A rock drilling bit having PDC radial bearings has journal and cone bearing surfaces with increased contact area to increase load support. The radius of curvature of the bearing pin journal and cone bearing surfaces are matched or conformed on the bearing pressure side. The conformal journal surfaces may be formed on the main journal bearing, the pilot pin radial bearing, or both surfaces. In addition, diamond inlays may be located on the bearing surfaces of the cone, the bearing pin, or both components.

BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates in general to rock drill bits and, in particular, to an improved system, method, and apparatus for conformal bearings in rock drill bits. 
     2. Description of the Related Art 
     Rolling cone earth boring bits have a bit body that typically has three bit legs which extend downward from the body. A bearing pin extends inward and downward from each bit leg. A conventional rock bit bearing pin is cylindrical and rotatably receives a cone. There are several varieties of bearing systems used to support the cone. These bearing systems typically consist of a combination of radial and thrust bearings that may be either scaled and lubricated or unsealed and open to the drilling fluid. The contacting wear surfaces may consist of wear-resistant metals or non-metals such as tungsten carbide and/or diamond, and may engage through sliding and/or rolling. Open bearings may contain ports to force drilling fluid through the bearing system to lubricate and cool the wear surfaces. 
     The cones have teeth or compacts on their exteriors for disintegrating earth formations as the cones rotate on the bearing pins. A sealed, grease-lubricated bearing drill bit contains a lubricant reservoir in the bit body that supplies lubricant to the bearing pins. A seal prevents debris from contaminating the bearing and also blocks the lubricant from leaking to the exterior. When operated in a borehole filled with liquid, hydrostatic pressure acts on the drill bit as a result of the weight of the column of drilling fluid. Each bearing pin has a pressure compensation system that is mounted in the lubricant reservoirs in the bit body. A lubricant passage extends from the reservoir of the compensator to an exterior portion of the bearing pin. The pressure compensation system has a communication port that communicates with the hydrostatic pressure on the exterior to equalize the pressure on the exterior with lubricant pressure in the passages and clearances within the drill bit. The viscous lubricant creates hydrodynamic lift as the cone rotates on the bearing pin so that the load is partially supported by lubricant fluid film and partially by surface asperity to surface asperity contact. 
     A polycrystalline diamond compact (PDC) bearing is a type of open bearing system. The bearing pin and cone contain discreet PDC elements placed in a circumferential array on the radial bearing and in a planar array on the thrust bearing. The PDC elements on the cone slidingly engage the PDC elements on the bearing pin. Drilling fluid is driven through the bearing to cool and to lubricate the bearing. In this type of bearing system, load is supported almost entirely by surface asperity contact. Drill bits of this nature operate under extreme conditions. Very heavy weights are imposed on the drill bit to facilitate the cutting action, and friction causes the drill bit to generate heat. In addition, the temperatures in the well can be several hundred degrees Fahrenheit. Improvements in cutting structure have allowed drill bits to operate effectively for much longer periods of time than in the past. Engineers involved in rock bit design continually seek improvements to the bearings to avoid bearing failure before the cutting structure wears out. 
     In conventional bits ( FIG. 1 ), even though the clearance  111  between the cavity  113  of the cone  15  and the bearing pin  117  is quite small, the high load imposed on the drill bit causes the axis  119  of the cone  115  to translate eccentrically relative to the axis  121  of the bearing pin  117 . The clearance  111  is smaller on the lower side of the bearing pin  117  than the clearance  123  (e.g., 0.006 in) on the upper side of the bearing pin  117 . At high loads, the clearance  111  between the lower side of bearing pin  117  and cone  115  is reduced to zero and surface asperity to surface asperity contact occurs. The different radii of bearing pin  117  and cone  115  cause the surface asperity to surface asperity contact to be concentrated in a small area on the lower side of bearing pin  117 . The concentrated contact load creates large stress and temperature gradients that can lead to bearing failure. 
     There has been a variety of patented proposals to address this issue. For example, U.S. Pat. No. 4,403,812 discloses the use of an elastomeric suspension around the ball bearing race to take up bearing play. This compliant suspension allows the bearing elements to self-align. Other techniques have called for pre-wearing the bearings to increase surface contact area, and modifying the PDC element size and shape. Although each of these designs is workable, an improved solution that overcomes the limitations of the prior art would be desirable. 
     SUMMARY OF THE INVENTION 
     Embodiments of a system, method, and apparatus for rock drilling bit comprises improved radial bearings that maximize the contact area between the journal and cone bearings and, thus, maximizes load support. The radius of curvature of the journal and cone bearing are matched or conformed to greatly increase the apparent contact surface area on the pressure side of the bearing. The conformal radial bearing matches the journal and cone bearing radius on the bearing pressure side. This design reduces thermal fatigue cracks compared to conventional, completely cylindrical designs. The conformal journal surfaces may be formed on the main journal bearing, the pilot pin radial bearing, or both surfaces. In addition, diamond inlays or particles may be located on the bearing surfaces of the cone, the bearing pin, or both components. 
     The foregoing and other objects and advantages of the present invention will be apparent to those skilled in the art, in view of the following detailed description of the present invention, taken in conjunction with the appended claims and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the features and advantages of the present invention, which will become apparent, are attained and can be understood in more detail, more particular description of the invention briefly summarized above may be had by reference to the embodiments thereof that are illustrated in the appended drawings which form a part of this specification. It is to be noted, however, that the drawings illustrate only some embodiments of the invention and therefore are not to be considered limiting of its scope as the invention may admit to other equally effective embodiments. 
         FIG. 1  is a sectional view of a conventional journal bearing for a rotating cone drill bit; 
         FIG. 2  is a quarter-sectional view of an earth boring drill bit constructed in accordance with the invention; 
         FIG. 3  is a sectional view of a bearing pin and cone on the drill bit of  FIG. 2  taken along the line  3 - 3  of  FIG. 2 , and is constructed in accordance with the invention; 
         FIG. 4  is a partial isometric view of an interior of a cone that is constructed in accordance with the invention; 
         FIG. 5  is a sectional end view of one embodiment of a bearing pin constructed in accordance with the invention; 
         FIG. 6  is a sectional side view of the bearing pin of  FIG. 5 , taken along the line  6 - 6  of  FIG. 5 , and is constructed in accordance with the invention; and 
         FIGS. 7-10  are schematic sectional views of some embodiments of drill bit bearing pins and cones constructed in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 2 , a bit  11  has a body  13  at an upper end that is threaded (not shown) for attachment to the lower end of a drill string. Body  13  has at least one bit leg  15 , typically three, which extend downward from it. Each bit leg  15  has a bearing pin  17  that extends downward and inward along an axis  16 . Bearing pin  17  has an outer end, referred to as last machined surface  19 , where it joins bit leg  15 . Bearing pin  17  has a main journal surface  18  and a nose  21  having a smaller diameter than surface  18  that is formed on its inner end. Nose  21  also has a pilot pin radial bearing surface  22  that is parallel to surface  18  relative to axis  16 . 
     A cone  23  rotatably mounts on bearing pin  17 . Cone  23  has a plurality of protruding teeth  25  or compacts (not shown). Cone  23  has a cavity  27  that is slightly larger in diameter than the outer diameters of bearing pin  17 . Cone  23  has a back face  29  that is located adjacent, but not touching, last machined surface  19 . If the bearing type is a sealed, lubricated bearing, a seal  31  is located in a seal cavity adjacent to the back face  29 . Seal  31  may be of a variety of types, and in this embodiment is shown to be as an o-ring. Seal  31  engages a gland or area of bearing pin  17  adjacent to last machined surface  19 . Other types of seals such as dual seals, seals with non-circular cross-sectional shapes, etc., also may be used. 
     Cone  23  may be retained in more than one manner. In the embodiment shown, cone  23  is retained on bearing pin  17  by a plurality of balls  33  that engage a mating annular recess formed in cone cavity  27  and on bearing pin  17 . Balls  33  lock cone  23  to bearing pin  17  and are inserted through a ball passage  35  during assembly after cone  23  is placed on bearing pin  17 . Ball passage  35  extends to the exterior of bit leg  15  and may be plugged as shown after balls  33  are installed. 
     A portion of cavity  27  slidingly engages journal surfaces  18  and  22 . In one embodiment, the outer end of journal surface  18  is considered to be at the junction with the gland area engaged by seal  31 , and the inner end of journal surface  18  is considered to be at the junction with the groove or race for balls  33 . Journal surfaces  18  and  22  serve as a journal bearing for loads imposed along the axis of bit  11 . 
     In sealed, lubricated bearings, a first lubricant port  37  is located on an exterior portion of journal surface  18  of bearing pin  17 . In one embodiment, first port  37  is located on the upper or unloaded side of journal surface  18  of bearing pin  17  between balls  33  and seal  31 . When viewed from nose  21  ( FIG. 2 ), the first port  37  is shown at zero degrees to vertical ( FIG. 3 ), which is top dead center. First port  37  could be on other areas of journal surface  18 , but may be located in the range from zero to 90 degrees. First port  37  is connected to a first passage  39  ( FIG. 2 ) via ball passage  35 . First passage  39  leads to a lubricant reservoir  41  that contains a lubricant. 
     Lubricant reservoir  41  may be of a variety of types. In one embodiment, an elastomeric diaphragm  43  separates lubricant in lubricant reservoir  41  from a communication port  45  that leads to the exterior of bit body  13 . Communication port  45  communicates the hydrostatic pressure on the exterior of bit  11  with pressure compensator  43  to reduce and preferably equalize the pressure differential between the lubricant and the hydrostatic pressure on the exterior. 
     The precise positioning between bearing pin  17  and cone  23  varies as the drill bit  11  is loaded during service, thereby creating eccentricity. The eccentricity is a result of the difference between the outer diameter of journal surfaces  18  and  22  and the inner diameter of cone cavity  27 .  FIG. 3  shows the annular clearance  51  greatly exaggerated for illustration purposes. In actuality, annular clearance  51  is quite small, typically being no more than about 0.006 inches on a side. Annular clearance  51  may be the same as in the prior art bits of this type. 
     Under load, there is a difference between axis  16  ( FIG. 2 ) of bearing pin  17  and the axis of cone  23 . A particular bit  11  will have a maximum theoretical eccentric distance between the axes of the pin and cone based on a maximum load. In operation, there is an actual eccentric distance between the axes based on the actual load. The eccentricity ratio is the actual eccentric distance under a given load divided by the maximum eccentric distance possible. Under high loads, there is some elastic deformation of bearing pin  17  and cone  23 . The eccentricity ratio of bit  11  during operation may vary between about 0.9 to slightly greater than 1.0. 
     Even though annular clearance  51  is very small, it is required to allow assembly of cone  23  on bearing pin  17  and to allow for differences in thermal expansion during service. The annular space  51  has a largest width or clearance point  51   a  at approximately 0° (i.e., top dead center). A minimum width or clearance span  51   b  extends on both sides of a position at approximately 180° due to the downward force imposed on the bit during drilling. 
     Assuming cone  23  rotates clockwise in  FIG. 3 , in one embodiment clearance  51  has a converging region  51   c  from 0° to the region of minimum clearance at approximately 90° where the annular space for the lubricant gradually gets smaller. Clearance  51  has a diverging region  51   d , from approximately 270° to 0° where the annular space for the lubricant gets gradually larger. The minimum clearance span  51   b  is effectively zero other than the lubricant film thickness between bearing pin  17  and cone  23 . During operation, at times the minimum clearance region  51   b  may reach zero, but normally does not remain at zero. The converging region  51   c  ends at minimum clearance span  51   b , and the diverging region  51   d  begins at minimum clearance span  51   b.    
     In one embodiment, the invention comprises an earth boring bit  11  ( FIG. 2 ) having a bit body  13  with at least one depending leg  15 . A bearing pin  17  extends from the leg  15  and has journal surfaces  18 ,  22  with shapes that are not perfectly circular designs with regard to their cross-sectional shapes (i.e., slightly rotationally asymmetrical about axis  16 , which may comprise the center axis of the bearing pin in some embodiments). A rotatable cone  23  has a cylindrical cavity  27 ,  28  that fits slidingly on and directly engages the journal surface of the bearing pin. The journal surface comprises a main journal bearing surface  18  on a proximal end of the bearing pin  17 , and a pilot pin radial bearing surface  22  on a distal end of the bearing pin  17 . Both the main journal bearing surface  18  and the pilot pin radial bearing surface  22  may incorporate designs that are not perfectly or completely circular in cross-section (i.e., they are non-circular or not quite rotationally symmetrical about axis  16 ). In some embodiments, the term “rotationally asymmetric” encompasses any bearing wherein a portion of the mating surfaces have a minimum clearance space as described herein. 
     Optionally, the invention may further comprise a material  42  ( FIG. 4 ) such as metal (e.g., powdered metallurgy), diamond inlays, diamond particles, tungsten carbide, polycrystalline diamond and diamond-enhanced carbide wear surfaces located on at least one of the journal surface of the bearing pin  17  and the cylindrical cavity of the cone  23 . 
     As shown in  FIGS. 5 and 6 , such materials  71  also may be formed on and/or incorporated into one or more surfaces  18 ,  22  of the bearing pin  17 . For example, the materials  71  may include those described above, including a plurality of polycrystalline diamond bearing elements or inserts (e.g., assembled into a ring on a steel carrier ring  73 ), wherein the bearing surfaces are formed or machined to different radii. This bearing surface configuration may embody any of the variations described herein. In the embodiment shown, the two radii, R 1  and R 2 , have the same center but R 1 <R 2 . 
     In the illustrated embodiment of  FIG. 3 , the journal surface  18  and/or  22  comprises a “pressure side” or direct contact surface (adjacent span  51   b ) formed at a first radius  61 . A “non-pressure side” or non-contact surface (adjacent areas  51   a, c, d ) is formed at a second radius  63  that is shorter than the first radius  61 . 
     The cylindrical cavity  27  defines a maximum potential contact area having an angular span of approximately 180° as shown in  FIG. 3 . In one embodiment, the direct contact surface  51   b  spans an angle  65  of at least 130° of said maximum potential contact area. The radial center  67  of the bearing pin  17  may be eccentric to a radial center  69  of the cone  23  (and the radial center of radius  61 ). This offset may comprise approximately the value of the radial bearing clearance. Alternatively, the bearing pin may be formed at two or more radii that originate from the same radial center. In still another alternative, only a portion  51   b  of the journal surface  18 ,  22  may be formed at the radius  61  that matches the radius of the cylindrical cavity  27 . 
     In alternate embodiments (e.g.,  FIGS. 7-10 ), the second radius may be equal to or greater than first radius  61 . For example,  FIG. 7  depicts an embodiment wherein the pressure and non-pressure sides of the pin have equal radii  75 ,  77  (e.g., 0.990 inches) that originate from different centers  79 ,  81 , respectively.  FIG. 8  illustrates an embodiment wherein pressure side radius  83  (e.g., 0.990 inches) is greater than non-pressure side radius  85  (e.g., 0.950 inches), but they originate from the same center  87 .  FIG. 9  depicts an embodiment where the pressure side radius  89  (e.g., 0.990 inches) is greater than the non-pressure side radius  91  (e.g., 0.950 inches), and they originate from different centers  93 ,  95 , respectively.  FIG. 10  illustrates an embodiment where the pressure side radius  97  (e.g., 0.990 inches) is less than the non-pressure side radius  99  (e.g., 1.030 inches), and they originate from different centers  101 ,  103 , respectively. 
     In another embodiment, the zero clearance conforming bearing surfaces are created in combination with spherical bearing surface curvature to further increase bearing contact area under conditions when the cone misaligns on the bearing pin. 
     While the invention has been shown or described in only some of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes without departing from the scope of the invention.