OPERATING PROCESS OF DRILLING HAMMERS WITH AN ANNULAR SPOOL VALVE

V. F. Gorbunov

The drilling hammers with an annular spool valve adopted for investigation of the operating process—PM-508, PR-20, and Bergmeister (BGM)—are hand-held types, but in weight and power they differ considerably from one another (Table 1).

Table 1

The drill-rotation mechanism of the PM-508 and PR-20 hammers has a separate helicoidal rod; the ratchet bushing is located behind the body of the annular spool valve. The BGM hammer is made according to a different structural scheme. Its spiral thread is made on the piston rod; therefore the ratchet bushing is brought out into the front part of the hammer. The use of the PM-508 hammer is expedient with a pneumatic support, as is also the case for the PR-20 and BGM hammers; however, it has no lugs on the body for fastening the fork of the pneumatic support. The BGM hammer may be used for drilling horizontal and upward-directed holes from a pneumatic support. In the latter case it is given a position almost coaxial with the pneumatic support, and the rod of the latter enters the recess of the rear head.

The investigation of the PM-508, PR-20, and BGM hammers was carried out on the test stand of the Tomsk Polytechnic Institute according to the method described by us earlier [1–2]. Drilling was performed in hornstone with hardness \(f = 14 \div 18\) on the scale of Prof. M. M. Protodyakonov. The bit diameter was taken as \(40\ \text{mm}\) and the drill-rod length as \(1.2\ \text{m}\); the air pressure was 4, 5, and 6 ati; the feed force ranged from \(20\ \text{kg}\) to the maximum.

Results of the Investigation

In investigating the PM-508, PR-20, and BGM hammers, the following quantities were determined: the pressure in the cylinder cavities, the piston stroke, the magnitude of the air cushions in the cylinder, the impact energy, the number of impacts, the impact power, the drilling speed, the air consumption, the volumetric work of rock destruction, the efficiency of the hammer, and other quantities. By analyzing oscillograms of the piston stroke \(S\) and the air pressures \(p_1\) and \(p_2\) in the cavities of the hammer, one can reveal the nature of the principal relationships in the working process of a drilling hammer of this design.

Let us consider the character of the change in air pressure in the cavities of the PR-20 hammer (Fig. 1). The pressure in the rear cavity of the hammer cylinder, \(p_1\), depends very little on the magnitude of the feed force. The pressure \(p_1\) reaches its maximum value at the moment corresponding to the extreme rear position of the piston in the hammer cylinder. During the forward motion of the piston, the pressure \(p_1\) decreases and, by the moment when air is exhausted to the atmosphere, reaches \(0.6\)–\(0.65\) of the line pressure. With further motion of the piston the pressure \(p_1\) drops sharply and, at the moment of impact of the piston against the drill rod, is equal to \(0.2\)–\(0.5\) of the line pressure (depending on the feed force). During the return motion of the piston, the pressure in the rear cavity begins to rise from the moment the exhaust port is covered by the piston. The magnitude of the maximum pressure is about \(90\%\) of the line pressure \(p_0\), and with an increase in the feed force there is a certain decrease in this value.

A sharper change in pressure is observed in the front cavity of the cylinder. At small feed forces, the pressure under the piston reaches 10–15 ati (Fig. 1, a). It is clear that this pressure is created by compression by the piston of the front cushion, on which a large part of the piston energy is expended. This occurs as a result of a large rebound of the hammer body from the limiting collar of the drill rod.

When the feed force is increased to the optimum, such pressure peaks \(p_2\) no longer occur (Fig. 1, b), and the maximum air pressure in the front cavity is equal to or slightly less than \(0.7 p_0\). The character of the change in pressure \(p_2\) with time becomes close to the character of the change in pressure \(p_1\). By the moment of impact of the piston on the drill rod, the magnitude of the pressure \(p_2\) usually does not exceed 1.5 ati and is somewhat less than the pressure in the rear cavity, as a result of which the transfer of the spool into the return-stroke position usually occurs after the collision of the piston with the drill rod. According to this

for the same reason, the initial velocity of the piston’s return motion is very small. For example, when the PR-20 hammer operates under optimal conditions, the piston covers the first 5 mm of the return stroke in 0.005–0.006 sec, which amounts to 20–25% of the cycle time. Obviously, there are possibilities for increasing the hammer’s impact frequency.

Fig. 1. Oscillograms of the operating process of the PR-20 drilling hammer at an air pressure of 5 atm and feed forces:
a—45 kg; b—85 kg; c—110 kg; time marks with a frequency of 500 cps.

The change in air pressure in the cavities of the BGM and PM-508 hammers occurs in essentially the same way as described above; therefore we shall not dwell on it in detail. It should only be noted that exceeding the optimal feed forces on the BGM hammer leads to a considerable reduction in piston stroke, as a result of which, in most cycles, the latter does not reach the drill shank and no impact occurs between them. When operating under such “heavy” conditions, exhaust of air from the rear...

the hammer cavity often does not occur at all, just as rotation of the drill also does not occur.

The magnitude of the piston stroke of the PR-20, PM-508, and BGM hammers varies with changes in feed force and air pressure (Fig. 2). In absolute value, the piston stroke of the PM-508 during drilling varies from 65 to 50 mm, which is 74–58% of the useful cylinder length. For the PR-20 and BGM hammers, the piston stroke during drilling varies respectively from 49 to 28 mm and from 31 to 13 mm, which is 83–47 and 74–31% of the useful cylinder length. The average reduction in piston stroke when the feed force is increased by 10 kg is, for the PM-508, PR-20, and BGM hammers, respectively, 2.5, 2.2, and 1.6 mm. An increase in air pressure by

Fig. 2. Change in piston stroke as a function of feed force and air pressure.

1 atm causes an increase in the piston stroke of the hammers by 2.5–4 mm, the larger value applying to hammers with a small piston stroke.

Comparison of the unit cycles of the piston stroke of the hammers when operating at optimal drilling conditions shows that, with transition to a higher air pressure and a corresponding increase in feed force, the piston stroke changes only insignificantly. For the PM-508 and PR-20 hammers, the change in piston stroke lies within the measurement error; for the BGM hammer it exceeds it somewhat [3]. The piston-stroke values of the hammers when operating at optimal conditions are given in Table 1.

Figure 3 gives a comparison of the unit cycles of the piston stroke of the hammers studied at an air pressure of 5 atm and optimal feed forces. According to the graph, the piston-stroke magnitudes of the PR-20 and BGM hammers differ by almost a factor of two, while the cycle time is approximately the same. This is a consequence of the large specific weight of the piston per unit area-

... the cross-sectional area of the cylinder of the BGM hammer (Table 2). In addition, the pressure in the front cavity of the BGM hammer increases slowly during the return stroke of the piston and acts for only a short interval of time, which also reduces the speed of the piston’s return stroke.

Table 2

The degree to which the design piston stroke is utilized in the hammers studied is also different. The ratio of the piston stroke obtained during drilling under optimum conditions, with an air pressure in the mains of 5 atm, to the design stroke is: for the PM-508 hammer, \(S/S_k = 0.59\); for the PR-20 hammer, 0.68; and for the BGM hammer, only 0.52. The better utilization of the cylinder length in the PR-20 hammer, as compared with the PM-508 and BGM hammers, is due to the reduction of the rear air cushion in the first hammer and to the relatively large front cushion in the BGM hammer.

Fig. 3. Comparison of single piston-stroke cycles of hammers at 5 atm and optimum feed forces.

The magnitude of the front cushion of hammers operating under optimum conditions is determined mainly by the ratio of the dimensions of the drill shank and other parts of the front portion of the hammer...

and for hammers PM-508, PR-20, and BGM amounts respectively to 20, 22, and 30% of the magnitude of the design stroke of the piston. The point of possible collision of the piston with the drill may correspond to different positions of the piston in the hammer cylinder. Thus, in the PM-508 hammer, with a drill shank length of 108 mm, the collision point corresponds to such a position in which the piston almost completely covers the front inlet port; in the PR-20 and BGM hammers it is at this moment covered only halfway. In our opinion, the covering of the inlet port in the PM-508 hammer adversely affects the duration of the reverse stroke of the piston; consequently, the ratio of the time of the reverse stroke to the time of the forward stroke of the piston in the PM-508 hammer is greater than in the PR-20 hammer.

Fig. 4. Change in impact energy depending on drilling regimes.

Changing the length of the piston stroke and the air pressure in the hammer cavities causes corresponding changes in the velocity of piston motion over the different portions of its stroke. In particular, this occurs at the moment of collision of the piston with the drill. The impact energy, as a function of the final velocity of motion of the piston and its mass, changes depending on the acting air pressure and the feed force (Fig. 4). At first, as the feed force increases, the energy rises, reaching a maximum value at feed forces somewhat smaller than the optimum ones. With further increase in the feed force, the impact energy decreases. A similar character of variation of impact energy as a function of feed force is observed in all hammers without exception and at all values of air pressure.

When the feed force is decreased, the piston stroke increases, while the piston energy decreases owing to the expenditure of kinetic energy on compressing the air cushion in the front cavity of the hammer cylinder. When the feed force is increased beyond the opti-

the maximum impact energy rapidly decreases due to a reduction in the active part of the piston stroke (from the rear end position to the opening of the exhaust port), and also due to deterioration of the exhaust of air from the front cavity of the cylinder, as a result of which the remaining air offers considerable counterpressure during the working stroke.

The value of the maximum impact energy, with a change in air pressure by 1 atm, changes in the investigated pressure range by an average of 40% (Table 3). At the same time, the piston stroke corresponding to the maximum impact energy in the PM-508 and PR-20 hammers changes hardly at all. In the BGM hammer, which has a very short stroke, an extension of the “optimum” piston stroke by 10–15% is observed with an increase in air pressure by 1 atm.

Table 3

The duration of the piston-stroke cycle of each hammer, as well as the magnitude of the stroke, depends on the air pressure and the feed force (Fig. 5). With increasing air pressure, the duration of one cycle decreases, and consequently the number of blows of the drilling hammer increases. According to the graph, the duration of the piston-stroke cycle of the PM-508 hammer is almost twice the cycle time of the PR-20 and BGM hammers. This is a consequence of the different ratio of the structural dimensions of the hammers (diameter and length of the cylinder, piston weight). The change in the number of blows of individual hammers with a change in air pressure occurs within a comparatively small range—7–10% per 1 atm. In considering the dependence of the duration of the piston-stroke cycle on the feed force, several different regions can be distinguished, in which the cycle time either increases or decreases. Let us consider, for example, the operation of the BGM hammer at an air pressure in the network of 6 atm.

During idle operation of the hammer, the cycle duration is 22 msec, which corresponds to 2730 blows per minute.

When drilling with feed forces of up to 30 kg, the cycle time increases to 23.6 msec (a reduction in the number of blows to 2540 per minute) because of the slowing of the piston return stroke as a result of less compression of the front air cushion and an increase in the reduced mass of the rotating parts. In the range of feed forces from 30 to 80 kg, the cycle duration decreases to 21.4 msec (\(n = 2800\) blows/min) as a result of a shortening of the piston stroke length. The resistance to the reverse motion of the piston under these conditions remains comparatively low, since the bit moves away from the bottom of the drill hole [4, 5]. With a further increase

Fig. 5. Change in the number of blows as a function of drilling conditions.

in the feed force, the cycle time increases somewhat because of the slowing of the piston return stroke. The shortening of the piston stroke can no longer compensate for the loss of time due to the speed of motion. Moreover, simultaneously with the slowing of the piston motion during its return stroke, there is a reduction in the mean speed of the working stroke owing to the shortening of the piston path at the portion where it had the maximum speed of motion. Finally, at very large feed forces for the BGM hammer, a reduction in the cycle time is again observed because the hammer changes over to an operating mode with a very short stroke, as described by us above.

For the PM-508 hammer, the character of the dependence of the number of blows on the feed force is similar to that described, except for the last portion, since the hammer cannot operate with a jammed drill. For the PR-20 hammer, a continuous decrease in cycle duration with increasing feed force is characteristic.

The power developed by the impact units of the hammers varies as a function of the feed force and air pressure as follows:

as well as the impact energy. When the feed force is increased up to a certain value, the hammer’s impact power increases, reaching a maximum at feed forces equal to the optimum, or slightly less. With a further increase in the feed force, the impact power decreases owing to a reduction in the impact energy. The maximum power values of the hammers investigated are given in Table 1.

It is of interest to compare the impact powers of the hammers and their own weights. Table 4 gives the values of the ratios of the weight of the drilling hammers to the powers developed by them at different air pressures.

From the data given in Tables 1 and 4, it is evident that the PR-20 hammer has the greatest absolute and relative power of the impact assembly. Per unit of its power there is 2.7–2.8 times less weight than in the PM-508 hammer, and 35–70% less than in the BGM hammer. Such relatively high indices of the PR-20 hammer can be explained by the successful choice of the ratio of piston weight, length of its stroke, and cylinder diameter (Table 2). In the BGM hammer the cylinder diameter is the same as in the PR-20 hammer; however, its impact energy is lower because of the small piston stroke, although the number of blows is almost the same owing to the greater piston weight.

Table 4

In accordance with the change in the impact power of the hammers, the drilling speed also changes. The character of the dependence of drilling speed on air pressure and feed force is examined in detail in the works of O. D. Alimov and I. G. Basov [4, 6], in which much attention is devoted to the study of this dependence. Therefore we shall not dwell on it here.

To determine the economy of operation of drilling hammers, the air consumption per minute was measured. At all pressures in the line, the air consumption of the PR-20 hammer is greater than that of the BGM and PM-508 hammers. This excess depends on the magnitude of the feed force, since the change in air consumption as a function of feed force is nonuniform for different hammers. In the PM-508 and PR-20 hammers, with an increase in feed force by 10 kg, the air consumption increases by only 0.03–0.05 m³/min, which amounts to 1–2% of the total consumption. In the BGM hammer this figure increases to 0.1–0.3 m³/min, or 3.5–4.5% per 10 kg of feed force. Po-

Consequently, if at low feed forces the BGM hammer consumes less air than the PM-508 and PR-20 hammers, then in the range of high feed forces its consumption is much greater than that of the PM-508 hammer and is equal to the air consumption of the PR-20 hammer (Fig. 6).

The power consumed by the hammers varies analogously to the air consumption; therefore, calculation of the consumed power is conveniently carried out by means of a nomogram constructed from the formulas for adiabatic compression of air in the compressor [3].

In accordance with the variation of drilling speed as a function of the feed force, the specific air consumption per linear meter of drilled blast-hole has a minimum at the optimum feed forces. When the operating conditions deviate from the optimum, the specific air consumption increases. The PR-20 hammer has the lowest specific air consumption per meter of blast-hole, equal to \(14\ \mathrm{m^3/m}\). For the BGM and PM-508 hammers the minimum specific air consumption is respectively equal to 18 and \(22\ \mathrm{m^3}\) per linear meter of blast-hole.

Fig. 6. Variation of consumed power as a function of drilling conditions.

The magnitude of the specific consumption very clearly demonstrates the influence of feed force on the efficiency of operation of drilling hammers. It should be noted, however, that by means of the specific air consumption we can evaluate the comparative economy of hammer operation only in one rock. For broader comparisons it is more convenient to use the efficiency coefficient of the hammers, calculated in accordance with the selected method [7].

The efficiency coefficients of the PR-20, BGM, and PM-508 hammers (Fig. 7) vary as a function of drilling conditions. The efficiency reaches its maximum value at feed forces close to the optimum. The PR-20 hammer has the highest efficiency, equal to 14–15%, whereas the PM-508 and BGM hammers have an efficiency not exceeding 10%.

The higher efficiency of the PR-20 hammer in comparison with the efficiency of the PM-508 and BGM hammers can be explained by a more successful choice of its design parameters. The comparatively low efficiency of the BGM hammer, which has an increased number of blows due to reduction of the piston stroke, in our opinion, can be signifi-

significantly, either by increasing the cylinder diameter or by reducing the weight of the piston. In the first case the impact energy and their frequency will be increased; in the second, only the number of impacts will rise. To increase the efficiency of the BGM hammer it is also necessary to change the position and dimensions of the exhaust port in order to ensure more economical operation of the air-distribution system.

Fig. 7. Change in hammer efficiency as a function of drilling conditions.

CONCLUSIONS

1. In the course of studying the drilling hammers PM-508, BGM, and PR-20, the nature of the change in the principal parameters of their working process was established as a function of the drilling conditions of the rock.

2. The feed force and the air pressure in the line are the most important factors determining the parameters of the working process of drilling hammers.

3. By selecting the appropriate feed forces it is possible to ensure such an operating regime of the drilling hammer in which maximum impact power and the best coefficient of efficiency are attained. The impact power of drilling hammers depends on the conditions of their operation and is the determining factor in increasing the drilling rate.

4. The increase in drilling rate with an increase in feed force is achieved through a change in the operating process of the hammer, as a result of which: (a) the number of strokes of the striker piston ending with its impact on the drill increases; (b) the energy of the piston before its collision with the drill (impact energy) increases;

c) the efficiency of energy transfer from the piston to the drill bit and from the bit to the destruction of the rock increases.

1. Air distribution by the annular flanged spool valve ensures a sufficiently high filling, with compressed air, of the cavities of drilling hammers. The improvement of the valve box carried out in the BGM hammer by eliminating the helical rod increases the degree of filling of the rear cavity of the cylinder with compressed air.

2. Among the tested types of drilling hammers with an annular spool valve, the PR-20 hammer should be recognized as the best; it has a higher efficiency and shows the greatest drilling speed.

3. The PR-20 drilling hammer possesses a better technical characteristic in comparison with the BGM and PM-508 hammers due to: a) a better ratio between the cylinder diameter and the piston-stroke length; b) the lower specific weight of the piston per unit area of the useful cross-section of the cylinder; c) good utilization of the cylinder volume owing to the reduction of air cushions.

4. The efficiency of the PR-20 drilling hammer is 6% higher than that of the PM-508 hammer, which is proportional to the increase in the number of blows, with the same basic arrangement of the hammers.

5. Increasing the cylinder diameter of the hammers causes a corresponding increase in the optimum feed force. As a result of the considerable optimum feed forces, operation of the PM-508, BGM, and PR-20 drilling hammers without feed devices is uneconomical. For their effective use it is necessary to work at the optimum feed forces indicated in Table 1.

6. The efficiency of drilling hammers varies depending on the feed force. The maximum values of efficiency were obtained at feed forces close to the optimum ones, and, for the given hammer design, do not depend on the air pressure (within the range from 4 to 6 atm gauge).

7. The design of the PM-508 hammer can be improved by: a) shortening the cylinder by reducing the front and rear air cushions, which will help increase the number of blows while preserving the energy of a single blow; b) reducing the weight of the piston, which will also increase the number of blows.

The design of the BGM hammer can be substantially improved by reducing the weight of the piston and by better placement of the air-inlet and exhaust ports.

References

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

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

2. V. F. Gorbunov. Experimental investigation of the operating process of pneumatic drilling hammers. Candidate dissertation, Tomsk, TPI, 1958.

3. I. G. Basov. Investigation of the influence of feed force on the drilling speed and operating regime of pneumatic drilling hammers. Candidate dissertation, Tomsk, TPI, 1958.

4. O. D. Alimov, P. M. Alabuzhev, V. F. Gorbunov, V. I. Kopytov. Investigation of the operating dynamics of the PM-508 drilling hammer. Article in the present collection.

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

6. O. D. Alimov, V. F. Gorbunov. On the efficiency coefficient of pneumatic drilling hammers. Article in the present collection.