A Study of the Influence of Feed Force and Air Pressure on the Drilling Speed and Operating Process of the RP-17 Pneumatic Drilling Hammer

O. D. ALIMOV, I. G. BASOV, V. F. GORBUNOV

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

To determine the optimal dimensions of the feeder of RP-17 pneumatic drilling hammers, experimental investigations were carried out in the laboratory of mining machines and mine transport. The investigations were conducted according to a previously developed methodology [1, 2]. The relationships obtained are presented below.

The study of the influence of feed force on drilling speed was carried out when drilling sandstone and granite at compressed-air pressures at the hammer of 4, 5, and 6 atm. As the drilling tool, a drill with a removable bit was used, the characteristics of which are given in Table 1. The drill holder was removed from the hammer.

Table 1

The change in drilling speed with the RP-17 hammer as a function of feed force in sandstone and granite with the same strength characteristics is of the same nature as when drilling with OM-506l and PM-508 hammers [2].

When drilling sandstone with the RP-17 drilling hammer and an air pressure of 4 atm, stable operation of the hammer is determined by the range of feed forces from 12.8 to 31 kg. Correspondingly, the drilling speed changes (Fig. 1) from 150 to 320 mm/min. With a further increase in feed force, the drilling speed decreases significantly, since the hammer then operates abnormally, unstably. At a compressed-air pressure at the hammer of 5 atm, the stable-operation zone is determined by feed forces of 16.5–35 kg. These feed forces correspond to speeds of 190–395 mm/min. At 6 atm ...

the drilling rate of sandstone varies from 300 to 515 mm/min depending on the feed force, which varied within the limits of 21–44.5 kg.

When drilling granite in the zone of stable operation, the hammer had the following drilling rates:

\[ \begin{aligned} p &= 4\ \text{atm} \qquad &F &= 22\text{—}30.5\ \text{kg} \qquad &V &= 50\text{—}80\ \text{mm/min},\\ p &= 5\ \text{atm} \qquad &F &= 23.5\text{—}40\ \text{kg} \qquad &V &= 64\text{—}112\ \text{mm/min},\\ p &= 6\ \text{atm} \qquad &F &= 26\text{—}45.8\ \text{kg} \qquad &V &= 70\text{—}175\ \text{mm/min}. \end{aligned} \]

The nature of the change in the drilling rates of granite (Fig. 1) is similar to the change in the drilling rates of sandstone.

Fig. 1. Dependence of drilling rate on feed force.
Vertical axis: drilling rate, mm/sec. Horizontal axis: feed force, kg. Legend: sandstone and granite at compressed-air pressures of 4 atm, 5 atm, and 6 atm.

In the zone of stable operation of the hammer, the drilling rate increases with increasing feed force. After a certain feed force is exceeded, the drilling rate begins to decrease. In this case, at first, in order to start the hammer it is necessary to reduce the feed force, and then, as the feed force is further increased, the hammer operates unstably and stalls.

Some deviation from this regularity was observed when drilling granite with a compressed-air pressure at the hammer of 5 atm. In this case the drilling rate increased even under regimes in which it was necessary to reduce the feed force when starting the hammer. This indicates insufficient power of the rotary device of the RP-17 hammer.

From Fig. 2 and Table 2 it is seen that the number of blows of the RP-17 drilling hammer, with an increase in feed force in the zone of stable operation of the hammer, increa-

increases only slightly, although the drilling rate increases. The increase in the drilling rate may be explained by an increase in the impact energy and by better conditions for transmitting the impact energy due to the reduced rebound of the drilling-hammer body, as described in more detail below.

Fig. 2. Dependence of the number of blows on feed force.

Tests of the RP-17 drilling hammer showed that, when drilling rock, a change in the feed force causes substantial changes in the angles of rotation of the drill.

In the zone of stable operation of the hammer, when drilling sandstone with a compressed-air pressure of 4 atm, the angles of rotation of the drill vary within the range 32.5–16.5°; correspondingly, the feed force increases from 12.8 to 31 kg. The minimum angle of rotation of the drill for \(p = 4\) atm, at which the drilling hammer, though unstably, still operates, is 11.5°. At

Table 2

5 ati correspond to feed-force limits of 16.5–35 kg, which restrict the stable operation of the hammer; the drill rotation angles are 31.5–17°.

If, at the working compressed-air pressure at the hammer of 4 and 5 ati, the drill rotation angles, as the feed force increased, decreased smoothly and differed only slightly at equal feed forces, then at a compressed-air pressure at the hammer of 6 ati the drill rotation angles change somewhat differently. In this case the zone of stable operation of the hammer was limited by feed forces of 21–47.5 kg. With an increase in feed force from 21 to 34.5 kg, a smooth decrease in the drill rotation angles from 38.5 to 32.5° is observed. When the feed force is increased above 34.5 kg, the rotation angles

Fig. 3. Dependence of the drill rotation angle on the feed force.

Table 3

the bit decrease sharply both in the zone of stable operation, which is limited by a feed force of 47.5 kg \((\alpha = 16.6^\circ)\), and in the zone of unstable hammer operation. The minimum angle of rotation of the bit at \(p = 6\) atm in the zone of unstable hammer operation is equal to \(9^\circ\).

When drilling granite with the RP-17 drilling hammer, the change in the angles of rotation of the bit as a function of the feed force (Fig. 3) for all operating compressed-air pressures at the hammer is analogous to that described above. From Table 3 and Fig. 3 it is evident that, at the feed force providing the maximum drilling rate in granite, the angles of rotation of the bit for all air pressures are greater than when drilling sandstone.

With a change in feed force, the average displacement of the drilling-hammer body from the limiting shoulder of the bit also changes. The nature of the change in the displacements \(H_{\mathrm{av}}\) as a function of feed force is shown in Fig. 4. From the curves in Fig. 4 and Table 4 it is evident that, with increasing feed force, the magnitude of the average displacement of the body from the limiting shoulder of the bit decreases both when drilling granite and when drilling sandstone.

Fig. 4. Change in the displacement of the drilling-hammer body from the limiting shoulder of the bit as a function of feed force.

In addition to all the relationships considered above, during the tests of the RP-17 drilling hammer the change in the amplitude of oscillation was also investigated

Table 4

hammer body per blow as a function of the feed force and the air pressure.

From the curves (Fig. 5) it is seen that the change in the mean amplitudes of vibration of the body of the drilling hammer \(A_{\mathrm{av}}\) when drilling sandstone has the same character at different specified compressed-air pressures. With an increase—

Fig. 5. Dependence of the amplitude of vibration of the drilling-hammer body per blow on the feed force.

—in the feed force, the amplitudes of vibration of the body decrease, and upon reaching a certain feed force, whose magnitude depends on the compressed-air pressure, the amplitude of vibration of the body remains constant, equal to 1 mm.

When drilling granite at compressed-air pressures of 4 and 5 ati, the amplitudes of vibration of the body of the drilling hammer differ only slightly in magnitude at equal feed forces, whereas at an air pressure of 6 ati the amplitudes of vibration increase almost twofold. For comparison of the amplitudes of vibration of the hammer body both when drilling granite and when drilling sandstone, Table 5 is given.

Table 5

From the recoil diagram (Fig. 6) it is evident that, unlike the drilling hammers PM-508 and OM-506, for the RP-17 drilling hammer the nature of the vibration amplitude per blow is somewhat different. For the PM-508 and OM-506 hammers, bends in the curve of vibration amplitude per blow are characteristic, beginning at a certain feed force, even in the zone of stable hammer operation. For the RP-17, such phenomena are not observed in the zone of stable hammer operation, and they are encountered only very rarely when operating with feed forces above the optimum.

Fig. 6. Recoil curves of drilling hammers during drilling.

Investigation of the Kinematics of the Percussion Unit of the RP-17 Hammer during Drilling

The kinematics of the percussion unit of the RP-17 hammer was investigated according to the same procedure as the investigation of the OM-506 hammer [1]. In the subsequent analysis of the obtained relationships we use the notation introduced by us earlier in the investigation of the OM-506 hammer [3].

The character of the dependence of the piston-stroke magnitude on air pressure and feed force (Fig. 7) when drilling granite is represented by straight lines. Some deviations from a rectilinear dependence occur when drilling sandstone, with air pressures of 4, 5, and 6 atm. When the feed force is increased by 10 kg, the piston stroke decreases by an amount \(\Delta S\). The value of \(\Delta S\) at different air pressures and when drilling various rocks is given in Table 6.

Comparison of the piston-stroke magnitudes at one and the same feed force shows that when the pressure is increased by 1 atm the piston stroke increases on average by 2–8 mm; moreover, in the zone of small feed forces the increase in piston stroke is insignificant, on the order of 2 mm, and during idle running it is about 1 mm. The magnitude of the piston stroke during idle running of the hammer at an air pressure of 4–6 atm is 60–62 mm, at a pressure of 3 atm—58 mm, and at a pressure of 2 atm—52 mm.

The piston-stroke magnitudes corresponding to the maximum drilling speed are given in Table 7 and, in most cases, are equal to 48 mm. The minimum possible piston stroke (obtained during hammer operation) is 37 mm. In this case the exhaust port does not open during the return stroke of the piston, and the front edge of the piston reaches only the front edge of the exhaust port.

Individual cycles with a piston stroke of 33–34 mm are observed. All these cycles have a stoppage of the piston in the rear position. The dwell time of the piston in the rear position ranges from 0.5 to the full cycle time during stable operation, and in individual exceptional cycles the dwell time is 3–4 times the duration of the cycle.

Table 6

Fig. 7. Dependence of the piston stroke on the feed force when drilling granite.
\(\square\) — 4 atm; \(\triangle\) — 5 atm; \(\bigcirc\) — 6 atm.

During idling of the hammer and at air pressures of 4, 5, and 6 atm, the compression ratio of the front buffer (Figs. 8, 9) is about 95%; that of the rear buffer is, respectively, 83, 85, 87%. When the hammer operates with a tool, the compression ratio of the front and rear buffers decreases as the feed force increases. When drilling sandstone, compression of the front buffer ceased at feed forces of 26, 38, and 50 kg, corresponding to air pressures of 4, 5, and 6 atm. With a further increase in feed force the piston does not completely cover channel “B.”

The compression of the front and rear buffers at identical feed forces increases with increasing air pressure. An increase in pressure by 1 atm gives an increase in compression of the front buffer of 25% when drilling sandstone, and of 10% when drilling granite. The compression of the rear buffer with an increase in pressure by 1 atm increases by the value \(B_3\%\), shown in Table 8.

Table 7

Fig. 8. Dependence of the piston-stroke magnitude and the degree of compression of the air buffers on the feed force when drilling granite. □ — 4 atm; △ — 5 atm; ○ — 6 atm.

Table 8

The minimum compression of the rear buffer is, when drilling sandstone, 62%, and for granite, 58%, which corresponds to piston strokes of 39 and 37 mm. Unstable operation of the hammer corresponds to a rear-buffer compression ratio \(B_{\mathrm{z}}\) of less than 70% for sandstone and \(B_{\mathrm{z}} = 64\%\) for granite.

Fig. 9. Dependence of the piston stroke and the degree of compression of the air buffers on the feed force when drilling sandstone. □ — 4 ati; △ — 5 ati; ○ — 6 ati.

The operating conditions of the hammer that ensure high drilling speeds correspond to compression of the air buffers as follows: rear buffer—73–80%, front buffer—15–20% for granite and 0–10% for sandstone.

The dependence \(B_{\mathrm{p}} = f(F)\) (Fig. 8) may be regarded as linear for all pressures, both when drilling sandstone and when drilling granite. The dependence \(B_{\mathrm{z}} = f(F)\) differs from a straight line. In the range of large feed forces, the degree of compression of the rear buffer decreases with increasing \(F\) more rapidly than in the range of small feed forces.

The general character of the change in impact energy as a function of feed force at various air pressures (Fig. 10) changes analogously to the change in drilling speed. As the feed force is increased up to a certain value, the impact energy increases to a maximum. With a further increase in feed force, the impact energy decreases sharply.

The feed forces corresponding to the maximum kinetic energies of the piston before its impact on the drill steel are close to, and in a number of cases coincide with, the feed forces that ensure the maximum drilling speed (Table 9). The maximum values of the kinetic energy of the piston before impact are given in Table 9.

Table 9

From the graph in Fig. 11 it is evident that the maximum impact energy corresponds to a piston stroke close to 48 mm; deviation from this value leads to a decrease in impact energy. With a decrease in the piston stroke (in the region of large feed forces), the impact energy decreases because of poor ex-

Fig. 10. Dependence of the energy of a single impact on the feed force when drilling granite. □ — 4 atm; △ — 5 atm; ○ — 6 atm.

haust from the front cavity of the hammer, as a result of which the greater part of the kinetic energy of the piston is spent on compressing the air in the front cavity. With an increase in the piston stroke beyond 48–50 mm (in the region of small feed forces), the portion of the working stroke increases in which the piston moves due to accumulated kinetic energy and, in addition, spends the acquired kinetic energy on compressing the front air cushion after the inlet channel “B” has already been closed.

Fig. 11. Dependence of the energy of a single impact on the piston stroke when drilling granite. □ — 4 atm; △ — 5 atm; ○ — 6 atm.

The duration of one cycle of piston motion \(t_c\) changes with changes in air pressure and feed force (Fig. 12). With an increase in air pressure by 1 ati, the cycle duration decreases by 1.5–3 milliseconds, i.e., by 3–10%, and a deviation from this is observed when drilling sandstone, when the cycle duration at a pressure of 5 ati is greater than at a pressure of 4 ati (at a feed force of 33 kg).

Fig. 12. Dependence of the cycle time of piston motion on the feed force.

With a change in feed force from 0 to 25 kg (for a pressure of 4 ati) and to 30, 35 kg for pressures of 6 and 5 ati, the cycle duration increases (for sandstone). When drilling granite, the increase in cycle duration extends up to feed forces of 35–40 kg. With a further increase in feed force, the cycle duration decreases up to a feed force of 60 kg for granite and 50 kg for sandstone. Increasing the feed force beyond these values leads to an increase in the duration of the cycles, but under such regimes the hammer stalls. The explanation for the change in cycle duration remains the same as in describing the nature of the change in \(t_c\) for the OM-506 hammer [3].

The minimum cycle duration when drilling granite is less than when drilling sandstone, owing to the greater rebound of the piston after impact. In this case the difference in the minimum values of \(t_c\) is approximately 3 milliseconds, which corresponds to a change in the number of blows per minute by 175.

The ratio of the duration of the return (idle) stroke \(t_x\) to the duration of the working stroke of the piston \(t_p\) changes as a function of the feed force. The character of this dependence (Figs. 13 and 14) is expressed by a convex curve, which has a maximum for sandstone and granite at feed forces that provide the maximum drilling rates.

In the range of small feed forces, when drilling sandstone, the ratio \(t_x/t_p\) increases from 1.1 (idle operation) to 1.8; 2.05; 2.2, respectively, for air pressures of 4, 5, and 6 ati. Then, with further increase in feed force, the value of the ratio \(t_x/t_p\) begins to decrease.

When drilling granite, analogous changes in the ratio

\[ \frac{t_x}{t_p}, \]

are observed, with the maximum of the curve equal to 1.7; 1.85; 1.95 for pressures of 4, 5, and 6 ati, respectively, i.e., 0.1–0.2 less than when drilling sandstone. At feed forces even slightly greater than optimal, the RP-17 hammer operates

unstable and quickly dies out. This is a significant difference between the RP-17 hammers and the OM-506, which in a number of cases continue to operate even when the optimum feed force is considerably exceeded.

The ratios $\dfrac{t_x}{t_p}$ corresponding to stable operation of the hammer increase with increasing feed force, since in this case an increase is observed—

Fig. 13. Change in the character of the working process of a drilling hammer as a function of feed force when drilling sandstone.
$\square$ — 4 atm; $\triangle$ — 5 atm; $\bigcirc$ — 6 atm.

Fig. 14. Change in the character of the working process of the hammer as a function of feed force when drilling granite.
$\square$ — 4 atm; $\triangle$ — 5 atm; $\bigcirc$ — 6 atm.

... an increase in the duration of the return stroke \(t_x\) and a reduction in the time of the working stroke \(t_p\).

When the feed force is increased beyond the optimum, the duration of the piston working stroke begins to increase, evidently because of the resistance of the air in the front chamber, owing to incomplete exhaust. In drilling granite this phenomenon is not observed; evidently, it occurs in the range of feed forces at which the hammer is already stalling (“does not pull”).

The decrease, in a number of cases, of the duration of the piston return stroke in the range of large feed forces can be explained by a shortening of the piston stroke when the resistance to rotation of the drill is relatively great. In this case the drill turns only after several cycles of the piston stroke, since the ratchet mechanism of the RP-17 hammer in these cases does not ensure rotation of the drill in each cycle. Cases were observed in which, during drilling of rock, the hammer operated but the drill did not rotate. Evidently, the piston stroke was so small that it did not ensure rotation of the rod by \(1/2\) tooth pitch of the ratchet bushing.

The change in the power of the impact unit (Fig. 15) is analogous to the change in drilling speed as a function of air pressure and feed force. The maximum values of the power of the RP-17 impact unit in most cases were observed at feed forces providing the maximum drilling speed. When the feed force is increased to the optimum (providing the maximum drilling speed), the impact power of the hammer increases to the value indicated in Table 10.

Fig. 15. Dependence of impact power on feed force when drilling granite. □ — 4 ati; △ — 5 ati; ○ — 6 ati.

Table 10

When the optimum feed force is exceeded, the impact power of the hammer decreases sharply.

With changes in the feed force and air pressure, the specific work of rock breaking also changes. The character of the dependence of the specific work of breaking on the feed force is shown in Fig. 16. The minimum specific work of breaking is almost the same for all compressed-air pressures, and it corresponds to feed forces of 35 kg when drilling sandstone and 50–60 kg when drilling granite. Under these conditions the hammer rebound from the limiting stop of the drill is very small, and consequently the coefficient of transfer of the piston-impact energy to the drill is the greatest.

Fig. 16. Dependence of the specific work of breaking on the feed force.
a — sandstone  b — granite
□ — 4 atm; △ — 5 atm; ○ — 6 atm.

With a considerable excess over the optimum feed force, the specific work of breaking increases. In this case the drilling rate decreases faster than the impact power of the hammer, which remains at a sufficiently high level owing to the larger number of blows and the considerable impact energy of the hammer.

Conclusions

1. The optimum feed forces for the RP-17 hammer should be selected from the condition of ensuring stable operation of the hammer. For drilling horizontal blastholes in rocks of different hardness with the RP-17 drilling hammer, the following feed forces may be recommended:

\[ \begin{aligned} \text{at } p &= 4 \text{ atm} \qquad & F &= 30 \text{ kg},\\ \text{at } p &= 5 \text{ atm} \qquad & F &= 37 \text{ kg},\\ \text{at } p &= 6 \text{ atm} \qquad & F &= 45 \text{ kg}. \end{aligned} \]

1. The most suitable feeder for the RP-17 hammer is a piston pneumatic feeder.

2. On the basis of the above analysis, it may be assumed that the operation of the RP-17 hammer can be substantially improved by a certain alteration of the design and dimensions of the impact unit. For example, the length of the hammer exhaust port is never used completely. Under rational operating conditions of the hammer, the exhaust port opens by 40–60%, and only during idle operation of the hammer does the exhaust port open by 85–90% of its cross-section. Consequently, the size of the exhaust port can be reduced by approximately 5 mm by bringing the rear edge closer to the front edge; this will increase the effective value of the piston working stroke. The stability of the hammer under large feed forces can be increased by arranging a tangential channel “B” (as in the OM-506 and PM-508 hammers). The inner opening of this channel should be located at a distance of approximately 25 mm from the rear edge of the exhaust port.

3. It may be assumed that the impact energy of the RP-17 hammer can be increased somewhat by reducing the exhaust ports (bringing the rear edge closer to the front edge by 4–5 mm) and thereby increasing the useful length of the piston stroke.

4. To improve the operation of the rotation device of the RP-17 drilling hammer, it is necessary to modify the ratchet device, replacing it with four ratchet pawls of the PA-23 hammer type.

5. It should be noted that the minimum volumetric work of destruction when drilling with the RP-17 hammer is the same as when drilling with the OM-506 hammer, and is equal to: for sandstone, 10 kg·m/cm³; for granite, 20 kg·m/cm³.

REFERENCES

1. Alimov O. D., Basov I. G., Gorbunov V. F., Serov Ya. A. Methodology for conducting experiments in the investigation of impact units of pneumatic drilling hammers and for processing experimental data. Izv. TPI, vol. 88, 1956.

2. Alimov O. D., Basov I. G., Gorbunov V. F. Methodology for laboratory investigation of the influence of feed force on the drilling speed of pneumatic drilling hammers. Izv. TPI, vol. 88, 1956.

3. Alimov O. D., Gorbunov V. F. Some results of an investigation of the OM-506 pneumatic drilling hammer. Izv. TPI, vol. 88, 1956.