On the Efficiency of Pneumatic Drilling Hammers
Abstract Generated abstract
This paper examines efficiency as a technical criterion for pneumatic drilling hammers, contrasting it with the more common use of drilling rate or air consumption in comparative testing. Using simultaneous laboratory recordings of piston displacement and air pressure in the cylinder cavities, the authors construct indicator and force diagrams to calculate indicated, mechanical, and effective efficiencies, and to analyze air use, impact energy, rotation work, and losses. Application to RH-754 and related hammer cycles suggests that shortening the air admission period can substantially increase indicated efficiency, although often at the cost of impact power, while cycles aimed at maximum power reduce efficiency. The study argues that hand-held hammers should prioritize specific power, but that valveless T-10 and T-10s designs show promising air-distribution schemes for developing more powerful and economical column-mounted hammers.
Full Text
ON THE EFFICIENCY OF PNEUMATIC DRILLING HAMMERS
O. D. Alimov, V. F. Gorbunov
The criterion for evaluating the technical data of pneumatic drilling hammers when carrying out comparative tests is, in most cases, the drilling rate. Sometimes, for comparison, the total air consumption by the pneumatic drilling hammer is used, and very rarely the efficiency. In determining the quality of electric impact machines, on the contrary, the efficiency of the machine is determined first of all. Comparisons of the efficiency of electric and pneumatic drilling hammers show that the latter are much worse in efficiency than electric ones. Therefore, at first glance it seems that work on improving pneumatic drilling hammers should be directed toward increasing their efficiency. However, carrying out such work is made difficult by the fact that, in industrial tests, it is impossible to determine the value of the efficiency of the hammers being tested. This is easier to accomplish under laboratory conditions.
Fig. 1. Oscillogram of piston motion and air pressure in the RH-754 hammer at 4 ati.
In an experimental investigation of the working process of pneumatic drilling hammers [1], we determined their efficiency by the following method. In the laboratory, while drilling rock, oscillograms of piston motion and press—
of the air pressure in the cylinder cavities. This makes it possible to construct indicator diagrams easily in the coordinates “air pressure—piston travel” (abbreviated \(p—S\)), i.e., diagrams of pressure along the path. Such diagrams make it possible to carry out a more detailed study of the design of the drilling hammer and to determine a number of parameters of its working process and, in particular, the mechanical and indicator efficiency of the hammer.
Let us consider the method of constructing and processing diagrams, using as an example the diagram of the RH-754 hammer at an air pressure in the network of 4 ati and at the optimum feed force. The diagram of piston travel and air pressure versus time (Fig. 1) was divided into a number of periods, the boundaries of which were marked by vertical lines. The magnitude of the piston travel corresponding to each of the boundary lines was the abscissa of the indicator diagram, while the air pressure in the rear \(p_1\) and front \(p_2\) cavities of the cylinder was the ordinate of the indicator diagram of the corresponding cavity (Fig. 2).
Fig. 2. Indicator diagrams of the RH-754 hammer.
The area of the diagram \(p_1—S\) represents the indicated work of the compressed air in the rear cavity of the hammer,
\[ L_1 = m_1 F_1\ \text{kgm}, \]
where \(m_1\) is the scale of the indicator diagram of the rear cavity, \(\mathrm{kgm/cm^2}\);
\(F_1\) is the area of the indicator diagram of the rear cavity, \(\mathrm{cm^2}\).
The area of the diagram \(p_z—S\) represents the indicator work performed by the air in the front cavity of the cylinder,
\[ L_2 = m_2 F_2\ \mathrm{kgm}, \]
where \(m_2, F_2\) are, respectively, the scale and the area of the indicator diagram of the front cavity of the cylinder.
The sum \(L_1 + L_2\) is the indicator work of the cycle.
From the indicator work and air consumption calculated in this way, the efficiency of the hammer is determined and its working cycle is evaluated. In analyzing the operation of pneumatic engines, the indicator, mechanical, and effective efficiencies are usually determined [2]. In our case, we have the indicator efficiency
\[ \eta_i = \frac{L_i}{L_{\mathrm{n}}}\,100\%, \]
where
\[ L_{\mathrm{n}} = \frac{W}{n}\,3500p_1\left[\left(\frac{p_2}{p_1}\right)^{0.286} - 1\right]\ \mathrm{kgm}, \]
is the energy of the compressed air consumed per cycle;
\(W\) is the air consumption of the drilling hammer, \(\mathrm{m^3/min}\);
\(n\) is the number of blows of the hammer per minute;
\(p_1\) is the air pressure at the compressor intake;
\(p_2\) is the air pressure in the line, ati.
The mechanical efficiency of the drilling hammer is determined by the ratio of the impact energy to the indicator work of the cycle,
\[ \eta_{\mathrm{mech}} = \frac{A_y}{L_i}\,100\%, \]
although such a definition can be admitted only conditionally, since the energy used to rotate the drill should also be counted as useful. In our case it has not been taken into account.
The effective efficiency of the drilling hammer may be determined as the product of the indicator and mechanical efficiencies, or as the ratio of the impact energy to the energy consumed per cycle,
\[ \eta_0 = \eta_i \eta_{\mathrm{mech}} = \frac{A_y}{L_{\mathrm{n}}}. \]
Continuing the analysis of the working process of the hammers, we constructed force diagrams for the working and return strokes of the piston. The force diagram of the working stroke was constructed as follows: the ordinates of the \(p_1—S\) diagram on the section \(1—6\) (Figs. 1—2) were multiplied by the useful cross-sectional area of the cylinder, which determined the scale of the ordinate axis for the force diagram of the working stroke,
\[ m_f = p_1 Q\ \mathrm{kg/division}, \]
where \(Q\) is the effective area of the piston on the valve-box side, \(\text{cm}^2\).
The scale of the abscissa axis remained the same as in the construction of the indicator diagrams.
The force diagram shows the character of the change in the force driving the piston along its path. For the RH-754 hammer, at an air pressure of \(4\) atm and with the optimum feed force (Fig. 3), the force driving the piston during the working stroke changes smoothly from \(150\)—\(160\) kg at the beginning of the stroke to \(50\)—\(55\) kg at its end, i.e., at the moment of collision of the piston with the drill steel. Since the force pressing the piston against the drill steel at the moment of their collision is still rather large, it is easy to explain the phenomenon we observed, namely that
Fig. 3. Force diagrams of RH-754.
the piston of the drilling hammer, after striking the drill steel, practically does not rebound from it and begins its backward motion after a brief stop at the end of the stroke. A similar phenomenon is also observed when the hammer operates at a higher air pressure, with the magnitude of the force acting on the piston at any moment of its stroke increasing correspondingly.
The area of the force diagram of the working stroke reflects the work performed by the compressed air during the working stroke of the piston. From the magnitude of the useful part of this work (the impact energy), one can determine the mechanical efficiency of the working stroke of the piston:
\[ \eta_{\mathrm{mech}} = \frac{A_y}{L_p}\cdot 100\%, \]
where \(L_p\) is the indicator work of the air during the working stroke of the piston.
This efficiency reflects the mechanical losses of energy and must be taken into account when determining the impact energy analytically or from indicator diagrams. From the force diagram it is easy to deter-
divide by the average air pressure along the piston path, necessary for determining the impact energy by calculation,
\[ p_{\mathrm{avg}}=\frac{L_p\,100}{QS}\ \text{atm}, \]
where \(Q\) is the cross-sectional area of the rear cavity of the cylinder, \(\text{cm}^2\);
\(S\) is the piston stroke, cm.
The force diagram of the return stroke of the piston was used by us to determine the energy going into rotation of the drill and compression of the rear air cushion.
The work performed by the compressed air during the return stroke of the piston is proportional to the area \(L'_x\). This work is expended on compressing the rear air cushion \(L''_x\), on overcoming frictional forces, and on rotating the drill. Assuming that the mechanical efficiency of the return stroke of the piston is approximately equal to the mechanical efficiency of the working stroke, one can calculate the work expended on rotating the drill
\[ L_{\mathrm{rot}}=\eta'_{\mathrm{mech}}(L'_x-L''_x)\ \text{kgm}. \]
The work calculated in this way can serve as the initial premise for determining the torque on the drill. However, the efficiency of the rotary mechanism is not known to us with sufficient accuracy, which makes it difficult to calculate the torque. For this same reason, we do not introduce the expenditure of energy on rotation of the drill into the formula for determining the effective efficiency of the hammer.
Fig. 4. Comparison of different working cycles of a drilling hammer.
From the foregoing it follows that, by applying the appropriate research method, it is possible to obtain data for determining both the total (effective) efficiency and the indicated and mechanical efficiencies of pneumatic drilling hammers.
As we indicated above, the efficiency of modern pneumatic drilling hammers, equal to 8–24%, is much lower than the efficiency of electric hammers. Let us consider the possibility of increasing the efficiency of drilling hammers by analyzing the indicator and force diagrams of the RH-754-1W hammer.
Figure 4 shows the actual and theoretical diagrams of the working stroke of the piston. With the existing design of the hammer and its operation at an air-line pressure of 4 atm and at the optimum feed force, the real diagram of the working stroke is represented by the cycle \(okld\). In this case the impact energy is ...
3.1 kgm, the indicator work of the air during the working stroke \(L_p = 3.8\) kgm, the mechanical efficiency \(\eta_{\text{mech}} = 0.8\), the indicated efficiency \(\eta_i = 23.6\%\), and the energy consumed per cycle \(L_{\text{p}} = 16\) kgm.
To obtain a higher efficiency of the hammer, it is necessary to ensure that the exhaust of air from the hammer cavity takes place at the lowest possible pressure [2], i.e., to make fuller use of the energy of the compressed air admitted into the cylinder. Proceeding from this, we constructed the cycle \(abcde\) (Fig. 4), in which admission of air into the cylinder cavity is provided over a short segment of the piston travel. The admitted air then expands almost to atmospheric pressure. In this case, naturally, the air consumption is reduced.
As a first approximation, it may be assumed that the energy consumed per cycle will be
\[ L_{\text{p}}' = L_{\text{p}} \frac{ab}{am} = 16 \frac{10}{32} = 5 \text{ kgm}. \]
With this supplied energy, owing to complete expansion of the air in the hammer cylinder, it is possible to obtain the impact energy
\[ A_y' = \eta_{\text{mech}} L_p' \text{ kgm}, \]
where \(L_p' = 3.0\) kgm is the indicator work according to the diagram;
\(\eta_{\text{mech}} = 0.8\) is the mechanical efficiency of the working stroke of the RH-754 hammer.
Substituting the values, we obtain
\[ A_y' = 0.8 \cdot 3.0 = 2.4 \text{ kgm}. \]
The indicated efficiency of the cycle is
\[ \eta_i = \frac{L_p'}{L_{\text{p}}'} = \frac{3.0}{5.0} = 0.6,\ \text{or } 60\%. \]
Consequently, by reducing the time of air admission into the hammer cavity, it is possible to increase its indicated efficiency more than twofold, but the impact energy and the number of blows will be somewhat reduced. An approximate calculation shows that such a change in the hammer cycle will lead to a 35% decrease in its impact power. In addition, it should be taken into account that it is much more difficult, from a design standpoint, to implement the new cycle \((abcde)\) than this has been done, for example, in the RH-754 hammer. Nevertheless, in our opinion, serious attention should be paid to a comprehensive study of the possibilities and economic advisability of designing drilling hammers with such a cycle.
The principal quality indicator of hand-held drilling hammers is the specific power per unit of their weight. In this connection, when designing hand-held hammers, the primary task should be the maximum possible increase in pow-
...the machine’s power with its comparatively small weight. Therefore, reducing the power and increasing the weight (by means of additional devices for cutting off the compressed air when it is admitted into the cylinder) is clearly disadvantageous from the operating standpoint. Consequently, the path of improving hand-held machines solely by increasing their efficiency is unacceptable in practice.
Another theoretical operating cycle of a drilling hammer, \(amde\) (Fig. 4), also deserves attention. In this case compressed air is supplied to the hammer cavity over the entire length of the piston stroke and at maximum filling of the cavity. The indicated work of the \(amde\) cycle will be \(L_p'' = 4.8\ \text{kgm}\). This will make it possible to obtain an impact energy of \(3.8\ \text{kgm}\), i.e., somewhat greater than that of the RH-754, per impact. The air consumption, obviously, must increase substantially. Let us assume that the air consumption increases twofold, which is proportional to the square of the increase in the indicated work of the cycle; then
\[ L_i'' = 2 \cdot 16 = 32\ \text{kgm}. \]
The indicated efficiency will be
\[ \eta_i'' = \frac{4.8}{32}\cdot 100 = 15\%, \]
which is only \(8.6\%\) less than that of the RH-754 hammer. Consequently, by modifying the indicator diagram of the RH-754 hammer, it is possible to increase its power by approximately \(30\%\).
From the calculation thus presented it is evident that the possible variants for creating new pneumatic drilling hammers are bounded by the two limiting diagrams shown in Fig. 4. Practically feasible diagrams, for the given dimensions of the hammer cylinder, will have an indicated efficiency within the range \(15\)—\(60\%\). Modern high-frequency drilling hammers with valve and spool air distribution have an operating cycle closer to the cycle constructed on the basis of obtaining maximum impact energy than to an economically expedient operating cycle. Only the diagrams of the valveless drilling hammers T-10 and T-10s have distinctive features: in these hammers a “cutoff” of compressed air in the cylinder cavities is effected after its brief admission. This has made it possible to increase substantially the indicated and effective efficiency of the hammers (Fig. 5).
At an air pressure in the main of \(5\ \text{ati}\), the indicated efficiency of the T-10s hammer is \(37\%\), i.e., one and a half times greater than that of the RH-754, PR-25, and other hammers with spool air distribution. Moreover, it is characteristic of the T-10 hammers that, as the air pressure in the main increases, their efficiency decreases. This occurs as a result of the lengthening of the piston stroke at high air pressures, as a result...
as a result of which the section \(ab\) (Fig. 4), over which air is admitted into the cylinder cavity, is lengthened. Consequently, when T-10 hammers operate at high air pressures (6–7 ati), their working cycle approaches the cycles of spool-valve hammers, with all the consequences that follow from this (a decrease in efficiency and a relative increase in impact power).
Using the T-10 and T-10s hammers as an example, the clear advisability is seen of designing hammers with a working cycle that provides for a shortening of the period of air admission into the hammer cavities. As can be seen from Fig. 5, the efficiency of the T-10s hammer at an air-line pressure of 5 ati exceeds, in magnitude, the efficiency of all the drilling hammers tested by us.
Fig. 5. Energy balance of drilling hammers at \(p = 5\) ati.
Legend: impact energy; drill-rotation energy; mechanical energy losses; exhaust to the atmosphere.
CONCLUSIONS
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The efficiency of pneumatic drilling hammers is one of the important indicators of their technical characteristics and should be taken into account when conducting comparative tests.
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The efficiency of drilling hammers can be calculated by the method set forth above in the case of simultaneous recording of diagrams of air pressure in the hammer cavities and piston displacement.
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In designing pneumatic drilling hammers, possible variants should be taken into account for obtaining, from a given design, maximum power or maximum efficiency. For hand-held drilling hammers, the first of these variants is preferable.
- The increase in the efficiency of the spool-less hammers T-10 and T-10s, as compared with spool-valve drilling hammers (without reducing power), has demonstrated the advisability of developing, on their basis, analogous air-distribution schemes for powerful and economical column-mounted drilling hammers.
REFERENCES
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V. F. Gorbunov. Investigation of the internal process of drilling hammers with a butterfly valve. Article in the present collection.
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A. S. Ilyichev. Mine Pneumatic Installations. Collected Works, vol. II, Ugletekhizdat, 1953.