Methodology for Experimental Investigation of the Working Process of Pneumatic Drilling Hammers

O. D. Alimov, V. F. Gorbunov

For the design of high-performance pneumatic drilling hammers, it is necessary to know the theory of the internal working process of the machines themselves and of the process of rock destruction. Meanwhile, despite the relative simplicity of the design of these machines and the long period of their use in mining, neither a sufficiently substantiated theory of the working process nor methods for calculating their principal design parameters have yet been developed. This situation is partly explained by the fact that the processes occurring both in the pneumatic machine of percussive-rotary action itself and at the point of contact between the tool and the rock are in a complex interrelationship [1, 2]. To reveal the basic regularities of these processes, it is necessary to carry out thorough theoretical and experimental investigations.

The main factors affecting the rate of drilling of rocks by the percussive-rotary method are the energy of a single blow, the frequency of blows, the angle of rotation of the drill between blows, and the feed force [1]. In turn, as parameters of the drilling hammer, these characteristics are interrelated and are derived from the cylinder diameter, the magnitude of the stroke of the piston-striker, the weight of the impacting parts, the pressure of the air supplied to the hammer, the air-distribution scheme, the design of the drill-rotation mechanism, the design and dimensions of the drilling tool, as well as from the mechanism of rock destruction and the method of removing drill cuttings from the borehole. To determine these dependencies in the experimental investigation of a drilling hammer of a given design, it is necessary, during the drilling process, to record the displacement of the piston-striker relative to the hammer body, the displacement of the hammer body and drill relative to the rock, the change in air pressure in the various cavities of the hammer, the speed of rotation of the drill, and, in a number of cases, the displacement of parts of the valve and rotation devices.

Below is given the methodology for investigating drilling hammers and a brief description of the apparatus developed for this purpose by the staff of the Department of Mining Machines and Mine Transport of the Tomsk Polytechnic Institute. The basis of the experimental installation was a stand (Fig. 1) previously created for studying the operating modes of pneumatic drilling hammers [5].

When testing drilling hammers having a number of blows up to 2000 per minute, the displacement of the piston was recorded mechanically [3]. When testing high-frequency hammers with more than 2000 blows per minute, an electrical method of recording piston displacement was used. The advantage of the second method of recording is the possibility of simultaneous re-

Fig. 1. Diagram of the experimental installation:

1—drilling hammer; 2—guide frame; 3—pneumatic piston feeder;
4—rock; 5—amplifier for strain gauges; 6—eight-loop oscillograph;
7—sensor for recording the stroke of the hammer piston; 8—sensor for recording the vibration diagram of the hammer body; 9—sensor for recording the angular displacement of the drill steel; 10—pressure gauge; 11—pressure sensors in the hammer cavities; 12—pressure gauge for measuring the pressure in the feeder;
13—float-type flowmeter; 14—pressure gauge for measuring the air pressure in the mains.

cording on one film of the oscillograph of several processes. In developing the electrical method for recording piston motion, several different sensors were tested. Common to all methods of recording piston motion [3] is the presence of a thin steel rod connecting the sensor with the hammer piston. One end of the rod was fastened in the central hole of the piston (Fig. 2), and the other was led out of the hammer through the rear cover and used for attaching the sensor elements. After preliminary testing of electromagnetic, strain-gauge, and potentiometric sensors, the latter was selected for carrying out the main investigations, as the most accurate and the simplest to manufacture and calibrate. When the potentiometric method is used, a slider 2 (Fig. 2) is fastened to the end of rod 1 brought out of the hammer; to this slider is screwed a metal elastic plate with a silver contact riveted to it. Rod 1 moves together with the pis-

by the hammer piston 6 and is restrained against lateral vibrations by a seal in the rear cover of the hammer 3 and by a bronze guide bushing 4, fastened to a special frame 5 made of angle steel. The frame is attached to the rear cover of the hammer. A wire resistance made of nichrome wire 0.3 mm in diameter is also mounted on this frame. On the side surface of the resistance a track is ground for the movement of the sliding contact. The electrical circuit of the recording device (Fig. 2) includes the contact switch \(K\), an additional resistance \(R\), the storage battery \(E\), and the oscillator circuit of the oscillograph, \(r_{\mathrm{sh}}\) and \(R_{\mathrm{d.sh}}\).

In selecting the elements of the circuit we were guided mainly by one requirement—to preserve the linearity of the recording, i.e.,

Fig. 2. Sketch and electrical circuit of the device for recording the piston stroke of the PR-25 drilling hammer.

direct proportionality between the ordinate of the diagram and the actual magnitude of the piston stroke, which greatly facilitates the analysis of the diagrams and the determination of conversion coefficients. To preserve the linearity of the diagrams, it is necessary that the resistance of the loop circuit be much greater than the resistance of the potentiometer

\[ (R_{\mathrm{d.sh}} + r_{\mathrm{sh}} \gg R). \]

The current passing through the resistance can be determined from the formula

\[ i_{\mathrm{c}} = \frac{E}{ R_{\mathrm{d}} + r_2 + \dfrac{(R_{\mathrm{d.sh}} + r_{\mathrm{sh}})\,r_1} {R_{\mathrm{d.sh}} + r_{\mathrm{sh}} + r_1} }. \]

The additional resistance of the main circuit \(R_{\mathrm{d}}\) provides regulation of the circuit current, which is necessary when changing the supply voltage \(E\) and when replacing the vibrator.

The current passing through the vibrator (loop) of the oscillograph, determined by the formula

\[ i_{\mathrm{sh}} = \frac{i_{\mathrm{c}} r_1}{R_{\mathrm{d.sh}} + r_{\mathrm{sh}}}, \]

varies directly in proportion to the resistance \(r_1\). And since this resistance directly reflects the amount of displacement of the potentiometer contact connected with the hammer piston, the loop current is directly proportional to the magnitude of the piston stroke. The absolute magnitude of the loop current is selected according to its sensitivity, with the calculation that, when the piston is displaced by the maximum possible amount, the deflection of the loop beam will not exceed the permissible value (with respect to the width of the film and the amplitude of beam deflection).

Calibration of the recording device is carried out with the hammer switched off. For this purpose, a scale with divisions (millimeter paper or a ruler) is placed on the potentiometer; along it, the positions of the potentiometer contact and, consequently, of the hammer piston are marked every 10 mm, starting from the front extreme position. Each position of the piston corresponds to a definite deflection of the beam of the oscillograph loop, which is recorded on the photographic film. From the marks made on the photographic film, a calibration graph is constructed (Fig. 3).

Fig. 3. Approximate calibration graph of piston-stroke diagrams.

The error in determining the magnitude of the piston stroke from the oscillogram depends on the actual length of the stroke, the recording scale, the accuracy of measuring the value from the oscillogram, and the accuracy of the transmitting and recording instruments [6, 7]

\[ \frac{\Delta S}{S}=\frac{\Delta h}{h}+\frac{\Delta l}{l}+E_{\mathrm{p}}, \]

where \(S\) is the magnitude of the piston stroke, mm;

\(\Delta l\) is the magnitude of the play of the potentiometer contact; it is determined practically after carrying out a series of tests and usually does not exceed 1 mm;

\(l\) is the mean value of the travel of the contact; it is taken equal to the magnitude of the piston stroke;

\(\Delta h\) is the accuracy of determining the ordinate of the oscillogram; one may take \(\Delta h=0.5\) mm, although in practice, with sufficient enlargement of the oscillogram, the ordinate can be determined more accurately;

\(h\) is the mean value of the ordinate of the oscillogram; for the MPO-2 oscillograph one may take \(h=20\) mm;

\(E_{\mathrm{p}}\) is the error of the measuring instruments; \(E_{\mathrm{p}}=0.25\text{--}1\%\) [10].

For an average piston stroke length in high-frequency hammers equal to 32 mm, the relative error in determining the magnitude of the stroke is 0.06, or 6%.

Processing the piston-stroke curves is most conveniently carried out with the aid of a layout of the air-distribution diagram of the hammer under investigation [3, 4]. The layout is made to the same scale as that in which the piston stroke is recorded on the oscillograph film. It is most expedient to record the piston stroke on the oscillograph film at full size. Such recording can be made with the aid of LKhTI, 3SO-101, or 8SO-114 oscillographs.

When recording the piston stroke on photographic film 35 mm wide and processing the diagrams without enlargement, in order to obtain the actual picture of the piston stroke the ordinate value is multiplied by the recording scale \(m_s = \dfrac{S}{h}\), which is determined during calibration.

The duration of the piston-stroke cycle and of its individual elements is determined from the time marks applied to the oscillogram by the oscillograph vibrator every 0.002 sec. In this case the error in determining the cycle time is 0.001 sec, or 3–6% (for \(n = 1700\text{--}3500\) blows/min).

On this basis, the number of blows of the OM-506 hammer \((n = 1700\) blows/min) can be counted with an accuracy of \(\pm 50\) blows/min, and that of the T-10 hammer \((n = 3200\) blows/min) with an accuracy of \(\pm 170\) blows/min. With mechanical recording of the piston-stroke diagram, the error in determining the duration of one cycle was 9%.

The velocity of piston motion in various positions is determined by the formula

\[ V = m_v \operatorname{tg}\alpha, \]

where \(m_v = \dfrac{m_s}{m_t}\) is the velocity scale, m/sec;

\(\alpha\) is the angle of inclination of the tangent to the piston-stroke curve at the point of collision;

\(m_t\) is the time scale of the diagram, sec/m.

The speed of film advance should expediently be selected so as to obtain an inclination of the piston path curve at the point of its impact on the drill close to 45°. In this case the error in measuring the angle of inclination of the curve will be minimal [3]. The velocity scale factor is determined separately for each film advance speed.

The methods for determining impact energy, impact power, volumetric work of destruction, and related quantities were adopted in a form similar to those described earlier [3].

Impact energy

\[ A_y = \frac{m V_{\sigma}^{2}}{2}\ \text{kgm}. \]

Impact power

\[ N_y = \frac{A_y n}{60 \cdot 75} \]

and the volumetric work of destruction

\[ A_v = k \cdot \frac{N_y}{V_b}, \]

where
\(m\) is the mass of the piston, kg·sec\(^2\)/m;
\(n\) is the number of blows per minute;
\(V_b\) is the drilling rate, mm/min;
\(k\) is the coefficient for converting the dimension l. s./mm/min into kgm/cm\(^3\).

For a more complete study of the interaction between the piston and the body of the drilling hammer, diagrams of the piston stroke and of the displacement of the hammer body relative to the rock were recorded simultaneously. The oscillations of the body were recorded by a potentiometric sensor mounted on the guides of the feeder. The moving contact of the potentiometer was fastened to the feeder carriage in which the drilling hammer was fixed. The potentiometer was made of the same dimensions as that used for recording the piston stroke, and it was supplied from a separate accumulator battery in order to prevent the circuits from influencing one another.

The availability of superimposed diagrams of the piston stroke and of the oscillations of the hammer body makes it possible to determine most accurately the energy of the piston before its collision with the drill steel. In addition, it becomes possible to record the position of the hammer at the moment of the piston blow against the drill steel and during rotation of the drill steel.

Calibration of the device for recording the recoil diagram did not differ from calibration of the device recording the piston stroke. A significant drawback of the electrical sensor of body oscillations was that it was impossible to record recoil at low hammer feed forces, when the backward motion of the latter is considerable [4]. Moreover, during drilling of rock the hammer continuously moved forward, as a result of which recoil could be recorded only when it was switched on for a short time. Therefore, recording of recoil diagrams by the electrical method was carried out only under optimal drilling conditions, while in the course of investigating various conditions (to determine the optimal ones) recoil was recorded by a mechanical method [5].

The pressure diagrams were recorded with the aid of wire sensors, an amplifier, and a loop oscillograph. The wire sensor consists of a constantan wire 30 μ in diameter, glued to thin paper in the form of loops 10–20 mm long. Lead conductors made of foil are soldered to the ends of the wire. The length of the wire loops forms the base of the sen-

sensor. The sensitivity of the sensor to deformation is determined from the expression [8]

\[ k_s=\frac{\Delta R/R}{\Delta l/l} \]

or

\[ k_s=1+2\mu, \]

where
\(\Delta R/R\) is the relative resistance of the sensor;
\(\Delta l/l\) is the relative elongation;
\(\mu\) is Poisson’s ratio; for a constantan sensor \(\mu=1.9 \div 2.1\).

The resistance of the sensor depends on the length and number of loops. In our case sensors with \(R=200\) ohms were used.

Fig. 4. Diagram of the pressure sensor.

To measure pressure in the cavities of the drilling hammer, sensor 1 was glued to a rectangular steel membrane 2 with a thickness of 1.5 mm (Fig. 4), which is clamped by nut 3 into housing 4. A textolite panel with terminals 6 for leading out the wires from the working 1 and compensating 5 sensors and grounding terminal 7 was fastened to the housing with screws. For sealing, an elastic but sufficiently dense rubber 9 was placed between the housing and the nut; it transmitted the pressure of the compressed air to membrane 2 and protected it from the abrupt temperature changes occurring in the hammer cylinder. The space in the sensor housing, beneath the membrane, somewhat increases the volume of air in the cavity under investigation. When investigating the pressure in the cavities of the cylinders of drilling hammers, this increase

the volume does not exceed 1–2% and has no substantial effect on the hammer’s working process.

When strain-gauge sensors are used with a loop oscillograph, an intermediate device—an amplifier—is necessary. A class-3 loop oscillograph requires an input current on the order of 100–20 ma, while the change in the sensor current when it is deformed by 0.5–1% does not exceed 0.1–0.05 ma [8]. To preserve linearity, it is recommended to work at deformations on the order of 0.01–0.1%. Consequently, the required signal amplification is 10–20 thousand times.

A three-channel electronic device was used as the amplifier (Fig. 5), designed and fabricated at the Department of Theoretical Foundations of Radio Engineering of the Tomsk Polytechnic Institute. The device was based on the circuit of a single-channel amplifier of the IMASH, Academy of Sciences of the USSR [8]. Each amplifier channel includes a bridge powered by a generator with a current of 5000 cps. The bridge has two variable resistances: one for balancing the amplitude, and the other for balancing the phase. The measuring diagonal of the bridge is connected to the amplifier input. A “ring circuit” with selenium rectifiers, responding to changes in phase, was used as the rectifier, making it possible to determine the sign of the deformation.

The device operates from an alternating-current mains supply with its own stabilization of the rectified voltage.

Two sensors are connected into the bridge of each channel, one of which is active—for measurements—and the second is a temperature compensator. The two other arms of the bridge may also be made up of sensors. In this case they are glued to a cantilever beam bent by a micrometer screw. The amplifier output is adapted for connection to cathode-ray and loop oscillographs.

As recording instruments, a three-loop oscillograph manufactured by Siemens & Halske was used (paper travel speed up to 1.1 m/sec), as well as an eight-loop oscillograph MPO-2 (film travel speed up to 5 m/sec) and an eight-loop oscillograph 8SO-114 (paper travel speed up to 14 m/sec).

Calibration of the pressure sensors was carried out statically by means of a calibration device consisting of a metal cylinder with openings for installing the sensor, a manometer, and an air-hose fitting. The calibration process proceeded as follows. Compressed air at various pressures was supplied to the cylinder, and the deflection of the loop beam from the zero mark was recorded on film by short-duration recording. After one sensor had been calibrated, a similar operation was carried out with another, with the zero mark remaining in the same place and the first sensor remaining connected. After calibration, a calibration graph was constructed, according to which in the subsequent—

by which the ordinates of the oscillograms were determined. The calibration process was carried out before and after each series of experiments.

Analysis of the pressure diagrams was carried out simultaneously with processing of the piston-stroke diagram. The magnitudes of the air pressures

Fig. 5. Circuit of a three-channel amplifier for wire transducers:

$C_1 = 600$ micromicrofarads; $C_2 = C_{15} = 5$ microfarads; $C_3 = 2000$ micromicrofarads; $C_4 = C_6 = C_{10} = C_{13} = 200$ micromicrofarads; $C_5 = C_7 = C_9 = C_{18} = C_{16} = 5000$ micromicrofarads; $C_8 = C_{11} = C_{17} = C_{20} = C_{21} = C_{22} = C_{24} = C_{12} = 20.0$ microfarads; $C_{19} = 250$ micromicrofarads; $C_{23} = 0.5$ microfarads.

$R_1 = R_3 = R_6 = 5$ kilohms; $R_2 = 2.5$ kilohms; $R_4 = R_{29} = R_{30} = 100$ ohms; $R_5 = 0.24$ megohms; $R_7 = R_9 = R_{11} = R_{14} = R_{17} = 1.5$ megohms; $R_8 = R_{13} = R_{26} = 0.3$ megohms; $R_{10} = R_{12} = 2$ megohms; $R_{16} = R_{21} = R_{22} = R_{23} = 1$ kilohm; $R_{15} = R_{20} = 1$ megohm; $R_{18} = 1$ kilohm; $R_{19} = 30$ kilohms; $R_{24} = 500$ ohms; $R_{25} = 600$ ohms; $R_{27} = 15$ kilohms; $R_{28} = 25$ kilohms; $R_{31} = 0.2$ megohms; $R_{32} = 0.5$ megohms.

$L_1 = 6Zh7$; $L_2 = 6Zh7$; $L_3 = 6S5$; $L_4 = 6S5$; $L_5 = 6N8S$; $L_6 = 6P3$; $L_7 = 5Ts4S$; $L_8 = 6Zh7$; $L_9 = SG-4S$; $L_{10} = 6P3S$; $S$ -- selenium rectifier; PVM -- input-bridge supply; PLK -- supply for the channel tubes.

in the front and rear cavities can be determined at any point of the oscillogram, and, consequently, at any position of the piston in the hammer cylinder. By dividing the entire length of the piston stroke into several sections, one can construct the resultant force diagram, from which it is easy to determine the resul-

tiring force acting on the piston at any moment of its stroke. Subsequently, using the formulas of theoretical mechanics, it is possible to determine the impulse of the force and the work expended on moving the piston over individual sections and over the entire cycle. This work is spent on overcoming friction during the motion of the piston and is transmitted to the drill in the form of impact and rotary impulses.

From the diagrams of pressure and piston travel one determines the maximum pressure in the cavities of the hammer, the pressure during exhaust, and the moment of valve transfer (cessation and beginning of admission in valveless hammers). Owing to the presence of time marks on the oscillograms, it is possible to measure

Fig. 6. Sensor of angular displacements of the drill.

the period of air admission, the period of its expansion, etc. The accuracy of determining the pressure value is characterized by the recording error of the electrical instruments and of the loop oscillograph.

To investigate the process of rotation of the drill when drilling rock, a sensor of angular displacements was manufactured and tested (Fig. 6). The sensor consisted of a sleeve 1 with a hexagonal opening corresponding to the dimensions of the drill, and a toothed wheel 2 pressed onto it. The toothed wheel had 72 teeth, between which gaskets of textolite were pressed in. The wheel was held against longitudinal displacements along the drill by the stops of bracket 3, on which an insulating panel 4 with spring contact 5 was fastened by screws. When the drill turned through every \(5^\circ\), the electrical circuit of the sensor closed and opened, sending an impulse to the loop of the oscillograph. Thus, on the oscillogram the angular displacement of the drill was marked every \(5^\circ\) (Fig. 7). It is obvious that such a sensor required no calibration, and the accuracy of its readings depended on careful manufacture.

... of its parts and good maintenance in order to preserve the reliability of the spring contact.

Simultaneous recording of the piston-stroke diagram, the angular displacements of the drill rod, and the recoil diagram (Fig. 7) made it possible to determine:

1) the position of the piston in the hammer cylinder, the position of the hammer body relative to the rock, and the piston velocity at the moment preceding rotation of the drill rod;

2) the magnitude of the angle of rotation of the drill rod per cycle and during the working and idle strokes of the piston;

3) the nonuniformity of rotation of the drill rod, etc.

For a complete analysis of the operation of the rotary device of a drilling hammer, it is also necessary to record the torque and the longitudinal oscillations of the drill rod when blows are delivered to it.

Fig. 7. Oscillogram of the working process of the PR-25 drilling hammer at an air pressure in the line of 4 atm and a feed force of 90 kg:

1—piston-stroke diagram; 2—diagrams of oscillations of the hammer body; 3—diagram of air pressure in the rear cavity of the hammer cylinder; 4—diagram of pressure in the front cavity; 5—diagrams of angular displacement of the drill rod; 6—time mark (ticks every 0.002 sec.)

The recording of the longitudinal displacements of the drill rod, in our opinion, can be carried out by means of a potentiometric sensor fastened to the drill rod itself. The contact of the potentiometer can be fastened to the rotary bushing of the hammer.

The recording of the torque developed by the hammer’s rotary device can be made with the same wire sensors glued directly to the drill rod or to the ratchet bushing of the rotary device, as was done by N. N. Esin [14] when optical sensors were used for these purposes.

The availability of the apparatus described makes it possible to investigate the characteristics and working process of pneumatic drilling and breaker hammers and, on this basis, to give recommendations for improving the designs of existing machines, as well as to obtain data for developing methods for their calculation.

References

1. O. D. Alimov, Investigation of the mechanism of rock destruction in percussion-rotary drilling and of the initial parameters of drilling hammers. Candidate’s dissertation, TPI, Tomsk, 1953.

2. O. D. Alimov, E. N. Zikeev, A. V. Engel. Laboratory and production tests of pneumatic drilling hammers. In: Calculations, design, and testing of mining machines, Collection of articles, vol. 2, Ugletekhizdat, 1955.

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

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

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

6. M. E. Blanter, Methodology for the investigation of metals and processing of experimental data. Metallurgizdat, 1952.

7. V. I. Soroko-Novitskii. Testing of tractor engines. Mashgiz, 1955.

8. N. A. Raevskii, Methods of experimental investigation of mechanical parameters of machines. Publishing House of the USSR Academy of Sciences, 1952.

9. N. N. Esin. Investigation of light drilling hammers. Candidate’s dissertation, ZSPhAN, Novosibirsk, 1952.

10. Phisics Scerving Stiense and Industry. 1956, vol. 3, No. 6, pp. 97—105.

Noted Misprints

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