Note: Descriptions are shown in the official language in which they were submitted.
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DRILL BIT BEARING CONTACT PRESSURE REDUCTION
TECHNICAL FIELD
This disclosure relates generally to equipment utilized
and operations performed in conjunction with subterranean
wells and, in an example described below, more particularly
provides for drill bit bearing contact pressure reduction.
BACKGROUND
Bearing failure is one of the main problems affecting
conventional drill bits used to drill subterranean wells.
Such failures will generally require that the drill bits be
retrieved for replacement, resulting in substantial loss of
time and money.
Bearing failures can be due to a variety of factors.
However, if the maximum contact pressure between bearing
surfaces could be substantially reduced, bearing failures
could also be reduced significantly.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an elevational view of a drill bit embodying
principles of the present disclosure.
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FIG. 2 is a cross-sectional view through one arm of the
drill bit of FIG. 1.
FIG. 3 is an enlarged scale schematic cross-sectional
view of a portion of an interface between bearing surfaces
of the drill bit.
FIG. 4 is graph of modeled contact pressure versus
distance along a journal for modified and unmodified
cone/journal surface interfaces.
FIGS. 5-14 are schematic cross-sectional views of
various strain energy-relieving configurations for
interfaces between bearing surfaces.
DETAILED DESCRIPTION
Representatively illustrated in FIG. 1 is a drill bit
10 which can embody principles of this disclosure. The
drill bit 10 is of the type known to those skilled in the
art as a roller cone bit or a three-cone bit, due to its use
of multiple generally conical rollers or cones 12 having
earth-engaging cutting elements 14 thereon.
Each of the cones 12 is rotatably secured to a
respective arm 16 extending downwardly (as depicted in FIG.
1) from a main body 18 of the bit 10. In this example,
there are three each of the cones 12 and arms 16.
However, it should be clearly understood that the
principles of this disclosure may be incorporated into drill
bits having other numbers of cones and arms, and other types
of drill bit configurations. The roller cone drill bit 10
depicted in FIG. 1 is merely one example of a wide variety
of drill bit types which can utilize the principles
described herein.
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Referring additionally now to FIG. 2, a cross-sectional
view of one of the arms 16 is representatively illustrated.
In this view it may be seen that the cone 12 rotates about a
journal 20 of the arm 16. Retaining balls 22 are used
between the cone 12 and the journal 20 to secure the cone on
the arm 16.
Lubricant is supplied to the interface between the cone
12 and the journal 20 from a chamber 24 via a passage 26. A
pressure equalizing device 28 ensures that the lubricant is
at substantially the same pressure as the downhole
environment when the drill bit 10 is being used to drill a
wellbore.
A seal 30 is used to prevent debris and well fluids
from entering the interface between the cone 12 and the
journal 20, and to prevent escape of the lubricant from the
interface area. As the cone 12 rotates about the journal
20, the seal 30 preferably rotates with the cone and seals
against an outer surface of the journal.
The seal 30 is retained in an annular groove 38 (also
known to those skilled in the art as a seal "gland") formed
radially outward from an inner cylindrical bearing surface
32 in the cone 12. The seal 30 seals against an outer
cylindrical bearing surface 44 on the journal 20.
Although the retaining balls 22 retain the cone 12 on
the journal 20, they do not resist the large forces exerted
on the cone during drilling. Instead, contact between the
surfaces 32, 44, as well as contact between pairs of other
bearing surfaces 34, 36, 40, 42, operate to resist the
enormous loads acting on the cone 12 as it rotates on the
journal 20 during drilling. Each pair of surfaces (32 and
44; 34 and 36; 40 and 42) functions as a type of plain
bearing which permits substantially unhindered rotation of
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the cone 12 about the journal 20, while transmitting large
forces between the arm 16 and the cone.
One problem with conventional drill bit bearing designs
is that, at transitions in the bearing surfaces, very high
contact pressures can be experienced. These very high
contact pressures cause considerable material loss on parts
due to sliding wear and, in extreme cases, can result in
premature failure of the bearing surfaces, thereby reducing
or destroying the effectiveness of the drill bit 10 in the
drilling operation, requiring replacement of the bit, and
thereby causing loss of time and money in a drilling
operation. Fortunately, these drawbacks of conventional
drill bit bearing designs can be minimized or eliminated by
employing the principles described in this disclosure.
Referring additionally now to FIG. 3, an enlarged scale
cross-sectional view of the seal groove 38 and adjacent
bearing surfaces 32, 44 is representatively illustrated. In
this view it may be seen that a transition in the surface 32
occurs at a radius 50 formed between the surface and a side
wall 52 of the groove 38. The wall 52 is oriented
orthogonal to an axis of rotation 53 (see FIG. 2) of the
cone 12 about the journal 20.
The present inventor has discovered via modeling
techniques that contact pressure between the surfaces 32, 44
is highly concentrated at the transition between the surface
32 and the side wall 52. More specifically, the modeling
techniques have revealed that, under certain circumstances,
contact pressure near a tangent 54 (i.e., the transition
between a) contact between the surfaces 32, 44 and b) lack
of contact between the surfaces) can be many times the
contact pressure away from the tangent. Note that, for
clarity of illustration and description, the surfaces 32, 44
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are depicted in FIG. 3 as being spaced apart somewhat, but
the surfaces would contact each other when forces 56 are
being transmitted between the cone 12 and journal 20.
The inventor's analysis has also revealed that contact
pressure on most of (e.g., -90% of) the surface area of
contact between the surfaces 32, 44 can be much less than
(e.g., -20% of) the contact pressure at the transition
between the surface 32 and the side wall 52. This
phenomenon is known as "edge loading" in the art of contact
mechanics.
The underlying reason for this circumstance is the
concentrated accumulation of strain energy in the structure
of the cone 12 adjacent the transition between the surface
32 and the side wall 52. If this strain energy could be
relieved, the contact pressure at the transition could be
reduced, resulting in the contact pressure being more evenly
distributed across the area of contact between the surfaces
32, 44.
One technique for relieving the strain energy at the
transition in the surface 32 is depicted in FIG. 3. Note
that material has been removed from the cone 12 to thereby
form an annular recess 58 extending axially from the side
wall 52. One of the beneficial results of the recess 58 is
a reduction in the stiffness of the structure 60 adjacent
the tangent 54.
This reduction in stiffness allows the structure 60 to
flex somewhat, thereby relieving strain energy. That is,
the strain energy in the structure 60 will be reduced
relative to what the strain energy in the structure 60 would
have been if it had been constructed similar to an adjacent
structure 74 of the cone 12 which does not have the recess
58 formed therein. Transmission of the forces 56 through
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the structure 60 will result in much greater deflection of
the structure 60, as compared to deflection of the adjacent
structure 74 due to transmission of the forces.
The recess 58 may be in the form of a groove, slit,
depression, etc. In the example of FIG. 3, the recess 58
extends completely around in the structure 60, so that the
stiffness of the structure is reduced circumferentially
about the bearing surface 32. In other examples, the
reduction in stiffness of a structure may not extend
completely around the interior or exterior of the structure.
Particularly where loading on the structure is typically
from one direction (for example, in non-rotating elements,
such as the journal 20), it may be desirable to reduce the
stiffness of the structure only on one side of the
structure.
In FIG. 4, a graph of contact pressure versus distance
along the journal 20 is representatively illustrated. This
graph represents one of the results of the inventor's
modeling efforts discussed above.
One curve 62 on the graph represents contact pressure
along the journal 20 with an unmodified cone 12, that is,
the cone without the recess 58 formed therein to reduce the
stiffness of the structure 60. Another curve 64 on the
graph represents contact pressure along the journal 20 with
the cone 12 modified as described above to relieve the
strain energy in the structure 60.
Note that the maximum contact pressure 66 for the
unmodified cone 12 is many times greater than the maximum
contact pressure 68 for the modified cone 12. A contact
pressure 70 at the remainder of the cone/journal interface
for the unmodified design (i.e., adjacent the relatively
high stiffness structure 74) appears to be somewhat less
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than a contact pressure 72 at the remainder of the
cone/journal interface for the modified design, but both of
these contact pressures 70, 72 are much less than the
maximum contact pressure 66 for the unmodified design.
The maximum contact pressure 68 for the modified design
at the transition on the surface 32 is only slightly more
than the contact pressures 70, 72 at the remainder of the
cone 12/journal 20 interface, and is much less than the
maximum contact pressure 66 for the unmodified design.
Thus, it is expected that drill bits constructed using the
principles described in this disclosure will have much
greater bearing longevity.
Although the recess 58 is depicted as being used in
FIG. 3 for reducing the stiffness of the structure 60
adjacent the tangent 54 between the surfaces 32, 44, it
should be understood that other means of reducing stiffness
at transitions can be used, without departing from the
principles of the present disclosure. These other means can
be used to permit the structure 60 to distort near the
transition (e.g., near the tangent 54) and thereby relieve
strain energy and reduce contact pressure between the
surfaces 32, 44. Such other means could include, for
example, hole(s) 78 (see FIG. 5), void(s) 80 (see FIG. 6),
reduced stiffness structure(s) 82 (see FIG. 7; wherein the
reduced stiffness may be due to various features, such as,
use of a reduced modulus material, lack of material, etc.),
reduced elastic modulus material(s) 84 (see FIG. 8), etc.,
and any combination of contact pressure reducing means.
If a reduced elastic modulus material 84 is used, in
some examples the elastic modulus of the material may vary
gradually. Thus, the reduced elastic modulus material 84
may have "gradient" modulus properties. Such a gradient
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elastic modulus material or functionally gradient material
(see FIG. 11) can be used to smooth out a transition in
stiffness, to thereby provide a gradual drop in contact
pressure between the surfaces 32, 44.
The material 84 can incorporate nano structures 76
therein to provide the reduced elastic modulus of the
material. As known to those skilled in the art, a nano
structure is a structure having a maximum size of 100 nm.
As used herein, the term "nano structure" can encompass nano
particles, nano tubes, and any other structures having a
size of 100 nm or less.
In FIG. 9, the structure 60 comprises a sleeve bearing
which is interference fit within the cone 12. Thus, the
bearing surface 32 which contacts the bearing surface 44 is
formed on an interior of the sleeve bearing. In this
example, the sleeve bearing has the radius 50 formed
thereon, so that a transition between contact and lack of
contact between the bearing surfaces 32, 44 occurs at the
tangent 54. The recess 58 is formed into the structure 60
to relieve strain energy at the transition between contact
and lack of contact between the surfaces 32, 44. The recess
58 reduces the stiffness of the structure 60 supporting the
surface 32 at the tangent 54, thereby reducing the maximum
contact pressure between the surfaces 32, 44.
In FIG. 10, the structure 60 comprises a floating
thrust bearing 86 disposed between bearing surfaces 40, 42
on the cone 12 and the journal 20. Note that the radius 50
is not formed on the structure 60 in this example, but the
recess 58 still reduces the stiffness of the structure 60 at
a transition between contact and lack of contact between the
bearing surfaces 40, 42 and respective bearing surfaces 88,
90 on the thrust bearing 86.
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The configuration of FIG. 10 demonstrates that the
principles of this disclosure may be implemented even though
the radius 50 and tangent 54 are not formed on the structure
60, and even though the structure is not part of the cone
12. This example also demonstrates that the principles of
this disclosure can be applied to various different types of
bearing surfaces.
The thrust bearing 86 could utilize any of the
techniques described herein for reducing contact pressure
between bearing surfaces. For example, nano structures 76,
holes 78, voids 80, reduced modulus materials 84,
functionally gradient materials 92, multiple materials 160,
162, etc. could be used in the thrust bearing 86, if
desired.
The recess 58 is depicted in FIGS. 3, 9 & 10 as being
annular-shaped. However, other shapes could be used in
keeping with the principles of this disclosure. The above
disclosure describes reducing stiffness of the structure 60
supporting the surface 32, but it should be clearly
understood that the principles of this disclosure can be
used for reducing the stiffness of any structure supporting
any of the other surfaces 34, 36, 40, 42, 44, or any other
surfaces, or any combination of surfaces.
Although the principles of this disclosure have been
described above as being used to reduce contact pressure at
the interface between the surfaces 32, 44 near the tangent
54, those principles can be applied at other locations in
the drill bit 10, and in other types of equipment. For
example, the principles of this disclosure could be used at
the interfaces between other pairs of surfaces, such as
between surfaces 34 and 36, and between surfaces 40 and 42.
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Note that the surfaces 32, 44 are interrupted by
annular recesses for receiving the retaining balls 22.
However, the surfaces 32, 44 continue on opposite sides of
the retaining balls 22, and the principles of this
disclosure can be utilized for reducing contact pressure
between the surfaces on either side of the retaining balls.
The above disclosure describes reducing the stiffness
of the structure 60 supporting the bearing surface 32 on the
cone 12. However, the principles of this disclosure can
also, or alternatively, be used to reduce the stiffness of
structures supporting bearing surface 44 on the journal 20,
bearing surface 42 on the journal, bearing surface 40 on the
cone, bearing surface 34 on the cone, or bearing surface 36
on the journal. If the techniques described in this
disclosure are used for reducing the stiffness of a
structure supporting a bearing surface on the journal 20,
then it is not typically necessary for the reduction in
stiffness to extend completely around the journal, since
maximum contact pressure is typically experienced on only
one side of the journal.
Referring additionally now to FIG. 11, another
configuration of the drill bit 10 is representatively
illustrated, in which a functionally gradient material 92
provides a reduced stiffness to the structure 60 and an
increased stiffness to the structure 74. The material 92
may have a reduced modulus at the structure 60 and an
increased modulus at the structure 74, thereby providing for
reduced contact pressure at the transition between contact
and lack of contact between the bearing surfaces 32, 44.
Referring additionally now to FIG. 12, another
configuration of the drill bit 10 is representatively
illustrated, in which the cone 12 has the functionally
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gradient material 92 incorporated therein, so that there is
a gradual transition from the reduced stiffness structure 60
to the increased stiffness structure 74 in the cone itself.
This will result in reduced contact pressure at the
transition between contact and lack of contact between the
bearing surfaces 32, 44. The functionally gradient material
92 could also, or alternatively be incorporated into the
journal 20, if desired.
Referring additionally now to FIG. 13, a cross-
sectional view of the journal 20 is representatively
illustrated for yet another configuration of the drill bit
10. In this configuration, the functionally gradient
material 92 is used on a lower side of the journal bearing
surface 44. It will be appreciated that this side of the
journal 20 receives the maximum contact pressure due to
forces applied to the journal, and so it may be desired to
only utilize the functionally gradient material 92 on the
lower side where it would be most advantageous for reducing
contact pressure between the bearing surfaces 44, 32.
In the configurations of FIGS. 11-13, the functionally
gradient material 92 can reduce contact pressure between the
bearing surfaces 32, 44 at the transition between contact
and lack of contact between the bearing surfaces 32, 44.
This is due to the material 92 providing a reduced stiffness
in the structure 60 and an increased stiffness in the
structure 74. For example, the material 92 may have a
reduced modulus at the structure 60 and an increased modulus
at the structure 74, thereby providing for reduced contact
pressure at the transition between contact and lack of
contact between the bearing surfaces 32, 44.
Such a change in stiffness and/or modulus can be
accomplished in other ways, in keeping with the principles
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of this disclosure. In one example depicted in FIG. 14,
multiple materials 160, 162 could be used, with one material
160 having a reduced stiffness and/or modulus in the
structure 60, and another material 162 having an increased
stiffness and/or modulus in the structure 74. The
transition from the first material 160 to the second
material 162 could be gradual (such as, by tapering from one
to the other as depicted in FIG. 14), and could be provided
in a separate bearing sleeve 164 or as part of either or
both of the components 12, 20. For example, the material
160 could be a less rigid material (such as silver, etc.)
and the material 162 could be a more rigid material (such as
hardened steel, etc.).
It may now be fully appreciated that the above
disclosure provides several advancements to the art of
reducing contact pressures in equipment such as drill bits.
The principles of this disclosure result in dramatic
reductions in maximum contact pressure between bearing
surfaces, and can do so without requiring that any
additional components be added to the equipment, and without
requiring that extensive redesign be implemented.
The principles of this disclosure can be applied in-
situ in a non-intrusive manner in some examples. The
resulting structures can also be easily inspected for
conformance to specifications.
Due to the reduced maximum contact pressure, a variety
of different types of lubricants can be used in the drill
bit 10. For example, oil could be used as a lubricant,
instead of conventional grease.
The above disclosure provides to the art an improved
method of reducing contact pressure between bearing surfaces
32, 44 of a drill bit 10. The method includes constructing
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a structure 60 which supports the first bearing surface 32
in contact with the second bearing surface 44, and reducing
contact pressure between the first and second bearing
surfaces 32, 44 by relieving strain energy in the structure
60.
Constructing the structure 60 may include forming the
structure as part of a cone 12 and/or a journal 20 of the
drill bit 10.
The reduced contact pressure may be due to a lack of
material supporting the structure 60 when the surfaces 32,
44 contact each other. The lack of material may be disposed
adjacent a wall 52 of a seal groove 38. The lack of
material may comprise a recess 58, a hole 78 and/or a void
80.
Constructing the structure 60 may include positioning
the structure 60 between the lack of material and the first
bearing surface 32.
The reduced contact pressure may be due to a reduced
stiffness structure 82 and/or a reduced elastic modulus
material 84 of the structure 60. The structure 60 may
comprise a functionally gradient material 92, a graduated
elastic modulus material 84 and/or may comprise nano
structures 76 therein. The structure 60 may comprise a
first material 160 having a reduced stiffness relative to a
second material 162 which supports the first bearing surface
32 in contact with the second bearing surface 44.
Also described by the above disclosure is a drill bit
10 which includes a first bearing surface 32 in contact with
a second bearing surface 44. There is a transition between
contact between the first and second bearing surfaces 32,
44, and lack of contact between the first and second bearing
surfaces. A structure 60 which supports one of the bearing
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surfaces 32, 44 has a reduced stiffness. In this manner, a
contact pressure between the first and second bearing
surfaces 32, 44 is reduced at the transition.
The transition may be at a tangent formed on the first
bearing surface 32. The transition may be positioned
adjacent a wall 52 of a seal groove 38.
The reduced stiffness of the structure 60 may be due to
a lack of material supporting the structure 60. The lack of
material may be disposed adjacent a wall 52 of a seal groove
38. The lack of material may comprise a recess 58, a hole
78 and/or a void 80.
The reduced stiffness of the structure 60 may be due to
a reduced elastic modulus material 84 of the structure 60.
A deflection of the structure 60 may be increased due
to the reduced stiffness of the structure 60 when forces 56
are transmitted between the first and second bearing
surfaces 32, 44.
The structure 60 may comprise a first material 160
having a reduced stiffness relative to a second material 162
which supports the one of the first and second bearing
surfaces 32, 44.
The above disclosure also describes a drill bit 10
which includes a first bearing surface 32 which contacts a
second bearing surface 44, the first bearing surface 32
being formed in a cone 12, the second bearing surface 44
being formed on a journal 20, the cone 12 being rotatably
mounted on the journal 20, and there being a transition
between contact and lack of contact between the first and
second surfaces 32, 44. The cone 12 and/or the journal 20
includes first and second structures 60, 74 which support
the first and second bearing surfaces 32, 44 in contact with
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each other. The first structure 60 has a reduced stiffness
relative to a stiffness of the second structure 74.
The reduced stiffness of the first structure 60 may
reduce contact pressure between the first and second bearing
surfaces 32, 44 at the transition. The transition may be
formed at a tangent 54 on the first bearing surface 32.
A deflection of the first structure 60 may be greater
than a deflection of the second structure 74 when forces 56
are transmitted between the first and second surfaces 32,
44.
The first structure 60 may comprise a first material
160, and the second structure 74 may comprise a second
material 162, and the first material 160 may have a reduced
stiffness relative to the second material 162.
A drill bit 10 described by the above disclosure can
include a thrust bearing 86 interposed between a cone 12 and
a journal 20 of the drill bit 10. A first bearing surface
88 or 90 on the thrust bearing 86 contacts a second bearing
surface 40 or 42 on at least one of the cone 12 and the
journal 20. There is a transition between contact and lack
of contact between the first and second bearing surfaces
88/40 and/or 90/42. A structure 60 supports the first
bearing surface 88 or 90, the structure 60 having a reduced
stiffness, whereby a contact pressure between the first and
second bearing surfaces 88/40 and/or 90/42 is reduced at the
transition.
The reduced stiffness of the structure 60 may be due to
a lack of material supporting the structure 60. The lack of
material may comprise a recess, a hole and/or a void.
The reduced stiffness of the structure 60 may be due to
a reduced elastic modulus material 84 of the structure 60.
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A deflection of the structure 60 may be increased due
to the reduced stiffness when forces 56 are transmitted
between the first and second bearing surfaces 88/40 and/or
90/42.
The structure 60 may comprise a functionally gradient
material 92. The structure 60 may comprise a graduated
elastic modulus material 84. The structure 60 may comprise
nano structures 76 therein. The structure 60 may comprise a
first material 160 having a reduced stiffness relative to a
second material 162 which supports the first bearing surface
32.
It is to be understood that the various examples
described above may be utilized in various orientations,
such as inclined, inverted, horizontal, vertical, etc., and
in various configurations, without departing from the
principles of the present disclosure. The embodiments
illustrated in the drawings are depicted and described
merely as examples of useful applications of the principles
of the disclosure, which are not limited to any specific
details of these embodiments.
Of course, a person skilled in the art would, upon a
careful consideration of the above description of
representative embodiments, readily appreciate that many
modifications, additions, substitutions, deletions, and
other changes may be made to these specific embodiments, and
such changes are within the scope of the principles of the
present disclosure. Accordingly, the foregoing detailed
description is to be clearly understood as being given by
way of illustration and example only, the spirit and scope
of the present invention being limited solely by the
appended claims and their equivalents.
