Note: Descriptions are shown in the official language in which they were submitted.
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CUTTER ELEMENT ADAPTED TO WITHSTAND TENSILE STRESS
FIELD OF THE INVENTION
The invention relates generally to earth-boring bits used to drill a borehole
for the ultimate
recovery of oil, gas or minerals. More particularly, the invention relates to
rolling cone rock bits having
cutting elements, and to a more durable structure and shape for such elements.
Still more particularly,
the invention relates to cutting elements having a borehole-engaging leading
compression zone that is
sharper than its trailing tension zone.
BACKGROUND OF THE INVENTION
An earth-boring drill bit is typically mounted on the lower end of a drill
string and is rotated by
rotating the drill string at the surface or by actuation of downhole motors or
turbines, or by both
methods. With weight applied to the drill string, the rotating drill bit
engages the earthen formation and
proceeds to form a borehole along a predetermined path toward a target zone.
The borehole formed in
the drilling process will have a diameter generally equal to the diameter or
"gage" of the drill bit.
A typical earth-boring bit includes one or more rotatable cutters that perform
their cutting
function due to the rolling movement of the cutters acting against the
formation material. The cutters
roll and slide upon the bottom of the borehole as the bit is rotated, the
cutters thereby engaging and
disintegrating the formation material in its path. The rotatable cutters may
be described as generally
conical in shape and are therefore sometimes referred to as rolling cones.
Such bits typically include a
bit body with a plurality of journal segment legs. The cone cutters are
mounted on bearing pin shafts
which extend downwardly and inwardly
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from the journal segment legs. The borehole is formed as the gouging and
scraping or crushing
and chipping action of the rotary cones remove chips of formation material
which are earned
upward and out of the borehole by drilling fluid which is pumped downwardly
through the drill
pipe and out of the bit.
The earth disintegrating action of the rolling cone cutters is enhanced by
providing the
cutters with a plurality of cutter elements. Cutter elements are generally of
two types: inserts
formed of a very hard material, such as tungsten carbide, that are press fit
into undersized
apertures in the cone surface; or teeth that are milled, cast or otherwise
integrally formed from
the material of the rolling cone. Bits having tungsten carbide inserts are
typically referred to as
"TCI" bits, while those having teeth formed from the cone material are known
as "steel tooth
bits." In each case, the cutter elements on the rotating cutters break up the
formation to form
new borehole by a combination of gouging and scraping or chipping and
crushing.
The cost of drilling a borehole is proportional to the length of time it takes
to drill to the
desired depth and location. The time required to drill the well, in turn, is
greatly affected by the
number of times the drill bit must be changed in order to reach the targeted
formation. This is
the case because each time the bit is changed, the entire string of drill
pipe, which may be miles
long, must be retrieved from the borehole, section by section. Once the drill
string has been
retrieved and the new bit installed, the bit must be lowered to the bottom of
the borehole on the
drill string, which again must be constructed section by section. As is thus
obvious, this
process, known as a "trip" of the drill string, requires considerable time,
effort and expense.
Accordingly, it is always desirable to employ drill bits which will drill
faster and longer and
which are usable over a wider range of formation hardness.
The length of time that a drill bit may be employed before it must be changed
depends
upon its rate of penetration ("ROP"), as well as its durability or ability to
maintain an acceptable
ROP. As is apparent, dull, broken or worn cutter elements cause a decrease in
ROP. The form
and positioning of the cutter elements (both steel teeth and TCI inserts) upon
the cone cutters
greatly impact bit durability and ROP and thus are critical to the success of
a particular bit
design.
Bit durability is, in part, also measured by a bit's ability to "hold gage,"
meaning its
ability to maintain a full gage borehole diameter over the entire length of
the borehole. Gage
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holding ability is particularly vital in directional drilling applications
which have become
increasingly important. If gage is not maintained at a relatively constant
dimension, it becomes
more difficult, and thus more costly, to insert drilling apparatus into the
borehole than if the
borehole had a constant diameter. For example, when a new, unworn bit is
inserted into an
undergage borehole, the new bit will be required to ream the undergage hole as
it progresses
toward the bottom of the borehole. Thus, by the time it reaches the bottom,
the bit may have
experienced a substantial amount of wear that it would not have experienced
had the prior bit
been able to maintain full gage. This unnecessary wear will shorten the bit
life of the newly-
inserted bit, thus prematurely requiring the time consuming and expensive
process of removing
the drill string, replacing the worn bit, and reinstalling another new bit
downhole.
To assist in maintaining the gage of a borehole, conventional rolling cone
bits typically
employ a heel row of hard metal inserts on the heel surface of the rolling
cone cutters. The heel
surface is a generally frustoconical surface and is configured and positioned
so as to generally
align with and ream the sidewall of the borehole as the bit rotates. The
inserts in the heel
surface contact the borehole wall with a sliding motion and thus generally may
be described as
scraping or reaming the borehole sidewall. The heel inserts function primarily
to maintain a
constant gage and secondarily to prevent the erosion and abrasion of the heel
surface of the
rolling cone. Excessive wear of the heel inserts leads to an undergage
borehole, decreased ROP
and increased loading on the other cutter elements on the bit, and may
accelerate wear of the
cutter bearing and ultimately lead to bit failure.
In addition to the heel row inserts, conventional bits typically include a
gage row of
cutter elements mounted adjacent to the heel surface but orientated and sized
in such a manner
so as to cut the corner of the borehole. Conventional bits also include a
number of additional
rows of cutter elements that are located on the cones in rows disposed
radially inward from the
gage row. These cutter elements are sized and configured for cutting the
bottom of the borehole
and are typically described as inner row cutter elements.
Each cutter element on the bit has what is commonly termed a leading face or
edge and
a trailing face or edge. The leading face or edge is defined as that portion
of the cutting surface
of the cutter element that first contacts the formation as the bit rotates.
The trailing face or edge
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is the portion of the cutter opposite the leading face or edge and is the last
portion of the cutter
element to contact the formation.
Similarly, it has been found that the stresses produced in each cutter element
during a
cutting cycle are not equal across the body of the cutter. More specifically,
wear studies on used
bits and computer modeling of cutting paths have shown that each cutter
element has a portion
that has been subjected to compressive stress in the direction of cutting
movement and another
portion that has been subjected to primarily tensile stress in the direction
of cutting movement.
It is frequently the case that the leading edge of a cutter element is also
the portion of the cutter
that is subjected to the greatest compressive stress in the direction of
cutting movement.
Similarly, it is often the trailing edge of a cutter element that is subjected
to the greatest tensile
stress in the direction of cutting movement.
The term "leading compression zone" will be used hereinafter to refer to the
portion of a
cutter element that is subjected to large compressive stress, and the term
"trailing tension zone"
will be used hereinafter to refer to the portion of a cutter element that is
subjected to large
tensile stress, regardless of whether the section so referred to is planar,
contoured or includes an
edge. Because the precise portion of the cutter element meeting each
definition varies not only
with bit design and cutter element design, but also with movement of the
rolling cone, it will be
understood by those skilled in the art that the terms "compression" and
"tension" are functional
and are each meant to be defined in terms of the operation of the drill bit
and cutter element
itself.
It has been found that, in a given cutter element, the trailing tension zone
is typically
subject to earlier failure than the leading compression zone, regardless of
whether those zones
are planar, contoured or have a defined "face" or "edge". This is particularly
true with respect
to heel row cutter elements. The predominant failure mode of the trailing
tension zone, and
ultimately of the whole cutter element, is the result of excessive friction
along the trailing
tension zone and of tensile stresses that are localized in the trailing
tension zone. Unlike the
leading compression zone, the trailing tension zone of the cutter element does
not play an active
role in shearing or reaming of the borehole wall, and is therefore subjected
to significantly
smaller compressive forces in the direction of its cutting movement (even
though this trailing
tension zone does experience compressive loading in the direction
perpendicular to the hole
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wall). Instead, as a result of frictional contact with the borehole wall, the
trailing section is
subjected to tensile loads, which induce stress. Inserts coated with
superabrasive materials, such
as polycrystalline diamond ("PCD") and polycrystalline cubic boron nitride
("PCBN"), are
adversely affected by the application of tensile stress, although uncoated
inserts can also suffer
damage on the unsupported trailing tension zone. Because diamond is relatively
brittle,
unsupported or poorly supported areas of diamond coating tend to crack and
break off, leaving
the insert unprotected. Diamond coated inserts are better suited to withstand
wear and frictional
heat compared to uncoated inserts, but are adversely affected by the
application of loads that
induce tensile stress.
SUMMARY OF THE INVENTION
The present invention provides a novel cutter element for an earth boring bit
that avoids
damage that is typically caused by tensile stresses in conventional cutter
elements. The present
cutter element includes a leading compression zone that is sharper than its
trailing tension zone.
By providing a trailing tension zone that is better supported and therefore
able to better
withstand tensile stress, the overall life of both the cutter element and the
drill bit are improved.
The present invention further provides an earth boring bit for drilling a
borehole of a
predetermined gage, the bit providing increased durability, ROP and footage
drilled (at full
gage) as compared with similar bits of conventional technology. The bit
includes a bit body and
one or more rolling cone cutters rotatably mounted on the bit body. The
rolling cone cutter
includes a generally conical surface, an adjacent heel surface, and preferably
a circumferential
shoulder therebetween. Each of the heel, conical and shoulder surfaces may
support a plurality
of cutter elements that are adapted to cut into the formation so as to produce
the desired
borehole.
According to the invention, the cutter elements may be hard metal inserts
having cutting
portions attached to generally cylindrical base portions which are mounted in
the cone cutter, or
may comprise steel teeth that are milled, cast, or otherwise integrally formed
from the cone
material. In either case, the present cutter elements are configured and
formed so as to reduce
tensile stresses on the trailing tension zone. This is accomplished by
increasing the angle at
which the trailing face of the cutter element intersects the wear face of the
cutter element, or by
increasing the radius between the two faces, or by a combination of both. This
design enables
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the cutter elements to withstand longer use, so as to enhance ROP, bit
durability and footage
drilled at full gage.
In one embodiment of the present invention, inserts are formed having
substantially
frustoconical, curved leading and trailing faces, which intersect the wear
face of the cutter
element at a curved edge. The insert is configured in accordance with the
principles of the
present invention such that the inside angle at which the curved leading face
intersects the wear
face is less than the inside angle at which the curved trailing face
intersects the wear face.
In another embodiment of the invention, the sides of the present insert may be
contoured, with the transitions between the leading and trailing faces and the
wear face being
rounded. In this embodiment, the leading compression zone is made sharper than
the trailing
tension zone by providing the leading compression zone with a smaller radius
of curvature than
the radius of curvature of the trailing tension zone.
In still another embodiment, a cutter element having contoured sides and
rounded
transitions and having a leading compression zone sharper than its trailing
tension zone also has
a beveled or relieved sub-zone within its trailing tension zone. More
specifically, a portion of
the cutter element that is subject to particularly great tensile stresses in
the direction of cutting
movement is reduced in a manner that still provides a well-supported cutting
face.
BRIEF DESCRIPTION OF THE DRAWINGS
For an introduction to the detailed description of the preferred embodiments
of the
invention, reference will now be made to the accompanying drawings, wherein:
Figure 1 is a perspective view of an earth boring bit constructed in
accordance with the
principles of the present invention;
Figures 1 A-C are enlarged schematic views of a single cutter element at
different stages
of engagement with a borehole wall;
Figure 1D is a plan view of a single rolling cone of the bit of Figure 1, the
view taken
along the bit axis (the "z" axis) from the pin end of the bit and showing a
projection of the cone
axis onto a plane perpendicular to the bit axis;
Figure lE is an enlarged view of a single cutter element from Figure 1D,
showing a
preferred alternative orientation of the leading compression zone and trailing
tension zone of a
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cutter element constructed in accordance with the principles of the present
invention with
respect to a projection of the cone axis;
Figure 2 is a partial section view taken through one leg and one rolling cone
cutter of
the bit shown in Figure 1;
S Figure 3 is a perspective view of a single cutter element constructed in
accordance with
the principles of the present invention;
Figure 4 is a front elevation of the present cutter element as viewed along
lines 4-4 of
Figure 3;
Figure S is a section view taken along lines 5-5 of Figure 3;
Figure 6 is a plan view of the cutter element shown in Figure 3 including
contour lines;
Figure 7 is a plan view of a first alternative embodiment of the present
cutter element
including contour lines;
Figure 8 is a plan view of a second alternative embodiment of the present
cutter element
including contour lines;
Figure 9 is a plan view of a third alternative embodiment of the present
cutter element
including contour lines;
Figure 10 is a perspective view of a fourth alternative embodiment of the
present cutter
element;
Figure 11 is a section view taken along lines 11-11 of Figure 10;
Figure 12 is a section view of a fifth alternative embodiment of the present
cutter
element;
Figure 13A is a section view of a sixth alternative embodiment of the present
cutter
element;
Figure 13B is a section view of a seventh alternative embodiment of the
present cutter
element;
Figure 14 is a section view of an eighth alternative embodiment of the present
cutter
element;
Figure 1 S is a section view of a ninth alternative embodiment of the present
cutter
element;
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Figure 16 is a perspective view of a steel tooth cone cutter incorporating the
cutter
element of the present invention;
Figure 17 is a side elevation of still another alternative embodiment of the
present cutter
element;
Figure 17A is a plan view of the cutter element of Figure 17, showing a
preferred
orientation of the cutter element with respect to a projection of the cone
axis;
Figure 18 is a front elevation of the embodiment shown in Figure 17;
Figures 19A,B,C are cross-sectional views taken along lines 19-19 of Figure
17,
showing alternative embodiments of the cross section of the cutter element
shown in Figure 17;
Figure 20 is a side view of another alternative preferred embodiment of
another cutter
according to the present invention; and
Figure 2 i is a plan view of the cutter element of Figure 20, showing a
preferred
orientation of the cutter element with respect to a projection of the cone
axis;
Figure 22 is an enlarged, partially cross-sectional view of a portion of the
cutting
structure of the cone cutter shown in Figure 16 and showing the cutter element
of Figures 20
and 21 positioned in a nestled gage row; and
Figures 23 and 24 are perspective and side views, respectively, of an
alternative
embodiment of the cutter element of Figure 20.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring first to Figure 1, an earth-boring bit 10 made in accordance with
the present
invention includes a central axis 11 and a bit body 12 having a threaded
section 13 on its upper
end for securing the bit to the drill string (not shown). Bit 10 has a
predetermined gage
diameter as defined by three rolling cone cutters 14, 1 S, 16 rotatably
mounted on bearing shafts
that depend from the bit body 12. Bit body 12 is composed of three sections or
legs 19 (two
shown in Figure 1 ) that are welded together to form bit body 12. Bit I 0
further includes a
plurality of nozzles 18 that are provided for directing drilling fluid toward
the bottom of the
borehole and around cutters 14-16. Bit 10 further includes lubricant
reservoirs 17 that supply
lubricant to the bearings of each of the cutters.
Referring now to Figure 2, in conjunction with Figure 1, each rolling cone
cutter 14-16
is rotatably mounted on a pin or journal 20, with an axis of rotation 22
orientated generally
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downwardly and inwardly toward the center of the bit. Drilling fluid is pumped
from the
surface through fluid passage 24 where it is circulated through an internal
passageway (not
shown) to nozzles 18 (Figure 1). Each cutter 14-16 is typically secured on pin
20 by locking
balls 26. In the embodiment shown, radial and axial thrust are absorbed by
roller bearings 28,
30, thrust washer 31 and thrust plug 32; however, the invention is not limited
to use in a roller
bearing bit, but may equally be applied in a friction bearing bit. In such
instances, the cones 14,
15, 16 would be mounted on pins 20 without roller bearings 28, 30. In both
roller bearing and
friction bearing bits, lubricant may be supplied from reservoir 17 to the
bearings by apparatus
that is omitted from the figures for clarity. The lubricant is sealed and
drilling fluid excluded by
means of an annular seal 34. It is again to be understood that the invention
is not limited to a
particular bearing or seal structure. The invention may likewise be employed
in unsealed bits
and in bits that have air cooled bearings.
The borehole created by bit 10 includes sidewall 5, corner portion 6 and
bottom 7, best
shown in Figure 2. Referring still to Figures l and 2, each rolling cone
cutter 14-16 includes a
backface 40 and nose portion 42 spaced apart from backface 40. Rolling cone
cutters 14-16
each further include a frustoconical surface 44 that is adapted to retain
cutter elements that
scrape or ream the sidewall of the borehole as rolling cone cutters 14-16
rotate about the
borehole bottom. Frustoconical surface 44 will be referred to herein as the
"heel" surface of
cutters 14-16, it being understood, however, that the same surface may be
sometimes referred to
by others in the art as the "gage" surface of a rolling cone cutter.
Extending between heel surface 44 and nose 42 is a generally conical surface
46 adapted
for supporting cutter elements that gouge or crush the borehole bottom 7 as
the cone cutters
rotate about the borehole. Conical surface 46 typically includes a plurality
of generally
frustoconical segments 48 (Figure 1) generally referred to as "lands" which
are employed to
support and secure the cutter elements as described in more detail below.
Grooves 49 (Figure 1 )
are formed in cone surface 46 between adjacent lands 48. Frustoconical heel
surface 44 and
conical surface 46 converge in a circumferential edge or shoulder 50. Although
referred to
herein as an "edge" or "shoulder," it should be understood that shoulder 50
may be contoured,
such as a radius, to various degrees such that shoulder 50 will define a
contoured zone of
convergence between frustoconical heel surface 44 and the conical surface 46.
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In the embodiment of the invention shown in Figures 1 and 2, each rolling cone
cutter
14-16 includes a plurality of wear resistant inserts 60, 70, 80. Inserts 60,
70, 80 include
generally cylindrical base portions that are secured by interference fit into
mating sockets drilled
into the lands of the rolling cone cutters, and cutting portions that are
connected to the base
portions and have cutting surfaces for cutting formation material that extend
from cone surfaces
44, 46 or shoulder 50. The present invention will be understood with reference
to one such
rolling cone cutter 14, cones 15, 16 being similarly, although not necessarily
identically,
configured.
As best shown in Figure l, rolling cone cutter 14 includes a plurality of heel
row inserts
60 that are secured in a circumferential row 60a in the frustoconical heel
surface 44. Cutter 14
preferably also includes a circumferential row 70a of nestled inserts 70
secured to cutter 14 in
locations along or near the circumferential shoulder 50, a circumferential row
80a of off gage
inserts 80 secured to cutter 14, and a plurality of inner row inserts 81, 82,
83 secured to cone
surface 46 and arranged in spaced-apart inner rows 81 a, 82a, 83a,
respectively. As understood
by those skilled in this art, heel inserts 60 generally function to scrape or
ream the borehole
sidewall 5 to maintain the borehole at full gage and prevent erosion and
abrasion of heel surface
44. Nestled inserts 70 and off gage inserts 80 function primarily to cut the
corner of the
borehole, in that they cooperate to cut both the sidewall and the bottom of
the hole. It is
preferred that these cutters 70, 80 be positioned such that nestled inserts 70
extend to full gage
and primarily perform sidewall cutting, while off gage inserts 80 are off gage
a predetermined
distance and primarily perform bottom hole cutting. Cutter elements 81, 82 and
83 of inner
rows 81a, 82a, 83a are employed primarily to gouge and remove formation
material from the
borehole bottom 7. Inner rows 81 a, 82a, 83a are arranged and spaced on
rolling cone cutter 14
so as not to interfere with the inner rows on each of the other cone cutters
15, 16. While the
present invention is described hereinafter in terms of a heel row insert 60
and nestled row inserts
70, it should be understood that the principle of the present invention can be
advantageously
applied to other cutter elements in other rows as well, although the
advantages of the invention
are presently believed most pronounced when employed in cutter elements whose
primary
function is reaming or sidewall cutting or cooperatively cutting the borehole
corner. Further,
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although it is preferred that inserts 80 be off gage to a predetermined
degree, the principles of
the present invention are equally applicable where inserts 80 extend to full
gage.
Figures 3-5 show a first preferred embodiment of the present invention,
comprising a
novel insert indicated generally by arrow 62. Insert 62 includes a cylindrical
base 61 and a
cutting surface 68. It should be noted that the base 61 is made in cylindrical
form largely
because it is the most practical. Other shapes of bases and corresponding
sockets could be
formed, but since it is more economical to drill circular holes in the cone
for receiving base
portion 61 of insert 62, cylindrical insert bases are generally preferred.
Base 61 includes a
longitudinal axis 61 a. Insert 62 is particularly well suited for use as a
heel row insert and will
be described as such hereinafter, it being understood that it will also have
utility in other
positions as well, including as nestled gage inserts 70, for example.
Cutting surface 68 of insert 62 includes a wear face 63 that is adapted to
extend
beyond heel surface 44 of cone 14, a curved leading face 65, and a curved
trailing face 67.
Wear face 63 can be slightly convex, concave or flat. Wear face 63 includes a
leading
compression zone 64 and a trailing tension zone 66, both generally indicated
in phantom in
Figure 3. Zone 64 and 66 are represented as generally crescent shaped regions
for illustration
purposes, although the actual shape of these zones is dependent on many
factors, such as bit
offset, journal angle, cone geometry, formation being drilled, etc. In any
event, wear face 63
further includes a center point 63a, defined as the point midway between the
leading
compression zone 64 and the trailing tension zone 66. Leading compression zone
64 and
leading face 65 are generally directly opposite trailing tension zone 66 and
trailing face 67 on
insert 62. It will be understood that the terms "leading compression zone" and
"trailing
tension zone" do not refer to any particularly delineated section of the
cutting face, but rather
to those zones in which the stresses (compressive and tensile, respectively)
are most highly
concentrated during cutting.
The application of loads inducing compressive and tensile stress in a cutter
element
can best be understood with reference to Figures lA-C, which schematically
show the
relationship of a conventional heel insert 116 with respect to the borehole
wall 5 as the insert
performs its scraping or reaming function. These Figures show the direction of
the cutter
element movement relative to the borehole wall 5 as represented by arrow 109,
this
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movement being referred to hereinafter as the "cutting movement" of the cutter
element. This
cutting movement 109 is defined by the geometric parameters of the static
cutting structure
design (including parameters such as cone diameter, bit offset, and cutter
element count and
placement), as well as the cutter element's dynamic movement caused by the
bit's rotation,
the rotation of the cone cutter, and the vertical displacement of the bit
through the formation.
As shown in Figure 1 A, as the cutting surface of insert 116 first approaches
and engages the
hole wall, the formation applies forces as represented by arrow 119 inducing
primarily
compressive stresses in the direction of cutting movement in the leading
portion of the insert.
As the cone rotates further, the leading portion of insert 116 leaves
engagement with the
formation and the trailing portion of the insert comes into contact with the
formation as
shown in Figure 1 C. This causes a reaction force from the hole wall as
represented by arrow
120, to be applied to the trailing portion of the insert, which produces
tensile stress in the
insert. With insert 116 in the position shown in Figure 1 C, it can be seen
that the trailing
portion of the insert, the portion which experiences significant tensile
stress, is not well
supported. That is, there is only a relatively small amount of supporting
material behind the
trailing portion of the insert that can support the trailing portion to reduce
the deformation
and hence the tensile stresses, and buttress the trailing portion. As such,
the produced tensile
stress will many times be of such a magnitude so as to cause the trailing
section of heel insert
116 to break or chip away. This is especially the case with inserts that are
coated with a layer
of super abrasive, such as polycrystalline diamond (PCD), which is known to be
relatively
weak in tension. Breakage of the trailing portion or loss of the highly wear
resistant super
abrasive coating, or both, leads to further breakage and wear, and thus
accelerates the loss of
the bit's ability to hold gage.
It will be understood that the views illustrated in Figures lA-C do not
necessarily
represent the cutter path from a uniform perspective. Figures lA-C represent
different
segments of the cutter path arranged so as to best illustrate the concepts
related to
compressive and tensile stresses relative to the direction of cutting
movement.
The orientation of leading compression and trailing tension zones 64, 66
relative to
cone axis 22 and the degree of each zone's arcuate extension around insert 62
are dependent
upon the design and geometry of rolling cone 14. The preferred relative
orientation of the
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leading compression and trailing tension zones within the bit has been
determined by the
study of cutter element wear patterns and by computer modeling of the cutting
paths taken by
cutter elements in the cone of a rolling cone bit. By way of illustration,
reference is now
made to figure 1D, in which these concepts are shown in a view looking down
the bit axis at
rolling cone 14. Figure 1D generally illustrates the leading compression and
trailing tension
zones of a cutter element 60, as divided by imaginary line 23. The portions of
cutter element
60, that are designated leading compression and trailing tension zones in
Figure 1D
correspond to the portions that have been determined to be subjected to
relatively large
compressive or tensile stress in the direction of cutting movement,
respectively, in most bits.
To quantify the relative orientation of the leading compression zone and to
establish a
method of measurement, Figure lE will serve as a frame of reference for the
following
discussion. Figure IE constitutes the projection of cutter element 60"
imaginary line 23, and
cone axis 22 onto a plane perpendicular to the bit axis. This projection is
taken with cutter
element 60, positioned at its furthermost point from the hole bottom. The
imaginary line
projection and the cone axis projection onto this plane are designated 23p and
22p,
respectively and form an angle ~, therebetween, as shown in Figure 1 E. To
achieve at least a
portion of the benefit of this invention, it will be understood that the value
of angle ~" as
measured relative to cone axis projection 22p, can range from zero degrees to
as much as 90
degrees, depending the precise configurations of the cutter element, cone and
bit. In a typical
preferred embodiment, angle ~, ranges from approximately 35 to 80 degrees, and
is most
preferably approximately 60 degrees. Correspondingly, a radial line through
the centerpoint
of the leading compression zone 64 forms an angle ~~ with respect to cone axis
projection
22p. In a typical preferred embodiment, angle ~~ ranges from approximately 10
to 55
degrees, and is most preferably approximately 30 degrees, as shown in Figure 1
E.
Heel cutter 62, one embodiment of the present invention, differs significantly
from
conventional inserts, as best described with reference to Figures 3-5.
Specifically, the
transition between wear face 63 and leading face 65 (leading compression zone
64) is much
sharper than the transition between wear face 63 and trailing face 67
(trailing tension zone
66). As used herein to describe a portion of a cutter element's cutting
surface, the term
"sharper" indicates that either (1) the angle defined by the intersection of
two lines or planes
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WO 97/48874 PCT/US97/10778
or (2) the radius of curvature of a contoured interface, is smaller than a
comparable
measurement on another portion of cutting surface to which it is compared.
In the embodiment shown in Figures 3-5, the relative sharpness of the leading
compression zone as compared to the trailing tension zone, is manifest in the
relative
magnitudes of inside angles a~ and aT (Figure 5), which measure the angles
between wear
face 63 and leading face 65 and between wear face 63 and trailing face 67,
respectively.
According to the embodiment shown in Figure 5, angles a~ and aT are
100° and 135°,
respectively. It will be understood that angles aL and aT can be varied, so
long as aT is
greater than aL.
It is preferred that the cutting surface 68 of insert 62 between leading face
65 and
trailing face 67 be "contoured" or "sculpted," such that the cutting surface
68 of insert 62 is
substantially free of any nontangential intersections. The term
"nontangential" is intended to
describe those interfaces that cannot be described as continuous curves.
Non-circular wear faces are most clearly shown in Figures 7-9, wherein it can
be seen
that wear face 63 need not be circular and that the principles of the present
invention can be
applied to an insert regardless of the relative circumferences of the leading
and trailing faces
of the insert. In Figure 7 curved leading face 65 has a greater radius of
curvature than curved
trailing face 67, in Figure 6 the leading and trailing radii of curvature are
equal and in Figure
8 curved trailing face 67 has a greater radius of curvature than that of
leading face 65. While
the embodiments shown in Figures 7 and 8 have ovoid wear faces 63, other
embodiments
incorporating the principles of the present invention could be made having
wear faces 63 of
other shapes. For example, Figure 9 shows an embodiment in which the leading
and trailing
faces intersect nontangentially. It will be understood by those skilled in the
art that each of
the inserts shown in Figures 7-9 could be formed so as to have the cross-
section shown in
Figure 5. Furthermore, the embodiments shown Figures 3-8 have leading and
trailing faces
65, 67 that comprise sections of cones, with each face being defined by a
straight line when a
cross section of the cutter is taken through its axis as in Figure 5. In the
alternative, leading
and trailing faces 65, 67 can be curved in two directions, in the manner shown
in Figures 10-
11, described below.
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CA 02257932 1998-12-10
_ WO 97148874 . PCT/US97/10778 -
The embodiments of the invention thus described are structured such that the
center
63a of wear face 63 is shifted toward the leading face 65 relative to the
cutter element's axis
61a. For example, as illustrated in Figure 3 the axis 61a of the cutter
insert, as defined by the
axis of its base, does not coincide with the center 63a of wear face 63.
Instead, axis 61a is
well behind center 63a. This is in contrast to previously known inserts, in
which the center
63a of the wear face 63 either coincides with the insert axis 61a or is
located behind the axis
toward the trailing tension zone. Further, the benefit of this geometry is
that the potentially
damaging tensile stress normally induced in the trailing portion of previously
known inserts,
the portion that is typically subject to the greatest tensile stress in the
direction of cutting
movement, is eliminated or reduced to a survivable level.
Referring now to Figures 10-11, a fourth preferred embodiment of the present
insert
uses rounded leading compression and trailing tension zones 64, 66
respectively and rounded
leading and trailing faces 65, 67 respectively. In Figures 10-12 and
subsequent Figures, items
common to the embodiment shown in Figures 3-S are indicated by like reference
numerals.
Because the leading compression and trailing tension zones 64, 66 are rounded,
the relative
sharpness of the leading compression and trailing tension zones is manifest in
the relative
magnitudes of r~ and rT (Figure 11 ), which are the radii of curvature of the
leading
compression and trailing tension zones, respectively. According to a preferred
embodiment,
radius rL and rT are .02 and .09 inches respectively. It will be understood
that radii r~ and rT
can be varied, so long as r, is smaller than rT. It will further be understood
that embodiments
exist, such as that shown in Figure 12, in which the zones 64, 66 are rounded
and leading
radius r~ is greater than rT, but the desired relative sharpnesses of the
leading compression and
trailing tension zones is maintained because of the relative magnitudes of
angles a~ and aT,
aL being less than a.T. It will be further understood that the present
invention does not require
that both zones be rounded, or both angled, so long as the leading compression
zone is
sharper than the trailing tension zone. For example, one or both zones 64, 66
can include a
chamfer, which can affect the sharpness of the transition by its depth.
Likewise, if the
curvature of the transition is not constant, but is elliptical or otherwise
curved, the curvature
of the transition may not be a pure radius. It will be understood that in such
instances, the
smallest radius of curvature for each transition may be used for comparative
purposes, or the
IS
CA 02257932 1998-12-10
WO 97/48874 . PCT/L1S97/10778
position of the center of the wear face with respect to the axis of the base
may be considered,
if that measurement is more direct.
Figures 13-15 illustrate that the advantages of the present invention can be
maintained
even where the insert is formed so as to have significant amounts of positive
or negative rake
S angle in the leading edge. Specifically, Figures 13A and 13B show cutter
elements having a
positive rake angle on its leading face 65. The embodiment of Figure 13B
includes a concave
surface on leading face 65. The embodiment shown in Figure 14 has a more
negative rake
angle than that shown in Figure 5, but still conforms to the principles of the
present invention,
as a~ is less than aT. Figure 1 S shows a cutter element having an extremely
aggressively
shaped leading face 65, similar to the leading edge of Figure 13A, but having
a radiused
intersection with wear face 63 to reduce stress and to diminish the
possibility of breakage.
Increasing the positive rake angle of the leading face 65 makes the cutting
action more
aggressive, which in turn increases ROP potential of the bit.
Referring now to Figures 17, 18 and 19A-C, an alternative construction of the
present
IS cutter element has an essentially chisel-shaped configuration. The chisel-
shaped insert 90 has
an outer wear face 92 generally oriented so as to face the borehole wall
during the portion of
the cutting cycle in which the cutter contacts the wall, an inner face 93
substantially opposite
the outer wear face, a crest 94, and leading and trailing faces 98, 99,
respectively. According
to the present invention and as shown in Figure 17A, chisel-shaped insert 90
is oriented in the
rolling cone so that its crest is perpendicular to a projection 22a of the
axis of the cone. Thus,
insert 90 further includes a crest compression zone 95 between leading face 98
and crest 94
and a crest tension zone 96 between trailing face 99 and crest 94. In
addition, the
intersections of the outer wear face 92 and inner face 93 with the leading and
trailing faces
98, 99 define four edges, identified as outer leading compression edge 100,
inner leading edge
102, outer trailing tension edge 104 and inner trailing edge 106. As described
above, the
crest compression zone 95 is sharper than crest tension zone 96. The insert of
this
embodiment can be made symmetrical, so that each pair of leading and trailing
edges 100/102
and 104/106 is substantially the same. Alternatively, as described with
respect to the previous
embodiments, this chisel-shaped insert 90 can be modified in a similar manner
such that the
outer trailing tension edge is adapted so as to further reduce the tensile
stress produced in the
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insert, as shown in Figures 19A-C. Figure 19A shows an embodiment in which
outer trailing
tension edge 104 is contoured with a larger radius of curvature than that of
outer leading
compression edge 100 and Figure 19B shows an embodiment in which the same
intersection
104 is made essentially planar by eliminating a portion of the insert at the
corner. Figure 19C
shows an embodiment in which the leading face 98 has a positive rake angle,
illustrated at
transition 100. Insert 90 is believed best employed in the position of nestled
gage row 70a,
although insert 90 may also be employed in other rows as well, including in
heel row 60a,
off gage row 80a, and conventional gage rows.
By changing the geometry of the trailing portion of a heel cutter insert 60 or
nestled
insert 70, for example, in the manner described above, the portion of the
insert placed in
greatest tensile stress in the direction of cutting movement during drilling
is removed. In this
manner, the tensile stresses that would otherwise be produced in the insert
can be relieved
without adversely affecting the amount of mechanical support provided to
leading
compression zone 64 by the body of cutter 62. It is this relationship that
results in the
improvement in cutter life and the desired features of the present invention.
The failure mode of cutter elements usually manifests itself as either
breakage, wear, or
mechanical or thermal fatigue. Wear and thermal fatigue are typically results
of abrasion and
friction as the elements act against the formation material. Breakage,
including chipping of the
cutter element, typically results from loads causing tensile stresses,
including impact loads,
although thermal and mechanical fatigue of the cutter element can also
initiate breakage. The
trailing edge of prior art inserts is subjected to a combination of abrasive
wear, frictional heat,
tensile stresses and impact forces from the cutting action. On tungsten
carbide inserts, the
frictional heat combined with rapid cooling by the drilling fluid can lead to
thermal fatigue,
initiating a network of micro cracks on the surface. Frictional forces on the
surface of the
trailing tension zone place the trailing portion of the insert under tensile
stress, causing the
cracks to propagate by mechanical fatigue leading to chipping or breakage.
Prior art inserts
coated with polycrystalline diamond (PCD} are especially prone to chipping and
breakage of the
trailing portion due to tensile stresses in the trailing tension zone and
impact forces from the
cutting action.
17
CA 02257932 2005-04-05
The present invention addresses the above failure modes by significantly
reducing the tensile
stress in the direction of cutting movement in the trailing portion of the
insert. The new geometry of the
trailing section provides structural support to better enable the insert to
withstand frictional forces that
cause tensile stress and impact forces that result from the cutting action.
Due to a lesser area being
presented to the formation by the trailing tension zone and a larger trailing
face exposed to drilling
fluid, the frictional heat is reduced and more efficiently dissipated and
therefore the potential of thermal
fatigue is reduced. Even if thermal fatigue should occur, the new geometry of
the present insert is
better suited to withstand the mechanical loading that causes the tensile
stress component and leads to
chipping and breakage. The new and improved geometry of the trailing portion
provides increased
opportunities for inserts with superabrasive coatings, such as PCD and PCBN,
since the principal
factors that cause the superabrasive coating to fail are greatly reduced.
The present cutter element is a departure from prior art mufti-cone bit cutter
elements that have
generally either required that the leading and trailing portions of the cutter
element be symmetrical, or
have provided a trailing portion that is sharper than the leading portion. In
other systems, attempts have
been made to reduce the tensile stresses and premature failure in the heel
rows inserts by inclining the
whole cutter element so that its trailing portion is at a greater distance
from the borehole wall than is its
leading portion. These prior art devices, however, have the adverse effect of
forcing the leading edge of
wear face 63 to do all of the work associated with scraping and/or reaming the
borehole sidewalk with
the result that the surface area of each cutter element is back-relieved and
not in full contact with the
borehole wall and therefore is not available to help resist abrasive wear
caused by contacting the
formation. In the present invention, the positioning of wear face 63 with
respect to the borehole wall is
maintained so that virtually the entire wear face 63 can operate on the
borehole sidewall.
A particularly preferred embodiment of the present invention includes use of
cutter elements in
accordance with the present invention in a bit having gage and off gage cutter
elements positioned to
separate sidewall and bottom hole cutting duty. A bit of this sort is fully
disclosed and described in
commonly owned copending application filed on April 10, 1996, Serial No.:
08/630,517, and entitled
Rolling Cone Bit with Gage and Off gage Cutter Elements Positioned to Separate
Sidewall and Bottom
Hole Cutting Duty, now issued as US Patent No. 6,390,210. The cutter elements
of the present
invention, having a relatively sharper leading section and relatively less
sharp trailing section, can be
used advantageously in place of any one or more of heel row cutter elements or
gage row cutter
18
CA 02257932 2005-04-05
elements, as described in the copending application. In addition, it will be
understood that the cutter
elements of the present invention can be used in bits that have more than one
heel row.
Referring now to Figures 20 and 21, another embodiment of an insert
constructed according to
the principles of the present invention comprises an insert 200. The
configuration of insert 200 makes it
especially suited for use in the nestled row, such as inserts 70 in Figure 1,
but it can likewise be
employed with benefit in other rows as well. Insert 200 includes a base 261
and a cutting surface 268.
As described previously, base 261 is preferably cylindrical and includes a
longitudinal axis 261a.
Cutting surface 268 of insert 200 extends beyond shoulder 50 of cone 14 and
includes a slanted or
inclined wear face 263, a frustoconical side surface 280 including a leading
face 265 and a trailing face
269, and a circumferential transition surface 267. Wear face 263 can be
slightly convex or concave, but
is preferably substantially flat. Wear face 263, although inclined as compared
to previous
embodiments, is oriented in the cone so as to hug the gage curve and resist
abrasive wear by projecting
a substantial area against the formation. As best shown in Figure 20, wear
face 263 is inclined at an
angle a with respect to a plane perpendicular axis 261a. As shown, /3
indicates the angle between axis
261a and the leading face 265 of surface 280. It will be understood that
leading face 265 can
alternatively have a positive rake angle, similar to those shown in Figures
13A, 13B and 15, discussed
above. Likewise, the surface 280, including leading face 265 and trailing face
269, need not be
frustoconical, but can be rounded or contoured in the manner illustrated in
Figures 10 and 11, and the
angle /3 between surface 280 and axis 261a need not be constant around the
circumference of the insert.
Circumferential transition surface 267 forms the transition from wear face 263
to leading face
265 on one side of insert 200 and from wear face 263 to trailing face 269 on
the opposite side of insert
200. Circumferential shoulder 267 includes a leading compression zone 264 and
a trailing tension zone
266 (Figure 21). It will be understood that, as above, the terms "leading
compression zone" and
"trailing tensile zone" do not refer to any particularly delineated section of
the cutting face, but rather to
those zones that undergo the larger stresses
19
CA 02257932 1998-12-10
_ WO 97!48874 . PCT/LTS97/I0778 -
(compressive and tensile, respectively) associated with the direction of
cutting movement. The
position of compression and tension zones 264, 266 relative to the axis of
rolling cone 14, and
the degree of their circumferential extension around insert 200 can be varied
without departing
from the scope of this present invention.
S Refernng briefly to Figures 21 and 1 E, in a typical preferred
configuration, a radial line
270 through the center of leading compression zone 264 lies approximately 10
to SS degrees,
and most preferably approximately 30 degrees, clockwise from the projection
22a of the cone
axis, as indicated by the angle 0 in Figure 21. A line 272 through the center
of trailing tension
zone 266 preferably, but not necessarily, lies diametrically opposite leading
center 270.
In accordance with the present invention, leading compression zone 264 is
sharper than
trailing tension zone 266. Because compression and tension zones 264 and 266
are rounded,
their relative sharpness is manifest in the relative magnitudes of r~ and rT
(Figure 20), which are
radii of curvature of the leading compression and trailing tension zones,
respectively and a,~ and
a.~;, which measure the inside angle between wear face 263 and leading and
trailing faces.
1S Shoulder 267 is preferably contoured or sculpted, so that the progression
from the smallest
radius of curvature to the largest is smooth and continuous around the insert.
For a typical
S/16" diameter insert constructed according to a preferred embodiment , the
radius of curvature
of surface 267 at a plurality of points c,~ (Figure 21) is given in the
following Table I.
Table I
Radius of
Point Curvature
(in.)
c, .050
c2 .050
c3 .120
cq .080
By way of further example, for a typical 7/16" diameter insert constructed
according to
the present invention, the radii at points c,_q are given in the following
Table II.
Table II
CA 02257932 1998-12-10
_ WO 97/48874 . PCT/US97/10778
Radius of
PointCurvature
(in.)
c, .050
cZ .050
c3 .160
c4 .130
An optimal embodiment of the present invention requires balancing competing
factors that tend
to influence the shape of the insert in opposite ways. Specifically, it is
desirable to construct a
robust and durable insert having a large wear face 263, an aggressive but
feasible leading
compression zone 264, and a large rT so as to mitigate tensile stresses in the
direction of cutting
movement in trailing tension zone 266. Changing one of these variables tends
to affect the
others. One skilled in the art will understand that the following quantitative
amounts are given
by way of illustration only and are not intended to serve as limits on the
individual variables so
i l lustrated.
Thus, by way of illustration, in one preferred embodiment, angle a is between
5 and 45
degrees and more preferably approximately 23 degrees, while angle (3 on the
leading side is
between 0 and 25 degrees and more preferably approximately 12 degrees.
According to a
preferred embodiment, the smallest radius of curvature r~ for a 5/16 inch
insert is .050 inches
and the largest radius of curvature rT is .120 inches. It will be understood
that radii r,, and rT
can be varied, so long as r~ is smaller than rT. It will further be understood
that embodiments
exist, similar to that shown in Figure 12, in which the zones 264, 266 are
rounded and trailing
radius rL is greater than rT, but the desired relative sharpnesses of the
leading compression and
trailing tension zones is maintained because of the relative magnitudes of
angles aL and aT,
oc~ being less than aT. It will be further understood that the present
invention does not require
that both zones be rounded, or both angled, so long as the leading compression
zone is
sharper than the trailing tension zone.
Insert 200 optionally includes a pair of marks 274, 276 on cutting surface
26$, which
align with the projection 22a of the cone axis. Marks 274, 276 serve as a
visual indication of
the correct orientation of the insert in the rolling cone cutter during
manufacturing. It is
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WO 97/48874 . PCTIUS97/10778 -
preferred to include marks 274 and 276, as the asymmetry of insert 200 and its
unusual
orientation with respect to the projection 22a of the cone axis would
otherwise make its proper
alignment counter-intuitive and difficult. Marks 274, 276 preferably
constitute small but visible
grooves or notches, but can be any other suitable mark. The insert 200 is
preferably used in the
nestled gage position indicated as 70 in Figure 1, but can alternatively be
used to advantage in
other cutter positions. In a preferred embodiment, marks 274 and 276 are
positioned 180
degrees apart.
Figure 22 shows an insert 200 in the nestled position on a steel tooth cone
and shows its
relationship to gage curve Sa. As understood by those skilled in the art of
designing bits, a "gage
curve" is commonly employed as a design tool to ensure that a bit made in
accordance to a
particular design will cut the specified hole diameter. The gage curve is a
complex
mathematical formulation which, based upon the parameters of bit diameter,
journal angle, and
journal offset, takes all the points that will cut the specified hole size, as
located in three
dimensional space, and projects these points into a two dimensional plane
which contains the
journal centerline and is parallel to the bit axis. The use of the gage curve
greatly simplifies the
bit design process as it allows the gage cutting elements to be accurately
located in two
dimensional space which is easier to visualize.
Wear face 263 hugs the gage curve Sa, meaning that wear face 263 follows the
contour
of the gage curve when viewed in rotated profile as shown in Figure 22. Wear
face 263 thus
provides a large area for frictional engagement. Use of the present cutter
elements in steel tooth
bits is described in greater detail below.
Referring now to Figures 23 and 24, an alternative embodiment 300 of insert
200
includes the features of insert 200, plus one or more relieved or beveled
trailing sub-zones.
Specifically, insert 300 includes a body 361, wear face 363, leading face 365,
transition surface
367 and compression and tension zones 364, 366, respectively. In the
embodiment shown,
transition surface 367 includes relieved sub-zones 368a, 368b that each
comprise a slightly
flattened region in the trailing tension zone 366. Relieved sub-zones 368a,
368b effectively
reduce the portion of trailing tension zone 366 that is subjected to the
largest tensile stress in the
direction of cutting movement by increasing the included angle between wear
face 363 and sub-
zone 368. In addition, wear face 363 may be slightly convex, as shown in this
embodiment,
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allowing further relief of the trailing portions of the cutter element. It
will be understood that
sub-zones 368 a, 368b also need not be flat, but can be slightly convex.
Alternatively, rather
than providing two distinct sub-zones 368a, 368b, insert 300 can include a
single continuous or
contoured sub-zone 368 that extends or covers the regions shown as sub-zones
368a, 368b in
Figure 23.
The various cutter elements and the present invention may be employed in steel
tooth
bits as well as TCI bits as will be understood with reference to Figures 16
and 22. As shown, a
steel tooth cone 130 is adapted for attachment to a bit body 12 in a like
manner as previously
described with reference to cones 14-16. When the invention is employed in a
steel tooth bit,
the bit includes a plurality of cutters such as rolling cone cutter 130.
Cutter 130 includes a
backface 40, a generally conical surface 46 and a heel surface 44 which is
formed between
conical surface 46 and backface 40, all as previously described with reference
to the TCI bit
shown in Figures 1-2. Similarly, steel tooth cone cutter 130 includes heel row
inserts 60
embedded within heel surface 44, and nestled row cutter elements 70, such as
inserts 200
disposed adjacent to the circumferential shoulder 50 as previously defined.
Although depicted
as inserts, nestled cutter elements 70 may likewise be steel teeth or some
other type of cutter
element. In addition to cutter elements 60, 70, steel tooth cutter 130
includes a plurality of gage
row cutter elements 120 generally formed as radially-extending teeth. Steel
teeth 120 include
an outer layer or layers of wear resistant material 120a to improve durability
of cutter elements
120.
Steel tooth cutters such as cutter 130 have particular application in
relatively soft
formation materials and are preferred over TCI bits in many applications.
Nevertheless, even in
relatively soft formations, in prior art bits in which the gage row cutters
consisted of steel teeth,
the substantial sidewall cutting that must be performed by such steel teeth
may cause the teeth
to wear to such a degree that the bit becomes undersized and cannot maintain
gage. The
benefits and advantages of the present invention that were previously
described with reference
to a TCI bit apply equally to steel tooth bits. Namely, any of heel row
cutters 60 and nestled
row cutters 70 can be configured in accordance with the principles set out
herein if it is desired
to reduce the effects of tensile stress in the direction of cutting movement
on the trailing tension
zone of the cutter elements.
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While various preferred embodiments of the invention have been shown and
described,
modifications thereof can be made by one skilled in the art without departing
from the spirit and
teachings of the invention. The embodiments described herein are exemplary
only, and are not
limiting. Many variations and modifications of the invention and apparatus
disclosed herein are
possible and are within the scope of the invention. Accordingly, the scope of
protection is not
limited by the description set out above, but is only limited by the claims
which follow, that
scope including all equivalents of the subject matter of the claims.
24
