GB2272029A - Bearings - Google Patents

Abstract

Sliding bearing materials comprise a backing metal having a porous metal layer formed on the surface thereof where a composition is impregnated and coated into pores and onto the surface. The composition contains from 5 to 30% by volume of lead having a relative surface area of 1,000 to 8,500 cmÇ/g, the remainder being PTFE. The composition can also contain from 0.5 to 30% by volume of at least one kind of solid lubricant such as a metal oxide, metal fluoride, graphite, a fibre material such as carbon fibre or glass fibre, and a ceramic material such as SiC. The total of the components, other than the PTFE can then total 5.5 to 50% by volume. The sliding material is made by impregnating the composition and baking in a non-oxidising (e.g. neutral or reducing) atmosphere.

Description

BEARINGS The present invention relates to a method of manufacturing a sliding material that may have excellent friction and wear characteristics. This application is divided from UK patent application no.

9104053.5 (GB-A-2241993).

Conventional sliding materials (and methods of manufacturing them) include a sliding material comprising a backing metal having a porous metal layer, in which pores are defined, formed on the surface thereof and the following compositions for impregnation coating as shown in, for example Japanese Patent Kokoku (Post Examination Publicaticns) No. 39-16950: (a) PTFE (polytetrafluoroethylene) and 20 Pb (metallic lead); or (b) PTFE and 20 PbO (lead oxide).

The Japanese Patent Kokoku publication above relates to coarse lead powder having a mesh of 300 B.S.S, as the type of metal lead used (see the example on lines 21 and 22 of the right hand column of page 1). The lead powder has a ratio of surface area to weight of 500 cm2/g. It was thus found that the lead powder, having a very small relative surface area, was used.

However, the sliding material and a method of manufacturing it according to the technology disclosed in the above prior art, in which the lead powder having a very small relative surface area as used, do not satisfy friction and wear characteristics when the sliding material is slid in a dry condition.

Thus one problem that is addressed, and intended to be solved by the present invention, is to.determine a size of metal lead so the above friction and wear characteristics may be improved.

According to a first aspect of the present invention there is provided a method of manufacturing a sliding material, the method comprising: (a) forming a porous metal layer on at least part of a surface of a backing metal; (b) impregnating the porous layer with a composition comprising from 5 to 30W by volume lead having a relative surface area of from 1,000 to 8,500 cra2/g, the balance being substantially PTFE; and (c) heating the backing material and the impregnating composition.

In (a) the porous metal layer may be formed by sintering. This may be done by employing metal or metal alloy in a particulate (such as a powder) form.

The porous metal layer is suitably bronze.

In (b) the impregnation process is suitably carried out by pressure. This may mean that the impregnating composition is forced into the pores. This may be achieved by passing the sliding material through rollers.

The heating stage in (c) (or baking stage) is suitably conducted in a non-oxidising atmosphere, so as to hinder or prevent oxidation of the lead to its oxide.

It is preferred that this is conducted in either a neutral atmosphere or in a reducing atmosphere. The process may additionally include: (d) which involves rolling the sliding material, such as using rollers, in order to obtain uniform thickness of the impregnated surface.

A particularly preferred method according to the present invention of manufacturing a sliding material comprises impregnating and covering, using the impregnating composition (such as that previously described) into the pores and onto the surface of a porous metal layer which has been formed on the surface of a backing metal (and usually also to the surface) and thereafter baking the backing metal with the composition for impregnation coating in a non-oxidising atmosphere such as a neutral atmosphere or a reducing atmosphere.

A second aspect of the present invention relates to a method of manufacturing a sliding material, the method comprising: (a) forming a porous metal layer on at least part of a surface of a backing metal; (b) impregnating the porous layer with a composition comprising lead (such as in particulate form), the balance being substantially PTFE; and (c) heating the backing metal and the impregnating composition in a non-oxidising atmosphere (such as a neutral or reducing atmosphere).

A second aspect of the present invention relates to a method of manufacturing a sliding material, the method comprising: (a) forming a porous metal layer on at least part of a surface of a backing metal; (b) impregnating the porous layer with a composition comprising lead having a relative surface area of from 1,000 to 8,500 cm2/q, the balance being substantially being PTFE; and (c) heating the backing metal and the Impregnating composition.

It will of course be appreciated that the invention also extends to sliding materials made by any of the methods of the present invention.

Preferred features and characteristics of one aspect of the present invention are as for another aspect mutatis mutandis.

The composition may contain only PTFE and lead (although this may include any inevitable impurities).

The composition preferably contains one or more solid lubricants. The lubricants are suitably provided at from 0.5 to 30% by volume. Thus, it is preferred that the total of the all components in the composition, other than the PTFE, are from 5.5 to 50% by volume.

Suitably the lead is in a particulate form, such as in the form of a powder.

The term "sliding material" used in the specification is to be understood as encompassing constructions that include bearings such as a plain bearing or bearing material or self-lubricating material that is suitable as anti-friction material for producing sliding members. These include bearing bushes, shoes, slide plates and sliding members suitable for use in clutches, brakes and collectors.

The terms "plain bearing", "bearing material" or "self-lubricating material" are to be understood as including a metal (such as steel) backing to which is bonded a layer of porous metal (e.g. bronze) which constitutes a matrix of porous metal which has had at and/or adjacent to its exposed surface impregnated with a mixture of PTFE and lead and any suitable solid lubricants.

The inventors have thus found that the problems mentioned earlier may be solved by using fine lead powder in place of conventional coarse lead powder, for example lead powder having a (large) relative surface area of from 1,000 to 8,500 cm2/g. This will be in place of conventional lead powder having a relative surface area of 500 cm2/g to try to solve the problems mentioned above.

A particularly preferred sliding materials made by the present invention thus comprises a backing metal having a porous metal layer, in which pores are defined, formed on the surface thereof, . the following composition being used for impregnation coating to impregnate and coat into the pores and onto the surface. The composition for impregnation coating thus comprises from 5 to 30% by volume of lead having a relative surface area of from 1,000 to 8,500 cm2/g, in an average particle size, the remainder of the composition being substantially composed of PTFE.

In a preferred embodiment the invention encompasses a method of manufacturing a sliding material comprising a backing metal having a porous metal layer, in which pores are defined, formed on the surface thereof, the following composition being used for impregnation coating to impregnate and coat into the pores and onto the surface thereof. The composition for impregnation coating comprises: (a) from 5 to 30% by volume of lead having a relative surface area of from 1,000 to 8,500 cm2/g (in an average particle size); and (b) from 0.5 to 30% by volume of a filler comprising a solid lubricant such as a metal oxide, a metal fluoride, a metal sulphide, silicon oxide, graphite, a fibre material such as carbon fibre and/or glass fibre or a ceramic material such as SiC, the remaining portion substantially composed of PTFE; and (c) the total of the respective components other than the PTFE being from 5.5 to 50% by volume.

When dispersed in PTFE, the lead (e.g. in the form of a powder) may have meritorious effect, causing the PTFE to be transferred to, and deposited on, a mating member (in general, steel or stainless steel) of the sliding material, which may provide low friction and wear characteristics. This is discussed in the above Japanese Patent Kokoku Publication No. 39-16950.

Nevertheless, commercially available lead powders have been classified (to respective similar sizes) and these powders of different sizes have been combined in various ways to obtain a ratio of surface area to weight of 5,000 cm2/g (in an average particle size) which is ten times that in the prior art of 500 cm2/g.

It has been found that such powder can greatly reduce the friction and is worn less in use. This may be due to the meritorious effects achieved by the transfer and deposition of PTFE being carried out more finely, more often and thus more effectively.

The reason why the relative surface area is preferred to be between 1,000 to 8,500 cm2/g is that when it is less than 1,000 cm2/g, the friction wear characteristics can be similar to those in the prior art, and metal (powder) having a relative surface area exceeding 8,500 cm2/g can be difficult to produce industrially.

If an additive amount exceeds 30% by volume it may be difficult to keep desirable cavitation-proofing properties even when a meltable fluorocarbon resin is added. Furthermore, an additive amount of less than 5% by volume may not provide the necessary friction and wear characteristics. Therefore the preferred additive amount ranges from 5 to 30% by volume. The metal lead (powder) is suitably made by gas atomisation, water atomisation and/or a stamp milling method.

It is preferred that the (impregnating) composition is substantially free of (and thus suitably does not contain any) PFA, EPE and/or FEP.

The impregnating composition preferably contains at least one lubricant, such as a solid lubricant. The addition of at least one solid lubricant, such as a metal oxide, metal fluoride, graphite or a metal sulphide (e.g. MoS2, WS2 or PbS) a fibre material such as carbon fibre, silicon oxide or a ceramic material, such as SiC, can improve the friction and wear characteristics. When the amount of the solid lubricant is less than 0.5% by volume, then the friction and wear characteristics may not be significantly effected, and the amount of solid lubricant may be insufficient to provide the desired characteristics. Furthermore, if the addition of the solid lubricant exceeds 30% by volume then this may make the sliding material brittle and impractical.

Thus, the solid lubricant is suitably present in the range of from 0.5 to 30% by volume.

When it is stated that metal oxides can be used as the solid lubricant then this includes both single compound oxides, such as oxides of Zn, Al, Sb, Y, In, Zr, Mo, Cd, Ca, Ag, Cr, Co, Ti, Mn, Sn, Ce, W, Bi, Ta, Fe, Cu, Pb, Ni, Te, Nb, Pt, V, Pd, Mg, Li, as well as mixtures thereof and composite metal oxides, such as the combinations of: CoO and A1203; Til2 and ZnO; PbO and TiC2; CoO and SnO2; MgO and Al2O3; ZrO2 and SiO2; CoO, Al203 and MgO; CoO, Al2O3 and Cr2O3; CoO, ZnO and MgO; Pb304, Sb2O3 and TiC2; Cr2O#, Sb203 and TiO2; Cr203, CuO and MnO2; CoO, Cr203 and Fe203; CoO, ZnC, NiO2 and TiC2; CoO, Cr2O3, MnO2 and Fe203.

Preferred lubricants include metal oxides, copper sulphides and metal fluorides, in particular when the metal is lead. However, particularly preferred lubricants include TiC21 MoS2 and PbF2.

Suitable metal fluorides include PbF2, AlF3, CdF2 and BaF2 (or a mixture thereof). The fibre material includes both natural fibres and artificial fibres such as carbon fibre, glass fibre, cotton (cellulose), asbestos, rockwool, potassium titanate fibre and aromatic polyamide fibre. Carbon fibre is preferred.

Preferred ceramic materials include carbides and nitrides such as SiC, TiC, TiN, B4C, BN, Si3N4, AlN, HfN, TaN, WC, TaC, VC and/or ZrC. Other compounds which are effectively equivalent in properties and/or composition to the above lubricants may be employed instead or in addition.

It should be appreciated that more than one solid lubricant may be present in the . (impregnating) composition. In such a case, it is often preferred that each individual solid lubricant is present at from 0.5 to 30% by volume.

The lead (e.g.- in the form of a powder) used in the present invention is preferably pure Pb (powder) which can be produced metallurgically. The lead suitably has an oxidised surface (as will often be the case with ordinary metals) and so is usually grey or black when observed from the outside and has a melting point of 3270C.

Although the backing metal is preferably steel (such as stainless steel) it should be realised that other metals (or metal alloys) can be used. Copper alloys, such as bronze, are also preferred. If bronze is employed, there may be no plating layer applied between the backing metal and the porous layer. However, this does not exclude the case when metal (or metal alloy) is placed between the backing metal and the porous layer (often known as a plating layer). If such a layer is provided, it is suitably a metal (e.g. Cu) or metal alloy.

The porous layer itself is metallic and may be made of either a single metal or a metal alloy, for example, bronze or lead bronze.

The lead present in the composition (and therefore on the surface of the backing metal) will thus generally be contained in a composition which is mainly composed of PTFE, but since the PTFE usually comprises small particles (such as in a baking step) , the composition will usually have permeability and thus the lead can become susceptible to oxidation. When friction and wear tests were carried out using a sliding member employing completely oxidised lead, it had performance and cavitation-proofing properties inferior to those sliding members according to the present invention. It is suggested that this may be due to an inferior wetting property between the PTFE and lead oxide.

Thus, in connection to methods of manufacturing sliding materials which will be described later, any heating or baking is preferably conducted in a non-oxidising atmosphere, to prevent the sliding member from being partially oxidised.

The present invention will now be described by way of example with reference to the following Examples, which are provided by way of illustration and are not to be construed as being limiting.

COMPARATIVE EXAMPLES 1 and 2 and EXAMPLES 3 TO 12 Bronze powder was porously sintered on a copper plated steel backing metal and thereafter the compositions for impregnation coating shown in Table 1 were impregnated and coated into the pores and on to the surface of the above porous bronze layer by passing the backing metal (sintered with bronze powder) through between rollers.

Next, the steel backing metal was baked in a Table 1

Example Composition for Relative Surface Distinction ; No. Impregnation Area of Metal Lead @ Coating (vol%) or Lead Oxide (cm2/g) Conventional PTFE-20Pb 500 Sliding Mate rial , 2 @ PTFE-20PbO 1,500 3 PTFE-5Pb 5,000 4 PTFE-1OPb @ 5,000 5 PTFE-20Pb 5,000 6 PTFE-30Pb 5,000 7 7 @ PTFo-20Pb 1,000 Sliding Mate rial of Pre- 8 PTFE-20Pb 3,000 sent Invention 9 PTFE-20Pb 7,000 10 PTFE-5Pb- 5,000 2PbF2 11 PTFE-10Pb- 5,000 10 Carbon Fib@e 12 PTFE-20Pb- 5,000 25PbO TiO2 3MoS2 Table 2

Example Composition for Result of Friction Distinction No. Impregnation and Wear Tests Coating (vol%) Wear Amount Friction ( m) Coefficient Conventional 1 PTFE-20Pb 50 0.143 Sliding Mate rial 2 PTFE-10PbO 45 0.195 3 PTFE-5Pb 33 0.120 4 PTFE-10Pb 30 0.127 5 PTFE-20Pb 27 0.130 6 PTFE-30Pb 25 0.135 7 PTFE-20Pb 30 0.125 Sliding Mate tial of Pre- 8 PTFE-20Pb 28 0.127 sent Invention 9 9 PTFE-20Pb 24 0.133 10 PTFE-5Pb- 29 0.125 2PbF2 11 PTFE-10Pb- 26 0.127 10 10 Carbon Fibre 12 PTFE-20Pb- 20 0.130 25PbO TiO2 3MoS2 Table 3

Test Conditions of Cavitation-Proofing Test Tester Used Dedicated Tester for Cavitation Specimen Size Longitudinal Length 40 x Lat@ral Length 40 x Thickness 1.5 (nun) Resonance 19 KHz IFrequency Output Power @ 600 W Liquid Used for Water Test Temperature of @ Room Temperature Test Liquid Gap between a 1 millimetre Hone and Specimen Hone Diameter 35 millimetres Testing Time 3 minutes conventional atmosphere or in a non-oxidising atmosphere at a temperature of 327 to .4000C and passed through rollers to form a uniform thickness. Thus, conventional prior art sliding materials (Example Nos.

1 and 2) and sliding materials of the present invention (Examples No. 3 to 12) were obtained.

Tables 1 and 2 show the compositions made and the results of various tests of Examples 1 to 12, while Table 3 shows the test conditions employed.

The baking was carried out in a neutral atmosphere or in a reducing atmosphere to prevent the lead powder in the composition (for impregnation coating) from being oxidised during baking.

As will be apparent from Tables 1 and 2, when the conventional sliding materials (Example Nos. 1 and 2) are compared with the sliding materials of the present invention (Examples Nos. 3 to 12), the friction and wear characteristics of the latter were greatly improved.

Thus, the sliding materials of the present invention may achieve improvement in the friction and wear characteristics, which can be very important as far as sliding characteristics are concerned.

Claims (15)

1. A method of manufacturing a sliding material, the method comprising: (a) forming a porous metal layer on at least part of a surface of a backing metal; (b) impregnating the porous layer with a composition comprising lead, the balance being substantially PTFE; and (c) heating the backing metal and the impregnating composition in a non-oxidising atmosphere.

2. A method of manufacturing a sliding material, the method comprising: (a) forming a porous metal layer on at least part of a surface of a backing metal; (b) impregnating the porous layer with a composition comprising lead having a relative surface area of from 1,000 to 8,500 cm2/g, the balance being substantially PTFE; and (c) heating the backing metal and the impregnating composition.

3. A method as claimed in claim 1 or 2 wherein the composition is provided with a solid lubricant.

4. A method as claimed in claim 3 wherein the solid 4. A method as claimed in claim 3 wherein the solid lubricant is a metal oxide, metal fluoride, graphite, metal sulphide, a fibre material, silicon oxide or a ceramic material.

5. A method as claimed in claims 3 or 4 wherein the solid lubricant is PbO2, TiC2, MoS2, PbF2 or carbon fibre.

6. A method as claimed in any of claims 2 to 5 wherein the solid lubricant is provided at from 2 to 4% by volume.

7. A method as claimed in any preceding claim wherein all the components in the compositicn, other than the PTFE, total from 5.5 to 50% by volume.

8. A method as claimed in any preceding claim wherein the lead is in particulate form.

9. A method as claimed in any preceding claim wherein the lead is in powder form.

10. A method as claimed in any preceding claim wherein the porous metal layer is made of bronze.

11. A method as claimed in any preceding claim wherein the porous metal layer is formed by sintering.

12. A method as claimed in any preceding claim wherein the porous metal layer is formed by sintering a metal or metal alloy in particulate form.

13. A method as claimed in any of claims 2 to 12 wherein the heating in (c) is conducted in a non-oxidising atmosphere.

14. A method as claimed in any of claims 2 to 18 wherein heating in (c) is conducted in a neutral or reducing atmosphere.

15. A sliding material made by a method of manufacture as claimed in any preceding claim.

15. A method of manufacturing a substantially as herein described with reference to Examples 3 to 12.