Investigation of the Operating Process of a High-Frequency Drilling Hammer with Valveless Air Distribution

O. D. Alimov, V. F. Gorbunov

In recent years the mining industry has received for operation new drilling hammers having an increased frequency of blows.

To obtain objective data on the advantages of high-frequency hammers over ordinary ones, comparative tests were carried out on drilling hammers of various designs [1]. As a result of the tests it was found that, among the high-frequency drilling hammers of foreign firms, the greatest drilling rate can be provided by the T-10 drilling hammer. This same hammer is also the most economical in air consumption. In contrast to other designs, the T-10 hammer is valveless, i.e., it has no separate valve mechanism for air distribution.

The design of the hammer also contains other original devices, which to some extent influence the quality of operation of the hammer. As a result of the tests carried out, a number of other questions arose as well, the resolution of which proved impossible without investigating the kinematics and dynamics of the processes occurring in pneumatic drilling hammers. The present work presents the results of an investigation of the operating process of the T-10 drilling hammer.

The T-10 hammer is intended for drilling horizontal and slightly inclined blastholes with pneumatic support, and also for drilling vertical blastholes with the use of a special device included in the machine set. To supply compressed air to the telescopic feed cylinder, there is a special channel 17 in the shank part of the hammer (Fig. 1). The body of the hammer (Fig. 1) consists of four principal parts: cast-iron cylinder 1, aluminum housing of the starting and air-distribution devices 2, steel holder of the rotary coupling 3, and steel rear cover 4. Bronze bushings 12 are installed at the joints of the rubbing parts. The striking unit of the hammer is made very carefully; the piston is polished. The design of the piston-striker is somewhat unusual. The piston ...

has a large diameter (100 mm) and a small height (17 mm). The rod 5 of the piston has straight splines for transmitting rotation to the rotary bushing 6, while the piston shank has a left-hand helical groove with a pitch of 900 mm and ports for air distribution.

During operation of the hammer, the piston is centered in the cylinder by means of the rod and the collars of the shank.

Unlike the existing standard operating scheme of rotary devices, rotation of the drill is effected during the forward (working) stroke of the piston. The rotation mechanism consists of a bimetallic ratchet bush 10, the inner part of which is made of bronze and the outer part of steel; ratchet pawls 11; a left-hand helical groove with a pitch of 900 mm on the piston shank; and a rotary coupling 6 made of high-speed steel. The piston rod is connected to the rotary coupling by means of a bronze splined bushing 19. The dimensions of the drill shank are 22 × 108 mm. The air distribution of the T-10 hammer is accomplished by connecting the air-distribution channels with ports 7 and 8 in the piston shank (Fig. 2, a).

Let us consider the operating principle of the T-10 hammer. To do this, we shall construct a diagram of the motion of the piston over one cycle and mark on this diagram the most characteristic positions of the piston in the hammer cylinder.

Fig. 1. T-10 drilling hammer of the “Tametall” firm.

Working stroke of the piston (forward stroke)

Position 1 is the extreme rear position of the piston, possible when the hammer is started (point 1 on the theoretical diagram of piston travel, Fig. 2, b). In this case compressed air from the line enters the rear cavity of the hammer cylinder. The front cavity of the cylinder is connected with the atmosphere. Owing to the pressure difference in the rear and front cavities of the cylinder, the piston moves forward.

Position 2—the beginning of the closing of channel 1 by collar 6. From this position, the cross-section of the inlet port begins to decrease (point 2).

Position 3—collar 6 has closed channel 1; the supply of compressed air to the rear cavity of the cylinder has ceased (point 3). The front edge of the piston begins to cover the exhaust port. The piston moves due to the expansion of the air cut off in the rear cavity.

Fig. 2. Diagram of the air distribution of the hammer with theoretical and working diagrams of piston travel.

Position 4—exhaust port 2 is closed by piston 11. In this position (point 4), the exhaust of air from the front cavity to the atmosphere ceases and compression of the remaining volume begins. In the rear cavity, the expansion of air continues.

Position 5—groove of piston shank 8 connects channels 1 and 3. During the working stroke of the piston in this position, air begins to be supplied to the front cavity of the cylinder (point 5) through channel 1, groove 8, channel 3, and port 4. As a result, the pressure in the rear and front cavities begins to equalize. At this moment it is most expedient for the piston rod and the drill shank to collide. In the T-10 hammer this occurs if the 108 mm shank is fully inserted into the hole of the rotating bushing. If there is no collision of the rod with the drill, the piston continues to move forward.

Position 6—the rear edge of piston 11 is at the level of the rear edge of exhaust port 2. Exhaust of compressed air from the rear cavity of the cylinder begins (point 6). The front edge of the piston head begins to cover port 4, reducing its cross-section and the supply of air into the front cavity.

Position 7—piston 11 has covered port 4; the shoulder of piston shank 5 has covered channel 3. Access of air from the mains to the front cavity ceases (point 7). The air enclosed in the remaining volume of the front cavity begins to play the role of an air buffer absorbing the kinetic energy of the piston.

Position 8—the extreme forward position of the piston in the hammer cylinder (point 8), possible when starting the hammer. Compressed air can enter the front cavity of the cylinder only through channel 13 and the lubrication channels, which, in our opinion, ensures withdrawal of the piston from the front “dead” position.

Return stroke of the piston

During the return stroke, the piston in position 7 opens access for air from the mains through channel 3 and port 4 into the front cavity of the cylinder (point 7′). From this moment the cross-section of intake port 4 begins to increase and exhaust port 2 begins to be covered. With further backward motion, the piston completely covers exhaust port 2 (point 6′), as a result of which the exhaust of air from the rear cavity of the cylinder ceases.

In position 5, the shoulder of piston 6 covers intake port 1 (point 5′). The supply of air into the front cavity of the cylinder ceases, and the further motion of the piston takes place due to the excess pressure of the compressed air in the front cavity. In the rear cavity of the hammer, at this moment a compression process takes place.

In position 4, air is exhausted from the front cavity to the atmosphere through port 2 (point 4′). The further motion of the piston takes place only due to the kinetic energy remaining in it. The degree of air compression in the rear cavity continues to increase.

In position 3, the rear cavity of the cylinder, through recess 7 in the piston shank, is connected with the compressed-air mains (point 3′). From this moment the backward motion of the piston is sharply slowed.

In position 2, exhaust port 2 is fully opened (point 2′). Position 1 is impossible during operation of the hammer.

The theoretically possible (design) value of the stroke of the piston of hammer T-10 is 51 mm. In this case, admission of air into the rear cavity of the hammer during the working stroke can be accompli-

... attains a maximum over a piston-stroke length equal to 20 mm. In practice, because a certain volume of air is present in the rear cavity of the cylinder, the length of the filling stroke will be smaller and can be determined experimentally. During the return stroke, admission of compressed air into the front cavity can take place over a length of up to 10 mm, and in practice is limited by the point at which the piston strikes the drill and by position 5 (Fig. 2, c).

The air-distribution design of the T-10 hammer, unlike other arrangements, does not permit air to pass directly from the mains to the atmosphere through the inlet ports and the cylinder. Kinetic energy is imparted to the piston both by supplying air from the mains into the cylinder cavities and by expansion of the cut-off air. Obviously, the air distribution of the T-10 hammer should be very economical in operation.

The starting device of the T-10 hammer consists of cock 13 (Fig. 1), with a handle and a detent for its positions. The cock can be set in eight positions, of which the principal ones are:

1) the hammer is switched off; only one passage, 13 (Fig. 2), is open for supplying air to the lubricating pocket and for moving the piston out of the front “dead” position;

2) collaring or operation at partial hammer power; passage 1 is opened to an incomplete cross-section for supplying air to the distributing device; passage 9 supplies air to the rear cavity of the cylinder to move the piston out of the rear “dead” position; passage 12 supplies air to the telescopic device;

3) normal operating position; passages 1, 12, 13 and passage 14 are open, the latter supplying air to tail cavity 10 for blowing out the borehole;

4) operation at partial power with intensive blowing of the borehole by compressed air; passages 1 (to an incomplete cross-section), 12, 13 and 14 are open.

The T-10 hammer can operate with blowing and with flushing, when water or air is supplied through a separate hose. In this case a fitting is screwed into opening 18 (Fig. 1), through which water or air enters central tube 9 and then the drill hole. When operating with flushing, rear cover 4 is recommended to be set according to the marks “vesi,” and when operating with blowing, according to “ilma.”

When the hammer is operated with blowing only from the main hose, it is recommended to remove the central tube from the hammer. This reduces the resistance to the movement of air through the axial opening of the piston.

Having considered the design and operating principle of the T-10 hammer, the following main features may be noted:

1) the hammer can be used both as a hand-held and as a telescopic hammer;

2) the design of the air distribution is based on valveless air distribution with cut-off of the compressed air both during the forward and during the return stroke of the piston;

3) the hammer’s rotation mechanism provides rotation of the drill bit during the forward (working) stroke of the piston;

4) alloys of nonferrous and ferrous metals are widely used in the design, including light alloys for the parts of the hammer body.

Results of the Investigation of the Operating Process

The investigation of the operating process of the T-10 hammer was carried out in the laboratory of the Department of Mining Machines and Mine Transport of TPI. As the mounting fixture, a stand for studying the influence of feed force on drilling speed [3] was used, with the addition of recording instruments. The investigation was conducted while drilling rock with a hardness of \(f = 14 \div 18\) on Prof. M. M. Protodyakonov’s scale, according to the method described in this collection [2]. In the course of the investigation, diagrams were recorded of the piston stroke, recoil, pressure in the front and rear cavities of the cylinder, as well as a diagram of the angular displacements of the drill bit. In analyzing the diagrams of the operating process, the following hammer parameters were determined: drilling speed, number of blows, magnitude of the piston stroke, impact energy, impact power, air pressure in the various cavities of the hammer at different piston positions in the cylinder, speed of piston motion at characteristic points, angle of rotation of the drill bit per blow and during the cycle, etc. The diagrams of the operating process were recorded under different operating conditions of the hammer, i.e., during idle running and during drilling with various feed forces.

Fig. 3. Pressure oscillogram.

The change in air pressure in the hammer cavities during operation under optimal conditions is shown in Fig. 3. The maximum value of the air pressure in the rear cavity of the hammer depends on the network pressure and does not depend on the feed force. At a compressed-air network pressure of 6 ati, the maximum pressure in the rear cavity is 4.0–4.3 ati, or 67–72% of the network pressure. Approximately the same pressure ratio is maintained when the hammer operates at 4 and 5 ati. In this case the maximum pressure...

the pressure in the rear chamber is, respectively, 2.7–2.9 atm gauge and 3.4–3.6 atm gauge. In the general case, for the given experimental conditions one may approximately take

\[ p_{1\max}=0.7p_0\ \text{atm gauge}, \]

where \(p_0\) is the compressed-air pressure in the network, atm gauge.

Fig. 4. Oscillograms of the working process:
\(a\)—at an air pressure of 4 atm gauge; \(b\)—at an air pressure of 5 atm gauge; \(c\)—at an air pressure of 6 atm gauge; \(d\)—idle operation at 4 atm gauge; \(e\)—diagram of piston travel and angular displacements of the drill at 4 atm gauge and optimum feed force; \(f\)—diagram of piston travel and oscillations of the hammer body at optimum feed force.

The pressure drop in the supply hose is 0.2–0.4 atm gauge for an air pressure of 4–6 atm gauge, i.e., only 5–7%. The difference between the values of \(p_{1\max}\) and \(p_0\), in our opinion, is explained by insufficient filling of the rear chamber of the hammer due to

short duration of the intake and the large cylinder diameter, which is characteristic of this scheme.

In Fig. 4, a, b, c, pressure diagrams in the hammer cavities under optimal drilling regimes are presented. The maximum values of the pressures \(p_1\) and \(p_2\) are approximately equal, but in pulse area they are clearly different. When the feed force is reduced, idle-stroke cycles appear, when the maximum pressure in the front cavity increases sharply (Fig. 4, d). At line pressures of 4, 5, and 6 ati and idle operation of the hammer, the maximum pressure in the front cavity is respectively equal to 10, 12.5, and 15 ati.

The magnitude of the piston stroke of the T-10 hammer when drilling hornstone with different feed forces, but at one and the same air pressure in the line, changes somewhat (Fig. 5). A decrease

Fig. 5. Dependence of the piston stroke (a) and rebound of the hammer body (b) on feed force and air pressure.

of the piston stroke with increasing feed force, in our view, occurs for two reasons:

1) with increasing feed force, the average rebound of the hammer body from the limiting collar of the drill decreases; consequently, the drill shank enters more deeply into the hammer’s turning coupling, and the piston strikes it closer to point 5 (Fig. 2, b);

2) with increasing feed force, the magnitude of the maximum pressure in the front cavity of the hammer and the magnitude of the force impulse during the reverse stroke of the piston decrease.

The stroke magnitude obtained during drilling fluctuates from 27 to 35 mm, and during idle operation of the hammer from 37 to 38 mm. When the air pressure is increased by 1 ati, the piston stroke increases on average by 2–3 mm, which amounts to 6–10%. A similar regularity was also observed in other hammers [5, 6]. It is interesting to note that the piston stroke magnitude of the T-10 hammer, corresponding to optimal drilling regimes, increases with an increase in air pressure (see the table and Fig. 7), whereas for the RP-17 and OM-506 hammers it changes almost not at all [5, 6]. Evidently, this is characteristic of hammers in which the rotation

the bit occurs during the direct (working) stroke of the piston, while during the return stroke the piston moves freely. In this case, when the air pressure in the line is increased, the pressure in the front cavity rises, and consequently so does the magnitude of the piston stroke. At the same time the cycle time decreases, as a result of which the piston velocity increases over individual portions and, in particular, at the moment before collision with the bit. The maximum velocity during the direct stroke is acquired by the piston approximately at point 5 (Fig. 2). In practice, collision of the piston with the bit during operation of the hammer in all regimes occurs later, i.e., after point 5, when the piston velocity begins to decrease. In view of this, the kinetic energy of the piston before collision with the bit is somewhat less than the maximum possible.

Table

The maximum impact energy was obtained at definite feed forces for each air pressure (Fig. 6). These

Fig. 6. Dependence of impact energy on feed force.

forces are usually somewhat smaller than the forces that ensure the maximum drilling rate [5, 6], but sometimes they also coincide with them. When the hammer operates with forces much smaller than the optimum feed force, collision of the piston with the bit occurs at points 6, 7 (Fig. 2), where the piston velocity decreases considerably because of compression of the air in the front cavity of the hammer. Consequently, the kinetic energy also decreases.

energy of the piston. In the case of operation at high feed forces, the blow occurs closer to point 5 (Fig. 2), but the velocity acquired by the piston decreases because of the greater expenditure of energy on turning the drill bit, since in this case the withdrawal of the body of the hammer from the limiting collar of the drill bit is small (Fig. 5).

Fig. 7. Dependence of impact energy on the magnitude of piston stroke.

The dependence of the impact energy on the magnitude of the piston stroke when operating under different regimes is given in Fig. 7. The transition to an elevated air pressure with a corresponding increase in the feed force up to the optimum value is accompanied by a lengthening of the piston stroke and, consequently, by an increase in the active part of the working stroke. At the same time, the velocity of the piston before its collision with the drill bit, and the impact energy, increase.

Fig. 8. Dependence of the number of blows on the feed force.

The number of blows of the hammer (Fig. 8) increases with increasing feed force. At one and the same air pressure, the fluctuation in the frequency of blows is about 3% and is explained by a change in the

the magnitude of the piston stroke. A decrease in the stroke (with an increase in the feed force) leads to a reduction in the duration of the forward and return strokes of the piston, and consequently also in the time of the complete cycle. It is interesting to note that the ratio of the times of the return and forward strokes of the piston under different operating regimes remains constant and equal to 1.2, which was not observed in hammers of the RP-17 and OM-506 types [5, 6].

The pressure of the compressed air has a great influence on the blow frequency. When the pressure is increased by 1 atm, the number of blows per minute rises by 150–200, which amounts to 5–7%. At the same time, the blow energy increases by 30–35%, and the impact power of the hammer by 35–42%.

The character of the change in impact power is similar to the change in blow energy (Fig. 9). The maximum values of impact power

Fig. 9. Dependence of impact power on feed force.

at 4, 5, and 6 atm are, respectively, 2.1, 2.9, and 3.8 hp. Per unit of power of the T-10 drilling hammer there is (at \(p_0 = 5\) atm) about 10 kg of weight, whereas for the OM-506 hammer this figure is almost 20 kg/hp. This indicates the clear advantage of high-frequency hammers over ordinary ones in power at the same weight.

It is interesting to note that fluctuations in impact power with changes in feed force for the T-10 hammer amount to 20–30%, while the drilling speed changes by a factor of 2–3. The latter is explained by the fact that, when the feed force changes, both the internal process of the hammer and the conditions of energy transfer for the destruction of the rock change.

The volumetric work of destruction (Fig. 10) has a minimum value at the optimum feed forces, when the magnitude of the rebound of the hammer body approaches zero [5]. In this case the most favorable conditions are created for transferring energy from the piston to the drill bit and from the bit to the rock being broken. The minimum value of the volumetric work of destruction for hornfels is about 25 kgm/cm\(^3\).

The regularity of the change in the angle of rotation of the drill bit per blow was established in laboratory and industrial investigations

(Fig. 11) and, in general, does not differ from similar relationships for other drilling hammers. In the present work we attempted to determine not the average angle of rotation of the drill, but the change in the rotational speed of the drill during the cycle. To determine this characteristic, diagrams of the angular displacements of the drill were recorded

Fig. 10. Dependence of the volumetric work of destruction on the feed force.

at an increased film-pulling speed (Fig. 4, д). Analysis of the diagram shows that, for the given design of the rotary device and at considerable feed forces, the drill rotates

Fig. 11. Dependence of the drill rotation angle on the feed force.

unevenly, and the slowing of rotation coincides in time with the extreme rear position of the piston. The nonuniformity of rotation increases with an increase in the feed force. During the forward stroke of the piston, the drill begins to rotate near point 4 (Fig. 2, б), when the piston takes up the clearances in the joints of the parts of the rotary mechanism. At this moment the piston velocity is already (when operating under optimum conditions)

80% of the velocity of collision with the drill, and the kinetic energy of the piston is equal to 67% of the impact energy.

In absolute magnitude, the velocity of the piston before the beginning of the drill rotation is 3.8, 4.4, and 5.1 m/sec for air pressures of 4, 5, and 6 atm. After imparting a rotational impulse to the drill, the piston travels a distance of about 20 mm. Over this section of the path the drill turns through 10–15° (which is 40–45% of the angle of rotation per cycle) and continues to rotate by inertia. The theoretically calculated angle of rotation of the drill over the piston-stroke length of 20 mm is only 8°. This proves that the energy for rotation of the drill is transmitted by a short-duration impulse. The rotation of the drill continues both during the impact of the piston upon it and during the return (idle) stroke of the piston. A similar phenomenon is also observed in the rotation devices of other hammers [7, 8], in which the impulse for rotation of the drill is transmitted during the return stroke of the piston, and the drill continues rotating during the forward stroke. Between individual turns the drill has stops lasting 4–6 msec (milliseconds), which amounts to 20–30% of the cycle time.

We assume that less energy will be required for rotation of the drill during the forward stroke than during the return stroke, since in the first case, at the moment when the rotational impulse is imparted to the drill, the hammer body moves backward (Fig. 4, g) and, as a result, a clearance is produced between the body and the drill collar. When, however, the drill is turned during the return stroke of the piston, the impulse for rotation of the drill is transmitted during the period when the hammer body is moving toward the face and, consequently, while the drill is being pressed against the bottom of the blasthole. Obviously, in this case more energy is required for turning the drill. Rotation of the drill during the forward stroke of the piston should also have a favorable effect on the process of rock destruction, since during impact the drill performs complex motions—translational, necessary for penetration, and rotational, facilitating chipping. However, for an exhaustive explanation of this question, special experiments are required with a study of the physics of the rock-destruction process. It should be noted that rotation of the drill during the forward stroke of the piston cannot have a favorable effect on the stability of operation of the drilling hammer when drilling fractured rocks. A characteristic feature of the T-10 hammer is that, in the event of the drill jamming in the blasthole, the impact energy decreases sharply and the hammer switches to an idle-stroke mode with a piston stroke length of about 17 mm (sufficient to produce exhaust from the cylinder cavities). In this case the drill may not rotate at all because of the presence of some play in the parts of the rotation device.

It must be noted that valveless drilling hammers had been developed earlier as well [9], but they did not give the proper result and were replaced by hammers with spool and valve air-

air-distribution mechanisms. In our opinion, this is explained by the fact that at that time the main direction was the creation of drilling hammers with a small piston diameter, while the required impact energy was achieved by means of a relatively greater piston stroke. Thus, for example, in hammers of the JB-5, OM-506, and RV-300 types, which until recent years had been considered the most advanced, the piston stroke was 50–56 mm, and the ratio of the piston stroke to the cylinder diameter was 0.8–0.85. With these parameters and with rotation of the piston on the return stroke, valveless hammers operated less steadily. The effort to increase drilling speed by raising the frequency of impacts without increasing the energy of each impact led to the creation of high-frequency hammers with a short piston stroke (40–30 mm) and an increased diameter (the ratio of the piston stroke to the diameter in the T-10 hammer is 0.32). This, in our opinion, made it possible successfully to apply valveless schemes.

CONCLUSIONS

1. In foreign practice, the T-10 hammer is one of the most powerful hand-held drilling hammers with a low ratio of weight to developed impact power. These high performance indices are achieved, on the one hand, by a considerable increase in the piston diameter and impact frequency, and, on the other hand, through the use of high-quality materials, including light alloys.

2. Unlike the overwhelming majority of modern drilling hammers, the T-10 hammer has a valveless air-distribution scheme with the use of air expansion in the working process. This ensures a lower specific air consumption and, consequently, greater economy.

3. The cross-section of the channels of the existing T-10 distribution device does not ensure sufficiently complete utilization of the cylinder during the working stroke of the piston. By increasing the cross-section of the channels and the time of air admission in T-10-type hammers, the impact power can be further increased without changing the basic design dimensions.

4. Rotation of the piston on the forward stroke, in combination with a valveless scheme and a short piston stroke in high-frequency drilling hammers, does not impair their operating qualities.

5. The optimum feed forces for the T-10 hammer, owing to the special features of its design, greatly exceed the feed forces for hand-held drilling hammers; as a result, a hammer of this design can be operated more successfully as a column-mounted or telescopic hammer.

6. Taking into account the positive qualities of the basic scheme of the T-10 hammer, it is expedient, on its basis and with due regard for the comments made [1], to create hand-held, column-mounted, and telescopic

drilling hammers. Hand-held hammers made according to this scheme, in comparison with T-10 hammers, should have a smaller optimal feed force even in the case of some reduction in hammer power. In designing column-type drilling hammers, however, it is necessary to increase the impact energy by 1.5–2 times while maintaining the same impact frequency.

REFERENCES

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

2. O. D. Alimov, V. F. Gorbunov. Methodology for the experimental investigation of the working process of pneumatic drilling hammers. Article in the present collection.

3. O. D. Alimov, I. G. Basov, V. F. Gorbunov. Methodology of laboratory investigation of the influence of feed force on the drilling speed of pneumatic drilling hammers. Izvestiya TPI, vol. 83, Tomsk, 1956.

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

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

6. O. D. Alimov, I. G. Basov, V. F. Gorbunov. Investigation of the influence of feed force and air pressure on the drilling speed and on the working process of the RP-17 pneumatic drilling hammer. Izvestiya TPI, vol. 88, Tomsk, 1956.

7. P. M. Alabuzhev, O. D. Alimov. The influence of feed force and the angle of rotation of the drill between impacts on the speed of percussive-rotary drilling of blast holes. Izvestiya TPI, vol. 75, Tomsk, 1954.

8. O. D. Alimov. On the calculation and design of drill-rotation mechanisms of pneumatic drilling hammers. Izvestiya TPI, vol. 78, Tomsk, 1955.

9. F. Shchipansky. Drifters with automatic resetting. Russian-German Bulletin of Science and Technology, No. 1, 1935.

Noted Misprints

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