Drill bit assembly for fluid-operated percussion drill tools

A drill bit assembly for fluid-operated percussion drill tools. The assembly comprises a percussion bit (1) having a head portion (3) formed with an axially extending stub shank (32). A drive bush (2) is releasably connectable to the bit head portion (3). Axially extending splines (35) on the drive bush (2) are slideably engageable with complementary splines (36) formed on a drive chuck (4) whereby rotational drive from the chuck (4) may be transmitted to the drive bush (2). The assembly further comprises engagement means on the chuck (4) adapted for connecting the chuck (4) to a drive means (5) of the fluid-operated percussion drill tool.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a drill bit assembly for fluid-operated percussion drill tools. In particular, the invention concerns a drill bit assembly for use with “down-the-hole” hammers.

BACKGROUND TO THE INVENTION

Some designs of conventional down-the-hole hammers and fluid-operated percussion drill tools comprise an external cylinder or outer wear sleeve, within which is mounted an inner cylinder which in turn engages with a backhead assembly. A sliding reciprocating piston co-operates with the inner cylinder and backhead assembly, which when air pressure is supplied through the backhead assembly, acts with a percussive effect on a drill bit retained within a chuck on the outer wear sleeve.

Typically the inner cylinder is mounted co-axially within the outer wear sleeve. A sliding piston is mounted for reciprocating movement within the inner cylinder and the outer wear sleeve, to strike a hammer bit mounted for sliding movement in a chuck located at the forward end of the outer wear sleeve, in well known manner. A foot valve is positioned above the bit.

Our prior patent application Publication No. WO 2004/031530, discloses a down-the-hole hammer in which the bit has an elongate shank portion which at its upper end has an annular strike face (or anvil) against which the piston impacts to impart a percussive force to the bit. A lower end of the bit shank is formed externally with a plurality of splines which are spaced around the circumference of the bit shank and extend in the axial direction. The splines slideably engage with complementary splines formed on the internal wall of an annular chuck. The chuck is screw-threadably connected to the bottom of the outer wear sleeve.

The bit is rotationally driven by the chuck, and rotational torque is transferred to the top portion of the elongate shank portion. That is, rotational torque is transferred to a part of the bit shank remote from the cutting face of the bit. This imparts significant bending moments on the upper part of the bit shank increasing the risk of breakage of the shank due to cracking. Drill bits are very expensive to produce, and to recover if they are lost down the drilling hole. That this is a significant problem with the drill bits of conventional down-the-hole hammers is evidenced by the fact that there are a number of patents directed to means of retaining a broken-off bit within the bit assembly so as to prevent it falling down the drill hole. Examples of these patents are U.S. Pat. No. 5,065,827, U.S. Pat. No. 4,003,442, WO 96/15349, WO 98/05476, WO 03/062585, WO 03/062586.

However, the inventions disclosed in these patents are directed to dealing with problems which occur after the bit shaft has fractured, and not to preventing the breakage in the first place.

Other disadvantages associated with conventional percussion drill tools, such as down-the-hole hammers, is that the bit has a long shank portion which is expensive to produce. As indicated above, rotational torque is transferred to the bit in the weaker shank area, and more remote from the cutting face of the bit. Also relatively expensive machining of the shank is required due to the necessity to form splines on the shank. In conventional hammers, when the bit head or cutting face is worn out, the shank can often be in good condition but because it is made integral with the cutting face, it must be discarded. The premature wearing out of the head/cutting face may occur where drilling is carried out in very abrasive rock or material which wears the tungsten carbide inserts in the cutting heard. With many conventional hammers, there is a need to provide foot valves in the bit. The foot valve is required as an integral part of the functioning of the hammer i.e. when the piston is in the strike position, the bottom lift chamber is sealed by the bore of the piston and the outside of the footvalve. If this were not the case then the piston would not lift. The footvalve is prone to occasional breakage leading to down-time.

OBJECT OF THE INVENTION

It is an object of the invention to provide a bit mounting system for a down-the-hole hammer, or other fluid operated percussion drill tool which avoids many of the advantages of the prior art systems as described above. It is also an object of the invention to provide a bit system in which the shank is shorter in length than conventional drill shanks. It is also an object of the invention to provide an improved coupling means for the bit assembly.

SUMMARY OF THE INVENTION

In one aspect the invention provides a drill bit assembly for fluid-operated percussion drill tools comprising a percussion bit having a head portion formed with an axially extending stub shank, a drive bush releasably connectable to the bit head portion, axially extending splines on the drive bush slideably engageable with complementary splines formed on a drive chuck whereby rotational drive from the chuck may be transmitted to the drive bush, and engagement means on the chuck adapted for connecting the chuck to a drive means of the fluid-operated percussion drill tool.

Preferably, the head portion has an external screw-thread which engages with a complementary internal screw-thread formed on an inner cylindrical wall of the drive bush; and the drive chuck is formed with a screw thread which is adapted to engage with a complementary screw thread on the lower end of a wear sleeve of the drill tool. Suitably, the axially extending splines are formed on an internal cylindrical wall of the bush and engage with complementary splines formed externally of the drive chuck.

In a second embodiment the chuck is formed in two parts. Preferably, a lower part of the chuck formed with splines for engagement with the splines of the drive bush, and an upper part has screw-threads for engagement with the wear sleeve. Preferably, the splines of the lower part are formed with a stop, e.g. in the form of a raised diameter, which prevents the bush end of the bit from falling from the assembly, in the event of a breakage during operation of the drill bit.

In another aspect, the invention provides a down-the-hole hammer comprising an external cylindrical outer wear sleeve, a sliding piston mounted for reciprocating movement within the outer wear sleeve to strike a percussion bit of a drill bit assembly located at the forward end of the outer wear sleeve, in which the drill bit assembly is as described above.

The drill bit assembly of the invention has a number of advantages over conventional systems. The bit having a short stub shaft is less expensive to manufacture than bit heads having long shafts. There is the added benefit of providing the strike energy closer to the bit cutting face. Also, the rotational torque is transferred through the strongest part of the bit closer to the cutting face.

Another advantage is that it is much cheaper to replace the bit in abrasive conditions, as compared to conventional drill bit assemblies where the bit head with attached long shaft and machined shaft has to be replaced. Furthermore, with the assembly of the invention there is no need to have a footvalve in the bit. The footvalve and piston cooperation of the prior art may be replaced in the invention by the nose of the piston8sealing in the bore of the chuck4. Nevertheless, it is optional whether a footvalve is used or not.

The systems described in WO 96/15349 and U.S. Pat. No. 4,003,442 (referred to above) have threadably detachable bit heads. This can lead to problems when the thread loosens because all force is transferred over the threads leading to breakages. In the present invention even if the threads loosen between the drive bush and the bit, the percussive forces are still transferred through the bit and not the threads.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to the drawings, an embodiment of a down-the-hole hammer of the invention comprises an external cylindrical outer wear sleeve5. An inner cylinder is mounted co-axially within the outer wear sleeve5. A sliding piston8is mounted for reciprocating movement within the inner cylinder and the outer wear sleeve5, to strike a hammer bit1located at the forward end of the outer wear sleeve5to exercise a percussive force to the drill bit. Rotational forces are transferred from the rotating outer wear sleeve5by means of a chuck4and connector means as described more fully below. The wear sleeve is threadably connected to a drill string which is connected to a rotation motor on a drilling rig at the surface.

Referring particularly toFIGS. 4 and 5, the head portion3of the bit assembly comprises the percussion bit1which is provided with tungsten carbide inserts31, in well know manner. The bit head portion3is formed with an axially extending stub shank32. The stub shank32is formed with a shoulder portion33abutting the bit1. The annular shoulder portion33is externally screw threaded. Rotational torque is applied to the bit head portion3through a drive bush2. A lower portion of the internal wall of the bushing2is formed with screw threads34which engage with the external screw threads of shoulder33to enable the bush2to be screwed onto the head portion3of the bit.

The upper internal wall portion of the drive bush2is machined internally to form axially extending internal splines35.

A hollow cylindrical chuck4is provided, on its lower external wall with a plurality of axially extending splines36which engage with the splines35of the drive bush2to transmit rotational drive from the chuck4to the drive bush2. The splines36terminate at their upper end with an annular shoulder38, which acts as a stop when the chuck4is screwed into the wear sleeve5as described below. Above the shoulder38, an number of slots7are cut through the wall of the chuck4to receive retaining pieces6. In the assembled bit assembly, the bit retaining pieces6engage with the underside of an annular shoulder37formed at the top end of the shank32. The chuck4is, in turn, screwed into the lower end of the wear sleeve5and, in doing so retains the bit retaining pieces6in slot7. In addition, the screw threaded engagement of the chuck4with the wear sleeve5enables rotational torque to be transmitted from the wear sleeve5through the chuck4, and drive bush2, to the bit1.

It will be appreciated that, alternatively, the splines35could be formed externally of the drive bush2, and the splines36of the chuck4formed internally of the chuck. Also, in an alternative arrangement of the chuck4the external screw threads may be replaced with internal threads so that the chuck can engage with complementary external screw threads on the wear sleeve5.

A reciprocating piston8is mounted for reciprocating movement within the inner cylinder25and the outer wear sleeve5to strike the top face of shoulder37to impart a percussive force to the bit. The splines35of the bush2slideably engage with the complementary splines36on the chuck4so that the bush2and head portion3are moved axially relatively to the chuck during the percussive action.

In an embodiment of the invention, which has a nominal4″ hammer which drills hole sizes of 10 cm and greater, the stub shaft32has a length in the range 90 mm to 140 mm, preferably about 130 mm. In contrast the long shank portions of conventional down-the-hole hammers, of similar drill size, typically have a length in the range 200 mm to 260 mm. Thus, with the present invention the length of the shaft32can be about 50% less than that of the shafts of conventional down-the-hole hammers.

The hammer cycle is as shown inFIGS. 1 to 3.FIG. 1shows the hammer in the off-bottom position. Piston8is permitting exhaust air to flush through bore9and bore10in bit1to the face flushing holes11.FIG. 2shows the hammer in the strike position. Pressurised air is supplied down chamber13, through piston grooves14and wearsleeve undercut15, into the pressure chamber12. This air is sealed off by the piston nose20and chuck bore21. Simultaneously, the top chamber16is open to exhaust through bores9,10and11. As a result the piston8lifts.FIG. 3shows the piston8at the top of stroke. Lift chamber12is open to exhaust through chuck4and bores10,11. Top chamber16is supplied with pressure air through ports19and channels17. The chamber16, is sealed by the distributor probe18. As a result the piston is forced down to strike the bit and repeat the cycle.

In an alternative embodiment, shown inFIG. 6, the chuck4is formed in two parts, namely a lower part40having the splines36, and an upper part41having the screw-threads for connection to the wear sleeve.

The two parts40,41of the chuck are connected by screw-threads42on the lower part40which engage with complementary screw-threads43on the upper part41.

This arrangement has the advantage of ease of assembly at the parts of the drill bit system. The parts can be assembled from above by sliding the splines36of the lower part40of the chuck4into engagement with the splines35of the bush2so that the lower part40fits within the bush2.

The retaining pieces6are held in slots7machined in the lower part40of the chuck4and held in position when the upper part41of the chuck4is screwed into engagement with the lower part40. The advantage of this is that there is a secondary retaining area for the assembly which is in the form of a raised diameter44at the bottom end of the splines36of the chuck4which serves to cooperate with the top ends of the splines35on the bush2.

The raised diameter portion (, or stop,)44ensures that, in operation of the hammer, the bush2cannot slide off the ends of the splines36. Thus, in the event of a breakage of the bit1in the region of the stub shank32or the shoulder37, or in the event of a failure of the retaining pieces6, the raised portion (, or stop,)44serves as a stop to prevent the bush2and the bit1from falling from the assembly.

From the foregoing, it will be apparent that numerous modifications and variations can be effected without departing from the true spirit and scope of the novel concept of the present invention. It will be appreciated that the present disclosure is intended to set forth the exemplifications of the invention which are not intended to limit the invention to the specific embodiments illustrated. The disclosure is intended to cover by the appended claims all such modifications as fall within the scope of the claims.