Abstract:
A lubricating nutating single cone drill bit  10  is disclosed. Bit  10  includes a bit shank  12  with an axially skewed bore  18  rotatably retaining a cutter body  22 . A plurality of cutter elements ( 46, 48 ) are affixed to the cutter body  22 . Bearings ( 26, 28, 30, 32 ) enable rotation of the cutter body  22  and lubricant is dispensed to the bearings from a lubricant chamber ( 76, 76 ′) by a plunger  80  driven by drilling fluid. A method to drill includes attaching the bit  10  to a drill string, engaging the bit  10  into a formation (WB), pumping a drilling fluid into a plunger  80  in the lubricant chamber  76  to dispense lubricant to the bearings ( 26, 28, 30, 32 ). The method can include pumping the drilling fluid through a fluid passage  90  in the bit  10  and out of a low pressure orifice  94  on the cutter body  22.

Description:
BACKGROUND 
   The present invention generally relates to drill bits for boring subterranean and sub sea formations. More particularly, the present invention relates to a lubricating nutating single cone drill bit having an axis of rotation skewed relative to the central axis of the bit body in the borehole providing low torque and allowing high compressive loading on the bit assembly. 
   A number of single cone drill bits have been proposed through the years to drill bore holes for mining, oil and gas exploration, and utility construction. It has been previously recognized that a single cone bit would offer superior design characteristics, such as bearing size permitting greater longitudinal compressive loading on the drill bit. Previous single cone drill bits however provided substantial scraping of the cutter elements causing abnormal wear and torque on the drill string assembly. 
   Each of the prior single cone drill bits were subject to excessive wearing of the cutting elements because at least during some portion of the rotation, the cutter elements were dragged by the circular motion of the bit on the journal across the formation face rather than moved in compressive engagement with the surface. These cutter elements are designed to have long use lives if used in compression, but have a tendency to break if subjected to side shear or scraping. A nutating single cone drill bit, for example the one disclosed in U.S. Pat. No. 6,892,828, incorporated by reference herein, can offer the advantage of long wearing cutter elements. Typically, traditional tri-cone bits must be repeatedly tripped out of the borehole due to excessively worn cutter elements. Since a nutating single cone drill bit allows for longer service life, the extended periods of down-hole use can be limited by the amount of available lubrication sufficient to maintain the bearings of the nutating single cone bit. Without sufficient lubrication, the bearing can fail prior to the cutter elements of the bit wearing, limiting the usefulness of the nutating single cone drill bit. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to a lubrication system for a nutating single cone drill bit. By employing at least one lubricant chamber in the bit shank, drilling fluid can drive a plunger in the chamber and thus dispense lubricant into the rotationally contacting bearing surfaces, for example, of a thrust, radial, or ball bearing. Further, a radial dynamic seal restricts contaminants from contacting the bearing surfaces and extends the life of the bearing to at least the useful life of the cutter or crushing elements on the drill bit body, thereby eliminating the need to trip the drill bit into and out of the well bore to replace lubrication in a bit whose cutter or crushing elements are not worn sufficiently to be removed from service. 
   In one embodiment, a lubricating nutating single cone drill bit can include a bit shank having a drill string connection on a proximal end and an axially skewed bore formed in a distal end thereof, a plurality of bearings rotatably retaining a cutter body in the axially skewed bore, the cutter body having a plurality of cutter elements on a distal end, and at least one lubricant chamber in the bit shank in communication with a first fluid port in the proximal end of the bit shank and containing a plunger displaceable by a drilling fluid to dispense a lubricant to the plurality of bearings. The bit can include a low pressure orifice on the distal end of the cutter body in communication with a fluid passage extending through the cutter body and the bit shank, the fluid passage in communication with a second fluid port in the proximal end of the bit shank adjacent the first fluid port. The cutter body can have a narrow journal portion at a proximal end and a thrust shoulder between the narrow journal portion and the distal end. 
   The plurality of bearings retaining the cutter body in the axially skewed bore can include any of the following bearings: 1) a first radial bearing disposed circumferential the narrow journal portion of the cutter body, 2) a second radial bearing disposed circumferentially about the distal end of the cutter body, 3) a thrust bearing disposed between the thrust shoulder of the cutter body and a respective thrust shoulder in the axially skewed bore of the bit shank, and 4) at least one ball bearing disposed between a first channel formed in the axially skewed bore and a second channel formed in the narrow journal portion of the cutter body. 
   A lubricating nutating single cone drill bit can include a first radial dynamic seal adjacent a distal end of the axially skewed bore between the cutter body and the bit shank and/or a second radial dynamic seal adjacent a proximal end of the axially skewed bore between the cutter body and the bit shank. 
   In another embodiment, the plurality of cutter elements can be affixed to the cutter body so that a tip of each cutter element is forward an intersection of a central axis of the bit shank and an axis of rotation of the cutter body and a first chordal distance to the tip of each cutter element from the axis of rotation of the cutter body is longer than a second chordal distance to said tip of each cutter element from the central axis of the bit shank. 
   In yet another embodiment, a lubricating nutating single cone drill bit can include a cutter body rotatably retained in an axially skewed bore of a bit shank by a plurality of bearings disposed between the cutter body and the bit shank, the bit shank having a drill string connection on a proximal end, a plurality of radial dynamic seals disposed in a gap between the axially skewed bore and the cutter body, at least one radial dynamic seal adjacent a proximal end of the cutter body and at least one radial dynamic seal adjacent a distal end of the cutter body to seal the plurality of bearings from contamination, and at least one substantially longitudinal lubricant chamber formed in the bit shank in communication with the gap and in communication with at least one fluid port in the proximal end of the bit shank. The plurality of bearings can include a thrust bearing disposed between a thrust shoulder formed on the cutter body and a respective thrust shoulder in the axially skewed bore of the bit shank. The bit can include a low pressure orifice on the distal end of the cutter body in communication with a fluid passage extending through the cutter body and the bit shank, the fluid passage in communication with a second fluid port in the proximal end of the bit shank. 
   A nutating single cone drill bit having a cutter body rotatably retained in an axially skewed bore of a bit shank having a connection to a drill string, can include the improvement of a lubricant chamber in the bit shank with a first fluid port to receive a drilling fluid to drive a plunger disposed therein, the lubricant chamber in communication with at least one bearing disposed between the bit shank and the cutter body, a first radial dynamic seal between a distal end of the cutter body and the bit shank, a second radial dynamic seal between a proximal end of the cutter body and the bit shank, and a low pressure orifice on the distal end of the cutter body in communication with a second fluid port in the bit shank formed adjacent the first fluid port through a fluid passage extending through the cutter body and the bit shank. The bit can include a second lubricant chamber and a respective plunger in communication with the bearing surface between the bit shank and the cutter body and in communication with a third fluid port in the bit shank. 
   In yet another embodiment, a method to drill a formation can include attaching the lubricating nutating single cone drill bit of claim  1  to a drill string to form an assembly, engaging the assembly into the formation, pumping the drilling fluid through the drill string into the first fluid port to drive the plunger and dispense the lubricant to the plurality of bearings, and rotating the drill string to drill the formation with the bit to produce a well bore. The method can include 
   removing the lubricating nutating single cone drill bit from the well bore, and replenishing the lubricant in the at least one lubricant chamber. The step of pumping the drilling fluid can include pumping the drilling fluid through a bit shank fluid passage into a cutter body fluid passage and out a low pressure orifice on the distal end of the cutter body to remove any cuttings from the formation from the well bore. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional schematic view of a lubricating nutating single cone drill bit, according to one embodiment of the invention. 
       FIG. 2  is a perspective end view of the lubricating nutating single cone drill bit of  FIG. 1  as seen along the lines  2 - 2 . 
   

   DETAILED DESCRIPTION 
   With reference to the figures wherein like reference numerals are used to refer to like parts,  FIG. 1  shows a cross-sectional schematic view of a lubricating nutating single cone drill bit  10 . The bit  10  includes a bit shank  12  with a drill string connection  14  adjacent the proximal end  16  and an axially skewed bore  18  formed in a distal end of the bit shank  12 . The bore  18  is axially skewed at an acute angle A relative to the axis of the bit shank  12 . The axis of rotation  24  of the cutter body  22  can be skewed about 10° from the central axis  20  of the bit shank  12 , for example, however any acute skew angle can be utilized, consistent with the disclosure made in U.S. Pat. No. 6,892,828. Cutter body  22  has an outside surface formed respectively to the axially skewed bore  18  to allow at least partial insertion therein. As the cutter body  22  rotates within the axially skewed bore  18 , at least one bearing can be disposed therebetween to aid in the rotation. The plurality of bearings in the illustrated embodiment includes ball bearings  26 , a thrust bearing  28 , and radial bearings ( 30 ,  32 ). The invention is not limited to the illustrated bearings and any type of bearings known to one of ordinary skill in the art of tribology can be used. 
   The cutter body  22  includes a narrow journal portion  34  at a proximal end  36  and a thrust shoulder  38  formed between the narrow journal portion  34  and the distal end  42 . An optional second shoulder  44  is further illustrated to allow cutter element  46 , for example, to create a well bore (WB) diameter capable of allowing the bit shank  12  to drill into the formation. A respective bit shank thrust shoulder  40  is formed in the axially skewed bore  18 . Thrust bearing  28  is disposed between the cutter body thrust shoulder  38  and the bit shank thrust shoulder  40 . Thrust bearing  28  can be selected to support a preferred amount of load on the bit  10 . Any of the bearings ( 28 ,  30 ,  32 ) can be a rubbing bearing or a rolling element bearing, as known to one of ordinary skill in the art. For example, thrust bearing  28  can be a rolling thrust bearing with ball, roller, or needle bearings. 
   The single cone cutter body  22  includes a plurality of cutter elements ( 46 ,  48 ) on the distal end  24  of the cutter body  22 . Preferably, every cutter element ( 46 ,  48 ) is affixed to the cutter body  22  so that a tip of each cutter element, for example, cutter element  48 , is forward an intersection  50  of a central axis  20  of the bit shank  12  and an axis of rotation  24  of the cutter body  22 . By having each cutter element ( 46 ,  48 ) tip forward a plane defined normal to the axis of rotation  24  of the cutter body  22  at the intersection  50  of the axis of rotation  24  of the cutter body  22  and the central axis  20  of the bit shank  12 , the preferred crushing, and not scraping, engagement of the well bore (WB) can be achieved. Similarly, a first chordal distance  52  to the tip of each cutter element (e.g. cutter element  48 ) from an axis of cutter body rotation  24  can be longer than a second chordal distance  54  to said tip of each cutter element  48  from the central axis  20  of the bit shank  12 . 
   To assemble the lubricating nutating single cone drill bit  10 , the thrust bearing  28  and the first  32  and second  30  radial bearings can be disposed between the cutter body  22  and the bit shank  12 . The thrust bearing  28 , the first radial bearing  32 , and/or second radial bearing  30  can be disposed on the cutter body  22  or in the axially skewed bore  18  before assembly. With the thrust bearing  28  and the first  32  and second  30  radial bearings disposed therebetween, the cutter body  22  is inserted into the axially skewed bore  18  of the bit shank  12 . The void between the proximal end  36  of cutter body  22  and the proximal end  69  of the axially skewed bore  18  can be any size, and is not limited to that shown. To allow rotation, preferably the cutter body  22  and the axially skewed bore  18  are sized relative to each other to provide a gap therebetween. After the cutter body  22  is inserted into the axially skewed bore  18 , the ball bearings  26  can then be added to limit axial movement of the cutter body  22 , and thus impede separation of the bit shank  12  and cutter body  22 . 
   The bearing race to house the ball bearings  26  is formed from a first channel  56  circumferentially formed in the narrow journal reception portion of the axially skewed bore  18  and from a second channel  58  circumferentially formed in the narrow journal portion  34  of the cutter body  22 . To allow the insertion of ball bearings  26  into the bearing race ( 56 ,  58 ) of the bit  10 , a retainer plug port  60  is formed in the lateral surface of the bit shank  12 . The retainer plug port  60  includes a narrow portion  62  for receiving a retainer plug  64 . Retainer plug  64  can be threadably engaged (not shown) with a respective threaded narrow portion  62  of the retainer plug port  60 . Narrow portion  62  is sized to allow ball bearings  26  to be disposed through the retainer plug port  60  into the bearing race ( 56 ,  58 ). Retainer plug  64  can have a socket cap screw with a hexagonal recessed drive  66 . Retainer plug  64  can be retained in a threaded or non threaded retainer plug port  60  by a snap ring  68  disposed in a groove formed in the upper portion of the retainer plug port  60 . The end of the retainer plug  64  forms a section of the first channel  56  in the axially skewed bore  18  and is preferably retained in a position so as to not interfere with the rolling of the ball bearings  26 . A seal  70  of any type is disposed between the retainer plug port  60  and retainer plug  64  to restrict the expulsion of any bearing lubricant. 
   A plurality of ball bearings  26  can then be added to the bearing race ( 56 ,  58 ) through the retainer plug port  60  and the retainer plug  64  installed to retain the ball bearings  26 . The number of ball bearings  26  utilized is design dependent, but is preferably a full-complement. 
   Lubricant can be added to the bearings ( 26 ,  28 ,  30 ,  32 ) at any time before, during, or after assembly. To retain the lubricant, and to restrict the ingress of any contaminants, the invention includes a plurality of seals ( 72 ,  74 ) between the cutter body  22  and the axially skewed bore  18 . In a preferred embodiment, the seals ( 72 ,  74 ) are radial dynamic seals. A radial seal is typically designed for an interference fit on the diameters between two concentric, or somewhat eccentric, cylinders, for example. As used herein, the term dynamic seal shall refer to a seal wherein at least one of the sealed surfaces is motive, for example, a rotating shaft. A radial dynamic seal ( 72 ,  74 ) can be any appropriate seal, including, but not limited to, an O-ring, square-ring, U-cup seal, shaft seal, etc. 
   Radial dynamic seals ( 72 ,  74 ) are typically installed in a groove in a housing (e.g. a groove in the axially skewed bore  18 ) and compress against a shaft (e.g. the cutter body  22 ). A first radial dynamic seal  72  is disposed adjacent a distal end of the axially skewed bore  18  between the cutter body  22  and the axially skewed bore  18 . A second radial dynamic seal  74  is disposed between the narrow journal portion  34  of the cutter body  22  and the axially skewed bore  18 . A radial dynamic seal ( 72 ,  74 ) can be disposed in a groove formed in the cutter body  22  (not shown), a groove in the axially skewed bore  18 , or a combination thereof. So arranged, a lubricant disposed in the gap between the cutter body  22  and the axially skewed bore  18  is retained by the radial dynamic seals ( 72 ,  74 ). This configuration protects the bearings ( 26 ,  28 ,  30 ,  32 ) from contamination, for example, from drilling fluid and cuttings. 
   To provide lubrication, the invention includes at least one lubricant chamber  76 . The lubricant chamber  76  is a bore formed in the bit shank  12 . The proximal end of the lubricant chamber  76  forms a first fluid port  78  in the proximal end  16  of the bit shank  12 . The distal end of the lubricant chamber  76  is in communication with the bearings ( 26 ,  28 ,  30 ,  32 ) and/or in communication with the gap formed between the cutter body  22  and the axially skewed bore  18  of the bit shank  12  and between the radial dynamic seals ( 72 ,  74 ), said gap providing the clearance to allow rotation of the cutter body  22 . 
   In a preferred embodiment, the distal end of the lubricant chamber  76  is formed so as to be in communication with the retainer plug port  60 . This can be achieved by drilling the lubricant chamber  76  at least partially into the retainer plug port  60 , or vice versa. As the retainer plug port  60  is in communication with the gap formed between cutter body  22  and the axially skewed bore  18 , the lubricant chamber  76  is consequently in communication with the gap, and thus the bearings ( 26 ,  28 ,  30 ,  32 ) disposed therein. The portion  65  of the retainer plug port  60  that is not filled with the retainer plug  64  can be bored out (not shown) during the forming of the lubricant chamber  76 , to fluidically connect the lubricant chamber  76  and retainer plug port  60 , and thus bearings ( 26 ,  28 ,  30 ,  32 ). Similarly, the lubricant chamber  76  can be drilled into the retainer plug port  60  at an area where the retainer plug  64  will be disposed (as shown). Any gap between the retainer plug  64  and the retainer plug port  60  can then allow for the lubricant to flow past the retainer plug  64 , which has a seal  70 , and into the gap formed between the cutter body  22  and the axially skewed bore  18 . 
   After a lubricant has been disposed in the lubricant chamber  76 , by any means known in the art, plunger  80  can be disposed in the lubricant chamber  76 . Plunger  80  can include a built-in sealing mechanism, or have a radial seal  82  and respective seal grove formed in the plunger  80 . To retain the plunger  80  within the lubricant chamber  76 , a snap-ring  84  can be disposed in a groove in the proximal end of the lubricant chamber  76 . The inner diameter of the snap ring is preferably sized to restrict the passage of the plunger  80 . All radial dynamic seals leak. By pressurizing the lubricant in the chamber  76  with substantially the same pressure as the drilling fluid, or more specifically, the pressure inside the drill string at first fluid port  78  imparted by the drilling fluid, there is essentially no pressure differential between the lubricant and environmental side of the radial seal  82  of plunger  80 . A lubricant chamber  76  so pressurized will not abruptly discharge the lubricant past the radial dynamic seals ( 72 ,  74 ) when the plunger  80  is acted on by the pressure of a circulating drilling fluid. For example, an unpressurized lubricant chamber  76  can lead to seal blow-out or failure as there is no additional force to counteract the high pressure imparted to the plunger  80 , and thus the lubricant, by the drilling fluid. The balancing of the pressure on chamber  76  thereby prevents the ingress of drilling fluid into the bearing surfaces, prolonging bearing life for this single cone nutating drill bit. 
   As shown more readily in the view of the proximal end  16  of the bit  10  in  FIG. 2 , a second lubricant chamber  76 ′ can further be formed in bit shank  12 . The second lubricant chamber  76 ′ can be in communication with at least one bearing ( 26 ,  28 ,  30 ,  32 ) by a channel formed between a distal end of the second lubricant chamber  76 ′ and a portion of the axially skewed bore  18 , for example, a location proximate to a bearing ( 26 ,  28 ,  30 ,  32 ) or through a second retainer plug and plug port (not shown) in the manner discussed above. The second lubricant chamber  76 ′ can be in communication with at least one bearing ( 26 ,  28 ,  30 ,  32 ) by boring both lubricant chambers ( 76 ,  76 ′) at a slightly acute angle so that the distal end of each lubricant chamber ( 76 ,  76 ′) is in communication with a single retainer plug port  60 , which itself is in communication with the ball bearing race ( 56 ,  58 ). 
   The nutating single cone drill bit  10  also includes a fluid passage  90  formed therethrough to allow the passage of drilling fluid. The fluid passage  90  including a first section  86  of fluid passage  90  through the bit shank  12  in communication with a fluid port  92  located on the proximal end  16  of bit shank. The first fluid passage section  86  is in communication with a port in proximal end  69  of the axially skewed bore  18 . The cutter body  22  has a second section  88  of the fluid passage  90  formed therein. The second fluid passage section  88  is in communication with a port in the proximal end  36  of the cutter body  22 . The distal end of the second fluid passage  88  is in communication with at least one low pressure orifice  94  formed on the distal end  42  of the cutter body  22 . Any fluid pumped through an attached drill string will thus flow into the fluid port  92 , through first fluid passage section  86 , into the axially skewed bore  18 . As radial dynamic seal  74  forms a fluidic seal between a proximal end  36  of the cutter body  22  and the axially skewed bore  18 , the fluid will flow into second fluid passage section  88  and be discharged from at least one low pressure orifice  94  formed in the cutter body  22 . 
   In use, the lubricating nutating single cone drill bit  10  is attached to a drill string at the drill string connection  14  on the proximal end  16  of the bit shank  12 . The bit shank  12  can include an optional bit breaker slot  96 , shown as a dotted line, formed in the outer surface to permit the engagement and disengagement of a bit  10  and a drill string. The nutating single cone drill bit  10  can then be engaged into a formation, as is known the art. The orientation of the cutter elements ( 46 ,  48 ) and the axially offset geometry of the cutter body  22  with respect to the axis  20  of the bit shank  12  enables a portion of cutter or crushing elements ( 46 ,  48 ) to contact the well bore (WB) while the adjacent section of cutter or crushing elements does not contact the well bore (WB). Such a configuration can minimize or eliminate the dragging of the cutter or crushing elements ( 46 ,  48 ) across the opposing face of the well bore (WB) and thereby reduce the wear experienced by the bit  10  overall. The rolling nutating action of the present bit  10  offers low resistance to the rotational movement of the drill string, and thus provides a much lower operating torque that allows for operation at a higher rotational speed as compared to a typical drill bit. Further, the superior crushing action of the cutter elements ( 46 ,  48 ) abates the need to use a high pressure orifice or jet, as is typical in the art. The present lubricating nutating single cone drill bit  10  offers the benefit of using low pressure orifices  94  in the cutter body  22  and/or operating at a lower pump pressure as the bit  10  does not drill or fluidically abrade the well bore (WB) with typical high pressure jetting, but merely uses the mud fluid flow to cool and lubricate the crushing or cutting action of the nutating bit  10  and carry off cuttings from the face of the wellbore WB. Fluid passage  90  and low pressure orifice  94  can be used to supply a drilling fluid at a lower pressure and/or flow rate to remove the cuttings from the well bore floor and/or cool the bit  10 . A high pressure jet can be used if so desired in replacement of low pressure orifice  94 . 
   The nutating single cone drill bit  10  can then be rotated and loaded to drill the formation as is known to one of ordinary skill in the art. The drilling fluid is pumped down the drill string and thus into contact with the proximal surface  16  of the bit shank  12 . The drilling fluid can force a plunger  80  into its respective lubricant chamber  76 . The speed of displacement of the plunger  80 , and thus the lubricant, can depend on many elements, including, but not limited to, the viscosity of the lubricant, the size of the gap between the cutter body  22  and the axially skewed bore  18 , the size of the lubricant chamber  76 , the spacing and/or number of bearings ( 26 ,  28 ,  30 ,  32 ), the number of lubricant chambers ( 76 ,  76 ′), the drilling fluid pressure at the plunger  80 , the pressure on the exterior of the bit  10 , the pressure of the lubricant in the lubricant chamber ( 76 ,  76 ′), the amount of leakage, if any, allowed by the radial dynamic seal ( 72 ,  74 ), etc. Seepage or leakage at the seals ( 72 ,  74 ), although not required, can aid lubrication by allowing a lubricant to continuously flow to the gap between the cutter body  22  and the axially skewed bore  18 , and thus the bearings ( 26 ,  28 ,  30 ,  32 ). Independent of any seepage or leakage, by utilizing a lubricant, for example, a viscous grease, and a lubricant chamber ( 76 ,  76 ′), bit  10  can be used for an extended period of time as compared to a bit with no lubricant chamber. The lubricant chamber ( 76 ,  76 ′) allows for longer periods of use of a nutating single cone drill bit without repacking lubricant and/or replacing any bearings ( 26 ,  28 ,  30 ,  32 ). 
   While the invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of the invention.