Self-lubricated bearing compositions and methods of making the same

A self-lubricated bearing contains a first member, a second member configured to slide against the first member, and a lubricating liner disposed in a sliding contact area between the first member and the second member. The lubricating liner contains at least one structural polymer and at least one lubricant with viscosity less than 5100 centistokes. A method is also provided for making the bearings, which includes adding the at least one lubricant with viscosity less than about 5100 centistokes to one or more components of the lubricating liner during the process of making the liner or bearing. The self-lubricated bearing compositions of the invention have surprisingly long service lives versus prior art self-lubricated bearings.

FIELD OF THE INVENTION

The present invention relates to bearings, and particularly, to bearings having lubricating liners.

BACKGROUND OF THE INVENTION

Prior art bearings have attempted to provide an environment with decreased friction. Unfortunately, such attempts have failed in that they have a shortened service life, thereby having excess maintenance requirements and short bearing reliability. U.S. Pat. No. 3,932,008, by McCloskey and Williams, describes a spherical bearing including a bearing liner having a low friction layer containing a resin matrix which includes therein a dispersion of self-lubricating particles including powdered or flock polytetrafluoroethylene (PTFE) particles and graphite fibers.

Self-lubricated bearings using PTFE as a sliding surface have been in use in the aerospace industry since the 1960's according to U.S. Pat. No. 4,666,318, assigned to Ampep, pic. Thin films of woven PTFE bearing surfaces are frequently reinforced with sintered bronze and textile yarns such as glass, graphite fibers, or high strength organic yarns of relatively high melting point. Woven structures are usually infused with resin systems such as phenol formaldehyde, epoxies or cyanoacrylates to bind the sliding surface into a dense structure. The inventors go on to teach improved bearings using a reinforced low-friction plastic element containing PTFE having a sliding surface and a counter face with a low surface roughness and high hardness.

U.S. Pat. No. 6,180,574 assigned to Rexnord Corporation describes a self-lubricating liner for bearings which includes a curable acrylate composition and a solid lubricant such as PTFE.

The abovementioned bearings, and other prior art bearings, have limited life. The prior art teaches bearings with self-lubricating liners containing fluoropolymer solid lubricants. New self-lubricated bearings are needed with improved service life to reduce maintenance requirements and cost as well as to increase the long term bearing reliability.

SUMMARY OF THE INVENTION

The present invention provides improved self-lubricated bearing compositions containing a first member, a second member configured to slide against the first member, and a lubricating liner disposed in the sliding contact area between the first member and the second member, wherein the lubricating liner contains at least one structural polymer and at least one lubricant with a viscosity less than 5100 centistokes.

The bearing liner can be any bearing liner, including but not limited to, composite liners formed from combinations of: woven and non-woven fabrics; binder resins; solid lubricants such as, but not limited to, polytetrafluoroethylene; and other additives. The bearing liner can also be a coated or molded liner made with: thermoplastic or thermoset resins; solid lubricants such as, but not limited to polytetrafluoroethylene; and other additives, reinforcements, stabilizers, catalysts, thixotropic agents, and colorants.

The present invention provides a way to substantially improve the performance of the prior art self-lubricated bearings by addition of a lubricant having a viscosity less than 5100 centistokes in an amount of from 0.1% to 10% by weight of the total liner. A method is also provided for making the bearings, where the method includes adding the at least one lubricant with viscosity less than 5100 centistokes to one or more components of the lubricating liner during the process of making the liner composition or bearing. A preferred method is provided whereby the at least one lubricant with viscosity below 5100 centistokes is vacuum impregnated into the self-lubricating liner after the liner is adhered to one of the bearing members. The self-lubricated bearings of the invention have surprisingly long service lives versus prior art self-lubricated bearings.

DETAILED DESCRIPTION

The present invention provides improved self-lubricated bearings containing a first member, a second member configured to slide against the first member, and a lubricating liner disposed in a sliding contact area between the first member and the second member.

The self-lubricating liner contains at least one structural polymer and at least one lubricant with a viscosity less than 5100 centistokes. In a preferred bearing of the invention the at least one lubricant with a viscosity less than 5100 centistokes contains a fluorinated polymer.

A method is also provided for making the bearings, where the method includes adding the at least one lubricant with viscosity less than 5100 centistokes to one or more components of the lubricating liner during the process of making the liner or bearing. A preferred method is provided whereby the at least one lubricant with viscosity below 5100 centistokes is vacuum impregnated into the self-lubricating liner after the liner is adhered to one of the bearing members. The self-lubricated bearings of the invention have surprisingly long service lives versus prior art self-lubricated bearings.

The self-lubricated bearing is shown inFIG. 1. The illustration provided byFIG. 1is meant to show the key elements of the invention and is by no means meant to limit the invention. The bearing contains a first member1(also referred to herein as a sliding member) configured to slide against a second member2(also referred to herein as a second sliding member) where a lubricating liner3is attached to first member1and located between the first member1and the second member2. The illustration ofFIG. 1shows a space between the three components in order to illustrate that the lubricating liner3is attached to the first sliding member1. In the bearing ofFIG. 1, the second sliding member2is also in contact with the lubricating liner3. The lubricating liner3moves with the first sliding member1, to which it is attached, while the second sliding member2slides along the surface of the lubricating liner3. The lubricating liner3may be attached to either one or the other sliding member1,2however the lubricating liner3can also be floating freely between the members1,2as long as other provisions are made to keep the lubricating liner3from moving out of the sliding contact area between the sliding members1,2.FIG. 1depicts the sliding members1,2as flat surfaces. It should be noted, however, that the sliding members1,2can be any shape as long as they are in sliding contact with each other, via the lubricating liner3.

FIG. 2provides an illustration of one common type of self-lubricated bearing of the invention, specifically, a rotating bearing,4. It should be noted that the bearing ofFIG. 2is intended to convey one type of object of the invention. These types of bearings can be found in many systems and actuators. A cross sectional view5of the rotating bearing4is also shown byFIG. 2. A ball7of the bearing has a convex outer surface (1st sliding member) in sliding contact with a self-lubricating liner8, which is attached to a race component6(2nd sliding member). The particular ball type shown byFIG. 2has a central bore so that it can be placed onto a shaft or rod and used in some construction. The self-lubricating liner8could also be attached to the ball7; however this is usually not done in practice as a matter of convenience.

FIG. 3is an illustration of a rod12in a bushing11, which is another non-limiting example of a type of self-lubricated bearing9provided in accordance with the present invention. A cross sectional view10of the rod12in the bushing11is also shown inFIG. 3. The rod12is in sliding contact with a self-lubricating liner13(the very thin dark area between11and12) disposed between the bushing11and the rod12. The self-lubricating liner13is adhered to the bushing11. The self-lubricating liner13contains a structural polymer and at least one lubricant with viscosity below 5100 centistokes.

FIG. 4illustrates a bushing or one sliding member15having the thin liner14attached to the inner surface of the sliding member15. The other sliding surface is not shown in this rendering. The liner14is attached to the inner surface of the bushing15, but could also be attached to the outer surface of the bushing15in the case where the other sliding component was a sleeve.

FIG. 5illustrates a flanged bushing16with lubricating liners17,18on two surfaces. This bushing is a component with two bearing component surfaces—the outside flange and the internal circular bushing surface. The mating sliding surfaces are not shown in this figure.

FIG. 6is a bearing wear versus life graph illustrating a prior art self-lubricated bearing and a bearing of the invention which is identical to the prior art bearing except that a fluoropolymer lubricant with viscosity less than 5100 centistokes was vacuum impregnated into the self-lubricating liner. These bearings were tested according to Mil-B-81819/1-2 (Reversing Load Test with Alkaline Water Contaminant) which is used to evaluate helicopter bearings. The Comparative Example self-lubricated bearing of the invention is commercially available and the Inventive Example 1 bearing is the same bearing as the Comparative example bearing except that a fluoropolymer lubricant with viscosity below 5100 centistokes was vacuum impregnated into it. Inventive Example 1 bearings had less than half the wear after 1600 hours of testing under MILB-81819/1 Test Condition 4. This is a surprising result because the Comparative Example Bearings already contain a fluoropolymer lubricant; however the fluoropolymer lubricant incorporated into Comparative Example 1 bearing has viscosity greater than 5100 centistokes. In one embodiment this invention can be used to greatly extend the maintenance intervals for helicopters, thereby reducing operating costs, since it provides superior bearing life.

FIG. 7illustrates a bearing or one sliding member15having the thin liner14attached to the inner surface of the sliding member15, where the bearing is helicopter main rotor700bearing.

FIG. 8illustrates a flanged bearing16with lubricating liners17,18on two surfaces, where the bearing is helicopter tail rotor800bearing.

Self-Lubricated Bearing Description

The principal self-lubricated bearing components of the invention are illustrated inFIG. 1. The first and second members may be constructed of any materials, and may be the same or different. As an example, the first and second members may be metal, polymer or ceramic based compositions or a combination of one or more of these compositions. In cases where the first or second members are some combination of metal and polymer, metal and ceramic, polymer and ceramic, or metal, polymer, and ceramic, the combination can be in the form of a composite structure where one or more components are intimately mixed with the other component or they may be in a layered structure where one or more components are layered on top of the other component (one non-limiting example being a ceramic coating over a metal substrate).

Often the first member is made of different materials than the second member in order to optimize bearing performance and reduce wear of the member as it slides along the lubricated liner. The first and second members often have specific surface finishes that may be the same or different. The first member may have a rougher surface finish to enhance adhesion of the lubricating liner in cases where the lubricating liner is adhered to the first member. In other articles of the invention the first and or second members may have a highly polished surface to reduce abrasive wear between the member and the lubricating liner.

The lubricating liner contains at least one structural polymer and at least one lubricant with viscosity less than 5100 centistokes. The structural polymer useful in the invention may be any thermoplastic or thermoset polymer. The structural polymer may be a homopolymer or copolymer. The structural polymer can be a mixture or alloy of two or more polymers.

The structural polymers may contain any organic or inorganic additives, reinforcements, fibers, woven fabrics, non-woven fabrics, or stabilizers, to provide strength, toughness, stiffness, heat resistance, thermal conductivity, dimensional stability, hardness, wear resistance, thixotropy, color, or other attributes. The structural polymers provide the matrix for the lubricating liner composition.

The lubricating liner compositions useful in the invention may also contain reinforcing fibers. Glass fibers, polyaramide fibers, and carbon fibers are some of the more common reinforcing fibers found in self-lubricated bearing liners. One or more solid lubricant particles, organic fibers, or inorganic fibers especially comprised of polytetrafluoroethylene or other lubricants such as graphite, molybdenum disulfide, and hexagonal boron nitride are often included in the lubricating liner compositions useful in the invention.

The lubricating liner useful in the present invention may be solid or may have porosity. The porosity may have a closed cell (isolated pores) or open cell (connected pores) structure. Cured phenolic resins often naturally have some porosity within their cured structure. The lubricating liner may have porosity engineered into the composition through the use of foaming agents.

The lubricating liner can also have porosity engineered into its structure through the use of extractible salts. One non-limiting example would be using sodium benzoate as an additive to a polymer liner composition. The sodium benzoate may be subsequently extracted with hot water after the liner is formed, thereby leaving an engineered reservoir. This engineered reservoir is then vacuum impregnated or otherwise filled with at least one lubricant with viscosity below 5100 est.

The structural polymer holds the liner composition together and gives it integrity. The structural polymer may also contain a woven or non-woven fabric of any composition to provide increased strength or lubricity (in the case where the fabric comprises polytetrafluoroethylene, polyethylene fibers, or other lubricating fibers).

The lubricating liner may also contain solid lubricant polytetrafluoroethylene film in a composite type of structure.

The self-lubricated bearings can be made by separately making the lubricating liner, adhering it to one of the sliding members, and then constructing the bearing. The lubricating liner can also be made directly between the sliding members by a molding process where the liner composition is injected between the members and hardened in place. The lubricating liner can also be coated onto one of the sliding members and then combined with the other sliding member. The lubricating liner can also be placed between the sliding members and not adhered to one or the other sliding member so long as provision is made to mechanically or otherwise prevent the lubricating liner from slipping or moving out from in between the sliding members.

The at least one lubricant with viscosity less than about 5100 centistokes (est) can be any lubricant such as silicone fluids, hydrocarbon fluids, synthetic fluids, low molecular weight fluoropolymers or mixtures of more than one of these. The at least one lubricant viscosity is measured at or below 100° C. where it is less than about 5100 centistokes. The at least one lubricant can also contain dispersed additives to further enhance performance such as antioxidants, stabilizers, anti-corrosion additives, or fine lubricant particles like polytetrafluoroethylene, ceramic, graphite, molybdenum disulfide or hexagonal boron nitride. The at least one lubricant is preferably a perfluoropolyether, which represents a class of perfluoropolymers that differ mostly in viscosity (molecular weight) and chemical functionality. Some examples of commercially available perfluoropolyethers useful in the present invention include, but are not limited to, Krytox® GPL 100 series and XP series containing anticorrosion additives available from the DuPont Company. The at least one lubricant can also be chemically functionalized to enhance compatibility with the structural polymer, if so desired. An example of a chemically functionalized perfluoropolyether would be Fluorogard FMS from the DuPont Company. Pertluoropolyethers are also available from other manufacturers such as Solvay Solexis.

The at least one lubricant with viscosity below about 5100 est can be added to the self-lubricating liner at any stage of manufacture of the bearing of the invention. The at least one lubricant with viscosity below about 5100 est can be added to one or more of the liner components before the liner is formed, molded, or otherwise assembled. The at least one lubricant with viscosity below about 5100 est can be added to the liner after it is formed and can be added to the liner before or after the liner is assembled with the first bearing member and the second bearing member. In bearings of the invention where the liner is molded, sprayed, or otherwise coated in place, such as with acrylate liners or epoxy liners, for example, the at least one lubricant with viscosity below about 5100 est can optionally be combined with the acrylate liner precursors or epoxy liner precursors before molding, spraying, or otherwise coating the liner into place.

In a preferred method, the at least one lubricant with viscosity below about 5100 est is added to the liner after the liner is adhered to one of the bearing members. In the most preferred method the at least one lubricant with viscosity below about 5100 est is vacuum impregnated into the liner after the liner is adhered to one of the bearing members.