Down-the-hole hammer with alternating pressure distributor chamber

Abstract

The present invention relates to a down-the-hole hammer comprising an outer wear sleeve and a sliding piston mounted for reciprocating movement within the outer wear sleeve to strike a percussion bit located at a forward end of the outer wear sleeve, wherein forward and rear drive chambers for the piston are disposed at a forward end and a rear end of the piston, respectively, such that pressurised air alternately supplied to the forward and rear drive chambers causes the reciprocating movement of the piston. The hammer further comprises an inner chamber and a control valve, wherein the control valve is arranged to alternately connect the inner chamber to a supply of pressurised air and to the forward chamber, and wherein an outer diameter of the inner chamber is smaller than an outer diameter of the rear chamber.

Description

Field of the Invention The present invention relates to down-the-hole hammers, and in particular, to a down-the-hole hammer having an alternating pressure distributor chamber. Background to the Invention A down-the-hole hammer typically comprises three pneumatic chambers. Pressurised air is supplied alternately to top and bottom chambers, which are arranged to drive a piston in a reciprocating manner. The top (or rear) chamber is arranged to drive the piston downwards (or forwards) against a drill bit during the hammer downstroke. The bottom (or forward) chamber is arranged to lift the piston away from the drill bit during the hammer upstroke. A third chamber, which may be referred to as a constant pressure chamber or distributor chamber, has two functions. It serves to distribute air to the top and bottom chambers during the hammer cycle. It also provides a constant pneumatic force that pushes the piston against the drill bit. This ensures that, even for horizontal drilling, the piston will be pushed against the drill bit at start-up of the hammer, avoiding a situation where the hammer fails to start up if the piston is in a mid-stroke position. Two examples of prior art hammers which include a constant pressure third chamber, are shown in Figures 1A and IB, respectively. In each case, the hammer 100 comprises an outer wear sleeve 101 with a piston 102 mounted for reciprocating motion therein, to act with percussive force on a drill bit 103 retained at a forward end of the wear sleeve. Each hammer comprises a top chamber 104, to drive the piston downwards, and a bottom chamber 105, to lift the piston away from the drill bit. Figure 1A shows a typical down-the-hole hammer in which an inner cylinder 108 is mounted within the wear sleeve, and the piston is, in turn, mounted within the inner cylinder. A foot valve 109 is provided at a rear end of the bit, which co-operates with the piston at the bottom of the stroke to prevent air from entering an exhaust bore of the bit. The constant pressure chamber 106 is disposed between the outer wear sleeve and the piston. Figure IB shows an alternative arrangement in which the constant pressure chamber 106’ is disposed between the piston and an air distributor 107, and air is exhausted from the hammer between the wear sleeve and a guide sleeve 110. A disadvantage associated with such prior art hammers is that the constant pressure or distributor chamber does not contribute pneumatic energy to the piston cycle. During the upstroke, the constant pressure chamber receives energy from the piston, since the piston is moving against the constant pneumatic force provided by the chamber. During the downstroke, the chamber returns the same energy back to the piston. Throughout the hammer cycle, the distributor chamber is delivering air to or receiving air from the supply line. Since such air flow always results in a pressure drop, the constant pressure chamber results in an overall energy loss from the system. Summary of the Invention The present invention relates to a down-the-hole hammer, comprising: an outer wear sleeve; a sliding piston mounted for reciprocating movement within the outer wear sleeve to strike a percussion bit located at a forward end of the outer wear sleeve, wherein forward (or bottom) and rear (or top) drive chambers for the piston are disposed at a forward end and a rear end of the piston, respectively, such that pressurised air alternately supplied to the forward and rear drive chambers causes the reciprocating movement of the piston; and an inner chamber and a control valve, wherein the control valve is arranged to alternately connect the inner chamber to a supply of pressurised air and to the forward chamber, and wherein an outer diameter of the inner chamber is smaller than an outer diameter of the rear chamber. The forward drive chamber may be referred to as a bottom chamber and the rear drive chamber may be referred to as a top chamber. The inner chamber may also be referred to as an alternating pressure distributor chamber. The term “forward” is used herein to indicate an end of the hammer towards the percussion bit, that is, a drilling end of the hammer. The term “rear” is used herein to indicate an end of the hammer away from the percussion bit, that is, an end of the hammer that is typically uppermost during drilling. Alternately connecting the inner chamber to the supply of pressurised air and to the forward chamber provides an alternating pressure in the inner chamber. As a result, the forward, rear and inner chambers all have an alternating pressure throughout the hammer cycle, thereby improving efficiency of the hammer. By providing a third alternating pressure chamber with a smaller diameter than that of the rear chamber, the overall size of the hammer can be kept small while achieving improved efficiency. In certain embodiments, an outer diameter of the control valve may also be smaller than the outer diameter of the inner chamber. This further ensures that the size of the hammer can be kept small while achieving improved efficiency. In certain embodiments, the outer diameter of the inner chamber may be substantially the same as an inner sealing diameter of the rear chamber. The inner sealing diameter of the rear chamber is the smallest diameter of the rear chamber which forms a sealing arrangement with the piston during normal operation of the device. An advantage of this arrangement is that it enables a simpler hammer layout. The pressurised air supplied to the forward chamber is supplied to the forward chamber from the inner chamber upon connection of the inner chamber to the forward chamber by the control valve. That is, unlike with the conventional hammers described above, the forward chamber is not receiving pressurised air directly from a supply line. Rather, at or around the time of piston impact, the control valve connects the inner chamber to the forward chamber to supply pressurised air from the inner chamber to the forward chamber to drive the piston upwards or rearwards. A central piston bore may extend longitudinally through the piston. The piston bore may extend through the entire length of the piston or a part thereof. According to a first embodiment of the invention, the inner chamber is disposed in a counterbore (of the central piston bore) in a rear end of the piston. The hammer may further comprise an air distributor disposed in the piston counterbore. The inner chamber may be disposed between an outer surface of the air distributor and an inner surface of the piston counterbore. The control valve may be disposed within the air distributor. According to another embodiment of the invention, the hammer may further comprise an inner cylinder mounted within the outer wear sleeve and an air distributor disposed at a rear end of the piston. The piston may comprise an extension portion at a rear end thereof. The extension portion of the piston may be mounted within the inner cylinder and the inner chamber may be disposed between an outer surface of the air distributor and an inner surface of the inner cylinder. In certain embodiments, the down-the-hole hammer may comprise a footvalve disposed at a rear end of a central exhaust bore of the percussion bit and configured to co-operate with a central bore of the piston. In this embodiment, the central piston bore of the piston may comprise two blind bores, each of which extends longitudinally through a part of the piston. The blind bores may comprise a forward piston bore, arranged to cooperate with the footvalve. The blind bores may also comprise a rear piston bore, arranged to co-operate with an air distributor of the hammer. The hammer may further comprise a first set of channels in the piston, extending between an outer surface of the piston and the forward piston bore, to provide fluid communication between the rear chamber and the exhaust bore of the percussion bit; and a second set of channels in the piston, extending between the rear piston bore and the outer surface of the piston, to provide fluid communication between the inner chamber and the forward chamber. Brief Description of the Drawings Figure 1A is a schematic representation of a conventional down-the-hole hammer, wherein a constant pressure chamber is disposed between the wear sleeve and the piston; Figure IB is a schematic representation of a conventional down-the-hole hammer, wherein a constant pressure chamber is disposed between the piston and the air distributor; Figure 2A is a schematic representation of a down-the-hole hammer according to a first embodiment of the invention, showing the piston in a downstroke; Figure 2B is a schematic representation of the hammer of Figure 2A, showing the hammer in an upstroke; Figure 3 A is a schematic representation of a down-the-hole hammer according to a second embodiment of the invention, showing the piston in a downstroke; Figure 3B is a schematic representation of the hammer of Figure 3 A, showing the hammer in an upstroke; Figure 4 is a cross-sectional view of a portion of a hammer according to the first embodiment, showing a valve arrangement; Figs 5A to 5E illustrate the valve cycle for the hammer of the first embodiment of the invention; Figure 6 is a cross-sectional view of a portion of a hammer according to the second embodiment, showing a valve arrangement; Figs 7A to 7E illustrate the valve cycle for the hammer of the first embodiment of the invention; Figure 8 is a cross-sectional view of a portion of a hammer according to a third embodiment, in which the bottom chamber is disposed between the wear sleeve and a footvalve; and Figure 9 is a cross-sectional view of a portion of a hammer according to a fourth embodiment, in which the bottom chamber is disposed between the wear sleeve and a footvalve. Detailed Description of the Drawings A down-the-hole hammer 200 according to a first embodiment of the invention is illustrated in Figures 2A and 2B. The hammer comprises an external cylindrical outer wear sleeve 201 and a sliding piston 202 mounted for reciprocating movement within the outer wear sleeve to strike a percussion bit 203 located at a forward end of the outer wear sleeve. A rear or top drive chamber 204 and a forward or bottom drive chamber 205 are disposed at rear and forward ends of the piston, respectively. Pressurised air is alternately supplied to the forward and rear drive chambers to cause the reciprocating movement of the piston. The hammer further comprises an inner chamber 211 and a control valve 212, wherein the control valve is arranged to alternately connect the inner chamber to a supply 213 of pressurised air and to the forward chamber 205. The pressurised air supplied to the forward chamber 205 is supplied to the forward chamber from the inner chamber 211 upon connection of the inner chamber to the forward chamber by the control valve. A central piston bore 217 extends longitudinally through the piston. The inner chamber 211 is disposed in a counterbore 214 in a rear end of the piston. The hammer further includes an air distributor 207 disposed in the piston counterbore 214. The inner chamber 211 is disposed between an outer surface of the air distributor and an inner surface of the piston counterbore. A possible valve arrangement for this embodiment is shown in Figure 4, in which the control valve 212 is disposed within the air distributor. As also shown in Figure 4, an outer diameter of the inner chamber 211 is smaller than an outer diameter of the rear chamber 204 and substantially the same as an inner sealing diameter of the rear chamber. Although not shown in Figures 2A and 2B, an outer diameter of the control valve 212 may also be smaller than the outer diameter of the inner chamber. Unlike the conventional hammer shown in Figure IB, the pressure in the inner chamber 211 of the hammer shown in Figures 2A and 2B varies over the course of the hammer cycle, and is controlled by control valve 212. Figure 2A shows the piston in a downstroke, that is, travelling towards the percussion bit 203. As shown schematically in Figure 2A, the inner chamber 211 is connected to the supply pressure line 213 via the control valve 212. The inner chamber is pressurised throughout the whole downstroke. At the time of piston impact on the bit 203, the inner chamber is fully pressurised and is at its largest volume within the piston cycle. The pressurised air in the inner chamber has sufficient energy to perform the upstroke of the piston. In order to begin the upstroke, the valve 212 connects the inner chamber to the forward chamber 205, as shown schematically in Figure 2B, so that the inner chamber and the forward chamber are at the same pressure. However, the pneumatic area of the forward chamber is larger than the pneumatic area of the inner chamber and so there is a net force driving the piston upwards, away from the bit. During the upstroke, the combined volume of the forward and inner chambers will increase and the pressure in both chambers will decrease (corresponding to the expansion phase). At the end of the upstroke, the valve 212 will disconnect the inner chamber from the forward chamber and connect the inner chamber to the supply pressure. Any remaining pressurised air in the forward chamber is exhausted out through the drill bit via splines on the bit shank, and the next downstroke begins. A down-the-hole hammer 300 according to a second embodiment of the invention is illustrated in Figures 3A and 3B. As in the first embodiment, the hammer comprises an external cylindrical outer wear sleeve 301 and a sliding piston 302 mounted for reciprocating movement within the outer wear sleeve to strike a percussion bit 303 located at a forward end of the outer wear sleeve. A rear or top drive chamber 304 and a forward or bottom drive chamber 305 are disposed at rear and forward ends of the piston, respectively. Pressurised air is alternately supplied to the forward and rear drive chambers to cause the reciprocating movement of the piston. A central piston bore 317 extends longitudinally through the piston. The hammer further comprises an inner chamber 311 and a control valve 312, wherein the control valve is arranged to alternately connect the inner chamber to a supply 313 of pressurised air and to the forward chamber 305. The pressurised air supplied to the forward chamber 305 is supplied to the forward chamber from the inner chamber 311 upon connection of the inner chamber to the forward chamber by the control valve. In this embodiment, the hammer further comprises an inner cylinder 308 mounted within the outer wear sleeve. The hammer also includes an air distributor 307 disposed at a rear end of the piston. The piston comprises an extension portion 315 at a rear end thereof. The extension portion of the piston is received within the inner cylinder and the inner chamber 311 is disposed between an outer surface of the air distributor, an inner surface of the inner cylinder and a rear end of the piston extension 315. A possible valve arrangement for this embodiment is shown in Figure 6, in which the control valve 312 is disposed at a rear end of the air distributor. As also shown in Figure 6, an outer diameter of the inner chamber 311 is smaller than an outer diameter of the rear chamber 304 and substantially the same as an inner sealing diameter of the rear chamber. An outer diameter of the control valve 312 is also smaller than the outer diameter of the inner chamber. The piston cycle for the hammer shown in Figures 3 A and 3B is similar to that described above with reference to Figures 2A and 2B. Figure 3A shows the piston in a downstroke. As shown schematically in Figure 3 A, the inner chamber 311 is connected to the supply pressure line 313 via the control valve 312. The inner chamber is pressurised throughout the whole downstroke. At the time of piston impact on the bit 303, the inner chamber is fully pressurised and is at its largest volume within the piston cycle. The pressurised air in the inner chamber has sufficient energy to perform the upstroke of the piston. In order to begin the upstroke, the valve 312 connects the inner chamber to the forward chamber 305, as shown schematically in Figure 3B, so that the inner chamber and the forward chamber are at the same pressure. As with the previous embodiment, the pneumatic area of the forward chamber is larger than the pneumatic area of the inner chamber and so there is a net force driving the piston upwards, away from the bit. During the upstroke, the combined volume of the forward and inner chambers will increase and the pressure in both chambers will decrease (corresponding to the expansion phase). At the end of the upstroke, the valve 312 will disconnect the inner chamber from the forward chamber and connect the inner chamber to the supply pressure. Any remaining pressurised air in the forward chamber is exhausted out around the bit and the next downstroke begins. Figure 4 illustrates a possible valve arrangement for the embodiment of the hammer shown in Figures 2A and 2B. As set out above, the control valve 212 is disposed within the air distributor. An outer diameter of the control valve 212 is smaller than the outer diameter of the inner chamber 211. A valve undercut 216 is connected to supply pressure so that there is a net pneumatic area pushing the valve forwards (to the right as shown in Figure 4). Piston bore 217 and distributor bore 218 are both connected to the bottom chamber (not shown in Figure 4). There is a pneumatic area pushing the valve rearwards (to the left as shown in Figure 4). A pilot chamber 219 of the valve is connected to the rear chamber 204 of the hammer via a pilot channel 220. The pilot pressure pushes the valve forward (to the right as shown in Figure 4). When the valve is pushed to the forward (right) end of the stroke position (referred to as a first position), as shown in Figure 4, the valve connects the inner chamber to supply pressure via a first port 221, corresponding to Figure 2A. When the valve is pushed to the rear end (left) of the stroke position (referred to as a second position), the first port 221 is closed and the inner chamber 211 is connected to the bottom chamber via a second port 222 and the piston bore 217. The valve cycle is described in more detail with reference to Figures 5A to 5E. In Figure 5A, the piston 202 is at its rearmost or uppermost position, and is about to start the downstroke. As shown by the arrows, the rear chamber 204 is receiving high pressure supply air via an undercut 223 in the distributor 207 and an undercut 224 in the piston. Although not shown in Figure 5A, the forward chamber is connected to the drill bit and is exhausting between the wear sleeve 201 and a guide sleeve or aligner 230 (shown in Figures 2A and 2B). The pilot chamber 219 is at high pressure (via the pilot channel 220) and the distributor bore 218 (and thus the forward end of the valve) is at low pressure. This means that there is a net pneumatic forward force (to the right as shown in the figures) on the valve 212 which has switched the valve to the first position, so that the inner chamber 211 is at high pressure. This means that both the rear chamber 204 and the inner chamber 211 are driving the piston towards the drill bit. In Figure 5B, the piston has travelled towards the drill bit. The rear chamber 204 is disconnected from the supply pressure and has started an expansion phase. The forward chamber (not shown) is no longer exhausting and is therefore also disconnected from the low pressure so that a compression stage has started. The inner chamber 211 is still receiving pressurised air via the first port 221 and remains fully pressurised. In Figure 5C, the piston has reached a position in which the rear chamber 204 has begun exhausting through an undercut 225 in the wear sleeve and an exhaust annulus 226 around the piston, as shown by the arrows. The pressure in the rear chamber is therefore dropping. The distributor 207 has left the piston bore 217, connecting the inner chamber 211 to the piston bore 217. The piston bore 217, distributor bore 218 and forward chamber (not shown) are initially pressurised by receiving air from the inner chamber when the distributor leaves the piston bore, causing the valve to switch which in turn opens the second port 222, as shown by the arrows. To ensure that the pressure level in the inner and the forward chambers is high enough, in the embodiment shown in Figures 5A to 5E, the piston undercut 224 is arranged to be wide enough to allow pressurised supply air to flow around the distributor, into the inner chamber and to the forward chamber, as shown by the dotted arrow. The pilot chamber 219 is now at low pressure and the distributor bore 218 is at high pressure, so that the valve has switched to the second position. The valve switching is arranged to take place just before or after the piston impact on the bit. The piston upstroke then begins. In Figure 5D, the piston 202 is travelling upwards or rearwards (to the left as shown in the figures), away from the drill bit. The rear chamber 204 is no longer connected to the exhaust annulus 226, so that a compression phase has begun. The forward chamber and the inner chamber 211 are both in an expansion phase. In Figure 5E, the piston 202 has reached the end of the upstroke. The rear chamber 204 is again connected to supply pressure as shown by the arrows. The forward chamber (not shown) is exhausting. The pilot chamber 219 is connected to supply pressure via the pilot channel 220, and the distributor bore 218 is at low pressure. The net pneumatic force has pushed the valve 212 back to the first position, so that the inner chamber 211 is pressurised and a new cycle begins. Figure 6 illustrates a possible valve arrangement for the embodiment of the hammer shown in Figures 3A and 3B. As set out above, the control valve 312 is disposed at a rear end of the air distributor 307. A high pressure chamber 327 of the valve is connected to supply pressure so that there is a pneumatic area pushing the valve forwards (to the right as shown in Figure 6). Piston bore 317 and distributor bore 318 are both connected to the forward chamber (not shown in Figure 6) so that there is a pneumatic area pushing the valve rearwards (to the left as shown in Figure 6). A pilot chamber 319 of the valve is connected to the rear chamber 304 via a pilot channel 320. The pilot pressure pushes the valve forward (to the right as shown in Figure 6). When the valve is pushed to the forward (right) end of the stroke position (referred to as a first position), as shown in Figure 6, the valve connects the inner chamber 311 to supply pressure via a first port 321, corresponding to Figure 3 A. When the valve is pushed to the rear end (left) of the stroke position (referred to as a second position), the first port 321 is closed and the inner chamber 311 is connected to the bottom chamber via a second port 322 and the piston bore 317. The valve cycle is described in more detail with reference to Figures 7A to 7E. In Figure 7A, the piston 302 is at its rearmost or uppermost position, and is about to start the downstroke. As shown by the arrows, the rear chamber 304 is receiving high pressure supply air via a port 328 in the inner cylinder 308. Although not shown in Figure 7A, the forward chamber is connected to the drill bit and is exhausting. The pilot chamber 319 is at high pressure (via the pilot channel 320) and the distributor bore 318 (and thus the forward end of the valve) is at low pressure. This means that there is a net pneumatic forward force (to the right as shown in the figures) on the valve 312 which has switched the valve to the first position, so that the inner chamber 311 is at high pressure. This means that both the rear chamber 304 and the inner chamber 311 are driving the piston towards the drill bit. In Figure 7B, the piston has travelled towards the drill bit. The rear chamber 304 is disconnected from the supply pressure and has started an expansion phase. The forward chamber (not shown) is also disconnected from the low pressure so that a compression stage has started. The inner chamber 311 is still receiving pressurised air via the first port 321 and remains fully pressurised. In Figure 7C, the piston has reached a position in which the rear chamber 304 has begun exhausting through an undercut 325 in the wear sleeve and an exhaust annulus 326 around the piston, as shown by the arrows. The pressure in the rear chamber is therefore dropping. The distributor 307 has left the piston bore 317, connecting the inner chamber 311 to the piston bore 317. The piston bore 317, distributor bore 318 and forward chamber (not shown) are initially pressurised by receiving air from the inner chamber, thereby causing the valve to switch, which in turn opens the second port 322. To ensure that the pressure level in the forward chamber is high enough to provide sufficient lift for the piston, in the embodiment shown in Figures 7A to 7E additional pressurised air may flow through the inner cylinder port 328, into the inner chamber 311 and to the forward chamber, as shown by the dotted arrow. The pilot chamber 319 is now at low pressure and the distributor bore 318 is at high pressure, so that the valve has switched to the second position. The valve switching is arranged to take place just before or after the piston impact on the bit. The piston upstroke then begins. In Figure 7D, the piston 302 is travelling upwards or rearwards (to the left as shown in the figures), away from the drill bit. The rear chamber 304 is no longer connected to the exhaust annulus 326, so that a compression phase has begun. The forward chamber and the inner chamber 311 are both in an expansion phase. In Figure 7E, the piston 302 has reached the end of the upstroke. The rear chamber 304 is again connected to supply pressure as shown by the arrows. The forward chamber is exhausting. The pilot chamber 319 is connected to supply pressure via the pilot channel 320, and the distributor bore 318 is at low pressure. The net pneumatic force has pushed the valve 312 back to the first position, so that the inner chamber 311 is pressurised and a new cycle begins. Figure 8 illustrates a down-the-hole hammer 800, similar to that shown in Figures 2A and 2B. However, in this embodiment, the central bore of the piston comprises a forward blind bore 833 and a rear blind bore 834. The forward chamber 805 is disposed between the wear sleeve 801 and a footvalve 829. The footvalve is disposed at a rear end of a central exhaust bore of the percussion bit 803 and configured to cooperate with the forward blind bore 833 of the piston. The control valve arrangement and cycle may be as shown in Figures 5A to 5E. In contrast to the arrangement shown in Figures 2A and 2B, the rear chamber 804 exhausts via channels 831 in the piston, and the forward blind bore 833. During the upstroke, the inner chamber 811 is connected to the forward chamber 805 via rear blind bore 834 and channels 832 in the piston 802. Figure 9 illustrates a down-the-hole hammer 900, similar to that shown in Figures 3 A and 3B. However, in this embodiment, the central bore of the piston comprises a forward blind bore 933 and a rear blind bore 934. The forward chamber 905 is disposed between the wear sleeve 901 and a footvalve 929. The footvalve is disposed at a rear end of a central exhaust bore of the percussion bit 903 and configured to co-operate with the forward blind bore 933 of the piston.. The control valve arrangement and cycle may be as shown in Figures 7A to 7E. In contrast to the arrangement shown in Figures 3 A and 3B, the rear chamber 904 exhausts via channels 931 in the piston, and the forward blind bore 933. During the upstroke, the inner chamber 911 is connected to the forward chamber 905 via rear blind bore 934 and channels 932 in the piston 902. 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

1. A down-the-hole hammer comprising:an outer wear sleeve;a sliding piston mounted for reciprocating movement within the outer wear sleeve to strike a percussion bit located at a forward end of the outer wear sleeve, wherein forward and rear drive chambers for the piston are disposed at a forward end and a rear end of the piston, respectively, such that pressurised air alternately supplied to the forward and rear drive chambers causes the reciprocating movement of the piston; andan inner chamber and a control valve, wherein the control valve is arranged to alternately connect the inner chamber to a supply of pressurised air and to the forward chamber, and wherein an outer diameter of the inner chamber is smaller than an outer diameter of the rear chamber.

2. A down-the-hole hammer as claimed in claim 1, wherein the outer diameter of the inner chamber is substantially the same as an inner sealing diameter of the rear chamber.

3. A down-the-hole hammer as claimed in claim 1 or claim 2, wherein an outer diameter of the control valve is smaller than the outer diameter of the inner chamber.

4. A down-the-hole hammer as claimed in any of claims 1 to 3, wherein the pressurised air supplied to the forward chamber is supplied to the forward chamber from the inner chamber upon connection of the inner chamber to the forward chamber by the control valve.

5. A down-the-hole hammer as claimed in any preceding claim, wherein the inner chamber is disposed in a counterbore in a rear end of the piston.

6. A down-the-hole hammer as claimed in claim 5, further comprising:an air distributor disposed in the piston counterbore;wherein the inner chamber is disposed between an outer surface of the air distributor and an inner surface of the piston counterbore.

7. A down-the-hole hammer as claimed in claim 6, wherein the control valve is disposed within the air distributor.

8. A down-the-hole hammer as claimed in any of claims 1 to 4, further comprising:an inner cylinder mounted within the outer wear sleeve; andan air distributor disposed at a rear end of the piston;wherein the piston comprises an extension portion at a rear end thereof, the extension portion of the piston mounted within the inner cylinder and wherein the inner chamber is disposed between an outer surface of the air distributor and an inner surface of the inner cylinder.

9. A down-the-hole hammer as claimed in claim 8, wherein the control valve is disposed at a rear end of the air distributor.

10. A down-the-hole hammer as claimed in any preceding claim, further comprising:a footvalve disposed at a rear end of a central exhaust bore of the percussion bit and configured to co-operate with a central bore of the piston;a first set of channels in the piston, extending between an outer surface of the piston and the piston bore, to provide fluid communication between the rear chamber and the exhaust bore of the percussion bit; anda second set of channels in the piston, extending between the piston bore and the outer surface of the piston, to provide fluid communication between the inner chamber and the forward drive chamber.1 l.A down-the-hole hammer substantially as hereinbefore described with reference to and / or as illustrated in any of Figures 2A to 8 of the accompanying drawings.

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