Some Results of Experimental Studies of Electropneumatic Percussion Machines

P. M. Alabuzhev

The need to create new designs of percussion machines set before the Tomsk Electromechanical Plant (TEMZ) the task of studying the working process of the electropneumatic hammers and perforators manufactured by the plant. For this purpose, in accordance with an agreement with TPI, as a matter of creative cooperation, a test stand was designed and built; its general view is shown in the attached photograph, Fig. 1. On the stand it is possible to investigate various designs of electropneumatic units of percussion machines under variable parameters determining the working process, without any substantial modification and without building the machine anew.

Fig. 1

Fig. 2

The diagram of the stand is presented in Fig. 2a. On the steel plate 14, fastened to columns, there are four main units: 1) an electric motor—1; 2) a crank-and-connecting-rod transmission (2—3—4); 3) a percussion unit, consisting ...

from the cylinder of hammer 7, connecting tube 8 (for the double-action hammer circuit), piston 5, striker 6, and tool 9; 4) the drill-rotation mechanism (10—11—12—13), while the rotational motion of the drill could also be effected by a separate direct-current electric motor.

Fig. 2b shows the circuit of a ZERT-type hammer with a double cylinder; Fig. 2c shows the circuit of a hammer with one cylinder, but with two-way air communication and internal air compensation; Fig. 2d gives the circuit of a hammer with external air compensation; Fig. 2e presents the circuit of an OMG-type hammer; Fig. 2f shows the circuit of a KNSH-type hammer. The diameter of the cylinders and strikers is \(D = 68\) mm. The weight of the stand (without the electric motor) is 170 kg. Feed is manual, and also by means of weights fastened to a flexible cable and passed over a pulley.

The optimal feed force \(P\), ensuring advancement of the hammer toward the face, is of the order of \(\simeq 75—90\) kg. On the stand it is possible to work in any position (under the terms of the contract, the tests were carried out with the hammer in the horizontal position). The experimental installation makes it possible to carry out laboratory studies under conditions close to production conditions

Fig. 3

Fig. 4

(operation on rock, possibility of penetration; use of factory-type drills and bits). On this stand, circuits of ZERT-type hammers, double-action hammers, and other circuits were studied. In addition, the stand can be used in refining designs and in studying a number of particular questions concerning the strength of individual parts, wear, impact efficiency, etc. In the present article we shall give some results of tests of hammers of the ZERT circuit. The theoretical and experimental studies of individual questions concerning the operation of hammers of this circuit have been partly covered in works [1; 2; 3].

Fig. 3 shows the variation of a number of quantities characterizing the working process of the hammer in the steady-state regime. Here the notation is as follows: \(s_p\) and \(s_b\) are the displacements of the piston and striker, recorded by us by an optical method [4]; \(l\) is the length of the air cushion; \(\omega\) is the angular velocity of the crankshaft, obtained with the aid of a direct-current tachogenerator; \(p\) is the air pressure between

by the striker and piston, recorded by a capacitive-type transducer and observed on the oscilloscope screen [7]. In this regime (No. 8 in the log), the weight of the striker per unit cross-sectional area of the cylinder is \(q = 0.066\ \text{kg}/\text{cm}^2\).

As a result of the tests carried out, the real possibilities of machines of this type were revealed.

On the basis of the selected best regimes (see Table 1), it is possible, using the theory of similarity [3], without particular difficulty to select the parameters of a machine with impact energy \(A = 1 \div 15\ \text{kgm}\) and with a number of blows \(z = 500 \div 1500\) per minute.

A relationship has been established between the parameters of the electropneumatic hammer in the best regimes when striking metal:

\[ \Pi = \frac{m p_0 g \pi D^2}{4 Q l_0 \omega^2} \simeq \frac{1}{3} \quad (\text{with a probable error of the result } = 6\%). \]

Here the following are denoted:

\(m\) — polytropic exponent (mean value \(m_{\text{av}} \simeq 1.25 \pm 0.07\));

\(p_0 = 1\ \text{kg}/\text{cm}^2\);

\(g = 9.8\ \text{m}/\text{s}^2\) — acceleration of free fall;

\(D\) — diameter of the hammer cylinder (in our case \(D = 68\ \text{mm}\));

\(Q\) — weight of the striker;

\(l_0\) — initial length of the air cushion;

\[ \omega = \frac{\pi n}{30} \]

— mean value of the angular velocity of the crankshaft.

The existence of the similarity criterion \(\Pi\) was shown by us in work [3]. The numerical value of the criterion \(\Pi = 0.33\) enables the designer to vary the choice of parameters of a machine dynamically similar to the given machine.

Fig. 5

The test results given in Table 1 may, for clarity, be represented in the form of dependences between dimensionless quantities (Figs. 4, 5). As is seen from Figs. 4 and 5, there are regions of values of optimal parameters, and each point of this region links together the values of the machine parameters in the optimal regime. From Fig. 4 it is seen that the most probable value of the ratio of the crank radius \(r\) to the initial ...

initial length of the air cushion, in the optimal regime, is equal to \(\dfrac{r}{l_0}=0.6\) (independently of the value of the unit weight of the striker). In this case the ratio of the striker velocity to the circumferential velocity of the crank pin is

\[ \frac{V_b}{r\omega}=1.6. \]

Consequently, for a given crank radius \(r\), crankshaft speed \(n\), and striker weight, it is not difficult to calculate the value of the impact energy on the striker:

\[ A=\frac{Q}{2g}(1.6\,r\omega)^2. \]

The maximum value of the ratio \(\dfrac{V_b}{r\omega}\) in our studies is equal to 2.1.

Table 1

Some of the best operating regimes of the hammer impact unit of the ZERT design on the stand at GPI, for combinations of the parameters \(r, l_0, Q, z\)

Here the notation is as follows:

\(r\) — crank radius;
\(l_0\) — initial length of the air cushion;
\(Q\) — weight of the striker;
\(z\) — number of impacts per minute;
\(N\) — power consumed from the electric network;

\[ A=\frac{Q\cdot V_b^2}{2g} \]

is the work during impact on the striker.

Eight operating regimes of the hammer of the ZERT design were studied by us more thoroughly; experimental graphs of the working process were recorded—\(s_p, s_b, \omega, p\)—and, by a statistical method, the following relationships were obtained, combined in Table 2.

In Fig. 6, in dimensionless coordinates \(\dfrac{V_b}{r\omega_{\mathrm{av}}}\) and \(\dfrac{s_b}{l_0}\), the so-called limiting cycle in the established operating regime of the machine is shown. \(k\) is the point representing the state of the system on the phase plane. The totality of all points \(k\) gives us the “phase portrait” of all states of the system in the established regime. Let us note that obtaining the limiting

the cycle theoretically for this nonlinear system is a very difficult problem.

Table 2

The probable error of the result is

$$ R=\pm \frac{2}{3}\sqrt{\frac{\sum \Delta \delta^2}{n(n-1)}}, \qquad \text{where } n=8. $$

The statistical-processing method is checked by the relation:

$$ \frac{l_0}{s_{b\,max}} \cdot \frac{r}{l_0} \cdot \frac{s_{b\,max}}{r} = 1 $$

In our case—$0.71 \cdot 0.60 \cdot 2.44 = 1.03 \quad (\pm 0.16)$

In bench tests it was found that the overall efficiency of the machine averages about 23% (in the best operating modes, the efficiency $\eta = 35\text{--}42\%$). Figure 7 gives the energy balance of the electropneumatic hammer in one of the best modes; the losses in the air cushion, depending on the clearances, can vary within the range of 14–26% per working cycle of the hammer.

In the course of carrying out the work, a methodology was developed for experimental investigations and for processing the materials [4; 5; 7], with determination of the error of the experimental results; the qualifications of the laboratory assistants and executors of the work (among whom TPI students were also included) were improved; experience was accumulated, the lack of which was especially felt at the beginning of the work. Equipment and original inertialess apparatus were developed for investigating rapidly occurring processes in time (an electronic commutator; a two-beam cathode oscillograph with commutators on direct-current amplifiers, on whose screen it is possible simultaneously to observe the variation of four processes; pressure sensors).

of the capacitive type; equipment for recording the motion of the striker, piston, etc.). The equipment we have built can be applied in related areas of engineering (internal-combustion engines, pneumatic hammers, forging hammers, air blowers, etc.) to the study of the working process of other machines, both in steady and transient operating regimes.

Fig. 6. Limiting cycle (in dimensionless coordinates).
Curve \(K\) depicting the motion on the phase plane.

Fig. 7. Energy balance of an electropneumatic hammer.

Supplied energy to the electric motor from the mains:

• useful work on the tool — 36%;
• energy losses:
• losses during impact on the tool;
• recovered energy during rebound of the striker;
• losses in the air cushion and compressor;
• losses in the reducer;
• losses in the engine–crank mechanism.

The results of the experiments confirmed the theoretical concepts and considerations expressed by us earlier [1; 3], and also refined the values of a number of coefficients and indices \((\eta, \Pi, m_{cp})\) necessary for calculation. The essence of the mechanism of the processes occurring in the percussion machine and in its individual elements is now more clearly represented—for example, the role of the rate of deformation of the air linkage, and the role of compensation of air and clearances in the operation of the hammer. It turned out that the working process of an electropneumatic percussion machine depends substantially on the magnitude and location of the compensation openings in the hammer cylinder (see Fig. 8). This circumstance, in turn, opens possibilities for increasing the efficiency of the machine and for regulating the impact energy, which is especially important during collaring.

In the course of experimentation, the interrelation of the working process of the percussion machine with external factors (the rock being broken and the feed force) was revealed. It was found that the initial parameters of a drilling hammer include the impact energy required for breaking the given rock, the angle of rotation of the drill between two impacts, and the feed force.

On the basis of the available material, the choice of the remaining parameters of a percussion machine \((r, l_0, D, Q, z)\) for certain specific conditions does not present significant difficulties. There are possibilities for the theoretical construction of an indicator diagram of pressure in the air cushion through the average pressure over the cycle:

\[ p_{cp}=\frac{Qz(1+e)\cdot V_{\sigma}}{15g\cdot D^2} \quad \text{(here \(e\) is the coefficient of restitution during impact),} \]

as well as data for the strength calculation of the percussion machine (crank-and-connecting-rod group, drill-rotation mechanism, etc.) and for determining the efficiency of individual elements of the machine.

The experimental studies carried out are not complete. In further work it is proposed:

a) to use a loop oscillograph for recording processes in percussion machines; for this purpose a converting device (for six curves) has already been designed, serving as a current amplifier between the sensors and the loop.

Fig. 8

b) to complete the structural design and adjustment of capacitive-type sensors for studying and recording the phenomenon of impact, as well as the torque on the drill;

c) to measure the temperature in the air cushion (over one operating cycle) and in individual parts of the machine;

d) to complete an attachment to the cathode oscillograph with the possibility of double electrical differentiation and integration of curves characterizing one or another phenomenon of the working process of the percussion machine, with the possibility of using magnetic recording;

e) to study the behavior and operation of the electric motor under impact-load conditions;

f) to initiate work on the study of various kinds of feeders; impact shock absorbers; vibration absorbers reducing vibration of the hammer body; to investigate the oscillation of a load on a spring in the presence of impacts on various materials, depending on the ratio between the frequencies of the disturbing force and the natural oscillations;

g) jointly with TEMZ, to initiate work on the study of the strength and durability of individual elements and parts under alternating and impact loads;

h) to carry out research on the question of impact efficiency.

References

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

2. Alabuzhev P. M. and Yudin I. V. Experimental investigation of an electropneumatic hammer. Proceedings of ZSFAN, issue 8, 1950.

1. Alabuzhev P. M. Application of the theory of similarity and dimensions to the study (modeling) of impact-action machines. Izv. TPI, vol. 73, 1952.

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

3. Alabuzhev P. M., Medlin E. K., Nevstruev E. N. On the methodology of investigating electromagnetic hammers. Izv. TPI, vol. 76, 1954.

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

5. Alabuzhev P. M. Application of electrical methods to the investigation of impact-action machines. Izv. TPI, vol. 78, 1954.