Some Results of a Study of the Operating Modes of Pneumatic Drilling Hammers

O. D. Alimov, I. G. Basov, P. A. Samoilov

In recent years, both in our country and abroad, new models of pneumatic drilling hammers have been developed. Most of them belong to the class of high-frequency drilling hammers. In comparison with those produced earlier, high-frequency hammers have an increased blow frequency—up to 3000–3500 blows per minute. Increasing the blow frequency while maintaining the former energy of each blow makes it possible to increase the power of the machine. However, as our studies show [1–4], the power of a pneumatic drilling hammer can be used most expediently only when it operates under certain modes (at a definite feed force, air pressure, etc.). The data necessary for selecting such modes are not given in the literature known to us.

In this connection, the Department of Mining Machines and Mine Transport of the Tomsk Polytechnic Institute carried out tests of pneumatic drilling hammers of various types. For testing, hammers were selected that had received the highest appraisal in domestic and foreign literature: the high-frequency hand-held drilling hammers PR-20 of the Krivoi Rog Machine-Building Plant; PR-25 of the Leningrad “Pnevmatika” plant; RH-754 of the Swedish firm “Atlas-Diesel”; and T-10 of the Finnish firm “Tampella.” The most widely used in domestic mining practice are the hand hammers PM-508, PR-35, and the column-mounted hammers KTsM-4 (Table 1).

The purpose of the study was to determine the optimal operating modes of drilling hammers and the drilling rates corresponding to them. On the basis of the experimental data obtained, the productivity and economy of the machines tested were evaluated.

The optimal drilling modes were determined:

a) when drilling rocks of various hardness ($f = 6 \div 8$ and $f = 14 \div 18$ according to the scale of Prof. M. M. Protodyakonov);

b) at compressed-air pressures of 4, 5, and 6 ati;

c) when drilling with different drilling tools (drill length 1–2 m, drill-bit diameter 40 and 46 mm).

Table 1

Technical data of various drilling hammers

Notes. The number of blows, the number of drill rotations, and the air consumption were determined when drilling sandstone ($f = 6 \div 8$). The air consumption of the RH-754, T-10, and PR-25 hammers was determined while they operated with separate flushing of the hole through a flushing device. The technical data for indicators 3—9 correspond to $p_m = 5$ atm.

Tests of drilling hammers were carried out according to a previously developed procedure [1]. Under laboratory conditions, the operating modes of pneumatic drilling hammers were investigated only when drilling hard rocks (hornfels $f = 18$, mica-amphibole schist $f = 14 \div 18$). Drilling of rocks of medium hardness was carried out at the Stepanov stone quarry near the city of Tomsk (sandstone $f = 6 \div 8$).

As has been established by a number of studies [2, 3], the feed force of the hammer against the face has a substantial influence on the drilling speed and on the stability of operation of drilling hammers, all other conditions being equal. This is characteristic of all modern pneumatic drilling hammers and is explained by the fact that, as a result of a change in the feed force, the conditions of collision of the striker piston with the drill are altered. Consequently, the working process of the hammer and the magnitude of the power transmitted by the striker piston to the drilling tool are changed.

At comparatively small feed forces the body of the hammer moves backward away from the limiting collar of the drill; as a result, the efficiency of impact-energy transmission in the “piston—drill—rock” system decreases [4]. As the feed force is increased, the clearance between the body of the drilling hammer and the limiting collar of the drill decreases, the operating mode of the machine improves, and the drilling speed increases (Fig. 1). The maximum drilling speed corresponds to a certain value of the feed force, which depends mainly on the type of hammer and on the pressure of the compressed air. A further increase in the feed force again leads to a decrease in drilling speed.

Thus, the drilling speed, as the feed force changes, first increases to a maximum and then decreases. Moreover, the decrease in drilling speed of the PR-35 and PM-508 hammers coincides with a deterioration in their operation (the hammers operate with interruptions), whereas the PR-25, PR-20, T-10, RH-754 hammers, as well as the KTsM-4, operate steadily under such conditions. This can be explained by the fact that high-frequency drilling hammers, like the KTsM-4, produce a significantly greater torque than ordinary hand hammers.

The test results showed that the quality of hand hammers should be evaluated not only by the maximum possible drilling speed. It is necessary to take into account the magnitudes of the feed forces at which these speeds are attained, as well as the ranges of rational feed forces. By the range of rational feed forces is meant the interval in which the drilling speed is at least 85% of the maximum and the hammer operates steadily (Fig. 2).

Fig. 1. Dependence of sandstone drilling speed ($f = 6 \div 8$) on feed force at an air pressure of 6 atm.

High-frequency pneumatic drilling hammers of the PR-25, PR-20, RH-754, and T-10 types, when drilling rocks of medium and high hardness within the zones of rational feed forces, can provide a drilling speed 1.26–2.30 times greater than hand drilling hammers of the PM-508 and PR-35 types, and 1.06–1.67 times greater than KTsM-4 column-type drilling hammers (Table 2). The greatest advantages of high-frequency drilling hammers are observed when drilling rocks of medium hardness. In these

Table 2

Optimal feed conditions and the corresponding drilling speeds for rocks of different hardness

cases, the increase in drilling speed with high-frequency hammers, as compared with the PM-508, PR-35, and KTsM-4, corresponds to an increase in the impact powers of these machines (Table 1). When drilling hard rocks ($f = 14 \div 18$), high-frequency hammers have no substantial superiority over the KTsM-4 column-type drilling hammers, which have lower power but greater energy per individual blow.

The advantages noted above of drilling hammers with an increased number of blows can be utilized only when the machines are operated at optimal feed forces. The magnitude of the optimal feed forces for the PM-508 hammers and

PR-35 at \(p_m = 4 \div 6\) atm is 45–75 kg, and for high-frequency hammers it is 70–150 kg. The driller cannot provide such feed forces when drilling horizontal blast-holes “by hand.” Drilling with high-frequency hammers at feed forces that one driller can provide without special devices (25–35 kg) gives no increase in drilling rate compared with the PM-508 and PR-35 hammers and in most cases leads to poorer results (Fig. 1). Thus, in order to obtain the greatest effect from high-frequency drilling hammers, it is advisable to operate them only with special feed devices (pneumatic supports and feeders) that provide the required feed force. In addition, from the standpoint of occupational hygiene, operation of high-frequency drilling hammers without feed devices is impermissible because of the significant vibrations.

Fig. 2. Determination of the range of rational feed forces.

The T-10 drilling hammer has drilling rates 2–2.5 times higher than the PM-508 and PR-35. But because, in order to obtain high drilling rates, it must be fed to the face with a force of 90–150 kg, it cannot be used as a hand-held machine, despite the low weight of the machine. Such large feed forces also cannot be produced when drilling from a pneumatic support without active intervention by the driller [2]. Moreover, the harmful effect of vibration on the worker’s health is not eliminated in this case. Therefore, T-10 drilling hammers and similar ones must be mounted on feed devices that exclude contact between the worker and the machine during drilling.

High-frequency drilling hammers have wider ranges of rational feed forces (Fig. 3). Therefore, when drilling with special feed devices, they, like the KTsM-4 hammers, require comparatively less precise adjustment of the feed force. This is a definite advantage of high-frequency hammers over ordinary ones.

From Fig. 3 it can be seen that the ranges of rational feed forces of the 7-10 and KTsM-4 hammers almost coincide, while the average drilling rates ...

of the former are significantly higher. This gave grounds to suppose that hammers of the T-10 and RH-754 types can be successfully used as column-mounted hammers when installed on screw-feed devices of the KTsM-4 type. To verify this assumption, special investigations were carried out, which showed that the character of the change in drilling speed as a function of feed force is the same both when drilling with a piston feed and with a screw feed. But the piston feed device provides higher drilling speeds.

The increase in the drilling speed of sandstone with a piston pneumatic feed amounted, for the T-10 hammer, to an average of 33%, and for the RH-754, 17%. The KTsM-4 hammer, when operating with a screw feed,

Figure labels:
“Type of drilling hammer”; “Regions of rational feed forces”; “Feed force, kg”; legend: “4 atm”; “5 atm”; “6 atm”.

Fig. 3. Diagram of the regions of rational feed forces when drilling sandstone \((f = 6 \div 8)\) with a bit 40 mm in diameter.

showed a lower drilling speed in rocks of medium hardness than the T-10 and R-754 hammers operating with the same feed device. Like the high-frequency hammers, the KTsM-4 provides higher drilling speeds when working with a piston feed device than with a screw feed. This indicates the advisability of creating and introducing into practice, first of all, piston feed devices. Such feed devices must ensure drilling of the blasthole to its full depth without resetting the drilling tool, which will considerably reduce the time spent on auxiliary operations and increase the productivity of drilling operations as a whole.

Screw feed devices, in comparison with piston devices, are more compact and in a number of cases more convenient in operation. Therefore, in the future it is necessary to carry out a detailed study of them with the aim of selecting such parameters under which they will ensure

would provide drilling speeds for the hammers no lower than those of piston hammers.

Increasing the air pressure leads to an increase in drilling speed, but only when the hammers operate under optimal conditions (Fig. 4). At feed forces that the driller can provide manually, the operation of high-frequency hammers at increased air pressure in most cases does not increase the drilling speed. On the contrary, because of the greater backward travel of the drilling hammer at higher air pressure, the drilling speed decreases, while considerable vibration of the machine and increased air consumption (Fig. 4) lead only to greater worker fatigue and to higher drilling costs.

Fig. 4. Dependence of drilling speed and specific air consumption on feed force when drilling sandstone \((f = 6 \div 8)\) with a PR-20 hammer.

Changing the air pressure has the most noticeable effect on the operation of the T-10 drilling hammer, which, when the air pressure is increased from 4 to 6 ati, increases the drilling speed by 1.2–2.5 times. At a pressure of 4 ati, the PR-25 and RH-754 hammers give the highest drilling speed. At a pressure of 6 ati, the T-10 drilling hammer has the best performance.

The transition to operation at increased air pressure, with optimal feed forces, is characterized for all drilling hammers by some decrease in the specific air consumption, i.e., by more economical operation (Fig. 5). This is explained

because under these conditions the drilling speed increases to a greater degree than the air consumption.

The most economical, as is seen from Fig. 5, is the T-10 drilling hammer, which has a spoolless air-distribution system. Good data on specific air consumption were also shown by the PR-25 and RH-754 drilling hammers. The PM-508 drilling hammer proved to be the least economical.

The magnitudes of the feed forces corresponding to the maximum drilling speeds increase in proportion to the increase in air pressure (Fig. 6). The strength of the rock has a considerably smaller effect on the magnitude of the optimum feed force. Its influence on the optimum feed forces of the PM-508, PR-25, and RH-754 hammers is not substantial and is somewhat more pronounced for the T-10 and KTsM-4 hammers (Table 2).

Fig. 5. Dependence of the minimum specific air consumption on its pressure:
1—PM-508; 2—KTsM-4; 3—PR-20; 4—PR-35; 5—RH-754; 6—PR-25; 7—T-10

Fig. 6. Dependence of the optimum feed force on air pressure:
△—sandstone \((f = 6 \div 8)\); □—micaceous-amphibole schist \((f = 14 \div 18)\); ○—hornstone \((f = 18)\).

In recent years, in the technical press, considerable attention has been paid to the question of the dependence of drilling speed on the diameter of the blast hole. In the published theoretical and experimental studies on this question, a number of contradictions are encountered [6]. These contradictions, in our opinion, are explained by the fact that different researchers tested drilling tools using different drilling hammers, while the latter, in turn, operated at different feed forces. A unified method for testing drilling tools for drilling hammers does not yet exist.

Taking into account the need to resolve the question of the dependence of drilling speed on the diameter of the drill bit, we carried out the corres-

corresponding studies. For hand-held drilling hammers (including high-frequency ones), the question was studied of how the drilling rate changes when the bit diameter is reduced from 46 to 40 mm, and for column-mounted hammers (type KTsM-4)—in the case of drilling blastholes with diameters from 40 to 80 mm.

Table 3 gives the drilling rates in sandstone \((f = 6—8)\) with single-chisel bits of diameter 40 and 46 mm, corresponding to the optimum feed forces.

Table 3

It follows from Table 3 that, when the diameter of a single-chisel bit is reduced from 46 to 40 mm, drilling rates increase by 10–80%, depending on the type of hammer and the air pressure. The greatest increase in drilling rate with a reduction in bit diameter is observed at an air pressure of 4 ati.

Studies of drilling blastholes with diameters from 40 to 80 mm using KTsM-4 hammers showed that, at optimum feed forces, the drilling rates of various rocks change, in the main, in inverse proportion to the square of the blasthole diameter (Fig. 7). Certain deviations from the indicated dependence are evidently explained by the stepwise character of rock breakage and by the stepwise change in the volumetric work of breakage [7].

Under quarry conditions, the dependence of drilling rate on feed force was studied when drill cuttings were removed from the blasthole by blowing and by flushing. The studies showed that the character of the change in drilling rate, as well as the absolute values of the maximum drilling rates and the corresponding feed forces, are the same both for drilling with blowing and for drilling with flushing. From this one may conclude that removing drill cuttings from the blasthole by flushing does not impair the drilling process, provided that the drilling machine operates at the optimum feed force and that the optimum amount of water is supplied into the blasthole. Optimum-

the required amount of water depends on the type of drilling machine and on the physicomechanical properties of the rock. Thus, for example, the maximum drilling rate in sandstone \((f = 6 \div 8)\) with the PR-25 hammer was attained with a water consumption for central flushing of 3 liters per minute. The design of the hammer provides this consumption at a supplied-water pressure of 1 atm.

The dependence of the drilling rate on the feed force when drilling downward blast-holes has the same character as when drilling horizontal blast-holes. The magnitude of the optimum feed forces, both when drilling horizontal and vertical blast-holes, with allowance for the weight of the drilling machines, is approximately the same, while the drilling rates of downward blast-holes are on average 26–30% lower than those of horizontal blast-holes. This can be explained by the fact that, when drilling vertical downward blast-holes, the drilling fines are removed less effectively from the bottom of the blast-hole, and a large part of the hammer’s impact power is expended on secondary crushing of them. Thus, in order to ensure high drilling rates for downward blast-holes, more intensive removal of drilling fines is necessary.

Fig. 7. Dependence of drilling rate on bit diameter.

On the basis of the study of the operating modes of drilling hammers under laboratory and production conditions, it may be concluded that high-frequency drilling hammers of the PR-25, PR-20, RH-754, and T-10 types, at optimum feed forces, can provide drilling rates 1.5–2.5 times greater than those of PR-35 and PM-508 hammers and are not inferior in drilling rate to KTsM-4 column-mounted hammers. But, as was noted above, they cannot be used as hand-held tools despite their low weight. In operation, these hammers must be mounted on special supporting-and-feeding devices. Drilling hammers of the PR-20 and RH-754 types are most expediently used for drilling from pneumatic supports, and hammers of the T-10 type for drilling from feeds mounted on a loading machine or drilling carriage by means of manipulators.

The studies carried out showed that operation of both conventional and high-frequency hammers at an air pressure of

5–6 atm will be advisable only if they are provided with the corresponding feeding devices. In designing high-frequency drilling hammers for drilling very hard rocks—for example, for the conditions of the Gornaya Shoriya mines—in addition to increasing the blow frequency to 3000–3500 per minute, it is necessary to increase the energy of each blow to 6–7 kgm, as against 3.5–5.0 kgm in high-frequency hammers of existing types. To reduce the time of auxiliary operations, the feed mechanisms of powerful high-frequency hammers must be long-stroke, i.e., have a feed equal to the depth of the hole, and be installed by means of manipulators on loading machines or drilling carriages.

The drilling speed that hammers can provide when operating under optimal conditions still does not fully characterize their productivity. The productivity of a drilling machine is determined by the number of linear meters of hole drilled during a shift. The productivity of the driller, in turn, depends on the number of machines he services. In this connection, we calculated the productivity of drilling hammers, taking into account possible methods of installing them at the face. For comparison, three variants of drilling operations were considered: hand drilling, drilling with pneumatic supports, and drilling with long-stroke feeders installed by means of manipulators on loading machines. The remaining operations of the driving cycle were assumed to be the same for all variants. The calculations were performed for driving a single-track crosscut by drilling-and-blasting operations under the following conditions: crosscut section in the clear 4.5 m² and in driving 6.8 m²; timber support of the working; a gassy mine; the rocks intersected by the crosscut have hardness \(f = 6 \div 8\); 20 holes are drilled in each cycle; the average hole length is 1.8 m; the hole utilization factor is 0.85; two drilling machines operate simultaneously at the face. All operations of the driving cycle are carried out by an integrated crew of 12 men. Work is conducted in three shifts, with a continuous week. It was assumed that the integrated norm is fulfilled at 100%. The cost of driving work in all variants was calculated without taking overhead costs into account.

When drilling holes with hand-held drilling hammers, the organization of drilling operations was taken to be the one most widespread at present in Kuzbass. Drilling at the face is carried out by two drilling machines, which are serviced by four drivage workers. When drilling holes with drilling hammers on pneumatic supports (second variant), it was assumed that each hammer is serviced by one drivage worker. In the case of drilling with drilling hammers with long-stroke feeders (third variant), there are two drilling hammers at the face, which are serviced by one worker. The results of the techno-economic calculation are given in Table 4.

Table 4

[[unclear: beginning of caption]] during drilling with various drilling hammers

On the basis of a technical and economic comparison of the operation of drilling hammers and methods of mounting them, the following conclusions may be drawn:

1. The highest productivity can be achieved by a drilling machine created on the basis of the high-frequency T-10 hammer, mounted on a long-stroke feeder. In this case the productivity of the machine can be increased by a factor of 2 in comparison with hand-held PM-508 drilling hammers (when operated by hand).

2. The productivity of drilling operations is influenced more strongly by the use of more advanced supporting devices than by increasing the air pressure from 4 to 6 atm. Operation of high-frequency hammers with long-stroke feeders makes it possible to reduce the time spent on auxiliary operations and to service two drilling machines with one worker. As a result, the productivity of the driller is increased by a factor of 7–10 in comparison with drilling by hand with a PM-508 hammer.

3. The use of high-frequency drilling hammers of the PR-25 or RH-754 type, mounted on pneumatic supports, can reduce the duration of the drifting cycle by 30% in comparison with drilling by hand with PM-508 drilling hammers. When drilling with T-10 hammers from a long-stroke feeder, the duration of the drifting cycle is reduced by 40%. This effect will be more significant at greater rock hardness, a larger volume of drilling work, and more advanced techniques for performing the other operations of the drifting cycle.

4. The cost of driving 1 linear m of a single-track crosscut when using high-frequency hammers and the corresponding feeding devices can be reduced by 30–50 rubles.

5. The use of high-frequency drilling hammers, long-stroke feeders, and an increase in air pressure from 4 to 6 atm reduces the cost of 1 linear m of crosscut in terms of energy consumption. Moreover, the cost of the crosscut in terms of energy consumption amounts to \(1/10\) of the wage costs and is less than the cost of the drilling tool.

REFERENCES

1. O. D. Alimov, I. G. Basov, V. F. Gorbunov. Methodology of a laboratory study of the effect of feed force on the drilling speed of pneumatic drilling hammers. Izvestiya TPI, vol. 88, Tomsk, 1956.

2. O. D. Alimov. The effect of feed force on the drilling speed of pneumatic drilling hammers. Izvestiya TPI, vol. 75, Tomsk, 1954.

3. O. D. Alimov, I. G. Basov. Rational feed forces for the PM-508 drilling hammer when drilling rocks of varying hardness. Izvestiya TPI, vol. 88, Tomsk, 1956.

1. O. D. Alimov, V. F. Gorbunov. Some results of a study of the pneumatic drilling hammer OM-506. Izvestiya TPI, vol. 88, Tomsk, 1956.

2. A. F. Sukhanov. Drillability and blastability of rocks. Gostoptekhizdat, 1940.

3. On the expediency of using larger-diameter blast holes. Mining Journal, No. 6, 1956.

4. O. D. Alimov. On the mechanism of rock destruction in rotary-percussive drilling by drilling hammers. Izvestiya TPI, vol. 75, Tomsk, 1954.

Noted Misprints

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