EXPERIMENTAL STUDIES OF THE DYNAMICS OF THE OPERATION OF THE PM-508 PNEUMATIC DRILLING HAMMER

P. M. Alabuzhev, O. D. Alimov, V. F. Gorbunov, V. I. Kopytov

Studies of the operating regimes and working process of electric and pneumatic drilling hammers [1–3] revealed a number of questions that could not be answered without carrying out an experimental investigation of the dynamics of the operation of drilling hammers. In the present work an attempt is made to answer these questions, and also to explain the nature of the variation in the forces acting on the body of the hammer. This was achieved by simultaneously recording on photographic film (with the aid of appropriate pickups and a loop oscillograph) the displacements of the piston, the drill, and the body of the hammer; the changes in the force between the rod of the pneumatic feeder and the body of the hammer, as well as between the body of the hammer and the drill; and the air pressure in the front and rear cavities of the hammer cylinder.

The experiments were carried out while drilling holes in hornfels with hardness \(f = 14 \div 18\) on the scale of Prof. M. M. Protodyakonov.

Experimental setup and procedure for conducting the tests

The procedure described in [4] was taken as the basis for carrying out the experiments. The drilling hammer was mounted on the stand described in [5]. The feed force of the hammer was provided by a pneumatic feeder.

To record the motion of the piston in the hammer cylinder and the motion of the hammer body relative to the rock, rheostat-type pickups (potentiometric pickups [6]) were used. The potentiometric pickup for recording the piston motion was fastened to the hammer body by means of a special carriage, and the moving contact—to one end of a steel rod, which was passed through an opening in the rear cover and fastened by its other end in the piston.

of the hammer. The reciprocating motion of the piston was transmitted through a rod to the movable contact of a potentiometric sensor.

Thus, when the piston moved in the hammer cylinder, the ohmic resistance changed and, consequently, so did the magnitude of the current in the electrical circuit of the potentiometric sensor. The alternating component of the current taken from the sensor by means of a loop oscillograph was recorded on photographic film in the form of a curve representing, as a function of time, the reciprocating motion of the piston in the hammer cylinder.

The recording of the oscillatory motion of the housing was carried out analogously to the recording of the piston motion. In this case the rheostat was attached to the guides of the hammer feed, and the movable contact (slider) was connected to the hammer housing. With sufficiently rigid fastening of the guides, feed mechanisms, and the rock specimen, it may be considered that, when recording the displacement of the hammer housing relative to the sensor guides, we are recording the motion of the hammer housing relative to the rock.

The motion of the drill was also recorded with the aid of a potentiometric sensor. The movable contact of the sensor was attached directly to the drill. The rheostat was secured on a special bushing with a hexagonal opening corresponding to the size of the drill, so that the latter could move freely within the bushing. Special stops fastened to the sensor guides did not allow the bushing to move along the drill. At the same time, the bushing could undergo rotational motion together with the drill. Thus it was possible to record the motion of the drill relative to the rock being destroyed, since the hammer-feed guides and, consequently, the bushing stops were immobile relative to the rock.

The potentiometric sensor consists of a cylinder made of insulating material (ebonite, dry wood), on which wire is tightly wound. In our experiments, constantan wire of diameter 0.2 mm was used. As the experiments showed, this wire is sufficiently resistant to abrasion during the motion of a sliding contact over it.

Each rheostat-type sensor was powered by a separate storage battery in order to eliminate mutual influence between the electrical circuits of the loops.

The sensor for registering the forces arising between the hammer housing and the drill was a wire resistance (strain gauge) [4—6] glued to a metal beam bent in the form of an arc. One end of the beam was attached to the hammer housing, while the other rested against the collar of the drill. The drill shank was lengthened by an amount equal to the projection of the bent beam.

Thus, all forces arising between the hammer housing and the drill collar were transmitted through the beam, the deformation

which caused a change in the ohmic resistance of the bonded strain gauge, which was an active sensor connected into a bridge circuit [6]. Steel of grade ShKh15 was used as the material of the beam for bonding the strain gauge.

The sensor for recording the force between the rod of the pneumatic feed mechanism and the hammer body, which we shall agree to call the feed force, was likewise a wire resistance bonded to a cantilever beam. The deformation of the beam caused a change in the ohmic resistance of the bonded strain gauge, which was also the active sensor of a bridge circuit [6]. One end of the beam was rigidly fastened to the hammer body, while the free end rested against the rod of the pneumatic feed mechanism. Thus the force of the pneumatic feed mechanism on the hammer body was transmitted through the cantilever beam, which was made of ShKh15 steel.

To record pressure diagrams, strain gauges bonded to a membrane were also used, as described in [4]. The assembled pressure sensor was mounted on the hammer body, in which a hole 6–8 mm in diameter was drilled. The dimensions of the membrane and the sensor housing may be chosen differently depending on the sensor base [6]. We used strain gauges with a base of 25 mm and a resistance of 530 ohms.

To amplify the signals coming from the sensors during deformation of the corresponding beams in the course of hammer operation, a three-channel amplifier [4] was used. The processes were recorded on motion-picture film by means of an eight-loop MPO-2 oscillograph.

Calibration of the sensors

The potentiometric circuit for recording the motions of the piston, the hammer body, and the drill steel was arranged so that there was a directly proportional relation between the change in current in the loop circuit and the displacement of the piston (body, drill steel). On the oscillograms, the variable ordinate of the curve characterizing the motion of the piston in time determined, on a certain scale, the position of the piston relative to the hammer cylinder. To satisfy the condition of linear dependence, it is necessary that the ohmic resistance of the loop circuit be much greater than the resistance in the sensor circuit [4, 6].

Calibration of the potentiometric sensor was carried out as follows. Moving the slider along the rheostat through equal-length intervals, i.e., manually moving the piston (body, drill steel) from a certain definite position, the deflection of the loop line on the oscillograph screen was recorded for each piston position. Measuring the magnitude of the loop deflection and knowing the magnitude of the displacement of the piston (body, drill steel), a calibration graph was constructed (Fig. 1).

Calibration of the sensors for recording the feed force, the forces between the hammer housing and the rod of the pneumatic feeder, and the pressure sensors was carried out with the hammer not operating.

By setting a series of values of the feed force (from 15 to 155 kg in steps of 15.5 kg), deviations of the loop line from the zero mark were obtained on the oscilloscope screen; the zero mark corresponded to the case when the feed force was equal to zero. By setting a definite feed force and measuring the deviation of the loop line, a calibration

Fig. 1. Calibration graphs of the feed force (a), piston travel (b), housing and drill-steel displacement (c), and air pressure (d) in the hammer cylinder:
1—displacement of the drill steel; 2—displacement of the housing.

curve was constructed; using it, by knowing the ordinate of the diagram recorded on the film, one can determine the actual value of the feed force (Fig. 1). From the calibration graph it is seen that, with the circuit parameters adopted by us, the deviation of the loop line over the range of feed forces from 15 to 150 kg was directly proportional to the feed force. At higher feed forces the linearity was somewhat disturbed; however, in our experiments the maximum feed forces did not exceed 130 kg.

Calibration of the transducers, for the purpose of checking the stability of the readings, was carried out before and after each series of experiments. The linear dependence between the magnitude of the feed force and the ordinate of the oscillogram makes it possible to determine the numerical variation of the feed force during the cycle of motion of the body, and also to calculate the magnitudes of the impulses of the forces under consideration.

The feed force \((P_{\text{п}})\) of the pneumatic feeder, at a known pressure of the compressed air in the line, is calculated from the formula

\[ P_{\text{п}} = p_0 S \ \text{kg}, \]

where \(S\) is the cross-sectional area of the piston of the pneumatic feeder, \(\text{cm}^2\);

\(p_0\) is the pressure of the compressed air per unit area of the piston of the pneumatic feeder, \(\text{kg}/\text{cm}^2\).

The calibration of the pressure transducers has been described by us in detail earlier [4]. The maximum relative error obtained when recording the travel of the piston, the body of the hammer and the drill, as well as the forces between the rod of the pneumatic feeder and the body of the hammer, calculated by formulas [4, 6, 7], does not exceed 10–15%.

Results of Experimental Investigations

In Fig. 2 are given several combined oscillograms most characteristic of the operation of the hammer in the regimes investigated (air pressure in the line and feed force). Drilling was carried out at air pressures in the line of 4, 5 and 6 ati, since drilling hammers in quarries, mines and ore mines operate mainly at these air pressures. To simplify the analysis of the oscillograms, each of the curves (Fig. 2) has a definite designation.

From the oscillograms presented one can draw a number of conclusions on such questions as the relationship between the travel of the piston and the hammer body, the travel of the hammer body and the variation of the feed force, the relationship between the travel of the hammer body and the travel of the drill, and the travel of the drill and the variation of the force between the drill and the body as a function of the air pressure in the hammer cylinder.

Relationship of the Travel of the Piston and the Hammer Body During One Cycle

After the blow of the piston on the drill (Fig. 2, point \(e\)), the hammer body (curve II) continues to move somewhat toward the feeder and reaches its greatest deflection after 0.002–0.004 sec following the blow (which corresponds to 0.05–0.10 of the cycle time), i.e., already during the return stroke of the piston.

Speaking of the deflections of the body, we mean its displacements relative to the rock during the cycle, i.e., we consider the amplitude of oscillations.

During the return stroke of the piston, the body moves forward and reaches the greatest forward position (for the given cycle) at the extreme rear position of the piston in the hammer cylinder, or as the piston approaches this position. Such an interrelation between the motion of the hammer body and that of the piston is characteristic for all the investigated conditions (air pressures in the line of 4, 5, and 6 atm and hammer feed forces on the face from 30 to 90 kg). Thus, at the moment of impact of the piston on the drill, the hammer body is practically motionless. Consequently, the velocity of the piston, determined from the diagram of the displacement of the piston relative to the hammer body, at the moment of impact on the drill may be considered as determined relative to the guides of the feeder or to the rock being broken.

Fig. 2. Oscillograms of the travel of the piston (I), the hammer body (II), and the drill (III); variation of the force \(P\) between the rod of the pneumatic feeder and the hammer body; variation of the force \(Q\) between the hammer body and the drill; variation of the pressure in the rear \(p_1\) and front \(p_2\) cavities of the hammer cylinder:

\(A\)—small feed forces \(P_n\); \(B\)—optimal feed forces \(P_n\).

Interrelation between the travel of the hammer body and the variation of the force between the feeder rod over one cycle

When considering the interrelation between the travel of the hammer body and the variation of feed over one cycle, one should simultaneously consider also the position of the piston in the cylinder, since in the final sys—

the latter is the determining factor in the change of the force over a cycle relative to its mean value \((P_{\text{п}})\).

In analyzing the dynamics of hammer operation, three zones of drilling regimes should be distinguished, depending on the feed force at a given network pressure [2, 5]. The first zone—the feed force is less than the optimum. The second zone—the feed force is equal to the optimum value. The third zone—the feed force is greater than the optimum, all other conditions being equal.

1. At small feed forces \(P_{\text{п}}\) (Fig. 2, A), after the blow on the drill the piston begins to move backward with acceleration, turning the drill. The body, as established above, begins to move forward with some delay, as if moving away from the feed rod. As a result, the feed force (curve \(P\)) falls below the mean value \(P_{\text{п}}\), sometimes to zero. At a certain position of the piston in the cylinder, its velocity begins to decrease (inflection of curve \(l\) during the return stroke of the piston). As a result, the velocity of motion of the hammer body forward also begins to decrease, and the feed rod (weighing 6 kg) runs onto the body, which causes an increase in the feed force. Having reached the extreme rear position, the piston begins to move forward with acceleration, which causes a corresponding accelerated motion of the hammer body backward, and this, in turn, causes an increase in the force \(P\) (the body “runs onto” the feed rod). The curve of the change in feed force over the cycle of the body stroke is represented approximately by a sinusoid with a period equal to the cycle time.

2. In the second case, when the feed force \(P_{\text{п}}\) is equal to the optimum value, the magnitude of the force over a cycle has two unequal peaks (up to \(1.5 P_{\text{п}}\)) and two drops in feed force (down to \(0.5 P_{\text{п}}\)).

3. In the third case, when the feed force \(P_{\text{п}}\) is greater than the optimum, the force \(P\) over a cycle has two almost equal peaks and two drops. In this case the greatest deviation of the force \(P\) from the mean value varies within the limits \(0.3 \div 0.5 P_{\text{п}}\).

The change in feed force over the piston stroke cycle in the second and third zones of drilling regimes may be represented by a sinusoid with a period equal to half the cycle period, and by a zero line coinciding with the line \(P_{\text{п}}\).

Thus, with an increase in the mean feed force \((P_{\text{п}})\), the maximum deviations of the force from its nominal value decrease.

At feed forces \(P_{\text{п}}\) somewhat greater than the optimum, the influence of contact between the drill and the body on the change in feed force during the cycle becomes evident. After the piston strikes the drill (point \(d\), Fig. 2, Б) and the drill strikes the rock (point \(a\)), the latter rebounds from the rock by 2–2.5 mm and strikes the body (point \(b\)) at the moment when the piston is making its return stroke and begins to rotate the drill. At this time the hammer body, from its extreme rear position, begins to move forward, pressing the drill against the rock, as a result of which an additional resistance to rota-

…the drill. These resistance forces affect the speed of motion of the piston during the return stroke, which is expressed in a change in the smoothness of curve 1 of the piston stroke (section \(m—n\), Fig. 2 B)*. The speed of the piston in this section decreases, which causes a decrease in the speed of motion of the body. The decrease in the speed of motion of the hammer at this moment is also aided by the impact of the drill on the body after rebounding from the rock. As a result of a certain braking of the motion of the hammer body, the feeder rod “catches up” with the body and the feed force increases sharply—the first peak.

Subsequently the piston speed again increases, which causes an increase in the speed of motion of the body, which now “moves away” from the rod; as a result the force drops sharply to \(0.5 P_{\mathrm{p}}\) at the extreme forward position of the hammer body. The next backward motion of the hammer body again causes an increase in the feed force—the second peak. After the motion of the piston in the forward stroke, and consequently also of the body, becomes uniform, the feed force decreases. At the moment when the piston strikes the drill, the hammer body, having almost reached its extreme rear position, stops and the feed force becomes equal to \(P_{\mathrm{p}}\). After the hammer body stops, the feeder rod continues for some time to move backward by inertia, “moving away” from the body, which causes the feed force to fall below \(P_{\mathrm{p}}\), etc.

Interrelation of the stroke of the hammer body and the stroke of the drill

During operation of the hammer in almost all the investigated regimes (pressure in the line 4–6 atm and feed forces from 30 to 90 kg), the drill, after being struck by the piston, strikes the rock and rebounds from it. The magnitude of the rebound in our experiments varied from 2 mm (large feed forces) to 6–12 mm (small feed forces).

At small feed forces the hammer body, relative to the feeder guides, in addition to the principal oscillations with a frequency equal to the number of blows, performs additional oscillations with a period approximately 8–10 times greater than the period of the principal oscillations. The amplitudes of these additional oscillations reach 10–15 mm and more. This complex character of the oscillation of the hammer body during operation is noted in works [2, 3, 5], in which the maximum amplitudes of these additional oscillations are called the “recession of the body” from the rock being broken. The character of the drill motion depends on the magnitude of this recession of the body.

After the piston strikes the drill (point \(d\), Fig. 2, A), the latter moves toward the rock (section \(d—a\)) and strikes it (point \(a\)).

* This feature of the piston-stroke curve was first pointed out in work [3].

Having struck the rock, the drill bit rebounds from it and strikes the hammer casing (point \(b\)), rebounds from the casing, and again moves toward the rock.

If, in this case, the departure of the hammer casing from the rock is large, the drill bit stops at some distance from the rock (approximately \(3\)—\(5\) mm). The next blow of the piston is delivered to the drill bit, which in most cases, when drilling horizontal boreholes, is not in contact with the rock. If there is no departure of the casing from the limiting collar of the drill bit, then at the moment when the piston strikes it the drill bit is in contact with the rock. This case is repeated after \(3\)—\(10\) cycles (blows) [3].

At feed forces equal to the optimal and greater ones, the departure of the casing from the drill-bit collar is small. As a result, the character of the motion of the drill bit in the interval of time between piston blows on it will be different than at small feed forces. After a blow by the piston (point \(d\), Fig. 2, Б), the drill bit, with some velocity, strikes the rock (point \(a\)), rebounds from it (up to \(3\) mm), strikes the hammer casing (point \(b\)), rebounds from the casing, and again strikes the rock (point \(c\)). After this, the drill bit, having rebounded from the rock, stops at some distance from it (\(1\)—\(2\) mm).

At very large feed forces, by the moment the piston strikes the drill bit the latter is pressed against the rock with some force, since the amplitude of oscillation of the hammer casing is small and there is no departure of the casing from the rock [3]. Thus, at the moment the piston strikes the drill bit, it may be assumed that the velocity of the drill bit is zero.

Relationship between the travel of the drill bit and the change in force between the drill bit and the hammer casing

During the rebound of the drill bit from the rock, a fairly strong impact of the drill-bit collar against the hammer casing occurs. The collision of the drill bit with the casing occurs in most cases at the moment of the maximum departure of the drill bit from the face, as indicated by the sharp increase in force between the hammer casing and the drill-bit collar (curve \(Q\), Fig. 2).

At small feed forces, the impact of the drill bit against the hammer casing corresponds to the moment when the casing begins to move backward (Fig. 2, А).

At optimal and large feed forces, in addition to the first peak, corresponding to the “pure” blow against the hammer casing, a second appears, arising as a result of the “running-up” of the hammer casing onto the drill-bit collar and the opposing motion of the drill bit during its rebound from the rock (Fig. 2, Б).

CONCLUSIONS

In considering the question of the character of the motion of the drill bit during one cycle, as well as the regularities of the change in the feed force of the hammer on the face, three zones of drilling regimes should be distinguished, depending on the magnitude of the mean value of the feed force: ma-

[[unclear: beginning of line]] forces, optimal and close to them, and greater forces, when the hammer operates unstably.

At small feed forces, after the piston strikes the drill steel, the drill steel moves toward the rock and strikes it. It then rebounds from the rock and strikes the body of the hammer hard. After this the drill steel rebounds from the body and stops at some distance from the rock. If the magnitude of the body’s retreat from the face is large, the drill steel may not reach the body during its first rebound from the rock.

At optimal feed forces and higher (the second and third drilling zones), during one cycle of piston travel the drill steel makes two blows against the rock and two blows against the hammer body. The second blow of the drill steel against the body may also be absent; this depends on the conditions of rock destruction (removal of dust and rock grains from the hole).

At the moment when the piston strikes the drill steel, the latter in all three drilling zones is a free, practically motionless rod located at some distance from the rock or touching it. At the moment when the piston strikes the drill steel, the hammer body may be regarded as motionless relative to the rock being broken. Consequently, the velocity of the piston, determined from the diagram of its motion, at the moment of striking the drill steel may be considered as determined relative to the rock.

The character of the change in feed force over one cycle depends on the magnitude of the mean feed force of the hammer on the face and on the mass of the rod of the pneumatic feeder. At small feed forces \(P_{\text{п}}\), during the return stroke of the piston the feed force falls almost to zero. Then, during the forward stroke of the piston, the feed force increases in comparison with its mean value over the cycle (Fig. 2, \(A\)). The curve \(P\), characterizing the change in feed force, is almost a sinusoid of period equal to the period of the cycle. The area of the impulse over the cycle, bounded by the straight-line segment \(P_{\text{п}}\), is equal to the area of the impulse bounded by the line \(P\).

With an increase in the mean feed force in the second and third zones of drilling regimes, the character of the change in feed force over the cycle is somewhat different: first, the magnitude of the deviation of the feed force from the mean value decreases to \(\pm 0.5 P_{\text{п}}\); second, during one cycle the feed force twice reaches a certain maximum and twice a minimum (Fig. 2, \(Б\)). The curve \(P\) is a sinusoid of period equal to half the cycle time.

This phenomenon can be explained by the following factor. Considering the hammer body and the piston in it as a free system, we may, on the basis of the theorem of theoretical mechanics on conservation of the center of mass of a system, assert that the greater the displacements made by the piston in the hammer cylinder, the greater the displacements that must also be made by the hammer body. At optimal and greater feed forces, the displacements of the piston in the hammer cylinder will be the smallest. Therefore the dynamic

the interaction between the pneumatic feed piston rod and the hammer body is reduced. The effect of the blows of the drill against the hammer body when it rebounds from the rock promotes a more frequent change in the feed force during the cycle.

REFERENCES

1. P. M. Alabuzhev. Investigation of the working process of electropneumatic percussion machines. Doctoral dissertation, Tomsk, TPI, 1954.

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

3. O. D. Alimov, V. F. Gorbunov. Some results of research on the pneumatic drilling hammer OM-506. Proceedings of TPI, vol. 88, 1956.

4. O. D. Alimov, V. F. Gorbunov. Methodology for the experimental investigation of the working process of pneumatic drilling hammers. Proceedings of TPI, vol. 106, Metallurgizdat, Sverdlovsk, 1958.

5. O. D. Alimov, I. G. Basov, V. F. Gorbunov. Methodology for a laboratory study of the influence of feed force on the drilling speed with pneumatic drilling hammers. Proceedings of TPI, vol. 88, 1956.

6. I. A. Raevskii. Methods for the experimental investigation of the mechanical parameters of machines. Publishing House of the Academy of Sciences of the USSR, 1952.

7. K. P. Yakovlev. Mathematical processing of measurement results. GPTI, 1950.