O. D. Alimov, N. P. Ryashentsev, A. V. Frolov

ON THE DEVELOPMENT OF ELECTRIC MACHINES OF ROTARY-PERCUSSIVE ACTION

In the mining industry, three methods of drilling blastholes are of practical application: percussive-rotary, rotary, and rotary-percussive.

Experimental studies [1, 2, 3] have shown that, when drilling rocks of medium and high strength, it is advisable to use machines of rotary-percussive action, which have advantages over others both in drilling speed and in tool wear resistance. At present, machines of percussive-rotary action [2] are known, produced by the firms Salzgitter, Hausherr, Nüsse and Gräfer, as well as the BU-1, BU-2, and SBU-2 machines of the Kuznetsk Machine-Building Plant. These machines differ from one another in the parameters and design of individual assemblies, but what they have in common is the presence of a pneumatic drive for the rotator and the percussion unit. The absence of a compressor facility at many mines and ore mines hinders the broad introduction of the progressive rotary-percussive method of drilling blastholes in rocks above medium strength. The creation of compressor stations at mines solely for servicing drilling machines is in many cases inexpedient and creates a number of serious inconveniences associated with operation of the power facilities.

All that has been said above prompts the development of electric drilling machines of rotary-percussive action. The main difficulty in creating such machines lay in the absence of a sufficiently simple and reliable design for an electric percussion unit.

The solution of this problem is being pursued by various organizations in two directions:

1. The development of electromechanical percussion units in which the rotary motion of the rotor of an electric motor is converted, by means of mechanical transmissions, into the reciprocating motion of the striker (hammer).

2. The development of electromagnetic percussion units in which rotary-translational motion is imparted to the striker directly by the drive motor.

The principal advantages of electromechanical percussion units are the availability of reliable and improved rotating electric motors and the ease of providing ventilation for the entire percussion unit. Their disadvantages are the complexity of the design as a whole, the large number of moving parts, and the increased cost of the percussion unit.

The main advantages of electromagnetic hammers are the simplicity of their design and the small number of moving parts, which apparently makes it possible to achieve greater reliability and durability of the impact unit and a significantly lower manufacturing cost in comparison with electromechanical impact units.

The main reasons hindering the industrial application of hammers created on the electromagnetic principle are overheating and the low energy of a single blow. A number of researchers believe that these shortcomings can be eliminated and that sufficiently powerful and advanced reciprocating electromagnetic (solenoid) machines can be created.

On this question let us cite the opinions of several specialists.

Prof. A. I. Moskvitin: “A properly designed solenoid hammer can be no heavier than a motor-driven one, and its efficiency will also be no lower. At the same time this hammer is considerably simpler and cheaper” [4].

N. A. Shmarov: “An electric-percussive hammer must have a simple design, the number of wearing parts must be minimal, and the hammer must be simple to manufacture, repair, and operate. These conditions are best satisfied by hammers built, conventionally speaking, on the solenoid principle. Obviously, the solution of the problem should proceed mainly along this path” [5].

Acad. A. M. Terpigorev: “Comparing electromechanical and solenoid hammers, we can draw the following conclusion. Electromechanical hammers use high-speed asynchronous three-phase motors already mastered by industry, having good efficiency and \(\cos \varphi\). However, the presence of a transmission greatly reduces the overall efficiency of the hammer, and its design becomes too complicated, which leads to an increase in the number of malfunctions during operation. Conversely, in solenoid hammers the transmission is completely absent, which simplifies and facilitates their design and makes it reliable in operation. At the same time, the reciprocating electric motor, not yet mastered by industry, has low efficiency, heats up strongly in operation, and develops insufficient power. However, this does not exclude the possibility that sufficiently powerful and reliable solenoid hammers will appear in the near future” [6].

Work carried out at the Department of Mining Machines of the Tomsk Polytechnic Institute (TPI) also indicates the possibility of creating a workable design of a powerful electric impact unit of the electromagnetic type. At TPI, a prototype of a multi-coil electromagnetic hammer of the MS-3 type with a synchronous sparkless switch [7, 8] was developed and manufactured; it is considerably simpler than electromechanical hammers and, in its technical characteristics, close to them.

The main shortcomings of the electric impact unit of the MS-3 type are the presence of a rotating switch, which requires systematic maintenance, and the irrational use of the magnetic system, manifested in the separate use of coils for the forward and return strokes.

Studies of the MS-3 showed that the efficiency of the electromagnetic drive can be significantly increased by choosing a rational design of the magnetic circuit and operating modes of the hammer.

As a result of the study of the MS-3, as well as electromagnetic hammers of the MS-15 and MS-16 types [7, 8, 9, 13], at the Department of Mining Machines of TPI an experimental model of a powerful impact ...

unit of the electromagnetic type, intended for drilling machines (Fig. 1).

The impact unit (Fig. 2, a) consists of a tool 1, on which the striker 2 delivers blows, a magnetizing coil 4, a yoke 5, and pole pieces 6.

Fig. 1. General view of the model of the impact unit.

pieces 6. The operating principle of the unit is as follows. Under the action of the elastic forces of the spring, the striker is in the leftmost position. When switch P is closed (Fig. 2, b), a pulsating current \(i\) begins to pass through the circuit: magnetizing coil, semiconductor rectifier V, and fuse P.

Fig. 2. Sketch of the impact unit (a), power-supply circuit (b), and oscillogram of the working process (c).

The striker, under the action of the electromagnetic forces of the coil, moves to the right. The dynamics of the hammer are calculated so that, as the striker approaches the position of magnetic equilibrium (indicated in Fig. 2, a by the dotted line, and in Fig. 2, b by the straight line \(ab\)), the flow of current in the coil is ...

the coil is switched off, but the striker, under the action of the accumulated kinetic energy, continues to move in the same direction, compressing spring 3 (Fig. 2, a). During the interval when the rectifier current is zero, the striker passes through the position of magnetic equilibrium, additionally compressing spring 3. By the beginning of the next half-wave of current in the coil, the striker occupies the extreme right-hand position. Under the action of the elastic forces of the spring and of the second half-wave of current, the striker moves in the direction of the tool. As the striker approaches the position of mean magnetic equilibrium, the current in the coil ceases to flow, but the striker, under the action of the accumulated kinetic energy, continues its motion and delivers a blow to the tool. The cycle then repeats.

From an analysis of the operating principle of the impact unit considered, it is evident that the frequency of the striker blows is synchronous with the frequency of the supply voltage and is determined as

\[ n=\frac{60\cdot f}{2p}\ \text{blows/min}, \tag{1} \]

where \(n\) is the frequency of the striker blows; \(f\) is the frequency of the supply voltage; \(2p\) is the number of voltage periods used to complete one full cycle of striker motion.

Consequently, changing the blow frequency is possible (for the given circuit for energizing the coil) only by changing \(f\) or \(2p\).

The model of the electromagnetic impact unit has the following technical characteristics:

    energy of a single blow, kgm — 4.0—4.5;
    blow frequency per min — 1500;
    type of current, single-phase 50 cps;
    voltage, V — 380;
    efficiency, % — 46—50;
    weight, kg — 47.

Of greatest interest is the determination of the dependence of the operating regime on design factors, such as: the displacement of the tool shank or the displacement of the buffer spring relative to the magnetic system of the impact unit. It has been established that, all other conditions being equal, the magnitude of the initial compression \(l_0\) of the spring by the striker has a substantial effect on the blow energy, the efficiency, and the stability of operation of an electromagnetic-type impact unit (Fig. 2, a). Here, by the magnitude of the initial compression of the spring we mean the distance between the rear end of the striker when it is in the blow position and the front turn of the buffer spring, if the latter were in its free state.

The dependence of the blow energy on the magnitude of the initial compression of the spring is characteristic of all electromagnetic impact units, regardless of their operating principle and energizing circuit, and is explained by the fact that the buffer spring is not only a shock absorber but also an elastic element that accumulates the kinetic energy of the striker’s motion during the idle stroke and returns the striker during motion toward the blow. When the parameters of the spring are changed, the working process of the impact unit changes, as does the magnitude of the power drawn from the mains and transmitted by the striker to the tool shank. The first stage of the experimental investigation of the created model of a powerful electromagnetic impact unit (Fig. 1) was the selection and establishment of the dependence of the blow energy \(A_y\) on the magnitude of the initial compression of the spring \(l_0\).

At small values of \(l_0\), the force of the striker’s preliminary compression is also small; therefore, at comparatively small values of the current in the coil, the striker begins to move and, before the half-wave of current has ceased to flow, reaches the position of magnetic equilibrium (Fig. 3). In this case the electromagnetic forces brake the motion of the striker, reducing its velocity and, consequently, the magnitude of the striker’s free travel. As a result, the striker stroke and the energy of a single blow decrease. With an increase in the initial compression of the spring, the braking action of the electromagnetic force decreases and the impact energy increases. The maximum impact energy corresponds to a definite value of the spring compression, which for the given design depends on the stiffness of the spring and on the magnitude of the initial working air gap \(\delta_0\). A further increase in the spring compression again leads to a decrease in the energy of a single blow, since in this case the striker approaches the position of magnetic equilibrium later than the current flow in the coil of the impact unit ceases, i.e., a decrease in the magnitude of the striker’s free travel is again observed. If by the region of stable operation of the impact unit one understands such an operating mode in which the energy of a single blow changes by no more than 10% relative to the maximum, then it may be said that changing the magnitude of the initial spring compression by \(\pm 25\%\) from the optimum value does not lead to a noticeable change in the operating mode of the impact unit.

Fig. 3. Dependence of impact energy on the magnitude of the initial compression of the spring.

The results of the experimental investigation shown in Fig. 3 correspond to such an operating mode of the impact unit in which there is no displacement of the tool shank relative to the magnetic system.

During operation of a drilling machine with this impact unit, the magnitude of the initial spring compression and the striker travel toward the tool will remain constant only when the feed force exceeds the maximum value of the electromagnetic forces of the coil. At smaller values of the feed force, it is practically impossible to strictly fix the magnitude of the initial spring compression and of the striker travel toward the tool (i.e., the displacement of the tool shank). Therefore, investigations of the operation of the impact unit in a mode in which the magnitude of the initial gap changes are of no less interest. The latter has a significant influence on the operating mode, since the distribution of the magnetic flux throughout the entire system of the electromagnetic unit depends on the magnitude of the gap.

Moreover, if one assumes that the electromagnetic tractive force is proportional to the product of the square of the ampere-turns and the derivative of the air-gap permeance with respect to the gap itself [11, 12], then, taking into account the results of the work performed [13], the following conclusions may be drawn.

1. At large values of the initial air gap, the electromagnetic tractive force is determined mainly by the magnitude of the current flowing in the coil. This is explained by the fact that the rate of change of the conductance of the air gap at large values of \(\delta_0\) is almost independent of the displacement of the striker.

2. At small values of the initial air gap \(\delta_0\), the rate of change of conductance increases considerably. Consequently, the electromagnetic tractive force will be determined not only by the magnitude of the current, but also by the rate of change and the magnitude of the conductance.

Let us consider three characteristic operating regimes of an electromagnetic impact unit, depending on the magnitude of the initial air gap.

A. For \(\delta_0 > \delta_0'\) (where \(\delta_0'\) is the value of the initial air gap corresponding to the optimum operating regime), the initial velocity of motion of the striker, determined by the magnitude of the electromagnetic force, is small, and the striker will approach the position of magnetic equilibrium later than the current in the coil ceases. The part of the path corresponding to the maximum value of the rate of change of conductance will be traversed by the striker only under the action of the braking forces of friction and of the spring, since at this time the current is zero. Consequently, the reserve of kinetic energy of the striker will be insufficient to provide the necessary initial gap for motion in the direction of impact. As a final result, the impact energy decreases. For \(\delta_0 \gg \delta_0'\), the striker may return to the position of magnetic equilibrium during the next half-wave of the current (Fig. 2, c). In this case the operation of the impact unit will be unstable, and the striker may fall out of synchronism.

B. For \(\delta_0 < \delta_0'\), the electromagnetic force, determined by the product of the square of the ampere-turns and the rate of change of the conductance of the air gap, reaches a considerable value, and the striker, acquiring a higher velocity, approaches the position of magnetic equilibrium earlier than the current flow in the coil ends. On leaving this position, the striker begins to be braked by the current, and the electromagnetic impact unit passes from the motor to the generator mode of operation [15], as a result of which the kinetic energy of the striker by the end of the half-period of current flow drops sharply. As in the case \(\delta_0 > \delta_0'\), the impact energy of the striker decreases, and for \(\delta_0 \ll \delta_0'\) an unstable operating regime is observed.

Thus, for an electromagnetic impact unit there exists a quite definite value of the initial air gap corresponding to the maximum value of the energy of a single impact.

In order to determine the most advantageous value of the initial air gap and to study its influence on the impact energy and the stability of operation of the impact unit, the dependences \(A_y = f(\delta_0)\) were recorded for various values of the preset length of the spring \(l_y\) (Fig. 4). For all values of the initial air gap \(\delta_0\) lying within the range \(21\text{–}28\ \mathrm{mm}\), the operating regime of the impact unit is stable. For \(21 > \delta_0 > 28\), unsatisfactory starting and cases of the striker stopping in the position of magnetic equilibrium are observed. With an increase in the initial air gap, the energy of a single impact increases for all the cited values of the preset length of the buffer spring (Fig. 4). The impact energy reaches its maximum value at \(\delta_0 = 26\ \mathrm{mm}\). With a further increase in the air gap, the impact energy \(A_y\) decreases.

The investigation of the model of the impact unit was also carried out in other possible operating regimes. The maximum value of the impact energy

during steady operation was 4.5 kgm, and the minimum values were 2.3 kgm, which, converted to impact power, amounts respectively to 1.1 kW and 0.57 kW.

Fig. 4. Dependence of impact energy on the magnitude of the initial air gap.

The results obtained provide grounds for recommending the electromagnetic-type impact unit above all for use as an impact unit for rotary-impact drilling machines. In addition, impact units may be used for vibrating

Fig. 5. Dependence of the efficiency of the impact unit on the magnitude of the initial air gap.

the buckets of rock-loading machines of periodic and continuous operation, which will make it possible to eliminate adhesion of rock to the bucket, increase the bucket filling coefficient, and, overall, increase the productivity of the machine.

It should be noted that the minimum efficiency of the electromagnet under the most unfavorable operating conditions was 34%. In the zone of stable operation, the efficiency varies within the range 45–49% (Fig. 5).

A comparison of the results of the experimental investigation presented in Figs. 3 and 5 shows that the parameters characterizing the operating mode of the impact unit with the greatest energy per single blow differ somewhat from the parameters corresponding to the maximum efficiency at one and the same value of the initial air gap. For example, at \(\delta_0 = 26\) mm, the maximum value of \(A_y\) corresponds to an initial spring compression of 6 mm, while the maximum efficiency corresponds to 5 mm; at \(\delta_0 = 23\) mm, the initial spring compression is respectively 7 and 9 mm.

It is characteristic that the change in the parameters characterizing the greatest value of the efficiency occurs in the direction of stable operation of the impact unit. It should be noted that the obtained values of the efficiency of the impact-unit model are not limiting, although they exceed the efficiency of some of the best examples of electromechanical-type impact units [14].

Conclusions

1. Experimental investigations of a model of an electromagnetic-type impact unit with free striker run-out showed that, on the basis of a half-wave rectification circuit and the given operating principle, it is possible to create a powerful and efficient impact-action machine.

2. The blow frequency of an electromagnetic impact unit connected according to a half-wave current-rectification circuit with free striker run-out can vary in proportion to the frequency of the supplied voltage. Variation of the blow frequency at a constant voltage frequency is also possible by tuning the unit to a special operating mode. However, this apparently will require changes in the design of the impact unit and the carrying out of additional investigations.

3. The operating mode is strongly influenced by the initial compression of the spring and the magnitude of the initial air gap. Variation of these parameters entails a change in the impact energy and the efficiency of the impact unit. Maximum values of the energy per single blow of 4.5 kgm and an efficiency of 47–48% were obtained at \(\delta_0 = 26\) mm, \(l_0 = 6\) mm, and a spring stiffness coefficient of 1.18 kg/mm.

4. The results obtained in investigating a model of an electromagnetic-type impact unit may serve as the basis for designing an electric rotary-percussion drilling machine and vibration units for vibrating the bucket of a rock-loading machine.

LITERATURE

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