Note: Descriptions are shown in the official language in which they were submitted.
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A PERCUSSIVE DOWNHOLE HAMMER AND
PISTON DESIGN FOR SUCH A HAMMER
This invention relates to a percussive downhole hammer and in particular to
the
design of a piston used in the hammer.
BACKGROUND OF THE INVENTION
Percussive hammers normally comprise a hammer casing within which a piston
reciprocates. The reciprocation force is provided by a fluid under pressure
which is
normally compressed air. During the reciprocation of the piston, it strikes a
drill bit
which is located within one end of the hammer.
A variety of different means are well known for controlling air flow to cause
the
piston to reciprocate. This requires high pressure fluid to be alternately
supplied to
either side of the piston to lift it from bottom dead centre and to force it
downwardly
to strike against the drill bit.
It is an aim of this invention to provide an improved design for the
distribution and
control of pressurised fluid around a piston.
SUMMARY OF THE INVENTION
In its broadest form, the invention comprises a piston for use in a downhole
percussive hammer comprising;
a first section,
a second section at the forward end of said piston having a diameter smaller
than that of said first section,
an axial hole extending through the length of said piston,
at least one first conduit extending from the upper end of said piston to a
position intermediate the ends of said first section,
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at least one external second conduit in the outer surface of said first
section
extending between but not to the upper and lower ends of said first section;
and
conduits extending from the lower end of said first conduit and the upper end
of said second conduit through said first section to said axial hole.
The hammer has an air control tube that locates within the central axial hole
of the
piston so that the piston may reciprocate back and forth along the control air
tube.
The control air tube has apertures for supply of high pressure fluid that
communicate
with the conduits to provide a supply of high pressure fluid to either an
upper or
lower chamber via the first and second conduits depending on the position of
the
piston. By this means, the piston is rapidly reciprocated within the hammer.
The outer surface of the lower end of the second section of the piston engages
into an
aperture within the hammer that forms a seal and consequently a lower chamber
which is between that seal and the lower end of the first section of the
piston.
Pressurised air that enters this chamber via the second conduits force the
piston
upwardly. As the piston moves upwardly, the forward end of the second section
disengages from the aperture and thereby allows air to exhaust from the lower
chamber by flowing through outlets located within the drill bit or on the
external
surface of the drill bit.
Likewise, high pressure air flows into an upper chamber between the upper end
of
the piston and the hammer which forces the piston downwardly. The air in this
second chamber exhausts through the centre of the piston when the conduits at
the
lower end of the first conduits pass the end of the control air tube.
The first conduits preferably comprise holes which extend from the end of the
first
section that are spaced from the outer surface of the first section. They
intersect with
conduits that extend from the axial hole in the centre of the piston.
Alternatively, the
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first conduits may comprise a groove or channel in the external surface of the
first
section.
Preferably, there is a plurality of first and second conduits which are spaced
circumferentially around the first section of the piston. In the case of both
the first
and second conduits comprising grooves or channels, they may alternate one
after
the other around the circumference of the piston. They may also be at an angle
to the
centre line of the piston (helical) to aid distribution of lubricant and to
also cause
rotation of the piston as it reciprocates up and down.
A peripheral groove may be located at the lower end of the second conduits
with the
second conduits opening into the peripheral groove. Further, the portion of
the first
section that is located between the peripheral groove and the lower end of the
first
section may comprise a sealing flange. The sealing flange may work in
conjunction
with a peripheral groove on the internal surface of the casing within which
the piston
locates so as to provide free movement of pressurised air from the second
conduits
into the peripheral groove and around the flange when the flange aligns with
the
circumferential groove in the casing.
During the upward movement of the piston, the sealing flange engages against
the
wall of the casing to seal the ends of the second conduits thereby maintaining
high
pressure fluid within the second conduits until such time as the sealing
flange again
aligns with the circumferential channel in the casing.
A further aspect of the invention comprises a piston for use in a downhole
percussive
hammer comprising:
an axial hole extending through the length of the piston,
at least one first groove or channel formed in the external surface of the
piston
that extends from the upper end of the piston to a position intermediate the
ends of
the piston,
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at least one second groove or channel formed in the external surface of the
piston that extends from a lower edge of the piston to a point below the upper
end of
the piston, and
conduits extending from the intermediate ends of each first and second
groove or charnel through the piston to the axial hole of the piston so as to
provide a
fluid conduit between the axial hole and each of the first and second grooves
or
channels.
Preferably, a piston according to this second aspect of the invention has a
first section
and a second section at the forward end of the piston where the diameter of
the
second section is smaller than that of the first section.
Further, the piston according to this aspect of the invention may have a
peripheral
groove extending around the circumference of the first section that is located
at the
lower end of the second groove or channel with the second groove or channel
opening into the peripheral groove. Further, the portion of the first section
between
the peripheral groove and the lower end of the first section may comprise a
sealing
flange that works in the same manner as described above for the first aspect
of the
invention.
In the case of a piston having a larger and smaller diameter portion, the
transition
between the large to small diameter is preferably by way of a tapered section.
This
avoids the use of an abrupt section change which may create a stress riser
that could
lead to fracturing of the piston in this region.
In order for the invention to be more fully understood, a preferred embodiment
will
now be described. However, it should be realised that the invention is not to
be
confined or restricted to the combination of features described in the
embodiment.
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BRIEF DESCRIPTION OF THE DRAWINGS
The embodiment is illustrated in accompanying drawings in which:
Fig. l shows a side view of a piston according to a first embodiment,
Figs. 2 to 4 show a part cross-section view of a piston within a hammer
illustrating
the fluid flow through the hammer when the piston is at bottom dead centre,
and
Fig. 5 shows a side view of a piston according to a second embodiment.
DETAILED DESCRIPTION
Figure 1 shows a side view of the piston 10 which comprises a first section of
larger
diameter 11 and a second section 12 of smaller diameter. The piston 10 has an,
axial
hole 13 that extends throughout the length of the piston 10 along its centre
line.
The first section of the piston 11 has a plurality of conduits 15 and 20
machined
within its outer surface. These conduits 15 and 20 are sufficiently deep
within the
surface of the piston to allow flow of compressed air along the conduits 15
and 20.
Preferably they are semicircular in cross-section but other cross-sections
such as
rectangular or elliptical may be used.
The first conduits 15 extend from the upper end of the piston 16 toward the
lower
end 17 of the first section 11. The conduits 15 do not extend beyond the lower
end 17.
Conduits 18 are formed by drilling holes from the end of the first conduits 15
through to the axial hole 13.
Second conduits 20 extend along the first section 11 between the lower end 17
of the
first section towards the upper end 16. However, the second conduits 20 stop
short of
the ends 16 and 17. Conduits 21 are formed by drilling a hole in the end of
the second
conduits 20 through to the axial hole 13.
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In this embodiment, a circumferential groove 23 is positioned towards the
lower end
of the first section 11. This forms a circumferential flange 24 at the lower
end of the
first section 11. A continuous seal around the circumference of the flange 24
is
formed when the flange is in contact with the internal surface of the hammer
casing
26. This feature will be explained in more detail below.
Referring to Figs. 2 to 4, the piston 10 is shown located within a hammer. The
hammer comprises a casing 26 which has a drive sub (not drawn) at its lower
end for
retaining a drill bit and a top sub (not drawn) at its upper end for attaching
drill tube.
An air control tube 27 locates within the axial hole 13 of the piston. The air
control
tube 27 has apertures 28 that supply high pressure air to control movement of
the
piston 10. As seen in Fig. 2, the conduits 21 of the second conduits 20 align
with the
apertures 28 to allow high pressure air to flow through the conduits 21 and
the
second conduits 20. Referring to Fig. 2, the circumferential flange 24 aligns
with a
circumferential groove 29 in the casing 26 to allow air to flow around the
flange 24
into the front chamber of the hammer.
The second section 12 of the piston 10 is engaged with sealing means 30 to
thereby
define a front chamber between the seal and the lower end of the first section
11. The
pressurised air forces the piston 10 upwardly until the lower end of the
second
section 12 disengages from the sealing means 30 to allow air to exhaust
through the
forward end of the hammer.
When the conduits 18 align with the apertures 28 in the air control tube 27,
high
pressure air flows through the conduits 18 and the first conduits 15 into the
upper
chamber to thereby force the piston 10 downwardly. In this manner, the piston
10 is
reciprocated rapidly in the hammer and percussively strikes against the end of
the
drill bit.
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As can be seen in Figs. 3 to 4, the circumferential flange 24 seals against
the inter
surface of the casing 26 thereby maintaining high pressure air within conduits
21 and
the second conduits 20. This means that the conduits 18 and the first conduits
20 will
take less time to pressurise to operating pressure when the lower chamber
opens.
Therefore, less air is consumed and the pressure increase in the lower chamber
occurs more rapidly that it would if conduit 18 and the second conduit 20 were
able
to reduce to exhaust pressure. This provides a significant advantage in the
performance of the hammer.
An alternative piston design is shown in Fig. 5. In this embodiment of the
invention,
the first conduits 15, instead of being grooves or channels in the outer
surface of the
first section 11 are holes that are drilled from the end 16 of the piston 10
to the
conduits 18. The conduits 18 are drilled from the external surface of the
first section
to the axial hole 13. The outer end of the conduit 18 seals against the casing
26 when
air flows through the air control tube 27 into the upper chamber. However,
this
embodiment maintains the advantage described above in that the circumferential
flange 24 seals against the inner surface of the casing 26 thereby maintaining
high
pressure air within the conduits 21 and the second conduits 20 during its
upward
and return stroke.
The piston 10 has a tapered portion 32 that provides transition from the first
section
11 to the second forward portion 12. This provides increased strength by
comparison
to an abrupt diameter change. Such an abrupt change would result in a
significant
stress raiser leading to failure or fracture at this juncture.
A significant advantage of the invention is that lubricant, entrained within
the
compressed air, can more readily access the internal surface of the casing 26.
This is
important to prolong the life of the hammer. In addition, the first and second
conduits 15 and 20 shown in Fig. 1 or the conduits 20 in Fig. 5 are at a
slight angle
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with respect to the longitudinal axis of the piston 10 to again ensure that
the
maximum internal surface area of the casing 26 is coated with lubricant upon
each
pass of the piston 10. The conduits 15 and 20 at this angle also cause the
piston 10 to
rotate as it reciprocates thereby further improving lubricant distribution.
As will be appreciated from the above description, the invention provides
significant
advantages by comparison to prior art air distribution methods. However, it
should
be appreciated by those skilled in the art that modifications may be made to
the
above embodiment without necessarily departing from the scope of the invention
as
described in this specification.
