On Contemporary Directions in the Development of High-Performance Pneumatic Drilling Hammers

O. D. Alimov, V. F. Gorbunov

At the present time, as over many preceding years, the principal machines for drilling blast holes in rocks of medium and high hardness are pneumatic percussion drilling machines—pneumatic drilling hammers (rock drills). In recent years, the growth in labor productivity of drilling workers has been restrained to a considerable extent by the low productivity of modern drilling hammers and by the considerable labor intensity of their servicing.

The productivity of a drilling hammer depends on several factors, the main one of which is the drilling speed. The latter, in turn, depends on the physicomechanical properties of the rock and on the amount of energy expended for its destruction. In percussion-rotary drilling, the main part of this energy is supplied in the form of blows. Therefore, the energy of each blow and their frequency are decisive factors in ensuring high drilling speeds. Proceeding from this, the principal work on creating more productive drilling hammers should be carried out in the directions of increasing the energy of a single blow and increasing their frequency.

In studying the mechanism of rock destruction it has been established [1, 2] that, in percussion-rotary drilling of blast holes with a diameter of 40–45 mm and a depth of 2–3 m, the drilling speed increases in proportion to the increase in the energy of each blow (in the range from 2.5 to 20–25 kgm) and to the frequency of blows up to 6000–7000 per minute. In modern drilling hammers these parameters are at the very beginning of the indicated range. Thus, the magnitude of the blow energy is only 3–8 kgm, and the number of blows in the fastest hammers does not exceed 3800 per minute.

It is comparatively not difficult to create designs of drilling hammers with a blow energy of 10–25 kgm. Such designs have been created, but they have not found practical application because of the absence of a sufficiently strong drilling tool (drill rods and drill bits). Thus, the maximum value of the blow energy of drilling hammers is limited by the strength of mod-

of contemporary drilling tools. Meanwhile, work in the direction of developing drilling tools more resistant to impact loads has been and is at present being carried out manifestly insufficiently.

The second reserve for increasing drilling speed is an increase in the frequency of blows. To make use of this reserve, designers are improving existing drilling hammers and, in a number of cases, are creating machines based on new fundamental schemes. This direction, in particular, is followed by most designers who have created the so-called high-frequency pneumatic drilling hammers. High-frequency (or, as they are called in other sources, high-speed, rapid-impact, fast-running) pneumatic drilling hammers have begun to be introduced at mining-industry enterprises in the last 2–3 years. A distinctive feature of these hammers is that they operate with 2500–3500 blows per minute instead of the 1200–1800 blows provided by ordinary pneumatic drilling hammers. Like the latter, high-frequency hammers operate on pneumatic energy at an air pressure of 4–6 atm and, by their principle of operation, belong to impact-rotary machines.

Among foreign designs, the quality of the high-frequency hammers produced by the following firms is most highly regarded: “Atlas-Copco” (hammers RH-754-1W, VVD-41), “Demag” (Bergmeister), “Tampella” (T-10 and T-10c). In the Soviet Union, high-frequency hammers are manufactured by the “Kommunist” plants (hammers PR-10, PR-20, PR-23, PRS-3m) and “Pnevmatika” (PR-18, PR-25l, PR-24l).

By weight category, the new high-frequency hammers are classified as hand-held (weighing from 10 to 28 kg) and telescopic. Of these, the lighter hammers are intended for carrying out auxiliary work and are used predominantly without feeding devices, while medium and heavy hand-held drilling hammers are used with supporting-and-feeding devices in the form of pneumatic supports [3–7]. There is no information in the literature on the creation of high-frequency column-type drilling hammers. The column-type hammers currently produced have a number of blows in the range of 1400–2000 blows/min. The increased productivity of high-frequency drilling hammers is achieved through a large number of blows. The energy of a single blow remains at the level of 3–5 kgm, i.e., of the same magnitude as in hammers with the usual blow frequency.

Increasing the number of blows by 70–100%, with the same value of blow energy, raises the impact power of the hammer almost twofold. The drilling speed can also be increased by the same amount when the hammers operate under optimal conditions.

Increasing the power of drilling hammers by increasing the blow frequency has made it possible to reduce the weight of the hammer, arriving at—

… per unit of power. Thus, for 1 hp of power of the OM-506 hammer, manufactured by the Pneumatika plant, there were 20 kg of weight, whereas in the high-frequency hammer PR-25L of the same plant the specific weight was reduced to 10 kg/hp.

An increase in the frequency of blows in most types of high-frequency hammers is achieved by increasing the cylinder diameter to 80–100 mm, reducing the specific weight of the piston per unit of its area, and shortening the piston stroke to 30–50 mm. Reducing the specific weight of the piston per square centimeter of its area from 50–55 to 30–35 g, at the same air pressure of 4–6 atm, led to an increase in the acceleration of the striker piston to 1600 m/sec². In turn, this made it possible to reduce the piston stroke and increase the frequency of blows while maintaining the same impact energy.

Whereas in hammers of the usual type the ratio of piston-stroke length to cylinder diameter was 1.15–0.85, in high-frequency hammers it has been reduced to 0.3–0.6. The smallest value of this ratio is characteristic of the T-10s hammer, which has a cylinder diameter of 100 mm and a piston stroke of 25 mm with a piston weight of 2.6 kg. The number of blows of this hammer at an air pressure of 6 atm reaches 3800 per minute.

To ensure reliable fixing of the relatively light striker piston in the cylinder of the T-10 hammer, the designers were compelled to use an original form of striker piston. Unlike conventional designs, the striker piston of the T-10 hammer has two rods, by means of which it rests on bushings inserted in the front and rear covers of the hammer.

For a further increase in the frequency of blows to 5500–6000 per minute, hammers are being developed with two-piston strikers [8, 9], with impact units having two striker pistons that alternately strike the shank of the drill steel, as well as other original designs.

An increase in the frequency of blows can also be achieved by converting existing hammers to operate at an air pressure of 10–12 atm and higher. The advisability of this is confirmed by certain studies [12]. However, the introduction of increased air pressure into mining practice will require the reconstruction of the entire pneumatic system of mines.

As noted above, most of the high-frequency hammers that have been developed are “hand-held” drilling hammers. Meanwhile, the question of the optimum number of blows for hand-held drilling hammers has not yet been resolved. The point is that, in order to realize fully the power of a high-frequency hammer, it must be fed against the face with feed forces considerably exceeding those that the driller can provide “by hand,” i.e., without additional devices. Modern devices for supporting and feeding hand-held drilling hammers, in the form of pneumatic supports, do not exclude additional—

of the expenditure of the driller’s physical strength. The considerable frequency of vibration of hand hammers, which is thereby transmitted to the hands and body of the driller, leads to rapid fatigue of the worker, and, under prolonged exposure, to disruption of the organism’s functional activity. There are no sufficient data in the literature for evaluating the influence of blow frequency on the magnitude of the optimum feed force of the hammer, or the influence of hammer vibration on workers’ health. Therefore, investigation of the operating regimes of high-frequency pneumatic drilling hammers and generalization of operating experience with them should help designers in resolving this question.

Nor can the question of selecting air-distribution schemes and mechanisms for one or another type of drilling hammer be considered clear. At present several types of air-distribution devices are used in high-frequency hammers: ball-type, with an annular flanged spool, with a butterfly valve, with a disk or plate valve, with a tubular spool, and, finally, valveless air distribution. Plants of the domestic machine-building industry manufacture high-frequency hammers with a butterfly valve (PR-25l), with a flanged spool (PR-24l), and with a plate spool (PR-10, PR-20). The advantages of one or another air-distribution mechanism can be determined only through a thorough experimental study of the internal operating process of the hammers. The need to carry out such studies is obvious.

Modern pneumatic hammers also differ in the mechanisms for rotating the drill steel. English and American firms adhere to the design of a rotation mechanism with a separate helical rod; the firms “Demag,” “Mellon,” and “Atlas-Copco” use rotation mechanisms with a helical thread on the rod of the striker piston and with the ratchet bushing located in the front part of the hammer; the firm “Tampella” places the ratchet bushing in the rear part of the hammer, and the helical thread is made on the tail extension of the piston. In this hammer the piston tail simultaneously performs the function of air distribution. It is interesting to note that rotation of the drill steel in the T-10 hammer is effected during the forward (working) stroke of the piston, which previously had been encountered only in the telescopic hammers BBD-45WR of the firm “Atlas-Copco,” used for installing anchor support in workings [6]. In our opinion, the successful use of the valveless scheme in the T-10 hammers is due to the use of a shortened piston stroke and to rotation of the drill steel during the working stroke of the piston.

Most high-frequency drilling hammers of domestic plants have rotation mechanisms with a separate helical rod, with the ratchet bushing located behind the spool box (PR-10, PR-20, PR-23, PR-24l). The PR-25l hammer of the “Pnevmatika” plant has a helical thread on

on the piston rod; the thread profile is semicircular, which ensures a tighter fit and increases the wear resistance of the piston and the rotary nut.

In order to increase the strength and wear resistance of high-frequency hammers, a wide variety of materials are used for manufacturing their parts—from alloy steels to light alloys and cast iron. Most parts are heat-treated.

In the latest hammer models, considerable attention is paid to the cleanliness of the surfaces of parts. All particular achievements in this direction are the secrets of firms and are not published in the literature. This holds back the improvement of drilling machines. The time has come to generalize the achievements of individual plants and, taking into account advances in adjacent fields of machine building, to create the necessary theoretical prerequisites and methodological aids for selecting materials for characteristic parts of drilling hammers and for choosing methods of their heat and surface treatment. This applies equally both to drilling hammers themselves and to drilling tools.

A problem that remained unsolved until recent years is that of reducing the noise of pneumatic hammers and reducing “recoil.” Meanwhile, from the standpoint of improving the working conditions of drill operators, solving these problems must be a matter of first priority.

Simultaneously with the creation of more advanced drilling hammers, work must be carried out on creating corresponding supporting-and-feeding devices. Investigations carried out by us [13] show that by using improved feed mechanisms for high-frequency hammers it is possible to achieve a considerably greater effect than by converting drilling hammers to increased air pressure.

The availability of reliable supporting-and-feeding devices will make it possible additionally to increase the power of drilling machines and at the same time to ease the labor of drill operators. These questions can be solved most simply in the creation of powerful column-mounted hammers. By such hammers we mean powerful high-frequency hammers mounted on long-stroke feeders that make it possible to drill a blasthole with one drill steel to a depth of 2.0–3.0 m. To ease the labor of drill operators, such feeders should be installed on loading machines or on special drilling carriages by means of manipulators. The control of hammers, feeders, and manipulators should be such that one worker, without significant physical exertion, can service several drilling hammers.

Modern measuring equipment has considerably increased the possibilities for in-depth experimental investigation of mining machines. The availability of various designs of high-frequency drilling hammers makes it possible to carry out their comparative

research and, on this basis, develop theoretical prerequisites for the proper selection of the principal parameters of high-performance drilling hammers.

References

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

2. O. D. Alimov, On the mechanism of rock destruction in percussion-rotary drilling. Izvestiya TPI, vol. 75, Tomsk, 1954.

3. L. D. Borisenko. Ore pneumatic drilling hammers. Ugletekhizdat, 1956.

4. Air leg-mounted Drill. Colliery Engineering. January, 1954, p. 37.

5. Abridged List of Products. Holman Bros. Ltd. Camborne, England.

6. Drilling Equipment. Colliery Engineering. May, 1956, pp. 203—205.

7. Speed Fimn—higt speed Drill. Water Power, 1954, vol. 6, No. 5.

8. A. K. Sidorenko. Prospects for the development of perforator drilling of deep boreholes in hard rocks. Mining Journal, No. 4, 1957.

9. E. A. Sorokin. Giprorudmash Institute. Scientific-research and design institutes for production. Mining Journal, No. 8, 1957.

10. S. F. Babenko, G. M. Koloberdyan, P. S. Khutornoy. PR-20 and PR-23 rapid-impact perforators. Mining Journal, No. 4, 1957.

11. A. G. Dedyura, I. E. Pogorelov. PR-10 rapid-impact perforator. Mining Journal, No. 4, 1957.

12. G. T. Yamkovoy, Investigation of the possibility of applying high pressure in perforator drilling. Collection of Scientific Papers, NIGRI, No. 1, 1957.

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