Self-lubricating composite friction parts

Abstract

A self-lubricating composite friction element is provided which may be subjected to temperatures at least equal to 250°C during operation. The lubricating composite friction element (1), which may be subjected to temperatures of at least 250° C. during operation, has, along its friction surface (2), a single layer of material consisting of weft and warp threads made of polytetrafluoroethylene, which material is impregnated with a heat-stable resin whose glass transition temperature is at least 250° C. This single layer is applied to a reinforcing layer (3).

Description

[Technical Field]

[0001] The present invention relates to a self-lubricating composite friction part for various applications, which does not utilize a lubricant between the opposing parts and therefore has a low coefficient of friction with the opposing parts and / or is involved in temperatures that can range above 250° C. and up to 300° C. or even peak at 320° C. Such a friction part may in particular be a joint or a slider.

[0002] To meet these constraints, it has already been proposed to cover the surfaces of machine parts with linings impregnated with resins that form a matrix, but these coverings do not make it possible to obtain a low coefficient of friction combined with good performance at high temperatures.

[0003] Thus, British Patent Specification No. 1,439,030 describes a friction covering, particularly for bearings or bearings, having a friction layer formed by weaving adjacent cords formed of strands with a low coefficient of friction containing a fluorocarbon-containing resin, the surface of which has irregularities including raised and depressed portions, and the strands and cords embedded in a plastic material. The strands are formed by fibers made of a material that does not chemically bond to any plastic material, such as PTFE, but these fibers are anchored in the covering, and the PTFE fibers can be mixed with cotton strands. In the above-mentioned embodiment, the fabric extends, helically bounded by a helical assembly of glass fibers, which is embedded in an epoxy or polyester resin. The layer formed by the glass fibers is thicker than the layer formed by the PTFE strands. It is understood that, particularly due to the properties of the resin used as the matrix, such a covering can hardly withstand temperatures of 200°C during operation.

[0004] In addition, in Japanese Patent Application Laid-Open No. 04-25669, the surface of PTFE fibers is activated to fix the fibers in a matrix, in which the fibers are mixed at a concentration of barely 5% at most. Furthermore, even with this configuration, it is not possible to obtain good mechanical performance at high temperatures and when the coefficient of friction is low.

[0005] By repeating certain teachings from U.S. Pat. Nos. 2,804,886 and 3,804,479, another type of friction layer has been proposed, comprising Teflon® filaments and adhesive or tacky Dacron® filaments, woven loosely enough to allow good impregnation with liquid resin, the layer being bounded by windings of resin-impregnated, glass-fiber-filled strips. The presence of adhesive filaments, such as Dacron®, in the material means that the covering cannot withstand temperatures above 200° C. during operation.

[0006] U.S. Pat. No. 4,666,318 discloses a self-lubricating covering for very specific use in the aeronautical field (low pressure and low amplitude) made of a plastic material containing PTFE and cooperating with a counter part having a roughness of 0.050 microns CLA or less and a hardness of 1000 VPN or more. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] British Patent No. 1,439,030 [Patent Document 2] Japanese Patent Publication No. 04-25669 [Patent Document 3] U.S. Patent No. 2,804,886 [Patent Document 4] U.S. Patent No. 3,804,479 [Patent Document 5] U.S. Patent No. 4,666,318 Summary of the Invention [Problem to be solved by the invention]

[0008] As can be seen, constructing a friction shroud that combines a low coefficient of friction with good mechanical performance (in particular good tear strength) and the ability to retain good frictional and mechanical properties up to operating temperatures of 250°C to 300°C (or even up to 320°C during transient operation) means that it is possible to combine, under industrially acceptable conditions and at reasonable cost, a resin that retains good mechanical performance above 250°C while exhibiting satisfactory adhesion, including adhesion at temperatures above this temperature threshold, with a lining element that has a particularly low coefficient and therefore does not a priori adhere very highly to this resin.

[0009] The object of the present invention is to meet this need. [Means for solving the problem]

[0010] To this end, the invention proposes a self-lubricating composite friction element capable of being subjected in operation to temperatures of at least 250°C, characterized in that the element has, along its friction surface, a single layer of fabric made of weft and warp threads of polytetrafluoroethylene, the fabric being impregnated with a heat-stable resin having a glass transition temperature of at least 250°C.

[0011] It is noteworthy that, unlike previously proposed solutions, the present invention teaches the use of a single layer of fabric, the warp and weft of which are all made of polytetrafluoroethylene (PTFE). Thus, the present invention recommends increasing the cross-section of the strands that make up the fabric (which tended to be several thin layers in more recently known solutions), promoting better anchoring with the resin, and using only PTFE strands (which tended to combine PTFE strands with different strands that have good adhesion to the resin within the same fabric). In fact, the use of only a single layer of PTFE fabric in the layer protecting the surface of the part offers the advantage that, if the strands are continuous throughout the thickness of this protective layer, the tear strength is increased, while the remaining spaces between the fabric strands promote better anchoring of this layer in the resin.

[0012] Such fabrics are said to be self-lubricating.

[0013] According to an advantageous feature of the invention, The fabric is a weave formed by the intersection of paired weft threads and paired warp threads, and the fabric is preferably a 2 / 2 twill weave, The weft or warp threads are formed by short fibers bonded together, the fabric has a thickness of at least 0.10 mm, advantageously at least 0.30 mm and preferably at least 0.50 mm, the weft and warp threads have a count of at least 100 dtex, preferably at least 400 dtex, The resin is a thermosetting polyimide. the part further comprises a reinforcing layer that bounds the fabric facing the friction surface, the reinforcing layer being impregnated with the same resin as the fabric; The part constitutes a bearing or a guide rail among various possible applications.

[0014] The product of the present invention can exhibit self-lubrication at temperatures exceeding 250°C during operation, ranging continuously up to 300°C, and even peaking at 320°C, while exhibiting an extremely low coefficient of friction (0.01-0.2) comparable to that of untreated PTFE (without additives or lining), but with a friction coefficient of 40 N / mm 2 This has the advantage of being able to withstand loads exceeding

[0015] By analogy, the invention proposes a method for the manufacture of a self-lubricating composite friction part of the type described above, characterized in that a strip of fabric, formed by weft and warp threads all composed of polytetrafluoroethylene, is wound helically on a mandrel according to a winding angle such that the strip lies end-to-end with itself after each turn, and the fabric has been impregnated with a heat-stable resin having a glass transition temperature of at least 250° C. This resin is advantageously a thermosetting polyimide.

[0016] The objects, features and advantages of the present invention will become apparent from the following description, given by way of non-limiting exemplary embodiments with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a perspective view of a friction component of the present invention. [Figure 2] 1 is a diagram of a preferred embodiment of weaving the friction layer of this friction element. [Figure 3] 1 is a cross-sectional view of this friction layer bounded by a reinforcing layer. [Figure 4] 4 is a simplified schematic diagram of a method of forming a friction part, such as the friction part of FIGS. 1-3. DETAILED DESCRIPTION OF THE INVENTION

[0018] The friction part of the present invention essentially comprises a friction layer with a free surface S facing the opposing part, and advantageously, the friction part further comprises a reinforcing layer bounding the friction layer facing the friction surface in order to enhance the mechanical performance of the friction layer.

[0019] In the embodiment of FIG. 1, the friction element is a bearing 1, which is adapted to receive a shaft (not shown) in its longitudinal bore 1A. In a variant, the friction element may be a slider which receives a rod in translational motion. The friction layer which bounds the friction surface (and therefore the inner surface) is designated by the reference numeral 2, and the reinforcing layer by the reference numeral 3. This layer 3 has in this case a thickness which is substantially greater than that of the friction layer; in fact, depending on the requirements, the friction layer may in practice have a thickness of the order of at most a few millimeters (in practice not more than 3 mm), and the reinforcing layer may be a few millimeters or even centimeters thick. However, it goes without saying that the reinforcing layer, if present, may have any relative thickness to the friction layer.

[0020] The function of the friction layer is to guide the opposing component, the shaft, with as little friction as possible, while maintaining its physical integrity during operation for as long as possible, including operation at operating temperatures of at least 250°C without interruption, and including operation with peak temperatures exceeding 300°C (e.g., up to the order of 320°C).

[0021] To do this, the friction layer consists of a single layer of woven strands (or threads) made of polytetrafluoroethylene (i.e., PTFE) coated with a matrix formed by a heat-stable resin having a glass transition temperature above the maximum continuous operating temperature, and therefore above at least 250°C or even as close as possible to 300°C.

[0022] The term polytetrafluoroethylene, or PTFE, in this case refers to various forms of this compound, including the expanded form known as "ePTFE."

[0023] The weave of the fabric, i.e., the relative arrangement of the strands that make up the fabric, is selected to create flow channels between the various strands that can be filled with a heat-stable resin. In fact, as will be appreciated, since PTFE has virtually no adhesive properties with other materials, the anchoring of the fabric within the matrix can only be achieved by intertwining the amorphous filaments made up of resin that fill the various flow channels present throughout the fabric, these filaments being connected to one another along the strands of PTFE located near the friction surface.

[0024] It will be appreciated that depending on the application, a compromise must be reached regarding the cross section and number of resin flow channels through the fabric: the more numerous and wider these flow channels, the better the anchoring of the strands in the PTFE, but the smaller the proportion of the friction surface formed by the PTFE strands. Conversely, the greater the proportion of the friction surface formed by the strands in PTFE, the better the friction strength of the friction part, but the weaker the anchoring of the fabric in the matrix.

[0025] It is believed desirable to have a flow channel for the resin at each intersection of the weft and warp yarns.

[0026] Among the common weaves, twill weave, or more precisely 2 / 2 twill weave formed by weaving paired weft threads with paired warp threads, appears to allow the construction of channels filled with resin between the strands of the network, the resin being dense enough to ensure good anchoring of the fabric despite the lack of adhesion between the strands and the resin, while providing a fairly large surface formed by PTFE on the opposing parts.

[0027] Such a 2 / 2 twill fabric is shown in Figures 2 and 3, where the weft threads are designated by reference numeral 5 and the warp threads by reference numeral 6, and are woven less tightly to leave gaps 7 free for the resin to pass through. Good results have been obtained with a 2 / 2 twill fabric.

[0028] More advantageously, the weft and warp threads are each formed by a single filament formed by PTFE fibers bonded to one another by twisting, which means that the filament, and thus the weft and warp threads, have an uneven surface that contributes to a good anchoring of the fabric in the matrix.

[0029] Good results have been obtained with such weaving of monofilaments formed by fibers having an average diameter of 0.1 to 0.14 mm.

[0030] These fibers preferably have a count greater than 400 dtex, advantageously at least 750 dtex, with very satisfactory test results being obtained with 833 dtex fibers.

[0031] Alternatively, the fabric may be formed by strands each made up of several filaments, either continuous or made up of staple fibers, as in the above-mentioned examples, and in such cases the count of the filaments may be low, for example of the order of 350-450 decitex or less for two-filament strands. Weaving can be carried out by assembling two or three separate strands, with or without twisting.

[0032] According to yet another variant, the surface of the filament is carefully roughened, for example by the formation of micro-notches.

[0033] The thickness of the fabric is at least 0.30 mm or even at least 0.5 mm, and can assume values ​​in the range of more than 1 mm, which determines the cross section of the strands to be used. The weft and warp threads are advantageously identical. These cross sections are those of disks in the example considered in Figures 2 and 3. In a variant not shown, this cross section is rectangular, for example, with a shape factor (ratio of the largest dimension to the smallest dimension) of preferably at least 2.

[0034] The heat-stable resin is advantageously chosen from thermosetting polyimides, cyanate ester-based resins or polyetherketones, in particular polyetheretherketone (PEEK) or polyetherketoneketone (PEKK). These resins have a minimum glass transition temperature above 280° C. Among the thermosetting polyimides, mention may be made of polybismaleimides or BMI.

[0035] It is not believed that incorporating fillers into heat stable resins is beneficial.

[0036] As will be appreciated, the reinforcing layer has the function of reinforcing the friction layer to maintain its shape despite pressure being applied between the friction part and the opposing part (including pressure conditions at high temperatures), and therefore the reinforcing layer is typically at most 13.10 -6 K -1 It is advantageously made of a material with a very low coefficient of thermal expansion (corresponding to steel), equal to 0.05 MPa, and it is within the ordinary skill of a person skilled in the art to define the geometry and configuration of this reinforcing layer as a function of the requirements. This reinforcing layer may in particular comprise strands or fibers (sometimes called rovings) of carbon, glass or aramid, free or combined in a fabric.

[0037] The fabric of the friction layer is advantageously available as a strip, which offers great freedom for shaping the friction layer, if necessary by using a preform that is the negative of the shape of the friction surface whose profile is to be obtained. The width of the strip can be chosen as a function of the requirements, this width being advantageously chosen between 5 mm and 2 m, for example between 1 cm and 10 cm, preferably between 1.5 cm and 3 cm.

[0038] In the case where the friction part is a bearing as described above, its manufacture can begin by winding a strip of such fabric around a mandrel whose outer diameter is equal to the inner diameter of the bearing to be manufactured, and winding the strip in a spiral manner to ensure edge-to-edge contact of successive turns formed on the mandrel (see FIG. 4). As will be appreciated, the width of the strip 10 determines the inclination of the weft and warp threads relative to the axis of the mandrel and thus the axis of the resulting bearing. In fact, the weft and warp threads are arranged longitudinally and transversely, respectively, relative to the strip of fabric.

[0039] As will be appreciated, the continuity of both weft and warp threads during friction contributes to maintaining good health during the operating life of the bearing, and for these strands, an inclination angle of between 40° and 60° relative to the longitudinal axis of the bearing is considered desirable in this regard. In the case of edge-to-edge windings, and also in the case of woven fabrics (but not in the case of strands), it is preferable to provide windings that follow an angle of between 65° and 89°.

[0040] To form the reinforcing layer, strands of carbon or glass or any other suitable material could be wound at any angle, with the optimum angle for strand winding believed to be between 40° and 60°, although this angle may vary depending on the application and the desired mechanical properties.

[0041] The strips of fabric on the one hand and the strands of the reinforcing layer on the other hand are advantageously impregnated beforehand with a heat-stable resin, although it will be appreciated that if the reinforcing layer is formed using the same resin as the friction layer, a subsequent heat treatment can help to firmly attach the matrices to one another.

[0042] Instead of being formed by winding a strip, the PTFE fabric may alternatively be constituted by a braided tubular sleeve.

[0043] As will be readily appreciated, in the case of a slider-type friction element, the friction layer can be formed simply by attaching the above-described fabric to the underlying reinforcing layer.

[0044] As an example, a friction bearing was formed as follows.

[0045] A 0.3 mm thick PTFE fabric was selected with a 2 / 2 twill weave in the form of a 3 cm wide strip, which was immersed in an impregnation bath maintained at 110 °C containing a heat-stable resin of the polybismaleimide (BMI) type with a glass transition temperature of 285 °C.

[0046] The impregnated strip was wrapped around the mandrel, taking care to cover the entire surface of the mandrel without overlap between successive turns, to form a single continuous layer over the entire surface of the mandrel, this single continuous layer forming the bearing (or bearings). Advantageously, the mandrel was also itself maintained at the temperature of the impregnation bath.

[0047] Then, strands of epoxy glass (called roving-type glass filaments) previously impregnated with the same resin were wound around it.

[0048] The subsequent polymerization cycle included treatment at 170°C for 4 hours (longer durations are possible), demolding, and an additional curing treatment at a temperature of 230-250°C for 4 hours (longer durations are possible).

[0049] It was noted that during subsequent machining, it was difficult to cut the PTFE fibers, which attests to the good performance of the assembly in terms of wear.

[0050] Tribological tests were carried out under the following conditions: - Amplitude axis vibration: 100° - Planned pressure: 80MPa -Average speed: 8mm / s -Average PV (pressure x velocity): 0.64 MPa m / s - Initial shaft / bearing clearance: 0.1~0.2mm - Bearing dimensions: Φint30 x Φext36 x Lg20 - Original grease: None - Opposed shaft of tested solution: 16NC6 case hardened - Maximum test duration: 1 month (350,000 cycles) - ambient temperature

[0051] While an increase in the coefficient of friction of up to 0.04 or even 0.08 has been observed for known bearings (made of polyester fabric coated with a resin containing PTFE or fabric formed by strands of polyester and strands of PTFE (maximum amount of PTFE less than 50%), the coefficient of friction for the bearings of the present invention has been shown to remain substantially constant at a value of barely 0.02 up to 350,000 cycles.

[0052] When the temperature change at the center of the shaft of the opposing part was monitored, it was noticed that this temperature increased to almost 50°C or even 60°C for the known bearing, while the temperature remained below 40°C for the bearing of the invention, which clearly reflects that less energy is dissipated for the bearing of the invention than for the known bearing.

[0053] Even so, it is noted that the overall wear of the bearing of the invention is greater than that of the known bearing, and that the opposing parts show very little wear, but this wear is only apparent and in fact reflects the existence of a comminution phenomenon of the friction layer under the applied contact pressure.

[0054] Additionally, the bearing of the present invention was tested under the following conditions. - Amplitude axis vibration: 100° - Planned pressure: 80MPa -Average speed: 8mm / s -Average PV (pressure x velocity): 0.64 MPa m / s - Initial shaft / bearing clearance: 0.1~0.2mm - Bearing dimensions: Φint30 x Φext36 x Lg20 - Original grease: None - Opposed shaft of tested solution: 16NC6 case hardened - Maximum test duration: 1 month (350,000 cycles) - Temperatures varying from 50°C to 280°C (ambient) - It was difficult to maintain the temperature constant between these temperature levels.

[0055] The coefficient of friction is shown to remain essentially constant with increasing temperatures up to 280°C.

[0056] These test results show that the bearings of the present invention have a satisfactory combination of extremely low temperature coefficient and good performance at temperatures up to and including 250°C and in the range of 250°C to 280°C.

Claims

1. A self-lubricating composite friction part capable of being subjected to temperatures of at least 250°C during operation, said self-lubricating composite friction part having, along its friction surface, a single layer of fabric formed of weft and warp threads, said weft and warp threads all consisting of polytetrafluoroethylene only, said fabric being a weave formed by the intersection of pairs of said weft threads and pairs of said warp threads, said weft threads and said warp threads each containing at least two filaments, each filament being formed by short fibers bonded to each other, said fabric being impregnated with a heat-stable resin having a glass transition temperature of at least 250°C; a method for producing a self-lubricating composite friction part, wherein one layer of fabric is formed by spirally winding a strip of fabric on a mandrel according to a winding angle such that said strip is in an end-to-end position with itself after each turn, said strip having a width selected between 1.5 cm and 3 cm; The woven fabric is formed from weft and warp yarns entirely composed of polytetrafluoroethylene, the woven fabric is formed by the crossing of pairs of the weft yarns and pairs of the warp yarns, the weft yarns and the warp yarns each containing at least two filaments, each filament being formed by short fibers bonded to each other, and the woven fabric is impregnated with a heat-stable resin having a glass transition temperature at least equal to 250°C, the fabric has a thickness of 0.3 mm, is a 2 / 2 twill weave and is in the form of a strip 3 cm wide, the strip is immersed in an impregnation bath maintained at 110°C containing a thermostable resin of polybismaleimide type with a glass transition temperature of 285°C, the impregnated strip is wound around the mandrel so as to cover the entire surface of the mandrel without overlap between successive turns of the strip, forming a single continuous layer over the entire surface of the mandrel, this single continuous layer forming at least one bearing, followed by winding a strand of epoxy glass previously impregnated with the same resin, the method further comprising a polymerization cycle comprising a treatment at 170°C for at least 4 hours, demolding and an additional curing treatment at a temperature of 230°C to 250°C for at least 4 hours.

2. The method of claim 1, wherein the wraps of the strip follow an angle of between 65° and 89° relative to the longitudinal axis of the mandrel.

3. 3. The method of claim 1 or claim 2, wherein the fibers are bonded to one another by twisting.

4. 4. The method according to claim 1, wherein the weft and warp yarns of the self-lubricating composite friction element have a count of at least 100 decitex.

5. The method of any one of claims 1 to 4, wherein the thermally stable resin is filler-free.

6. 6. The method according to claim 1, wherein the self-lubricating composite friction part further comprises a reinforcing layer that bounds the woven fabric facing the friction surface, and the reinforcing layer is impregnated with the same resin as the woven fabric.

7. The reinforcing layer has a thickness of at most 13.10 -6 K -1 7. The method of claim 6, wherein the thermal expansion coefficient is equal to

8. The method according to any one of claims 1 to 7, wherein the weft threads and the warp threads are identical.

9. 9. The method according to claim 1, wherein the cross section of the weft and warp threads is the cross section of a disk.

10. 9. The method according to claim 1, wherein the cross section of the weft and warp threads is rectangular with a shape factor of at least 2.

11. The method according to any one of claims 1 to 10, wherein the self-lubricating composite friction element has a coefficient of friction of 0.01 to 0.

2.

12. The self-lubricating composite friction part has a friction coefficient of 40 N / mm 2 The method according to any one of claims 1 to 11, wherein the material is resistant to a load exceeding 1000 kJ / min.

13. 13. The method according to claim 1, wherein the self-lubricating composite friction element is a bearing, and the weft yarns and the warp yarns are inclined at 40° to 60° with respect to the longitudinal axis of the bearing.

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