Patent Publication Number: US-8534390-B2

Title: Wear resistant material for the shirttail outer surface of a rotary cone drill bit

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is related to co-pending U.S. application for patent Ser. No. 12/896,406 filed Oct. 1, 2010 entitled “Wear Resistant Material at the Shirttail Edge and Leading Edge of a Rotary Cone Drill Bit”, the disclosure of which is hereby incorporated by reference to the maximum extent allowable by law. 
     BACKGROUND 
     1. Technical Field of the Invention 
     The present invention relates to earth boring bits, and more particularly to those having rotatable cutters, also known as rotary cone drill bits. 
     2. Description of Related Art 
     Reference is made to  FIGS. 1 and 2 , wherein  FIG. 1  illustrates an isometric view of a prior art rotary cone drill bit  10  and  FIG. 2  illustrates a cross-sectional view of a portion of the prior art rotary cone drill bit  10  of  FIG. 1 . A leg  12  depends from a body portion  14  of the drill bit  10 . The leg  12  includes a bearing shaft  16  which extends in a downward and radial inward direction. The bearing shaft  16  includes a cylindrical bearing surface  18 . A cutter cone  20  is mounted to the bearing shaft  16  and supported for rotation by the bearing surface  18 . In an alternative implementation, the cutter cone  20  is supported for rotation on the bearing shaft  16  by a set of roller bearings. The shape and configuration of the cone  20 , as well its rotatable attachment to the bearing shaft  16 , is well known in the art. In sealed bearing implementations, the bearing (journal or roller) between the cone  20  and bearing shaft  16  is lubricated by a lubricant (such as a grease) that fills regions adjacent to the bearing as well as other passages  21  in the rotary cone drill bit in a manner well known by those skilled in the art. This lubricant is retained within the rotary cone drill bit through the use of, for example, a resilient seal in the form of an o-ring  22  positioned in a seal gland  24  between the inner cylindrical surface  26  near the base of the cone  20  and the outer cylindrical surface  28  near the base of the bearing shaft  16 . 
     The outer surface  30  of the leg  12  terminates at a semicircular edge  32  proximal to the cone  20 . The region of the leg  12  associated with the surface  30  is known in the art as the “shirttail region,” and the edge  32  is known in the art as the “shirttail edge.” The shirttail edge  32  is provided where the terminal portion of the surface  30  transitions to an inside radial surface  34  oriented parallel to the base of the cone  20  (and perpendicular to the bearing shaft  16 ) and positioned at the base of the bearing shaft  16 . On a rotary cone drill bit  10 , one of the primary forms of bit failure can be traced back to shirttail wear. In one form of such shirttail wear, the shirttail edge  32  wears down, the radial extent of the inside radial surface  34  is decreased by this wear, and the resilient o-ring  22  seal in sealed bearings is exposed. If the bearing is instead an open (non-sealed or air) bearing, the wearing of the shirttail edge may expose the air bearing. Another form of shirttail wear includes wear of the shirttail outer surface  30  at locations away from the shirttail edge. 
     The prior art teaches two methods for delaying shirttail wear.  FIG. 3  illustrates a first method in which a layer of welded hardfacing material  40  is applied to the surface  30  extending along at least a portion of the shirttail edge  32 . The hardfacing material is typically a deposit of tungsten carbide hardmetal  40  applied to the surface  30 . The material is typically pelletized tungsten carbide carried in a nickel welding medium. This solution does not work well when the rotary cone drill bit is run in a highly abrasive rock formation because the hardfacing material  40  wears down too quickly. It is primarily the welding medium, typically nickel, which accounts for the relative poor performance of the weld on material.  FIG. 4  illustrates a second method in which tungsten carbide inserts  42  are press-fit into holes  44  formed in the surface  30  near the shirttail edge  32 . While these inserts  42  provide better abrasion resistance (in comparison to the use of hardfacing material), the inserts  42  do not provide protection for the shirttail edge  32 . The reason for this is that the holes  44  must be located at some appreciable distance from the shirttail edge  32  in order for the press-fit to function properly and peripherally retain the inserts  42 . For example, a separation d 1  of at least 0.125 inches is typically provided from the edge of the hole  44  to the shirttail edge  32 . Thus, the method of  FIG. 4  functions to primarily protect the shirttail region near to, but not exactly at, the shirttail edge  32 . Furthermore, in order to be suitably retained, the press-fit inserts  42  must typically have a thickness t (with a corresponding depth of the hole  44 ) such that a ratio of the thickness of the insert to a diameter d′ of the insert (where the inserts are round) or width w of the insert (with other shapes) exceeds about 0.5 (i.e., t/d′≧0.5; or t/w≧0.5) 
     A need accordingly exists in the art to provide an improved method of protecting the shirttail edge  32 . 
     With reference once again to  FIGS. 1 and 2 , the outer surface  30  of the leg  12  in the shirttail region laterally terminates at a leading shirttail edge  50  and a trailing shirttail edge  52 . The leading shirttail edge  50  is especially susceptible to wear during operation of the rotary cone drill bit  10 . The prior art again teaches two methods for delaying wear of the leading shirttail edge  50 .  FIG. 5  illustrates a first method in which a layer of welded hardfacing material  40  is applied to the surface  30  extending along at least a portion of the leading shirttail edge  50 . The hardfacing material is typically a deposit of tungsten carbide hardmetal  40 . The material is typically pelletized tungsten carbide carried in a nickel welding medium. This solution does not work well when the rotary cone drill bit is run in a highly abrasive rock formation because the hardfacing material  40  wears down too quickly. It is primarily the welding medium, typically nickel, which accounts for the relative poor performance of the weld on material.  FIG. 6  illustrates a second method in which tungsten carbide inserts  42  are press-fit into holes  44  formed in the surface  30  near the leading shirttail edge  50 . While these inserts  42  provide better abrasion resistance (in comparison to the use of hardfacing material), the inserts  42  do not provide protection for the leading shirttail edge  50 . The reason for this is that the holes  44  must be located at some appreciable distance from the leading shirttail edge  50  in order for the press-fit to function properly and peripherally retain the inserts  42 . For example, a separation d 2  of at least 0.125 inches is typically provided from the edge of the hole  44  to the leading edge  50 . Thus, the method of  FIG. 6  functions to primarily protect the shirttail region near to, but not exactly at, the leading shirttail edge  50 . Furthermore, in order to be suitably retained, the press-fit inserts  42  must typically have a thickness t (with a corresponding depth of the hole  44 ) such that a ratio of the thickness of the insert to a diameter d′ of the insert (where the inserts are round) or width w of the insert (with other shapes) exceeds about 0.5 (i.e., t/d′≧0.5; or t/w≧0.5) 
     A need thus exists in the art to provide an improved method of protecting the leading shirttail edge  50 . 
     SUMMARY 
     In an embodiment, a rotary cone drill bit comprises: a body, a leg depending from the body, a bearing shaft extending from the leg and a cone mounted to the bearing shaft. The leg terminates at a shirttail edge adjacent a base of the cone. A bottom surface of a hard material plate is attached to a substantially conforming surface of the leg in a position along the shirttail edge. 
     In an embodiment, a rotary cone drill bit comprises: a body, a leg depending from the body, a bearing shaft extending from the leg and a cone mounted to the bearing shaft. A bottom surface of a hard material plate is attached to a substantially conforming surface of the leg (for example, at a surface along an edge of the shirttail region subject to wear). 
     In an embodiment, a rotary cone drill bit comprises: a body, a leg depending from the body, a bearing shaft extending from the leg and a cone mounted to the bearing shaft. The leg includes a location that is subject to wear during operation of the bit. A bottom surface of a hard material plate is attached to a substantially conforming surface of the leg at the location subject to wear. 
     In any of the foregoing embodiments, the conforming surface to which attachment is made may comprise: a floor surface formed in or by an outer shirttail surface of the leg, a floor surface formed in or by an outer surface of the leg adjacent the shirttail edge, a floor surface of an opening formed in the outer shirttail surface of the leg, or a floor surface of an opening formed in the leg adjacent the shirttail edge. 
     In any of the foregoing embodiments, a material for attaching the hard material plate may comprise a flowable adhesive material interposed between the bottom surface of the hard material plate to the substantially conforming surface of the leg. That material may comprise, for example, a brazing material. 
     In any of the foregoing embodiments, the hard material plate may comprise polycrystalline diamond compact, or be made of a material such as solid tungsten carbide or a polycrystalline cubic boron nitride compact, or comprise a diamond impregnated segment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features and advantages of the invention will become clear in the description which follows of several non-limiting examples, with references to the attached drawings wherein: 
         FIG. 1  illustrates an isometric view of a prior art rotary cone drill bit; 
         FIG. 2  illustrates a cross-sectional view of a portion of a leg of the prior art rotary cone drill bit of  FIG. 1 ; 
         FIG. 3  illustrates application of a layer of hardfacing material extending along at least a portion of the shirttail edge; 
         FIG. 4  illustrates the use of tungsten carbide inserts near the shirttail edge; 
         FIG. 5  illustrates application of a layer of hardfacing material extending along at least a portion of the leading edge of the shirttail; 
         FIG. 6  illustrates the use of tungsten carbide inserts near the leading edge of the shirttail; 
         FIG. 7  illustrates an isometric view of a rotary cone drill bit including protection mechanisms for the shirttail edge and the leading edge of the shirttail; 
         FIG. 8  illustrates a cross-sectional view of a portion of a leg of a rotary cone drill bit which includes an embodiment of a shirttail edge protection mechanism; 
         FIGS. 9 and 10  illustrate cross-sectional views of a portion of a leg of a rotary cone drill bit which include embodiments of a mechanism for protecting the leading edge of the shirttail; 
         FIG. 11  illustrates an isometric view of a rotary cone drill bit including protection mechanisms for the shirttail edge and the leading edge of the shirttail; 
         FIG. 12  illustrates a cross-sectional view of a portion of a leg of a rotary cone drill bit which includes an embodiment of a shirttail edge protection mechanism; and 
         FIGS. 13 and 14  illustrate cross-sectional views of a portion of a leg of a rotary cone drill bit which include embodiments of a mechanism for protecting the leading edge of the shirttail. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Reference is now made to  FIG. 7  which illustrates an isometric view of a rotary cone drill bit  110  including protection mechanisms for the shirttail edge and the leading edge (also referred to as the lateral leading edge) of the shirttail. A leg  112  depends from a body portion  114  of the drill bit  110 . The leg  112  includes a bearing shaft (not shown, see  FIG. 8  reference  116 ) which extends in a downward and radial inward direction. A cutter cone  120  is mounted to the bearing shaft and supported thereon for rotation. The outer surface  130  of the leg  112  terminates at a semicircular edge  132  proximal to the cone  120 . The region of the leg  112  associated with the surface  130  is known in the art as the “shirttail region,” and the edge  132  is known in the art as the “shirttail edge.” The outer surface  130  of the leg  112  laterally terminates at a leading shirttail edge  150  and a trailing edge  152  of the shirttail. The lateral leading edge  150  and lateral trailing edge  152  of the shirttail comprise extensions of the shirttail edge  132  extending along the length of the leg  112 . Although illustrated for example as including a sealed bearing system, it will be understood that the present invention is applicable to both sealed and non-sealed (air) bearing bits. 
     To protect the shirttail edge  132 , a plurality of openings  134  are provided in the outer surface  130  of the leg  112  at locations extending along (adjacent to, but not coincident with) the shirttail edge  132 , and a hard plate insert  136  is adhered to a floor surface within each opening  134 . See, also,  FIG. 8 . To protect the lateral leading edge  150  of the shirttail, a plurality of openings  138  are provided in the outer surface  130  of the leg  112  at locations extending along (adjacent to, but not coincident with) the leading edge  150  of the shirttail, and a hard plate insert  140  is adhered to a floor surface within each opening  138 . See, also,  FIG. 9 . To generally protect the outer surface  130 , a plurality of openings  142  are provided in the outer surface  130  of the leg  112  at locations inward of and adjacent to the openings  138 , and a hard plate insert  144  is adhered to a floor surface within each opening  142 . See, also,  FIG. 10 . The openings may be milled or cast into the outer surface  130  of the leg  112  at desired positions, specifically positions on the leg which are susceptible to wear during operation of the bit. 
     Although all three protection mechanisms are illustrated in  FIG. 7 , it will be understood that any one or more of the illustrated protection mechanisms may be selected for use on the rotary cone drill bit  110 . Although  FIG. 7  primarily illustrates the use of circular inserts, it will be understood that the inserts can have any desired shape (including polygonal shapes, oval shapes (as shown), and the like). Furthermore, as shown in  FIG. 7 , the inserts can be of different sizes, perhaps with size selection depending on placement position. 
     It will be noted that, unlike prior art implementations, the inserts need not be press-fit into the openings, and further the inserts do not rely on a peripheral frictional retention mechanism. Rather, the inserts are adhered within the openings in a manner to be described. There is a significant advantage to the use of adhesion over press-fit retention with respect to the inserts. The use of adhesion for insert retention permits the openings in which the inserts are received to be placed closer to the shirttail edge  132  and leading edge  150  than would be possible with a press-fit installation. Additionally, two adjacent openings may be placed closer to each other than would be possible with a press-fit installation. 
     Reference is now made to  FIG. 8  which illustrates a cross-sectional view of a portion of a leg of a rotary cone drill bit which includes an embodiment of a shirttail edge protection mechanism. In this embodiment, the opening  134  is provided in the outer surface  130  of the leg  112  near (adjacent to, but not coincident with) the shirttail edge  132 . The opening  134  may be milled or cast into the outer surface  130  of the leg  112 . The opening  134  is defined by a floor surface  160  and a peripheral wall  162  (the floor surface may, for example, be flat). The hard plate insert  136  is adhered within the opening  134 , but does not require frictional retention with respect to the peripheral wall  162 . In a preferred embodiment, a bottom surface of hard plate insert is adhered to the floor surface  160  of the opening  134 . The bottom surface of the insert conforms to the floor surface of the opening (and thus, for example, may be a flat surface). The means for adhering the bottom surface to the floor surface may, for example, comprise any suitable adhering material which is interposed between the substantially conforming (for example, parallel) surfaces including adhesive material flowable between the substantially conforming surfaces by capillary action such as a brazing material, solder, adhesives, resins, and the like (see, for example, U.S. Patent Application Publication No. 2009/0038442, the disclosure of which is hereby incorporated by reference). Because of drawing scale, the adhesive material is not explicitly shown in  FIG. 8 , but it will be understood that the adhesive material is present between the conforming bottom surface and floor surface. The adhesive material preferably has a substantially uniform thickness between the conforming bottom surface and floor surface. The hard plate insert  136  has a thickness such that when adhered within the opening  134 , a top surface  166  of the plate insert  136  is substantially flush with, or slightly exposed beyond, or slightly recessed below, the outer surface  130  of the leg  112 . The hard plate insert  136  is made of a material or combination of materials which are more abrasion resistant than the material used to make the leg and shirttail of the bit. In a preferred implementation, the hard plate insert is made of a material such as solid tungsten carbide, polycrystalline diamond compact (PDC), polycrystalline cubic boron nitride compact, impregnated diamond segment, and the like. These materials are superior to the traditional weld on tungsten carbide hardfacing known in the prior art because they are denser and are not as susceptible to abrasion and erosion. 
     The shirttail edge  132  is provided where the terminal portion of the surface  130  transitions to an inside radial surface  192  oriented parallel to the base of the cone  120  (perpendicular to the bearing shaft  116 ) and positioned at the base of the bearing shaft  116 . The hard plate inserts  136  function to protect against wearing of the shirttail edge  132  and erosion of the inside radial surface  192 . The depth of the opening  134  is limited by its position proximal to the shirttail edge  132  (and thus close to the radial surface  192 ). If a thin plate insert  136  is used, the opening  134  can be moved very close to the shirttail edge  132  without reaching the surface  192 . For example, a separation d 3  (where d 3 &lt;d 1 ) of 0.050 to 0.120 inches could be used from edge of the opening  134  to the shirttail edge  132  (with an insert thickness in the range of 0.050 to 0.500 inches). It is at the conforming floor surface  160  where adhesion (for example, through brazing) is made to the hard plate insert  136 . In this way, the adhesive material, unlike prior art techniques, is not externally exposed and subject to possible wear. The conforming surfaces where adhesion takes place may curve, for example, with the radius of the bit, or have any selected curved configuration. Although a sealed bearing system is illustrated, it will be understood that protection in accordance with the present invention is applicable to both sealed and non-sealed (air) bearing bits. 
     The hard plate inserts  136  have a thickness t and width w (wherein the width is measured in a direction perpendicular to the shirttail edge  132 ). The hard plate inserts  136  are thin inserts. In this case, a ratio of the thickness t of the insert to a width w of the insert is less than 0.5 (i.e., t/w&lt;0.5). More particularly, the ratio of the thickness t of the insert to the width w of the insert is substantially less than 0.5 (i.e., t/w&lt;&lt;0.5). Even more particularly, the ratio of the thickness t of the insert to the width w of the insert is less than 0.2 (i.e., t/w&lt;0.2), and may even be less than 0.1 (i.e., t/w&lt;0.1). This is permitted because the hard plate inserts  136  are retained by adhesion to their bottom surface and not their peripheral edge (as is the case with the press-fit inserts used in the prior art (see,  FIG. 4 )). 
     Reference is now made to  FIG. 9  which illustrates a cross-sectional view of a portion of a leg of a rotary cone drill bit which includes an embodiment of a protection mechanism for the lateral leading edge of the shirttail. In this embodiment, the opening  138  is provided in the outer surface  130  of the leg  112  near (adjacent to, but not coincident with) the leading shirttail edge  150 . The opening  138  may be milled or cast into the outer surface  130  of the leg  112 . The opening  138  is defined by a floor surface  170  and a peripheral wall  172  (the floor surface may, for example, be flat). There is a greater degree of freedom at this position with respect to selecting the depth of the opening  138 . The hard plate insert  140  is adhered within the opening  138 , but does not require a frictional retention with respect to the peripheral wall  172 . In a preferred embodiment, a bottom surface of hard plate insert is adhered to the floor surface  170  of the opening  138 . The bottom surface of the insert conforms to the floor surface of the opening (and thus, for example, may be a flat surface). The means for adhering the bottom surface to the floor surface may, for example, comprise any suitable adhering material which is interposed between the substantially conforming (for example, parallel) surfaces including adhesive material flowable between the substantially conforming surfaces by capillary action such as a brazing material, solder, adhesives, resins, and the like (see, for example, U.S. Patent Application Publication No. 2009/0038442, the disclosure of which is hereby incorporated by reference). Because of drawing scale, the adhesive material is not explicitly shown in  FIG. 9 , but it will be understood that the adhesive material is present between the conforming bottom surface and floor surface. The adhesive material preferably has a substantially uniform thickness between the conforming bottom surface and floor surface. The hard plate insert  140  has a thickness such that when adhered within the opening  138 , a top surface  176  of the plate insert  140  is substantially flush with, or slightly exposed beyond, or slightly recessed from, the outer surface  130  of the leg  112 . The hard plate insert  140  is made of a material or combination of materials which are more abrasion resistant than the material used to make the leg and shirttail of the bit. In a preferred implementation, the hard plate insert is made of a material such as solid tungsten carbide, polycrystalline diamond compact (PDC), polycrystalline cubic boron nitride compact, impregnated diamond segment, and the like. These materials are superior to the traditional weld on tungsten carbide hardfacing known in the prior art because they are denser and are not as susceptible to abrasion and erosion. Again, the adhesive material is this implementation is not externally exposed and subject to possible wear. The conforming surfaces where adhesion takes place may curve, for example, with the radius of the bit, or have any selected curved configuration. 
     The hard plate inserts  140  have a thickness t and width w (wherein the width is measured in a direction perpendicular to the leading edge  150 ). The hard plate inserts  140  are thin inserts. In this case, a ratio of the thickness t of the insert to a width w of the insert is less than 0.5 (i.e., t/w&lt;0.5). More particularly, the ratio of the thickness t of the insert to the width w of the insert is substantially less than 0.5 (i.e., t/w&lt;&lt;0.5). Even more particularly, the ratio of the thickness t of the insert to the width w of the insert is less than 0.2 (i.e., t/w&lt;0.2), and may even be less than 0.1 (i.e., t/w&lt;0.1). This is permitted because the hard plate inserts  140  are retained by adhesion to their bottom surface and not their peripheral edge (as is the case with the press-fit inserts used in the prior art (see,  FIG. 4 ). 
     Reference is now made to  FIG. 10  which illustrates a cross-sectional view of a portion of a leg of a rotary cone drill bit which includes an embodiment of a protection mechanism for the lateral leading edge of the shirttail. In this embodiment, the opening  142  is provided in the outer surface  130  of the leg  112 . The opening  142  may be milled or cast into the outer surface  130  of the leg  112 . The opening  142  is defined by a floor surface  180  and a peripheral wall  182  (the floor surface may, for example, be flat). There is a greater degree of freedom at this position with respect to selecting the depth of the opening  142 . The hard plate insert  144  is adhered within the opening  142 , but does not require a frictional retention with respect to the peripheral wall  182 . In a preferred embodiment, a bottom surface of hard plate insert is adhered to the floor surface  180  of the opening  142 . The bottom surface of the insert conforms to the floor surface of the opening (and thus, for example, may be a flat surface). The means for adhering the bottom surface to the floor surface may, for example, comprise any suitable adhering material which is interposed between the substantially conforming (for example, parallel) surfaces including adhesive material flowable between the substantially conforming surfaces by capillary action such as a brazing material, solder, adhesives, resins, and the like (see, for example, U.S. Patent Application Publication No. 2009/0038442, the disclosure of which is hereby incorporated by reference). Because of drawing scale, the adhesive material is not explicitly shown in  FIG. 10 , but it will be understood that the adhesive material is present between the conforming bottom surface and floor surface. The adhesive material preferably has a substantially uniform thickness between the conforming bottom surface and floor surface. The hard plate insert  144  has a thickness such that when adhered within the opening  142 , a top surface  186  of the plate insert  144  is substantially flush with, or slightly exposed beyond, or slightly recessed from, the outer surface  130  of the leg  112 . The hard plate insert  144  is made of a material or combination of materials which are more abrasion resistant than the material used to make the leg and shirttail of the bit. In a preferred implementation, the hard plate insert is made of a material such as solid tungsten carbide, polycrystalline diamond compact (PDC), cubic boron nitride, and the like. Again, the adhesive material is this implementation is not externally exposed and subject to possible wear. The conforming surfaces where adhesion takes place may curve, for example, with the radius of the bit, or have any selected curved configuration. 
     The hard plate inserts  144  have a thickness t and width w (wherein the width is measured in a direction providing the smallest w value). The hard plate inserts  144  are thin inserts. In this case, a ratio of the thickness t of the insert to a width w of the insert to is less than 0.5 (i.e., t/w&lt;0.5). More particularly, the ratio of the thickness t of the insert to the width w of the insert is substantially less than 0.5 (i.e., t/w&lt;&lt;0.5). Even more particularly, the ratio of the thickness t of the insert to the width w of the insert is less than 0.2 (i.e., t/w&lt;0.2), and may even be less than 0.1 (i.e., t/w&lt;0.1). This is permitted because the hard plate inserts  144  are retained by adhesion to their bottom surface and not their peripheral edge (as is the case with the press-fit inserts used in the prior art (see,  FIG. 4 ). 
     It will be noted that the openings and plate inserts may be of any selected geometry and arrangement thus allowing for the application of protection to complex surfaces of the bit. 
     The opening  138  can be moved very close to the leading edge  150 . For example, a separation d 4  (where d 4 &lt;d 2 ) of 0.000 to 0.120 inches could be used from edge of the opening  138  to the leading edge  150 . Furthermore, the openings  134 ,  138  and  142  can be moved very close to each other because the inserts are retained by bottom surface adhesion. For example, separations as small as, or smaller than, d 3  or d 4  could be used between openings  134 ,  138  and  142 . 
     The illustration of protection being applied using openings and plate inserts along the shirttail edge and/or leading shirttail edge and/or shirttail outer surface is by way of example only, it being understood that the protection mechanisms described can be applied to any edge or surface of the bit susceptible to wear. 
     Reference is now made to  FIG. 11  which illustrates an isometric view of a rotary cone drill bit  210  including protection mechanisms for the shirttail edge and the leading edge of the shirttail. A leg  212  depends from a body portion  214  of the drill bit  210 . The leg  212  includes a bearing shaft (not shown, see  FIG. 14  reference  216 ) which extends in a downward and radial inward direction. A cutter cone  220  is mounted to the bearing shaft and supported thereon for rotation. The outer surface  230  of the leg  212  terminates at a semicircular edge  232  proximal to the cone  220 . The region of the leg  212  associated with the surface  230  is known in the art as the “shirttail region,” and the edge  232  is known in the art as the “shirttail edge.” The outer surface  230  of the leg  212  laterally terminates at a leading shirttail edge  250  and a trailing edge  252  of the shirttail. The leading edge  250  and a trailing edge  252  of the shirttail comprise extensions of the shirttail edge  232  extending along the length of the leg  212 . The shirttail region further includes a leading side surface  254  which is adjacent the outer surface  230  of the leg  212  at the leading shirttail edge  250 . Although illustrated for example as including a sealed bearing system, it will be understood that the present invention is applicable to both sealed and non-sealed (air) bearing bits. 
     To protect the shirttail edge  232 , a plurality of hard plates  236  are adhered to a floor surface  231  provided in or by the curved outer surface  230  of the leg  212  at locations along (adjacent to, but not coincident with) the shirttail edge  232 . See, also,  FIG. 12 . To protect the leading edge  250  of the shirttail, a plurality of hard plates  240  are adhered to a floor surface  231  provided in or by the curved outer surface  230  of the leg  212  at locations along (adjacent to, but not coincident with) the leading edge  250  of the shirttail. See, also,  FIG. 13 . To generally protect the outer surface  230 , a plurality of hard plates  244  are adhered to a floor surface  231  provided in or by the curved outer surface  230  of the leg  212  at locations inward of and adjacent to the plates  240 . See, also,  FIG. 14 . The hard plates may be located at desired positions, specifically positions on the leg which are susceptible to wear during operation of the bit. Although all three protection mechanisms are illustrated in  FIG. 11 , it will be understood that any one or more of the protection mechanisms may be selected for use on the rotary cone drill bit  210 . The floor surfaces  231  are preferably machined or cast into the curved outer surfaces of the shirttail region. 
     Although  FIG. 11  primarily illustrates the use of circular plates, it will be understood that the plates can have any desired shape (including polygonal shapes, oval shapes (as shown), and the like). Furthermore, as shown in  FIG. 11 , the plates can be of different sizes, perhaps with size selection depending on placement position. 
     Reference is now made to  FIG. 12  which illustrates a cross-sectional view of a portion of a leg of a rotary cone drill bit which includes an embodiment of a shirttail edge protection mechanism. In this embodiment, the bottom surface  260  of the hard plate  236  is adhered to a substantially conforming floor surface  231  provided in or by the curved outer surface  230  of the leg  212  near (adjacent to, but not coincident with) the shirttail edge  232 . The floor surface may, for example, be a flattened surface formed in the outer surface. The means for adhering the bottom surface to the floor surface may, for example, comprise any suitable adhering material which is interposed between the substantially conforming (for example, parallel) surfaces including adhesive material flowable between the substantially conforming surfaces by capillary action such as a brazing material, solder, adhesives, resins, and the like (see, for example, U.S. Patent Application Publication No. 2009/0038442, the disclosure of which is hereby incorporated by reference). Because of drawing scale, the adhesive material is not explicitly shown in  FIG. 12 , but it will be understood that the adhesive material is present between the confirming bottom surface and floor surface. The adhesive material preferably has a substantially uniform thickness between the conforming bottom surface and floor surface. The thickness of the plate  236  may range from 0.050 to 0.500 inches. The hard plate  236  is made of a material or combination of materials which are more abrasion resistant than the material used to make the leg and shirttail of the bit. In a preferred implementation, the hard plate is made of a material such as tungsten carbide, PDC, polycrystalline cubic boron nitride compact, impregnated diamond segment, and the like. These materials are superior to the traditional weld on tungsten carbide hardfacing known in the prior art because they are denser and are not as susceptible to abrasion and erosion. The conforming surfaces where adhesion takes place may curve, for example, with the radius of the bit, or have any selected curved configuration. The shirttail edge  232  is provided where the terminal portion of the surface  230  transitions to an inside radial surface  292  oriented parallel to the base of the cone  220  (perpendicular to the bearing shaft  216 ) and positioned at the base of the bearing shaft  216 . The hard plates  236  function to protect against wearing of the shirttail edge  232  and erosion of the inside radial surface  292 . It is at the floor surface  231  on the outer surface  230  where adhesion (for example, through brazing) is made to the bottom surface  260  of the hard plate  236 . In this way, the adhesive material, unlike prior art techniques, is not externally exposed and subject to possible wear. Although a sealed bearing system is illustrated, it will be understood that protection in accordance with the present invention is applicable to both sealed and non-sealed (air) bearing bits. 
     The hard plates  236  have a thickness t and width w (wherein the width is measured in a direction perpendicular to the shirttail edge  232 ). The hard plates  236  are thin inserts. In this case, a ratio of the thickness t of the plate to a width w of the plate is less than 0.5 (i.e., t/w&lt;0.5). More particularly, the ratio of the thickness t of the plate to the width w of the plate is substantially less than 0.5 (i.e., t/w&lt;&lt;0.5). Even more particularly, the ratio of the thickness t of the plate to the width w of the plate is less than 0.2 (i.e., t/w&lt;0.2), and may even be less than 0.1 (i.e., t/w&lt;0.1). 
     Reference is now made to  FIG. 13  which illustrates a cross-sectional view of a portion of a leg of a rotary cone drill bit which includes an embodiment of a protection mechanism for the leading edge of the shirttail. In this embodiment, the bottom surface  270  of the hard plate  240  is adhered to a substantially conforming floor surface  231  provided in or by the curved outer surface  230  of the leg  212  near (adjacent to, but not coincident with) the leading shirttail edge  250 . The floor surface could, for example, be a flattened surface formed in the outer surface. The means for adhering the bottom surface to the floor surface may, for example, comprise any suitable adhering material which is interposed between the substantially conforming (for example, parallel) surfaces including adhesive material flowable between the substantially conforming surfaces by capillary action such as a brazing material, solder, adhesives, resins, and the like (see, for example, U.S. Patent Application Publication No. 2009/0038442, the disclosure of which is hereby incorporated by reference). Because of drawing scale, the adhesive material is not explicitly shown in  FIG. 13 , but it will be understood that the adhesive material is present between the conforming bottom surface and outer surface. The adhesive material preferably has a substantially uniform thickness between the conforming bottom surface and floor surface. The thickness of the plate  240  may range from 0.050 to 0.500 inches. The hard plate  240  is made of a material or combination of materials which are more abrasion resistant than the material used to make the leg and shirttail of the bit. In a preferred implementation, the hard plate is made of a material such as tungsten carbide, PDC, polycrystalline cubic boron nitride compact, impregnated diamond segment, and the like. Again, the adhesive material is this implementation is not externally exposed and subject to possible wear. The conforming surfaces where adhesion takes place may curve, for example, with the radius of the bit, or have any selected curved configuration. 
     The hard plates  240  have a thickness t and width w (wherein the width is measured in a direction perpendicular to the leading edge  250 ). The hard plates  240  are thin inserts. In this case, a ratio of the thickness t of the plate to a width w of the plate is less than 0.5 (i.e., t/w&lt;0.5). More particularly, the ratio of the thickness t of the plate to the width w of the plate is substantially less than 0.5 (i.e., t/w&lt;&lt;0.5). Even more particularly, the ratio of the thickness t of the plate to the width w of the plate is less than 0.2 (i.e., t/w&lt;0.2), and may even be less than 0.1 (i.e., t/w&lt;0.1). 
     Reference is now made to  FIG. 14  which illustrates a cross-sectional view of a portion of a leg of a rotary cone drill bit which includes an embodiment of a protection mechanism for the leading edge of the shirttail. In this embodiment, the bottom surface  280  of the hard plate  244  is adhered to a substantially conforming floor surface  231  provided in or by the outer surface  230  of the leg  212  adjacent the plates  240 . The floor surface could, for example, be a flattened surface formed in the outer surface. The means for adhering the bottom surface to the floor surface may, for example, comprise any suitable adhering material which is interposed between the substantially conforming (for example, parallel) surfaces including adhesive material flowable between the substantially conforming surfaces by capillary action such as a brazing material, solder, adhesives, resins, and the like (see, for example, U.S. Patent Application Publication No. 2009/0038442, the disclosure of which is hereby incorporated by reference). Because of drawing scale, the adhesive material is not explicitly shown in  FIG. 16 , but it will be understood that the adhesive material is present between the conforming bottom surface and floor surface. The adhesive material preferably has a substantially uniform thickness between the conforming bottom surface and floor surface. The thickness of the plate  244  may range from 0.050 to 0.500 inches. The hard plate  244  is made of a material or combination of materials which are more abrasion resistant than the material used to make the leg and shirttail of the bit. In a preferred implementation, the hard plate is made of a material such as tungsten carbide, PDC, polycrystalline cubic boron nitride compact, impregnated diamond segment, and the like. Again, the adhesive material is this implementation is not externally exposed and subject to possible wear. The conforming surfaces where adhesion takes place may curve, for example, with the radius of the bit, or have any selected curved configuration. 
     The hard plates  244  have a thickness t and width w (wherein the width is measured in a direction providing the smallest w value). The hard plates  244  are thin inserts. In this case, a ratio of the thickness t of the plate to a width w of the plate is less than 0.5 (i.e., t/w&lt;0.5). More particularly, the ratio of the thickness t of the plate to the width w of the plate is substantially less than 0.5 (i.e., t/w&lt;&lt;0.5). Even more particularly, the ratio of the thickness t of the plate to the width w of the plate is less than 0.2 (i.e., t/w&lt;0.2), and may even be less than 0.1 (i.e., t/w&lt;0.1). 
     It will be noted that the hard plates may be of any selected geometry thus allowing for the application of protection to complex surfaces of the bit. 
     The illustration of protection being applied using plates at the shirttail edge and/or leading shirttail edge and/or shirttail outer surface is by way of example only, it being understood that the protection mechanisms described can be applied to any edge or surface of the bit susceptible to wear. 
     Although preferred embodiments of the method and apparatus have been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it will be understood that the invention is not limited to the embodiments disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the spirit of the invention as set forth and defined by the following claims.