Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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--1--
IMPROVED BEARING SYSTEM FOR A ROLLER CONE ROCK BIT
BACKGROUND OE THE INVENTION
Field of the Invention
The present invention relates generally to
the field of earth boring bits and, more particularly,
to roller cone rock bits.
Descrietion of the Prior Art
Roller cone rock bits generally comprise a
main bit body which can be attached to a rotary drill
string. The bit body usually includes two or three
legs which extend downward. Each leg has a journal
extending at a downward and inward angle. A roller
cone, with cutters, "teeth", or ridges positioned on
its outer surface, is rotatively mount-ed on each jour-
lS nal. During drilling, the rotation of the drill stringproduces rotation of each roller cone about its journal
thereby causing the cutter elements to engage and dis-
integrate the rock.
Because of their aggressive cutting action
and resultant faster penetration rates, roller cone
rock bits have been widely used for oil, gas, and geo-
thermal drilling operations. However, certain problems
exist which limit the useful life and effectiveness of
roller cone rock bits- The useful life of a rock bit
--1-- O
~5~;~
is an especially critical consideration when viewed in
light of the great expense in time and money required
to remove and replace the entire drilling s-tring be-
cause of bit failure.
The bearings used between the journals and
the cones are the source of significant problems. These
bearings operate in an extremely hostile environment
due to high and uneven loads, elevated temperatures and
pressures, and the presence of abrasive grit both in
the hole cuttings and the drilling 1uid. This is par-
ticularly true when drilling deep holes. In addition,
some rock bits such as those used in geothermal explora-
tion are subject to corrosive chemical environments.
Another factor which can lead to early bearing failure
15 is the inability of the bearings to withstand changes
in the moment of forces directed against the roller
cone. As the inserts on the gage row gradually wear
down, the sides of the hole become less defined. As a
result, the forces from the side of the hole increase.
These increased side forces tend to push the cone off
its original axis of rotation, thereby "pinching" the
bearings in their races and contributing to early bear-
ing failure.
Unfortunately, these extreme conditions often
cause failure of the roller cone bearings before any
part of the bit, even before the cone's cutters. In
addition, as the bearings wear, they can allow for more
"wobble" of the cones. As a result, a roller cone bit
with worn bearings does not track as well in the hole
and has a reduced penetration rate. Also, these limits
on the bearing's capacity in turn limit both the load
which can be applied to the bit as well as the angular
velocity at which the bit can be rotated, thereby estab-
lishing constraints on achievable penetration rates and
feasible cutter designs.
the earlier roller c
ng structure was rel ti
Patent Nos. 1 649 858
cone rOck bit with a f
Ype bUshing located b t
h bearingS had a
ut so did the other c
r~ as harder and lon
mented tungsten Carbid
the ther compOnent
as these new bits wer
per hOles through hard
made to improve the b
igher 1Oads for lOnge
iournal and r 1 ~ the tYPical bearingS b t
ler cone Consist of c b
friction bearingS Th
ball or rOller beari
ion, absorb radially di
ain the rOller cone o th
ringS are used bOth fo "
tiDg Sureaces are di
oe the journal aDd ab
aS well as eor radi 1 b
aCes are di5posed par 11
nd absorb radially di
and Cool ants are freqU
life of bearings One d
te the bearings In bit
Cessary to seal the b
g eaSe and keep out th d
MSt of these bits
ir and pressure comp
as Well as the high t
r in drilling de~p hol
~4~
3~397,928; 3 476 195
f the se f ea tureS add
iCation usually re
d or Pinned~ on th
hiCh the legs are weld
ion~ because of th h
ure on the sealS and l
1 Speed at which th
Perated is ften li i
harder, more we~ alSO increased by
r resistant materi 1
P y carburizing ha d
Y for the bearing su f
CmPlexity and co t
lS tion U S p ~ U-S- Patent No 4 054
nt No 4, lso, 301 tea h
g Polycrystalline diam d
nose thrust bearin
tent No. 4,260 203 t
20 Plycrystalli hrUSt bearing surfa
amnd Although the
aring Surfaces may b
ther materials the d
nt for certain prope ti
25 the in diamond In particul
Vation that Plycry t
ns in compression
aUse the '203 design h
et Perpendicular to
Polycrystal 1 ine di a
That is, as the thr t
, the radial bea i
the Patent does not di
cone on the iurnal th
hod With ball bearingS hi
POint in the bearing
U.S. Patent No. 4,145,094 shows a somewhat
aring System for roll
se ball bearings to r t i
hat design, the cone is
eCtron beam Welding an
member onto the journal after it has been inserted into
the cone~ thereby "plugging" the cone onto the journal.
SVM~ ~ OF THE INvENTIoN
fore the seneral object
n to Provide an improved b
rock bit which wi 11 perf
~ at higher rotati
higher applied loads.
, the present invention i
proved bearing system for roller cones on roller cone
partiCularly, the inventi
bit with a frusto-coni
on the journal which m t
aring surface on the r 11
ncludes a means for pre l
pression between these t
faces.
In the preferred embodiment, the compression
an elongate pin which i
one~ extends through th
has a feature at the oth
g urface contacting a thr t
face on the leg of the bit. In simplest form, this pin
i s threadably attached
bolt rotates within the journal, and there is a pair of
type bearings between th
an Utside Surface of th
5~2
. ~
6-
preferred embodiment th
ongate pin which is att h
r leg, extends through th
the Other end with a th
with a thrust bearing f
on the outside of the cone.
In the preferred embodiment, the journal,
hrUSt bearings Compri S
he Preferred embOdiment
g a stream of the drilli
es This results in b f
ing and lubricating of the bearings.
ESCRIPTIoN OF THE DRAWIN
ent inventiOn will be m
the following detailed d
mbdimentS, read in
accompanying drawings, wherein:
is a perspective view and
cross section of a typical prior art roller cone rock
bit;
Figure 2 is a sectional view illustrating a
first preferred embodiment roller cone bit of the pre-
sent invention with a frusto-conical bearing inter~ace
between the journal and the roller cone;
s a perspective view of f
conically shaped journal bearing suitable for use in
he embodiment of Figure 2
Figure 4 is a schematic diagram showing a
of a pair of frusto-coni
p ation and resolution of f
thereon.
Figure 5 is a sectional view of a second pre-
ment wherein the retaini
taChed to the leg of th bi
~$48~
AILED DEscRIp~IoN
the drawings~ Figur
a tYPical prior artis Comprised of a m i
13 adaPted to be thr d
g (not shown). Extendi
legS 14, each of whi h h
ren A cone 16 which h
Y mUnted onto each iou
10 1 he iournal 15 by a s t
ce cut intO the iourn l
n fabrication~ the ball b
e through passage 19 Th
led by plug 20 and th
15 b to the ball bearing 18
t Communicates with p
Wi th lubri cant reser
24 is lcated with th
g er PreSsures and temp
s- An elastomerlc se 1 2
and the journal and s
he drilling debris out
aring 27 serves to ab b
ia11Y to the one whi
aring 28 serve the sa
frCes direct
a SeCtinal vie
iment of the present i
ro 11 er cone rock bi t
bit body 31 which has an end 32 which is adapted to be
y attached to a drill st i
extends down from the main bit body. The preferred
inCludes two Other le
would be equally spaced around the bit body. Bits have
ith one, two~ three O
;2
-8-
he Scope of thiS inve ti
e leg 34 and extends t
A roller cone 36 is r t
1 35 Cutters 37 ar
engagins the bOttom f
PreSent invention is th
rUcture~ the rOller
Y al 1 owing for deeper p k
al terna ti ve embOdime t
10 eintegral tee~h or ann 1
Gage i nsert s 38 a
ne to engage ma
the important
iameter of the hOle
lS tom the Side of the h 1
Y arrow C increase and th
ff its original axi
on the jOurnal 35 is
P bearing 41. It is n t
e frusto-conically sh
the curved side surf
P or bttom. This be
3 and fUrther d
preferred embodiment
25 tnts are discussed in
St a Portion of th
nal bearing surface
ond As used herein th
mond and its abbrevi ti
mPrising diamond cry t l
high preSsure and t
of randomly oriented
y directly bonded to adj
PrPerties of Plycr t
(pcC8~) are quite simil
~2~4SS:~ .
PCD, much of the discussion of PCD will also apply to
ns that will be discus d
preferred material A m ti
conical bearing 42 is mounted within the roller cone
36- In the preferred embodiment, the mating surface of
this roller cone bearing 42 also comprises polycrystal-
diamond
is thread2b1y attached t i
ne 36. The bolt 43 wi11
36. In one preferred b
a locking pin 61 may be inserted through the passage 62
n place, thereby preventi
ve to the cone which w
een the thru t b
The bolt may also be p
e cone 36 using Other m th
welding, application of locking compounds, peening,
etc.
In yet another embodiment, the inventor dis-
Urprising reSult that wh
bearing 41 and roller cone bearing 42 are comprised of
PDC, it is possible to allow the bolt to remain un-
by be self-tightening with
the bearings bind- In particular, instead of causing5 tl~e bearings to bind, the self-tightening of the bolt
fiCial function of maint i
f compression against e h
wear- ACcordingly it
in the preferred embodiments to permit t30 tightening of the bolt- It is deemed within the ordi-
art to select a pitch fwhich would avoid placing too much stress on the bolt
and maximize the benefits of this self-tightening action
3 eXtends through a hol 5
nd has a head 47 which i
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hole 54. This head 47 is adapted to receive a tool for
tightening such as a hex or allen wrench. Adjacent to
the head 47 of the bolt 43 is a thrust bearing or washer
46 which mates with a second thrust bearinq 45 which is
in turn adjacent to the leg 34. Each of these thrust
bearings 46 and 45 may either be bonded in their posi-
tion ~i.e. to the head 47 or leg 34 respectively) or
may be non-attached. As can be seen, a recess 56 in
the leg 34 is adapted to receive the thrust bearinys 45
and 46 along with the head 47. It may also be desir-
able to secure a cover over this recess 56 or fill it
with a filler material such as epoxy for protection
during drilling. In the preferred embodiment, the mat-
ing bearing surfaces of the thrust bearings 45 and 46
comprise polycrystalline diamond. Also, in the pre-
ferred embodiment, the thrust bearings have a substan-
tially planar interface. In alternative embodiments it
may be desirable to shape the head 47 and bearings 45
and 46 to provide a frusto-conical interface between
the thrust bearings.
It will be observed that a space 57 is left
between the distal end surface S9 of the journal bear-
ing 41 and the internal surface 58 of the roller cone
36. Also, the proximate end surface 60 of the roller
cone 36 does not contact the journal 35 or the leg 34.
These two features are important because they allow the
roller cone bearing 42 to be freely compressed against
the journal bearing 41. In other words, in the pre-
ferred embodiment, the frusto-conical journal bearing
is the only surface on the journal or leg which pre-
vents the roller cone from moving axially in the direc-
tion indicated by arrow B.
The inventor has discovered that, in using
this arrangement, the frusto-conical bearings and the
bolt with its thrust washers cooperate to produce at
--10--
i5~
ant benefitS~ First ti
put the frusto-conic 1 b
In particular the b 1
Put it in tension and th
nes in a Stat
PreSsion is highly be fi
r cone b~aring
is the inventorls ob
y strong in compreSsi
n- Therefore~ if on
st for it to be pre 1 d
order to minimize th t
xperience. In othe
ng is less likely t
15 t may be deleterious t th
PeCially true for thi
dUring drilling the PC
ads from multiple dir
is confisuration impr
20 Thi aintain itS original
e of the fact that th
niCal bearing5 and th
gUlar cross-section t k
the pposite Sides f
aring interface and th
gS make up three Side
When force is appli d
Y direction, there is a
es to WhiCh the forc
rmal component. ~or e
reased forces from th
( n the direction ir,di
thrust bearing interf
one bearing doeS not
ff of the journ 1 b
-11-
5~i2
-12-
r embodiment, the distal
5 doe5 contact the su f
nS are carefully
faces 58 and 59 to cont
cient compression betw
e bearings. In this b
inClude PCD Over at 1
nd 59. Allowing the t
ct WOuld provide prot ti
bearing against excessive axial loads encountered dur-
e same result can also b
1 Surface 60 of the c
the leg 34 of the bit- Providing this ~backstop~ to
j rnal and roller cone b
loads may be desirable i
ogenous formations or it
tring- These embodime t
ing the exact amount of
bolt could be tigh~en d
surfaces contact and then backed off.
nOted that the preferr d
thi5 baCkstop~ to pr t
cone bearing against a i 1
prised to discover that th
~5 system with this particular goemetry does operate well
1 loads. As discussed i
with Figure 4, it was th
large axial loads would cause the frusto-conical bear-
not provided with a se
bearing to absorb these axial loads. To the contrary,
the inventor has found that the high axial loads areal to the bearings cper ti
ther embodiment, instead
bolt to retain the cone 36 on the journal 35, a post is
d to the cone in the sa
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s~
-13-
instead integral with the cone. This post has threads
on the end which extend into the recess 56 of the leg
34. A nut is then threaded onto that post with two
thrust washers between it and the leg. In yet another
embodiment, a hole passes through both the journal and
the cone and a bolt with threads on both ends is in-
serted therethrough. A nut is threaded onto each end
and thrust bearings are placed between each nut and the
leg or an outer distal surface of the cone. In this
embodiment it may be desirable to allow both nuts to
self-tighten. This could result in better adjustment
of the tension on the bolt through varying stages of
wear on the elements of the bit.
Another important advantage of the preferred
embodiment of the present invention is that it offers a
bit which can be fabricated with greater ease. First,
the fabrication of typical prior art roller cone rock
bits involves a complex "pinning" step where the ball
bearings are inserted through a hole into a race in
order to retain the cone on the journal. In contrast,
the roller cones of the preferred embodiment can be
attached by simply inserting the retaining bolt through
the leg and threading it into the cone.
Second, in typical prior art fabrication~
~5 because of the complexity of the pinning operation and
spatial limitations, each leg is usually forged sepa-
rately. After a cone is pinned on each leg, the legs
are welded onto a main bit body. (See U.S. Patent No.
4,266,622 for a discussion of the problems inherent in
this process.) In contrast, because the cones of the
present invention are attached in a simpler process, it
may be possible to form the legs as an integral part of
the bit body and yet be able to attach the cones.
Also, if spatial limitations prevent attaching the cones
over the jcurnals when the legs are already in place,
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the preferred embodiment can be modified slightly. In
particular, the journals can be made separate from the
legs. In this way, the journals can be inserted into
the cones, the cones and journals slid into their posi-
tion, and then tne retaining bolt inserted through theleg into the cones. The journals can be indexed with
the leg to insure correct positioning and retention on
the leg.
A further advantage gained by this simplifi-
cation of the fabrication process is that it Will makereconditioning of worn out bits possible. At present,
the standard practice in the drilling industry is to
discard the entire bit when any part of it fails since
the cones or bearings cannot be replaced with the weld-
ed legs approach. With the present invention, it ispossible to replace the cones without destroying and
refabricating the whole bit. Therefore, it will be
economical to use each of the parts of the bit to its
fullest extent. Indeed, preliminary tests show that it
will be the bearings that have the longest life expec-
tancy. Thus, the bearings can be removed when the bit
body and cutters are worn out and put into a new bit.
An advantage which also stems from this sim-
plification of the bit fabrications is that it will now
be possible to service roller cone bits in the field.
That is, because the cones may be removed and attached
in a relatively simple operation and with standard
tools, it will be possible to change cones at the drill-
ing site. In addition to replacing worn out cones, it
30 will also be possible for a driller to maintain an in-
ventory of cones with different types of cutters and
thereby be better able to tune the cutting characteris-
tics of the bit in response to the formation in which
he is drilling. Under present technology, the driller
is required to maintain a costly inventory OI entire
bits to accomplish the 5ame result.
-14-
Although, preliminary tests show that the
bearing of the present invention performs well at high-
er loads and rotational speeds without any lubrication,
it is considered desirable in the preferred embodiment
to cool and lubricate the journal and roller cone bear-
ings 41 and 42 as well as the thrust washers 46 and 45.
This is accomplished in the preferred embodiment by
providing a passage 52 which communicates at one end
with a central cavity 51 in the bit body 31 which in
turn communicates with a source of drilling fluid in
the drill string. A grate or screen 53 is located in
the central cavity 51 to prevent large particles from
entering the passage 52. The other end of the passage
52 cornmunicates with the hole 54 in the journal. In
- 15 this way, a stream of the drilling fluid can pass over
the journal and roller cone bearings and the thrust
washers 45 and 46. It was a surprising result that a
drilling fluid such as typical drilling mud could work
well as both a lubricant and coolant for the bearing of
the preferred embodiment. Drilling mud typically con-
tains high quantities of abrasive silicate particles.
Many bits are designed to keep the mud away from the
bearings. With the present invention, when the bearing
is comprised of polycrystalline diamond, these silicate
particles are actually ground by the polycrystalline
diamond surfaces and result in fine particles which
function as a lubricant on the diamond bearing sur- -
faces. Accordingly, the preferred embodiment does not
require seals to keep the drilling mud away from the
bearings, but rather uses the drilling mud as a lubri-
cant and coolant.
~ igure 3 is a perspective view of the frusto-
conically shaped journal bearing 41 of the preferred
embodiment. This bearing is shaped to mate with a re-
verse shaped roller cone bearing (rlot shown). In the
--15-
-16-
most preferred embodiment, the journal bearing com-
prises a base member 71 which holds polycrystalline
diamond inserts 72 in holes 73. Each insert 72 con-
sists of a layer of polycrystalline diamond 74 bonded
directly to a cemented tungsten carbide back 75. These
- carbide-backed inserts 72 are formed by sintering a
mass of diamond crystals adjacent to the pre-cemented
carbide piece using ultra high pressure and tempera-
tures.
10The height, radius, and slope of the base
member 71 are dictated by the various design parameters
such as the size and shape of the cone. In particular,
the dimensions of the frusto-conical bearings must be
selected with the following considerations in mind.
The bearing must fit within the cone and allow a suffi-
cient wall thickness for the roller cone. The dimen-
sions of the bearing must also allow for a sufficient
thickness of the journal which also has the hole 54
passing through it. Because of the tremendous amount
of wear on the gage row of the bit, it is important
that the frusto-conical bearings do not extend to the
exterior of the bit. That is, the part of the bearing
closest to the gage row should be contained within the
hole of the cone. In addition, the slope of the bear-
ing also affects the retention of the cone on the jour-
nal as it is subjected to the multi-directional forces
from the side and bottom of the hole. With a large
angle between the bearing surface and the axis of rota-
tion the forces from the side and bottom of the hole
will have more of a shear component at the interface of
the journal and roller cone bearing than those same
forces would have with a smaller angle. For the pre-
ferred embodiment, the angle is 20 between the bearing
surface and the axis of rotation.
-16-
-17-
General mechanical principles tend to lead
those skilled in the art to conclude that the frusto-
conical geometry would be inappropriate for friction
bearings which experience high forces directed axially.
A frusto-conical friction bearing with high axial loads
would act much like a wedge, in which the force normal
to the face of the wedge would exceed the applied down-
ward force. Figure 4, which is a~schematic cross sec-
tion of a pair of frusto-conical friction bearings,
illustrates this point. As the roller cone bearing 82
is pushed onto the journal bearing''81 with the axial
force represented by arrow Fa, the.~rce to resist that
motion is represented by arrow Fr. ~owever, because
the two bearing surfaces are not attaehed,- the onl-y
counteractive force must be normal to the two surfaces
and is represented by arrow Fn. Because the normal
force Fn is at an angle to the force Fr, it must be of
a greater magnitude than the force~Fr. As a result,
the normal force Fn (the force which determines the
frictional force between the two b~arings) is magnified.
In trigonometric terms, ~é applied force F
and the normal force Fn of the frust'o,-conical journal
bearing 81 against the frusto-conical roller cone bear-
ing 82 are related by the following equations:
l) Fn = Fa sin 0, à~d~[F = F i 0
2) Ff = Fn = Fa ~sin 0~
where ~ is the angle between the outer surface of the
bearing and the a~is of rotation-of the roller cone,
is the coefficient of friction of the bearing material,
and Ff is the frictional force resisting the rotation
of the roller cone bearing 82 on t~e journal bearing
81. With respect to the forces normal to the surface
of the bearing, the applied force Fa is magnified by a
factor of l/sin a . As ~ approaches 0 (i.e. as the
bearing becomes steeper) l/sin ~ goes to infinity, and
12~
-18-
the normal force Fn becomes infinitely large. The
practical result is that the normal forces in a frusto-
conical friction bearing experiencing high axial loads
would become so large as to exceed the frictional force
Ff and cause the bearing to seize. A surprising result
of the present invention is that, even with a high load
of precompression, no such seizing occurs when a
material such as polycrystalline diamond is used for
the bearing material.
Referring again to Figure 3, the position,
size and number of inserts in each bearing is selected
so as to insure that the polycrystalline diamond sur-
faces support the loads between the roller cone and the
journal. As shown, each insert 72 protrudes slightly
from the outer surface of the base member 71. In the
depicted embodiment, there are three annular rows of
inserts on each bearing. On the journal bearing, there
are ten inserts in the row closest to the leg, eight
inserts in the middle row, and eight inserts in the
distal row. On the roller cone bearing, there are nine
inserts in each of the three rows. This particular
arrangement was selected so as to provide a proper
amount of overlapping of the polycrystalline diamond
surfaces at any given point of rotation of the cone.
In another preferred embodiment, there are two annular
rows of inserts in each bearing, and the angular spac-
ing between inserts is chosen such that there are dif-
ferent numbers of inserts in corresponding rows of the
cone and journal bearings so as to provide for smooth
operation. One bearing may contain closely-spaced in-
serts while the inserts in the other bearing may be
fairly wide spaced. Naturally, the gap between the
inserts on one of the bearings must be smaller than the
diameter of the inserts on the other bearing. Alterna-
tive embodiments include designs wherein there areeither fewer or more inserts as well as more rows.
-18-
Polycrystalline diamond (PCD) is most prefer-
red as the bearing surface material. Although PCD is
extremely wear-resistant, it is relatively brittle,
i.e. it has relatively low tensile strength. As a re-
S sult, PCD would not be expected to perform well in theroller cone bit bearing application where such high and
uneven impact loads are experienced. However, the in-
ventors discovered that this problem could be solved by
maintaining the PCD bearings in a state of compression.
That is, the inventor realized that the extremely hlgh
compressive strength of PCD could be used to offset its
low tensile strength. When the bearings are maintained
in a state of compression, the tensile forces are
greatly reduced. Therefore, the pre-compressed PCD can
survive the high impact loads exerted on the journal
and roller cone bearings.
Another reason that one would not naturally
think to use polycrystalline diamond as a bearing is
that most uses of PCD to date have been for cutting,
grinding or abrading operations. That is, it would be
though that PCD is too rough or abrasive to be success-
ful as a bearing. However, it has been found that when
two PCD surface are well polished and fit together
well, that the coefficient of friction is actually
quite low. The inventor has measured value as low as
O.OOS over wide ranges of loads and speeds ranging up
to 40,000 lbs. axial pre-compression and 1,000 r.p.m.
In fact, most likely due to its high compressive
strength, the coefficient of friction remains low over
an impressive range of applied loads- This low coeffi-
cient of friction at high applied loads is very impor-
tant in relation to the frusto conical geometry which
is discussed with respect to Figure 4.
Another advantage which was discovered in
using PCD for the bearings of the preferred embodiment
-19-
20-
is PCD's high thermal conductivity. In particular, it
is important for the bearings to be able to dissipate
the heat which builds up during use.
Still another advantage of the preferred em-
bodiment using PCD is its relative inertness. That is,
in most bearings which are subjected to high loads at
high temperatures, there is a problem of welding of the
c~ntacting surfaces. To avoid this problem, many such
bearin~s are made from dissimilar metals, a solution
which can introduce new problems related to dissimilar
coefficients of thermal expansion etc. In contrast,
when PCD is used in the present invention, the fact
that diamond is relatively unreactive, obviates these
problems.
Alternative embodiments may employ materials
other than polycrystalline diamond for the bearing sur-
face of the frusto-conical roller cone and journal
bearings. Polycrystalline cubic boron nitride exhibits
many of the same properties as PCD and additionally
possesses much better thermal stability and as such may
be substituted in the bearing of the present invention.
In addition, particular ceramic or cermet materials may
be found to possess the requisite properties to be of
use in the bearing system.
Both the bacX 75 and the polycrystalline
diamond layer 74 are pre-shaped to conform to the cur-
vature and slope of the outer surface of the journal
bearing. The PCD layer 74 is shaped during the ultra-
high pressure/temperature pressing cycle by being
sintered adjacent to the carbide back 75 which has pre-
viously been correctly shaped. Likewise, the inserts
in the roller cone bearing (not shown) are pre-shaped
to conform to the curvature and slope of the inside
surface of the roller cone bearing. The pre-shaping o~
the diamond layer to as near the required shape as
-20-
-21
possible, as opposed to starting with flat or other
nonconforming shapes, has been found to be important
for three reasons. First, polycrystalline diamond is
extremely wear resistant. Accordingly, it would re-
quire large amounts of time and effort to grind or cutthe polycrystalline diamond to fit the final shape.
Second, because the polycrystalline diamond
is a relatively brittle material, it is important that
before the polycrystalline inserts are allowed to wear
against each other, they present smooth surfaces. In
other words, if the polycrystalline diamond pieces ex-
perience point to point contact, they would be likely
to chip or crack. As a result, it is important to have
the polycrystalline diamond pieces conforming to the
curvature and slope of the bearing interface before
use.
Third, the inventor has discovered that the
final finishing of the polycrystalline diamond surfaces
can be accomplished by simply running the two bearings
against each other at high speeds and at high loads.
This simplification of the finishing process would not
ba possible if the polycrystalline diamond surfaces
were not already close to their final shapes.
In addition to the preferred method of using
PCD inserts set into frusto-conical base members, there
exist alternative methods of forming PCD bearing
surfaces with the frusto-conical shape of the present
invention. Theoretically, it would be desirable to
produce a bearing for the present invention with a
single piece of PCD, with or without a carbide backing,
which could be used for the journal or roller cone
bearing. However, using present high pressure
technology, it is not possible to produce pieces of PCD
of a sufficient size.
A feasible alternative is to produce several
PCD segments or "tiles" which could be fit together to
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produce contiguous surfaces of PCD for the bearings of
the invention. These tiles could be used to cover the
entire frusto-conical surfaces, or alternatively could
be arranged in annular rows or other configurations
which would provide sufficient mating PCD s~lrfaces to
support the loads between the roller cone and journal
while facilitating rotation of the roller cone about
the iournal.
Another alternative method for forming the
PCD bearing surfaces of the present invention is to use
PCD in a high concentration matrix. In particular, it
is possible to fill a mass of PCD chunks or grit with a
suitable metal or the like to produce a unitary piece
which possesses properties similar to a piece of solid
PCD. One advantage is that this high concentration
matrix PCD can be produced in much larger pieces than
is possible with solid PCD. Accordingly, a single
piece PCD bearing surface could be produced.
Yet another alternative for forming the PCD
bearing surface would involve coating base member with
PCD. That is, it is possible to coat a iournal bearing
base member or a roller cone bearing base member with a
layer of PCD through such techniques as flame spraying,
electroless plating, etc. In this way, PCD surfaces
can be applied to the bearings of the invention.
Although these other alternatives for incor-
porating the PCD into the journal and roller cone bear-
ings are available, it is noted that the preferred em-
bodiment, i.e., pre-shaped PCD inserts held in frusto-
conical bases, has exhibited certain surprising andimportant advantages. Naturally, in light of its rela-
tively high cost of production, it is economical to use
only as much PCD as necessary. It was thought however,
that a contiguous PCD surface would be required to pro-
vide sufficient smoothness for rotation and load carry-
ing capability. Surprisingly, the bearing constructed
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accordin~ to the preferred embodiment has exhibited ~~ '
remarkable smoothness in rotation and load carrying ~c
capability. A further advantage of using discrete P~D
inserts in the bearing is that it allows for improve'a
cooling and lubrication of the PCD surfaces. With th'e
sligh~ protrusion from the base member, the drillin~'
mud can flow around each insert 72. Also, because the
PCD inserts do not present a contiguous surface, the~
drilling mud can pass directly over the bearing sur-
faces.
Figure 5 shows an embodiment of the inventionwhich is identical to that depicted in Figure 2 except
that the retaining bolt 93 in Figure 5 is threaded into
the leg 34 as opposed to being threaded into the cone
as in Figure 2. The head 98 of the bolt 93 contacts a
thrust bearing 96 which mates with a thrust bearing 97
which contacts the bottom surface of a recess 106 in
the roller cone 36. Being so attached, the retaining
bolt 93 of this embodiment will not rotate with the~
cone 36. However, friction between the mating bearing
surfaces on the thrust bearings 95 and 96 and the pit'ch
of the threa~s 94 produce a self-tightening effect as
with the embodiment depicted in Figure 2. Also, as
with the embodiment of Figure 2, it is desirable to
maintain the bolt 93 in tension so that the journal
bearing 41 and the roller cone bearing 42 are constant-
ly pushed against each other resulting in compression
of the bearing surfaces.
In a further alternative embodiment not
shown, the retaining bolt is made with threads on both
ends. A pair of thrust bearings and a nut are fitted'
on both ends. These thrust bearings could be planar as
shown in Figure 2 and 5, frusto-conical, or other
shapes which would produce the best result. The direc-
tion and pitch of each set of threads is selected so a's
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to provide sel~-tightening on both ends. This embodi-
ment may be desirable to reduce stress to the retaining
bolt.
Although the discussion of the invention has
S included only threaded bolts to retain the roller cone
on the journal and to maintain compression between the
roller cone and journal bearing surfaces, other embodi-
ments exist. For example, pins which are welded and/or
heat shrunk into place may be used to serve the same
two functions. In addition, although the roller cones
depicted all use cutting inserts to engage the mate-
rial, it should be understood that the bearing system
of the present invention is likewise suitable for other
types of roller cones such as those which use steel
teeth or annular ridges to engage and disintegrate
material. Furthermore, although much of the discussion
has focused on the use of polycrystalline diamond on
the bearing surfaces, it is considered within the scope
of the invention to use other materials for the bearing
surfaces, such as polycrystalline cubic boron nitride,
ceramic or cermet materials, which may perform suitably
under these conditions. Certainly, these and other
modifications which are within the ordinary skill in
the art to make are considered to lie within the scope
of the invention as defined by the following claims.
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