CA2251446A1

CA2251446A1 - Mine prop

Info

Publication number: CA2251446A1
Application number: CA 2251446
Authority: CA
Inventors: Clifford A. Mccartney
Original assignee: Strata Products USA Inc
Current assignee: Strata Products USA Inc
Priority date: 1997-10-23
Filing date: 1998-10-22
Publication date: 1999-04-23
Also published as: GB2330596B; GB9822522D0; AU8927198A; GB2330596A; AU749241B2

Abstract

The invention concerns a mine prop which includes an elongate timber post.
The post has a longitudinal axis and opposed ends and is substantially square in cross-section over a major portion of its length. Diagonally opposed corners of the post are trimmed off, at an inclination to the axis, at one end of the post. This forms a yield pod, defined by the other diagonally opposed corners of the post, a diagonally extending end and a pair of diagonally opposed planar surfaces inclined to the axis, at the trimmed end. The yield pod is adapted to yield along a diagonal transverse to its diagonally extending end when a compressive load of sufficient magnitude is applied axially to the post.

Description

~ CA 022~1446 1998-10-22 "MINE PROP"

FIELD OF THE INVENTION

THIS invention relates to a mine prop, i.e. an elongate mine support which is used to support the roof of a mine working above the floor thereof.

BACKGROUND TO THE INVEN'rION

It is desirable for a mine support to have the facility to yield vertically under the loads imposed thereon as a result of closure or convergence of the mine working in which the support is installed, i.e. movement of the roof and floor towards one another. Yieldability of the mine support enables it to accommodate the closure of the mine working without punching into the roof or floor. In the case of an elongate mine support, i.e. prop, arranged with its long axis upright, appropriate yieldability of the prop can also avoid catastrophic buckling or snapping of the prop and total loss of the mine support function.

Many proposals for yieldable mine props have been put forward in the past and are currently in use with greater or lesser degrees of success. Typical examples are described in US patents 4,520,980 (Townsend), 4,915,339 (Botha) and 5,318,387 (Pienaar et al) and South African patent 83/8454 (Botha). In each of these examples of the prior art, the prop has a round cross-section timber pole, modified in different ways at one end, as its major, basic component.

~ CA 022~1446 1998-10-22 A disadvantage of these props is their tendency, when yielding under load, to deviate unacceptably from an upright orientation as a result of sideways brushing of the modified end.

It is accordingly an object of the present invention to provide a mine prop which exhibits more controlled yielding.

SUMMARY OF THE INVENTION

According to the present invention there is provided a mine prop comprising an elongate timber post which has a longitudinal axis and opposed ends, the post being substantially square in cross-section over a major portion of its length, diagonally opposed corners of the post being trimmed off at an inclination to the said axis at least at one end of the post so that the post has, at that end, a yield pod defined by the other diagonally opposed corners of the post, a diagonally extending end and a pair of diagonally opposed planar surfaces inclined to the said axis, the pod being adapted to yield, along a diagonal transverse to the said diagonally extending end, when a compressive load of sufficient magnitude is applied axially to the post.

In some embodiments of the invention, the trimmed off corners of the post are removed from the remainder of the post. In other embodiments, the trimmed off corners, although separate from the remainder of the post, are held against the said planar surfaces by appropriate tie means, typically wire ties, which embrace the post and the trimmed off corners.

CA 022=,1446 1998-10-22 BRIEF DESCRIPTION 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 the timber post of a mine prop according to the invention;

Figure 2 shows a view of the post in the direction of the arrow 2 in Figure l;

Figure 3 shows a partial cross-section at the line 3-3 in Figure 2;

Figures 4 and 5 show successive stages in the loading of a mine prop incorporating the post of Figures 1 to 3;

Figure 6 graphically illustrates the performance of a mine prop of the type seen in Figures 4 and 5;

Figure 7 shows a view, similar to that of Figure 1, of an alternative post; and Figure 8 graphically illustrates the performance of a mine prop ' incorporating the post seen in Figure 7.

CA 022~1446 1998-10-22 DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

The timber post 10 seen in Figures 1 to 3 consists of a length of timber, in this case yellow poplar. As will be particularly evident from Figures 2 and 3, the timber length has been machined to have a substantially square cross-section with four longitudinally extending corners 12, 14, 16 and 18. At one end 20 the post is cut off at right angles to the longitudinal axis 22 of the post to form an end surface 24.

At the other end the post is symmetrically modified. This is achieved by forming planar saw cuts through the timber material at diagonally opposed corners of the post. As will be apparent from the Figures, the saw cuts are inclined to the axis 22 and extend from the end of the post to the corners 12 and 16 at some distance from the end. The saw cuts form a tapered or wedge-shaped yield pod 26 which is defined by a narrow end surface 28, planar surfaces 30 which are inclined to the axis 22, and the rem~ining corners 14 and 18 of the post. The end surface 28 extends diagonally between the corners 14 and 18 as illustrated.

Figures 4 and 5 show a mine prop 34, according to the invention, which includes the post 10 and a footboard or base 36 which is, in this case, a rectangular block of yellow poplar. The base 36 is laid on the floor 38 of a mine working 40 and the post 10 is arranged at an upright attitude with the end surface 28 of the yield pod bearing on the upper surface of the base and with the opposite, upper end surface 24 bearing against the roof 42 of the mine working. In practice, the post is trimmed to the appropriate length for installation in the illustrated manner.

CA 022~1446 1998-10-22 Alternatively a headboard, which may of similar configuration to the base 36, can be interposed between the surface 24 and the roof and/or wedges are hammered into position between the surface 24 or headboard to ensure that there is firm contact with the roof.

Figure 4 shows the mine prop 34 on initial installation and Figure 5 shows the same prop after some convergence between the floor 38 and roof 42 has taken place. Such convergence will typically be due to normal closure of the mine working, i.e. relative movement of the roof towards to the floor with passage of time as a result of descent of the roof and/or heaving of the floor.

The yield pod 26 provides the mine prop with a yielding capability to accommodate the convergence or closure of the mine working. As shown in Figure 5, the axial compressive load applied to the post 10 has resulted in yielding of the pod to one side. In practice, the sideways yield takes place along the diagonal illustrated in Figure 2 with the numeral 44, i.e. in a direction transverse to the end surface 28. The yielding of the pod in this way results in axial shortening or contraction of the post 10.

Figure 6 graphically represents the performance of a prop such as the prop 34 when axially loaded in a press. The vertical axis represents the load (in kips) applied axially to the prop and the horizontal axis represents the vertical displacement or contraction undergone by the post l O. It will be seen that after minim~l contraction the prop was able to support a load of approximately 40 kips, and that it thereafter supported loading of that approximate magnitude through a contraction of approximately 12 inches.
In other words, the prop was able constantly to support a load of approximately 40 kips despite being caused to shorten in length by 12 inches.

CA 022~1446 1998-10-22 The prop which was used in the test represented by Figure 6 had a yellow poplar post 10 with square dimensions of 6 inches and an overall length of 72 inches. The base 36 had dimensions of 10 x 20 x 4 inches. The planar surfaces 30 were inclined at an angle of 8~ to the axis of the post and the width of the end surface 28, i.e. the narrow dimension thereof, was 1.5 inches.

Figure 7 illustrates an alternative post 50. In this case, corner pieces 52 are trimmed off by saw cuts in exactly the same way as in the embodiment of Figure 1. However in this case the separated corner pieces 52 are subsequently tied in their original positions by means of wire ties 54.

Figure 8 illustrates the performance, in a press, of a mine prop similar to the prop 34 but incorporating the post 50 instead of the post 10. The post S0 in this test had identical dimensions to those given above for the test post 10.
In this case it will be seen that the prop was able to withstand a load of approximately 40 kips over the major part of its contraction. Some load was shed during contraction of 4 to 6 inches. It is believed that one reason for this is the fact that the wire ties 54 in the test were relatively heavy and strong and accordingly not conducive to allowing the corner pieces 54 to release from, i.e. move relative to, the remainder of the post S0 during yielding.

It is considered preferable in practice to employ tie means which are capable of yielding at least to some extent, or ties which are frangible under sufficient load, to secure the sawn off corner pieces to the remainder of the post. These may, for instance, wire ties of relatively low tensile strength or flat steel or polymer bands.

- CA 022~1446 1998-10-22 Load shedding such as that seen in the range 4 to 6 inches in Figure 8 could also be attributable, at least to some extent, to natural variances and localised weaknesses in the timber.

An important feature of the invention is the fact that the symmetrical trimming of the corners of the post results in yielding of the pod 26 across a diagonal. Since the diagonal dimension is the greatest transverse dimension of the post, the resistance to yielding in this direction is greatest. Thus while modification of the post end is undertaken with the specific intention of promoting yielding this is done in such a way that the greatest yielding resistance is attained.

The tests represented by Figures 6 and 8 showed that the square section timber posts 10, 50 exhibited admirable lateral stability during the yielding process. This stability exceeded that which can be obtained with asymmetrically or symmetrically modified timber posts of round cross-section.

From Figures 1 to 3 it will be appreciated that the yield pod is symmetrical about a plane which contains the untrimmed corners, i.e. the corners 14 and 18, and also about a plane which contains the trimmed corners, i.e. the corners 12 and 16.

Claims

1. A mine prop comprising an elongate timber post which has a longitudinal axis and opposed ends, the post being substantially square in cross-section over a major portion of its length, diagonally opposed corners of the post being trimmed off at an inclination to the said axis at least at one end of the post so that the post has, at that end, a yield pod defined by the other diagonally opposed corners of the post, a diagonally extending end and a pair of diagonally opposed planar surfaces inclined to the said axis, the pod being adapted to yield, along a diagonal transverse to the said diagonally extending end, when a compressive load of sufficient magnitude is applied axially to the post.

2. A mine prop according to claim 1 in which the trimmed off corners of the post are absent.

3. A mine prop according to claim 1 in which the trimmed off corners, although separate from the remainder of the post, are present and are held against the said planar surfaces.

4. A mine prop according to claim 3 wherein the trimmed off corners are held against the said planar surfaces by tie means.

5. A mine prop according to claim 4 wherein the tie means are wire ties which embrace the post and the trimmed off corners.

6. A mine prop according to claim 4 wherein the tie means are frangible or yieldable under imposed loading of sufficient magnitude.

7. A mine prop according to claim 1 wherein the yield pod is symmetrical about a plane containing the opposed corners which are not trimmed off and also about a plane containing the opposed corners which are trimmed off.

8. A mine prop according to claim 7 wherein the diagonally extending end of the yield pod is defined by a narrow, flat surface lying in a plane normal to the axis of the post.

9. A mine prop according to claim 8 wherein the post is arranged upright in a mine working having a floor and a roof, the mine prop including a timber board placed against either the floor or the roof, the end of the yield pod of the post bearing against the board and the opposite end of the post bearing against the other of the floor or roof.