N. P. RYASHENTSEV, A. P. TRONOV

SOME RESULTS OF EXPERIMENTAL STUDIES OF ELECTROMAGNETIC HAMMERS

(Presented by the Department of Mining Machines and Mine Transport)

The first electromagnetic hammers did not become widespread because of the rapid burning of the contacts of the mechanical interrupters of the power-supply circuit, which served to commutate the current in the windings of the electromagnets [1, 2]. Later, the mechanical interrupters were replaced by gasotrons, which in turn were replaced first by cuprous rectifiers and then by germanium and silicon diodes [3].

The use of semiconductor rectifiers made it possible to improve substantially the parameters of existing hammers, to create new power-supply circuits, and to develop hammers with qualitatively new working processes [4].

At present, electromagnetic hammers made by the firms “Sintron” (USA), “Simbi” (Italy), AEG, and “Millers-Falls” (FRG) are widely used in industry and construction. A prototype of the EK-1M electromagnetic hammer has been manufactured in Czechoslovakia. In the USSR, production of the MS-15 and MS-18/36 hammers is being mastered [5].

An analysis of the power-supply circuits and working processes of electromagnetic hammers makes it possible to outline ways of improving machines of this class. The present work is based on studies of AEG, Sintron, and MS-15 hammers as machines that employ the most characteristic methods of obtaining the reciprocating motion of the striker [6].

The investigations were carried out according to the procedure set forth in the literature [7, 8, 9].

The AEG hammer (Fig. 1, a) operates from a constant voltage obtained by means of a special substation, the circuit of which is shown in Fig. 1, b. The reciprocating motion of the striker is produced by two coils, switched on alternately by a commutator controlled by the striker [2].

At the moment when the striker strikes the shank of the tool (point a of the striker travel curve S in Fig. 2), the switch closes the circuit of the reverse-stroke coil, owing to which the striker begins to move toward the spring (segment a—b). The forward-stroke coil is switched on somewhat earlier than the compression of the spring by the striker ends (segment b—v). After stopping at point v, the striker, under the action of the elastic forces of the spring and the forces developed by the forward-stroke coil, rushes toward the tool and delivers a blow at point a. The cycle then repeats. The hammer makes 1500 blows/min; cos φ is fairly high—0.64. Features

Fig. 1. AEG hammer.
a) Section of the hammer. 1 — tool, 2 — anvil, 3 — magnetic core,
4 — forward-stroke coil, 5 — striker, 6 — return-stroke coil,
7 — switch rod, 8 — front spring of the switch, 9 — switch contacts,
10 — rear spring of the switch, 11 — shock-absorber cup,
12 — shock-absorber spring, 13 — switch.
b) Hammer power-supply circuit. Tr — transformer, FSC — forward-stroke coil, RSC — return-stroke coil.

Fig. 2. Oscillograms of the working process of the AEG hammer.
\(S\) — striker travel, \(I_{\mathrm{RSC}}\) — current in the return-stroke coil, \(I_{\mathrm{FSC}}\) — current in the forward-stroke coil, \(v\) — striker velocity, \(\varepsilon\) — acceleration, and resultant force acting on the striker.

the power-supply circuit of the AEG hammer (the presence of capacitors serving to reduce the arc when the switch contacts open, and of the resistance \(R_1\), owing to which the return-stroke coil remains energized at all times, regardless of the position of the switch) leads to the appearance of parasitic currents in the coils, causing electromagnetic braking of the striker both on the forward and on the return stroke.

Electromagnetic braking of the striker by the operating current flowing in the forward-stroke coil can be eliminated if the return-stroke coil is disconnected and the forward-stroke coil is connected when the striker is at point \(v\). This can be achieved by appropriate adjustment of the switch. This latter circumstance is one of the principal advantages of the AEG hammer.

In hammers of the Sintron firm (Fig. 3, a), two coils switched on alternately are likewise used to produce the reciprocating motion of the striker. However, unlike the AEG hammer, the latter operate from an alternating-voltage supply; the alternate switching-on of the coils is accomplished by means of two rectifiers (Fig. 3, b).

Fig. 3. Sintron hammer.
a) Section of the hammer. 1 — front flange, 2 — bushing, 3 — tube, 4 — magnetic core, 5 — armature, 6 — forward-stroke coil, 7 — striker, 8 — return-stroke coil, 9 — piston, 10 — handle, 11 — buffer spring, 12 — cup, 13 — switch, 14 — panel with rectifiers, 15 — fan blades, 16 — fan motor, \(T_1\)–\(T_8\) — locations for embedding thermocouples for taking the hammer heating curves.
b) Hammer power-supply circuit. \(KPX\) — forward-stroke coil, \(KOX\) — return-stroke coil, \(B_1B_2\) — rectifiers, \(D\) — fan motor, \(Vkl.\) — switch.

It follows from the oscillograms (Fig. 4) that, regardless of whether the ram rebounds after impact (segment \(d—e'—\) on the ram-travel curve \(S\)), or moves together with the tool (segment \(d—e—n\)), the drawing of the ram into the return-stroke coil begins from the moment when the current in the forward-stroke coil ceases to flow (at point \(a\)). The interval

Fig. 4. Oscillograms of the working process of the Sintron hammer.
\(S\)—ram travel; \(U_{\text{r.c.}}, I_{\text{r.c.}}\)—voltage and current in the return-stroke coil;
\(U_{\text{f.c.}}, I_{\text{f.c.}}\)—voltage and current in the forward-stroke coil; \(v\)—curve of the change in ram velocity; \(\varepsilon\)—curve of the change in ram acceleration and of the resultant force acting on the ram.

of time from the moment of impact to the start of the ram’s motion is about 30% of the time of the entire working cycle. At the end of the return stroke (point \(b\)) the ram strikes the spring and compresses it (segment \(b—c\)). Then, under the action of the elastic forces of the spring and the electromagnetic forces developed by the forward-stroke coil, the ram moves toward the tool and delivers an impact at point \(d\). Thus, the presence of two coils and two valves in the hammer supply circuit makes it possible to use both half-waves of current from the supply network; in this case the hammer makes 3000 impacts/min. The latter circumstance has a favorable effect on the average value of \(\cos \varphi\) of the hammer over the working cycle, which was of the order of 0.6–0.7. A significant drawback is the large overlap of currents, caused by the presence of inductance and by the adopted supply circuit: for more than 50% of the time of the entire working cycle, current flows simultaneously in both coils. This substantially reduces the efficiency of utilization of the energy supplied to the hammer. Since the coils of the AEG and Sintron hammers are switched on alternately, it is possible to reduce the weight of the hammers if the same coils are used for the forward and return strokes [7].

The desire to increase the ratio of useful power to the weight of the electromagnetic hammer led to the creation of supply circuits that ensure the use of the same coils for the forward and return strokes of the ram. Thus, resonant hammers [3] and hammers with free ram run-out [2–3] were proposed.

Figures 5 and 6 show the section, power-supply circuit, and oscillograms of the operating process of the MS-15 hammer with free run-up of the striker [10]. Point \(a\) of the striker travel curve \(S\) (Fig. 6) corresponds to the moment at which the striker impacts the shank of the tool. The first half-wave of current after the impact carries the striker toward the spring. By the time the first half-wave of current has finished flowing, the striker reaches the position of magnetic equilibrium: the ends of the striker are under poles \(b\)—\(v\) and \(k\)—\(m\). The next half-wave of current is not passed by the valve; therefore the striker continues to move toward the spring, providing the working clearance on the side of the tool (section \(v\)—\(g\)). Approaching the spring (point \(g\)), the striker compresses it (section \(g\)—\(d\)). Then, under the action of the elastic forces of the spring and the electromagnetic forces of the solenoid caused by the next half-wave of current, the striker rushes toward the tool (section \(d\)—\(e\)—\(k\)), passes the pole at section \(k\)—\(m\) (the second half-wave of current in the coil is no longer flowing by this time), and delivers a blow at point \(a\), providing the working clearance on the side of the spring. Thus, in hammers with free run-up of the striker, the same coil is used for the forward and return strokes of the striker. The hammer makes 1500 blows/min, since the complete working cycle is performed during two periods of the applied voltage. Obviously, the transition to a half-wave rectification circuit makes it possible to avoid overlapping currents, but does not ensure the hammer’s operating process without braking of the striker in one of the sections of its travel. In this case, electromagnetic braking of the striker occurs in section \(g\)—\(d\) (Fig. 6). To implement a brake-free operating mode, ...

Fig. 5. MS-15 hammer.
a) Section of the hammer. 1 — tool, 2 — key, 3 — chuck, 4 — rubber pivot washers, 5 — nut, 6 — bushing, 7 — front flange, 8 — housing, 9 — striker, 10 — coil, 11 — guide tube, 12 — pole, 13 — yoke, 14 — locking pin, 15 — piston, 16 — buffer spring, 17 — cup, 18 — switch.
b) Hammer power-supply circuit. \(P\) — fuse, \(VG\) — germanium rectifier, \(Vkl.\) — switch, \(K\) — coil.

of the hammer operation, it is necessary that the ram pass through separate portions of the working stroke as follows:

1) the path \(a—b—v\) during the time \(t_{\text{p}}\) of flow of the first half-wave of current;
2) the distance \(v—g—d\) during the time \(t_{\text{n}}\), during which no current flows in the coil;
3) the distance \(d—e—k\) during the time \(t_{\text{p}}\) of flow of the second half-wave of current.

Fig. 6. Oscillograms of the working process of the MS-15 hammer.
\(S\) — ram stroke, \(U, I\) — voltage and current in the coil, \(P_c\) — power consumed from the network, \(V\) — curve of the change in ram velocity, \(F_p\) — curve of the change in the resultant force acting on the ram.

Conditions 1–3 are incompatible, since in real machines \(t_{\text{p}} = t_{\text{p2}} > t_{\text{n}}\), and the speed of the forward stroke exceeds the speed of the return stroke of the ram. Consequently, braking of the ram on one of the portions of its stroke is a law of the working process of hammers with a freely moving ram, designed according to the scheme of Fig. 5. In the MS-15 hammer the braking time of the ram amounts to more than 25% of the time of the entire working cycle. The average value of \(\cos \varphi\) for one hammer is low and is 0.3–0.4. When two hammers operate from one network, the overall \(\cos \varphi\) approaches the value of \(\cos \varphi\) of the Sintron hammer, if both half-waves of current are used [3].

Thus, a characteristic feature of the working processes of electromagnetic hammers with rectifiers (Sintron and MS-15) is the inevitability of electromagnetic braking of the ram, which has an adverse effect on the energy balance of the working cycle. Thus, for example, in the MS-15 hammer the kinetic energy of the ram during the return stroke, at the moment when it is under the poles (points \(b, v\), Fig. 6), is equal to 0.29 kgm; in the spring, however (portion \(g—d\), Fig. 6), 0.056 kgm is accumulated, i.e., less than 20% of the mechanical energy obtained during the return stroke. The remaining part of the mechanical energy is converted into magnetic energy and is also dissipated as heat. Therefore the question of a rational

constructing an operating cycle that excludes, in particular, electromagnetic braking of the striker is of primary importance.

A comparison of the technical parameters of the hammers shows that the MS-15 hammer has a specific power 3.4 times greater than that of the AEG hammer at the same weight of these machines (Table 1). At the same time, the Sintron and MS-15 hammers, which use different methods of obtaining the reciprocating motion of the striker, have practically the same power. In this connection the question of the value of the principles for obtaining the reciprocating motion of the striker required additional studies [11]. These contradictions are aggravated by the fact that most of the design parameters of the AEG, Sintron, and MS-15 electromagnetic hammers are commensurable (Table 2). The hammers have practically identical total cross sections of the coil windows, cross sections of the strikers and magnetic cores, commensurable weights of active materials, and values of ampere-turns.

Table 1

Table 2

At the same time, the geometric dimensions and shapes of the electromagnets are completely different. The striker of the AEG hammer is rectangular in shape, the coil winding height is 18 mm, the coil length is 56 mm, and the magnetic core consists of two Ш-shaped packets. The striker of the Sintron hammer is cylindrical, the coil winding height is 14 mm, the coil length is 77 mm, and the magnetic core is made in the form of eight Ш-shaped packets assembled in a star configuration. The striker of the MS-15 hammer is cylindrical; the shape of the coil-window section is trapezoidal, with bases of 46 mm and 90 mm and a height of 30 mm; the magnetic core consists of a cylindrical yoke and conical poles (Figs. 1, 3, 5).

Obviously, in view of the absence of a theory of electromagnetic hammers, the authors of these machines, when choosing the geometric dimensions of the electromagnets, were forced to be guided only by design considerations.

This partly explains the fact that hammers having commensurate weights of active materials provide different useful powers (32 W, 250 W, and 110 W).

Thus, the designs and power-supply circuits of the electromagnetic hammers considered can be considerably improved. By using silicon controlled rectifiers in hammer power-supply circuits, it is possible completely to eliminate the overlap of currents in the coils of “Sintroi”-type hammers and to achieve a non-braking mode of operation of hammers with free striker overtravel.

Another reserve for increasing the efficiency, \(\cos \varphi\), and specific power of electromagnetic hammers consists in finding the optimal geometrical dimensions and electromechanical parameters of traction electromagnets, the existence of which was proved in [12] and confirmed by the present investigations. The need to conduct experiments in order to select rational parameters of the electromagnets of impact-action machines and to improve their working processes is evident.

REFERENCES

1. A. I. Moskvitin. Electric hammers. Elektrichestvo, No. 20, 1933.

2. A. I. Moskvitin. Electric reciprocating machines. Publishing House of the Academy of Sciences of the USSR, Moscow–Leningrad, 1950.

3. A. I. Moskvitin. An electric hammer for the coal industry. Elektrichestvo, No. 4, 1939.

4. Inventor’s certificates No. 42962, 1933, No. 132578, No. 139622, 1960.

5. N. P. Ryashentsev, V. V. Ivanov. Solenoid hammers for installation work. Technical-Economic Bulletin, Kemerovo Council of National Economy, No. 10, 1960.

6. E. M. Timoshenko. Candidate dissertation. Tomsk Polytechnic Institute, 1963.

7. A. I. Moskvitin. Electric or solenoid hammer. Elektrichestvo, Nos. 5, 6, 1934.

8. N. P. Ryashentsev, E. M. Timoshenko. Solenoid hammers. Proceedings of TPI, vol. 108, 1959.

9. N. P. Ryashentsev. Investigation of the dynamics of solenoid hammers. Proceedings of TPI, vol. 108, 1959.

10. N. P. Ryashentsev, E. M. Timoshenko. Experimental investigation of the working process and energy characteristics of a solenoid hammer with free striker overtravel. Proceedings of TPI, vol. 123, 1963.

11. N. P. Ryashentsev, A. P. Tronov. On methods of obtaining reciprocating motion of the striker in electromagnetic impact-action machines. The article is included in the present collection.

12. N. P. Ryashentsev, E. M. Timoshenko, A. V. Frolov, A. P. Tronov. On the parameters of a solenoid hammer with free striker overtravel. Proceedings of TPI, vol. 123, 1963.