On the Establishment of Optimal Modes of Electric Rotary Drilling of Blast Holes in the Rocks of the Prokopyevsk Deposit

M. K. Tsekhin

(Submitted by the scientific seminar of the departments of bedded-deposit development, ore-deposit development, mine construction, safety engineering, and mine ventilation)

Drilling operations during the driving of horizontal workings in rock at the mines of the Prokopyevsk hard-coal mine are a decisive link in the complex of drifting operations. Occupying half or more of the cycle time, they determine both the rate and the cost of carrying out these workings.

In the course of the investigation, the unsatisfactory condition of drilling operations at the mines of the mine was revealed: low rates of drilling blast holes in rock, high consumption of cutters and drill steel, irrational use of the power of the motors of electric drills, imperfection of the dry method used for removing drilling dust from the blast hole, and so forth. It was also established that the principal cause of the unsatisfactory condition of drilling operations is the inefficient use of column-type electric drills. The operating mode of these drills often does not correspond to the mining-and-geological conditions under which the drilling process is carried out.

Increasing the efficiency of drilling operations at the mines of the mine is thus connected with the task of finding rational operating modes for column-type electric drills when drilling blast holes in rocks of different hardness.

Investigations concerning rotary drilling of blast holes and boreholes and, in particular, increasing the efficiency of column-type electric drills are not new. On the contrary, because of their urgency, they have attracted and continue to attract the attention of many Soviet and foreign scientists and engineers [1, 2, 3, 4, 5].

However, among researchers there is still no unified opinion regarding the ways of accelerating drilling operations in electric rotary drilling of blast holes in various rocks. Questions concerning the influence of axial force, the number of revolutions, and cutter feed per revolution on drilling speed, cutter wear, and the load on the electric-drill motor remain essentially unresolved. The question of the expediency of using water flushing of blast holes also remains unclear. All this makes it difficult to establish optimal modes of drilling blast holes by column-type electric drills with mechanical spindle feed under production conditions.

The study of the regularities of the process of electric rotary drilling and, on their basis, the establishment of optimal modes for drilling blast holes in rocks characteristic of the Prokopyevsk deposit was the principal objective of the investigation. The solution of this problem makes it possible to increase the labor productivity of miners and the rate of driving workings, and has a significant influence on the intensity and economy of mining the deposit.

The main experimental investigations aimed at establishing the regularities of the drilling process and identifying the optimal operating modes of electric drills were carried out in the field drift of the Kharakternyi seam and in crosscut No. 11 of the M. I. Kalinin mine of the Prokopyevskugol trust. Additional observations, connected mainly with establishing optimal drilling modes and selecting rational sets of blast holes, were conducted in crosscut No. 2 of the Chernaya Gora mine and in the workings of the pit-bottom yard of the K. E. Voroshilov mine.

For the investigation, the rocks most characteristic in strength and structure of the Prokopyevsk deposit were selected: from the hard-to-drill coarse-grained quartz sandstone of the roof of the Kharakternyi seam to weak carbonaceous argillite, inclusive.

For drilling blast holes in the above-mentioned rocks, new core electric drills of the EBK-2M type, not previously in use, were employed. The kinematic diagram of the drill is shown in Fig. 1.

Fig. 1. Kinematic diagram of the EBK-2M electric drill.
1—electric motor; 2—gears of the rotor pair; 3—intermediate pair; 4—spindle-rotation pair; 5—spindle-feed pair; 6—intermediate shaft; 7—friction mechanism; 8—spindle; 9—chuck for fastening the drill bit; 10—spindle-rotation bushing; 11—spindle-feed nut.

The spindle speed was varied from 125 to 600 rpm by replacing the rotor and intermediate pairs of gears of the reducer.

The spindle feed per revolution was regulated from 0.9 to 4.4 mm/rev by replacing the feed pair with gears manufactured for this purpose in the mechanical workshops of the Prokopyevskugol trust.

The limiting axial feed force of the spindle was varied from 200 to 1100–1200 kg by compressing the spring of the friction mechanism, specially prepared for the purposes of the investigation. For calibrating it to the specified maximum possible axial force, a spring dynamometer and a stand manufactured for this purpose with an oil pressure gauge were used.

The load on the motor of the electric drill was measured with a VIT-type wattmeter. The machine drilling time was determined with a stopwatch.

In the experimental drilling of blastholes with flushing, use was made of a hollow spindle, hollow hexagonal drill rods, and a pumping unit.

Each experiment was usually repeated no fewer than five times. In individual cases the number of repetitions of an experiment was brought up to 7–8. As a result of each observation, the following were established:

a) the number of linear units \((m, cm)\) of blastholes drilled by the bit before it became dull (bit serviceability);

b) the average values of drilling speed and load on the electric-drill motor for the period from the start of drilling to the cessation of penetration of the bit into the rock.

The final result of each experiment was established as the arithmetic mean of all observations recognized as valid.

In the course of the investigations, carried out under underground working conditions, it was possible, with a known accuracy, to reveal the principal regularities of the process of rotary drilling of blastholes in the rocks of the Prokopyevsk deposit.

It was established that the drilling speed in pure machine time, at different values of axial thrust, increases with an increase in the number of spindle revolutions. However, this increase lags somewhat behind the increase in the number of revolutions, which is especially characteristic of the process of drilling blastholes in hard sandstones (Fig. 2).

Fig. 2. Character of the dependence of the drilling speed of blastholes \((V)\) on the number of revolutions \((n)\) in coarse-grained quartz sandstone at different axial thrusts \((P)\) and a specific feed of \(0.9\) mm/rev.

The optimum value of the number of revolutions, from the point of view of obtaining the greatest speed in pure machine time, increases as the strength and abrasiveness of the rock decrease, and falls as the magnitude of axial pressure decreases.

The load on the electric-drill motor also increases with an increase in the number of spindle revolutions (Fig. 3). This increase, within the investigated range (up to 600 rev/min), lags somewhat behind the increase in the number of revolutions.

The serviceability of the cutter decreases as the number of revolutions increases (Fig. 4). Its decrease is especially noticeable when drilling blast holes in coarse-grained quartz sandstone, which is distinguished by high abrasiveness.

An increase in axial force, within the limits permitted by the mechanical strength of the cutter and the drilling rig, causes an increase in drilling speed, cutter serviceability, and power consumption. In this case, the increase in the indicated quantities somewhat outpaces the increase in axial pressure.

An increase in the specific spindle feed per revolution, regardless of the number of revolutions adopted, increases the drilling speed in terms of net machine time (Fig. 5). At the same time, the specific energy consumption of the blast-hole drilling process decreases.

Fig. 3. Character of the dependence of the load on the motor of the electric drill \((W)\) on the number of revolutions \((n)\) in coarse-grained quartz sandstone at different axial forces \((P)\) and a specific feed of \(0.9\ \mathrm{mm/rev}\).

The serviceability of the cutter reaches a maximum at a certain value of the specific feed that is optimal for the given rock. With a decrease in the hardness and abrasiveness of the rock being drilled, the value of the spindle specific feed that is most advantageous from the standpoint of obtaining the greatest cutter serviceability increases.

Fig. 4. Character of the change in cutter serviceability \((S)\) with the number of revolutions \((n)\) in coarse-grained quartz sandstone at different axial forces \((P)\) and a specific feed of \(0.9\ \mathrm{mm/rev}\).

It has been established that the energy consumption of blast-hole drilling decreases with an increase both in axial force and in the specific spindle feed.

With an increase in the number of revolutions, it has its smallest value at 200–300 rpm when drilling blast holes in hard coarse-grained sandstone, and approaches a minimum at 600 rpm in the process of drilling fine-grained sandstone of medium strength (Fig. 6).

Fig. 5. Nature of the change in the drilling speed of blast holes as a function of the number of revolutions in coarse-grained quartz sandstone at different specific spindle feeds

It has also been established that the indicators of drilling speed in terms of net machine time cannot serve as the sole criterion in selecting the optimum mode of drilling blast holes, especially in hard, highly abrasive rocks. From the graph shown in Fig. 7, which reflects the expenditure of total time for drilling one linear meter of blast hole in hard coarse-grained sandstone, it is evident how uneconomical it is to drill under these conditions with a specific feed of 3.2 mm/rev at 300 spindle revolutions per minute, although in terms of net drilling time this mode is unquestionably advantageous. Also uneconomical under these conditions proved to be the mode of 600 spindle revolutions per minute with a specific feed of 0.9 mm/rev.

When selecting optimum drilling modes, it proved expedient also to take into account changes in the cost of one linear meter of blast hole according to the principal variable indicators (the drillers’ basic wages and cutter depreciation).

Fig. 6. Change in the energy intensity of drilling blast holes as a function of the number of revolutions, with a specific spindle feed of 0.9 mm/rev and feed forces of 1100 kg—for coarse-grained sandstone—and 850 kg—for fine-grained sandstone.

As can be seen from the graph (Fig. 8), the mode with a number of revolutions per minute \(n = 300\) and a specific spindle feed per revolution \(h = 3.2\ \mathrm{mm}\), for strong coarse-grained sandstone, proved extremely irrational also from the standpoint of the cost of drilling operations. Also disadvantageous in this respect was the mode at \(n = 300\ \mathrm{rev/min}\) and \(h = 4.4\ \mathrm{mm/rev}\) for fine-grained sandstone of medium strength, although precisely in these modes we obtain the highest drilling-rate indices in terms of net machine time.

In the course of the investigations it was found that the strength, hardness, and abrasiveness of the rocks jointly influence the main indices of rotary drilling and determine the drillability of the rock. Neither the temporary compressive resistance nor the hardness, without taking into account the influence of abrasiveness, can serve as indicators of the drillability of the sandy and sandy-clayey rocks of the Prokopyevsk deposit, which possess high abrasiveness.

Fig. 7. Character of the change in the total time for drilling 1 running m of blasthole in coarse-grained sandstone with an increase in the number of revolutions, at an axial force of 1100 kg and specific feeds of 0.9, 2.0, and 3.2 mm/rev.

It has been established that the use of flushing the blastholes with water (or with aqueous solutions of rock-strength reducers) ensures an increase in the efficiency and safety of drilling operations and is

Fig. 8. Dependence of the cost of 1 running m of blasthole on the number of revolutions when drilling blastholes in coarse-grained sandstone with a feed force of 1100 kg.

an extremely important measure. Flushing increases the drilling rate and improves the serviceability of the cutters, reduces the energy consumption of the process and the cost of drilling operations. It makes it possible, with great effect, to drill blast holes regardless of their angle of inclination and length, at any specific spindle feed that is maximally advantageous for the given rock. With the introduction of flushing, it becomes possible to replace screw-type drill rods with more advantageous and safer rods of hexagonal cross-section, and the dustiness of the near-face air by silicosis-hazardous dust is reduced.

The investigations carried out have shown that the principle of mechanical spindle feed, limited by optimal bounds, is expedient for the conditions of drilling blast holes in the rocks of the Prokopyevsk deposit. The rationality of applying this principle is determined by:

a) the automatic regulation of axial pressure when the properties of the rocks being drilled and the degree of cutter dulling change;

b) the possibility, effective and safe for the cutter and the drilling rig, of applying an increased feed force (up to 1000–1200 kg) when drilling bedded rocks of different strengths, so frequently encountered in the mines of the ore field.

At the same time, in the course of the investigation it was established that the EBK-2M electric drills of 2.7 kW power now used in the mines of the ore field, operating without water flushing of the blast holes, with a feed force not exceeding 600 kg and with a specific spindle feed of 0.9 mm/rev, are of low productivity and require design improvement. The need was revealed to adapt the currently manufactured EBK-2M drills for flushing, and to improve the design of their friction mechanism and reducer, taking into account the possibility of setting the drill to effective drilling modes.

As a result of the investigations carried out and the analysis of the revealed regularities of electric rotary drilling of blast holes in sandy and sandy-clayey rocks, it has been established that the efficiency indicators of this process are determined to a considerable extent by the correspondence of the drilling mode employed to the physico-mechanical properties of the rocks and, in particular, to their strength, hardness, and abrasiveness.

The study of the process of drilling blast holes in the rocks most characteristic of the Prokopyevsk deposit made it possible to develop a classification of them by drillability [8] and to establish, for the corresponding rocks, optimal operating modes of electric drills with water flushing of the blast holes.

For each rock having definite drillability indices, such an operating mode of the drill has been established in which a high rate, both in net drilling time and in total drilling time, corresponds to the minimum energy consumption and the lowest expenditure of funds for drilling a unit length of blast hole.

When drilling blast holes with flushing by design-improved EBK-2M electric drills with a motor power of 2.7 kW, the modes shown in Table 1 are proposed as rational.

Analysis of the investigations carried out also shows that the highest results in drilling blast holes with column-type electric drills can be achieved if the power of the electric motors on these drills is brought up to 3.2–3.5 kW. In this case, in the modes indicated above, the axial force and specific spindle feed remain the same. As for the number of revolutions, they should be somewhat increased: up to two hundred—for rocks of the first

and second categories, up to three hundred—for rocks of the third category, and up to four hundred—for rocks of the fourth and fifth categories.

The introduction of the proposed optimal drilling modes, in comparison with those used at the mine, makes it possible to increase the speed

Table 1

of drilling in net machine time by a factor of 2.4—12.5, increase the service life of cutters by a factor of 1.1—7.2, reduce the energy intensity of the drilling process by a factor of 1.5—4.5, and reduce the cost of one meter of drilled blasthole by an average factor of 4.9. Solely by increasing the technical drilling speed, without taking into account the existing possibilities for reducing the time spent on preparatory, concluding, and auxiliary operations, the total time expenditures for drilling work in the drifting cycle can be reduced by 33—45%.

References

1. Mikhailov V. G. Drilling of Blastholes, Metallurgizdat, 1947.

2. Schreiner L. A. Physical Foundations of the Mechanics of Rocks, Gostoptekhizdat, 1950.

3. Buchev V. K. Drilling-and-Blasting Operations, Ugletekhizdat, 1953.

4. Imas A. D., Azarkh V. L. Determination of Drilling Modes for Rocks, Ugletekhizdat, 1952.

5. Alpan H. S. Factors affecting the speed of penetration of Bits in electric drilling, “The Institution of Mining Engineers.” Transactions. Vol. 109. London. 1950.

6. Sievers, Hans. Die Bestimmung des Bohrewiderstandes von Gesteinen, “Glückauf,” Heft 37 38, 1950.

7. Barth, Rudolf. Untersuchungen zur Ermittlung der erforderlichen Maschinenleistung und der Beanspruchung des Geräts Bein drehenden. Bohren grosser Bohrlöcher in Karbongesteinen, “Glückauf,” Heft 3 4, 1951.

8. Tsekhin M. K. On the question of classifying the rocks of the Prokopyevsk deposit by drillability, Proceedings of the Tomsk Polytechnic Institute named after S. M. Kirov, Order of the Red Banner of Labor, vol. 93, 1958.