GB2274895A - Rock bolt. - Google Patents

Abstract

To reduce the friction between a nut (20) (or bolthead) and a force transmission member (16, 18) located on a rock bolt (12, 14), a ball bearing (24) is interposed between them. The bearing (24) comprises hardened guide plates (26) separated by rolling balls (28). Deflection of the mine roof causes the balls (28) to punch into the plates (26) so as to bring the plates (26) into contact to act as a solid washer. The plates (26) may be surface or fully hardened. The rock bolt (12, 14) may a steel rod or a flexible cable. <IMAGE>

Description

ROCK BOLT BACKGROUND OF THE INVENTION The invention relates to an improved rock bolt construction of the type used, for example, for supporting the roof and walls of an underground coal mine against collapse.

In coal mines, rock bolts are inserted into the mine roof or walls at intervals to stabilise the mine surface against collapse. The bolts consist of a shaft which is threaded toward its lower end, a domed plate and domed washer slidable along the shaft, and a nut. In use, a hole is drilled in the roof by a bolting rig and a combination of either single or two speed resin capsules are forced upwards into the hole by the bolt. The bolt is then forced spinning further into the hole, to break the resin capsules and mix the two components of the resin which hardens to glue the bolt into the roof. The nut is then tightened by the rig to hold the plate and washer, and optionally any strap or the like, in compression against the roof.

The upwards force applied by the nut is limited by the torque applied to the nut by the rig, which is typically between 137-277 N.m (100-200- ft.lbf) dependent upon the type of machine used.

Friction between the nut and the washer reduces the conversion of the rig torque into axial tension in the bolt, thus reducing the compressive force applied to the mine surface. The systems used by rock bolt manufacturers to reduce the friction include the use of a bell-shaped plate to reduce the contact area between the nut and the plate (no washer is used) or a fluoropolymer friction washer positioned between the nut and the washer.

In his paper presented in 1990 to the 92nd Annual General Meeting of the Canadian Institute of Mining, Stephen C. Tadolini of the U.S. Bureau of Mines measured under laboratory conditions the tension to torque ratios of three different roof bolt arrangements - a donut plate, a bell prate and a donut plate with friction washer. Tadolini also made in situ measurements of an arrangement using a flat plate and no friction washer. Tadolini found that the tension to torque ratio of the arrangement with the friction washer was over twice that of the donut plate. He recommended that the idealized torque-tension relationships of the bolting systems used underground be established under laboratory conditions so that the recommended installation torque for each system could be determined.

In his later U.S. Patent No. 5 127 769, Tadolini seeks to avoid the limitations of friction on bolt tension altogether by the installation of a roof bolt with upwards thrust and no screw tightening.

The laboratory results of the abovementioned Tadolini paper suggest that, with the use of friction washers, bolt tensions of 4 tonnes force or higher are achievable within the torque limitations of commercial bolting rigs. However, the present Inventors have found that there is a large discrepancy between the laboratory and in situ results for the tension achievable in roof bolts with friction washers.

SUMMARY OF THE INVENTION The present invention aims to provide a rock bolt construction which facilitates the application of greater forces to the mine surfaces in order to provide increased mine stability, while allowing the use of essentially standard mine bolting equipment and techniques.

The present invention provides a rock bolt having a shaft, a force transmission member for applying a compressive force to a mine surface anq a tightening member on the shaft for rotation to effect screw tightening of the bolt to cause the force transmission member to apply said compressive force, characterised by the provision of a rolling element bearing between the tightening member and the force transmission member for transferring thrust therebetween.

BRIEF DESCRIPTION OF THE DRAWINGS Further preferred embodiments of the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic elevation of a first preferred rock bolt installed in a mine roof; Figure 2 is a detail, partly in section, of the lower part of Fig. 1; Figures 3 and 4 are schematic representations of the operation of a second preferred bearing assembly; and Figures 5 and 6 are schematic representations of the operation of a third preferred bearing assembly.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Figs. 1 and 2, the rock bolt 10 consists generally of a shaft with an upper, unthreaded portion 12 and a lower, threaded portion 14, on which is mounted a domed washer 16 and domed plate 18 assembly.

A nut 20 is threaded onto the lower portion 14. The upper end of the shaft is bonded in the hole by fast set and slow set resins 22a and 22b in the normal manner.

The bolt shaft may be rigid, for example a a steel rod, or may be a flexible cable. In either case, the shaft must be capable of being placed in axial tension by tightening the nut to force the plate 18, and optionally any strap 23, against the mine surface.

A thrust bearing 24 is positioned between the nut 20 and the domed washer 16 to transfer the force from the nut to the washer. The bearing consists of a pair of annular bearing guide plates 26 generally perpendicular to the shaft axis, separated by a number of rolling elements such as balls 28.

The bearing guide plate material should be of sufficient strength and hardness that plate cracking or failure, or significant bearing penetration, does not occur during rotation of the bearing while the rock bolt is being pre-loaded to the desired axial tension. The Applicant has found that steel of a Rockwell Hardness of 60 or more, and of sufficient thickness to withstand the dynamic loading, is a suitable material for the guide plates. Due to the high value of Rockwell Hardness of the guide plates, a brittle, explosive type of failure may occur, which may eject fragments. To counteract the safety risk of explosive failure, the bearing assembly of Fig. 2 incorporates an outer housing 30 for retaining any fragments.

The Applicant has found that the rock bolt construction incorporating the rolling element thrust bearing greatly increases the tensile load which may be applied to the bolt using conventional roof bolting equipment and techniques. For example, with a roof bolt according to the invention, the nut may be tightened to apply a tensile force of about 10 tonnes using a standard hydraulic rig applying a torque of about 277 N.m (200ft.lbf). The torquewavailable from the bolting machine is more efficiently used in generating roof bolt loading as the coefficient of friction between the nut and washer is dramatically reduced. It is believed that bolt forces of this magnitude will result in improved clamping of the coal or rock beds together so that the small gaps normally present between the coal beds are substantially eliminated.The individual beds then act as one, thus significantly increasing roof stability.

In conventional roof bolts, even if plastic friction washers are used, the nut cannot be tightened to generate 10 tonnes bolt load using standard roof bolting equipment, without requiring interruption of the usual procedure and final tightening of the bolt using very large torque wrenches or torque multipliers. This is not practical in commercial mining operations, and the long-term reliability of torque multipliers is also questionable.

The Applicant measured the bolt tension achievable at an applied torque of 277 N.m (200ft.lbs) under normal underground installation conditions for roof bolts according to the invention and for roof bolts with the prior art friction washers manufactured by ANI Arnall of Newcastle, Australia. These measurements were compared to laboratory values.

The manufacturer's published value of the pre-load tension achieved with a roof bolt with a fluoropolymer friction washer is about 64kN (6.5 tonnes) at a torque of 277 N.m., compared to only 30kN (3.1 tonnes) for steel-on-steel contact between the nut and the plate.

The Applicant's laboratory tests show that a bolt according to the subject invention will give a pre-load of 10 tonnes under similar torque conditions.

The Applicant then measured the pre-load achievable with both the prior art and the subject roof bolts in underground conditions using a hydraulic roof bolting rig at nut torques of around 277 N.m. The resulting pre-load ranged up to 2.6 tonnes for the prior art bolt and up to 7.5 tonnes for the bolt incorporating the ball bearing. These in situ measurements give values about 20% less than the real in situ value, due to modifications to the bolt construction in order to accomodate the instrumentation. After adjustment of both measured pre-load values to compensate for this and other factors, the results show that the roof bolts with friction washers give a pre-load of 3.25 tonnes, while the inventive roof bolt gives a pre-load of 9.5 tonnes.

Thus, it can be seen that there is a large discrepancy between the laboratory and insitu results for the prior art bolts, which discrepancy is substantially eradicated by the present invention. The Applicant believes that the poor in situ results for the friction washers may be due to the heat generated on the sliding surfaces, due to the combination of the high spin speed of conventional roof bolting rigs with high load, causing surface fusion which therefore increases the coefficient of friction and decreases the achievable pre-load.

In contrast, however, the roof bolt construction according to present invention is well suited to existing roof bolting equipment and techniques, so that the benefits of greater roof stability may be obtained with no substantial decrease in efficiency compared to standard operations.

Figs. 3 and 4 illustrate the operation of a preferred bearing assembly which is adapted to reduce or eliminate the problem of explosive failure.

Fig. 3 shows the assembly before and during the dynamic pre-loading of the bolt by tightening of the nut.

The rolling element bearing assembly consists of a pair of closely spaced annular guide plates 26 separated by balls 28 which run along race surfaces 34. In this embodiment, the guide plates are made from mild steel which has been case hardened to form an outer surface crust 32 with a Rockwell Hardness of about 62 (denoted by the cross hatching). The assembly acts as a thrust bearing, with the guide plates free to rotate independently and the vertical thrusts transferred by point contact between the guide plates and the balls.

Fig. 4 illustrates the bearing assembly after dynamic loading to the desired pre-load and then further static loading of the bolt by deflection of the mine roof. This extra load causes the balls to punch locally through the hardened crust of the race surface 34 of one or both guide plates and into the mild steel core 36. The mild steel behaves plastically under the ball loading, thus preventing explosive failure of the plates. This deliberate partial failure of the plates, at loads greater than the desired dynamic pre-load but less than the yield strength of the bolt shaft, brings the two guide plates into contact so that the thrust is now distributed both through the balls and through direct contact of the guide plate surfaces.Thus, after the partial failure the guide plates no longer act as a bearing but instead operate as a solid washer of greatly increased stiffness and strength, the latter being preferably greater than the yield strength of the bolt shaft.

By constructing the bearing assembly so that contact between the guide plates occurs as soon as possible after the balls punch through the hardened steel case, the load/deflection stiffness of the entire system is maintained. Thus, once the guide plates come into contact as shown in Fig. 4, the bolt head arrangement retains its integrity and continues to load as though the bearing arrangement were not in the system at all.

The Applicant's tests confirm that the axial load/deformation characteristics of the bolt head arrangement are not affected by the presence of the bearing.

The assembly of Figs. 3 and 4 is easily manufactured, as the use of mild steel allows the guide plates to be punched out and subsequently case hardened to the correct hardness and thickness to achieve the partial 'failure' as described above at the desired level of compressive load. The Applicant has found that this bearing construction is suitable for partial failure loadings up to about 10 tonnes.

Figs. 5 and 6 illustrate an alternative bearing construction, which also undergoes partial 'failure' to act as a solid washer. In this alternative embodiment, the guide plates are very closely spaced and are formed throughout of hardened steel. This construction relies on the elastic deformation of the guide plates 26 and/or the balls 28 under very high compressive load to bring the guide plate surfaces into load bearing contact. It is believed that this construction may allow even greater loadings.

While particular embodiments of this invention have been described, it will be evident to those skilled in the art that the present invention maybe embodied in other specific forms without departing from the essential characteristics thereof. The present embodiments and examples are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

For example, while the embodiments are illustrated with reference to bolts of the type in which the bolt is grouted into the hole and tightened by a nut, the teachings of the invention are also applicable to rock bolts of the type in which the bolt is screwed into an anchor. In the latter case, the bolt head serves as the tightening member.

Claims (13)

1. A rock bolt having a shaft, a force transmission member for applying a compressive force to a mine surface, and a tightening member on the shaft for rotation to effect screw tightening of the bolt to cause the force transmission member to apply said compressive force, characterised by the provision of a rolling element bearing between the tightening member and the force transmission member for transferring thrust therebetween.

2. A rock bolt according to claim 1 wherein the bearing comprises a pair of annular guide plates substantially perpendicular to the shaft, said guide plates being separated by a plurality of rolling elements.

3. A rock bolt according to claim 2 wherein said rolling elements are balls.

4. A rock bolt according to claim 2 wherein the bearing is adapted to partially fail above a predetermined level of compressive load so that the bearing subsequently acts as a solid washer.

5. A rock bolt according to claim 4 wherein said predetermined level of compressive load is less than the axial yield strength of the rock bolt.

6. A rock bolt according to claim 5 wherein the bearing is adapted to undergo said partial failure during static loading of the bolt by deflection of the mine surface following dynamic loading of the bolt by said screw tightening.

7. A rock bolt according to claim 4 wherein said partial failure comprises deformation of the bearing such that said guide plates come together in a direct load transfer relationship.

8. A rock bolt according to claim 7 wherein said deformation includes penetration of said rolling elements into a race surface of at least one of the guide plates.

9. A rock bolt according to claim 8 wherein said guide plates have a hardened outer surface at least at said race surface, and said penetration comprises penetration through said hardened surface.

10. A rock bolt according to claim 9 wherein said guide plates are formed of case hardened mild steel.

11. A rock bolt according to claim 7 wherein said deformation comprise elastic deformation of said guide plates and/or said rolling elements.

12. A rock bolt according to claim 11 wherein said guide plates are formed of fully hardened steel.

13. A rock bolt substantially as herein described with reference to the accompanying drawings.