Plain Bearings

This invention relates to plain bearings and methods of making them, and according to the present invention a plain bearing has a bearing lining consisting of Poly-phenylene sulphide. Poly-phenylene sulphide is a crystaline material prepared by the reaction of p-dichlorobenzene and sodium sulphide in a polar solvent. The linear polymer so formed has a fair mechanical strength but it can be cured by heating in air to be cross-linked and to be tough, ductile, and extremely insoluble. In this specification the term `polyphenylene sulphide` includes such polymers with varying degrees of cross-linking in dependence on the heat treatment used.

Preferably the bearing has a metal backing, for example of steel or of aluminium or an aluminium alloy and in the case of a bearing with steel backing the upper surface of the backing can be aluminised or can have bonded to it possibly by roll-bonding a strip of aluminium foil because polyphenylene sulphide adheres very readily to aluminium and aluminium alloys. Manganese dioxide or lead oxide may be mixed with the poly-phenylene sulphide to assist cross-linking and oxidising the backing surface to improve the bond. Alternatively or in addition the titanium dioxide can improve the adhesion of the poly-phenylene sulphide.

The bearing may be for wet or dry use, that is to say, it may be arranged to operate with oil, grease, or other appropriate lubricant, and in such a case lubricant pockets can be provided in the lining possibly by indenting into the backing, and can be charged with sufficient lubricant to last the life of the bearing. Alternatively the bearings may be used as so called dry bearings by including polytetrafluoroethylene with the poly-phenylene sulphide layer.

The poly-phenylene sulphide may include fillers for improving the mechanical properties or the lubricating properties for example, copper, or bronze powders, asbestos, molybdenum disulphide, lead powder, lead oxide powder, cadmium, nickel, cobalt, or boron nitride.

From another aspect the invention may be considered to reside in a plain bearing having an overlay.

A soft overlay is often applied to the bearing surface of plain bearings such as the crankshaft bearings of internal combustion engines by the electrodeposition of such alloys as lead/tin, lead/tin/copper and lead/indium. The function of the overlay is to improve the seizure resistance of the bearing, to embed dirt particles carried by the lubricating oil and in some cases to provide protection for the bearing against corrosion by the oil. The function of the tin or indium content of the overlay is to make the overlay itself resistant to corrosion. The thickness of the overlay must also be closely controlled, usually within the limits of .+-.0.01 mm.

Overlay plating is used on a range of bearing materials of the kind where there is a steel back, an intermediate alloy layer of copper/lead, lead/bronze, aluminium/tin or aluminium/silicon, and an overlay over this intermediate layer. Preparation of the intermediate alloy surface prior to electrodeposition of the overlay is a multistage process involving one or more chemical etching stages and in the case of the aluminium alloys the electrodeposition of a tin interlayer of nickel or copper. The electrodeposition of overlays is thus a complex process requiring close control; it is for this reason a costly process which accounts for a significant proportion of the total cost of the bearing.

The common electrodeposited overlays have a further disadvantage in that during operating at engine temperature loss of tin or indium by diffusion into the intermediate alloy or into the nickel or copper interlayer can take place. When the overlay becomes denuded of tin or indium the overlay becomes subject to corrosion by the engine oil. Corrosion can result in severe cases in the partial or complete disappearance of the overlay.

A layer of poly-phenylene sulphide may be applied over such a plain bearing as a protective overlay.

The invention relates to all kinds of plain bearings whether thrust or journal bearings including plain shell bearings for such applications as engine shaft bearings and also plain bearings consisting of flat relatively slidable metal surfaces of which many examples occur in the world of engineering. A journal bearing shell can be made by coating a flat strip with poly-phenylene sulphide and then forming the strip into a part cylindrical shell.

The poly-phenylene sulphide may be applied to the backing possibly in powder form or possibly in the form of a thin sheet or possibly by spraying, for example, electro-static spraying, spraying in solution, plasma or flame spraying or even by application from a fluidised bed.

The bonding to the backing can be simply effected by heating the lining material and backing sufficiently to melt the lining material, and additional heat treatment can be applied to it and a good bond can be achieved. Thus bonding to an aluminium alloy for example, a magnesium/aluminium, silicon/aluminium or tin/aluminium alloy can be effected by heating to at least 370.degree. C in air for at least 10 minutes. Alternatively bonding can be effected by a rolling process and indeed a rolling process can also be used to form indentations in the lining to define the lubricant pockets discussed above.

Heat treatment of the poly-phenylene sulphide can cause an increased degree of cross-linking of the polymer which can improve its mechanical properties.

In general the poly-phenylene sulphide layer will not soften in use at temperatures below 270.degree. C which is adequate as a bearing surface operating temperature for very many applications.

The bearing can be bonded, formed and cured in operations which produce quite severe deformations in the plastic/metal laminate without failure of the bond and without loss of manufacturing tolerance, and that makes the poly-phenylene sulphide particularly suitable for the type of bearing which requires such formation.

Another method of keying the poly-phenylene sulphide to the backing is by causing the layer of metal powder to adhere to the backing and then impregnating it with the poly-phenylene sulphide, for example, by rolling. As poly-phenylene sulphide is dimensionally stable it is possible to form bearings of the final size in that manner but it is possible to machine the bearing to the final size for example by boring.

Also where a particular thickness of poly-phenylene sulphide is required it may be built up in one or more layers.

The invention may be carried into practice in various ways and some examples will now be described.

A 1 mm thick strip of an alloy of 4.0 to 4.9% magnesium by weight, 0.5 to 1.0% manganese, with the balance aluminium, is brushed under a rotating wire brush and has applied to it poly-phenylene sulphide powder with a particle size in the range 90 to 350 microns, the powder having been lightly cross-linked by heat treatment in air. The powder is spread by a reciprocating blade to a thickness of 2 mm and is then melted by heating at 300.degree. C from the aluminium side. Then the melted layer is transferred to an oven and cured for one hour at 375.degree. C achieving a final layer thickness in the range 0.3 to 0.4 mm. Bearing brushes are formed from such a strip blank.

A low carbon steel strip 1.5 mm thick is linished and has bonded to it a brushed commercially pure aluminium foil 0.1 mm thick and bonding is effected by rolling with 43% reduction. The upper aluminium surface is brushed and then the lining is applied to it by the method described in Example I. In either case the temperature at which the finished bearing begins to distort can be raised by a long curing step, for example curing at 375.degree. C for 16 hours.

This method is the same as in Example I except that after the polymer has melted the strip is cooled rapidly and formed into a bush which is then heated at 375.degree. C for one hour.

This methos is the same as Example III except that instead of cooling the strip rapidly the following heat treating cycle is followed: heating for one minute between 300.degree. and 375.degree. C, holding for five minutes at 375.degree. C, and heating for 15 minutes at 375.degree. to 440.degree. C.

This is the same as Example I except that the strip is a 1 mm thick strip of an alloy consisting of 0.8 to 1.5% manganese by weight with the balance being aluminium. After curing the strip is passed under an indenting roll to give a pattern of 2 mm by 1 mm rectangular indents 0.4 mm deep. The blank is formed into part-cylindrical portions with the indents extending axially and the indents are filled with grease prior to use.

This is the same as Example V up to indentation, after which a blank is rolled in low molecular weight polyethylene wax and formed into a bush.

A drawn tube 30 mm inside diameter and 35 mm outside diameter and 50 mm long of an alloy consisting of 3.1 to 3.9% magnesium by weight with the balance being aluminium is degreased and heated to 520.degree. C. It is then dipped into a fluidised bed of lightly cross-linked poly-phenylene sulphide powder which melts on the surface. Unmelted powder is removed by blowing or sucking and the coated tube transferred to an oven at 310.degree. C. The temperature is raised to 375.degree. C for one hour and the tube is machined to final size.

A shell consisting of a 60% aluminium/40% tin alloy has its surface brushed and applied to a cleaned surface of a steel backing. The backing and lining are heated to 400.degree. C and then treated as in Example VII.

Poly-phenylene sulphide powder which is not cross-linked is mixed with DQ3 powder in a high speed mixer in equal proportions, and spread by a reciprocating blade in a layer 1 mm thick on a brushed surface of a 1 mm thick strip of alloy consisting of 4.0 to 4.9% magnesium, 0.5 to 1.0% manganese, the balance being aluminium. The powder is melted at 300.degree. C by heating from the aluminium side and consolidation bonding is effected with a cooled roll after which the blank is cured for one hour at 375.degree. C cooled and formed into a bush.

75 parts of poly-phenylene sulphide powder not cross-linked is melted and combined with 25 parts of DQ3 powder and cominuted to a particle size between 90 and 350 microns and it is then applied to a strip as in Example VII.

In the above two examples, DQ3 powder consists of:-

PTFE: 26.8% by weight;

Pb.sub.3 O.sub.4 : 65.0%

PbO.sub.2 : 3.7%

Tin/Bronze powder: 4.5%

This is the same as Example VIII but applied to an alloy consists of 10.3% to 11% silicon, 0.9% to 11% copper, the balance being aluminium.

In the examples a temperature of 375.degree. C has been referred to. In fact the temperature and time of treatment can be varied in accordance with requirements between about 316.degree. C for three hours and about 427.degree. C for 10 minutes.