On Trends in the Design of Metal Fire Pits

E. A. Bober

(Presented by the scientific seminar of the departments of development of bedded deposits, development of ore deposits, and construction of mining enterprises.)

One of the labor-intensive production processes in coal mining is roof control. The improvement of methods of roof control can be traced through the example of the “Kapitalnaya 1” mine of the Molotovugol trust.

In the postwar period, at this mine, partial stowing was used in seams with a thickness of 0.8–1.0 m; in seams with a thickness of 1.4 m and more, roof control was carried out by the method of complete caving.

Partial stowing is a very labor-intensive, heavy production process that is difficult to mechanize. Caving onto timber organ supports is a less labor-intensive production process, but the operation of lowering the roof is dangerous. In addition, during lowering a large quantity of support timber is lost.

The use of metal fire pits, and subsequently of clusters made of metal props, was the first step toward eliminating the indicated shortcomings. Thanks to the introduction of metal fire pits at the mine, partial stowing was completely displaced and replaced by complete caving.

Effectiveness of the Use of Metal Support

The use of metal support has a favorable effect on a number of indicators characterizing the production process in the longwall. The use of clusters made of metal props and fire pits significantly increases labor safety and contributes to the uninterrupted operation of the faces.

During 1953, in the longwalls of the “Kapitalnaya 1” mine where roof control was carried out with the use of metal fire pits or clusters made of SDP props, as can be seen from the data in Table 1, there were no cases of longwall collapse.

Table 1

An analysis of the causes of industrial injuries at the mine showed that in those longwalls where roof control was carried out using metal fire pits or bushes, there were no accidents due to roof falls. Industrial injuries among workers engaged in roof control sharply decreased.

The saving of support timber from the introduction of metal fire pits or bushes per 1000 tons of mine output amounted to:

a) in longwalls on seams 0.9–1.0 m thick, where, before the transition to metal support, waste-rock strips had been used—on average 9.5–10.0 m³;

b) in longwalls on seams 1.4–1.6 m thick, where complete caving with setting on wooden organ support had previously been used—18–21 m³.

Metal fire pits and bushes, unlike waste-rock strips and wooden organ support, do not create any difficulties in preparing the working front in longwalls. Previously it was necessary to deliver a larger quantity of timber, blasting operations were used in laying out waste-rock strips, etc. The elimination of these operations with metal support contributes to a considerable extent to rhythmic operation of the longwalls.

The experience of the “Kapitalnaya 1” mine showed that, under the same conditions, the number of workers engaged in roof control, owing to the reduction in the volume of work, decreased significantly:

a) when using metal fire pits or bushes, compared with waste-rock strips, almost threefold;

b) when using bushes made of metal props, compared with metal fire pits, by 1.3 times.

The use of metal support for roof control reduces the labor input of the work. The data of Table 2 show the actual expenditure of basic wages per 1 ton, in rubles.

Table 2

From the data in the table it is evident that the labor input when using wooden organ support or metal fire pits is almost the same. But replacing wooden organ support with metal fire pits gives a saving of support timber of up to 20 m³ per 1000 tons of output.

When bushes made of metal props are used, the labor input is lower than when metal fire pits are used. But for the use of bushes, strong side rocks must be present. It should be noted that the use of bushes made of SDT metal props at the “Kapitalnaya 1” mine is as yet not widespread; therefore the advantages of metal support described above mainly apply to metal fire pits.

Thus, the practice of using metal fire pits for roof control has confirmed their high economic efficiency and technical expediency.

Principal Structural Elements and the Nature of Deformation of Fire-Pit Supports

The elements of metal fire-pit support are manufactured at the mine from the following materials:

a) from mine rails weighing 18–24 kg per running meter, in the form of sections 0.8–1.0 m long;

b) from I-beams Nos. 16–20, in sections 0.8–1.0 m long;

c) from channel beams No. 20, in the form of sections 0.9–1.1 m long.

The accumulated experience in the use of fire-pit support at the mine has shown that fire pits made up of sections of mine rails, during prolonged operation, undergo almost no deformation, and their consumption is due, as a rule, to losses in the caved space.

In addition, laying fire pits made of rail sections is a labor-intensive process, especially in seams 1.5–2 m thick. As the seam thickness increases, the labor productivity of the fire-pit workers drops sharply. The weight of a fire pit made of mine rails is almost twice as great as the weight of a fire pit made of sections of No. 20 I-beams.

In order to reduce the above-mentioned shortcomings, at the Kapitalnaya 1 mine fire pits made from sections of I-beams and channel beams were used; compared with mine rails, with an insignificant increase in the weight of 1 running meter, they have almost twice the height.

To give greater stability to elements of fire-pit support made from I-beams, four sections of angle iron were welded between their flanges at the support points.

Experience with this design revealed its substantial shortcomings. In the presence of high rock pressure, and also during dismantling of the fire pit, the welded joints are often destroyed; transverse bending and bending of the I-beam flanges occur more often. It is necessary that the reinforced parts of the I-beam in the fire pit be located strictly opposite one another, which under production conditions is not always feasible, especially in dipping seams. In the presence of eccentric compression, the I-beam flanges bend, and the fire pit loses its stable state. Finally, reinforcement of I-beams by welding angle iron to their ends is associated with additional expenditures that increase the cost of fire-pit support. It should also be noted that the use of welding changes the structure of the metal, and this reduces the strength of the structural elements of the fire pit.

The practice of using sections of I-beams has shown that the web thickness of I-beams Nos. 16–20, respectively 6.0 and 7.0 mm, does not satisfy the requirements of fire-pit support.

The use in longwall No. 65 of fire-pit support made from sections of No. 20 channel beams showed that this profile is unsuitable for fire pits, since under roof pressure they quickly develop transverse bending. As a result, this metal fire-pit structure becomes unstable.

Thus, fire pits made of mine rails are metal-intensive, which affects the labor productivity of the fire-pit workers, while fire pits made of I-beams do not have high resistance.

Main Trends in the Design of Metal Fire Pits

Proceeding from the experience of using metal fire pits, it is necessary to find such a design for them that would:

a) help increase the productivity of the pitmen;
b) possess high resistance;
c) have high stability with the lowest possible coefficient of structural quality¹;
d) ensure optimal yielding.

The main paths in the design of metal fire pits that meet the need for growth in the productivity of pitmen should proceed in the following directions: 1) the creation of a special profile for pit support; 2) the creation, on the basis of a special profile and with the use of forced feed for their movement, of designs of metal semi-collapsible or non-collapsible mobile fire pits.

It is necessary to create such a design of metal fire pit as would be light and convenient in operation, and would also possess high resistance.

An attempt [4] to improve the existing design of the metal fire pit by replacing the middle part between its supports with rods made of pipes 1–1¼ inches in diameter was not successful. It has been noted [2; 3; 6] that the proposed design has a substantial shortcoming: the lightening of the structure was achieved at the expense of reducing the load-bearing capacity of the fire pit, which is impermissible. The most important indicator of the suitability of a metal fire-pit design is its resistance to rock pressure.

It was noted above that the materials used, especially pieces of I-beams and channel beams, do not possess sufficient resistance. The recommendation [1,50] to use heavier rails for pit support is, in our opinion, unacceptable, since this leads to a reduction in the productivity of the pitmen and to increased consumption of metal.

The proposal [2] to use two channels welded together for pit support in order to increase the wall thickness should also be considered unacceptable, since in such a design the thickness of the flanges will not change. Welding between the flanges of pieces of angle iron (stiffening ribs) to give them stability, as practice has shown, does not justify itself.

In view of the above, we consider it expedient to find a special profile for pit support. Such a profile should combine the advantages of rails and I-beams. This will make it possible to solve the problem of ensuring the necessary resistance of the pit support and reducing its metal content.

To impart high stability to the design of a metal fire pit, it is necessary to use the metal more rationally, especially in the designs of mobile fire pits. It is necessary to create such a design of metal fire pit in which the metal would work mainly in compression. In this way the minimum coefficient of structural quality will be achieved.

Finally, one of the requirements imposed on the designs of metal cribs is their yielding capacity. It is well known that metal cribs are used in roof control by the method of complete or partial caving. In this case it is required that the crib be a rigid structure with a known and definite yielding capacity, measured in mm. If the crib is a rigid structure, then during its operation the load will fall mainly on the crib support, while the face-side support will be underloaded. This cannot be considered expedient. If, on the other hand, the crib has considerable yielding capacity, the opposite picture may be observed: the main load will be taken up by the face-side support, and the cribs will be underloaded. This too is inexpedient.

Thus, the requirement that the crib support and the face-side support work jointly is essential. And only when the roof caves should the main load be taken up by the cribs, for which purpose they must possess high resistance.

From all this it follows that crib support must have an optimal yielding capacity. This becomes especially important when metal yielding props are used as face-side support.

LITERATURE

1. Bochkarëv V. G. Roof control in longwalls by means of metal cribs. Ugletekhizdat, 1948.

2. Ilkovich S. M. On the article by P. Onishchenko, “On metal cribs of a lightweight type.” Ugol’, No. 7, 1950.

3. Malyshev A. S. On the article by P. Onishchenko, “On metal cribs of a lightweight type.” Ugol’, No. 3, 1950.

4. Onishchenko P. On metal cribs of a lightweight type. Ugol’, No. 9, 1949.

5. Parusimov V. F., Novikov K. P. Systems for mining coal seams of the Kuzbass. Ugletekhizdat, 1950.

6. Popov V. L. On the article by P. Onishchenko, “On metal cribs of a lightweight type.” Ugol’, No. 3, 1950.

1. The “coefficient of structural quality” is the ratio of the actual weight of a structure to that theoretical weight which would be necessary if all the material in the structure worked only in pure compression [1]. ↩