AU694351B2 - Improvements in tensioning systems for rock bolts - Google Patents

Description

I

Our Ref: 595458 P/00/011 Regulation 3:2

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT t *I 0 o £0 oo0 a 0 0# Applicant(s): Address for Service: Industrial Rollformers Pty Limited 8 Tarlington Place Smithfield New South Wales 2164

AUSTRALIA

DAVIES COLLISON CAVE Patent Trade Mark Attorneys Level 10, 10 Barrack Street SYDNEY NSW 2000 Invention Title: Improvements in tensioning systems for rock bolts The following statement is a full description of this invention, including the best method of performing it known to me:- 5020 -1- IMPROVEMENTS IN TENSIONING SYSTEMS FOR ROCK BOLTS This invention relates to improvements in tensioning systems for rock bolts used in supporting a mine roof.

Rock bolting is a well known practice that utilises a rod which is anchored at one end by grout or a chemical anchor in a hole drilled deep into the mine roof and is threaded at the exposed other end. A retaining plate is passed over the threaded rod end and the plate is clamped against the mine roof by a nut on the threaded end of the rod. The purpose of rock bolting is to support the roof of a mine thereby minimising the chance of rock falls and caveins.

The frictional resistance to rotation of the nut, as a result of direct contact between the metal of the nut and the metal of the retaining plate, or an intermediate washer, is a barrier to the application of optimum tensile loads to the rod.

In one known arrangement devised to minimise friction, a washer of low friction plastic material is disposed between the nut and the retaining plate. During nut tightening direct contact between the nut and the retaining plate is eliminated until the compressive load on the washer crushes the washer and it disintegrates. The use of the plastic washer allows tensile load which can be applied to the rod to be increased over that which is possible with an arrangement where there is no washer.

In another known arrangement the nut and the retaining plate are separated by a thrust bearing formed of opposed bearing races separated by rollers or balls. It has been found that if the rollers or balls of the thrust bearing are left in a fully hardened condition, as would be the case for normal industrial use, the load which can be applied to the bolt is limited by the failure point of either the races or the rollers/balls. The failure point occurs short of the limit to which rods have the potential to be tensioned, but nevertheless is much higher than can be achieved with the plastic washer arrangement. When the failure point does occur the rollers/balls and/or races fail suddenly and shatter explosively, thereby presepnting a safety n -2hazard.

In an endeavour to overcome the shatter problem an arrangement has been proposed whereby the thrust bearing is provided with a unitarily formed housing assembly for retaining shatter fragments. However, formation of such a housing assembly around the thrust bearing complicates production of the bearing and increases the unit cost. In another attempt to overcome the shatter problem, thrust bearings of diminished rating are used in which either the rollers/balls or races are not fully hardened. Although the use of such thrust bearings will deliver greater bolt tension than is possible with the plastic washer arrangement the races or rollers/balls of the thrust bearings fail by crushing before a desired tension can be developed in the rod.

This invention has been devised to provide a means whereby thrust bearings with fully i hardened rollers/balls and/or races can be used (so as to achieve acceptable tension loading Sa 15 on the rod) without the safety problems associated with the shattering of the rollers/balls and/or races under load.

In accordance with the present invention, there is provided a tensioning system for a bolt fixed in a mine wall, the tensioning system comprising a tensioning member adapted to be 20 rotatably mounted on a free end of the bolt and arranged for screw tightening for tensioning a the bolt, a force transmission assembly adapted to be positioned between the tensioning member and the mine wall, for applying a compressive force to a surface of the mine wall G 4i j upon screw tightening of the tensioning member, and a bearing element between the tensioning member and the force transmission assembly for transferring thrust therebetween, wherein the bearing element is recessed in either or both the force transmission assembly or the tensioning member to such a degree that the bearing element is substantially encased by the force transmission assembly and tensioning member during tensioning of the bolt.

Preferably, the force transmission assembly includes a washer element and a load plate, the washer element being recessed to accommodate at least a part of the bearing element.

In another aspect, the invention provides a tensioning member, in the form of a nut, for use -3in the abovedescribed system, the nut being recessed to accommodate at least a part of the bearing element.

In another aspect, the invention provides a force transmission assembly including a washer, for use in the abovedescribed system, the washer being recessed to accommodate at least a part of the bearing element.

Recessing either or both the tensioning member or a washer of the force transmission assembly can be readily achieved by either precast moulding or counter-boring.

In another aspect, the system can be utilised in a novel nut assembly for tensioning a cable bolt, as described below.

A cable bolt is generally installed in a mine as a dowel. Installation involves firstly drilling a hole in a rock formation and subsequently inserting a cable therein (cable lengths of 4 to metres are common). Grout can then be pumped up the hole to fill the annulus between the cable and the walls of the hole.

In some applications a plate and "barrel and wedge" are installed on the cable. Once the cable is anchored (grouted), a pulling device, generally hydraulic, is attached to stretch the cable.

Then the barrel and wedge are installed to stop the cable relaxing when the pulling device is released. The installation process is time consuming and therefore expensive.

In accordance with another aspect of the present invention, there is provided a tensioning 2ystem as above described for tensioning a cable bolt, wherein thle tensioning member Sttt includes a threading device adapted to be screwed along the cable whilst cutting a thread therein.

The threading device cqn, therefore, simply be screwed along the cable bolt by a conventional driving mechanism to tension the cable, the bearing element providing maximum transfer of torque from the tensioning member for tensioning of the cable.

p:\wpdocs\jmp\indrol1\20a 1-"111,w 1 1 ::~ili -l!l~l~l-ll-_~Z~ltllflll.ll(i;i .if^l~ii*L._ii.l-; -4- Preferably, the threading device includes a housing having a frusto-conicular recess extending axially thereof whereby to provide an opening at each end of the housing for the cable to pass through, and threading elements being arranged within the recess of the housing to threadably engage the cable.

Preferably, the threading elements are wedge-shaped and have a tapered end corresponding to the internal profile of the frusto-conicular recess. Preferably, the elements are provided with a restraining means to inhibit relative movement therebetweer in an axial direction with respect to the housing.

Preferably, the device is arranged to thread the cable in the same direction as a twist of the cable such that tensioning of the cable tends to unwind and thereby straighten the cable. The cable bolt, tensioned in this manner, will function to some extent as a pre-loaded spring and can accommodate limited movement of a rock mass supported by the bolt.

The invention is now more fully described, by way of non-limiting example only, with reference to the accompanying drawing, in which: Figure 1 is a sectional elevation of a known arrangement for tensioning a bolt; Figure 2 is a sectional view of a tensioning system in accordance with the invention; Figure 3 is a sectional view of a tensioning system ii -ccordance with another aspect of the invention; Figure 4 is a sectional view of a tensioning member in accordance with another aspect of the invention; Figure 5 is a cross-sectional view of a tensioning system in accordance with another aspect of the invention, for tensioning a cable bolt; and Figure 6 is an exploded view of part of the system of Figure Fig. 1 is a sectional elevation of a typical tensioning arrangement as used hitherto with direc' contact between nut 1, engaged with rod 2, and a domed washer 3 engaged with a seat 3 in a domed zone 5 of a retaining plate 6 supporting an area of mine roof 7. The interengagement between the seat 4 and the domed surface of the washer 3 allows for angular positioning and alignment during the nut tightening process. As will be understood the

A

surface of the mine roof to be engaged by the retaining plate will seldom be in a plane exactly normal to the axis of the hole drilled for the rod 2 so there needs to be some provision for such relationships. The "ball and socket" type arrangement between the seat 4 of the retaining plate 6 and the domed surface of the washer 3 provides a simple way of accommodating the above problem.

Fig.2 is a sectional elevation of one form of the present invention generally replicating the arrangement of Fig.l. The difference resides in the provision of a counterbore 8 in the rear surface 9 of the domed washer 3. The counterbore 8 is sufficiently deep to receive at least the major portion of the thickness of a thrust bearing 10 comprised of two hardened steel race plates 11 and 12 with circular tracks in which there is disposed a plurality of hardened steel balls Commercially available thrust bearings can be used. Other forms of commercially available thrust bearings which include an encircling cover for the exclusion of foreign matter can also be used.

Preferably the counterbore 8 is deep enough to receive the whole thickness of the bearing but, as illustrated, a depth such that the escape of any ball fragments resulting from the attering of the balls during the nut tightening process is adequate.

Referring to Fig.3, this is a sectional elevation of another construction incorporating the invention. In this arrangement the retaining plate 26 is provided with a domed portion 27 with a hole 28 therethrough and a washer 29 with a cupped surface to match the dome 27 is provided. There is a counterbore 32 around the hole 30 through the washer 29 to accommodate the thrust bearing In a further arrangement illustrated in Fig. 4, the nut 1 is provided with a counterbore 31 with the required dimensions to substantially totally receive the thrust bearing In yet another arrangement both the washer and the nut may be counterbored so that the thrust bearing is substantially encased therebetwei n during tensioning.

-6- As can be appreciated the counterbored recess or recesses may instead be formed by suitable pre-cast moulding.

In any event, formation of the recesses in either or both the washer or the nut, to thereby provide a safety measure for containment of an exploded thrust bearing is substantially easier and .nore cost effective than provision of a housing assembly unitarily formed with the berAing, as used in the above described known form of bearing. Also, failure of the bearing will result in the washer and nut moving into engagement so that load can be transferred over a greater surface area than available via the bearing alone to thereby increase the overall strength of the arrangement.

Another application of the above described tensioning system is for tensioning a cable bolt.

In particular, Figures 5 and 6 show a tensioning member 40 threadably engaging a cable bolt 42 to thereby tension the cable and apply a compressive force against mine roof 44 via a force transmission assembly 45, comprising a dome ball washer 46 and roof plate 48. The dome ball washer 46 and the tensioning member 40 are provided with recesses 49 and 50 in adjacent ends 51, 52 so as to receive a thrust bearing 54 therebetween. The tensioning member 40 is formed of a threading device 55 which includes a housing 56 having an axially extending frusto-conical aperture 58 formed therein. The housing is arranged such that a narrow end 60 of the opening faces the thrust bearing 54 and a wide end 62 faces toward a free end of the cable 42. Wedge shaped elemer's 64 are arranged in the opening 58 and have tapered portions 65 to correspond with the frusto-conical shape of the opening 58. The ends 66 of the elements remote from the tapered portions 65, or possibly the housing itselft, are suitably configured to be received in a conventional driving mechanism or spanner. A restraining member (not shown) such as a ring of wire or rubber passing around the elements 64 is provided to inhibit relative axial movement between the elements. The internal surface 68 of each of the elements has a thread formation along the length thereof and is preferably formed of hardened steel.

The above described tensioning system is readily employed by passing a free end of an anchored cable through the force transmission assembly 45, the thrust bearing and the opening formed in the housing 58. Application of torque to the elements 64 causes the p:\wdoc\map\iilroU'.2Oa r I I. 2, IE ;222 -7surfaces 68 to cut a thread into the cable while simultaneously rotating along the cable. The frusto-conical shape of the opening 58 and the tapered portions 65 formed on the elements 64 serve to lock the components and thereby the tension member 40 against reactive forces directed away from the mine roof Any particular grade of thread may be used on the components but the thread is preferably fine to reduce torque required for maximum induced tension in the cable. Also, a fine thread provides more contact and better load holding characteristics. The depth of the thread is also calculated according to the minimum torsional power required to produce suitable tension in the cable.

The direction that the tensioning member 40 threads along the cable is preferably in the same direction as the twist of the cable itself As such, when the member 40 tensions against the load plate 48, the cable will, to some extent, unwind and straighten, therefore slightly increasing in the overall length of the cable. This provides the advantage that the cable acts has a highly preloaded spring to accommodate limited movements of the mine roof 44. Also, utilising the thrust bearing allows for maximum transition of machine torque into tensioning of the cable.

The above described tensioning system provides a substantial advantage over the known form cable tensioning in that a simple self-threading tensioning member can be used instead of hydraulic machinery and the tensioning member can be readily driven by pre-existing or conventional driving means.

Finally, it is to be understood that the inventive concept in any of its aspects can be incorporated in many different constructions so that the generality of the preceding description is not to be superseded by the particularity of the attached drawings. Various alterations, modifications and/or additions may be incorporated into the various constructions and arrangements of parts without departing fiom the spirit or ambit of the invention.

I

Claims (9)

1. A tensioning system for a bolt fixed in a mine wall, the tensioning system comprising a tensioning member adapted to be rotatably mounted on a free end of the bolt and arranged for screw tightening for tensioning the bolt, a force transmission assembly adapted to be positioned between the tensioning member and the mine wall, for applying a compressive force to a surface of the mine wall upon screw tightening of the tensioning member, and a bearing element between the tensioning member and the force transmission assembly for transferring thrust therebetween, wherein the bearing element is recessed in either or both the force transmission assembly or the tensioning member to such a degree that the bearing element is substantially encased by the force transmission assembly and tensioning member during tensioning of the bolt.

2. A tensioning system as claimed in claim 1, wherein the force transmission assembly includes a washer element and a load plate, the washer element being recessed to accommodate at least a part of the bearing element.

3. A tensioning member for use in the tensioning system claimed i, claim 1 or 2, said member being recessed to accommodate at least part of the bearing element.

4. A force transmission assembly for use in the tensioning system claimed in claim 1, including a washer, the washer being recessed to accommodate at least a part of the bearing element.

A tensioning system as claimed in claim 1 or 2, wherein the bolt is a cable bolt and wherein the tensioning member includes a threading device adapted to be screwed along the cable whilst cutting a thiead therein.

6. A tensioning system as claimed in claim 5, wherein the threading device includes a housing having a frusto-conicular recess extending axially thereof whereby to provide an opening at each end of the housing for the cable bolt to pass through, the wider end of the I P:\WPDOCS\AMK\SPEC\595458 15/5/98 -9- recess being arranged to face away from the force transmission assembly, and threading elements being arranged within the recess of the housing to threadably engage the cable.

7. A tensioning system as claimed in Claim 6, wherein the threading elements are wedge- shaped and have a tapered end corresponding to the internal profile of the frusto-conicular recess.

8. A tensioning system as claimed in Claim 6 or 7, wherein the device includes a restraining means to inhibit relative movement between the elements in an axial direction with respect to the housing.

9. A tensioning system is claimed in any one of Claims 5 to 8, wherein the device is arranged to thread the cable in the same direction as a twist of the cable such that tensioning of the cable tends to unwind and thereby straighten the cable. Dated this 15th day of May 1998 INDUSTRIAL ROLLFORMERS PTY LIMITED SBy Their/ or /Its Patent Attorneys S 20 DAVIES COLLISON CAVE A e e U~I~ I_ ABSTRACT A .nsioning system for a bolt (42) fixed in a mine wall the tensioning system comprising a tensionin, member (40) adapted to be rotatably mounted on a free end of the bolt and arranged for screw tightening for tensioning the bolt, a force transmissio. ,ssembly adapted to be positioned between the tensioning member and the mine wall, for applying a compressive force to a surface of the mine wall upon screw tightening of the tensioning member, and a bearing element (10) between the tensioning member and the force transmission assembly for transferring thrust therebetween, wherein the bearing element ;F S, 10 recessed in either or both the force transmission assembly or the tensioning member to su.-m a degree that the bearing element is substantially encased by the force transmission assembly Sand tensioning member during tensioning of the bolt. Preferably, the bolt is a cable bolt and the tensioning member includes a threading device (55) adapted to be screwed along the cable whilst cutting a thread therein. i p:\wpdocsma