A drive collar is disclosed for use in a percussion drilling apparatus of the type for boring into the earth. Embodiments of the drive collar include a generally tubular, one-piece body further having an inner surface, an outer surface, a first end and a second end. Embodiments further include a threaded section on the outer surface, a retention mechanism on the inner surface, a plurality of splines on the inner surface; and a shoulder on the outer surface.

CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     Not Applicable.  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not Applicable.  
       BACKGROUND  
       [0003]     1. Technical Field  
         [0004]     This disclosure 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 between 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 of this arrangement, the drill string rotation is thereby transferred to the hammer bit itself. Experience has demonstrated for an eight inch diameter hammer bit that a rotational speed of approximately 20 rpm for an impact frequency of 1600 bpm (beats per minute) 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. This porting admits fluid in a first space between the piston and top sub to drive the piston towards the drill bit support, and thereafter 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 this conventional hammer assembly and drill bit by the attached drill string which, in turn, is powered by a rotary table typically mounted on the rig platform or by a 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 two components.  
         [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. If a portion of the drill bit completely fractures, it can become separated from the rest of the percussion drill assembly. In such a case, mere removal of the drill assembly from the borehole by withdrawing (or “tripping”) the drillstring will not extract the fractured portion of the drill bit. Instead, the fragment must be removed by a separate and time-consuming procedure, adding still further cost. It is therefore desirable to retain any fractured portions of the drill bit with the rest of the percussion drill assembly, thereby allowing the fractured portion to be extracted simultaneously with the withdrawal of the drillstring from the borehole.  
         [0010]     The embodiments of the present invention described herein provide opportunities for improvement in retaining the drill bit in the event of a fracture. These and various other characteristics and advantages will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments of the invention, and by referring to the accompanying drawings.  
       SUMMARY OF THE PREFERRED EMBODIMENTS  
       [0011]     Embodiments of the present invention comprise a drive collar with a retention mechanism for use in a percussion drilling apparatus of the type for boring into the earth. In certain embodiments, the drive collar comprises a one-piece or unitary generally tubular body with a threaded section on the outer surface and proximal to a first end of the body and a retention mechanism on the inner surface proximal to the second end of the body. Embodiments further comprise a plurality of splines on the inner surface and a shoulder on the outer surface disposed at a location between the threaded section and the retention mechanism. The retention mechanism may comprise different configurations, such as a threaded section or a retaining ring. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     For a more detailed description of the preferred embodiments, reference will now be made to the accompanying drawings, wherein:  
         [0013]      FIG. 1  is a cross-section of a prior art percussion hammer drilling assembly;  
         [0014]      FIG. 1A  is an enlarged partial cross-section view of the prior art percussion hammer drilling assembly of  FIG. 1 ;  
         [0015]      FIG. 2  is cross-section of a percussion hammer drilling assembly made in accordance with principles of the present invention;  
         [0016]      FIG. 3  is an enlarged partial cross-section view of the embodiment of  FIG. 2 ;  
         [0017]      FIG. 4  is an enlarged partial cross-section view of the embodiment of  FIG. 2  with the components shown as they appear during one stage of the assembly;  
         [0018]      FIG. 5  is a partial cross-section view similar to  FIG. 4 , but showing the components of the percussion hammer drilling assembly as they appear during operation; and  
         [0019]      FIG. 6  is a partial cross-section view of an alternative embodiment of a percussion hammer drilling assembly made in accordance with principles of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0020]     Referring first to  FIGS. 1 and 1 A, a cross-section of a typical prior art percussion drilling assembly  200  is shown 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 . Driver sub  240  is slideably engaged with a bit  260 . Captured between case  230  and driver sub  240  is a retainer  239 , which extends to a position below driver sub  240 . Top sub  220  further comprises a check valve  225  and a feed tube  235  that extends from check valve  225  to a piston  254  that is slideably engaged with a guide sleeve  255 .  
         [0021]     During operation, drillsting  210  rotates, thereby rotating percussion drilling assembly  200 . In addition, piston  254  travels back and forth in an axial direction so that it cyclically impacts bit  260 . A series of splines  265  on bit  260  engage driver sub  240  and 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.  
         [0022]     As previously described, bit  260  is slideably engaged with driver sub  240  and is therefore free to move axially with respect to driver sub  240 . A bit retaining ring  257  retains bit  260  within drilling assembly  200  and prevents bit  260  from sliding out of the end of drilling assembly  200 . As explained more fully below, if bit  260  fractures below bit retaining ring  257 , retainer  239  prevents a fractured portion of bit  260  from falling out of the end of drilling assembly  200 . This prevents the fractured portion from separating from the rest of drilling assembly  200  and allows the fractured portion to be extracted from the borehole by withdrawing drilling assembly  200 .  
         [0023]     A more detailed view of driver sub  240  and retainer  239  is shown in  FIG. 1A . In this view, retainer  239  is shown to comprise an upper portion  237  and a lower portion  238 . Upper portion  237  has a retainer shoulder  236  that engages a driver shoulder  263 , while lower portion  238  has retainer threads  280 , which must be threaded past bit threads  281  during assembly. A key  282  is also disposed between a driver keyway  283  and a retainer keyway  284 . Driver sub  240  comprises a plurality of splines (not visible in  FIG. 1A ) that engage bit splines  265  and driver sub threads  290  which threadably engage casing  230 .  
         [0024]     The assembly shown in  FIG. 1A  is typically assembled by sliding driver sub  240  onto bit  260  so that bit splines  265  engage driver sub splines (not visible in  FIG. 1A ) and an end  295  of driver sub  240  contacts a bit shoulder  296 . A key  282  is placed in a driver keyway  283  and retainer  239  is placed onto bit  260  so that retainer threads  280  engage bit threads  281 . Retainer  239  is rotated relative to bit  260  so that retainer threads  280  are disengaged from bit threads  281  and retainer  239  can be moved axially towards an end  297  of bit  260 . Retainer  239  is aligned with driver sub  240  so that key  282  aligns with retainer keyway  284  and retainer  239  is then moved towards end  297  until retainer shoulder  236  engages driver sub shoulder  263 .  
         [0025]     Driver sub threads  290  are threadably engaged with case  230  so that upper portion  237  of retainer  239  is captured between the end of case  230  and driver sub shoulder  263 . In typical applications, driver sub  240  is threadably engaged with case  230  so that upper portion  237  is placed under a compressive stress. The cyclical forces generated in a percussion drilling assembly can therefore lead to stress or fatigue fractures near retainer shoulder  236  and driver sub shoulder  263 . Geometrical constraints also make it difficult to enlarge the cross-sectional thickness of upper portion  237  or retainer shoulder  236  to reduce the likelihood of such failures. For example, upper portion  237  must slide axially past driver sub threads  290  during assembly, so the thickness of upper portion  237  cannot be increased inwardly. In addition, the geometry of bit  260  dictates the size of the bore being drilled, and thereby provides a limitation on the maximum outer diameter that can be utilized for retainer  239 .  
         [0026]     Referring now to  FIG. 2 , drilling assembly  300  comprises a drive collar  241  that incorporates features of both a driver sub and a retainer. The other features and components of drilling assembly  300  are equivalent to those of drilling assembly  200  described in the discussion of  FIGS. 1 and 1 A. A more detailed view of drive collar  241  is depicted in  FIGS. 3-5 , which depict a cross-section view of drive collar  241  on bit  260 .  
         [0027]     In the embodiment of  FIG. 3 , bit  260  comprises a top portion  269  and a plurality of splines  265  that engage splines  266  (visible in the section view of  FIG. 3A ) in drive collar  241 . In addition, bit  260  comprises a threaded section  264  and a reduced diameter portion  267 . In this embodiment, drive collar  241  is a generally cylindrical or tubular body extending from an upper portion  245  at one end to a lower portion (or extension)  247  at the opposing end. Drive collar  241  includes an outer surface  248  and an inner surface  249 . Drive collar  241  further includes a central portion  246  between upper and lower portions  245  and  247 . Outer surface  248  of drive collar  241  comprises an upper threaded section  242  that threadably engages case  230 . Therefore, as case  230  rotates, both drive collar  241  and bit  260  will also rotate. Drive collar  241  also comprises a shoulder  243  on outer surface  248  and a threaded section  244  on inner surface  249  of extension  247 . In the embodiment shown in  FIGS. 3-5 , upper portion  245 , central portion  246 , and lower portion  247  are manufactured by casting, molding, forging or similar manufacturing processes to form a single piece of material so that drive collar  241  is a unitary piece or one-piece body. In other embodiments, different components (such as upper portion  245  and lower portion  247 ) can be connected by welding or similar processes to form a unitary piece. As used herein, the terms “unitary piece” or “one-piece body” are defined as a component consisting of a single member or multiple members that are non-releasably connected.  
         [0028]     As shown in  FIG. 4 , drive collar  241  and bit  260  are initially assembled by engaging threaded section  244  of drive collar  241  with threaded section  264  of bit  260 . At this point, splines  265  are not engaged with drive collar splines  266 , so that drive collar  241  can be rotated relative to bit  260 . Threaded section  244  is then threaded past threaded section  264 , allowing splines  265  to be aligned with drive collar splines  266 . After the splines are aligned and engaged, bit  260  can be further inserted into drive collar  241  so that reduced diameter portion  267  is received within extension  247 . After assembly, threaded section  244  is proximal to (but not threadably engaged with) reduced diameter portion  267 , allowing bit  260  to move axially with respect to drive collar  241 . Case  230  is also threadably engaged with drive collar  241 , so that the end of case  230  engages upper shoulder  243 .  
         [0029]     Referring now to  FIG. 5 , the bit retention properties of drive collar  241  are displayed. In  FIG. 5 , bit  260  has suffered a fracture  270 , so that a fractured portion  275  of bit  260  is separated from an upper portion  277  of bit  260  that is above fracture  270 . However, as fractured portion  275  moves farther from upper portion  275 , threaded section  244  of drive collar  241  contacts threaded section  264  of bit  260 . In the this embodiment, threaded sections  244  and  264  are configured so that the rotation of drive collar  241  relative to fractured portion  275  during operation will not cause threaded section  264  to threadably engage threaded section  264 . For example, when viewed from above during operation, if drive collar  241  rotates clockwise, then threaded sections  244  and  264  are configured so that they only threadably engage when bit  260  is rotated counter-clockwise relative to fractured portion  275 . This prevents threaded section  264  from threadably engaging and traveling past threaded section  244 . Therefore, threaded section  244  acts as a retention mechanism, allowing drive collar  241  to capture fractured portion  275  and prevent it from separating from drilling assembly  300  (shown in  FIG. 2 ). As a result, fractured portion  275  can be withdrawn from the borehole by merely removing drill string  210  and drilling assembly  300  and a separate “retrieval” procedure for the broken component is not required. This captive arrangement can save considerable time and expense in comparison to removing a fractured portion  275  of drill bit  260  from a borehole.  
         [0030]     Comparing the embodiment shown in  FIGS. 2-5  with the conventional arrangement shown in  FIGS. 1 and 1 A, drive collar  241  eliminates retainer shoulder  236  and driver sub shoulder  263  common in such assemblies. For example, driver collar  241  eliminates retainer shoulder  236  and driver sub shoulder  263 . This in turn eliminates stress risers created by shoulders  236  and  263  and thereby reduces the likelihood of component failures as was experienced in prior art systems utilizing drive collars and retainers. In addition, because drive collar  241  is now a unitary or one-piece component, as compared to the conventional arrangement having a separate drive collar and retainer, the cross-sectional thickness of driver collar  241  in lower portion  247  can be made greater. This is an area that is often prone to erosion due to the high velocity of cuttings and air in the bore hole. Increasing the cross-sectional thickness in lower portion  247  also reduces the stress levels and increases the ability to sustain erosion, and further reduces the likelihood of fracturing drive collar  241 . A one-piece assembly also is able to retain a bit shank with a relatively large spline diameter within the envelope of a given bore hole.  
         [0031]     An alternative embodiment of the present invention is shown in  FIG. 6 . In this embodiment, a bit  360  is retained within drive collar  341 . Bit  360  comprises a plurality of splines  365  that engage splines (not visible in  FIG. 6 ) in drive collar  341 . Bit  360  also comprises a shoulder area  364  and a reduced diameter portion  367 . In this embodiment, drive collar  341  comprises a shoulder  343 , an extension  347 , and an upper threaded section  342  that threadably engages case  230 . In the embodiment of  FIG. 6 , drive collar  341  comprises a retaining ring  344  on extension  347 . As explained below, retaining ring  344  acts as a retention mechanism in the event that bit  360  is fractured. Retaining ring  344  can comprise various configurations, such as a snap ring inserted into a groove in extension  347 , or a split ring bolted to extension  347 .  
         [0032]     Retaining ring  344  is installed onto extension  347  after bit  360  has been inserted in drive collar  341 . Therefore, retaining ring  344  does not obstruct shoulder area  364  during insertion of bit  360  into drive collar  341 . After bit  360  is fully inserted into drive collar  341  and splines  365  are engaged with the drive collar splines, retaining ring  344  can be installed. Retaining ring  344  projects within extension  347  so that, in the event bit  360  fractures, retaining ring  344  will prevent shoulder area  364  from passing through the end of extension  347 . In this manner, the fractured portion of bit  360  will be retained, allowing removal of the fractured bit portion by withdrawing the drillstring from the borehole.  
         [0033]     Another alternative embodiment of the present invention is shown in  FIG. 7 . In this embodiment, bit  360  is retained within drive collar  441 . In this embodiment, drive collar  441  comprises a shoulder  443 , an extension  447 , and an upper threaded section  442  that threadably engages case  230 . In the embodiment of  FIG. 7 , drive collar  341  comprises a plurality of pins  444  on extension  447 . Similar to retaining ring  344  described in the discussion of  FIG. 6 , pins  444  act as a retention mechanism in the event that bit  360  is fractured.  
         [0034]     Pins  444  can be inserted in holes in extension  447  after bit  360  has been inserted in drive collar  441 . Therefore, pins  444  do not obstruct shoulder area  364  during insertion of bit  360  into drive collar  441 . Pins  444  can be fastened to extension  447  in one of many different methods known in the art, such as threaded engagement or welding. Pins  444  project within extension  447  so that, in the event bit  360  fractures, pins  444  will prevent shoulder area  364  from passing through the end of extension  447 . In this manner, the fractured portion of bit  360  will be retained, allowing removal of the fractured bit portion by withdrawing the drillstring from the borehole.  
         [0035]     While various preferred embodiments of the invention have been showed and described, modifications thereof can be made by one skilled in the art without departing from the spirit and teachings of the invention. The embodiments herein are exemplary only, and are not limiting. Many variations and modifications of the invention and apparatus disclosed herein are possible and within the scope of the invention. For example, retention mechanisms other than a threaded section, a ring, or a pin may be used on the extension of the drive collar. 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