WO2003087538A1

WO2003087538A1 - A rock bolt

Info

Publication number: WO2003087538A1
Application number: PCT/ZA2003/000050
Authority: WO
Inventors: Karel Francois Bredenkamp; Peter Frank Reinhold Farcroft Altounyan
Original assignee: Rock Mechanics Technology Ltd
Current assignee: Rock Mechanics Technology Ltd
Priority date: 2002-04-12
Filing date: 2003-04-14
Publication date: 2003-10-23
Anticipated expiration: 2004-10-12
Also published as: NZ535955A; ZA200408149B; AU2003262192B2; AU2003262192A1

Abstract

A rock bolt.(1) comprising an elongate body having torque limiting means at an outer end (3) of the body, the torque limiting means having at least one deformable protusion (2) that deforms under a predetermined torque. There is provided for the deformation of the deformable protusion (2) to prevent further rotation of the rock bolt (1) by a torque application apparatus. A further feature of the invention provides for the deformable protusion (2) to be shaped to receive a complementary shaped chuck on a torque application apparatus to facilitate transferring torque to the rock bolt (1). Further features of the invention provide for the deformable protusion to be protusions extending in an axial direction from an end surface of the rock bolt (1).

Description

A ROCK BOLT

FIELD OF THE INVENTION

This invention relates to a rock bolt and more particularly, but not exclusively, to a rock bolt and a torque limiting means for use when installing the rock bolt and which torque limiting means also provides a visual indication of the correct installation of the rock bolt.

BACKGROUND TO THE INVENTION

A well-known and commonly applied method of installing rock-bolts involves the use of a two-component adhesive that is placed in a pre-drilled hole in a rock face. The two components of the adhesive are located in two separate compartments of a plastics material capsule. Insertion and rotation of the bolt in the hole ruptures the capsule and mixes the two components of the adhesive.

Two alternative methods of installing a rock bolt are used. The first method involves the use of a bolt having a circular cross-section with a textured or knurled outer surface. The operatively inner end of the bolt is pushed into the hole containing a two compartment adhesive capsule. The bolt ruptures the capsule for mixing of the two components previously held in the two compartments. The outer end of the bolt may be formed to a non-circular section in order to mate with a matching socket on a rotation means. The rotation may be simultaneous with the insertion, or be commenced after insertion. After a specified time, judged to be sufficient to mix the two components of the adhesive, the rotation is stopped and the rotation means is removed. It is recognized that the rotation time must be sufficient to allow thorough mixing of the two components of the adhesive, but must not be prolonged after the adhesive has started to set, otherwise the chemical bonds that produced the "set" would be disrupted mechanically. This method, although simple and using an inexpensive bolt, has the following disadvantages:

a) the process is critically time-dependent and assumes no variation in factors such as temperature and age of the adhesive which are known to have a major effect on the mixing and setting times; and b) there is no visible indication that the bolt has been installed correctly and is well bonded to the rock and the bolt.

The second rock bolt installation method involves the use of a bolt having a threaded portion at its outer end. Figures 5 - 8 show the prior art apparatus for use in this method. A drive-nut (7) is fixed by means of a shear-pin (8) to the outer end of the bolt (9). The shear-pin (8) may be a "half pin" or a "full pin" as shown in figures 6 and 7 respectively. The bolt is pushed and rotated into the hole containing the adhesive in the manner described above, up to the point at which the rotation is stopped. Commonly, two adhesives with different setting times are used in the hole, the faster setting adhesive being placed in the distal portion of the hole. The faster adhesive is then allowed to set for a specified minimum time, after which the rotary torque is re-applied. If, and only if, the adhesive has set, this re-application will cause the shear- pin to break and will cause the nut to run a distance up the threaded portion to force a load-bearing plate (10) having a centrally located hole through which the bolt extends, into contact with the rock face, thus applying a degree of outwardly directed tension to the bolt. If, in spite of the setting of the adhesive, the bolt is not secure in the hole, the nut will pull the bolt out of the hole as it is tightened and this fault will be revealed by the final position of the nut on the threaded portion of the bolt. This method offers much more control of variables in the setting process and is to an extent self-checking in terms of correct installation. The length of the bolt extending out of the rock face also serves as an indication of the integrity of the installation. Its disadvantage is the increased cost associated with the provision of the threaded portion, and the nut and the shear-pin assembly, which must be of good engineering quality and tolerance in order to perform consistently and reliably. In addition, there still remains the requirement for timing the installation process, which requires special training and supervision. OBJECT OF THE INVENTION

It is an object of this invention to provide a rock bolt that, at least partially, alleviates some of the abovementioned difficulties.

SUMMARY OF THE INVENTION

In accordance with this invention there is provided a rock bolt comprising an elongate body having torque limited means at an outer end of the body, the torque limiting means having least one deformable protrusion that deforms under a predetermined torque.

There is provided for the deformation of the deformable protrusion to prevent further rotation of the rock bolt by a torque application apparatus.

A further feature of the invention provides for the deformable protrusion to be shaped to receive a complementary shaped chuck on a torque application apparatus to facilitate transferring torque to the rock bolt.

Further features of the invention provide for the deformable protrusion to be protrusions extending in an axial direction from an end surface of the rock bolt; for the protrusions to extend across the end on diameter of the rock bolt; alternatively for the protrusions to be located at intervals on the circumference at the outer end of the rock bolt.

This invention extends to a method of installing a rock bolt comprising the steps of: locating an adhesive in a hole in a rock body; inserting a rock bolt in the hole; rotating the rock bolt in the hole to mix the two component adhesive using a torque limiting device until the torque limiting device stops further rotation of the bolt as a result of the setting of the two component adhesive around the rock bolt.

The method includes the step of designing the torque-limiting device to limit the torque applied to the bolt at a predetermined value.

There is provided for the predetermined value of the torque to be determined from at least one or more of the group consisting of the following: the diameters of the bolt and of the hole; the length of the bolt located in the adhesive; and the type of adhesive used.

The method also includes a positive visual indication that the torque-limiting device has been invoked.

These and other features of the invention are described in more detail below. BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described below, by way of example only, and with reference to the accompanying drawings in which:

Figure 1 : shows an end view of an outer end of a rock bolt;

Figure 2: shows a side view of the outer end of the rock bolt of figure 1 ;

Figure 3: shows an end view of an outer end of a second embodiment of a rock bolt;

Figure 4: shows a side view of the outer end of the rock bolt of figure 3;

Figure 5: shows a partial side view of an end of a prior art rock bolt having a nut and shear-pin assembly;

Figure 6: shows an end on view of the rock bolt of Figure 5 having a "half-pin" shear-pin assembly;

Figure 7: shows an end on view of another alternative of the rock bolt of figure 5 in which the rock bolt includes a "full-pin" shear-pin assembly; and Figure 8; shows a side view of the rock bolt of figure 5 installed in a rock face.

DETAILED DESCRIPTION OF THE DRAWINGS

With reference to the drawings; a rock bolt is generally indicated by reference numerals 1 and 4.

The rock bolt 1 of figures 1 and 2, on its outer end 3 includes axially extending deformable protrusions 2 for receiving a complementary shaped chuck of a torque application apparatus, or rotation means.

The rock bolt 4 of figures 3 and 4, on its outer end and on its circumferential surface, has a torque limiting means in the form of deformable protrusions 6 for receiving a complementary shaped chuck of a torque application apparatus.

In use, the chuck engages with the protrusions 2 and 6 to transfer torque to the bolts 1 and 4. The protrusions are designed such that they deform at a predetermined torque. Upon deformation the chuck disengages from the protrusions and will no longer transfer torque to the bolt.

The torque application apparatus is removed from the bolt and at least part of the deformed protrusion remains on the bolt to provide a visible indication that rotation of the bolt was discontinued at a predetermined torque. In some cases, some parts of the protrusions may be sheared from the rest of the protrusion, but the fact that the protrusions gave way at a predetermined torque would be visible.

The apparatus described above requires the use of fast setting resin but obviates the use of shear-pin assemblies and the precision timing installation processes.

The fast setting resin used with the apparatus described above is necessary to ensure that the resin is not damaged before break-out occurs. If the resin does not set fast enough it will be destroyed while it is setting. The change over from mixing to setting must be rapid so that break-out occurs without destroying the resin while in an almost set state.

One method of using the rock bolt is now described and explained below by way of example only.

Field and laboratory measurements and experiments have shown that, although an over-long mixing time does cause some disruption of the setting of the adhesive in the hole, there are conditions under which this has only a minor effect on the final quality of the installation of the rock bolt. The adhesive used during the field and laboratory experiments was a two- compartment resin capsule, sold under the name "Fasloc®". The larger compartment contains a mastic polyester resin and inert fillers, mainly limestone. The small compartment contains further inert limestone fillers, water and benzoyl peroxide, which is the activator for the polyester resin. In use, the resin capsule is inserted in the pre-drilled hole, followed by the bolt. When the bolt is inserted and spun, it ruptures the compartments of the capsule and mixes the contents. The polyester resin is activated by the benzoyl peroxide and hardens after a pre-determined latent period. The latent period is necessary for uniform mixing of the contents to take place.

The hardening characteristics of the polyester resin can be described in terms of two times: gel time and set time. The gel time is the time, measured from start of mixing, at which the first stiffness becomes sensible. The set time is a later time, at which the mastic resin has become sensibly rigid. The intervening period is the cure time. The test method is described in, for example, South African Standard SABS 1534: 1991.

Conventionally, it is considered poor practice to disturb the resin in the cure time, as the disturbance permanently damages the partially set resin.

The efficacy of the spin-to-breakout method relies on a very rapid increase in strength of the resin after the gel time, that is a short cure time. The increase must be so rapid that the limiting torque of the break-out device is reached before significant disturbance to the setting resin mastic occurs. Optimally, the cure time should be as short as possible but a practical lower limit is determined by the necessary latent period, as cure time is approximately proportional to gel time for polyester resins.

The "Fasloc®" resin used had typical gel and set times of 9 seconds and 13 seconds respectively. These times were determined according to the test method of SABS 1534:1991. In mine use both times are shortened, and the interval reduced by the temperature rise caused by f ctional heating during mixing.

The fully set mastic had typical shear strengths of 25 - 26 megapascals, measured according to SABS 1534:1991.

Furthermore, under these conditions, the process of applying tension to the bolt during the setting time (as in the second method referred to above) can cause relatively greater damage to the quality of the installation.

These discoveries lead to the conclusion that it would be possible to install a bolt satisfactorily by continuing the mixing by rotation until the setting adhesive had developed a specified mechanical resistance to further mixing, at which point a torque-limiting device, conveniently based on the shearing of a component, would stop the mixing process.

The conditions under which the technique may be successful have been established for different configurations and different adhesives. This condition is defined in terms of the relationship between bolt diameter and hole diameter (the adhesive annulus) and the physical properties of the adhesive.

Having secured the bolt in the hole by this method, it would only be detrimental in this case to apply tension to the bolt, as is customary in the second method described above. The provision of a threaded portion, matching nut and load-plate therefore becomes not only un-necessary but also undesirable, thus reducing the cost of the unit.

The method by which the "shear" condition is achieved includes the incorporation into the lower or operatively outer end of the bolt of a visible "tell-tale" mechanism or deformable protrusion as described above, which provides confirmation that the required torque has been generated during the installation of the bolt. As shown in figure 1 and 2, and as an example only, the lower end of the bolt is formed into a star-shape, the points of which would mate with the means of rotation, and would shear off at a predetermined torque corresponding to the optimum mechanical resistance of the setting adhesive. This, or a similar device could be formed with the bolt during manufacture without recourse to accurate machining, or attached subsequently as a simple casting or molding.

To avoid nut/bolt inaccuracies, the breakout must transmit torque directly to the bolt. The breakout must transmit the thrust of the bolter (means of rotation) onto the bolt. The breakout must not leave any broken pieces or shards in the spanner, as they will interfere with the next installation (the operator cannot be expected to clean out any residue). The spanner must also engage with the nut, to tighten it. The breakout must be able to withstand normal handling during transport, without loss of function. The breakout must be capable of manufacture to consistent break-out torques in the range of 5 Nm about the required break-out torque.

This alternative and improved method thus provides the time-dependent element, and its associated uncertainty and need for special training, from the installation procedure and also provides a confirmation of correct installation, while retaining the simplicity and hence low cost of the individual bolt.

It is envisaged that the apparatus and method described herein will provide for the improved installation of bolts and allow for the use of relatively inexpensive bolts. The apparatus described herein will also provide a visible indication of the correct installation of a rock bolt.

The invention is not limited to the precise details described herein. For example, the method can be used to install a variety of bolts or rods or any other object in any hole or bore or to another object in a number of different applications without departing from the scope of the invention. Furthermore, many configurations of deformable protrusions may be used without departing from the scope of the invention.

Claims

1. A rock bolt comprising an elongate body having torque limiting means at an outer end of the body, the torque limiting means having at least one deformable protrusion which deforms under a predetermined torque.

2. A rock bolt as claimed in claim 1 in which the deformation of the deformable protrusion prevents further rotation of the rock bolt by a torque application apparatus.

3. A rock bolt as claimed in any one of the preceding claims in which the deformable protrusion is shaped to receive a complementary shaped chuck on a torque application apparatus to facilitate transferring torque to the rock bolt.

4. A rock bolt as claimed in any one of the preceding claims in which the deformable protrusions are protrusions extending in an axial direction from a circumferential surface at one end of the rock bolt.

5. A rock bolt as claimed in claim 4 in which, the protrusions extend at least partially across the end-on diameter of the rock bolt.

6. A rock bolt as claimed in any one of claims 1 to 4 in which the deformable protrusions are located at intervals on the circumference at the outer end of the rock bolt.

7. A method of installing a rock bolt comprising the steps of:

locating an adhesive in a hole in a rock body;

inserting a rock bolt in the hole;

- rotating the rock bolt in the hole to mix the adhesive using a torque limiting means until the torque limiting means stops further rotation of the bolt as a result of the setting of the adhesive.

8. A method as claimed in claim 7 including the step of setting the torque-limiting device to limit the torque applied to the bolt to a predetermined value.

9. A method as claimed in claim 8 in which the predetermined value of the torque is determined from at least one or more of the group consisting of the following: the diameters of the bolt and of the hole; the length of the bolt located in the adhesive; and the type of adhesive used.

10. A method as claimed in any one of claims 7 to 9 in which the torque limiting on the rock bolt shaving device causes a visual indication that the torque-limiting device has been used on that rock bolt.

11. A method as claimed in any one of claims 7 to 10 in which the rotation of the rock bolt is continuous.

12. A method as claimed in any one of claims 7 to 11 in which the torque limiting means includes at least one deformable protrusion.

13. A method as claimed in claim 12 in which the deformable protrusion deforms on application of a predetermined torque thereto.

14. A method as claimed in claim 13 in which at least part of the deformable protrusion remains on the rock bolt or remains associated with the rock bolt after deformation.

15. A method as claimed in any one of claims 12 to 14 in which the deformable protrusion is shaped to receive a complementary shaped chuck on a torque application means to facilitate transferring torque to the rock bolt.

16. A method as claimed in any one of claims 12 to 15 in which the deformable protrusions are protrusions extending in an axial direction from a circumferential surface at one end of the rock bolt.

17. A method as claimed in any one of claims 12 to 15 in which the protrusions extend at least partially across the end-on diameter of the rock bolt.