Note: Descriptions are shown in the official language in which they were submitted.
104439S
- Background of the Invention
.
This invention relates to composite materials. ~ -
More specifically, this invention relates to composite
friction materials which exhibit high, stable, coef~icients
of friction over a wide temperature range.
The elastomeric materials heretofore proposed for
use as friction materials have generally proven to be un-
satisfactory when exposed to high ambient working temperatures
such as encountered, for example, in clutch and brake
applications in heavy duty service vehicles. Typically,
such materials have been based on heat-hardenable resins
such as phenol-aldehyde resins which tend to heat-decompose
under the high peak and bulk temperature conditions created ~
by the sustained and/or heavy loading forces experienced in -
the clutch and brake systems of these vehicles while
operating. As a result of this decomposition, the physical
properties of these materials typically deteriorate, and the
consequent softening of the material and dispersal of the
products of heat decomposition generally interfere with the -
20 functioning of the friction unit. Furthermore, many times ~;
after friction material comprising a partially heat-decomposed ~'
heat-hardenable resin has cooled, the material will exhibit
a less coefficient friction than did the original material. `
These conditions, as well as other problems
associated with these and similar friction materials,
result in a loss of efficiency in the friction unit and - -
unreliability in the service vehicle, which is highly -
undesirable.
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Many attempts have been made to obviate the
problems associated with the polymers in general use as
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~ 1044395
friction material basis. Many different resins have been
experimented with, in attempts to obtain a friction material
which possesses a high, stable coefficient of friction over
a wide temperature range. Modification of the heat-hardenable
resins with other polymeric materials has been attempted.
Many of these friction material formulations have not performed
well. Other formulations have required multi-step procedures -
which are costly in terms of labor and frequently in terms
of the materia~ used in these formulations.
Importantly, also, many o~ these known friction
materials require a bonding agent to affix them to the ; -
backing plate or "core" portion of the friction element.
This requirement severely restricts the scope of the molding
methods and mold configurations employable in forming these
friction elements. In injection molding, for example,
the bonding agent is subject to scuffing during the molding ~;~
process, which deactivates or destroys the bond and renders
this molding process useless with these friction elements.
In general, where bonding agents must be utilized, only
compression molding and relatively simple mold configurations
can be employed in the process of molding the friction
element. ;
In order to obtain a friction material with a
usefully high coefficient of friction which is stable over
a wide temperature range, the industry has most usually ~--
used nonresilient inorganic friction materials such as
sintered bronze. Although the friction characteristics
of this and similar metallic materials have been generally
satisfactory under high temperature conditions, the high ~;
modulus or lack of resiliency of these materials and their
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104439S
resultant inability during operation to conform to the
friction element mating surface and absorb adequate energy
result in relatively high wear rates and shortened life.
Furthermore, great care must be taken in the type of oil
used in conjunction with such friction materials during
use to ensure that the desired coefficient of friction is
not impaired.
It is therefore an object of this invention to
provide a friction material composition with a high, stable -
10 coefficient of friction over a wide range of dynamic ~ -
operating conditions.
It is a further object of this invention to
provide a friction material composition with high dynamic
and static coefficients of friction over a wide temperature
range.
It is a further object of this invention to
provide a friction material composition which can readily
. :. .
be bonded to a metal core material. ~:
It is an additional object of this invention to
20 provide a friction material composition which can be ~ -
compression molded, and which can be molded in conjunction
with complex mold configurations.
. -: ....
It is also an object of this invention to provide
a conformable, long-wearing friction material composition
with a high, stable coefficient of friction over a wide
temperature range.
Other objects and purposes of this invention will
be apparent to those skilled in the art from the disclosure
: .. ..
contained herein. --
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104~39S
Brief Summary of the Invention
According to one aspect of the invention there is
provided as an article of manufacture a friction material
formed of 20% to 60% by weight of a matrix comprising a
fluoroelastomer intimately intermixed with a polyacrylate
elastomer in a weight ratio of fluoroelastomer to poly- .
acrylate which falls within the range from about 11:1 to
about 1:11, at least 5~ by weight of said 20% to 60% by .';
weight total matrix being said fluoroelastomer, from about
12 to about 40~ by weight of carbon black; and from about
20% to about 60~ by weight of vitreous or ce~.amic particles
of from about .0001" to about .005" in effective diameter
intermixed with and dispersed throughout said friction ~ ~
material in mechanically held nonbonded relation to provide ,.:, .
a friction surface of said article of manufacture including : :
the said matrix and said particles.
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-: 104439S
~.ccording to another aspect of the invention there is ~,,
provided a friction material composition comprising 20% to 6Q% by ,;~
weight of a matrix consisting essentially of a copolymer of hexa-
fluoropropylene and vinylidene fluoride intimately intermixed with
a polyacrylate elastomer in a weight ratio of fluoroelastomer to
polyacrylate which falls within the range from about 11:1 to about
. . .
1:11, at least 5% by weight of said 20~ to 60~ by weight total ,~ '
matrix being said fluoroelastomer, from about 12 to about 40% by ' ' '
weight of carbon black; and from about 20 to about 60% by weight
of vitreous or ceramic particles having a minimum size of .0001"
which are dispersed throughout the matrix in mechanically held . ,
nonbonded relation and assuring a relatively hi~h surface con- ''~
centration of the particles to provide a friction surface of ~,; '
said friction material including said matrix and said particles.
l'he fluoroelastomer-polyacrylate matrix has excellent
properties of thermal stability, and at-the same time provides a ,,' ,~',
relatively low modulus resilient matrix which permits the friction ''~
.. . . .
material to conform readily to inherently rapid changes between it '' '
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and its mating surface, thereby distributing dynamic stresses,and ,~' '
20 energy absorption over a much larger true friction-surface area ~ '- '
than is permitted with high modulus metallic or other non-resilient ,
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~laxim~lm energy absorption rates of from about 3 to -
about 5 HP/in of fluoroelastomer friction material are typical
In comparison with these hiyh modulus materials, such a low
modulus matrix significantly increases the load-carrying
capabilities of the friction element of which it is a part,
and further, possesses superior wear characteristics when
compounded with particles of vitreous or ceramic material as
herein disclosed.
The particles of vitreous or ceramic material are
compounded with the matrix (intermixed with and dispersed
throughout the matrix in mechanically held nonbonded relation
to said matrix) in sufficient quantities to produce a relatively
high concentration of those particles on the frictional surface
of the matrix. In addition to acting as the friction-producing
agent in the friction material, these particles further serve to
strengthen the support matrix and lessen compression set or
~ermanent deformation under applied loads.
The compounded friction material is then applied to
the core of the friction element, for example as disclosed in
20 U.S. Patent No. 4,036,668 by William D. Brandon of common
assignment herewith.
Description of the Drawing
Figure 1 illustrates in greatly enlarged-view a section
through the improved friction material ~isposed adjacent a
substantially flat reaction plate with a relatively -
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- thin energy absorbing oil film therebetween and with a metal ~ -
backing plate bonded thereto.
Figure 2 illustrates in greatly enlarged view
the surface of the improved friction material a~ter this
surface has been "worn in". The ~itreous or ceramic part-
icles are noted to have flattened upper sur~aces produced
upon "wearing in" of the material.
Detailed Description of the Invention
This invention provides a fluoroelastomer-polyacry-
late based material having vitreous or ceramic particles
.: .
dispersed therethrouqh. This composite material exhibits
tensile strengths comparable with the fluoroelastomer-
polyacrylate matrix material alone, but howe~er exhibits -
better set and stress relaxation resistance than the matrix ^ ~ -
alone. ~ - ~
.... ... .
The fluoroelastomers useful in this invention as -
part of the matrix material are exemplified by Viton E60C,
(E . L . Dupont Inc., Wilmington, Delaware), a copolymer o~
hexafluoropropylene and vinylidene fluoride, and a similar
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copolymer Fluorel FC2170 (The 3M Company, of Minneapolis,
Minnesota). Preferably, Viton~E60C or Fluorel FC2170 are
employed to form the matrix of the friction material.
The polyacrylates useful in this invention as
part of the matrix material are thermosetting elastomers of
acrylic acid and its esters having a repeating structural
formula -CH2 - CH (COOR) - where R is hydrogen or a low
molecular weight alkyl group having one to six carbon atoms, -~ !,
e.g., methyl, ethyl, or one of the propyl, butyl, pentyl,
or hexyl isomers. Generally, the ethyl, propyl and butyl
30 esters are preferred. Mixed polyacrylates, i.e., those - ;
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where some of the R groups are different than others, e.g.,
some are ethyl and some are butyl are quite usable. Poly-
acrylates wherein R is ethyl and also wherein some R is
ethyl and some R is butyl have very desirable properties.
The monomers generally polymerize easily in the presence
of heat, light or catalysts, e.g., benzoyl peroxide or the ;
like. The inclusion of acrylic anhydride, glycol esters ~ -
or acrylic or methacrylic acid or acrylamide is advantageous ~-
in assuring that the resulting resin is insoluble and '
thermosetting. The presence of some acrylonitrile may also
be desirable to adjust resiliency.
To form the composite material of the invention,
the matrix is compounded with particles of a vitreous or
ceramic hard material. These particles are preferably
in the form of very small beads, fibers or other irregular
shapes.
Although the useful size of these particles
may vary somewhat according to the nature of the material
and other factors, glass fiber particles of from about
.0001" to about .005" in diameter, and preferably about
0.0005" in diameter, will yield the desired results. Such~ ~`
particles advantageously have a length to diameter ratio
of from about 3 to about 10,000. They have melting points ~-
in the very general neighborhood of 1400F. Other useful -
particles include ceramics such as commercially available
Fiberfrax which have melting points very generally in the
neighborhood of 3200F. The glass fiber or other particles ~`
may also be compounded in the form of chips, fibers,
spheres or other convenient shapes, although fibérs are
30 generally preferable. ,
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~04~39S
The particles are compounded with the fluoro-
elastomer-polyacrylate matrix at a rate sufficient to give
and maintain a high surface concentration of particles in
the finished friction facing. Preferably, about 20 to about
50~ by weight of fiberglass particles to about 20 to about
60% by weight of matrix are admixed to provide a randomly
irregular macroscopic surface finish on the friction --
material. It may in some instances, however, be desirable
to exceed these proportions, depending on the frictional` ~ -
characteristics desired in the finished material.
Generally, the matrix will include fluoroelastomer
and polyacrylate in weight ratio from about 1:11 to about 11:1.
The preferred weight ratio will fall within the range from ~
about 1:5 to about ~:1. Further, such matrix will generally A7_~ ",'.'.,,, .'
include at least about 5 weight per cent tof the 20-60 weight - ~
per cent total matrix) of fluoroelastomer. Thus, matrices -~-
having from about 5 weight percent fluoroelastomer and about
55 weight percent polyacrylate to matrices having from about
55 weight percent fluoroelastomer to about 5 weight percent
polyacrylate are contemplated as falling within the scope
of the invention.
It is contemplated that carbon black will be
incorporated into the compound matrix, conveniently at the
same time the vitreous or ceramic and the fluoroelastomer
and polyacrylate portions of the matrix are incorporated.
The additive is preferably added in amounts of about 12
to about 40% by weight of carbon black to about 20 to about
60% by weight of matrix. Additionally, accelerators,
stabilizers, and curing agents, inter alia, commonly used
3a in fluoroelastomer and/or polyacrylate products, will usually
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10~439S
f be compounded with the matrix.
The vitreous or ceramic particles, carbon black,
and other additives are incorporated into the matrix by
conventional mixing techniques, for example, in a Banbury
mixer. Ideally, the vitreous or ceramic should be concentra-
ted near the surface, or the frictionally active portion, of
the matrix. However, in practiaality this is difficult to
achieve, and satisfactory results are obtained by intimately
incorporating the particles throughout the matrix to obtain -`
a random orientation of the particles through the matrix.
The matrix may be bonded to a core of steel or
other metal by the process of U.S. Patent 4,036,668 noted
above. Broadly, this process comprises incorporating CaO into
the matrix prior to curing, and then at high *emperatures
curing the matrix in pressed contact with the core material.
Conveniently, the CaO may be incorporated into the matrix
at the same time as are the particles and other additives
noted above.
Generally, the friction material of the invention
can be formed by high shear blending together of the solid
particles of the fluoroelastomer component having its -
appropriate accelerators, stabilizers and curing agents
previously dispersed therethroughout with solid particles
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of the polyacrylate component having its appropriate
accelerators, stabilizers and curing agents previously
. . .
dispersed therethroughout to form a homogeneous uniform
intimate codispersion o the fluoroelastomer and the
: . . .
polyacrylate components. The carbon black, vitreous or
ceramic particles and CaO components can be either previously
blended with each of the fluoroelastomer component and the
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1~44395 ~:
polyacrylate component or can be added during the step of ~ -
blending together the fluoroelastomer with the polyacrylate.
Usually at least the carbon black will ~e previously dispersed
throughout both of the fluoroelastome~ and polyacrylate
components since each of these components are commercially
available in such a form.
It should be noted that the accelerators, stabili~
zers and curing agents of the fluoroelastomer are generally
different than the accelerators, stabilizers and curing
agents of the polyacrylate. Thus it is rather surprising
that a composite friction material using a mixture of these ~
two components is sufficiently structurally sound to exhibit -
good frictional and structural characteristics under heavy
frictional usage. Because of the considerably different ~-
accelerators, stabilizers and curing agents generally used
with the fluoroelastomer and polyacrylate components it
is believed that at most a minor a~ount of cross-linking
occurs between the two polymer systems. Yet, the resulting
friction material has both good frictional and structural
characteristics as previously mentioned. ~he friction
material further exhibits a good thermal operating range -
although containing significant amounts of the non- `
halogenated polyacrylate component.
Conventional molding techni~ues, such as compres-
sion transfer or injection molding, are utilized for forming ~
the matrix/backing plate friction element. In applying the ~ -
friction material to the backing plate of the friction
element, it is usually desirable to apply the friction ~
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material to the plate in an amount sufficient to obtain
3a a finished thickness of friction material of from about ;~
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~044395
0.020 to about 0.250 inches, especially in applications
where the material is utilized in clutches.
The friction material of this invention exhibits
a high, stable, dynamic coefficient o~ friction through a
wide range of sliding speeds and normal loads against a ~t
wide variety of opposing faces and finishes. For example,
dynamic friction coefficients (~D) of from about 0.14 to
about 0.06 at from about 2,000 to 11,000 ft/min sliding
speed and from about 50 to about 680 psi of face pressure
on gross area typically can be expected in friction elements
comprised of the friction material of this invention.
Additionally, good static ("breakaway") coe~ficients
of friction from about 0.17 to about 0.26 are characteristic
of this friction material.
~he friction material of this invention is capable -
of operating against mating surfaces of a variety of types,
for example, hard or soft steel, cast iron, sintered metals, ~-
and ground, deburred or lapped surfaces. However, the mating ~ -
surface finish may adversely affect the friction character-
., ~ .
2a istics of the friction material if this surface is too ~-~
roughly or too finely finished. Generally, a mating ~
surface finish of about 10 to about 45 m~ will result in -~ --
satisfactory performance of the friction material. ';
The friction material of this invention is furthex
;
characterized by low wear and dimensional stability can be
expected during extended dynamic operation. Furthermore, -
the material exhibits a relatively flat torque curve that
"wrings in" about 10 - 50% above the dynamic torque. -
The friction material of this invention will
respond according to test results over a wide operating
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4~395
surface temperature range. In general, the material can
be expected to maintain optimum response levels at bulk
temperatures below about 470~ F; i.e., where the average
_ ~ ,.. ~. .
surface temperature of the friction material between
operations of the friction element is below about 475 F.
Maximum peak temperatures, however, may be as high as
from about 560 F to about 680 F before performance of
the friction material is substantially affected.
In general, effective performance of the friction ~ --
material contemplates operation of the friction element
under oil cooled operating conditions. However, a much
.
wider selection of oils may be effectively employed with ~-
the friction material than with, for example, bronze.
In preparing friction elements utilizing the
friction material of this invention, it will generally be
.
found that after demolding, few if any of the vitreous or
ceramic particles will be present on the frictional surface
of the material. The thin elastomer coating covering the
part~icles must therefore be worn off to expose the particles
and hence to obtain a stable coefficient of friction for
the element. This may either be done in situ, allowing the -
rubber coating to be worn off during an initial break-in ~ -i
period of the friction element in the service vehicle, or
by pregrinding of the friction material before installation i
of the element. The amount of matrix material which must ~ ~
be removed to obtain a desirably stable coefficient of -
friction for the material as a whole will of course vary ;;
according to the specific formulation. However, it is -
generally advantageous to sufficiently expose a major
3~ portion of the underlying particles to a point where these
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1044395
particles are in contact with the grinding or mating
surface. -
During early use, these particles are ground to
a point where they appear to be well-worn, as shown in
FIGS. 1 and 2, to obtain a stable coefficient of friction.
The particles appear to be mechanically bonded, i.e.,
mechanically held but not chemically bonded in the matrix.
The flouroelastomer-polyacrylate elastomer matrix
of the present invention has the distinct advantage of being ~-
10 reboundably deflecting at normal use temperature, as for -
example when in use as part of a clutch plate. Normal use
temperature, as discussed previously include average
surface temperatures between operations of generally below
about 475 F. Generally, normal use temperatures will be
at least about 180 F in frictional operation. Thus, the -
matrix is reboundably deflecting and resilient in the
temperature range from about 180 F to about 475~ F. The -
term reboundably deflecting as used herein means that the
particles which are at the surface of the material are -~
pushéd or deflected thereinto during contact with a mating
reaction plate as in a clutch but then rebound back as the -
material resumes its natural or unstressed state when the -
mating reaction plate is removed from contact therewith. :-
It is clearly of great advantage to have a friction material
that i9 reboundably de~lecting at its use temperature yet
i9 not easily or quickly worn away (due to the particles
since this allows for a controlled and relatively smooth ~- ~
change in friction as pressure is applied between the - ~ -
material and a mating reaction plate plus long wearing
30 characteristics. -
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104439S
The structure and operation of the friction
material of the present invention may be still better
understood by reference to the figures of the drawing
wherein like numbers denote like parts throughout. A
friction material 10 in accordance with the present in~ention --
is illustrated as including a matrix 12 of previously ~ -
mentioned composition with particles 14 suspended mechanica-
lly therein in non-bonded relationship thereto. The friction ~ ;
material 10 is bonded to a metal backing plate 16. In
operation, the surface of the friction material 10 removed
from the backing plate 16 faces a mating plate 18 with a -
. ,.- .,
fluid layer 20 generally therebetween. In operation in a
clutch, the mating plate 18 and the facing surface of the
friction material 10 are forced towards one another while
rotating relative to one another. Thus, a high shear is
. ~.. ~., .
introduced in the fluid layer 20-whereby a substantial
amount of the energy of clutch engagement is absorbed. On
contact, a top portion 22 of each protruding one of the
particles 14 touches the mating plate 18 and the particles ~ -
.. :~ .:~, .
20 14 thus tend to flatten as illustrated in Figure 2. Also, -~
~,. . .. .
due to the reboundably deflecting character of the matrix ; -
12 at use temperature the protruding particles 14 are
pushed down against the matrix 12. On release of pressure,
as when the mating plate 18 is moved away from the friction ;~
material 10, the particles 14 spring up under the impetus
of the resilient matrix 12 and return-generally to their
original protuberance above said matrix 12. Thus, relatively
smooth clutch engagement occurs, first via the fluid 20
shear, then via the particles 14 during their partial
retraction into the matrix 12 and finally via the direct
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1~44395
support contact of the particles 14 with the mating plate
18.
The following example is provided only to further
illustrate specific friction material compositions of this
invention and pertinent frictional characteristics thereof, .~
without limiting the invention in any manner: : :
Example .~
Ingredients Amount (Parts by Weight) : :
- : :'
Formula IFormula II - :
Viton*E60 50 Parts70 Parts -
Polyacrylate 50 Parts30 Parts
Fiberfrax 30 Parts110 Parts .
Carbon Black 50 Parts57 Parts
Accelerators ) ~: .
Stabilizers ) Minor amounts Minor amounts :;.
Curing Agents ) . ~.
CaO 5 Parts 5 Parts ~
The clutch plate having Formula I affixed thereto : :
exhibited dynamic coefficients of friction of 0.114 at 7000
fpm (feet per minute) and 50 psi, 0.086 at 5000 fpm and 250 - ~- :
psi and~0.071 at 7000 fpm and 250 psi. The failure point of : -
this clutch plate was above ll,000 fpm. The wear of the :
friction material on this plate at 7000 fpm and 250 psi was
1 mil. Wear was measured by a screening test comprising 120
to 200 cycles of break in at 5000 fpm and 100 psi. (until the
dynamic coefficient of friction stabilized) followed by a ~. :
cycle of 15 clutch engagements each at 50 psi, 150 psi and
250 psi at 3000 fpm, then a cycle of 15 clutch engagements `;.:
each at 50 psi, 150 psi and 250 psi at 5000 fpm, and then a
30 * Trademark - 15 - ; :
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cycle of 15 clutch engagements each at 50 psi, 100 psi, 150 - -
psi, 200 psi and 250 psi at 7000 fpm. The clutch was
periodically checked at 5000 fpm and 100 psi to assure that
no significant change in dynamic coef~icient of friction
had occured. Wear values obtained were generally good to
about - 0.3 mil.
The clutch plate having Formula II affixed thereto
exhibited dynamic coefficients of friction of 0.089 at
7000 fpm and 50 psi, 0.088 at 5000 fpm and 250 psi and 0.081 ~ ~ -
at 7000 fpm and 250 psi. The failure point of this clutch
plate was not measured. The rate of wear of the friction - ~-
material on this plate at 7000 fpm and 250 psi was O mil.
In each case the Formulas were compounded by
mixing in a Banbury mixer to achieve an even dispersion `~
of the additives into the matrix and of the two components
(the fluoroelastomer and the polyacrylate) of the matrix ~ -~
into one another with random orientation of the (Fiberfrax) -
.. ..
particles.
Each Formula mixture was applied to a steeI
backing plate and pressed to this plate into the derired
pattern under about 2,500 psi. Formula I then was cured
for 30 minutes at 335 - 340F and Formula II for 10
minutes in the same temperature range. Sufficient mixture
material was applied to each plate to give a thickness of
material, when cured, of .050 inches/face. The cured -
elastomer and backing plate, i.e., clutch disc, were then
postcured at 450F for 3 hours.
The friction material possessed a Shore A Hardness
of 90 - 95, and an ultimate tensile strength of 1,900 to
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2,100 psi. The friction material was found to have an :
excellent thermal stability. : -
While the invention has been described in connection
with specific embodiments thereof, it will be understood
that it is~capable of further modifications, and this ~ ~-
application is intended to cover any variations, uses or
adaptations of the invention following, in general, the
principles of the invention and including such departures
from the present disclosure as come within known or ~ ~ :
customary practice in the art to which the invention
pertains and.as may be applied to the essential features -.
hereinbefore set forth, and as fall within the scope of
the invention and the limits of the appended claims.
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