AU696277B2 - A bearing plate assembly - Google Patents

Description

II- WO 95/03477 PCTAU94/0419 1 A BEARING PLATE ASSEMBLY The present invention relates to a bearing plate assembly for a rock bolt.

Bearing plates, washers, support plates, load transfer plates, support brackets, domed plates, cup washfirs, and other devices (hereinafter referred to as "bearing plate assemblies") are all devices which are used with rock bolts to transfer loads in a rock bolt to the rock and/or to support straps, mesh or other devices between the rock and a nut or end fitting on the rock bolt.

The bearing plate assemblies are normally made of steel, but wood, plastic and fibre glass plates are also available.

A significant number of known bearing plate assemblies are designed to transfer the tensile load generated in a rock bolt to the rock surrounding the end of the rock bolt. The bearing plate assemblies usually do this by having a large surface area in contact with the rock compared to the surface area in contact with the nut or end fitting on the rock bolt. This means that for any given tensile load in the rock bolt, the stress at the nut/assembly interface is much higher than the streme at the assembly/rock interface. This stress distribution has the following inherent disadvantages.

Firstly, if the bearing plate assemblies are supporting a W-strap or mesh the relatively low contact stresses at the assembly/rock interface can allow the Wstrap or mesh to slide between the bearing plate assemblies and the rock, thus causing the W-strap or mesh to tear on the rock bolt.

Il q I lr a 1 P c- -I-1 PCT/AU94/00419 WO 95/03477 2 Secondly, the high stresses at the nut/assembly interface can cause the bearing plate assemblies to deform at this point thus reducing its ultimate capacity.

Furthermore, a number of known bearing plate assemblies initially contact the rock only on the peripheral edges of the bearing plate assemblies, with the result that there are extremely high contact stresses at this position, and then deform or bend as the load increases, which in turn increases the contact area with the rock. The high initial contact stresses at the assembly/rock interface can severely damage W-straps or other support systems under the bearing plate assemblies such that in extreme cases the bearing plate assemblies can push through the support systems causing the support systems to tear and fail. This has obvious design disadvantages.

In addition, the shape of known bearing plate assemblies is almost always square or rectangular in plan view. This is due partly to production efficiencies in that there is less wastage using square or rectangular shaped bearing plate assemblies compared to circular shaped bearing plate assemblies and partly to ensure a neat fit between other devices such as the ridges on W-straps.

Ho;wever, all bearing plate assemblies installed with rock bolts are subjected to a rotational force either during the installation process or during tightening the nut. The diagonal dimension of a square or rectangular bearing plate assembly is greater than its side dimension and as a consequence rotational force to tighten the nut can cause the square or rectangular bearing plate assembly to incorrectly fit over the ridges on a W-strap or other support strap. Square or rectangular bearing plate assemblies universally suffer from this disadvantage whereas the circular bearing plate assemblies do not have this problom.

1 111 WO 95/03477 PCT/AU94/00419 3 Moreover, known square or rectangular bearing plate assemblies have straight sides and hence straight edges. As the roof or rock deforms between rock bolts it can force a W-strap or mesh to bend over these straight edges which in effect become knife-edges. The knife-edges combined with high contact stresses can also cause the strap or mesh to tear and fail. This is also a major disadvantage of known bearing plate assemblies.

In addition, known bearing plate assemblies are designed to adjust to uneven rock surfaces. This is achieved by the following methods each of which has disadvantages.

1. The use of a domed ball which fits into an upturned hole in a bearing plate that contacts a rock surface. The system works well but the domed ball creates extremely high contact stresses on the plate because the domed ball only contacts with the plate at a tangent point. The high stresses can cause the steel to deform in the plate and the domed ball can be pulled through the hole in the plate. Alternatively, the high contact stresses cause radial tensile stresses which can .ause the plate to split from the hole to the edge of the plate.

2. The use of a relatively thin, easily deformable plate which deforms to adjust for the angular differences between the plate and the rock. This is the simplest system but the crudest and provides poor rock support and creates shear forces on the end of the rock bolt.

An object of the present invention is to provide 1 4 M ~~CI-l r 111.- WO 95/03477 PCT/AU94/00419 4 a bearing plate assembly which alleviates the disadvantages of the known bearing plate assemblies described in the preceding paragraphs.

According to a first aspect of the present invention there is provided a bearing plate assembly for a rock bolt retained in a rock formation, the bearing plate assembly comprising: a bearing plate for contacting a surfdce of the rock formation, the bearing plate having an opening for receiving the rock bolt therethrough; a washer adapted to be positioned between the bearing plate and a tensioning nut for the rock bolt, the washer having an opening for receiving the rock bolt therethrough, the width of the opening being selected so that there is a gap between the rock bolt and the washer when the washer is positioned so that the rock bolt extends through the opening; and a keyblock adapted to close the gap thereby to increase the resistance of the washer to deformation when load is applied to the rock bolt.

It is preferred that the bearing plate and the washer be hemispherical.

It is preferred that the width of the opening in the bearing plate be selected so that the bearing plate can receive the rock bolt through the opening at a range of angles to the surface of the rock formation.

II 1 WO 95/03477 PCT/AU94/00419 5 It is preferred that the opening in the bearing plate be circular.

It is preferred particularly that the hemispherical bearing plate and the hemispherical washer have the same radius so that there is substantial contact between the bearing plate and the washer.

It is preferred that the bearing plate comprise a rim for contacting the surface of the rock formation with a relatively small surface area of contact.

The term "relatively small area of contact" is understood herein in the context that known bearing plate assemblies generally have relatively large surface areas of contact with a rock formation compared with the total surface areas of the plates from which the known bearing plate assemblies are generally formed.

It is preferred that the rim be the only contact surface between the bearing plate assembly and the rock formation.

It is preferred particularly that the rim be circular.

It is preferred particularly that the rim be curved in a radial section.

It is preferred particularly that the rim define a smooth contact surface.

In one arrangement it is preferred that the keyblock comprise a bore for receiving the rock bolt therethrough.

In one particular embodiment it is preferred that

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IC-- WO 95/03477 PCT/AU9410Q0419 6 the keyblock be integrally formed with the tensioning nut and that the keyblock and the tensioning nut be internally threaded to mate with a threaded profile on the rock bolt.

In another particular embodiment it is preferred that the keyblock and the tensioning nut be separate and that the diameter of the bore of the keyblock be selected so that the keyblock can slide over the rock bolt.

In an alternative arrangement it is preferred that the keyblock and the tensioning nut be integrally formed as a part of the rock bolt or a screw bolt.

It is noted that the term "screw bolt" is understood herein to cover self-drilling rock bolts and self-tapping rock bolts.

It is preferred that the opening of the washer be defined by a frustoconical surface and that the keyblock comprise a first frustoconical face for contacting the .rustoconical surface of the washer.

In addition, or alternatively, it is preferred that the keyblock comprise a second frustoconical face for contacting the outer surface of the washer.

According to a second aspect of the present invention there is provided a bearing plate assembly for a rock bolt retained in a rock formation, the bearing plate assembly comprising: a bearing plate having a rim for contacting the surface of the rock formation with a relatively small surface area of contact as described above and an opening for receiving the rock bolt therethrough; and I I L _r m b WO 95/03477 PCT/AU94/00419 7 a washer adapted to be positioned between the bearing plate and a tensioning nut, the washer having an opening for receiving the rock bolt therethrough.

It is preferred that the rim be the only contact surface between the bearing plate assembly and the rock formation.

It is preferred particularly that the rim define a smooth contact surface.

It is preferred that the bearing plate and the washer be hemispherical.

It is preferred that the width of the opening in the bearing plate be selected so that the bearing plate can receive the rock bolt through the opening at a range of angles to the surface of the rock formation.

It is preferred that the opening in the bearing plate be circular.

It is preferred particularly that the hemispherical bearing plate and the hemispherical washer have the same radius so that there is substantial contact between t\he bearing plate and the washer.

It is preferred that the width of the opening of the washer be selected so that there is a gap between the rock bolt and the washer when the washer is positioned so that the rock bolt extends through the opening.

With such an arrangement, it is preferred that the bearing plate assembly further comprise a keyblock adapted to close the gap between the rock bolt and the washer to increase the resistance of the washer to WO 95/03477 PCT/AU94/00419 8 deformation when loading is applied to the rock bolt.

In one arrangement it is preferred that the keyblock comprise a bore for receiving the rock bolt therethrough.

In one particular embodiment it is preferred that the keyblock be integrally formed with the tensioning nut and that the keyblock and the tensioning nut be internally threaded to mate with a threaded profile on the rock bolt.

In another particular embodiment it is preferred that the keyblock and the tensioning nut be separate and that the diameter of the bore in the keyblock be selected so that the keyblock can slide over the rock bolt.

In an alternative arrangement it is preferred that the keyblock and the tensioning nut be integrally formed as a part of a rock bolt or screw bolt.

It is preferred that the opening of the washer be defined by a frustoconical surface and that the keyblock comprise a frustoconical face for contacting the frustoconical surface of the washer.

In addition, or alternatively, it is preferred that the keyblock comprise a second frustoconical face for contacting the outer surface of the washer.

The present invention is described hereinafter by way of example with reference to the accompanying drawings in which: Figure 1 is a partially sectional side elevation of a preferred embodiment of a bearing plate assembly of the present invention for a rock bolt grouted in a drilled hole in a mine roof;

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WO 95/03477 PCT/AU94/00419 9 Figure 2 is an enlarged perspective view of the bearing plate of the bearing plate assembly shown in Figure 1; Figure 3 is a partially sectional side view of the bearing plate in Figure 2; Figure 4 is a plan view of the bearing plate in the direction of the arrows 4 in Figures 2 and 3; Figure 5 is an enlarged perspective view of the washer of the bearing plate assembly shown in Figure 1; Figure 6 is a partially sectional side view of the washer in Figure Figure 7 is a plan view of the washer in the direction of the arrows in Figures 5 and 6; Figures 8 and 9 are partially sectional side views, in simplified form, of the bearing plate assembly shown in Figure 1 illustrating that the position of the washer on the bearing plate is adjustable; Figures 10 to 13 are a series of graphs illustrating the results of compressions tests on the components of the bearing plate assembly shown in Figures 1 to 9 and prior art bearing plate assemblies; Figure 14 is a perspective view of a variation of the keyblock of the bearing plate assembly shown in Figure 1; Figure 15 is a side view of the keyblock in Figure 14; Figure 16 is a plan view of the keyblock in the L3L- WO 95/03477 PCT/AU94/00419 10 direction of the arrow 16 in Figure Figure 17 is a plan view of the keyblock in the direction of the arrow 17 in Figure Figure 18 is a another variation of the assembly shown in Figure Figure 19 is a another variation of the assembly shown in Figure longitudinal sectional view of keyblock of the bearing plate 1; longitudinal sectional view of keyblock of the bearing plate Figure 20 is a perspective view of another variation of the keyblock of the bearing plate assembly shown in Figure 1; Figure 21 is a side view of the keyblock in Figure Figure 22 is direction of the arrow Figure 23 is direction of the arrow a plan view of the keyblock in the 22 in Figure 21; a plan view of the keyblock in the 23 in Figure 22; Figure 24 is a partially sectional side view of an integrally formed assembly of the washer and the keyblock; Figure 25 is a plan view of the assembly shown in Figure 24 in the direction of the arrow 25 in Figure 24; and Figure 26 a partially sectional side elevation of a preferred embodiment of a bearing plate assembly of the present invention for a screw bolt in a mine roof.

I 1 111 1II~ 1Lanrrr~ I-~ WO 95/03477 P)CT/A'h94/004191 11 The bearing plate assembly of the present invention can be used in conjunction with rock bolts having threaded profiles which are tensioned by tensioning nuts or with screw bolts.

The bearing plate assembly of the present invention can be used directly against a rock formation or in conjunction with W-straps or other support systems which contact the rock formation.

The bearing plate assembly of the present invention can be formed from any suitable material such as steel or plastics materials.

With reference to Figure 1, the general assembly of a preferred embodiment of a bearing plate assembly for a rock bolt 3 having a threaded profile 5 is shown in the figure retained against an exposed face 21 of a rock formation 11 by means of the rock bolt 3 and a tensioning nut 13 on the threaded profile 5. The rock bolt 3 is retained in the rock formation 11 in a conventional manner by means of grout in resin 7 is a drilled hole 9 in the rock formation 11.

The bearing plate assembly comprises: a hemispherical bearing plate a hemispherical washer 17; and a keyblock 19.

It is noted that the term "hemispherical" as used herein is not limited to configurations that are strictly hemispherical and extends generally to configurations which are often described in the art as "domed".

I "pa I e~ WO 95/03477 ICI/AU94/0O419 12 With further reference to Figure 1 and with reference to the more detailed drawings of the rock bolt assembly in Figures 2 to 9, the bearing plate 15 and the washer 17 have the same radius so that the washer 17 rests on the bearing plate 15 and consequently there is substantial contact between the two components.

In addition, the bearing plate 15 and the washer 17 have centrally located openings 20 (Figures 2 to 5) so that the bearing plate 15 and the washer 17 can be positioned as shown in Figure I with the rock bolt 3 extending through the openings It is noted that the rock bolt 3 as shown in Figure 1 is positioned to extend perpendicularly to the exposed face 21 of the rock formation 11 and, as a consequence, the bearing plate 15 and the washer 17 are symmetrically positioned with respect to the rock bolt 3.

It is also noted that the width ol the opening of the bearing plate 15 is selected so that the rock bolt 3 can be positioned at a range of angles with respect to the exposed face 21 of the rock formation 11. Specifically, with reference to Figures 8 and 9, in situations where it is necessary that the rock bolt 3 extend from the rock formation at an angle other than an angle perpendicular to the exposed surface 21, the bearing plate 15 and the washer 17 may be positioned without difficulty in relation to the angled rock bolt 3 by relative sliding movement of the bearing plate 15 and the washer 17. It can readily be appreciated that in each position shown in Figures 8 and 9 there is the same surface area of contact between the bearing plate 15 and the washer 17 as in the arrangement shown in Figure 1.

With particular reference to Figures 1 and 3, the bearing plate 15 is formed with a circular rim 23 which WO 95/03477 PCTIAI1J94/004 19 13 defines the contact surface of the bearing plate 15 with the exposed surface 21 of the rock formation 11. The rim 23 is curved in radial section (as illustrated in these figures) and presents a smooth relatively small contact surface to the exposed face 21 of the rock formation 11.

In addition, the construction of the rim 23 is such that the surface area of contact does not increase significantly as load is applied to the rock bolt 3.

With particular reference to Figures 1, 8, and 9, the keyblock 19 comprises a sleeve 27, a head 29 at one end of the sleeve 27, and a centrally located bore 31 that has a diameter which is slightly larger than that of the rock bolt 3 so that the keyblock 19 can slide over the bore 31 with minimum clearance. As can best be seen in Figures 8 and 9, the head 29 of the keyblock 19 comprises a first frustoconical surface 35 which contacts the hemispherical outer surface 37 of the washer 17 and a second frustoconical surface 39 which contacts the frustoconical edge 41 which defines the opening 20 in the washer 17. The keyblock 19 so formed closes the annular gap between the washer 17 and the rock bolt 3 so that the washer 17 has increased resistance to deformation when load is applied to the rock bolt 3.

The performance of the above described components of the bearing plate assembly shown in Figures 1 to 9 is illustrated in Figures 10 to 13. The figures present the results of compression tests carried out by placing the sample to be tested between two platens and compressing the platens together whilst measuring the applied load versus displacement of the platens.

The results of a compression test carried out on the washer 17 only is shown in Figure 10. As can be seen from the figure, the washer 17 began to fail at a load of approximately 23 tonnes at a displacement of approximately

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WO 95/03477 PC'4ICAU94/00419 14 3mm and progressively collapsed thereafter.

Another compression test was carried out on the bearing plate 15 and the washer 17 (without the keyblock 19). With reference to Figure 11, the first levelling of the curve at approximately 23 tonnes marks the collapse of the washer 17 onto the bearing plate 15. Thereafter, increasing load was required to deform the combined washer 17/bearing plate 15 with a maximum load of 60 tonnes recorded at the point of final collapse of the unit.

Figure 12 illustrates the results of a series of compression tests on assemblies of the bearing plate 15 and the washer 17 (curves marked 1, 2, 4) and an assembly of the bearing plate 15/washer 17/keyblock 19 (curve In the test illustrated by the curve 4, the washer 17 was arranged at an angle of 150 to the bearing plate 15. The curves 1, 2 and 3 are similar save for the curve 3 being displaced to the right, which indicates that the keyblock 19 increased the resistance of the bearing plate 17/keyblock 19 assembly to deformation under load.

Figure 13 illustrates the results of a series of compression tests on the bearing plate 15/washer 17 assembly described in the figure as "BHP 6mm thick cup and saucer" and 3 prior art bearing plate assemblies described in the figure as "Standard domed plate 8mm thick", "Flat plate 3.5mm thick", and "Hemispherical plate 3.5mm thick".

The results indicate that the load-bearing capacity of the bearing plate 15/washer 17 assembly is significantly higher than the prior art bearing plate assemblies. This is an important outcome when considered in the context that the load-bearing capacity of rock bolts, in commercial use typically is in the order of 30 tonnes, i.e. at least tonnes higher than the prior art bearing plate assemblies.

In other words, the prior art bearing plate assemblies fail before the rock bolts and, as a consequence, are a WO 95/03477 PCT/AU94/00419 15 limitation on such rock bolt systems and on the use of higher load capacity rock bolts.

With reference to Figures 14 to 17, a variation of the keyblock 19 shown in the figures is identical to that shown in Figures 1 to 9 with the exception that the keyblock 19 has a shorter sleeve 27.

With reference to Figure 18, a variation of the keyblock 19 shown in the figure is similar conceptually to the keyblocks 19 shown in Figures 1 to 9 but is different in detail in that the keyblock 19 is integrally formed with the nut 13 and comprises: an enlarged head 41, which is formed as a hex nut, and a threaded bore 43 to mate with the threaded profile of the rock bolt 3.

With reference to Figure 19, a variation of the keyblock 19 shown in the figure is identical to that shown in Figure 18, with the exception that the contact between the keyblock 19 and the washer 17 is restricted to the "second" frustoconical surface 39 which contacts the frustoconical edge 41 which defines the opening 20 in the washer 17.

With reference to Figures 20 to 23, a variation of the keyblock 19 shown in the figures is identical to that shown in Figures 14 to 17 with the exception that there is only the "second" frustoconical surface 39 to contact the frustoconical edge 41 which defines the opening in the washer 17 and there is no "first" frustoconical surface 35 to contact the outer hemispherical surface 37 of the washer 17.

WO 95/03477 WPCT/AU94I/00O4:t9 16 With reference to Figures 24 and 25, a variation of the assemblies of the washer 17 and the keyblock 19 described previously, which is particularly suited to be moulded in plastics material, comprises an assembly, generally identified by the numeral 81 in the figures. The assembly 81 comprises: a hemispherical section 83 which defines the "washer" components, and a sleeve 85 with a central bore 87 that is dimensioned to receive a rock bolt 3 with minimum clearance which defines the keyblock component.

With reference to Figure 26, the general assembly of a preferred embodiment of a bearing plate assembly is shown positioned in relation to a screw bolt 51 in a rock formation 55. The screw bolt 51 comprises a threaded profile 53 and an integrally formed hex nut 54.

The bearing plate assembly comprises the bearing plate 15 and the hemispherical washer 17 shown in Figures 1 to 9 and 14 to 23.

In addition, the bearing plate assembly further comprises a keyblock 57 of the general construction shown in Figures 1 to 10 with first and second frustoconical surfaces 35, 39 which contact the washer 17 but which are integrally formed as part of the screw bolt 51. It is noted that, alternatively, the bearing plate assembly may comprise a keyblock of the general construction shown in Figure 19 integrally formed as part of the screw bolt 51.

The preferred embodiments of the bearing plate assembly shown in the figures have the following advantages over known bearing plate assemblies.

WO 95/03477 PCT/AU94/00419 17 1. The bearing plate 15 is circular in plan view so that the bearing plate assembly will always fit between any ridges or grooves in ancillary support systems and the orientation of the bearing plate assembly is irrelevant to functional performance.

2. The bearing plate assembly comprises two hemispherical segments, i.e. the bearing plate 15 and the washer 17, such that the contact stresses are kept to relatively low levels within the segments since the contact area always remains constant irrespective of angular orientation of the segments.

3. The circular rim 23 of the bearing plate presents a small contact area with the rock formation or W-straps or other support systems and the contact surface is extremely smooth such that it is almost impossible to tear the support systems under the bearing plate 4. The hemispherical shape of the bearing plate 15 and the washer 17 resist compressive forces.

In the case of the arrangements shown in Figures 1 to 9 and 14 to 23, the keyblock 19 fills the gap between a rock bolt 3 and the washer 17 so that deformation of the washer 17 cannot occur into the opening in the bearing plate 6. The bearing plate 15 and the washer 17 can

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WO 95/03477 PCr/AU94/00419 18 be stamped from steel plate to reduce production costs.

7. As illustrated by the arrangement shown in Figures 24 and 25, the washer 17 and the keyblock 19 can be integrally formed either by forging from steel or by moulding from plastics material to reduce production costs.

Many modifications may be made to the preferred embodiments described above without departing from the spirit and scope of the present invention.

By way of example, whilst the hemispherical bearing plate 15 and t1i hemispherical washer 17 are described as separate components, it can readily be appreciated that the present invention is not so limited and it is within the scope of the present invention to retain the bearing plate 15 and the washer 17 together for ease of handling. This can be achieved by plastic clips or glue or by any other suitable means such as magnetism.

In addition, whilst the hemispherical bearing plate 15 and the hemispherical washer 17 are described as having the same radius, it can readily be appreciated that the present invention is not so limited and it is within the scope of the present invention that the bearing plate 15 and the washer have difference radii.

In addition, whilst several of the preferred embodiments comprise the first and second frustoconical surfaces 35, 37 on the keyblock 19, it can readily be appreciated that the present invention is not restricted to such an arrangement and extends to arrangements in which the keyblock comprises the first frustoconical surface only and the second frustoconical surface only.

WO 9$/03477 PCT/AU94/00419 19 Finally, whilst the preferred embodiments of the bearing plate assembly include a keyblock, and in this context the keyblock 19 is an essential feature of one aspect of the invention, its can readily be appreciated, particularly having regard to the experimental results in Figures 11 and 12, that the present invention is not so limited and in a second aspect of the invention the essential components are a bearing plate and washer of the types shown in the figures.

Claims (4)

1. A bearing plate assembly for a rock bolt retained in a rock formation, the bearing plate assembly comprising: a hemispherical (as defined herein) bearing plate having: a rim which in use is the only contact surface between the bearing plate assembly and the rock formation or between the bearing plate assembly and support straps, mesh or other devices that are intermediate the rock formation and the bearing plate assembly, the rim being curved in a radial section so that the rim forms a relatively small surface area of contact, and (ii) a circular opening for receiving the rock bolt therethrough, the width of the opening being i 25 selected so that the bearing plate can receive the rock bolt through the opening at a range of angles to the surface of the rock formation; and a h ,ispherical (as defined herein) washer adapted to be positioned between the bearing plate and a tensioning nut, the washer having an opening for receiving the rock bolt therethrough, and the radius of the washer being the same as that of the bearing plate so that there is substantial contact H:\ara\Xeop\ 7

3401.97 bhp.dot ai/07/98 I C- -II 21 between the bearing plate and the washer.

2. The assembly defined in claim 1 further a keyblock adapted to close the gap between the and the washer to increase the resistance of the deformation when loading is applied to the rock comprises rock bolt washer to bolt.

3. The assembly defined in claim 2, wherein the keyblock comprises a bore for receiving the rock bolt therethrough.

4. The assembly defined in claim 2, wherein the keyblock is integrally formed with the tensioning nut and the keyblock and the tensioning nut is internally threaded to mate with a threaded profile on the rock bolt. S S SS S The assembly keyblock and the tensioning 20 diameter of the bore in the the keyblock can slide over 6. The assembly keyblock and the tensioning 25 part of a screw bolt. defined in claim 3, wherein the nut are separate and the keyblock is selected so that the rock bolt. defined in claim 2, wherein the nut are integrally formed as a 7. The assembly defined in any one of claims 2 to 6, wherein the opening of the washer is defined by a frustoconical surface and the keyblock comprises a 30 frustoconical face for contacting the frustoconical surface of the washer. Dated this 27 TH day of July 1998 BHP ENGINEERING PTY LTD By Its Patent Attorneys GRIFFITH HACK Fellows Institute of Patent Attorneys of Australia. HM\~haron\Kvep\ePOci\73 4 Ol .97 bhp.doc 27/O7/9