AU779367B2 - An anchor device for use in mining - Google Patents

Description

AN ANCHOR DEVICE FOR USE IN MINING Field of the Invention This invention relates to an anchor device for engaging in a bore and, more particularly, but not exclusively, to an anchor device for use in mining applications.

Background of the Invention One form of anchor device used in the mining industry is known as a friction bolt. It comprises a tapered substantially tubular body, with a C-shaped transverse crosssection. The C-shaped cross-section enables the body to be radially compressed to fit within a bore such that when inserted into the bore, elastic compression of the body causes a radially outward force to be exerted upon the sides of the bore to thereby 15 anchor the bolt in place. The bolt may be fitted with a load plate or the like which is fixed in place by a ring secured to a free end of the bolt.

It has been determined by the Applicant that the load bearing capacity of such a bolt is limited since force transfer between the bolt and the bore has been found to occur at 20 three contact points on the C-shaped cross-section. Previous perception was that the bolt engaged the bore wall about the entire circumference of the bolt but closer "5 analysis shows that the body of the bolt actually hinges about its geometric centre when the arms are deflected inwardly to provide only three contact points: one point S. at each free end of the cross-section and another contact point midway on the cross- 25 section between the free ends.

Other prior art includes Australian Patent Application No. 68704/94 and Fig. 7 of US Patent No. 5,192,146 which both disclose non-cylindrical friction bolts which have 3, 4 or more regions of contact with the interior wall of the hole into which they are inserted. Such frictional bolts suffer from the disadvantage that bending of the bolt is restricted to a small number of locations. Then it is easy for the elastic limit of the bolt material to be exceeded during such bending. As a consequence plastic deformation may take place thereby lowering the stored energy of the installed bolt, and thus its holding ability.

United States Patent No. 5,192,146 (Figs 1-6) discloses a slit cylindrical friction bolt where the slit constitutes a substantial proportion of the bolt circumference. Since the part cylindrical surface only makes an interference fit with the bore hole at three locations, the material adjacent the slit has been removed to save weight and material costs. There is no disclosure of non-elastic plastic) deformation of the bolt.

United States Patent No. 4,472,087 discloses various arrangements which enable the friction bolt to be compressed prior to entering the hole. Instead of the hole itself compressing the bolt as it is inserted, a support plate keeper rings (30, 36), a washer plate an internal longitudinal clip (40) or an external spiral strip (41) are used to compress the bolt as it is inserted into the hole. The compressed bolt than expands after insertion. Interior and exterior shallow grooves in the cylindrical wall of the bolt are provided to weaken same and make it more compressible. Such a 15 strategy is only of assistance in soft coal and is not relevant to "hard rock" metalliferous mining.

o The above prior art seeks to address the same problem in different ways. The present invention seeks a still further solution utilising a different inventive concept.

Object of the Invention •o••o S" The present invention seeks to provide an anchor device of alternative construction. to t 25 Summary of the Invention In accordance with the present invention, there is provided an anchor device for frictional engagement with the interior of a hole drilled in a rock or the like, said device comprising a longitudinally slit hollow substantially cylindrical member rolled from a strip of metal in which the longitudinally extending edges of said strip of metal form the side edges of the longitudinally extending slit and wherein a longitudinally extending groove is formed in said device substantially opposite said slit by bending of the material of said strip of metal, said groove opening away from said slit.

Brief Description of the Drawin s The invention is described in more detail, by way of example only, with reference to the accompanying drawings, in which: Fig. I is an enlarged cross-sectional view of a prior art friction bolt in a compressed configuration within a bore; Fig. 2 is an enlarged cross-sectional view of an anchor device according to the conreferred embodiment of the present invention, the device being in a compressed configuration within a bore; Fig. 3 is a graph illustrating pressure profiles for the device of Fig. 2 and the bolt of 15 Fig. 1; and Fig. 4 is a side view of the device shown in Fig. 2.

0 9 Detailed Descri tion of a Preferred Embodiment Referring firstly to a Figure 1, a prior art of friction bolt 1s shown as comprising a body 2 with a 'C'-shaped section terminating in free ends 3, 4 which are arranged to .d r n n deflect about a single 00 0 WO 01/46558 PCT/AU00/01583 -4locus 5, which corresponds to a geometric centre 6 of the body. The bolt is thereby arranged to frictionally engage a wall 7 of a bore 8 at only three points 9, 10, 1 1, which reduces the load bearing capacity of the bolt 1. Further, the distance between the free ends 3, 4 and the locus is usually maximised which maximises bending moments applied to the free ends 3,4 of the body 2. That in turn can limit the contact force which may be applied between the bolt I and the bore, prior to failure of the bolt due to excessive bending moments being applied.

Referring now to Figure 2, an anchor device 20 of the present invention is shown as including a body 21 with a transverse cross-section defined by two arms 22,23 coupled to an intermediate section 24. The device 20 has an inset portion 25 defined by a concave section 26 of the intermediate section 24. Laterally spaced contact portions 27,28 are provided at either side of the portion 25, for engagement with a wall 29 of bore 30. The portion 25 is substantially diametrically opposite a gap 52 provided between the arms 22,23 and each contact portion 27,28 is arranged generally opposite a respective free end of the arms. The contact portions 27,28 substantially correspond to respective locations 31,32 where the arms 22,23 join with the section 24. The locations 31,32 also form or are substantially congruent with loci 33, about which the arms 22,23 deflect.

The device 20 thereby provides four contact points for engagement with the wall 29; two points adjacent free ends 34,35 of the arms 22,23 and two further points associated with the portions 27,28. The increased number of contact points, as compared to the bolt 1 of Figure 1, allows for greater load bearing capacity, as reflected in experimental results, shown in Figure 3. That Figure shows a plot of contact force versus radial angle, where 180* represents the locus 5 of the bolt 1 and the corresponding position of the device 20. The curves 40, 41 and 42 show sustainable contact forces about respective points 9, 10 and 11 of the bolt 1, while the curves 43, 44, 45 and 46 show sustainable contact forces about the free ends 34,35 and the contact portions 27,28, respectively.

Aside form providing an increased number of contact points, the device 20 also reduces the length of the arms 22,23, as a result of introduction of the intermediate section 24, as compared to the bolt 1 and that has the affect of reducing the bending moment for equivalent force WO 01146558 PCT/AUOO/01583 application, which in turn increases the comparative load bearing limit of the device 20, prior to failure.

Further, it should be noted that the inset portion 25, although shown as being somewhat concave, is still reasonably flat, in order that the portion 25 does not itself function as a hinge point or section of weakness in the device. To ensure that is the case, the portion 25 is formed such that a relative angle between an imaginary line 61,62, drawn along any two parts of the section 25 is less, and preferably substantially less, than The above advantages may be utilised to reduce material required to produce a device having similar load bearing capabilities as the bolt 1 or alternatively a device may be provided using a similar amount of material with enhanced load bearing characteristics.

Referring lastly to Figure 4, the device 20 can be seen as being formed as an elongate friction bolt 50 with a tapered end 51 and an axially extending gap 52 defined between the free ends 34,35 of the arms 22,23. The device 20 may, however, instead comprise only a portion of the bolt, such as the tapered end 51 or, alternatively, be in the form of an attachment device which is simply fitted to an existing bolt or formed at an end thereof, in order to provide improved load bearing characteristics.

Expressed in a different manner to the above, the comparative load bearing advantage of the device of the invention may be seen as resulting from the shape of the bolt cross-section, and the resulting location and orientation of the four contact regions between the bolt and the bore wall, with the bolt cross-section functioning as an array of three relatively short, stiff beamcolumns. During installation then, the free ends 34,35 of the cross-section close together pivoting at 33 with the arms 22,23 maintaining the original pre-installation radii. In effect then the arms 22,23 behave as short curved beams whereas in the prior art bolt configuration the curved sections are longer and less stiff. Thc curved scction 25 is also iquite rgid, which is the result of being of a relatively short length and having three small radii resulting from cold working of the metal during a roll forming process. The roll forming process takes the metal into the plastic or permanent deformation zone. Further, the curvature of each arm of the prior WO 01/46558 PCT/AUO0/01583 -6art configuration is around 175 degrees whereas the profile of the device 20 is around 155 degrees. This reduced curvature and resulting shorter arm length means that for the same deflection during installation (hole diameter does not change) a greater contact force or pressure is achieved at the ends 34,35. Bending moments at the contact portions 27,28 are greater than what is achieved at the single point I 1 of the prior art configuration. This is supported by Figure 3 which shows higher local stress areas at portions 27,28. Resolving forces at the ends 34,35 into tangential and radial components and solving for forces at 27 and 28 results in higher contact forces or pressures at contact portions 27,28 than the equivalent at the prior art configuration at location 1i. The sum of the four contact forces produced by the bolts of the invention is 30% to 60% higher than the sum of the three contact forces which are produced by prior art bolts of the same material, wall thickness and insertion depth which suit the same hole size. Accordingly, the pull-out force for a bolt formed in accordance with the invention will be correspondingly higher than that for the prior art "circular section" bolts.

The anchor device has been described, by way of non-limiting example only, and many modifications and variations may be made thereto without departing from the spirit and scope of the invention as described.

Claims (5)

1. An anchor device for frictional engagement with the interior of a hole drilled in a rock or the like, said device comprising a longitudinally slit hollow substantially cylindrical member rolled from a strip of metal in which the longitudinally extending edges of said strip of metal form the side edges of the longitudinally extending slit and wherein a longitudinally extending groove is formed in said device substantially opposite said slit, by bending of the material of said strip of metal, said groove opening away from said slit.

2. The device is claims in claim 1 wherein said groove is substantially arcuate when viewed in cross-section.

3. The device as claimed in claim 1 or 2 wherein said groove has a pair of longitudinally extending shoulders each of which extends along a corresponding side of said groove, and wherein on insertion of said device into a cylindrical hole, said device bears against said hole at substantially four locations being each of said S 15 shoulders and a region located intermediate each shoulder and the adjacent side edge *O of said slit. *0 The device as claimed in claim 3 wherein each said intermediate region is of substantial circumferential extent. The device as claimed in any one of claims 1-4 and having a tapered tip. "00 20 6. The device as claimed in any one of claims 1-5 and having a ring formed at "one end thereof.

7. The device as claimed in any one of claims 1-6 wherein said groove extends at least partially along said device. oo 0 S:

8. An anchor device for frictional engagement with the interior of a hole drilled 25 in a rock or the like, said device being substantially as herein described with reference to Figs. 2-4 of the drawings. Dated this 18 h Day of November 2004 INDUSTRIAL ROLL FORMERS PTY LTD by: FRASER OLD SOIN Patent Attorneys for the Applicant