AXIAL PISTON MACHINE HAVING A SEAL RING WHICH IS SPHERICAL IN SECTIONS

Abstract

The invention relates to an axial piston machine in which pistons carry out a stroke movement in cylinders and in which the pistons have a seal ring receptacle for a seal ring. In order to improve robustness, wear resistance, friction and stick-slip behavior, according to the invention, the seal ring is spherical, wherein the curvature radius of the seal ring, which is spherical in regions, substantially corresponds to half the diameter of the cylinder inner wall.

Claims

1. An axial piston machine in which pistons in cylinders execute a stroke movement, and in which the pistons have a sealing ring seat for a sealing ring, wherein the sealing ring seat is operatively configured such that it permits a movement of the sealing ring transverse to a longitudinal axis of the piston, wherein the sealing ring is spherical in shape at least in a region which effects a seal during the stroke movements on inner walls of the cylinder, and wherein the radius of curvature of the sealing ring is formed in a spherical shape in certain regions and corresponds substantially to half a diameter of the cylinder.

2. The axial piston machine according to claim 1, wherein the sealing ring is made of non-deformable material.

3. The axial piston machine according to claim 1, wherein the sealing ring comprises a rigid material which is resistant to wear.

4. The axial piston machine according to claim 1, wherein the sealing ring is made of metal or a metal alloy.

5. The axial piston machine according to claim 1, wherein the sealing ring is made of oxide ceramic or non-oxide ceramic.

6. The axial piston machine according to claim 1, wherein the sealing ring seat comprises a pin having a pin diameter and the sealing ring has a central inner opening corresponding to the pin, and wherein the an inner diameter of the sealing ring is greater than the pin diameter.

7. The axial piston machine according to claim 1, wherein a cross-section of the sealing ring is operatively configured such that, at a high operating pressure, a deformation of the sealing ring by the an operating pressure largely compensates for a widening of the inner wall of the cylinder by the operating pressure.

8. The axial piston machine according to claim 7, wherein a central inner opening of the sealing ring has a circumferential bead-like recess.

9. The axial piston machine according to claim 7, wherein a central inner opening of the sealing ring has a stepped profile.

10. The axial piston machine according to claim 7, wherein the piston is configured to operatively enable a pressure equalization between the a piston interior and an interior of the sealing ring.

11. The axial piston machine according to claim 10, wherein a horizontal clearance between an inner diameter of the sealing ring and a pin of the sealing ring seat, and a vertical clearance of the sealing ring within the sealing ring seat are selected to be at least great enough that they operatively enable a the pressure equalization between the piston interior and the interior of the sealing ring.

12. The axial piston machine according to claim 10, wherein the pressure equalization between the piston interior and the interior of the sealing ring is enabled by one or more openings in the a cover securing the sealing ring in the sealing ring seat against a movement along the longitudinal axis of the piston and/or one or more pressure equalization bores, which extend from the an upper side of the a pin of the sealing ring seat into the interior of the sealing ring.

13. The axial piston machine according to claim 6, wherein the sealing ring is made of a zirconium oxide ceramic.

14. The axial piston machine according to claim 1, wherein the sealing ring is secured in the sealing ring seat with a cover against a movement along the longitudinal axis of the piston.

15. The axial piston machine according to claim 14, wherein the cover is attached to the piston by means of a screw or by clamping or by pressing.

16. The axial piston machine according to claim 1, wherein the piston is fastened to a piston plate by a first end.

17. The axial piston machine according to claim 16, wherein a piston diameter in the a region between the sealing ring seat and the first end tapers increasingly.

18. The axial piston machine according to claim 17, wherein the piston has the a shape of a truncated cone in the region between the sealing ring seat and the first end.

19. The axial piston machine according to claim 1, wherein the cylinders are distributed over a cylinder barrel around a cylinder barrel axis, and wherein the pistons are distributed over a piston plate around a piston plate axis, and wherein a rotation of the cylinder barrel about the cylinder axis and a rotation of the piston plate about the piston plate axis are synchronized with each other and the synchronization does not take place by a torque transmission via the pistons.

20. The axial piston machine according to claim 1, wherein the cylinders comprise piston bore axes distributed on a first circular line around a cylinder barrel axis, and wherein the pistons comprise piston longitudinal axes distributed on a second circular line around a piston plate axis, and wherein a diameter of the second circular line is greater than the a diameter of the first circular line.

21. The axial piston machine according to claim 1, wherein the axial piston machine is a floating piston machine.

22. The axial piston machine according to claim 1, wherein the axial piston machine is a swashplate machine.

23. A method for producing a sealing ring according to claim 1, wherein a solid sphere is selected as the starting product, and in that two spherical segments are removed parallel to a great circle of the solid sphere to form a spherical disk.

24. The method for producing a sealing ring according to claim 23, wherein a central bore is made through the an axis of rotation of the spherical disk.

Description

[0033] The invention is now described and explained in more detail on the basis of exemplary embodiments depicted in the drawings. The figures show:

[0034] FIG. 1 shows a schematic drawing of an axial piston machine with the pistons designed according to the invention, in a neutral position

[0035] FIG. 2 shows a schematic drawing of an axial piston machine with the pistons designed according to the invention, in a pivoted position

[0036] FIG. 3 shows a frustoconical structure of a piston

[0037] FIG. 4 shows a cylindrical structure of a piston

[0038] FIG. 5 shows a frustoconical piston with the installed sealing ring

[0039] FIG. 6 shows an exemplary embodiment of a symmetrical sealing ring

[0040] FIG. 7 shows an exemplary embodiment of an asymmetrical sealing ring

[0041] FIG. 8 shows an exemplary embodiment of a symmetrical sealing ring with an inner bead

[0042] FIG. 9 shows an exemplary embodiment of a sealing ring with a stepped inner side

[0043] FIG. 10 shows an exemplary embodiment of a sealing ring with a continuous widening of its inner diameter in its upper region

[0044] FIG. 11 shows a piston with a sealing ring with a bead-like inner recess and a pressure equalization bore

[0045] FIG. 12 shows a piston with a sealing ring with a stepped internal profile and a pressure equalization bore

[0046] FIG. 1 and FIG. 2 show the schematic structure of a so-called floating piston machine representative of the construction and function of axial piston machines. FIG. 1 and FIG. 2 show the same floating piston machine in different working states. The structure and function of a floating piston machine are known well enough to the person skilled in the art that in FIG. 1 and FIG. 2 only the interaction of a piston 2 with a cylinder barrel 7, a piston plate 8 and a swashplate 9 is described. The piston plate 8 is supported on the swashplate 9 and is rotatably mounted thereon. FIG. 1 shows the floating piston machine 1 in a neutral state in which the swashplate 9 and cylinder barrel 7 are aligned parallel to one another, whereas FIG. 2 shows the floating piston machine 1 in a state in which the swashplate 9 and the cylinder barrel are not aligned parallel to one another.

[0047] In the exemplary embodiment, a plurality of cylinders 3 is distributed in a circular shape and uniformly around a cylinder barrel axis 70 of a cylinder barrel 7. In the exemplary embodiment, the cylinders 3 are designed as piston bores 3, and are herein referred to as such. However, it is clear to the person skilled in the art that a cylinder 3 can also be manufactured in a manner other than by a piston bore. In order to prevent harmonic vibrations, an odd number of piston bores 3 is usually chosen. Each piston bore 3 has a connecting bore 33 on the upper side 71 of the cylinder barrel 7, via which a pressure medium can be supplied to or discharged from the piston bores 3 on the so-called high-pressure side of the floating piston machine 1.

[0048] The cylinder barrel 7 is mounted such that a rotation about the cylinder barrel axis 70 is allowed. In order to transmit torques, a shaft 72 is arranged on the cylinder barrel 7, which shaft-in an operating mode of the floating piston machine as a pump-provides a drive shaft and-in an operating mode of the floating piston machine as an engine-provides an output shaft. In the exemplary embodiment described, the distance R from a piston bore axis 30 to the cylinder barrel axis 70 is 45 mm, and the piston bores 3 each have an inner diameter D of 15 mm. In order to illustrate the invention better, the figures are not true to scale and provide details in part in a greatly enlarged manner.

[0049] The pistons 2 are rotationally symmetrical. The axis of symmetry of the piston 2 is also referred to below as the longitudinal axis 20 of the piston 2. FIG. 3 shows the basic structure of a piston 2 with a piston head 21 at its upper end and a piston foot 22 at its lower end. In the context of a piston 2, the directional indication “upward” refers to a movement of the piston 2 within the piston chamber 31 in the direction of the piston head 21, while the directional indication “downwards” denotes a movement of the piston 2 within the piston chamber 31 in the direction of the piston foot 22. The piston head 21 typically has a larger diameter than the piston foot 22. The piston 2 can therefore have the shape of a truncated cone in its central region 24, according to FIG. 2. It is important that the diameter of the piston head 21 is selected such that the piston head 21 does not come into contact with an inner wall 32 of the piston bore 3 at any time of the operation of the piston machine. In this respect, the piston 2 can also be designed in its central region 24 in the form of a cylinder, as is shown in FIG. 4.

[0050] The piston plate 8 is designed as a circular disk; a piston plate axis 80 extends perpendicular to the piston plate 8 through the center point of the circular disk. The piston plate 8 is rotatably mounted so that the piston plate 8 can rotate about the piston plate axis 80. The swashplate 9 is also designed as a circular disk, wherein a swashplate axis 90 extends perpendicular to the swashplate 9 through the center point of the circular disk. In the neutral state of the floating piston machine 1, the piston plate axis 80 and the swashplate axis 90 are in a line with the cylinder barrel axis 70.

[0051] In the following, a plane which extends perpendicular about the cylinder barrel axis 70, as a cylinder barrel plane 75, and a plane which extends perpendicular to the piston plate axis is referred to as the piston plate plane 85. In the neutral state, the cylinder barrel plane 75 and the piston plate plane 85 are oriented parallel to one another. When the cylinder barrel 7 rotates, the distance between the bottom 72 of the cylinder barrel 7 and the upper side 81 of the piston plate 8 remains constant in the neutral position. Due to the constant distance, the pistons 2 do not perform a stroke movement. This distance between the bottom 72 of the cylinder barrel and the upper side 81 of the piston plate 8 is therefore referred to below as the neutral distance S0.

[0052] In this exemplary embodiment, the piston plate 8 is designed to be pivotable relative to the cylinder barrel plane 85. When the swashplate 9 is pivoted, it should be ensured that the cylinder barrel axis 70 and the swashplate axis 90 intersect at an angle α at a pivot point X. Since the piston plate 8 slides on the swashplate 9 and thus the piston plate 8 and the swashplate 9 always remain oriented parallel to one another, the consequence is that, because of a geometric law, the angle α at which the cylinder barrel plane 75 and the piston plate plane 85 intersect corresponds to the pivot angle α. The pivot angle α also corresponds to the angle at which the piston axes 20 are tilted relative to the cylinder bore axis 30. At a pivot angle α = 0°, the neutral position, the pistons 20 are aligned parallel to the piston bore axes 30.

[0053] At a pivot angle α not equal to 0°, one half of the piston plate 8 is tilted away from the cylinder barrel 7, and the other half of the piston plate is inclined toward the cylinder barrel 7, so that during a rotation the distance between the cylinder barrel bottom 72 and the piston plate upper side 81 changes continuously. During a rotation, the piston plate 8, proceeding from the middle distance, passes through a maximum distance S.sub.max after a quarter rotation of the circle; after a further quarter rotation of the circle, the upper side 81 of the piston plate 8 returns to the middle distance; after a further quarter rotation of the circle, the upper side 81 of the piston plate 8 passes through a minimum distance S.sub.min from the bottom of the cylinder barrel 7, and after a further quarter rotation of the circle, the piston plate 8 returns to its starting point. In order to illustrate these positions in FIG. 2, these distances and the two pistons/piston chambers are shown for an even number n of piston bores.

[0054] Since the piston foot 22 of the pistons 2 is fixedly connected to the piston plate 8, the pistons 2 are compelled to perform these up and down movements during a rotation of the cylinder barrel 7 and the piston plate 8. During the upward movement, the piston chamber 31, which is sealed by the sealing ring 5 with respect to the inner side of the housing, becomes smaller until the piston 2 reaches a top dead center OT, where it changes its stroke movement direction. The top dead center OT of the piston 2 is the same as the position in which the piston plate 8 has reached the minimum distance S.sub.min. In the subsequent downward movement, the size of the piston chamber increases until the piston 2 reaches a bottom dead center UT, where the downward stroke movement changes once again into an upward stroke movement. The bottom dead center UT is the same as the position in which the upper side 81 of the piston plate 8 is at a maximum distance S.sub.max from the bottom 72 of the cylinder barrel 7.

[0055] The piston foot 22 is advantageously shaped as a cylinder because the piston foot 22 can accordingly be received by a passage bore in the piston plate 8. Since, adjacent to the piston foot 22 the piston is either widened as a truncated cone or forms a step to the larger cylindrical central part 24, the piston 2 is supported on the piston plate upper side 81 in order to divert the forces acting on the piston head 21 in the piston chamber 31 into the piston plate 8.

[0056] If the central part 24 has no widening relative to the piston foot 22, this support can alternatively be achieved in that the seats for the piston foot 22 are designed as blind holes, and each piston foot 22 is supported in a blind hole. The piston feet 22 are fixed against any type of movement, for example, by a press-fit in the passage bore or the blind hole. Alternatively, a connection can also be in the form of a positive fit or friction fit, for example by pressing, shrinking, a threading, or welding.

[0057] FIG. 5 shows a piston 4 with a sealing ring 5 mounted in a sealing ring seat 4. In this case, the sealing ring seat 4 has a pin 23 which is centered on the piston head 21 and which accommodates a central opening 51 of the sealing ring 5. The inner diameter d.sub.i of the center opening 51 is significantly greater in this case than the diameter dz of the pin 23. A movement of the sealing ring 5 in the direction of the longitudinal axis 20 of the piston 2 is limited by a cover 6 which is mounted on the pin 23.

[0058] FIG. 6 shows a sealing ring 5 in its simplest embodiment in terms of manufacture. The sealing ring 5 of FIG. 6 is a spherical disk, wherein the spherical disk has the same heights h/2 upward and downward from an equatorial plane 58 of the sealing ring. The equatorial plane 58 includes the great circle on the peripheral surface 52 of the sealing ring which is perpendicular to the sealing ring axis 50. Because the same heights h/2 of the sealing ring are the same on both sides of the equatorial plane, it is therefore a symmetrical sealing ring 5. The diameter d.sub.a of the sealing ring, which ideally is somewhat smaller than the piston diameter d, is the result of the radius of curvature r.

[0059] We first consider the case where the piston plate plane 85 is oriented parallel to the cylinder barrel plane 75, and the cylinder barrel axis 70 coincides with the piston plate axis 80 and the swashplate axis 90—that is to say, the neutral position. When the cylinder barrel 7 and the piston plate 8 rotate in the neutral position, the pistons 2 do not perform a stroke movement because no relative movements occur in the direction of the piston bore axes 30. Thus, no vertical forces, i.e., forces parallel to the cylinder barrel axis 70, act on the sealing ring 5.

[0060] In the view of FIG. 2, we can see the situation in the case of a position of the swashplate 9 inclined relative to the cylinder barrel 7 by a pivot angle α <> 0°. A rigid piston head describes an elliptical path during a rotation of the cylinder barrel 7 within the piston bore 3, and the apexes of the main axis of this elliptical path are passed through at the top dead center OT and bottom dead center UT. In the situation as shown in FIG. 2, when it reaches the top dead center OT, the piston 2 would protrude beyond the part of the inner wall 32 of the piston bore which has the least distance from the cylinder barrel axis 70 — that is to say, lies closer to the cylinder barrel axis 70. In contrast to this, the piston 2 would project beyond the part of the inner wall 32 of the piston bore 3 which has the greatest distance from the cylinder barrel axis 70 when the piston 2 reaches its bottom dead center UT. In the illustration of FIG. 2, both pistons 2 would thus press against the respective right cylinder walls 31. For a rigid piston head 21 and a rigid cylinder barrel 7, this would inevitably lead to the piston head 21 jamming in the piston bores 3.

[0061] This jamming is counteracted in two ways in the floating piston machine 1 according to the invention. Firstly, the piston plate 8 is mounted displaceably on the swashplate 9. The pressures of the piston chambers 31 are transmitted via the rigid pistons 2 to the piston plate 8, and displace the piston plate 8 on the swashplate 9. This can be seen in FIG. 2, where the piston plate axis 80 is now located to the left of the swashplate axis 90. On the other hand, the sealing ring 5, because it is displaceably accommodated in the sealing seat 4, can deflect the forces acting on the sealing ring 5 from the inner walls 32 of the piston bore 3 transverse to the piston longitudinal axis. The inner diameter d.sub.i of the sealing ring 5 and the diameter dz of the pin are ideally matched to one another in such a manner that the resulting clearance δ.sub.Q is great enough that the sealing ring 5 can follow the elliptical path in cooperation with the displacement of the piston plate 8 on the swashplate 9, without jamming. When this clearance is correctly tuned, a torque can be transmitted from the cylinder barrel 7 via the sealing ring 5 to the piston plate 8, such that the piston plate is entrained by the cylinder barrel 7. Alternatively, however, the piston plate 8 can be synchronized with the cylinder barrel 7 by way of a gearing, for example, as a result of which greater freedom is achieved with respect to the inner sealing ring geometry and the pin 23.

[0062] By means of the partially spherical peripheral surface 52 of the sealing ring 5 with a radius of curvature r which substantially corresponds to half the piston bore diameter D/2, the piston bore inner wall 32 and sealing ring 5 contact each other in a circular line, the sealing circle 59, irrespective of how strongly the piston longitudinal axis 20 is tilted relative to the piston bore axis 30, and therefore how deeply the piston 2 dips into the piston bore 3 in its stroke movement. As a result, the plane in which the sealing circle 59 lies is always perpendicular to the piston bore axis 30. As a result, the wear in the contact between the sealing ring and the piston bore is reduced, and the axial piston machine is more efficient and more robust. The service life of the metallic sealing ring 5 is thus significantly higher than an elastically designed sealing ring according to the prior art.

[0063] In the following, the circular line on which the piston bore axes 30 are distributed about the cylinder barrel axis is designated as a piston bore reference circle, and the diameter of the piston bore reference circle is designated as the piston bore reference circle diameter Dz. The piston feet 22 and in particular the piston longitudinal axes 20 of the individual pistons 2 intersect the piston plate 8 perpendicularly, and are distributed uniformly about the piston plate axis 80 on a circular line, which is referred to below as the piston reference circle. The diameter of the piston reference circle is hereinafter referred to as the piston reference circle diameter D.sub.K.

[0064] In one embodiment, the pistons 2 are arranged on the piston plate 8 such that the longitudinal axes 20 of the pistons 2 and the longitudinal axes 30 of the respective piston bores 3 coincide in the neutral position. Thus, the piston reference circle diameters D.sub.K and the piston bore reference circle diameters Dz are identical. If the distance R of the piston bore axes 20 from the cylinder barrel axis 70 is 45 mm as mentioned above, the piston bore reference circle diameter Dz is calculated as Dz = 2R = 90 mm, and the piston reference circle diameter D.sub.K is also calculated as 90 mm.

[0065] However, it has been shown that the piston reference circle diameter D.sub.K can also in particular also be selected to be greater than the piston bore reference circle diameter Dz. In a second embodiment, the piston reference circle diameter D.sub.K is equal to 90.4 mm. A piston reference circle diameter D.sub.K which is greater than the piston bore reference circle diameter Dz has the advantage that the floating piston machine can be constructed more compactly, because, with the same clearance δ.sub.Q, a greater pivot angle α can be achieved. A piston reference circle diameter D.sub.K which is larger compared to the piston bore reference circle diameter Dz is made possible by the sealing rings 5, which are mounted in a manner allowing displacement transverse to the piston axis 20 and compensate for the greater piston axis distance D.sub.K by the sealing rings 5 moving in the sealing ring seat 4.

[0066] In a further embodiment shown in FIG. 8, the inner wall of the sealing ring 5 is provided with an inner bead 54, so that the sealing ring 5 has, for example, a constant material thickness over its height h in the vertical direction. The background for a geometry of the sealing ring deviating from the pure ring shape is the following:

[0067] If a piston chamber 31 of the cylinder barrel 7 is connected to the highpressure side via the connecting bores 33, this high pressure (up to 350 bar or more) acts on the inner wall 32 of the bore of the cylinder barrel 7 which forms the piston chamber 31. It has been shown that this internal pressure force can lead to a widening or deformation of the corresponding piston bore 3, despite the solid design of the cylinder barrel 7. Such a one-sided widening would lead to an increase in the gap 34 between the piston bore 3 and the sealing ring 5. In order to compensate for this disadvantage, the invention proposes to design the sealing ring 5, with respect to its geometry, in such a way that, when the inner side of the sealing ring 5 is subjected to radial pressure forces, the sealing ring can expand accordingly, and thus the gap 34 between the piston bore 3 and the sealing ring 5 remains ideally constant over the entire range of the operating pressure. The clearance δ.sub.Q, as well as δ.sub.H allows the pressure to find its way into the region behind the sealing ring or into the space between the pin 23 and the inner diameter 5. Since the working pressure in the piston chamber 31 acts on the inner geometry of the sealing ring 5 at the same height, the sealing ring 5 is correspondingly widened with a correspondingly adapted wall thickness and/or adapted cross-sectional profile.

[0068] In a first variant, this can be achieved by the sealing ring 5 having a bead-like recess 54 on its inner side 53. This bead-like recess 54 can, for example, be designed in such a manner that the sealing ring 5 has approximately the same horizontal thickness z over its vertical profile h. As a result of this uniform horizontal thickness z, the sealing ring can be deliberately weakened in order thus to yield to a pressure acting on the inner side of the sealing ring by widening, i.e., by enlarging its outer diameter d.sub.a.

[0069] In an alternative embodiment of the sealing ring, as shown in FIG. 7, a decrease in the sealing ring wall thickness is achieved by the sealing ring 5 being asymmetrical. That is, the height h.sub.2 of the sealing ring measured upward from its equatorial plane 58 is greater than the height h.sub.1 of the sealing ring measured downward from its equatorial plane 58. In this way, the lower wall thickness z.sub.2 of the sealing ring 5 at its upper end with respect to the wall thickness z.sub.1 of the sealing ring at its lower end is deliberately permitted in order to allow yielding to the high pressure of the pressure medium in the piston interior. The desired widening of the sealing ring can be tuned accordingly via the upper height h.sub.2.

[0070] In a further embodiment shown in FIG. 9, the inner diameter of the sealing ring is stepped. The inner diameter d.sub.2 is made larger in the upper part—that is, the part which faces the cover of the piston 2—than the inner diameter d.sub.i in the lower part. As an alternative to an approximately constant sealing ring cross-sectional thickness z according to the embodiment shown in FIG. 6, the sealing ring 5, due to the lesser material thickness z.sub.2 yields to a higher operating pressure in its upper region, while the sealing ring 5, due to the higher material thickness z.sub.1 in its lower region, largely retains its shape, and thus the adaptation between the inner ring diameter d.sub.i and the pin diameter d.sub.z is not altered. The desired widening of the sealing ring in its upper region can in particular be set by the upper diameter d.sub.2 and the height at which the step between the upper and lower regions is arranged.

[0071] In an alternative embodiment, which is shown in FIG. 10, the inner diameter of the sealing ring expands continuously upward over its height, as a result of which the wall thickness of the sealing ring 5 decreases as the height increases, and can thus even more easily yield to the pressure of the sealing ring in the interior space 57. In its lower region, the sealing ring 5 extends over a first height h.sub.1 downward from the equatorial plane, and in its upper region extends upward over a second height h.sub.2. Depending on how much expansion is required, the widening of the interior 57 of the sealing ring 5, as shown at the equatorial plane 58, can, however, also begin only above or alternative also below the equatorial plane 58. For this purpose, both a sealing ring 5 designed to be symmetrical, in which the first height h.sub.1 is equal to the second height h.sub.2, and also, as shown in FIG. 10, an asymmetrically designed sealing ring 5, in which the first height h.sub.1 is different from the second height h.sub.2, can be used. A geometry-optimized design of the ring geometry as a function z(h) over the height of the sealing ring 5 can, if necessary, also be determined sufficiently precisely, for example, by means of corresponding deformation analyses with the finite element method.

[0072] Since the widening of the piston inner wall 32 depends on many factors, such as the material used for the cylinder barrel 7, the piston bore diameter d, the wall thicknesses between two adjacent piston bores 3, to name the most important ones, no general formula can be specified here. In laboratory tests, however, it has been shown that, at operating pressures of 350 bar, the widening of the piston bore 3 in the dimensioning selected in the exemplary embodiment can be between 10 .Math.m and 30 .Math.m—in special individual cases, also greater or less than this. A method for determining the cross-sectional thickness z of the sealing ring therefore consists in initially determining the deformation of the piston bore 3 at the highest intended operating pressure in a first step. In a test series, sealing rings 5 with different cross-sectional thicknesses z are subjected to the highest intended operating pressure, and the resulting increase in diameter Δd of the sealing ring 5 is determined. The sealing ring geometry is then selected—that is, in this case, the sealing ring 5 with the cross-sectional thickness z at which the difference Δd between the measured piston inner wall diameter d+ Δd under load at the highest operating pressure and the sealing ring diameter d.sub.i+ Δd.sub.i under load at the highest operating pressure corresponds to the selected clearance between the piston inner wall 32 and the sealing ring 5.

[0073] Alternatively or additionally to a pressure equalization via the vertical and the horizontal clearance of the sealing ring 5 in the sealing ring seat 4, a pressure equalization between the piston interior 31 and the interior 57 of the sealing ring 5 can also be achieved by one or more openings in the cover 6. FIG. 11 shows an embodiment of a piston 2 with a sealing ring 5 with a bead-like recess on the inner wall 54 of the sealing ring 5. In this exemplary embodiment, a pressure equalization between the piston interior 31 and the interior 57 of the sealing ring 5 is provided by one or more pressure equalization bores 9, which extend downward from the upper side of the cover 6 through the pin 23 and then in the radial direction of the pin 23. Such a pressure equalization is suitable both for sealing rings 5 with a continuous profile of the sealing ring thickness z and, as shown in FIG. 12, for sealing rings with a stepped inner profile. In this exemplary embodiment, a pressure equalization between the piston interior 31 and the interior 57 of the sealing ring 5 is also provided by one or more pressure equalization bores 9, which extend downward from the upper side of the cover 6 through the pin 23 and then in the radial direction of the pin 23.