Abstract:
A tensioning device, in particular for a highly dynamic endless drive, such as a chain drive or a belt drive for an internal combustion engine, comprises a tensioner housing, a tensioning piston displaceably arranged in the tensioner housing, a pressure chamber formed between the tensioner housing and the tensioning piston and a pressure medium inlet leading into the pressure chamber and including a non-return valve. In order to meet also new demands, in the case of which oil pumps with having a low feed pressure are used, the present device discloses that a valve body of the non-return valve is defined by a leaf spring diaphragm. The device additionally relates to a non-return valve as well as an endless drive.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to foreign German patent application No. DE 102013004850.8, filed on Mar. 5, 2013, the disclosure of which is incorporated by reference in its entirety. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a tensioning device, in particular for a highly dynamic endless drive, such as a chain drive or a belt drive for an internal combustion engine, comprising a tensioner housing, a tensioning piston displaceably arranged in the tensioner housing, a pressure chamber formed between the tensioner housing and the tensioning piston and a pressure medium inlet leading into the pressure chamber and including a non-return valve, a valve body of the non-return valve being defined by a spring diaphragm. 
       BACKGROUND 
       [0003]    Such tensioning devices are used e.g. for timing chain drives on an internal combustion engine and they normally press a pivotably arranged tensioner blade with its slide surface against a timing chain. The tensioning device is normally connected to the engine oil hydraulic system of the internal combustion engine and has hydraulic fluid supplied thereto via a pressure medium inlet. In many cases a helical compression spring and a filler body for reducing the filling volume of the pressure chamber are additionally provided within the pressure chamber between the tensioning piston and the tensioner housing. Due to the fast running endless drive, the tensioning device is subjected to substantial highly dynamic loads that necessitate a fast switching non-return valve which operates efficiently throughout its service life. The valves used are mainly ball check valves in which a helical compression spring presses a valve body configured as a ball into an opening position so that hydraulic fluid can flow via the pressure medium inlet and the non-return valve into the pressure chamber. However, as soon as the pressure in the pressure chamber exceeds a certain limit, the non-return valve will close. Hydraulic fluid can then escape from the pressure chamber only via possibly existing leakage paths, which determine the damping characteristics of the tensioning device. In addition, also plate or disk-shaped valve bodies are known for such cases of use. In modern internal combustion engines controlled oil pumps are increasingly used. These oil pumps control the supply pressure so as to achieve low specific consumption. This means that hydraulic chain tensioners must be capable of operating even if the supply pressure should be low. 
         [0004]    A tensioning device of the type in question is known from DE 11200603102 T5. Other examples of spring diaphragms are described in DE 4030717 A1, DE 102006055466 A1 and EP 0473261 A2. In contrast to the conventional translational movement of a rigid valve body, the spring and the valve body form here a unit and execute a movement after the fashion of a cantilevered bending beam. Configuring the valve body and the spring in common as a spring diaphragm offers the possibility of establishing a closure system with much smaller masses, which has a comparatively soft spring characteristic and ensures nevertheless a fast and reliable opening and closing of the non-return valve. The use of the spring diaphragm therefore allows the provision of large inflow cross-sections in combination with small moving masses as well as low opening pressures. 
       SUMMARY OF THE INVENTION 
       [0005]    It is therefore the object of the present invention to provide a tensioning device which remains capable of operating even if the supply pressures should be low and which is nevertheless robust as well as cost-efficient. 
         [0006]    According to the present invention, this object is achieved by a tensioning device of the above-mentioned type according to claim  1 . The spring diaphragm comprises here a holding area, which is arranged such that it is at least secured against rotation preferably relative to the tensioner housing, and two elastically arranged closure areas extending away from said holding area and covering each a respective inlet opening in the closed condition. This allows a plurality of design possibilities, which also influence the mass to be moved and the spring rate accomplished. 
         [0007]    In addition, a reception bore with a concave inner surface is provided, the inlet openings terminating at the concave inner surface, and the closure areas of the spring diaphragm having a curved shape, which is adapted to the reception bore, and abutting on the concave inner surface of the reception bore in the closed condition. Up to now, the flow into the pressure chamber often took place coaxially with the tensioning-piston axis, so that the valve body also moved along this direction. Therefore, the inlet opening was normally arranged in a flat surface at the bottom of the tensioner housing or valve housing and was sometimes provided with an edge that was chamfered or adapted to the shape of the valve body. In contrast to this, the spring diaphragm is here in close contact with a concave inner surface of the reception bore and has a shape that is adapted thereto. This also provides the possibility of elegantly positioning such a spring diaphragm in the reception bore. For example, the holding area may be retained in the reception bore simply by form-fit engagement therewith. Normally, the spring diaphragm is, however, installed in the reception bore under a certain pretension or it is press-fitted therein. The reception bore may be arranged especially in the tensioner housing or in a valve housing. 
         [0008]    By using two inlet openings, the flow cross-section can be enlarged, the spring diaphragm having then preferably a symmetric structural design. For accomplishing a desired incoming flow also the contour of the closure areas may be adapted accordingly. 
         [0009]    In order to avoid excessive loads on the spring diaphragm, a substantially central stop means is provided, which limits the opening stroke of the closure areas. The closure areas can therefore only execute a limited stroke and are then prevented from still further by the stop means. If the spring diaphragm has a symmetric structural design, the stop means is configured such that, when the inlet opening is being opened, the closure areas first move towards one another and prevent one another then from executing any further movements. 
         [0010]    To this end, the reception bore is preferably cylindrical in shape and the closure area has a curved shape adapted thereto. Depending on the respective structural design, the closure area can therefore move into close, large-area contact with the inner wall of the reception bore and thus efficiently seal the inlet opening. 
         [0011]    According to a further embodiment, the use of a spring diaphragm also offers the possibility of defining the inlet openings and/or the reception bore by the tensioner housing and installing the spring diaphragm directly in said tensioner housing. It is therefore not absolutely necessary to provide a separate valve housing, whereby a reduction of costs can be achieved. 
         [0012]    Nevertheless, the non-return valve may include a valve housing having the spring diaphragm arranged therein according to another embodiment, said valve housing being arranged in the pressure medium inlet of the tensioning device. The use of a valve housing can simplify the mounting of the non-return valve in the tensioner housing. This variant will be particularly suitable in the event that these components are produced separately and assembled subsequently. In some cases, it may, however, also be possible to produce the inlet openings more accurately in a valve housing, in particular in the area of the reception bore. The reason for this is that the sectional edges of a reception bore and an inlet opening in a valve housing are not positioned as deeply in a blind hole bore as in the case of a tensioner housing. 
         [0013]    In addition, anti-loss and/or rotation-lock means retaining the spring diaphragm in its position may be provided. In order to allow the spring diaphragm and its components to be easily positioned relative to the inlet openings, adequate means which facilitate mounting are provided. These means may e.g. be projections or recesses, such as grooves etc., on the spring diaphragm or on the tensioner housing or valve housing. 
         [0014]    Preferably, the spring diaphragm may be produced from a stamp-bending part, preferably a spring steel sheet. Embodiments making use of a single sheet-metal blank, so that weak spots originating from additional connections can be avoided, are here particularly advantageous. 
         [0015]    According to a preferred further embodiment, the spring diaphragm is made of plastic material. Materials suitable for this purpose are well-established heavy duty plastics, which may also be fiber reinforced in some cases. In particular the use of an overload protection, e.g. in the form of a stop means, will, however, allow the use of other materials as well. 
         [0016]    Especially in the case of a variant using plastic materials, it will be of advantage, when, starting from the holding area, the thickness of the closure area decreases substantially continuously towards the free end of the closure area. It is thus possible to accomplish a sufficiently high strength in the actual load area and a soft spring characteristic, so that a spring diaphragm with a high number of strokes will obtain a long service life. 
         [0017]    In addition, the closure surface of a closure area may have formed therein a flow directing contour in the form a depression extending beyond the opening cross-section of the inlet opening. Depending on the respective structural design, the closure areas of the spring diaphragm abut in large-area sealing contact. For guaranteeing reliable flowing-off also in the case of small opening strokes, this flow directing contour, which provides a larger flow-off cross-section even in the case of small opening strokes, may be used. The effect of this kind of measure will also be advantageously enhanced, when the thickness of the closure area decreases towards the free end. 
         [0018]    Furthermore, the present invention relates to a non-return valve for a tensioning device according to one of the claims  1  to  9 . The non-return valve is characterized in that a valve body of the non-return valve is defined by a spring diaphragm. Especially in the sphere of endless drives, in particular timing drives of an internal combustion engine, ball and plate valves have mainly been used up to now. The reason for this is that these valves generally exhibit a high degree of reliability when used in highly dynamic processes. However, they partially fail to satisfy the demands entailed by new internal combustion engine concepts. 
         [0019]    In addition, the present invention also relates to an endless drive, in particular timing drive of an internal combustion engine, comprising a drive pulley, a driven pulley, an endless drive means coupling said drive pulley and said driven pulley, and a tensioning device according to one of the claims  1  to  9  for tensioning the endless drive means. In a timing drive of an internal combustion engine, the drive pulley may be a crankshaft wheel, in particular a crankshaft sprocket, and the at least one driven pulley may be a camshaft wheel, in particular a camshaft sprocket. The tensioning device is then normally a chain tensioner that is connected to the engine oil hydraulic system and applies pressure to a tensioner blade, which, in turn, abuts on a timing chain. The timing chain may have a great variety of different structural designs, such as a sleeve-type chain, a roller chain and a toothed chain. In addition to the timing drive, also auxiliary drives of an internal combustion engine may be provided with such a tensioning device. The material used for the chains is normally steel. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    In the following, embodiments of the present invention will now be explained in more detail making reference to drawings, in which: 
           [0021]      FIG. 1  shows a schematic front view of a timing chain drive, 
           [0022]      FIG. 2  shows a first embodiment of a chain tensioner in a full section view, 
           [0023]      FIG. 3  shows the non-return valve according to  FIG. 2  in a perspective representation, 
           [0024]      FIG. 4  shows the non-return valve according to  FIG. 3 , the valve housing being shown in a full section view, 
           [0025]      FIG. 5  shows the valve diaphragm according to  FIG. 4  in a perspective front view, 
           [0026]      FIG. 6  shows the valve diaphragm according to  FIG. 5  in a side view, 
           [0027]      FIG. 7  shows the valve diaphragm according to  FIG. 5  in a top view, 
           [0028]      FIG. 8  shows a second embodiment of a valve diaphragm in a perspective view, 
           [0029]      FIG. 9  shows a second embodiment of a non-return valve with the valve diaphragm according to  FIG. 8 , the valve housing being shown in a full section view, 
           [0030]      FIG. 10  shows a fragmentary view of a further embodiment of the chain tensioner with a valve diaphragm according to  FIG. 8 , the chain tensioner housing being shown in a full section view, and 
           [0031]      FIG. 11  shows a further embodiment of a non-return valve in a schematic sectional top view. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    The timing chain drive  1  for an internal combustion engine shown in  FIG. 1  essentially comprises a crankshaft sprocket  2 , two juxtaposed camshaft sprockets  3 . 1  and  3 . 2 , a timing chain  4  wrapped around these sprockets, a chain guide  5  fixed to the engine block, a tensioner blade  6  pivotably arranged on the engine block and a chain tensioner  7  whose tensioning piston  8  presses against the tensioner blade  6 . In the present case, the chain tensioner  7  is configured as a so-called screw-in chain tensioner, which is screwed into a wall  9  on the engine case. The chain tensioner  7  may, however, also be configured as a flange- or attachment-type chain tensioner. The crankshaft sprocket  2  drives the two camshaft sprockets  3 . 1  and  3 . 2  by means of the timing chain  4 . In the course of this process, the tight span of the chain  4  slides along the chain guide  5  and the slack span slides along the tensioner blade  6 . The chain tensioner  7  must apply a sufficiently strong force to the tensioner blade  6  so that reliable tensioning of the timing chain  4  will be guaranteed over the whole operating range of the internal combustion engine. Highly dynamic processes will here take place in the interior of the chain tensioner  7 , which also provides a damping function. 
         [0033]    In the following, a more detailed structural design of a chain tensioner embodiment will be explained more precisely with the aid of  FIG. 2 . 
         [0034]    The chain tensioner  7 , which is shown in a full section view in  FIG. 2 , comprises a screw-in housing  10  with a hexagon head  11  and an abutment flange  12 , a supply portion  13  and a threaded portion  14 . The abutment flange  12  and the supply portion  13  have provided between them an annular groove  15  for arranging therein a sealing ring so as to seal the supply portion  13  from its surroundings. The tensioner housing, which is substantially cylindrical in the front area thereof, has a guide bore  16  configured as a blind bore with a bottom surface  17 . The guide bore  16  serves to receive the tensioning piston  8  therein and to guide it in a longitudinally displaceable manner, the guided portion  18  of said tensioning piston  8  being guided in the reception bore such that a leakage gap is defined. The tensioning piston  8  has a head  20 , which, except for a vent hole  19 , is closed and which includes a pressing surface  21  that presses against the tensioner blade  6 . A pressure chamber  22  is formed between the tensioner housing  10  and the tensioning piston  8 , said pressure chamber  22  extending partially into the interior of the substantially hollow tensioning piston  8 . In addition, the pressure chamber  22  has arranged therein a helical compression spring  23  and a mushroom-shaped filling element  24  reducing, on the one hand, the filling volume of the pressure chamber  22  and comprising, on the other hand, a vent groove  26  on its head  25 , which is in flow communication with the vent hole  19 . In the supply portion  13  two diametrically arranged supply bores  27 . 1  and  27 . 2  are provided, which extend at an oblique angle downwards and which establish a connection to the reception bore  16 . At the bottom of the reception bore  16 , a non-return valve  28  is installed, which comprises a valve housing  29  and a valve diaphragm  30 . 
         [0035]    Making reference to  FIGS. 3 to 7 , the structural design of this non-return valve  28  will now be explained in more detail. The valve housing  29  comprises a first cylindrical portion  31  and a second cylindrical portion  32 , which is configured in a flange-like manner. The diameter of the first cylindrical portion  3  is therefore smaller than that of the second cylindrical portion  32 . The valve housing  29  has provided therein an axial reception bore  33 , which is configured conically after the fashion of an insertion aid in its area  35  facing the first end face  34 . This area  35  is followed by a cylindrical central area  36  merging with a further conically tapering area  37 , which, in turn, ends in a cylindrical outlet bore  38 . Two inlet bores  39 . 1  and  39 . 2 , which are arranged diametrically in the first cylindrical portion, end in the central area  36  of the reception bore  33 , said inlet bores  39 . 1  and  39 . 2  forming two respective inlet openings  40 . 1  and  40 . 2  on the central area  36 . In addition, the valve housing  29  includes a reception groove  41  on the end face  34  as well as on the outer circumferential surface of the first cylindrical portion  31 , said reception groove  41  being displaced by substantially 90° relative to the inlet bores  39 . The valve housing  29  is preferably made of a steel material, it may, however, also consist of some other metal or of plastic material. 
         [0036]    The reception bore  33  has now inserted therein the valve diaphragm  30 , which is separately shown in  FIGS. 5 to 7 . The valve diaphragm  30  is formed from a sheet metal blank, preferably from a spring steel, by means of a stamp-bending process. The valve diaphragm  30  is formed such that a great variety of different functions is provided simultaneously. The main components of the valve diaphragm  30  are, on the one hand, the centrally arranged holding area  42  and the closure areas  43 . 1  and  43 . 2  provided thereon in a wing-like manner. On the other hand, the lower end of the holding area  42  has additionally formed thereon a stop means  44 . The holding area  42  is substantially formed by a straight, centrally arranged sheet metal strip, which projects beyond the closure areas  43  at its upper end and which is bent back in a U-shape after the fashion of a locking clip. The U-web  45  and the free U-leg  46  come to lie in the retaining groove  41  when the valve diaphragm  30  is arranged in the valve housing  29 , so that an anti-loss and rotation-lock arrangement is defined. The depth of the retaining groove  41  is slightly larger than the sheet metal thickness of the valve diaphragm  30 . The free U-leg  46  is slightly bent inwards and has on its end a portion which, in turn, is slightly bent outwards as an insertion aid, so that the free U-leg  46  acts as a spring arm. 
         [0037]    The closure areas  43 . 1  in  43 . 2  extend like wings laterally away from the central holding area  42 . The rear portions, which directly adjoin the holding area  42 , each include a window  47 . 1  and  47 . 2 , so that only an upper and a lower sheet metal strip remain. The windows  47 . 1  and  47 . 2  have rounded corners, and the corners located closer to the holding area  42  have a larger radius. The windows essentially influence the spring characteristics of the closure areas  43 . 1  and  43 . 2 , so that their size is chosen in accordance with the desired spring characteristics. The front portions of the closure areas  43 . 1  and  43 . 2  are configured as full-area contact portions  48 . 1  and  48 . 2 . These contact portions  48 . 1  and  48 . 2  are closure elements covering, i.e. closing the actual inlet openings  40 . 1  and  40 . 2  in the valve housing  29 . The area of the contact portions  48 . 1  and  48 . 2  is therefore larger than the cross-sectional area of the inlet openings  40 . 1  and  40 . 2 . The closure areas  43 . 1  and  43 . 2  are arcuate in shape with a curvature, so that, after having been inserted in the valve housing  29 , they will be in close contact with the concave inner wall of the reception bore  33 . Insertion into the reception bore  33  can take place under slight pretension, so that the closure areas  43 . 1  and  43 . 2  are bent open a bit wider in the non-mounted condition and are then inserted into the reception bore  33  under pretension. The respective free ends of the closure areas  43 . 1  and  43 . 2  have formed thereon end portions, which are bent back in a U-shape and the free U-legs  49 . 1  and  49 . 2  of which serve as a contact surface. 
         [0038]    The stop means  44  is, when seen in a top view ( FIG. 7 ), approximately T-shaped. Its central leg  50  is arranged on the lower end of the holding area  42  and extends then, after a certain distance, at an oblique angle upwards, so that the stop means  44  reaches the area of the free ends of the closure areas  43 . The free ends of the front crosspiece  51  of the stop means  44 , which adjoins the leg  50 , are bent substantially perpendicularly towards the interior of the valve diaphragm  30 , so that a respective stop lug  52 . 1 ,  52 . 2  is formed on either side. In the installed condition of the valve diaphragm  30 , the stop lug  52 . 1  is spaced at a predetermined distance from the stop surface of the free U-leg  49 . 1 , said predetermined distance defining the opening stroke to the associated closure area  43 . 1 . This applies in the same way to the opposite side with the stop lug  52 . 2  and the stop surface of the free U-leg  49 . 2 . As can be seen from  FIG. 2 , the non-return valve  28  shown in  FIG. 3  is installed upside down in the valve housing, so that the end face  34  abuts on the bottom surface  17  of the tensioner housing  10  and the helical compression spring  23  presses against the opposed end face of the valve housing  29 . Hence, also the U-web  45  of the holding area  42  is located between the bottom surface  17  and the retaining groove  41 . This measure has the effect that the valve diaphragm  30  is reliably anchored in position in the valve housing  29 . In view of the difference in diameter between the first cylindrical portion  31  and the second cylindrical portion  32  of the valve housing  29 , an annular channel  53  is formed between the tensioner housing  10  and the valve housing  29 , said annular channel  53  allowing the hydraulic fluid to flow from the supply bores  27 . 1  and  27 . 2  to the inlet bores  39 . 1  and  39 . 2 . 
         [0039]    In the following, the operating mode of the above-described chain tensioner  7  will be explained in more detail. After the starting process of the internal combustion engine, pressure builds up in the system and hydraulic fluid flows via the supply bores  27 . 1  and  27 . 2  into the annular channel  53  and from there into the inlet bores  39 . 1  and  39 . 2 . Due to the fact that the hydraulic pressure built up within the pressure chamber  22  has not yet reached a substantial level, the closure areas  43 . 1  and  43 . 2  bend inwards and uncover the inlet openings  40 . 1  and  40 . 2  so that hydraulic fluid will flow into the reception bore  33  and, via the cylindrical outlet bore  38 , into the pressure chamber  22  until pressure balance occurs between the supply pressure and the pressure in the pressure chamber  22 . The closure areas  43 . 1  and  43 . 2  then swing back and close the inlet openings  40 . 1  and  40 . 2  again. Such a chain tensioner  7  operates in a highly dynamic way and, when an internal combustion engine is in operation, the chain tensioner passes numerous oscillation states due load and speed changes. Damping is accomplished in the case of such chain tensioners  7  e.g. due to the fact that part of the hydraulic fluid flows off from the pressure chamber  22  through the leakage gap formed between the guided portion  18  of the tensioning piston  8  and the reception bore  16 . When the tensioning piston  8  is to be extended again later on, the spring  23  forces the tensioning piston  8  outwards and hydraulic fluid flows in, thus compelling the non-return valve  28  to open once more. During retraction of the tensioning piston  8 , the non-return valve  28  closes. A very high number of these processes recurs during operation in a highly dynamic manner, which means that the closure areas  43 . 1  and  43 . 2  are subjected to high alternating bending loads. In order to avoid excessive stress peaks, in particular in the holding area  42 , the stop means  44  is provided, through which the opening stroke is limited. On the basis of the structural design of the non-return valve  28  shown, comparatively high flow rates of the hydraulic fluid can be accomplished even in the case of low opening pressures, since comparatively large opening cross-sections are provided, which are closed by only small moving masses. 
         [0040]    It would also be imaginable to form a stamp-bending part that can be installed directly in the tensioner housing  10  without making use of an intermediate valve housing  29 . 
         [0041]    In the following, a further embodiment of a non-return valve  28  according to the present invention will be explained in more detail making reference to  FIGS. 8 and 9 . Only the essential differences will be discussed in the following, so that reference is additionally made to the above description. 
         [0042]    The valve housing  29  is configured without a retaining groove  41  and has a differently configured reception bore  33 . Said reception bore  33  has a larger first cylindrical portion and a smaller second cylindrical portion, so that a supporting step  54  is formed, which is provided with a centering projection  55  that is semicircular in cross-section in  FIG. 9  and displaced relative to the two inlet bores  39  by substantially 90°. The valve housing  29  may be consist of a steel material or of some other metal or of plastic material. 
         [0043]    The associated valve diaphragm  30  is, in the present case, not formed from a sheet metal blank, but produced as a plastic molding, which has preferably been produced by injection molding. Also a fiber-reinforced plastic material may here be used. For reasons of stability, the closure areas  43  are configured as continuous components, which do therefore not include any windows. In addition, the thickness of the closure areas  43  is much larger in the region following the holding area  42  than at the opposite free ends of the closure areas  43 . As can be seen from the representation, the closure areas  43 . 1  and  43 . 2  decrease in thickness continuously. The holding area  42  is configured as a web projecting arcuately towards the interior of the valve diaphragm  30  and including a concave indentation  56 . 1 ,  56 . 2  at the upper end as well as at the lower end thereof. The lower concave indentation  56 . 2  is positioned, in a substantially accurately fitting manner, on the centering projection  55 , whereby locking against rotation is accomplished. The stop means  44  is defined by two webs  57 . 1  and  57 . 2 , which are arranged on the inner side of the closure areas  43  and the free end faces of which are spaced apart at a predetermined distance and determine the opening stroke limitation. On the outer side of the closure areas  43  a respective flow directing contour in the form of an oval, elongate depression  58 . 1  and  58 . 2  is provided, whose rear end begins on the level of inlet opening  40  and whose front end ends in spaced relationship with the free end of the closure areas  43 . The pressure applied to the respective depressions  58 . 1  and  58 . 2  will thus also act on a front section of the closure areas  43 , which is configured as a section of reduced thickness, whereby the closure areas  43 . 1  and  43 . 2  can be opened more easily. 
         [0044]    Making reference to  FIG. 10 , a further embodiment of the non-return valve  28  according to the present invention will now be explained in more detail. In this embodiment, the identical valve diaphragm  30  according to  FIG. 8  is installed directly in the tensioner housing  10  of the chain tensioner  7 . To this end, the lower area of the tensioner housing  10  has a slightly different structural design in comparison with the first embodiment. The two supply channels  27 . 1  and  27 . 2  are positioned on a slightly lower level and terminate slightly above the bottom surface  17 . The valve diaphragm  30  is inserted in a bore section of the reception bore  16 , whose height corresponds essentially to the height of the valve diaphragm  30 . The reception bore  16  widens thereabove on the level of the supply portion  13  thus forming a first step having a retaining ring  59  press-fitted therein. The retaining ring  59  has on the lower side thereof a centering projection  60  engaging the upper recess  56 . 1  of the valve diaphragm  30  in a substantially accurately fitting manner and securing the valve diaphragm  30  thus against rotation relative to the retaining ring  59 . Hydraulic fluid, which flows in through the inlet channels  27 . 1  and  27 . 2 , flows past the closure areas  43  on the outer side thereof and into the interior of the non-return valve  28  and from there through the central opening of the retaining ring  59  upwards into the pressure chamber  22 . The retaining ring  59  is positioned such that it does not impede the movement of the closure areas  43 . 
         [0045]    According to  FIG. 11 , a further embodiment of the non-return valve  28  according to the present invention will now be explained. In the following, only the essential differences existing in comparison with the preceding embodiment will be discussed, so that reference is additionally made to the above description. 
         [0046]    The valve diaphragm  30  essentially resembles the valve diaphragm  30  according to  FIG. 8 . As can clearly be seen, reinforcement ribs  61 . 1  and  62 . 2  are provided as a variant on the webs  57 . 1  and  57 . 2  for stabilizing them as well as for stiffening the front portions of the closure areas  43 . 1  and  43 . 2 . The valve housing is here made of plastic material and produced by means of injection molding. The section plane is on the level of the inlet bores  39 . 1  and  39 . 2 . The lock against rotation is defined by a radially protruding projection  62 , which engages the arcuately configured holding area  42  from behind. 
         [0047]    The non-return valve  28  according to the present invention allows a cost-efficient and robust design.