Note: Descriptions are shown in the official language in which they were submitted.
-CA 02598813 2007-08-23
HIGHLY REINFORCED ELASTOMER FOR USE IN DOWNHOLE STATORS
FIELD OF THE INVENTION
This invention relates generally to Moineau style power sections useful in
subterranean drilling motors, and more specifically relates a drilling motor
including an
improved elastomer material.
BACKGROUND OF THE INVENTION
Moineau style hydraulic motors and pumps are conventional in subterranean
drilling and artificial lift applications, such as for oil and/or gas
exploration. Such motors
make use of hydraulic power from drilling fluid to provide torque and rotary
power, for
example, to a drill bit assembly. While downhole drilling motors fall into the
general
category of Moineau-type motors, they are generally subject to greater working
loads,
temperatures, and more severe chemical and abrasive environments than Moineau
motors
and pumps used for other applications. As such, the demands on drilling motor
components (rotor and stator components) typically far exceed the demands on
the
components of other Moineau-type motors and pumps. For example, drilling
motors may
be subject to a pressure drop (from top to bottom across the motor) of up to
1500 psi at
temperatures of up to about 200 degrees C. Furthermore, a conventional stator
may
exceed 25 feet in length. Achieving suitable processability (e.g.,
flowability) in order to
injection mold the elastomer materials tends to be difficult at such lengths.
Moreover,
many rubber compounds are known to deteriorate in the presence of
hydrocarbons.
The power section of a typical Moineau style motor includes a helical rotor
disposed within the helical cavity of a corresponding stator. When viewed in
circular
cross section, a typical stator shows a plurality of lobes in the helical
cavity. In most
conventional Moineau style power sections, the rotor lobes and the stator
lobes are
preferably disposed in an interference fit, with the rotor including one fewer
lobe than the
stator. Thus, when fluid, such as a conventional drilling fluid, is passed
through the
helical spaces between rotor and stator, the flow of fluid causes the rotor to
rotate relative
to the stator (which may be coupled, for example, to a drill string). The
rotor may be
coupled, for example, through a universal connection and an output shaft to a
drill bit
assembly. Rotation of the rotor therefore causes rotation of the drill bit in
a borehole.
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.CA 02598813 2007-08-23
Conventional stators typically include an elastomeric helical cavity component
bonded to an inner surface of a steel tube. The helical cavity component in
such
conventional stators is made substantially entirely of elastomer (rubber) and
provides a
resilient surface with which to facilitate the interference fit with the
rotor. The elastomeric
material typically includes a Nitrile Butadiene Rubber (NBR) or a variation of
NBR
referred to as Hydrogenated Nitrile Butadiene Rubber (HNBR) (which is also
referred to
in the art as Highly Saturated Nitrile (HSN)). NBR and HNBR elastomers are
commonly
used owing to their chemical resistance, processability, mechanical
properties, dynamic
properties, and high temperature resistance.
The chemical and dynamic properties of NBR and HNBR elastomers are
controlled, in part, by the acrylonitrile (ACN) content of the elastomer.
Conventional
elastomers used in downhole drilling motors include about 30-40% ACN.
Elastomers
having less than about 30% ACN typically have compromised chemical resistance,
while
elastomers having more than about 40% ACN typically have inadequate dynamic
properties.
One drawback with conventional stators including an all elastomer helical
cavity
component is that a tradeoff in elastomer properties has been required. One
such tradeoff
has been between the resilience (rigidity) of the elastomer and its
processability (its
flowability during injection molding). For example, U.S. Patent 6,905,319 to
Guo states:
"processability is generally inversely related to the stiffness of the rubber.
This is
particularly true in injection-mold processes. ... Typically, a stiffer
compound will
demand much more processing power and time, thereby increasing manufacturing
costs"
(column 4, lines 4-12). Despite the potential advantages of using a stiffer
elastomer, Guo
discloses an elastomer having a hardness of about 74 on the Shore A scale
(ASTM
D2240). Guo's teaching is consistent with conventional wisdom in the art,
which suggests
that rigid elastomers (e.g., those having a Shore A hardness of about 90 as
well as other
mechanical properties described in more detail below) are not suitable for use
in downhole
stators due to inherently poor processability. The elastomeric materials in
conventional
stators typically have a hardness (Shore A) in the range from 65-75.
One significant drawback with conventional stators is that the elastomer
helical
cavity component deforms under torque loads (due to the low rigidity of the
elastomer).
This deformation creates a gap on the unloaded side of the stator lobe,
thereby allowing
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CA 02598813 2007-08-23
drilling fluid to pass from one cavity to the next without producing any work
(i.e., without
causing rotation of the rotor). This is known in the art as "RPM drop-off."
When the
torque reaches a critical level, substantially all of the drilling fluid
bypasses the stator
lobes and the rotor stalls.
Stators including a comparatively rigid helical cavity component (e.g.,
fabricated
from an elastomer lined metal or composite material) have been developed to
address this
problem. The use of rigid stator materials has been in part due to the above
described
conventional wisdom in the art and to the poor processability of known, high
modulus
rubbers. U.S Patents 5,171,138 to Forrest and 6,309,195 to Bottos et al., for
example,
disclose stators having helical cavity components in which a thin elastomer
liner is
deployed on the inner surface of a rigid, metallic stator former. The use of
such rigid
stators is disclosed to preserve the shape of the stator lobes during normal
operations (i.e.,
to prevent lobe deformation) and therefore to improve stator efficiency and
torque
transmission.
While rigid stators have been disclosed to improve the performance of downhole
power sections (e.g., to improve torque output), fabrication of such rigid
stators is complex
and expensive as compared to that of the above described conventional
elastomer stators.
Most fabrication processes utilized to produce long, internal, multi-lobed
helixes in a
metal reinforced stator are tooling intensive (such as helical broaching)
and/or slow (such
as electric discharge machining). As such, rigid stators of the prior art are
often only used
in demanding applications in which the significant added expense is
acceptable.
Other reinforcement materials have also been disclosed. For example, U.S.
Patent
6,183,226 to Wood et al. and U.S. Patent Publication 20050089429, disclose
stators in
which the helical cavity component includes an elastomer liner deployed on a
fiber
reinforced composite reinforcement material. The fabrication of composite
reinforced
stators has also proven difficult. For example, removal of the tooling (the
stator core)
from the injected composite has proven difficult due to the close fitting
tolerances and the
thermal mismatches between the materials.
Comparatively rigid (resilient) elastomer helical cavity components are also
known
in the art (e.g., having a Shore A hardness of about 90). However, as
described above,
such rigid elastomers typically suffer from poor processability and poor
dynamic
properties, which tends to result in more difficult and costly stator
fabrication and a
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'CA 02598813 2007-08-23
shortened service life of the stator. Therefore, there exists a need for a
downhole stator
having an improved elastomeric material. In particular, there exists a need
for an
elastomeric material having improved rigidity while maintaining suitable
processability
and other properties such as dynamic properties and temperature and chemical
resistance.
SUMMARY OF THE INVENTION
The present invention addresses one or more of the above-described drawbacks
of
conventional downhole drilling motors. Aspects of this invention include a
stator for use
in a downhole drilling motor. The stator includes an internal helical cavity
component
fabricated from an improved elastomeric material formulated to provide both
high
resilience and good processability. For example, in one exemplary embodiment
the
elastomer material includes at least 15 parts by weight of a phenolic resin
plasticizer per
100 parts by weight of the nitrile rubber. The phenolic resin plasticizer
preferably further
includes a hexa cross linking agent. In another exemplary embodiment, the
elastomer
material includes rheological parameters ML in a range from about 1.0 to about
4.01b=in
and MH in a range from about 75 to about 1101b=in. ML and MH are
representative of a
minimum and maximum torque as determined according to ASTM D2084 at 380
degrees
F with no preheat.
Exemplary embodiments of the present invention advantageously provide several
technical advantages. For example, exemplary embodiments of the invention
advantageously reduce the above described tradeoffs associated with elastomer
material
selection (in particular in regard to resilience and processability). As such,
stators in
accordance with this invention may exhibit improved efficiency (and may thus
provide
improved torque output) as compared with conventional stators without
substantially
increasing manufacturing costs. Moreover, stators in accordance with this
invention may
provide comparable torque output with stators including rigid metallic lobes,
but at
significantly reduced expense. An additional benefit of exemplary embodiments
of the
invention is higher temperature capability due to reduced internal heat
generation in the
center of the lobe. Reduced heat generation also tends to reduce elastomer
breakdown in
the lobes and thereby prolong service life of the stator.
In one aspect, this invention includes a Moineau stator for a drilling motor.
The
stator includes an outer tube and a helical cavity component deployed
substantially
coaxially in the outer tube. The helical cavity component provides an internal
helical
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CA 02598813 2007-08-23
cavity and includes a plurality of internal lobes. The helical cavity
component further
includes an elastomeric material, the elastomeric material including (i) a 33-
3 nitrile
butadiene rubber having about 30 percent by weight acrylonitrile and a Mooney
viscosity
of about 30, (ii) at least 60 parts by weight carbon black per 100 parts by
weight of the
nitrile rubber, and (iii) at least 15 parts by weight phenolic resin
plasticizer per 100 parts
by weight of the nitrile rubber, the phenolic resin plasticizer further
including a hexa cross
linking agent.
In another aspect, this invention includes a Moineau stator for a drilling
motor.
The stator includes an outer tube and a helical cavity component deployed
substantially
coaxially in the outer tube. The helical cavity component provides an internal
helical
cavity and includes a plurality of internal lobes. The helical cavity
component is
fabricated from an elastomeric material, the elastomeric material including a
nitrile rubber
having from about 30 to about 40 percent acrylonitrile. The elastomeric
material further
includes rheological parameters ML in a range from about 1.0 to about 4.01b=in
and MH in
a range from about 75 to about 1101b=in, wherein ML and MH are representative
of a
minimum and maximum torque as determined according to ASTM D2084 at 380
degrees
F with no preheat.
In still another aspect, this invention includes a method for fabricating a
stator.
The method includes providing an elastomeric compound including a nitrile
rubber having
from about 30 to about 40 percent acrylonitrile. The elastomeric compound
further
includes rheological parameters ML in a range from about 1.0 to about 4.01b=in
and MH in
a range from about 75 to about 110 lb=in, wherein ML and MH are representative
of a
minimum and maximum torque as determined according to ASTM D2084 at 380
degrees
F with no preheat. The method further includes injecting the elastomeric
compound into a
tubular stator housing to form a helical cavity component, the helical cavity
component
providing an internal helical cavity and including a plurality of internal
lobes.
The foregoing has outlined rather broadly the features and technical
advantages of
the present invention in order that the detailed description of the invention
that follows
may be better understood. Additional features and advantages of the invention
will be
described hereinafter, which form the subject of the claims of the invention.
It should be
appreciated by those skilled in the art that the conception and the specific
embodiments
disclosed may be readily utilized as a basis for modifying or designing other
structures for
CA 02598813 2007-08-23
carrying out the same purposes of the present invention. It should also be
realize by those
skilled in the art that such equivalent constructions do not depart from the
spirit and scope
of the invention as set forth in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of the present invention, and the advantages
thereof, reference is now made to the following descriptions taken in
conjunction with the
accompanying drawings, in which:
FIGURE 1 depicts a conventional drill bit coupled to a Moineau style drilling
motor utilizing an exemplary stator embodiment of the present invention.
FIGURE 2 is a circular cross sectional view of the Moineau style stator as
shown
on FIGURE 1.
FIGURE 3 plots RPM versus pressure drop for an exemplary embodiment of a
downhole drilling motor in accordance with the invention. The exemplary
drilling motor
of this invention is compared with conventional drilling motors; one including
an
elastomeric helical cavity component and another including a rigid metal
reinforced
helical cavity component.
DETAILED DESCRIPTION
As described above, conventional Moineau drilling motors have used an
elastomeric helical cavity component bonded to a steel housing. However, due
to the
behavior of the selected elastomer material in various competing conditions,
there have
been inevitable tradeoffs in the choice of a desired elastomer material. Such
tradeoffs
typically result in the selected elastomer having at least one less-than-
optimal material
property (e.g., lower-than-desired resilience, suboptimal processability,
and/or inadequate
dynamic properties) and as described above, these tradeoffs tend to compromise
various
stator fabrication and/or performance metrics.
Lower than desired elastomer resilience results in inadequate torque
transmission. As described above, elastomeric materials with insufficient
resilience
undergo excessive deformation at high torque loads (due to the low rigidity of
the
elastomer), which allows drilling fluid to pass from one cavity to the next
without
producing any work. The result is a loss in rotor RPM (and therefore drill bit
RPM). In
severe conditions the rotor can stall in the stator. Several material
properties may be
measured to determine the resilience of an elastomeric material. Such
properties include,
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elastic modulus (e.g., at tensile strains of 25 and 100%), compression modulus
(e.g., at
compressive strains 5, 10, and 15%), and hardness (Shore A).
While increased elastomer resilience is known to reduce RPM drop-off (thereby
improving torque transmission), it is also known to degrade elastomer
processability. As
described above in the Background section, conventional wisdom in the downhole
drilling
industry suggests that resilient elastomer materials are not suitable for
downhole stators
due to inherently high viscosity (poor flowability of the pre-cured elastomer)
at
conventional injection molding temperatures. The processability of the
elastomer is
particularly important in longer and/or smaller diameter stators. Longer
stators (e.g.,
greater than 20 feet) are often used in an attempt to minimize RPM drop off.
Smaller
diameter stators (e.g., less than four inch diameter) are commonly used in
side tracking or
other coiled tubing applications. It is known to those of skill in the art
that increasing
stator length and decreasing lobe diameter significantly increase the required
pressure and
time (and therefore expense) required to fabricate a stator via injection
molding.
One measure of processability commonly used in the art is a property referred
to
as Mooney viscosity (e.g., measured according to ASTM D1646). Mooney
viscosities in
the range from about 20 to about 60 are sometimes considered to provide
suitable
processability. However, such measurements can be difficult and time
consuming.
Rheological properties can also be used to determine both the processability
and the
resilience (rigidity) of an elastomer. For example, the minimum torque, ML, as
determined via ASTM D2040, tends to be a good indicator of elastomer
processability,
while the maximum torque, MH, tends to be a good indicator of elastomer
resilience. An
elastomer typically has good processability (suitable flowability at
conventional injection
molding temperatures) when ML is in the range from about 1.0 to about 4.0
lb=in when
measured at 380 degrees F with no preheat. High elastomer resilience (for
reducing RPM
drop-off) is typically indicated when MH is in the range from about 75 to
about I 10 lb=in
as also measured at 380 degrees F with no preheat. Conventional stators
typically have an
MH of about 55 lb=in or less.
Often increasing the resilience of an elastomer also degrades the dynamic
properties of the elastomer. Such degradation of the dynamic properties is
known to cause
localized heating of the elastomer lobes due to the viscoelastic behavior of
the elastomer
(and its poor thermal conductivity). This in turn can result in thermal
degradation of the
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elastomer and ultimately in failure of the stator (due to a phenomenon
referred to in the art
as "chunking" in which the stator lobes become embrittled and subsequently
crack and
tear apart). The dynamic properties are typically determined in the art by
measuring a
quantity referred to as tanS, which is the ratio of the loss (or viscoelastic)
modulus to the
storage (or elastic) modulus. Increasing tanS typically indicates increasing
viscoelastic
behavior and therefore degraded dynamic properties. While there is no
universally agreed
upon industry standard measurement technique for determining tan8, the
Applicant has
found that a 250 degree F tanS value as determined in an RPA, after cure
temperature
sweep at a frequency of 10 Hz and a strain of 7% provides a suitable
indication of the
dynamic properties of a stator elastomer for use in a downhole stator. TanS
values of less
than about 0.25 typically indicate suitable dynamic properties; however, the
Applicant has
also found that stators employing highly resilient elastomers can accommodate
somewhat
compromised dynamic properties via reducing the strain in the interference fit
between
rotor and stator.
With reference now to FIGURES 1 and 2, one exemplary embodiment of a
Moineau style power section 100 according to this invention is shown in use in
a
downhole drilling motor 60. Drilling motor 60 is coupled to a drill bit
assembly 50 in a
configuration suitable for drilling a subterranean borehole, such as in an oil
and/or gas
formation. Drilling motor 60 includes a helical rotor 150 deployed in the
helical cavity of
Moineau style stator 105. The rotor 150 it operatively positioned in the
cavity to
cooperate with the plurality of lobes. Applying fluid pressure to the cavity
causes the
rotor 150 to rotate in cooperation with the lobes in order to allow
pressurized drilling fluid
that is introduced at an upper end of the stator 105 to be expelled at the
lower end and
subsequently exhausted from the drill bit into a borehole. Rotation of rotor
150 causes
drill bit 50 to rotate in the borehole.
With reference now to FIGURE 2, power section 100 is shown in circular cross
section, as shown by the section lines on FIGURE 1. Moineau style stator 105
includes an
outer stator tube 140 (e.g., a steel tube) retaining an elastomeric helical
cavity portion 110.
Helical cavity portion 110 is shaped to define a plurality of helical lobes
120 (and
corresponding grooves) on an inner surface thereof. In the exemplary
embodiment shown,
the differing helical configurations on the rotor and the stator provide, in
circular cross
section, 4 lobes on the rotor and 5 lobes on the stator. It will be
appreciated that this 4/5
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-CA 02598813 2007-08-23
design is depicted purely for illustrative purposes only, and that the present
invention is in
no way limited to any particular choice of helical configurations for the
power section
design.
With continued reference to FIGURES 1 and 2, helical cavity component 110 is
fabricated from an improved elastomeric material that, despite the teachings
and
conventional wisdom in the art, is formulated to be both rigid and
processable. In one
exemplary embodiment the elastomer material includes rheological parameter ML
in the
range from about 1.0 to about 4.01b=in and parameter MH in the range from
about 75 to
about 1101b=in as determined via ASTM D2040 at 380 degrees F with no preheat.
In
other exemplary embodiments ML may be in the range from about 1.0 to about
3.51b=in or
even 1.0 to 3.01b=in at 380 degrees F with no preheat. Advantageous
embodiments may
also include one or more of the mechanical properties in one of the ranges
shown in Table
1.
TABLE I
Elastomeric Property Preferred Most Preferred
Range Range
25% Tensile Modulus (psi) > 400 550 - 750
100% Tensile Modulus (psi) > 800 900 - 1200
5% Compression Modulus (psi) > 100 110 - 150
10% Compression Modulus (psi) > 200 225 - 325
15% Compression Modulus (psi) > 300 350 - 475
Hardness (Shore A) > 85 88 - 94
In one exemplary embodiment, elastomer formulations including Nysyn 33-3
nitrile butadiene rubber (having 33 percent acrylonitrile and a Mooney
viscosity of 30), at
least 15 parts of a phenolic resin plasticizer per 100 parts nitrile rubber,
and at least 60
parts carbon black per 100 parts nitrile rubber have been found to have both
desirable
resilience and processability (e.g., ML in the range from about 1.0 to about
4.0 and MH in
the range from about 75 to about I 10). Such formulations have also been found
to have
desirable dynamic properties (e.g., a 250 degree F tan8 value of less than
about 0.25).
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Table II lists exemplary formulations A, B, C, and D in accordance with the
present invention as well as a prior art formulation STD. It will be
appreciated that this
invention is not limited by the precise formulations listed in Table II. The
artisan of
ordinary skill will readily recognize that the various components in those
formulations
may be substituted with suitable equivalents. In the exemplary embodiments
shown,
Akrochem P55 phenolic resin is utilized. It will be appreciated that the
invention is not
limited to any particular phenolic resin. It will also be understood that
Akrochem P55 also
includes from about 6.5 to about 8.5 percent of a hexa cross-linking agent.
TABLE II
Formulation STD A B C D
NYSYN 33-3 100.00 100.00 100.00 100.00 100.00
ASD 75 - 75% Sulfur in NBR 4.80 4.80 4.80 4.80 4.80
911 C- 85% ZnO in NBR 5.00 5.00 5.00 5.00 5.00
Stearic Acid 1.00 1.00 1.00 1.00 1.00
Agerite Resin D 3.00 3.00 3.00 3.00 3.00
DUSANTOX 6 PPD 2.00 2.00 2.00 2.00 2.00
N774 Ultra Carbon Black 60.00 60.00 60.00 80.00 100.00
Cumar - R13 15.00 -- -- -- 15.00
Akrochem P55 Phenolic Resin 10.00 15.00 25.00 25.00 10.00
Diisodecyl Phthalate 10.00 15.00 10.00 10.00 10.00
Paraplex G25 5.00 7.50 5.00 5.00 5.00
50% PVI in SBR 1.00 1.00 1.00 1.00 1.00
PB(OBTS)75 2.00 2.00 2.00 2.00 2.00
PB(TMTM)75 0.15 0.15 0.15 0.15 0.15
TOTAL 218.95 216.45 218.95 238.95 258.95
Table III lists characteristic properties measured for the formulations listed
in
Table II. These properties were determined in accordance with the test
methodologies
listed in Table IV.
=CA 02598813 2007-08-23
TABLE III
Elastomeric Property STD A B C D
Tensile Strength (psi) 2294 2093 2120 1749 2209
Ultimate Elongation (psi) 381 303 252 259 294
25% Tensile Modulus (psi) 210 323 511 695 366
100% Tensile Modulus (psi) 478 701 991 1093 873
5% Compression Modulus (psi) 56 84 108 122 --
10% Compression Modulus (psi) 111 170 224 276 --
15% Compression Modulus (psi) 171 261 344 423 --
Tear Strength (lb/in) 203 219 237 234 194
Hardness (Shore A) 75 84 88 91 88
Rheological Parameter ML (lb=in) 2.3 2.8 3.0 3.3 3.4
Rheological Parameter MH (lb=in) 63 73 88 80 68
TanB at 250 F 0.15 0.18 0.20 0.23 0.24
TABLE IV
Elastomeric Property Test Method
Tensile Strength (psi) ASTM D412, Die C
Ultimate Elongation (psi) ASTM D412, Die C
25% Tensile Modulus (psi) ASTM D412, Die C
100% Tensile Modulus (psi) ASTM D412, Die C
5% Compression Modulus (psi) ASTM D575
10% Compression Modulus (psi) ASTM D575
15% Compression Modulus (psi) ASTM D575
Tear Strength (lb/in) ASTM D624 Die C
Hardness (Shore A) ASTM D2240
Rheological Parameter ML ASTM 2084, 380 F no preheat
Rheological Parameter MH ASTM 2084, 380 F no preheat
Tan8 at 250 F RPA Aftercure, 10 Hz, 7% strain
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CA 02598813 2007-08-23
With reference now to FIGURE 3, the performance of three exemplary drilling
motors is contrasted at a flow rate of 600 gallons per minute. The three
drilling motors
were each sized and shaped in accordance with Dyna-Drill Model No. DD675783.0
having a length of 125 inches, an outer diameter of 6.75 inches, and a 7/8
inch lobe. The
drilling motors differed only in the materials used to fabricated the helical
cavity
component of the respective stators: (i) the conventional elastomer stator
being fabricated
with elastomer STD in Table II, (ii) the stator in accordance with this
invention being
fabricated with elastomer C shown in Table II, and (iii) a prior art stator
having a Rigid,
metallic helical cavity component with an elastomeric liner deployed on an
inner surface
thereof.
FIGURE 3 plots RPM versus pressure drop (psi) from the top to the bottom of
the stator. As shown, the drilling motor including elastomer C in accordance
with this
invention advantageously undergoes significantly reduced RPM drop off as
compared to
that of conventional drilling motor STD. For example, at a pressure drop of
1000 psi
drilling motor C (including elastomer C) exhibits an RPM drop of only about 45
rpm
versus an RPM drop off of about 105 rpm for the conventional stator (including
elastomer
STD). The performance of drilling motor C even compares favorably with prior
art
drilling motors including a stator with an elastomer lined, rigid metallic
helical cavity
component (an RPM drop off of 45 rpm versus 30 rpm at 1000 psi).
Exemplary embodiments of this invention advantageously obviate the need for
the above described tradeoff in elastomer rigidity and processability.
Moreover,
exemplary embodiments of this invention may even obviate the need for stators
having
rigid, metallic helical cavity components (except perhaps in the most
demanding
applications).
Although the present invention and its advantages have been described in
detail,
it should be understood that various changes, substitutions and alternations
can be made
herein without departing from the spirit and scope of the invention as defined
by the
appended claims.
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