Patent Publication Number: US-6209666-B1

Title: Percussive down-the-hole hammer and a piston and drill bit therefor

Description:
FIELD OF THE INVENTION 
     The present invention relates to a percussive down-the-hole hammer and a piston and drill bit therefor. 
     PRIOR ART 
     During drilling with down-the-hole hammers under ground, such as in tunnels, the dust generated by the drilling operation often is bound together by the use of water mixed into the pressurized air driving the hammer and flushing the dust away. The down-the-hole hammer is provided with a plastic foot valve located in a central passageway in a drill bit anvil and projecting from the impact surface of the anvil. The foot valve is repeatedly enclosed by a central bore of the reciprocating piston to transfer spent pressurized driving air through the drill bit. When drilling downwardly, water is deposited on the impact surface between successive impacts such that each impact will create a jet stream of water away from the impact surface. The part of the jet stream traveling radially inwardly, however, will cut into the plastic foot valve and finally the valve will break such that the hammer will stop impacting. 
     OBJECTS OF THE INVENTION 
     An object of the present invention is to provide a drill bit for a down-the-hole hammer which provide for extended lifespan of the foot valve, 
     Another object of the present invention is to provide a drill bit for a down-the-hole hammer that will have a longer life between service than hitherto known hammers. 
     These and other objects of the drill bit and the down-the-hole hammer according to the present invention will become apparent from the following detailed description of a preferred embodiment thereof in connection with the accompanying drawings. 
     SUMMARY OF THE INVENTION 
     The present invention relates to an air actuated down-the-hole hammer for rock drilling. The hammer comprises a generally cylindrical casing which defines an axis. A drill sub is mounted to a rear end of the casing. A drill chuck is mounted to a front end of the casing. A drill bit is mounted in the drill chuck and includes a front cutting face and a rear anvil portion. The anvil portion includes a rearwardly facing anvil surface. The drill bit includes a first central passageway extending through the anvil surface. A piston is mounted in the casing behind the drill bit. The piston includes a forwardly facing impact surface and a second central passage extending through the impact surface and aligned with the first central passage. A foot valve extends partially in the first central passage and partially in the second central passage when the impact surface impacts the anvil surface, for transferring pressurized air from the second central passage to the first central passage. The piston is mounted for axial reciprocation toward and away from the drill bit, causing the impact surface to impact the anvil surface during a forward stroke of the piston whereby at least some fluid disposed between the impact surface and the anvil surface is forced radially inwardly toward the foot valve. Either the impact surface or the anvil surface includes a projection extending around a radially inner peripheral edge thereof for deflecting the radially inwardly forced fluid in a direction having an axial component to minimize an impact force against the foot valve. 
     The present invention also relates to a percussive drill bit which includes the projection, and also relates to a piston which includes the projection. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The objects and advantages of the invention will become apparent from the following detailed description of a preferred embodiment thereof in connection with the accompanying drawing in which like numerals designate like elements, and in which: 
     FIG. 1 shows a down-the-hole hammer according to the present invention in a longitudinal section; 
     FIG. 2 shows, in the left hand portion of that figure, a foot valve and portions of a drill bit according to the present invention and a piston, in a longitudinal section, and the right hand part thereof discloses a prior art solution; 
     FIG. 3 shows an enlarged view of the prior art portion of FIG. 2; 
     FIG. 4 shows an enlarged view of the left hand portion of FIG.  2  and the drill bit according to the present invention; and 
     FIG. 5 is a view similar to FIG. 4 showing another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
     In FIG. 1 there is shown a preferred embodiment of a down-the-hole hammer  10  according to the present invention. The hammer  10  comprises an outer cylindrical casing  11  connectable to a rotatable drill pipe string, not shown, through which compressed air is conducted. A hammer piston  16  reciprocates in the cylindrical casing  11 , and compressed air is directed alternately to the upper (rear) and lower (front) ends of the piston to effect its reciprocation in the casing, each downward stroke inflicting an impact blow upon the anvil  30  of a drill bit  13  extending upwardly within the lower portion of the cylindrical casing. The piston comprises a passageway  31  for pressurized air. The percussive down-the-hole hammer further comprises a top sub  14 , a check valve  35 , a control or fluid feed tube  15 , a foot valve  20 , a retaining means  33  and a driver sub  12 . The down-the-hole hammer  10  is of conventional design except for the shape of the anvil  30  of the drill bit  13 . Usually the addition of water into the pressurized air for avoiding dust problems amounts to about 4 to 40 liters of water per minute. 
     The foot valve  20  (see FIG. 2) is of generally cylindrical basic shape and is made of plastics, such as nylon. The foot valve comprises a hollow tube  21  provided with a circumferential ridge  22  of a diameter larger than the diameter of the remainder of the tube  21 . The ridge  22  is provided to keep the foot valve in the drill bit by being pressed into a corresponding circumferential groove  17  in a drill bit passageway  18 . The foot valve  20  extends generally equally far into the drill bit and the piston  16  when the piston front surface  23  (impact surface) impacts on the drill bit rear surface  24  (anvil surface). The impact surface  23  connects to a circumferential chamfer  29  located at the orifice of the passageway  31  of the piston. A center portion of the anvil surface  24  lies in a plane P oriented perpendicular to the axis. 
     Turning now to the prior art disclosure of FIG. 3, it is previously known to chamfer or smoothen the intersection I of the anvil surface  24  and the drill bit passageway  18 . During drilling, when the piston  16  impacts the drill bit, the deposited water on the impact surface will create a jet stream of water, some of which travels in a radially inward direction indicated by the arrow in FIG.  3 . That part of the jet stream has an angle of attack of about 90° with respect to the outer surface of the foot valve and will cut into the plastic foot valve (as shown by a notch N in FIG. 3) and finally the valve will break. When the foot valve is broken there will not be any lower chamber present where pressurized air can assemble to lift the piston but instead the air will be transferred immediately through the drill bit passageway  18  and the hammer will not work. 
     Now looking at FIG. 4 there is presented a solution to the problem of jet stream damage to the foot valve. The intersection of the anvil surface  24  and the drill bit passageway  18  is provided with a jet stream deflector in the shape of a circumferential projection or lip  25  integrated with the drill bit  13 . The projection  25  extends beyond the plane P. The jet stream of water during drilling will be deflected by the lip  25  in a direction D indicated by the arrow in FIG. 4 in such a manner that the energy of the stream is reduced to about one half. In addition, the attack angle of the stream with respect to the foot valve will be obtuse such that only about half of the remaining energy of the stream is transferred onto the foot valve. The lip  25  can be of many alternative shapes but in the preferred embodiment the lip extends generally parallel to the chamber  29  and has a conical cross-section formed by a radially inwardly facing entrance surface  26 , a curved peak  27  and a radially outwardly facing deflection surface  28 , whereby the direction D has axially upward (rearward) and radially inward directional components. The deflection surface  28  forms an angle a with the center line CL of the drill bit  13 . The angle a is acute and is preferably at least 45°. 
     A drill bit according to the present invention will provide for an extended lifespan of the foot valve. Furthermore, a down-the hole hammer according to the present invention will have a more reliable function than hitherto known hammers. 
     Although the present invention has been described in connection with preferred embodiments 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. For example, the projection  25 ′ could be integrated with the impact surface  23 ′ of the piston and the chamfer  29 ′ formed in the anvil surface  24 ′ of the drill bit, as shown in FIG.  5 .