SOLENOID HAMMERS

N. P. Ryashentsev, E. M. Timoshenko

There are very many diverse proposals, patents, and authors’ certificates concerning solenoid hammers, but only a small number of them have been produced in series. Therefore, this article describes only solenoid-type hammers that differ fundamentally in design and in the method of changing the direction of motion of the striker.

The first designs of solenoid hammers operated on direct current with alternate switching of the coils for the forward and return strokes of the striker. In 1852 a patent was taken out for a direct-current electric machine with reciprocating motion of the striker. This machine consisted of two solenoids with alternate switching of the traction coils. Typical representatives of such hammers are the hammers designed by Pokrovsky, Schiemann, Thom, Kozyrev, and Starikov [2].

The hammer designed by Pokrovsky (Fig. 1, a) consisted of coils 1, 5, and 7, through which direct current passes. Cores 2, 8, and 9, mounted on crosshead 3 together with striker 6, were drawn into these coils. Coils 5 and 7 were intended for the working stroke, and coil 1 for the return stroke of the striker. When current flowed in coils 5 and 7, under the action of electromagnetic forces cores 8 and 9 were drawn in, and striker 6 delivered a blow to the working tool. At the moment of impact of the striker, the switch cut off the current from coils 5 and 7 and switched on coil 1, which drew in core 2 and returned the striker to its rear extreme position. Then the coils for the working stroke of the striker were switched on again, and the cycle was repeated.

The main disadvantages of the Pokrovsky hammer were the presence of a switch, whose contacts burned out within several minutes and failed; the absence of a closed magnetic circuit in the hammer; and the limited stroke of the striker. For these reasons the hammer did not find application in industry. In the hammers designed by Schiemann it was proposed to switch—

switching the coil on by sections. This considerably complicated the switch, leaving unresolved the question of the duration and reliability of its operation.

In 1879 Werner Siemens, and in 1891 Deprez, proposed a synchronous hammer (Fig. 1, б) intended for drilling shot-holes. The hammer consisted of three coils, inside which a steel striker—the core—moved. In the hammer designed by Werner Siemens the two outer coils were fed with alternating current from reduced-frequency generators, while the middle coil was fed with direct current, giving the striker a constant polarity. The hammer designed by Deprez received power from a generator according to the circuit shown in Fig. 1, б.

Fig. 1. Designs of solenoid hammers:
а—Pokrovsky’s; б—Werner Siemens’s; в—Morzene’s; г—V. Ya. Starikov’s.

The generator for feeding the hammer coils was a direct-current machine with rotating brushes 5 and 6 and a fixed brush 7. The voltage supplied to the coils was determined by the rotational speed of the generator armature, and the frequency of the alternating current in coils 1 and 3 by the rotational speed of brushes 5 and 6. Despite the original idea underlying the hammers of Siemens and Deprez, they were unsuccessful. The main shortcomings of these hammers were the very great heating of the windings and the presence of two currents—alternating and direct. This greatly complicated and increased the cost of the installation, since it required a quiet-running, expensive, and bulky low-frequency generator. Despite these shortcomings, these hammers were the first to be used in practice (for drilling shot-holes) and existed until 1898, when they were replaced by a more advanced type of hammer of the Morzene system.

In Fig. 1, v, the construction of Morvzen’s hammer and the circuit for its supply are shown. This hammer had two coils, one of which was intended for the forward stroke, and the other for the reverse stroke of the striker. The coils were supplied from a special generator, on the shaft of which a synchronous changeover switch was mounted, connecting one coil during the passage of one half-wave of alternating current, and the other coil during the passage of the other half-wave of the current. Owing to this, the striker was drawn now into one coil, now into the other. Instead of a third excitation coil, its function here was performed by the residual induction of the striker, since the polarities of the coils were opposite. With the hammer, made with mica insulation of the coils, up to 4000 spike meters were tried. However, the hammer did not become widely used because of strong heating, the presence of reduced-frequency generators, and the unreliability of the switch. The entire converter installation required careful maintenance and supervision.

In 1895 a hammer was built that operated on the “transformer principle.” The hammer was based on the principle of the dynamic interaction of two current-carrying systems, not electrically connected with each other, but connected by a common alternating magnetic flux. On this principle a hammer of Starikov’s design was developed (Fig. 1, g). It consisted of fixed coils of the primary winding 2 and 5, a core 1, onto which laminated copper rings 4 were press-fitted, serving as the secondary winding of the transformer [3].

The hammer operates as follows. When current passes through coil 2, the latter creates an alternating magnetic flux, which closes through core 1 and the steel body of the hammer, crossing the copper rings 4 in the process. This flux creates in coil 4 (as in a transformer) a secondary e.m.f. The magnetic fields created by the currents in the coils will have opposite directions, as a result of which coil 4 together with core 1 will be repelled. When the core moves to the rear extreme position, the latter switches the supply of the coils. Coil 5 receives power, the electromagnetic process is repeated, and core 1 moves in the opposite direction. The phenomenon of repulsion of the coils occurs exclusively due to leakage fluxes, but since the latter are very small, the forces arising between the coils are also very small, and as a consequence weak blows are obtained and the efficiency does not exceed 2%. In addition, a drawback of this design is the difficulty of manufacturing a monolithic striker-core satisfying the impact conditions of operation, and the rapid (1.5–2 min.) burning of the switch contacts.

The creation of a simple and reliably operating switch presented great difficulties, since the hammer coil had...

has a large inductance, and an arc is formed at the switch contacts, which, at a high switching frequency, causes very rapid burning of the contacts. Methods of combating the arc used in low-current contacts (shunting with a capacitor [5], lowering the operating voltage, making the contacts from expensive alloys), various active methods of arc extinction, and also rational electromechanical calculation [1], are not successful because of the high power of the arc [6] and the frequency of its repetition. For example, Schiemann [4] and N. N. Maksimov [6] proposed sectional switching-on and switching-off of the hammer coils, while V. V. Moskovkin [8] developed a special lever switch. Despite the original design of the switch, the problem of the duration and reliability of its operation remained unsolved. The switch designed by Schiemann and N. N. Maksimov also burned out rapidly, and this was the main reason for the lack of adoption of hammers with a switch. Concerning switches for solenoid hammers, Prof. K. N. Shmargunov writes that analysis of the indicated designs leads to the conclusion that any attempt to design a hammer with a mechanical switch will inevitably be doomed (at the present level of technology) to failure because of the rapid burning of the contacts [4]. It is interesting to note that up to the present time there is no simple, reliable, and long-lasting switch controlled by the hammer striker.

In view of the fact that hammers with current switches controlled by the striker did not become widespread because of the rapid burning of the contacts, Thom proposed the design of a hammer with a synchronous switch [4, 7]. The action of the synchronous switch is based on the fact that, being mechanically unconnected with the striker, it is controlled by the AC mains voltage and opens the supply circuit of the solenoids at the moment when the current in it passes through zero or is close to zero.

The design of Thom’s hammer is shown in Fig. 2, a. From the drawing presented it is clear that two coils, 1 and 2, are mounted on a horseshoe-shaped magnetic core made of laminated steel. The hammer striker is fastened to armature 3 and bears against a cylindrical spring. Switching of the solenoids was carried out by vibrator 4, operating on the principle of vibrators used in radio engineering. When current is passed through coils 1 and 2, armature 3 is attracted to the magnetic core; at the same time the striker delivers a blow to the working tool, compressing the spring. When the supply to the coils is switched off, the spring returns the core-armature to its initial position, preparing a repeated blow. When the solenoids are supplied with current at a frequency of 50 periods per second, the hammer will make 3000 blows per minute.

The design of Schöller’s hammer is similar to Thom’s hammer, except that the switch in Schöller’s hammer operated on the principle of a synchronous rotating commutator.

The shortcomings of these hammers are that they have additional, rather complicated equipment: a vibrator in the hammer of the Tom system and a synchronous commutator in the Schuler-system hammer, which not only complicate and increase the cost of the hammer, but also do not ensure reliable and prolonged operation. In practice, here too, sparking and burning of the switch contacts were observed, since the phase of the solenoid current depends on many variable factors, while the moment at which the contacts open does not depend on the phase of the solenoid current. The insignificant impact energy, low efficiency, and strong heating of the windings and steel limited the possibilities for industrial application of the hammer.

Instead of synchronous switches, low-speed generators of reduced current frequency were used [10—12]. But these generators also did not become widespread, since they were bulky and expensive, and the entire installation proved very complex and required systematic supervision of its operation.

The appearance of semiconductor rectifiers made it possible to design solenoid hammers without mechanical current switches. Representatives of such designs are the Syntron hammer, developed and produced by National Electric MfgCo, Pittsburgh, and the hammer of C. S. Weyandt [9].

The design and electrical circuit of the Syntron hammer are shown in Fig. 2, b. As can be seen from the figure, the hammer consists of coils 1 and 2, a magnetic core 3 made of laminated steel, a steel striker 4 moving in a bushing 5 made of nonmagnetic material; in the walls of the bushing the poles of the magnetic core are arranged flush with its inner surface.

To reduce eddy-current losses, longitudinal slots are made in the bushing. The hammer casing 6 was made of magnetically conductive material and closed the magnetic flux along the outside of the solenoid. Spring 7 and spring plates 8 and 9 are intended to reduce recoil. The hammer coils (1, 2) were connected to the mains according to a single-half-period rectification circuit, as a result of which they were energized alternately and the striker was drawn now into one coil, now into the other. The frequency of oscillation of the striker occurs synchronously with the frequency of the current in the mains. At a current frequency of 50 cycles per second, the hammer made 3000 blows per minute.

The Syntron hammer was tested in the VEI laboratory and showed the following characteristics: useful power of the hammer at the working tool 46 W, work per blow less than 0.1 kgm, efficiency of the entire installation 17.6%, efficiency of the hammer itself 32.3%.

In this hammer design the energy of a single blow is too small, and it is wholly insufficient for breaking even soft rock or coal. The hammer was used for artistic and engraving work on stone, etc. Heavy hammers—

hammers of this type became very hot, although artificial cooling was also provided for them.

The hammer did not find industrial application because of its low impact energy and considerable overheating.

If the replacement of special low-frequency generators and switches by semiconductor rectifiers greatly reduced the cost and simplified the hammer installation, it still proved impossible to get rid entirely of the additional apparatus. Bevy proceeded along the path of further simplifying solenoid hammers, proposing a synchronous solenoid hammer with a single coil

Fig. 2. Designs of solenoid hammers:
a—Thom; b—Sintroid; c—Bevy; d—N. M. Batuev.

(Fig. 2, c), operating without rectifiers and switches. This hammer consisted of a coil 1, surrounded by a magnetic circuit 2, a striker 3, making a small stroke 4, and a nut 5, which serves to regulate the preliminary compression of spring 6.

The hammer operated according to the following principle. When the maximum amplitude of the current flowed, the coil drew in the striker and compressed the spring. When the amplitude of the current decreased, the striker, under the action of the elastic forces of the spring, moved in the direction of the working stroke and struck the working tool. Consequently, during one period of alternating current the striker delivered two blows to the working tool; at a current frequency of 50 cycles per second the hammer delivered 6000 blows per minute. Despite the simplicity of its design, the hammer did not find application because of its insignificant impact energy and large overheating.

In 1932 N. M. Batuev [13] proposed a solenoid hammer operating on single-phase alternating current without a switch (Fig. 2, c). The currents flowing in coils 1 and 2 are shifted relative to one another by approximately 90°, so that at the moment when in one coil the current passes through a maximum, in the other coil the current value is approximately zero. Such a phase shift of the current in the coils was achieved by connecting an active resistance 4 into the circuit of one coil 1 and a reactive resistance 3 into the circuit of the other coil 2. Both resistances remain permanently connected in the circuit during operation of the hammer.

In 1934 N. M. Batuev developed an electric hammer [14] with two coils connected to a three-phase network according to an open-delta scheme. In addition, the author proposed connecting semiconductor rectifiers into the circuit of one of the coils.

The above circuits were intended to simplify the electric percussion hammer.

Solenoid hammers of the design of engineer N. M. Batuev did not become widespread because of the great heating caused by the simultaneous flow of current in the coils during the working and idle strokes of the striker.

Taking into account that the low efficiency of the hammer is due to large losses in the striker and the solenoid, Prof. K. N. Shmargunov developed the KNSh-7 solenoid hammer, operating on the principle of the hammer of the Werner Siemens design [14]. To increase the attractive forces of the solenoid, the author proposed floating bushings and a striker with longitudinal slots. The hammer design and the solenoid supply circuit are shown in Fig. 3, a. Three coils are installed in an aluminum casing; the outer coils 6 and 12 are supplied with alternating current, connected in parallel and turned toward one another with like poles. With a change in the direction of the current, the polarity of the outer solenoids changes; the middle coil 9 is supplied with direct current and serves to maintain the constant polarity of the solid-metal striker 10 with longitudinal grooves. To increase the tractive force of the outer solenoids, floating bushings 5 and 13 are installed. Springs 3 and 15 serve to reduce recoil. From the striker 10, by means of a rod through a special bushing, rotation is imparted to the fan 2, which is covered on the outside by 1. The energy of the return stroke of the striker is used to rotate the fan. To reduce the resistance to the magnetic flux, all three solenoids are closed into the magnetic circuit 8 with the poles of coils 4, 7, 11, and 14.

The hammer operates as follows. Direct current, obtained by connecting semiconductor rectifiers according to the circuit shown in Fig. 3, a, imparts constant polarity to the striker. Alternating current, flowing through the windings of the outer solenoids 6 and 12, changes the polarity of the outer coils twice during one period of current variation. In this case the striker under

under the action of magnetic forces, it is attracted to the pole once and repelled from it once. Consequently, at a current frequency of 50 cycles per second the striker makes 3000 blows per minute. Despite improvements in the design of the hammer—increasing the attractive force in the solenoid by using floating bushings and a striker with longitudinal slots—the hammer did not find industrial application, since the energy of a single blow was insignificant, while the overheating of the hammer exceeded the permissible level. When the impact energy was increased, the efficiency of the hammer decreased still further.

Fig. 3. Designs of solenoid hammers:
a—K. N. Shmargunov; b—A. I. Moskvitin.

In 1933, engineer A. I. Moskvitin [1] developed a resonant hammer, the design of which is shown in Fig. 3, b. The hammer consisted of coils 3, surrounded by a magnetic core 5. Under the action of the magnetic flux of the coil, the slotted striker 6 moves, delivering blows to the working tool 8. Spring 1 serves to increase the initial velocity of the striker and operates in resonance with its motion. The internal parts of the hammer are closed by caps 2 and 7, which are tightened with bolts 4.

The principle of operation of this hammer is as follows. When current flows in the coils, the striker tends to assume the position of magnetic equilibrium, but it assumes this position not immediately, but after making several oscillations about the middle position. The number of oscillations of the striker depends on the pulling force and the mass of the striker. When the frequency of the oscillations coincide

of the striker with the frequency of the current feeding the coil, resonant operation of the hammer begins. The number of impacts of the striker may be equal to the line frequency or a multiple of it. In 1940, engineer A. I. Moskvitin created a resonant hammer with characteristics close to those of the Sintron hammer. At 3000 impacts per minute, the work of a single impact amounted to 0.3 kgm. Despite the fact that the efficiency of the hammer was 32%, because of its insignificant energy the hammer did not gain industrial use.

Attempts to create a solenoid hammer for performing mining work presented great difficulties. Hammers without current interrupters worked tolerably well, but produced a weak blow, had low efficiency, and heated up rapidly. In order to increase the energy and efficiency of the hammer, designers again returned to hammer designs with interrupters.

The further development of solenoid hammers is described in works [15, 16].

As a result of the brief review carried out, solenoid hammers may be divided into the following groups:

1. By current: hammers of direct, alternating sinusoidal, and pulsating current.

2. By power-supply circuits: hammers with interrupters or with special thyratron generators, semiconductor rectifiers, and without switching devices.

3. By the location of the interrupter: hammers with an interrupter controlled by the striker, and with a remote interrupter of asynchronous or synchronous switching.

From the analysis of solenoid hammers it has been established that the principal reasons for the lack of widespread use of solenoid hammers were the following:

1. Great heating of the hammer, caused by large electrical losses in the solenoid.

2. Extremely small energy of a single impact at the working tool. Increasing the impact energy in direct-current hammers (hammers of the designs of Pokrovskii, Starikov, Schiemann, Schüler, Depelle, Morven, etc.) was not permitted by the interrupters existing at that time; and in hammers of the designs of Sintron, Bevi, Moskvitin, Shmargunov, Batuev, and others, as the impact energy is increased the already low efficiency decreases further, and, in addition, the overall dimensions, weight, and overheating of the hammer increase.

LITERATURE

1. A. I. Moskvitin. Electrical Machines of Reciprocating Motion. Academy of Sciences of the USSR, 1950.

2. N. A. Shmarov. Electropercussive Hammer for the Coal Industry. ONTI, NKTP, 1935.

3. Inventor’s certificate for invention No. 33499 of 26.II.1932. V. Ya. Starikov. Description of an electric impact tool.

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

5. Ein neuer electromagnetisches schlagwerzeug. Die werkzeug maschine No. 22, 1939.

1. Inventor’s certificate No. 111529 of 24.5.1932. N. N. Maksimov. Electric impact tool.

2. Inventor’s certificate No. 18324 of 12.8.1929. G. I. Semenov. Electric hammer.

3. Inventor’s certificate No. 27352 of 3.7.1932. V. V. Moskovkin. Electric hammer.

4. Patentschrift 13929 of 8.5.1926. C. S. Weyandt. Central Electric Tool Company.

5. Patentschrift 463667 of 30.5.1926. Electrisches schlagwerkzeug.

6. Brevet d’invention 476142 of 3.9.1914. International Electric Tool Company. Dispositif de commande de mouvements.

7. Patentschrift 465412 of 30.8.1928. Siemens—Shuckertwerke Akt.—Ges. in Berlin—Siemensstadt. Anlage zur speisung electrisch detriebenen Werkzeuge.

8. Inventor’s certificate No. 31877 of 21.6.1932. N. M. Batuev. Electric chipping hammer.

9. Inventor’s certificate No. 40278 of 30.11.1934. N. M. Batuev. Electric hammer.

10. P. M. Alabuzhev, N. P. Ryashentsev. On the problem of creating solenoid hammers. Izvestiya TPI, vol. 106, Metallurgizdat, Sverdlovsk, 1958.

11. N. P. Ryashentsev. Synchronous sparkless switch. Izvestiya TPI, vol. 106, Metallurgizdat, 1958.

Noted Misprints

Order 29