Abstract:
A roller cone drill bit includes a bit body adapted to be rotated about a longitudinal axis. At least one leg depends from the bit body, and a journal is cantilevered from the leg. A roller cone is rotatably mounted on the journal. A cone retention member is disposed between a first slot in the journal and a corresponding second slot in the roller cone. An access hole runs through the journal to the first slot, penetrating the first slot at a location away from the top dead center of the first slot.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The invention relates generally to roller cone drill bits. More particularly, the invention relates to a system for retaining a roller cone on a bearing journal of the drill bit. 
     2. Background Art 
     Roller cone drill bits are generally made of a plurality of legs that are welded together to form a unitary structure. FIG. 1 shows an example of a drill bit leg  2  which defines a shirttail portion  4  and a cantilevered journal  6 . The journal  6  forms a main bearing surface  8  for a roller cone  10 . The roller cone  10  has cutting elements  12  which are adapted to deform earth formation as the drill bit leg  2  is rotated within a borehole. The roller cone  10  is retained on the journal  6  by a ball lock system which includes balls  16  that are retained between ball races  18  and  20  on the journal  6  and the roller cone  10 , respectively. To assemble and lock the roller cone  10  to the journal  6 , the balls  16  are inserted between the journal  6  and the roller cone  10  through a ball hole  22  which is drilled through the shirttail portion  4  and the journal  6 . The ball hole  22  intersects the top dead center  24  of the ball race  18 . The balls  16  are retained between the journal  6  and the roller cone  10  by welding a ball plug  25  in the shirttail side of the ball hole  22 . 
     The drill bit leg thus described retains a roller cone on a journal using a ball lock system. However, there are other methods of retaining a roller cone on a journal, for example, segmented cone retention rings disposed in a slot on the journal and subsequently threadedly locked to the roller cone using a hole to gain access to a device which prevents rotation of the rings. Generally, any cone retention system that includes a hole penetrating a ball race or other slot on the journal will induce localized stresses in the ball race or slot. In particular, when the hole intersects a high stress region on the ball race, localized stresses which develop around the intersection of the ball race with the ball hole during operation of the drill bit may be sufficient to initiate cracks in the ball race and, possibly, break the journal. It would, however, be desirable to access the ball race or other slot on the journal without initiating cracks in the journal. 
     SUMMARY OF THE INVENTION 
     A roller cone drill bit comprises a bit body adapted to be rotated about a longitudinal axis. The bit body has at least one leg depending from it. A journal is cantilevered from the leg. A roller cone is rotatably mounted on the journal. A cone retention member is disposed between a first slot in the journal and a corresponding second slot in the roller cone. An access hole runs through the journal to the first slot. The access hole intersects the first slot at a location away from the top dead center of the first slot. 
     Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross section of a prior art drill bit leg. 
     FIG. 2 shows a perspective view a roller cone drill bit. 
     FIG. 3 is a cross section of one of the drill bit legs shown in FIG.  2 . 
     FIG. 4 shows a perspective view of the bearing pin shown in FIG.  3 . 
     FIG. 5 is a cross section of the journal shown in FIG. 3 along lines A—A. 
     FIG. 6 shows threaded split rings for retaining a cone on a journal. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2 depicts a roller cone drill bit  30  which comprises a bit body  32  that is adapted to rotate about a longitudinal axis L. Three legs  36  extend downwardly from the bit body  32 . The legs  36  are spaced 120 degrees apart along the circumference of the bit body  32 . The upper end of the bit body  32  includes a threaded pin  38  which can be coupled to another tool, usually a drill string (not shown). A roller cone  40  is rotatably coupled to each leg  36 . The roller cones  40  have cutting elements  42  which deform earth formation as the drill bit  30  is rotated about the longitudinal axis L. Although the drill bit  30  is shown as having three legs  36 , it should be clear that the invention is equally applicable to a drill bit having only one leg or any other suitable number of legs. Also, the invention is independent of the type of cutting elements on the roller cones  40 . 
     FIG. 3 shows a partial cross section of one of the legs  36  shown in FIG.  2 . The leg  36  terminates in a shirttail portion  44 . A bearing pin  46  extends from the shirttail portion  44 . The bearing pin  46  includes a journal  50 , an axial thrust face  52 , and a nose pin  54 . The journal  50  forms a main bearing surface  56  for the roller cone  40 . The roller cone  40  has a bearing surface  58  which provides a bearing for the main bearing surface  56 . The nose  54  forms a bearing surface  60  which is retained within a complementary surface  62  within the roller cone  40 . Lubricant is fed between the bearing surfaces  56  and  58  through one or more lubrication ports (not shown) in the journal  50  to minimize friction between the bearing surfaces. Friction between the bearing surfaces  56  and  58  may also be minimized by placing a low-friction bearing material, such as a low-friction pad  64 , a roller bearing (not shown), a ball bearing (not shown), or other type of anti-friction bearing between the bearing surfaces. The lubrication ports (not shown) in the journal  50  communicate with a lubrication passage  66  which is connected to receive lubricant from a grease reservoir  67  (shown in FIG. 2) in the upper part of the leg  36 . A seal  68  is provided to retain the lubricant between the bearing surfaces  56  and  58 . However, it should be clear the invention is also applicable to non-sealed bearings. 
     The roller cone  40  is retained on the journal  50  by balls  70 . Ball races  72  and  74  are defined in the bearing surfaces  56  and  58  to hold the balls  70 . The balls  70  are fed between the ball races  72  and  74  through a ball hole  76  that runs through the leg  36  and the journal  50  to the ball race  72 . The balls  70  are retained between the ball races  72  and  74  by welding a ball plug  80  in the shirttail side of the leg  36 . The ball hole  76  intersects the ball race  72  at a location away from the top dead center  78  of the ball race  72 . The top dead center  78  is the uppermost point on the ball race  72  in the direction of the longitudinal axis L (shown in FIG.  2 ). Preferably, the ball hole  76  intersects the ball race  72  at an angle 2.5 degrees or more from the top dead center  78  of the ball race  72 . Although the ball hole  76  is shown as a straight hole, it should be clear that the ball hole may comprise two or more non-parallel, intersecting holes. FIG. 4 shows a perspective view of the bearing pin  46  with the ball hole  76  intersecting the trailing side  73  of the ball race  72 , i.e., the side of the ball race away from the rotational direction of the drill bit  30 . The ball race  72  could also intersect the leading side of the ball race  72 , i.e., the side of the ball race  72  facing the rotational direction of the drill bit  30 . 
     Referring back to FIG. 2, when the drill bit  30  is rotated about the longitudinal axis L and forced against earth formation, a rotary motion is induced in each of the roller cones  40  about its respective rotational axis R. Typically, the rotational axis R of each roller cone  40  is offset a distance from the longitudinal axis L so that the cutting elements  42  can scrape earth formation in a direction inward of the drill bit  30 . This inward scraping action of the cutting elements  42  results in a reaction force F 1  from the earth formation to the roller cone  40  in the direction outward of the drill bit  30 . Also, the earth formation applies a reaction force F 2  to cutting elements  42  in the direction upward of the longitudinal axis L, which causes compressive loading on the bottom portion  82  (shown in FIG. 4) of the journal  50 . In addition, as the drill bit  30  rotates about longitudinal axis L during drilling operation, most of the cutting elements  42  will scrape the formation in the direction of bit rotation, causing a reaction force F 3  in circumferential direction against bit rotation. 
     The reaction forces acting on the roller cone  40  are transmitted to the journal  50 , making the top dead center  78  a high stress region. However, because the ball hole  76  intersects the ball race  72  in a location away from the top dead center  78 , i.e., away from the high stress region, the probability of initiating cracks in the ball race  72  is reduced. Generally, the further away the ball hole  76  is from the top dead center  78 , the lesser is the probability of initiating cracks in the ball race  72 . To avoid compressive loading on the ball plug  80 , it is desirable that the ball hole  76  intersects the ball race  72  in the upper half of the ball race  72 , as illustrated in FIG.  5 . The upper half of the ball race  72  in FIG. 5 is that part which is above line A—A′. Also, as a result of the forces acting on the ball race  72  during drilling operation, the leading side  75  of the ball race  72  has potential high stress. Thus, it is desirable that the ball hole  76  intersects the ball race  72  in the trailing side  73  of the ball race  72 . 
     The invention is advantageous in that the probability of initiating cracks in the ball race  72  is substantially reduced when the ball hole  78  intersects the ball race  72  at a location away from the top dead center of the ball race. When the ball hole  76  intersects the ball race  72  at an angle of 45 degrees away from the top dead center  78 , the highest stress on the ball race  72  drops by roughly 45 percent. When the ball hole  76  intersects the ball race at an angle of 90 degrees away from the top dead center  78 , the highest stress on the ball race  72  drops by roughly 70 percent. In addition, the stress in the throat area  84  (shown in FIG. 3) of the leg  36 , i.e., the juncture between the leg  36  and the journal  50 , is reduced by roughly 10-20 percent when the ball hole intersects the ball race  72  at 90 degrees away from the top dead center  78 . It is preferable that the ball hole  76  intersects the ball race  72  at an angle of at least 10 degrees from the top dead center  78  to provide meaningful reduction on stress about the ball hole  76 . More preferably, the ball hole  76  intersects the ball race  72  at an angle of at least 25 degrees from the top dead center  78  to provide a substantial reduction in stress about the ball hole  76 . 
     The invention has been described with respect to a drill bit which uses ball bearings to retain a roller cone on a journal. However, the invention is equally applicable to other types of cone retention systems. For example, FIG. 6 shows a cone retention system which includes split ring segments  86  and  88  that are disposed in a slot  90  on the bearing surface of a journal  92 . The journal  92  is similar to the journal  50  shown in FIGS.  3 - 5 , except that the slot  90  is designed to accept the split ring segments  86  and  88  instead of a series of ball bearings. The outer surfaces  94  and  96  of the split segments  86  and  88  include threads which are adapted to interlock with a similar threaded surface on the inner surface of a roller cone (not shown). 
     To assemble the drill bit, the split ring segments  86  and  88  are arranged in the slot  90  of the journal  92 , as illustrated in FIG.  6 . Then a tool (not shown) is inserted through an access hole  98 , similar to the ball hole  76  (shown in FIGS.  3 - 5 ), into the slot  100  in the split segment  86 . The tool locks the split ring segments  86  and  88  down, allowing the roller cone to be slipped over the journal  92  and turned to threadedly engage the split segments  86  and  88 . The access hole  98  is located away from the top dead center  99  of the slot  90  as previously disclosed for the ball hole  76  in FIGS.  3 - 5 . 
     It will be apparent to those skilled in the art that the foregoing description is only an example of the invention, and that other embodiments of the invention can be devised which will not depart from the spirit of the invention as disclosed herein. Accordingly, the invention shall be limited in scope only by the attached claims.