AU2004203876B9 - Yielding rock bolt - Google Patents

Description

P/00/011 Regulation 3.2 AUSTRALIA PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT Name of Applicant: FERO STRATA SYSTEMS PTY LTD Address of Applicant: 22 Chisholm Crescent, KEWDALE WA 6105 Actual Inventor: David MALTBY Address for Service: Griffith Hack, Patent and Trade Mark Attorneys, 6th Floor, 256 Adelaide Terrace, Perth, Western Australia, 6000. Standard Complete Specification for the invention entitled: YIELDING ROCK BOLT Details of Associated Provisional Applications: 2003904384 dated 18 August 2003 The following is a full description of this invention, including the best method of performing it known to me:- - 2 Yielding Rock Bolt Field of the Invention 5 The present invention relates to a yielding rock bolt of the type typically used for stabilisation of ground in underground mining. Background of the Invention 10 Various ground control schemes are used in underground mining for the purposes of preventing rock falls and wall stabilisation. These schemes include use of rock bolts, cable bolts, plates and mesh. 15 The present Inventor has previously developed a yielding rock bolt described in International application No. PCT/AUO2/01223 which generally comprises a multi-strand cable which includes an axial king wire and a hollow 20 sleeve surrounding the cable. The hollow sleeve has an internal diameter less than the outer diameter of the cable. The cable is grouted into a hole formed in a body of rock with the toe of the hole located in stable, non fractured rock. An end of the cable protrudes from the 25 free face of the rock body and is retained by a clamp. In the event of relative movement say along a fracture line between bodies of rock which create a tensile force in the cable, the cable is drawn through the sleeve thereby transferring load from the rockface to the bolt. 30 Summary of the Invention According to the present invention there is provided a yielding rock bolt comprising: 35 H:\rosg\Keep\Speci\Complete\P54049 - Yielding Rock Bolt - Fero Strata Systems 16/08/04 - 3 an elongate bar having opposite first and second ends and a screw thread commencing at said first end and extending towards the second end; and, 5 a nut provided with a through hole having a circumferential surface and a plurality of spaced apart teeth protruding from said circumferential surface, said teeth configured to engage with said screw thread to facilitate screwing of said nut along said bar and to cut 10 through said screw thread in response to a tensile force of determinate magnitude applied to said bar between said second end and a position on said bar where said nut engages said thread. 15 In one embodiment said teeth are spaced along a helical path extending axially along said circumferential surface. Preferably said teeth are evenly spaced circumferentially along said helical path. 20 In one embodiment, said teeth are aligned axially along said circumferential surface. However in an alternate embodiment, said teeth are staggered so that no two teeth are axially aligned in mutually adjacent turns of said helical path. 25 Preferably said nut comprises at least two spaced apart teeth for each complete turn of said helical path. In one embodiment, all of said teeth are of the same arc 30 length. However in an alternate embodiment, said nut comprises teeth of at least two different arc lengths. 35 Preferably circumferentially adjacent teeth are spaced by a distance substantially the same as the arc length of the teeth. H:\rosg\Keep\Speci\Complete\P54049 - Yielding Rock Bolt - Fero Strata Systems 16/08/04 - 4 Preferably said teeth are made of a harder material than said thread. 5 Preferably said bar and nut are relatively dimensioned so that when said bar is inserted into, and fixed in, a hole in a body of hard material with said first end protruding from a free face of said hard material, and said nut is screwed along said bar from said first end to a position 10 where said nut abuts said free face, a tail portion of said bar which includes said first end and extends beyond said nut whereby on application of said tensile force by relative displacement of volumes of said hard material, said bar is pulled through said nut with said teeth 15 cutting through said thread so that said tail portion reduces in length providing a visual indication of said relative displacement of said volumes of said hard material. 20 According to the present invention there is further provided a method of detecting relative displacement of two or more volumes of hard material in a body of hard material, said body having a free face and a hole formed through said free face into said two or more volumes, said 25 method comprising: providing an elongated bar having opposite first and second ends and a screw thread commencing at said first end and extending towards the second end; 30 inserting said bar into said hole with said second end first and leaving a tail portion of said bar including said first end extending from said free face; 35 screwing a nut along said thread to abut said free face, said nut provided with a through hole having a circumferential surface and a plurality of spaced apart H:\rosg\Keep\Speci\Complete\PS4049 - Yielding Rock Bolt - Pero Strata Systems 16/08/04 - 5 teeth protruding from said circumferential surface, said teeth configured to engage said screw thread to facilitate screwing of said nut along said bar, and to cut through said screw thread in response to a tensile force of 5 determinate magnitude applied to said bar between said second end and a position of said bar where said nut engages said thread; said bar and nut arranged so that said tail portion of 10 said bar extends beyond said nut; and, monitoring for a change in length of said tail portion to detect displacement of one of said volumes of hard material from another of said volumes of hard material. 15 In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as 20 "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention. 25 It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country. 30 Brief Description of the Drawings An embodiment of the present invention will now be described by way of example only with reference to the 35 accompanying drawings in which: H:\rosg\Keep\Speci\Complete\P54049 - Yielding Rock Bolt - Fero Strata Systems 16/08/04 - 6 Figure 1 is a schematic representation of an embodiment of the present yielding rock bolt in use; Figure 2 is an end view of a nut incorporated in the 5 yielding rock bolt; Figure 3 is a view of Section AA of the nut shown in Figure 2; 10 Figure 4 is a cross-sectional view of a nut in accordance with a second embodiment of the present invention; and, Figure 5 is a cross-sectional view of a nut in accordance with a third embodiment of the present invention. 15 Detailed Description of Preferred Embodiments Referring to the accompanying drawings and in particular Figures 1-3, it can be seen that a yielding rock bolt 10 20 in accordance with the present invention comprises an elongate bar 12 and a nut 14. The bar 12 has opposite first and second ends 16 and 18 respectively and a screw thread 20 commencing at the first end 16 and extending helically about an outer circumferential surface of the 25 bar 12 towards the second end 18. If desired, the screw thread 20 may extend for the whole length of the bar 12 from the first end 16 to the second end 18. The nut 14 is provided with a through hole 22 having a 30 circumferential surface 24 and a plurality of spaced apart teeth 26 protruding radially inward from the circumferential surface 24. The teeth 26 are configured to engage with the screw thread 20 to facilitate screwing of the nut 14 along the bar 12. The teeth 26 are further 35 configured to cut through the screw thread 20 in response to a tensile force T of determinate magnitude applied to H:\rosg\Keep\Speci\Complete\P54049 - Yielding Rock Bolt - Fero Strata Systems 16/08/04 - 7 the bar 12 between the second end 18 and a position on the bar 12 where a nut 14 engages the thread 20. Figure 1 depicts the bolt 10 in a typical application. In 5 this application, the bolt 10 is inserted into a hole 28 formed in the body of hard material such as a rock body 30. A number of fracture lines or faults 32 exist in the body 30. The hole 28 is drilled of a length so that the toe 34 of the hole 28 is located in an unfractured and 10 stable volume 36 of the rock body 30. The bar 12 is inserted into the hole 28 with the second end 18 first and located in the stable rock volume 36. The portion of the length of the bar 18 within the hole 28 15 is fixed to the rock body 30 by a setting compound such as grout 38 although other settable compounds such as resins can be used. The bar 12 is further dimensioned and otherwise arranged so that when the nut 14 is screwed along the thread 20 to a position where it abuts a free 20 face 40 of the rock body 30 or, an intervening plate 42 adjacent the free face 40, a tail portion 44 of the bar 12 extends beyond the nut 14. The bar 12 may be pretensioned by appropriate torquing of 25 the nut 14. However in addition, further tensile force T is applied to the bar 12 upon relative movement of various volumes of the rock body 30. For example, this may occur if the rock volumes 46 and 48 were to move away from each other along a common fracture line 32. 30 Provided that the tensile force T induced by this movement is of a sufficient magnitude, it will result in the teeth 26 cutting through the thread 20 and effectively drawing the bar 12 through the nut 14. Consequently the length of 35 the tail portion 44 will reduce. This reduction in the length of the tail 44 can be monitored either visually or H:\rosg\Keep\Speci\Complete\P54049 - Yielding Rock Bolt - Fero Strata Systems 16/08/04 - 8 otherwise to then provide an indication of movement of respective volumes of rock in the rock body 30. The tensile force T required to draw the bar 12 through 5 the nut 14 is determined by the thread 20 and the configuration and spacing of the teeth 26. In short, the greater the contact area between the teeth 26 and the thread 20, the greater the tensile force T required in order for the teeth to cut through the thread and thus for 10 the bar 12 to be drawn through the nut 14. In the specific embodiment shown in Figures 1 and 2, the teeth 26 are arranged in a helical path 50 centered about a central axis 52 of the hole 24. There are eight teeth 15 for each turn of the helical path 50 with the teeth 26 being evenly spaced circumferentially and arranged so that teeth in adjacent turns of the helical path 50 are in axial alignment. Circumferentially adjacent teeth 26 are separated by gaps 54. In this particular embodiment the 20 gaps 54 are of substantially the same arc length as the teeth 26. The gaps 54 from regions into which metal can flow as the teeth 26 cut through the thread 20. 25 The nut 14 is also provided with a spherical seat 56 which locates in a complementary recess in plate 42 to assist in alignment of the nut 14, plate 42 and bar 12. 30 Figure 4 depicts an alternate embodiment of the nut 14' for which like reference numbers are used to denote like features. The nut 14' differs from the nut 14 depicted in Figures 2 and 3 in terms of the arc length of the teeth 26 and their relative axial alignment. In the nut 14', the 35 teeth 26 are of a greater arc length than the teeth 26 in the nut 14 and in addition, teeth 26 in adjacent turns of H:\rosg\Keep\Speci\Complete\P54049 - Yielding Rock Bolt - Fero Strata Systems 16/08/04 - 9 the helical path 50 are staggered relative to each other rather than being axially aligned. Figure 5 depicts a further embodiment of the nut 14'' in 5 which the teeth 26 are of a different configuration to the teeth depicted in the nuts 14 and 14' having a greater circumferential or arc length than their axial length; and with the degree of staggering between teeth 26 and adjacent helical turns more pronounced. 10 From the above description, it should be apparent to those skilled in the art that embodiments of the present yielding rock bolt possesses several features hitherto unknown in the prior art. In particular, the bolt 10 15 provides users and operators with a very simple means for detecting seismic activity and other relative motions between volumes of rock in a rock body. In particular, embodiments of the yielding rock bolt 10 allow operators to visually detect such activity either by sight only in 20 recognising a change in the length of the tail portion 44 or alternately by additional use of a measuring device such as a ruler. It further allows operators to continually monitor the stability of the rock body by taking say length measurements of the tail portion 44 on a 25 regular basis. Further, the amount of load that can be transferred onto the bar 12 before the bar commences to yield may be varied by simply interchanging nuts 14 with different teeth 30 geometry and/or spacing. Now that embodiments of the present invention have been described in detail it will further be apparent to those skilled in the relevant arts that numerous modifications 35 and variations may be made without departing from the basic inventive concepts. In particular, the geometry and configuration of the teeth 26 can be almost infinitely H:\rosg\Keep\Speci\Complete\P54049 - Yielding Rock Bolt - Fero Strata Systems 16/08/04 - 10 varied to provide the desired load transferring characteristics to the bolt 10 provided that adjacent teeth 26 are spaced to form a gap into which the material from which the bar 12 is made can flow as it is being 5 drawn through the nut 14. Also, various combinations of materials can be used or hardening processes employed for the teeth 26 and/or thread 20 to provide desired load bearing characteristics, such as but not limited to phosphating and nitrocarburising. 10 All such modifications and variations together with others that would be obvious to a person of ordinary skill in the art are deemed to be within the scope of the present invention the nature of which is to be determined from the 15 above description and the appended claims. H:\rosg\Keep\Speci\Complete\P54049 - Yielding Rock Bolt - Fero Strata SyStems 16/08/04

Claims (14)

1. A yielding rock bolt comprising: an elongate bar having opposite first and second ends and 5 a screw thread commencing at said first end and extending towards the second end; and, a nut provided with a through hole having a circumferential surface and a plurality of spaced apart 10 teeth protruding from said circumferential surface, said teeth configured to engage with said screw thread to facilitate screwing of said nut along said bar and to cut through said screw thread in response to a tensile force of determinate magnitude applied to said bar between said 15 second end and a position on said bar where said nut engages said thread.

2. A yielding rock bolt according to claim 1 wherein said teeth are spaced along a helical path extending 20 axially along said circumferential surface.

3. A yielding rock bolt according to claim 2 wherein said teeth are evenly spaced circumferentially along said helical path. 25

4. A yielding rock bolt according to claim 2 or 3 wherein said teeth are aligned axially along said circumferential surface. 30

5. A yielding rock bolt according to claim 2 or 3 wherein said teeth are staggered so that no two teeth are axially aligned in mutually adjacent turns of said helical path. 35

6. A yielding rock bolt according to any one of claims 2-5 wherein said nut comprises at least two spaced apart teeth for each complete turn of said helical path. H:\rosg\Keep\Speci\Complete\P54049 - Yielding Rock Bolt - Fero Strata Systems 16/08/04 - 12

7. A yielding rock bolt according to any one of claims 1-6 wherein all of said teeth are of the same arc length.

8. A yielding rock bolt according to any one of claims 5 1-6 wherein said nut comprises teeth of at least two different arc lengths.

9. A yielding rock bolt according to claim 7 wherein circumferentially adjacent teeth are spaced by a distance 10 substantially the same as the arc length of the teeth.

10. A yielding rock bolt according to any one of claims 1-9 wherein said teeth are made of a harder material than said thread. 15

11. A yielding rock bolt according to any one of claims 1-10 wherein said bar and nut are relatively dimensioned so that when said bar is inserted into, and fixed in, a hole in a body of hard material with said first end 20 protruding from a free face of said hard material, and said nut is screwed along said bar from said first end to a position where said nut abuts said free face, a tail portion of said bar which includes said first end extends beyond said nut whereby, on application of said tensile 25 force by relative displacement of volumes of said hard material said bar is pulled through said nut with said teeth cutting through said thread so that said tail portion reduces in length providing a visual indication of said relative displacement of said volumes of said hard 30 material.

12. A method of detecting relative displacement of two or more volumes of hard material in a body of hard material, said body having a free face and a hole formed through 35 said free face into said two or more volumes, said method comprising: N:\Perth\Cases\Patent\5OOO-50999\P50340.AU.1\Specis\P50340.AU.1 Specification 2009-7-27.doc 27/07/09 - 13 providing an elongated bar having opposite first and second ends and a screw thread commencing at said first end and extending towards the second end; 5 inserting said bar into said hole with said second end first and leaving a tail portion of said bar including said first end extending from said free face; 10 screwing a nut along said thread to abut said free face, said nut provided with a through hole having a circumferential surface and a plurality of spaced apart teeth protruding from said circumferential surface, said teeth configured to engage said screw thread to facilitate 15 screwing of said nut along said bar, and to cut through said screw thread in response to a tensile force of determinate magnitude applied to said bar between said second end and a position of said bar where said nut engages said thread; 20 said bar and nut arranged so that said tail portion of said bar extends beyond said nut; and, monitoring for a change in length of said tail portion to 25 detect displacement of one of said volumes of hard material from another of said volumes of hard material.

13. A yielding rock bolt substantially as herein described with reference to and as illustrated in the 30 accompanying drawings.

14. A method of detecting relative displacement of two or more volumes of a hard material substantially as herein described with reference to and as illustrated in the 35 accompanying drawings. H:\rosg\Keep\Speci\Complete\P54049 - Yielding Rock Bolt - Fero Strata Systems 16/08/04 - 14 Dated this 16th day of August 2004 FERO STRATA SYSTEMS PTY LTD 5 By His Patent Attorneys GRIFFITH HACK Fellows Institute of Patent and Trade Mark Attorneys of Australia H:\rosg\Keep\Speci\complete\P54049 - Yielding Rock Bolt - Fero Strata Systems 16/08/04