On the Development of a Rational Range of Pneumatic Drilling Hammers
Abstract Generated abstract
This paper addresses the lack of standardization in Soviet pneumatic drilling hammers, which complicated both design selection and practical use under differing mining conditions. Drawing on experimental investigations of domestic and foreign hammers, the authors argue that optimal feed force, determined mainly by air pressure and piston area rather than rock hardness when cuttings are removed, should guide classification together with hammer weight and method of operation. They propose five principal groups of drilling hammers, specify limiting piston areas and indicative design parameters for different air-distribution schemes, and recommend adjustments for medium-hard and hard rocks. The paper also notes that high-frequency hammers require vibration-reducing design measures to protect operators.
Full Text
ON THE DEVELOPMENT OF A RATIONAL RANGE OF PNEUMATIC DRILLING HAMMERS
O. D. Alimov, I. G. Basov, V. F. Gorbunov
For drilling blast holes under various production conditions, drilling machines are required that differ in their method of use, power, and weight. Mining practice has established the necessity of currently having three principal groups of drilling hammers: hand-held hammers, which are used by the worker without additional devices for supporting the machine in the required position and for feed; powerful column-mounted hammers—for carrying out a large volume of drilling work in rocks of medium and high hardness; and telescopic hammers—for drilling blast holes directed from below upward.
Factories in the USSR manufacture drilling hammers of all three groups. As for the weight and power of the drilling hammers included in these groups, there is no unified opinion on this matter either in the literature or among practitioners. As a result, various factories in the USSR are presently producing, in series and in separate batches, 7 different types of hand-held drilling hammers alone, differing from one another in power, weight, and structural design.
There are as yet no sufficiently objective comparative data on the quality of drilling hammers with different parameters and on the most appropriate conditions for their application. The absence of a clear differentiation of drilling hammers by standard sizes, depending on the field of application and the method of their operation, is a difficulty both for designers in selecting the parameters of hammers intended for operation under particular mining-engineering conditions, and for operating personnel in selecting machines of the required qualities. All this increases the production of drilling machines and leads to cases of their irrational use. An urgent need has arisen for the standardization of drilling hammers manufactured by factories
USSR. The first step toward this should be the establishment of a rational range of drilling hammers according to the indicators characterizing them.
The principal indicator characterizing the quality of a given type of drilling hammer is the drilling rate, which depends mainly on the impact power of the machine. As our investigations have shown [1—4], the completeness with which the power of a drilling hammer is utilized depends to a considerable degree on the feed force, which can substantially change the operating regime and the drilling rate of the hammer.
As the feed force is increased, the drilling rate first rises, reaches a maximum, and then decreases. This occurs, as is evident from investigations [3, 4], as a result of the change, depending on the feed force, of such hammer parameters as the energy and frequency of blows, the number of revolutions of the drill steel, and the rebound of the hammer body from the limiting collar of the drill steel, and hence of the coefficient of efficiency of transmission of impact power in the “piston—drill steel—rock” system.
Investigations of various types of drilling hammers of domestic and foreign manufacture—RPM-17, PM-508, PR-20, PR-25, RH-754, T-10—when drilling rocks of various hardness from \(f = 4\) to \(f = 20\) and at air pressures of 4—6 ati showed that the optimum feed force, i.e., the force ensuring the maximum drilling rate with stable hammer operation, is in direct proportional dependence on the air pressure and the area of the piston on which compressed air acts during its forward stroke, and does not depend on the hardness of the rock, provided that drill cuttings are removed from the hole in a timely and complete manner [3].
Changing over to operation of the hammer at an increased air pressure must be accompanied by a corresponding increase in the feed force; otherwise the drilling rate not only will not increase, but may even decrease considerably.
As a result of experiments [3] it was found that the value of the optimum feed force for modern pneumatic drilling hammers with a piston area from 25 to 70 cm\(^2\) can, with sufficient accuracy, be determined from the expression
\[ F_{\mathrm{op}} = \frac{pQ}{3.56} \pm Cp \pm G \sin \alpha, \]
where
\[ \begin{aligned} p &\text{— air pressure in the line, ati;}\\ Q &\text{— area of the piston on which the compressed air acts during its forward stroke, cm}^2;\\ G &\text{— weight of the hammer, kg;}\\ \alpha &\text{— angle of inclination of the hole to the horizontal, deg;}\\ C = kQ &= 4 \text{— constant coefficient, found experimentally.} \end{aligned} \]
In striving to increase the power of a drilling hammer without substantially changing its weight, designers increase the number
impacts by reducing the piston stroke and increasing its diameter. This leads to the fact that the operation of the hammers is associated with such feed forces that cannot be produced by the muscular strength of the driller. It is known that a driller over a prolonged period can provide a feed force of no more than 25–30 kg [5], whereas hammers with an increased number of blows, such as PR-20, PR-25, RH-754, and T-10, at air pressures from 4 to 6 atm, for the most rational use of their power, must be operated with feed forces from 65 to 140 kg. Naturally, with manual feed these drilling hammers give a very low drilling speed and, despite their comparatively small weight of 20–28 kg, are extremely fatiguing for the driller.
As a result of experiments it was established that, by hand, drilling hammers can be operated with an optimal feed force of no more than 25–35 kg. More powerful hammers, having larger values of the optimal feed force, must be equipped with special devices for supporting them and providing feed. For the operation of powerful high-frequency pneumatic drilling hammers at the present time, pneumatic supports are most widely used. However, as studies [3] have shown, at air pressures of 4–6 atm with pneumatic supports it is expedient to operate only those high-frequency hammers whose optimal feed forces in this case do not exceed 130 kg. For more powerful hammers it is necessary to create other, in design, mounting-and-feed devices.
Thus, the optimal feed force is such a characteristic of a drilling hammer which, along with its weight, makes it possible to establish the maximum possible power of the machine for one or another method of its operation. In our opinion, drilling hammers, depending on the field of their application, weight, and power, as well as on the method of providing the optimal feed force, should be subdivided and manufactured by industry in five main groups.
The first group comprises light hand-held drilling hammers, having a weight of no more than 15–18 kg and operating at optimal conditions with forces of up to 20 kg, which the driller can provide manually over a prolonged period. These hammers should be intended for auxiliary drilling work in construction, for drilling blastholes in rocks of medium hardness when driving mine workings of small cross section, for drilling blastholes in confined places, etc.
The second group comprises hand-held drilling hammers for drilling downward blastholes in rocks of medium hardness and hard rocks during shaft sinking and in open work. The parameters of these machines should be selected on the assumption that the magnitude of their feed force at the face is provided by the hammer’s own weight of 25–30 kg and by an additional load weighing up to
20 kg, fixed on the hammer after collaring, and the additional force of 25–30 kg created by the driller. Thus, the optimal feed force of these hammers should not exceed 70–80 kg.
The third group consists of hand-held drilling hammers weighing not more than 25 kg, used for drilling horizontal and inclined blastholes when driving mine workings in rocks of medium hardness and hard rocks. The parameters of these hammers must be calculated on the basis of providing the necessary feed forces, not more than 130 kg, by pneumatic supports. At the same time, the pneumatic support must not be bulky, and its weight should not exceed 25 kg.
The fourth group consists of telescopic pneumatic drilling hammers intended for drilling upward blastholes in hard rocks during stoping of ore and the driving of raises. The parameters of such hammers should be selected on the basis of obtaining the greatest possible power, but at the same time their weight together with the feed device should not exceed 45–50 kg, since otherwise operation of such machines will be difficult.
The fifth group consists of column-mounted drilling hammers intended for drilling blastholes in hard rocks when driving underground mine workings, during stoping of ore, and in open-pit operations. Studies [2] have shown that column-mounted drilling hammers should expediently be operated only when installed on feed devices mounted by means of manipulators on loading machines or on special drilling carriages. With this method of operation, the weight of column-mounted drilling hammers has no substantial effect on labor productivity; therefore, the parameters of such machines must be calculated on the basis of obtaining the maximum power of the machine, the magnitude of which is at present limited by the durability of the drilling tool.
Both the power of a drilling hammer and the optimal force of its feed depend on the piston area on which the compressed air acts during the forward stroke of the piston, and on the air pressure. The piston area is thus a design parameter that can be used in selecting the drilling hammer required for operation under particular mining and technical conditions.
For the five above-listed groups of drilling hammers, proceeding from the mining and technical conditions characterizing them and from the values of the optimal feed forces at an air pressure of 5 atm, we have determined limiting values of the piston area that can be used for selecting the parameters of a rational range of pneumatic drilling hammers (Table 1).
Studies of a large number of various drilling hammers, carried out by the Department of Mining Machines and Mine—
Table 1
| Types of drilling hammers | Piston area on which compressed air acts during its forward stroke, cm² |
|---|---|
| I. Light hand-held | Up to 25 |
| II. Hand-held for drilling downholes | 25—35 |
| III. Hand-held, used with pneumatic supports | 25—55 |
| IV. Telescopic | 55—80 |
| V. Column-mounted | 80—100 |
... of transport of the Tomsk Polytechnic Institute in recent years also make it possible to put forward proposals on the most expedient air-distribution schemes and the principal design parameters of drilling hammers of the indicated groups.
Investigations of the working process of the best domestic and foreign models of drilling hammers [4] have shown that the most acceptable air-distribution schemes for modern hammers are: 1) with a butterfly valve; 2) with an annular flanged spool; 3) spoolless.
Air distribution with a butterfly valve is simple to manufacture and ensures good filling of the hammer-cylinder cavities with compressed air, as a result of which it is possible to obtain a relatively high impact power of the hammer with a small volume of its cylinder.
The air-distribution scheme with a flanged spool usually has somewhat smaller flow sections of the air-conducting passages; therefore the filling of the hammer-cylinder cavities with compressed air is less than in hammers with a butterfly valve. At present, mining-machine-building plants produce high-frequency drilling hammers mainly with a flanged spool. A transition to the manufacture of hammers with a butterfly valve is inevitably associated with large capital expenditures due to the restructuring of the technological process. Therefore, in the recommended range of drilling hammers (Table 2) we consider it expedient to retain hammers with a flanged spool. In economy of operation these hammers are not inferior to butterfly-valve hammers, and in terms of power they can be made within limits acceptable for practice.
Spoolless pneumatic drilling hammers are recommended by us for production because of their high efficiency in terms of compressed-air consumption. As investigations of the spoolless hammers T-10 and T-10s have shown, their efficiency is 1.5—2 times higher than that of hammers with valve and spool air-distribution schemes. Therefore hammers with spoolless air distribution should expediently be manufactured in various standard sizes (Table 2).
Table 2
Parameters of drilling hammers with various air-distribution schemes at \(p = 5\ \mathrm{atm}\)
| Type of drilling hammer | Rock category | Flank spool \(D_{\mathrm{ц}}\) | Flank spool \(S\) | Flank spool \(G_{\mathrm{п}}\) | Flank spool \(A_{\mathrm{у}}\) | Flank spool \(n\) | Flank spool \(N_{\mathrm{у}}\) | End valve \(D_{\mathrm{ц}}\) | End valve \(S\) | End valve \(G_{\mathrm{п}}\) | End valve \(A_{\mathrm{у}}\) | End valve \(n\) | End valve \(N_{\mathrm{у}}\) | Valveless scheme \(D_{\mathrm{ц}}\) | Valveless scheme \(S\) | Valveless scheme \(G_{\mathrm{п}}\) | Valveless scheme \(A_{\mathrm{у}}\) | Valveless scheme \(n\) | Valveless scheme \(N_{\mathrm{у}}\) |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Manual light | For rocks of medium hardness | 60 | 25 | 1,6 | 1,8 | 2500 | 1,0 | 55 | 30 | 1,3 | 1,8 | 2400 | 1,0 | 65 | 25 | 1,8 | 1,6 | 2600 | 0,92 |
| Manual light | For hard rocks | 60 | 40 | 1,6 | 2,8 | 2000 | 1,25 | 55 | 50 | 1,5 | 3,0 | 1700 | 1,1 | 65 | 35 | 1,8 | 2,4 | 2200 | 1,2 |
| Manual, for drilling downward blast holes | For rocks of medium hardness | 70 | 30 | 1,7 | 2,6 | 2500 | 1,45 | 70 | 30 | 1,7 | 2,5 | 2400 | 1,3 | 75 | 25 | 1,8 | 2,3 | 3000 | 1,5 |
| Manual, for drilling downward blast holes | For hard rocks | 70 | 50 | 1,7 | 4,0 | 1850 | 1,65 | 70 | 50 | 1,7 | 4,6 | 2000 | 2,1 | 75 | 35 | 1,8 | 3,3 | 2600 | 1,9 |
| Manual, for drilling boreholes | For rocks of medium hardness | 80 | 25 | 1,8 | 3,0 | 3000 | 2,0 | 80 | 25 | 1,8 | 3,2 | 3200 | 2,3 | 90 | 25 | 2,0 | 3,5 | 3600 | 2,8 |
| Manual, for drilling boreholes | For hard rocks | 80 | 45 | 1,8 | 5,2 | 2100 | 2,3 | 80 | 40 | 1,8 | 5,0 | 2500 | 2,8 | 90 | 35 | 2,1 | 4,7 | 2850 | 3,0 |
| Telescopic | For rocks of medium hardness | 90 | 30 | 2,0 | 4,4 | 2950 | 2,8 | 90 | 30 | 2,0 | 4,7 | 3050 | 3,2 | 100 | 30 | 2,2 | 5,0 | 3300 | 3,7 |
| Telescopic | For hard rocks | 90 | 35 | 2,0 | 5,0 | 2700 | 3,0 | 90 | 35 | 2,0 | 5,5 | 2850 | 3,5 | 110 | 35 | 2,4 | 6,5 | 3370 | 4,8 |
| Column-mounted | For rocks of medium hardness | 120 | 20 | 3,0 | 5,7 | 4000 | 5,0 | 120 | 20 | 2,8 | 6,0 | 4200 | 5,6 | 120 | 20 | 2,7 | 5,0 | 4500 | 5,0 |
| Column-mounted | For hard rocks | 120 | 30 | 3,0 | 7,8 | 3200 | 5,6 | 120 | 25 | 3,0 | 7,0 | 3600 | 5,6 | 120 | 30 | 2,7 | 7,5 | 3700 | 6,2 |
\(D_{\mathrm{ц}}\) — cylinder diameter, mm; \(S\) — piston stroke, mm; \(G_{\mathrm{п}}\) — total piston mass, kg; \(A_{\mathrm{у}}\) — impact energy, kgm; \(n\) — number of impacts per minute; \(N_{\mathrm{у}}\) — power, hp.
The parameters of the drilling hammers given in Table 2 were calculated from formulas known from the literature, corrected on the basis of the results of the investigation of many types of drilling hammers [4, 6, 7]. The impact energy was determined from the equation
\[ A_y = k_a pSQ \ \text{kgm}, \]
where \(k_a = 0.5\text{–}0.55\) is an empirical coefficient allowing for throttling losses of air pressure and the mechanical efficiency of the drilling hammer;
\(p\) — air pressure in the network, atm;
\(S\) — working stroke of the piston, m;
\(Q\) — useful cross-sectional area of the rear cavity of the hammer cylinder, \(\text{cm}^2\).
The number of blows of drilling hammers was determined from the equation
\[ n = \frac{60 k_p}{1 + \gamma}\sqrt{\frac{pQ}{mS}} \ \text{blows/min}, \]
where \(k_p = 0.6\) is an empirical coefficient of mechanical losses and air-pressure losses;
\(m\) — mass of the piston, \(\text{kg sec}^2/\text{m}\);
\(\gamma\) — ratio of the time of the return stroke of the piston to the time of the working stroke when the hammers operate under optimum conditions.
Investigations have shown that, when hammers operate under optimum conditions, the time of the return stroke of the piston is 1.4–2.1 times greater than the time of the working stroke [4]. For modern drilling hammers with a flanged spool valve and a butterfly valve, one may take \(\gamma = 1.7\); for spoolless hammers with rotation of the drill during the working stroke of the piston, \(\gamma = 1.4\).
Depending on the strength of the rock, we propose to establish certain differences in the structural dimensions and parameters of drilling hammers. For drilling rocks of medium strength, \(f = 6 \div 12\), most widely encountered in coal mines, hammers are recommended that have a comparatively low energy per single blow and an increased blow frequency. For hard rocks, \(f = 14 \div 20\), often encountered in mines of ferrous and nonferrous metallurgy, hammers with increased impact energy are recommended.
Investigations [3] have shown that high-frequency drilling hammers, even at optimum feed forces, have amplitudes of oscillation of the hammer body that considerably exceed the limits permitted by sanitary standards. In this connection, to eliminate the harmful effect of vibration on the worker, the designs of such hammers must provide for elastic handles or dynamic vibration absorbers.
We believe that the recommendations given above can serve as the basis for creating a unified range of standard sizes of pneumatic drilling hammers for the coal and mining industries of the USSR.
REFERENCES
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