Rational Feed Forces of the PM-508 Rock Drill When Drilling Rocks of Different Strength

O. D. Alimov and I. G. Basov

(Presented by Prof. P. M. Alabuzhev, Doctor of Technical Sciences)

As has been established by a number of studies [1–3], the feed force of a rock drill at the face has a substantial effect on the drilling rate, the energy intensity of the fracture process, tool wear, the stability of drill operation, and, overall, the productivity of drilling operations. However, to date there have been no special studies of individual drills aimed at substantiating the feed devices required for them when drilling rocks of different strength and at different air pressures.

In the literature [4] there is an opinion that the stronger the rock and the greater the pressure of the supplied air, the greater the optimum feed force should be. Specific data on expedient feed forces in one case or another are not given.

In the present work, some results are presented of experimental studies of optimum feed forces when drilling horizontal blastholes with the PM-508 drill in rocks of different strength.

The experimental studies were carried out in the rock-fracture laboratory of the Tomsk Polytechnic Institute on a special installation [5].

In order to determine the magnitudes of the optimum feed forces and the limits of their variation when drilling rocks of different strength, the following rocks were selected:

1. Rocks of high strength, \(f = 14—18\) according to M. M. Protodyakonov, that is, those in which blasthole drilling is carried out mainly by the percussive-rotary method. Diabase, \(f = 16—18\), and granite, \(f = 14—16\), were adopted as such rocks.

2. Rocks of low strength, \(f = 4—5\), in which drilling blastholes by the percussive-rotary method is not recommended, since the drilling rate in this case is considerably lower than with rotary drilling. Sandstone, \(f = 4—5\), was adopted as such a rock.

3. Rocks having relatively low strength, but that are “viscous.” Marble, \(f = 6—8\), was taken as such a rock.

As the drilling tool, drill steels of length \(l = 1.0\) m were used, with replaceable single-chisel drill bits of diameter \(d = 43\) mm, tipped with VK-15 hard-alloy plates.

The blastholes were drilled horizontally to a depth of 400–500 mm at air pressures of 4, 5, and 6 atm. Before each test the blasthole was blown out. During drilling, the drilling fines were removed by blowing. The small depth of the blasthole made it possible to remove the drilling fines with almost constant intensity. Thus, in the course of the study it was possible to determine the optimum feed forces when drilling rocks of different strength, at

at different air pressures, without taking into account the influence on the drilling process of the intensity of removal of drill cuttings from the hole.

The optimal feed force was taken to be that which ensured drilling at the highest speed and stable operation of the drill hammer, not requiring the driller’s intervention every minute.

In processing the experimental data, the number of blows, the angle of rotation of the drill between two blows, and the rebound of the hammer body from the limiting collar of the drill were determined. The change in drilling speed as a function of feed force (Fig. 1), when drilling various rocks and at different air pressures, has one and the same character. With an increase in feed force, the drilling speed increases up to a certain limit, and then gradually decreases.

The beginning of the decrease in drilling speed, as a rule, coincides with a deterioration in the stability of operation of the hammer: the hammer begins to operate as if “choking,” and then stops altogether. The change in drilling speed in the region of unstable hammer operation is shown in the graphs by dashed lines.

The highest drilling speed in all cases was obtained when drilling sandstone. Lower drilling speeds were obtained, respectively, for stronger rocks—granite and diabase. Marble is an exception: its drilling speed is the same as that of the considerably stronger granite.

When the hammer feed force on the face is changed from 20 to 80 kg, the drilling speed increases by a factor of 2–3. The greater increase in drilling speed is observed at the higher air pressure—6 atm.

Fig. 1. Dependence of drilling speed on feed force. ×—diabase, △—granite, □—marble, ○—sandstone.

From Fig. 1 it is evident that the change in drilling speed as a function of the air pressure in the rock drill can be evaluated most correctly by comparing the drilling speeds corresponding to the optimum feed forces.

If the drilling speed at an air pressure of 4 atm is taken as unity, then the increase in drilling speed as a function of air pressure, for the case considered by us, may be expressed by a coefficient \(K\), equal to the ratio of the drilling speed at a given pressure to the drilling speed at 4 atm. The values of the coefficients of increase in the drilling speed of blastholes in rocks of different strength by the PM-508 rock drill are, at 5 atm, \(1.6 \div 1.7\), Fig. 2, and at 6 atm, \(2 \div 2.5\); that is, despite the different physico-mechanical properties of the rocks being drilled, the coefficients vary within small limits.

Fig. 2. Dependence of the coefficient of increase in drilling speed \(K\) on air pressure.

Conclusions on the influence of air pressure on drilling speed and on the stability of operation of the drill can be made more fully only on the basis of an additional study of the working process occurring in the rock drill.

As can be seen from Fig. 3, the magnitude of the feed force ensuring the maximum drilling speed increases with an increase in air pressure. When the air pressure is increased from 4 to 6 atm, that is, by a factor of 1.5, the feed force corresponding to the maximum drilling speed increased from 40–50 kg to 65–75 kg, that is, by a factor of 1.6–1.5. Consequently, the feed force ensuring the maximum drilling speed increases in direct proportion to the increase in the air pressure supplied to the drill.

A considerably smaller influence on the magnitude of the feed force ensuring the maximum drilling speed is exerted by changes in the physico-mechanical properties of the rock being drilled. Thus, when the strength changes from \(f = 4\) to \(f = 16—18\), that is, by more than four times, the feed force changed by 6 kg; the difference in the values of the feed forces ensuring the maximum spe-

the drilling rate in all cases was no more than 6–10 kg, which corresponds to two to three times the magnitude of the root-mean-square error in measuring the feed force.

Thus, in the investigations carried out, no substantial influence of the rock strength on the magnitude of the feed force ensuring the maximum drilling rate was found. Some change in the magnitude of the feed forces when drilling different rocks apparently depends not so much on the strength of the rock as on other physico-mechanical properties that determine the different coefficient of friction of the drill bit blade against the rock and, consequently, affect the moment of resistance to rotation of the drill. The latter has a substantial effect on the stability of the working process of the hammer.

Fig. 3. Dependence of the optimal feed force on air pressure.
×—diabase, △—granite, □—marble, ○—sandstone.

The opinion existing in practice that, when drilling less strong rocks, a smaller feed force is necessary is apparently based on the fact that drilling of less strong rocks proceeds at a higher rate, as a result of which, with one and the same intensity of flushing or blowing, there is more drill cuttings in the hole; this increases the resistance to rotation of the drill and, at a smaller feed force, leads to unstable operation of the drilling hammer. To eliminate the influence of this factor, when drilling less strong rocks it is necessary to increase the pressure and, consequently, the flow rate of the flushing liquid. As has been established by special investigations [6] (Fig. 4), by increasing the pressure of the flushing liquid while working at one and the same air pressure, the drilling rate can be increased considerably.

On the basis of the experiments we conducted, the feed force \(F\) ensuring the maximum drilling rate as a function of the air pressure when drilling horizontal holes with the PM-508 hammer is expressed as \(F = k \cdot p\), where \(F\) is the feed force ensuring the maximum drilling rate, in kg; \(p\) is the pressure of the air supplied to the hammer, atm; \(k\) is the coefficient of proportionality.

On the basis of experimental data, the value of the coefficient of proportionality varies from 10 to 12.5. If, however, only those rocks in which holes are drilled mainly by the percussive-rotary method are taken into account, then the coefficient of proportionality varies only from 12.0 to 12.5. A comparison of the optimal forces obtained when drilling horizontal, inclined, and vertical holes shows that the optimal value of the feed force decreases (in the case of drilling downward) or increases (when drilling upward) by an amount equal to the component of the hammer weight perceived by the feed device of the feeder, that is, by the amount \(G \sin \alpha\), where \(G\) is the weight of the hammer in kg, and \(\alpha\) is the angle between the direction of the hole and the horizontal.

If the influence of the shape and dimensions of the drilling tool is not taken into account, which has not yet been investigated, then, for determining the optimal feed force of the PM-508 drilling hammer, the following formula may be proposed:

\[ F = kp \pm G \sin \alpha, \]

where

\(k\) — coefficient of proportionality, \(k = 12\text{–}12.5\);

\(p\) — air pressure, atm;

\(G\) — weight of the hammer, kg;

\(\alpha\) — angle of inclination of the drill hole to the horizontal.

Fig. 4. Dependence of drilling speed on flushing-water pressure.

The feed-force values corresponding to the maximum drilling speed of horizontal holes are, in most cases, limiting forces; exceeding them leads to unstable operation of the hammer.

When designing feeders, the calculated feed force should be taken as 10–15 kg less than the force that provides the maximum drilling speed. In this case, in all the cases considered above (Fig. 1), it is possible to provide a drilling speed close to the maximum and the required stability of hammer operation, and thereby a high productivity of the machine.

From Figs. 5 and 6 it is evident that when the feed force is changed, the number of hammer blows per minute and the angle of rotation of the drill bit between two blows change. This

occurs mainly because of a change in the rebound of the hammer body from the limiting collar of the drill steel; as a result, the working process inside the hammer and the moment of resistance to rotation of the drill steel in the blasthole change.

Fig. 5. Dependence of the number of blows on the feed force.
×—diabase, △—granite, □—marble, ○—sandstone.

The nature of the change in the magnitude of the rebound of the hammer body from the limiting collar of the drill steel (Fig. 7), depending on the feed force, is the same in all cases.

Fig. 6. Dependence of the angle of rotation of the drill steel on the feed force.
×—diabase, △—granite, □—marble, ○—sandstone.

At one and the same feed force, the magnitude of the recoil increases with increasing air pressure, and this difference is the greater, the smaller the feed force. As a result of the experiments, no substantial influence of the rock on the magnitude of the recoil was found; thus, for example, the magnitudes of recoil of the drill hammer body when drilling sandstone at \(f=4\) and diabase at \(f=18\), with air pressures of 5 and 6 atm, are identical. A somewhat greater magnitude of recoil was observed in most cases when drilling marble. From Table 1 it is evident that the maximum drilling rates in rocks of different strength, at a given air pressure, correspond to definite magnitudes of recoil. These magnitudes are the greater, the higher the air pressure.

Fig. 7. Recoil of the body of the drilling hammer from the drill-steel stop as a function of feed force:
\(\times\)—diabase, \(\Delta\)—granite, \(\square\)—marble, \(\circ\)—sandstone.

Table 1

| Rock | \multicolumn{2}{c}{Air pressure in atm, \(p=4\)} | \multicolumn{2}{c}{Air pressure in atm, \(p=5\)} | \multicolumn{2}{c}{Air pressure in atm, \(p=6\)} |
|---|---:|---:|---:|---:|---:|---:|
| | \(H^1\) | \(H^2\) | \(H^1\) | \(H^2\) | \(H^1\) | \(H^2\) |
| Sandstone | 1.5 | 1.5–4 | 1.6 | 1.8–5 | 1.6 | 2.1–[[unclear: upper value]] |
| Marble | 1.7 | 1.5–4 | 1.8 | 1.8–5 | 2 | 2.1–[[unclear: upper value]] |
| Granite | 1.5 | 1.5–4 | 1.9 | 1.8–5 | 2.7 | 2.1–[[unclear: upper value]] |
| Diabase | 1.5 | 1.5–4 | 1.8 | 1.8–5 | 2 | 2.1–[[unclear: upper value]] |
| | \(H^1_{\mathrm{avg}} = 1.5\) | | \(H^1_{\mathrm{avg}} = 1.8\) | | \(H^1_{\mathrm{avg}} = 2.1\) | |

\(H^1\)—magnitude of recoil corresponding to the maximum drilling rate, mm;
\(H^2\)—magnitude of recoil corresponding to stable operation of the hammer, mm;
\(H^1_{\mathrm{avg}}\)—arithmetic mean value of \(H^1\) for rocks of different strength.

The change in the magnitudes of recoil corresponding to the maximum drilling rate and to stable operation of the hammer, as a function of air pressure, is shown in Fig. 8.

The increase in the rebound required to ensure the maximum drilling speed when the air pressure is increased can be explained by the fact that, first, at higher air pressure the penetration of the tool per blow increases, and consequently a greater rebound of the drill is also required in order to ensure rotation of the drill; second, when the air pressure is increased, the energy of the piston before impact with the drill increases, and for its more complete transmission to the drill rod a greater travel of the rod and striker during impact is necessary.

Labels in the figure: vertical axis—rebound of the drill, mm; horizontal axis—air pressure, atm; upper region—operation of the hammer with a large air passage through the drill; hatched region—stable operation of the hammer during drilling; lower region—unstable operation of the hammer.

Fig. 8. Operating regimes of the hammer at different air pressures.

The recoil characteristics of the hammer at different air pressures, feed forces, and during the drilling of different rocks are given in Fig. 7. The number of blows of the hammer per minute during drilling (Fig. 5) varies as a function of the air pressure and the feed force. The strength of the rock being destroyed has a much smaller influence on the number of blows. Thus, for example, when drilling granite, diabase, and marble, the difference in the number of blows does not exceed the measurement error. Some exception is represented by sandstone, during the drilling of which the number of blows per minute in some cases was 100–120 less than under analogous feed regimes when drilling other rocks. This difference does not exceed 6.0% of the number of blows developed by the hammer when drilling other rocks and corresponds to two to three times the root-mean-square error of measuring the number of blows.

Thus, in the study carried out on the operation of the PM-508 drilling hammer, no substantial influence of differences in rock strength on the number of blows developed by the drilling hammer during drilling was found.

The widespread opinion that the number of blows of a drilling hammer should change significantly when drilling rocks of different strength is evidently explained by the simple transfer of this conclusion from the results of studies of the operation of demolition hammers, without sufficient analysis of the essential differences in the working processes of demolition and drilling hammers.

Figure 9 gives the dependence of the variation in the mean values of the number of blows and the rebound magnitudes when drilling different rocks on the feed force. In the region of unstable regimes the dependences are shown by a dashed line. From Fig. 9 it is seen that the number of blows changes more significantly when the feed force is increased up to the optimum value, and subsequently remains ap—

constant. As the feed force increases, the frictional moment of the drill bit against the face increases; therefore the number of revolutions of the bit and the angle of rotation of the bit between two blows decrease substantially (Fig. 6). The decrease in the angle of rotation is greater the less strong the rock is. The PM-508 rock drill operates steadily when the recoil of the hammer from the face ensures rotation of the bit between blows through an angle of not less than \(\alpha = 28—37^\circ\) at \(4\) atm, \(\alpha = 32—38^\circ\) at \(5\) atm, and \(\alpha = 32—42^\circ\) at \(6\) atm.

Fig. 6. Dependence of the average recoil value and the number of blows on the feed force.

The angles of rotation of the drill bit corresponding to the maximum drilling speed are given in Table 2.

With a decrease in the angles of rotation below the optimum, unstable operation of the rock drill is observed, and at \(\alpha = 24—25^\circ\), when working with a helical rod with a thread pitch of 650 mm, the hammer stalls. At ve-

the magnitude of the angle of rotation, in addition to the feed force and the physicomechanical properties of the rock being drilled, is substantially affected by the intensity with which the destroyed fines are removed from the blasthole; however, to determine this effect it is necessary to conduct a special study.

Table 2

Conclusions

1. The feed force has a substantial effect on the drilling rate of a rock drill.

2. The optimal feed force increases in proportion to the increase in air pressure (within the range of air-pressure variation from 4 to 6 ati).

3. The strength of the rock does not have a substantial effect on the magnitude of the optimal feed force, the number of blows, or the amount by which the hammer moves away from the collar of the drill steel during drilling.

LITERATURE

1. Buchev V. K. Drilling and blasting operations. Metallurgizdat, 1950.

2. Sidorenko A. K. Automation of the feed of pneumatic rock drills. Mining Journal, No. 11, 1954.

3. Trofimov P. F. Automatic feeders for perforators. Geology and Mining, No. 11, 1946.

4. Handbook of mining and ore work. Metallurgizdat, 1952.

5. Alimov O. D., Basov I. G., Gorbunov V. F. Methodology for laboratory investigation of the influence of feed force on the drilling rate with pneumatic rock drills. Proceedings of TPI, vol. 88, 1956.