"Method and apparatus for forming recesses in a beaπng surface"
The invention relates to methods and apparatus for forming recesses in a beaπng
surface, and particularly in the beaπng surfaces of air and other fluid film beaπngs
As is well known, the performance of many types of air and other fluid film
beaπngs can be enhanced by the formation of suitably shaped and orientated grooves or
other recesses in the beaπng 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 beaπng type and
requirements Improvements are particularly beneficial on "self acting" bearings where
the pressure in the lubπcating fluid is generated by rotation of the bearing itself, but can
also be significant on beaπngs where the lubricating fluid is externally pressurised
The recesses may be of vaπous kinds, but "herπngbone" grooves on the journal
beaπngs and spiral grooves on thrust beaπngs 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, l e 5-50μm 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 7- 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 of the 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 of the 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 1 1. 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 5-50μm 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 of the journal 10 within the bearing itself serves to pump the lubricating fluid
in the gap and for this purpose the journal 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 journal in which the grooves 12 are formed may be of any diameter for
example a journal for a computer disc drive may have a diameter of only about 3 mm,
whereas a 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 50-300μm.
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
internal 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 journals 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/mm2 removes material at approximately 25μm/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 of the above arrangements the depths of the 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.