Abstract:
The invention relates to a cup-shaped hydraulic piston ( 10 ) made from rolled sheet metal, which comprises an open end ( 12 ), a side wall ( 16 ) with a cylindrical outer surface ( 18 ) and an inner surface ( 22 ), as well as a piston head ( 14 ). The side wall ( 16 ) at its outer surface ( 18 ) is provided with an annular groove ( 20 ) in a portion adjoining the open end ( 12 ). To ensure a lightweight style of construction combining high stability with ease of manufacture, the part of the inner surface ( 22 ) adjoining the open end ( 12 ) and extending axially over the annular groove ( 20 ) has a cylindrical shape, and the thickness of the side wall ( 16 ) decreases monotonically from the portion ( 24 ) of the inner surface ( 22 ) up to the piston head ( 14 ).

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of International Application No. PCT/EP02/08887 filed Aug. 8, 2002, the disclosures of which are incorporated herein by reference, which claims priority to Russian Patent Application No. 2001-122026 filed Aug. 8, 2001, the disclosures of which are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   The invention relates to a hydraulic piston made from rolled sheet metal, to a method of manufacturing such a piston and to a blank forming an intermediate product of said method. Pistons of this type are used e.g. in hydraulically actuated vehicle brakes. 
   Pistons of the described type have to satisfy mutually conflicting requirements: on the one hand, they are to be light so as to have a low inertia, on the other hand, they have to be stable enough to prevent radial deformation of the side wall of the piston in the event of the base of the piston being subjected to a high load, such as occurs e.g. during a powerful braking operation because of the high hydraulic pressure in the brake system. If the last condition is not met, a total loss of function of the piston may occur. 
   Hydraulic pistons usually have the shape of a cylindrical cup with an open end. In a portion of the outer surface of the side wall adjoining the open end of the piston there is often an annular groove, which is used to fasten a protective sleeve. The region of the annular groove is particularly at risk of load-induced material deformation because the wall is weakened by the groove. 
   From WO 91/12445, and corresponding U.S. Pat. No. 5,231,916, both of which are incorporated by reference herein, a piston manufactured by a drawing method is known, in which the wall thickness in the critical groove region is not smaller than the wall thickness in the other regions of the piston. Two critical regions are mentioned in WO 91/12445, namely, on the one hand, the region of the groove and, on the other hand, the inner region of a spring arrangement. The stability requirements in these two regions determine the thickness of the piston wall. 
   From the Russian patent 2 163 987 a drawing method for piston manufacture is known, which enables a piston to be manufactured with a stable, yet thin wall. The piston is a hollow structure with an open end. Its outer wall is cylindrical in shape and has a groove for fastening a protective sleeve. The inner surface of the piston is cylindrical in a region adjoining the open end and extending in axial direction internally over the annular groove. In a further inward lying portion of the piston the inside diameter is greater than in the region adjoining the open end. Consequently, the thickness of the side wall is greater at the open end than in the vicinity of the piston head, this being beneficial in terms of the ability to withstand axial forces in the critical region of the groove. It is however complicated to manufacture such a piston having two inner surfaces of differing inside diameter and identical outside diameter, especially as the inner surface with the smaller inside diameter lies at the open end of the piston. 
   SUMMARY OF THE INVENTION 
   The underlying object of the invention is to provide a hydraulic piston, which combines high stability with a low weight and at the open end has a wall thickness sufficient to guarantee the problem-free transmission of axial forces. The piston is moreover to be easy to manufacture. A further object is to indicate an intermediate product that arises within the framework of manufacture of the hydraulic piston. 
   In contrast to cylindrical hydraulic pistons known from the prior art, the thickness of the side wall of the piston decreases monotonically from the part of the inner surface of the side wall adjoining the open end and having a cylindrical shape, to the piston head. In other words, from the cylindrically shaped portion of the inner surface of the side wall to the piston head the inside diameter of the hydraulic piston according to the invention increases monotonically. Here, by the term “monotonic” is meant, in particular, a continuous decrease of the side wall thickness and/or a continuous increase of the inside diameter. A piston thus configured presents excellent stability with regard to the required types of load and is moreover easy to manufacture. 
   According to a preferred embodiment, the piston head is curved inwards, in particular in a concave manner. Such a curvature of the piston head guarantees a good ability to withstand the usual working loads. 
   Preferably, when the piston head is curved inwards, the radially outer part of the piston head has the shape of a truncated cone and the central or middle part of the piston head has the shape of a spherical cap. The extension of the truncated-cone-shaped part of the piston head along its profile is in said case preferably not greater than three times the wall thickness of the side wall in its portion adjoining the piston head. A partly truncated-cone-shaped, partly spherical-cap-shaped configuration of the piston head is particularly advantageous for the shaping, by stamping, of the outer surface of the side wall in the region of transition of the side wall to the piston head. 
   The manufacturing method according to the invention, in its simplest form of implementation, comprises the steps of punching a disk-shaped round blank out of a piece of rolled sheet metal, deep-drawing the disk-shaped round blank into a cup shape by means of a bottom die and a punch, stamping the cup-shaped blank to form the piston head and the cylindrical outer surface of the hydraulic piston, and incorporating an annular groove into the outer surface of the hydraulic piston. It is apparent that the manufacturing method according to the invention entails only very little cutting, e.g. for producing the annular groove. In principle, the piston is however manufactured from a rolled sheet metal by drawing and stamping operations. 
   In preferred developments of the manufacturing method according to the invention, the deep-drawing is effected in a plurality of steps. According to an embodiment, the disk-shaped round blank is pressed firstly by means of a first punch through a first circular die opening and then by means of a second punch through a second circular die opening, the diameter of which is smaller than the diameter of the first die opening. In the first deep-drawing step a still relatively flat, cup-shaped blank is obtained, the inside diameter of which corresponds to the outside diameter of the punch used. As a result of the reshaping of material, the thickness of the sheet metal varies. After the deep-drawing step this thickness is greater at the open end than in a transition region between side wall and head since because of the drawing operation the material thickness decreases there. The wall thickness reduction from the open end to the transition region between side wall and head occurs continuously. As a result of the second deep-drawing step the cup shape becomes narrower and higher and a further reduction of the wall thickness occurs in the region of transition from the side wall to the head. The inside diameter of the cup-shaped blank after the second deep-drawing step is determined by the outside diameter of the punch used in the second deep-drawing step. Preferably, the punches used in the first and in the second deep-drawing step are cylindrical. 
   According to a particularly preferred development of the method according to the invention, the cup-shaped blank is pressed by means of a third punch through a third circular die opening, the diameter of which is smaller than the diameter of the second die opening, wherein the third punch has a first cylindrical portion emanating from its free end and adjoined by a second cylindrical portion, the diameter of which is greater than the diameter of the first cylindrical portion and smaller than the diameter of the third die opening. By means of this third deep-drawing step, firstly, the diameter-to-height ratio of the cup-shaped blank is altered further in the direction of the desired final shape, wherein a further decrease of the wall thickness in the transition region between the side wall and the piston head occurs, and, secondly, in the vicinity of the open end of the blank there is formed in the side wall an inner step, which is very advantageous for subsequent machining steps. 
   The step in the inner surface is preferably used, after deep drawing into a cup shape, to form an inwardly curved piston head by means of a first stamping operation in that a step-shaped punch comes into engagement with the step in the side wall of the blank and presses the blank into a bottom forming die. 
   This first stamping operation is preferably followed by a fourth deep-drawing step, in which the cup-shaped blank is pressed once more by means of a step-shaped punch, which comes into engagement with the step in the side wall of the blank, through a fourth circular die opening, the diameter of which is smaller than the diameter of the third die opening. In this fourth drawing step the cylindrical outer surface of the side wall is formed. 
   According to a particularly preferred development of the method according to the invention, subsequent to forming of the cylindrical outer surface of the side wall a second stamping operation is effected by means of a bottom forming die and a step-shaped punch, which comes into engagement with the step in the side wall of the blank, in order to form a transition region between the piston head and the side wall. In said case, the region of the bottom die touching the piston head is advantageously cap-shaped in the centre and truncated-cone-shaped at the edge. 
   Forming of the transition region between the piston head and the side wall is preferably followed by a third stamping operation by means of a bottom forming die and a, once more, step-shaped punch, which comes into engagement with the step in the side wall of the blank, in order to lend the piston head its final configuration. 
   After forming of the annular groove, at least the outer surface may be subsequently machined. The subsequent machining of the outer surface may comprise one or more steps. The subsequent machining preferably comprises at least one of the steps: grinding, coating and polishing. 
   An intermediate product according to the invention, which arises within the framework of manufacture of the hydraulic piston, is manufactured by punching a disk-shaped round blank out of a piece of rolled sheet metal, by deep-drawing the disk-shaped round blank into a cup shape using a bottom die and a punch, as well as by stamping the cup-shaped blank to form the piston head and the cylindrical outer surface of the hydraulic piston. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  a first embodiment of a hydraulic piston according to the invention in longitudinal section, 
       FIG. 2  a deep-drawing step effected in the framework of manufacture of the piston of  FIG. 1 , 
       FIG. 3  a stamping step effected in the framework of manufacture of the piston of  FIG. 1 , 
       FIG. 4  a punching step during manufacture of the piston of  FIG. 13 , 
       FIG. 5  a first deep-drawing step during manufacture of the piston of  FIG. 13 , 
       FIG. 6  a second deep-drawing step during manufacture of the piston of  FIG. 13 , 
       FIG. 7  a third deep-drawing step during manufacture of the piston of  FIG. 13 , 
       FIG. 8  a first stamping step during manufacture of the piston of  FIG. 13 , 
       FIG. 9  a fourth deep-drawing step during manufacture of the piston of  FIG. 13 , 
       FIG. 10  a second stamping step during manufacture of the piston of  FIG. 13 , 
       FIG. 11  a third stamping step during manufacture of the piston of  FIG. 13 , 
       FIG. 12  an almost finished piston in longitudinal section, and 
       FIG. 13  a second embodiment of a hydraulic piston according to the invention in longitudinal section. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows a first embodiment of a hydraulic piston  10  for a hydraulic brake of a motor vehicle in longitudinal section. The piston  10  is hollow and has an open end  12 . It comprises a piston head  14 , which is integrally connected to a side wall  16 . 
   The side wall  16  has a cylindrical outer surface  18 , let into which is an annular groove  20 , which is situated in an end portion of the side wall  16  adjacent to the open end  12 . 
   The side wall  16  further has an inner surface  22 , which is cylindrical in a region  24  adjoining the open end  12  and extending axially over the annular groove  20 . From the end of this region  24  to the piston head  14  the thickness of the side wall  16  continuously decreases, while the cylindrical outer shape is retained. 
   The dimensions of the hydraulic piston  10  may be selected in accordance with the intended application. The piston  10  may be, for example, 50.5 mm high and designed for an operating pressure of at least 160 bar. The wall thicknesses a, b and c are then so selected that the piston  10  particularly in the region of the annular groove  20  is stable under axial loads without increasing an optimum wall thickness c, which ensures a resistance to both axial thrust forces and radial compressive forces. For the described piston size, the dimension a may be e.g. 1.9 mm, the dimension b 3.9 mm and the dimension c 3 mm. Starting from the open end  12 , the cylindrical region  24  of the inner surface  22  extends approximately 16 mm in axial direction. The region of the continuous wall thickness reduction then extends over a total of 31 mm in axial direction. 
     FIG. 2  illustrates a deep-drawing step for manufacture of the piston  10 . Before this deep-drawing step, a disk-shaped round blank  26  is punched out (not shown) of rolled sheet metal. This blank  26  is pressed by means of a cylindrical punch  28  through a bottom die  30  having a circular die opening  32 . The dimension b is defined by the clearance between the punch  28  and the bottom die  30 .  FIG. 2  shows, in the left half, the initial state and, in the right half, the state after effecting the deep-drawing step. The decrease of the wall thickness of the side wall  16  from the open end  12  in the direction of the piston head  14  is apparent. In this state the outer surface  18  of the side wall  16  still does not have a cylindrical shape. 
   The deep-drawing step illustrated in  FIG. 2  need not be effected all at once but may be divided into a plurality of deep-drawing steps. In a first deep-drawing step, then, a still relatively flat and wide cup-shaped blank  26  is first produced, which in subsequent deep-drawing steps becomes progressively narrower and higher in order to come closer and closer to the ultimately desired shape. 
     FIG. 3  shows a subsequent stamping step, in which the now cup-shaped blank  26  is pressed by means of a further punch  34  into a further bottom die  36  in order to lend the outer surface  18  the cylindrical shape and the piston head  14  an inwardly curved shape. To achieve this, the punch  34  has a concave end face and the bottom die  36  has a convex base. By means of the stamping step shown in  FIG. 3  the outer surface  18  and the piston head  14  are simultaneously brought into the desired shape. 
     FIG. 13  shows a second embodiment of a hydraulic piston  10 , which differs from the embodiment illustrated in  FIG. 1  in that the piston head  14  in its middle region is spherical-cap-shaped and in a radially outer region is truncated-cone-shaped. 
     FIGS. 4 to 11  show a method that is eminently suitable for manufacturing the piston of  FIG. 13 . Each of  FIGS. 4 to 11  shows, on the left, an initial state and, on the right, the state reached on completion of the respective machining step. 
     FIG. 4  firstly shows a punching step, in which a disk-shaped, round blank  40  is punched by means of a punch  44  and a bottom die  46  out of a rolled sheet metal  42  of the quality 08Yu and in the present case has a thickness S 0  of 3.6 mm and a diameter D of 124 mm. 
     FIG. 5  shows a first deep-drawing step, in which the disk-shaped blank  40  is pressed by means of a first punch  48  through a first circular die opening  50  of a bottom die  52  and hence formed into a cup shape. In the present case, the first punch  48  has a diameter d 1  of 72 mm, while the first die opening  50  has a diameter D 1  of 80 mm. The outside diameter of the now cup-shaped blank  40  at its open end is therefore 80 mm and the wall thickness S 1  in the region of its open end is 4 mm. As a result of the drawing operation the wall thickness decreases continuously in the direction of the head until it reaches a value S 2  of approximately 3.5 mm in the transition region between the side wall and the head. 
     FIG. 6  shows a second deep-drawing step, in which the blank  40  is brought from an outside diameter of 80 mm to an outside diameter of 65 mm in that a second cylindrical punch  54  presses the cup-shaped blank  40  through a second circular die opening  56  of a bottom die  58 . The outside diameter d 2  of the second punch  54  is 57 mm, while the diameter D 2  of the second die opening  56  is 65 mm. By means of this deep-drawing step a cup-shaped blank  40  is obtained, which has an inside diameter of 57 mm and an outside diameter that decreases from 65 mm at the open end continuously in the direction of the head. The thickness of the side wall accordingly decreases from a value S 3  of 4 mm to a value S 4  of 3.45 mm in the region of the transition between the side wall and the head. 
     FIG. 7  shows a third deep-drawing step, in which the cup-shaped blank  40  is brought from an outside diameter of 65 mm to an outside diameter of 56.5 mm in that a third cylindrical punch  60  presses the blank  40  through a third circular die opening  62  of a bottom die  64 . The outside diameter d 3  of the third punch  60  is 48.6 mm, while the diameter D 3  of the third die opening  62  is 56.5 mm. Once more, as a result of the drawing operation the wall thickness of the side wall of the blank  40  decreases in the direction of its head, close to the open end of the blank  40  said wall thickness is defined by the clearance between the side wall and the bottom die  64  and has a value S 5  of 3.95 mm, while close to the head it has a value S 6  of 3.4 mm. As is apparent from  FIG. 7 , the third punch  60  comprises a first cylindrical portion, which forms its free end and has the already mentioned outside diameter d 3  of 48.6 mm, and an adjoining, second cylindrical portion having an outside diameter of 53 mm. At the end of the third deep-drawing step this second, larger-diameter portion of the third punch  60  forms a step  41  (see  FIG. 12 ) in the inner surface of the side wall of the blank  40  by reducing the thickness of the side wall close to the open end. The radial extension of the formed step is 2.2 mm. 
     FIG. 8  shows a first stamping step, in the course of which the head of the blank  40  is curved inwards. For this purpose, a step-shaped punch  66  of a complementary design to the inner surface of the blank  40  presses the blank  40  into a bottom forming die  68  in that the punch  66  comes into engagement with the step  41  formed in the side wall of the blank  40 . The inside diameter D 4  of the forming die opening is 56.5 mm, with the result that the outside diameter of the blank  40  is not further reduced by said opening. The base of the forming die opening is formed by a punch-shaped ejector  70 , of which the surface facing the blank  40  is spherical-cap-shaped. Thus, the head of the blank  40  is deformed inwards to a concavity k 1  of 8 mm. In said case, the part of the side wall adjacent to the head is drawn slightly radially inwards. 
     FIG. 9  shows a fourth deep-drawing step, in which the outside diameter of the blank  40  is brought from 56.5 mm to 54.3 mm in that a step-shaped punch  72 , which comes into engagement with the step  41  formed on the inner surface of the blank  40 , presses the blank  40  through a fourth circular die opening  74  of a bottom die  76 . A first cylindrical portion forming the free end of the punch  72  in said case has an outside diameter d 4  of 46.4 mm, while the fourth die opening  74  has a diameter D 5  of 54.3 mm. By means of this fourth deep-drawing step the outer surface of the side wall of the blank  40  is formed into a cylinder having a diameter of 54.3 mm. Because of the cylindrical portion having the diameter d 4  and forming the free end of the punch  72 , the inner surface of the blank  40  also remains cylindrical in this region. In the adjoining region the inside diameter of the blank  40  increases continuously as far as the head, because the wall thickness of the side wall decreases continuously. 
     FIG. 10  shows a second stamping step, in which the transition region between the side wall and the head of the blank is formed. For this purpose, a likewise step-shaped punch  78  by coming into engagement with the shoulder  41  provided on the inner surface of the blank  40  presses the blank  40  into the opening of a bottom forming die  80 , the base of which is formed by a punch-like ejector  82 . The surface of the ejector  82  facing the blank  40  is spherical-cap-shaped in the centre and truncated-cone-shaped at the edge. In said manner, the transition region between the side wall and the head of the blank  40  is applied against the forming die opening and forms a transition radius R 1  of 2 mm. The concavity k 1  of 8 mm is retained. The inside diameter of the forming die opening is 54.3 mm, i.e. the external dimension of the blank  40  is not altered. 
     FIG. 11  shows a third stamping step, in which the head of the blank  40  receives its final shape. For this purpose, a step-shaped punch  84  designed for engagement with the shoulder  41  in the blank  40  presses the blank  40  into the opening of a bottom forming die  86 , the base of which is formed by a punch-shaped ejector  88 . By means of the ejector  88  the concavity is increased to a value k 2  of 9 mm. The spherical-cap-shaped configuration of the head of the blank in the centre, and the truncated-cone-shaped configuration of the head at the edge is in said case retained. The transition radius between the side wall and the head of the blank  40  increases to a value R 2  of 3.4 mm. 
     FIG. 12  shows the almost finished hydraulic piston as a blank in longitudinal section. It is possible to see the cylindrical portion  24  of the inner surface  22 , the side wall thickness decreasing continuously from the portion  24  to the head  14 , a central region  90  of the head  14  with a spherical-cap-shaped configuration, and a truncated-cone-shaped edge region  92  of the head  14  that extends over a distance d, which is not greater than three times the value of wall thickness of the side wall  16  in the vicinity of the head  14 . Neither the inner surface  22  nor the cylindrical outer surface  18  require further machining. 
   To complete the hydraulic piston  10 , the part of the side wall  16  forming the step or shoulder is removed (see  FIG. 13 ) and the annular groove  20  is worked into the outer surface  18 , e.g. by cutting. 
   Usually, at least the outer surface  18  of the hydraulic piston  10  is subsequently machined after forming the annular groove  20 . The subsequent machining may comprise e.g. a grinding operation as well as subsequent coating with a corrosion- and abrasion-resistant material. The coated outer surface  18  may then be subjected to a polishing operation. 
   In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiments. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.