Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a sacrificial compensating
ring for a down hole hammer.
In general, in the art of reverse circulation drilling, a
relatively small annular clearance typically about 3mm is
provided between the drill tube and the bore of the hole
being drilled. The clearance must be kept relatively small
so 'that air preferentially travels around the drill bit
over the cutting face and up inside the drill bit and not
through the annular clearance. Same leakage of air through
the clearance is inevitable but is intended to be kept
small.
As the cutting face bores the hole, it becomes blunt and
wears. The drill bit must then be removed from the hole
and the cutting face of the bit ground to make the cutting
face sharp. As a result, the drill bit becomes smaller in
diameter. When the bit diameter is very close to that of
the drill tube, the bit is no longer serviceable and must
be discarded. Otherwise the drill tube may become damaged
through contact with the wall of the hole.
The drill tube is relatively expensive and the drill bits
are comparatively expensive and this leads to two
conflicting requirements.
On one hand the clearance between the drill tube and the
bore must be kept as small as is practical to minimise
:a
leakage and maximise sample recovery. This means the drill
bit must be very close in diameter to the drill tube, but
not so close so as to lead to damage of the drill tube.
Thus, the usable bit life is reduced. On the other hand,
to increase bit life means the clearance between the drill
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tube and the bore must be as large as is practical to
enable the drill bit to be reground more times and so used
for longer. Thus, the drill.bit must be larger in diameter
compared to the drill tube. Consequently, more air is able
to leak through the annular clearance and the sample
recovery rate drops. This is because much more of the
sample is blown out of the hole around the drill tube and
the volume of compressed air carrying the sample up inside
the drill tube is reduced.
Prior art devices have attempted to overcome this trade off
by blowing air downwardly in the annular clearance adjacent
the drill bit. This has only been partially successful and
results in increased air consumption and sample
contamination.
The present inventian provides a sacrificial compensating
ring far a down hole hammer that attempts to overcome the
above stated problems.
The sacrificial compensating ring of the present invention
is designed to provide an effective seal between the bore
of the hole and the drill tube and to wear down at a
similar rate as the drill bit to maintain such seal as the
drill bit wears. The seal created by the use of the
compensating ring allows an acceptable sample recovery rate
to be maintained without sample contamination or increased
air consumption. The sacrificial compensating ring allows
use of larger drill bits than would otherwise be the case
for a given diameter drill tube.
Sacrificial in the context of the present invention means
that the compensating ring is intended to be consumed in
CA 02021391 2003-06-02
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drilling much the same as the drill bit is.
In accordance with one aspect of the present invention there is
provided a sacrificial compensating ring when used with a
reverse circulation down hole hammer having an outer sleeve, a
drive sub held in the outer sleeve and a drill bit retained in
the drive sub, the compensating ring comprising: an inner
surface and an outer surface, the diameter of at least a first
portion of the outer surface being greater than that of the
outer sleeve and substantially the same as that of the drill
bit, the sacrificial compensating ring adapted for connection
between the outer sleeve and the drive sub in such a manner so
as to longitudinally space the outer sleeve from the drive sub,
and having wear characteristics similar to that of the drill
bit, whereby, in use, the compensating ring wears at a similar
rate to the drill bit.
In accordance with another aspect of the present invention there
is provided a down hole hammer comprising a drill bit retained
in a drive sub, the drive sub being held in an outer sleeve and
a sacrificial compensating ring located longitudinally
intermediate the outer sleeve and the drive sub, the sacrificial
compensating ring comprising an inner surface and an outer
surface, the diameter of at least a first portion of the outer
surface being greater than that of the outer sleeve and
substantially the same as that of the drill bit, the sacrificial
compensating ring adapted for connection between the outer
CA 02021391 2003-06-02
-4a-
sleeve and the drive sub in such a manner so as to
longitudinally space the outer sleeve from the drive sub, and
having wear characteristics similar to that of the drill bit,
whereby, in use, the compensating ring wears at a similar rate
to the drill bit.
The present invention will now be described, by way of example,
with reference to the accompanying drawings, in which:
Figure 1 is a sectional side view of a down hole hammer
incorporating a sacrificial compensating ring in accordance with
the present invention; and
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Figure 2 is a sectional side view of the down hole hammer
of Figure 1 incorporating a sacrificial compensating ring
in accordance with another embodiment of the present
invention.
Shown in the drawings is a reverse circulation dawn hole
hammer 10 comprising a drill bit 12 having a cutting face
14 and a plurality of apertures 16 extending into a central
aperture 18. The drill bit 12 is retained in a drive sub
20 by slip split rings 22 in known mariner. The drive sub
..10 20 is held in an outer sleeve 24 typically by way of a
threaded portion 26 of the outer sleeve 24.
The outer sleeve 24 surrounds porting means (not shown) of
known type to enable a piston (not shown] to reciprocally
strike the bit 12 in known mannex:.
Longitudinally intermediate of the outer sleeve 24 and the
drive sub 20 is a sacrificial compensating ring 28 having a
body 29 comprising a first inner surface 30 substantially
conforming to the drive sub 20 and a second outer surface
32.
The body 29 comprises a base 34 extending transversely to
a longitudinal axis of the down hole hammer 10 and a bevel
36 inclined to a longitudinal axis of the down hole hammer
10 (and hence to the axis of the compensating ring 28).
Typically, such inclination is of the order of 15°. As
shown the bevel 36 tapers inwardly to be flush with the
outer sleeve 24. As may also be seen, the body 29 is
partially proud of the outer sleeve 24, typically by about
3mm to l6mm, where the drill bit 12 is between about 46 and
160mm in diameter,, and the drill tube is about 140mm in
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diameter, for example. It is to be noted that most
drilling equipment is measured in imperial means and the
range of drill bit 12 sizes is between 5.75 to 6.25 inch
diameter for a 5.5 inch diameter drill tube. Ereferably,
the diameter of the compensating ring 28, as measured at
the second outer surface 32, is substantially the same as
that of the drill bit 12.
In Figure 2 there is shown a second embodiment of a
sacrificial compensating ring 98 in accordance with the
present invention. The compensating ring 98 is similar to
the compensating ring 28 and like numerals denote like
parts.
The compensating ring 98 further comprises an annular body
100 extending from a circumferential edge 102 of a body 104
of the compensating ring 98. The flange 100 is disposed
with an interior surface 105 overlying the outer sleeve 24
and directed away from the drill bit 12.
The flange 100 preferably has a diameter substantially the
same as that of the second outer surface 32. The flange
100 typically has no mechanical interference or location
with the outer sleeve 24 of the hammer 10. That is, the
flange 100 typically is not threaded onto the outer sleeve
24.
,, The bevel 36 of the outer surface 32 is replaced with a
bevel 106 located at a free end of the flange 100. The
bevel 106 is otherwise of identical formation and purpose
as the bevel 36 in the first mentioned embodiment.
The body 29, 104 is typically about 25mm long measured in
the longitudinal direction of the down hole hammer 10. The
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flange 100 is typically about 90mm long measured in the
longitudinal direction of the down hole hammer 10 starting
at the edge 102 of the base 104. . All being dimensions
consistent with drill bit diameter of between 146mm to 160
mm - which is typically for 5.75, 6.00 and 6.25 inch drill
bits 22.
Typically, the compensating ring 28, 98 has initially an
outside diameter about 3mm smaller (0.125 inches) than the
diameter of the drill bit 12 so as to avoid drag otherwise
caused by the compensating ring 28, 98, at the outset of
drilling.
Typically, the compensating ring 28 is used for sealing to
smaller holes 108 (i.e. 146mm, 5.75 inch) and the
compensating ring 98 is used for sealing to larger holes
108 (i.e. 152 or 160mm, 6.00 or 6.25 inch).
Typically, the compensating ring 28, 98 is made of a heat
treated alloy steel with a hardness similar to the outer
sleeve 24 but~softer than the drive sub 20. The wear
characteristics of the compensating ring 28, 98 are
preferably similar to those of the drill bit 12.
In use, the compensating ring 28, 98 is placed on the drive
sub 20 and the drive sub 20 is tightly threaded onto the
threaded portion 26 of the outer sleeve 24. In the case of
the compensating ring 98, the flange 100 is disposed so
that its inner surface 105 overlies and contacts the outer
sleeve 24 adjacent the drive sub 20. In both cases the
bevel 36, 106 is uppermost and provides a taper to the
outer sleeve 24.
The down hole hammer 10 with the compensating ring 28, 98
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fitted is then drilled, drill bit 12 first, to form a hole
108 (Figure 2) or inserted, drill bit 12 first, into the
hole 108. The down hole hammer 10 is operated in known
manner and the cutting face 14 produces rock chips.
Compressed air passes out of the down hole hammer 10, in a
manner known in the art and shown by arrows 110, and
carries the rock chips through the apertures 16, into the
central aperture 18 and thence to be recovered.
The body 29 of the compensating ring 28, and the flange 100
and body 104 of the compensating ring 98, both being of
substantially of the same diameter as that of the drill bit
12, are close to or in contact with the bore of the hole
108 and thus present a substantially positive barrier or
seal to the leakage of air. Rock chips carried by air thus
7.5 tend not to pass this barrier or seal and thus are unlikely
to contact the outer sleeve 24. 'This is particularly true
. of shallow holes 108 and of relatively soft ground. Hence
wear of the outer sleeve 24 is reduced.
The outer sleeve 24 surrounds porting means for the hammer
10 and is very expensive compared t~ the drill bit. A
larger annular clearance between the outer sleeve 24 and
the bore of the hole 108 is able to be used as a result o~
this substantially positive barrier and so the outer sleeve
., 24 is better protected from wear. The drill bit 12 is able
to be reground more times, thereby increasing bit life.
The incidence of wearing of the drive sub 20 is also
reduced.
As is known, the bit 12 decreases in diameter after being
reground. The compensating ring 28, 98 is now of larger
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diameter than the bit 12 but will wear to a diameter
substantially the same as that of the bit 12.
Alternatively, the compensating ring 28, 98 may also be
reground to substantially the same diameter as the bit 12.
The inclined bevel 36, 106 directs rock chips away from the
sleeve 24 when the down hole hammer 10 is removed from the
hole 108 and thereby reduces the likelihood of catching or
jamming of the down hole hammer 10 in the hole 108.
Typically, the compensating ring 28, 98 is not threaded or
otherwise fixed onto the drive sub-20 or the outer.casing
24 (in the case of the latter). This is because
difficulties in tightening and loosening of the drive sub
compensating ring 28, 98 and outer sleeve 24 could occur
if it was so ffixed.
15 The present invention enables a larger drill bit to be used
whilst still maintaining an acceptable barrier or seal
against air leakage between the outer sleeve 24 and the
hole 108. Bit life is increased and wear of the outer
sleeve 24 and the drive sub 20 is decreased. Also a
20 greater return of sample chips via the central aperture 1,8
is achieved.
Also, for the compensating ring 98, the flange 100 extends
over substantially all of the threaded portion 26 between
the drive sub 20 and the outer sleeve 24 and so it deters
operators from placing hydraulic break out tools, such as
hydraulic stilsens, upon the threaded portion 26.
Accordingly, the incidence of compressing the thread is
reduced and hence break out is made easier.
Further, the flange 100 also protects the outer sleeve 24
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adjacent the drive sub 20 and hence extends its life.
The compensation ring 28, 98 of the present invention is a
relatively inexpensive, relatively short life, sacrificial
element to protect and prolong the life of the down hole
hammer 10 adjacent the drill bit 12 and achieves good
sealing between the hole and the drive sub 20 to attain
better collection of chippings and less contamination
thereof and reduces loss of chippings between the down hole
hammer 10 and the hole 108.
Modifications and variations such as would.be apparent to a
skilled addressee are deemed within the scope of the
present invention. For example, it could be used for
larger down hole hammers 10.
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