The present invention relates to a plastics based bearing material and a bearing having a lining of the material.

In some bearing applications, such as in food processing machinery for example, it is frequently necessary to employ bearings having self lubricating properties. It is also necessary, due to ever more strict legislation, that the bearing materials themselves are unleaded. A well known bearing material comprising polytetrafluoroethylene (PTFE) and lead has excellent self lubricating properties and possesses a low wear rate and long operating life.

However, the presence of lead has rendered this material unsuitable for use in applications such as those specified above.

One material which is currently used comprises a molybdenum disulphide filled PTFE matrix which, whilst conforming to the legislation, possesses a relatively short life compared to the lead containing PTFE material. It is believed that the long operating life and low wear rate of the lead containing material is due to the effect of lead in promoting a tenacious transfer film to the co-operating shaft. It is further believed that the molybdenum disulphide filled material does not possess this adhesive transfer-film forming ability.

We have now found an lead-free material which does have a longer life than known molybdenum disulphide containing materials.

According to a first aspect of the present invention, a bearing material comprises a PTFE matrix having incorporated therein 1 to 10 vol.% of graphite; 5 to 20 vol.% of copper-based alloy powder and 4 to 25 vol.% of zinc powder.

The copper-based alloy powder is preferably bronze of, for

example, 11 wt.% tin-balance copper, phosphor bronze or brass.

Preferably, the copper-based alloy powder should all pass through a 300 B.S mesh sieve, which corresponds to a maximum particle size of about 50 micro metres.

Similarly, it is also preferred that the zinc powder also passes through a 300 B.S mesh sieve.

In a preferred composition range the material has from 2 to 7 vol.% of graphite; 10 to 20 vol% of copper-based alloy and 5 to 20 vol.% of zinc.

In a still more preferred composition, the material has from 3 to 7 vol% of graphite; 13 to 17 vol.% of copper-based alloy powder and 8 to 12 vol. of zinc.

It is believed that the good performance of this material is due to the zinc helping to promote an adherent transfer film to the co-operating shaft.

According to a second aspect of the present invention, there is provided a bearing having a lining thereon comprising a PTFE matrix and having incorporated therein 1 to 10 vol.% of graphite, 5 to 20 vol.% of a copper-based alloy and 4 to 25 vol.% of zinc.

In one embodiment of a bearing according to the present invention the bearing lining material may be provided as a layer on a strong metal backing such as steel or bronze, for example.

The metal backing may also be provided with a further adherent layer of a powdered material which acts as a mechanical bonding medium to retain the bearing lining material . The powdered material may comprise a layer of generally rounded bronze particles sintered to the metal backing surface, for example.

In order that the present invention may be more fully understood examples will now be described by way of illustration only with reference to the accompanying drawing (Figure 1) which is a histogram depicting the life in hours of a bearing having a lining of a bearing material according to the present invention compared with a known material tested under the same conditions.

Bearings having a lining of a bearing material according to the present invention were manufactured by the method set out in the following description.

An aqueous dispersion of PTFE, Fluon GSD5 ( a trade mark of the ICI company) is mixed with Foliac X6236 (trade

mark of the Rocol company) graphite, 11 wt% tin-bronze powder of -300 B.S mesh sieve size and zinc powder from the BDH company. After mixing the material is allowed to settle and excess water drained away followed by partial drying of the resulting paste. The paste is impregnated into a layer of bronze particles which are sintered onto a steel strip. Impregnation is effected by a spreading die which distributes the paste onto the strip surface, followed by impregnation by a roller into the surface to leave a surface layer of the lining material about 30 micrometres thick. The impregnated strip is heated to drive off residual moisture and to cure the PTFE matrix at about 380°C.

Bearings having a lining comprising 5 vol.% graphite, 13 vol.% bronze and 10 vol.% zinc in a PTFE matrix were manufactured from the strip produced as described above. Bearings were also produced from a known molybdenum disulphide filled PTFE material also having a steel backing.

The bearings produced were subjected to dry running endurance tests. Each bearing was cylindrical in form with an internal bore of 20 mm and an axial length of 15 mm. The bearings were subjected to a load of 53.4 kgs (specific load 1.78 N/mm ) and a shaft

rotational speed of 750 rev/min. which produced a PV (presure x velocity) factor of 1.4. The tests were terminated when a total wear of 0.25 mm had taken place in the lining.

The bearings and material according to the present invention were tested on two different testing machines under nominally the same test schedule; these are shown in the histogram as "LF4(a)" and "LF4 (b)". The known molybdenum disulphide filled material tested on the same test machines under the same schedules is designated as

"MoS ₂ (a)" and "MoS ₂ (b)".

It may be seen from the histogram that, although there is a scatter of results in the wear life, the performance of the material according to the present invention is significantly better than that of the known material.