FIELD OF THE INVENTION

This invention relates to a down hole hammer and in one aspect, the invention particularly relates to air control means between a piston and hammer barrel for the down hole hammer.

BACKGROUND OF THE INVENTION

A principal design concern is the air control means used to cause reciprocating motion of a piston within a hammer. The piston strikes the anvil end of a drill bit which in turn creates the percussive force for drilling through rock.

The components of down hole hammers normally comprise a hammer barrel within which the piston and other components locate. Those components include a top sub which includes air distribution means, a drive sub at the drill bit end of the hammer which is normally threadably engaged with the hammer barrel and a drill bit that is retained within the drive sub. In known air control means, the rear and forward end of the pistons form a sealing engagement with the inside surface of the hammer barrel. In some instances, barrel liners are provided for cooperation with the upper end of the piston to provide air conduits between the liner and the hammer barrel.

In one design, the inner surface of the forward end of the hammer barrel has a continuous circumferential channel which cooperates with the forward end of the piston to allow transfer of air through the peripheral channel. The piston in turn has a number of longitudinal and radially spaced channels extending rearwardly from a circumferential sealing surface or sealing band. When this sealing surface is located adjacent the peripheral channel, air is able to flow through the longitudinal channels into the peripheral channel in the hammer barrel and past the forward end of the piston.

While the sealing surface is adjacent the peripheral channel, surfaces between the longitudinal channels remain in contact with the inside surface of the hammer barrel. This continues to support the piston while the sealing surface is adjacent the peripheral channel.

This arrangement results in a relatively long section of the piston which is at a diameter that equals the inner diameter of the hammer barrel so as to provide a bearing and sealing surface engagement.

This results in significant drag and also presents the problem of bending that can result from longitudinal bending of the hammer barrel. This longitudinal bending can occur when the hammer transits through broken ground or faults which are at any angle to the direction of drilling.

The increased bearing and surface areas reduce the performance of the hammer while the bending with these large surface areas result in accelerated wear to the hammer components.

It is an object of this invention to provide a down hole hammer capable of substantially overcoming one or more of these problems thereby providing a hammer of improved efficiency.

SUMMARY OF THE INVENTION In a first aspect, the present invention accordingly provides a down hole hammer comprising; a hammer barrel with a drill bit retained at one end, a piston that reciprocates within the hammer barrel and repeatedly impacts against said drill bit,

fluid control means to cause reciprocation of said piston by control oi high pressure fluid wherein part of said fluid control means includes: a plurality of elongate channels on an inner surface of said hammer barrel spaced radially around said inner surface, and a circumferential sealing surface at or towards a forward end of said piston that co-operates with said elongate channels to assist in control of said high pressure fluid flow, there being clearance between any portion of said piston forward of said circumferential sealing surface and the inner surface of said hammer barrel.

The use of a plurality of channels on the inner surface of the hammer barrel obviates the need for portions extending behind the sealing surface to be at the same diameter as the inner surface of the hammer barrel. Instead, support is provided to the piston by the sealing surface engaging the portions of the hammer barrel inside surface that are located between the channels. As a result, the length of the sealing surface is greatly reduced by comparison to prior art arrangements. In addition, there is no need for portions forward of the sealing surface to engage the inside surface of the hammer barrel to provide support.

Accordingly, the drag forces are significantly reduced. Further, forces applied to the piston as a result of bending of the hammer barrel are also reduced resulting in less wear by comparison to prior art hammers described above.

Preferably, said elongate channels are parallel and evenly spaced.

Optionally, said elongate channels are angled with respect to the longitudinal axis of said hammer barrel.

Preferably, a cross section of each of said elongate channels is substantially rectilinear.

This is to maximise the volume of the channels to increase airflow efficiency. Shaping the channels in this way, by comparison to using semi-circular cross section shaped channels, means that, for the same cross sectional area compared to a semi circular channel, the channelled width can be reduced. This in turn maximises the width between adjacent channels so as to provide greater support area for the piston.

Preferably, the down hole hammer further comprises a first circumferential channel at the forward end of said elongate channels positioned so that said elongate channels open into said first circumferential channel.

Preferably, the down hole hammer further comprises a second circumferential channel at the rear end of said elongate channels positioned so that said elongate channels open into said second circumferential channel.

These circumferential channels which can be located at one or either ends of the channels in the hammer barrel assist in providing even fluid distribution prior to fluid flowing into or out of the channels.

In a second aspect, the present invention accordingly provides a piston for use with a down hole hammer, wherein said down hole hammer comprises, a hammer barrel with a drill bit retained at one end, fluid control means for control of a high pressure fluid within said hammer including a plurality of elongate channels on an inner surface of said hammer barrel spaced radially around said inner surface, said piston arranged to reciprocate within said hammer barrel to repeatedly impact against said drill bit and having a circumferential sealing surface at or towards a forward end of said piston that co-operates with said elongate channels to assist in control of said high pressure fluid flow, there

being clearance between any portion of said piston forward of said circumferential sealing surface and the inner surface of said hammer barrel.

In a third aspect, the present invention accordingly provides a hammer barrel for use with a down hole hammer having a drill bit retained therein at one end, said hammer further comprising, a piston that reciprocates within the hammer barrel and repeatedly impacts against said drill bit, fluid control means to cause reciprocation of said piston by control of a high pressure fluid wherein part of said fluid control means includes, a plurality of elongate channels on an inner surface of said hammer barrel spaced radially around said inner surface, and a circumferential sealing surface at or towards a forward end of said piston that co-operates with said elongate channels to assist in control of said high pressure fluid flow, there being clearance between any portion of said piston forward of said circumferential sealing surface and the inner surface of said hammer barrel.

BRIEF DESCRIPTION OF THE DRAWINGS In order to more fully understand the invention, preferred embodiments will now be described. However, it should be realised that the invention is not to be confined or restricted to any of these embodiments and that variations will be readily apparent to skilled addressees and such variations should be considered to be within the scope of the invention.

The embodiments illustrated in the accompanying drawings in which:

Figure 1 shows side and end views of a piston,

Figure 2 shows a cross section view of an assembled hammer,

Figure 3 shows a part cross section view of a piston and hammer barrel where the piston is at impact position in relation to the drill bit,

Figure 4 shows a cross section of Figure 3 about lines AA,

Figure 5 shows a cross section of an assembled hammer, Figure 6 shows a cut away perspective view of the inside features of a hammer barrel, and

Figure 7 is a part cut away perspective view of a hammer barrel showing a second embodiment of channels on the inside surface of the hammer barrel.

DESCRIPTION OF PREFERRED EMBODIMENT

The hammer described in this embodiment is a normal circulation hammer.

However, the invention will be equally applicable to a reverse circulation hammer.

Figure 1 shows a piston 10 in accordance with the invention. The piston 10 has a rear sealing surface 11. The circumferential sealing surface towards the forward end of the piston 10 comprises a forward sealing surface 12. The impact surface 13 on the forward end of the piston impacts against the anvil end of a drill bit 14.

Figure 2 shows an assembled view of a hammer which includes a hammer barrel 17 and a drive sub 18 which is threadably engaged into the forward end of the hammer barrel 17. The drive sub 18 retains the drill bit 14. The hammer also includes a top sub 19 threadably engaged into the upper end of the hammer barrel 17, an air distributor 20 and a liner 21. The rear sealing surface 11 of the piston 10 engages within the liner 21. Air control means is provided between the liner 21 and the rear sealing surface 11 to control the flow of air as required to enable reciprocation of the piston 10.

Towards the forward end of the hammer barrel 17, there are a number of longitudinal channels 22 radially spaced around the inner periphery of the hammer barrel 17. The surfaces 24 between the channels 22 form a bearing surface engaged by the sealing surface 12 (as best seen in Figure 6). Figure 2 shows the piston 10 in

its lowest position where the forward sealing surface 12 of the piston 10 is located adjacent the longitudinal channels 22. In this position, air is able to flow through the channels 22 past the forward sealing surface 12 to enter the lower chamber 23. This high pressure air acts to force the piston 10 towards the rear end of the hammer barrel 17.

Air movement through the longitudinal channels 22 is prevented once the forward sealing surface 12 engages the inner surface of the hammer barrel 17 above the longitudinal channels 22. While the forward sealing surface 12 is adjacent the longitudinal channels 22, the forward sealing surface 12 engages the inner surface of the hammer barrel 17 which is located between the longitudinal channels 22 and is therefore supported within the hammer barrel 17.

As will be seen from the diagrams, a portion of the piston 10 which is between the forward sealing surface 12 and the impact surface 13 has a diameter less than the inner diameter of the hammer barrel 17. Similarly, the diameter of the piston 10 between the rear sealing surface 11 and the forward sealing surface 12 is smaller than the inner diameter of the hammer barrel 17. Accordingly, it is only the forward sealing surface 12 which supports the forward end of the piston 12 within the hammer barrel 17.

Due to the surfaces 24 located between the longitudinal channels 22 which are radially spaced within the hammer barrel 17, the forward end of the piston 10 is always supported and therefore there is no need for support surfaces behind the forward sealing surface 12 to support the piston 10 when the forward sealing surface 12 is adjacent the longitudinal channels 22. In turn, the forward sealing surface 12 can be relatively short by comparison to prior art pistons described above.

Figure 7 shows a second embodiment for the longitudinal channels 22. In this embodiment, instead of the longitudinal channels 22 having a circular or arcuate cross section, they have a more rectilinear cross section. The sidewalls 26 of each channel 22 are either substantially perpendicular to the inner wall of the hammer barrels 17 or parallel as would be the case when the channel 22 is cut by a milling cutter. There is a rounded transition between the sidewalls 26 and the base 27 of each channel 22.

Further, a circumferential channel 28 connects to the upper end of each longitudinal channel 22. The width of the circumferential channel 28 is less than the width of the forward sealing surface 12 and therefore is smoothly passed by the forward end of the piston 10. The use of the circumferential channel 28 provides a large opening area for supplied air to each of the longitudinal channels 22 and allows for evening of the distribution of airflow through the longitudinal channels 22.

Further, the lower side 29 of the circumferential channel 28 can be angled f orwardly so as to provide a sloping surface on that side of the circumferential channel 28 which aids shock wave transmission through the tool and reduces the entry coefficient for the fluid flow into the longitudinal channels 22. Also, each side of the circumferential channel 28 may be substantially parallel with each longitudinal channel 22 joining the circumferential channel 28 with a radius. The transition between the side of the circumferential channel 28 is rounded similarly to the longitudinal channels 22 to prevent stress induced cracks that might otherwise form in a right angled transition.

As will be seen from the above description, the invention provides a down hole hammer design which the inventors believe is significantly different from the prior art and which will function to enhance both performance of the hammer and also improve its durability. It will also be noted that, although the embodiment described above is a normal circulation hammer, the invention will still be equally

applicable to reverse circulation hammers.

It will be understood that the term "comprise" and any of its derivatives (eg. comprises, comprising) as used in this specification is to be taken to be inclusive of features to which it refers, and is not meant to exclude the presence of any additional features unless otherwise stated or implied.

Although a preferred embodiment of the present invention has been described in the foregoing detailed description, it will be understood that the invention is not limited to the embodiment disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the scope of the invention as set forth and defined by the following claims.