ON A PROFILE FOR FIBERGLASS SUPPORTS

E. A. Bober

Among the many types of stowing support, dismantlable portable crib supports are finding application. In the Donbass, as of January 1, 1957, 213 longwalls were equipped with them [4]; in the Kuzbass, by the end of 1957, 1,830 units had been installed.

At present, sections of mine rails are used mainly for laying crib supports. As practice has shown, cribs made of sections of mine rails are almost not deformed during prolonged operation, and their consumption is due chiefly to losses in the collapsed space. Consequently, from the standpoint of load-bearing capacity, cribs made of rails satisfy the requirements imposed on stowing supports. However, on the other hand, they have a substantial shortcoming. Laying cribs from rail sections is a labor-intensive process. As the seam thickness increases, the weight of the crib increases, which leads to a reduction in the labor productivity of the crib layers.

In order to avoid the shortcomings inherent in cribs made of rail sections, attempts were made to use sections of I-beams and channel beams Nos. 16–20 for their laying; compared with rails, with an insignificant increase in the weight of 1 linear meter, these have almost twice the height. Because of this, fewer sections are placed in the crib, and its weight is reduced by 1.5–2 times. To give greater stability to the elements of crib support made of I-beams and channel beams, sections of angle iron were welded between the flanges at the bearing points.

Experience in the use of cribs made of I-beams and channel beams has shown that they cannot withstand the loads arising in longwalls. In the presence of high rock pressure, longitudinal bending occurs in the webs of the elements, and the flanges also bend. As a result, the structure becomes unstable. Consequently, with the greater height of the profiles used, which is positive, the wall thickness of 6–7 mm (Table 1) proves insufficient.

Table 1

Thus, buntons made from mine rails are metal-intensive, which affects the labor productivity of bunton installers, while buntons made from I-beams and channel beams do not possess the necessary bearing capacity.

It should also be noted that at some mines in the Kuzbass, buntons were used that were laid out from sections of trough-shaped profile (the back of an arched support made from a special profile for preparatory workings).

Thus, the practice of laying buntons from various rolled profiles has shown that none of them, in essence, satisfies the requirements imposed on individual shaft supports (lightness, strength, etc.). In this connection, it has been proposed [1; 8] to proceed along the path of creating a special profile for bunton supports. Such a profile should contribute to reducing the weight of the bunton, provide its high bearing capacity, and allow speed, simplicity, and accuracy in assembling the structure during installation. In this respect, we developed and manufactured from sheet steel, by bending, three modifications of the special profile (Fig. 1), which were tested as reduced-size models. Fig. 2 shows the moment of testing a model bunton made from profile B.

Fig. 1. Models of metal profiles tested under laboratory conditions

Fig. 2. Testing of a model made from profile B. Load 5775 kg.

Laboratory tests showed that a bunton assembled from profile B withstands a considerably greater load (Fig. 3) than buntons made from profiles A and Б. This is facilitated by: a) the accuracy of assembly of the structure, preventing the appearance of transverse-bending deformations; b) the presence of a “hinged” connection in the supports, ensuring close contact of adjacent sections. On the basis of experience in the use of bunton supports, laboratory tests of the special profile, and tests of certain profiles at the DII [5], it may be concluded that the loss of bearing capacity occurs due to longitudinal bending of the profile wall. Therefore, to determine the bearing capacity of the structure one may use the theory of longitudinally compressed plates [6; 7], likening the walls of the profile to plates with freely supported edges.

Let us take the following possible dimensions of the full-scale specimen of the special profile. Height—175 mm; wall thickness—10 mm; length of the working part of the wall—200 mm; width at the top—100 mm, width at the bottom—180 mm; length of a segment, depending on the seam thickness—800–1000 mm; height of the straight part of the walls—150 mm; height of the profile in the weakened part—80 mm. The weight of one segment with a length of 0.8 m will be about 22 kg, and with a length of 1.0 m—25 kg. A crib made of such a profile is lighter than a crib made of rails. Its weight for a seam thickness of 1.2 m, taking account of the locking beams, will be about 350 kg, as against 530 kg for rails of 24 kg per linear meter. With the adopted parameters of the special profile, the maximum load taken up (destructive load) on the crib will be about 350 t; this exceeds the destructive load on a crib made of mine rails.

Fig. 3. Characteristics of the tested models of metal cribs

Thus, the use of such a shaped profile would make it possible to reduce the consumption of metal for laying the crib, to ensure sufficient load-bearing capacity of it; ultimately, this would help to increase the labor productivity of the workers engaged in roof control.

However, it should be noted that the weight of mine support is a most important factor affecting the labor intensity of support work. Therefore, when metal is used for making individual portable supports, it does not appear possible to reduce the labor intensity of support work radically, since the support remains heavy, especially when it is intended for use in seams more than 1.5–2.0 m thick. For seams of greater thickness there are essentially no acceptable designs of metal support, apart from carrying out experimental work.

A substantial influence on reducing the labor intensity of support work will be exerted by the use of light and strong materials. The All-Union Scientific Research Coal Institute (VUGI) has carried out work on the search for new structural materials [2]. In general, the most effective from the standpoint of specific strength (the ratio of temporary resistance to volumetric weight) are glass-fiber materials on a resin base. At present a new type of high-strength glass-fiber anisotropic material—SVAM [3]—has been developed.

Structural properties of SVAM

SVAM is a homogeneous anisotropic material. It is hydrophobic, not subject to decay or corrosion. From this point of view, satisfac—

satisfy the requirements imposed on the material of mine timbering. This material has high strength. In terms of specific strength it even exceeds the specific strength of hardened steel 30KhGSA (Table 2).

Table 2

Characteristics of structural materials

Note. 1. The ratio of longitudinal and transverse layers is indicated in parentheses.
2. In the numerator the specific strength in tension is indicated; in the denominator, that in compression.

From the data in the table it follows that the strength of CBAM depends on the ratio of longitudinal and transverse layers. With an increase in the number of longitudinal layers, the tensile strength of the material increases significantly. The homogeneity and anisotropy of CBAM make it possible, by combining the orientation of the layers, to obtain the most rational designs.

CBAM have a high modulus of elasticity; Hooke’s law is valid for them almost up to failure. These materials make it possible to create very rigid structures. An important property of CBAM is their rather high specific impact toughness, which favorably distinguishes them from other plastics, for example from laminated plastics, which have a high ultimate compressive strength with very low specific impact toughness. This is especially important, since under mine conditions one has to deal with impact application of load (for example, when installing supports).

Thus, the characterization given on the basis of published data [2; 3] makes it possible to conclude that this material can be used for making timbering for extraction workings. Investigations by VUGI have established that CBAM in mine timbering can be used in the form of tubes, slabs, plates, beams, and other open sectional profiles in such structures as posts, caps, ties, chocks, pedestals, etc.

Possible design of chock timbering made of CBAM

Having considered in general terms the properties of CBAM, let us touch upon a possible design of chock timbering made of these materials. It was noted above that, among the tested metal profile models, model B possesses the best properties. This shaped profile permits precise assembly. This leads to the fact that the structural elements work

in compression and there are no transverse-bending stresses. This thereby makes it possible to lighten the elements by weakening the middle part of the segment. In addition, with inclined incidence the process of erecting the set is facilitated: it is not necessary to drive in additional props in order to prevent the segments from slipping in the direction of incidence. A necessary condition is the use of locking beams to relieve the set from pressure. Otherwise the structure becomes non-dismountable.

Taking into account the structural properties of GFAM, we consider it possible to use these materials for making elements of support sets in the form of a shaped profile, the general appearance of which is shown in Fig. 4.

Fig. 4. General appearance of a shaped profile for support sets

A collapsible portable set made from such a shaped profile will be 5–6 times lighter than a set made of rails. At the same time, by adopting the corresponding parameters of the proposed special profile in its general form, tentatively indicated above, it is possible to give the structure sufficient load-bearing capacity. Ultimately, this will make it possible to increase productivity significantly and to ease the working conditions of workers engaged in roof control.

REFERENCES

1. Bober E. A. On the question of directions in the design of metal sets. Proceedings of the Tomsk Polytechnic Institute, vol. 84, Tomsk, 1956.

2. Bilik Sh. M., Panov A. D. Mine supports made of fiberglass. Ugol’, No. 10, 1957.

3. Burov A. K., Andreevskaya G. A. Glass-fiber anisotropic materials and their technical application. Publishing House of the Academy of Sciences of the USSR, 1956.

4. Davidyants V. T. Roof control by complete caving. Ugletekhizdat, 1957.

5. Lipkovich S. M. On Onishchenko’s article “On lightweight metal sets.” Ugol’, No. 7, 1950.

6. Timoshenko S. P. Strength of Materials, vol. 2 (translated from English). Gostekhizdat, 1946.

7. Timoshenko S. P. Stability of Elastic Systems (translated from English). Gostekhizdat, 1946.

8. Shcherbak N. I., Denisov V. V. Experience in the use of metal supports in the entries of the Kapital’naya-1 mine. Kemerovo, 1956.