Note: Descriptions are shown in the official language in which they were submitted.
CA 02217751 1997-10-08
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O-RING SEAL WITH LUBRICANT ADDITIVES
FOR ROCK BIT BEARINGS
Field of the Invention
This invention relates to an O-ring seal for retaining the lubricant around
the journal
bearings in a rock bit or drill bit for drilling oil wells or the like. More
particularly, this
invention relates to an O-ring seal comprising one or more lubricant additives
that reduce
friction, improve wear resistance, reduce abrasion, and reduce stick slip
between the O-ring seal
and interfacing rock bit surfaces to enhance the service life of the O-ring
seal and rock bit.
Background of the Invention
Heavy-duty drill bits or rock bits are employed for drilling wells in
subterranean
formations for oil, gas, geothermal steam, minerals and the like. Such drill
bits have a body
connected to a drill string and a plurality, typically three, of hollow cutter
cones mounted on the
body for drilling rock formations. The cutter cones are mounted on steel
journals or pins integral
with the bit body at its lower end. In use, the drill string and bit body are
rotated in the bore
hole, and each cone is caused to rotate on its respective journal as the cone
contacts the bottom
of the bore hole being drilled. As such a rock bit is used for drilling in
hard, tough formations,
high pressures and temperatures are encountered.
When a drill bit wears out or fails as a bore hole is being drilled, it is
necessary to
withdraw the drill string for replacing the bit. The amount of time required
to make a round trip
for replacing a bit is essentially lost from drilling operations. This time
can become a significant
portion of the total time for completing a well, particularly as the well
depths become great. It
is therefore quite desirable to maximize the service life of a drill bit in a
rock formation.
Prolonging the time of drilling minimizes the time lost in "round tripping"
the drill string for
replacing the bits. Replacement of a drill bit can be required for a number of
reasons, including
wearing out or breakage of the structure contacting the rock formation.
One of the consistent problems in drill bits is the inconsistency of service
life.
Sometimes bits are known to last for long periods, whereas bits which are
apparently identical
operated under similar conditions may tail within a short lifetime. One cause
of erratic service
life is failure of the bearings. Bearing failure can often be traced to
failure of the seal that retains
lubricant in the bearing. Lubricant may be lost if the seal fails, or abrasive
particles of rock may
work their way into the bearing surfaces, causing excessive wear.
Rock hit 0-rings are being called on to perform service in environments which
are
extremely harsh. Modern bits are being run at exceptionally high surface
speeds, sometimes
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more than X00 feet per minute, with cone speeds averaging in the range of from
200 to 400
revolutions per minute. One face of the O-ring is exposed to abrasive drilling
fluid and mud.
The life of the O-ring may be significantly degraded by high temperatures due
to friction (as well
as elevated temperature in the well bore) and abrasion.
In order to provide a consistently reliable O-ring seal for maintaining the
lubricant within
rock bits, it is known to make the O-ring seal from a resilient elastomeric
composition displaying
a desire degree of chemical resistance, heat resistance, and wear resistance.
O-ring seals known
in the art are constructed from resilient elastomeric materials that, while
displaying some degree
of chemical, heat, and wear resistance, ultimately limit the service life of
the rock bit by wearing
away along the surface during use.
Attempts to have been made to improve O-ring properties of wear resistance by
adding
lubricant additives to the elastomeric composition. U.S. Patent No. 5,402,858,
for example,
discloses an O-ring seal formed from an elastomeric material comprising low-
friction wear
resistant particles distributed therein. The wear resistant particles were
selected from the group
including copper, bronze, brass, nickel, cobalt, cemented tungsten carbide,
and titanium carbide.
It has been discovered that while such example O-ring seal constructions
displayed reduced
properties of stick slip, they were not completely effective at reducing
friction and abrasion
between the interfacing O-ring seal and rock bit surface.
It is therefore desirable to provide a consistently reliable O-ring seal for
maintaining the
lubricant within a rock bit, that has a long useful life, is resistant to
crude gasoline and other
chemical compositions found within oil wells, has high heat resistance, is
highly resistant to
abrasion, has a low coefficient of friction against the adjacent seal surfaces
to minimize heating
and wear, and that will not readily deform under load and allow leakage of the
grease from
within the bit or drilling mud into the bit.
Summary of the Invention
There is. therefore, provided in practice of this invention an improved O-ring
seal for rock
bit bearings comprising a body formed from an elastomeric material having one
or more
lubricant additive uniformly distributed throughout. The lubricant additives
are selected from
the group consisting of polytetrafluoroethylene, hexagonal boron nitride,
flake graphite, ultra-
high molecular weight polyurethane, and mixtures thereof.
O-ring seals made from elastomeric compositions of this invention comprise in
the range
of from about 85 to 99 percent by volume elastomeric material, and in the
range of from about
1 to 15 percent by volume of the lubricant additives based on the total volume
of the
composition.
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O-ring seals made from elastomeric compositions of this invention have a
reduced
coefficient of friction, display reduced wear, stick-slip, abrasion, and
improved temperature
stability when compared to O-ring seals formed from conventional elastomeric
materials not
having such lubricant additives.
The invention provides a journal seal for use with a rotary cone rock bit
comprising a
flexible and resilient seal body formed from an elastomeric material selected
from materials
consisting of high-saturated nitrite etastomers, nitrite-butadiene rubber,
highly-saturated nitrile-
butadiene rubber, and mixtures thereof, and, a seal dynamic surFace along one
surface area of the
seal body, and a seal static surface along another surface area of the seal
body each formed from
the elastomeric material, wherein the elastomeric material fixrther comprises
a lubricant additive
to reduce friction and stick slip at the seal dynamic surface that is formed
from hexagonal boron
nitride, wherein the hexagonal boron nitride has an average particle size in
the range of from about
0.1 to 20 micrometers. The seal may comprise in the range of from 1 to 15
percent by volume
lubricant additive based on the total volume of the combined elastomeric
material and lubricant
additive. The lubricant additive may be disposed uniformly throughout the
elastomeric material
forming the seal body, and dynamic and static sealing surfaces.
The invention also provides a journal seal for use with a rotary cone rock bit
comprising
a flexible and resilient seal body formed from an elastomeric material
selected from materials
consisting of highly-saturated nitrite elastomers, nitrite-butadiene rubber,
highly-saturated nitrile-
butadiene rubber, and mixtures thereof, and, a seal dynamic surface along one
surface area of the
seal body, and a seal static surface along another surface area of the seal
body each formed from
the elastomeric material, wherein the elastomeric material further comprises a
lubricant additive
to reduce friction and stick slip at the seat dynamic surface formed from
flake graphite, and
wherein the flake graphite has an average particle size of about five
micrometers. The flake
graphite may comprise in the range of from about 88 to 99 percent by weight
carbon, and in the
range of from about 1 to 12 percent by weight ash. The seal may comprise in
the range of from
1 to 15 percent by volume lubricant additive based on the total volume of the
combined
elastomeric material and Lubricant additive. The lubricant additive may be
disposed uniformly
throughout the elastomeric material forming the seal body, and dynamic and
static sealing
surfaces.
The invention also provides a rotary cone rock bit for drilling subterranean
formations
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comprising a bit body including a plurality of journal pins each extending
from a leg portion of
the bit and having a bearing surface, a cutter cone rotatably mounted on each
journal pin and
including a bearing surface, a pressure-compensated grease reservoir in
communication with such
bearing surfaces, a grease in the grease reservoir and adjacent the bearing
surfaces, and, a dynamic
annular seal for retaining the grease between the bearing surfaces comprising
a body portion and
a surface portion both being flexible and resilient, and both being formed
from an elastomeric
composition selected from materials consisting of highly-saturated nitrite
elastomers, nitrile-
butadiene rubber, highly-saturated nitrite-butadiene rubber, and mixtures
thereof, wherein the
elastomeric composition additionally comprises a lubricant additive formed
from flake graphite
and wherein the surface portion engages a journal pin and cone, wherein the
flake graphite has
an average particle size of about five micrometers. The annular seal may
comprise in the range
of from 1 to 15 percent by volume lubricant additive based on the total volume
of the elastomeric
composition and lubricant additive. The lubricant additive may be distributed
uniformly
throughout the elastomeric composition. The flake graphite may comprise in the
range of from
about 88 to 99 percent by weight carbon, and in the range of from about 1 to
12 percent by
weight ash.
The invention also provides a rotary cone rock bit for drilling subterranean
formations
comprising a bit body including a plurality of journal pins each extending
from a leg portion of
the bit and having a bearing surface, a cutter cone rotatably mounted on each
journal pin and
including a bearing surface, a pressure-compensated grease reservoir in
communication with such
bearing surfaces, a grease in the grease reservoir and adjacent the bearing
surfaces, and, a dynamic
annular seal for retaining the grease between the bearing surfaces comprising
a body portion and
a surface portion both being flexible and resilient, and both being formed
from an elastomeric
composition selected from materials consisting of highly-saturated nitrite
elastomers, nitrile-
butadiene rubber, highly-saturated nitrite-butadiene rubber, and mixtures
thereof, wherein the
elastomeric composition additionally comprises a lubricant additive formed
from hexagonal boron
nitride having an average particle size in the range of from about 0.1 to 20
micrometers, and
wherein the surface portion engages a journal pin and cone. The annular seal
may comprise in the
range of from 1 to 1 S percent by volume lubricant additive based on the total
volume of the
elastomeric composition and lubricant additive. The lubricant additive may be
distributed
uniformly throughout the elastomeric composition.
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The invention also provides a rotary cone rock bit for drilling subterranean
formations
comprising a bit body including a plurality of journal pins each extending
from a leg portion of
the bit and having a bearing surface, a cutter cone rotatably mounted on each
journal pin and
including a bearing surface, a pressure-compensated grease reservoir in
communication with such
bearing surfaces, a grease in the grease reservoir and adjacent the bearing
surfaces, and, a dynamic
O-ring seal for retaining the grease between the bearing surfaces comprising a
resilient and flexible
body having a static sealing surface along a first body portion and a dynamic
sealing surface along
a second body portion, wherein the body and static and dynamic sealing
surfaces are formed from
a resilient elastomeric composition comprising materials consisting of highly-
saturated nitrite
elastomers, nitrite-butadiene rubber, highly-saturated nitrite-butadiene
rubber, and mixtures
thereof, the elastomeric composition further comprising a lubricant additive
uniformly distributed
therethrough that is made of flake graphite having an average particle size of
about five
micrometers, and wherein the elastomeric composition comprises in the range of
from 1 to 15
percent by volume lubricant additive based on the total volume of the
composition. The flake
graphite may comprise in the range of from about 88 to 99 percent by weight
carbon, and in the
range of from about 1 to 12 percent by weight ash.
The invention also provides a rotary cone rock bit for drilling subterranean
formations
comprising a bit body including a plurality of journal pins each extending
from a leg portion of
the bit and having a bearing surface, a cutter cone rotatably mounted on each
journal pin and
including bearing surface, a pressure-compensated grease reservoir in
communication with such
bearing surfaces, a grease in the grease reservoir and adjacent the bearing
surfaces, and, a dynamic
O-ring seal for retaining the grease between the bearing surfaces comprising a
resilient and flexible
body having a static sealing surface along a first body portion and a dynamic
sealing surface along
a second body portion, wherein the body and static and dynamic sealing
surfaces are formed from
a resilient elastomeric composition comprising materials consisting of highly-
saturated nitrite
elastomers, nitrite-butadiene rubber, highly-saturated nitrite-butadiene
rubber, and mixtures
thereof, the elastomeric composition further comprising a lubricant additive
uniformly distributed
therethrough, wherein the lubricant additive is hexagonal boron nitride having
an average particle
size in the range of from about 0.1 to 20 micrometers, and wherein the
elastomeric composition
comprises in the range of from 1 to 1 S percent by volume lubricant additive
based on the total
volume of the composition.
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Brief Description of the Drawings
These and other features and advantages of the present invention will become
appreciated
as the same becomes better understood with reference to the drawings wherein:
FIG. 1 is a semi-schematic perspective of a rock bit containing an O-ring seal
constructed
according to the principles of this invention;
FIG. 2 is a partial cross-sectional view of the rock bit of FIG. 1;
FIG. 3 is a cross-sectional view of an O-ring seal, constructed according to
principles of
this invention, having a symmetric axial cross section; and
FIG. 4 is a cross-sectional view of an alternative embodiment of an O-ring
seal,
constructed according to principles of this invention, having an asymmetric
axial cross section.
20
3U
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.
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Detailed Description
A rock bit employing an O-ring seal constructed according to principles of
this invention
comprises a body 10 having three cutter cones 11 mounted on its lower end, as
shown in FIG.
1. A threaded pin 12 is at the upper end of the body for assembly of the rock
bit onto a drill
string for drilling oil wells or the like. A plurality of tungsten carbide
inserts 13 are pressed into
holes in the surfaces of the cutter cones for bearing on the rock formation
being drilled. Nozzles
15 in the bit body introduce drilling fluid into the space around the cutter
cones for cooling and
carrying away formation chips drilled by the bit.
O-ring seals are generally thought of as comprising a cylindrical inside and
outside
diameter, and a circular cross section. Accordingly, for purposes of reference
and clarity, the
figures used to describe the principles and embodiments of this invention have
been created to
illustrate an O-ring seal having a generally circular cross section. However,
the principles of
this invention are also meant to apply to O-ring seals having non-circular
cross sections. Such
O-ring seals can be configured having either symmetric or asymmetric non-
circular cross
sections. It is, therefore, to be understood that the principles of this
invention may apply to O-
rings having a circular or non-circular cross sections.
FIG. 2 is a fragmentary, longitudinal cross-section of the rock bit, extending
radially from
the rotational axis 14 of the rock bit through one of the three legs on which
the cutter cones 11
are mounted. Each leg includes a journal pin extending downwardly and
radially, inwardly on
the rock bit body. The journal pin includes a cylindrical bearing surface
having a hard metal
insert 17 on a lower portion of the journal pin. The hard metal insert is
typically a cobalt or iron
based alloy welded in place in a groove on the journal leg and having a
substantially greater
hardness that the steel forming the journal pin and rock bit body.
An open groove 18 is provided on the upper portion of the journal pin. Such a
groove
may, for example, extend around 60 percent or so of the circumference of the
journal pin, and
the hard metal insert 17 can extend around the remaining 40 percent or so. The
journal pin also
has a cylindrical nose 19 at its lower end.
~0 Each cutter cone 11 is in the form of a hollow, generally-conical steel
body having
cemented tungsten carbide inserts 13 pressed into holes on the external'
surface. For long life,
the inserts may be tipped with a polycrystalline diamond layer. Such tungsten
carbide inserts
provide the drilling action by engaging a subterranean rock formation as the
rock bit is rotated.
Some types of bits have hard-faced steel teeth milled on the outside of the
cone instead of
carbide inserts.
The cavity in the cone contains a cylindrical bearing surface including an
aluminum
hronze insert 21 deposited in a groove in the steel of the cone or as a
floating insert in a groove
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in the cone. The aluminum bronze insert 21 in the cone engages the hard metal
insert 17 on the
leg and provides the main bearing surface for the cone on the bit body. A nose
button 22 is
between the end of the cavity in the cone and the nose 19 and carries the
principal thrust loads
of the cone on the journal pin. A bushing 23 surrounds the nose and provides
additional bearing
surface between the cone and journal pin. Other types of bits, particularly
for higher rotational
speed applications, have roller bearings instead of the journal bearings
illustrated herein. It is
to be understood that O-ring seals constructed according to principles of this
invention may be
used with rock bits comprising either roller bearings or conventional journal
bearings.
A plurality of bearing balls 24 are fitted into complementary ball races in
the cone and on
the journal pin. These balls are inserted through a ball passage 26, which
extends through the
journal pin between the bearing races and the exterior of the rock bit. A cone
is first fitted on
the journal pin, and then the bearing balls 24 are inserted through the ball
passage. The balls
carry any thrust loads tending to remove the cone from the journal pin and
thereby retain the
cone on the journal pin. The balls are retained in the races by a ball
retainer 27 inserted through
the ball passage 26 after the balls are in place. A plug 28 is then welded
into the end of the ball
passage to keep the ball retainer in place. The bearing surfaces between the
journal pin and the
cone are lubricated by a grease. Preferably, the interior of the rock bit is
evacuated, and grease
is introduced through a fill passage (not shown). The grease thus fills the
regions adjacent the
bearing surfaces plus various passages and a grease reservoir, and air is
essentially excluded
from the interior of the rock bit. The grease reservoir comprises a cavity 29
in the rock bit body,
which is connected to the ball passage 26 by a lubricant passage 31. Grease
also fills the portion
of the ball passage adjacent the ball retainer, the open groove 18 on the
upper side of the journal
pin, and a diagonally extending passage 32 therebetween. Grease is retained in
the bearing
structure by a resilient seal in the form of an O-ring 44 between the cone and
journal pin.
A pressure compensation subassembly is included in the grease reservoir 29.
The
subassembly comprises a metal cup 34 with an opening 36 at its inner end. A
flexible rubber
bellows 37 extends into the cup from its outer end. The bellows is held into
place by a cap 38
with a vent passage 39. The pressure compensation subassembly is held in the
grease reservoir
by a snap ring 41.
When the rock bit is filled with grease, the bearings, the groove 18 on the
journal pin,
passages in the journal pin, the lubrication passage 31, and the grease
reservoir on the outside
of the bellows 37 are filled with grease. If the volume of grease expands due
to heating, for
example, the bellows 37 is compressed to provide additional volume in the
sealed grease system,
thereby preventing accumulation of excessive pressures. High pressure in the
grease system can
damage the O-ring seal 44 and permit drilling fluid or the like to enter the
bearings. Such
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CA 02217751 2003-09-12
material is abrasive and can quickly damage the bearings. Conversely, if the
grease volume
should contract, the bellows can expand to prevent low pressures in the sealed-
grease system,
which could cause flow of abrasive and/or corrosive substances past the O-ring
seal.
The bellows has a boss 42 at its inner end which can seat against the cap 38
at one end of
the displacement of the bellows for sealing the vent passage 39. The end of
the bellows can also
seat against the cup 34 at the other end of its stroke, thereby sealing the
opening 36. If desired,
a pressure relief check valve can also be provided in the grease reservoir for
relieving over-
pressures in the grease system that could damage the O-ring seal. Even with a
pressure
compensator, it is believed that occasional differential pressures may exist
across the 0-ring of
up to 150 psi (550 kilopascals). To maintain the desired properties of the O-
ring seal at the
pressure and temperature conditions that prevail in a rock bit, to inhibit
"pumping" of the grease
through the O-ring seal, and for a long useful life, it is important that the
O-ring seal be resistant
to crude gasoline and other chemical compositions found within oil wells, have
a high heat and
abrasion resistance, have low rubbing friction, and not be readily deformed
under the pressure
and temperature conditions in a well which could allow leakage of the grease
from within the
bit or drilling mud into the bit.
Suitable elastomeric materials useful for forming O-ring seal constructions of
this
invention include those selected from the group of carboxylated elastomers
such as carboxylated
nitriles, highly saturated nitrile (HSN) elastomers, riitrile-butadiene rubber
(HBR), highly
saturated nitrile-butadiene rubber (HIv'BR) and the like. Particularly
preferred elastomeric
materials are HNBR and HSN. An exemplary HNBR material is set forth in the
examples below.
Other desirable elastomeric materials include those HSN materials disclosed in
U.S. Patent No.
5,323,863, and a proprietary HSN manufactured by Smith International, Inc.,
under the product
name HSN-8A. It is also to be understood that the HNBR material set forth in
the example, and
the HSN materials described above, are but one example of elastomeric
materials useful for
making O-ring seals according to this invention, and that other elastomeric
materials made from
different chemical compounds and/or different amounts of such chemical
compounds may also
be used.
It is desired that such elastomeric materials have a modulus of elasticity at
100 percent
elongation of from about 400 to 2,000 psi (3 to 12 megapascals), a minimum
tensile strength of
3 ~ from about 1,000 to 7,000 psi (6 to 42 megapascals), elongation of from
100 to 500 percent, die
C tear strength of at least 100 lb/in. (1.8 kilogram/millimeter), durometer
hardness Shore A in
the range of from about 60 to 95, and a compression set after 70 hours at
100° C of less than
about 18 percent, and preferably less than about 16 percent.
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An exemplary elastomeric composition may comprise per 100 parts by weight of
elastomer (e.g., HSN, HNBR and the like), furnace black in the range of from
20 to 50 parts by
weight, peroxide curing agent in the range of from 7 to 10 parts by weight,
zinc oxide or
magnesium oxide in the range of from 4 to 7 parts by weight, stearic acid in
the range of from
0.5 to 2 parts by weight, and plasticizer up to about 10 parts by weight.
O-ring seals constructed according to principles of this invention also
include one or more
lubricant additives that are distributed uniformly throughout the elastomeric
material, and that
are selected from the group consisting of polytetrafluoroethylene (PTFE), hBN,
flake graphite,
ultra-high molecular weight polyurethane (UHMWPE), and mixtures thereof. The
lubricant
additives are selected to provide an added degree of low friction and wear
resistance to the
elastomeric component of the O-ring seal material. The lubricant additives are
also selected to
reduce the amount of stick-slip that occurs between the O-ring seal surface
and an adjacent
rotating rock bit surface.
Stick-slip refers to a mechanism of failure in a rock bit O-ring seals. As the
elastomer of
the O-ring seal moves along the metal surface of the leg or cone, the O-ring
seal material
momentarily sticks to the metal surface. Almost instantly the elastomer then
slips relative to the
metal. This making and breaking of bonds between the elastomer and metal
dissipates energy
and causes frictional heating. Furthermore, if too strong a bond is formed
between the elastomer
and metal, some of the elastomer may be removed from the O-ring, thereby
degrading the O-ring
surface and roughening the rock bit surface. The lubricant additives described
above have been
selected because of their ability to minimize the amount of sticking between
the elastomer and
metal without changing the bulk properties of the main body of the O-ring.
Particularly preferred lubricant additives are hBN and flake graphite. hBN is
particularly
preferred because of its low coefficient of friction, good thermal
conductivity, and high
temperature stability. hBN also provides excellent properties of high-
temperature mold release.
A preferred hBN has a particle size in the range of from about 0.1 to 20
micrometers. A
particularily preferred hBN is commercially available, for example, from
Advanced Ceramic
s0 Corp., of Cleveland, Ohio as Grade HCP hBN having an average particle size
in the range of
from about one to ten micrometers. hBN is a particularly preferred lubricant
additive because
it provides a superior degree of lubrication when placed in contact with steel
without producing
harmful side effects, e.g., abrasive, side effects to the journal or cone.
Flake graphite is also particularly preferred because of its low coefficient
of friction and
low coefficient of thermal expansion, and because of its softness and low
abrasion. Flake
graphite, rather than other forms of graphite such as artificial graphite,
amorphous graphite, and
crystalline graphite vein, is selected for use as a lubricant additive for
making O-ring seals of this
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CA 02217751 1997-10-08
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invention because of its morphology of weak interlayer bonds of the crystal
that allows for easy
slippage of the planes, thereby giving flake graphite a pronounced softness
and increased
properties of lubrication not present in such other forms of graphite. Flake
graphite is a mined
mineral that has plate form and acts as single-crystal graphite.
A preferred flake graphite has in the range of from about 88 to 99 percent by
weight
carbon, about 1 to 12 percent by weight ash, and may have an average particle
size of 20x50
mesh, 30x60 mesh, 50x200 mesh, 80x325 mesh, 325 mesh, or about five
micrometers. A
particularly preferred flake graphite has carbon content of approximately 96
percent by weight,
an ash content of approximately 4 percent by weight, and has an average
particle size of
approximately 325 mesh.
In addition to their friction reducing properties, it has been discovered that
hBN and flake
graphite can be used as a partial substitute for carbon black in the
elastomeric material to provide
strength thereto, to reduce the coefficient of friction of the elastomeric
composition, and to
reduce the amount of abrasive wear caused to the interfacing rock bit surface
by the elastomeric
material. The hBN and flake graphite lubricant additives have the unique
effect of both
increasing the wear resistance and coefficient of friction of the elastomeric
material while
making the elastomeric material less abrasive against the mating journal
surface.
O-ring seal compositions of this invention comprise in the range of from about
85 to 99
percent by volume elastomeric material, and in the range about 1 to 15 percent
by volume of the
lubricant additive based on the total volume of the seal composition. A seal
composition
comprising less than about one percent by volume of the lubricant additive
would contain an
insui~icient amount of the lubricant additive to provide a desired reduction
in the friction, wear,
abrasion, and stick-slip characteristics of the elastomeric material. A seal
composition
comprising greater than about 15 percent by volume of the lubricant additive
would contain an
amount of lubricant additive that could interfere with or adversely effect
desired mechanical
properties of the elastomeric material.
Seal compositions of this invention are prepared by combining the elastomeric
seal
component with the lubricant additive by conventional solid mixing techniques,
such as by mill
process and the like, until the lubricant additive are uniformly distributed
throughout the
elastomeric material. The O-ring seal is formed and cured according to
conventional industry
practices for forming and curing O-rings seals made from conventional
elastomeric materials,
e.g., by high temperature mold process. A feature of O-ring seals formed from
elastomeric seal
3 ~ compositions of this invention is that the presence of the lubricant
additive aids the release of the
just-formed O-ring seal from the mold during the high-temperature molding
process.
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Referring to FIG. 3, an exemplary embodiment of an O-ring seal 48 of this
invention
comprises a body 50 having a symmetric axial cross section, the body being
formed from the
elastomeric material 52 and the lubricant additive 54 uniformly distributed
throughout.
Refernng to FIG. 4, an alternative embodiment of an O-ring seal 55 of this
invention comprises
a body 56 having an asymmetric axial cross section, the body being formed from
the elastomeric
material 58 and the lubricant additive 60 uniformly distributed throughout.
The completed O-ring seal is placed into position in the rock bit with
portions of the seal
surface in contact with respective surfaces of the cone and the journal. In an
exemplary rotary
cone rock bit, the O-ring seal is disposed within the cone and includes a
static seal surface that
is in contact with the cone, and a dynamic seal surface that is placed
adjacent a journal pin. As
the cone is rotated about the journal pin, the dynamic seal surface slides
over the adjacent journal
pin surface, producing friction at the dynamic seal surface. The presence of
the lubricant
additive in the elastomeric seal composition reduces amount of friction
produced at the dynamic
seal surface, and reduces the amount of stick-slip by minimizing the
occurrence of the "sticking"
portion and maximize the "slipping" portion of the stick-slip phenomena. The
reduction of stick-
slip serves to reduce material loss from the surface of the O-ring seal and,
thus extends the
service life of the O-ring seal and rock bit.
O-ring seals constructed from elastomeric compositions prepared according to
principles
of this invention may be better understood with reference to the following
examples.
Preparation of Control Elastomeric Material
An elastomeric material was prepared by combining approximately 100 parts by
weight
HNBR, 1.5 parts by weight antioxidant, 30 parts by weight carbon black, 5
parts by weight zinc
oxide, 0.5 parts by weight stearic acid, 10 parts by weight plasticizes, 4
parts by weight
vulcanizes, and 10 parts by weight curative agent. A seal was formed from the
elastomeric
material as a control, and the control was tested stick slip amplitude and
average coefficient or
friction. The test results are provided in Table 1 below.
Example No. 1- Elastomer with hBN Lubricant Additive
An elastomeric seal composition was prepared using the control elastomeric
material
described above and adding to it approximately 40 parts per weight hBN. The
resulting
elastomeric seal composition comprised approximately 20 percent by weight hBN
(15 percent
by volume hBN).
Examgle No. 2- Elastomer with Flake Graphite Lubricant Additive
-10-
CA 02217751 1997-10-08
31542/GTL/S61
An elastomeric seal composition was prepared using the control elastomeric
material
described above and adding to it approximately 25 parts per weight flake
graphite. The resulting
elastomeric seal composition comprised approximately 14 percent by weight
flake graphite (10
percent by volume flake graphite).
Table 1
Test Specimen Average Coefficient Stick Slip Amplitude
of
Friction
Control (w/o lubricant0.007 0.0016
additive)
Example 1 (20% by 0.006 0.0004
wt hBN)
Example 2 0.006 0.0007
( 14% by wt flake
graphite)
The test data presented in Table 1 supports the claim that O-ring seals
constructed
according to principles of this invention have an average coefficient of
friction that is up to about
0 15 percent lower than that of the elastomeric material alone, and have a
stick-slip amplitude that
is up to about 75 percent lower than that of the elastomeric material alone.
Depending on the
particular amount of and type of lubricant additive selected, the coefficient
of friction can be up
to about 20 percent lower, and the stick-slip amplitude can be up to about 98
percent lower, that
of elastomeric materials alone. The combined reduction in the coefficient of
friction and stick-
slip amplitude occurs without adversely effecting the desired properties of
the elastomeric
materials, such as modulus, percent elongation, compression set, Shore A
hardness and the like,
thereby enhancing the service life of the seal.
Although, limited embodiments of O-ring seal compositions for rock bit
bearings have
been described and illustrated herein. Many modifications and variations will
be apparent to
those skilled in the art. Accordingly, it is to be understood that within the
scope of the appended
claims, O-ring seals for rock bit bearings according to principles of this
invention may be
embodied other than as specifically described herein.
