GB2168737A - Improved bearing system for a roller cone rock bit - Google Patents

Abstract

A roller cone rock bit is disclosed with an improved bearing system. The improvement comprises a journal bearing (41) which is substantially frusto-conically shaped and a roller cone bearing (42) which is reverse shaped to mate with the journal bearing. The invention also includes a means (43) for maintaining the journal and roller cone bearing in compression against each other. In the preferred embodiment, the journal and roller cone bearings comprise polycrystalline diamond and the compression is maintained by a threaded bolt (43) which is tightened to the point of putting the bolt in tension. In one preferred embodiment, the bolt is threaded into the cone and rotates in the center of the journal. In another preferred embodiment, the bolt is threaded into the leg (34) of the journal and the cone rotates about it. In both of these embodiments, thrust bearings (45, 46) are located between the head of the bolt and the leg or cone surface contacted. The bolt's threads can either be locked in place, or allowed to self- tighten as the cone rotates during drilling. The invention has application in extending the life of bits used in oil, gas and geothermal drilling operations. <IMAGE>

Description

SPECIFICATION Improved Bearing System for a Roller Cone Rock Bit The present invention relates generally to the field of earth boring bits and, more particularly, to roller cone rock bits.

Roller cone rock bits generally comprise a main bit body which can be attached to a rotary drill string. The bit body usually includes two or three legs which extend downward. Each leg has a journal extending at a downward and inward angle. A roller cone, with cutters, "teeth", or ridges positioned on its outer surface, is rotatively mounted on each journal. During drilling, the rotation of the drill string produces rotation of each roller cone about its journal thereby causing the cutter elements to engage and disintegrate the rock.

Because of their aggressive cutting action and resultant faster penetration rates, roller cone rock bits have been widely used for oil, gas, and geothermal drilling operations. However, certain problems exist which limit the useful life and effectiveness of roller cone rock bits. The useful life of a rock bit is an especially critical consideration when viewed in light of the great expense in time and money required to remove and replace the entire drilling string because of bit failure.

The bea.rings used between thejournals and the cones are the source of significant problems. These bearings operate in an extremely hostile environment due to high and uneven loads, elevated temperatures and pressures, and the presence of abrasive grit both in the hole cuttings and the drilling fluid. This is particularly true when drilling deep holes. In addition, some rock bits such as those used in geothermal exploration are subject to corrosive chemical environments. Another factor which can lead to early bearing failure is the inability of the bearings to withstand changes in the moment of forceps directed against the roller cone. As the inserts on the gage row gradually wear down, the sides of the hole become less defined. As a result, the forces from the side of the hole increase.These increased side forces tend to push the cone off its original axis of rotation, thereby "pinching" the bearings in their races and contributing to early bearing failure.

Unfortunately, these extreme conditions often cause failure of the roller cone bearings before any part of the bit, even before the cone's cutters. In addition, as the bearings wear, they can allow for more "wobble" of the cones. As a result, a roller cone bit with worn bearings does not track as well in the hole and has a reduced penetration rate. Also, these limits on the bearing's capacity in turn limit both the load which can be applied to the bit as well as the angular velocity at which the bit can be rotated, thereby establishing constraints on achievable penetration rates and feasible cutter designs.

In some of the earlier roller cone bit designs, the bearing structure was relatively simple. For example, U.S. Patent Nos. 1,649,858 and 1,909,078 both show a roller cone rock bit with a frustoconically shaped friction type bushing located between the journal and the cone. Such bearings had a relatively short life expectancy, but so did the other components of the early bits. However, as harder and longer lasting materials such as cemented tungsten carbide began to be used for some of the other components of the roller cone bits, and as these new bits were used to drill increasingly deeper holes through harder materials, several changes were made to improve the bearing's capacity to handle higher loads for longer periods of time.

At present, the typical bearings between the journal and roller cone consist of combinations of anti-friction and friction bearings. The anti-friction bearings (such as ball or roller bearings) are used to facilitate rotation, absorb radially directed forces, and often to retain the roller cone on the journal.

The friction bearings are used both for "thrust" bearings where the mating surfaces are disposed perpendicular to the axis of the journal and absorb axially directed forces as well as for radial bearings where the mating surfaces are disposed parallel to the axis of the journal and absorb radially directed forces.

Lubricants and coolants are frequently used to increase the life of bearings. One design uses a grease to lubricate the bearings. In bits using such a lubricant, it is necessary to seal the bearings in order to preserve the grease and keep out the drilling mud and rock cuttings. Most of these bits also incorporate a lubricant reservoir and pressure compensator to allow for lubricant loss as well as the high temperatures the bit may encounter in drilling deep holes. For example, see U.S. Patent Nos. 3,397,928; 3,476,195; and 4,061,376. Naturally, each of these features adds complexity in bit design and fabrication usually requiring the cones to first be mounted or "pinned" on the journals on separate legs after which-the legs are welded onto a main bit body.In addition, because of the harmful effects of high temperature on the seals and lubricants, the maximum rotational speed at which these sealed bearing bits are operated is often limited.

Bearing life is also increased by utilizing harder, more wear-resistant materials. For example, most bits now employ carburizing, hard facing, or special metal inlays for the bearing surfaces, all of which increase the complexity and cost of fabrication. See, for example, U.S. Patent No.4,054,426. In addition, U.S. Patent No. 4,190,301 teaches the use of a pair of opposing polycrystalline diamond compacts for the "nose" thrust bearing.

Also, U.S. Patent No. 4,260,203 teaches the use of radial and thrust bearing surfaces consisting of polycrystalline diamond. Although the use of polycrystalline diamond bearing surfaces may be an improvement over the use of other materials, the design of the '203 bit does not account for certain properties inherent in polycrystalline diamond. In particular, it has been the inventor's observation that polycrystalline diamond, althoughvery strong in compression, is not very strong in tension.

Because the '203 design has both radial and axial bearings set perpendicular to each other it will subject the polycrystalline diamond to substantial tensile forces. That is, as the thrust bearings are put in compression, the radial bearings are put in tension. Also, the patent does not disciose a method of retaining the cone on the journal other than the conventional method with ball bearings which appears to leave a weak point in the bearing system.

U.S. Patent No. 4,145,094 shows a somewhat simplified bearing system for roller cone rock bits which does not use ball bearings to retain the cone on the journal. In that design, the cone is retained on the journal by electron beam welding an annular thrust member onto the journal after it has been inserted into the cone, thereby "plugging" the cone onto the journal.

According to the present invention there is disclosed a roller cone rock bit comprising: a main bit body with at least one downwardly extending leg, said leg having a journal thereon, said journal having a journal bearing comprising a substantially frusto-conically shaped bearing surface; a roller cone rotatively mounted on each journal, said roller cone having a roller cone bearing with a surface which mates with the frusto-conically shaped bearing surface ofthe journal bearing; and means for maintaining the two bearings in a state of compression against each other The invention also discloses a means for preloading and maintaining compression between these two bearing surfaces.An advantage of the present invention is to provide an improved bearing system for a roller cone rock bit which will perform for longer periods of time, at higher rotational speeds and with higher applied loads.

In the preferred embodiment, the compression means comprises an elongate pin which is attached at one end to the cone, extends through the center of the journal, and has a feature at the other end with a thrust bearing surface contacting a thrust bearing surface on the leg of the bit. In simplest form, this pin is a bolt which is threadably attached in the cone, the bolt rotates within the journal, and there is a pair of thrust washer type bearings between the head of the bolt and an outside surface of the leg.

In a second preferred embodiment, the means comprises an elongate pin which is attached at one end to the journal or leg, extends through the cone, and has a feature at the other end with a thrust bearing surface to mate with a thrust bearing surface located on the outside of the cone.

In the preferred embodiment, the journal, roller cone, and thrust bearings comprise polycrystalline diamond. The preferred embodiment also includes means for passing a stream of the drilling fluid over the bearing surfaces. This results in beneficial cooling and lubricating of the bearings.

The present invention will be more clearly understood from the following detailed description of the specific embodiments, by way of example only read in conjunction with the accompanying drawings, wherein: Figure 1 is a perspective view and partial cross section of a typical prior art roller cone rock bit, Figure 2 is a sectional view illustrating a first preferred embodiment roller cone bit of the present invention with a frusto-conical bearing interface between the journal and the roller cone, Figure 3 is a perspective view of a frusto-conically shaped journal bearing suitable for use in the embodiment of Figure 2, Figure 4 is a schematic diagram showing a sectional view of a pair of frusto-conical bearings with the application and resolution of forces exerted thereon, Figure 5 is a sectional view of a second preferred embodiment wherein the retaining means is attached to the leg of the bit.

Referring to the drawings, Figure 1 is a perspective view of a typical prior art roller cone rock bit 11. The bit is comprised of a main bit body 12 which has an end 13 adapted to be threadably attached to a drill string (not shown). Extending from the main bit body are three legs 14, each of which have a journal 15 formed thereon. A cone 16 which has cutters 17 is rotatively mounted onto each journal 15. The cone is retained on the journal 15 by a set of ball bearings 18 which fill a race cut into the journal and the inside of the cone. In fabrication, the ball bearings are loaded into the race through passage 19. This passage is subsequently sealed by plug 20 and then serves to supply lubricant to the ball bearings 18 and roller bearings 21 as it communicates with passage 22 which in turn communicates with lubricant reservoir 23.A pressure compensator 24 is located with the reservoir 23 to account for higher pressures and temperatures as well as lubricant loss. An elastomeric seal 25 is located between the cone and the journal and serves to keep the lubricant in and the drilling debris out of the bearing system. Thrust bearing 27 serves to absorb and transmit dorces directed axially to the one while roller bearings 21 and annular bearing 28 serve the same function with respect to the forces directed radially to the cone.

Figure 2 is a sectional view showing a first preferred embodiment of the present invention. The embodiment is a roller cone rock bit comprising a main bit body 31 which has an end 32 which is adapted to be threadably attached to a drill string 30. A leg 34 extends down from the main bit body.

The preferred embodiment also includes two other legs not shown which would be equally spaced around the bit body. Bits have been designed with one, three, three or more legs; such bits are within the scope of this invention. Ajournal 35 is formed on the leg 34 and extends at a downward and inward angle. A roller cone 36 is rotatively mounted on the journal 35. Cutters 37 are mounted in the roller cone for engaging the bottom of the hole. One advantage of the present invention is that by simplifying the bearing structure, the roller cone may be made thicker, thereby allowing for deeper pockets for the cutters 37. In alternative embodiments the roller cones may have integral teeth or annular ridges for engaging material. Gage inserts 38 are mounted on the gage row of the cone to engage material on the side of the hole and serve the important function of maintaining a constant diameter of the hole. As this gage row wears, forces from the side of the hole in the direction indicated by arrow C increase and otherwise tend to push the cone off its original axis of rotation.

Mounted on the journal 35 is a frusto-conically shaped bearing 41. It is noted that when using the phrase "frusto-conically shaped", it is intended to refer to the curved side surface of a frustum and not the top or bottom. This bearing 41 is also illustrated in Figure 3 and further details of its structure in the preferred embodiment as well as alternative embodiments are discussed in connection with that figure. At least a portion of the curved outer surface of the journal bearing surface consists of polycrystalline diamond. As used herein, the term "polycrystalline diamond" and its abbreviation "PCD" refer to a material comprising diamond crystals which have been sintered at ultra high pressure and temperature resulting in a mass of randomly oriented crystals which are substantially directly bonded to adjacent crystals.Also because the properties of polycrystalline cubic boron nitride (pcCBN) are quite similar to those of PCD, much of the discussion of PCD will also apply to pcCBN. For reasons that will be discussed later, PCD is the most preferred material. A mating frusto-conical bearing 42 is mounted within the roller cone 36. In the preferred embodiment, the mating surface of th is roller cone bearing 42 also comprises polycrystalline diamond.

A bolt 43 is threadably attached at its end 44 to the roller cone 36. The bolt 43 will accordingly rotate with the cone 36. In one preferred embodiment, a locking pin 61 may be inserted through the passage 62 to lock the bolt in place, thereby preventing rotation of the bolt relative to the cone which would otherwise result from friction between the thrust bearing surfaces 45 and 46. The bolt may also be prevented from rotating in the cone 36 using other methods such as welding, application of locking compounds, peening, etc.

In yet another embodiment, the inventor discovered the surprising result that when the journal bearing 41 and roller cone bearing 42 are comprised of PDC, it is possible to allow the bolt to remain unlocked and thereby be self-tightening, without having the bearings bind. In particular, instead of causing the bearings to bind, the selftightening of the bolt serves the beneficial function of maintining the bearings in a state of compression against each other while compensating for wear.

Accordingly, it may be desirable in the preferred embodiments to permit this self-tigthening of the bolt. It is deemed within the ordinary skill in the art to select a pitch for the threads which would avoid placing too much stress on the bolt and maximize the benefits ofthis self-tightening action.

The bolt 43 extends through a hole 54 in the journal 35 and has a head 47 which is widerthan the hole 54. This head 47 is adapted to receive a tool for tightening such as a hex or allen wrench. Adjacent to the head 47 of the bolt 43 is a thrust bearing or washer 46 which mates with a second thrust bearing 45 which is in turn adjacent to the leg 34. Each of these thrust bearings 46 and 45 may either be bonded in their position (i.e. to the head 47 or leg 34 respectively) or may be non-attached. As can be seen, a recess 56 in the leg 34 is adapted to receive the thrust bearings 45 and 46 along with the head 47. It may also be desirable to secure a cover over this recess 56 orfill it with a filler material such as epoxy for protection during drilling. In the preferred embodiment, the mating bearing surfaces of the thrust bearings 45 and 46 comprise polycrystalline diamond.Also, in the preferred embodiment, the thrust bearings have a substantially planar interface.

In alternative embodiments it may be desirable to shape the head 47 and bearings 45 and 46 to provide a frusto-conical interface between the thrust bearings.

It will be observed that a space 57 is left between the distal end surface 59 of the journal bearing 41 and the internal surface 58 of the roller cone 36.

Also, the proximate end surface 60 of the roller cone 36 does not contact the journal 35 or the leg 34.

These two features are important because they allow the roller cone bearing 42 to be freely compressed against the journal bearing 41. In other words, in the preferred embodiment, the frustoconical journal bearing is the only surface on the journal or leg which prevents the roller cone from moving axially in the direction indicated by arrow B.

The inventor has discovered that - using this arrangement, the frusto-conical bearings anci the bolt with its thrust washers cooper 7 produce at least two important benefits. First, tightening the bolt allows one to put the frusto-conical bearings in pre-compression. In particular, the bolt is tightened sufficiently to put it in tension and thereby put the journal and roller cones in a state of compression.

This state of compression is highly beneficial when the journal and roller cone bearings comprise polycrystalline diamond. It is the inventor's observation that PCD, although very strong in compression, is relatively weak in tension.

Therefore, if one is to use it as a bearing, it is best for it to be pre-loaded with compressive forces in order to minimize the tensile forces the PCD will experience. In other words, the pre-compressed bearing is less likely to exhibit bearing "chatter" which may be deleterious to the PCD structure. This is especially true for this roller cone bit application where during drilling the PCD bearing faces uneven loads from multiple directions.

Secondly, this configuration improves tile cone's ability to maintain its original axis of rotation. This is true because of the fact that the combination of the frusto-conical bearings and the thrust bearings presents a triangular cross-section taken through the axis of rotation (i.e. the opposite sides of the roller cone/journal bearing interface and the interface between the thrust bearings make up three sides of a triangle). This means that when force is applied to the roller cone from any direction, there is a part of one of the bearing surfaces to which the force will have a substantial normal component.For example, as the cone experiences increased forces from the side of the hole due to gage wear (in the direction indicated by arrow C in Figure 2), the thrust bearing interface is compressed and the roller cone bearing does not pinch and is not allowed to lift off of the journal bearing.

In another embodiment, the distal surface 59 of the journal 35 does contact the surface 58 of the cone 36. The dimensions are carefully selected so as to allow the surfaces 58 and 59 to contact yet still allow for sufficient compression between the journal and roller cone bearings. In this embodiment, it is desirable to also include PCD over at least a portion of surfaces 58 and 59. Allowing the two surfaces 58 and 59 to contact would provide protection for the bearing against excessive axial loads encountered during drilling. The same result can also be achieved by ailowing the heel surface 60 of the cone 36 to contact the leg 34 of the bit.

Providing this "backstop" to protect the journal and roller cone bearings against excessive axial loads may be desirable in bits intended for use in heterogenousformations orwith high loads from the drill string. These embodiments may also be useful in setting the exact amount of pre-load on the bearings as the bolt could be tightened to where the surfaces contact and then backed off.

It should be noted that the preferred embodiment does not have this "backstop" to protect the journal and roller cone bearing against axial loads. The inventor was surprised to discoverthatthis bearing system with this particular geometry does operate well with extreme axial loads. As discussed in more detail in connection with Figure 4, it was thought that the large axial loads would cause the frustoconical bearing to bind if not provided with a separate thrust bearing to absorb these axial loads.

To the contrary, the inventor has found that the high axial loads are beneficial to the bearings operation.

In yet another embodiment, instead of using a bolt to retain the cone 36 on the journal 35, a post is either attached to the cone in the same position or is instead integral with the cone. This post has threads on the end which extend into the recess 56 of the leg 34. A nut is then threaded onto that post with two thrust washers between it and the leg. In yet another embodiment, a hole passes through both the journal and the cone and a bolt with threads on both ends is inserted therethrough. A nut is threaded onto each end and thrust bearings are placed between each nut and the leg or an outer distal surface of the cone. In this embodiment it may be desirable to allow both nuts to self-tighten. This could result in better adjustment of the tension on the bolt through varying stages of wear on the elments of the bit.

Another important advantage of the preferred embodiment of the present invention is that it offers a bit which can be fabricated with greater ease. First, the fabrication of typical prior art roller cone rock bits involves a complex "pinning" step where the ball bearings are inserted through a hole into a race in order to retain the cone on the journal. In contrast, the roller cones of the preferred embodiment can be attached by simply inserting the retaining bolt through the leg and threading it into the cone.

Second, in typical prior art fabrication, because of the complexity of the pinning operation and spatial limitations, each leg is usually forged separately.

After a cone is pinned on each leg, the legs are welded onto a main bit body. (See U.S. Patent No.

4,266,622 for a discussion of the problems inherent in this process). In contrast, because the cones of the present invention are attached in a simpler process, it may be possible to form the legs as an integral part of the bit body and yet be able to attach the cones. Also, if spatial limitations prevent attaching the cones over the journals when the legs are already in place, the preferred embodiment can be modified slightly. In particular, the journals can be made separate from the legs. In this way, the journals can be inserted into the cones, the cones and journals slid into their position, and then the retaining bolt inserted through the leg into the cones. The journals can be indexed with the leg to insure correct positioning and retention on the leg.

A further advantage gained by this simplification of the fabrication process is that it will make reconditioning of worn out bits possible. At present, the standard practice in the drilling industry is to discard the entire bit when any part of it fails since the cones or bearings cannot be replaced with the welded legs approach. With the present invention, it is possible to replace the cones without destroying and refabricating the whole bit. Therefore, it will be economical to use each of the parts of the bit to its fullest extent. indeed, preliminary tests show that it will be the bearings that have the longest life expectancy. Thus, the bearings can be removed when the bit body and cutters are worn out and put into a new bit.

An advantage which also stems from this simplification of the bit fabrications is that it will now be possible to service roller cone bits in the field. That is, because the cones may be removed and attached in a relatively simple operation and with standard tools, it will be possible to change cones at the drilling site. In addition to replacing worn out cones, it will also be possible for a drillerto maintain an inventory of cones with different types of cutters and thereby be better able to tune the cutting characteristics of the bit in response to the formation in which he is drilling. Under present technology, the driller is required to maintain a costly inventory of entire bits to accomplish the same result.

Although, preliminary tests show that the bearing of the present invention performs well at higher loads and rotational speeds without any lubrication, it is considered desirable in the preferred embodiment to cool and lubricate the journal and roller cone bearings 41 and 42 as well as the thrust washers 46 and 45. This is accomplished in the preferred embodiment by providing a passage 52 which communicates at one end with a central cavity 51 in the bit body 31 which in turn communicates with a source of drilling fluid in the drill string. A grate or screen 53 is located in the central cavity 51 to prevent large particles from entering the passage 52. The other end of the passage 52 communicates with the hole 54 in the journal. In this way, a stream of the drilling fluid can pass over the journal and roller cone bearings and the thrust washers 45 and 46.It was a surprising result that a drilling fluid such as typical drilling mud could work well as both a lubricant and coolant for the bearing of the preferred embodiment. Drilling mud typically contains high quantities of abrasive silicate particles. Many bits are designed to keep the mud away from the bearings. With the present invention, when the bearing is comprised of polycrystalline diamond, these silicate particles are actually ground by the polycrystalline diamond surfaces and result in fine particles which function as a lubricant on the diamond bearing surfaces.

Accordingly, the preferred embodiment does not require seals to keep the drilling mud away from the bearings, but rather uses the drilling mud as a lubricant and coolant.

Figure 3 is a perspective view of the frustoconically shaped journal bearing 41 of the preferred embodiment. This bearing is shaped to mate with a reverse shaped roller cone bearing (not shown). In the most preferred embodiment, the journal bearing comprises a base member 71 which holds polycrystalline diamond inserts 72 in holes 73. Each insert 72 consists of a layer of polycrystalline diamond 74 bonded directly to a cemented tungsten carbide back 75. These carbide-backed inserts 72 are formed by sintering a mass of diamond crystals adjacent to the pre-cemented carbide piece using ultra high pressure and temperatures.

The height, radius, and slope of the base member 71 are dictated by the various design parameters such as the size and shape of the cone. In particular, the dimensions of the frusto-conical bearings must be selected with the following considerations in mind. The bearing must fit within the cone and allow a sufficient wall thicknessforthe roller cone.

The dimensions of the bearing must also allow for a sufficient thickness of the journal which also has the hole 54 passing through it. Because of the tremendous amount of wear on the gage row of the bit, it is important that the frusto-conical bearings do not extend to the exterior of the bit. That is, the part of the bearing closest to the gage row should be contained within the hole of the cone. In addition, the slope of the bearing also affects the retention of the cone on the journal as it is subjected to the multi-directional forces from the side and bottom of the hole. With a large angle, between the bearing surface and the axis of rotation the forces from the side and bottom of the hole will have more of a shear component at the interface of the journal and roller cone bearing than those same forces would have with a smaller angle.Forthe preferred embodiment, the angle is 20 between the bearing surface and the axis of rotation.

General mechanical principles tend to lead those skilled in the art to include that the frusto-conical geometry would be inappropriateforfriction bearings which experience high forces directed axially. Afrusto-conical friction bearing with high axial loads would act much like a wedge, in which the force normal to the face of the wedge would exceed the applied downward force. Figure 4, which is a schematic cross section of a pair of frustoconical friction bearings, illustrates this point. As the roller cone bearing 82 is pushed onto the journal bearing 81 with the axial force represented by arrow F,, the force to resist that motion is represented by arrow Fr.However, because the two bearing surfaces are not attached, the only counteractive force must be normal to the two surfaces and is represented by arrow Fn. Because the normal force F0 is at an angle to the force Fr, it must be of a greater magnitude than the force Fr. As a result, the normal force F0 (the force which determines the frictional force between the two bearings) is magnified.

In trigonometric terms, the applied force Fa and the normal force F0 of the frusto-conical journal bearing 81 againstthefrusto-conical roller cone bearing 82 are related by the following equations: 1) Fn=Fa sin 0, and [Fn=Fa sin 0] 2) Ff=Fn=Fa sin 0, where 6 is the angle between the outer surface of the bearing and the axis of rotation of the roller cone, is the coefficient of friction of the bearing material, and Ff is the frictional force resisting the rotation of the roller cone bearing 82 on the journal bearing 81.With respect to the forces normal to the surface of the bearing, the applied force Fa is magnified by a factor of 1/sin 6. As (3 approaches 0 (i.e. as the bearing becomes steeper) 1/sin 6goes to infinity, and the normal force F0 becomes infinitely large. The practical result is that the normal forces in a frusto-conical friction bearing experiencing high axial loads would become so large as to exceed the frictional force Ff and cause the bearing to seize. A surprising result of the present invention is that, even with a high load of precompression, no such seizing occurs when a material such as polycrystalline diamond is used for the bearing material.

Referring again to Figure 3, the position, size and number of inserts in each bearing is selected so as to insure that the polycrystalline diamond surfaces support the loads between the roller cone and the journal. As shown, each insert 72 protrudes slightly from the outer surface of the base member 71. In the depicted embodiment, there are three annular rows of inserts on each bearing. On the journal bearing, there are ten inserts in the row closest to the leg, eight inserts in the middle row, and eight inserts in the distal row. On the roller cone bearing, there are nine inserts in each of the three rows. This particular arrangement was selected so as to provide a proper amount of overlapping of the polycrystalline diamond surfaces at any given point of rotation of the cone. In another preferred embodiment, there are two annular rows of inserts in each bearing, and the angular spacing between inserts is chosen such that there are different numbers of inserts in corresponding rows of the cone and journal bearings so as to provide for smooth operation. One bearing may contain closely-spaced inserts while the inserts in the other bearing may be fairly wide spaced. Naturally, the gap between the inserts on one of the bearings must be smaller than the diameter of the inserts on the other bearing.

Alternative embodiments include designs wherein there are either fewer or more inserts as well as more rows.

Polycrystalline diamond (PCD) is most preferred as the bearing surface material. Although PCD is extremely wear-resistant, it is relatively brittle, i.e. it has relatively low tensile strength. As a result, PCD would not be expected to perform well in the roller cone bit bearing application where such high and uneven impact loads are experienced. However, the inventors discovered that this problem could be solved by maintaining the PCD bearings in a state of compression. That is, the inventor realized that the extremely high compressive strength of PCD could be used to offset its low tensile strength. When the bearings are maintained in a state of compression, the tensile forces are greatly reduced. Therefore, the pre-compressed PCD can survive the high impact loads exerted on the journal and roller cone bearings.

Another reason that one would not naturally think to use polycrystalline diamond as a bearing is that most uses of PCD to date have been for cutting, grinding or abrading operations. That is, it would be thought that PCD is too rough or abrasive to be successful as a bearing. However, it has been found that when two PCD surfaces are well polished and fit togetherwell, that the coefficient of friction is actually quite low. The inventor has measured value as low as 0.005 over wide ranges of loads and speeds ranging up to 40,000 Ibs. axial precompression and 1,000 r.p.m. In fact, most likely due to its high compressive strength, the coefficient of friction remains low over an impressive range of applied loads.This low coefficient of friction at high applied loads is very important in relation to the frusto-conical geometry which is discussed with respect to Figure 4.

Another advantage which was discovered in using PCD for the bearings of the preferred embodiment is PCD's high thermal conductivity. In particular, it is important for the bearings to be able to dissipate the heat which builds up during use.

Still another advantage of the preferred embodiment using PCD is its relative inertness. That is, in most bearings which are subjected to high loads at high temperatures, there is a problem of welding of the contacting surfaces. To avoid this problem, many such bearings are made from dissimilar metals, a solution which can introduce new problems related to dissimilar coefficients of thermal expansion etc. In contrast, when PCD is used in the present invention, the fact that diamond is relatively unreacted, obviates these problems.

Alternative embodiments may employ materials other than polycrystalline diamond for the bearing surface of the frusto-conical roller cone and journal bearings. Polycrystalline cubic boron nitride exhibits many of the same properties as PCD and additionally possesses much better thermal stability and as such may be substituted in the bearing of the present invention. In addition, particular ceramic or cermet materials may be found to possess the requisite properties to be of use in the bearing system.

Both the back 75 and the polycrystalline diamond layer 74 are pre-shaped to conform to the curvature and slope of the outer surface of the journal bearing.

The PCD layer 74 is shaped during the ultra-high pressure/temperature pressing cycle by being sintered adjacent to the carbide back 75 which has previously been correctly shaped. Likewise, the inserts in the roller cone bearing (not shown) are pre-shaped to conform to the curvature and slope of the inside surface of the roller cone bearing. The pre-shaping of the diamond layer to as near the required shape as possible, as opposed to starting with flat or other non-conforming shapes, has been found to be important for three reasons. First, polycrystalline diamond is extremely wear resistant.

Accordingly, it would require large amounts of time and effortto grind or cut the polycrystalline diamond to fit the final shape.

Second, because the polycrystalline diamond is a relatively brittle material, it is important that before the polycrystalline inserts are allowed to wear against each other, they present smooth surfaces. In other words, if the polycrystalline diamond pieces experience point to point contact, they would be likely to chip or crack. As a result, it is important to have the polycrystalline diamond pieces conforming to the curvature and slope of the bearing interface before use.

Third, the inventor has discovered that the final finishing of the polycrystalline diamond surfaces can be accomplished by simply running the two bearings against each other at high speeds and at high loads. This simplification of the finishing process would not be possible if the polycrystalline diamond surfaces were not already close to their final shapes.

In addition to the preferred method of using PCD inserts set into frusto-conical base members, there exist alternative methods of forming PCD bearing surfaces with the frusto-conical shape of the present invention. Theoretically, it would be desirable to produce a bearing for the present invention with a single piece of PCD, with or without a carbide backing, which could be used forthe journal or roller cone bearing. However, using present high pressure technology, it is not possible to produce pieces of PCD of a sufficient size.

A feasible alternative is to produce several PCD segments or "tiles" which could be fit together to produce contiguous surfaces of PCD for the bearings of the invention. These tiles could be used to cover the entirefrusto-conical surfaces, or alternatively could be arranged in annular rows or other configurations which would provide sufficient mating PCD surfaces to support the loads between the roller cone and journal while facilitating rotation of the roller cone about the journal.

Another alternative method for forming the PCD bearing surfaces of the present invention is to use PCD in a high concentration matrix. In particular, it is possible to fill a mass of PCD chunks or grit with a suitable metal orthe like to produce a unitary piece which possesses properties similar to a piece of solid PCD. One advantage is that this high concentration matrix PCD can be produced in much larger pieces than is possible with solid PCD.

Accordingly, a single piece PCD bearing surface could be produced.

Yet another alternative for forming the PCD bearing surface would involve coating base member with PCD. That is, it is possible to coat a journal bearing base member or a roller cone bearing base member with a layer of PCD through such techniques as flame spraying, electroless plating, etc. In this way, PCD surfaces can be applied to the bearings of the invention.

Although these other alternatives for incorporating the PCD into the journal and roller cone-bearings are available, it is noted that the preferred embodiment, i.e. pre-shaped PCD inserts held in frusto-conical bases, has exhibited certain surprising and important advantages. Naturally, in light of its relatively high cost of production, it is economical to use only as much PCD as necessary.

It was thought however, that a contiguous PCD surface would be required to provide sufficient smoothness for rotation and load carrying capability. Surprisingly, the bearing constructed according to the preferred embodiment has exhibited remarkable smoothness in rotation and load carrying capability. A further advantage of using discrete PCD inserts in the bearing is that it allows forimproved cooling and lubrication of the PCD surfaces. With the slight protrusion from the base member, the drilling mud can flow around each insert 72. Also, because the PCD inserts do not present a contiguous surface, the drilling mud can pass directly over the bearing surfaces.

Figure 5 shows an embodiment of the invention which is identical to that depicted in Figure 2 except that the retaining bolt 93 in Figure 5 is threaded into the leg 34 as opposed to being threaded into the cone as in Figure 2. The head 98 of the bolt 93 contacts a thrust bearing 96 which mates with a thrust bearing 97 which contacts the bottom surface of a recess 106 in the roller cone 36. Being so attached, the retaining bolt 93 of this embodiment will not rotate with the cone 36. However, friction between the mating bearing surfaces on the thrust bearings 95 and 96 and the pitch of the threads 94 produce a self-tightening effect as with the embodiment depicted in Figure 2.Also, as with the embodiment of Figure 2, it is desirable to maintain the bolt 93 in tension so that the journal bearing 41 and the roller cone bearing 42 are constantly pushed against each other resulting in compression of the bearing surfaces.

In a further alternative embodiment not shown, the retaining bolt is made with threads on both ends. A pair of th rust bearings and a nut are fitted on both ends. These thrust bearings could be planar as shown in Figure 2 and 5, frusto-conical, or other shapes which would produce the best result. The direction and pitch of each set of threads is selected so as to provide self-tightening on both ends. This embodiment may be desirable to reduce stress to the retaining bolt.

Although the discussion of the invention has included only threaded bolts to retain the roller cone on the journal and to maintain compression between the roller cone and journal bearing surfaces, other embodiments exist. For example, pins which are welded and/or heat shrunk into place may be used to serve the same two functions. In addition, although the roller cones depicted all use cutting inserts to engage the material, it should be understood that the bearing system of the present invention is likewise suitable for other types of roller cones such as those which use steel teeth or angular ridges to engage and disintegrate material.

Furthermore, although much of the discussion has focused on the use of polycrystalline diamond on the bearing surfaces, it is considered within the scope of the invention to use other materials for the bearing surfaces, such as polycrystalline cubic boron nitride, ceramic or cermet materials, which may perform suitably under these conditions.

Certainly, these and other modifications which are within the ordinary skill in the artto make are considered to lie within the scope of the invention as defined by the following claims.

Claims (20)

1. A roller cone rock bit comprising: a main bit body with at least one downwardly extending leg, said leg having a journal thereon, said journal having a journal bearing comprising a substantially frusto-conically shaped bearing surface; a roller cone rotatively mounted on each journal, said roller cone having a roller cone bearing with a surface which mates with the frusto-conically shaped bearing surface of the journal bearing; and means for maintaining the two bearings in a state of compression against each other.

2. A roller cone rock bit as in Claim 1, wherein said compression means comprises a pin which rotates within the journal, said pin having a first end attach i to the roller cone, and a second end including a bearing which mates with a bearing on the leg.

3. A roller cone rock bit as in Claim 2, wherein the first end of the pin is threadably attached within the roller cone.

4. A roller cone rock bit as in Claim 3, wherein the bearing of the second end of the pin comprises a first thrust washer with a bearing surface, and the bearing on the leg comprises a second thrust washer with a bearing surface, said first and second thrust washer bearing surfaces mating and being maintained in compression against each other.

5. A roller cone rock bit as in Claim 1, wherein said retaining means comprises a pin about which the roller cone rotates, and has a first end attached to the journal, and a second end including a bearing which mates with a second bearing surface on the roller cone.

6. A roller cone rock bit as in Claim 5, wherein the first end of the pin is threadably attached within the journal.

7. A roller cone rock bit as in Claim 6,wherein the bearing of the second end of the pin comprises a first thrust washer with a bearing surface, and the second bearing on the roller cone comprises a second thrust washer with a bearing surface, said first and second thrust washer bearing surfaces mating and being maintained in compression against each other.

8. A roller cone rock bit as in Claims 1,2,4,5, or 7, wherein the journal bearing and the roller cone bearing are comprised of polycrystalline diamond.

9. A roller cone rock bit as in Claims 4 or 7, wherein the first and second thrust washer bearing surfaces are comprised of polycrystalline diamond.

10. A roller cone rock bit as in Claims 1,2,4,5, or 7 further comprising means for passing a stream of drilling fluid over the journal and roller cone bearing surfaces.

11. A roller cone rock bit as in Claim 10, wherein the means for passing a stream of drilling fluid over the journal and roller cone bearing surfaces comprises a passage through the bit which communicates at one end with a source of drilling fluid from a drill string and communicates at another end with said journal and roller cone bearings, and an exit passage.

12. A roller cone rock bit comprising: a main bit body with at least one downwardly extending leg, each leg having a journal thereon, said journal having a journal bearing with a substantially frusto-conically bearing outer surface comprising polycrystalline diamond; a roller cone rotatably mounted on the journal, the roller cone having a roller cone bearing with a surface comprised of polycrystalline diamond which mates with the substantially frusto-conically shaped bearing surface of the journal bearing; said journal defining a hole which includes the axis of rotation of the roller cone; a bolt which passes through said hole in the journal and has a first end which is threadably attached to the roller cone, and has a second enlarged end positioned outside of said hole;; a first thrust bearing which is adjacent to said enlarged end and through which said bolt passes said first thrust bearing having a first th rust bearing surface; a second thrust bearing which is adjacent to said leg and through which said bolt passes, said second thrust bearing having a second thrust bearing surface which mates with said first thrust bearing surface; and said bolt being maintained in tension so as to place said journal bearing and roller cone bearing in compression against each other, and to place said first and second thrust bearing surfaces in compression against each other.

13.Arollercone rock bit comprising: a main bit body with at least one downwardly extending leg, each leg having a journal thereon, said journal having a journal bearing with a substantially frusto-conically shaped bearing surface comprising polycrystalline diamond; a roller cone rotatably mounted on the journal, the roller cone having a roller cone bearing with a bearing surface comprised of polycrystalline diamond which mates with the substantially frustoconically shaped bearing surface of the journal bearing, said roller cone defining a hole which includes the axis of rotation of the roller cone; a bolt which passes through said hole in the roller cone and has a first end which is threadably attached to the journal, and has a second enlarged end positioned outside of said hole;; a first th rust bearing which is adjacent to said enlarged end and through which said bolt passes said first thrust bearing having a first thrust bearing surface; a second thrust bearing which is adjacent to said roller cone and through which said bolt passes, said second thrust bearing having a second thrust bearing surface which mates with said first thrust bearing surface; and said bolt being maintained in tension so as to place said journal bearing.and roller cone bearing in compression against each other, and to place said first and second thrust bearing surfaces in compression against each other.

14. A roller cone rock bit comprising: a main bit body with at least one downwardly extending leg, each leg having a journal thereon, said journal having a journal bearing with a substantially frusto-conically shaped bearing surface comprising polycrystalline diamond; a roller cone rotatably mounted on the journal, the roller cone having a roller cone bearing with a bearing a surface comprised of polycrystalline diamond which mates with the substantially frustoconically shaped bearing surface of the journal bearing, said roller cone defining a hole which includes the axis of rotation of the roller cone; said journal defining a hole which includes the axis of rotation of the roller cone;; a bolt which passes through said hole in the journal and said hole in the roller cone, said bolt having a first end with a first nut threaded thereon and a second end with a second nut threaded thereon; a first set of thrust bearings through which the first end of the bolt passes and which are positioned between said first nut and the leg; a second set of thrust bearings through which the second end of the bolt passes and which are positioned between the second nut and the roller cone; and said first and second nuts being sufficiently tightened to place said bolt in tension, thereby placing said journal bearing and roller cone bearing in compression against each other.

15. A roller cone rock bit as in Claim 12, 13 or 14, wherein said bolt is self-tightening during drilling.

16. A rolier cone rock bits in Claims 12, or 14 further comprising means for passing a stream of drilling fluid over the journal and roller cone bearing surfaces.

17. A roller cone rock bit as in Claims 1,4,7, 12, 13, or 14, wherein said frust-conical journal bearing is comprised of a base member with a frusto-conically shaped outer surface, and a plurality of polycrystalline diamond inserts which are held in place by said base member.

18. A roller cone rock bit as in Claim 17, wherein said polycrystalline diamond inserts comprise a polycrystalline diamond portion directly bonded to a backing of a cemented carbide.

19. A roller cone rock bit as in Claim 18, wherein said polycrystalline diamond inserts are molded to shape substantially conforming to the slope and curvature of the frusto-conically shaped journal bearing.

20. A roller cone rock bit substantially as a herein before described with reference to and as shown in Figures 2 to 4 of the accompanying drawings or as modified in Figure 5.