IES20090129A2 - A drill bit assembly for fluid-operated percussion drill tools - Google Patents

Abstract

The present invention relates to a drill bit assembly for fluid-operated percussion drill tools. The assembly comprises a percussion bit (1) having ahead portion (3) formed witrh an axially extending stub shank (32). Axially extended splines (36) on the stub shank (32) are slideably engageable with complementary splines (35) formed on a drive chuck (4) whereby rotational drive from the chuck (4) may be transmitted to the stub shank (32). Bit retaining means (41,42) are provided at the chuck adapted for engagment with complementary retaining means (37) at a spline portion of the stub shank to retain the stub shank in the drill bit assembly. Engagement means on the chuck (4) are adapted for connecting the chuck (4) to a drive means (5) of the fluid-operated percussion drill tool. The bit retaining means comprises at least one opening (42) through the wall of the drive chuck and a corresponding at least one bit retaining pin (41). The complementary retaining maens comprises an annular shoulder (37), such that when a bit assembly is assembled, the bit retaining pins are recieved in the openings and engage with the underside of the annular shoulder to retain the stub shank in the drill bit assembly. The width of the at least one opening in the wall of the drive chuck decreases towards the inner wall of the drive chuck and the at least one bit retaining pin corresponds to the shape of the at least one opening, such as that radial movement of the pin into the opening is limited by the shape of the opening. <Figure 8>

Description

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-thehole” 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 2004/013530, discloses a down-thehole 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 retained in the hammer assembly by means of a bit retaining ring, which si ts above the chuck and cooperates with an annular ts the bit from falling out of the assembly in operation.

In operation the bit shank comes under forces due to the percussive action of the hammer, and rotational torque which is provided by the chuck. 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 US 5,065,827, US 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.

Another disadvantage associated with conventional percussion drill tools, such as downthe-hole hammers, is that the bit has a long shank portion which is expensive to produce. The long shank portion is required in order to provide a splined shank portion of sufficient length to give enough support for transfer of rotational torque, and an area above the splines for retaining the bit. In conventional hammers, when the bit head or cutting face is wom 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 head. 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.

Our granted European Patent No. 1910 640 describes a drill bit assembly for fluidoperated percussion drill tools which overcomes a number of the problems discussed above. The assembly comprises a percussion bit having a head portion formed with an axially extending stub shank. The stub shank is provided with axially extending splines, IE 0 9 ο 1 2 9 which are slideably engageable with complementary splines formed on a drive chuck. Rotational drive from the chuck may be transmitted to the stub shank by means of the splines. Bit retaining means at the chuck are adapted for engagement with complementary retaining means at a spline portion of the stub shank to retain the stub shank in the drill bit assembly. Engagement means on the chuck are adapted for connecting the chuck to a drive means of the fluid-operated percussion drill tool.

This arrangement has a number of advantages over conventional systems. Because the means to retain the bit within the chuck has been moved to the splined portion of the stub shank, the present invention allows for a shortened shank. In addition, splined support for transfer of rotational torque is provided both above and below the bit retaining means. Where the complementary retaining means comprises a plurality of recesses, each of which is formed entirely within one of the splines on the stub shank, splined support for transfer of rotational torque is provided over the entire length of the splines.

However, there are also a number of disadvantages associated with this assembly. All bit/chuck splines wear during use due to the axial sliding motion of the bit splines on the complimentary chuck splines. In some cases, particularly in softer drilling conditions where the axial movement is greater than in more competent ground, the wear can be extreme. In traditional systems, the way to overcome this is to increase the number of the splines. This means that the width of each of the bit splines must be reduced for a given diameter bit. (n one embodiment of the above-referenced patent, the bit retaining means comprises circular openings through the wall of the drive chuck through which ball-nose pins are inserted, and the complementary retaining means comprises a plurality of recesses, each of which is formed entirely within one of the splines on the stub shank. Thus, each spline must be made sufficiently wide to accommodate the recess within its width, as well as an allowance on either side to account for spline wear. This effectively limits the number of splines that can be provided on a bit of a given diameter.

Another problem which arises is the issue of wear between the bit retaining means at the chuck and the complementary retaining means on the bit. The surface contact area between the bit retaining means and the complementary retaining means must be sufficient to prevent extreme wear between the components. Again, this can be particularly bad in conditions of greater axial movement between the bit and chuck splines, where the bit can be frequently driven onto the retaining means at a high velocity.

In the ball-nose pin embodiment of the above-referenced patent, the only way to increase the surface contact area for bit retention is to increase the depth of the recesses provided in the splines. This requires a corresponding increase in the protrusion of the ball-nose pin from the hole in the chuck. The longer this protrusion, the greater the shearing force experienced by the pin as the bit is driven at high velocity onto the pins. This shearing force causes wear between the outside of the pin and the inside of the hole in the chuck and can ultimately lead to failure of either the pin or the chuck.

Another disadvantage associated with the ball-nose pin embodiment is that sufficient diametric clearance must be provided between the pins and the bit, due to the high temperatures that can be induced in the bit from high impact energies being translated and the friction caused by the axial movement between it and the chuck. As a result, the pins can vibrate in the holes and again cause wear between the holes and pins.

In another embodiment of the above-referenced patent, the bit retaining means comprises a plurality of part-annular slots through the wall of the drive chuck and a corresponding plurality of part-annularly shaped bit-retaining pieces. The complementary retaining means comprises an annular shoulder.

There are a number of disadvantages associated with the annular slot system. The chuck design is relatively weaker when compared to the pin system, as the pin holes spread the load more evenly throughout the chuck and provide a stronger shape. The annular slot system is also more difficult to manufacture than the pin system, especially the part-annular bit retaining pieces.

Another disadvantage associated with both the ball-nose pin and annular slot embodiments is that the bit retaining pieces are held in position by the wear sleeve. The hammer is subject to external wear due to high pressure air and cuttings flushing at high velocity around the outside of the hammer. This effectively sandblasts (he outside diameter causing extreme wear. As this wear occurs, the cross-sectional area of the wear sleeve reduces, thereby reducing the strength of the sleeve in the area of the bit retaining means and increasing the possibility of damage in this area due to the bit retaining pieces being forced out against the wear sleeve walls as the bit drops out of the hammer. Also, when the bit and chuck assembly is being screwed into the hammer, the bit retaining pieces can sometimes fall out. In addition, if the chuck were to unscrew even by a small amount in operation, the bit retaining pieces could fall out with the result that the bit is dropped down the hole.

Summary of the Invention In a first 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; axially extending splines on the stub shank slideably engageable with complementary splines formed on a drive chuck whereby rotational drive from the chuck may be transmitted to the stub shank; bit retaining means at the chuck adapted for 20 engagement with complementary retaining means at a spline portion of the stub shank to retain the slub shank in the drill bit assembly; and engagement means on the chuck adapted for connecting the chuck to a drive means of the fluid-operated percussion drill tool; wherein the bit retaining means comprises at least one opening through the wall of the drive chuck and a corresponding at least one bit retaining pin and the complementary retaining means comprises an annular shoulder, such that when the bit assembly is assembled, the bit retaining pins are received in the openings and engage with the underside of the annular shoulder to retain the stub shank in the drill bit assembly; characterised in that the width of the at least one opening in the wall of the drive chuck decreases towards the inner wall of the drive chuck; and the at least one bit retaining pin corresponds to the shape of the at least one opening, such that radial movement of the pin into the opening is limited by the shape of the opening.

In an embodiment of the invention, the at least one opening in the wall of the drive chuck is tapered such that the width of the opening decreases towards the inner wall of the drive chuck and the at least one bit retaining pin is tapered to correspond to the shape of the at least one opening, such that radial movement of the pin into the opening is limited by the shape of the opening.

In a preferred embodiment, the at least one opening in the wall of the drive chuck is ball-nosed in shape, and the at least one bit retaining pin is a ball-nose pin. Suitably, the opening may be machined using a ball-nose cutter. The assembled position of the ballnose pins may be the same as the position of the ball-nose cutter at full depth.

In alternative embodiments, the at least one opening may be frustal in shape. Correspondingly, the at least one bit retaining piece may be conical or pyramidal in shape (or frustoconical or frustopyramidal). Any other tapered shape may also be used.

There are a number of advantages associated with the above arrangement. The contact surface area between the bit retaining means and the complementary bit retaining means is significantly improved as compared with the prior art, thereby preventing component wear. In addition, because radial movement of pin into the chuck opening is limited, the pin is held away from the bit, so that vibration of the pin in the opening is eliminated.

In a preferred embodiment, the drill bit assembly comprises a plurality of openings evenly spaced about the circumference of the drive chuck and a corresponding plurality of bit retaining pins. Spacing openings and pins evenly around the chuck ensures that the chuck is stronger than in the previous annular slot system.

In an embodiment of the invention, the width of the or each opening at the inner wall of the drive chuck is greater than the width of each spline formed on the drive chuck. This serves to increase the surface contact area between the pin and the opening.

In an embodiment of the invention, the underside of the annular shoulder is provided with recesses which correspond in shape to the shape of the bit retaining pins. For example, where the bit retaining pins are ball-nose pins, the underside of the annular ^090129 shoulder may be provided with ball-nose recesses. This maximises the surface contact area of the retaining surfaces upon drop-out of the bit.

According to an optional feature of the invention, the drill bit assembly comprises a flexible member provided at the chuck. In one embodiment, the flexible member comprises a flexible ring coaxially mounted on the drive chuck around the bit retaining means. In an alternative embodiment, the flexible member comprises a flexible layer provided at an outer end of the or each bit retaining pin. For example, a flexible disc may be bonded, moulded or otherwise attached to the outer end of each pin. The to flexible ring or discs may be formed from rubber, polyurethane or other suitable material. The flexible member acts as a shock absorber between the chuck and the outer wear sleeve upon drop-out of the bit.

According to a further optional feature of the invention, the drill bit assembly further comprises a replaceable ring coaxially mounted on the drive chuck. Preferably, the ring provides a sliding fit with the chuck. The replaceable ring may be mounted on the chuck by sliding the replaceable ring over the chuck. The ring may be retained in position between the chuck and an outer wear sleeve. Preferably, the ring may be held in place on the chuck between an external annular shoulder on the chuck and a lower end of the wear sleeve. The replaceable ring addresses the problem of excessive wear on the wear sleeve in the of the bit retaining means. When the replaceable ring becomes worn due to high pressure air and cuttings flushing at high velocity around the outside of the hammer, it may be replaced more easily and cheaply than replacing the entire wear sleeve.

According to another aspect of the invention, there is provided 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; axially extending splines on the stub shank slideably engageable with complementary splines formed on a drive chuck whereby rotational drive from the chuck may be transmitted to the stub shank; bit retaining means at the chuck adapted for engagement with complementary retaining means at a spline portion of the stub shank to retain the stub shank in the drill bit assembly; and engagement means on the chuck adapted for connecting the chuck to a drive means of If Ο 9 Ο 1 2 9 the fluid-operated percussion drill tool; characterised in that the assembly further comprises a flexible member provided at the chuck.

In one embodiment, the flexible member comprises a flexible ring coaxially mounted on 5 the drive chuck around the bit retaining means. In an alternative embodiment, the flexible member comprises a flexible layer provided at an outer end of the or each bit retaining pin. For example, a flexible disc may be bonded, moulded or otherwise attached to the outer end of each pin. The flexible ring or discs may be formed from rubber, polyurethane or other suitable material. The flexible member acts as a shock absorber between the chuck and the outer wear sleeve upon drop-out of the bit.

According to a further aspect of the invention, there is provided 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; axially extending splines on the stub shank 15 slideably engageable with complementary splines formed on a drive chuck whereby rotational drive from the chuck may be transmitted to the stub shank; bit retaining means at the chuck adapted for engagement with complementary retaining means at a spline portion of the stub shank to retain the stub shank in the drill bit assembly; and engagement means on the chuck adapted for connecting the chuck to a drive means of 20 the fluid-operated percussion drill tool; characterised in that the assembly further comprises a replaceable ring coaxially mounted on the drive chuck.

Preferably, the ring provides a sliding fit with the chuck. The replaceable ring may be mounted on the chuck by sliding the replaceable ring over the chuck. The ring may be 25 retained in position between the chuck and an outer wear sleeve. Preferably, the ring may be held in place on the chuck between an external annular shoulder on the chuck and a lower end of the wear sleeve. The replaceable ring addresses the problem of excessive wear on the wear sleeve in the of the bit retaining means. When the replaceable ring becomes worn due to high pressure air and cuttings flushing at high 30 velocity around the outside of the hammer, it may be replaced more easily and cheaply than replacing the entire wear sleeve.

IE Ο 9 Ο 1 2 9 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 5 described above.

The hammer may be a conventional down-the-hole hammer or a reverse circulation down-the-hole hammer. to Brief Description of the Drawings Two embodiments of a down-the-hole hammer having a bit coupling system in accordance with the invention will now be described with reference to the accompanying drawings, wherein: Figure 1A is a sectional side elevation of a lower portion of a down-the-hole hammer according to a first embodiment of the invention, showing the piston in an off-bottom position; Figure 1B is a sectional side elevation of the lower portion of the down-the-hole hammer of Figure 1 A, showing the piston in the strike position; Figure 2 is a cross-sectional view of the down-the-hole hammer of Figure 1, taken along 20 the line X-X; Figure 3A is a perspective view of the percussion bit and bit retaining pins of the downthe-hole hammer of Figure 1, in the off-bottom or drop-out position; Figure 3B is a perspective view of the percussion bit and bit retaining pins of the downthe-hole hammer of Figure 1, in the strike position; Figure 4 is a perspective view of the drive chuck of the down-the-hole hammer of Figure 1; Figure 5Λ is a sectional side elevation of a lower portion of a down-the-hole hammer according to a first embodiment of the invention, showing the piston in an off-bottom position; Figure 5B is a sectional side elevation of the lower portion of the down-the-hole hammer of Figure 5 A, showing the piston in the strike position; Figure 6 is a cross-sectional view of the down-the-hole hammer of Figure 5, taken along the line X-X; IE 0 9 Ο 1 2 9 Figure 7 is an exploded perspective view of the lower portion of the down-the-hole hammer of Figure 5; and Figure 8 is an exploded perspective view of the lower portion of the down-the-hole hammer of Figure 1.

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 sleeve 5. An inner cylinder is mounted co10 axially within the outer wear sleeve 5. A sliding piston is mounted for reciprocating movement within the inner cylinder and the outer wear sleeve 5, to strike a hammer bit located at the forward end of the outer wear sleeve 5 to exercise a percussive force to the drill bit. Rotational forces are transferred from the rotating outer wear sleeve 5 by means of a chuck 4. The wear sleeve is threadably connected to a drill string which is 15 connected to a rotation motor on a drilling rig at the surface.

Referring particularly to Figures 1 to 4 and Figure 8, the head portion 3 of the bit assembly comprises the percussion bit 1 which is provided with tungsten carbide inserts 31, in a well-known manner. The bit head portion 3 is formed with an axially extending 20 stub shank 32. The stub shank 32 is formed with a spline portion, which includes a lower annular shoulder portion 33, an upper annular shoulder portion 37 and an intermediate portion 50. The upper and lower annular shoulder portions 33, 37 are provided with a plurality of axially extending splines 36. The intermediate portion 50 is not provided with splines. Rotational torque is applied to the bit head portion 3 through 25 the chuck 4. The hollow cylindrical chuck 4 is machined internally to provide a plurality of axially extending internal splines 35 on its internal wall which engage with the splines 36 of the shank 32 to transmit rotational drive from the chuck 4 to the drill bit.

An upper part of the chuck 4 is externally screw threaded. The chuck 4 is also provided with an external annular shoulder 38, which acts as a stop when the chuck 4 is screwed into the wear sleeve 5. Above the shoulder 38, a number of substantially circular openings 42 are cut or machined through the wall of the chuck 4 to receive retaining IE 09 0 1 29 pins 41. As shown in Figure 2, the openings 42 in the wall of the drive chuck are ballnosed in shape. Thus, the openings 42 are tapered such that the width of each opening decreases towards the inner wall of the drive chuck 4. In the embodiment shown, the retaining pins 41 are ball-nose pins. Thus, the pins are tapered to correspond to the shape of the openings, such that the radial movement of the pin into the opening is limited by the shape of the opening. The openings may be machined using a ball-nose cutter. The assembled position of the ball-nose pins is the same as the position of the ball-nose cutter at full depth.

In the assembled bit assembly, the ball nose pins 41 engage with the underside of the annular shoulder 37 formed at the top end of the shank 32 to retain the stub shank in the drill bit assembly. The underside of the annular shoulder 37 is provided with ball-nose shaped recesses 47 (which correspond in shape to the shape of the bit retaining pins). The recesses 47 are shaped to accommodate the pins so as to maximise the contact area between the pins and the bit in the drop-out position (as shown in Figure 3 A). A flexible ring 51 is coaxially mounted on the drive chuck around the bit retaining means. A replaceable ring 52 is coaxially mounted on the drive chuck. The replaceable ring 52 is slid onto the chuck so that it contacts the upper surface of shoulder 38. The chuck 4 is screwed into the lower end of the wear sleeve 5 and, in doing so retains the ball nose pins 41 in openings 42. The replaceable ring 52 is held in place between shoulder 38 and the lower end of the wear sleeve 5. In addition, the screw threaded engagement of the chuck 4 with the wear sleeve 5 enables rotational torque to be transmitted from the wear sleeve 5 through the chuck 4 to the bit 1.

A reciprocating piston is mounted for reciprocating movement within the inner cylinder and the outer wear sleeve 5 to strike the top face of shoulder 37 to impart a percussive force to the bit. The splines 35 of the chuck 4 slideably engage with the complementary splines 36 on the shank 32 so that the head portion 3 is moved axially relatively to the chuck during the percussive action.

An alternative embodiment of the invention is shown in Figures 5 to 7. As shown in the drawings, each of the bit retaining pins 41 is provided with a flexible disc 51 at an outer end thereof, in place of the flexible ring of the first embodiment. 12 As shown in the drawings, bit retaining pins are provided at alternate spline positions on the drive chuck 4. In the embodiments shown, the intermediate portion 50 of the stub shank 32 is not provided with splines. However, in alternative embodiments, splines may extend over the intermediate portion 50 at the spline positions where bit retaining pins are not provided.

The words “comprises/comprising” and the words “having/including” when used herein with reference to the present invention are used to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.

Claims (18)

Claims

1. A drill bit assembly for fluid-operated percussion drill tools comprising: a percussion bit (I) having a head portion (3) formed with an axially extending stub shank (32); 5 axially extending splines (36) on the stub shank (32) slideably engageable with complementary splines (35) formed on a drive chuck (4) whereby rotational drive from the chuck (4) may be transmitted to the stub shank (32); bit retaining means (41,42) al the chuck adapted for engagement with complementary retaining means (37) at a spline portion of the stub shank to retain the stub shank in the drill bit 10 assembly; and engagement means on the chuck (4) adapted for connecting the chuck (4) to a drive means (5) of the fluid-operated percussion drill tool; wherein the bit retaining means comprises at least one opening (42) through the wall of the drive chuck (4) and a corresponding at least one bit retaining pin (41) and the complementary retaining means comprises an annular shoulder (37), such that when 15 the bit assembly is assembled, the bit retaining pins (41) are received in the openings (42) and engage with the underside of the annular shoulder (37) to retain the stub shank (32) in the drill bit assembly; characterised in that the width of the at least one opening (42) in the wall of the drive chuck decreases towards the inner wall of the drive chuck (4); and 20 the at least one bit retaining pin (41) corresponds to the shape of the at least one opening, such that radial movement of the pin into the opening is limited by the shape of the opening.

2. A drill bit assembly as claimed in claim 1, wherein the at least one opening (42) in 25 the wall of the drive chuck is tapered such that the width of the opening decreases towards the inner wall of the drive chuck (4) and the at least one bit retaining pin (41) is tapered to correspond to the shape of the at least one opening, such that radial movement of the pin into the opening is limited by the shape of the opening. 30

3. A drill bit assembly as claimed in claim 1 or claim 2, wherein the at least one opening in the wall of the drive chuck is ball-nosed in shape, and the at least one bit retaining pin is a ball-nose pin. /£ ο 9 ο 1 2 9

4. A drill bit assembly as claimed in any of claims 1 to 3, comprising a plurality of openings evenly spaced about the circumference of the drive chuck and a corresponding plurality of bit retaining pins.

5. 5. A drill bit assembly as claimed in any preceding claim, wherein the width of the or each opening at the inner wall of the drive chuck (4) is greater than the width of each spline (35) formed on the drive chuck.

6. A drill bit assembly as claimed in any preceding claim, wherein the underside of the 10 annular shoulder is provided with recesses (47) which correspond in shape to the shape of the bit retaining pins.

7. A drill bit assembly as claimed in any preceding claim, further comprising a flexible member (51) provided at the chuck.

8. A drill bit assembly as claimed in claim 7, wherein the flexible member comprises a flexible ring coaxially mounted on the drive chuck around the bit retaining means.

9. A drill bit assembly as claimed in claim 7, wherein the flexible member comprises a 20 flexible layer provided at an outer end of the or each bit retaining pin.

10. Λ drill bit assembly as claimed in any preceding claim, further comprising a replaceable ring (52) coaxially mounted on the drive chuck. 25

11. .A drill bit assembly for fluid-operated percussion drill tools comprising: a percussion bit (1) having a head portion (3) formed with an axially extending stub shank (32); axially extending splines (36) on the stub shank (32) slideably engageable with complementary splines (35) formed on a drive chuck (4) whereby rotational drive from the chuck (4) may be transmitted to the stub shank (32); bit retaining means (41.42) at 30 the chuck adapted for engagement with complementary retaining means (37) at a spline portion of the stub shank to retain the stub shank in the drill bit assembly; and engagement means on the chuck (4) adapted for connecting the chuck (4) to a drive means (5) of'the fluid-operated percussion drill tool; characterised in that the assembly further comprises: a flexible member (51) provided at the chuck.

12. A drill bit assembly as claimed in claim 11, wherein the flexible member comprises a flexible ring coaxially mounted on the drive chuck around the bit retaining means.

13. A drill bit assembly as claimed in claim 11, wherein the flexible member comprises a flexible layer provided at an outer end of the or each bit retaining means.

14. A drill bit assembly for fluid-operated percussion drill tools comprising: a percussion bit (1) having a head portion (3) formed with an axially extending stub shank (32); axially extending splines (36) on the stub shank (32) slideably engageable with complementary splines (35) formed on a drive chuck (4) whereby rotational drive from the chuck (4) may be transmitted to the stub shank (32); bit retaining means (41,42) at the chuck adapted for engagement with complementary retaining means (37) at a spline portion of the stub shank to retain the stub shank in the drill bit assembly; and engagement means on the chuck (4) adapted for connecting the chuck (4) to a drive means (5) of the fluid-operated percussion drill tool; characterised in that the assembly further comprises: a replaceable ring (52) coaxially mounted on the drive chuck.

15. A down-the-hole hammer comprising an external cylindrical outer wear sleeve (5), a sliding piston (8) mounted for reciprocating movement within the outer wear sleeve (5) to strike a percussion bit (1) of a drill bit assembly located at the forward end of the outer wear sleeve (5) wherein the drill bit assembly is an assembly as claimed in any one of the preceding claims.

16. A down-the-hole hammer as claimed in claim 15, wherein the hammer is a reverse circulation down-the-hole hammer.

17. A drill bit assembly for fluid-operated percussion drill tools substantially as described herein with reference to and/or as illustrated in Figures 1 to 4 and 8, or Figures 5 to 7 of the accompanying drawings. IE Ο 9 Ο 1 2 9

18.A down-the-hole hammer substantially as hereinbefore described with reference to and/or as illustrated in Figures 1 to 4 and 8, or Figures 5 to 7 of the accompanying drawings.