Abstract:
An earth boring bit has a seal cavity that is eccentric to agitate drilling fluid and avoid mud packing. The bit body having bit legs, each having a bearing pin. A cone rotatably mounts on the bearing pin. Teeth are formed or mounted on the cone for cutting the earth formation as the bit rotates. The cone has a backface adjacent the bit leg and an entrance portion to the cavity that intersects the backface. A seal assembly is in stationary engagement with the bearing pin and sliding engagement with the cone. An annular seal gland is located between an outer diameter portion of the bearing pin and the entrance portion of the cavity. The seal gland has a width that varies so that as the cone rotates, the width of the seal gland at any point along the outer diameter portion of the bearing pin changes at least once per revolution of the cone.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims priority to provisional application Ser. No. 60/392,814, filed Jul. 1, 2002. 

   FIELD OF THE INVENTION 
   This invention relates in general to earth-boring roller cone drill bits, and in particular to features for reducing mud packing around the seal gland. 
   BACKGROUND OF THE INVENTION 
   Rolling cone earth boring bits have been used for many years for drilling wells. The bit has a body with at least one leg, usually three. A bearing pin depends from each bit leg, extending inward and downward toward the axis of rotation of the bit body. A cone with teeth on its exterior mounts rotatably to each bearing pin. 
   The cone has a cavity that fits over the bearing pin. The cavity has an entrance portion or mouth adjacent the junction of the bit leg and the bearing pin. A seal is located at the entrance to seal the cavity form cuttings and drilling mud. Most well drilling rolling cone bits are filled with a lubricant that is sealed by the seal at the cavity mouth. 
   Many different seals have been used in the past as well as today. One type has a rigid seal ring that extends around the bearing pin and is urged by an elastomeric energizer ring into sliding engagement with a seal face in the cone cavity. The seal face rotates with the cone, while the seal ring and energizer ring are stationary with the bearing pin. An elastomeric excluder ring may be located between the bit leg and the outer ends of the energizer ring for keeping drilling mud and cuttings from the seal. While successful, sometimes mud packing occurs at the seal, causing damages to the seal. 
   SUMMARY OF THE INVENTION 
   In this invention, the annular seal cavity that contains the seal is made non-uniform. The volume of the seal cavity differs when measured around the outer diameter of the bearing pin, with at least one point having a greater volume or lesser volume than other points in the seal cavity. Consequently, as the cone rotates, the volume changes cyclically, creating a pumping action to circulate drilling fluid from the seal cavity. 
   In one embodiment, the difference in volume is accomplished by changing the radial width of the seal cavity from one point to another. This is preferably done by making the mouth eccentric relative to the bearing pin axis. The mouth is preferably circular, but has its axis offset from the bearing pin axis. 
   In one embodiment, the eccentric groove of the mouth is spaced radially from the seal assembly. In another embodiment, the eccentric groove of the mouth is located outward from the seal and is closely spaced to a seal boss formed on the bearing pin. 
   In a third embodiment, the backface of the cone contains a plurality of vanes. The vanes rotate in close proximity to a last machined surface formed on the bearing pin to provide a positive pressure. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a partial sectional view of one roller cone and bearing pin of a bit constructed in accordance with this invention. 
       FIG. 2  is an enlarged view of a portion of the bit of  FIG. 1 . 
       FIG. 3  is a sectional view of the bit of  FIG. 1 , taken along the line  3 — 3  of  FIG. 2 , with the seal assembly omitted. 
       FIG. 4  is a sectional view similar to  FIG. 3 , but showing the cone further rotated from the position of  FIG. 3 . 
       FIG. 5  is a partial sectional view of an alternate embodiment of a bit constructed in accordance with this invention. 
       FIG. 6  is a partial sectional view of another alternate embodiment of a bit constructed in accordance with this invention. 
       FIG. 7  is an end view of the cone of the bit of  FIG. 6 , the cone shown removed and with the teeth omitted. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , bit  11  has three legs  13 , only one of which is shown. Each bit leg  13  has a depending bearing pin  15 , which is a cylindrical member that points downward and inward relative to an axis of rotation of bit  11 . A cone  17  is rotatably carried on bearing pin  15 . Cone  17  has a cone body  19  and a plurality of teeth  21 . Teeth  21  are shown to be steel teeth that are milled from body  19 , but they could also be hard metal inserts, such as tungsten carbide, inserted into holes within cone body  19 . Cone body  19  has a bearing cavity  23  that fits over bearing pin  15 . In this embodiment, a seal insert ring  25  is press-fitted within cavity  23  and rotates with cone  17 . 
   Cone  17  has a backface  27  that is located on the outer end of cone  17  and faces bit leg  13 . Backface  27  is placed very close to but not touching a last machined surface  29  formed at the base of bearing pin  15  where it joins bit leg  13 . Last machined surface  29  also includes a depending portion on the lower side of bit leg  13 , called a shirttail. A seal assembly  31  is located at the base of bearing pin  15  for sealing lubricant within cone cavity  23 . 
   Referring to  FIG. 2 , seal assembly  31  in this embodiment includes a metal ring  33  that is stationary with bearing pin  15  and engages seal insert  25  in rotating sliding contact. An elastomeric energizing ring  35  is stationarily located in an annular recess  36  on bearing pin  15  and provides a bias force to urge metal ring  33  against the seal face of insert  25 . An elastomeric excluder ring  37  is deformed between last machined surface  29  and the outer ends of metal ring  33  and energizing ring  35  for preventing entry of drilling mud and debris. Numerous other seal assemblies are suitable in lieu of seal assembly  31 . 
   Cone  17  is retained on bearing pin  15  by a plurality of locking balls  41 , shown in  FIG. 1 , although other types of retention are possible, such as snap rings. Lubricant passages  43  supply lubricant from a reservoir (not shown) to locking balls  41  and to the bearing spaces in sliding contact between cone  17  and bearing pin  15 . 
   Cone cavity  23  has an entrance portion or mouth  40  that is an annular surface located radially outward from seal assembly  31  relative to an axis  47  of bearing pin  15 . An annular seal cavity  39 , which contains seal assembly  31 , is defined by annular recess  36  on the outer diameter of bearing pin  15  and cone mouth  40 . Seal cavity  39  has a radial width, measured between recess  36  and mouth  40 . Seal cavity  39  also has an axial depth, measured from last machined surface  29  to the outer end of seal insert ring  25 . The volume of seal cavity  39 , measured at any point around recess  36  is the radial width times the axial depth. This volume is not constant around recess  36 , rather varies continuously. 
   In the preferred embodiment, the varying volume is due to a varying radial width of seal cavity  39 . As shown in  FIGS. 2–4 , mouth  40  is eccentric relative to bearing pin axis  47 . In the preferred embodiment, mouth  40  is circular, but has an axis  48  that is offset from bearing pin axis  47 . The eccentricity results in varying radial width for seal cavity  39 . The width varies from a minimum radial width W 1 , located on the upper side in  FIGS. 2 and 3  to a maximum radial width W 2 , located on the lower side in  FIGS. 2 and 3 . The minimum radial width point W 1  is 180 degrees from the maximum radial width point W 2 . The change in width is gradual from minimum width point W 1  to maximum width point W 2 . 
   Mouth  40  is spaced radially outward of the outer diameter of metal seal ring  33 , defining an annular clearance between the outer diameter of metal seal ring  33  and mouth  40 . The annular clearance forms a part of seal cavity  39 . Because of the eccentricity, this annular clearance also varies in width also. 
   The dotted lines  49  of  FIGS. 3 and 4  indicate mouth  40  if it were conventional, which is circular and concentric to bearing pin axis  47 . Mouth  40  is preferably made eccentric by increasing the diameter slightly and shifting its axis  48  relative to the axis of rotation  47 . As cone  17  rotates, minimum width portion W 1  and maximum width portion W 2  rotate past any selected point on seal assembly  31  once per revolution. 
   In operation, when bit  11  is rotated, cone  17  will rotate concentrically about bearing pin axis  47  as indicated by the arrows of  FIGS. 3 and 4 .  FIG. 4  shows cone body  19  rotated approximately 90° from the position of  FIG. 3 . The minimum and maximum width points W 1  and W 2  of cone mouth  40  thus have rotated 90° from the position shown in  FIG. 3 . As cone  17  rotates, seal cavity  39  changes in width, and thus volume, at each point on the circumference of metal seal ring  33 . The wider width point W 2  will rotate with cone  17  about bearing pin  15  as well as the lesser width point W 1 . 
     FIG. 5  shows a bit  51  that is similar to bit  11 , however, it does not use a metal face seal  33  ( FIG. 2 ). Bit  51  has a bit leg  53  and bearing pin  55 . Cone  57  has an annular recess or seal gland  59  at the entrance to its cavity that receives an elastomeric seal  61 , preferably an O-ring. Seal  61  is deformed between seal gland  59  and the outer diameter of bearing pin  55  close to bit leg  53 . Seal gland  59  extends to a mouth of the cavity of cone  57  at backface  65 . 
   An eccentric grooved portion  63  is formed at the mouth of the cone cavity, which is at the corner between seal gland  59  and backface  65 . Eccentric portion  63  is preferably a circular groove that has an axis offset from the axis of rotation of cone  57 . A seal boss  64  is machined on bit leg  57  radially inward from eccentric portion  63 . The radial gap between the cylindrical wall of eccentric portion  63  and seal boss  64  varies around the circumference of seal boss  64 . This change in radial width results in minimum and maximum radial width portions rotating around seal boss  64  as cone  57  rotates. The change in radial width between eccentric portion  63  and seal boss  64  results in a cyclic change in volume in seal gland  64 . This creates a pumping action tending to pump drilling mud from seal gland  59  outward to reduce mud packing. Eccentric portion  63  thus functions in the same manner as the eccentric mouth  40  of the first embodiment to reduce mud packing. 
     FIG. 6  shows another embodiment. Bit  69  has a bit leg  71 , from which depends a bearing pin  73 . A cone  75  rotates concentrically on bearing pin  73 . Cone  75  has a seal gland  77  that contains a seal  79 . Seal  79  is shown as an elastomeric O-ring, but it could be other types. Cone  75  has a backface  81  surrounding mouth  82  of the cavity of cone  75 . As in the other embodiments, backface  81  is closely spaced to but not touching last machined surface  83 . Last machined surface  83  is contained in a plane perpendicular to the axis of bearing pin  73 . The lower portion of last machined surface  83  is referred to as a shirttail. 
   In this embodiment, a plurality of curved vanes  85  are formed on backface  81  as shown in  FIG. 7 . Vanes  85  extend from cavity mouth  82  to the outer diameter of backface  81  and are spaced from last machined surface  83  by a gap. Each vane  85  curves in a manner to create a differential pressure between the seal cavity surrounding gland  77  and the exterior, with the pressure being greater in the seal cavity  77 . This pressure tends to cause drilling fluid to circulate out of seal gland  77 . Vanes  85  cause the volume of the seal cavity, which includes seal gland  77  and the gap between backface  85  and last machined surface  83 , to change at any selected point on last machined surface  83  due to rotation of cone  75 . 
   Other configurations to provide a variable volume for the seal cavity are feasible. For example, the cone mouth could be noncircular but rotate about the bearing pin axis. The cone mouth could be oblong or have one or more recess portions within it that result in a noncircular inner diameter at the entrance portion of the cone cavity. These recess portions could be formed at different radial distances from the seal assembly. The change in seal cavity volume need not occur gradually as in the first embodiment. There could be more than one maximum width and minimum width portions, as provided by the vanes of the third embodiment. Rather than gaps between the cone and bearing pin differing in radial dimension, recess portions could extend axially from the backface of the cutter at different distances to change the volume of the seal cavity at different points around the circumference of the cavity mouth. In that instance, the mouth could have a circular diameter that is concentric with the bearing pin axis. 
   The invention has significant advantages. This rotational change in volume of seal cavity  39  causes agitation of the drilling mud that locates around seal assembly  31  ( FIG. 1 ). The agitation discourages mud packing in this area. The dynamic pressure within the seal gland changes as the cone rotates, causing a positive pressure in the seal gland over the exterior to circulate drilling fluid from the seal gland. This agitation is accomplished without any addition components. 
   While the invention has been shown in only three of its forms, it should be apparent to those skilled in the art that it is not so limited but is susceptible to various modifications without departing from the scope of the invention.