SELF DRILLING ROOF BOLT

Abstract: The present invention relates to an elongate roof bolt having an axially extending central bore and defining a leading end at one end thereof. The leading end includes a pair of axially extending abutment elements formed by machining, each of which includes an abutment face for engagement with a drill tip on opposite sides thereof, to support the drill tip during drilling. The drill tip extends across said leading end and is secured to the abutment elements by welding, soldering or brazing. The bore opens out of said bolt at or adjacent said leading end.

Description: SELF DRILLING ROOF BOLT The present invention relates to a bolt of the type which is commonly known as rock or a roof bolt and in particular, the invention relates to a self-drilling roof bolt. Roof bolts are commonly employed in the underground mining industry to support the walls of rock excavations against fragmentation or collapse. The normal procedure for installing a roof bolt involves drilling a pilot hole into the rock wall and inserting the bolt therein. The bolt is fixed within the hole normally by a settable glue, such as a resin or mortar cement, or the bolt may include means to directly interlock with the surface of the pilot hole, such as a thread or an expanding sleeve. A wide variety of roof bolts exist and one particular form is tubular, having a central bore formed lengthwise through the bolt. Advantageously, the central bore provides a conduit for the introduction of water during insertion of the bolt into the pilot hole. Water is employed to flush away rock segments, finings and dust, and to reduce frictional resistance between the bolt and the rock, and excessive temperature generation. The central bore also provides a conduit for the injection of cement mortar or resin into the pilot opening to secure the bolt therein. The above described methods of installing a roof bolt thus employ a two stage process, in which the pilot hole is first drilled, after which the roof bolt is inserted into the pilot hole and secured therein by the appropriate means. This type of roof bolt therefore has no drilling capability. Roof bolts having a drilling capability, and thereby referred to as "self-drilling roof bolts", are known, but such bolts are not in common use in the mining industry. This is despite such roof bolts requiring less installation steps when compared to roof bolts of the earlier described kind, because a separate drill to form the pilot hole is not required. Thus, the self-drilling type of roof bolt can be installed more quickly, thereby reducing the period in which the excavated rock face may be unstable and likely to fragment or collapse. Self-drilling roof bolts have not become popular, and one reason for this is because the bolt must be modified to include a reaming or drilling bit at the leading end thereof in order to enable the bolt to drill the pilot hole and that increases considerably the cost of the bolt. Such bits are expensive because they are normally subject to very high wear, given the abrasive material they are required to penetrate and thus, it is normal for the drill bit of the bolt to be hardened, and that is normally facilitated by the provision of a tungsten carbide tip. In one particular form of tubular roof bolt, described in Australian Patent No. 672428, the roof bolt is formed by rolling or die drawing a pair of elongate members into half cylindrical sections, and connecting them along their longitudinal edges by welding to form a bolt rod. In this form of roof bolt, the central bore adjacent the leading end thereof is threaded, to accept the threaded shank of a tungsten carbide rotary drill bit. Rotary drill bits are well known and Applicant makes a range of such bits which include a steel body supporting the tungsten carbide tip, which is brazed thereto. These bits however, are relatively expensive when compared to the rod they are connected to. In each of the above described roof bolts, the addition of the tungsten carbide bit to the leading end of the rock bolt, involves sufficient expense to make general use of a roof bolt formed in this manner, prohibitive. However, it nevertheless remains the case, that the installation advantages of a self-drilling roof bolt outweigh those of the non-self-drilling type. It is an object of the present invention to provide a self-drilling roof bolt which is less costly to manufacture than roof bolts of that kind described above and so to make their use in the mining industry more attractive. According to the invention there is provided an elongate roof bolt having an axially extending central bore and defining a leading end at one end thereof, said leading end including a pair of axially extending abutment elements formed by machining, each of which includes an abutment face for engagement with a drill tip on opposite sides thereof, to support the drill tip during drilling, said drill tip extending across said leading end and being secured to the abutment elements by welding, soldering or brazing, said bore opening out of said bolt at or adjacent said leading end. In a first aspect of the invention, the arrangement is further such that the central bore remains open at or adjacent the leading end of the rod when the drill tip is connected thereto. The roof bolt of the invention thus maintains the advantages of a centrally bored bolt, but additionally provides for self-drilling with associated advantages, without requiring attachment of a complete drill bit. The invention thus eliminates the need for separate roof bits and drill rods. The drill tip is preferably of planar form and extends diametrically across the leading end of the rod across the open end of the central bore. The abutment elements are spaced apart on opposite sides of the central bore and the thickness of the drill tip is less than the diameter of the central bore, so that the bore is open on either side of the drill tip. This permits an increase in the pumping speed of the glue used to secure the bolt in the hole drilled by the rod, as well as the rate of flushing during drilling, compared to an arrangement having the hole covered (but not closed) by the drill tip. In a second aspect of the invention, the drill tip extends across and covers the opening of the central bore at the leading end of the rod. In this arrangement, branching conduits extend from the central bore and open out of the side wall surface of the rod, adjacent the leading end. A single branching conduit may be provided, although it is preferable to have at least two such conduits opening into opposite sides of the rod. The branching conduits may extend laterally to the central bore, or at an angle to the central bore. Moreover, the branching conduits may have any suitable cross- section but preferably will be circular and have a diameter which is either the same as, or greater or larger than that of the central bore. The above arrangement of branching conduits may equally be applied to the first aspect of the invention described above. That is, the roof bolt may have the central bore open at the leading edge thereof, but may also employ branching conduits extending to sidewall surfaces. Such an arrangement may promote greater pumping and flushing speeds, compared to an arrangement that does not include the branching conduits. In a roof bolt according to the invention, the drill tip extends across the central bore and preferably extends beyond the outer periphery of the rod. Alternatively, the opposite ends of the drill tip may terminate within the outer periphery of the rod, or extend to or adjacent the outer periphery. In the above arrangements the drill tip is also supported axially against the leading end of the bolt and preferably that support is provided against a base surface of the drill tip located adjacent to the leading end of the bolt and preferably the axial support is provided on either side of the central bore. The axial support of a base surface of the drill tip may extend between an edge of the central bore and an outer edge of the bolt and a planar surface may be machined for that purpose. The machined surface may be a platform distinct from other surface sections of the leading end. Fixed connection of the drill tip to the bolt can be achieved in any suitable manner and for example, a tungsten carbide tip may be fixed by brazing. Welding may be possible with other types of tip material. The drill tip can be fixed to any suitable part of the leading end of the bolt and preferably is fixed to both the abutment elements and the axial supporting surface. The rod of the roof bolt may be formed in any suitable manner and may for example be formed with a continuous or discontinuous thread extending along its length. Longitudinal flat sections may be located on opposite sides of the rod. If a thread and/or flat sections are provided, these may extend the full length of the rod, or may extend only over a section or sections of the rod. In one embodiment, the thread is terminated prior to the leading end of the rod. It is preferred that the leading end of the rod is formed to provide clearance behind the drill tip in the normal direction of rotation of the bolt. Thus, each of the abutment elements may be formed to extend axially to support substantially the full height of the rear face of the drill tip, but to slope axially downwardly away from the rear face to provide the desirable clearance. The leading end may also be formed to provide clearance in front of the oppositely facing cutting edges of the drill tip. That frontal clearance may extend from a radial outer edge of the drill tip inwardly to the other of the abutment elements supporting the opposite side or trailing face of the drill tip. The frontal clearance may vary in depth and in one form of the invention a curved or scalloped profile is applied which extends from a minimum depth adjacent the radial edge, to a maximum depth approximately adjacent the mid-section of the drill tip. It is preferred that the frontal clearance be of a continuous smooth curve, in order to minimize stress concentration, although it is nevertheless possible to provide frontal clearance by two or more curved sections, or by adjacent curved and straight sections. The profile of the frontal clearance may enlarge the opening of the central bore axially to facilitate increased flushing and glue pumping speed. The opening of the central bore can be so enlarged on either side of the drill tip by applying the same frontal clearance profile on either side thereof. The attached drawings show an example embodiment of the invention of the foregoing kind. The particularity of those drawings and the associated description does not supersede the generality of the preceding broad description of the invention. Figure 1 is a perspective view of a roof bolt 10 according to the invention while Figures 2 to 4 show different views of the same bolt. The bolt 10 is constructed in any suitable manner to define a central bore and the previously described methods of rolling or die drawing to achieve this are each appropriate, but not the only methods available. The rod 10 is formed with a discontinuous thread profile, comprising a thread 11, separated into opposite threaded rod portions 12 and 13, by a pair of oppositely located longitudinally extending flats 14 and 15. This type of rod profile is shown in Australian Patent No. 672428, although it should be appreciated that other rod profiles are equally within the scope of the present invention. The rod 10 has a leading end 16 which is machined to accept for connection thereto, a drill tip 17, such as a tungsten carbide drill tip of a known form. The drill tip 17 is of planar form defining a pair of opposite planar faces 18 and 19 and having a pair of inclined cutting edges 20 depending from a central channel or recess 21. The cutting edges 20 are formed according to known principles and a roof bolt formed in the manner shown is operative as a rotary drill and not a percussive drill. The planar faces 18 and 19 of the drill tip 17 extends into abutting engagement with a pair of axially extending abutment elements or wings 22 and 23. The abutment elements 22 and 23 are machined to engage flush against the respective planar faces 18 and 19 of the drill tip 17. Thus, the elements each include a planar face 24 for that purpose. Figure 5 shows the roof bolt 10 with the drill tip 17 removed and the planar faces 24 are more clearly evident from that figure. In the plan view of Figure 4, the elements 22 and 23 are shown to be generally triangular and to extend to faces 25 that are machined in a curved or scalloped manner. The faces 25 are machined in a scalloped manner to provide frontal clearance for the drill tip 17. The degree of frontal clearance can vary and Figures 6 and 7 show different profiles that have the same clearance purpose. In each of Figures 6 and 7, the face 25 includes a curved section 27 extending between two linear sections 28 and 29. Other profiles can equally be adopted. The end faces 26 of the elements 22 and 23 (see Figure 5) are inclined downwardly in the same direction as the cutting edges 20 of the drill tip 17 to provide cutting clearance behind the drill tip. That clearance permits the drill tip to move forward into the rock surface being drilled without the abutment elements 22 and 23 engaging that surface and hindering forward movement. As shown in Figure 2, the face 25 intersects the central bore 31, further exposing the open end 32 of the bore 31. This also enhances flushing and glue pumping speed. The bore 31 may have a diameter which is equal to the diameter of the bore of the rod as formed prior to machining the leading end 16 thereof, or it may be enlarged as necessary. Preferably the bore 31 is enlarged, and this may be achieved by drilling out the original rod bore. The bore may be enlarged for the full lengthwise extent of the rod, or for only a portion thereof, such as at the leading end only. The leading end of the rod 10 is furthermore machined in a manner that supports the drill tip 17 axially against axial drilling forces imparted on the cutting edges 20 during drilling. The support surface 30 shown in Figure 2 forms a supporting platform to support the drill tip 17 along the underneath surface thereof. The extent of axial support can vary from the outer peripheral edge of the rod to the inner edge of the central bore 31, or can be a lesser amount as may be acceptable. In this respect, drill tips are generally quite brittle and maximum support is desirable, although not necessarily essential. The drill tip 17 is thus supported both axially and rotationally and is connected to the leading end of the roof bolt generally by brazing. Most importantly, in the roof bolt of the invention, it is not necessary to employ a drill bit as in known self- drilling roof bolts, so that the expense of the bolt is considerably reduced. Figure 8 shows a still further embodiment of the invention, in which the thread 11' is terminated well before the leading end 16' of the bolt 10'. The unthreaded section 33 of the bolt 10' further enhances flushing and glue pumping speed, by reducing resistance to flow. Figures 9 to 16 show a roof bolt which is principally arranged for percussive drilling. The roof bolt 100 includes a pair of abutment elements 101 and 102 which define a recess 103 for accommodating a drill tip 104. The drill tip is a percussive drill tip, which is shaped to have a chisel-like form, as opposed to a cutting edge of the kind shown in Figures 1 to 8. The roof bolt 100 operates in a similar manner to the roof bolt 10 of Figures 1 to 8 and thus is arranged to have the central bore 105 open at the leading end 106 as shown in Figures 9 and 12. Additionally however, the roof bolt 100 includes a lateral branching conduit 107 which extends laterally to the central bore 105 and opens out of the flat surface 108. The roof bolt 200 shown in Figures 13 to 16 includes many of the features shown in the roof bolt 100, and therefore, like parts will be identified by the same reference numeral plus 100. The drill tip 204 has a greater thickness T than the thickness t of the tip 104 as shown in Figure 10. Thus, because of that thickness T (which is greater than the diameter of the central bore), the central bore is not able to be open at the leading end 206. Thus, the main difference between the roof bolts 100 and 200, is that the drill tip 204 extends over and fully covers the open end of the central bore at the leading end 206 thereof. The roof bolt 200 does however, include at least one branching conduit 207 that extends laterally from the central bore and therefore opens the central bore into the flat surface 208 adjacent the leading edge 206. The invention described herein is susceptible to variations, modifications and/or additions other than those specifically described and it is to be understood that the invention includes all such variations, modifications and/or additions which fall within the spirit and scope of the above description.

Claims: Claims: 1. An elongate roof bolt having an axially extending central bore and defining a leading end at one end thereof, said leading end including a pair of axially extending abutment elements formed by machining, each of which includes an abutment face for engagement with a drill tip on opposite sides thereof, to support the drill tip during drilling, said drill tip extending across said leading end and being secured to the abutment elements by welding, soldering or brazing, said bore opening out of said bolt at or adjacent said leading end. 2. An elongate roof bolt according to claim 1, said drill tip having a thickness which is less than the width of the central bore and said central bore thereby opening at said leading end on at least one side of said drill tip. 3. An elongate roof bolt according to claim 2, wherein said drill tip extends across said leading end centrally of said central bore, so that said bore opens at said leading end substantially equally on either side of said drill tip. 4. An elongate roof bolt according to any one of claims 1 to 3, further including one or more branching conduits extending from said central bore to open adjacent said leading end of said bolt. 5. An elongate roof bolt according to claim 1, said drill tip having a thickness which is greater than the width of said central bore to close said central bore at said leading end, said central bore opening adjacent said leading end through one or more branching conduits extending from said central bore. 6. An elongate roof bolt according to any preceding claim, wherein said abutment elements support said drill tip along substantially its full length across said leading end and against opposite side surfaces thereof. 7. An elongate roof bolt according to any one of claims 1 to 5, wherein said abutment elements are disposed on opposite sides of said drill tip and support said tip only at opposite ends thereof. 8. An elongate roof bolt according to any preceding claim, wherein said drill tip is supported along at least a portion of a base surface thereof against a planar surface of said leading end which is formed by machining. 9. An elongate roof bolt according to claim 8, said planar surface being formed on a platform which is machined into said leading end. 10. An elongate roof bolt according to claim 8 or 9, said planar surface extending from opposite edges of said central bore to outer edges of said leading end. 11. An elongate roof bolt according to any preceding claim, said roof bolt including a thread extending along the outer surface of the bolt and being terminated at or adjacent said leading end. 12. An elongate roof bolt according to any one of claims 1 to 5 or any one of claims 7 to 11 , a clearance space being provided in said leading end on at least one side of said drill tip adjacent a respective abutment element so as to expose a portion of at least one side face of said drill tip. 13. An elongate roof bolt according to claim 12, said leading end being shaped to extend axially downwardly from at least one of said abutment elements to provide said clearance space. 14. An elongate roof bolt according to claim 13, said leading end being shaped to provide said clearance space from one side edge of said drill tip to said abutment element. 15. An elongate roof bolt according to claim 14, said profile of said clearance space being curved or scalloped from a minimum depth adjacent an outer edge of said leading end to a maximum depth adjacent the mid section of said drill tip.

Inventors: JOHNSON ERIC, MCLEAN MICHAEL, WEAVER STEVEN, JOHNSON, ERIC, MCLEAN, MICHAEL, WEAVER, STEVEN
Applicants: SANDVIK AB, SANDVIK AB (PUBL)
Application year: 2000
Main classification: E21B10
IPC codes: E21D21