METHOD FOR PRODUCING A HYDROSTATIC FLUID BEARING WITH
CELLSt
_BackgrDund oL the invcention _
5 The present description relates to the field of
hydrostatic fluid bearings with cells for supporting
rotary shafts, in particular in rocket engine turbopumps
for putting fluid under pressure.
As stated in French patent application
10 No. 2 809 782, such a bearing comprises an annular stator
in which a rotor is received, e.g. a rotor constituting
the shaft of a turbopump. In its annular inside surface,
the stator has cells that are fed Hith fluid via orifices
that lead to its outer surface. Thus, the rotor is, held
15 by being suspended in the stator on a thin layer of fluid
under pressure as introduced via the orifices in the
cells.
More precisely, French patent application
No. 2 809 782 relates to a hydrostatic fluid bearing
20 having a cylindrical stator that includes on its inside
surface a ring that is constituted by assembling a metal
plate having orifices that co-operate Hith the fluid
insertion orifices, and a layer of self-lubricating
composite material that includes cells that are arranged
25 around the orifices.
Such a bearing presents a layer of composite
material on its inside surface that is of a thickness
enabling it to Hear without running the risk of
irreparable damage to the shaft or the bearing in the
30 event of shaft-bearing contacts, thus also making it
possible to have transient stages Hithout assistance.
The method of fabricating such a bearing, as
described in French patent application No. 2 809 782 and
as set out briefly belo1-1 1-1ith reference to accompanying
35 Figures 1 to 3, comprises steps of assembling a layer of
t Translation of the title as established ex officio.
• I 2
self-lubricating composite material 3 on a surface
plane metal plate 2 of dimensions corresponding to
development of the inside surface of the stator,
-machining cells 5 in the- thickness of the layer ofof
a
the
5 composite material in the assembly 1, curving the
assembly 1 into the form of a split ring 6, inserting the
assembly in the form of a split ring 6 against the inside
surface of the stator 4, and machining orifices 7 through
the thickness of the stator and the inserted assembly
10 into the cells 5.
That method is effective and reliable.
Nevertheless, it requires the use of a plane metal plate
that is to receive the composite material. The length of
the plate needs to be calculated \·lith great accuracy, on
15 the basis of the development of the cylindrical surface.
Furthermore, curving the plate into the shape of a split
ring prior to putting it into place in the stator is a
step that is difficult to perform, and that affects the
shape of the cells. It should also be observed that the
20 orifices are machined through a sand\•lich comprising the
base material of the stator, the material of the plate,
and the composite material. These three materials have
different hardnesses, so that machining is relatively
complex.
25
Object and summary of the invention
There thus exists a need for a method of fabricating
a hydrostatic fluid bearing having a stator with a
cylindrical inside surface that includes cells, which
30 method is substantially exempt from the above-mentioned
drawbacks.
Thus, in an aspect, the invention provides a method
of fabricating a hydrostatic fluid bearing comprising a
stator having a cylindrical inside surface that includes
35 cells, the method comprising:
depositing a coating of metal material on the
inside surface of the body of the stator;
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\: ,~,, :
3
· impregnating said coating with a self-lubricating
composite material;
· machining internal cells in the thickness of the
-coating, and machining orifices leading into the c::ells.-
5 Thus, with the invention, the metal material is
deposited directly on the inside surface of the stator
and it is impregnated \'lith self-lubricating composite
material while it is already coating that surface. This
avoids having recourse to a plane plate that needs to be
10 cut out to the right dimensions and curved, while also
making it possible to have a coating of composite
material that is of significant thickness. The metal
coating adheres effectively to the inside surface of the
stator body and the composite material is itself secured
15. to the- metal coating by being caused to penetrate int-o·
its thickness by impregnation. Thus, the selflubricating
composite coating formed by the metal coating
impregnated with self-lubricating composite material and
occupying the same significant thickness as the metal
20 coating, is secured effectively to the inside surface of
the stator body. Furthermore, the cells are machined in
the self-lubricating composite coating and can be very
accurate in shape, being unaffected by an operation, such
as curving, that is performed after they have been
25 machined. The cells may be machined on a turning center
having a 5-axis milling spindle and an angle head, in
particular an angle head of ultra-high speed (UHS) type
with precision numerical control. In addition, machining
of the orifices is simplified compared with the above-
30 mentioned prior art, since the curved intermediate plate
is omitted.
35
Optionally, the metal material comprises bronze.
Optionally, the self-lubricating composite material
comprises polytetrafluoroethylene.
Optionally, after impregnating the coating and in
particular while machining the internal cells, the bore
of the coated inside surface is subjected to re-boring.
4
With the invention, this step of rectifying the bore
formed by the inside surface of the body of the stator,
can be performed when the coating is in place on the
surface, and with great accuracy. This rs~boring~ can-be-
5 performed 1vith a cutting tool or by regrinding the inside
surface of the body of the stator. Furthermore, the
regrinding may be performed in the same operation as
machining the cells, without disassembly. Optionally,
automatic deburring of the edges of the cells may be
10 performed in this operation, by deburring tools.
Optionally, the coating is deposited by thermally
spraying particles of metal material against the inside
surface of the stator.
This technique is found to be particularly
15 appropriate ·for performing the method:of· the invention.
Firstly it is flexible in use, in particular insofar as
it makes it possible to vary the thickness of the
coating. Specifically, with this technique, the metal
material is sprayed onto the inside surface of the stator
20 using a spray gun that moves relative to the surface. In
order to increase the thickness of the coating, it
suffices to act on the relative speed between the gun and
the stator, or indeed to perform one or more additional
passes with the gun.
25 Furthermore, surprisingly, the inventors have
observed that this technique serves to enhance
impregnation of the coating 1vith the composite material,
since the coating obtained using this technique presents
a certain amount of porosity, with its pore volume
30 fraction possibly lying in the range 1% to 10%.
Optionally, the coating is deposited in an
environment that is less oxidizing than air, in
particular in a closed enclosure in which there is an
inert gas.
35 This makes it possible to avoid oxidation at the
interface betHeen the coating and the coated surface. By
·\vay of example, this inert gas may be nitrogen.
5
Optionally, the coating is deposited by making at
least one internal layer of coating against the
cylindrical inside surface and making at least one top
lay€lr ~of coating-against- said internal layer, ~the top-
5 layer presenting porosity that is greater than the
porosity of the internal layer.
The internal layer(s) serve to provide bonding
between the coating and the coated surface. It is
therefore advantageous for them to have little porosity,
10 e.g. a pore fraction of about 1% or even. less. In
contrast, it is advantageous for the top.layer(s) to have
greater porosity, thereby enhancing their impregnation
with the self-lubricating material.
Optionally, deposition of the coating is followed by
15 heat treatment, that precedes ·impregnating ·the· coat·ing.
This heat treatment serves to relax stresses in the
coating or betlveen the coating and the inside surface of
the body of the stator and serves to make the coating
more uniform prior to being impregnated. This heat
20 treatment may be quenching.
Optionally, the coating is impregnated by applying
the composite material to the coating and pressing it
against the coating.
Optionally, the coating is impregnated by applying
25 the self-lubricating composite material to the coating in
the form of a paste including a self-lubricating
component.
Brief description of the drawings
30 Other characteristics and advantages of the
invention appear from the following description of an
embodiment of the invention given by way of non-limiting
example and Hith reference to the accompanying dra1vings,
in which:
35 · Figures 1 to 3, described above, show the prior
art method;
•
I 6
· Figure 4 is a fragmentary isometric view of the
body of a stator, prior to depositing a coating on its
inside surface;
- ·-Figure 5 shows th€! coating -being deposited by -- -
5 spraying;
· Figure 6 shows the application of the selflubricating
composite material on the coating;
· Figure 7 shows the composite material being
pressed against the coating;
10 · Figure 8 shows the stage of machining cells and
machining orifices leading into the cells; and
· Figure 9 is a fragmentary isometric view of the
stator.
15 - Det.ailed description of the invention
Like the prior art method described with reference
to Figures 1 to 3, the method of the present invention
applies to a stator body 4. This body is shown in
Figure 4. It comprises an annular cylindrical body
20 having a cylindrical inside surface 4A that defines a
central bore 4'. For example, the body 4 may be made out
of a hard alloy, in particular having a Brinell hardness
number (HB) greater than 300, or even 330 HB, e.g. an
alloy known by the name Inconel 718.
25 Figure 5 shows a coating 10 being deposited on the
30
cylindrical inside surface of the body 4. Specifically,
this deposition is performed by spraying using a spray
gun 12 having its head pointing towards the surface 4A.
In Figure 5, the body 4 is held in a support 14
having jaws 14' that clamp onto the body. The body 4 and
the gun 12 are movable relative to each other. For
example, the gun 12 may be movable in translation T
parallel to the axis A of the inside surface of the body
4, while the support 14 may be movable in rotation R
35 above that axis. These movements are controlled by an
electronic control unit (ECU) 16 so as to be synchronized
and while also taking account of the delivery rate of the
7
gun, in order to obtain the desired thickness for the
coating 10.
This device for depositing the coating may be
aTranged ·inside an enclosure- 18 in which the atmospheTe -
5 is not oxidizing, or indeed less oxidizing than ambient
air. In particular, the enclosure may be filled with an
inert gas, such as nitrogen.
The metal material used for forming the coating 10
may be aluminum, copper, stainless steel, nickel, or
10 indeed an alloy based on th0.se materials, in particular
an alloy of aluminum and nickel. The material may
advantageously be bronze.
15
The metal material may be sprayed by thermal
spraying. For example, this may be high velocity oxyfuel
· (HVOF) spraying or supersonic cJlame · spraying_• or·
plasma spraying in which the particles of material are
melted and accelerated by a gaseous plasma, or indeed it
may comprise dynamic cold spraying in which particles are
accelerated and projected in the powder state against the
20. surface 4A of the body 4.
25
The particles of metal materials used for spraying
may for example have an equivalent diameter lying in the
range 5 micrometers (pm) to 40 pm, in particular when the
spray technique used is dynamic cold spraying.
In order to obtain the desired coating thickness,
the coating may be deposited as a plurality of superposed
layers, e.g. by a plurality of successive passes of the
gun 12. Under such circumstances, it is advantageous for
porosity to vary as a function of the layer so that
30 porosity increases on going a1·1ay from the wall 4A of the
body 4. For example, the volume fraction of the pores in
the internal layer in contact 1·lith the surface 4A of the
body 4 may be about 1%, whereas the volume fraction of
the top layer closest to the axis A may be about 10%.
35 Once the coating 10 has been made, it is impregnated
with a self-lubricating composite material.
8
To do this, a layer of self-lubricating composite
material 2rr is initially applied to the coating 10, as
shown in Figure 6. This material may be applied in t·he
-form of. a-paste, -e.g ... using an appLicator-22 that is
5 movable relative to the stator 4. For example, the
stator is held by the support 14 of Figure 5 or by an
analogous support, and it may be moved in rotation R,
while the applicator 22 may be moved in translation T.
Thereafter, the composite material 20 is pressed
10 against the coating 10. '!'his is shown in Figure 7, in
which it can be seen that while the stator body 4 is
carried by jaws 14' analogous to those of Figure 4, an
expandable mandrel 24 is inserted in the central bore 4'
of the body. The mandrel has sectors 24A that, starting
15 from a rest configuration enabling the mafrdrel·. to ,-be ,
inserted in the bore 4', are suitable for being moved
radially outwards in order to deploy the mandrel 24.
This expansion exerts pressure on the layer of composite
material 20 and tends to cause it to penetrate into the
20 pores of the coating 10. Furthermore, it is possible to
perform the pressing as a plurality of successive stages,
between 1vhich the mandrel 24 and the stator body are
·moved angularly relative to each other so that the zones
occupied by the gaps bet1·1een the sectors 24A in one stage
25 are covered by the sectors in a later stage.
Application of composite materials to the coating 10
may be preceded by applying heat treatment to the
coating, in particular quenching.
The composite material paste is thus hardened while
30 performing the pressing. During pressing, the material
is confined radially. In order to avoid the pressed
composite escaping axially from the stator, it is
possible to use confinement rings 26 or the equivalent
that are pressed against the axial ends of the stator
35 body and that cover the coating 10 and the layer of
composite material 20 axially.
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The self--lubricating composite material may comprise
polytetrafluoroethylene (PTFE). For application-to the
coating, this material may for example be put into the
__ form of a_ paste.
5 Figure 8 shows the machining step that follows the
impregnating step. In the machining step, internal cells
28 are machined in the thickness of the coating 10 by
using a cutter 30 mounted on a numerically controlled UHS
angle head or the like. During this machining, the
10 stator body continues to be held by jaws 14' or the like.
The depth of the cells 28 is less than the thickness .of
the coating 10 impregnated 11ith composite material, this
depth and this thickness being measured radially. By way
of example, the depth of the cells lies in the range
15 0. 3 rnillimeterw (nun) to 0. 6 rrun, v1hile the- thickness .