WAYS TO INCREASE THE IMPACT FREQUENCY OF HANDHELD PNEUMATIC DRILLS

V. F. Gorbunov

Studies of the operating conditions and working process of modern drilling hammers, carried out by the Department of Mining Machines and Mine Transport of the Tomsk Polytechnic Institute [1—5], have shown that an increase in the power of drilling hammers is achieved mainly by increasing the number of blows. The impact energy of all the hammers tested, at an air pressure of up to 6 atm, does not exceed 6 kgm.

An increase in impact frequency is achieved through the following design features of drilling hammers: increasing the cylinder diameter; shortening the piston stroke length; reducing the piston weight per unit area of the cylinder cross section; and shortening the time of the piston return stroke. Let us consider the influence of these factors on the number of blows of drilling hammers.

Increasing the Cylinder Diameter

For the high-frequency drilling hammers studied, the cylinder diameter lies within the range of 75—100 mm. In this case the useful cross-sectional area of the cylinder is 42—70 cm² (Table 1).

Table 1

Experimental data [3—5] show that drilling hammers with an increased cylinder diameter possess greater impact power than hammers with a small cylinder diameter. Increasing the cylinder area at the same mean air pressure leads to an increase in the forces moving the piston.

If the weight of the piston does not change, then as the cylinder area increases the specific weight of the piston per unit area decreases. The piston moves with greater acceleration and acquires the required velocity over a comparatively short section of travel (Table 2).

Table 2

In connection with the increase in acceleration during the working stroke of the piston and the shortening of the travel distance, the time of the working stroke is sharply reduced. Simul-

At the same time, the time of the return stroke of the piston decreases, and, consequently, the number of blows increases.

In the investigated designs of drilling hammers with valve and spool air distribution, by increasing the cylinder diameter the number of blows was raised from 1400–1700 per minute in the PM-508 hammer to 2600–2800 in the RH-754, PR-20, and PR-25l hammers. At the same time, the blow energy of the indicated hammers, with a comparatively short piston stroke, even increased in comparison with the blow energy of the PM-508 hammer (Table 2). Spoolless drilling hammers T-10 and T-10s have an even higher blow frequency with the same specific weight of the piston as compared with the RH-754, BJ-20, and other hammers (Table 2). We shall discuss this phenomenon in greater detail below.

It should be noted that increasing the cylinder diameter of drilling hammers (and the cross-sectional area of the cylinder) simultaneously with the positive factors leads to certain technical complications in servicing these machines. Obtaining high impact power in high-frequency hammers is possible only by creating the corresponding feed force [3, 5], which can be ensured only by using sufficiently advanced feed mechanisms. The absence of such mechanisms may nullify all the advantages of drilling hammers with a large cylinder diameter.

Reduction of the Piston Stroke Length

When the useful cross-sectional area of the cylinder is increased, as we indicated above, it is possible to shorten the piston stroke without reducing the blow energy. In all high-frequency hammers, with the exception of the BGM hammer, this provision reflects the essence of their constructive improvement. The piston stroke of high-frequency hammers lies within the range of 19–42 mm (Table 2). The ratio of piston stroke to cylinder diameter is, for most high-frequency hammers, 0.35–0.45, and for the BGM and T-10s hammers even 0.28–0.32. If, for the T-10s hammer, the indicated reduction of piston stroke with a larger cylinder cross-section in comparison with all hammers does not reduce the blow energy and impact power, the same cannot be said of the BGM hammer. Reducing the piston stroke length to 19–24 mm with a comparatively small cylinder cross-sectional area in the BGM hammer leads to an increase in blow frequency (Table 2); however, the impact power of this hammer is small, owing to the low blow energy.

Thus, shortening the piston stroke of a drilling hammer, from the standpoint of increasing its impact power, is expedient up to certain values calculated from the condition of maintaining the blow energy at the level of 4–6 kgm.

Reducing the Weight of the Piston

The weight of the piston of a drilling hammer does not affect the magnitude of the impact energy, since the latter is determined by the force acting on the piston and by the length of its travel. However, the efficiency of energy transfer from the piston to the drill depends on the ratio of their weights; therefore, when designing drilling hammers, the piston weight should not be too small. In practice, the piston weight of hand-held drilling hammers of the medium and heavy weight categories lies within the range of 1.6–2.5 kg (Table 1). In carrying out experimental studies, we did not observe any substantial fluctuation in hammer productivity as a function of piston weight. A more significant influence on impact power—and consequently on the productivity of the drilling hammer—is exerted by the ratio of the piston weight to the useful cross-sectional area of the cylinder, i.e., the specific weight of the piston (Tables 1, 2). A reduction in the specific weight of the piston, all other conditions being equal, leads to an increase in impact frequency and, consequently, in impact power. As is evident from the data in Tables 1–2, high-frequency hammers, with the exception of the BGM hammer, have a piston specific weight in the range of 36–40 g/cm², despite considerable variation in the total piston weight (from 1.6 to 2.57 kg). For the BGM hammer, the piston weighing 2.05 kg is clearly too heavy; by reducing it to 1.7–1.8 kg while keeping the stroke length unchanged, it is possible to improve significantly the technical characteristics of the BGM hammer.

Reducing the Duration of the Return Stroke of the Piston

Experimental investigations [3–5] show that the duration of the return stroke of the piston is substantially greater than the time of the forward stroke. When determining the impact frequency of a drilling hammer, it is necessary to take into account both the time of the forward stroke and that of the return stroke of the piston. A study of the drilling hammers BJ-20, RH-754, and T-10 showed that, with an equal ratio of piston strokes and their specific weights, the impact frequency of the indicated hammers differs by 25–40%, or by 600–1000 impacts/min. The air pressure in the cylinder of the hammers during the working stroke differs by no more than 10% (under optimum operating conditions and equal line pressure). Therefore, the large difference in impact frequency can be attributed to the return stroke of the piston. At an air pressure of 5 atm, the ratio of the duration of the return stroke to the time of the forward stroke is, for the hammers T-10, RH-754, and BJ-20, respectively, 1.2, 1.65, and 2.1 (Fig. 1, Table 3).

In accordance with this, the number of impacts of the hammers (Fig. 2), corresponding to the optimum feed forces, varies from 3200 impacts/min for the T-10 hammer to 2250 impacts/min for the BJ-20 hammer. Consequently, reducing the ratio \(t_x : t_p\) in the T-10 hammer made it possible to increase the number of impacts by 25% in comparison with RH-754 and by 40% in comparison with the BJ-20 hammer.

Table 3

The long duration of the return stroke of the piston of the BJ-20 hammer is explained by the imperfection of its air-distribution device. The front cavity of the cylinder is poorly filled with compressed air owing to the inertia of the ball-type distribution and the small flow areas of the passages. Added to this is the early exhaust of air into the atmosphere. In the RH-754 hammer the indicated shortcomings are absent; the filling of the front cavity with compressed air is sufficiently high [6], and therefore the duration of the return stroke of the piston is somewhat less than in the BJ-20 hammer and, at optimum feed forces, is \(t_x = (1.6—1.7)\, t_p\).

Fig. 1. Effect of feed force on the ratio of the piston return-stroke time to the forward-stroke time.

Changing the principle of operation of the rotation mechanism by carrying out the rotation of the drill during the working stroke of the piston made it possible, in the T-10 hammer, to substantially reduce the return-stroke time. It should be noted that the duration of the working stroke of the piston did not increase (Table 3). Thus, carrying out the rotation of the drill during the working stroke of the piston made it possible, in the T-10 hammer, in comparison with the RH-754 hammer, to increase the number of blows by 25%. In accordance with the increase in the number of blows, the impact power of the hammer also increased.

In the contemporary literature, proposals are appearing for increasing the number of blows of drilling hammers to more than 3500 per minute. Some investigators [7] propose creating two-piston hammers in which blows on the drill are delivered alternately by each of the pistons. These recommendations are hardly applicable to hand-held drilling hammers, owing to the difficulty of manufacturing convenient hammer designs of low weight. For hand-held drilling hammers, in our opinion, the more acceptable principle is the po-

increasing the number of impacts, developed by staff members of the Siberian Branch of the Academy of Sciences of the USSR [6]. In the pneumatic-hammer scheme they propose there are likewise two pistons of equal weight, moving in counterphase, but only one of them delivers the blow to the drill steel. The second piston serves to accumulate kinetic energy and transmit it to the first at the moment of their collision. According to the authors, the impact frequency of the hammer will increase by approximately 1.5 times in comparison with a single-piston hammer under other equal conditions (compressed-air pressure, piston area, piston mass, impact energy).

Fig. 2. Effect of the ratio $t_x : t_p$ on the number of impacts of drilling hammers.

The viability of the proposed designs can be determined only by thorough investigation of prototypes.

In currently existing hand-held drilling hammers, however, only the four methods of increasing impact frequency considered above have been implemented; taken together, they have made it possible to raise the number of impacts of the T-10s hammer to 3600–3700 per minute at an air pressure of 6 ati.

REFERENCES

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

2. O. D. Alimov, I. G. Basov, P. A. Samoilov. Some results of investigating operating regimes of pneumatic drilling hammers. Izvestiya TPI, vol. 106, Metallurgizdat, 1958.

3. O. D. Alimov, V. F. Gorbunov. Investigation of the working process of a high-frequency drilling hammer with valveless air distribution. Izvestiya TPI, vol. 106, Metallurgizdat, 1958.

4. O. D. Alimov, V. F. Gorbunov. Some results of investigating the OM-506 pneumatic drilling hammer. Izvestiya TPI, vol. 88, Tomsk, 1956.

5. V. F. Gorbunov. Investigation of the internal process of drilling hammers with a butterfly valve. Article in the present collection.

6. B. V. Sudnishnikov, N. N. Esin. A new principle for increasing the impact frequency of pneumatic hammers. Izvestiya Sibirskogo otdeleniya AN SSSR, No. 7, 1958.

7. A. K. Sidorenko. Prospects for the development of percussion drilling of deep boreholes in hard rocks. Gornyi zhurnal, No. 4, 1957.