On the Development of Solenoid Hammers

P. M. Alabuzhev, N. P. Ryashentsev

The rapid growth of mechanization in the national economy is accompanied by the introduction of various mechanisms. In particular, pneumatic hammers and percussion-rotary drilling machines have found wide application in the mining, coal-mining, construction, metalworking, and machine-building industries, in track work on railways, in road building, and in earthworks for municipal and agricultural purposes.

The pneumatic hammer has come into wide use owing to its reliability in operation and the simplicity and low cost of its design. However, pneumatic hammers have a number of substantial shortcomings, the chief of which is that they cannot be used where there is no pneumatic energy, or no converter of other forms of energy into pneumatic energy. A second shortcoming is the high cost of pneumatic energy, caused by enormous losses in compressed-air piping, in the process of energy conversion, and by the low efficiency of the hammer. Pneumatic equipment is especially uneconomical in the mining and coal industries, where pneumatic hammers (perforators) are used in large numbers. The unprofitability of compressed-air installations is especially evident now, when an entire mine or pit is electrified, while the presence of two forms of energy creates a number of operational inconveniences and lowers the efficiency of the mine’s energy system. In addition, electric energy (in comparison with pneumatic energy) has a number of advantages: it makes it possible to automate production, introduce dispatch control, and deliver energy to any point of a mine, pit, plant, construction site, collective farm, and so forth.

A third shortcoming of pneumatic drilling machines and hammers is the need for precise machining of the contacting surfaces of the cylinder with the striker and of the air-distribution system. Slight wear of the indicated surfaces leads to a considerable decrease in the efficiency of the hammer.

All that has been said above has prompted numerous authors and design organizations to develop electric impact devices as cheaper to operate and possessing a number of advantages inherent in electric machines. In the USSR, the desig-

The design of electric hammers has been pursued since 1928; however, up to the present time, domestic industry has not produced electric hammers and impact-rotary drilling machines with a solenoid drive, since meeting the requirements imposed on them, both in terms of their operating qualities and in terms of their design features, presents great difficulties. For production, an impact-action machine is needed that would satisfy the following basic requirements: the impact unit must develop at the tool an impact energy equal to the impact energy of pneumatic hammers; heating must not exceed 60–53°C during prolonged operation; the design must be simple, with a minimum number of parts, reliable and durable in operation, with minimum weight and a convenient form of construction. The last requirement is imposed especially strictly for hand hammers. In addition to the indicated requirements, the hammer must satisfy all necessary sanitary-hygienic working conditions and labor-safety requirements. Meeting these conditions presents considerable difficulties.

At present, the design of electric hammers is proceeding in two directions. The first is along the path of creating an electromechanical hammer, and the second a solenoid hammer. By a solenoid hammer, somewhat conventionally, we mean a hammer in which the reciprocating motion of the core—the striker—is produced under the action of electromagnetic forces without mechanical transmission.

The electromechanical group of hammers is characterized by the presence of an electric motor, the rotary motion of which, with the aid of various mechanical transmissions, is converted into impact energy. The advantage of this group of hammers over solenoid hammers is the fact that the drive is a widely used electric motor (operating in steady-state mode), the theory and design of which have been well developed. The motor of such a hammer has a high efficiency and $\cos \varphi$.

However, the indicated group of hammers has a number of substantial disadvantages, including: complexity of design; a large number of relatively precisely manufactured parts of the mechanical transmission, which increase the weight and reduce the reliability of operation of the entire construction; and also the dynamic loads arising during operation of the hammer, as a result of which the mechanical transmission quickly fails.

The principal advantages of solenoid hammers are: absence of mechanical transmission, simplicity of construction, absence of precisely manufactured parts, and consequently also the insignificance of their cost in comparison with electromechanical ones.

The principal disadvantage of solenoid hammers is their low efficiency and $\cos \varphi$, as a result of which solenoid hammers strongly

heat up during operation. The efficiency of hammers of medium and high power can be significantly increased when they operate at a reduced blow frequency. In this case, however, a special generator is required, which makes the circuit extremely complicated, quite apart from the fact that the manufacture of reliably operating switches for electric hammers is an extremely difficult matter. Attempts to create switches of various designs have not been successful because of the rapid burning of the contacts. Investigations carried out by A. M. Moskvitin [1, 2] show that, in general, the efficiency of electromechanical hammers is no higher than that of solenoid hammers, and is even somewhat lower, while the weight per unit of delivered power for both hammers is of the same order. Moreover, the efficiency of electromechanical hammers can be increased only slightly, whereas for a solenoid hammer, by choosing a rational design and power-supply circuits, the efficiency can be increased several times and brought up to 55–65%. The design of a solenoid hammer is considerably simpler and cheaper than that of an electromechanical one. All this indicates that solenoid hammers not only can compete with electromechanical hammers, but also have greater prospects for further development.

Solenoid hammers operate more effectively at a reduced blow frequency, since here the negative effects of the counter-electromotive force of the striker, eddy currents, and hysteresis losses have less influence. In order to realize a hammer design with a reduced blow frequency, the authors developed a sparkless switch* and hammers of the MS-1 and MS-2 types, producing 1500 blows per minute.

The MS-1 hammer is a more advanced design in comparison with previously proposed hammers [1–5], owing to the two-stage action of the electromagnetic forces of the coils on the striker. This made it possible to create a hammer with more stable operation of the striker, since throughout the entire stroke of the striker electromagnetic forces of greater magnitude act on it than in two-coil designs. The design of the MS-1 hammer is shown in Fig. 1, a. The coil-supply circuit is shown in Fig. 1, b, and the expanded voltage diagram in Fig. 1, c. From the diagram it is evident that for one blow cycle the time of two current periods is used, i.e., the striker makes 1500 blows per minute.

Let us consider the operation of the hammer with the striker in the middle position, as indicated in Fig. 1, a. The supply of coil K2 (Fig. 1, b, c), and

* “Synchronous sparkless switch.” Article in the present collection.

** Student V. T. Lopatin took part in the manufacture of the MS-1 hammer.

Consequently, the action of the electromagnetic forces of this coil has ceased, but at this same moment the current of the half-period of phase C, supplying coil K1 through switch P, increases. After the action of the electromagnetic forces of coil K1 ceases, the striker strikes the working tool. By this time—

Fig. 1. Solenoid hammer MS-1:
a—design; b—power-supply circuit; c—voltage diagram; 1—working tool; 2—cushioning washer; 3—support flange; 4—magnetically conducting housing; 5—working coils; 6—damper sleeve; 7—floating sleeves; 8—spring retaining nut; 9—damper-sleeve spring; 10—guide nonmagnetic tube; 11—stepped striker.

the coil K2 receives power from the half-periods of phases A and B, and the striker, under the action of the electromagnetic forces of coil K2, tends to assume the middle position relative to coil K2. However, even before the striker moves into the middle position, the supply to coil K2 is cut off, with a simultaneous increase in current and, consequently, in the electromagnetic forces of coil K3. The striker continues moving to the right, but on its path it encounters the damper sleeve, through which the striker compresses the damper-

spring. At the moment when the current in coil K3 decreases, the striker approaches the guide bushing. The striker is held in the extreme right position until the force of the spring overcomes the electromagnetic forces of coil K3. As soon as the electromagnetic forces of the coil become less than the compressive forces of the spring, it begins to return the stored energy to the striker, moving it in the direction of the working stroke. At this moment the electromagnetic forces of coil K2 are added to the forces of the spring. Then the cycle is repeated.

From the foregoing it is evident that, for the striker to traverse the first half of its path during its initial motion from the extreme positions, more time is required than for traversing the second half of its path. The power-supply circuit presented (Fig. 1, a, b, c) satisfies these conditions: the duration of action of the electromagnetic forces of the middle coil K2 is twice that of these forces in the end coils K1 and K3.

In the proposed hammer design, two electromagnetic forces displaced in time act on the striker: the first from the middle coil, the second from one of the end coils. This allows the hammer to operate in the region of larger tractive forces of the solenoid characteristic than in the operation of a two-coil hammer. Owing to the alternating action on the striker of two electromagnetic forces in one direction, stable operation of the hammer is achieved without remagnetization of the magnetic circuit and of the hammer striker. In contrast to previously proposed designs [1, 3], the present design makes it possible to increase the impact energy of the striker, reduce losses in the hammer and, consequently, its heating. Compactness, simplicity of manufacture, the two-stage action of electromagnetic forces on the striker, an increase in impact energy and in the hammer efficiency, etc., provide a positive solution to the problem of creating a solenoid hammer.

The MS-2 hammer is a more advanced design in comparison with the MS-1 hammer, owing to the two-stage action of the electromagnetic forces of the coils on a multistage striker. During the entire stroke, electromagnetic forces of considerably greater magnitude act on the striker than in the designs proposed earlier. This is achieved not only by the two-stage action of the electromagnetic forces, but also by their repeated application to the striker, which consists of magnetic and nonmagnetic links.

The MS-2 solenoid hammer (Fig. 2), depending on its intended use, is designed for an impact energy of up to 8 kgm at 1500 blows per minute. The hammer may be cooled by water or naturally.

Operation of the MS-2 hammer. Depending on the number of stages of the striker and its power-supply circuit, the hammer can produce different values of impact energy. Let us consider the operation of the hammer at an impact energy of up to 2 kgm and 1500 blows per minute, without

Fig. 2. Solenoid hammer MS-2.

1 — floating bushings; 2 — cylindrical magnetically conductive housing with openings for circulation of cooling water; 3 — coil for the forward stroke of the striker; 4 — magnetic part of the striker; 5 — coil pole; 6 — nonmagnetic link of the striker; 7 — core of the coil for the return stroke of the striker; 8 — coil for the return stroke of the striker; 9 — cylinder of the damping device; 10 — piston; 11 — commutation openings.

allowance for the phase shift due to the inductance of the load. When the hammer operates according to the coil-supply circuit shown in Fig. 3, the following order of energizing the coils is observed as the striker moves from the leftmost extreme position:

K2 → K2 → K3 → K2 → K2 → K1
      ↘ K3           ↘ K1

then the cycle is repeated. According to the voltage diagram given for the coils (Fig. 3, b), at the initial moment the middle coil K2 is switched on; it is supplied with rectified current first from phase C, then from phase A. The duration of energization of the middle coil K2 lasts for 300° (from point a to point c). Under the action of the electromagnetic forces of the middle coil, the striker begins to move to the right; together with the striker the rod with the piston moves, and the latter begins to compress the air contained in the cylinder.

After 120° following the energization of the middle coil K2 (at point B), the sparkless switch P (Fig. 3, a) energizes the end coil of the return stroke K3, which is supplied from the rectified current of phases A and B. In this case, the electromagnetic forces of the end coil of the return stroke are added to the electromagnetic forces of the middle coil. The combined action of these forces continues up to point c, and the action of the electromagnetic forces of the end coil K3 up to point d. During the entire time the striker moves to the right, the air in the cylinder is compressed by the piston (Fig. 2).

It can be seen from the voltage diagram that, even before the action of the electromagnetic forces of coil K3 ceases, the action of the forces of coil K2 begins to increase. When the electromagnetic forces of coil K2 and the forces of pressure on the piston from the compressed air become greater than the electromagnetic forces of coil K3 and the stored energy of the moving masses, the motion of the striker changes to the reverse direction. Now, under the action of the electromagnetic forces of the middle coil and the compressed air, the striker moves in the working direction. After switch P energizes the end coil K1, at point e the electromagnetic forces of the end coil of the forward stroke K1, supplied from phases A and B, are added to the indicated forces. At point n the action of the electromagnetic forces of the middle coil ceases. The action of the forces of the compressed air almost ceases. The striker continues its motion only under the action of the electromagnetic forces of the end coil of the forward stroke K1.

At the moment when the electromagnetic forces of the end coil K1 sharply decrease and the electromagnetic forces of the middle coil K2 increase, the striker strikes the working tool. The cycle then repeats.

From the foregoing it is clear that the striker begins its motion independently of its initial position relative to the coils. Owing to the use of a reliably operating sparking power switch for the end coils, and to the selection of definite coil-connection circuits for a specified characteristic and a definite striker weight, it appears possible to design a hammer that will operate in a resonant mode.

From the diagram presented (Fig. 3, b) it is clear that sparkless operation of the switch for sequentially switching on the supply to the end coils of the hammer is ensured if phase \(A\) is switched in the period \(c—e\), and phase \(B\) in the period \(d—m\). This basic condition for sparkless operation of the switch is provided for in its design.

Fig. 3. Power-supply circuit and voltage diagram of the hammer coils:
\(a\)—power-supply circuit; \(b\)—voltage diagram.

Main advantages of the MS-2 hammer

The proposed hammer design makes it possible:

1) to carry out the reciprocating motion of the striker without remagnetization of the magnetic circuit, which significantly reduces hysteresis and eddy-current losses and, consequently, increases the tractive force of the solenoid and reduces heating of the hammer;

2) to create a multistage hammer according to a specified tractive characteristic, ensuring an increase in the efficiency of the hammer;

3) to increase the tractive force of the hammer striker and the impact energy through repeated application of electromagnetic forces to the striker;

4) to increase the tractive force with a smaller striker diameter, which in turn makes it possible to reduce considerably the consumption of copper for manufacturing the coils, to reduce the heat losses in them and, consequently, to increase the efficiency of the hammer;

5) to create a hammer with small transverse dimensions.

Preliminary experimental tests of the manufactured models of the MS-1 and MS-2 hammers at the Tomsk Polytechnic Institute confirm the validity of the considerations stated above.

Work on the investigation of the MS-1 and MS-2 hammers is continuing.

References

1. A. N. Moskvitin. Electric Machines of Reciprocating Motion. Publishing House of the Academy of Sciences of the USSR, 1950.

2. A. N. Moskvitin. Electric or solenoid hammer. Elektrichestvo, Nos. 5 and 6, 1934.

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

4. N. A. Shmarov. Electric Percussion Hammers for the Hard-Coal Industry. Gostekhizdat of Ukraine, 1935.

5. P. M. Alabuzhev. A Study of the Working Process of Electric Impact Machines. Tomsk, Poligrafizdat, 1954.