AU2014202882A1

AU2014202882A1 - Hybrid Rotary Cone Bit

Info

Publication number: AU2014202882A1
Application number: AU2014202882A
Authority: AU
Inventors: Graeme Thomas Evans; Brian Thomas Sanfead
Original assignee: Tricon Drilling Solutions Pty Ltd
Current assignee: Tricon Drilling Solutions Pty Ltd
Priority date: 2013-05-31
Filing date: 2014-05-27
Publication date: 2014-12-18
Anticipated expiration: 2034-05-27
Also published as: AU2014202882B2

Abstract

Abstract A rotary roller cone bit 11 has carriers 13 welded together along weld lines 15. Each carrier 13 includes a leg 27 extending downward, spaced from the central axial extent of the bit 11, supporting a rotary cutting cone 29. Each carrier 13 has a horizontal ledge 25. The assembly is strengthened by a triangular plate 39 is welded to the ledge 25 of each carrier 13 and the abutting leg 27 of the adjacent carrier 13, and above this is located a rectangular plate 45, which bridges the underlying weld line 15, and is fillet welded 47 all around to the underlying carriers 13. An annular bush 49 is welded into a recess 37 in the threaded of the threaded shaft 19 at the top 17 of the bit 11. . *z s bLa Fi.

Description

1 P/00/011 28/5/91 AUSTRALIA Patents Act 1990 ORIGINAL COMPLETE SPECIFICATION STANDARD PATENT Name of Applicant: Tricon Drilling Solutions Pty Ltd Actual Inventor(s): Brian Thomas Sanfead Graeme Thomas Evans Address for service is: Golja Haines & Friend PO Box 1417 West Leederville Western Australia 6901 Attorney Code: IJ Invention Title: Hybrid Rotary Cone Bit The following statement is a full description of this invention, including the best method of performing it known to me:- 2 Hybrid rotary roller cone bit Technical Field [0001] This invention relates to the field of drilling, and in particular to a rotary roller cone drill bit for a percussive drilling assembly for rock drilling. Background Art [0002] The following discussion of the background art is intended to facilitate an understanding of the present invention only. It should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was part of the common general knowledge as at the priority date of the application. [0003] There are two types of drilling bits that are used for rock drilling. Typically, a fixed cutter drill bit with no moving parts is provided with wear surfaces which protrude from the surface, are used in a percussive down-the-hole hammer drill. This type of drill is rotatably mounted on a drill stem which includes a hammer mechanism. The hammering action impacts the wear surfaces against the rock, breaking the rock up into chips. Compressed air or fluid circulation is used to drive the percussive action, and carries the debris to the surface via the drilled hole, upon exhausting the bit. [0004] An alternative in the market is known as a rotary roller cone drill bit or rotary roller cone bit. This bit has rotating conical heads fitted with wear studs. Typically there are two or three rotating conical heads provided at the base of the rotary roller cone bit. The rotating conical heads are mounted on axes which converge in front of the bit, so that the conical heads together present a crushing/cutting plane lying across the axial extent of the bit. As the drill stem is rotated, the conical heads also rotate, impacting different wear studs around the conical heads at the bottom of the bore hole, which allows the wear studs to recover from localised heating before they are again presented against the rock work face. The impacting wear studs causes rock to fracture, and again, the chips are carried to the surface by compressed air. [0005] Normally rotary roller cone bits are used with a rotary cutting action only, using between 2,500 and 8,000lbs per inch of bit diameter to cut efficiently. For 3 a 9 inch diameter rotary roller cone bits this equates to 22,500 to 72,000lbs of downforce. By comparison, when a down-the-hole hammer drill is used with a fixed cutter drill bit, a downforce of between 500 lbs to 1000 lbs per inch of drill diameter is used, equating in a 9 inch drilling bit to 4,500 to 9,000 lbs of downforce. [0006] Rotary roller cone bits are not generally used with down-the-hole hammer drill mechanisms (in percussion drilling), as the mechanism of the rotary roller cone bit cannot withstand the forces impacted by the down-the-hole hammer drill mechanism during extended periods sustained during normal drilling practice or operations. [0007] Rotary roller cone bits are manufactured from identical parts comprising a carrier and a rotatable cone which is mounted on a bearing for rotation thereon, and a port for admitting compressed air at the drill face. Depending on the number of conical heads, in the case of a bit having two heads there are two carriers and in the case of a bit having three heads, there are three carriers. Each carrier has a convex external arcuate surface with a substantially flat longitudinal extent, the convex external arcuate surface extending across an arc of 180 degrees for a two headed bit or 120 degrees for a three headed bit, measured with respect to a notional central longitudinal axis, and radial surfaces that extend inwardly from the edges of the arcuate surface toward the notional central longitudinal axis. On the opposite surface of each carrier there is provided a blind concave internal arcuate surface at the top, which leads to a port for the supply of compressed air or fluid circulation to bail the cuttings from the hole. [0008] The radial surfaces match each other so that the two or three carriers as the case may be, can be mated. The carriers mate so that the convex external arcuate surfaces form a cylindrical rod configuration, and the concave internal arcuate surfaces form an axial passageway for carriage of compressed air and lubricant, and to receive a check valve. Each carrier includes a leg extending downwardly formed integrally therewith, and a ledge located next to the base of the leg carrying a port to supply compressed air or drilling mud to the cutting surfaces. The ledge of one carrier sits next to the base of the leg of an adjoining carrier when the carriers, two or three as the case may be, are mated. [0009] The leg is provided for mounting a rotatable cone is mounted facing with its apex extending toward the notional central longitudinal axis. When the carriers are mated, the rotational axes of the rotatable cones extend and intersect 4 or coincide approximately with the notional central longitudinal axis, at a position below the bearing mounting points in the legs for the rotatable cones. [0010] At the top of each carrier is a surface which carries a machined external thread when the carriers are joined. Inside the top of each carrier is a round cylindrical recess, in which a check valve is placed. [0011] The external edge surfaces of the carriers may be ground away where they meet, and when they are brought together, they are welded along the edges of the arcuate external surfaces, and also where the thread is to be machined. After welding the completed assembly is machined in the threaded area and this operation removes any proud welding material, but otherwise the assembly welds are left au natural. [0012] In the completed assembly, the arrangement of the legs forms an inverted-U-channel recess. The region of the completed assembly where the ledge is located has a reduced radial extent. Both the inverted-U-channel recess and the reduced radial extent in that region, allows escape of debris from the drilling operation, up the side of the convex external arcuate surfaces. [0013] The problem with these rotary roller cone bits is that that have proven to be prone to failure at the welds that hold together the carriers, under stress of down-the-hole hammer drilling, when combining both percussive and rotary drilling actions. [0014] Throughout the specification unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. Summary of Invention [0015] It has long been accepted that rotary roller cone bits are subject to failure during use, yet there has been a view that little could be done to deal with this problem and the attendant costs it brings. The inventors have determined that with some structural changes to the fabrication of the rotary roller cone bit, a rotary roller cone bit can have its mean time before failure increased, and be more reliably used with a hammer drill mechanism in rock drilling. This can lead to drilling efficiency gains and costs savings.

5 [0016] In accordance with one aspect of the invention there is provided a rotary roller cone bit comprising at least two carriers welded together, each carrier including a leg extending downward spaced away from the central axial extent of the bit and disposed at the circumferential periphery for supporting a rotary cutting/crushing cone on an axis disposed inwardly and downwardly, each carrier including adjacent to the top of the leg and extending to the edge of the bit, a ledge disposed in a plane across the axial extent of the bit, the ledge carrying a port to admit compressed air or fluid, where at the top of the carriers there is provided shoulder from which extends a threaded shaft for mounting the bit, where the threaded shaft has a central cylindrical recess located axially therein; where the adjacent carriers are welded at least from the shoulder to the ledge, and wherein a plate is included extending downward from the ledge to the leg welded thereto down the leg to increase the effective length of the weld and to further brace the leg against the adjacent ledge. [0017] The plate may be integrally formed with the ledge, extending downward therefrom, or in a preferred form the plate may be provided as a separate component which is welded thereto, across the ledge, in addition to being welded down the leg of the adjacent carrier. [0018] The plate being included from the ledge to the leg, welded thereto down the leg acts to increase the effective length of the weld (from shoulder to ledge) and being attached to the ledge either by being integrally formed therewith or being welded thereto, acts to further brace the leg against the adjacent ledge. [0019] Preferably the plate is a triangular plate. [0020] Preferably the triangular plate is welded to the ledge and leg along its non-hypotenuse edges. [0021] Preferably the bit includes a cylindrical member located in the central cylindrical recess and welded in the central cylindrical recess in the inside and at the top thereof. This provides further strengthening. The welded bush ties the leg assemblies together in the bore instead of just in two or three places as they are normally (depending on the number of carriers). This anchoring of the assembly at the extremity in the thread which transmits the torque provides a substantial improvement. [0022] Preferably the cylindrical member comprises a cylindrical bush.

6 [0023] Preferably the cylindrical member extends down to a floor inside the central cylindrical recess, and is welded thereto around the inside periphery of the cylindrical member. The cylindrical member will include ports to allow the flow of air and coolant/lubricant to the moving parts of the bit. [0024] A an alternative, preferably a further reinforcing member in the form of a plug is provided, where the plug extends in slide fit manner through a floor inside the central cylindrical recess, substantially in axial alignment therewith, and is welded to the carriers on the outside thereof. The further reinforcing member may be a further cylindrical member located along the same axis as the central cylindrical recess. [0025] As a further alternative, the cylindrical member comprises a cylinder with a closed end, where the closed end extends through a floor inside the central cylindrical recess, and is welded to the carriers on the outside thereof. This arrangement in effect provides the cylindrical bush and the further cylindrical member described above, as a single piece. At least one port, and preferably two ports, three ports or four ports may be provided in the wall of the cylinder near but spaced from both the closed end and the threaded shaft, in order to allow transit of compressed air or fluid. [0026] Preferably the bit includes a rectangular plate located across each weld extending between shoulder and ledge, about mid way down, and fillet welded all around to the bit. [0027] Preferably the rectangular plate is welded spaced from and above the triangular plate. [0028] Preferably the rectangular plate is welded spaced from the triangular plate and roughly parallel to a horizontal edge of the triangular plate. [0029] In accordance with a second aspect of the invention there is provided a rotary roller cone bit comprising at least two carriers welded together, each carrier including a leg extending downward spaced away from the central axial extent of the bit and disposed at the circumferential periphery for supporting a rotary cutting/crushing cone on an axis disposed inwardly and downwardly, each carrier including adjacent to the top of the leg and extending to the edge of the bit, a ledge disposed in a plane across the axial extent of the bit, the ledge carrying a port to admit compressed air or fluid, where at the top of the carriers 7 there is provided shoulder from which extends a threaded shaft for mounting the bit, where the threaded shaft has a central cylindrical recess located axially therein; where the adjacent carriers are welded at least from the shoulder to the ledge, and wherein the bit includes a cylindrical member located in the central cylindrical recess and welded in the central cylindrical recess in the inside and at the top thereof. [0030] The welded cylindrical member ties the leg assemblies together in the bore instead of just in two or three places as they are normally (in the case of a bit having two or three carriers respectively). This anchoring of the assembly at the extremity in the thread which transmits the torque provides a substantial improvement. [0031] Preferably the cylindrical member comprises a cylindrical bush fillet welded in the central cylindrical recess. [0032] Preferably a further reinforcing member in the form of a plug is provided, where the plug extends in slide fit manner through a floor inside the central cylindrical recess, substantially in axial alignment therewith, and is welded to the carriers on the outside thereof. The further reinforcing member may be a further cylindrical member located along the same axis as the central cylindrical recess. [0033] Alternatively the cylindrical member comprises a cylinder with a closed end, where the closed end extends through a floor inside the central cylindrical recess, and is welded to the carriers on the outside thereof. This arrangement in effect provides the cylindrical bush and the further cylindrical member described above, as a single piece. At least one port, and preferably two ports, three ports or four ports may be provided in the wall of the cylinder near but spaced from both the closed end and the threaded shaft, in order to allow transit of compressed air or fluid. [0034] Preferably a plate is included extending downward from the ledge to the leg welded thereto down the leg to increase the effective length of the weld and to further brace the leg against the adjacent ledge. [0035] The plate may be integrally formed with the ledge, extending downward therefrom, or in a preferred form the plate may be provided as a separate component which is welded thereto, across the ledge, in addition to being welded down the leg of the adjacent carrier.

8 [0036] The plate being included from the ledge to the leg, welded thereto down the leg acts to increase the effective length of the weld (from shoulder to ledge) and being attached to the ledge either by being integrally formed therewith or being welded thereto, acts to further brace the leg against the adjacent ledge. [0037] Preferably the plate is a triangular plate. [0038] Preferably the triangular plate is welded to the ledge and leg along its non-hypotenuse edges. [0039] Preferably the bit includes a rectangular plate located across each weld extending between shoulder and ledge, about mid way down, and fillet welded all around to the bit. [0040] Preferably the rectangular plate is welded spaced from and above the triangular plate. [0041] Preferably the rectangular plate is welded spaced from the triangular plate and roughly parallel to a horizontal edge of the triangular plate. Brief Description of Drawings [0042] Three embodiments of the invention will now be described with reference to the drawings in which: Figure 1 is a perspective view from above of a rotary roller cone bit according to both embodiments; Figure 2 is a perspective view from below above of the rotary roller cone bit of figure 1; Figure 3 is a side elevation of the rotary roller cone bit of figure 1; Figure 4 is a further side elevation of the rotary roller cone bit of figure 1; Figure 5 is a top plan view of the rotary roller cone bit of figure 1, according to the first embodiment; Figure 6 is a cut away/part cross section side elevation of the rotary roller cone bit of figure 4, according to the first embodiment; and 9 Figure 7 is a cut away/part cross section side elevation of the rotary roller cone bit as per figures 4 and 6 but according to the second embodiment; Figure 8 is a cut away/part cross section side elevation of the rotary roller cone bit as per figures 4 and 6 but according to the third embodiment. Description of Embodiments [0043] All three embodiments of the invention are a rotary roller cone bit 11 comprising three carriers 13 welded together along weld lines 15 which extend vertically from the top 17 of a threaded shaft 19, down past a shoulder 21 to the a point 23 proximal to a ledge 25. The threaded shaft 19 and the shoulder 21 have been machined after the welding process, so the weld line in these regions is not visible and cannot be discerned in the drawings. As can be seen in the drawings, the threaded shaft 19 is of conical configuration. [0044] Each carrier 13 includes a leg 27 extending downward, spaced away from the central axial extent of the bit 11 and disposed at the circumferential periphery of the bit 11. Each leg 27 supports a rotary cutting cone 29 on a rotational axis disposed inwardly and downwardly so that the rotational axes intersect at a point in space 31 located below the bit 11 (see figure 3). The rotary cones 29 are mounted on bearings (not shown) which in turn are mounted on a pin (also not shown). [0045] Each carrier 13 has a ledge 25 disposed in a plane across if not normal to the axial extent of the bit 11, located adjacent to the top of the leg 27 and extending toward the outer circumferential edge 33 of the bit 11. Each ledge 25 has a port 35 to admit compressed air or fluid for the flushing of debris from the drill hole formed by the bit 11 in operation. An acetyl plastic nozzle 36 is slide-fit into the port 35, and secured by a pin 36a which is driven through the side of the carrier 13 above the ledge 25. The pin 36a engages a circumferential groove which extends around the outside of the nozzle 36 (in contact with the port 35), and this engagement prevents the nozzle 36 falling out under the vibration experienced by the bit 11, when in use. The nozzle 36 has a central aperture through which compressed air or fluid is discharged. The central aperture is sized to cause a restriction in order to maintain a pressure within the rotary roller cone bit 11 of about 30 PSI which ensures that there is sufficient cooling of the bearings on which the rotary cones 29 are mounted.

10 [0046] The shoulder 21 is located at the top of the carriers 13, and the threaded shaft 19 extends above the shoulder 21, the threaded shaft 19 being provided to mount the bit 11 into a corresponding female threaded portion of a hammer drill mechanism (not shown). [0047] The threaded shaft 19 is hollow, with a central cylindrical recess 37 located extending axially therein. In the first embodiment illustrated in figure 5, the join lines between the carriers 13 can be seen in a floor 38 at the bottom of the recess 37. [0048] The above described arrangement is strengthened against failure, possibly a combination of failure under both shear and tensional stresses, imparted during percussion and rotation of the hammer drill to which the bit 11 is attached, by the addition of a triangular plate 39 which is of substantially right angle triangular configuration and 10 mm thick, and is located on the ledge 25 of one carrier 13 and abutting the leg 27 of the adjacent carrier 13 (the one carrier 13 is on the left and the adjacent carrier 13 is on the right in figures 1, 2 and 3). The triangular plate 39 is joined to the ledge 25 with a horizontal fillet weld 41, and joined to the leg 27 with a vertical fillet weld 43. The effect of the vertical fillet weld 43 is to increase the effective length of the axial weld between the adjacent carriers 13. The effect of the horizontal fillet weld 41 is to provide additional bracing. [0049] There are three such triangular plates welded as described located spaced 120 degrees from each other about the central axis of the bit 11, to brace the adjacent carriers 13. [0050] While the triangular plates are added as a separate piece, in an alternative embodiment the carriers 13 may each be formed with the triangular plate 39 formed integral with and extending down from the ledge 35, in position to allow welding to the leg 27 of the adjacent carrier 13. [0051] In addition to this, located above each triangular plate 39 is located a rectangular plate 45, also 10 mm thick, which bridges the underlying weld line 15, and is fillet welded 47 all around, to the underlying carriers 13. It is thought that the welding of this plate to the carriers contributes to the shear strength across the weld 15.

11 [0052] In the first embodiment, the final reinforcing is provided by a cylindrical member in the form of an annular bush 49 which is slide fit into the recess 37 and fillet welded 51 all around to the inside of the recess 37 at the top 17 of the bit 11. The recess 37 at the top of the bit was provided to house a check valve, used to prevent back flow of debris into the DTH mechanism. As a consequence of the annular bush 49 being fitted as described, the check valve is located within the DTH mechanism. The dimensions of the bush have been selected so as to not unduly restrict flow of compressed air or lubricant. [0053] Referring to figure 7, in the second embodiment, a cylindrical member is provided in the form of a cylinder 55 which is closed at the bottom 57 so as to form a container 59. A hole is bored through the floor 38 inside the recess 37, to receive the bottom of the cylinder 55, and the cylinder is welded 61 all around its lowermost circumference, where it protrudes through the floor 38, the welding being accomplished from the outside, underneath the drill. The welding is carried out before the rotary cones 29 are fitted,in order to provide access from the outside, for the welding to be completed. However, it is also possible to carry out this welding procedure on a fully assembled bit, as a retro-modification, but it requires some manoeuvring around the rotary cones 29 in order to complete the welding. The cylinder 55 is also fillet welded 51 all around to the inside of the recess 37 at the top 17 of the bit 11. The fitting of a cylinder 55 instead of an annular bush 49 provides additional strength due to presence of the weld 61 between the bottom of the cylinder 55 and the floor38. [0054] Three apertures in the form of ports 63 are provided in the wall of the cylinder 55, spaced equidistant around the circumference, to permit the communication of compressed air or fluid from the DTH mechanism, through to the ports 35, the direction of air/fluid flow being indicated by arrows at 65. [0055] Referring to figure 8, in the third embodiment, a cylindrical member is provided in the form of a cylindrical tube 67 which is open at the bottom and comes into contact with the inside of the floor 38, and is welded 69 from the inside of the cylindrical tube 67, all around its inside circumference, to the floor 38 inside the recess 37. The cylinder 55 is also fillet welded 51 all around to the inside of the recess 37 at the top 17 of the bit 11. The fitting of a cylindrical tube 67 instead of an annular bush 49 provides additional strength due to presence of the weld 69 between the bottom of the cylindrical tube 67 and the floor38.

12 [0056] The structure of the embodiments of the rotary roller cone bit 11 described above allows it to be more reliably used in percussion drilling, whereas without the additional plates 39 and 45 and the annular bush 49 or cylinder 55 as the case may be, the assembly welds have proven to be the weak point that led to premature bit failure when used in percussion drilling. [0057] The placement of the triangular plates 39 and rectangular plates 45 on the carriers 13 is in a region of the bit 11 that is of reduced radius from the outer circumferential edge 33 of the bit. This reduced radius provides clearance between the bit 11 at this position and the wall of the hole that the bit has drilled, for debris from the drilling operation to pass through entrained in the compressed air or fluid exhaust from the drill, that is admitted through the ports 35 in the ledges 25. The plates 39 and 45 extend toward the circumference defined by the circumferential edge 33, but do not reach that edge. The arcuate extent of the plates around the drill is such that there remains sufficient space for debris entrained in exhaust compressed air or fluid circulation to pass and be flushed out of the hole. [0058] It should be appreciated that the scope of the invention is not limited to the particular embodiment described herein.

Claims

1. A rotary roller cone bit comprising at least two carriers welded together, each carrier including a leg extending downward spaced away from the central axial extent of the bit and disposed at the circumferential periphery for supporting a rotary cutting/crushing cone on an axis disposed inwardly and downwardly, each carrier including adjacent to the top of the leg and extending to the edge of the bit, a ledge disposed in a plane across the axial extent of the bit, the ledge carrying a port to admit compressed air or fluid, where at the top of the carriers there is provided shoulder from which extends a threaded shaft for mounting the bit, where the threaded shaft has a central cylindrical recess located axially therein; where the adjacent carriers are welded at least from the shoulder to the ledge, and wherein a plate is included extending downward from the ledge to the leg welded thereto down the leg to increase the effective length of the weld and to further brace the leg against the adjacent ledge.

2. A rotary roller cone bit as claimed in claim 1 wherein the plate is integrally formed with the ledge, extending downward therefrom.

3. A rotary roller cone bit as claimed in claim 1 wherein the plate is provided as a separate component which is welded to the ledge, across the ledge, in addition to being welded down the leg of the adjacent carrier.

4. A rotary roller cone bit as claimed in any one of the preceding claims wherein the plate is a triangular plate.

5. A rotary roller cone bit as claimed in claim 4 wherein the triangular plate is welded to the ledge and leg along its non-hypotenuse edges.

6. A rotary roller cone bit as claimed in any one of the preceding claims wherein the bit includes a cylindrical member located in the central cylindrical recess and welded in the central cylindrical recess in the inside and at the top thereof.

7. A rotary roller cone bit as claimed in claim 6 wherein the cylindrical member comprises a cylindrical bush. 14

8. A rotary roller cone bit as claimed in claim 7 wherein the cylindrical member extends down to a floor inside the central cylindrical recess, and is welded thereto around the inside periphery of the cylindrical member.

9. A rotary roller cone bit as claimed in any one of the preceding claims wherein a further reinforcing member in the form of a plug is provided, where the plug extends in slide fit manner through a floor inside the central cylindrical recess, substantially in axial alignment therewith, and is welded to the carriers on the outside thereof.

10. A rotary roller cone bit as claimed in claim 9 wherein the further reinforcing member is a further cylindrical member located along the same axis as the central cylindrical recess.

11. A rotary roller cone bit as claimed in claim 6 or 7 wherein the cylindrical member comprises a cylinder with a closed end, where the closed end extends through a floor inside the central cylindrical recess, and is welded to the carriers on the outside thereof.

12. A rotary roller cone bit as claimed in any one of the preceding claims wherein the bit includes a rectangular plate located across each weld extending between shoulder and ledge, about mid way down, and fillet welded all around to the bit.

13. A rotary roller cone bit as claimed in claim 12 wherein the rectangular plate is welded spaced from and above the triangular plate.

14. A rotary roller cone bit as claimed in claim 12 or 13 wherein the rectangular plate is welded spaced from the triangular plate and roughly parallel to a horizontal edge of the triangular plate.

15. A rotary roller cone bit comprising at least two carriers welded together, each carrier including a leg extending downward spaced away from the central axial extent of the bit and disposed at the circumferential periphery for supporting a rotary cutting/crushing cone on an axis disposed inwardly and downwardly, each carrier including adjacent to the top of the leg and extending to the edge of the bit, a ledge disposed in a plane across the axial extent of the bit, the ledge carrying a port to admit compressed air or fluid, where at the top of the carriers there is provided shoulder from which extends a threaded shaft for mounting the bit, where the threaded shaft has 15 a central cylindrical recess located axially therein; where the adjacent carriers are welded at least from the shoulder to the ledge, and wherein the bit includes a cylindrical member located in the central cylindrical recess and welded in the central cylindrical recess in the inside and at the top thereof.

16. A rotary roller cone bit as claimed in claim 15 wherein the cylindrical member comprises a cylindrical bush fillet welded in the central cylindrical recess.

17. A rotary roller cone bit as claimed in claim 15 wherein the cylindrical member extends down to a floor inside the central cylindrical recess, and is welded thereto around the inside periphery of the cylindrical member.

18. A rotary roller cone bit as claimed in claim 15, 16 or 17 wherein a further reinforcing member in the form of a plug is provided, where the plug extends in slide fit manner through a floor inside the central cylindrical recess, substantially in axial alignment therewith, and is welded to the carriers on the outside thereof.

19. A rotary roller cone bit as claimed in claim 18 wherein the further reinforcing member is a further cylindrical member located along the same axis as the central cylindrical recess.

20. A rotary roller cone bit as claimed in claim 15 or 16 wherein the cylindrical member comprises a cylinder with a closed end, where the closed end extends through a floor inside the central cylindrical recess, and is welded to the carriers on the outside thereof.

21. A rotary roller cone bit as claimed in any one of claims 15 to 20 wherein a plate is included extending downward from the ledge to the leg welded thereto down the leg to increase the effective length of the weld and to further brace the leg against the adjacent ledge.

22. A rotary roller cone bit as claimed in claim 21 wherein the plate is integrally formed with the ledge, extending downward therefrom.

23. A rotary roller cone bit as claimed in claim 21 wherein the plate is provided as a separate component which is welded to the ledge, across the ledge, in addition to being welded down the leg of the adjacent carrier. 16

24. A rotary roller cone bit as claimed in claim 21, 22, or 23 wherein the plate is a triangular plate.

25. A rotary roller cone bit as claimed in claim 24 wherein the triangular plate is welded to the ledge and leg along its non-hypotenuse edges.

26. A rotary roller cone bit as claimed in claim 25 wherein the bit includes a rectangular plate located across each weld extending between shoulder and ledge, about mid way down, and fillet welded all around to the bit.

27. A rotary roller cone bit as claimed in claim 26 wherein the rectangular plate is welded spaced from and above the triangular plate.

28. A rotary roller cone bit as claimed in claim 27 wherein the rectangular plate is welded spaced from the triangular plate and roughly parallel to a horizontal edge of the triangular plate.