Study of Rotary-Percussive Drilling

O. D. Alimov, I. G. Lyapichev, Ya. A. Serov

Methodology for Studying Rotary-Percussive Drilling

At present, in blasthole-drilling technology, two drilling methods are used mainly: rotary drilling (with drill bits) and percussive-rotary drilling (with rock drills). The use of the rotary drilling method is most effective in soft rocks. It provides the highest speed and the lowest energy consumption of drilling. In design and manufacture, rotary drilling machines are quite simple, and in operation they are durable. Thanks to these advantages, such machines are widely used in the coal and construction industries. A number of researchers \[1, 2, 3\] assert that by the rotary method, with a feed force of 800–1500 kg, even hard rocks can be drilled at rates customary for the percussive-rotary drilling method. However, the wear of a tool tipped with hard alloy is then 20–30 times greater than in percussive-rotary drilling.

The percussive-rotary method is used for drilling blastholes and boreholes in rocks of medium and high hardness. Its main advantage over rotary drilling is the significantly lower wear of the tool.

This drilling method has found its greatest application in the mining industry.

The desire to combine the advantages of rotary and percussive drilling in rocks of medium and high hardness led to the creation of machines for rotary-percussive drilling. The essence of this method is that the drilling tool rotates continuously and is fed to the bottom with a certain feed force. In addition, blows are additionally delivered to the drilling tool by a percussion unit installed on the same machine. The first proposal for the creation of such machines was made by E. F. Epshtein \[1\].

The rotary-percussive method of drilling has recently found industrial application in a number of Western European countries. According to foreign literature, rotary-percussive drilling machines provide drilling rates 2–3 times higher than electric drills and pneumatic rock drills.

light. It is noted that it is economically expedient to use rotary-percussive drilling machines only when they operate under drilling conditions specified for the given rocks and with the use of a special tool. The literature [3–6] does not provide sufficiently substantiated data for selecting drilling conditions and drilling tools for such machines. Nor are such data available in our practice, although designs of similar machines are already being developed. Therefore, it became necessary to conduct special studies to determine the most expedient modes of rotary-percussive drilling, the design of the drilling tool, and the parameters of the drilling machine. The program of these studies should include drilling various rocks under different conditions. Moreover, the rocks should be chosen as the most characteristic of those mining and geological conditions in which the machines are expected to be used.

The principal factors affecting the rate of rotary-percussive drilling—the feed force and character of feed, the number of revolutions of the drill, the frequency and energy of impacts, the design of the drilling tool, and the method of removing drill cuttings from the blasthole—must be investigated over a wide range of their variation. The design of the experimental installation must ensure the constancy of these factors during each test, and the measuring equipment must make it possible to record the amount of power consumed by the impact unit, by the feed mechanism, and by the unit for rotating the drill, the consumption of air and water, tool wear, and other parameters necessary for determining the technical characteristics and operating conditions of the drilling machine.

For the most complete and objective assessment of the advantages and disadvantages of the rotary-percussive drilling method in comparison with other methods, it is expedient to conduct comparative tests of these methods. Such studies must be carried out under laboratory and production conditions.

The aim of laboratory studies should be: to study the mechanism of rock destruction during drilling; to determine the rate and energy consumption of drilling under various combinations of the number of revolutions of the drill, feed force, impact energy and frequency, and the geometry of the drilling tool.

Studies under production conditions are carried out to determine the wear resistance of the drilling tool when operating under various modes, and to select the most expedient method of removing cuttings. As a result of production studies, recommendations on drilling modes, the drilling tool, and the technical characteristics of the machine are finally refined.

Below is given a description of the experimental installation, the procedure for conducting the tests, and the results of experimental

studies carried out by the Department of Mining Machines and Mine Transport of the Tomsk Polytechnic Institute in order to determine the advisability of applying rotary percussive drilling under the conditions of driving development workings in Kuzbass.

To determine the optimum drilling regimes applicable to Kuzbass conditions, the investigations were conducted while drilling two of the most characteristic rocks. The rocks were represented by sandstone with a strength, according to the scale of Prof. M. M. Protodyakonov, of \(f = 6 \div 8\) and \(f = 8 \div 12\). The strength of the rocks was determined by crushing cubes and by the Shore method. Drilling was carried out in the laboratory and at the Stepanov quarry near the city of Tomsk.

Taking into account the recommendations of various investigators and the differing conditions of application of percussion-rotary drilling machines, the principal parameters of the drilling regimes were varied within the following ranges: feed force from 0 to 1600 kg; number of blows from 1600 to 3000 per minute; drill rotation speed: 196 and 408 rpm; impact energy from 3 to 5 kgm. Holes 42 mm in diameter were drilled with drills from 1 to 2.5 m long. Removal of the broken rock from the hole was accomplished by flushing or blowing. The most expedient pressure and flow rate of water or air for removing drill cuttings were established experimentally in the course of the investigation.

All drilling regimes were tested with several types of detachable drill bits, the selection of which for the tests was made on the basis of the following considerations. With some approximation it may be considered that the rotary percussive method of rock destruction during drilling is a product of two known methods: rotary and percussive-rotary. On the basis of many years of experience, for these methods of destruction a tool has been developed which, in the main, satisfies operating requirements. The requirements for a rotary-drilling tool include the smallest cutting-edge sharpening angle, with sufficient strength and resistance to abrasive wear. The geometry of such a cutter is shown in Fig. 1, b. The cutting-edge sharpening angle is \(\beta = 65^\circ\). To increase resistance to abrasive wear, the cutters are fitted with VK-8 hard-alloy plates.

For the tool used in percussive-rotary drilling, the foremost requirement is strength of the blade under impact loading, as well as relatively high resistance to abrasive wear. For this purpose the sharpening angle of the drill bit is increased to \(\beta = 100 \div 110^\circ\) (Fig. 1, a). The bit is reinforced with hard-alloy plates with an increased cobalt content, of types VK-11 and VK-15.

As regards the geometry and quality of the drilling tool for rotary percussive drilling, sufficiently substantiated proposals...

Fig. 1. Tools tested in rotary and rotary-percussive drilling:

a—dental bit No. 1; b—bit No. 3; c—bit No. 2; d—template for checking the sharpening shape of bit No. 2.

at present. Cutters for rotary-percussive drilling, evidently, must satisfy both the above-mentioned requirements and others.

It may be assumed that the rational geometry of a tool for this method of drilling should be intermediate between the geometry of cutters for rotary and percussive-rotary drilling. In this range, a large number of different bits may be proposed, with various sharpening and blade shapes, made of different materials.

Testing a large number of bits when drilling various rocks would be a very laborious and lengthy task without sufficient scientific substantiation and sequence. Therefore, in order to resolve the question posed concerning the advantages of rotary-percussive drilling, in a first approximation the number of drill bits selected for testing should be reduced to a minimum. In our opinion, in comparative tests of rotary, percussive-rotary, and rotary-percussive drilling, the bits widely used for the first two drilling methods should be used first of all (Fig. 1, a and b). When drilling with these bits under the corresponding conditions, the principal parameters will be obtained: drilling rate and cutter durability, which characterize these drilling methods. When the same bits are tested under rotary-percussive drilling conditions, the limiting values of speed and wear resistance under the new operating conditions will be revealed. These data will make it possible, in a first approximation, to decide the question of the advantages of one or another method of destruction, as well as of the design of the drilling tool intended for rotary-percussive drilling.

The drilling tool created on the basis of these recommendations must undergo additional comparative tests, the results of which may serve as the basis for a final comparison of the technical and economic indicators of the various drilling methods.

Proceeding from these considerations, for preliminary comparative tests the authors adopted two types of bits: a single-chisel bit, most widely used at present in percussive-rotary drilling (bit No. 1), Fig. 1, a, and a rotary-drilling bit (bit No. 3), Fig. 1, b, used in drilling with PEB-2m type drills. The geometric parameters of bits No. 1 and No. 3 are given in Table 1. The same table gives data for bit No. 2, which is recommended on the basis of the investigations carried out for rotary-percussive drilling of rocks of medium hardness, \(f = 6 \div 10\).

For investigations of various drilling regimes, an experimental installation was designed and built, consisting of a test stand with impact, rotary, and feed ...

Table 1

assemblies, recording devices, a frame for mounting the stand and the rock specimens, two ZIF-ShVKS-5 mine mobile compressors for obtaining compressed air, and a “Prokhodka 500” fan.

The schematic diagram of the stand and of the device for securing the rock in carrying out laboratory studies is shown in Fig. 2.

Fig. 2. Diagram of the stand for studying rotary-percussive drilling.

The stand consists of frame 1, piston pneumatic feed unit 2, impact unit 5, and rotary unit 9 with an electric motor. The stand frame 1 (Figs. 2, 3) is made of two channels. The rear part of the frame rests on the bracket of the strut 3 (Fig. 2) or on shims (Fig. 3). The front part of the stand frame is secured by means of a collar on a crosspiece fastened with clamps to the posts of the welded frame 8 (Fig. 2). The design of the collar makes it possible to rotate the stand frame about the vertical and horizontal axes of the crosspiece. Along the guides of the frame moves carriage 4 (Figs. 2, 3) with

with percussion mechanism 5. As the percussion mechanism on carriage 4, any pneumatic drilling hammer may be installed: from the light RP-17 type to the heavy KTsM-4 hammer.

In carrying out our investigations, OM-506 and RH-754 hammers were used as the percussion unit; the pawls of the rotary mechanism had first been removed from these hammers. The technical characteristics of the hammers at air pressures of 4 and 6 ati are given in Table 2. As the feed unit, a piston pneumatic feeder was used,

Table 2

which, in comparison with other types of feeders, has a number of advantages: a wide range of variation of the feed force, simplicity of feed-force regulation, and its constancy during each experiment. The piston feeder 2 (Figs. 2, 3) feeds the carriage with the necessary feed force toward the face. Carriage 4 transmits the feed force to drill 6 through a ball bearing. To limit the movement of carriage 4 in the event of breakage of drill 6 or spalling of the rock, safety stop 7 (Fig. 2) is provided. The dimensions of the piston feeder made it possible to regulate the feed force within the range from 0 to 1600 kg (at an air pressure of 6 ati). The air pressure supplied to the feeder and to the hammer is regulated by throttle valves and monitored by pressure gauges.

In the front part of the stand frame 1, a mechanism for rotating drill 9 (Figs. 2, 3) is mounted; for this purpose a column-type electric drill EBK-2 was used. The replaceable gears of the drill reducer made it possible to vary the number of revolutions of the drill, while the asynchronous motor ensured a practically constant number of drill revolutions when the torque changed. The body of the electric drill is suspended on ball bearings, the axes of which coincide with the axis of rotation of the drill. When torque is transmitted to the drill, the body of the electric drill deflects in the direction opposite to the rotation of the drill. This displacement is resisted by a torque meter, which records the magnitude of the torque on the drill. In the experiments, spring or hydraulic torque meters were used.

Fig. 3. General view of the rotary-percussive drilling test stand.

Fig. 4. Impact unit of the rotary-percussive drilling test stand.

A bracket 13 (Figs. 2 and 4) with a pencil is fastened to the carriage of the percussion unit; on the rotating drum 12 it records the oscillations of the drilling hammer and the depth of the blast-hole drilled during the test. Drum 12 is set in motion by electric drill 11.

The number of revolutions of the drill during a test is recorded by a UGN-1 type revolution counter, and the duration of the test by a stopwatch. The power consumed by the rotary unit is recorded by a self-recording wattmeter, and the air consumption by an air meter.

Rock samples were delivered to the laboratory for drilling; their dimensions \((1.5 \div 2 \times 1.0 \div 1.3 \times 1.0 \div 1.3\ \text{m})\) made it possible to conduct a sufficient number of tests when drilling blast-holes to a depth of \(0.7—0.8\ \text{m}\).

The rock subjected to drilling is secured in a special frame by jacks. As was found in preliminary tests, removal of drill cuttings from the blast-hole during drilling under laboratory conditions is most conveniently and effectively carried out by blowing. Air for blowing was supplied into the blast-hole by means of a side coupling and an opening in the drill. The dust formed during drilling and blowing of the blast-hole was extracted by a “Prokhodka 500” fan. To protect the gear wheels of the rotary mechanism from clogging with drill cuttings, air under a pressure of \(1.5—2\ \text{atm}\) was also supplied into the reducer housing.

Technical characteristics of the TPI test stand for studying rotary and rotary-percussive drilling

Procedure for conducting tests and processing experimental data

Before the tests are begun, the blast-hole is collared to a depth of \(6—8\ \text{cm}\) by the rotary-percussive method, with a small feed force and an air pressure in the hammer of the percussion unit of about \(3\ \text{atm}\). In conducting studies of rotary drilling, collaring was likewise carried out by the rotary-percussive method, since the collaring process was thereby facilitated. To determine the depth of the blast-hole drilled during the test, before the beginning of the test, with the pencil of the recording device ...

an initial line is drawn on the paper fastened to the drum. After this, the drill-rotation motor, the drilling hammer, and the stopwatch are switched on simultaneously. At the end of the experiment the rotary unit, the hammer, and the stopwatch are likewise switched off simultaneously, after which the final line is drawn. The distance between these lines is equal to the depth of the hole drilled during the experiment. Each experiment was carried out 3–4 times with the selected drilling hammer and a specified pressure of the air supplied to the impact unit and to the feeder, with a specified number of drill revolutions and a specified type and size of tool.

During the experiment the following quantities were measured: the air pressure in the hammer; the air consumption of the impact unit; the duration of the experiment; the air pressure in the feeder cylinder; the depth of the hole drilled during the experiment; the number of drill revolutions during the experiment; the resistance to drill rotation; and the power consumed by the rotary mechanism.

The measurement results were entered in an observation log, in which each experiment was numbered. These same experiment numbers were indicated on the recording diagrams for hole depth, power, and torque.

From the observation results the following quantities were determined:

1. Feed force

\[ P = pF - \Delta P \ \text{kg}; \]

where \(F\) is the area of the feeder piston, \(\text{cm}^2\);

\(p\) is the air pressure in the feeder, atm;

\(\Delta P\) is the force required to overcome friction during displacement of the piston and carriage, determined experimentally.

1. Drilling speed

\[ V = \frac{60S}{t}\ \text{mm/min}, \]

where \(S\) is the depth of the hole drilled during the experiment, mm;

\(t\) is the duration of the experiment, sec.

On the basis of the experimental data, graphs were plotted showing the dependence of drilling speed on feed force for all methods of drilling when using various tools and methods of removing drill cuttings. Such dependencies make it possible to represent the influence of each of the factors listed above.

For a more complete representation of the advantage of rotary-percussive drilling over other methods in terms of drilling speed, graphs were plotted for the dependence of the increase in drilling speed \(\Delta V\) on feed force. The increase in drilling speed is understood as the excess of the speed of rotary-percussive drilling over the drilling speed provided by another method,

\[ \Delta V = V_{\text{r.p.}} - V_{\text{b}}, \]

where \(V_{\mathrm{r.p.}}\) is the rate of rotary-percussive drilling;
\(V_{\mathrm{d}}\) is the rate of rotary or percussive-rotary drilling.

Of the latter two, the method selected for comparison was the one which, in drilling the given type of rock, provided the highest drilling rate. For example, when drilling rocks with strength up to \(f \leq 10\), it is most expedient to compare rotary-percussive drilling with rotary drilling. Such relationships were established for drilling with different tools while varying the number of revolutions of the bit and the methods of removing drilling cuttings from the blasthole. As a result of such processing of the experimental data, it is possible not only to reveal the advantages of one method over another in terms of drilling rate, but also to determine the range of the most expedient feed forces for rotary-percussive drilling. Final conclusions concerning the optimum feed forces, bit speeds, type of tool, and method of removing drilling cuttings can be made with allowance for drilling rate, energy consumption, and wear resistance of the drilling tool.

To determine the energy consumption of the breakage process for a given drilling method and regime, the power expenditures for rotation of the bit, feed, and the impact unit were determined. The power for rotation of the bit was determined by the formula

\[ N_{\mathrm{rot}}=\frac{M n_{\mathrm{rev}}}{97600}\ \text{kW}, \]

where \(M\) is the torque on the bit, determined from the moment meter and calibration graph;
\(n_{\mathrm{rev}}\) is the number of revolutions of the bit per minute, according to the revolution counter, or by the formula

\[ N_{\mathrm{rot}}=a_{1} C \eta_{1}\eta_{2}\ \text{kW}; \]

where \(a_{1}\) is the readings of a type D-351 self-recording wattmeter, mm;
\(C\) is the instrument constant (in our case, with a UTT-4 current transformer, \(C=90\));
\(\eta_{1}\) is the efficiency of the motor, determined from the load characteristic;
\(\eta_{2}\) is the efficiency of the reducer.

The power transmitted to the bit by the feeder is

\[ N_{\mathrm{f}}=\frac{PV}{1000\cdot102\cdot60}\ \text{kW}. \]

The power transmitted to the bit by impacts was taken as equal to the power of the hammer,

\[ N_{\mathrm{h}}=\frac{A n_{\mathrm{imp}}}{102\cdot60}\ \text{kW}, \]

where \(A\) is the impact energy, kgm;
\(n_{\mathrm{imp}}\) is the number of impacts per minute, determined from the data of preliminary tests.

The volumetric work of destruction was determined for each experiment by the formula

\[ A_v=\frac{N_{\mathrm{vr}}+N_{\mathrm{p}}+N_{\mathrm{m}}}{F_{\mathrm{sh}}V}, \]

where \(F_{\mathrm{sh}}\) is the area of the blasthole, \(\mathrm{cm}^2\);
\(V\) is the drilling speed, \(\mathrm{cm/min}\).

On the basis of the calculation results, graphs were plotted showing the dependence of the volumetric work of destruction on the feed force for various numbers of drill revolutions, types of tool, and methods of removing drill cuttings.

Because of the presence of a number of differences between drilling under laboratory and production conditions, experiments to determine drilling speed and energy consumption were repeated under production conditions. It was found that the difference in test conditions does not have a significant effect on the dependences, obtained in the laboratory, of drilling speed, energy consumption, and other parameters on feed force. However, under production conditions it was possible to study in greater detail the wear resistance of the bit and the influence of the method of removing drill cuttings by air flushing and water flushing on the principal indicators of the drilling process.

To determine these dependences, the experiments were carried out in an open pit, where the physicomechanical properties of the rocks were characteristic of the Kuzbass mines. The same installation and research methodology were used for the experiments as in the laboratory investigations. Blastholes were drilled in a rock mass to a depth of \(2.5\ \mathrm{m}\). Drill cuttings were removed from the blasthole by flushing with water or by air blowing. In each case, an appropriate pressure and flow rate of the flushing liquid and air were established, ensuring complete removal of drill cuttings from the blasthole.

To determine the wear resistance of the tool when operating under different drilling regimes, the increase in the width of the dulling land of the bit was determined as a function of the number of meters of blasthole drilled, together with the corresponding change in drilling speed. The width of the dulling land was measured with a Brinell magnifier at 24-fold magnification.

Research results

In rotary drilling of sandstone with hardness \(f=8\text{--}12\), for all types of tested bits (Figs. 5 and 6), the drilling speed increases as the feed force is increased. Moreover, at feed forces from 100 to \(300\ \mathrm{kg}\), the speed changes only slightly. With a further increase in feed force, the drilling speed in most cases increases in proportion to the increase in feed force. Characteristic of rotary drilling is the insignificant increase in drilling speed at small feed forces—

feed force (100–300 kg) can be explained by the fact that, in this case, the pressure at the contact point between the tool and the rock is insufficient for volumetric destruction of the rock. As the number of revolutions of the drill increases from 196 to 408 per minute, the rate of rotary drilling increases considerably.

In rotary-percussive drilling of sandstone with hardness \(f = 8 \div 12\), the dependence of the drilling rate on the feed force is expressed by a convex curve (Figs. 5 and 6). In contrast to rotary drilling, the most intensive increase in the drilling rate with bits No. 2 and No. 3 is observed at small feed forces (100–700 kg). This can be explained by the fact that even at small feed forces the blows on the drill already ensure penetration of the cutter into the rock. For the subsequent effective destruction of the blasthole bottom by chipping due to rotation of the drilling tool, a smaller axial force is then required than for the initial penetration.

Fig. 5. Dependence of the drilling rate on the feed force:
rock—sandstone \(f = 8 \div 12\); hammer OM-506;
1—bit No. 1; 2—bit No. 2; 3—bit No. 3; \(n_b = 196\) rpm; ○—rotary drilling; rotary-percussive drilling; △—\(p_m = 4\) atm; □—\(p_m = 6\) atm.

At feed forces above 700–1000 kg, the advantage of rotary-percussive drilling over rotary drilling is less substantial. Evidently, in this case the share of the dynamic load in creating the necessary specific pressure is already less significant, and the decisive importance for the penetration of the tool into the rock belongs to the static load from the constant feed force. However, the increase in drilling rate from additional blows is retained even at considerable feed forces. The decrease in the rate of rotary-percussive drilling, as compared with rotary drilling, at feed forces greater than 1000 kg, noted by some investigators [6], was not observed by us. At the same feed force, the rate of rotary-percussive drilling with bits No. 2 and 3 is 2–3 times higher than with bits No. 1. This can be explained by the fact that, for bits No. 1 to penetrate the blasthole bottom to a certain depth, considerably greater forces are required.

in comparison with bits Nos. 2 and 3. When the number of drill rotations is increased from 196 to 408 per minute, the drilling rate of sandstone (Fig. 7) increases, with the rotary method, by a factor of 1.5–2, and with the rotary-percussive method, by a factor of 1.5–2.5. The greatest increase in drilling rate, both for the rotary and for the rotary-percussive methods, corresponds to a small feed force, when the absolute values of the drilling rates are small.

Fig. 6. Dependence of drilling rate on feed force:

porous sandstone \(f = 8 \div 12\); hammer OM-506; 1—bit No. 1; 2—bit No. 2; 3—bit No. 3; \(n_b = 408\) rpm; ○—rotary drilling; rotary-percussive drilling; △—\(p_m = 4\) atm; □—\(p_m = 6\) atm.

Increasing the air pressure supplied to the impact unit, and consequently increasing the number of blows, has the greatest effect on the rate of rotary-percussive drilling in all cases at high feed forces.

Increasing the number of blows (using the RH-754 hammer as the impact unit instead of the OM-506) leads to an increase in the rotary-percussive drilling rate (Tables 3 and 4). Moreover, the absolute value of the increase in drilling rate is approximately the same for all feed forces and is 450–500 mm/min at a drill rotation speed of 196 revolutions and 700–850 mm/min at 408 revolutions per minute. It follows from this that the use of impact units with an increased blow frequency is especially effective at high drill rotation speeds. It should be noted that the cutting edges of bit No. 3 \((\gamma = 0^\circ)\), tipped with VK-6 hard alloy, when hammers of the RH-754 type are used as the machine’s impact units, quickly fail because of breakage of the hard-alloy inserts.

The increase in the rate of rotary-percussive drilling compared with rotary drilling rises with increasing feed force (Fig. 8), reaches a maximum, and then decreases. The increa-

Table 3

Note. Drilling was carried out with bit No. 2 at 196 revolutions of the drill per minute. In the numerator are given the values at an air pressure in the hammer of the impact unit of 4 atm, and in the denominator—6 atm.

Table 4

Note. Drilling was carried out with bit No. 3 at 408 revolutions of the drill per minute. In the numerator are given the values at an air pressure in the hammer of the impact unit of 4 atm, and in the denominator—6 atm.

The increase in drilling speed rises with an increase in the drilling speed due to increasing the number of drill revolutions from 196 to 408 per minute, to the use of the RH-754 hammer instead of the OM-506 hammer as the impact unit, and to a change in the geometry of the tool.

The maximum increment in speed in rotary-percussive drilling with bits No. 2 and 3 is attained at a feed force

Fig. 7. Dependence of drilling rate on feed force and drill rotation speed:
rock—sandstone, \(f = 6\text{–}8\); KN-754 hammer; bit No. 2; removal of drill cuttings by flushing;
1—\(n_b = 408\) rpm; 2—\(n_b = 196\) rpm;
\(\circ\)—rotary drilling; \(\triangle\)—rotary-percussive drilling; \(p_m = 4\) atm.

Vertical axis: drilling rate, mm/min.
Horizontal axis: feed force, kg.

Fig. 8. Dependence of the increment in rotary-percussive drilling rate on feed force:
rock—sandstone, \(f = 8\text{–}12\); OM-506 hammer;
1—bit No. 1; 2—bit No. 2; 3—bit No. 3; \(n_b = 408\) rpm;
\(\triangle\)—\(p_m = 4\) atm.; \(\square\)—\(p_m = 6\) atm.

Vertical axis: increment in drilling rate, mm/min.
Horizontal axis: feed force, kg.

500–600 kg. The change in the increase in drilling speed with bit No. 1 is less intensive than when drilling with bits Nos. 2 and 3, but it covers a much wider range of feed forces.

The greatest increase in drilling speed with bit No. 1 occurs at a feed force of 1000–1200 kg, i.e., twice as high as when drilling with bits Nos. 2 and 3. This can be explained by the fact that bits Nos. 2 and 3 have a smaller included angle and, consequently, a smaller area of contact with the rock. As a result, penetration of bits Nos. 2 and 3 into the bottom of the blasthole is achieved at considerably lower forces than for bit No. 1. The absolute values of the increases in drilling speed with bits Nos. 2 and 3 are considerably higher than when drilling with bit No. 1.

When drilling sandstone \((f = 8 \div 12)\), complete removal of drill cuttings from the blasthole by air flushing required an air pressure of 3 atm. In this case the air consumption was \(0.5\ \mathrm{m}^3/\mathrm{min}\). For complete removal of drill cuttings when drilling sandstone with \(f = 6 \div 8\), it was necessary to increase the pressure of the air supplied for flushing the blasthole to 4 atm; in this case the air consumption was \(0.7\ \mathrm{m}^3/\mathrm{min}\).

For complete removal of drill cuttings from the blasthole by flushing at the highest drilling speeds, a water consumption of 14–15 l/min is required. The design of the flushing device of the experimental installation provided this flow rate at a water pressure of 5 atm. A further increase in the pressure of the supplied water did not increase the drilling speed.

Fig. 9. Dependence of drilling speed on feed force and the method of removing drill cuttings from the blasthole:
rock—sandstone \(f = 6 \div 8\); hammer RH-754; bit No. 2; \(n_b = 408\ \mathrm{rpm}\); 1—drilling with air flushing; 2—drilling with water flushing; ○—rotary drilling; △—rotary-percussive drilling, \(p_m = 4\) atm.

The investigations carried out made it possible to compare the effectiveness of various methods of removing drill cuttings and to establish their influence on the speed of rotary and rotary-percussive drilling. As can be seen from Fig. 9, when drill cuttings are removed by air flushing—

with blowing, the speed of rotary drilling is 20–48% higher than when drill cuttings are removed by flushing. For rotary-percussive drilling this increase is 32–49%. The widely held view that flushing, compared with other methods, increases the speed of rotary drilling is not confirmed by our experiments. Evidently, blowing, as compared with flushing, provides better removal of drill cuttings from the cutting edges of the drilling tool and thereby increases the drilling speed.

The power consumed by the drill-rotation motor increases continuously with increasing feed force (Figs. 10 and 11).

Fig. 10. Dependence of the power of the rotator transmitted to the drill on the feed force:

rock—sandstone \(f = 8 \div 12\); hammer of impact unit OM-506; \(1\)—bit No. 1; \(2\)—bit No. 2; \(3\)—bit No. 3; \(n_b = 196\) rpm; ○—rotary drilling; rotary-percussive drilling; △—\(p_m = 4\) atm; □—\(p_m = 6\) atm.

With an increase in the pressure of the air supplied to the hammer from 4 to 6 atm, the power consumed by the rotator changes only slightly. In all cases, rotary-percussive drilling requires somewhat greater power than rotary drilling.

With an increase in the number of revolutions of the drill from 196 to 408 per minute, the power consumed by the rotator increases approximately twofold under all rotary-percussive drilling regimes. Increasing the number of blows (using the RH-754 hammer instead of the OM-506 as the impact unit) leads to an increase in the power consumed by the rotator of approximately 10–40%, with the greatest increase in rotator power in this case occurring at 196 drill revolutions per minute.

The geometry of the drilling tool tested does not have a substantial effect on the magnitude of the power consumed.

Fig. 11. Dependence of the power of the rotary drive transmitted to the drill on the feed force: sandstone rock, \(f = 8\text{–}12\); pneumatic hammer of the OM-506 impact unit; 1—bit No. 3; \(n_b = 408\) rpm; ○—rotary drilling; rotary-percussive drilling: △—\(p_m = 4\) atm; □—\(p_m = 6\) atm.

Fig. 12. Dependence of the total power transmitted to the drill by the rotary drive, hammer, and feeder on the feed force in rotary-percussive drilling:

sandstone rock, \(f = 8\text{–}12\); RH-754 hammer; 2—bit No. 2; 3—bit No. 3; \(n_b = 408\) rpm; △—\(p_m = 4\) atm; □—\(p_m = 6\) atm.

The total power consumed by the rotary drive, the percussion unit, and the feed mechanism (Fig. 12) increases as the feed force is increased. Increasing the pressure of the air supplied to the percussion unit and increasing the impact frequency increase the total power consumption. This increase amounts to from 15 to 40%. At the highest drilling rates obtained in the present study, the total power expenditure was 7–9 kW. The energy consumption of rotary-percussive drilling, as of rotary drilling, depends strongly on the geometry of the drilling tool and on the feed force (Fig. 13). A minimum energy consumption of 7 kgm/cm³ was obtained when using bit No. 2 and feed forces of 1000–1200 kg. Under the same drilling regimes, but when bit No. 1 was used, the energy consumption increased by a factor of 2–3 and reached 15–20 kgm/cm³. Increasing the number of revolutions of the drill from 196 to 408 rpm did not lead to any substantial change in the energy consumption of the destruction process. The energy consumption of rock destruction in rotary-percussive drilling practically does not exceed the energy consumption of rotary drilling.

Fig. 13. Dependence of the energy consumption of drilling on the feed force:

rock—sandstone \(f = 8 \div 12\); hammer OM-506; 1—bit No. 1; 2—bit No. 2; 3—bit No. 3; \(n_b = 196\ \text{rpm}\); ○—rotary drilling; rotary-percussive drilling: △—\(p_m = 4\ \text{atm}\); □—\(p_m = 6\ \text{atm}\).

The wear resistance of drilling bits was investigated under the following regimes of rotary-percussive and rotary drilling: number of drill revolutions per minute 196 and 408, feed forces 500, 750, and 1000 kg, impact hammer RH-754 operating at an air pressure of 4 atm; removal of drilling fines was carried out by blowing or flushing. Drilling was carried out in the most widespread coal-measure rock—sandstone of hardness \(f = 6 \div 8\).

The criterion of wear adopted was the average width of the blunting area on the rear face of the main cutting edge. The shape

cutting face of bit No. 2 ensured uniform wear along the entire length of the main cutting edge (Fig. 1). It is evident from Fig. 14 that bit wear in both rotary-percussive and rotary drilling increases with an increase in the feed force and in the number of drill meters. Moreover, the rate of increase of wear with an increase in the number of drill meters in rotary-percussive drilling is less than in rotary drilling. As tool wear increases, the drilling rate decreases (Fig. 15). At identical feed forces, rotary-percussive drilling provides substantially less bit wear than rotary drilling. When the feed force is increased, bit wear increases in both rotary and rotary-percussive drilling.

Fig. 14. Dependence of the average width of the bit dulling land on the drilling method, feed force, and number of meters drilled:

rock—sandstone \(f=6\div 8\); RH-754 hammer; bit No. 2; \(n_b=408\) rpm; removal of drill cuttings by flushing; \(O\)—rotary drilling; \(\Delta\)—rotary-percussive drilling; \(p_m=4\) atm.

If the permissible wear of the cutting face is taken to be an average dulling-land width of \(0.8—1.0\) mm, when the decrease in drilling rate does not exceed \(20—30\%\) of the initial rate, then in rotary-percussive drilling and at a feed force of \(500—700\) kg the bit ensures drilling of \(16—26\) linear m of blasthole. Under the same conditions, bits in rotary drilling ensure drilling of only \(6—12\) linear m. The increase in bit life is correspondingly \(118—116\%\). Similar dependences were found in drilling with other drill rotation speeds and with various methods of removing drill cuttings from the blasthole.

On the basis of the results of the study, the following operating conditions may be recommended as optimal for rotary-percussive drilling of rocks of hardness \(f\) from 6 to 10: feed force \(700—1200\) kg, drill rotation speed \(300—400\) rpm, power of the rotary-drive motor \(4\) kW, impact energy \(5—6\) kgm at an impact frequency of \(3000\div 3500\) per minute. Drilling under these conditions provides the greatest advantage of rotary-percussive drilling over rotary drilling in drilling rate, energy consumption, and tool wear resistance.

Table 5

Table 5 gives a comparison of various drilling methods by the principal indicators for sandstone hardness $f = 6 \div 8$. The basis for compiling the table was the results of our investigations, as well as the results of comparative tests of pneumatic drilling hammers manufactured in the USSR and abroad [7]. The latter were carried out simultaneously with the study of rotary-percussive drilling. The tests showed that the highest drilling rate in rocks of hardness $f$ from 6 to 12 can be provided by RH-754 type hammers when operating with pneumatic feeds, and by T-10 hammers mounted on special feeders. The data on drilling with RH-754 and T-10 hammers presented in Table 5 characterize the capabilities of modern drilling hammers.

Fig. 15. Dependence of drilling rate on the drilling method, feed force, and number of drilled drill-meters:

rock—sandstone $f = 6 \div 8$; hammer RH-754; bit No. 2; $n_b = 408$ rev/min; removal of drill cuttings by flushing; ○—rotary drilling; △—rotary-percussive drilling; $p_m = 4$ atm.

According to the data of Table 5, the drilling rate by the rotary-percussive method is 4.5–6.7 times greater than the drilling rate with pneumatic drilling hammers, and 1.3 times greater than the rate in rotary drilling. In terms of tool wear resistance, rotary-percussive drilling occupies an intermediate position between percussive-rotary and rotary drilling. The energy consumption of rotary and rotary-percussive drilling is the same and is 2 times less than in percussive-rotary drilling.

Table 6 gives the recommended technical characteristics of a rotary-percussive drilling machine intended for drilling blast holes when driving development workings in rocks of hardness \(f = 6 \div 10\).

Table 6

It should be noted that in all designs of rotary-percussive drilling machines known to us, only pneumatic motors and pneumatic drilling hammers are used as the drive. This is due to the fact that pneumatic drilling hammers are at present the most serviceable machines of percussive action. The presence of a single pneumatic form of energy for the impact unit and the rotator simplifies the design of the machine and its power supply. But, as our studies have shown, considerable power—7–9 kW—is required to drive rotary-percussive machines. The air consumption for a pneumatic motor of such power is 8–10 m³/min. This will complicate and increase the cost of using such machines in modern coal mines. Therefore, along with the creation of rotary-percussive machines with pneuma-

...with an electric drive, it is advisable to build several experimental prototypes of machines with an electric drive, as well as with a combined pneumatic-electric drive.

REFERENCES

1. E. F. Epstein, Theory of Drilling—Cutting of Rocks by Hard Alloys. GONTI, 1939.

2. V. G. Mikhailov, Drilling of Blast Holes. Metallurgizdat, 1947.

3. R. Jahn. Das Drehschlagbohren. “Glückauf,” No. 37/38, 1954, pp. 1086–1093.

4. F. Patzold. La foration vibree. “Revue de l’Industrie Minerale,” 1953, No. 602.

5. W. D. Lacabanne and E. P. Pfleider. Rotary Percussion Blasthole Machine may Revolutionize Drilling. “Mining Engineering,” September 1955, pp. 850–855.

6. B. G. Fish. Percussive—Rotary Drilling. “The Mining Magazine,” March 1956, pp. 133–142.

7. O. D. Alimov, I. G. Basov, P. A. Samoilov. On the advisability of using high-frequency pneumatic drilling hammers. Kolyma, No. 9, 1957.