Abstract:
A drill bit is disclosed for use in a percussion drilling apparatus of the type for boring into the earth. Embodiments of the drill bit comprise an elongate body with a first end, a second end, and a longitudinal surface extending between the first and second ends. Embodiments further comprise a plurality of splines on the longitudinal surface of the drill bit, with at least one of the plurality of splines comprising an apex, a root, and a planar surface between the apex and root. In preferred embodiments, the planar surface is oriented at an angle between 10 and 45 degrees to a plane extending from a center of the drill bit to a midpoint of the apex.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     Not Applicable.  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not Applicable.  
       BACKGROUND  
       [0003]     1. Technical Field  
         [0004]     The disclosure herein generally relates to earth boring bits used to drill a borehole for applications including the recovery of oil, gas or minerals, mining, blast holes, water wells and construction projects. More particularly, the disclosure relates to percussion hammer drill bits.  
         [0005]     2. Description of the Related Art  
         [0006]     In percussion hammer drilling operations, the bit impacts the earth in a cyclic fashion while simultaneously rotating. In such operations, the mechanism for penetrating the earth is of an impacting nature rather than shearing. Therefore, in order to promote efficient penetration by the bit, the cutting elements of the bit need to be “indexed” to fresh earthen formations during each impact. This need is achieved by rotating the drill string a slight amount between each impact of the bit to the earth and incorporating longitudinal splines which key the bit body to a cylindrical sleeve (commonly known as the driver sub or chuck) at the bottom of the hammer assembly. As a result, the drill string rotation is thereby transferred to the hammer bit itself. Experience has demonstrated for an eight inch hammer bit that a rotational speed of approximately 20 rpm for an impact frequency of 1600 bpm (beats per minute) typically results in efficient drilling operations. This rotational speed translates to an angular displacement of approximately 4 to 5 degrees per impact of the bit against the rock formation.  
         [0007]     An example of a typical hammer bit connected to a rotatable drill string is described in U.S. Pat. No. 4,932,483, incorporated herein by reference. The downhole hammer comprises a top sub and a drill bit separated by a tubular housing incorporating a piston chamber therebetween. A feed tube is mounted to the top sub and extends concentrically into the piston chamber. A piston is slideably received within the housing and over the feed tube. Fluid porting is provided in the feed tube and the piston to sequentially admit fluid in a first space between the piston and top sub to drive the piston towards the drill bit support and to a second space between the piston and the drill bit support to drive the piston towards the top sub.  
         [0008]     Rotary motion is provided to the hammer assembly and drill bit by the attached drill string powered by a rotary table typically mounted on the rig platform or top drive head mounted on the derrick. The drill bit is rotated through engagement of a series of splines on the bit and driver sub that allow axial sliding between the components but do not allow significant rotational displacement between the hammer assembly and bit.  
         [0009]     Due to the forces transmitted between the splines, as well as the cyclic nature of the stress created, mechanical failure of the splines can force an operator to remove the drill bit from operation for repair or replacement, thereby increasing maintenance and operation costs. Typically, the mechanical failure of the splines is initiated by galling between the splines of the drill bit and the driver sub or fatigue cracking in the splines of either component.  
         [0010]     As a result of the significant costs associated with premature failure of the drill bit, it is desirable to increase the mechanical reliability of percussion hammer drilling systems. It is also desirable to optimize the design characteristics of the percussion bit (such as the bit length and weight) to maximize the transfer of energy between the piston and the bit, thereby increasing the rate of penetration for the bit.  
         [0011]     The embodiments described herein provide opportunities for improvement in percussion bit service life and rate of penetration. These and various other characteristics and potential advantages will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments, and by referring to the accompanying drawings.  
       SUMMARY OF THE PREFERRED EMBODIMENTS  
       [0012]     Embodiments described herein comprise a drill bit for use in a percussion drilling apparatus of the type for boring into the earth. Preferred embodiments comprise an elongate body with a first end, a second end, and a longitudinal surface extending between the first and second ends. Preferred embodiments further comprise a plurality of splines on the longitudinal surface of the drill bit, with at least one of said plurality of splines comprising an apex, a root, and a planar surface between the apex and root. In certain preferred embodiments, the planar surface is oriented at an angle between 10 and 45 degrees to a plane extending from a center of the drill bit to a midpoint of the apex. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     For a more detailed description of the preferred embodiments, reference will now be made to the accompanying drawings, wherein:  
         [0014]      FIG. 1  is a cross-section of a percussion hammer drilling assembly;  
         [0015]      FIG. 2  is an elevational view of a percussion bit made in accordance with principles of the present invention;  
         [0016]      FIG. 3  is a section view of the embodiment of  FIG. 2 ;  
         [0017]      FIG. 4  is a detailed view of the section view of  FIG. 3   
         [0018]      FIG. 4A  is a detailed section view of an alternative embodiment of a percussion bit made in accordance with principles of the present invention;  
         [0019]      FIG. 5  is an elevational view of a percussion bit and a drive collar made in accordance with principles of the present invention;  
         [0020]      FIG. 6  is a section view of the embodiment of  FIG. 5 ;  
         [0021]      FIG. 7  is an elevational view of a prior art percussion bit; and  
         [0022]      FIG. 8  is a section view of the prior art bit of  FIG. 7 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]     Referring first to  FIG. 1 , a cross-section of a typical prior art percussion drilling assembly  200  is attached to a drillstring  210 . Assembly  200  comprises a top sub  220  threadably connected to a case  230 , which is threadably connected to a driver sub  240 . A bit  260  is slideably engaged with driver sub  240 , and a retainer sleeve  250  is disposed between case  230  and bit  260 . Top sub  220  further comprises a check valve  225  and a feed tube  235  that extends from check valve  225  to a piston  245  that is slideably engaged with a guide sleeve  255 .  
         [0024]     During operation, drillsting  210  rotates, thereby rotating percussion drilling assembly  200 . In addition, piston  245  travels back and forth in an axial direction so that it cyclically impacts bit  260 . A series of engaged splines  265  on bit  260  and driver sub  240  allow bit  260  to slide axially relative to driver sub  240  while also allowing driver sub  240  to rotate bit  260 . As described previously, this allows the cutting elements (not shown) of bit  260  to be “indexed” to fresh rock formations during each impact of bit  260 , thereby improving the efficiency of the drilling operation.  
         [0025]     Referring now to  FIG. 2 , a percussion bit  10  for earth-boring applications comprises an elongate body  20  with a drillstring end  12  (nearest a drillstring, not shown) and an insert end  14 . A plurality of generally axially-aligned splines  30  are disposed circumferentially about the outer surface of elongate body  20  between a threaded portion  40  and a recessed portion  50 . Elongate body  20  further comprises a collar  60  near drillstring end  12  and a flared portion  70  near insert end  14 .  
         [0026]     As shown in  FIG. 3  (a section view taken along line  3 - 3  of  FIG. 2 ) elongate body  20  comprises a cavity  25  disposed longitudinally through elongate body  20 . In the embodiment of  FIG. 3 , cavity  25  has a generally circular cross section with an inner wall  27  circumscribing a central axis  29 . As shown in  FIG. 3 , the cross-section of each spline  30  comprises an apex or peak  31 , as well as a pair of angled surfaces  33  between peak  31  and a root  32 , thereby creating a generally “V”-shaped spline  30 . In certain embodiments, peak  31  may be relatively flat or formed with a radius, and does not comprise a sharply pointed surface. In the embodiment shown in  FIG. 3 , each root  32  comprises a single curved surface  37  between a pair of angled surfaces  33 .  
         [0027]     Referring now to  FIG. 4 , a detailed view of a portion of the view in  FIG. 3  is shown. Although still depicting a section view, the section lines have been removed in  FIG. 4  to more clearly illustrate the details of splines  30 . In the embodiment shown in  FIG. 4 , angled surfaces  33  are generally planar and each is disposed at an angle A measured relative to a radius  47  drawn from central axis  29  to the midpoint  35  of peak  31 . In the embodiment shown in  FIG. 4 , angle A is approximately 30 degrees, so that the angle between the pair of angled surfaces  33  defining a spline  30  is about 60 degrees. In other embodiments, angle A can range from 10 degrees to 45 degrees, so that the angle between angled surfaces  33  is 20 to 90 degrees. Also shown in the embodiment of  FIG. 4 , root  32  comprises a single curved portion  37  with a radius R that intersects a pair of adjacent angled surfaces  33 . In the embodiment of  FIG. 4 , radius R is 0.080 inches, while other embodiments may comprise a root radius with different values. Referring now to an alternative embodiment shown in  FIG. 4A , root  32  may comprise a pair of curved surfaces  39 , joined by a substantially planar surface  36 . In still other embodiments, the surface of root  32  designated in  FIG. 4A  as surface  36  may be radiused, rather than substantially planar, and would have a radius that is substantially greater than the radius of a curved surface  34 .  
         [0028]     Referring now to  FIG. 5 , a drive collar (or driver sub)  25  is shown engaging splines  30  of drill bit  10 . During operation, drive collar  25  imparts a rotational force to drill bit  10  and causes drill bit  10  to rotate slightly during each cycle of operation. As shown in  FIG. 6  (a section view taken along line  6 - 6  of  FIG. 5 ), drive collar  25  comprises a plurality of splines  80  which intermesh and engage splines  30  of elongate body  20  of bit  10 . During operation, splines  80  and  30  allow longitudinal or axial sliding between drive collar  25  and drill bit  10 , but restrict rotational movement between drive collar  25  and drill bit  10 . Therefore, as drive collar  25  rotates, it translates its rotational movement to drill bit  10 , thereby causing drill bit  10  to rotate as well.  
         [0029]     Referring now to  FIG. 7 , a typical prior art percussion bit  110  comprises an elongate body  120  with a drillstring end  112  and an insert end  114 . A plurality of splines  130  are disposed on elongate body  120  between a threaded portion  140  and a recessed portion  150 . Elongate body  120  further comprises a collar  160  near drillstring end  112  and a flared portion  170  near insert end  114 .  
         [0030]     As shown in  FIG. 8 , a section view taken along line  8 - 8  of  FIG. 5 , elongate body  120  comprises a cavity  125  disposed longitudinally through elongate body  120 . In the embodiment of  FIG. 8 , cavity  125  has a generally circular cross section with an inner wall  127  circumscribing a central axis  129 . As shown in  FIG. 8 , each spline  130  comprises a peak  131  and with a pair of side surfaces  133  adjacent to a root  132 .  
         [0031]     Unlike the embodiment of  FIGS. 2-6 , the prior art embodiment shown in  FIGS. 7-8  comprises splines  130  that are generally rectangular or square in cross-sectional shape. More particularly, splines  130  comprise peaks  131  that have a pair of side surfaces  133  that are generally planar. In addition, adjacent to each spline  130  is a pair of roots  132  with a bottom surface  137  that may be straight or curved. A radiused portion  135  is formed between each bottom surface  137  and side surface  133 .  
         [0032]     Comparing the embodiments shown in  FIGS. 3 and 8 , the number of splines  30  that are disposed around elongate body  20  is greater than the number of splines  130  that are disposed around prior art elongate body  120 . The number of splines  30  is increased in  FIG. 3  even though the minor diameter d 1  of elongate body  20  (as measured from a first root  32  to a second root  32  that is disposed 180 degrees from the first root) is equivalent to the minor diameter d 2  of elongate body  120 . In addition, the width (depicted as dimension W 1  in  FIG. 4 ) of a base portion  38  of spline  30  is equivalent to the width W 2  of a base portion  138  of spline  130  shown in  FIG. 8 . Base portion  38  is the portion of spline  30  that is closest to central axis  29 . In the embodiment of  FIG. 4 , the width W 1  is measured across the base of the spline  30  between the points where angled surface  33  meets root  32 . In the embodiments of  FIGS. 3 and 8 , the minor diameter d 1  and d 2  is 3.618 inches. In other embodiments, the minor diameter of the drill bit may be a different value.  
         [0033]     In the embodiment of  FIG. 3 , the number of splines  30  is therefore greater than the number of splines  130  in the embodiment of  FIG. 8 , even though each embodiment has an equivalent bit minor diameter and spline base width. The increased number of splines  30  in  FIG. 3  is due to the fact that root  32  is not as wide as root  132 . Because splines  30  are subjected to (and sometimes fail as a result of) rotational or torsional stresses during operation, increasing the number of splines  30  can increase the torque capacity and fatigue strength of bit  10 . While it would be possible to increase the number of square or rectangular splines on a drill bit by decreasing the width of the root between the splines, this would also decrease the width of the spline on a driver sub that engages the drill bit. Decreasing the width of the splines on the driver sub would therefore reduce the torque capacity and fatigue strength of the driver sub splines and counteract the benefits gained from increasing the number of splines on the drill bit. By incorporating V-shaped splines on both the drill bit and driver sub, the number of splines on each component can be increased, and the base width of each spline can be maintained.  
         [0034]     In addition, in the embodiment of  FIGS. 24 , root  32  is configured of a single curved surface with a single radius R, as contrasted with the embodiment of  FIG. 8  where two separate radii  135  are separated by the generally flat bottom surface  137  of root  132 . Incorporating a larger single radius, as in the embodiment of  FIGS. 24 , as opposed to two smaller radii, reduces the stress concentration in root  32  and potentially increases the torque capacity and fatigue strength of bit  10  as well.  
         [0035]     Furthermore, by increasing the number of splines  30 , the overall length L of splines  30  (as shown in  FIG. 2 ) may also be reduced. Increasing the number of splines  30  increases the area of bit  10  that is subjected to torsional forces during operation. Therefore, by increasing the number of splines  30 , the length L of splines  30  can be reduced while still maintaining a torque load area equivalent to that of designs with fewer (but longer) splines. The reduction in the length of splines  30  can result in a reduction of the overall length of drill bit  10 , thereby reducing the weight of drill bit  10 . A reduction in weight of drill bit  10  can lead to an increased transfer of energy from the hammer (not shown) to drill bit  10 , resulting in more efficient drilling operations.  
         [0036]     While various preferred embodiments have been showed and described, modifications thereof can be made by one skilled in the art without departing from the spirit and teachings herein. The embodiments herein are exemplary only, and are not limiting. Many variations and modifications of the apparatus disclosed herein are possible and within the scope of the invention. For example, the angle of the planar surfaces on each side of the spline may vary from those depicted in the embodiments shown. Accordingly, the scope of protection is not limited by the description set out above, but is only limited by the claims which follow, that scope including all equivalents of the subject matter of the claims