The present invention relates to low-friction products, and more particularly to bearing liners having low-friction properties.

Devices such as bearings and bushings for carrying moving loads frequently employ low-friction liners or gaskets for contacting the load. In many applications, the frictional characteristics of the liner material have a substantial impact on the acceptability of a particular bearing or bushing for a given application. Accordingly, liner selection is frequently a critical consideration in the design of load carrying devices.

The use of fabric bearing liners is known. For example, U.S. Patent No. 3,815,468 - Matt et al. (Matt I) discloses bearing liners comprised of nylon and polytetrafluoroethylene (PTFE) yarn braided together. The fabric is said to preferably contain from 20 to 30% by volume of PTFE thread. Matt I teaches that the PTFE should be in multifilament form, preferably with little or no twist in the filaments. U.S. Patent No. 4,040,883 - Matt et al. ( ^r tt II) discloses the use of a similar material in a knitted construction. According to Matt II, the fabric product contains from about 35 to about 45% by volume of PTFE filaments.

It is seen, therefore, that low-friction materials, such as PTFE, have been suggested for use in fabric bearing liners of both knitted and braided construction. The prior art has generally taught, however, that the use of PTFE yarn in such products should be carefully controlled so as to avoid cold flow of that material during use. For example, U.S. Patent

No. 2,804,886 - White (White I) indicates that the prior art has generally attempted to include low-friction materials in such fabrics in a manner designed "to resist cold flow during use..." (col. 1, lines 33-35). In fact, this patent notes that cold flow of the low-friction material will cause failure in bearing applications (col.

2, lines 50-53), and that fibrous PTFE is preferable to sheet material because it is resists cold flow (col. 2, lines 60-65).

U.S. Patent No. 2,885,248 - White (White II) also teaches that bearing liners containing low-friction materials, such as PTFE, should be configured so as to avoid cold flow of that material. The bearing liners disclosed in White II comprise a plurality of canvas-like layers impregnated with a suitable thermosetting material. The layers are shaped as desired, and a top layer of Teflon cloth is bonded to the backing layers during the molding process. According to the patent, the solid backing body was used because it "prevents any deforming or flow of the [Teflon] material" (col. 4, lines 1-5). White II also indicates that other arrangements were considered but rejected because they did not avoid the cold flow of the low-friction material employed (col. 4, lines 10-14).

SUMMARY OF T - INVENTION

The present invention provides low-friction products having the desirable characteristic of being less costly than many of the heretofore used materials. In particular, the amount of _.ow-friction material used in the present products may be greatly reduced relative to prior materials without a concomitant reduction in frictional efficiency. Applicants have found that this result is achieved by ignoring the teachings of the prior art discussed above. That is, the products of the present invention generally comprise a substrate and low- friction polymeric resin supported by said substrate such that the application of moving loads to the product causes cold flow of at least that portion of the polymeric resin in contact with the load. According to a preferred embodiment of the present invention, the low- friction polymeric resin comprises monofilament of fluorocarbon polymer.

Another aspect of the present invention resides in the use of such products as components in bearing assemblies, and preferably as liners in bearing assemblies used to support moving loads. Such liners preferably comprise a supporting fabric and a shaped body of low-friction polymeric resin supported by said fabric, wherein at least that portion of the shaped body in contact with the load undergoes cold flow upon the application of said loads to said bearing liner. According to certain embodiments, the amount of cold flow exhibited by the shaped body is sufficient to substantially increase, for example by about 1000 percent or more, the area of surface contact between the load and the shaped body.

The present invention also provides methods for supporting moving loads. These methods comprise providing a bearing liner according to the present

invention and causing cold flow of at least a portion of the low-friction polymeric resin contained therein by applying the load to the bearing liner. The product used according to the present methods preferably comprises a braided fabric comprising multifilament yarn and fluorocarbon polymer monofilament, said fabric having a multifilamentrmonofilament weight ratio of from about 75:5 to about 50:40.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an expanded view in elevation of a braided fabric bearing liner according to the present invention.

FIG. 2 is a simplified view in elevation, partly broken away in longitudinal section, of a bearing assembly according to the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The products of the present invention generally comprise a substrate and a low-friction polymeric resin. The term substrate is used herein to refer to means for supporting the low-friction polymeric resin. In addition, it is desirable that the substrate also comprise means for providing the products with shape, strength and form. The substrate may be flexible or rigid, depending upon the particular application, and may take a large variety of shapes and sizes. For example, the substrate may comprise a rigid material having a square or rectangular shape. While all such variations in size, shape and rigidity are within the scope of the present, the substrate preferably comprises a flexible material in tubular form. When the products of the present invention are used as bearing liners, it is preferred that the substrate be readily bondable, by adhesive resins and the like, to the support member of

the bearing assembly.

As will be appreciated by those skilled in the art, the functional requirements of the present substrates can, in general, be satisfied by a large number of materials and constructions. For example, it is contemplated that the substrate may comprise sheet material of paper, cardboard, resinous solid and the like. It is preferred, however, that the substrate comprise a flexible fabric comprised of fibrous material, such as natural, semi-synthetic and synthetic yarns. The fabric may be formed by interleaving the filamentary material according to well-known techniques, such as weaving, knitting or braiding the fibers, with braiding being preferred. A large number of resinous materials are believed to be adaptable for forming the fabric substrates of the present invention. The substrate is preferably comprised of filaments of synthetic or semi- synthetic resin. Such materials include cell' ose polyacetates, polyacrylates, polymethacrylaτes, polyamides, polyolefins, polyurethanes , polyfluorocarbons, and mixtures of these, with textured polyester filaments being preferred.

An important aspect of the present invention resides in the particular characteristics of the low- friction polymeric resin supported by the substrates of the present invention. As used herein, the term low- friction polymeric resin generally refers to materials having a coefficient of static friction on steel of about 0.5 or less. In addition to imparting anti-friction properties to the product, the present low-friction polymeric resins are preferably selected so as to undergo substantial cold flow under conditions of use. For the purpose of convenience, such low-friction resins are sometimes referred to herein as cold flow resins. As the term is used herein, cold flow refers to the inelastic

deformation exhibited by certain materials when those materials are compressed or stretched beyond certain limits. For example, the products of the present invention are especially adaptable for use as liners for bearings and bushings used to support moving loads. When the present products are used as bearing liners, the moving load causes the portion of the low-friction polymeric resin in contact with the load to cold flow. It is believed that this cold flow of the low-friction material, in turn, causes an increase in the effective area of surface contact between the load and the low- friction material. While applicant does not intend to be bound to or limited by any particular theory, it is believed that this increase in the effective surface are of the low-friction material is largely responsible for the desirable anti-fiction characteristics of the present products. It is preferred that the amount of cold flow exhibited in use be substantial. As the term is used herein, substantial cold flow refers to cold flow which causes at least about 100% increase in the amount of surface contact between the low-friction material and the load. According to certain embodiments, the surface contact increases by at least about 500%, more preferably by at least 1000% and even more preferably by at least about 2000%.

As those skilled in the art will understand, several factors affect the cold flow characteristics of the low-friction polymeric resins of the present invention. For example, the particular resin used, its physical form and the manner of its incorporation into the substrate all may have an impact upon whether or not cold flow will occur to the desired extent for a given moving load. All operative combinations of these factors are within the scope of the present invention. Furthermore, it is believed that those skilled in the art

will be able to readily determine without undue experimentation whether any particular combination of factors produces a product within the scope of the present invention. With respect to the bearing liners of the present invention, it is contemplated that such products will generally be exposed to moving loads of about 50 pounds or greater. Accordingly, the products of the present invention preferably include low-friction polymeric resin which exhibits cold flow when exposed to moving loads of about 50 pounds or greater. As the term is used herein, moving load refers to any load members which undergo rotating, vibrating and translating motion and load members having various combinations of the above motions. In contrast to the present invention, the prior art teaches that cold flow of the low-friction material used in fabric bearing liners is detrimental and should be avoided. The disadvantage of the prior art limitation on cold flow is herein overcome, in part, by the use of substrates which provide sufficient structural integrity and support to allow cold flow of the low- friction material without detriment to the overall structural integrity of the product. While the substrates of the present invention may themselves be comprised of materials having anti-friction characteristics approaching those of low-friction polymeric resins, this is not generally necessary.

Accordingly, the present products preferably comprise substrate in an amount effective to provide the product with structural integrity. The present products also preferably include cold flow polymeric resin in an amount effective to impart the desirable low-friction characteristics to the present invention. Applicants have found that these requirements are generally satisfied by products comprising from about 50% to about

95% by weight of substrate and at least about 5% by weight of cold flow polymeric resin, although it is contemplated that in certain embodiments less than about 5% by weight of cold flow resin may be used. As will be appreciated by those skilled in the art, products which require only small amounts of low-friction material are desirable from the cost-of-materials perspective. It is preferred, therefore, that the present products include from about 5% to about 40% by weight of cold flow polymeric resin, with from about 5% to about 20% being more preferred, and from about 5% to about 15% being even more preferred. Thus, the weight ratio of substrate to cold flow polymer is preferably from about 95:5 to about 50:40, more preferably from about 95:5 to about 50:20, and even more preferably from about 95:5 to about 50:15.

Many low-friction resins are known and available to those skilled in the art for use as cold flow resins in the products of the present invention. However, it is highly preferred that the cold flow polymeric material be selected from the group comprising fluorocarbon polymers, polyamides and polyolefins, with fluorocarbon polymers being most preferred. As is well known, fluorocarbon polymers are a class of paraffinic polymers which have some or all of the hydrogen replaced by fluorine. The cold flow resins are thus preferably selected from group comprised of polytetrafluoroethylene (PTFE), fluorinated ethylene propylene cσpolymer (FEP), perfluoroalkoxy (PFA) resin, polychlortrifluoroethylene (PCTFE), ethylene-trichlortrifluoroethylene copolymer (ECTFE), ethylene-tetrafluoroethylene copolymer (ETFE), polyvinylidene fluoride (PVDF), polyvinyl fluoride (PVF), and mixtures of these. While all of the above fluorocarbon polymers, including homopolymers, copolymers, block and graft copolymers of any of the

above, may be used, the cold flow resins of the present invention preferably comprise PFA resin. PFA resin is a copolymer of tetrafluoroethylene and perfluorinated vinyl ether having the general structural formula: CF ₂ CF ₂ CF CF ₂ CF ₂

O

The cold flow polymer resins of the present invention are preferably provided as shaped bodies. While it is contemplated that a wide variety of shaped bodies are adaptable for u^e according to the present invention, it is preferred that the present resins have a monofilament structure. As the term is used herein, monofilament iβfers to single, individual filaments. This is in contrast to the term ultifilament, which refers to chords, threads and yarns of about 15 or more closely associated filaments. The monofilaments preferably have an outer diameter of from about 0.004 to about 0.06 inch. Further, the monofilaments preferably have a ratio of stress at the breaking point to stress at yield of about 3 or greater, and even more preferably about 7 or greater. In part because of its ready availability in monofilament form, the present products preferably comprise monofilaments of PFA. Applicants have found that the monofilament form, especially when incorporated in the preferred braided fabric substrate of the present invention, aids in obtaining the desirable cold flow of the present low-friction materials. As explained above, this cold flow is believed to contribute significantly to the beneficial properties of the present invention.

According to a preferred embodiment of the present invention, as illustrated, for example, in FIG.

1, the substrate comprises a braided fabric of multifilament synthetic fiber. The multifilament synthetic fiber is preferably selected from the group comprising polyester fiber, polyamide fiber, PTFE fiber and mixtures of these. Such multifilament fiber preferably have a denier of from about 200 to about 2000, with about 600 being more preferred. The low-friction character of the product is preferably imparted, by monofilament of low-friction resin. The monofilament, which is supported by being braided together with the substrate, preferably comprises PFA monofilament having an outer diameter of from about 0.004 to about 0.06 inch, and even more preferably from about 0.01 to about 0.02 inch. With particular reference now to FIG. 1, a low- friction bearing liner 10 of the general type described immediately above is illustrated. Liner 10 comprises a tubular fabric constructed of multifilament synthetic yarn 1 braided together with strands of low-friction polymeric monofilament 2. According to the braiding pattern shown in the illustration, tubular bearing liner 10 comprises two ends of multifilament yarn 1 for every end of low-friction monofilament 2. It will be appreciated by those skilled in the art, however, that the particular braiding pattern and the relative amount of monofilament shown in the FIG. 1 are not limiting but are illustrative only. In fact, as described in the example presented hereinafter, the advantages of the present invention are most fully achieved when the relative amount of monofilament is less than that shown in FIG. 1.

Applicants have found that braided fabric products of the type discussed in connection with FIG. 1 are especially well adapted for promoting cold flow of the low-friction resin. In particular, a braided fabric

substrate provides means for supporting the low-friction resin so as to enhance the cold flow of that material in use, particularly when the low-friction resin comprises a monofilament having the dimensions and properties described above. Furthermore, such configurations have been found to provide products which require relatively little low-friction resin to achieve desirable anti¬ friction properties. Accordingly, the braided fabric products of the present invention preferably comprise from about 50% to about 95% by weight of multifilament synthetic resin braided together with from about 5% to about 40% by weight of low-friction monofilament.

An important aspect of this invention resides in applicants' discovery that the present products are readily adaptable for use in bearing assemblies, especially as bearing liners in assemblies for supporting moving loads. Applicants have found that when used as bearing liners, the products of the present invention provide the structural intensity and the frictional efficiency required for such applications. As stated above, the present products are capable of achieving these results using low-friction polymeric resin in amounts that are substantially below those required by many of the prior art products. As is well known to those skilled in the art, low-friction resins typically add significantly to the cost of bearing liners, and products which minimize the use of this expensive material without sacrificing performance are highly desirable. With reference now to FIG. 2, bearing assembly

10 according to the present invention supporting a moving load 11. In the case of a journal bearing, bearing assembly 10 is mounted for predominantly rotary sliding motion with respect to cylindrical shaft 11. The double headed heavy arrow 15 connotes the desired rotary nature

of sliding action with respect to the adjacent inwardly exposed face of bearing assembly 10. Bearing assembly 10 comprises support member 12 having an inner surface generally conforming in shape and size to the outer surface of shaft 11. A braided fabric bearing liner 13 as described above is adhered to the inner surface of support member 12. In the process of manufacturing the bearing assembly, the inside annular surface of supporting member 14 is coated with a bonding agent, such as adhesive resin, and the braided fabric product 13, which has an outer surface sized and shaped to closely mate with the inner surface of support member 12, is placed in contact with the bonding agent and thereby consolidated with the support member. The use of braided fabric according to the present invention provides a feature which is highly advantageous at this point in the manufacturing process. In particular, less void space is generally associated with patterns of braided fabric as compared to patterns of knitted fabric. This characteristic inhibits the undesirable flow of the bonding agent to the inner, bearing surface of the fabric during the assembly process. The presence of such bonding agent at the inner surface is generally not desirable since it will negatively affect the frictional characteristics of the bearing assembly.

The methods of the present invention relate to using the present products to support moving loads. These methods include the step of providing a bearing assembly of the type described above. As will be appreciated by those skilled in the art, at least a portion of the cold flow resin in the bearing is in contact with the surface of the load member. In the case of cold flow resin comprising substantially cylindrical monofilament, the surface contact between the monofilament and the load will initially be low. When

the bearing assembly is operated and exposed to the moving load, cold flow of the monofilament occurs and the area of surface contact between the load and the cold flow resin increases dramatically. The following example, which is illustrative and not limiting of the present invention, shows the unexpected and beneficial results achieved by the present products.

Example 1 A bearing liner having an inside diameter of about 1 inch was braided using a braiding apparatus of the general type shown in FIG. 2 of U.S. Patent No. 4,836,080, which is incorporated herein by reference. The braiding apparatus produces a repeating pattern of the general type disclosed in FIG. 1. The braided fabric consisted of about 91.25 wt. percent of textured polyester (600 denier), about 3.25 wt. percent PTFE and about 5.5 wt. percent of PFA. The PFA was in monofilament form and had a coefficient of static friction on steel of about 0.04 and a ratio of tensile stress at break to tensile stress at yield of about 7. The fabric was produced on an apparatus having 120 carriers. Three ends of textured polyester multifilament yarn (600 denier) were placed on 90 carriers, two ends of the same polyester yarn and one end of PTFE multifilament yarn (400 denier) were placed on 20 carriers, and two ends of the same polyester yarn and one end of PFA monofilament (11 mil.) were placed on 10 carriers. The monofilament carriers were evenly spaced around the deck of the braider, with five going in one direction and five going in the opposite direction. The braiding liner was incorporated into a bearing assembly of the type described in connection with FIG. 2. The assembly thus formed was found to perform satisfactorily relative to prior art products using much greater percentages of

multifilament yarn of fluorocarbon polymer resin.

The foregoing is descriptive of the features of the present invention but is not limiting thereof, it being understood that the scope of the present invention is defined only by the claims which follow.