On the Design of Electropneumatic Impact Machines

P. M. ALABUZHEV and O. D. ALIMOV

One of the possibilities for significantly increasing the economy of drilling hammers is the creation of various types of electric drilling hammers, of which, at present, the most promising are hammers operating on the electropneumatic principle. Descriptions of the circuits and designs, as well as studies of the working process of hammers of this type, are covered rather fully in the literature [1–14].

At the Tomsk Electromechanical Plant (TEMZ), a number of designs of electropneumatic chipping and drilling hammers were designed and manufactured; however, none of them has yet proved equal, in terms of productivity and operating capacity, to modern pneumatic chipping and drilling hammers.

In Table 1 we present the principal technical characteristics of electric drilling hammers manufactured at different times by TEMZ. For comparison, the table also gives the technical data of modern pneumatic drilling hammers OM-506m and KTsM-4 when operating at an air pressure of 5 atm.

As can be seen from Table 1, the principal and sole advantage of the electric drilling hammers SPK-1, PBR-1, ZERGRB, in comparison with pneumatic ones, is the considerably higher overall efficiency of the hammer (owing to the elimination of additional energy losses in the compressor and air lines that occur when pneumatic drilling hammers are used). However, this advantage of electric impact machines has not yet been realized in practice because of a considerable number of shortcomings, the main ones being: insufficient drilling speed, lack of the necessary strength of parts, and also insufficient stability in operation of the impact and rotary units.

On the basis of familiarity with materials on the manufacture and testing of electric drilling hammers at TEMZ and of the investigations we have carried out, we have arrived at the following conclusions:

1. The main reason for the failures in creating electropneumatic drilling hammers is the absence of a sufficiently developed theory and methods for calculating the working process and selecting the basic parameters of these machines. As a result, the machines were created without sufficient theoretical substantiation.

2. The desire to make an electropneumatic drilling hammer necessarily light (hand-held) limited the weight and, consequently, the power of the electric motor, the energy, and the number of blows. To create an electropneumatic drilling hammer equal in its principal indices (energy and number of blows, drilling speed) to modern hand-held pneumatic drilling hammers, it is necessary to equip it with an electric motor of 3–5 kW power. If one takes into account that the weight

Table 1

such an electric motor will be equal to 30–60 kg and, in turn, in the design of the hammer it will constitute about \(1/2\) of the total weight, then the difficulties of creating, on the electropneumatic principle, a light and highly productive hand-held drilling hammer become clear. Reducing the weight of the hammer by reducing the weight of individual parts and, consequently, reducing their margin of strength, in the absence of data on the magnitude of the forces arising in the parts during operation of the hammer, does not lead to the desired results. The hammers turn out simply to be insufficiently strong and, because of the large number of failures during preliminary tests, often do not even leave the factory premises. According to data from TEMZ, the greatest number of failures during factory tests of experimental models of electropneumatic drilling hammers are failures of the gear wheels, the connecting-rod and crank mechanism, the turning mechanism, and the hammer body (at the places where axles and shafts are fastened).

The quantities that substantially limit the improvement of the choice of parameters and the increase in durability of hand-type electric impact machines are recoil and heating of the machine. The question of reducing the recoil of a comparatively light hand-held hammer (weighing 8–10 kg), while achieving an impact energy \(A_b = 3.5\) kg·m, presents considerable difficulties. Here, apparently, it is necessary partly to balance the reciprocating masses and, in addition, to introduce elastic elements that absorb the energy of the machine body [3].

As regards heating of the hammer, the designer must pay attention to the selection of the optimum volume of air in the sleeve between the striker and the piston, proceeding from thermodynamic considerations. During a cycle of hammer operation, the air is heated during compression and cooled during expansion, but heating nevertheless predominates. According to our investigations [2], the efficiency of the air cushion is of the order of \(\simeq 85\%\). In addition to heating from the compressed air, there is heating from the rubbing parts of the crank-and-connecting-rod mechanism. The overall efficiency of a hammer of the ZERT scheme is, in the best case, of the order of \(\simeq 40\%\). Consequently, 60% of the energy is lost as heat. From this follows the conclusion: to reduce heating (overheating) of the hammer, heat removal is needed, i.e., cooling is necessary—air or water cooling.

It seems to us that at present it is more expedient to concentrate attention on manufacturing a powerful column-type electropneumatic drilling hammer reliable in operation (equivalent in its performance to a pneumatic drilling hammer of the KTsM-4 type), where the questions of recoil and heating, although still relevant, recede to the background, since elastic elements can be introduced into the system without particular difficulty, as can water cooling with subsequent use of the water for flushing the blast hole; moreover, the problem of the weight of the design in this case is not as acute as in a hand tool. On the path to creating a powerful drilling hammer, primary attention must be given to the problem of strength, wear resistance, and durability of the parts. It is necessary to develop a technology for manufacturing strong and durable parts operating under severe conditions—variable and impact loads.

Full-scale tests of individual units of the machine (including the electric motor) and modeling acquire enormous importance [4, 5, 8, 12].

On the basis of extensive experimental and theoretical studies of the working process of electropneumatic impact machines, carried out over a number of years at the Tomsk Polytechnic Institute and partially covered in works [1–7], data are available for a substantiated choice of the initial parameters of a drilling impact machine. Such initial parameters include: the impact energy necessary for breaking the rock \(A_b\), the angle of rotation of the drill between impacts \(\varphi\), and the feed force of the machine against the face [8, 9]. It is appropriate to note that, in impact-rotary

in percussive drilling under the optimal hammer operating regime, vibrations of the hammer body must necessarily occur in order to make it possible to provide a clearance between the drill bit blade and the rock; this, in turn, is regulated by the corresponding feed force \[8, 10\].

At present, greater prospects exist for the creation of powerful, highly productive column-type electropneumatic drilling hammers and especially machines for percussive-rotary drilling of large-diameter boreholes, as well as dynamic planes with an active cutting edge using impact. For these cases it is possible to create a strong and reliable electropneumatic impact machine with high efficiency, which in operation can indeed recoup the somewhat increased cost of its manufacture. Such machines may be used for carrying out major works and preparatory workings, in drilling deep boreholes for block caving, and also in quarry operations.

LITERATURE

1. Alabuzhev P. M. Investigation of the working process of an electric chipping hammer with an elastic (air) coupling. Izv. TPI, vol. 61, no. 1, 1947.

2. Alabuzhev P. M. and Yudin I. P. Experimental investigation of an electropneumatic hammer. Tr. Zap.-Sib. filiala AN SSSR, no. 8, 1950.

3. Alabuzhev P. M. On the question of the oscillatory motion of the body of an electric chipping hammer with a crank-and-connecting-rod mechanism. Izv. TPI, vol. 68, no. 1, 1951.

4. Alabuzhev P. M. Application of the theory of similarity and dimensions to the investigation (modeling) of impact machines. Izv. TPI, vol. 73, 1952.

5. Alabuzhev P. M. and Alimov O. D. Determination of impact energy in impact machines. Izv. TPI, vol. 76, 1954.

6. Alabuzhev P. M. Application of a grapho-analytical calculation method to the investigation of the working process of an electropneumatic hammer. Izv. TPI, vol. 75, 1954.

7. Alabuzhev P. M., Alimov O. D., Tsukanov A. G. On the efficiency of impact in drilling hammers. Izv. TPI, vol. 75, 1954.

8. Alabuzhev P. M. and Alimov O. D. Influence of feed force and the angle of rotation of the drill between impacts on the rate of percussive-rotary drilling of blast holes. Izv. TPI, vol. 75, 1954.

9. Alimov O. D. Investigation of the mechanism of rock destruction in percussive-rotary drilling and of the initial parameters of drilling hammers. Author’s abstract. Tomsk, 1953.

10. Alimov O. D. Influence of feed force on the drilling rate with pneumatic drilling hammers. Izv. TPI, vol. 75, 1954.

11. Batuev N. M. Electric hammers. In: Collected Works of VNII Stroydormash—Investigation of Machines and Mechanisms for Construction and Road Works, 1, Mashgiz, 1950.

12. Batuev N. M. Energy characteristics of electric hammers and ways of increasing their productivity. In: Collected Works of VNII Stroydormash, VI—Investigation of Vibrators and Electric Hammers, Mashgiz, 1953.

13. Kucherov P. S. Theory of the calculation of electropneumatic impact tools. In: Collected Works of the Institute of Mining, no. 2 (11), Publishing House of the Academy of Sciences of the Ukrainian SSR, 1952.

14. Shmargunov K. N. Electric Hammers, Mashgiz, 1950.

1. The numerator gives design data; the denominator, experimental data. The energy and number-of-blows data for pneumatic drilling hammers are given at 4–7 atm. ↩