U.S. patent application number 11/704299 was filed with the patent office on 2007-06-21 for progressive cavity pump/motor stator, and apparatus and method to manufacture same by electrochemical machining.
This patent application is currently assigned to Lehr Precision, Inc.. Invention is credited to Tom Chamberlain, Terry Lievestro, John Reynolds.
Application Number | 20070140883 11/704299 |
Document ID | / |
Family ID | 34316853 |
Filed Date | 2007-06-21 |
United States Patent
Application |
20070140883 |
Kind Code |
A1 |
Lievestro; Terry ; et
al. |
June 21, 2007 |
Progressive cavity pump/motor stator, and apparatus and method to
manufacture same by electrochemical machining
Abstract
Electrochemical machining is used to generate the helical lobe
profiles of the stator of a progressive cavity pump or motor. A
thin, elastomeric liner, of uniform thickness is bonded either to
the interior of the stator, or to the exterior of the rotor. Where
the elastomeric liner is to be bonded to the interior of the
stator, bonding is improved by electrically etching the interior of
the stator during the electrochemical machining process to produce
a roughened surface.
Inventors: |
Lievestro; Terry; (West
Chester, OH) ; Reynolds; John; (Maineville, OH)
; Chamberlain; Tom; (West Chester, OH) |
Correspondence
Address: |
BAKER & HOSTETLER LLP
WASHINGTON SQUARE, SUITE 1100
1050 CONNECTICUT AVE. N.W.
WASHINGTON
DC
20036-5304
US
|
Assignee: |
Lehr Precision, Inc.
|
Family ID: |
34316853 |
Appl. No.: |
11/704299 |
Filed: |
February 9, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10887189 |
Jul 8, 2004 |
7192260 |
|
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11704299 |
Feb 9, 2007 |
|
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60510107 |
Oct 9, 2003 |
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Current U.S.
Class: |
418/45 ;
418/48 |
Current CPC
Class: |
B23H 9/005 20130101;
Y10T 29/49242 20150115; B23H 3/00 20130101; B23H 9/00 20130101;
F04C 2/1075 20130101; F04C 2230/101 20130101; Y10T 29/49272
20150115 |
Class at
Publication: |
418/045 ;
418/048 |
International
Class: |
F01C 5/00 20060101
F01C005/00; F01C 1/10 20060101 F01C001/10; F04C 5/00 20060101
F04C005/00 |
Claims
1. A progressive cavity fluid mechanism, adapted for use as a pump
or motor, comprising a stator housing and a rotor, said rotor
having an exterior surface formed with helical lobes, and said
stator housing comprising a unitary, tubular, rigid element having
a circular, cylindrical exterior, and an interior surface also
having helical lobes, said mechanism also comprising a flexible
layer having a uniform thickness on one of said interior and
exterior surfaces.
2. (canceled)
3. (canceled)
4. (canceled)
5. (canceled)
6. (canceled)
7. (canceled)
8. (canceled)
9. (canceled)
10. (canceled)
11. (canceled)
12. (canceled)
13. (canceled)
14. (canceled)
15. (canceled)
16. An apparatus for use in machining a tubular workpiece or the
like, comprising: a cathodic electrode shaped to machine helical
lobes in the tubular workpiece; a drive bar arranged to move said
cathodic electrode along a linear path, while simultaneously
rotating the cathode about an axis parallel to said linear path,
wherein the drive bar is connected to the cathodic electrode by a
machine taper, said taper having a frusto-conical exterior surface
mating with a frusto-conical interior surface in one of said drive
bar and said cathodic electrode; a electrical power supply
connected to the cathodic electrode, and connectible to a tubular
workpiece disposed along said linear path and arranged so that said
cathodic electrode can pass axially along the interior of said
tubular workpiece, whereby said power supply can establish an
electric current through said cathodic electrode and said
workpiece, as the cathodic electrode passes along the interior of
the workpiece; and a flow path for directing electrolyte past the
cathodic electrode, between the cathodic electrode and the
workpiece; wherein said flow path includes a space defined between
a portion of the drive bar and the workpiece, behind the cathodic
electrode as the cathodic electrode moves along said linear path;
and including an electrical conductor, connected to said electrical
power supply and exposed to said space, whereby an electric current
is established, through electrolyte within said space, between said
conductor and said workpiece.
17. An apparatus according to claim 16, in which said space is
further defined by a seal on said drive bar, spaced rearward from
said cathodic electrode, and sealingly engageable with the
workpiece.
18. An apparatus according to claim 16, in which a part of said
drive bar is covered by an insulating sleeve, and in which said
electrical conductor exposed to said space defined between a
portion of the drive bar and the workpiece is a part of said drive
bar not covered by said insulating sleeve.
19. An apparatus according to claim 16, including a rear guide on
said drive bar, located behind the cathodic electrode as the
electrode moves along said linear path, said rear guide engageable
with, and slidable on, the finished interior of the workpiece, for
supporting the weight of the cathodic electrode as it moves through
the workpiece, said rear guide being forward of said space defined
between a portion of the drive bar and the workpiece, and at least
one passage allowing flow of electrolyte past said guide, from said
space toward the cathodic electrode.
20. An apparatus according to claim 19, in which said at least one
passage allowing flow of electrolyte past said guide is formed on
the exterior of the rear guide, between the rear guide and the
workpiece.
21. An apparatus according to claim 19, in which said rear guide is
formed to provide space allowing electrolyte to pass between the
rear guide and the drive bar in order to transfer heat away from
the cathode to drive bar interface.
22. An apparatus according to claim 19, in which said rear guide is
formed with a plurality of passages allowing electrolyte to pass
between the rear guide and the drive bar in order to transfer heat
away from the cathode to drive bar interface.
23. An apparatus for use in machining a tubular workpiece or the
like, comprising: a cathodic electrode shaped to machine helical
lobes in the tubular workpiece; a drive bar arranged to move said
cathodic electrode along a linear path, while simultaneously
rotating the cathode about an axis parallel to said linear path; a
electrical power supply connected to the cathodic electrode, and
connectible to a tubular workpiece disposed along said linear path
and arranged so that said cathodic electrode can pass axially along
the interior of said tubular workpiece, whereby said power supply
can establish an electric current through said cathodic electrode
and said workpiece, as the cathodic electrode passes along the
interior of the workpiece; and a flow path for directing
electrolyte past the cathodic electrode, between the cathodic
electrode and the workpiece; wherein said flow path includes a
space defined between a portion of the drive bar and the workpiece,
behind the cathodic electrode as the cathodic electrode moves along
said linear path; a rear guide on said drive bar, located behind
the cathodic electrode as the electrode moves along said linear
path, said rear guide engageable with, and slidable on, the
finished interior of the workpiece, for supporting the weight of
the cathodic electrode as it moves through the workpiece, said rear
guide being forward of said space defined between a portion of the
drive bar and the workpiece, and at least one passage allowing flow
of electrolyte past said guide, from said space toward the cathodic
electrode; and an electrical conductor, connected to said
electrical power supply and exposed to said space, whereby an
electric current is established, through electrolyte within said
space, between said conductor and said workpiece; said space being
sufficiently long, in the direction of said linear path, that the
electric current established in said space can etch, and thereby
roughen the finished interior surface of the workpiece after
machining thereof by said cathodic electrode.
24. An apparatus according to claim 23, in which the drive bar is
connected to the cathodic electrode by a machine taper, said taper
having a frusto-conical exterior surface mating with a
frusto-conical interior surface in one of said drive bar and said
cathodic electrode.
25. An apparatus according to claim 23, in which said space is
further defined by a seal on said drive bar, spaced rearward from
said cathodic electrode, and sealingly engageable with the
workpiece.
26. An apparatus according to claim 23, in which a part of said
drive bar is covered by an insulating sleeve, and in which said
electrical conductor exposed to said space defined between a
portion of the drive bar and the workpiece is a part of said drive
bar not covered by said insulating sleeve.
27. An apparatus according to claim 23, in which said rear guide is
formed to provide space allowing electrolyte to pass between the
rear guide and the drive bar in order to transfer heat away from
the cathode to drive bar interface.
28. An apparatus according to claim 23, in which said rear guide is
formed with a plurality of passages allowing electrolyte to pass
between the rear guide and the drive bar in order to transfer heat
away from the cathode to drive bar interface.
29. An apparatus for use in machining a tubular workpiece or the
like, comprising: a cathodic electrode shaped to machine helical
lobes in a tubular workpiece; a drive bar arranged to move said
cathodic electrode along a linear path, while simultaneously
rotating the cathode about an axis parallel to said linear path; a
electrical power supply connected to the cathodic electrode, and
connectible to a tubular workpiece disposed along said linear path
and arranged so that said cathodic electrode can pass axially along
the interior of said tubular workpiece, whereby said power supply
can establish an electric current through said cathodic electrode
and said workpiece, as the cathodic electrode passes along the
interior of the workpiece; and a flow path for directing
electrolyte past the cathodic electrode, between the cathodic
electrode and the workpiece; wherein said flow path includes a
space defined between a portion of the drive bar and the workpiece,
behind the cathodic electrode as the cathodic electrode moves along
said linear path; a rear guide on said drive bar, located behind
the cathodic electrode as the electrode moves along said linear
path, said rear guide engageable with, and slidable on, the
finished interior of the workpiece, for supporting the weight of
the cathodic electrode as it moves through the workpiece, said rear
guide being forward of said space defined between a portion of the
drive bar and the workpiece, and at least one passage allowing flow
of electrolyte past said guide, from said space toward the cathodic
electrode, wherein said rear guide is formed to provide space
allowing electrolyte to pass between the rear guide and the drive
bar in order to transfer heat away from the cathode to drive bar
interface; and an electrical conductor, connected to said
electrical power supply and exposed to said space, whereby an
electric current is established, through electrolyte within said
space, between said conductor and said workpiece.
30. An apparatus according to claim 29, in which the drive bar is
connected to the cathodic electrode by a machine taper, said taper
having a frusto-conical exterior surface mating with a
frusto-conical interior surface in one of said drive bar and said
cathodic electrode.
31. An apparatus according to claim 29, in which said space is
further defined by a seal on said drive bar, spaced rearward from
said cathodic electrode, and sealingly engageable with the
workpiece.
32. An apparatus according to claim 29, in which a part of said
drive bar is covered by an insulating sleeve, and in which said
electrical conductor exposed to said space defined between a
portion of the drive bar and the workpiece is a part of said drive
bar not covered by said insulating sleeve.
33. An apparatus according to claim 29, in which said at least one
passage allowing flow of electrolyte past said guide is formed on
the exterior of the rear guide, between the rear guide and the
workpiece.
34. An apparatus according to claim 29, in which said rear guide is
formed with a plurality of passages allowing electrolyte to pass
between the rear guide and the drive bar in order to transfer heat
away from the cathode to drive bar interface.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims priority from U.S. provisional
application 60/510,107, filed Oct. 9, 2003.
FIELD OF THE INVENTION
[0002] This invention relates to the manufacture of progressive
cavity fluid mechanisms, that is, progressive cavity pumps and
progressive cavity motors, and more particularly to a novel stator
structure, and to an apparatus and process for producing the
stators of such pumps and motors. The stators of progressive cavity
mechanisms are typically very long, some being up to seven meters
in length.
BACKGROUND OF THE INVENTION
[0003] U.S. Pat. Nos. 1,892,217 and 2,028,407, to R. J. L. Moineau,
disclose a gear mechanism for use as a progressive cavity pump or
motor. In a typical application of progressive cavity technology,
the drilling of subterranean wells, a progressive cavity motor is
used as a downhole motor to convert the energy of a flowing
drilling fluid to mechanical power to rotate a drill bit.
[0004] In a progressive cavity pump or motor an interference fit
between the external profile of the rotor and the internal profile
of the stator provides a seal isolating the cavities of the pump or
motor from adjoining cavities. The seal resists the fluid pressure
which results from the mechanical pumping action, or from the
conversion of fluid motion to mechanical energy in a motor. Because
of the requirement for an interference fit between the rotor and
stator, one or both of these components must be covered with a
resilient, or dimensionally forgiving, material which also allows
the pump or motor to pass or transfer abrasive particles and other
objects carried along with the fluid. Historically, the resilient
material has been provided on the interior of the stator.
[0005] The resilient material used for the stator introduces
weaknesses into the operation of the pump or motor and shortens its
operating life. Common elastomers have a temperature tolerances
below that of most of the other components in the pump or motor,
which are made of metal.
[0006] Mechanical resistance of the elastomer is also a concern
because of the high fluid pressures generated in the cavities of
the pump and motor. These high pressures, and the resulting
reactive forces, result in a significant deflection and stress in
the elastomer, particularly at the locations of the interferences
between the rotor and stator. The friction resulting from the large
forces existing between the rotor and stator generates a large
amount of heat, which is deleterious to the desired characteristics
of the elastomer, and thus deleterious to the performance and life
of the pump or motor.
[0007] The stator is conventionally constructed by molding an
elastomer, having the desired helical interior profile, within a
cylindrical steel tube or housing. Due to the helical profile of
the stator's internal surface, the radial thickness of the molded
elastomer, between its inner surface and the inner surface of the
metal tube, varies. If the heating of the elastomer is excessive,
its properties will degrade. Elastomers are generally highly
insulative, and thus inherently restrict conduction of the heat
generated at the interface of the rotor and stator to the thermally
conductive metal tube, where the heat can be dissipated, usually
with the aid of a cooling system such as a liquid cooling system or
exposed fins. The radially thicker sections of the elastomer are
more insulative, and thus degrade faster than the radially thinner
sections. Additionally, the high pressures produced during the
operation of the pump or motor can deflect the thicker sections of
elastomer to the extent that the interference between the elastomer
and the rotor is overcome, and contact with the rotor is lost. This
loss of contact results in a reduced operating efficiency,
characterized by decreased speed in the case of a motor, and by
decreased flow in the case of pump. In addition, heat generated by
the operation of the pump or motor, in some cases acting in
conjunction with heat from the environment in which the pump or
motor operates, can distort the shape of the molded elastomer on
the interior of the metal tube. Elastomers have a high coefficient
of thermal expansion compared to the other materials used in the
construction of a progressive cavity pump or motor. As a result of
the varying thicknesses and relatively high thermal expansion of
the elastomer, the radially thick sections tend to exhibit greater
distortion than the thinner sections. The distortion results in a
geometric stator profile drastically different from the intended
profile, and hinders the operation of the pump or motor. The
distortion of the stator profile can generate additional heat,
which in turn causes further distortion of the stator profile.
Because of such distortion the stator contributes rapidly to its
own degradation and ultimate failure.
[0008] As a result of the previously mentioned degradation, the
interior of the thicker sections also can become brittle, allowing
a stator lobe to break or "chunk out" of the stator profile. In
addition, the pressure acting in the chambers formed by the stator
and rotor may exceed the strength of the elastomer, causing the
stator lobe to deflect from its original shape, and may also cause
a break or "chunk out". These effects also degrade the efficiency
of the pump or motor.
[0009] U.S. Pat. No. 6,309,195 describes a Moineau motor having a
stator with a constant wall thickness. The stator is manufactured
by a mechanical forming process in which the metal is bent locally
to form a constant wall thickness in the outer steel structure, and
in which the interior wall is covered by a thin wall elastomer. The
dimensions of the stator produced by this forming method are
limited, and more tolerance is required in the thickness of the
thin wall elastomer. The patent alludes to the difficulty in
maintaining the required twist tolerance. The outside of the casing
is also contoured, making it more difficult to handle with the
equipment commonly used to handle tubular articles in the drilling
process. Machining of the outer wall of the casing to eliminate the
contours would cause the wall thickness of the casing to be
excessively small at some locations and comparatively thick at
other locations.
[0010] Electrochemical machining has been used for various
purposes. For example, U.S. Pat. No. 6,413,407 describes a process
and apparatus for electrochemical machining (ECM) of flutes in the
interior of a tube for use in a petroleum cracking furnace.
However, so far as we are aware, ECM has not been used successfully
in the production of the lobes in the interior of a stator of a
progressive cavity device.
SUMMARY OF THE INVENTION
[0011] In accordance with this invention, ECM is used to generate
the lobe profiles of the stator of a progressive cavity device. The
invention overcomes many of the problems identified in the prior
art for progressive cavity pumps and motors, including excessive
heat build-up and the ability to hold tolerances. A motor having a
stator made in accordance with the invention is particularly well
suited for use as a downhole motor in a well to drive a drill
bit.
[0012] In order to still have a compliant seal between the stator
and rotor, a thin layer of constant thickness elastomer is still
required. The desired inside profile of the stator, offset by the
desired thickness of the elastomer layer, is formed in a circular,
cylindrical inner wall of a tubular, metal workpiece serving as a
stator blank. The surface finish of the inner profile must allow
for bonding of the elastomer forming the profile that contacts the
rotor. The constant thickness of the thin elastomer layer
significantly reduces the adverse effects experienced in the case
of an elastomer lining having a varying thickness. Because the
elastomer layer can be relatively thin throughout, its insulating
effect is also reduced, allowing for better heat transfer to the
rigid metallic housing.
[0013] To achieve a surface finish suitable for bonding of the
elastomer to the interior wall of the stator, a distinct ECM
process is used, differing from the process described in U.S. Pat.
No. 6,413,407, where a seal and a flush system were used to protect
the finish of machined flutes formed by electrochemical machining
of the interior of a tube for use in a petroleum cracking furnace.
The machine used in the present invention is similar to that in
U.S. Pat. No. 6,413,407, but the seal and flush system are
eliminated, allowing controlled exposure of electrolyte to the
finish-machined surface. This results in a rougher surface finish,
which increases the effectiveness of the elastomer-to-metal bond. A
device behind the cathode of the machine is used as a supporting
guide only, instead of as a seal.
[0014] In the conventional ECM apparatus, the cathodic electrode is
mounted by means of a threaded joint. The threaded joint is very
cost effective, but is limited in its ability to transfer large
amounts of electrical current from the drive tube to the cathode.
The concentricity of the drive bar and electrode is also limited in
its precision, and can vary. On the other hand, the concentricity
requirement for progressive cavity pumps and motors requires more
precision than is possible with a threaded joint.
[0015] This invention addresses the problems of electrical current
transmission and concentricity by using a tapered adapter similar
to those used for mounting rotating cutting tools in conventional
machine tools. This taper can be precision machined to improve
concentricity, and the increased surface contact area reduces
resistance heating of the electrical joint.
[0016] The invention also provides a method of manufacturing
progressive cavity stators by electrochemical machining and an
alternative method by which electrolyte flows through the
system.
[0017] The progressive cavity pumps and motors in accordance with
the invention include a rotor rotatably disposed in a stator which
has a suitably profiled interior surface but which can have an
exterior shape in the form of a circular cylinder, or any other
desired shape. Either the stator or the rotor is covered by a thin
layer of elastomer or other flexible material for the purpose of
providing a seal between the stator and rotor.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a radial section view of a conventional
progressive cavity pump or motor in accordance with the prior art,
having an elastomeric lining of variable thickness inside a rigid
metallic housing;
[0019] FIG. 2 is a cut-away isometric view of the progressive
cavity pump or motor of FIG. 1;
[0020] FIG. 3 is a cut-away isometric views of the stator of the
progressive cavity pump or motor in FIG. 1, showing the helical
configuration of the internal lobes of the stator;
[0021] FIG. 4 is a radial section of a progressive cavity pump in
accordance with the invention, showing the constant thickness of
the elastomer lining of the stator;
[0022] FIG. 5 is a cut-away isometric views of the progressive
cavity pump or motor of FIG. 4;
[0023] FIG. 6 is an isometric view of the stator of the progressive
cavity pump or motor of FIG. 4;
[0024] FIG. 7 is a longitudinal section of an entry manifold of the
apparatus for electrochemical machining of a tubular workpiece to
produce the stator illustrated in FIG. 6, showing the initial
position of a fluted cathode entering the proximal end of the
workpiece;
[0025] FIG. 8 is a longitudinal sectional of the exit manifold at
the distal end of the workpiece;
[0026] FIG. 9 is an isometric view of the cathode, entry manifold,
and workpiece, as illustrated in FIG. 7;
[0027] FIG. 10 is a longitudinal sectional view showing the cathode
tool during electrochemical machining inside a workpiece;
[0028] FIG. 11 is a longitudinal sectional view showing a cathode
tool during electrochemical machining inside a workpiece, using the
alternate embodiment of an etch chamber;
[0029] FIG. 12 is a schematic radial section a stator workpiece,
showing the area to be machined by the electrochemical machining
process; and
[0030] FIG. 13 is a radial section of an alternative progressive
cavity pump in accordance with another embodiment of the
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0031] As shown in FIGS. 1-3, the conventional Moineau motor 20
comprises a helically lobed rotor 22 disposed within a stator, the
stator comprising a metal tube 24 having a circular, cylindrical
interior wall 26 and a circular, cylindrical exterior wall 28, the
interior wall having a molded elastomer liner 30 formed with
helical lobes cooperable with the rotor to provide moving fluid
chambers as the rotor rotates. As seen in all three of FIGS. 1-3,
the thickness of the elastomer liner varies because of the presence
of the lobes.
[0032] A Moineau Motor 32 in accordance with the invention,
illustrated in FIGS. 4-6, comprises a helically lobed rotor 34
disposed within a stator comprising a tube 36 and a flexible liner
40. The tube 36 is composed of steel or a similar structural
material. The tube has an interior wall 38 having helical lobes
formed by electrochemical machining, and a circular, cylindrical
exterior. Using a suitable mold or core (not shown), the molded
liner 40, of rubber or other suitable elastomer, is bonded to the
interior wall of the stator after electrochemical machining. The
elastomer liner, which has a uniform thickness, defines the
interior wall of the stator. The helically lobed interior wall of
the stator cooperates with the helically lobed rotor 34 to define a
set of fluid chambers, which move axially as the rotor rotates
within the stator.
[0033] In the operation of the motor 32, a pressure differential
exists between each adjoining fluid chamber. In the operation of
the motor, the hydraulic force acting on the rotor as a result of
the pressure of a driving liquid causes the rotor to rotate about
its longitudinal axis. Mechanical transfer of the rotation motion
of the rotor to a drill bit can be accomplished through any of a
variety of mechanisms known to those skilled in Moineau motor
design. The sliding motion of the rotor at the rotor/stator
interface generates frictional heat.
[0034] The lobed interior wall 38 of the stator mechanically
support the elastomer liner 40, strengthening the elastomer liner,
and allowing it to withstand operating loads and stresses greater
than those which can be withstood by elastomer liners of
conventional Moineau motors. The lobes on the interior of the metal
stator tube also provide the metal tube with an increased surface
area enhancing the transfer of heat generated at the rotor/stator
interface. Thus, frictional heat generated at the rotor/stator
interface is conducted through a relatively insulative, but thin,
elastomer layer, over a relatively large area, to a stator tube
having a high thermal conductivity, from which the heat is
dissipated to the environment. The relatively low, and uniform,
thickness of the elastomeric liner 40 allows for a nearly even
transfer of heat around the circumference of the liner. The nearly
even transfer of heat results in a highly uniform temperature
distribution, which prevents thermal distortion of the elastomeric
liner and resulting disturbance to the proper operation of the
motor.
[0035] The advantages of the Moineau motor described above would,
of course, be equally beneficial in a Moineau-style pump, which is
essentially the inverse of the motor.
[0036] Because the elastomeric liner 40 is bonded to the rigid
interior wall 38 of the stator, surface preparation of wall 38 is
important. The elastomer will bond better to a rougher surface.
U.S. Pat. No. 6,413,407 describes an electrochemical machining
process in which every effort is made to yield the smoothest
possible surface finish. To avoid degradation of the finish by
further action of electrolyte after the bore is machined to the
desired dimensions, an aft inner guide, fixed to the aft end of the
electrode provides a seal behind the electrode, sealing the tool to
the workpiece behind the electrode as it moves through the
workpiece. Water or another suitable fluid is then introduced
behind the aft guide to flush away stray electrolyte.
[0037] The process and apparatus used in the production of the
stator are, in most respects similar to the process and apparatus
described in U.S. Pat. 6,413,407, and thus the disclosure of that
patent is incorporated by reference. In contrast with the process
and apparatus described in U.S. Pat. No. 6,413,407, in accordance
with this invention, in order to achieve a rougher surface finish
of the interior wall of the rigid stator tube, the length of time
during which the finish machined interior wall of the tube is
exposed to electrolyte is increased and electrical current is
allowed to continue to etch the finished interior surface, thereby
achieving a roughening effect on the surface finish to improve
bonding of the subsequently molded flexible layer.
[0038] As shown in FIG. 7, electrolyte is introduced through port
42 into a proximal inlet flow box 44. As depicted in FIG. 10, while
electrochemical machining of the workpiece 46 progresses, the
electrolyte passes over the length of the drive bar 48, between the
drive bar and the finish machined portion 47 of the workpiece 46,
and through slots 49 formed in the wall of a central opening of
rear guide 50 which receives the drive bar. The slots 49 are
preferably disposed parallel to one another, at intervals around
the circumference of the central opening of the rear guide 50, as
shown in FIG. 9. The flow of electrolyte through these slots allows
for cooling of the cathode/drive bar interface. The electrolyte
then passes over the cathode 52 in the proximal to distal
direction, i.e., in the direction of cathode travel, past the front
guide 80, and down the length of the unfinished bore 56 of the
workpiece 46, into the distal exit flow box 58, where it is then
discharged into an electrolyte return. The inlet flow box 44 must
have an interior diameter, as shown in FIG. 7, equal to the major
dimension of the finished profile of the workpiece, in order to
support the weight of the cathode assembly before the rear guide
enters the workpiece.
[0039] The negative output terminal of a DC power supply,
preferably capable of delivering up to 30,000 Amperes at 25 volts,
is connected to the workpiece, and the positive terminal is
connected through a slip ring assembly to the drive bar.
[0040] The exit flow box, shown in FIG. 8, must have sufficient
internal space to accept the cathode assembly as the cathode passes
through the distal end of the workpiece. It must also be connected
to the electrolyte flow system.
[0041] As shown in FIG. 10, the rear guide 50 directs electrolyte
flow and supports the weight of the cathode assembly mounted on the
drive bar 48, but does not serve a sealing function. Accordingly
electrolyte remains in the space behind the cathode as machining
progresses. An insulating sleeve 60 on the drive bar is cut back to
location 62 to expose an annular area 64 of the drive bar of
sufficient length to allow an electric current between the guide
bar and the workpiece to exert an etching action on the finished
interior wall of the workpiece.
[0042] The cross sectional area which needs to be to be removed in
machining the stator of a progressive cavity pump or motor is large
enough to require thousands of amperes of current. 30,000 amperes
is sufficient for most such applications. However conducting
electrical current at such a high level between the cathode and the
drive bar is difficult in conventional ECM equipment. In the
apparatus of the invention, as shown in FIGS. 7 and 10, a standard
machine tool taper 66, similar to a CAT 50, is used both to locate
the cathode and guide assembly on the drive bar, and to conduct
current from the drive bar to the cathode. The taper has two
frusto-conical exterior surfaces mating respectively a
frusto-conical interior surfaces in the drive bar and the cathodic
electrode, to provide precise alignment and also to provide a large
contact area for carrying the very high electric current required
in electrochemical machining. For the sake of maintenance, an
internal connector or clamping device, 68 is used to mount the
cathode 52 on the drive bar 48. The connector 68 may be constructed
of a hard metal having very good electrical conductivity, such as
UNS-C18200. Although a double taper, as shown, is preferred, a
single taper, formed as an integral part of the drive bar, or as an
integral part of the cathodic electrode could be used as an
alternative.
[0043] A cooling liquid flows through the drive bar as in the
conventional ECM apparatus. However, in this case, the cooling path
can be isolated from the electrochemical machining process. Thus,
as shown in FIG. 10, coolant flows along passages 74 formed by
flats machined in the surface of the drive bar and the interior
wall of the insulating cover 60, then inward through radial
passages 72 in the drive bar, and then in the reverse direction
through the central passage 70 in the drive bar. The coolant can
also flow in the opposite direction. This allows for a better
temperature control of the drive bar at locations remote from the
cathode. Electrolyte flow under the rear guide 50, through slots
49, is then used to conduct heat away from the exposed part 64 of
the drive bar 48 so that relatively little heat needs to pass
through the insulating cover 60 on the drive bar, which acts as not
only as an electrical insulator, but also as a thermal
insulator.
[0044] At the proximal end of the drive bar, a double flow rotary
coupling (not shown) is used to inject cooling water into the
chambers between the insulating cover and the flats on the outside
of the drive bar. At the distal end of the drive bar, the coolant
is directed to the central passage 70 of the bar, and is then
allowed to exit the center of the bar at the proximal end through
the double flow rotary coupling. O-ring seals 78 under the
insulating cover at the distal end, and similar O-ring seals at the
proximal end, ensure that the cooling liquid is maintained in the
cooling chambers without contamination from the electrolyte.
[0045] The front guide 80, shown in FIGS. 7, 9, and 10, has a
circular exterior to guide the cathode through the tubular
workpiece before the lobes are machined in it, and to support the
weight of the cathode assemble as it transverses the workpiece
during the machining cycle. The front guide has longitudinal slots
82 cut through it to allow electrolyte to pass from the cathode to
the exit flow box, one such slot 82, being shown in FIG. 10. The
front guide is mounted on the cathode clamping device 54, and a
threaded plug 84 is used to retain the front guide.
[0046] In an alternative embodiment of the invention, shown in FIG.
11, a second seal 86 is provided behind the cathodic tool 88 to
form an etching chamber 90. In this alternative embodiment, the
electrolyte is channeled through the center of the drive bar used
to push the cathode through the workpiece. The electrolyte is also
channeled into a 360 degree slot adjacent seal 86. The electrolyte
is forced through slots 91 formed in the rear seal 92, and across
the cathodic tool 88 as in the preferred embodiment. This
alternative embodiment provides better control over the amount of
time during which the machine-finished finish interior surface of
the workpiece is exposed to post-machining etching. In a further
alternative embodiment, not illustrated, the etching solution can
be isolated from the electrolyte used in machining the lobes of the
stator, in which case the composition of the etching solution can
be different from that of the electrolyte. Optionally, a second
cathodic tool may be positioned in the etch chamber to direct the
etching to particular regions within the etch chamber.
[0047] In the electrochemical process of this invention, a
salt-based electrolyte such as a water based sodium chloride (NaCl)
or sodium nitrate (NaNO.sub.3) solution may be used. This process
breaks down water into H.sub.2 and OH ions that will bond with the
metal ions, usually Fe, to form FeOH that precipitates out of the
solution and can be filtered. The amount of current required to
remove the metal electrochemically is directly proportional to the
volume of metal removed in a given time interval. Therefore, the
area 93, shown in FIG. 12, and the feed rate of the cathode
determine the volume of metal removed and the amount of power
required for the process. The maximum feed rate found practical to
date is one inch per minute, because of the limiting dissolution
rate of the metal. Increase in feed is possible by increasing the
length of the cathode to increase the surface area being
dissolved.
[0048] The process is generally limited by resistance heating. It
has been found that a 30,000 Ampere power supply is adequate for
existing stator sizes. If larger stators are needed, larger power
supplies and other conductors will be required.
[0049] A typical stator machining process in accordance with the
invention uses a NaNO.sub.3 electrolyte, at a concentration of 2.2
pounds per gallon of water, at a neutral PH, and at an inlet
pressure of 330 psi, and an outlet pressures of 80 to 150 psi. The
electrolyte is introduced at a temperature of 105.degree.
F..+-.1.degree. F. The voltage used is approximately 20 volts,
although it may be varied from 10V to 25V. The feed rates vary from
stator to stator, but an average part is produced at a feed rate of
approximately 0.55 inches per minute. The feed rates will normally
vary from 0.15 inches per minute on larger parts to 1 inch per
minute on smaller parts. Cathode taper angles vary from 5.degree.
to 15.degree., but in most cases a 10.degree. taper is
preferred.
[0050] In summary, in accordance with the invention, a novel stator
for a progressive cavity fluid mechanism is provided, in which the
elastomer layer can be of uniform thickness, and very thin, so that
it is less subject to damage resulting from thermal effects, and in
which the exterior shape of the stator housing can be a simple
circular cylinder, or any other desired shape. The stator is
produced to precise dimensions by electrochemical machining, and,
in the same process, the interior finish of the stator can be
etched to promote secure bonding of the thin elastomer layer to the
machined interior wall of the stator.
[0051] In an alternative embodiment, as shown in FIG. 13, a thin
elastomer layer 94 of uniform thickness can be formed on the rotor
96, in which case it is unnecessary, and undesirable, to etch the
interior surface 98 of the stator 100. Even in this alternative
embodiment, many of the advantages of electrochemical machining of
the stator can be realized.
* * * * *