AU2002366651B2 - A percussive downhole hammer and piston design for such a hammer - Google Patents

Description

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, 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 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, 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 channel 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. BRIEF DESCRIPTION OF THE DRAWINGS

The embodiment is illustrated in accompanying drawings in which:

Fig. 1 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. 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. As can be seen in Figs. 3 to 4, the circumferential flange 24 seals against the inter surface of the casing 26 thereby mamtaining 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 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.

Claims (14)

THE CLAIMS DEFINING THE F VENTION ARE AS FOLLOWS:

1. 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, 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.

2. A piston according to claim 1 further comprising a peripheral groove extending around the circumference of said first section, said peripheral groove located at the lower end of said second conduits with said second conduits opening into said peripheral groove.

3. A piston according to claim 2 wherein the portion of said first section between said peripheral groove and the lower end of said first section is a sealing flange.

4. A piston according to any one of the preceding claims wherein there are at least three said first and second conduits each spaced around said first section.

5. A piston according to any one of the preceding claims wherein said first conduits comprise holes formed between the upper end of said first section that intersects with said conduits that extend from said axial hole.

6. A piston according to any one of claims 1 to 4 wherein said first conduits comprise a groove or channel in the outer surface of said first section.

7. A piston according to any one of the preceding claims further comprising a tapered portion between said first and second sections that tapers from a diameter that is larger than said second section to a diameter that equals said second section.

8. 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, 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 channel 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.

9. A piston according to claim 8 further comprising a first section and a second section at the forward end of said piston having a diameter smaller than that of said first section.

10. A piston according to claim 9 further comprising a peripheral groove extending around the circumference of said first section, said peripheral groove located at the lower end of said second groove or channel with said second groove or channel opening into said peripheral groove.

11. A piston according to claim 10 wherein the portion of said first section between said peripheral groove and the lower end of said first section is a sealing flange.

12. A piston according to any one of claims 8 to 11 wherein there are at least three said first and second grooves or channels each spaced around said first section.

13. A piston according to any one of claims 8 to 12 further comprising a tapered portion between said first and second sections that tapers from a diameter that is larger than said second section to a diameter that equals said second section.

14. 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 hole extending from the upper end of said piston to a position intermediate the ends of said first section that is spaced inwardly from the outer surface of said first section, 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.