GB1591804A

GB1591804A - Bearing surface

Info

Publication number: GB1591804A
Application number: GB706/77A
Authority: GB
Original assignee: BP PLC
Current assignee: BP PLC
Priority date: 1977-01-10
Filing date: 1977-01-10
Publication date: 1981-06-24
Also published as: NL7800247A; JPS5390548A; FR2376966A1; DE2800597A1

Description

(54) BEARING SURFACE (71) We, THE BRITISH PETROLEUM COMPANY LIMITED, of Britannic House, Moor Lane, London, EC2Y 9BU, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a bearing having at lest one bearing surface which has lubricating properties per se and which does not need lubricating with conventional lubricants such as lubricating oils or greases.

There are a number of circumstances in which it is impossible or undesirable to use conventional lubricants for lubricating bearing surfaces.

One example of a ncn-conventionally lubricated bearing is a gas bearing.

Gas bearings are bearings in which the moving surfaces are kept separated by gas contained in a narrow gap, the gap usually being of the order of 1-1000 microns.

Such bearings have several potential advantages over normal liquid or grease lubricated bearings, including reduced friction and freedom from vibration; they are useful, for example, in gyroscopes and other sensitive equipment. Although no conventional lubrication is required during normal running, the bearing surfaces need to have lubricating properties to reduce friction and prevent damage on start-up and shut-down. This reduced friction minimises power requirements at start-up and gives rapid attainment of operational speed -and separation of the surfaces. A coefficient of static friction of below 0.2 may be desirable. At the same time the bearing surface has to be resistant to wear and abrasion since any particles removed from the surface could easily clog the narrow gap between the surfaces.

Another situation where conventional lubrication of bearings is not possible when the bearings have to operate at below atmospheric pressure. Sometimes (e.g. in space vehicles, satellites, etc.) the bearings are required to operate in a vacuum as low as 10-8 torr. In these circumstances the bearing surface may be in continuous contact but the same requirements of a low coefficient of friction and resistance to wear and abrasion are required as for gas bearings.

According to the present invention a bearing, operated without a conventional lubricant, which bearing is used at below atmosphere pressure and/or in which the moving surfaces of the bearing are separated by gas during operation, characterised in that the bearing has at least one bearing surface comprising graphite having a ratio of basal nlane surface- area to edge surface area, as mesured by the ratio of adsorption of n-dotriacontane from n-heptane to the heat of adsorption of n-butanol from nheptane, of at least 5:1.

The ratio of basal plane surface area to edge surface area is preferably at least 10:1 and may be as high as 99:1. As indicated above, the measure of the basal plane surface area is obtained by measuring the heat of adsorption of n-dotriacontane from n-heptane and the measure of the edge surface area is obtained by measuring the heat of adsorption of n-butanol from n-heptane. A suitable apparatus for measuring these heats of adsorption is a Micro Flow-Calorimeter and the apparatus and technique for using it are described in "Chemistry and Industry", March 20, 1965, pages 482489.

Graphites having the above minimum ratios are referred to as oleophilic graphites and may he produced by grinding graphite in an organic liquid in the substantial absence of air. e.g. in a grinding mill completely filled with liquid. British Patent Snecification No. 116S785 describes and claims a method of preparing a dispersion of oleophilic graphite which comprises grinding a natural or synthetic graphite below the surface of an organic liquid which distils below 5000C till a surface area of 20 to 800 square metres/gram is obtained. The oleophilic graphite may be separated from the dispersion in a variety of ways.

British Patent Specification No. 132737/ describes and claims, inter alia, the production of oleophilic graphite in an agitator mill full of organic liquid, the disper sion of oleophilic graphite and organic liquid having, during grinding, a viscosity of from 85 to 1000 centipoises at 250C, measured at a shear rate of 511 S-l.

British Patent Specification No. 1247333 describes and claims solid shaped bodies consisting wholly or partly of an oleophilic graphite. The bodies may be formed by compaction of the graphite at, for example, a pressure of from 10 to 50 tons per square inch and the bodies may contain a reinforcing metal powder.

Finally, British Patent Specification No. 1292818 describes and claims mixtures of oleophilic graphite and a metal having a ratio of heat of adsorption of n-dotriacontane from n-heptane to heat of adsorption of n-butanol from n-heptane of at least 3.5:1 and a surface area of at least 5 square metres/gram. The mixtures may be produced by grinding the metal and a natural or synthetic graphite in the manner descriped above and the mixture may be compressed to form solid compacts also as described above.

Oleophilic graphites may have BET surface areas of from 20 to 800 m2/g, more usually 30 to 200 m2g. The BET surface area is determined according to the theory of Brunaur, Emmett and Teller [J. Am. Chem. Soc., 60, 309 (1938)].

By contrast, graphites ground in air normally have ratios of heats of adsorption, as defined above, of not more than 3:1, and conventional carbon blacks produced by partial combustion have ratios of heats of adsorption of not more than 1:1.

It is, of course, well known that graphite maye have lubricating properties per se, but, as indicated above, oleophilic graphite particles have particular physico-chemical qualities that distinguish them from conventional graphite particles and that make them particularly suitable for use as a bearing surface. These qualities include the ability to cohere to each other and to metal particles to give coherent solid bodies or thin films which have good resistance to abrasion and wear.

The bearing surface of a bearing according to the present invention may be the surface of a solid compact of oleophilic graphite, with or without a metal, or it may be a thin film of oleophilic graphite, with or without a metal, deposited on another surface e.g. a metal surface.

One or both of the surfaces of a bearing according to the present invention may be a surface which comprises oleophilic graphite.

Where a bearing according to the invention has a surface which is the surface of a solid compact of oleophilic graphite it is desirable that the other bearing surface should be another oleophilic graphite surface or a surface of comparable hardness.

This reflects the fact that oleophilic graphite compounds are not as hard as, for example, steel and may exhibit some wear if in contact with steel or hard metal surfaces. Wear may be reduced in such circumstances, however, by using a compact of oleophilic graphite particles and metal particles, as discussed in more detail hereafter.

Alternatively, in certain circumstances, wear may be acceptable provided there is a suitable thickness of the solid compact and the compact is held against the other surface by suitable loading, e.g. spring loading, or some mechanical equivalent thereof.

Where wear may occur and may be unacceptable, the surface may be a thin film of oleophilic graphite. The surface on which the thin film is deposited may be a metal, e.g. steel, and is preferably of comparable hardness to the other bearing surface.

The thin coating of graphite may be applied to the bearing surface by forming a solid compact of oleophilic graphite and rubbing the compact against the surface under load. The thickness of the film may be from 0.01 to 1000 microns. It has been found that thin coherent films of this order of thickness can be formed by simple rubbing contact at 0.1 to 1000 kg load for from 1 minute to 10 hours, preferably 0.25 to 10 kg load for from 15 minutes to 2 hours.

The oleophilic graphite bearing surface either as part of a solid compact or as a thin film on another surface may contain from 20 to 100% wt oleophilic graphtie and 80 to 0% weight of metal in the form fine particles. Powdered metal produced separately from the graphite and subsequently mixed with it may be used as described in British Patent Specification No. 1247333, but preferably mixtures prepared by grinding graphite and metal together are used as described in British Patent Specification No. 1292818. The metal may be a metal from groups 1B, 2B, 3, 4, 5 and 8 of the Periodic Table according to Mendeleef, particularly from group 4, and more particularly lead.

In a particularly preferred embodiment the bearing has at least one bearing surface which may be from 20 to 100% wt oleophilic graphite and from 80 to 0% wt lead, more particularly from 25 to 40% oleophilic graphite and 75 to 60% wt lead.

Experiments have shown that oleophilic graphite has good resistance to wear at atmospheric pressure or above but that this resistance decreases at sub-atmospheric pressure.

Lead shows the reverse trend, i.e. good wear resistance under high vacuum but relatively poor resistance at atmospheric pressure. The use of oleophilic graphite and lead in combination therefore allows an optimum formulation to be produced for any given pressure conditions and, in particular, it gives a specific formulation which has good durability at pressures ranging from atmospheric to as low as 0.01 torr.

In another embodiment of the invention, particularly suitable for bearings operating vacuum, oleophilic graphite particles, with or without metal particles, may be mixed into plastic compositions. The plastic composition may be of any known type but is preferably one having self-lubricating properties itself. eg. pvc, nylon, polyethylene. The plastic composition may be formed into a solid body having a bearing surface which will then have particles of oleophilic graphite and, if necessary, metal particles dispersed in it. The oleophilic graphite and optionally metal may be used in total or partial replacement of the filler normally used in plastic compositions and may be present in an amount of from 20 to 50% wt by weight of the compostiion.

It will be apparent that nothing turns on the geometrical configuration of the bearing and whether it contains balls, rollers, cones, etc. In the case of gas bearings they may be internally or externally pressurised by any suitable gas as outlined in, for example, the Design of Aerostatic Bearings by J. W. Powell (Machinery Publishing Co. Ltd., 1970).

The following comparative examples relate to oleophilic graphite surfaces which may be used in bearings of the present invention.

Example 1.

Preparation of an oleophilic graphite compact.

Nuclear grade synthetic graphite powder supplied by Graphite Products Ltd.

was ground for two hours with steel balls in n-heptane in a vibratory ball mill manu factured by Pilamec Ltd. driven by a 0.25 hp motor and operating at 2800 rev/min.

After separation of the graphite from the grinding liquid, the graphite was found to have the characteristics shown in Table 1 below.

TABLE 1

Heat of adsorption Heat of adsorption BET surface of n-dotriacontane of n-butanol from Ratio of area from n-heptane n-heptane heats of m2/g cal /g cal /g adsorption 51 0.55 0.05 Il The oleophilc graphite powder so produced was formed into a solid compact in a hydraulic press at a pressure of 60 tons/in2.

Example 2.

Coating of surface and comparative friction and wear tests.

Friction and wear tests were carried out on a wear machine of the type described in "Some surface properties of Gas Bearing Steels and their lubrication by Fatty Acids" by A. J. Groszek, C. W. Parkes and A. J. Patterson presented at the Sixth International Gas Bearing Symposium, University of Southampton, March 1974.

The test conditions used were: Load 0.75 Kg Sliding speed 400 cm/min Duration 1 hr (i.e. a total sliding distance of 24,000 cm) The following bearing surfaces were used, all in an atmosphere of air.

(A) Stellite 3 and FHM stainless steel with melissic acid lubricant applied to both bearing surfaces as described in the reference above.

["Stellite" is a Registered Trade Mark and FHM stainless steel is a Firth Vickers stainless steel supplied by Baigent and Bird Limited].

(B) The oleophilic graphite composite of Example 1 and FHM stainless steel.

(C) Stellite 3 and the FHM stainless steel used in (B) which had become coated with oleophilic graphite during the test.

The following measurements were made: Initial coefficient of friction.

Film durability. This was measured as the distance travelled before the lubricant film broke down (breakdown was determined by the point at which the friction trace rose sharply from a low value).

Final coefficient of friction.

Billet wear rate, averaged over the duration of the test.

The results are shown in Table 2 below TABLE 2

Stellite 3 alloy/ | Oleophilic graphite Stellite 3/FHM FHM stainless compact/FHM. (lubricant steel (lubricant (No additional oleophilic melissic acid) lubricant) graphite) Initial a 0.13 0.14 0.18 Film Durability 2000 cm above 24000 cm above 24000 cm Final a 1.22 1 0.19 0.15 Billet 8.9 x 10-14 188 x 10-14 0.7 x 10-'4 Wear Rate cm3 cm~l g-1 cm3 cm-' g-l cm3 ctn' g-1 In the table, column 1 shows that the melissic acid lubricant film had only limited durability and that the final coefficient of friction was high as a result. Wear did not occur until the film had broken down, but the average wear rate was considerable since the film broke down fairly quickly.

When one of the bearing surfaces was the graphite compact (column 2) the coefficient of friction remained low throughout the run. The film durability was also good since the oleophilic graphite bearing surface was the surface of a relatively thick solid compact. The billet wear rate was, however, relativelv high reflecting the different hardness of the FHM stainless steel and the oleophilic graphite.

This wear of the oleophilic graphite compact did, in fact, transfer some of the graphite to the other FHM stainless steel bearing surface as a thin film having a thickness of 1-2 microns. This thin film was found to be coherent and to be adhering strongly to the stainless steel and this oleophilic graphite coated stainless steel was used against Stellite 3 (column 3). Good results were obtained; the coefficient of friction remained low, the film lasted the full distance travelled and the wear rate was also low.

Example 3.

Preparatiu. of a series of oleophilic graphite-lead compacts.

A series of runs were made to produce ground mixtures of oleophilic graphite and lead in varying proportions. The starting graphite used was nuclear grade synthetic graphite powder supplied by Graphite Products Ltd. The starting lead used was lead powder supplied by British Drugs Houses T,td. 100% graphite, 100% lead and varying proportions of each were ground for 8 hours in a vibratory ball mill manufac mred by Pilamec Ltd. driven by a 0.25 ho motor and operating at 2800 rev/min.

The mill was filled with n-heptane and the initial temperature of operation was 250C.

After separation of the solids from the grinding liquid, the graphite, lead and mixtures were found to have the characteristics set out in Table 3 below.

TABLE 3

Composition of Heat of Heat of ground product adsorption of adsorption of n-dotriacontane n-butanol from Ratio of from n-heptane n-heptane heats of Lead Graphite cal/g cal/g adsorption 0 100 1.40 0.07 20 20 80 1.19 0.08 15 50 50 0.85 0.07 12 70 30 1 0.74 0.11 6.7 80 20 0.70 9.06 11.7 95 5 0.22 0.07 3.1 100 0 < 0.02 < 0.02 The 100% graphite, the 100% lead and the mixtures were formed into solid compacts in a hydraulic press at a pressure of 60 tons/in2.

Example 4.

Coating of surfaces and comparative friction and wear tests.

The solid compacts produced in Example 3 were used to form thin films on the surface of a FHM stainless steel cylinder. The films were formed by rubbing for one hour under a load of 0.75 kg and had a thickness of 1-2 microns.

The steel cylinders having the thin films on them were tested using the apparatus and test conditions of Example 2. The other bearing surface against which the thin films were tested was Stellite 3 alloy. The tests were carried out in a chamber which could be evacuated and each film was tested in air at atmospheric pressure and under a vacuum of 0.01 torr. The results are shown in Table 4 below.

TABLE 4

Film Composition Durability Wear % wt distance, cm cm9 g-l cm~l x 10.14 Air Air Lead Graphite atmospheric Vacuum atmospheric Vacuum pressure 0.01 torr pressure 0.01 torr 0 100 > 24,000 75 1.2 138 20 80 > 24,000 95 0.73 145 50 50 > 24,000 330 1.14 167 70 30 > 24,000 > 24,000 1.42 0.20 80 20 13,630 > 24,000 28 0.15 95 5 5,380 > 24,000 65 0.21 100 0 4,800 > 24,000 40 0.46 Table 4 shows that a film of 100% oleophilic graphite had good durability and low wear in air at atmospheric pressure, but poor durability and high wear under vacuum. The film with. 100% lead showed exactly the reverse characteristics. The mixtures showed intermediate characteristics3 the durability of air decreasing and the durability in vacuum increasing as the proportion of lead increased. The wear showed generally the reverse characteristics with increase in lead content. There was, however, a very marked change on going from a 50:50 to a 70:30 lead:oleophilic graphite mixture with the result that the 70:30 mixture showed good durability and good wear characteristics in both air and vacuum.

FHM stainless steel used without the fi!m of graphite or graphite/lead against Stellite in air had a durability distance of less than 400 cm and a wear of 150.

WHAT WE CLAIM IS: 1. A bearing, operated without a conventional lubricant, which bearing is used at below atmospheric pressure and/or in which the moving surfaces of the bearing are separated by gas during operation characterised in that the bearing has at least one bearing surface comprising graphite having a ratio of basal plane surface to edge surface area, as measured by the ratio of the heat of adsorption of n-dotriacontane from n-heptane to the heat of adsorption of n-butanol from n-heptane, of at least 5:1.

2. A bearing as claimed in claim 1 wherein the ratio of heats of adsorption is from 10:1 to 99:1.

3. A bearing as claimed in claim 1 or 2 wherein the bearing has at least one bearing surface which is a thin film comprising oleophilic graphite having a thickness of 0.01 to 1000 microns.

4. A bearing as claimed in claim 1 or 2 wherein the bearing has at least one bearing surface which is the surface of a solid compact comprising oleophilic graphite.

5. A bearing as claimed in any of claims 1 to 4 wherein the bearing surface contains from 20 to 100% wt of graphite and 80 to 0% wt of metal in the form of fine particles.

6. A bearing as claimed in claim 5 wherein the metal is a metal from Groups 1B, 2B, 3 4, 5 and 8 of the Periodic Table according to Mendel ef.

7. A bearing as claimed in claim 6 wherein the metal is lead.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

TABLE 4

Film Composition Durability Wear % wt distance, cm cm9 g-l cm~l x 10.14 Air Air Lead Graphite atmospheric Vacuum atmospheric Vacuum pressure 0.01 torr pressure 0.01 torr 0 100 > 24,000 75 1.2 138 20 80 > 24,000 95 0.73 145 50 50 > 24,000 330 1.14 167 70 30 > 24,000 > 24,000 1.42 0.20 80 20 13,630 > 24,000 28 0.15 95 5 5,380 > 24,000 65 0.21 100 0 4,800 > 24,000 40 0.46 Table 4 shows that a film of 100% oleophilic graphite had good durability and low wear in air at atmospheric pressure, but poor durability and high wear under vacuum. The film with. 100% lead showed exactly the reverse characteristics. The mixtures showed intermediate characteristics3 the durability of air decreasing and the durability in vacuum increasing as the proportion of lead increased. The wear showed generally the reverse characteristics with increase in lead content. There was, however, a very marked change on going from a 50:50 to a 70:30 lead:oleophilic graphite mixture with the result that the 70:30 mixture showed good durability and good wear characteristics in both air and vacuum.

FHM stainless steel used without the fi!m of graphite or graphite/lead against Stellite in air had a durability distance of less than 400 cm and a wear of 150.

WHAT WE CLAIM IS: 1. A bearing, operated without a conventional lubricant, which bearing is used at below atmospheric pressure and/or in which the moving surfaces of the bearing are separated by gas during operation characterised in that the bearing has at least one bearing surface comprising graphite having a ratio of basal plane surface to edge surface area, as measured by the ratio of the heat of adsorption of n-dotriacontane from n-heptane to the heat of adsorption of n-butanol from n-heptane, of at least 5:1.
2. A bearing as claimed in claim 1 wherein the ratio of heats of adsorption is from 10:1 to 99:1.
3. A bearing as claimed in claim 1 or 2 wherein the bearing has at least one bearing surface which is a thin film comprising oleophilic graphite having a thickness of 0.01 to 1000 microns.
4. A bearing as claimed in claim 1 or 2 wherein the bearing has at least one bearing surface which is the surface of a solid compact comprising oleophilic graphite.
5. A bearing as claimed in any of claims 1 to 4 wherein the bearing surface contains from 20 to 100% wt of graphite and 80 to 0% wt of metal in the form of fine particles.
6. A bearing as claimed in claim 5 wherein the metal is a metal from Groups 1B, 2B, 3 4, 5 and 8 of the Periodic Table according to Mendeléef.
7. A bearing as claimed in claim 6 wherein the metal is lead.
8. A bearing as claimed in claim 7 wherein the bearing surface contains from 25

to 40 /O wt of graphite and 75 to 60% wt of lead.
9. A bearing as claimed in claim 1 having at least one bearing surface substantially as described in the Examples.