Bearing material

Details Bearing material Bearing material

2001-02-05

2002-10-08

Barr, Michael (Department: 1762)

Coating processes

With post-treatment of coating or coating material

Solid treating member or material contacts coating

C427S355000, C427S370000, C427S375000, C427S384000

Reexamination Certificate

active

06461679

ABSTRACT:

The present invention relates to a plastics bearing material, to that material applied to a strong backing material to form bearings and to a method for the production thereof.
Many different types of plastics bearing materials comprising a plastics matrix and having various fillers and applied to a strong backing material such as steel having a porous bonding interlayer composed of bronze particles sintered to the steel are known. One such material comprises polytetrafluoroethylene (PTFE) having therein lead particles, the material being impregnated into the porous bronze interlayer described above to leave a thin layer, generally less than 25 &mgr;m, above the upper surface of the bronze interlayer. The material is made by mixing an aqueous dispersion of the PTFE with the filler material together with an organic lubricant such as toluene; coagulating the dispersion to form a so-called “mush” and decanting off the water; spreading the wet mush on the backing material; applying pressure to the mush so as to impregnate the mush into the porous layer; heating to drive off the residual water and lubricant; and, finally heating the material at a temperature above the melting point of the PTFE to sinter the PTFE particles together. The need to drive off the residual water limits the thickness of the layer which may be formed above the porous interlayer due to the resultant blistering which occurs when thicker layers are attempted. However, even when the surface layer is limited to the generally accepted 25 &mgr;m or so, microscopic examination of the sintered bearing material reveals porosity in the bearing material itself.
Generally, such porosity does not normally matter in most engineering applications for which this type of material is used since the load application is usually static, i.e. applied in one direction, and at loads well within the capability of the material to withstand.
More recently such plastics bearings have been used in engineering applications where the load is dynamic, i.e. the load application direction constantly changes and the load applied to the axial length of a generally cylindrical bearing bush -is non-uniform in that edge-loading at the ends of the bush occurs. One such application is in hydraulic gear pumps which are used in many different applications including automotive vehicles. With the increasing complexity and sophistication of all types of vehicles, such gear pumps may be used in many different applications in one vehicle and may include for example the engine oil pump; power steering pump; pumps to adjust seat position and many more. The construction of such pumps generally comprises two intermeshing gears which are each supported on stub shafts at each axial end, the stub shafts themselves being supported in bearing bushes of the type described above which are held in a housing forming the body of the fluid pump. Plastics bearing materials used in these applications have been failing. The mode of failure appears to be due to the fact that the oil, for example, being pumped between the meshing gear teeth exerts a high load tending to push the gears away from each other and thus causing bending and deflection of the supporting stub shafts in their supporting bearing bushes causing side-edge loading at the bush ends. The result of this deflection induced loading is to cause some of the bearing material per se to creep over the chamfer on the end face at the point of greatest loading and a crack is initiated adjacent the chamfered edge. The crack then propagates into the bearing bush bore due to the oil pressure differential between the ends of the bearing bush. Oil under pressure then washes through the crack so formed and erosion of the bearing lining occurs. The cause of the initial creeping of the polymer material over the chamfered end edge has been identified as a lack of sufficient strength in the lining material itself, due in part to the porosity present in the plastics lining material.
An improved bearing material in this particular application of gear pumps is described in GB-B-2 196 876. The material described comprises tetrafluoroethylene resin and tetrafluoroethylene-hexafluoropropylene copolymer and/or tetrafluoroethylene-perfluoroalkylvinylether copolymer with a filler of lead-tin metal alloy, the material being impregnated into a porous bronze sintered interlayer on steel as described above. Whilst this material constitutes a distinct improvement over other known materials in gear pump applications, it suffers from the disadvantage that it contains lead which is ecologically undesirable especially when the time comes for engines and components utilising lead containing bearings to be scrapped.
It is an object of the present invention to provide a bearing material having improved flow and cavitation erosion resistance and wear and fatigue resistance whilst retaining low friction properties comparable with existing materials and to avoid the use of lead.
It is a further objective to provide such a bearing material at a manufacturing cost comparable with or lower than existing materials.
According to a first aspect of the present invention, there is provided a bearing material comprising a matrix of polytetrafluoroethylene having dispersed therein in volume %: a melt-processable fluoro-polymer in the range from 2 to 10; an inorganic particulate filler material in the range from 10 to 30; and, up to 5 of a ceramic particulate material.
The melt-processable fluoro-polymer may be only melt processable fluoro-polymer which is available in aqueous dispersion and may be selected, for example, from one or more of the group comprising; monofluoroalkoxy (MFA); fluorinated ethylene propylene (FEP); and perfluoro alkyl vinyl ether (PFA). However, MFA is preferred.
Preferably, the melt-processable fluoro-polymer is present in the range from 4 to 8 vol %.
The melt-processable fluoro-polymer increases the toughness of the plastics material matrix and improves cavitation erosion resistance.
The inorganic particulate filler may include at least one material selected from the group comprising: calcium fluoride; magnesium fluoride; strontium fluoride; metal oxides including for example, iron oxide, aluminium oxide, titanium dioxide; and, metal hydroxides such as aluminium hydroxide. Any material known in the prior art as being suitable as a filler material for plastics bearing materials may be employed. However, calcium fluoride is the preferred material.
Preferably, the particle size of the inorganic filler material may lie in the range from 0.1 to 10 &mgr;m. More preferably, the particle size may lie in the range from 0.5 to 5 &mgr;m.
Preferably, the inorganic filler is present in the range from 15 to 25 vol %.
The ceramic particulate filler may include at least one material selected from the group comprising: alumina; silica; zirconia and diamond, for example. However, alumina is the preferred material.
The ceramic particulate filler material is preferably present as particles of less than 100 nanometers and more preferably, of less than 50 nanometers (i.e. less than 0.1 &mgr;m and more preferably less than 0.05 &mgr;m).
Preferably the content of ceramic material lies in the range from 0.5 to 3.5 vol %.
Although it is possible for materials per, se to be duplicated in the inorganic filler and in the ceramic filler (e.g. alumina could be present as both the inorganic filler and the ceramic filler), the particle sizes of the two constituents are different as noted above. It is believed that the inorganic filler helps to support the load and improves wear resistance of the bearing whereas the ceramic filler increases material strength and improves cavitation erosion resistance of the polymer matrix.
According to a second aspect of the present invention, there is provided a method for the manufacture of a bearing material, the method comprising the steps of: mixing an aqueous dispersion of polytetrafluoroethylene with 2 to 10 vol % of an aqueous dispersion of a melt-processable fluoro-polymer, 10 to 30 vol % of a particulate inorganic filler mat

Affiliated with

Also associated with

Rating

Bearing material does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Bearing material, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Bearing material will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2990990