WO1996012089A1

WO1996012089A1 - Bail-type expanding shell rockbolt

Info

Publication number: WO1996012089A1
Application number: PCT/GB1995/001807
Authority: WO
Inventors: Johannes Hendrik Johnson
Original assignee: H.M. Licence Company (Proprietary) Limited; Brown, Keith, Edwin, Frank
Priority date: 1994-10-14
Filing date: 1995-07-31
Publication date: 1996-04-25
Also published as: ZA956219B; AU3181295A

Abstract

The bail-type expanding shell rockbolt comprises a shell with a bail (10) including depending bail arms (20), and leaves (14) which are adapted to engage the wall of a hole which has been drilled into a rock formation. The bail arms (20) are fixed to internal surfaces of the leaves (14) at positions (28) which are spaced from the inner ends of the leaves to allow the inner ends of the leaves (14) to be splayed apart from one another. As a result of the fixture of the bail arms (20) to the internal surfaces of the leaves there is no reduction in the effective area of the leaves.

Description

"BAIL-TYPE EXPANDING SHELL ROCKBOLT'

BACKGROUND TO THE INVENTION

THIS invention relates to a bail-type expanding shell rockbolt.

Various type of bail-type expanding shell rockbolts are known. Rockbolts of this type have a shell with a number of part-cylindrical leaves with serrated outer surfaces and tapered inner surfaces. The leaves are held together as a unitary construction by means of a bail. A tapered plug with an internally threaded passage is retained in a non-rotatable manner by the bail. A threaded rod extends through the central cavity defined by the leaves and is threaded into the plug.

In use the rockbolt is inserted, bail end first, into an oversize hole predrilled in a rock formation. Thereafter the threaded rod is tensioned by appropriate rotation thereof. This draws the plug rearwardly between the leaves. The cooperating tapers on the plug and leaves cause the leaves to move outwardly into contact with the wall of the predrilled hole and this anchors the rockbolt in the hole. In one widely used arrangement the bail has two diametrically opposed arms which are fixed to the external surfaces of opposing leaves at the outer end of the shell. The arms of the bail locate in longitudinally extending slots in the external surfaces of the leaves. Arrangements of this type suffer from the disadvantage that the slots in the leaves reduce the effective area of the leaves and hence reduce the area of contact with the wall of the hole. This leads to a reduction in anchoring efficiency. A further disadvantage of slotted leaves is the resultant weakening thereof, which may allow an undesirably high level of leaf deformation under load when the rockbolt is installed.

In another widely used arrangement, the ends of the arms of the bail are fixed externally to the inner ends of the leaves. To facilitate fixture the inner ends of the leaves are unserrated. Once again, the effective area of contact with the rock is reduced, leading to a reduction in anchoring efficiency.

Typical examples of known bail-type expanding shell rockbolts having some or all of the features mentioned above are described in GB 2145793 A; US 4,100,748; US 4,337,012; US 4,516,886; US 4,753,559; US 4,861,198; US 5,009,549; US 5,018,908 ; US 5,042,961; US 5,052,861; US 5,087,160; US 5,094,577; US 5,098,227; US 5,219,248; US 5,275,512 and US 5,344,257.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided a bail-type expanding shell rockbolt comprising a shell which includes a bail having a plurality of bail arms and a plurality of leaves which are adapted to engage the wall of a hole which has been drilled into a rock formation, wherein the bail arms are fixed to internal surfaces of the leaves at positions which are spaced from the inner ends of the leaves to allow the inner ends of the leaves to be splayed apart from one another.

In this specification the term "inner end" refers to the operatively inner end of the relevant component, i.e. that end of the component which is situated innermost when the apparatus is inserted into a hole drilled in a rock formation. Similarly, the term "outer end" refers to the operatively outer end of the relevant component, i.e. that end situated closest to the mouth of the drilled hole. The terms "inwardly", "outwardly" and so forth have corresponding meanings.

In one embodiment, the shell comprises a pair of leaves and the bail has two arms which are fixed to the respective leaves. In other embodiments there may be more than two leaves, the bail in such cases have one bail arm for each leaf. In some cases, each bail arm is fixed to its leaf at a position approximately mid- way along the length of the leaf. In other cases, each bail arm is fixed to its leaf at a position more towards the outer end of the leaf than the inner end of the leaf.

The shell of the rockbolt may also comprise a plug which has a tapered external surface to cooperate with a taper provided on the internal surfaces of the leaves, and a threaded rod extending between the leaves and threaded through a passage in the plug, the plug having external grooves in which the bail arms locate to prevent rotation of the plug when the rod is rotated relative to the plug. Conveniently, the ends of the bail arms are welded to internal surfaces of the leaves.

According to another aspect of the invention there is provided a method of installing a bail-type expanding shell rockbolt in a hole drilled in a rock formation, the bail-type expanding shell rockbolt being of a type in which the shell comprises a plurality of leaves, a bail including a plurality of bail arms fixed to internal surfaces of the leaves at positions spaced from the inner ends of the leaves, a tapered plug which is engaged non-rotatably with the bail arms and which has a tapered external surface to cooperate with a taper on the internal surfaces of the leaves, and a threaded rod extending between the leaves and threaded through a passage in the plug, the method comprising the steps of:

splaying the inner ends of the leaves apart from one another to allow the plug to move outwardly and assume a position between the inner ends of the leaves,

inserting the inner end of the rockbolt into the drilled hole, and

rotating the rod in the appropriate sense to draw the plug further between the leaves, thereby urging the leaves outwardly into engagement with the wall of the hole.

BRIEF DESCRD7TION OF THE DRAWINGS

The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings in which:

Figure 1 shows a perspective view of a bail-type expanding shell rockbolt according to the invention;

Figure 2 shows a cross-section at the line 2-2 in Figure 1 ;

Figure 3 illustrates the manner in which the bail is fixed to the shells in the rockbolt seen in Figure 1.

DESCRD7TION OF A PREFERRED EMBODIMENT

Figure 1 shows a bail-type expanding shell rockbolt having a shell which includes a U-shaped bail 10, a steel plug 12 and a pair of steel leaves 14. The rockbolt also includes an elongate, threaded, steel rod 16.

The bail 10 in this example is made of strip steel and has a bight portion 18 and a pair of longitudinally extending arms 20. The plug 12 is formed with a central, threaded passage 22 and has a frustoconical outer surface 24. Diametrically opposed grooves are formed in the outer surface 24 of the plug 12 and the bail arms 20 locate snugly in the grooves, as shown in Figure 2, with the result that the plug cannot rotate relative to the bail. As illustrated, the plug is also formed with diametrically opposed ridges 25 which also act to prevent rotation of the plug.

The leaves have part-circular serrations 26 on their outer surfaces and a smooth conical taper, matching the taper of the plug 12, on their inner surfaces. As illustrated, the rod 16 is threaded through the central passage 22 in the plug.

Referring to Figure 3, it will be seen that the outer ends of the bail arms 20 are spot- welded at locations 28 to the internal surfaces of the leaves 14. The locations 28 are situated towards the outer ends of the leaves but in some cases, they may be roughly mid-way along the lengths of the leaves. This configuration allows the inner ends of the leaves to be splayed outwardly, as illustrated by the arrows 30, such outward splaying being accompanied by slight bending of the bail arms 20.

In operation of the rockbolt, the inner ends of the leaves 14 are slightly outwardly splayed as described above. The increased spacing between the leaves allows the plug 12 to move in an outward direction to a position in which it locates between the inner ends of the leaves, as illustrated in broken outline in Figure 3. With the rod 16 engaged in threaded fashion with the plug, the rockbolt can be inserted, bail end first, into an oversize, predrilled hole in a rock formation.

Thereafter the rod 16 is rotated with the result that the plug is drawn further, in an outward direction, between the leaves 14. This urges the leaves radially outwardly into firm contact with the wall of the predrilled hole. Further rotation of the rod tensions the rod and anchors the rockbolt firmly in the hole.

It is believed that the manner of fixture of the bail arms 20 to the leaves 14 has two main advantages: the fixture of the bail arms to the internal surfaces of the leaves means that there is no need to reduce the contact area provided by the serrated outer surface of each leaf. It is therefore believed that the illustrated arrangement can give rise to enhanced anchoring efficiency when compared to prior rockbolts in which the bail arms are fixed externally to the leaves and located in slots formed in the external surfaces of the leaves.

the facility to splay the inner ends of the leaves apart from one another enables the plug 12 to locate at the outset between the leaves. This in turn means that the plug needs to move through only a short longitudinal distance in order to urge the leaves apart from one another to their rock-engaging positions. Once again it is anticipated that this feature will lead to enhanced installation and anchoring efficiency.

In the embodiment described above the shell of the rockbolt has only two leaves and the bail has a simple U-shape. In other cases, there may be three or more leaves. In the case of a shell with three or more leaves, the bail may comprise separate strip steel arms bent to an appropriate -shape and welded to one another on the axis of the rockbolt. This is considered preferable to stamping the bail structure out of steel sheet, since there is less wastage of steel material.

As stated above, the bail arms locate in grooves in the plug. Apart from preventing relative rotation between the plug and the bail, the location of the bail arms in the grooves also ensures that the taper surfaces of the plug align properly with the taper surfaces of the leaves. In a refinement of the rockbolt the bail may include an enlarged central portion, on the axis of the rockbolt, which is formed with a circular recess. The recess serves to centralise a coil spring which is located about the rod 16 and which acts between the plug 12 and the bail, urging the plug outwardly away from the bight portion of the bail. This has the advantage that when the rockbolt is first inserted into the drilled hole to the required depth, the spring positively urges the plug into position between the leaves. This ensures that there is immediate contact between the leaves and the wall of the hole, and enables rapid tensioning of the rockbolt to take place.

As yet another feature, the centre of the bail, on the axis of the rockbolt, may have a weakened zone which will be ruptured or which will break away when the rod 16 is rotated far enough through the nut during installation.

Claims

1.

A bail-type expanding shell rockbolt comprising a shell which includes a bail having a plurality of bail arms and a plurality of leaves which are adapted to engage the wall of a hole which has been drilled into a rock formation, wherein the bail arms are fixed to internal surfaces of the leaves at positions which are spaced from the inner ends of the leaves to allow the inner ends of the leaves to be splayed apart from one another.

2.

A bail-type expanding shell rockbolt according to claim 1 wherein the apparatus comprises a pair of leaves and the bail has two arms which are fixed to the respective leaves.

3.

A bail-type expanding shell rockbolt according to either one of the preceding claims wherein each bail arm is fixed to its leaf at a position approximately mid-way along the length of the leaf.

4.

A bail-type expanding shell rockbolt according to either one of claims 1 or 2 wherein each bail arm is fixed to its leaf at a position more towards the outer end of the leaf than the inner end of the leaf.

5.

A bail-type expanding shell rockbolt according to any one of the preceding claims and comprising a plug which has a tapered external surface to cooperate with a taper provided on the internal surfaces of the leaves, and a threaded rod extending between the leaves and threaded through a passage in the plug, the plug having external grooves in which the bail arms locate to prevent rotation of the plug when the rod is rotated relative to the plug.

6.

A bail-type expanding shell rockbolt according to any one of the preceding claims wherein the ends of the bail arms are welded to internal surfaces of the leaves.

7.

A method of installing a bail-type expanding shell rockbolt in a hole drilled in a rock formation, the bail-type expanding shell rockbolt being of a type in which the shell comprises a plurality of leaves, a bail including a plurality of bail arms fixed to internal surfaces of the leaves at positions spaced from the inner ends of the leaves, a tapered plug which is engaged non-rotatably with the bail arms and which has a tapered external surface to cooperate with a taper on the internal surfaces of the leaves, and a threaded rod extending between the leaves and threaded through a passage in the plug, the method comprising the steps of:

inserting the inner end of the rockbolt into the drilled hole, and

8.

A bail-type expanding shell rockbolt substantially as herein described with reference to the accompanying drawings.