PRECISION ROLLER BEARING FOR ROCK BITS

Abstract: A sealed bearing roller cone rock bit (10) is disclosed having precision roller bearings (44) with very close fits. The bearing package includes highly refined and hardened surface finishes, special roller bearing metal materials and lubricants with specific lube viscosities.

Description: This invention relates to sealed bearing rotary cone rock bits. More particularly, this invention relates to precision roller bearings for seal bearing rotary cone rock bits. Roller bearings for rotary cone rock bits have been in the public domain for many years. Roller bearing "air" bits are typically used in open pit mines where a multiplicity of shallow holes are formed in the earth formations. These types of roller bearing rotary cone bits usually are non-sealed bits that rely on a gaseous fluid to lubricate the bearings and to flush the bearings of debris during use of the bit in a borehole. These types of mining bits utilize cylindrical roller bearings that are loosely confined (large bearing clearances) by bearing races formed between a journal and rotary cutter cone. These open-type bearings encourage the flushing of detritus from the bearing cage by the gaseous fluid pumped under pressure through a connecting drill string. Prior art US Patents 2,030,442, 3,990,525, 4,255,000, 4,359,113 and 4,386,668 represent rock bits that operate under the foregoing parameters. Sealed bearing rotary cone rock bits that incorporate cylindrical type roller bearings are typically run in a deep well borehole with drilling fluid or "mud" to move the debris from the borehole bottom of the rig floor. The following US Patents 4,249,781, 4,256,193, 4,263,976 and 4,428,687 teach sealed lubricated roller bearings for rotary cone rock bits. The prior art does not teach a precision roller bearing having very close fits suitable for a rock bit. The bearing package of the present invention includes highly refined surface finishes that complement extremely hard roller bearing elements fabricated from special bearing metals. A specific lubricant is utilized having lube viscosities that are unique to the bearing combination to be described hereinafter. According to the present invention there is provided a sealed bearing rotary cone rock bit comprising: a rock bit body having a first pin end and a second cutter end, the body having at least one leg extending from the body toward the second cutter end, the leg having a journal adapted to support a rotary cutter cone rotatively secured thereon, the cone having a cylindrical cavity to accept the journal; and a roller bearing assembly between a bearing surface on the journal and the cylindrical cavity in the cone comprising a multiplicity of cylindrical bearings therebetween, and characterized in that the roller bearing assembly includes an inner race mounted on the journal bearing surface, and an outer race in the cylindrical cavity in the cone with the cylindrical bearings therebetween and wherein the diametrical clearance between an outside surface formed by an inner race, an outer surface defined by the multiplicity of cylindrical bearings and an inside surface formed by the outer race being between plus 100 micrometers and minus 50 micrometers, and the diametrical clearance between an outer surface formed by the journal bearing and an inside surface in the inner race being between zero and 175 micrometers. The utilization of tight roller bearing fits for rock bits wherein there is a minimum clearance between an inner race, the roller bearings, and an outer race adjacent, for example, a cone mouth opening, is particularly unique. Moreover, a bearing combination wherein bearing surface finishes less than 0.5 Ra micrometer coupled with high cleanliness steel bearing elements with a Rockwall C hardness of 58, the bearings being lubricated with grease having a viscosity greater than 100 centipoise at 100°C is especially unique in the rock bit industry. An advantage then, over the prior art, is a bearing combination that provides a sealed precision roller bearing having tight fits with highly refined and hardened surface finishes in combination with lubricants having specific lube viscosities. Sealed bearing rotary rock bits embodying the present invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which: Fig. 1 is a perspective view of a typical rotary cone rock bit; Fig. 2 is a partially broken away cross-section of a roller cone mounted to a journal bearing cantilevered from a leg of a rock bit taken through 2-2 of Fig. 1; Fig. 3 is a view taken through 3-3 of Fig. 2 illustrating the roller bearings mounted within a cone mouth of the roller cone; and Fig. 4 is a partially broken away cross-section of an alternative roller bearing assembly for a sealed bearing roller cone rock bit. Fig. 1 illustrates a typical rotary cone rock bit generally designated as 10 which consists of a rock bit body 12, a pin end 14 and a cutting end, generally designated as 16. The cutting end comprises rotatable cutter cones 22 that are attached to a leg portion 18 near shirttail 20. Each of the cones 22 has, for example, a multiplicity of cutter inserts 24 retained by the cone 22. Drilling fluid commonly known as "mud" is directed into a plenum chamber (not shown) formed by body 12 through pin end 14. The fluid or mud is then directed from the chamber out nozzles 28. Turning now to Fig. 2 the leg 18 of the bit 10 defines a journal 34 cantilevered from the shirttail end 20 of leg 18. A spindle 36 is formed on the end of the main journal 34, the spindle acting to stabilize the rotary cone 22 mounted on the journal 34. The cone 22 forms a cone mouth 40 which transitions into a bearing retaining groove or race 42. The cone 22 is secured to the journal 34 by a multiplicity of cone retention balls 39. A ball race 38 is formed within the cone 22 which registers with a similar ball race 31 formed in the journal 34. Balls are inserted through a ball hole 49. When the ball races 31 and 38 are full of ball bearings 39, a ball hole plug 43 is inserted in the ball hole 49 to close out the ball race 31. A lubricant is supplied to the bearings through a lubricant reservoir 26 (Fig. 1) down lube hole 52 into the ball plug hole 49. A groove 53 is provided in the ball plug 43 to enable the lubricant to reach the bearings confined between the cone 22 and the journal 34. cone 22. The O.D. 54 of the inner bearing race 48 is sized to tightly engage the surface 45 of roller bearing 44 thereby tightly engaging the cylindrical bearings with the race 42 formed in cone 22. The diametrical clearance between surface 54 and surface 42 is between plus 100 micrometers and minus 50 micrometers inch. The preferred range is plus or minus 25 micrometers. The material surface hardness of all of the bearing elements including the outer surface of the inner race 48, the roller bearings 44, and the outer race 42 formed in cone 22 is within a Rockwell C hardness range between 58 Rc and 65 RC. A preferred range for the material hardness is 60 to 62 Rockwell C. The surface finish of all of the bearing elements are as follows: for the inner race outer surface 54, the surface should be less than 0.8 Ra (roughness average) micrometers, the inner surface of the outer race 42 should be less than 0.8 Ra micrometers and the surface of the roller bearings should be less than 0.6 Ra micrometers. It would be desirable that the surface finish of all of the bearing elements be within 0.25 Ra micrometers one from the other. The preferred range of surface finish of the bearing elements are as follows: inner surface of the outer race should be less than 0.5 Ra micrometers, the outer surface of the inner race should be less than 0.4 Ra micrometers and the roller bearing should be less than 0.3 Ra micrometers. The clearance between the outside surface 35 of journal 34 and the inside diameter 55 of the inner race 48 is preferably 100 micrometers. A particularly preferred clearance is about 25 micrometers. The ends 47 of roller bearings 44 are prevented from engaging or binding against the leg hub 57 and axial surface 56 formed in cone 22 by the length of the inside bearing race 48. A seal is provided between the cone mouth 40 and the journal bearing 34. The seal 50 rides on a seal gland 41 formed in the rotary cone 22. The inside diameter of the seal 50 rides in a complementary seal gland 51 formed at the base of the journal 34. Additionally, the roller bearings 44 are preferably crowned along their axial length at 46 thus preventing stress risers at either end of the roller bearings which may exist if the cone 22 should become misaligned with the journal 34 or heavily loaded. By crowning the roller bearings at 46, stress risers are minimized within the bearing package. It would be equivalent to crown the O.D. of the inner bearing race and the I.D. of the outer bearing race. Moreover, the bearing elements are preferably fabricated from a high cleanliness steel which assures uniform quality of the elements. The steel should be free of inclusion or contaminants. In determining the nonmetallic inclusion content of steel, a number of recognized methods are used. For example, the 1985 Annual Book of ASTM STANDARDS , Volume 03.03 Metallography; Nondestructive Testing , sets forth on pages 61 through 74, several of these methods. Typical nonmetallic inclusions in steel are: A) sulfide, B) alumina, C) silicate, and D) globular oxides. A "J. K." (JERKONTORET) chart categorizes these inclusions in columns and rates the inclusions from 1 to 5; 1 being low in content and 5 is high in content. An acceptable J. K. rating would be a range from 1 to 3. The preferred J. K. rating, of course, would be 1 for the steel used to fabricate the bearing elements of the present invention. The lubricant, or grease, utilized to lubricate the bearings should have a lubricant viscosity of greater than 100 centipoise, a preferred lubricant would have a viscosity of 500 to 5,000 centipoise (Cp) at 100 degrees Centigrade. An example of a lubricant having these properties is manufactured by Dow Chemical Company of Midland, Michigan, and has a product designation of FS-surfaces and a highly refined surface finish, the longevity of the rock bit in a borehole is considerably enhanced. Referring now to Fig. 3, the cross-section taken through Fig. 2, illustrates the cone 22 mounted to journal 34 with the bearing 44 confined between hardened and polished surface 42 in cone 22 and the O.D. hardened surface 54 of inner race 48. The preferred fit, again, between the hardened surface 54, the surface 45 of roller bearings 44, and the hardened surface 42 of cone 22 is preferably plus or minus 25 micrometers. The diametrical clearance between surface 56 of journal 34, and I.D. 55 of bearing race 44, is preferably about 25 micrometers. The ball plug hole 49 is also illustrated in this view showing the ball plug 43 and the grease passages 48 formed in the ball plug 43 to allow lubricant from the lubricant reservoir to the bearing package. Turning now to an alternative embodiment illustrated in Fig. 4, a rotary cone 122 is mounted to a journal 134 which is cantilevered from a leg 118 adjacent shirttail portion 120. The leg portion 118, is shown with a lubricant reservoir system generally designated as 126. This type of lubricant reservoir is typical of the preferred embodiment of Figs. 1, 2 and 3. The lubricant reservoir system represents the state of the art and typically includes a flexible diaphragm 130 mounted within a reservoir chamber which is held in place by a cover cap 128. The diaphragm is exposed to the exterior of the rock bit so that it may be pressure compensated between the interior cavities of the bit and the outside environment of the bit. Lubricant 131 is forced into a space beyond the interior of the diaphragm and is directed through lubricant passage 132 found in the leg 118 towards a ball hole plug 133. Groove 156 in a ball plug 155 allows the lubricant to pass into the ball race 138 in cone 122 so that the lubricant can be routed to the cone retention ball race 138, balls 139 and the roller bearing package. A cone mouth 140, is formed in the rotary cone 122 which leads into a bearing retention groove 141 formed in cone 122. A bearing package which includes an inner race 142, roller bearings 145, and outer bearing race 147, completes the bearing package. The outer bearing race 147 is designed to be interference fitted or press fitted within bearing grove 141, formed in cone 122. The diametrical clearances between the outside diameter surface 144, of inner race 142, the surface 146 of roller bearing 145 and the inner or I.D. surface 148 of outer race 147 is between a plus 100 micrometers to minus 50 micrometers. The preferred range, or clearance, is plus or minus 25 micrometers. Again, the roller bearings 144 may be crowned at 154 to prevent stress risers at the cylindrical ends of each of the roller bearings, thus accommodating for slight misalignments between the cone 122 an the journal 134. The roller bearings are prevented from binding on their ends by the length of the inner and outer bearing races 142 and 147. After the bearing package is press fitted within circular recess groove 141, the bearing is retained within the cone by a bearing retainer ring 150. The ring 150 is preferably pressed within the annular bearing retainer groove 151 positioned within the cone mouth 140 of cone 122. The annular bearing retainer ring has formed on its inside diameter a seal gland 153 to support a resilient O-ring 152 placed between the seal gland 153, and a complementary gland formed at the base of the journal bearing 134. The annular bearing retaining ring is preferably metallurgically bonded to the cone 122 at the intersection of the O.D. of the ring and the annular groove 151. It would be equivalent, of course, to thread the O.D. of the bearing retainer 150 (not shown). The threaded ring would then be secured within complementary threads formed in the cone 122 (not shown). The clearance between the surface 135 of journal 134, and the I.D. 143 of inner bearing race 142, should be about 100 micrometers. It would be equivalent to eliminate the inner bearing race 48 of Fig. 2 and finish surface 35 of journal 34 as set forth in the foregoing specification. Roller bearings 44 would then be confined within surface 45 formed by the cone 22 and surface 35 formed by journal 34 without departing from the intent of the present invention.

Claims: A sealed bearing rotary cone rock bit comprising: a rock bit body (12) having a first pin end (14) and a second cutter end (16), the body having at least one leg (18) extending from the body (12) toward the second cutter end (16), the leg (18) having a journal (34) adapted to support a rotary cutter cone (22) rotatively secured thereon, the cone (22) having a cylindrical cavity to accept the journal (34); and a roller bearing assembly between a bearing surface on the journal (34) and the cylindrical cavity in the cone (22) comprising a multiplicity of cylindrical bearings (44) therebetween, and characterized in that the roller bearing assembly comprises an inner race (48) mounted on the journal bearing surface, and an outer race (42) in the cylindrical cavity in the cone (22) with the cylindrical bearings (44) therebetween and wherein the diametrical clearance between an outside surface (54) formed by an inner race (48), an outer surface (45) defined by the multiplicity of cylindrical bearings (44) and an inside surface (42) formed by the outer race being between plus 100 micrometers and minus 50 micrometers, and the diametrical clearance between an outer surface (35) formed by the journal bearing (34) and an inside surface (55) in the inner race (48) being between zero and 175 micrometers. A rotary cone rock bit as set forth in Claim 1, characterised in that the clearance between the outside surface (54) of the inner race (48), the surface (45) of the cylindrical roller bearings (44) and an inside surface of the outer race (42) is less than plus or minus 2.5 micrometers. A rotary cone rock bit as set forth in either of Claims 1 or 2, characterised in that the bearing surfaces of the metal bearing elements of the roller bearing assembly have a surface finish between 0.5 and 0.8 Ra micrometers. A rotary cone rock bit as set forth in Claim 3, characterised in that the surface finish is less than 0.5 Ra micrometers. A rotary cone rock bit as set forth in any of the preceding claims, characterised in that the metal used to form said bearing elements is steel that has a surface hardness range between 58 and 65 Rockwell C. A rotary cone rock bit as set forth in Claim 5, characterised in that the surface hardness range is between 60 and 62 Rockwell C. A rotary cone rock bit as set forth in any of the preceding claims, characterised in that the bearing elements are fabricated from a steel having high cleanliness with a J. K. rating from 1 to 3. A rotary cone rock bit as set forth in Claim 7, characterised in that the steel cleanliness J. K rating is 1. A rotary cone rock bit as set forth in any of the preceding claim, characterised in that each of the cylindrical bearings (44) is crowned along its axial length, a substantially central portion along the axial length of the cylindrical roller bearing being larger in diameter than respective ends of the cylindrical bearing (44). A rotary cone rock bit according to any one of Claim 1 to 8 characterised in that the inner and outer roller bearing races (48 and 42) are crowned along their axial length, the diameter of a substantially central portion of said outer surface of the inner race (48) being greater than the diameter of opposite ends of the inner race (48), the diameter of a substantially central portion of the inner surface of the outer race (42) being less than the diameter at each end of the outer race (42). A rotary cone rock bit as set forth in any of the preceding claims, characterised in that a lubricant used to lubricate the roller bearing assembly has a lube viscosity of greater than 100 centipoise at 100°C. A rotary cone rock bit as set forth in any of the preceding claims, characterised in that the assembly misalignment between the journal bearing and the roller cone (22) is less than .001 radian. A rotary cone rock bit according to any preceding claim, characterised in that the outer race (42) in the cone (22) comprises an outer bearing race insert (147) in the cavity in the cone (22).

Inventors: CROCKETT, DAVID PRESTON, ERPENBECK, GERRARD PAUL, HOOPER, MICHAEL E.
Applicants: SMITH INTERNATIONAL, INC.
Application year: 1988
Main classification: E21B10
IPC codes: E21B10, F16C33