rotary piston compressor

Abstract

A rotary piston compressor (1) for compressing a gas, in particular carbon dioxide, in which a side wall surface (7) of a housing side wall (4) and a flat sealing surface (8, 9) of each of a housing cover (5, 6) surround a working chamber (10), a rotary piston (3) is rotatably supported on an eccentric body (11) in the working chamber (10), a flat seal receiving passage (18) is formed in each of piston bottom surfaces (15, 16) of the rotary piston (3), and a flat seal ( In order to press each of the flat seals (19) against the respective planar sealing surfaces (8, 9), peripheral openings (21) are formed in the piston peripheral surface (17) of the rotary piston (3), and the peripheral openings (21) are connected in a pressure-transmitting manner to each of the flat seal receiving passages (18) via pressure through-pipes (22), which are formed inside the rotary piston (3) and open into each of the flat seal receiving passages (18) on the side of each of the flat seals (19) opposite the sealing surface (20).

Description

[Technical Field]

[0001] The present invention relates to a rotary piston compressor for compressing gas, in particular carbon dioxide, the rotary piston compressor having a working housing and a rotary piston, the working housing having a housing side wall and two housing covers arranged on opposite sides of the housing side wall, the side wall surfaces of the housing side wall and one planar sealing surface of each of the housing covers surrounding a working chamber arranged in the working housing, the rotary piston being rotatably supported on an eccentric body in the working chamber, and the rotary piston compressor having a gas inlet for introducing the gas to be compressed into the working chamber. and a gas outlet with an overpressure outlet valve for leading the compressed gas out of the working chamber, wherein the rotary pistons have two piston bottom surfaces facing one of the flat sealing surfaces of the housing cover and a piston circumferential surface facing the side wall surface of the housing side wall, wherein one flat seal receiving passage is formed in each of the piston bottom surfaces, and a flat seal is arranged in each of the flat seal receiving passages, and each flat seal has one sealing surface for abutting against one of the flat sealing surfaces of the housing cover. [Background technology]

[0002] Rotary piston compressors themselves have been known for a long time, and are shown, for example, in U.S. Pat. Nos. 4,105,375 and 4,118,157.

[0003] The rotary piston of the rotary piston compressor mentioned at the beginning is shown in WO 2020 / 159394. In the technology disclosed in the above-mentioned specification, spring elements are arranged in the flat seal receiving passages to press the flat seal against each flat sealing surface of the housing cover of the working housing. In practice, these springs usually only generate a small pressing force and usually only serve to maintain contact with the flat sealing surface. In the prior art, the actual sealing is usually caused by gas pressure acting on the seal, and the gas that generates the gas pressure reaches the flat seal via the gap dimension between the flat sealing surface of the housing cover and the bottom surface of the piston. Summary of the Invention [Problem to be solved by the invention]

[0004] The object of the present invention is to propose an improvement which provides a good seal with a flat seal, especially when the gas pressure in the working chamber is relatively high. [Means for solving the problem]

[0005] To achieve this object, the present invention proposes, starting from a rotary piston compressor of the type mentioned at the outset, that peripheral openings are formed in the piston peripheral surface of the rotary piston in order to press the sealing surface of each flat seal against the respective flat sealing surface, and that the peripheral openings are connected in pressure-transmitting manner to the respective flat seal receiving passages via pressure-through guide lines, which are formed inside the rotary piston and open into the respective flat seal receiving passages on the side of each flat seal facing away from the sealing surface.

[0006] In other words, the present invention no longer assumes that gas pressure reaches the flat seal through the gap dimension. Rather, the present invention proposes that peripheral openings are intentionally provided on the piston periphery and that these peripheral openings are connected in a pressure-transmitting manner to the flat seal receiving passage via a pressure-through guide line formed inside the rotary piston. Pressurized gas from the working chamber can pass through the peripheral openings and the pressure-through guide line opening into the flat seal receiving passage and directly act on the flat seal in the flat seal receiving passage, thereby pressing the flat seal against the respective flat sealing surfaces of the respective housing covers.

[0007] This solution according to the invention has the advantage that only a few components are required to press the sealing surface of each flat seal against the respective planar sealing surface of each housing cover. Therefore, the spring elements in the planar seal receiving passages used in the prior art of the type mentioned at the beginning can be completely omitted in the present invention. However, in particular, the present invention allows gas pressure from the region of the working chamber into which the circumferential openings of the rotary piston open into the corresponding region of the planar seal receiving passage to press the sealing surface of each flat seal against the respective planar sealing surface of the housing cover. This automatically adapts the pressing pressure to the pressure currently present in this region of the working chamber. This proves to be advantageous, particularly when particularly high pressures are achieved in the working chamber when compressing gas using the rotary piston compressor according to the invention.

[0008] A particularly suitable field of use for the rotary piston compressor of the present invention is to compress or compress carbon dioxide, which can then be used as an environmentally friendly refrigerant or heat transfer medium in a cooling or heating circuit. In this case, a working pressure of at least 80 bar, preferably at least 100 bar, must be achieved to compress the carbon dioxide. This allows the carbon dioxide to be used as a refrigerant for cooling equipment and air conditioning systems, or as a heat transfer medium for building heating and heat pumps. The rotary piston compressor of the present invention is primarily interested in compressing carbon dioxide. However, the rotary piston compressor of the present invention can, of course, also be used to compress other gases.

[0009] In this context, gases are all gases that are gaseous under normal conditions, i.e., at a temperature of 20°C and a pressure of 1013.25 mbar. When compressing or compressing the respective gas by the rotary piston compressor according to the invention, the gas, in particular carbon dioxide, can be brought into a completely transcritical or supercritical state, in which the gas is both liquid and gaseous at the same time. Nevertheless, in the context of the present invention, the term gas is used for the sake of simplicity.

[0010] In the rotary piston compressor according to the present invention, the rotary piston is rotatably supported on an eccentric body. Therefore, the rotary piston compressor according to the present invention can also be called a rotary piston compressor based on the change principle. The rotary piston can also be called a rotating piston or simply a rotor. The rotary piston compressor itself can also be called a rotary piston compressor. The flat seal can also be called a piston bottom seal.

[0011] The pressure-conducting line in the rotary piston is preferably tubular. It can be configured, for example, as a bore or as a series of bores that open into one another inside the rotary piston. However, there are other possibilities for configuring the pressure-conducting line in the rotary piston.

[0012] Preferably, the peripheral opening is formed in the peripheral surface of the piston in each case spaced apart from the piston bottom surface.

[0013] There are various possibilities for manufacturing the flat seals and placing them in the respective flat seal receiving passages. To this end, a first group of solutions, which can be implemented particularly cost-effectively, proposes manufacturing the flat seals directly in the respective flat seal receiving passages. Thus, a preferred variant provides, for example, that the flat seals are injection-molded into the respective flat seal receiving passages as injection-molded parts. In other words, in this variant, the flat seals are manufactured directly in the respective flat seal receiving passages using an injection molding method, and are thus also placed in the respective flat seal receiving passages. However, another variant provides that the flat seals are printed into the respective flat seal receiving passages as 3D-printed parts. In this variant, the flat seals are manufactured directly in the respective flat seal receiving passages using a printing process, and are thus also simultaneously placed in the respective flat seal receiving passages. Yet another variant provides that the flat seals are pressed into the respective flat seal receiving passages as molded press parts.

[0014] However, it is also possible to manufacture the flat seals first and then place them in the flat seal receiving passages after manufacture, i.e., to manufacture the flat seals as insert members in advance and insert them into the respective flat seal receiving passages as insert members.

[0015] According to a preferred embodiment of the present invention, the pressure-through guide lines are covered by caps in the region of their openings into the respective flat seal receiving passages. The use of corresponding caps to cover the openings of the pressure-through guide lines into the respective flat seal receiving passages is particularly advantageous when the flat seals are formed directly into the flat seal receiving passages, for example, by injection molding or 3D printing. The caps prevent the openings of the pressure-through guide lines from being accidentally closed during the manufacturing process of the flat seals. However, corresponding caps can also be used when the flat seals are prefabricated as inserts and inserted into the respective flat seal receiving passages as inserts. The term "covering the openings" using caps does not imply that the caps pressure-tightly close the openings of the pressure-through guide lines. The caps simply serve as a covering. When a corresponding gas pressure is present in the pressure-through guide lines, gas can pass completely past the caps into the flat seal receiving passages, thereby pressing the sealing surface of the flat seal against the respective flat sealing surface of the respective housing cover. However, in another embodiment, the caps can be omitted.

[0016] In order to minimize the number of components, a preferred embodiment of the invention provides that a flat seal is formed integrally in one of the flat seal receiving passages formed in one of the piston bottom surfaces. In other words, in this embodiment, there is always exactly one flat seal in the flat seal receiving passage. The flat seal in this flat seal receiving passage is therefore formed integrally.

[0017] In the rotary piston compressor according to the invention, it is advantageously provided that the side wall surfaces of the housing side walls are completely or at least partially trochoidally shaped when viewed in a cutting plane parallel to the planar sealing surface of the housing cover.

[0018] The rotary piston preferably has two or more corner regions, and preferably a radial seal is arranged in each of the corner regions to seal the rotary piston against the side wall of the housing. It is particularly preferred that the piston bottom surface is likewise defined by a defining line in each region between two of the corner regions of the rotary piston, the defining line being formed as an envelope curve of a family of trochoidal curves.

[0019] There are various possibilities that can be combined with one another for pressing the radial seals against the side wall surfaces of the housing side walls. Thus, in the rotary piston compressor according to the invention, it may be provided, for example, that a respective elastic element facing the rotary piston for pressing the radial seal against the side wall surface of the housing side wall is integrally molded into the radial seal. In a preferred variant, instead or additionally, flat seals are used for pressing the radial seals against the side wall surface of the housing side wall. In such a variant, it may be provided that the flat seals each have a contact surface for pressing the radial seal against the side wall surface of the housing side wall. These contact surfaces of the flat seals may each be configured as bevels and can act on the corresponding bevels of the radial seals.

[0020] As will be further shown in the description of the drawings below, when a gas is compressed in a working chamber using a rotary piston compressor, different partial volumes separated from one another by the rotary piston and its corner regions are created, in which different gas pressures exist during operation and the magnitude of the gas pressures changes continuously during operation. Depending on the instantaneous position of the rotary piston, a partial volume of the working chamber is created into which gas is drawn, while on the other side of the rotary piston, gas is compressed at that moment. That is, both a low-pressure side and a high-pressure side are simultaneously created on different sides of the rotary piston. To prevent gas from overflowing from the currently formed high-pressure side to the currently formed low-pressure side via the peripheral openings, the pressure guide passages, and the flat-seal receiving passages, in a preferred embodiment of the invention, flat seals are sealed in the corner regions of the rotary piston on the side opposite the respective sealing surfaces, respectively, with the flat-seal receiving passages in which they are received. To achieve this, it can be provided, for example, that the flat seal preferably has a sealing web on the side opposite the sealing surface, which is arranged in a corresponding sealing web receptacle in the flat seal receiving passage, so that the area of ​​the flat seal receiving passage between two adjacent corner areas of the rotary piston can be sealed against the adjacent area of ​​the flat seal receiving passage.

[0021] As is known for rotary piston compressors, the rotary piston compressor according to the invention can also be configured with various boost ratios, which in this case refer to the ratio of the number of trochoidal curves forming the side wall surface of the housing side wall to the number of corners of the rotary piston. In the rotary piston compressor according to the invention, the boost ratio is preferably 1:2, 2:3 or 7:6.

[0022] In the rotary piston compressor according to the invention, the gas inlet and / or the gas outlet can be guided through the housing wall, but alternatively, the gas inlet and / or the gas outlet can be guided through an eccentric body, or a combination thereof is also possible.

[0023] The planar seal and / or the optional radial seal are preferably made of a polymer or a polymer with a dry lubricant and / or reinforcing fibers. Examples of polymers that can be used include polyetheretherketone, polyamideimide, polyoxymethylene, polyketone, polyamide, or polyethylene terephthalate. Examples of dry lubricants that can be used include polytetrafluoroethylene or molybdenum disulfite. Examples of reinforcing fibers include glass fibers or carbon fibers.

[0024] In a preferred embodiment, the housing side wall and the housing cover each have a substrate made of aluminum alloy or cast iron. Preferably, this substrate is coated with a coating that forms the planar sealing surfaces of the side wall and the housing cover. The coating may be, for example, a nickel-phosphorus layer, an aluminum oxide layer, or a dry-lubricating lubricant layer. Combinations of at least two of these layers are also possible. The coatings may be applied directly to the substrate. However, an open-porous adhesive layer may also be present on the substrate, and the coating is then applied to this adhesive layer. In the case of a substrate made of aluminum alloy, the adhesive layer may be, for example, an open-porous aluminum oxide layer, such as anodized aluminum or a non-compressible hard anodized aluminum. Another variant of the support or adhesive layer is an open-porous plasma-chemically oxidized aluminum layer. In the case of a substrate made of cast iron, the support or adhesive layer may be formed, for example, by phosphating or sandblasting.

[0025] Unless otherwise intended, the term "one" as used herein should be understood to mean "at least one."

[0026] Further features and details of preferred embodiments of the invention are explained, by way of example, in the following description of the drawings and on the basis of various implementation variants of the invention. [Brief explanation of the drawings]

[0027] [Figure 1] 1 shows a rotary piston compressor according to the invention with a thickness increase ratio of 1:2 as well as its variants. [Figure 2] 1 shows a rotary piston compressor according to the invention with a thickness increase ratio of 1:2 as well as its variants. [Figure 3] 1 shows a rotary piston compressor according to the invention with a thickness increase ratio of 1:2 as well as its variants. [Figure 4] 1 shows a rotary piston compressor according to the invention with a thickness increase ratio of 1:2 as well as its variants. [Figure 5] 1 shows a rotary piston compressor according to the invention with a thickness increase ratio of 1:2 as well as its variants. [Figure 6] 1 shows a rotary piston compressor according to the invention with a thickness increase ratio of 1:2 as well as its variants. [Figure 7] 1 shows a rotary piston compressor according to the invention with a thickness increase ratio of 1:2 as well as its variants. [Figure 8] 1 shows a rotary piston compressor according to the invention with a thickness increase ratio of 1:2 as well as its variants. [Figure 9] 1 shows a rotary piston compressor according to the invention with a thickness increase ratio of 1:2 as well as its variants. [Figure 10] 1 shows a rotary piston compressor according to the invention with a thickness increase ratio of 1:2 as well as its variants. [Figure 11]1 shows a rotary piston compressor according to the invention with a thickness increase ratio of 1:2 as well as its variants. [Figure 12] 1 shows a rotary piston compressor according to the invention with a thickness increase ratio of 1:2 as well as its variants. [Figure 13] 1 shows a rotary piston compressor according to the invention with a thickness increase ratio of 1:2 as well as its variants. [Figure 14] 1 shows a rotary piston compressor according to the invention with a thickness increase ratio of 1:2 as well as its variants. [Figure 15] 1 shows a rotary piston compressor according to the invention with a thickness increase ratio of 1:2 as well as its variants. [Figure 16] 1 shows a rotary piston compressor according to the invention with a thickness increase ratio of 1:2 as well as its variants. [Figure 17] 1 shows a rotary piston compressor according to the invention with a thickness increase ratio of 1:2 as well as its variants. [Figure 18] 1 shows a rotary piston compressor according to the invention with a thickness increase ratio of 1:2 as well as its variants. [Figure 19] 1 shows a rotary piston compressor according to the invention with a thickness increase ratio of 1:2 as well as its variants. [Figure 20] 1 shows a rotary piston compressor according to the invention with a thickness increase ratio of 1:2 as well as its variants. [Figure 21] 1 shows a rotary piston compressor according to the invention with a thickness increase ratio of 1:2 as well as its variants. [Figure 22] FIG. 1 shows a rotary piston compressor according to the invention with a thickness increase ratio of 2:3. [Figure 23] FIG. 1 shows a rotary piston compressor according to the invention with a thickness increase ratio of 2:3. [Figure 24] FIG. 1 shows a rotary piston compressor according to the present invention with a pressure boost ratio of 7:6. [Figure 25] FIG. 1 shows a rotary piston compressor according to the present invention with a pressure boost ratio of 7:6. [Figure 26] FIG. 1 shows a rotary piston compressor according to the present invention with a pressure boost ratio of 7:6. [Figure 27] FIG. 1 shows a rotary piston compressor according to the present invention with a pressure boost ratio of 7:6. [Figure 28] FIG. 1 shows a rotary piston compressor according to the present invention with a pressure boost ratio of 7:6. [Figure 29] FIG. 1 shows a rotary piston compressor according to the present invention with a pressure boost ratio of 7:6. [Figure 30] FIG. 1 shows a rotary piston compressor according to the present invention with a pressure boost ratio of 7:6. [Figure 31] FIG. 1 shows a rotary piston compressor according to the present invention with a pressure boost ratio of 7:6. [Figure 32] FIG. 1 shows a rotary piston compressor according to the present invention with a pressure boost ratio of 7:6. [Figure 33] FIG. 1 shows a rotary piston compressor according to the present invention with a pressure boost ratio of 7:6. [Figure 34] FIG. 1 shows a rotary piston compressor according to the present invention with a pressure boost ratio of 7:6. [Figure 35] FIG. 1 shows a rotary piston compressor according to the present invention with a pressure boost ratio of 7:6. [Figure 36] FIG. 1 shows a rotary piston compressor according to the present invention with a pressure boost ratio of 7:6. [Figure 37] FIG. 1 shows a rotary piston compressor according to the present invention with a pressure boost ratio of 7:6. [Figure 38] FIG. 1 shows a rotary piston compressor according to the present invention with a pressure boost ratio of 7:6. [Figure 39] FIG. 1 shows a rotary piston compressor according to the present invention with a pressure boost ratio of 7:6. DETAILED DESCRIPTION OF THE INVENTION

[0028] FIG. 1 shows an exploded view of a first embodiment of a rotary piston compressor 1 according to the present invention. The rotary piston compressor 1 has a pressure boost ratio of 1:2. The rotary piston compressor 1 comprises a working housing 2 and a rotary piston 3. The working housing 2 also comprises a housing side wall 4 and housing covers 5 and 6 arranged on opposite sides of the housing side wall 4. In this embodiment, these components of the working housing 2 are connected to one another by screws 37 and nuts 38. However, this is of course not required, and other types of connection are also possible.

[0029] The side wall surface 7 of the housing side wall 4 and the planar sealing surfaces 8, 9 of each housing cover 5, 6 enclose a working chamber 10 arranged in the working housing 2. The rotary piston 3 is rotatably supported on an eccentric 11 in the working chamber 10. In this first embodiment, the eccentric 11 is fixedly mounted on a drive shaft 30. As can be seen in FIG. 2, the drive shaft 30 terminates in a connecting pin 31 that protrudes from the rotary piston compressor 1 in the assembled state. A motor can be connected to the connecting pin 31 to rotate the drive shaft 30, and thus the eccentric, about a rotation axis 60. In this embodiment, the eccentric 11 is fixedly connected to the drive shaft 30 and the connecting pin 31, so that rotation of the drive shaft 30 about the rotation axis 60 automatically rotates the eccentric 11 accordingly.

[0030] In this embodiment, outer teeth 32 are also connected to the drive shaft 30 in a rotation-proof manner. These outer teeth 32 engage with inner teeth 33, which are connected to the rotary piston 3 in a rotation-proof manner. Via this thread engagement, the rotary piston 3 rotates together in the working chamber 10 when the connecting pin 31 or the drive shaft 30 rotates accordingly. In this case, the rotary piston 3 is rotatably supported on an eccentric 11 in the working chamber 10.

[0031] The drive shaft 30 is rotatably supported in the working housing 2 via a bearing 34 and a fixing ring 36. The bearing 34 may be a ball bearing, a plain bearing, etc. In the illustrated embodiment, the bearing 34 in the housing cover 5 is a ball bearing, and the bearing 34 in the housing cover 6 is a plain bearing. However, this is of course not necessary and may be implemented differently.

[0032] A compensation weight 35 is mounted on the drive shaft 30 below the housing cover 6 and thus outside the working housing 2, in a non-rotatable manner, and compensates for imbalances caused by eccentricity of the rotary piston 3. In the illustrated embodiment, the working housing 2 is surrounded by an outer shell 39 of the rotary piston compressor 1, although this is of course not essential.

[0033] The rotary piston compressor 1 of this first embodiment is provided with a gas inlet 12 for introducing the gas to be compressed into the working chamber 10, and a gas outlet 13 equipped with an overpressure outlet valve 14 for discharging the compressed gas from the working chamber 10. This can be seen particularly well in Figures 3 to 6, as will be explained further below.

[0034] The rotary piston 3 has two piston bottoms 15 and 16 facing one of the planar sealing surfaces 8 and 9 of the housing covers 5 and 6, respectively, and a piston circumferential surface 17 facing the side wall surface 7 of the housing side wall 4. A flat seal receiving passage 18 is located in each of the piston bottoms 15 and 16. A flat seal 19 is arranged in each of the flat seal receiving passages 18, and each flat seal 19 has a sealing surface 20 that presses against one of the planar sealing surfaces 8 and 9 of the housing covers 5 and 6. This will be explained in more detail with reference to the following drawings. In any case, the rotary piston compressor 1 of this first embodiment also has a peripheral opening 21 formed in the piston circumferential surface 17 of the rotary piston 3 to press the sealing surface 20 of each flat seal 19 against the respective planar sealing surface 8 and 9. The peripheral opening 21 is connected to each flat seal receiving passage 18 via a pressure through-pipe 22 in a pressure-transmitting manner. Here, a pressure through-guide passage 22 is formed inside the rotary piston 3 and leads to each flat seal receiving passage 18 on the side of each flat seal 19 opposite the sealing surface 20. This will be further described below, particularly with reference to Figures 7, 8 and 11 to 13.

[0035] It should be noted that the inner teeth 33 of the rotary piston 3 of this first embodiment are shown in Figures 1, 7 and 9, but not in Figures 3 to 6, 11, 14 and 18. The omission of the inner teeth 33 in the figures cited here is purely for simplification of the drawings, and does not mean that the inner teeth 33 are actually absent.

[0036] Figure 2 shows a side view of the assembled rotary piston compressor 1 of this first embodiment, with the cutting plane AA marked. Figures 3 to 6 show somewhat simplified cross-sectional views about this cutting plane AA, respectively. To explain the working mode of the rotary piston compressor 1, various positions of the rotary piston 3 during rotation about the drive shaft 30 or the longitudinal axis of the drive shaft 30 are shown. Arrow 42 indicates the direction of rotation of the rotary piston 3 within the working chamber 10.

[0037] 3 to 6, the side wall surface 7 of the housing side wall 4 is formed completely trochoidally in this embodiment when viewed in a cross section parallel to the planar sealing surfaces 8 and 9 of the housing covers 5 and 6. The rotary piston 3 has two corner regions 25. Each of these corner regions 25 is equipped with a radial seal 26 for sealing the rotary piston 3 against the side wall surface 7 of the housing side wall 4. The piston bottom surfaces 15 and 16 are each defined by a demarcation line 27 in the region between each of the corner regions 25 of the rotary piston 3. This demarcation line 27 is the envelope of a family of trochoidal curves. The rotary piston 3 divides the working chamber 10 into a low-pressure side 40 and a high-pressure side 41 by the corner regions 25 and the radial seals 26 arranged in the corner regions 25. On the low-pressure side 40, gas is introduced or sucked into the working chamber 10 via the gas inlet 12 as the rotary piston 3 rotates. On the high-pressure side 41, the volume of which decreases with increasing rotation of the rotary piston 3, the previously sucked gas is compressed or compressed, so that the gas pressure on the high-pressure side 41 continuously increases as the rotary piston 3 rotates. When the desired gas pressure or compression is achieved on the high-pressure side 41, the overpressure outlet valve 14 opens, allowing the compressed gas to escape or be discharged from the working chamber 10 via the gas outlet 13. By adjusting or selecting the corresponding overpressure outlet valve 14, it is possible to adjust how strongly the gas is compressed by the rotary piston compressor 1 before it escapes via the gas outlet 13. In other words, it is possible to define or adjust the degree to which the gas is compressed in the rotary piston compressor 1.

[0038] 3 to 6 illustrate four different positions of the rotary piston 3 during rotation and thus during the compression process described. This functional form of the rotary piston compressor 1 is known per se and does not require further explanation. The arrow 43 shown in FIGS. 3 to 6 indicates the gas that is still to be compressed, which flows in or is sucked in via the gas inlet 12. The arrow 44 indicates the already compressed gas that flows out via the gas outlet 13.

[0039] FIG. 7 shows a vertical cross-section of the rotary piston compressor 1 of this first embodiment along the vertical plane BB shown in FIG. 2 or along the rotational axis 60 of the drive shaft 30. This cross-section BB cuts through the peripheral openings 21 and the pressure through-guide lines 22, which are important for the present invention. FIG. 8 shows an enlarged view of region D in FIG. 7. In these two cross-sectional views, it is clear that each of the piston bottoms 15 and 16 has a flat seal receiving passage 18, within which a flat seal 19 is located. Each flat seal 19 has a sealing surface 20, which presses against one of the flat sealing surfaces 8 and 9 of the housing cover 5 and 6, respectively, for sealing purposes. As shown in detail in FIG. 8 , a peripheral opening 21 is formed in the piston peripheral surface 17 of the rotary piston 3, and the peripheral opening 21 is connected to the flat seal receiving passage 18 via a pressure through-guide line 22 so as to transmit pressure. The pressure through-guide line 22 is formed inside the rotary piston 3 and communicates with each flat seal receiving passage 18 on the side of each flat seal 19 opposite the seal surface 20. In a preferred embodiment, the pressure through-guide line 22 is tubular as shown in the drawing. Specifically, in this embodiment, the pressure through-guide line 22 is formed as a series of holes that open into each other inside the rotary piston 3. The peripheral opening 21 is located on the piston peripheral surface 17, spaced apart from the piston bottom surfaces 15 and 16. The pressure direction arrow 47 shows how the gas present in the working chamber 10 under the corresponding pressure passes through the peripheral opening 21 and the pressure-through guide channel 22 and applies pressure to the flat seal 19 on the side facing away from the sealing surface 20, which is thereby pressed against the respective flat sealing surface 8 or 9. The gas pressure in the working chamber 10 is thereby used to press the sealing surface 20 of the flat seal 19 against the corresponding flat sealing surface 8 and 9. This makes it possible to achieve a very good seal even when the pressure in the working chamber 10 is very high.8, a cap 24 is located in the flat seal receiving passage 18 in the region of the opening 23 of the pressure through-flow conduit 22, covering the pressure through-flow conduit 22. The cap 24 is designed so as not to interfere with pressure transmission. In the event of a corresponding increase in pressure in the pressure through-flow conduit 22, gas can flow, preferably by the cap 24, into the flat seal receiving passage 18 on the side of the flat seal 19 facing away from the sealing surface 20, thereby pressing the sealing surface 20 of the flat seal against the respective planar sealing surfaces 8-9.

[0040] As explained at the beginning, the cap 24 can be omitted in principle. However, if the planar seal 19 is injected into each planar seal receiving passage 18 as an injection-molded part or printed into each planar seal receiving passage 18 as a 3D-printed part, as realized in this embodiment, the cap 24 prevents undesired closure of each opening 23 during the formation or manufacturing of the planar seal 19.

[0041] FIG. 9 shows a cross-section of the rotary piston compressor 1 of this first embodiment along the cutting plane CC indicated in FIG. 3. This cutting plane CC passes through the corner region 25 of the rotary piston 3 and the radial seal 26 arranged in this corner region 25. FIG. 10 shows an enlarged view of region E of FIG. 9. In FIG. 10, it is first possible to see how the radial seal 26 presses against the side wall surface 7 of the housing side wall 4 to seal against the rotary piston 3. To generate the necessary sealing pressure, two measures are implemented in this embodiment. On the one hand, the radial seal 26 is integrally formed with an elastic element 28 facing the rotary piston 3. This elastic element 28 presses the radial seal 26 against the side wall surface 7 of the housing side wall 4. On the other hand, the flat seal 19 also presses each radial seal 26 against the side wall surface 7 by its contact surface 57. In Figures 10, 14 and 16, the elastic element 28, which is molded onto the radial seal 26 on the side opposite the radial sealing surface 49, is formed as a kind of leaf spring. Figure 17 shows a variant in this regard, in which the elastic element 28 is formed as a corresponding ridge on the side of the radial seal 26 which is located opposite the radial sealing surface 49.

[0042] 10, it should be noted that the contact surface 57 of the flat seal 19 is preferably formed as a bevel 45 in order to press each radial seal 26 against the side wall surface 7. As can be clearly seen in FIG. 10, the radial seal 26 preferably has a corresponding bevel 46 against which the contact surface 57 or bevel 45 of each flat seal 19 acts.

[0043] FIG. 11 shows a perspective view of the rotary piston 3. The rotary piston 3 is provided with a flat seal 19 arranged in a flat seal receiving passage 18 on the piston bottom surface 15. A corresponding flat seal 19 is also formed on the opposite piston bottom surface 16, which is not visible in FIG. 11. The demarcation lines 27 of the piston bottom surfaces 15 and 16 are also clearly visible in FIG. 11. These demarcation lines 27 are formed as the envelope of a family of trochoidal curves between the corner regions 25 of the rotary piston 3. The piston circumferential surface 17 is provided with a peripheral opening 21 according to the invention. The cross section FF shown in FIG. 12 again shows the pressure-transmitting connection according to the invention between the peripheral opening 21 and the flat seal receiving passage 18 via one of the pressure through-passages 22. With the exception of the housing cover 5, which is not shown in FIG. 12, the view shown in FIG. 12 corresponds to the already described view shown in FIG. 8, and therefore reference can be made thereto. However, it should be noted here once again that in this embodiment the flat seals 19 are each injection molded as injection molded parts into the respective flat seal receiving passages 18. Naturally, the flat seals 19 could equally well be formed in the flat seal receiving passages 18 as 3D printed parts or as compression molded parts, as already mentioned above.

[0044] FIG. 13 illustrates an alternative to the above-described embodiment. In this embodiment, the flat seal 19 is prefabricated as an insert and inserted into each flat seal receiving passage 18 as an insert. The web 58 of the flat seal 19 serves as a guide when gas pressure is applied to the flat seal 19 in the direction 47, in accordance with the present invention, through the peripheral opening 21, the pressure guide channel 22, and the flat seal receiving passage 18 on the side opposite the sealing surface 20, thereby pressing the sealing surface 20 of the flat seal 19 against the planar sealing surface 8 of the housing cover 5 or the planar sealing surface 9 of the housing cover 6. In the alternative shown in FIG. 13, no cap 24 is provided to cover the opening 23. However, a corresponding cap 24 can of course also be used to cover the opening 23 in this variant shown in FIG. 13.

[0045] Figure 14 shows an exploded view of the rotary piston 3. In this exploded view, both the flat seal 19 and the radial seal 26 are shown separated from the rotary piston 3. Figure 14 also clearly shows that the flat seal 19, which is arranged in one of the flat seal receiving passages 18, is preferably formed as an integral part of itself. Figure 14 also once again shows the opening 23 of the pressure through-guide line 22 in the flat seal receiving passage 18.

[0046] FIG. 15 is a side view of one of the flat seals 19, viewed from the direction 59 indicated in FIG. 14 . In this side view, the sealing web 48 of the flat seal 19 can be seen below the already-described beveled surface 45. This sealing web 48 serves to seal the flat seal 19 to the flat seal receiving passage 18, which receives the flat seal 19 on the side opposite the respective sealing surface 20, in each corner region 25 of the rotary piston 3, as will be explained further below. This type of sealing can, of course, be achieved in other ways, for example, by gluing, clamping, etc. However, this sealing is particularly preferably achieved by arranging the above-mentioned sealing web 48 in a seal web receiving groove 50, which is recessed in the flat seal receiving passage 18 in each corner region 25. See FIGS. 18 to 21 for this. FIG. 18 shows a plan view of one of the piston bottom surfaces 15 of the rotary piston 3, as well as the section line GG and the section plane HH. As can be seen in Figure 19, the section line GG is located in the region of the sealing web receiving grooves 50. In the section plane H, the rotary piston 3 is cut in the region of the radial seal receiving passages 51 into which the radial seals 26 are inserted. Figure 21 shows the same cross section as Figure 19, but in which one flat seal 19 is arranged in each flat seal receiving passage 18. Here too, it can be seen how the sealing webs 48 of each flat seal 19 are arranged in the respective sealing web receiving grooves 50 in order to achieve the desired sealing effect.

[0047] As already explained at the beginning, the flat seal 19 and the radial seal 26 are preferably made of a polymer with a dry lubricant and / or reinforcing fibers. The housing side wall 4 and the housing covers 5 and 6 have a base body preferably made of an aluminum alloy or cast iron. A coating 29 is preferably applied to each base body to form the side wall surface 7 of the housing side wall 4 and the flat sealing surfaces 8 and 9 of the housing covers 5 and 6. This is also preferably the case in this first embodiment. For details and preferred embodiments of such a coating 29, please refer to the explanations already given at the beginning.

[0048] 22 and 23 also show an exploded view of a second embodiment of the rotary piston compressor 1 according to the invention. This second embodiment is broadly identical to the first embodiment, so only the differences will be mentioned here. The main difference is that a 2:3 boost ratio is realized. The rotary piston 3 of this embodiment also has three corner regions 25. The number of gas inlets 12 and gas outlets 13 and overpressure outlet valves 14 are adapted accordingly, as are the shapes of the side wall surfaces 7 and the flat seals 19. However, the above applies in other respects, with adapted shapes as necessary, so further explanation in this regard will be omitted. Furthermore, it should be noted that the internal toothing 33 shown in FIG. 22 is not shown in FIG. 23 either. In any case, the demarcation line 27 of the piston bottom surfaces 15 and 16, extending between the corner regions 25, has the shape of an envelope curve of a trochoidal family of curves. The side wall surface 7 of the housing side wall 4 is also in this embodiment completely trochoidal in cross section parallel to the planar sealing surfaces 8 and 9 of the housing covers 5 and 6. The arrangement according to the invention of the peripheral openings 21 and the pressure through-guide lines 22 in the rotary piston 3 corresponds to the first embodiment and does not need to be repeated again.

[0049] 24 to 39 show a third embodiment of the rotary piston compressor 1 according to the present invention. This third embodiment is a variant with a pressure boost ratio of 7:6. The rotary piston 3 of this rotary piston compressor 1 therefore has six corner regions 25. In the areas of the piston circumferential surface 17 located between the corner regions 25, the defining lines 27 that define the piston bottom surfaces 15 and 16 are also configured as envelope curves of a family of trochoidal curves. The side wall surface 7 of the housing side wall 4 has a trochoidal arch in a cross-section plane 7 parallel to the planar sealing surfaces 8 and 9 of the housing covers 5 and 6.

[0050] In contrast to the previously described embodiments of the rotary piston compressor 1 according to the invention, in this third embodiment the eccentric 11, on which the rotary piston 3 is rotatably supported in the working chamber 10, does not rotate as in the first two embodiments, but is instead arranged stationary in the outer shell 39 of the rotary piston compressor 1. In this embodiment, the rotary piston 3 rotates together with the working housing 2, and thus together with the housing side wall 4 and both housing covers 5, 6, about an axis of rotation 60 extending through the eccentric 11, while the eccentric 11 remains stationary. To achieve this, the rotary piston compressor 1 of this third embodiment has a rotor 53 that is non-rotatably connected to the working housing 2 by means of screws 37 and nuts 38, and which cooperates with a stator 54 that is fixedly connected to the outer shell 39 of the rotary piston compressor 1. The rotor 53 and the stator 54 form a drive motor which rotates the working housing 2 together with the rotary piston 3 which is mounted on an eccentric 11 in the working chamber 10 of the working housing 2 .

[0051] Another difference between the two previously described embodiments and the third embodiment is that the gas inlet 12 and the gas outlet 13 pass through the eccentric 11 in this third embodiment and not through the housing wall 4, as in the first-mentioned embodiment. Correspondingly, the rotary piston 3 is also provided with overflow openings 55 passing through the piston circumferential surface 17. Through these overflow openings 55, gas can enter the corresponding part of the working chamber 10 from the gas inlet 12 in the eccentric 11 and be discharged from there again in compressed form via the gas outlet 13.

[0052] The gas inlet 12 and gas outlet 13 passing through the eccentric body 11 open in this third embodiment into a valve cover 52. The valve cover 52 is externally mounted on the outer shell 39 of the rotary piston compressor 1, and both the gas inlet 12 and the gas outlet 13 lead into or out of the rotary piston compressor 1.

[0053] Except for the differences noted above and below, the description of the first embodiment can be essentially referred to, and this applies in particular to the configuration according to the invention of applying pressure to a flat seal 19 arranged in a flat seal receiving passage 18 in the piston bottom surfaces 15 and 16 to press its sealing surface 20 against the flat sealing surfaces 8 and 9 of the housing covers 5 and 6.

[0054] FIG. 25 shows a side view of the rotary piston compressor 1 of the third embodiment shown in FIG. 24 in an exploded view. A cutting plane II is marked in FIG. 25. FIGS. 26 to 32 show cross sections along the cutting plane II at various instants during operation of the rotary piston compressor 1, and thus during rotation of the working housing 2 including the rotary piston 3 around the corresponding axis of rotation 60 extending through the eccentric body 11, as shown in FIGS. 24 and 25. To better understand the rotational movement of the rotary piston 3 and the working housing 2 shown in FIGS. 26 to 32, a point 61 is marked on the rotary piston 3 in FIGS. 26 to 32. This is merely a diagrammatic aid, allowing a better understanding of the instantaneous position of the rotary piston 3 in the various views shown in FIGS. 26 to 32.

[0055] 26 to 32 therefore show various intermediate positions during the rotation of the working housing 2 and the rotary piston 3 about the rotation axis 60. The gas inlets 12 and gas outlets 13 are clearly visible in the eccentric body 11. The arrows 43 indicate the gas to be compressed, which has flowed into the respective partial regions of the working chamber 10, currently functioning as the low-pressure side 40, via the respective gas inlets 12 and corresponding overflow openings 55. The arrows 44 indicate the compressed gas which has already been compressed and is being forced into the respective gas outlets 13 from the corresponding high-pressure side 41 of the working chamber 10. The position of the rotary piston 3 as seen in FIGS. 26 to 32 reveals that the partial volumes of the working chamber 10, designated in each figure as the low-pressure side 40, are connected to the gas inlets 12 via the corresponding overflow openings 55 in the rotary piston 3, allowing gas to flow in. Then, in the partial volume of the working chamber 10, which is respectively referred to as the high-pressure side 41 and which no longer has a connection with the gas inlet 12, the gas is compressed by the corresponding relative movement between the rotary piston 3 and the working housing 2, so that it can flow in compressed form into the gas outlet 13 when the corresponding partial volume of the working chamber 10 is connected to the gas outlet 13 via a corresponding overflow opening 55 provided in the rotary piston 3.

[0056] FIG. 33 shows a plan view of a third embodiment of a rotary piston compressor 1 according to the present invention. The cutting planes JJ and KK are also indicated in FIG. 33. FIG. 34 shows a cross section through the cutting plane JJ. In FIG. 34, it is clearly visible how the gas inlet 12 passes through the valve cover 52 and the eccentric body 11. Similarly, it is clearly visible how the gas outlet 13 passes through the eccentric body 11 and the valve cover 52. The overpressure outlet valve 14, located in the region of the valve cover 52 at the gas outlet 13, is also shown. In this embodiment, the overpressure outlet valve 14 is a spring-loaded closing body that opens when the compressed gas coming from the working chamber 10 or the high-pressure side 41 is at a desired pressure, which can be set by a corresponding design of the overpressure outlet valve 14.

[0057] In the cross-sectional view shown in Figure 34, flat seals 19 can also be seen arranged in corresponding flat seal receiving passages 18 in the piston bottom surfaces 15 and 16, and these flat seals 19 press their sealing surfaces 20 against the flat sealing surfaces 8 or 9 of the housing covers 5 and 6 so as to seal accordingly.

[0058] FIG. 35 shows a cross-section along the cutting plane KK of FIG. 33, where the peripheral openings 21 and pressure guide channels 22 according to the present invention are arranged. The corresponding detail L of FIG. 35 is shown enlarged in FIG. 36. As can be clearly seen in FIG. 36, in this embodiment, the piston peripheral surface 17 of the rotary piston 3 also has peripheral openings 21 for pressing the sealing surface 20 of each flat seal 19 against the respective planar sealing surface 8 or 9. The peripheral openings 21 are connected to the flat seal receiving passages 18 via pressure guide channels 22. The pressure guide channels 22 are formed inside the rotary piston 3 and open into the respective flat seal receiving passages 18 on the side of each flat seal 19 opposite the sealing surface 20. Similarly, in FIG. 36, the direction of pressure application to the flat seal 19 on the side opposite the sealing surface 20 is indicated by an arrow 47. In other words, in both the first two embodiments and in this third embodiment, the gas under pressure in the working chamber 10 can act on the side of the flat seal 19 opposite the sealing surface 20 through the peripheral opening 21 and the pressure-passing guide channel 22 passing through the rotary piston 3, thereby pressing the sealing surface 20 of the flat seal 19 against the corresponding flat sealing surface 8 or 9 of the corresponding housing cover 5 or 6.

[0059] 36 also shows the cap 24, and the function of the cap 24 has already been explained. In this embodiment, the cap 24 can also be omitted, as a matter of course.

[0060] FIG. 37 shows a perspective view of the rotary piston 3 of this embodiment of the rotary piston compressor 1. In this perspective view, it is easy to see how the flat seal receiving passages 18 are formed in the piston bottom surface 15 and how the flat seals 19 are arranged within the flat seal receiving passages 18. The overflow openings 55 and the peripheral openings 21 arranged in the piston peripheral surface 17 are also easy to see. FIG. 38 shows an exploded view of the rotary piston 3, in which the flat seals 19 have been removed from each flat seal receiving passage 18 in each piston bottom surface 15 and 16. Thus, in FIG. 38, the openings 23 in the flat seal receiving passages 18 can be seen, which are connected to the corresponding peripheral openings 21 via the corresponding pressure through-pipes 22.

[0061] In the assembled state, the inwardly facing seal webs 48 integrally molded on each of the flat seals 19 are located in corresponding seal web receiving grooves 50 in the rotary piston 3. As in the other embodiments, the seal webs 48 ensure that each of the flat seals 19 is sealed against the flat seal receiving passages 18 that receive it, in the corner regions 25 of the rotary piston 3, on the side opposite the respective sealing surfaces 20.

[0062] As can be particularly clearly seen in Figures 37, 38, and 39, the flat seal 19 of this embodiment has each sealing corner region 56 with a correspondingly rounded contact surface 57. These rounded contact surfaces 57 allow each corner region 25 of the rotary piston 3 to seal against the corresponding rounded corner section 62 of the side wall surface 7 of the housing side wall 4 when it engages therein. The corner sections 62 are shown as such in Figure 24. As each sealing corner region 56 rolls over the corner section 62, the rounded contact surface 57 of the sealing corner region 56 always sealingly abuts the corresponding corner section 62, and thus the side wall surface 7, at least at one point between both end points X and Y shown in Figure 39. [Explanation of symbols]

[0063] 1 rotary piston compressor 2 Working housing 3 rotary piston 4 Housing side wall 5 Housing cover 6 Housing cover 7 Side wall 8 Flat sealing surface 9 Flat sealing surface 10 Workroom 11 Eccentric body 12 Gas inlet 13 Gas outlet 14 Overpressure Outlet Valve 15 Piston bottom 16 Piston bottom 17 Piston circumference 18 Flat seal receiving passage 19 Flat seal 20 Seal Surface 21 Peripheral opening 22 Pressure through-pipe guideway 23 Aperture 24 Cap 25 corner area 26 Radial seal 27 Demarcation Line 28 Elastic Elements 29 Coating 30 Drive shaft 31 connecting pins 32 Lateral dentition 33 Medial dentition 34 Bearings 35 Compensation Weight 36 Position fixing ring 37 Screw 38 Nut 39 outer shell 40 Low pressure side 41 High pressure side 42 Rotation direction 43 Inflowing gas 44 Leaking gas 45 Slope 46 Slope 47 Direction of pressure application 48 Seal Web 49 Radial sealing surface 50 Seal web accommodation groove 51 Radial seal receiving passage 52 Valve cover 53 Rotor 54 Stator 55 Overflow opening 56 Seal corner area 57 Contact surface 58 Web 59 directions 60 Rotation axis 61 points 62 Corner division

Claims

1. A rotary piston compressor (1) for compressing a gas, the rotary piston compressor (1) having a working housing (2) and a rotary piston (3), the working housing (2) having a housing side wall (4) and two housing covers (5, 6) arranged on opposite sides of the housing side wall (4), a side wall surface (7) of the housing side wall (4) and a planar sealing surface (8, 9) of each of the housing covers (5, 6) surrounding a working chamber (10) arranged in the working housing (2), the rotary piston (3) being rotatably supported on an eccentric body (11) in the working chamber (10), the rotary piston compressor (1) having a gas inlet ( a gas outlet (13) with an overpressure outlet valve (14) for discharging the compressed gas from the working chamber (10); the rotary piston (3) has two piston bottom surfaces (15, 16) facing one of the planar sealing surfaces (8, 9) of the housing cover (5, 6) and a piston peripheral surface (17) facing the side wall surface (7) of the housing side wall (4), a planar seal receiving passage (18) is formed in each of the piston bottom surfaces (15, 16), and a planar seal (19) is arranged in each planar seal receiving passage (18), and the planar seal (19) has a respective sealing surface (20) for contacting one of the planar sealing surfaces (8, 9) of the housing cover (5, 6); a peripheral opening (21) formed in the piston peripheral surface (17) of the rotary piston (3) for pressing the sealing surface (20) of each of the flat seals (19) against each of the planar sealing surfaces (8, 9); the peripheral opening (21) is connected to each of the flat seal receiving passages (18) via pressure through-pipes (22), the pressure through-pipes (22) being formed inside the rotary piston (3) and opening into each of the flat seal receiving passages (18) on the side of each of the flat seals (19) opposite the sealing surface (20); A rotary piston compressor (1) in which the rotary piston (3) has two or more corner regions (25), The flat seals (19) are sealed against the flat seal receiving passages (18) that house the flat seals (19) on the opposite side of the sealing surfaces (20) in the corner regions (25) of the rotary piston (3). A rotary piston compressor (1) characterized in that:

2. 2. The rotary piston compressor (1) according to claim 1, wherein the pressure through-guide line (22) is tubular and / or formed as a single bore and / or as a series of bores opening into one another inside the rotary piston (3).

3. 3. The rotary piston compressor (1) according to claim 1, wherein the peripheral opening (21) is formed in the piston peripheral surface (17) at a distance from the piston bottom surface (15, 16).

4. 4. The rotary piston compressor (1) according to claim 1, wherein the flat seals (19) are injection-molded into the respective flat seal receiving passages (18) as injection-molded parts, or the flat seals (19) are printed into the respective flat seal receiving passages (18) as 3D-printed parts, or the flat seals (19) are press-fitted into the respective flat seal receiving passages (18) as molded press parts.

5. 4. The rotary piston compressor (1) according to claim 1, wherein the flat seals (19) are each prefabricated as inserts and inserted into the respective flat seal receiving passages (18) as inserts.

6. 6. The rotary piston compressor (1) according to claim 1, wherein the pressure through-guide lines (22) are each covered by a cap (24) in the region of their openings (23) into the respective flat seal receiving passages (18).

7. 7. The rotary piston compressor (1) according to claim 1, wherein each of the flat seals (19) is integrally formed in one of the flat seal receiving passages (18) formed in one of the piston bottom surfaces (15, 16).

8. 8. The rotary piston compressor (1) according to claim 1, wherein the side wall surface (7) of the housing side wall (4) is formed completely or at least partially in a trochoidal shape when viewed in a cutting plane parallel to the planar sealing surfaces (8, 9) of the housing covers (5, 6).

9. A rotary piston compressor (1) as described in any one of claims 1 to 8, wherein one radial seal (26) is arranged in each of the corner regions (25) to seal the rotary piston (3) against the side wall surface (7) of the housing side wall (4).

10. 10. The rotary piston compressor (1) according to claim 9, wherein the piston bottom surface (15, 16) is defined by a defining line (27) in each region between two of the corner regions (25) of the rotary piston (3), and the defining lines (27) are each formed as an envelope curve of a family of trochoidal curves.

11. A rotary piston compressor (1) as described in claim 9 or 10, wherein each of the radial seals (26) is integrally molded with one elastic element (28) facing the rotary piston (3) for pressing the radial seal (26) against the side wall surface (7) of the housing side wall (4).

12. 12. The rotary piston compressor (1) according to claim 9, wherein the flat seals (19) each have a contact surface (57) for pressing each radial seal (26) against the side wall surface (7) of the housing side wall (4).

13. 13. The rotary piston compressor (1) according to claim 1, wherein the flat seal (19) is made of a polymer or a polymer with a dry lubricant and / or reinforcing fibers.

14. A rotary piston compressor (1) as described in any one of claims 9 to 11, wherein the radial seal (26) is made of a polymer or a polymer having a dry lubricant and / or reinforcing fibers.

15. 15. The rotary piston compressor according to claim 1, wherein the housing side wall (4) and the housing cover (5, 6) each have a base body made of an aluminum alloy or cast iron, and a coating (29) applied to the base body to form the side wall surface (7) of the housing side wall (4) and the planar sealing surfaces (8, 9) of the housing cover (5, 6), the coating (29) being a nickel-phosphorus layer, an aluminum oxide layer, a dry-lubricating lubricant layer, or a combination of at least two of these layers.

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