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.
FIG. 2 is a cross-sectional view through one arm of the
drill bit of FIG. 1.
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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 the cone 12 about the
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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
1
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illustration and description, the surfaces 32, 44 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 the structure 60 will
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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 GO, 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 than a
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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
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have "gradient" modulus properties. Such a gradient 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
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bearing surfaces 40, 42 and respective bearing surfaces 88, 90
on the thrust bearing 86.
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
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interfaces between other pairs of surfaces, such as between
surfaces 34 and 36, and between surfaces 40 and 42.
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.
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Referring additionally now to FIG. 12, another
configuration of the drill bit 10 is representatively
illustrated, in which the cone 12 has the functionally
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 GO and an increased modulus at the structure 74,
thereby providing for reduced contact pressure at the
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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 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.
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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 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.
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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
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 GO 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
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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 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.
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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.
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
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illustration and example only, the scope of the present
invention being limited solely by the appended claims.
