Methodology for Laboratory Investigation of the Effect of Feed Force on the Drilling Rate of Pneumatic Rock Drills

O. D. Alimov, I. G. Basov, V. F. Gorbunov

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

On the basis of practical data and our investigations [3], it is known that rock drills, in the course of their operation, work far from optimal regimes. In most cases this deficiency occurs because of the absence of feeding devices with appropriate parameters.

Determining the principal optimal parameters of feeders is made difficult by the absence of a substantiated method for testing rock drills. Below we present a methodology for laboratory investigation of the effect of feed force on the drilling rate of pneumatic rock drills.

By the optimal parameters of a feeder are meant the technical data of the feeding device that provide such forces and such a character of feeding the drill in the direction of the face that:

1) the maximum possible drilling rate is achieved, all other conditions being equal;

2) the expenditure of great physical effort by the worker-driller to hold the machine in the position required for drilling and to feed the machine in the direction of the face is eliminated;

3) the need for constant intervention by the worker in controlling the machine is removed.

At the present time, in our opinion, this problem can be solved more simply by means of experimental investigations, for which it is necessary:

1. To investigate the influence of feed force on the drilling rate of OM-506 and PM-508 rock drills. To determine the forces and the character of feeding that provide the greatest drilling rates at different air pressures and different strengths of rock.

2. To investigate how the internal process of pneumatic rock drills changes when the feed force is changed, and to determine the feed forces that ensure stable operation of the impact and rotation units of the drill during drilling.

3. To investigate the influence of feed force on tool wear.

4. To determine the possible dimensions of automatic feeders, proceeding from the most effective arrangements of blastholes when driving various workings.

5. To summarize the experience available in the design, investigation, and operation of feeders for rock drills.

On the basis of a comparison of the data obtained in solving the tasks set out above, it is possible to give sufficiently well-founded recommendations for the design of feeders for rock drills.

In the present work we describe the methodology for solving the first task of the problem—the investigation of the influence of feed force on drilling rate.

Taking into account the operating conditions of drilling hammers, the influence of feed force on drilling speed must be determined with consideration of the following factors:

1) at air pressures supplied to the hammer of 4, 5, and 6 atm;

2) when drilling rocks of different hardness, within the range from \(f = 4\text{--}6\), according to Professor Protodyakonov’s table, to \(f = 16\text{--}18\);

3) when drilling blastholes directed at different angles to the horizon: from holes directed upward at an angle of \(45^\circ\) to holes directed vertically downward;

4) when drilling with tools of different sizes and shapes: a) with drill-bit crown diameter \(d = 40, 60,\) and \(80\) mm, of chisel and cross-shaped form; b) with drill-rod length from 0.5 to 4 m;

5) with different intensity of removal of cuttings from the blasthole: by blowing and by flushing;

6) the feed force must be applied to the body of the drilling hammer;

7) the experimental installation must make it possible to vary the feed force over wide limits and to maintain this force at different speeds of motion of the drilling-hammer body during drilling. Since the drilling speed and the stability of operation of pneumatic drilling hammers are substantially affected by the impact energy, the number of impacts, the angle of rotation of the drill between two impacts, and the character of motion of the hammer body relative to the face, the design of the installation must provide the corresponding measuring devices. The measuring devices must not affect the working process of the drilling hammer or the process of rock destruction during drilling.

It is advisable to conduct the experimental investigations both under laboratory conditions and under production conditions.

In view of the complexity of carrying out such investigations, we believe that laboratory investigations should first be conducted, and then tests of the hammers under production conditions.

Experimental installation

The installation for investigating the influence of feed force on the drilling speed of pneumatic drilling hammers consisted of a mine mobile air-compressor station ZIF-ShVKS-5 for obtaining compressed air (Fig. 1) and a stand for installing the hammer and fastening the rock during drilling.

The technical characteristics of the compressor are as follows:

1. Compressor capacity: 5 m\(^3\)/min.
2. Working pressure up to 7 atm.
3. Power of the compressor electric motor MA-140, \(N = 34\) kW.

Stand for testing drilling hammers

For installing and feeding the hammer during the experiment and for fastening the rock, a special stand is used, the schematic diagram of which is shown in Fig. 2.

The design of the stand makes it possible to drill blastholes at different angles to the horizon; to record, during the experiment, the recoil diagram of the drilling hammer, the length of the blasthole drilled during the experiment; to determine the number of impacts and the number of revolutions of the drill during the experiment; and to determine the drilling speed of various rocks at different air pressures and different feed forces. With additional devices on the stand it is possible to record

simultaneously the diagram of the motion of the striker piston, the diagram of oscillations of the hammer body, and the diagram of drill motion during drilling.

Fig. 1. ZIF—ShVKS-5 mine mobile air-compressor station.

Fig. 2. Diagram of the stand for testing pneumatic rock drills.

The hammer 1 under test (Fig. 2) is secured by clamping bars in the carriage 2, which moves on wheels 3 in the grooves of the guide frame 4. The guide frame 4 is fastened by means of a clip to the cross-member 5, secured by clamps on the uprights of the welded frame of the stand 6. The design of the clip makes it possible to move the guide frame along the cross-member, and also to rota-

rotate the guide frame relative to the vertical axis of the cage. The cage can rotate about the axis of the crosshead, which makes it possible to set the hammer at an angle to the base of the frame. The guide frame is fixed in a definite position by tie bolts. The crosshead can be moved up and down along the frame posts and secured in the required position by means of clamps.

The hammer is supplied with compressed air from an air receiver through rubber hose 7 with air meter 8 and main-line oiler 9 connected in series.

The pressure of the air supplied to the hammer is varied by valve 10 and monitored by pressure gauge 11.

To create the axial feed force on the hammer, a pneumatic pressure device is used, consisting of cylinder 12 and a piston with rod 13. Cylinder 12 is connected by rubber hose 14 to a pipeline leading from the compressor station. Cylinder 12 has an opening communicating the cylinder with the atmosphere. By throttling the air from the feeder cylinder through an opening 2.5 mm in diameter, a constant pressure is achieved and, consequently, a constant feed force during the drilling process.

To eliminate the possibility of a change in air pressure in the feed cylinder due to its compression during oscillations of the hammer body, the piston feeder is connected to the network through a hose of increased diameter \(d = 52\) mm and length \(l = 4\) m. Oscillations of the hammer body at each blow cannot significantly change the pressure in the feeder, since, with a recoil amplitude of 2–3 mm, the degree of additional compression of the air buffer, taking into account the large-diameter air-supply hose, will amount to less than

\[ \frac{1}{1000} \]

of the initial volume.

The feed force is changed by changing the air pressure in cylinder 12 by means of valve 15 and is monitored by pressure gauge 16.

To record the diagram of oscillations of the hammer body during drilling, a rotating drum 17 is mounted on cylinder 12; it is driven by a belt transmission from an electric motor with gear reducer 18.

A rod 19 with pencil 20 is attached to the body of the hammer being tested; this pencil records the oscillations of the hammer body over time on a sheet of paper fastened to the drum.

The drilling hammer is mounted on the test stand without a drill holder.

The rock specimen 21 intended for drilling is placed in the frame of the stand and secured by means of jack 22.

Technical characteristics of the stand

1. Diameter of the feeder piston, mm — 55.5.
2. Area of the feeder piston, cm² — 23.7.
3. Stroke of the feeder piston, mm — 372.
4. Diameter of the drum for recording recoil diagrams, mm — 170.
5. Circumference of the drum for recording recoil diagrams, mm — 535.
6. Time for one revolution of the drum, sec — 1.75.
7. Dimensions of the internal cavity of the stand frame (height, width, length), mm — \(1475 \times 1040 \times 1910\).
8. Weight of the carriage, kg — 6.050.
9. Weight of the feeder rod and piston, kg — 4.930.

The dust produced during drilling is removed from the room by means of a “Prokhodka-500” fan.

Procedure for Conducting the Experiment

Before beginning the recording of the quantities under study, a hole was collared to a depth of 3–4 cm. Preliminary drilling was carried out at a low working pressure (about 4 atm) and a small forced-feed force. After collaring, the pressure of the air supplied to the hammer and to the feeder was adjusted to the specified values while the hammer was operating. The hammer was then switched off.

The diagram of the oscillations of the hammer body was recorded in the following order: with the hammer switched off and with the axial force on the hammer removed, pencil 20 (Fig. 2) drew the initial line 1 (Fig. 3) on paper fixed to the rotating drum 17. The hammer was then switched on again, simultaneously starting the stopwatch. During the experiment, with the pencil pressed against the paper on the rotating drum, the curve “body path—time” 2 was traced. At the end of the recording of curve 2, the hammer was switched off simultaneously with the stopping of the stopwatch, after which line 3 was drawn.

Fig. 3. Recoil diagram of a pneumatic rock drill.

The amount of advance of the bottom of the hole during the experiment was determined by measuring the distance between lines 1 and 3. Curve 2 was recorded in the middle of the experiment and before the end of drilling. At large feed forces and a considerable drilling rate, the recording was made throughout the entire experiment.

Upon completion of each experiment, the experiment number was entered in a previously prepared observation log, and the number was also placed on the recoil diagram. In addition, the log recorded the drilling time, the air pressure at the hammer during its operation, the air pressure in the feeder, and the number of revolutions of the drill, which was determined by winding a cord around the drill or with the aid of a special device (Fig. 4).

Before each experiment, the hole was blown out and thereby freed of fragments of the broken rock. Removal of drilling cuttings during the drilling process was carried out by blowing.

Each experiment was conducted at a definite pressure of the air supplied to the hammer (4, 5, or 6 atm) and at a definite air pressure in the feeder. As the experiments proceeded, the pressure in the feeder was increased in steps of 0.5 atm (0.5—1.0—1.5—2.0, etc.) until the hammer ceased to operate. At each feed force, no fewer than 4 experiments were carried out. During the testing, the following forces were recorded: the force at which the hammer operates most steadily; the force at which the hammer cannot be started without first pulling the hammer back from the face; and the force at which the hammer stalls.

Device for Measuring the Number of Revolutions of the Drill

One of the principal factors affecting the process of destruction of rocks in percussive-rotary drilling is the angle of rotation of the drill between two blows.

To determine the angle of rotation of the drill when investigating the operating regimes of rock drills during drilling, simultaneous ...

Form of the observation log

measurement of the number of blows and the number of drill revolutions. To determine the number of blows per minute, sufficiently well-developed measuring instruments are available in the form of vibration tachometers. To determine the rotational speed of the drill, the use of ordinary tachometers is difficult, since, in addition to intermittent rotation, the drill performs a reciprocating motion at high speed. In view of the absence of special instruments, the rotational speed of the drill in investigations of the operation of rock drills was measured with the aid of a cord. The essence of this method is that, before drilling begins, the end of a thin cord is fastened to the drill. During operation of the hammer the cord is wound onto the drill; the number of turns of the cord wound on the drill corresponds to the number of its revolutions during the test. By timing the operation of the hammer with a stopwatch and measuring the number of turns of the cord on the drill after drilling, the necessary data are obtained for calculating the average value of the angle of rotation of the drill between two blows during the test.

Despite the simplicity of the principle, measuring the number of drill revolutions by means of a cord takes a great deal of time because of the time spent counting the turns when unwinding the cord from the drill.

In the course of testing the OM-506 and PM-508 rock drills, students of the Tomsk Polytechnic Institute, B. P. Proskurin and V. V. Voronin, developed a simple device for determining the number of drill revolutions during a test. The general appearance of the device is shown in Figs. 4 and 5.

The device (Fig. 5) consists of bushing 1, gear wheel 2, a body with handle 3, and stop 4. Bushing 1 rotates freely in the guides of the body. By means of an internal groove the bushing can be coupled with a hexagonal drill. A tooth fixed on the outer side of bushing 1, as it rotates, engages with the oblique teeth of wheel 2 and, at each turn, rotates the gear wheel. The gear wheel rotates freely about axis 5.

The angle of inclination and the length of the teeth of wheel 2 are selected in such a way that at each passage of the tooth of the bushing the wheel is turned by one tooth. In order to eliminate the possibility of the wheel rotating by inertia at the time when the tooth is not engaged with the wheel, the wheel is braked by means of spring stop 4. For the same purpose, the wheel ...

It is advisable to make it of a light material. In the first specimen of the device, a textolite gear wheel from the distributor shaft of a GAZ-51 automobile was used, with the number of teeth \(z = 50\). The graduations marked on the gear wheel correspond to the number of teeth of the wheel, i.e., from zero to 50.

Fig. 4. Device for measuring the number of revolutions of the drill.

The device is connected to the drill in such a way that the drill passes through the slot of the bushing; then the bushing is turned somewhat relative to the device so that the mark on the jaws of the guide bushing coincides with the mark “0” on the bushing. The zero division of the gear wheel is set opposite the pointer 6, fixed on the axle of wheel 2. The hammer is switched on. As the drill rotates, it rotates the bushing and the gear wheel. After the time required for the experiment has elapsed, the hammer is switched off. The number located opposite pointer 6 indicates the whole number of revolutions of the drill during a time equal to the duration of the experiment, while the number coinciding with the mark on the guide bushing shows fractions of a revolution.

If it is assumed that the drill of a pneumatic hammer rotates at a speed of \(300 \div 50\) revolutions per minute, then the divisions available on the device are sufficient for conducting an experiment over a period of from 10 to 60 sec., which is sufficient for determining other operating-regime characteristics with the required accuracy. If it is necessary to determine the number of revolutions over a longer interval of time, it is necessary to note how many complete revolutions the zero mark of the wheel makes.

A properly functioning device does not create significant resistance to the rotation of the drill and, as special experiments have shown, does not affect the change in the operating regime of the drilling hammer.

Owing to the use of the indicated device, the time required to conduct an experiment is reduced by a factor of 2–3.

Fig. 5. Instrument for measuring the number of revolutions of the drill steel.

Processing of Experimental Data

From the “housing travel—time” diagram (Fig. 3), the following quantities are determined:

1) the duration of the test;
2) the drilling rate over the test, in mm/min;
3) the number of blows of the striker piston per min;

4) the mean value of the recoil of the rock-drill body from the limiting collar of the drill steel, in mm;

5) the mean value of the amplitude of oscillation of the body per blow, in mm.

To determine the drilling rate for a test, the distance is measured between the initial line 1 (Fig. 3), recorded before the rock drill is switched on, and line 3, drawn after operation of the rock drill. In this way the depth of advance of the hole during the test, \(S\), is determined. The drilling rate is determined as the quotient obtained by dividing the depth of the hole drilled during the test by the test time \(t_{\mathrm{op}}\), determined with an error of 2% [2]:

\[ V=\frac{S\cdot 60}{t_{\mathrm{op}}}\ \mathrm{mm/min}. \]

The relative error of calculation is

\[ \frac{\Delta V}{V}=\frac{\Delta S}{S}+\frac{\Delta t_{\mathrm{op}}}{t_{\mathrm{op}}}, \]

where \(\Delta S\) is the absolute error in measuring the value of the hole drilled during the test with a ruler, equal to 1 mm;

\(S\) is the mean value of the hole drilled during the test; in turn, it depends on the duration of the test and the hardness of the rock. Figure 6 shows the magnitude of the relative error in determining the drilling rate as a function of the duration of the test for different rocks—sandstone with hardness \(f=4\text{--}6\) and granite with \(f=16\text{--}18\) according to Protodyakonov.

From the graph it follows that the relative error in determining the drilling rate does not exceed 5% when the test duration is 5–10 sec for rocks of hardness not greater than \(f=18\). Consequently, depending on the hardness of the rock, it is necessary to choose the duration of the test. For example, for sandstone of hardness 4–6, a test duration of 5 sec ensures calculation of the drilling rate with an accuracy of \(\pm 5\%\).

The number of blows of the rock drill is determined by the formula:

\[ n_{\mathrm{ud}}=\frac{60\cdot a}{t_1}\ \mathrm{blows/min}, \]

where \(a\) is the number of oscillations of the hammer body per blow, taken from the diagram on a segment of curve 2 equal to the circumference of the drum;

\(t_1\) is the time of one revolution of the drum, sec.

Fig. 6. Variation in the magnitude of the relative error in determining the drilling rate as a function of the test duration.

1 — when drilling rocks, \(f=16\text{--}18\),
2 — when drilling rocks, \(f=4\text{--}6\).

The relative error in calculating the number of blows was determined by us [2] and is 2%.

The average value of the displacement of the body of the drilling hammer from the limiting collar of the drill is determined by the formula:

\[ H_{\mathrm{avg}}=\frac{\sum_{i=1}^{i=n} H_i}{n}, \]

where \(H_i\) is the individual displacement of the body of the drilling hammer from the limiting collar of the drill, corresponding to each blow, mm,

\(n\) is the number of measurements.

The measurement of the individual displacements \(H_i\) is made from the line of advance of the face 4 (Fig. 3) to the lower humps of curves 2. The line of advance of the face 4 is marked on the basis of the assumption that the drilling rate during the test is constant, and is drawn on the diagram from the point of intersection of curve 2 with line 3. The angle of inclination of the penetration line to line 3 is determined by the distance \(X\), which is calculated by the formula: \(X=\frac{S}{n_1}\),

where \(S\) is the depth of the blasthole drilled during the test, mm,

\(n_1\) is the number of drum revolutions during the test, determined from the formula:

\[ n_1=\frac{t_{\mathrm{op}}}{t_1}, \qquad \text{then } X=\frac{S\cdot t_1}{t_{\mathrm{op}}}. \]

The segment \(X\) is laid off perpendicular to zero line 3 at a distance from the point of intersection of the curve with line 3 equal to the circumference of the drum \(L\) (Fig. 3).

If the length of the paper tape does not make it possible to lay off the segment \(L\), a smaller segment may be taken, correspondingly reducing the segment \(X\) to the value

\[ X_1=X\frac{L_1}{L}. \]

The amplitude of oscillations of the hammer body is found by direct measurement, followed by averaging over the test according to the formula:

\[ A_{\mathrm{avg}}=\frac{\sum_{i=1}^{i=n}\frac{A_i}{2}}{n}\ \text{mm}, \]

where \(\frac{A_i}{2}\) is the amplitude of an individual oscillation, mm,

\(n\) is the number of measurements.

The number of drill revolutions per minute is determined from the formula

\[ n_b=\frac{n'_b\cdot 60}{t_{\mathrm{op}}}, \]

where \(n'_b\) is the number of drill revolutions during the test.

The angle of rotation of the drill between blows is determined by the following formula:

\[ \alpha=\frac{n_b\cdot 360}{n_{\mathrm{ud}}}\ \text{deg/blow}, \]

where \(n_b\) is the number of drill revolutions per minute,

\(n_{\mathrm{ud}}\) is the number of drill blows per minute.

The relative error in determining the angle of rotation of the drill is

\[ \frac{\Delta \alpha}{\alpha} = \frac{\Delta n'_{\delta}}{n'_{\delta}} + \frac{\Delta t_{op}}{t_{op}} + \frac{\Delta n}{n}, \]

where \(\Delta n'_{\delta}\) is the absolute error in determining the number of revolutions of the drill during the experiment. We take \(\Delta n_{\delta} = 0.5\) revolution when determining the number of revolutions of the drill with a string, and \(\Delta n_{\delta} = 0.25\) when measuring with an instrument.

The magnitude of the relative error in determining the angle of rotation of the drill between blows can be found from Fig. 7.

Fig. 7. Dependence of the relative error in measuring the angle of rotation of the drill on the method of measurement and the duration of the experiment.
1 — measurement with an instrument with an accuracy of 0.25 revolution; 2 — measurement with a string with an accuracy of 0.5 revolution. \(a\) — drill rotation speed 300 rev/min; \(b\) — drill rotation speed 80 rev/min.

It is evident from the graph that the permissible relative error in determining the angle of rotation of the drill—5%—can be achieved with an experiment duration of 5–7 sec in the case of hammer operation at small feed forces. When the hammer operates with a considerable feed force, in order to obtain an accuracy of 5% in determining the angle of rotation of the drill, it is necessary to increase the duration of the experiment to 12 sec (when measuring \(n'_{\delta}\) with an instrument) and to 20 sec (when measuring \(n'_{\delta}\) with a string).

After the experiments have been carried out, the mean results are determined, the root-mean-square error and the coefficient of variation are calculated.

From the mean results obtained, graphs are constructed for the principal parameters of the hammer operating conditions as functions of the feed force, and the influence of the feed force is analyzed.

On the basis of this methodology, at the request of Glavuglemash and the Pneumatika plant, laboratory tests were carried out on drilling hammers of the types OM-506, PM-508, and RP-17 [3].

As a result of the investigations it was established that the feed force has a substantial influence on the drilling rate of pneumatic drilling hammers. With a change in feed force, the drilling rate, under otherwise identical conditions, changes by a factor of 2–3.

References

1. Sidorenko A. K. Automation of the feed of pneumatic drilling hammers. Mining Journal, No. 11, 1954.

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

3. Alimov O. D., Basov I. G., Gorbunov V. F. Laboratory investigations of the effect of feed force on the drilling rate with pneumatic drilling hammers OM-506, PM-508, and RP-17. Work report, 1955.