PERCUSSIVE DOWN-THE-HOLE HAMMER FOR ROCK DRILLING, AND A DRILL BIT USED THEREIN

Abstract: A down-the-hole percussive hammer for rock drilling includes a cylindrical casing and a drill bit disposed at a front end of the casing. The drill bit includes a forwardly facing cutting surface and a center longitudinal passage extending forwardly through a rearwardly facing rearwardmost end surface of the drill bit. The passage includes a rearwardly facing impact surface. A piston is mounted in the casing longitudinally behind the drill bit for reciprocation in a longitudinal direction. The piston includes a front portion sized to enter the center passage of the drill bit and strike the impact surface of the drill bit during each forward stroke of the piston. The impact surface of the drill bit is spaced forwardly from the rearwardmost end surface of the drill bit by a distance of at least ten percent of a total longitudinal length of the drill bit.

Description: PERCUSSIVE DOWN-THE-HOLE HAMMER FOR ROCK DRILLING, AND A DRILL BIT USED THEREIN Technical Background The present invention relates to a percussive down-the-hole hammer for rock drilling, and a drill bit used therein. Description of the Prior Art A prior art drill bit for a down-the-hole hammer is disclosed in U.S. Patent No. 6,062,322. The drill bit comprises an extended anvil portion on which a piston impacts repeatedly to advance the down-the-hole hammer through the rock. However, when constructing a large diameter hammer having a diameter of at least 10 inches, the drill bit becomes relatively large and expensive. It would be desirable to shorten the drill bit and thus provide a more compact hammer, which is relatively simple to manufacture, while still providing for a high efficiency. Objects of the Invention One object of the present invention is to provide an efficient down-the- hole hammer which is compact, relatively easy to manufacture, and which contains a minimum of parts. An additional object is to provide a drill bit for a down-the-hole hammer, which is economical to produce. Summary of the Invention A first aspect of the present invention relates to a down-the-hole percussive hammer for rock drilling. The hammer comprises a generally cylindrical casing, and a drill bit disposed at a front end of the casing. The drill bit comprises a front portion which protrudes from the casing and includes a forwardly facing cutting surface, and a center longitudinal passage extending forwardly through a rearwardly facing rearwardmost end surface of the drill bit. The passage communicates with the front surface and includes a rearwardly facing impact surface spaced forwardly from the rearwardmost end surface. The hammer further includes a top sub mounted in an upper portion of the casing, and a hollow feed tube mounted to the top sub and extending downwardly along a longitudinal center axis of the casing. The feed tube defines a center passage adapted to conduct pressurized air. The hammer also includes a piston mounted in the casing longitudinally behind the drill bit for reciprocation in a longitudinal direction. The piston includes an axial throughhole slidably receiving the feed tube, and a front portion sized to enter the center passage of the drill bit. The front portion of the piston includes a front end defining a forwardly facing striking surface for striking the impact surface during each forward stroke of the piston. Preferably, the impact surface is spaced from the rearwardmost end surface of the drill bit by a distance of at least ten percent of a total longitudinal length of the drill bit. The invention also pertains to the drill bit per se. Description of the Drawings These and other objects of the present invention will become apparent from the following detailed description of preferred embodiments thereof in connection with the accompanying drawings, wherein: FIGS. 1 A, 1 B, 1C and 1 D show a down-the-hole hammer according to the present invention in a longitudinal section in first, second, third and fourth positions, respectively; FIG. 2 shows a drill bit according to the present invention in a longitudinal section; FIG. 3 is a top perspective view of the drill bit; and FIG. 4 is a fragmentary view of a check valve in an open state. Detailed Description of a Preferred Embodiment of the Invention In Figs. 1 A, I B, 1C and 1 D there is shown a preferred embodiment of a down-the-hole hammer 10 according to the present invention. The hammer 10 comprises a reversible outer cylindrical casing 11 which, via a top sub 14, is connectable to a rotatable drill pipe string, not shown, through which compressed air is conducted. The top sub has an external screw thread 14a connected to the casing 11. The inner wall of the casing 11 is almost free from air passage-defining grooves and is thus strong and relatively simple to manufacture. A hammer piston 16 reciprocates in the cylindrical casing 11, and compressed working air is directed alternately to the upper and lower ends of the piston to effect its reciprocation in the casing. Each downward stroke of the piston inflicts an impact blow upon a drill bit 13 mounted within a driver sub 12 at the lower portion of the cylindrical casing 11. The piston has a wide upper or rear portion 16a and a narrow lower or front portion 16b. The upper portion 16a slidably engages the inner wall of the casing 11. Each of the portions 16a and 16b has a cylindrical basic shape and the lower, cylindrical portion 16b has a reduced diameter, thereby causing an intermediate end face or downwardly facing shoulder surface 22 to be formed on the upper portion 16a, which surface is preferably perpendicular to the center line CL of the hammer. The construction of the piston is based on the idea that the mass distribution of the piston 16 is such that when the piston impacts the drill bit, initially a relatively small mass, i.e., the portion 16b, is applied to the drill bit 13. Subsequently, the application of a larger mass, i.e., the portion 16a, follows. It has turned out that by such an arrangement, much of the kinetic energy of the piston is transmitted into the rock via the drill bit as discussed in U.S. Patent 6,131,672, which is hereby incorporated by reference in the present description regarding the construction of the piston per se. An inner cylindrical wall 37 of the piston defines a central passageway 31 and is arranged to slide upon a coaxial control tube or feed tube 15 that is fastened to the top sub 14. The feed tube 15 is hollow and includes radial air outlet ports 20a and radial air re-entry ports 20b, as will be discussed later in more detail. The upper portion 16a of the piston is provided with several groups of passageways for the transportation of pressurized air. A first of those groups of passageways includes passageways 24 (see Fig. 1C), each of which includes a longitudinally extending portion 24a and a radially extending portion 24b. The longitudinally extending portion is spaced from an outer peripheral side surface 138 of the piston and communicates with the upper end face 19 of the piston. The radially extending portion 24b opens into the inner wall 37 of the piston at a location spaced longitudinally from the upper end face 19. Two second passageways 180 in the piston communicate with the shoulder 22 and are not spaced from the outer peripheral side surface 138 of the piston. Rather, a longitudinally extending recess formed in the outer peripheral side surface 138 of the piston defines each of the second passageways 180. Thus, there are two such recesses arranged diagonally opposite one another. An upper end of each recess is spaced downwardly from the upward end face 19. Each recess is formed by a secant extending through the outer side surface 138. Two third passageways 25 are formed in the piston, each having a radially extending portion 25a and a longitudinally extending portion 25b. Each longitudinally extending portion 25b is defined by a groove formed in the outer side surface 138 of the piston. The lower end of the longitudinal portion 25b is spaced above an upper end of a respective second passage 180, whereby a radially outwardly projecting rib 184 is formed therebetween. The rib includes an outer face formed by the outer peripheral side surface 138 of the piston. The longitudinal portion 25b is situated above the rib 184 and is in longitudinal alignment with a respective one of the second passageways 180. Each radially extending portion 25a opens into the inner wall 37 of the piston and is situated above the radially extending portion 24b of the first passageway. The casing 11 has an annular groove 112 formed in an inner surface 114 thereof. The groove 112 is arranged to become aligned with the rib 184 when the air outlet apertures 21 of the feed tube 15 are aligned with the third passageways 25 (see Fig. 1C), whereby air is able to flow around the rib 184 and reach the bottom chamber 26b. The drill bit 13 has a shank 70 and a head 71, see Figs. 2 and 3. The head is provided with a front cutting surface 72 comprising numerous cemented carbide buttons 73. The shank 70 is provided with splines 74 at the mid portion thereof. The splines 74 are intersected by an annular groove 36a made for cooperation with radially inwardly projecting retainers 33 to retain the drill bit in the casing while allowing axial reciprocation therein. The retainers are sandwiched between the top of the bottom sub 12 and a downwardly facing shoulder 79 of the casing 11. A rear portion 30 of the drill bit protrudes radially relative to said groove 36a thereby forming a forwardly facing stop shoulder 75 and an annular notched jacket surface 76 (see Fig. 3). A central passageway 39 is formed in the shank 70 to allow air to be transferred therethrough to outlet channels 39d (see Fig. 2), which are inclined downwardly at an acute angle relative to the center axis of the hammer to conduct air to the front cutting surface 72. The central passageway 39 comprises a downwardly tapering upper portion 39b connecting to a cylindrical portion 39c that in turn connects to a lower portion 39a of lesser diameter than the cylindrical portion. The lower portion 39a connects to a recess bottom 77 extending above a cavity having a concave floor 39e. The longitudinal length L of the drill bit is less than an outer diameter D of the front cutting surface. The recess bottom 77 is spaced from a rearwardmost end of the drill bit by a distance L' which should be greater than ten percent of the length L, but more preferably is greater than twenty percent of the length L, and most preferably is greater than thirty percent of the length L. The recess bottom 77 defines an impact surface that is to be engaged by a front end 27 of the piston 16. An outer diameter D1 of the impact surface 77 equals the diameter of the passageway portion 39a and is at least twenty percent of the outer diameter D of the front cutting surface 72, more preferably at least thirty percent of the diameter D, and most preferably at least forty percent of the diameter D. The recess bottom 77 defines an impact surface that is to be engaged by a front end 27 of the piston 16. The lower part of the lower portion 16b of the piston will constantly be situated within the central passageway 39 of the shank 70. The outer wall 40 of the lower portion 16b will slide against an inner wall of the lower portion 39a of the central passageway 39 to form a seal therebetween. The rear portion 30 of the drill bit 13 is disposed within a ring member 48 situated above the retainers 33. A bottom chamber 26 is continuously formed between the piston 16 and the drill bit 13. During a downward stroke of the piston, the lower portion 16b of the piston reaches a position shown in Fig. 1 B whereby the bottom recess 39e of the central passageway 39 is closed off. At that moment, the air outlet apertures 21 in the feed tube are also closed. Thus, the bottom chamber assumes a configuration 26a which is closed to the outside, whereupon the air in the bottom chamber begins to be compressed as the piston descends farther. Eventually, the piston strikes the drill bit 13 (see Fig. 1C), whereby the bottom chamber assumes a configuration 26b. It should be noted that the tapering upper portion 39b and the cylindrical portion 39c are of generally larger diameter than the lower portion 16b of the piston to form walls of said bottom chamber. The pressurized air is constantly delivered to a central bore 41 of the top sub 14 while the hammer is in use. The bore 41 connects to a cylindrical restriction 42 that in turn connects to an expanded center cavity 43. The feed tube 15 extends into the center cavity 43. Disposed on the upper portion of the tube 15 is a check valve defined by a hollow rubber sleeve 35. An upper portion of the sleeve is sandwiched between the feed tube and a wall of the central bore. That is, a radially extending top lip of the sleeve opposes a downwardly facing surface 41a' of the central bore, and a side of the sleeve opposes a radially inwardly facing surface 41a of the central bore (see Fig. 4). A lower portion of the sleeve extends over the air outlet ports 20a to stop water or air from passing through the hammer the wrong way, i.e., in an upward direction through the feed tube. A central plug 46 disposed in the feed tube carries seal rings 46a and blocks direct travel of air from the outlet ports 20a to the re-entry ports 20b, requiring the air to flow into the cavity 43 in order to reach the re-entry ports 20b. Thus, when air is allowed to pass through the hammer the correct way, i.e., downwardly, the resilient sleeve 35 will expand elastically due to a pressure differential between the interior of the tube 15 and the cavity 43 to enable air to pass through the air outlet ports 20a (see the right- hand side of Fig. 4) into the surrounding cavity 43 and then back into the feed tube 15 through the air re-entry ports 20b arranged axial ly below the air outlet ports 20a. Ideally, the sleeve 35 opens only once during a drilling session, and closes during periods when the air supply is terminated. A portion of the feed tube extends through a seal ring 41b mounted in a reduced-diameter portion 41c of the center bore 41, to seal against the forward passage of air past the portion 41 b along an outer surface of the feed tube. The feed tube is mounted to the top sub by means of a lateral pin 44 extending diametrically all the way through the top sub 14, i.e., through aligned radial bores respectively formed in the lower threaded portion of the top sub, the central plug 46 and the upper portion 47 of the tube 15. The pin 44 thus secures the plug 46 within the feed tube. The hammer functions as follows with reference to Figs. 1 A to 1C. Fig. 1C shows the impact position of the piston 16. The forward end 27 of the piston has just impacted on the recess bottom 77 of the bit 13. A shock wave will be transferred through the bit forwardly from the recess bottom 77 to the cemented carbide buttons at the front surface of the bit, thereby crushing rock material. The steel material of the drill bit situated rearwardly of the recess bottom 77 will be subjected to tension such that the inertia thereof will prolong the application of force to the bottom 77 from the striking surface 27. Thus, a reflecting shock wave in the piston will not be large. The hammer is simultaneously rotated via the drill string, not shown. The piston will then move upwardly due to rebound from the bit and due to the supply of pressurized air from the air outlet apertures 21 of the control tube 15 via the passageways 25 and 180 (see Fig. 1 C). The piston will close the apertures 21 while moving upwardly such that no more pressurized air will be emitted through the apertures 21. Accordingly, the sleeve 35 will close, thereby closing the passage 41 (see Fig. 1B), since the airflow is blocked. The piston 16 is still moving upwardly due to its momentum and due to the expanding air in the bottom chamber. This piston movement will continue until the force acting downwardly upon the top surface 19 of the piston becomes greater than the force acting upwardly on the intermediate end face 22 of the piston. In the meantime, neither the top chamber 32 nor the bottom chamber 26a communicates with the supply of air or the outlet channels (see Fig. 1 B). In the position shown in Fig. 1A the bottom chamber 26 has been opened to the exterior since the inner wall 39a of the drill bit 13 and the outer wall 40 of the lower portion 16b of the piston no longer engage one another. Thus, the air will rush from the bottom chamber through the drill bit 13 for blowing away drill dust. The top chamber 32 is now supplied by pressurized air via the apertures 21 and the passageway 24a, 24b. The piston, however, is still moving upwardly such that eventually the apertures 21 become closed from the passageway 24a, 24b while the pressure of the compressed air in the closed top chamber 32 is boosted to a level about equal to the pressure of the supply air being delivered to the control tube 15. At this stage the piston stops its upward movement. A downward movement is then started due to the spring force of the compacted air in the closed top chamber 32. The downward movement is accelerated by air pressure added by the opening of the air supply to the top chamber 32 when the apertures 21 become aligned with passageways 24a, 24b. The piston will continue its downward movement until the surface 27 of the elongated lower portion 16b impacts on the bit 13 as shown in Fig. 1C. The above-described cycle will continue as long as the pressurized air is supplied to the hammer or until the anvil portion 30 of the drill bit comes to rest on the bit retainers 33 as shown in Fig. 1 D. The latter case can occur when the bit encounters a void in the rock or when the hammer is lifted. Then, to avoid impacts on the retainers 33, the supply of air will not move the piston but will rather exit through the apertures 21 and to the front exterior of the hammer. However, when the hammer again contacts rock, the bit 13 will be pushed into the hammer to the position of Fig. 1C and drilling is resumed provided that pressurized air is supplied. Further in accordance with the present invention the design of the drill bit provides a weight saving of about 200 kg on a 20" diameter hammer since the hammer can be made shorter and a bit-mounting structure can be avoided. The drill bit, that is the prime wear part of the hammer, can be made about 100 kg lighter for a 20" hammer. Such a hammer in accordance with the present invention with an "internal" impact can still be very efficient, about 90%. It will be appreciated that the sleeve 35, which prevents a backflow of fluid and debris, does not have to be replaced when the top sub has to be replaced. Also, all of the operating air can be displaced through the center bore 41 of the top sub. Although the present invention has been described in connection with a preferred embodiment thereof, it will be appreciated by those skilled in the art that additions, deletions, modifications, and substitutions not specifically described may be made without departing from the spirit and scope of the invention as defined in the appended claims.

Claims: WHAT IS CLAIMED IS: 1. A down-the-hole percussive hammer for rock drilling, comprising: a generally cylindrical casing; a drill bit disposed at a front end of the casing, the drill bit comprising: a front portion protruding forwardly from the casing and including a forwardly facing cutting surface, and a center longitudinal passage extending forwardly through a rearwardly facing rearwardmost surface of the drill bit, the passage communicating with the front surface and including a rearwardly facing impact surface spaced forwardly from the rearwardmost end surface; a top sub mounted in an upper portion of the casing; a hollow feed tube mounted to the top sub and extending downwardly along a longitudinal center axis of the casing and defining a center passage adapted to conduct pressurized air, and a piston mounted in the casing longitudinally behind the drill bit for reciprocation in a longitudinal direction, the piston including an axial through-hole slidably receiving the feed tube, and a front portion sized to enter the center passage of the drill bit, the front portion including a front end defining a forwardly facing striking surface for striking the impact surface during each forward stroke of the piston. 2. The hammer according to claim 1 wherein the impact surface is spaced from the rearwardmost end surface of the drill bit by a distance greater than ten percent of a total longitudinal length of the drill bit. 3. The hammer according to claim 2 wherein the distance is at least twenty percent of the total longitudinal length. 4. The hammer according to claim 2 wherein the distance is at least thirty percent of the total longitudinal length. 5. The hammer according to claim 2 wherein the impact surface has an outer diameter of at least twenty percent of an outer diameter of the cutting surface. 6. The hammer according to claim 2 wherein the impact surface has an outer diameter of at least thirty percent of an outer diameter of the cutting surface. 7. The hammer according to claim 1 wherein the center passageway of the drill bit includes a rear portion tapering forwardly from the rearwardmost end surface of the drill bit, a first cylindrical portion extending forwardly from a forward end of the rear portion, a second cylindrical portion of smaller diameter than the first cylindrical portion and extending forwardly from a forward end of the first cylindrical portion and disposed between the impact surface and the first cylindrical surface, and a cylindrical cavity extending forwardly from the impact surface and being of smaller diameter than the second cylindrical surface, the front portion of the piston having an outer diameter substantially the same as a diameter of the second cylindrical portion. 8. The hammer according to claim 7 further including outlet channels extending at acute angles from a lower end of the cavity to the front cutting surface. 9. The hammer according to claim 1 wherein a total longitudinal length of the drill bit is less than an outer diameter of the cutting surface. 10. A drill bit adopted for use in a down-the-hole percussive hammer, comprising: a forward portion having a forwardly facing cutting surface, a rear portion including a rearwardly facing rearwardmost end surface, a forwardly facing stop shoulder disposed forwardly of the rear end surface, and a center longitudinal passage extending forwardly through the rear end surface, the passage communicating with the cutting surface and including a rearwardly facing impact surface spaced forwardly from the rearwardmost end surface. 11. The drill bit according to claim 10 wherein the impact surface is spaced from the rearwardmost end surface of the drill bit by a distance greater than ten percent of a total longitudinal length of the drill bit. 12. The drill bit according to claim 11 wherein the distance is at lest twenty percent of the total length. 13. The drill bit according to claim 11 wherein the distance is at least thirty percent of the total length. 14. The drill bit according to claim 11 wherein the impact surface has an outer diameter of at least twenty percent of an outer diameter of the cutting surface. 15. The drill bit according to claim 11 wherein the impact surface has an outer diameter of at least thirty percent of an outer diameter of the cutting surface. 16. The drill bit according to claim 10, wherein the center passageway of the drill bit includes a rear portion tapering forwardly from the rearwardmost end surface of the drill bit, a first cylindrical portion extending forwardly from a forward end of the rear portion, a second cylindrical portion of smaller diameter than the first cylindrical portion and extending forwardly from a forward end of the first cylindrical portion and disposed between the impact surface and the first cylindrical surface, and a cylindrical cavity extending forwardly from the impact surface and being of smaller diameter than the second cylindrical surface, the front portion of the piston having an outer diameter substantially the same as a diameter of the second cylindrical portion. 17. The drill bit according to claim 16 further including outlet channels extending at acute angles from a lower end of the cavity to the front cutting surface. 18. The drill bit according to claim 11 wherein a total longitudinal length of the drill bit is less than an outer diameter of the cutting surface. 19. The drill bit according to claim 10 wherein a total longitudinal length of the drill bit is less than an outer diameter of the cutting surface.

Inventors: BECCU RAINER, BECCU, RAINER
Applicants: SANDVIK AB
Application year: 2001
Main classification: E21B4
IPC codes: E21B4, E21B10