You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to an air actuated down-the-hole hammer for rock drilling, a drill bit and a foot valve to be used in the down-the-hole hammer. 
       BACKGROUND OF THE INVENTION 
       [0002]    When drilling with down-the-hole hammers under ground, such as in tunnels, the dust generated by the drilling operation often is bound 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 passage in a drill bit anvil and projecting from an impact surface of the anvil. The foot valve is repeatedly enclosed by a central bore of a reciprocating piston to transfer spent pressurized driving air through the drill bit. An example of such a technique is disclosed in U.S. Pat. No. 6,125,952, the subject matter of which is hereby incorporated by reference for disclosure of the down-the-hole hammer technique. During drilling, the piston can unseat the foot valve from the bit shank body when it begins the upward stroke due to wear from long run deep hole drilling applications, tolerance differences from different manufacturers, and the debris that may come between the piston bore and the outer diameter of the foot valve. As used herein, “deep hole drilling” means drilling for oil and gas which can range anywhere from 100 to 3,000 meters. The run hours on the hammer, bit and foot valve can exceed 36 continuous hours, and under warm temperatures down hole of around 135 degrees Celsius. It is disadvantageous to trip that much pipe back out of the hole just to change a plastic foot valve. Presently, the cost exceeds approximately $12,000.00 a day for rig time. 
         [0003]    The present invention aims at obviating the above-mentioned disadvantages of previously known devices. Therefore, an object of the invention is to provide a drill bit and foot valve for a percussive down-the-hole hammer which provides an extended lifespan of the foot valve. 
         [0004]    Another object of the invention is to provide a drill bit for a down-the-hole hammer that will prevent the dislocation of the foot valve from the bit body, especially during deep hole drilling conditions. 
         [0005]    Yet another object of the invention is to provide a foot valve for a down-the-hole drill bit that will have a longer life between service than in hitherto known percussive down-the-hole hammers. 
         [0006]    Still another object of the invention is to provide a drill bit and a foot valve that are less sensitive to tolerance faults. 
       SUMMARY OF THE INVENTION 
       [0007]    In an embodiment, the invention provides an air-actuated down-the-hole hammer for rock drilling, including a generally cylindrical casing defining an axis, a drill sub mounted to a rear end of the casing, a drill chuck mounted to a front end of the casing, and a drill bit mounted in the drill chuck and including 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 passage extending through the anvil surface, the passage forming a cavity adjacent to the rear anvil surface. A piston is mounted in the casing behind the drill bit and includes a forwardly facing impact surface and a second central passage extending through the impact surface and aligned with the first central passage. The piston is mounted for axial reciprocation toward and away from the drill bit whereby the impact surface impacts the anvil surface during a forward stroke of the piston. 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 foot valve and the first central passage include insertion stop means and major extraction stop means for the foot valve, such that the insertion stop means are provided at a distance from the major extraction stop means. 
         [0008]    In another embodiment, the invention provides a percussive drill bit for use in a down-the-hole hammer, including a front drilling face, a rear anvil surface, and a central passage extending through the anvil surface along a center axis of the drill bit, the passage forming a cavity adjacent to the rear anvil surface, the cavity including an undercut defined by a, at least partly, tapered portion and upper and lower ends. 
         [0009]    In yet another embodiment, the invention provides a foot valve that is used in a drill bit passage in an air-actuated down-the-hole hammer for rock drilling, including a generally cylindrical tube defining a center line, an upper end and a lower end, the foot valve including insertion stop means and major extraction stop means, such that the insertion stop means is provided at a distance from the major extraction stop means. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate the presently preferred embodiments of the invention, and together with the general description given above and the detailed description given below, serve to explain features of the invention. 
           [0011]      FIG. 1  is a down-the-hole hammer according to the present invention in a cross-sectional view; 
           [0012]      FIG. 2A  is a drill bit according to the present invention in a side elevational view; 
           [0013]      FIG. 2B  shows the drill bit at line  2 B- 2 B in  FIG. 2A ; 
           [0014]      FIG. 2C  shows the drill bit in  FIG. 2A  in a perspective view; 
           [0015]      FIG. 3A  shows a portion of the drill bit according to the present invention, a portion of a piston, and a foot valve according to the present invention, in a cross-sectional view; 
           [0016]      FIG. 3B  shows an enlarged area, encircled in  FIG. 3A ; 
           [0017]      FIG. 4A  shows a foot valve according to the present invention in a side view; and 
           [0018]      FIG. 4B  shows the foot valve in an end view. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0019]    In  FIG. 1  there is shown a down-the-hole hammer  10  according to the present invention. The hammer  10  includes 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 and lower ends of the piston to effect its reciprocation in the casing, each downward stroke inflicting an impact blow upon an anvil  30  of a drill bit  13  extending upwardly within the lower portion of the cylindrical casing. The piston includes a passage  31  for the pressurized air. The percussive down-the-hole hammer further includes 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 . 
         [0020]    The foot valve  20  is of generally cylindrical basic shape and is made of plastics, such as Nylon® or Delrin®, as illustrated in  FIGS. 4A and 4B . The foot valve is a hollow tube  21  provided with a circumferential ridge  22  defining the greatest diameter of the foot valve  20 . The foot valve  20  extends further into the drill bit than into the piston  16  when the piston front surface  23  impacts on the drill bit rear surface  24 . The average wall thickness of the foot valve  20  is greater at the portion entering the drill bit than the portion entering the piston. That is, the average wall thickness of a lower portion of the foot valve is greater than an upper portion of the foot valve. 
         [0021]    The ridge  22  is provided to keep the foot valve in the drill bit by pressing the ridge into a corresponding annular groove, or fourth tapered portion  17 , in a cavity in a drill bit passage  18 . The cavity is formed adjacent to the anvil. The ridge  22  is formed between a first tapered portion  25  and a second tapered portion  26 . The tapered portions  25 ,  26  slope away from the ridge  22 . 
         [0022]    The first tapered portion  25  is provided adjacent to a first valve portion  27 , which is preferably cylindrical. The first valve portion  27  is received by the piston passage  31  to close off a bottom air vent of the hammer when the piston descends upon the portion  27 . The first valve portion  27  has a relatively small wall thickness. The first tapered portion  25  inclines at an angle β of approximately 1.5-2.5 degrees relative to a center line of the foot valve  20  or a center line CL of the hammer  10 , i.e. the first portion  25  tapers axially towards the piston  16 . The first tapered portion  25  may have an interference fit with a complementary first cavity portion or third tapered portion  29  in the drill bit passage  18 . The inclination of the first cavity portion  29  is chosen similar to the angle β. The interference fit resists pull out and counters the pulling forces from the piston  16  during its upward cycle. It also adds more wall thickness to the foot valve to make it a stronger area. 
         [0023]    The second tapered portion  26  inclines at an angle a of approximately 3-6 degrees relative to the center line of the foot valve  20  or the center line CL of the hammer  10 , i.e. the second portion  25  tapers axially towards the drill bit  13 . That is, the second tapered portion  26  inclines in the opposite direction relative to the angle β. The angle α can be 3-6 degrees depending on the diameter or size of the foot valve. The angle α reduces the amount of force required to install the foot valve into the drill bit passage  18 , thereby reducing the amount of deformation and/or damage that can be caused to the foot valve during the installation process and will further reduces the possibility of premature foot valve failure. 
         [0024]    The angle β provides added retaining force at the foot valve into the bit body by creating a downward pulling force as well as accounting for any tolerance differences in the manufacturing processes thereof. 
         [0025]    An external, annular shoulder  28  can be formed at the junction of the tapered portions  25 ,  26 . The shoulder  28  has a generally radial extension. The shoulder is snaps into and cooperates with an annular collar  32 , or upper end of an undercut, in the passage  18 , which in turn provides the strongest holding feature and ensures against extraction. The shoulder  28  and the collar  32  may also be called major extraction stop means. 
         [0026]    A guide portion  34  is provided at the lower end of the foot valve. The guide portion  34  is substantially cylindrical and acts as a lower guide bushing in cooperation with a lower guide surface  36  of the passage  18 . The guide portion and the guide surface have the same measurements in diameter and are preferably not press fit as are other features discussed above. The guide portion  34  and the guide surface  36 , each connects to a nose end  37  and a second collar  38  or lower end of an undercut, respectively. The nose end  37  and the second collar  38  prevent further insertion of the foot valve into the passage  18  as the foot valve will bottom out in the passage. The nose end  37  and the second collar  38  can be rounded or chamfered. The nose end  37  and the second collar  38  may also be called insertion stop means. The insertion stop means are provided at a distance, at least 12 mm, from the major extraction stop means. An undercut is thus defined in the passage  18  by the tapered portion  17 , the guide portion  36  and upper  32  and lower ends  38 . 
         [0027]    Mounting of the foot valve is done as follows. The foot valve  20  is aligned with the drill bit passage  18  such that a guide portion  34  may be inserted into the first cavity portion  29  of the passage. The nose end  37  enters into the passage more easily if the nose end is rounded or chamfered. The second tapered portion  26  may abut against the passage when the entire guide portion has entered the passage  18 . The force needed for insertion of the foot valve gradually increases due to the increasing diameter of the portion  26  until the ridge  22  enters the passage  18 . The force needed for further insertion of the foot valve gradually decreases due to the increasing diameter of the cavity portion  29  upon which the ridge rides until the ridge snaps into the collar  32  in the passage  18 . If the foot valve is pushed a small additional distance, less than 5 mm, the insertion stop means  37 ,  38  will define the maximum possible inserted length L of the foot valve. The inserted length L of the foot valve into the drill bit is typically 40 to 70 mm, and about 60 mm in the shown example. The distance L 3  between the major extraction means  28 ,  32  and the impact surface  24  of the drill bit is 15-30 mm. The distance L 2  between the insertion stop means  37 ,  38  and the upper portion of the guide portions  34 ,  36  is 20-40 mm. When the hammer is in use and the piston travels upwardly, the piston passage  31  will tend to grip the foot valve and lift it from the drill bit passage. However, such a motion of the foot valve will be stopped primarily by the major extraction stop means, i.e. the shoulder  28  and the collar  32 . Also, the first tapered portion  25  and the first cavity portion  29  in the drill bit passage  18  aid to some extent in preventing the extraction. 
         [0028]    A drill bit and a foot valve according to the present invention will provide for an extended lifespan of the hammer before service is needed. Furthermore, a down-the hole hammer according to the present invention will have a more reliable function and more economical usage than hitherto known hammers. In addition, the down-the-hole hammer is less sensitive to tolerance faults. 
         [0029]    The terms “lower”, “upper”, “upwardly” and similar terms used herein refer to the normal position of a hammer situated vertically in a hole. 
         [0030]    While the invention has been disclosed with reference to certain preferred embodiments, numerous modifications, alterations, and changes to the described embodiments are possible without departing from the sphere and scope of the invention, as defined in the appended claims and their equivalents thereof. Accordingly, it is intended that the invention not be limited to the described embodiments, but that it have the full scope defined by the language of the following claims.

Summary:
The present invention relates to a percussive down-the-hole hammer, a drill bit and a foot valve. The drill bit includes a body having a front drilling face, a central passage extending from a rear end of the body in a direction towards the drilling face to conduct flushing medium to the drilling face. The rear end includes an impact surface surrounding the passage. The central passage is adapted to receive a foot valve to transfer pressurized air through the drill bit. The foot valve and the central passage include insertion stop means and major extraction stop means for the foot valve. The insertion stop means are provided at a distance from the major extraction stop means.