Self-lubricating sliding material and sliding bearings made from it

Abstract

The invention relates to a sliding material 2 comprising a matrix comprising at least 30 wt.% polyimide (4) and 4 wt.% to 40 wt.% lithopone.

Description

[0001] The present invention relates to a sliding material in which lithopones are embedded in a polyimide matrix. The addition of lithopones to a polyimide matrix improves dry-running performance. This sliding material makes it possible to produce sliding bearings with improved properties and, in particular, without perfluorinated and polyfluorinated alkyl compounds, so-called PFAS, which are also known by the English names "perfluoroalkyl or polyfluoroalkyl substances". The main advantage lies in avoiding PFAS, which comprise very stable compounds or at least very stable degradation products, some of which are also suspected of being carcinogenic or environmentally harmful.

[0002] Several sliding materials, such as polytetrafluoroethylene (PTFE), are already known. In the field of lubricated plain bearings, the well-known bearing bronzes and various self-lubricating sintered bearings are available. Depending on the size, expected load, and the possibility of lubrication, a specialist can choose from various types of plain bearings.

[0003] It is desirable to have a bearing material available that exhibits the best possible wet and dry running properties. It is also desirable to minimize the use or release of pollutants during the manufacturing, operation, disposal, or recycling of a bearing. Furthermore, it is desirable to have a bearing material that combines a low coefficient of friction, good resistance to temperature loads, and good formability.

[0004] According to a first aspect of the present invention, a sliding material is provided in which lithopones are embedded in a matrix (essentially) of polyimide and 4 wt.% to 40 wt.%, wherein the polyimide content of the sliding material is at least 30 wt.%. In a preferred embodiment of the sliding material, it contains between 10 wt.% and 30 wt.% lithopones. In a further embodiment of the sliding material, the lithopone content is between 15 wt.% and 122 wt.%. In this basic embodiment of the sliding material, it comprises at least 30 wt.% polyimide as well as lithopones as a solid lubricant, which can significantly reduce sliding friction compared to pure polyimide.

[0005] The term "matrix" here refers to a base material in which other components, such as lithopone, are embedded. Lithopone is a white pigment containing barium sulfate or barite (BaSO4) and zinc sulfite or sphalerite or zinc blende (ZnS), and is also marketed as "Pigment White 5." However, lithopone is used here not for its coloring properties, but for its properties as a dry lubricant. The term polyimide refers to a polyimide material that can be produced with reasonable purity while adhering to economic constraints, and where small amounts of unintended components are tolerated. The basic design of the material is geared towards the use of lithopone as a lubricant in a sliding layer.

[0006] The base material of the matrix is ​​polyimide, predominantly consisting of polyimide, but preferably consisting of polyimide and unavoidable impurities. The term "unavoidable impurities" is also to be understood in an economic sense. Further components may be embedded in the matrix.

[0007] The sliding material can contain lithopone with a ZnS content between 10 and 70 wt%, with BaSO4 and unavoidable components forming the remainder. However, it is preferred that the ZnS content be between 20 and 60 wt%, with BaSO4 and unavoidable components as the remainder. For example, a ZnO (zinc oxide) content of 0.02 to 2 wt% is considered unavoidable for economic reasons.

[0008] Depending on their zinc sulfide content, different types of lithopone are distinguished, with their brightness increasing with rising zinc sulfide content (approximately 10% for yellow seal lithopone to approximately 60% for silver seal lithopone). For example, lithopone with a zinc sulfide content of about 30% is called "red seal lithopone," and lithopone with a zinc sulfide content of about 40% is called "green seal lithopone."

[0009] In another embodiment of the sliding material, the matrix further incorporates embedded molybdenum disulfide (MoS2), wherein the MoS2 is present in an amount between 25 wt.% and 55 wt.% MoS2 in the sliding material, with the remainder being made up by lithopones and polyimide and any other additives to reach 100 wt.%.

[0010] It is also intended to incorporate a weight fraction of between 30 wt.% and 50 wt.% MoS2 into the sliding material, with the sum of the components again amounting to 100 wt.%. In a further exemplary embodiment, the sliding material comprises a proportion of 35 wt.% to 45 wt.% MoS2. All proportions are intended to add up to 100 wt.%.

[0011] In another exemplary embodiment of the sliding material, the polyimide matrix further incorporates embedded graphite, which can also be present in an amount between 12 wt.% and 30 wt.% C in the sliding material. Here, the graphite can be used in addition to or instead of the MoS₂ in the sliding material. The weight percentages of the polyimide and the lithopones limit the maximum proportion of graphite and / or MoS₂. Again, the sum of the wt.% of all components equals 100.

[0012] Especially when combining lithopone and MoS2, a MoS2 to lithopone ratio of 1.5 to 2.5 in the sliding material is desirable. Ratios of 1.7 to 2.3 parts MoS2 to one part lithopone can also be used in the sliding material. So far, good wet and, above all, dry running properties have been achieved with ratios of 1.9 to 2.1 parts MoS2 to one part lithopone. It appears that the properties complement each other particularly well in the aforementioned MoS2 to lithopone ratios.

[0013] In this formulation, due to the minimum 30 wt% polyimide content, the amount of lithopone is limited to a maximum range of 17.1 to 24 wt%, and the amount of MoS2 to a maximum range of 46 to 52.9 wt%. At higher polyimide values, the proportions of lithopone and MoS2 decrease accordingly. With a polyimide content of 40.1 wt% in the sliding material, and a lithopone to MoS2 ratio of 2.26, the respective wt percentages are 18.5 wt% lithopone and 41.4 wt% MoS2.

[0014] In another exemplary embodiment of the sliding material, it is PFAS-free. By avoiding so-called per- and polyfluoroalkyl substances (PFAS), materials that are suspected of being carcinogenic, for example, can be avoided. "PFAS-free" means that PFAS is present only to the extent of unavoidable impurities. An example of a well-known substance from the PFAS group is PTFE (polytetrafluoroethylene), which is widely used in both industry and household applications.

[0015] In another embodiment of the sliding material, the lithopone embedded in the polyimide matrix has a particle size between 0.3 and 10 µm. In the application area preferred by the applicant, particle sizes between 0.5 and 6 µm have proven advantageous for the lithopone. It has also been found that a particle size of the lithopone between 1 and 3 µm, embedded in the polyimide, also exhibits improved dry-running properties. The particle sizes relate, among other things, to the thickness of the sliding layers to be produced from the present sliding material.

[0016] In another embodiment of the sliding material, the MoS2, with a particle size between 0.5 and 30 µm, is embedded in the polyimide matrix of the sliding material. In yet another embodiment, the particle size of the MoS2 is between 1 and 20 µm. It is also possible to use MoS2 with a particle size between 2 and 12 µm in the polyimide matrix of the sliding material. These particle sizes are particularly important with regard to the subsequent thickness of a sliding layer made from this material, since smaller particles provide poorer lubrication and larger particles can cause problems during the formation of the sliding layer.

[0017] According to a further aspect of the present invention, a sliding layer is provided which comprises or consists of the sliding material described above. The thickness of the sliding layer depends on the respective area of ​​application or the specific use case. The sliding layer can preferably be applied to a substrate or carrier material and serve as part of a sliding bearing.

[0018] According to a further aspect of the present invention, a sliding bearing is provided comprising the sliding material or sliding layer described above. In a simple version, the sliding bearing can be machined from a solid material or from the sliding material itself, either as a bushing or a flanged bushing. However, it is also possible, for example, to produce flanged bushings by casting. It may be possible to injection-mold sliding surfaces made of polyimide onto injection-molded parts. The sliding bearing material can also be bonded to a substrate or applied in another way. It is also possible to provide the sliding material, for example, as a film or foil and bond it to a substrate. Alternatively, the sliding material can be cast or condensed directly onto a substrate.It is possible to manufacture plain bearings from the sliding material by sintering.

[0019] In another embodiment of the plain bearing, the sliding layer or sliding material is applied to a conventional bearing material. This can be achieved by coating a sliding surface of the bearing material with the sliding material. For example, a bearing bronze, as a conventional bearing material, can be coated with the sliding material of the present invention. In one embodiment, it is provided that a bearing comprising a porous bearing bronze is coated on the sliding surface of the bearing with a layer of the sliding material described above. This embodiment represents an improvement on conventional sintered bearings, wherein preferably a perforated or porous sliding layer of the sliding material is applied to the sintered bronze in order to maintain the self-lubricating property of a sintered bearing. Here, the sliding layer serves to give an otherwise maintenance-free and self-lubricating sintered bearing improved dry-running capability.

[0020] In another exemplary embodiment of the plain bearing, the sliding layer has a thickness between 15 and 500 µm. However, thinner sliding layers with a thickness between 20 and 350 µm are also used. It is planned to use a sliding layer with a thickness between 30 and 180 µm in bearings made of a bearing metal. The sliding layer has a lower hardness than a conventional bearing metal; therefore, the sliding layer made of the sliding material should be as thin as possible to avoid unnecessarily increasing the elasticity of the bearing. A sliding layer can be made of a relatively soft material, provided the layer thickness is small enough and the layer is supported by a sufficiently stable structure.

[0021] In an exemplary embodiment of the plain bearing, the bearing bronze is designed as a sintered bearing bronze.

[0022] In another embodiment of the plain bearing, a porous bearing bronze forms the bearing material, and this porous bronze has an average pore size that is at least twice the thickness of the sliding layer. Here, even with a thin sliding layer applied as a lacquer or film, it can be ensured that the running surface has pores through which a lubricant can penetrate the pores of the porous bearing bronze into the gap between the sliding surfaces, thus lubricating the plain bearing.

[0023] In another embodiment of the plain bearing, the conventional bearing material is applied to a support structure made of iron, preferably steel, or the bearing material is supported by a support structure made of iron or steel. Here, the expensive bearing material, which includes copper as a main component, can be replaced by a less expensive ferrous alloy, thereby increasing the bearing's stability. This advantage, particularly in the case of porous bearing bronzes, is offset by the disadvantage that the total pore volume in the bearing bronze of such a composite bearing is reduced due to the smaller volume of porous bronze used. A reduced total pore volume also means that the bearing can hold less lubricant in the pores, thus increasing the risk of premature lubricant depletion.This disadvantage can be reduced, if not completely eliminated, by the improved dry-running properties of the polyimide sliding material.

[0024] In an exemplary embodiment of a sliding bearing, the supporting structure has a thickness between 0.5 and 2 mm, and the porous bearing bronze has a thickness between 300 and 400 µm, while the sliding layer has a thickness between 30 and 250 µm.

[0025] The invention is illustrated below with reference to exemplary figures of preferred embodiments, which are not to scale and are not limiting. Fig. Figure 1 shows a sectional view through a basic embodiment of a sliding material according to the invention, comprising a polyimide matrix with embedded lithopone particles. Fig. Figure 2 shows a sliding material with a polyimide matrix containing embedded lithopone particles and embedded MoS2 particles. Fig. Figure 3 shows a sectional view through a basic embodiment of a bearing according to the invention in the form of a bearing bushing. Fig. Figure 4 shows an axial view of a flanged bearing bushing made of bearing bronze with an internal coating of the sliding material according to the invention. Fig. Figure 5 shows an axial view of a composite bearing bushing.

[0026] In the following, the same or similar reference symbols are used in both the description and the figures to refer to the same or similar elements or components.

[0027] Fig. Figure 1 shows a sectional view through a basic embodiment of a sliding material 2 according to the invention, comprising a polyimide matrix 4 with embedded lithopone particles 6. The figure shows the sliding material as a cross-section through a sliding layer. The proportion of lithopone 4 is between 4 and 40 wt.% of the polyimide, with the polyimide constituting at least 30% of the weight of the sliding material 2. This sliding material represents the basic embodiment, in which the lithopone is present as particles and is used as a dry lubricant. Separate particles of barite (BaSO4) and sphalerite (ZnS) are not used; rather, each particle in the matrix is ​​formed by lithopone.

[0028] Fig. Figure 2 shows a cross-sectional view through a sliding material 2 with a polyimide matrix 4 containing embedded lithopone particles 6 and embedded MoS2 particles 8. The sliding material 2 is shown here as a sliding layer. In the cross-section, the MoS2 particles 8 are depicted as significantly larger than the lithopone particles 6. It is also possible to embed other dry lubricants such as carbon or graphite in the polyimide matrix. In this configuration, the MoS2 particles in the matrix increase the lubricity and dry-running capability of the sliding material 2. The configuration of the sliding material of Fig. 2 can be used as a dry sliding bearing.

[0029] Fig. Figure 3 shows a sectional view through a basic embodiment of a bearing according to the invention in the form of a bearing bushing. Here, a bearing bushing 10 is formed entirely from the sliding material 2. Due to the lithopone as a dry lubricant, this bearing bushing can be used for light and small bearings. The elasticity of the polyimide in combination with the large thickness of the bearing bushing does not permit its use with high loads or high rotational speeds.

[0030] Fig. Figure 4 shows an axial view of a flanged bearing bushing 12 made of bearing bronze 16 with an inner coating of the sliding material 2 according to the invention. The flanged bearing bushing 12 can be designed as a more or less conventional flanged sintered bearing bushing 12. The flanged bearing bushing 12 has a coating of the sliding material 2 according to the invention on its inner bearing surface. Here, the coating with the sliding material 2 provides the bearing with improved dry-running capability should the lubricant in the sintered bearing lose its lubricating properties. In conjunction with a sintered bearing, a similarly porous sliding layer made of the sliding material should be used to allow a liquid lubricant in the sintered bearing to enter the bearing gap. Ideally, the bearing gap is lubricated by a liquid lubricant from the sintered bearing.During start-up, the lithopone and possibly the MoS2 in the sliding layer, acting as a dry lubricant, can significantly reduce friction in the bearing.

[0031] Fig. Figure 5 shows an axial view of a composite bearing bushing. The bearing bushing comprises a layer of bearing bronze 16, which here is designed as a porous bearing bronze. Compared to the design of the Fig. 4. A significantly thinner layer. The bearing bronze layer can be made thinner here because it is supported by a backing layer of iron or steel. The thinner bearing bronze layer reduces the bearing's material costs and simultaneously increases the stability of the sliding surface, since iron, and especially steel, has a higher strength than the bearing bronze material. The reduced thickness of the bearing bronze decreases the bearing's elasticity, as a larger portion of the bearing bushing consists of a more rigid material. However, the reduced thickness of the porous bearing bronze also means that fewer pores are available to hold a liquid lubricant. A smaller amount of liquid lubricant can negatively impact long-term lubrication and maintenance-free operation.Since the design of the figure increases the risk of the bearing running dry, this risk is significantly reduced by the dry lubricants lithopone and possibly MoS2 in the porous sliding layer.

[0032] The execution of the Fig. 4 and Fig. Five of these components can also be used with non-porous bearing materials, for example in forced-lubricated bearing shells, to improve emergency running properties and prevent bearing damage in the event of oil pressure loss. The improved bearings can help prevent bearing damage in the event of a late-detected oil pressure loss.

[0033] Particularly preferred is a polyimide content between 39.5 and 40.5 wt.%, a molybdenum disulfide content between 41 and 42 wt.% and a lithopone content between 18 and 19 wt.%.

[0034] The features of individual embodiments can also be combined with one another. The present invention is not limited to the exemplary embodiments shown in the figures.

Claims

[1] Sliding material (2) comprising a matrix comprising at least 30 wt% polyimide (4) and 4 wt% to 40 wt% lithopone (6), preferably 10 wt% to 30 wt% lithopone (6), and further preferably 15 wt% to 22 wt% lithopone (6). [2] Sliding material (2) according to claim 1, characterized by , that the lithopone BaSO4 ZnS (6) has a ZnS content between 10 and 70 wt.%, preferably between 20 and 60 wt.% and further preferably between 25 and 35 wt.% ZnS, remainder BaSO4, wherein the lithopone (6) may also contain between 0.02 and 2 wt.% ZnO. [3] Sliding material (2) according to claim 1 or 2, characterized by , that it further comprises MoS2 (8) in the polyimide (4) matrix, preferably in an amount between 25 wt.% to 55 wt.% MoS2 (8), preferably 30 wt.% to 50 wt.% MoS2 (8), and further preferably 35 wt.% to 45 wt.% MoS2 (8). [4] Sliding material (2) according to claim 3, characterized by, that in the sliding material (2) in the polyimide matrix (4) there is a ratio between MoS2 (8) and lithopone (6) of between 2.5 / 1 and 1.5 / 1, preferably between 2.3 and 1.7 to 1 and more preferably between 2.1 and 1.9 to 1. [5] Sliding material (2) according to any one of claims 1 to 4, characterized by that it is PFAS-free. [6] Sliding material (2) according to any one of claims 1 to 5, wherein the lithopone (6) embedded in the polyimide matrix (4) has a particle size between 0.3 to 10µm, preferably between 0.5 and 6µm, and more preferably between 1 and 3µm. [7] Sliding material (2) according to any one of claims 3 to 6, wherein the MoS2 embedded in the polyimide matrix (4) has a particle size between 0.5 to 30µm, preferably between 1 and 20µm, and more preferably between 2 and 12µm. [8] Sliding layer comprising a sliding material (2) according to any one of claims 1 to 7. [9] Plain bearing (10, 12, 14) comprising a sliding material (2) according to any one of claims 1 to 7 and / or a sliding layer according to claim 8. [10] Plain bearings (12, 14) according to claim 9, wherein the sliding layer according to claim 8 has a thickness between 15 and 500µm, preferably between 20 and 350µm, and more preferably between 30 and 180µm. [11] Plain bearing (12, 14) according to claim 10, wherein in the plain bearing (12, 14) the sliding layer is applied to a conventional bearing material (16), preferably a bearing bronze (16), more preferably a porous bearing bronze (16). [12] Plain bearings (12, 14) according to claim 11, wherein the porous bearing bronze (16) is a sintered bearing bronze. [13] Plain bearings (12, 14) according to claim 11 or 12, wherein the bearing material (16) is a porous bearing bronze and has a pore size that is at least twice as large as the thickness of the sliding layer. [14] Plain bearings (12, 14) according to claim 11, 12 or 13, wherein the conventional bearing material (16) is applied to a support structure (18) made of iron, preferably steel. [15] Plain bearings (12, 14) according to claim 14, wherein the support structure (18) has a thickness between 0.5 and 2 mm, the porous bearing bronze has a thickness between 300 and 400 µm, and the sliding layer has a thickness between 30 and 150 µm.

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