Note: Descriptions are shown in the official language in which they were submitted.
1058608
ROCK BIT BEARING STRUCTURE
Background of the Invention
This invention relates to rotary rock bits, and more
specifically to improved manufacturing and bearing structure
for the rotary cutter cones of the drill bit.
Rotary type drilling bits, such as are used in drilling
rock for oil, utilize cutters which are subject to very high
vertical loads from the weight of the drill string. The bear-
ings for rotatably supporting the cutters are subjected to
severe loading conditions which may cause thè drill bit to fail
even before the cutters wear out. The friction type journal
bearing is superior over anti-friction ball or roller type
bearings for withstanding high radial loads, but friction type
bearings require continuous lubrication to prevent excessive
wear and premature failure. This has led to the development
of rock
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bits which include a lubricant reservoir which is in communication
with the bearing area between the rotary cutter cone and the
journal bearing shaft. Such a lubrication system is described,
for example, in U~S. patent:No:3~9l7~o28 which shows a lubrication
system utilizing a reservoir which is pressure-compensated. In
such a system with a pressure compensating reservoir, a lubricant
passage leads from the compensator to the surface of a journal
bearing on the shaft. A flat is often cut on the upper, unloaded
side of the journal bearing where the lubricant passage intersects
it. Use of a pressure compensator permits the lubrication system
to be completely sealed against drilling mud and other contamina-
ting materials while permitting the lubrication system to adjust
to changes in pressure due to expansion and volatilization of the
lubricant in response to the elevated temperatures and pressures
to which the lubricant is subjected during operation.
To further improve wear of the journal bearing, it has
been the practice for several years to provide a bearing metal
on the journal leg or shaft which extends around the lower third
of the circumference of the shaft in the region of maximum load.
During drilling this fraction of the bearing is loaded and the
balance has only light, if any, loads. An arcuate slot was milled
in the bearing surface of the lower third of the shaft and filled
with hard bearing metal by weld deposition. The surface of the
bearing metal was then ground to a radius about 0.002 inch larger
than the journal bearing surface and with an axis of curvature
about 0.002 inch lower than the axis of the journal. Thus, the
lower part of the bearing surface on the journal is offset down-
wardly a small amount. This eccentricity minimizes excessive
squeeze on the O-ring seal on the loaded lower side of the
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journal and does not require an eccentric O-ring groove for the
same purpose. Milling of the arcuate slot for the bearing metal
necessitates a separate machining operation and does add to the
cost of manufacture.
In U.S. Patent No. 3,746,405 there is shown a rock bit in
which an arcuate slot is milled in the upper unloaded half of the
journal shaft, the lower half being case hardened. The space
formed by the slot and surrounding sleeve is used to store a
small quantity of lubricating grease. However, this arrangement
requires a separate milling operation to form a slot around only
a part of the periphery of the shaft. Further, the quantity of
grease that can be stored is very limited and is not compensated
for the effects of large pressure and temperature variations.
Summary of the Invention
The present invention is directed to an improved rock bit
construction and method of manufacture.
In accordance with the present invention there is provided
a rock bit comprising a drill bit body having a journal bearing
spindle shaft, a rotary cutter cone rotatably supported on the
shaft, the shaft having a cylindrical bearing portion, the
cutter cone having a cylindrical bore extending around the
bearing portion for forming a friction bearing, the bearing
portion of the shaft having a shallow groove extending around the
periphery thereof, bearing metal applied in the groove over less
26 than about half the circumference of the groove and having a load
bearing outer surface, a pressure compensating grease reservoir
Ln the bit body, and a passage connecting the grease reservoir
to the groove.
Also in accordance ~ith the present invention there is
provided a rock bit comprising a bit body; a journal bearing
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shaft on the bit body; a rotary cutter cone rotatably supported
on the journal bearing shaft and including a cylindrical bore
extending around a portion of the journal bearing shaft for
forming a friction bearing surface; a bearing portion on the
journal bearing shaft opposite the cylindrical bore, said bearing
portion including a pair of axially spaced apart cylindrical
bearing surfaces of hardened steel, a circumferential groove
between the cylindrical bearing surfaces, and a bearing metal
applied in the groove in only the lower load bearing fraction
of the journal bearing shaft and having a load bearing outer
surface; a lubricant reservoir in the bit body; means in the
lubricant reservoir for compensating for pressure changes; and
a lubricant passage between the reservoir and the unfilled portion
of the circumferential groove.
Further in accordance with the present invention there is
provided a method of making a friction type bearing for a rotary
rock bit comprising the steps of: turning a journal bearing shaft,
forming a cylindrical bearing surface on the shaft, forming a
groove around the circumference of the bearing surface, filling
only a portion of the groove with a bearing metal, and machining
an outer load bearing surface on the bearing metal.
Further in accordance with the present invention there is
provided a method of making a rotary rock bit having a friction
type bearing for a cutter cone comprising the steps of: turning
a journal bearing shaft including a pair of axially spaced apart
cylindrical bearing surfaces and a circumferential groove
therebetween; filling a fraction of the circumference of the
groove with bearing metal; and removing excess bearing metal to
form a load bearing surface on the outer surface thereof.
-- 4
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, The rotary cutters are supported by journal bearings
utilizing pressure compensated lubricant reservoirs and, in
brief, the journal bearing has a shallow groove in the bearing
surface which extends around the full circumference of the
journal shaft. Thus, the groove can be formed at the same
time the journal shaft is being turned on a lathe. Bearing
metal is applied in the groove over the lower load bearing
third of the circumference, leaving the groove open over the
remaining two-thirds of the circumference. Lubricant from a
pressure compensating lubricant reservoir in the rock bit
body is connected through a passage which opens into the
unfilled portion of the groove, the lubricant passing through
the groove directly to the maximum load bearing arc of the
journal in the region of the bearing metal.
' ? !~ .
,;
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Description of the Drawings
For a more complete understanding of the invention
reference should be made to the accompanying drawings, wherein:
FIG. 1 is a longitudinal vertical sectional view through
a portion of a drill bit body;
FIG. 2 is a partial sectional view of a drill bit body
showing the journal leg with the lubrication passages; and
FIG. 3 is a cross-sectional view of the bearing journal
taken substantially on the line 3-3 of FIG. 2.
Detailed Description
Referring to the drawings in detail, the numeral 10
indicates one steel leg of a rock bit body. Normally three
such legs are arranged around the vertical axis and welded
together to form a circular drill bit. The drill bit body
includes an upper shank portion 12 having drill bit attachment
threads 14. The outer surface 16 of the rock bit body is
generally cylindrical in shape and su~tends 120 of arc.
Thus when the three legs are joined they form a complete
cylindrical outer surface for the drill bit body. The lower
end of the leg 10 is formed with an inwardly projecting
spindle shaft or bearing journal, indicated at 18. The axis
of the spindle shaft 18 extends downwardly and inwardly~towards
the vertical axis of revolution of the rock bit. The drill bit
body adjacent the inner end of the spindle shaft 18 is machined
with a flat surface 20 which ioins a generally cylindrical
bearing portion 22 of the spindle shaft by a radius 24.
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A conventional rotary cutter 26 of generally conical shape
has a cylindrical bore 28 which forms a friction bearing
by which the cutter is journaled on the spindle shaft 18.
The cutter can either have integral teeth or carbide inserts
S as are conventional in the rock bit art. Conventionaly the
lower end of the shaft 18 has a reduced diameter friction
pin 30. A hard metal friction bushing32 is mounted in the rotary
cutter core 26 while a trust button 34 is positioned at the end
of the bore in the cutter cone and engages the end of the
friction pin 30. The bearing journal is hardened steel and often
includes hard facing bearing surfaces 31.
To hold the cutter 26 on the shaft 18, ball bearings 36
are provided which roll in complementary race portions 38 and
40 in the center bore of the cutter and in the shaft 18,
respectively. The balls are inserted in the race after the
cutter is positioned on the shaft 18 by inserting the balls.
through a passage 42 connecting the race with the outside sur-
face 16 of the drill bit body. After the balls are inserted
through the passage 42, a retainer plug 44 i9 inserted and
welded in the passage to retain the balls in the race. The ball
retainer plug 44 is provided with a groove 46 through which a
lubricant is supplied to the ball bearings, as hereinafter
described.
Lubrication is provided by a lubricant, such as grease,
stored in a pressure compensated reservoir in a cavity 47 in the
rock bit body. A rubber "boot" 48 in the general form of a
bellows is captive between a metal cup 49 in the cavity and a
plug 50. The interior of the cup 49 and cavity 47 are filled
with grease for the bearings of the rock bit. The grease
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communicates with the bearings by way of a passage 51
through the rock bit body to the lubricant groove 46
in the ball retainer plug 44. The exterior of the rubber
boot communicates with drilling fluid in the well bore during
use of the rock bit by way of passages 52 in the reservoir
plug 50.
Loss of lubricant, as well as admission of drilling
fluid and other foreign material into the bearings, is pre-
vented by an O-ring 53 which engages the radius 24 at the base
of the shaft 18 and engages a groove 54 formed at the outer
end of the bore 28 in the cutter 26. The O-ring 53 provides
a rotary pressure seal between the bearing region of the cutter
and shaft and the exterior of the drilling bit.
Pressure changes between the grease inside the reservoir
and bearings and drilling fluid outside the rock bit occur as
the bit is operated in changing environments of temperature, load
and external pressure. Changes occur in the effective volume of
the grease which could cause either loss of grease from the
system or intrusion of foreign matter into the bearings despite
the O-ring 53 if it were not for the pressure reservoir. Such
volume changes are accommodated by the flexible rubber boot 48
which can move inwardly or outwardly as required and still main-
tain a sealed system without undue loading on the O-ring 53.
As thus far described, the rock bit is of substantially
known design such as described in the above-identified;U.S. Patent
3,917,028. The present invention includes an improved combined
friction-type journal
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bearing and lubrication therefor. When the shaft or pin 18 is
machined by turning it on a lathe about the axis 56, a shallow
groove 58 (60 to 65 mils deep) is formed around the periphery
of the cylindrical portion 22 of the shaft 18. The axial
extent of the groove is slightly less than the axial length of
the cylindrical portion, thereby leaving a narrow bearing shoulder
at each side of the groove. The lower part of the groove 58
(less than about half the circumferential length of the groove)
is then filled with a bearing material 60 such as a type 4 hard
facing composition as set forth in the Metals Handbook, Vol. 6,
page 153. Preferably the hard facing alloy is a nickel or cobalt
base alloy having a higher hardness than the hardened steel of
the balance of the cylindrical bearing surface 22. Other
materials suitable for rock bit journal bearings can also be
used such as aluminum bronze. The bearing metal may be applied
by arc welding, flame welding, or other conventional technique.
Excess bearing metal on the outer surface of the journal bearing
shaft is then ground or otherwise machined off almost flush with
the cylindrical portion 22 of the shaft to have a slight
eccentricity as hereinabove described, allowing the cutter to
be journaled on the shaft without undue squeeze on the O-ring 53
and leaving an unfilled lubricant distribution groove extending
about two-thirds of the way around the top of the shaft. A
lubrication passage 62 in the pin or shaft 18 connects the upper
portion of the distribution groove 58 to the passage 42. Thus a
portion of the lubricating grease from the reservoir 48 passes
through the slot 46 in the ball retaining plug 44 and up the
passage 62 into the groove 58 to lubricate the friction bearing.
The bearing metal in the load bearing fraction of the journal
bearing shaft is preferably chamfered at either end to form a
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beveled transition at the arcuate ends adjacent the remaining
unfilled groove.
It has been found that by forming a groove which extends
around the full circumference of the shaft and filling only the
load bearing lower third of the groove with the bearing metal,
enhanced lubrication of the interface between the hard facing
material of the shaft and the cutter bore is provided. It is
believed that previously insufficient lubricant was reaching the
load bearing portion of the journal bearing. Prior grease
reservoir techniques each failed to assure adequate lubrication
under all operating conditions. It has been found that under
similar conditions, bearing life of the rock bit has been
extended by more than 5%, while at the same time the manufactur-
ing costs have been reduced as compared with previous cutter
bearing designs. Furthermore, the footage of hole drilled per
bit is increased and greater rotational speeds can be used with-
out premature bearing failure. A bearing is considered to fail
prematurely when one bearing on a three cone rock bit fails
while the other two are still in excellent condition or when the
bearings fail far below the expected average run under the same
conditions. Complaints of bit failure have decreased signifi-
cantly with the new journal bearing.
The remaining bearing surface 22 on either side of the
groove 58 on the unloaded side of the shaft 18 is sufficient to
keep the cutter centered and to receive light loads on the
unloaded side experienced, for example, when the drill is with-
drawn from the bore. At the same time, grease is retained in
the distribution groove and circulates to either end of the
groove to lubricate the hard faced load bearing interface 60.
-- 10 --
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The combination of the bearing metal on the load bearing
fraction of the sealed friction bearing with the groove and
connected sealed reservoir of grease assures an ample supply
of grease at the load bearing interface. The pressure compen-
sated reservoir in combination with the lubricant distributiongroove permits lubrication to be maintained under the wide range
of pressure and temperature conditions encountered in deep
drilling operations and minimizes lubricant starvation at the
load bearing interface. Forming a full circumferential groove
and filling a part with bearing metal significantly reduces
manufacturing costs.
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