THE EFFECT OF FEED FORCE ON THE DRILLING SPEED OF PNEUMATIC DRILLING HAMMERS

O. D. Alimov

Drilling blastholes with pneumatic drilling hammers has up to the present time been one of the labor-intensive and difficult processes in carrying out drilling-and-blasting operations.

One of the pressing tasks facing mining machine building is the intensification of drilling-and-blasting operations through further mechanization and automation of the process of feeding the drilling machine to the face during drilling. In connection with this, a considerable number of various automatic feeding devices have been proposed and are being proposed—pneumatic, vibrational, differential, and other automatic feeders [1], as well as various pressure regulators for feeders at the face [2]. Considerably less attention has been paid to experimental and theoretical study of the influence of the magnitude and character of the feed of drilling hammers on drilling speed and on the stability of operation of drilling hammers.

In determining the necessary axial force for feeding a pneumatic drilling hammer to the face, in most cases reference is made to the work of A. K. Sidorenko [1], on the basis of which it is considered that the optimum feed force of a drilling hammer to the face should be selected from the condition of complete pressing of the drill against the face during drilling, and when drilling rocks of medium and low strength also from the condition of ensuring chipping of the rock during rotation of the drill after its penetration into the rock. To determine the optimum feed force, the following empirical formula is recommended:

\[ Q=\frac{P_1+P_2}{4}\pm Q\sin a, \tag{1} \]

where \(P_1\) is the force moving the piston during the forward stroke, kg,
\(P_2\) is the force moving the piston during the return stroke, kg,
\(Q\) is the weight of the hammer body, kg,
\(a\) is the angle formed by the direction of the axial force with the horizontal plane.

Since the resistance to rotation of the drill must not exceed the torque developed by the drill-rotation mechanism, it is recommended that the axial pressure be no greater than

\[ Q'=\frac{M_{kr}-P_c r_2}{f r_2}, \tag{2} \]

where \(M_{kr}\) is the maximum torque developed by the rotation device of the drilling hammer, kg·cm,
\(P_c\) is the force necessary for chipping the rock, kg,
\(r_2\) is the reduced radius of the drill bit head, cm,
\(f\) is the coefficient of friction of the drill bit head against the rock.

The absence in the literature of sufficient data for determining the quantities entering formulas (1) and (2), as well as data confirming the validity of this theory, creates considerable difficulties in using it in practice for choosing the designs and parameters of the feed mechanisms of pneumatic drilling hammers. On the other hand, the studies of rotary-percussive drilling carried out by us [3; 4; 6] showed that the most expedient use of impact energy for rock destruction occurs not when the drill bit blade is constantly pressed against the rock, but when a certain withdrawal of the hammer body from the limiting collar of the drill is ensured and the drill rebounds from the rock. In this case the maximum drilling speed, the easiest turning of the drill in the borehole, and the least tool wear are ensured.

To determine the effect of feed force on the drilling speed of modern pneumatic drilling hammers, tests were conducted on the PM-508 and PR-30k drilling hammers.

The drilling hammers were tested in the pneumatic laboratory of the Tomsk Electromechanical Plant. The design of the installation ensured recording of the oscillation amplitude of the drilling-hammer body, the longitudinal displacements, and the changes in the angular velocity of rotation of the drill. During the experiments the number of blows, the number of revolutions of the drill, and the drilling speed were measured as functions of the feed force. The feed force was produced by a piston pneumatic feeder and was varied by changing the air pressure. Drilling was carried out without flushing, in diabase with a hardness according to Professor Protodyakonov’s scale of \(f 14 \div 16\), using drills \(0.7\ \text{m}\) long with single-chisel drill bits \(45\ \text{mm}\) in diameter, tipped with VK-15 hard-alloy plates.

The experiments showed that the feed force has a substantial effect not only on the drilling speed, but also on the stability of operation of pneumatic drilling hammers.

The dependence of the drilling speed, the average withdrawal of the hammer body from the limiting collar of the drill, the angle of rotation of the drill between blows, and the number of blows per minute on the feed force for the PM-508 and PR-30k hammers at various air pressures is presented in Figs. 1 and 2.

As can be seen from Figs. 1 and 2, when the feed force is increased up to a certain limit, the drilling speed increases. The increase in drilling speed corresponds to a decrease in the average magnitude of the hammer withdrawal from the limiting collar of the drill, \(H_{\mathrm{sr}}\), and to a decrease in the angle of rotation of the drill between blows, \(\alpha\). The average magnitude of the withdrawal of the body from the limiting collar of the drill, \(H_{\mathrm{sr}}\), and the angle of rotation of the drill between blows, \(\alpha\), at the same feed force increase with increasing working air pressure.

The maximum drilling speeds correspond to an average withdrawal of the body of \(H_{\mathrm{sr}} = 1 \div 6\ \text{mm}\). At the same time, as the air pressure increases from 4 to 7 atm, and consequently as the impact energy increases, the value of the average withdrawal \(H_{\mathrm{sr}}\) corresponding to the maximum drilling speed increases from 1 to 6 mm.

The angle of rotation of the drill, as the feed force is increased, decreases in steps, which may be explained by a structural feature of the turning mechanism of the drilling hammers.

The maximum drilling speeds for one and the same hammer correspond to different angles of rotation, these angles being larger the higher the working air pressure and, consequently, the energy of each blow. Increasing the feed force above the optimum is accompanied by a decrease of \(H_{\mathrm{sr}}\) to zero and by a further decrease in the angle of rotation of the drill between blows, which in all cases leads to a decrease in drilling speed.

When the angle of rotation of the drill between blows decreases below a certain value \(\alpha'\) (Figs. 1, 2), characteristic of each hammer, the operating condition of the hammer

Fig. 1

Fig. 2

becomes unstable and the hammer “chokes”—it stops working. The number of blows when drilling with the PM-508 hammer (Fig. 1) increases with increasing feed force gradually, up to the complete stoppage of the hammer. The change in feed force has a stronger effect on the number of blows when drilling with the PR-30k hammer (Fig. 2), which can be explained by the somewhat increased ratio, in this hammer, of the piston stroke to its diameter.

Thus, the experimental investigations carried out show that the change in drilling speed with pneumatic drilling hammers when the feed force is changed is determined mainly by two factors: the average value of the rebound—$H_{cp}$, and the angle of rotation of the drill bit between blows. The power developed by pneumatic drilling hammers can be used most expediently for rock destruction during drilling only with optimal ratios of the blow energy $A_{y\partial}$, the average value of the rebound—$H_{cp}$, and the angle of rotation of the drill bit between blows $\alpha$.

Experiment No. 113
$P = 11.9$ kg
$V = 12$ mm/min.

Experiment No. 114
$P = 23.8$ kg
$V = 78$ mm/min.

Experiment No. 115
$P = 35.1$ kg
$V = 84$ mm/min.

Experiment No. 116
$P = 47.5$ kg
$V = 90$ mm/min.

Experiment No. 117
$P = 59.5$ kg
$V = 67$ mm/min.

Experiment No. 118
$P = 71$ kg
$V = 54$ mm/min.

Experiment No. 119
$P = 83$ kg
$V = 42$ mm/min.

Experiment No. 121
$P = 107$ kg
$V = 48$ mm/min.

Time: 1 sec
$P$ — feed force
$V$ — drilling speed

Fig. 3

Such a ratio, when drilling with pneumatic drilling hammers with dependent rotation of the drill bit, where there are no special devices for changing the number of blows and the revolutions of the drill bit, can in each particular case be attained only by changing the feed force. Therefore, in studying drilling hammers, special attention must be given to determining the influence of the feed force on the rebound of the drilling hammer from the limiting collar of the drill bit and on the change in the angle of rotation of the drill bit between blows.

Figures 3 and 4 show diagrams of the rebound of the PM-508 drilling hammer, recorded during drilling with different feed forces $P$ at air pressures of 4 atm (Fig. 3) and 7 atm (Fig. 4). As can be seen from Figs. 3 and 4, with

with an increase in the feed force, both the magnitude of the amplitude and the character of the oscillation of the hammer body change.

In this case, what changes chiefly is the period of oscillation of the hammer over several blows, \(T\), and the magnitude of the maximum departure from the zero line, \(H_{\max}\). To a considerably lesser extent, the magnitude of the amplitude of oscillation of the hammer body during one blow changes.

At certain feed forces the magnitude of the amplitude of oscillation and of the hammer’s departure from the limiting collar remains almost constant (test No. 114, Fig. 3, and No. 161, Fig. 4), and during drilling the hammer body is in a “floating” state, that is, it seldom presses the drill against the bottom of the borehole.

The maximum drilling speed at different air pressures, and consequently also at different impact energies, corresponds to oscillations of the body with different periods \(T\) (Figs. 5, 6) and, accordingly, to different

Fig. 4

values of the mean departure of the drilling-hammer body from the limiting collar of the drill, \(H_{\mathrm{av}}\); for example, for the PM-508 hammer, Fig. 5, at an air pressure of \(4\) atm the maximum drilling speed corresponds to oscillation of the body with a period \(T\) comprising oscillations over 2–3 blows; at a pressure of \(5\) atm, 3–4 blows; and at a pressure of \(6\)–\(7\) atm, 4–5 and more blows.

In hammers of greater power, PR-30k, Fig. 6, the maximum drilling speeds at an air pressure of \(6\)–\(7\) atm in most cases corresponded to a constant value of the departure \(H_{\mathrm{av}}\).

The magnitude and character of the oscillation of the drilling-hammer body determine, to a considerable extent, the character of the drill’s motion.

Figure 7 presents a diagram of the motion of the drill bit when drilling horizontal blastholes with a PM-508 hammer at an air pressure of 5 atm under various feed forces. As can be seen from Fig. 7, after striking the rock the drill bit

Drilling hammer PM-508

Experiment No. 116

\(P_b = 4.0;\quad P = 47.5;\quad V = 90\)

Experiment No. 107

\(P_b = 5.0;\quad P = 59.5;\quad V = 105\)

Experiment No. 145

\(P_b = 6.0;\quad P = 59.5;\quad V = 144\)

Experiment No. 162

\(P_b = 7.0;\quad P = 59.5;\quad V = 219\)

movement of the body

→ drilling time

\(P_b\) — air pressure, atm.
\(P\) — feed force, kg.
\(V\) — drilling speed, mm/min.

Fig. 5

usually rebounds from it and strikes the drill-steel shank against the body of the drilling hammer. Therefore, the magnitude of the rebound of the drill bit is determined by the position of the hammer body at the given moment, that is, by the magnitude of the recoil \(H\). The maximum spe—

Drilling hammer PR-30K

Experiment No. 22

\(P_b = 4.0;\quad P = 47.5;\quad V = 104\)

Experiment No. 223

\(P_b = 5.0;\quad P = 71.5;\quad V = 156\)

Experiment No. 183

\(P_b = 6.0;\quad P = 71.5;\quad V = 217\)

Experiment No. 206

\(P_b = 7.0;\quad P = 83.5;\quad V = 252\)

movement of the body

→ drilling time

\(P_b\) — air pressure, atm.
\(P\) — feed force, kg.
\(V\) — drilling speed, mm/min.

Fig. 6

corresponds to cases in which, at the beginning of the striker’s (piston’s) blow on the drill steel, the bit of the latter is at a distance of 1–3 mm from the bottomhole. The magnitude of the optimum clearance between the drill steel and the bottomhole is greater, the greater the impact energy.

When the magnitude of the recoil of the body \(H_{\mathrm{av}}\) is reduced to zero, the clearance between the bit and the rock is likewise reduced to zero; this leads to a decrease in drilling speed even in the case where the angle of rotation of the drill steel between blows remains constant, Fig. 1. These data once again confirm that, when conditions are present which ensure, during drilling, a certain recoil of the hammer body from the collar of the drill steel and a recoil of the drill steel from the rock, not only is the drill steel turned more easily, but the impact energy is also used more favorably for breaking the rock [3]. However, with very large recoils of the hammer from the collar of the drill steel, more than 5–6 mm, the internal working process of the hammer deteriorates; a considerable part of the impact energy is spent in overcoming frictional forces during the motion of the drill steel in the shank, and therefore the drilling speed decreases.

1—Motion of the drill steel relative to the rock.
2—Motion of the drill steel relative to the hammer body.
3—Feed force.

Fig. 7

The possibility of operation of a pneumatic percussion machine in such regimes, when the body of the machine only rarely “sits” on the limiting collar of the tool and, even throughout the entire operating time of the hammer, remains in a “floating” state, was first shown theoretically by B. V. Sudnishnikov [5] in studying the rebound of pick hammers. However, since such a regime is not characteristic of the operation of a pick hammer, B. V. Sudnishnikov confined himself only to a brief description of this possible case of motion. From what has been stated above it is clear that, for drilling hammers, theoretical and experimental investigation of such a regime deserves serious attention, since, on the basis of revealing the regularities between the magnitude of the recoil of the hammer body from the limiting collar of the drill—

...of the drill, by the energy and number of blows, the number of revolutions, the feed force, the friction of the drill bit against the face, and the design parameters of the machine, it is possible to create a basis for calculating the mechanisms for drill rotation and feed in drilling hammers.

On the basis of laboratory tests of drilling hammers carried out at the Tomsk Electromechanical Plant, we constructed graphs of the optimal feed forces as a function of air pressure for hand-held drilling hammers, Fig. 8. Here a comparison is also given of the optimal feed forces in drilling horizontal blast-holes, obtained experimentally, with calculated data computed by the formula of A. K. Sidorenko. As is evident from Fig. 8, the values of the feed forces calculated by A. K. Sidorenko’s empirical formula agree sufficiently well with the experimental values only for light drilling hammers of the types RPM-17, PA-23

Air pressure, atm

— according to experimental data

– – – according to calculated data.

Fig. 8

and give a large discrepancy for more powerful and heavier hammers of the PR-35 and PR-30k types.

More precisely, the optimum feed force of the hammer in each specific case can be determined on the basis of the equation of motion of the hammer body during drilling, taking into account the data given above on the optimum values of the clearance of the hammer body from the limiting shoulder.

As is seen from Fig. 8, the optimum feed forces of most hand-held drilling hammers correspond to 50–90 kg. Naturally, when drilling “by hand,” the driller cannot provide such forces. Therefore, in practice, when drilling with hand-held drilling hammers, various devices are used that increase the feed force. In mines and shafts in recent years, drilling with hand-held drilling hammers using pneumatic supports of various kinds has become widespread. This has considerably eased the work of drillers, reduced the influence of recoil, increased productivity, and also opened up the possibility of creating more powerful “hand-held” drilling hammers of the PR-30k type, weighing 30 kg or more, which in their productivity are close to column-mounted drilling hammers of the KTsM-4 type. However, as even a simplified analysis shows, existing pneumatic supports do not ensure drilling under optimum conditions.

Fig. 9

The principle of operation of a pneumatic support consists in the fact that the force \(Q\), Fig. 9, developed by the pneumatic support under the action of compressed air, is balanced by two components: the vertical component \(Q_1\), equal to the weight of the hammer, and the horizontal component \(Q_2\), equal to the feed force of the hammer on the face \(P\). If, for calculation, the weight of modern hand-held drilling hammers is taken as about 30 kg, then a pneumatic support, without additional efforts by the driller, can provide, depending on the angle \(\gamma\), various feed forces \(P\), Fig. 10. From a comparison of Fig. 10 with Fig. 8 it is evident that, by its kinematics, a pneumatic support can provide the optimum feed force only at \(\gamma = 30^\circ\) and when the hammer operates at an air pressure \(P = 4\) atm. To achieve the optimum feed force when drilling with an air pressure of 6–7 atm, the driller must apply an additional force in the horizontal direction \(P'\) or in the vertical direction \(Q_1'\), Fig. 10. An increase of the load in the vertical direction requires corres—

…of the corresponding increase in the advancing force of the pneumatic support \(Q_1\), Fig. 10. The variation of the advancing force of the P-1 pneumatic supports of the Leningrad “Pnevmatika” plant, and of the PPK-17 and PPK-21 of the Krivoy Rog “Kommunist” plant, as a function of air pressure, is shown in Fig. 11. From a comparison of the data in Figs. 10 and 11 it is evident that, when drilling horizontal blast holes with hand hammers mounted on a pneumatic support, by applying an additional force of 20–40 kg the driller can provide the optimum feed force only when the inclination of the support \(\gamma\) is not more than 30–35°. However, even in this case drilling with hand drilling hammers on pneumatic supports under optimum conditions is not always possible, since skewing of the drill in the blast hole creates considerable resistance to rotation of the drill, which leads to unstable operation of the drilling hammer long before the optimum conditions are reached. Therefore, drilling with hand hammers on pneumatic supports is in most cases carried out with feed forces that are known to be

Fig. 10

Fig. 11

smaller than the optimum ones and, consequently, under less productive conditions. All this requires, in addition to improving the existing pneumatic supports, the creation of more advanced supporting and feeding devices for hand drilling hammers.

There are no data in the technical literature on the optimum feed forces for telescopic and column-type drilling hammers. However, there is every reason to suppose that the basic laws governing the effect of feed force on drilling speed, established for hand drilling hammers, can be extended to column-type and telescopic drilling hammers. As our experiments, carried out during industrial tests of drilling hammers at the Sokolny mine, have shown, the KTsM-4 column-type drilling hammers operate steadily and give the maximum drilling speed when the drill rebounds from the rock during drilling by 2–3 mm and when the angle of rotation of the drill between blows is \(\alpha = 40^\circ\), Fig. 12.

In view of the fact that stable operation of the drilling hammer under optimum conditions is possible only if the body of the hammer has the necessary clearance from the drill collar, and the latter is possible only in the presence of a certain vibration of the body, then, along with a device for smooth regulation

feed force; in the designs of feeding devices it is necessary to provide for the presence of an elastic element. The absence of the necessary elastic elements can explain the failures in introducing into production a number of mechanical, automatic, and other feeders of original design,

Movement of the drill during drilling

drill hammer — KTsM-4
hole depth — 1.8 m
drill weight — 12.5 kg
hard rock — \( f = 8\text{–}10 \)
air pressure — 6 atm

a) under an unstable regime

drill rotation angle

direction of drilling

b) under a stable regime

drill rotation angle

blow of the striker on the drill
blow of the drill on the hammer body
blow of the drill on the rock
drill rotation angle between blows
gap between the drill blade and the rock

Fig. 12

since the latter could not ensure a stable operating regime of pneumatic drilling hammers during drilling, and, when drilling with electromechanical drilling hammers with an independent drive for drill rotation—of the SPK-1 type—they led to considerable wear of the drill blade.

REFERENCES

1. Sidorenko A. K. Automation of percussive drilling. Gornyi zhurnal, No. 4, 1941.

2. Trofimov P. F. Automatic feeders for rock drills. Collection of scientific works, No. 16. Metallurgizdat, 1947.

3. Alimov O. D. Investigation of the mechanism of rock destruction in percussive-rotary drilling and of the initial parameters of drilling hammers. Author’s abstract, Tomsk, Poligrafizdat, 1953.

4. Alabuzhev P. M., Alimov O. D., Tsukanov A. G. On the efficiency of the blow in percussive-rotary drilling. Izvestiya TPI, vol. 75, 1954.

5. Sudnishnikov B. V. Some questions in the theory of impact-action machines. ZS FAN, Novosibirsk, 1949.

6. Alabuzhev P. M., Alimov O. D. The influence of feed force and the angle of drill rotation between blows on drilling speed. Izvestiya TPI, vol. 75, 1954.