Title of Invention

"A POWER-METALLURGY MANUFACTURED SHOCK-ABSORBER PISTON BODY AND A METHOD OF MANUFACTURING THEREOF"

Abstract

A powder-metallurgy manufactured shock-absorber piston body for a piston-cylinder arrangement, comprising a one-piece piston body (6) which is provided on its peripheral surface, in a region adjacent to a piston end face (4.1), with a circumferentially-extending peripheral web (12) which projects beyond the peripheral surface and to which are connected longitudinally extending supporting webs (10) parallel to one another with mutual spacing and extending so far as the other piston end face (5.1), characterized in that at least some of said webs (10) being provided with at least one transverse groove (11.1) between the two piston end faces (4.1, 5.1) wherein two respective adjacent supporting webs (10) form a groove-type recess (11), the recess (11) being open in the longitudinal direction at their ends remote from the peripheral web (12), wherein a sleeve-type seal (9) made of heat deformable sealing material is mouldable onto the piston body (6) in such a way that both the peripheral web (12) and the supporting webs (10) are moulded into the material of the sleeve-type seal (9) over at least part of their height. Fig. 3

Full Text

The present invention relates to a powder-metallurgy manufactured shock-absorber piston body and a method of manufacturing thereof EP-A-0 658 611 discloses a shock-absorber piston having a piston body which is pro^/lded on its peripheral surface with a circumferential web, to which is connected a plurality of axially extending webs on one side of the circumferential web. A seal made of thermoplastic plastics is injected onto this piston body in an injection-moulding process, wherein in particular the axially extending webs with their grooves lying therebetween have the function of reliably anchoring the sealing material and guiding the piston, the grooves being completely filled with sealing rriaceriai. The seal, applied by injection moulding, permits close Lolerances in order to avoid wihat is referred to as "blow-by" and thus produces a reliable seal between the associated cylinder chambers. The method of manufacturing injection-moulded seals of this type is relatively costly, A piston-cylinder arrangement, the piston body of which has a plurality of circumferential grooves on its peripheral surface, is known from US-A-3 212 411. To apply the seal, a cup-shaped preform made of PTFE (polytetrafluoroethylene) is provided which is initially placed loosely onto the piston body. The piston body thus prepared is then initially pressed into a forming and sizing cylinder heated to a high temperature, wherein the PTFE material is pressed into the grooves on the peripheral surface of the piston body under the influence of heat. Next, the piston body with the pressed-on seal is cooled In a correspondingly formed cooling cylinder. The grooves are completely filled with the sealing material, thereby producing a positive-locking connection between the seal and the peripheral surface of the piston body. For use as a shock-absorber piston, the preform base, which covers the end face of the piston body on one side, then also has to be removed. EP-A-682 190 discloses a shock-absorber piston, the manufacture of which differs from the above-described method essentially only in that a punch-cut annular disc is used instead of a cup-shaped preform, for applying the seal. This annular disc is placed over one end of the piston body. The piston body thus prepared is again pressed into a heated forming and sizing cylinder, the annular disc being placed in the form of a band around the peripheral surface of the piston body and then being pressed under the influence of heat into the grooves extending in the circumferential direction of the piston body. Next, the piston provided with its pressed-on seal is passed through a cooling tube. In this case too, the sealing material practically completely fills the grooves so that the seal is positively connected to the peripheral surface of the piston body. Both the methods described above have the disadvantage that considerable pressures can be necessary for forming and completely pressing the sealing material into the grooves on the peripheral surface of the piston body, and the sealing material forming the seal undergoes severe deformation, which adversely affects the structure of the sealing material. A piston for a piston-cylinder arrangement, in particular a shock-absorber piston, is known from DE-A-198 47 342, comprising a piston body which is provided on its peripheral surface, in a region adjacent to one end of the piston, with a circumferentially-extending peripheral web which projects beyond the peripheral surface and to which are connected longitudinally extending supporting webs arranged parallel to one another with mutual spacing and extending towards the other end of the piston, wherein two respective adjacent supporting webs define a groove-type recess, the recesses being open in the longitudinal direction at their ends remote from the peripheral web, and comprising a sleeve-type seal which is made of a heat-deformable sealing material and is moulded onto the piston body in such a way that both the peripheral web and the supporting webs are moulded into the material of the seal over only part of their height. This knoAvn solution has shown that, for a good sealing action between the sleeve-type seal of the piston on the one hand and the outer contour of the piston body on the other hand, it is not necessary for the seal to rest tightly against the piston body over the whole periphery. For a number of applications, it is sufficient if the sealing sleeve only rests firmly against the relatively narrow peripheral web In the circumferential direction. Surprisingly, it has also proved to be the case that, for a satisfactory and reliable connection between the seal and the piston body, it is not necessary for the groove-t5rpe recesses between the longitudinally extending supporting webs to be completely filled with the sealing material. Consequently, sufficient space remains, into which the sealing material can pass in the event of expansion due to temperature increases, while the piston is still satisfactorily guided by the longitudinally extending supporting webs. The arrangement of only one peripheral web, projecting beyond the peripheral surface, in order to fix the sleeve-type seal may not always be sufficient. Consequently, it is desirable to provide two end peripheral webs which are connected by the longitudinally extending supporting webs arranged parallel to one another with mutual spacing. However, the powder-metallurgical manufacture of a piston body of this type by pressing a green compact from a sinterable metal powder Avith subsequent sintering gives rise to considerable problems from the point of view of shaping technology, and thus it was proposed in DE-A-101 08 246 to divide the piston body into two partial bodies each having a peripheral web at one end, from which the longitudinally extending, mutually spaced supporting webs extend. The two partial bodies can both be formed into green compacts from a sinterable metal powder and then sintered without great difficulty. The complete piston is then assembled from the two finished sintered parts so that the peripheral webs are each arranged at the piston end remote from the parting plane. By chamfering the supporting webs at their end facing the parting plane, it is then possible to provide an additional transverse groove. The disadvantage of this design lies in the costly joining process for the two partial bodies if these partial bodies have to be joined together in a defined mutual arrangement. This is the case e.g. with piston bodies for shock-absorber pistons because the partial channels provided in the two partial bodies have to be precisely aligned. The object of the invention is to provide a piston. In particular a shock-absorber piston, which can be manufactured more easily. According to the invention, this object is achieved by a one-piece piston body having the features of claim 1. A method of manufacturing the piston body according to the invention is set out in claim 3. Further configurations of the invention are set out in the following description of embodiments and the sub-claims. The present invention relates to a power-metallurgy manufactured piston body for a piston-cylinder arrangement, in particular a shock-absorber piston, comprising a one-piece piston body (6) which is provided on its peripheral surface, in a region adjacent to a piston end face (4.1), with a circumferentially-extending peripheral web (12) which projects beyond the peripheral surface and to which are connected longitudinally extending supporting webs (10) arranged parallel to one another with mutual spacing and extending so far as the other piston end face (5.1), characterized in that at lease some of said webs (10) being provided with at least one transverse groove (11.1) between the two piston end faces (4.1, 5.1) wherein two respective adjacent supporting webs (10) define a groove-type recess (11), the recess (11) being open in the longitudinal direction at their ends remote from the peripheral web (12), wherein a sleeve-type seal (9) made of heat deformable sealing material is mouldable onto the piston body (6) in such a way that both the peripheral web (12) and the supporting webs (10) are moulded into the material of the sleeve-type seal (9) over at least part of their height. The present invention also relates to a method of manufacturing a one-piece piston body for a piston-cylinder arrangement, in particular a shock-absorber piston, which is provided on its peripheral surface, in a region adjacent to a piston end face (4.1), with a circumferentially-extending peripheral web (12) which projects beyond the peripheral surface and to which are connected longitudinally extending supporting webs (10) arranged parallel to one another with mutual spacing and extending as far as the other piston end face (5.1), wherein two respective adjacent supporting webs (10) define a groove-type recess (11), the recesses (11) being open in the longitudinal direction at their ends remote from the peripheral web, characterized in that in a first step a green compact, which has the peripheral web and the longitudinally extending supporting webs, is pressed from a sinterable metallurgical powder, in that in a second step the green compact is finally sintered to form a blank (6.1), in that in a third step transverse grooves (11.1) are formed in at least some of the supporting webs (10) by cold forming by means of radially guided impressing tools (14) and by material displacement, and in that in a fourth step the blank (6.1), which is thus provided with transverse grooves (11.1), is sized by being pressed with sizing tools to its final form. The invention will be further described with reference to schematic drawings of an embodiment, wherein: Fig. 1 shows a partial section, in the axial direction, through a piston-cylinder arrangement for a shock absorber; Fig. 2 shows a side view, partly in section, of a finished sintered blank for a shock absorber; Fig. 3 shows a schematic horizontal section through the blank along the line III-III in fig. 2, Avithout a sealing sleeve; Fig. 4 shows a side view of a ready formed piston body, partly in section, with a sealing sleeve applied; Fig. 5 shows an enlarged view of a partial section in accordance with fig. 4; Fig. 6 shows an installation for forming transverse grooves and sizing a piston-body blank as a step of the manufacturing method; Fig. 7 shows the installation according to fig. 6 during the forming and sizing step; Fig. 8 shows the Installation according to fig. 6 at the end of the forming and sizing step; Fig. 9 shows a vertical section through the forming stage of the tool according to fig. 6 with an inserted blank during the forming process; Fig. 10 shows an enlarged view of the design of the Impressing tool; Fig. 11 shows a plan view of the forming stage shown In fig. 9 with one arrangement of the impressing tools. Fig. 1 shows, in the form of a functional diagram, an axial section through a shock absorber which joins together two parts which are movable relative to one another, for example a vehicle axle and a vehicle frame. The shock absorber has a cylinder part 1 which is connected to one of the two parts which are movable relative to one another. A piston 2 is guided in the cylinder 1 and is fixed to a piston rod 3 which is fastened by its free end to the other one of the two parts which are movable relative to one another. In this case, the cylinder 1 is closed at both ends and filled with a hydraulic fluid so as to form a double-acting piston-cylinder arrangement, the piston separating two cylinder chambers 4, 5 from one another. The piston body 6 of the piston 2 has a plurality of passage channels 7, 8 extending side by side. The passage channels 7, 8 are each closed at their outlet end, the function of which is still to be explained, by a throttle valve 7.1 and 8.1 respectively. The arrangement here is such that in each case a plurality of passage channels 7 and a plurality of passage channels 8 are arranged in an alternating maanner about the cylinder axis. The peripheral surface of the piston 2 is provided with a sleeve-type seal 9 which ensures a sealing action between the cylinder chamber 4 and the cylinder chamber 5. Upon movement of the piston 2 into the cylinder chamber 4, the fluid is forced through the passage channels 7 counter to the restoring force of the throttle valves 7.1. The passage openings 8 are kept closed by the pressure of the fluid chamber 4, said pressure acting on the throttle valves 8.1. Upon movement in the opposite direction, the passage channels 7 are closed by the throttle valves 7.1, while the fluid can flow back through the passage channels 8 out of the cylinder chamber 5 and into the cylinder chamber 4. As a piston body of this type moves in a reciprocating manner, as indicated hereinabove, and as, under high loads, the sleeve-type seal 9 is correspondingly loaded, also during the return movement, an end peripheral web at one end of the piston body may no longer be sufficient for securely fixing the sleeve-type seal against loading in the axial direction. In order to be able to manufacture a piston body of this type in one piece so that additional transverse grooves are available for fixing the sleeve-type seal, a green compact is first pressed from sinterable metal powder in the form shown in flg. 2 and described in further detail hereinbelow, and then finally sintered to form a blank 6.1. The blank 6.1 Is provided on its peripheral surface with a plurality of longitudinally extending supporting webs 10 which each define corresponding groove-type recesses 11 and which extend from the end face 4.1, with which a peripheral web 12 is associated, to the other end of the blank 6.1 in the region of the end face 5.1. The peripheral web 12 and the longitudinally extending supporting webs 10, which form an outer surface of the piston body 6, are at the same level. For better illustration, this outer surface is hatched. The supporting webs 10 and the corresponding grooves 11 extend paraxially. Fig. 3 shows the piston body 6 in a horizontal section along the line HI-III in fig. 2 so that the structure of supporting webs 10 and grooves 11 can be seen. Fig. 4 shows the piston body 6, In partial section and in side view without the seal 9, in its final form after the formation of transverse grooves 11.1 in the surface of the longitudinal webs 10. The transverse grooves 11.1 are formed in the sintered blank 6.1 by an impressing process, as will be described in further detail hereinafter, and the blank 6.1 is then sized after the impressing process. The seal 9 is then applied to the finished, sized piston body 6. The sleeve-type seal 9 arranged on the periphery of the piston body 6 is made of a heat-deformable plastics, preferably PTFE. In the embodiment shown here, the sleeve-type seal 9 has been moulded onto the piston body 6 by hot forming. During the hot forming of the sealing material, which can comprise a prefabricated annular disc or a prefabricated tube section, the supporting webs 10 and the peripheral web 12 are moulded into the material of the sleeve-type seal 9 over only part of their height in order to leave a certain amount of space between the material of the seal 9 and the base of the groove-type recesses 11 so that, during formation of the seal 9, the sealing material can flow freely and without force into the groove 11. During this forming process, the cylindrical outer surface 13 of the seal 9 is simultaneously sized so that the desired tolerances with respect to the inner diameter of the cylinder 1 can be observed. Since in particular when a piston-cylinder arrangement of this type is used as a shock absorber, the whole system heats up during operation, this remaining space in the base of the groove also permits expansion of the sealing material into the groove within certain limits so that wear to the seal is reduced on the cylindrical peripheral surface of the seal 9 adjacent to the edges. The piston 2 is supported so as to be overall practically tilt-free over its height. The seal 9 is supported against a respective one of the groove ends on the inside of the peripheral web 12. Fig. 5 shows the positioning of the seal 9 on a greatly enlarged scale in a partial section corresponding to fig. 4. The seal 9 here is made of a homogeneous material which, during the above-described hot-forming process, is partly moulded into the longitudinally extending grooves 11 and the transverse grooves 11.1, while the longitudinally extending supporting webs 10 and the peripheral web 12 are correspondingly moulded into the material. For better illustration, the material moulded into the longitudinally extending groove 11 is cross-hatched in order to make it clear that the longitudinally extending grooves 11 are not completely filled. In order not to overstress the material of the seal 9 in the longitudinally extending groove 11 in the transition region B to the peripheral web 12 during the hot-forming process, it is provided that the longitudinally extending groove 11 in this region has a decreasing, i.e. reduced depth in the. direction of the peripheral web 12. The resulting reduction in the axial supporting surface for the sealing material on the peripheral web 12 is compensated for by the additional, axially acting supporting surfaces of the transverse grooves 11.1, giving rise to the advantage that, in this region which is highly stressed during later operation, the material structure is less stressed by the hot-forming process and the material therefore has greater stability. The method according to the invention for manufacturing a piston body of the type shown in fig. 4 will be described in further detail with reference to figs. 6, 7 and 8. After a green compact according to fig. 2 has initially been pressed from a sinterable metallurgical powder in a first step and then finally sintered to form a blank 6.1, a blank 6.1a is taken from a stock SI of blanks 6.1, as shown in fig. 6, and supplied to a pressing installation P which comprises a first pressing station I and a second pressing station II. In addition to the usual upper die 15.1 and lower die 16, the first pressing station I is provided with radially guided impressing tools 14, by means of which the transverse grooves 11.1 are formed in the supporting webs 10, as will be explained in further detail herelnbelow. The blank 6.1b, which is provided with transverse grooves in the preceding impressing step in the pressing station I, is then supplied to the second pressing station II, in which the thus prepared blank 6. lb is sized accordingly both with respect to the piston end faces and with respect to the outer circumference of the peripheral web and the supporting webs to produce a finished piston body 6. This process is shown in fig. 7. In the pressing station II, a lower die 16.2 is associated with the movable upper die 15.2 and has a moulding surface which corresponds to the moulding surface of the blank 6.1a or 6.1b to be inserted. In the first pressing station I, the blank 6.1a is in practice only held by the movable die 15.1 and the fixed counter-die 16.1, while in the second pressing station II the pressing die 15.2 is subjected to the action of pressure so that final sizing deformation of the Inserted blank 6.1b takes place in order to produce the finished piston body 6. As shown in fig. 8, the pressing dies 15 are then withdrawn and the blank 6.1b, which is now provided with transverse grooves, and the finished sized piston body 6 are pushed out of the pressing mould by an ejection tool 17 at both pressing stations so that the blank 6.1b can be transported to the pressing station II and the finished piston body 6 can be deposited in a store S2, from which the finished piston bodies are then removed for application of the sleeve-type seal 9. Fig. 9 shows the arrangement with an inserted blank 6.1a in a vertical section through the first pressing station I. The retracted upper die 15.1 is not shown here. The impressing tools 14 are pressed into the material of the supporting webs 10 and the transverse grooves 11.1 are formed. This can be seen from the enlarged view in fig. 10. The radially advanceable impressing tools 14 are formed in the manner of sliders and, as can be seen from figs. 6, 7, 10 and 11, are radially advanceable towards the blank 6.1a. In the embodiment shown here, three impressing tools 14 are provided and are arranged with the same angular spacing relative to one another. Depending upon size, the arrangement of more than three impressing tools can be provided. The arrangement of only two diametrically opposing impressing tools is in principle also possible. As can be seen from the enlarged view according to fig. 10, each impressing tool 14 has two parallel, blade-like impressing edges 18 for forming two respective transverse grooves 11.1 in the supporting webs 10 of a blank 6.1. Material is displaced by the impressing edges 18 when pressure is exerted on the impressing tools 14, thereby cold forming the sintered material as shown in fig. 7, and the transverse grooves 11.1 are formed. As can be seen from fig. 10, this cold-forming process only takes place over part of the overall height of the supporting webs 10, with the result that the transverse grooves 11.1 formed have a smaller depth than the grooves 11. The forming and sizing process can also be carried out with only one pressing station, which in structure corresponds substantially to the pressing station I. Only the upper die and the lower die are formed so as to correspond to the upper die 15.2 and the lower die 16.2. Accordingly, in the first step the blank 6. la is fixed by the upper die and the lower die only so that the transverse grooves 11.1 can be formed. Next, the impressing tools 14 are withdrawn and the pressure on the upper die and lower die is increased and the blank sized and only then removed from the mould as a finished piston body. WE CLAIM: 1. A powder-metallurgy manufactured shock-absorber piston body for a piston-cylinder arrangement, comprising a one-piece piston body (6) which is provided on its peripheral surface, in a region adjacent to a piston end face (4.1), with a circumferentially-extending peripheral web (12) which projects beyond the peripheral surface and to which are connected longitudinally extending supporting webs (10) parallel to one another with mutual spacing and extending so far as the other piston end face (5.1), characterized in that at least some of said webs (10) being provided with at least one transverse groove (11.1) between the two piston end faces (4.1, 5.1) wherein two respective adjacent supporting webs (10) form a groove-type recess (11), the recess (11) being open in the longitudinal direction at their ends remote from the peripheral web (12), wherein a sleeve-type seal (9) made of heat deformable sealing material is mouldable onto the piston body (6) in such a way that both the peripheral web (12) and the supporting webs (10) are moulded into the material of the sleeve-type seal (9) over at least part of their height. 2. A powder-metallurgy manufactured shock-absorber piston body as claimed in claim 1, wherein the depth of the groove-type recesses (11) is smaller in a region (B) adjacent to the peripheral web (12) than in the region of the end face (5.1) remote from the peripheral web (12). 3. A powder-metallurgy manufactured shock-absorber piston body as claimed in claim 1 or 2, wherein at least two transverse grooves (11.1) are provided. 4. A powder-metallurgy manufactured shock-absorber piston body as claimed in any one of claims 1 to 3, wherein the transverse groove (11.1) has a smaller depth than the groove-type recess (11). 5. A powder-metallurgy manufactured shock-absorber piston body as claimed in any one of claims 1 to 4, wherein a sleeve-type seal (9) made of a heat-deformable plastics, preferably PTFE, is moulded onto the piston body by hot forming, wherein the seal (9) does not completely fill the groove-type recess (11), and the transverse grooves (11.1) are completely filled. 6. A method of manufacturing a shock-absorber piston for a piston-cylinder arrangement as claimed in claim 1, wherein in a first step a green compact, which has the peripheral web and the longitudinally extending supporting webs, is pressed from a sinterable metallurgical powder, in that in a second step the green compact is finally sintered to form a blank (6.1), in that in a third step transverse grooves (11.1) are formed in at least some of the supporting webs (10) by cold forming by means of radially guided impressing tools (14) and by material displacement, and in that in a fourth step the blank (6.1), which is thus provided with transverse grooves (11.1), is sized by being pressed with sizing tools to its final form. 7. A method as claimed in claim 6, wherein at least two impressing tools (14) radially pressable against the blank and each having at least one blade-like impressing edge are used. 8. A method as claimed in claim 6 or 7, wherein a pressing installation (P) with at least two pressing stations (I, II) is used, wherein a blank (6.1a) is provided with transverse grooves (11.1) in the first pressing station (I), and a blank (6.1b) provided with transverse grooves (11.1) is simultaneously sized in the second pressing station (II). 9. A method as claimed in any one of claims 6 to 8, wherein in the third step for forming the transverse grooves (11.1), the blank (6.1a) is in practice only held by a movable upper die (15.1) and a fixed lower die (16.1) while the transverse grooves (11,1) are being formed in the supporting webs (10) by the radially guided impressing tool (14) over a smaller depth than the depth of the groove-type recess (11). 10. A method as claimed in any one of claims 6 to 9, wherein in the fourth step the blank (6.1b) is sized, with respect to the piston end faces (4.1; 5.1) and the outer circumference of the peripheral web (12) and the supporting webs (10), by a movable under die (15.2), on which pressure is exerted, and a fixed lower die (16.2), while the radially guided impressing tool (14) has advanced into the transverse grooves (11.1).

Documents:

1229-delnp-2005-abstract.pdf

1229-delnp-2005-claims.pdf

1229-delnp-2005-complete specification (as files).pdf

1229-delnp-2005-complete specification granted.pdf

1229-DELNP-2005-Correspondence Others-(22-03-2011).pdf

1229-delnp-2005-correspondence-others.pdf

1229-delnp-2005-correspondence-po.pdf

1229-delnp-2005-description (complete).pdf

1229-delnp-2005-drawings.pdf

1229-delnp-2005-form-1.pdf

1229-delnp-2005-form-18.pdf

1229-delnp-2005-form-2.pdf

1229-DELNP-2005-Form-27-(22-03-2011).pdf

1229-delnp-2005-form-3.pdf

1229-delnp-2005-form-5.pdf

1229-delnp-2005-gpa.pdf

1229-delnp-2005-pct-210.pdf

1229-delnp-2005-pct-301.pdf

1229-delnp-2005-pct-304.pdf

1229-delnp-2005-pct-308.pdf

1229-delnp-2005-pct-332.pdf