Investigation of the internal process of drilling hammers with a butterfly valve
Abstract Generated abstract
This paper examines the working process of hand-held pneumatic drilling hammers with butterfly-valve air distribution, using the PR-25l and RH-754 models as comparative cases. Experiments on a test stand measured cylinder pressures, piston motion, feed force, air pressure, drilling speed, impact energy, blow frequency, and efficiency while drilling hard hornfels under varied operating regimes. The results show that feed force and line pressure strongly affect piston stroke, air-buffer behavior, impact energy, and productivity, with maximum performance occurring at optimum feed forces of about 110 to 120 kg at higher pressures. The PR-25l produced somewhat higher impact power and drilling speed, while the RH-754 showed slightly better overall efficiency and lower air consumption, suggesting that improvements to PR-25l machining and air-distribution details could enhance its characteristics.
Full Text
INVESTIGATION OF THE INTERNAL PROCESS OF DRILLING HAMMERS WITH A BUTTERFLY VALVE
V. F. Gorbunov
For the investigation of the internal process of hammers with a butterfly valve, hand-held pneumatic drilling hammers PR-25l (from the “Pnevmatika” plant) and RH-754 (Atlas-Copco, Sweden) were selected; these are hand-held machines intended for operation with pneumatic supports. In these hammers the butterfly valve is located directly behind the cylinder, which shortens the air-supply channels in comparison with their length, for example, in hammers with a flanged spool valve. The drill-rotation mechanism is located in the front part of the hammer and is of the type of rotation mechanisms without a separate helical rod. The splines on the piston rod and in the ratchet-bushing cage are made semicircular. The ratchet pawls are arranged on two sides of the ratchet bushing; moreover, in the RH-754 hammer two pawls operate through 0.5 tooth, while in the PR-25l hammer four pawls operate through 0.25 tooth. The differences in the designs of the hammers consist mainly of the following.
-
The PR-25l and RH-754 hammers have cylinder diameters of 85 and 75 mm, design piston strokes of 51 and 60 mm, and piston weights of 2.2 and 1.6 kg.
-
The PR-25l hammer has a lubrication reservoir of larger volume.
-
The distribution of blowing air and water for flushing is carried out by a separate cock located in the rear head in the RH-754 hammer, and in a break in the supply hose in the PR-25 hammer (of the type of the OM-506 blowing device). There are also other minor differences.
Figure 1 gives the air-distribution diagram of the PR-25 hammer and the theoretical diagram of piston travel (close to the diagram for the hammer running idle). The points on the diagram indicate the characteristic positions of the piston-striker in the cylinder.
hammer. Using the procedure we have developed for processing the piston-stroke diagram [1, 3], it is easy to analyze the interaction of the air-distribution mechanism and the percussion unit of the hammer. Therefore we do not give a detailed description of the operating principle of the PR-25 and RH-754 drilling hammers.
The test stand used in investigating the working process of the RH-754 and PR-25l drilling hammers consisted of a pneumatic piston feeder on a bracing screw column, a compressor,
Fig. 1. Air-distribution diagram of the PR-25l hammer.
rock, and recording instruments [3]. In the course of the experiments the following quantities were recorded: the air pressure at the hammer, the pressure in the feeder, the drilling time, the drilled depth of the hole, the piston-stroke diagram of the hammer, and diagrams of the air pressure in the cavities of the hammer cylinder (as a function of time). In addition, for individual operating conditions, diagrams of oscillations of the hammer body and a diagram of angular displacements of the drill were recorded. The diagrams were recorded on an 8SO—114 loop oscillograph.
Both drilling hammers were tested under the same conditions, on the same test stand. Drilling was carried out in hornfels with hardness \(f = 14—18\) according to the scale of Prof. M. M. Protodyakonov. The drill used was made of hexagonal steel \(D = 25\) mm, leng-
1.2 m long, the diameter of the single-chisel bit was 40 mm, the diameter of the air hose 25 mm, and its length 5 m. Drilling was carried out without flushing and without intensive blowing of the borehole.
Results of the investigation
It is known that the operation of a pneumatic drilling hammer with a tool differs sharply from its operation at idle [1, 2, 4]. Depending on the operating conditions (air pressure, feed force), the hammer changes its characteristic and productivity. The best operating indices of the hammer are achieved at a certain optimum feed force, which depends on the air pressure in the network and on the type of hammer.
The investigation of the working process of the PR-25L and RH-754 hammers was carried out under various operating conditions—at air pressures of 4.5 and 6 ati and feed forces from 45 kg to the maximum. In processing the experimental results, the following indices were determined: the magnitude and character of the change in air pressure in the cavities of the hammer cylinder; piston stroke; magnitudes of the air buffers in the cylinder; duration of the piston-stroke cycle (number of blows); ratio of the durations of the return and forward strokes of the piston; piston velocity before impact on the drill steel; energy of a single blow; impact power of the hammer; drilling speed; volumetric work of destruction of the rock; efficiency of the hammer, and other quantities.
Fig. 2. Oscillogram of the air pressure in the cylinder of the RH-754 hammer:
a—front cavity; b—rear cavity.
The change in air pressure in the cavities of the RH-754 hammer over a cycle occurs differently depending on the hammer operating condition. During idle operation, the pressure in the rear cavity (Fig. 2, b, curve 1), in terms of its maximum value, is equal to or somewhat exceeds the network pressure \(p_0\) and decreases as the piston moves forward. At the moment when air is exhausted from the rear cavity, the pressure is \(0.5 \div 0.6\,p_0\). When operating at optimal-
regimes, or regimes close to them, the value of the maximum pressure in the rear cavity is somewhat less than the line pressure, while at the moment of exhaust it is \(0.65\text{–}0.7\,p_0\), i.e., higher than during idle operation of the hammer (Fig. 2, b, curve 2). This evidently occurs because of the difference in piston velocities. During idle operation of the hammer, the piston approaches the exhaust port with greater speed, as a result of which the pressure drop in the rear cavity occurs more rapidly than under optimal regimes.
The pressure in the front cavity of the hammer cylinder, \(p_2\), changes still more sharply (Fig. 2, a). During idle operation, the energy of the piston, stored during the forward stroke, is expended in compressing the air buffer, the pressure in which reaches 11–14 atm with an air pressure in the line of 3–6 atm.
Fig. 3. Change in piston stroke as a function of drilling regimes.
During drilling, the forward stroke of the piston is limited by its impact on the drill bit; therefore the compression of the front buffer decreases as the axial thrust on the hammer increases (the feed force). At small feed forces, pressure jumps of up to 6–8 atm are still observed on the pressure diagram (curve 2, Fig. 2, a), while in the region of optimal forces the peak is completely smoothed out (curve 3). The maximum value of the pressure \(p_2\) at optimal feed forces does not exceed 0.7 of the line pressure, i.e., 2.8, 3.5, and 4.2 atm at line pressures of 4, 5, and 6 atm, respectively.
The character and magnitude of the air pressure in the cavities of the PR-25l hammer are almost entirely similar to the corresponding values for the RH-754 hammer. The differences consist only in the fact that the air-pressure diagrams in the PR-25l hammer are characterized by a sharper indication of the moments of exhaust and air admission, and also by somewhat elevated values of compression of the front buffer.
The piston strokes of the RH-754 and PR-25l hammers are approximately equal, despite the fact that the useful length of the cylinder of the PR-25l hammer is 9 mm shorter than that of the RH-754 hammer. The dependence of piston stroke on feed force and air pressure is shown in Fig. 3. With an increase of the air pressure in the line by 1 atm,
and with a constant feed force the piston stroke of the PR-25l hammer increases by 4–5 mm, while that of the RH-754 hammer increases by only 3–3.5 mm.
The feed force also has a substantial effect on the piston stroke. A change of 10 kg causes a change in the piston stroke of the PR-25l hammer by an average of 1.8 mm, and of the RH-754 hammer by an average of 1.0 mm. Consequently, external conditions affect the operation of the PR-25l hammer to a greater degree than the operation of the RH-754 hammer.
It is interesting to note that the change in the magnitude of the piston stroke as a function of operating conditions is even sharper in hammers with the usual blow frequency. For example, the change in the piston stroke of the OM-506l hammer [1] as a function of pressure occurred at a rate of 4–7 mm/ati, and as a function of feed force—4.5 mm per 10 kg. For the T-10 hammer, which has rotation of the drill steel during the working stroke of the piston, these quantities were respectively 2–3 mm/ati and 0.4–0.7 mm/10 kg, i.e., much smaller than in other hammers [4].
When the feed force and air pressure are varied, the piston stroke of the PR-25l and RH-754 hammers changes respectively from 25 to 45 mm and from 27 to 42 mm. However, when hammers operate with feed forces greatly exceeding the optimum, cycles with a piston stroke of 18–20 mm are encountered. In this case, no air exhaust from the front chamber through the exhaust port occurs, and the impact energy in this cycle is very small. Cycles with a return stroke of the piston stretched out in time are also encountered. This occurs as a result of the heavy load on the rotation system. During the return stroke, the piston encounters considerable resistance when its helical splines run up onto the projections of the rotation sleeve, as a result of which its motion is slowed. Similar phenomena were noted by us in the investigation of other hammers [1–2].
Let us consider the change in the piston stroke under optimum drilling conditions (Table 1). With a change in air pressure—
Table 1
| Hammer type | Air pressure, ati | Optimum feed force, kg | Piston stroke corresponding to \(F_{\mathrm{opt}}\), mm | Impact energy corresponding to \(F_{\mathrm{opt}}\), mm | Size of rear air buffer, mm | Size of front air buffer, mm |
|---|---|---|---|---|---|---|
| PR-25l | 4 | 85 | 32 | 3.2 | 6 | 13 |
| PR-25l | 5 | 100 | 33 | 4.3 | 6 | 12 |
| PR-25l | 6 | 120 | 34 | 5.8 | 6 | 11 |
| RH-754 | 4 | 75 | 32 | 3.1 | 13 | 15 |
| RH-754 | 5 | 90 | 35 | 4.1 | 11 | 14 |
| RH-754 | 6 | 110 | 37 | 5.3 | 10 | 13 |
by 1 atm, but with the optimum regime maintained, the stroke of the piston of the PR-25 hammer changes by 1 mm. The stroke of the piston of the PR-25l hammer under optimum drilling regimes changes owing to a decrease in the front buffer, whereas in the RH-754 hammer the rear buffer also changes, its value being almost twice as large as in the PR-25l hammer.
Changes in the feed force from zero to the maximum at one and the same pressure cause, above all, a decrease in the front air buffer. In the range of small feed forces the value of the rear buffer changes hardly at all. At optimum and near-optimum feed forces the magnitudes of both the rear and the front air buffers change simultaneously. In the range of large feed forces
Fig. 4. Change in the impact energy of hammers as a function of drilling regimes.
the front buffer remains almost unchanged, owing to the very small displacement of the hammer body from the limiting collar of the bit \[1, 2, 4\], while the shortening of the piston stroke, in view of the great resistance to rotation of the bit, leads to an increase in the rear air buffer.
A change in the length of the piston stroke of the hammers leads to a change in impact energy. When the PR-25l hammer operates under different regimes, the impact energy varies within the limits 2.2–5.9 kgm, and for the RH-754 hammer from 1.9 to 5.3 kgm. The character of the change in impact energy as the feed force is increased from zero to the maximum (Fig. 4) is the same for both hammers. In the range of small feed forces the impact energy increases as a result of a decrease in the degree of compression of the front buffer. This range of feed forces may be called “transitional,” in the sense of the transition from the idle operating regime of the hammer to drilling at the most productive optimum regime.
At optimum feed forces the impact energy reaches a maximum. The length of piston stroke corresponding to the maximum
impact energy, and was called by us “optimal.” With a further increase in the feed force, the impact energy decreases as a result of shortening of the piston stroke at the expense of its active part. An increase in air pressure by 1 ati causes an increase in impact energy of 1–1.5 kgm (Table 1), with a more rapid increase being observed when the pressure changes from 5 to 6 ati than from 4 to 5 ati.
In view of the change in the magnitude of the piston stroke at the same air pressure, the frequency of impacts naturally also changes. When the feed force is increased by 10 kg, the number of blows of the hammers PR-25l and RH-754 increases by 35–40 blows/min. The number of blows corresponding to the optimal feed forces was obtained when the working process was studied as identical for both types of hammers, namely—2400, 2600, 2800 blows/min at air pressures of 4, 5, and 6 ati, respectively. The maximum number of blows was obtained at an air pressure of 6 ati and a feed force of 186 kg and is, for the PR-25l hammer, 3000 blows/min, and for the RH-754 hammer, 3050 blows/min, which corresponds to the data of the Atlas-Copco firm [5].
Fig. 5. Change in impact power as a function of drilling conditions.
In accordance with the change in impact energy and the number of blows when the feed force is varied, the power of the rock drill is also variable (Fig. 5). The change in impact power of the hammers at one and the same pressure amounts to from 0.5 to 1.1 h.p.; a change in pressure by 1 ati also causes a change in impact power by 0.5–1.1 h.p., with the larger value referring to the optimal conditions. The absolute value of the impact power of the PR-25l hammer is somewhat greater than that of the RH-754 hammer, owing to the greater energy of a single blow acquired by the piston as a result of the increase in cylinder diameter. The maximum value of the impact power of the PR-25l hammer—3.6 h.p.—was obtained at an air pressure of 6 ati and a feed force of 124 kg. In the hammer
...the RH-754 at the same air pressure, the impact power does not exceed 3.4 hp (with a feed force of 110 kg).
The productivity of drilling hammers, at the same number of blows, depends on the magnitude of the impact energy. In view of this, the drilling speed of the PR-25l hammer is somewhat higher than that of the RH-754 hammer [6]. A comparison of the efficiency of utilization of the impact power of the hammers in terms of the volumetric work of destruction shows that the expenditures of developed power for destroying a unit volume of rock are the same for both types of hammers; moreover, the minimum volumetric work of destruction ranges within 25–30 kgm/cm³. If, however, the hammer operates under non-optimal conditions, the volumetric work of destruction increases two or more times; for example, when the PR-25l hammer operates at an air pressure of 6 atm and a feed force of 60 kg, the volumetric work of destruction of hornfels is 54 kgm/cm³.
However, the total energy expenditures for the operation of the drilling hammer and the destruction of rock in the PR-25l and RH-754 hammers are not quite the same. In the PR-25l hammer the air consumption is, on average, 0.4 m³/min greater than in the RH-754 hammer (Table 2). The impact power of the PR-25l hammer increases to a lesser degree, as a result of which the efficiency of the RH-754 hammer is 1–1.5% higher.
Table 2
| Indicators | Hammer type | Hammer type |
|---|---|---|
| Indicators | RH-754 | PR-25l |
| Weight of hammer, kg | 22.5 | 23.8 |
| Cylinder diameter, mm | 75 | 85 |
| Piston weight, kg | 1.6 | 2.2 |
| Air consumption, m³/min | 3.2 | 3.6 |
| Impact energy, kgm | 4.1 | 4.3 |
| Number of blows, blows/min | 2600 | 2600 |
| Piston stroke, mm | 35 | 33 |
| Impact power, hp | 2.3 | 2.5 |
| Optimum feed force, kg | 90 | 100 |
| Drilling speed in hornfels \((d_k = 40\ \text{mm})\), mm/min | 300 | 330 |
| Volumetric work of destruction, kgm/cm³ | 28 | 27 |
| Specific air consumption, m³/running m | 10.6 | 10.9 |
Analysis of the indicator and force diagrams of the working cycle of the hammers at optimum feed forces and an air pressure in the network of 5 atm (Fig. 6) shows that the indicated efficiencies of the PR-25l and RH-754 hammers do not differ appreciably (Table 3). However, because of the somewhat poorer mechanical efficiency of the PR-25l hammer, its overall efficiency is lower than the efficiency of the RH-754 hammer [7]. The character of the force diagrams of the working and return strokes of the piston is approximately the same for both hammers owing to the identity of the design of the impact assemblies and of the air-distribution systems. When the hammers operate under optimum conditions, at the end of the working stroke of the piston there is always...
Table 3
| Indicators | Hammer type | Hammer type |
|---|---|---|
| PR-25l | RH-754 | |
| Energy consumed per cycle, kgm | 32 | 28 |
| Indicated work of the cycle, kgm | 7.2 | 6.6 |
| Impact energy, kgm | 4.3 | 4.1 |
| Effective efficiency, % | 13.4 | 14.6 |
| Indicated efficiency, % | 22.0 | 23.0 |
| Mechanical efficiency, % | 60 | 63 |
| Mean indicated pressure, ati | 3.2 | 3.6 |
the force pressing the piston against the drill shank, as a result of which, after collision with the drill, the piston stops, and in
Fig. 6. Indicator and force diagrams of hammers at 5 ati and operation under optimal conditions.
connection with this the initial speed of its return stroke is equal to zero. A similar phenomenon is observed in all drilling hammers having valve and spool air distribution [8].
In analyzing the force diagrams of the working stroke it was found that the mean indicated pressure during the working stroke of the piston in the PR-25l hammer is somewhat lower than in the RH-754 hammer (Table 3). Evidently, the air-distribution mechanism of the PR-25l hammer can be somewhat improved by adjusting the dimensions of the valve and the air-supply passages. Some improvement in the technical characteristics of the PR-25l hammer can also be achieved by more careful manufacture of its parts, in particular the starting device, the piston-striker, and the elements of the rotary mechanism.
CONCLUSIONS
-
The PR-25l hammer has somewhat greater power in comparison with the RH-754 hammer, owing to which its productivity is higher.
-
In the PR-25l hammer the cylinder cavity is used better as a result of the shortened length of the rear air cushion.
-
Air distribution by means of a butterfly valve ensures the supply of air into the rear cavity of the cylinder at a pressure almost equal to the network pressure, as a result of which, over a comparatively short piston stroke, the impact energy is equal to or greater than that of hammers with a lower blow frequency.
-
For effective use of the RH-754 and PR-25l hammers it is necessary to provide them with reliable supporting-feeding devices that ensure a feed force on the hammer of up to 110–120 kg.
-
Further improvement of the technical characteristics of the PR-25l hammer can be achieved by improving the quality of machining of its parts.
LITERATURE
-
O. D. Alimov, V. F. Gorbunov. Some results of the study of the OM-506 pneumatic drilling hammer. Izvestiya TPI, vol. 88, Tomsk, 1956.
-
O. D. Alimov, I. G. Basov, V. F. Gorbunov. Investigation of the influence of feed force and air pressure on drilling speed and on the working process of the RP-17 pneumatic drilling hammer. Izvestiya TPI, vol. 88, Tomsk, 1956.
-
O. D. Alimov, V. F. Gorbunov. Methodology for experimental investigation of the working process of pneumatic drilling hammers. Izvestiya TPI, vol. 106, Metallurgizdat, 1958.
-
O. D. Alimov, V. F. Gorbunov. Investigation of the working process of a high-frequency drilling hammer with spool-less air distribution. Izvestiya TPI, vol. 106, Metallurgizdat, 1958.
-
Air leg—mounted Drill. “Colliery Engineering,” No. 1, 1954.
-
I. G. Basov. Investigation of the influence of feed force on drilling speed and the operating regime of pneumatic drilling hammers. Candidate dissertation, Tomsk, TPI, 1958.
-
O. D. Alimov, V. F. Gorbunov. On the coefficient of efficiency of pneumatic drilling hammers. Article in the present collection.
-
V. F. Gorbunov. The working process of drilling hammers with an annular spool valve. Article in the present collection.