This invention is concerned with a method for manufacturing a plain bearing.

For many years, plain bearings of the general type described in US Patent Specification NO. 2,689,380 have been in common use. Such bearings comprise a layer of sintered metallic material, e.g. bronze, formed on a backing material such as steel. The layer of sintered metallic material has an overlay of polytetrafluoroethylene which is forced into the intersticies of at least a surface portion of the layer of sintered metallic material and forms a low friction surface of the bearing. Such bearings may be formed by applying particles of polytetrafluoroethylene to the surface of the layer of sintered metallic material and applying heat and pressure to the particles so that the polytetrafluoroethylene is forced into the intersticies as described above.

In the conventional method of manufacturing a plain bearing as described above, a dispersion of polytetrafluorethylene is coagulated in the presence of a filler, e.g. lead powder, and a high proportion of toluene. This produces a wet "mush" containing toluene. This mush is applied to the surface of the sintered metallic layer, the toluene acting as a lubricant to assist the flow of the polytetrafluoroethylene/lead into the intersticies of the

sintered material. This method, however, suffers from the disadvantage that the mush is extremely shear-sensitive, in that there is a tendency for the particles of polytetrafluoroethylene to fibrillate when they are subjected to relative movement. Thus, even mild working of the mush causes thin fibrilles of polytetrafluoroethylene to be drawn out across the shearing surfaces resulting in the mush quickly becoming very stiff and unusable since it does not possess the good flow characteristics required to enable it to impregnate the intersticies of the sinter. In order to avoid fibrillation of the mush, the mush is applied very carefully by hand to the sintered layer and attempts to avoid the necessity for such a hand operation by introducing automatic handling of the mush have so far, so far as is known, been unsuccessful.

It is an object of the present invention to provide a method of manufacturing a plain bearing of the type described above in which the above-mentioned hand operation can be eliminated.

The invention provides a method of manufacturing a plain bearing comprising applying particles of polytetrafluoroethylene to the surface of a layer of sintered metallic material and applying heat and pressure to the particles so that the polytetrafluoroethylene is forced into the intersticies of at least a surface portion of the layer of sintered metallic material and forms a low friction surface thereon, characterised in that the particles of polytetrafluoroethylene applied are encapsulated in a coating of a material which prevents fibrillation of the polytetrafluoroethylene during application of the particles but which breaks down during said application of heat and pressure to allow such fibrillation.

In a method according to the invention, the polytetrafluoroethylene particles are prevented from fibrillating until after they have been applied to the sintered metallic material so that handling of the particles by automatic application means does not cause such fibrillation. Furthermore, as the particles are protected from fibrillation, the particles can be applied as a dry powder which is not possible in the conventional process since the mush of the conventional process when dried is unworkable. This enables the use of toluene as a lubricant to be dispensed with since toluene has been selected as a material which can form a stable emulsion with the polytetrafluoroethylene and as having coagulation characteristics which allow it to coagulate with the polytetraflurorethylene mush leaving toluene within the mush. The lubricant can be added after the dry powder has been applied, for example, in the form of kerosine. Thus, the polytetrafluoroethylene can be stored in powder form and used when required with the addition of liquid lubricant.

Preferably, the coating is formed from an acrylic co- polymer, for example an acidic cross-linked acrylic emulsion co-polymer.

The polytetrafluoroethylene particles may also contain a filler, for example, lead, molybdenum disulphide, graphite, bronze, or zinc.

The polytetrafluoroethylene particles may contain an anti-discolouring agent particularly where a filler of lead is used. The anti-discolouring agent may be designed to prevent the formation of lead oxide.

There now follows a detailed example of a method manufacturing a plain bearing according to the invention which has been selected by way of example and not by way of limitation of the invention.

In the illustrative example, a dispersion of 75% polytetrafluoroethylene solids in water was poured in to a Hobart mixer fitted with a K-paddle. The mixer was operated at its slowest speed. To this dispersion in the mixer, a 28% solid aqueous solution of acidic cross-linked acrylic emulsion co-polymer was added. Mixing continued for 4 minutes. Then, sufficient lead powder to produce a 20% by volume lead metal content in the final lead and polytetrafluoroethylene mixture was dispersed in a minimum quantity of water with the minimum quantity of a non-ionic surfactant to give a free flowing liquid. The liquid was added to the mixture in the mixer as rapidly as possible to prevent the heavy lead from separating out.

Next, a discolouring agent was added to the mixture to prevent the lead metal from forming oxides during drying. The discolouring agent in question was sodium nitrite stirred into an ammonia solution.

Mixing was continued for about 2 minutes producing a dark-coloured, thick, cream-like material. This material was allowed to stand for 24 hours and then the supernatant liquid was decanted. The resulting thick gellatinous mass was then dried in an oven resulting in a dry powder containing 20% by volume of lead and polytetrafluoroethylene particles which were each completely encapsulated in a thin skin of the acrylic resin. This powder was found to be capable of being poured, stirred or mechanically handled with no fibrillation or flow difficulties.

The dry powder was mixed with kerosine which acted as a hydro-carbon lubricant and the material was applied to the surface of a sintered metallic material in the conventional manner. Heat and pressure were applied so that the coating of acrylic resin was broken down and burnt off and the polytetrafluoroethylene and lead mixture was able to penetrate the intersticies of the metallic layer and fibrillation was able to occur to give the necessary strength to the polytetrafluoroethylene layer.