running area. The after-running device is made separately from the main section and is connected without play to the main section to form the piston. As a result, a piston that is easy to manufacture is created and which, with reference to the after-running device, has an improved functionality compared with prior art.

Description:
DESCRIPTION OF THE PRIOR ART  
         [0001]    The present invention relates to a piston for the variable delimitation of a pressure chamber in a housing of a hydraulic cylinder. In particular, the invention relates to a piston for master cylinders of hydraulic clutch actuating or brake systems in motor vehicles, as used extensively in the automotive industry.  
           [0002]    These pistons are used to generate a pressure in the pressure chamber with an axial relative displacement in relation to the housing of the hydraulic cylinder. This pressure is possibly applied to a clutch slave cylinder hydraulically connected to the hydraulic cylinder, which is actively connected to the clutch to disengage a clutch. The piston considered here, also known as a plunger piston because of its design, has a main part with a running surface for (at least) one sealing element and an after-running device. The sealing element is attached to the housing of the hydraulic cylinder and serves to seal the pressure chamber in an operating position of the piston, i.e. with a piston displaced in the direction of the pressure chamber, in co-operation with the running surface. In a normal position, i.e. a position of the piston drawn to a stop, the after-running device connects the pressure chamber to an after-running area which, in turn, is connected to an after-running tank.  
           [0003]    Prior art does not lack proposals on how the after-running device should be designed. For example, a piston is known which consists entirely of plastic for economic reasons (DE 38 16 608 A1), whose end on the pressure chamber side is provided with slots running in the longitudinal direction, which form the after-running device in a simple manner. However, this type has the disadvantage that with a relative displacement between plastic piston and sealing element, a noise is generated, which is undesirable in the automotive industry, which appears to be caused by the surface structure of the plastic.  
           [0004]    Therefore, it has been suggested that to make the piston, a plastic body is covered with a thin-walled metal tube, at least in the area of the running surface (DE 37 13 248 C2) or is provided with a piston shank sleeve shaped from metal (see the generic DE 195 23 215 A1 for example), which has a closed floor on the pressure chamber side. In the case of these pistons, the after-running device is formed by sniffer grooves extending in the axial direction, which are incorporated in the surface of the piston shank sleeve on the end of the piston shank sleeve facing the pressure chamber. This is normally done without machining, i.e. using an embossing process.  
           [0005]    An embossing process of this type does represent an economic production method, but it is also associated with disadvantages. For example, a sharp, dimensionally precise outlet of the sniffer grooves distributed on the perimeter cannot be guaranteed. Consequently, the grooves may have a different length. So that this does not affect the function of the after-running device when the piston is in its normal position, the sealing element has to be positioned in the housing of the hydraulic cylinder with very big tolerances. However, this means that the piston has to travel longer distances before pressure can be built up in the pressure chamber, which ultimately leads to an undesirable loss of pedal stroke. There is also the fact that as the result of the embossing process, an anti-corrosion surface coating applied to the piston shank sleeve may be damaged and detached, which leads to undesirable leakages in operation sooner or later. The same applies to metal-coated surfaces of plunger pistons otherwise made 100% in plastic.  
           [0006]    Finally, pistons are known which are made from a solid material, like an aluminum alloy, the equalisation grooves provided on the end on the pressure chamber side being made by groove milling cutters. However, the equalisation grooves made in this way require considerable deburring to prevent damage to the sealing element in operation. As in the case of the piston designs described above, there is also the risk that the running surface of the piston is damaged if the piston has to be held or clamped to make the equalisation grooves.  
           [0007]    The object of the invention is to create a piston for hydraulic cylinders that is easy to make and which, with reference to the after-running device, has an improved functionality compared with the prior art described.  
         SUMMARY OF THE INVENTION  
         [0008]    According to the present invention, there is provided a piston for the variable delimitation of a pressure chamber in a housing of a hydraulic cylinder, in particular of a clutch master cylinder for motor vehicles, the piston having an operating position and including a main part, on which a running surface is provided for a sealing element on a housing side, which, in the operating position of the piston, seals the pressure chamber, and the piston further including an after-running device which, in a normal position of the piston, connects the pressure chamber to an after-running area; wherein the after-running device is made separately from the main part and is connected to the main part without play to form the piston.  
           [0009]    Through this two-part design of the piston, the after-running device can be made with small tolerances in a simple manner without the risk of damaging the running surface provided on the main part of the piston and having to be after-worked before the after-running device is connected to the main part of the piston and therefore without creating further dimensional differences. As a result, the sealing element can be positioned in the housing of the hydraulic cylinder advantageously with smaller tolerances and the hydraulic cylinder is thereby improved with reference to pedal stroke losses. A further advantage of the two-part piston design is that it allows economic modular solutions. For example, it is possible to use the same after-running device on main parts of different lengths in order to make pistons which allow a stroke corresponding to the requirements concerned.  
           [0010]    In one advantageously simple embodiment of the piston, the after-running device can be made as an annular part with an essentially U-shaped cross-section. The play-free connection between the after-running device and the main part is preferably made using a clip connection, which allows an easy assembly of the piston.  
           [0011]    If the outside diameter of the running surface is slightly bigger than the outside diameter of the after-running device, and a sloping transition section is provided on the main part between the running surface and the after-running device, the sealing lip of the sealing element in the normal position of the piston is advantageously relieved slightly in contact with the after-running device, whereas with a movement of the piston from the normal position to an operating position via the transition section, the sealing lip is carefully expanded.  
           [0012]    In a preferred embodiment of the piston, the after-running device has a radially outer annular section and a radially inner annular section, which are connected to each other via an annular disc section on the end. In this case, the outer annular section of the after-running device can have a cylindrical outside perimeter surface on which the sealing element rests in the normal position of the piston and which is provided with several equalisation grooves distributed over the perimeter, which extend from the free end of the outer annular section in the axial direction in order to ensure, in the normal position of the piston, the connection between the pressure chamber and the after-running area under the sealing element or its sealing lip. Preferably, the equalisation grooves on the outside perimeter surface of the outer annular section extend over the entire length of the outside perimeter surface, which allows easy manufacture, among other things.  
           [0013]    In addition, the outer annular section of the outer running device can have a cylindrical inside perimeter surface, by means of which the after-running device radially centers in an advantageously simple manner on a centering shoulder of the main part and which is provided with several equalisation grooves distributed over the perimeter, which extend from the free end of the outer annular section in the axial direction and whose axial length is greater than the width of the centering shoulder in order to allow a hydraulic connection over the centering shoulder. In addition, the free end of the outer annular section of the after-running device forms an annular shoulder, as described in patent claim  10 , with which the after-running device is supported without play on the main section in the axial direction in a simple manner and which is provided with several connecting grooves which run in the radial direction. The connecting grooves on the annular shoulder can connect the equalisation grooves on the outside perimeter surface to the equalisation grooves on the inside perimeter surface of the outer annual section. The annular disc section of the after-running device can be provided with at least one connecting duct extending in the axial direction.  
           [0014]    It is evident that according to the embodiment of the after-running device described above, a connection between the pressure chamber and the after-running area can not only be achieved via an outer area of the after-running device if the piston is in the normal position, but also advantageously via an inner area of the pressure chamber via the connecting duct in the annular disc section, the equalisation grooves on the inside perimeter surface of the outer annular section and the connecting grooves on the annular shoulder of the outer annular section. Through this embodiment of the after-running device, the undesirable “residual pressure build” can be avoided in the pressure chamber in a simple and reliable manner.  
           [0015]    If the position of the sealing element as already discussed above is improved with reference to pedal stroke losses, i.e. the stroke, also referred to as the over-running or valve closing stroke, which the piston has to cover starting from the normal position until the sealing element with its sealing lip is released from the after-running device to separate the pressure chamber and the after-running area, is minimised, there is basically the risk of a “residual pressure build” in the pressure chamber. This then means that the sealing lip of the sealing element pressed against the running surface of the main section in an operating position of the piston because of the pressure in the pressure chamber, particularly at high temperatures of the hydraulic fluid, may be pressed against the counter surface in the normal position of the piston, too, and therefore (partly) closes the passage to the after-running area with a minimised over-running stroke. As a result, a residual pressure in the pressure chamber is not reduced, or only with a time lag, in the direction of the after-running area after the piston returns to the normal position. In the case of hydraulic clutch operations, this can mean that the clutch abrades with excessive wear of the clutch lining or only a reduced torque is transmitted.  
           [0016]    The connection described above between the pressure chamber and the after-running area via the inner area of the after-running device also provides a remedy here in a simple manner. Depending on the design and position of the sealing lip of the sealing element, and as a function of the residual pressure in the pressure chamber, a pressure equalisation is created via this inner connection in relation to the area of the compressed sealing lip facing away from the pressure chamber, thereby creating a partial pressure equalisation at the sealing lip or lifting the pressed sealing lip away from the counter surface, so that a pressure equalisation in relation to the after-running area is allowed as a result. In other words, through the inner connection created with the after-running device, in the normal position of the piston a hydraulic pre-tension of the sealing lip at the sealing element can be reliably eliminated or avoided.  
           [0017]    The inner annular section of the after-running device can be slotted several times to form spring tabs, each of the spring tabs having a lug on the end projecting radially inwards, which can be engaged with a radial groove made in a fixing shoulder of the main section. Therefore, the above clip connection is created in a simple manner. Preferably, in this case, the lug is provided with a sloping surface on its side facing the pressure chamber, which excludes an otherwise undesirable axial play that might exist.  
           [0018]    The after-running device can be made in an advantageous way as a one-piece plastic injection molding, which is not only cost-effective, but also guarantees the functionally desirable small tolerances in a simple manner and without the need for reworking.  
           [0019]    Finally, the main part can be a solid body of preferably an NF metal to which a tubular sleeve forming the running surface or a coating, is applied. However, it is also possible for the main part to be an essentially pot-shaped body preferably made from sheet steel, which, if appropriate, surrounds a lining, preferably in plastic. As a result, the known running surface designs, optimised from the point of view of noise behaviour and economics, can be maintained. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    The invention is now described in more detail below on the basis of a preferred embodiment, with reference to the attached drawing, where:  
         [0021]    [0021]FIG. 1 shows a longitudinal section view of a hydraulic cylinder with a multi-part piston according to the invention in its normal position, which has a main part and an after-running device connected to it without play,  
         [0022]    [0022]FIG. 2 an enlarged, truncated longitudinal section view of the hydraulic cylinder compared with FIG. 1 in the area at the sealing elements which, for the sake of simplicity, are shown in the undeformed state,  
         [0023]    [0023]FIG. 3 a perspective representation of the after-running device according to FIG. 2 and  
         [0024]    [0024]FIG. 4 an enlarged section view of the after-running device according to FIG. 2, only the upper half of the rotation symmetrical after-running device being illustrated. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]    [0025]FIGS. 1 and 2 show a piston  10 , a so-called plunger piston to be more precise, which, in a housing  12  of a clutch master cylinder illustrated as an example of a hydraulic cylinder  14 , variably delimits a pressure chamber  16 . The piston  10  has a main part  18 , in the embodiment illustrated, in the form of a solid body consisting of a non-ferrous metal, possibly an aluminum alloy, on which is provided a running surface  20  for a primary sealing element  22  on the housing side. In an operating position of the piston  10  that is not illustrated, in which it is located above the running surface  20 , the primary sealing element  22  seals the pressure chamber  16  in relation to an after-running area  24 , so that a pressure can be built up in the pressure chamber  16  as the result of a stroke of the piston  10  to the left in FIG. 1. The piston  10  also has an after-running device  26 , described in more detail below, which, in the normal position of the piston  10  shown in FIG. 1 and  2 , connects the pressure chamber  16  to the after-running area  24  in order to create a pressure equalisation between the pressure chamber  16  and the after-running area  24 , allow an after-running of hydraulic fluid from the after-running area  24  into the pressure chamber  16  and, if necessary, facilitate the escape of air from the pressure chamber  16  via the after-running area  24 . The essential thing is that the after-running device  26  illustrated in detail in FIG. 3 and  4  is made separately from the main part  18  and is connected to the main part  18  without play to form the piston  10 , as is described in more detail below.  
         [0026]    Starting from its right-hand end shown in FIG. 1, the housing  12 , consisting of plastic, has a graduated blind hole  28 , on whose left-hand end in FIG. 1 a pressure connection  30  is provided, via which the hydraulic cylinder  14  can be connected to a slave cylinder (not illustrated). The housing  12  is also essentially provided in the center with a fixing flange  32  and an after-running connection  34 , via which the hydraulic cylinder  14  can be connected to an after-running tank (not illustrated). The after-running connection  34  has an after-running duct  36  which opens into the blind hole  28  in the after-running area  24 .  
         [0027]    The blind hole  28  of the housing  12  essentially forms four function sections with a diameter reducing from right to left in FIG. 1. Starting from its right-hand end in FIG. 1, the blind hole  28  actually has a first cylindrical section  38 , which changes to a second cylindrical section  42  via a small annular shoulder and a conical transition section  40  which is followed by a third cylindrical section  44  via a bigger annular shoulder.  
         [0028]    A guide sleeve  46  for the piston  10  is secured in the first cylindrical section  38  of the blind hole  28 . Between the guide sleeve  46  and the housing  12 , the hydraulic cylinder  14  is sealed in relation to the atmosphere by means of a static seal in the form of an O-ring  48 . On its right-hand end in FIG. 1, the guide sleeve  46  forms a stop  50  for the piston  10  which prevents the piston  10  from pulling out of the housing  12 . On its left-hand end in FIG. 1, the guide sleeve  46  has a shoulder  52  on the inside perimeter which is used to accommodate a secondary sealing element  54 , the dynamic sealing lip of which is permanently in contact with the running surface  20  in order to seal the outer running area  24  in relation to the atmosphere and environment.  
         [0029]    The conical transition section  40  of the blind hole  28  is used to center a supporting ring  56  for the primary sealing element  22  whose right-hand end in FIG. 1 and  2  rests on an annular shaped end of the guide sleeve  46  and is thereby fixed in the axial direction of the housing  12 . Because of the conical seat of the supporting ring  56  on the conical transition section  40  of the blind hole  28 , an annular gap  58  of a defined width occurs between the inside perimeter surface of the supporting ring  56  and the running surface  20  of the piston  10 , as can be seen from FIG. 2. The annular gap  58  communicates directly with the after-running area  24 , which is connected to the after-running duct  36  via grooves  60 , which extend initially in a radial direction and then in an axial direction over the end and the outside perimeter of the supporting ring  56 . In this case, the grooves  60  also extend through an annular shoulder  62  of the supporting ring  56 , which makes sure that the secondary sealing element  54  cannot interrupt the hydraulic connection between the after-running area  24  and the after-running duct  36 .  
         [0030]    The primary sealing element  22  is then positioned on the second cylindrical section  42  of the blind hole  28  and is held in this position by the supporting ring  56 . Finally, the third cylindrical section  44  of the blind hole  28  delimits the pressure chamber  16 .  
         [0031]    A piston rod  64  is actively coupled on the right-hand end in FIG. 1 of the piston  10  by means of an insert  66  attached to the main part  18 . In the normal position of the piston  10  illustrated, the insert  66  is in contact with the stop  50  of the guide sleeve  46 . The piston rod  64  extends through a dust collar  68  which is attached to the guide sleeve  46 .  
         [0032]    On the left-hand end of the piston  10  in FIG. 1 and  2 , the after-running device  26  is attached to the main part  18  via a clip connection, as will be described. The outside diameter of the running surface  20  of the main part  18  is slightly bigger than the outside diameter of the after-running device  26 , the main part  18  having a sloping or conical transition section  70  between the running surface  20  and the after-running device  26 . A cylindrical centering shoulder  72  of reduced diameter for the after-running device  26  follows the transition section  70  of the main part  18 . The main part  18  ends with a cylindrical fixing shoulder  74 , again with a reduced diameter, for the after-running device  26 , which has a radial groove  76  and is provided with a joining slope  78  on the end in order to facilitate the joining of the after-running device  26  to the main part  18 . Finally, the fixing shoulder  74  delimits the pressure chamber  16  in the housing  12  with its flat end.  
         [0033]    [0033]FIG. 3 and  4  illustrate in more detail the preferably one part injection-molded plastic after-running device  26 . It can be clearly seen that the after-running device  26  is designed as an annular section with an essentially U-shaped cross-section. More precisely, the after-running device  26  has a radially outer annular section  80  and a radially inner annular section  82  which are connected to each other via an annular disc section  84  on the end.  
         [0034]    The outer annular section  80  of the after-running device  26  has an essentially cylindrical outside perimeter surface  86 , with which, in the normal position of the piston  10  illustrated in FIG. 2, the dynamic sealing lip of the primary sealing element  22  is in contact. The outside perimeter area  86  is provided with several—twelve in the example embodiment illustrated—equalisation grooves  88  distributed uniformly over the perimeter which, starting from the free end of the outer annular section  80  in FIG. 3 and  4 , extend in the axial  
         [0035]    direction of the after-running device  26 , namely over the entire length of the outside perimeter surface  86 , whose end facing the pressure chamber  16  in the installed state is slightly bevelled.  
         [0036]    In addition, the outer annular section  80  has a cylinder inner perimeter surface  90 , by means of which the after-running device  26  is centered radially on the centering shoulder  72  of the main part  18 , as shown in FIG. 2. The inside perimeter surface  90  is also provided with several—twelve in the embodiment example illustrated—equalisation grooves  92  distributed uniformly over the perimeter, which, starting from the free end in FIG. 3 and  4  of the outer annular section  80 , extend in the axial direction of the after-running device  26 . As can be seen clearly from FIG. 2, the axial length of the equalisation groove  92  on the inner perimeter surface  90  of the outer annular section  80  is bigger than the width of the centering shoulder  72  of the main part  18 .  
         [0037]    Finally, the free end of the outer annular section  80  forms an annular shoulder  94  with which the outer running device  26  rests without play on the main part  18  in the axial direction. The annular shoulder  94  is provided with several—twelve in the example embodiment illustrated—uniformly distributed connecting grooves  96  which run in a radial direction. As FIG. 3 shows in particular, the connecting grooves  96  in the annular shoulder  94  connect the equalisation grooves  88  on the outside perimeter surface  86  to the equalising groove  92  on the inside perimeter surface  90  of the outer annular section  80 .  
         [0038]    According to FIG. 4, the annular disc section  84  of the after-running device  26  is also provided with at least one connecting duct  98  extending in the axial direction of the after-running device  26 . The connecting duct  98  is used, in the mounted state of the after-running device  26  on the main part  18 , to guarantee a connection between the pressure chamber  16  and an annular chamber  100 , which is delimited by the main part  18  and the after-running device  26 .  
         [0039]    As FIG. 3 and  4  show in particular, the inner annular section  82  of the after-running device  26  has several slots to form spring tabs  102  of the clip connection. Each of the eight spring tabs  102  shown in the example embodiment illustrated has a lug  104  projecting radially inwards on the end, which in the mounted state of the after-running device  26  on the main part  18 , is engaged with the radial groove  76  provided on the fixing shoulder  74  of the main part  18 . As is evident from FIG. 4 in particular, each lug  104  is provided with a slope  106  on its side facing the pressure chamber  16  in order to equalise any play and to hold the after-running device  26  with its annular shoulder  94  flush on the main part  18 .  
         [0040]    It can be seen that the after-running device  26  designed in this way can be easily attached to the main part  18 . To achieve this, the after-running device  26 , which is made separately from the main part  18 , is moved onto the main part  18  in the axial direction until the lugs  104  of the spring tabs  102  come into contact with the joining bevel  78  on the fixing shoulder  74  of the main part  18 . With an additional relative displacement between the after-running device  26  and the main part  18 , the spring tabs  102  spring up elastically until, towards the end of the fitting movement, lugs  104  of the spring tabs  102  clip into the radial groove  76  of the fixing shoulder  74  of the main part  18 , as the annular shoulder  94  of the after-running device  26  makes play-free contact with the main part  18 .  
         [0041]    It can also be seen from the above description that in the normal position of the piston  10  illustrated in FIG. 1 and  2 , a hydraulic connection exists between the pressure chamber  16  and the after-running area  24 . This is guaranteed via the equalisation grooves  88  on the outside perimeter surface  86  of  
         [0042]    the after-running device  26  under the dynamic sealing lip of the primary sealing element  22  and, in the case of the dynamic sealing lip of the primary sealing element  22  being unfavorably pressed against the outside perimeter surface  86  of the after-running device  26 , at least via the connecting duct  98 , the annular chamber  100 , the equalisation grooves  92  on the inside perimeter surface  90  and the connecting grooves  96  on the annular shoulder  94  of the after-running device  26  and also via the annular gap  58  between the running surface  20  and the supporting ring  56 .  
         [0043]    The hydraulic connection described between the pressure chamber  16  and the after-running area  24  is interrupted as soon as the piston  10  is moved in the direction of the pressure chamber  16 , i.e. to the left in FIG. 1 and  2 , to an operating position, the sealing contour of the dynamic sealing lip of the primary sealing element  22  arriving via the running surface  20  of the piston  10  and sealing the pressure chamber  16  in relation to the after-running area  24  so that a pressure can be built up in the pressure chamber  16 .  
         [0044]    In the example embodiment illustrated, the main part  18  of the piston  10  is a solid body of a non-ferrous metal whose outside perimeter directly forms the running surface  20  of the piston  10 . However, depending on the requirements concerned, this solid body can also consist of another material and/or be provided on the outside perimeter with a tubular sleeve or a coating which forms the running surface. It is also possible for the main part to be an essentially pot or beaker-shaped body in sheet steel, for example, which, if necessary, surrounds a lining preferably in plastic. In any case, in its installed state, the end of the main part facing the pressure chamber must have a geometry which allows a play-free connection with the after-running device.  
         [0045]    The above description of the invention was made using a master cylinder of a hydraulic clutch operation for motor vehicles as an example. However, it is obvious to a person skilled in the art that the after-running device described can also be used on pistons of master cylinders for hydraulic brake systems in motor vehicles, for example.