Note: Descriptions are shown in the official language in which they were submitted.
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PNEUMATIC DRILL
FIELD OF THE INVENTION
This invention relates to pneumatic drills and more particularly to the kind
of drills known
as "down-the-hole" drills.
BACKGROUND TO THE INVENTION
Drills of this kind usually include a pneumatically powered piston
reciprocating to impact
on a drill bit. The gauge of the drill bit must provide a hole large enough
for the passage
through the drilled rock of the operating mechanism. The operating mechanism
can
then be located at the working surface and the compressed air to power the
operating
mechanism conveyed down the hole through a drill stem to the operating
mechanism.
This air must be caused to act intermittently to drive the piston and to
return the piston
to a position at the start of its power or impact stroke. It is also necessary
that the air be
exhausted from the mechanism. Preferably this air is used to clear drilled
rock from the
working surface particularly when the bit is raised from the working surface
and allowed
to fall onto its retaining rings in the bit chuck.
These well known and widely used mechanisms are necessarily compact and the
air
passages confined. It is an ongoing challenge to improve the efficiency of the
supply of
air to these drills while maintaining the strength of the component parts in
the down-the-
hole working conditions against failure and unacceptable wear rates.
A particular down-the-hole drill of the kind referred to is disclosed in the
specification of
my South African Patent No. 2005/03406 and that specification is included in
this
document in its entirety by reference.
The drill disclosed in the above specification uses a chamber divider to
direct the air
suppiy from the backhead end into spaced cutouts in the inner wall of the wear
sleeve
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and around the piston. The latter forms at least one shoulder exposed to air
pressure
during operation of the drill. This gives rise to difficulties in balancing
the air pressure
forces to reciprocate the piston. More particularly unless adequate exhausting
from the
chamber formed to provide the return stroke of the piston is obtained the
piston will
continue to reciprocate when the bit is withdrawn from the working surface.
This is
explained more fully below. The cutouts in the wall of the cylinder also makes
its
necessary overall wall thickness such that it can withstand the operating
stresses at its
parts of minimum material thickness. This limits the dimensions of the piston
that can be
used in a drill designed to drill a specific hole size. Any use that can be
made of the
space occupied by wall thickness material having excess strength
characteristics
relative to the basic minimum requirement enables an overall increase in
effectiveness
of the drill to be achieved.
OBJECT OF THE INVENTION
It is an object of the present invention to address the problems referred to
and mitigate
their effects on the overall efficiency of such down-the-hole hammer drill
assemblies.
SUMMARY OF THE INVENTION
In accordance with the invention there is provided a down-the-hole pneumatic
hammer
drill assembly comprising: a backhead secured to a wear sleeve for an impact
piston; a
drill bit mounted in a chuck in the wear sleeve for limited reciprocation
remote from the
backhead and providing an anvil for the piston; the piston having a stem of
reduced
diameter for impacting the bit; the stem guided in a piston stem bush; and an
exhaust
flow passage provided passed the piston stem bush into a bore through the bit
to vent
air from a chamber around the piston stem when the bit is lifted from a
working surface.
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The invention further provides for the exhaust flow passage to be provided
through the
piston stem bush; or for the exhaust flow passage to be provided by a recess
partway
along the piston stem.
A further feature of one aspect of the invention provides for the drill
assembly to include
fluid paths formed by cut-outs in the walls of the piston and the wear sleeve
with a
longitudinal annular passage through the wear sleeve for the supply of
pressurized fluid
to drive the piston, the annular passage extending from the backhead end of
the
assembly and having spaced apart ports communicating with an air supply
passage
through the backhead at one end and with the interior of the wear sleeve at
the other
end.
Further features of the invention provide for the annular passage to extend
between the
outer surface of a part of the wear sleeve having a reduced diameter and the
inner
surface of a shroud radially spaced from the surface of reduced diameter and
secured
to the wear sleeve; and for the shroud to be releasably located in position by
attachment
of the backhead to the end of the wear sleeve.
Still further features of invention provide for the annular passage to extend
between the
inner surface of part of the wear sleeve having an increased diameter and the
outer
surface of a skirt depending from the backhead and for the skirt to be screw-
threaded to
engage within a screw-thread at the end of the wear sleeve for attachment of
the
backhead to the wear sleeve.
In accordance with still another aspect of the invention there is provided for
the drill
assembly to include fluid paths formed by cut-outs in the walls of the piston
and the
wear sleeve with a longitudinal annular passage through the wear sleeve for
the supply
of pressurized fluid to drive the piston, the annular passage extending from
the
backhead end of the assembly between the outer surface of a part of the wear
sleeve
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having a reduced diameter and the inner surface of a shroud radially spaced
from the
surface of reduced diameter and secured to the wear sleeve.
In accordance with yet another aspect of the invention there is provided for
the drill
assembly to include fluid paths formed by cut-outs in the walls of the piston
and the
wear sleeve with a longitudinal annular passage through the wear sleeve for
the supply
of pressurized fluid to drive the piston, the annular passage extending from
the
backhead end of the assembly between the inner surface of part of the wear
sleeve
having an increased diameter and the outer surface of a skirt depending from
the
backhead; and in which the skirt is screw-threaded to engage within a screw-
thread at
the end of the wear sleeve for attachment of the backhead to the wear sleeve.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other features of this invention will become apparent from the
following
description of embodiments of the invention described below with reference to
the
accompanying drawings in which:
Figure 1 shows a longitudinal cross-section through the drill assembly and the
insert
shows a modification;
Figure 2 shows a longitudinal cross-section through an alternative embodiment
of a
drill assembly; and
Figure 3 shows an enlarged view of part of Figure 2.
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DETAILED DESCRIPTION OF THE INVENTION
In the embodiment illustrated in Figure 1 compressed air is introduced into
the drill
assembly (1) through an inlet passage (2) in the backhead (3). The air is then
passed
through distribution passages (4) into a passage (5) around the circumference
of the
backhead (3).
A wear sleeve or cylinder (6) has at its operatively upper, outer end an
annular recess
providing reduced wall thickness. A longitudinally extending, annular passage
(7) is
formed by securing a shroud (8) at its ends over the reduced wall thickness
portion of
the wear sleeve (6). This securing will include seals (9) and may be effected
by welding
or the like. In this embodiment, a screw-thread attachment of the backhead (3)
to the
wear sleeve (6) is used to secure the shroud (8) in position.
The backhead includes a known assembly of a check valve (10) and control rod
(11).
Figure 1 shows the location of the necessary cut-outs in the inner wall of the
wear
sleeve (6) and along the outer wall of a piston (12). The cutouts provide the
necessary
flow paths for the compressed air through the assembly to reciprocate the
piston (12)
which strikes the anvil at the operatively upper end of the bit (13).
In the condition shown, the check valve (10) is closed and the control rod
(11)
withdrawn from the piston (12). When compressed air is applied, the check
valve (10)
opens against its spring bias to allow flow through passages (4) into the
passage (5) in
the backhead (3) and thence through inlet ports (7A) to outlet ports (7B) via
annular
passage (7) in the wear sleeve (6).
The passage (7) feeds the interior of the wear sleeve (6) and thus the chamber
(14)
formed between the upper cutouts in the piston (12) and wear sleeve (6)
through outlet
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ports (7B). This chamber (14) remains pressurized while the check valve (10)
is open
and alternately provides pressure to both ends of the piston (12) for power
and return
strokes as is set out below.
While the drill (1) is operating, the head or operatively upper end of the
piston (12)
never moves down the wear sleeve (6) passed the outlet ports (7B). The piston
(12) has
a reduced diameter in a lower region of chamber (14) and is stepped outwardly
to form
a shoulder at (15). Cutouts (16) in the enlarged step (15) are provided as
shown. The
shoulder (15) together with the bottom of the cutouts (16) provides a stepped
surface
area on the piston (12) which in use will be under the influence of
pressurized air in the
chamber (14). This surface area will be referred to as shoulder (15).
An axial bore (17) is provided through the piston (12) and the piston has a
stem (18) of
reduced diameter at its operatively lower free end. The free end of the stem
(18) forms
the striking face which acts on the anvil of the bit (13).
A piston stem bush (20) fitted in the wear sleeve (6) acts as a guide for the
stem (18) of
the piston (12) and upper end of the bit (13) when the latter is in the impact
zone during
operation of the drill.
The reduced diameter of the piston stem (18) forms a second chamber (21) in
the
assembly (1) and a usual form of chuck (22) with retaining ring (23) is
located in the
bottom of the wear sleeve (6). This allows limited free axial movement of the
bit (13)
when it is not in contact with a working surface.
In the described embodiment of this invention shown in Figure 1, exhaust
venting ports
(24) and (25) are shown from the chamber (21) around the stem (18) through the
piston
stem bush (20). An alternative arrangement of piston stem bush (20) is
illustrated in the
insert shown. In this latter arrangement recesses in the form of slots (27)
are provided
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partway along the piston stem (18) to provide the venting passage from the
second
chamber (21) when the bit (13) is in its extended position.
In operation the compressed air supplied through the annular passage (7)
enters the
chamber (14) and from there passes into the second chamber (21).
The pressure in the chamber (21) acts on the bottom surface of piston (12) and
urges it
upwards. In this travel (a) on the piston (12) passes (b) in the bore of the
wear sleeve
(6) to prevent the air from entering the chamber (21). The piston stem (18)
withdraws
from the piston stem bush (20) and the air in the chamber (21) exhausts
through bore
(28) in the bit (13) to atmosphere.
Point (c) in the bore (17) of the piston (12) passes (d) on the control rod
(11) thereby
sealing off the upper chamber (26) formed around the control rod (11) from
exhausting
through the bore (17) in the piston (12). As (e) on the piston passes (f) in
the bore of the
wear sleeve (6) compressed air from the pressurized chamber (14) enters the
upper
chamber (26). The air pressure from this chamber (26) acts on the upper end of
the
piston (12) and combines with pressure applied to the shoulder (15) on the
piston (12)
to provide the propulsion of the piston to strike the anvil of the bit (13).
At this time, (c) in the bore of the piston (12) has moved downwardly passed
(d) on the
control rod (11) and chamber (26) can exhaust through the bores in the piston
(12) and
bit (13) to atmosphere.
When the drill is raised in the bore hole, the bit (13) drops in the chuck
(22) and falls
against the retaining rings (23). As a result the piston (12) follows the bit
(13) and (g) on
the piston passes (h) in the wear sleeve (6) to seal chamber (21) from chamber
(14).
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Simultaneously, downward movement of bit (13) has opened the ports (24) and
(25)
through the piston stem bush (20) to release the pressure from the chamber
(21) and
allowing exhaust through the bore (28) in the bit (13). It is important to
release the
pressure from chamber (21) when the drill (1) is in this flushing condition.
Otherwise, the
piston (12) will bounce back and continue reciprocating.
Also (i) on the piston (12) has passed (j) in the bore of the wear sleeve (6)
to open ports
(7B) into chamber (26) and thus permitting a pressurized air supply to flow
through the
piston (12) and the bit (13) to atmosphere and flush the bore hole. The
flushing mode
will only take place when the piston (12) is inactive and the ports (7B) can
flush through
(17) and (28).
Referring again to the insert in Figure 1, it will be appreciated that
downward movement
of the bit (19) when it is lifted off the working face at the bottom of the
drill hole causes
the piston (12) to move downward and this is accompanied by location of the
slots (27)
on the stem (18) across the stem bush (20). In this way, the necessary venting
of the
chamber (21) is also achieved.
Once the bit (13) is replaced onto the working surface and moved back into the
wear
sleeve (6) the reciprocating action of the piston will restart.
As mentioned, while the piston (12) is stroking, chamber (14) is continually
pressurized
and alternately supplies upper chamber (26) and lower chamber (21). Compressed
air
on the shoulder (15) acts continuously against the lifting of the piston (12)
during the
return stroke. The smaller surface the area of the shoulder (15), the smaller
will be the
force acting against the return stroke. To achieve this, as compared with
existing drill
assemblies, one solution is use of the shroud described above. Referring now
to
Figures 2 and 3, an alternative construction of a drill assembly (31) which
also enables
this result to be obtained is illustrated and described below.
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The drill (31) has a backhead (32) with an axial inlet (33) for operating air
at one end.
The backhead (32) has inclined, radially extending distribution passages (34)
and the
usual spring biased inlet control valve assembly (35)
A control rod (36) extends from the backhead (32) into a wear sleeve (37) for
the
operating assembly. This assembly consists of a pneumatically operated piston
(38)
forming the hammer for a drill bit (39). The drill bit (39) is retained in a
well known
chuck assembly (40) mounted in the end of the wear sleeve (37) remote from the
backhead (32).
A piston stem bush (41) is mounted in the wear sleeve (37) above the bit
retaining ring
(42). This ring (42) permits a limited free fall of the bit (39) when the
latter is raised from
the working position shown in Figure 2. The stem bush (41) has an annular
exhaust
passage (43) extending longitudinally to meet radial outlet ports (44). When
the bit (39)
is raised from the working surface and moved outwardly from the wear sleeve
(37) in
the usual manner to achieve flushing, operating air will discharge from the
assembly
(31) via the axial passage (45) through the bit (39).
It will be appreciated that the annular exhaust passage (43) may alternatively
be
provided between the outside of an upper portion of the stem bush (41) having
a
reduced diameter and the wear sleeve (37) instead of extending through the
stem bush
(41) as shown.
The wear sleeve (37) and piston (38) have cut-outs respectively to provide
flow
passages for the pneumatic operation of the piston (38) on the bit (39). This
embodiment also has a chamber (46) in which pressurized air continuously acts
on
shoulder (47) of the piston (38). The operation of the assembly has already
been
described with respect to the first embodiment shown in Figure 1.
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What is important in this embodiment is the flow path (51) provided for inlet
pressurized
air into the operating assembly.
The backhead (32) is screw-threaded into engagement in the end of the wear
sleeve
(37) and to enable this to be done an externally screw-threaded skirt (49)
extends from
the body of the backhead (32) into the wear sleeve (37). An annular passage
(51) is
provided between the inner surface of the wear sleeve (37) and the skirt (49).
The
distribution passages (34) provide inlet ports (51A) adjacent the upper end of
the skirt
(49) which open into the annular passage (51). Outlet ports (51B) adjacent the
lower
end of the skirt (49) open from the passage (51) into the interior of the wear
sleeve (37)
and into cutouts at the upper end of the piston (38). These components can be
seen
more clearly in Figure 3.
To maintain an adequate wall thickness to the wear sleeve (37) the operatively
upper
end is made this enough to accommodate the provision of an internal screw-
thread to
receive the external screw-thread on the skirt (49).
In this embodiment, the outside diameter of the skirt (49) is initially
provided to be larger
than what would normally fit through the internal thread (37A) at the top of
the wear
sleeve (37). The outside of the skirt (49) is then threaded to engage in the
top of the
wear sleeve (37), as shown. The thread on the skirt (49) serves the purpose of
adding
thickness to the wall of skirt (49). The thread depth in the embodiment shown
is 3mm.
The thread depth is added to the wall thickness of the skirt (49) making it
much
stronger. This allows for a larger bore within the skirt (49) and a better
ratio of the
surface areas on the piston (38). At the same time a smaller surface area on
shoulder
(47) is achieved which improves the piston upward speed of the piston (38) to
create
more blows per minute and thus better performance.
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The drill assembly of the invention gives an improved ratio of the surfaces on
the piston
(38) that result in a positive return stroke and ensure the cessation of
movement of the
piston (38) when the bit (39) moves into an inoperative position. The
construction also
maintains satisfactory wall strength for the threaded skirt (49) while still
maintaining the
described compact construction.
The down-the-hole drills described above mitigate the problems referred to and
provide
efficient easily manufactured products.
