GB2319741A

GB2319741A - Method and apparatus for forming recesses in a bearing surface

Info

Publication number: GB2319741A
Application number: GB9724881A
Authority: GB
Inventors: Frank Peter Wardle
Original assignee: Loadpoint Ltd
Current assignee: Loadpoint Ltd
Priority date: 1996-11-28
Filing date: 1997-11-26
Publication date: 1998-06-03
Also published as: AU5126898A; GB9624728D0; WO1998023405A1; GB9724881D0

Abstract

A method of forming recesses 12 in an electrically conductive bearing surface, such as a surface of an air or fluid film bearing, comprises locating in spaced proximity to the bearing surface 19, 20 the surface of an electrode 15, 16 having a number of exposed conductors 30 in a configuration corresponding to the required configuration the recesses, filling the space 23 between the surfaces with an electrolyte, and passing an electric current between the bearing surface and the electrode for a sufficient period to form recesses of the required depth in the bearing surface.

Description

"Method and apparatus for forming recesses in a bearing surface" The invention relates to methods and apparatus for forming recesses in a bearing surface, and particularly in the bearing surfaces of air and other fluid film bearings.

As is well known, the performance of many types of air and other fluid film bearings can be enhanced by the formation of suitably shaped and orientated grooves or other recesses in the bearing surfaces. In general these recesses are designed to pump the air or other lubricating fluid so as to improve its distribution and/or increase its pressure. Such performance benefits may result in increased load carrying capacity, increased stiffness or improved stability depending upon the bearing type and requirements. Improvements are particularly beneficial on "self acting" bearings where the pressure in the lubricating fluid is generated by rotation of the bearing itself, but can also be significant on bearings where the lubricating fluid is externally pressurised.

The recesses may be of various kinds, but "herringbone" grooves on the journal bearings and spiral grooves on thrust bearings are widely used and are typical of the type of recesses to which the present invention is directed.

To be effective, the above-mentioned herringbone and spiral grooves must be shallow in depth, narrow in width and closely spaced. Typically depths are similar to the bearing clearance, i.e. 5-50pm and there may be 12-64 grooves around the circumference depending upon the bearing diameter. The grooves can be formed on either bearing surface, i.e. in the case of a journal bearing the grooves may be formed in the bore of the housing or on the outside diameter of the shaft depending upon the application.

Many methods of forming such grooves have been tried but none has been entirely satisfactory. Chemical etching and ion beam etching are slow and expensive; milling is also slow and requires subsequent deburring; laser machining is expensive; coining/rolling is relatively quick, but it plastically deforms the surface and so requires a subsequent finish machining operation. Also, some or all of the above processes may be unsuitable for use with some materials, such as certain metal alloys, and may also be impractical with very small bearings.

In order that the forming process should be economic and suitable for mass production there are a number of basic requirements, as follows: (a) The process should be capable of forming a wide range of patterns on any size or shape of bearing.

(b) The process should be capable of meeting a tolerance of +/- 15% on recess depth.

(c) The time taken by the forming process should be short, typically less than 3 seconds, to enable it to fit into a flowline manufacturing situation and keep pace with other requirements of manufacturing cycle times.

(d) The process should be a final forming operation and not require subsequent machining or deburring.

(e) The process should be stable over long periods of operation and should be tolerant of parameter variations.

(f) Capital and maintenance costs of the equipment should be reasonable when considered as part of a mass production facility.

The methods and apparatus according to the present invention set out to overcome the limitations of the above-mentioned prior art processes while at the same time meeting the above requirements.

According to the invention there is provided a method of forming one or more recesses in an electrically conductive bearing surface, the method comprising locating in spaced proximity to said bearing surface the surface of an electrode having one or more exposed conductors in a configuration corresponding to the required configuration of said recess or recesses, filling the space between said surfaces with an electrolyte, and passing an electric current between the bearing surface and the electrode for a sufficient period to form a recess or recesses of the required depth in the bearing surface.

The bearing surface may comprise a bearing surface for use in an air or other fluid film rotating bearing.

Although the invention is particularly applicable to the bearing surfaces of air and other fluid film rotating bearings, the term "bearing surface" is intended to include any surface which, in use, moves in close proximity to another surface with a thin film of air or other fluid between them. For example, the surface of a piston or a cam may be regarded as a bearing surface within the context of the present invention.

Preferably the electrolyte is caused to flow through the space between the surfaces.

In the case where the bearing surface is substantially cylindrical, the electrode may comprise an annular body encircling an axial portion of the bearing surface.

Alternatively, the bearing surface may extend transversely to the axis of rotation thereof, for example it may be in the form of a disc.

The conductors may be arranged in a configuration corresponding to the required overall configuration of recesses on the bearing surface, all required recesses then being formed simultaneously in a single operation.

Alternatively, the conductors may be arranged in a configuration corresponding to only a part of the required overall configuration of recesses on the bearing surface, the bearing surface being subjected to a number of successive forming operations, each of which forms a different part of the required overall configuration of recesses. For example, each operation may form only a single recess of a required plurality of recesses.

Each of the successive forming operations may be carried out using the same electrode, the relative positions of the bearing surface and electrode being varied for each operation. Alternatively, each ofthe forming operations may be carried out using a different electrode.

In any of the above methods, the distance of the surface of an electrode from the bearing surface may be substantially constant, whereby the corresponding recess formed in the bearing surface is of substantially uniform depth. Alternatively, the distance of the surface of an electrode from the bearing surface may be different in different regions of the electrode, whereby the corresponding recess formed in the bearing surface varies in depth.

The invention includes within its scope apparatus for carrying out any of the methods referred to above and comprising an electrode having one or more exposed conductors arranged in a configuration corresponding to a required configuration of one or more recesses on a bearing surface, means for locating the bearing surface in spaced proximity to the exposed conductors on the electrode, means for filling the space between said surfaces with an electrolyte, and means for passing an electric current between the bearing surface and the electrode.

Preferably means are provided to cause the electrolyte to flow through the space between the surfaces.

The areas of the electrode between the exposed conductors may be substantially filled with an electrically insulating material. The exposed surfaces of the conductors are preferably substantially flush with the surface of the surrounding electrically insulating material, so that the surface of the electrode is substantially smooth.

The conductors may be formed to project from the surface of a body of electrically conductive material, the electrically insulating material then being a settable flowable material which is applied to said surface around the conductors and then solidified. For example, substantially the whole of said body may be encapsulated in the settable electrically insulating material. The surface of the conductors and insulating material may be machined smooth after the material has solidified.

The electrode may comprise an annular body which, in use, encircles an axial portion of the bearing surface. There may be provided two axially spaced electrodes which, in use, encircle two different portions of the bearing surface. In this case, means may be provided for delivering the electrolyte into a region between the two electrodes so as to flow in opposite axial directions past both electrodes. Preferably, the flow of electrolyte is generally helical.

The conductors on the electrodes may be in the form of a circumferential array of generally parallel elongate strips inclined at an angle to the longitudinal axis of the electrodes.

In an alternative embodiment, for use where the bearing surface extends transversely to the axis of rotation thereof, the electrode may be in the form of a surface transverse to the direction of the axis of the bearing surface and located in the apparatus.

The conductors on the electrode may be in the form of an array of curved elongate strips arranged around the electrode and extending inwardly from the outer periphery thereof and towards the centre of the electrode.

In any of the above arrangements the conductors may be arranged in a configuration corresponding to only a part of the required overall configuration of recesses on a bearing surface, means being provided to locate the bearing surface in a plurality of alternative positions with respect to the electrode.

The following is a more detailed description of embodiments of the invention, by way of example, reference being made to the accompanying drawings in which: Figure 1 is a diagrammatic view of a journal for use in an air bearing, Figure 2 is a diagrammatic representation of apparatus in accordance with the present invention for forming the grooves in the journal of Figure 1, Figure 3 is a diagrammatic section through one ofthe electrodes of the apparatus of Figure 2, and Figure 4 is a diagrammatic representation of an air bearing surface for use as a thrust bearing.

Figure 1 shows a typical design of journal 10 for use in an air bearing supporting a shaft 11. The journal 10 is, in use, located within a surrounding bore having an internal bearing surface which is closely spaced from the outer surface of the journal 10.

Typically the bearing clearance might be 5-50,um. The journal is supported within the surrounding bearing surface by a film of air or other lubricating fluid disposed within the clearance between the journal and surrounding bearing surface. In the present example the rotation ofthejournal 10 within the bearing itself serves to pump the lubricating fluid in the gap and for this purpose thejournal 10 is formed at each end thereof with an array of shallow grooves 12.

The grooves are in the form of parallel elongate strips inclined at an angle to the longitudinal axis of the journal and extending from one end of the journal a short distance towards the opposite end. The grooves are inclined rearwardly with respect to the direction of rotation of the journal in use, as indicated by the arrow 13. This is commonly referred to as a "herringbone" arrangement. In some cases the inclined grooves may be of sufficient length to meet in the middle of the journal, thus appearing as a series of chevrons.

The joumal in which the grooves 12 are formed may be of any diameter for example journal for a computer disc drive may have a diameter of only about 3mm, whereas journal for a car engine bearing might have a diameter of up to four inches.

Figure 2 shows diagrammatically apparatus for forming the grooves in the journal 10 by a process in accordance with the invention.

Referring to Figure 2, the apparatus comprises a generally cylindrical housing 14 in which are mounted similar upper and lower annular electrodes 15, 16 separated by an annular spacer 17.

A shaft 18 formed with two spaced bearing journals 19, 20 is supported vertically in annular discs 21, 22 mounted at the upper and lower ends of the housing 14, the shaft being lowered into the housing from the top. The vertical spacing between the electrodes 15, 16 is such that they are aligned axially with the journals 19, 20. The discs 21, 22 also locate the shaft radially so as to provide a uniform small annular gap 23 between the outer surface of each journal and the surrounding inner surface of the associated electrode. This gap is typically 50-30011m.

The annular spacer 17 is formed with a peripheral channel 24 to which an electrolyte, such as a neutral salt solution, is delivered under pressure. The electrolyte flows around the channel 24 and into the interior of the spacer 17 through circumferentially spaced ports 24a. The ports 24a are inclined generally tangentially to the spacer 17, so that the electrolyte flows helically upwardly and downwardly through the annular gaps 23 between the journals and the electrodes and out through an upper drainage port 25 and a lower drainage port 26. The electrolyte flow and distribution should be smooth and uniform.

The shaft 18, which must be formed from an electrically conductive material, is connected as indicated at 27 to the positive pole of an electrical supply whereas the electrodes 15, 16 are connected, as indicated at 28, to the negative pole.

The construction of each of the electrodes 15, 16 is shown in greater detail in Figure 3.

Each electrode comprises an annular main body 29 formed from electrically conducting metal and formed on its inner peripheral surface with projecting conductor strips 30 in a configuration matching the required herringbone pattern of grooves to be formed on the journal.

The metal body 29 is encapsulated with thermo-setting electrically insulating material 31 the thickness of which at the inner periphery is such that it is flush with the conductors 30 to provide a smooth surface to the inner bore of the electrode. The provision of this smooth surface enhances the smooth flow of the electrolyte over the surface of the electrode.

The metal core 29 is moulded into the thermo-setting insulating material 31 so that the two become mechanically interlocked. Initially the conductors 30 may be oversize and project further into the central bore of the core than is required, so that the encapsulated core may be subsequently machined to provide a smooth continuous intemal bore surface having an exact predetermined diameter and with the surfaces of the conductors 30 precisely flush with the surface of the insulating material.

In operation an electric current, which may be DC or unipolar pulses, is passed between the journals 19, 20 and their surrounding electrodes 15, 16, while electrolyte is pumped through the gaps between the joumals and electrodes. During this operation there are no moving parts except for the flow of electrolyte.

Current passing between each electrode and the shaft has the action of removing material from the journals directly opposite the electrode conductors, thereby forming the required groove, on the journals. The process is easily controlled, with the groove depth being directly proportional to the product of current and time. Typically current density may be between 0.25 and 4 amps/mm2 and it then takes only a few seconds to form the complete set of grooves on each journal. As a guide to machining speed, the current density of 1 amp/nun2 removes material at approximately 2511m/s.

The housing arrangement is such that the shaft 18 may be introduced vertically between the electrodes 15, 16 from the upper end of the housing. The arrangement is therefore particularly suitable for production line operation where a succession of shafts are entered and withdrawn from the apparatus to have the grooves formed.

A hot water rinse is necessary to remove electrolyte from the workpiece surface after removal from the apparatus.

The provision of a smooth and flush internal surface on the electrodes avoids generation of unnecessary disturbances in the electrolyte flow which can result in uneven forming of the grooves. The clearly defined edges between the conductors 30 and the insulation 31 helps minimise stray electric current and hence gives a more accurate groove form on the journals and provides for better process control. The mechanically interlocking insulation and conductors makes for a durable electrode construction. It should be noted that conventional insulating coatings often do not maintain good adhesion in the presence of electro chemical interactions.

Even though the described electrode design maximises precision, there is still likely to be a difference between the shapes of the conductors and the shapes of the resulting grooves in the journals due to current diverging away from the conductor edges. Generally this leaves a groove slightly larger in size than the conductor. This effect may be compensated for by making the conductors slightly smaller in size than the required grooves.

The described electrode construction is by way of example only, and other forms of construction may be used to produce grooves when the required level of precision is low or only a small number of components is to be formed. For example, electrodes without insulation can be used, the conductors then protruding from the surface of the bore in the electrode.

In the arrangement described, the conductors are arranged in a configuration corresponding to the required overall configuration of recesses on the bearing surface, so that all the recesses are formed simultaneously in a single operation. However, the conductors might also be arranged in a configuration which corresponds to only a part of the required overall configuration of recesses on the bearing surface. For example, the electrode may have only a single conductor which puts a single groove in the bearing surface in each operation of the apparatus. In this case, the arrangement is such that the bearing surface may be located in a series of alternative positions with respect to the electrode, successive forming operations being carried out in the successive positions of the bearing surface. The grooves in the bearing surface may thus be formed one at a time, or a few at a time, depending on the number of conductors on the electrode.

Although such arrangement is preferred, it is also possible to have a series of electrodes each of which is arranged to form a single groove, or a number of grooves, in the bearing surface, at different locations, so that the overall groove pattern is built up in the successive operations.

The above description applies specifically to the grooving of bearing journals on a shaft, but it will be appreciated that the invention is also applicable to a full range of bearing geometry, including thrust bearings in the form of annular discs as shown in Figure 4. In this case the spiral grooves 32 extending inwardly from the outer periphery of the disc 33 are formed by corresponding annular electrodes having an annular surface with appropriately located conductors which are located a short distance from the surface of the disc. In another arrangement the grooves may be in the form of a series of asymmetrical chevrons extending over substantially the total area of the disc. The invention may also be applied to the forming of grooves or other recesses on tapered bearings, spherical bearings and linear bearings.

A disc thrust bearing 34 is provided on the shaft 18 in the arrangement of Figure 2. The spiral grooves on the disc 34 may be formed simultaneously with the grooves 12 on the journals 19 and 20, by providing suitably arranged conductors on the upper surface of the electrode 15. Alternatively, the grooves in the disc 34 may be formed using another electrode in a separate operation.

In any ofthe above arrangements the depths ofthe grooves in the bearing surface may be varied by arranging for the distance of the surface of an electrode from the bearing surface to differ in different regions of the electrode Where the gap between the electrode and the bearing surface is narrow, the material removal rate will be high and conversely where the gap is large, material removal rate will be low. The variation in the width of the gap may be effected by varying the extent to which the conductors protrude from the electrode. Given a stationary electrode and a stationary bearing surface, it is possible to calculate the shape of the electrode profile which is necessary to generate the required profile in a given time. In this case the profile on the electrode will not in general be the same as that required on the bearing surface.

Although the process has been described as forming recesses in just one of the opposed surfaces of the bearing, recesses can be formed in either or both of the opposing surfaces. For example, the surface of the bore in which the journals 19, 20 are to be housed may also be formed with grooves, instead of or in addition to the grooves on the bearing journals.

By suitable provision and location of electrodes, any number of bearings may be formed at the same time. For example, a shaft may require two journal bearings to support radial loads and two thrust bearings to provide axial location. In this case the housing may be constructed to house four electrodes so that all four bearings may be formed at once.

Since the tool for forming the grooves is an electrode which does not wear it will give repeatable groove forms over long periods of operation. Electrodes will generally only deteriorate through handling damage or incorrect use of the process. Also, there are no cutting forces to distort the workpiece surface, so that the grooving is a final machining operation, and no subsequent finishing operations are required. The process naturally leaves small corner radii so there is no need for deburring after grooving.

Because the electrode lasts so long, it is cost effective to machine complex or very fine groove forms on the electrode. The electrode can be machined by a wider variety of processes than is practical for mass produced components.

Since the electrolyte is a neutral salt solution there are no harmful chemical reactions. Also, the process operates at low voltages, typically 20 volts. The process is therefore basically safe to operators and to the environment.

Claims

1. A method of forming one or more recesses in an electrically conductive bearing surface, the method comprising locating in spaced proximity to said bearing surface the surface of an electrode having one or more exposed conductors in a configuration corresponding to the required configuration of said recess or recesses, filling the space between said surfaces with an electrolyte, and passing an electric current between the bearing surface and the electrode for a sufficient period to form a recess or recesses of the required depth in the bearing surface.

2. A method according to Claim 1, wherein the bearing surface comprises a bearing surface for use in an air or fluid film rotating bearing.

3. A method according to Claim 1 or Claim 2, wherein the electrolyte is caused to flow through the space between the surfaces.

4. A method according to any of the preceding claims, wherein the bearing surface is substantially cylindrical, and the electrode comprises an annular body encircling an axial portion of the bearing surface.

5. A method according to any of Claims 1 to 3, wherein the bearing surface extend transversely to an axis of rotation thereof.

6. A method according to any of the preceding claims, wherein the conductors are arranged in a configuration corresponding to the required overall configuration of recesses on the bearing surface, all required recesses being formed simultaneously in a single operation.

7. A method according to any of Claims 1 to 5, wherein the conductors are arranged in a configuration corresponding to only a part of the required overall configuration of recesses on the bearing surface, the bearing surface being subjected to a number of successive forming operations, each of which forms a different part of the required overall configuration of recesses.

8. A method according to Claim 7, wherein each operation forms only a single recess of a required plurality of recesses.

9. A method according to Claim 7 or Claim 8, wherein each of the successive forming operations is carried out using the same electrode, the relative positions of the bearing surface and electrode being varied for each operation.

10. A method according to Claim 7 or Claim 8, wherein each of the forming operations is carried out using a different electrode.

11. A method according to any ofthe preceding claims, wherein the distance ofthe surface of an electrode from the bearing surface is substantially constant, whereby the corresponding recess formed in the bearing surface is of substantially uniform depth.

12. A method according to any of Claims 1 to 10, wherein the distance of the surface of an electrode from the bearing surface is different in different regions of the electrode, whereby the corresponding recess formed in the bearing surface varies in depth.

13. Apparatus for forming one or more recesses in an electrically conductive bearing surface comprising an electrode having one or more exposed conductors arranged in a configuration corresponding to a required configuration of one or more recesses on the bearing surface, means for locating the bearing surface in spaced proximity to the exposed conductors on the electrode, means for filling the space between said surfaces with an electrolyte, and means for passing an electric current between the bearing surface and the electrode.

14. Apparatus according to Claim 13, wherein means are provided to cause the electrolyte to flow through the space between the surfaces.

15. Apparatus according to Claim 13 or Claim 14, wherein the areas of the electrode between the exposed conductors are substantially filled with an electrically insulating material.

16. Apparatus according to Claim 15, wherein the exposed surfaces of the conductors are substantially flush with the surface of the surrounding electrically insulating material, so that the surface of the electrode is substantially smooth.

17. Apparatus according to Claim 15 or Claim 16, wherein the conductors are formed to project from the surface of a body of electrically conductive material, the electrically insulating material being a settable flowable material which is applied to said surface around the conductors and then solidified.

18. Apparatus according to Claim 17, wherein substantially the whole of said body is encapsulated in the settable electrically insulating material.

19. Apparatus according to Claim 17 or Claim 18, wherein the surface of the conductors and insulating material is machined smooth after the material has solidified.

20. Apparatus according to any of Claims 13 to 19, wherein the electrode comprises an annular body which, in use, encircles an axial portion of the bearing surface.

21. Apparatus according to Claim 20, wherein there are provided two axially spaced electrodes which, in use, encircle two different portions of the bearing surface.

22. Apparatus according to Claim 21, wherein means are provided for delivering the electrolyte into a region between the two electrodes so as to flow in opposite axial directions past both electrodes.

23. Apparatus according to any of Claims 20 to 22, wherein the conductors on the, or each, electrode are in the form of a circumferential array of generally parallel elongate strips inclined at an angle to the longitudinal axis of the electrode.

24. Apparatus according to any of Claims 13 to 19, wherein the electrode is in the form of a surface transverse to the direction of the axis of a bearing surface which, in use, is located in the apparatus.

25. Apparatus according to Claim 24, wherein the conductors on the electrode are in the form of an array of curved elongate strips arranged around the electrode and extending inwardly from the outer periphery thereof and towards the centre of the electrode.

26. Apparatus according to any of Claims 13 to 25, wherein the conductors are arranged in a configuration corresponding to only a part of the required overall configuration of recesses on a bearing surface, means being provided to locate the bearing surface in a plurality of alternative positions with respect to the electrode.

27. Apparatus for forming one or more recesses in an electrically conductive bearing surface and substantially as hereinbefore described with reference to the accompanying drawings.