PROCEEDINGS
OF THE TOMSK ORDER OF THE RED BANNER OF LABOR
POLYTECHNIC INSTITUTE named after S. M. KIROV

Vol. 108  INVESTIGATION OF DRILLING MACHINES  1959

STUDY OF THE DYNAMICS OF SOLENOID HAMMERS

N. P. Ryashentsev

On the basis of an analysis of previously proposed and existing solenoid hammers [1], it was established that the common drawback of these hammers is that they have an extremely low power at the working tool. In direct-current hammers (hammers of the designs of Pokrovskii, Starikov, Shiman, Schuler, Depel, Morven, etc.), the switches existing at that time did not allow the impact energy to be increased; while in hammers of the designs of Sintron, Bevi, Moskvitin, and Shmargunov, as the impact energy is increased, the dimensions and weight increase, the efficiency decreases, and overheating of the hammer increases. Taking this into account, we developed a multistage solenoid hammer with a synchronous sparkless switch [2, 4], which provides the impact energy necessary for carrying out mining operations. However, in order to create a productive, economical, and reliable solenoid impact mechanism (hammer), it is necessary to pay attention to its dynamics. An analysis of the working process of the MS-3 hammer is also necessary because the creation of a solenoid hammer as a hand tool is ultimately a problem of the weight of the structure as a whole, of increasing the energy of a single blow, and of the hammer efficiency, etc. Such reserves can be revealed only by analyzing the working process of the existing Sintron* and MS-3 hammers.

The design of the tested MS-3 hammer is shown in Fig. 1 and consists of the following principal parts: working tool 1, stop nut 2, coil poles 3, spacer bushings 4, cylindrical body 5, coils 6, stepped striker 7, consisting of magnetic links (shown in the drawing with a smaller diameter), floating bushing 8, stop pin 9,

* The Sintron hammer is described in the article “Solenoid Hammers” in the present collection.

damping springs 10, adjusting nut 11, and water-supply device 12. The circuit for switching on the hammer coils is shown in Fig. 1, b, and the voltage diagram and oscillogram in Fig. 1, v and g.

The tests of the MS-3 hammer were carried out in accordance with the procedure described in works [1, 3], i.e., the travel of the striker and the current were recorded as functions of time. The connection diagram for the device recording the striker travel is given in Fig. 2, a. The sensor is attached to the end part of the hammer body and consists of a rod, one end of which is fixed to the striker, while at the other end a movable contact is mounted, which slides along a resistance (potentiometer) P. The potentiometer is an active resistance in the form of a slide rheostat.

Fig. 1. Solenoid hammer MS-3:
a — section of the MS-3 hammer; b — circuit for switching on the coils; v — voltage diagram; g — voltage oscillogram.

The parameters of the potentiometer are determined from the condition that the nonlinear variation of the current in the circuit should not exceed 1% at the extreme positions of the striker. The circuit is adjusted so that there is a proportional dependence between the striker travel (the displacement of the sliding contact) and the current in the loop circuit of the oscilloscope. The connection diagram of the hammer coils and oscilloscope loops for recording current and voltage is shown in Fig. 2.

As can be seen from the diagrams presented, the electrical method of measuring the striker travel as a function of time makes it possible to investigate the dynamics of the hammer fairly completely in rapidly changing processes. This is especially important for solenoid hammers, in the study of which it is necessary to record the mechanical quantity—the striker travel as a function of time—and the electrical quantity—the character of the variation

current feeding the coils. All the more so since these diagrams must be processed jointly.

Joint recording of two or more variable parameters is very conveniently carried out on the eight-loop oscillograph MPO-2, whose design, in addition to recording, also permits visual observation of the quantities being recorded. The latter is necessary for a preliminary check of the nature of the variation of the quantities being recorded before fixing them on photographic film or paper.

The general view of the experimental setup is shown in Fig. 3. In order to make a comparative assessment of the working process of the MS-3 hammer, let us analyze its dynamics and compare certain characteristic indices with the Sintron hammer, which in its time was the most widely used.

The working process of the Sintron hammer was described by Prof. A. I. Moskvitin [1]. The MS-3 hammer was investigated by us at the Tomsk Polytechnic Institute.

In Fig. 4 are shown oscillograms of the path \(a\), curves of the change of current in the coils \(b\), a graph of the velocity of motion of the striker \(v\), and the acceleration (tractive force) \(g\) of the Sintron and MS-3 solenoid hammers. By analyzing together the curves of the striker path \(S\) and the change of current in the coils of the working stroke \(i_p\) and return stroke \(i_x\) of the striker, it is easy to establish the interrelation between the mechanical (motion of the striker) and electrical (current of the solenoids) phenomena. Differentiating the curve of the path \(S\) with respect to time, we obtain the velocity graph, from which the striker velocity at the moment of impact is determined. Knowing the mass and velocity of the striker, we determine the energy of a single blow of the hammer. Differentiating with respect to time the velocity curve of the striker, we find the acceleration of the striker or, on another scale, the tractive force of the striker.

Fig. 2. Connection diagrams of instruments in the study of the dynamics of the MS-3 solenoid hammer:
\(a\)—diagram of the instrument for recording the striker stroke; \(b\)—diagram for connecting the hammer coils.

The curve of the striker path depends on the change of current in the coils. Therefore these curves will be considered together.

For the Sintron hammer, after the blow (point \(d\)) the striker enters the return-stroke coil not with acceleration, but with deceleration. After stopping for 6–7 msec, the striker, with acceleration, begins to be drawn into the return-stroke solenoid (from point \(v\)). During the working stroke the striker moves almost without acceleration, and after reflection from the

of the spring (at point $c$) there is even a noticeable deceleration. The current curves in the coils ($i_1$ and $i_2$, Fig. 4, a) show that the current in one coil flows not for a half-period, as might have been expected, but for considerably longer (transient processes in circuits with self-induction). Consequently, for some time the current flows simultaneously through the coil of the forward stroke and the coil of the return stroke of the striker. Such overlapping of currents is one of the main causes of the braking forces.

The next cause of braking forces in the Sintron hammer is the equal duration of energizing the coil of the working and return strokes of the striker. From the design of the Sintron hammer we concluded that the striker makes the working stroke under the action of the elastic forces of the spring and the electromagnetic forces of the coil, while the return stroke is made only under the action of electromagnetic forces. Consequently, taking into account the efficiency of the damper spring, the time of the return stroke of the striker should be 70–80% greater than the time of the working stroke.

Fig. 3. General view of the experimental setup.

Such a mode of coil energization cannot be established from the Sintron electrical circuit, since it provides power to the coils for more than a half-period. There is no interval of time when current would not flow in both coils; on the contrary, there is an overlap of currents in the coils of the working and return strokes of the striker.

The eddy currents induced in the striker and in the guide tube also exert a braking action on the striker and cause additional heating of the hammer.

Fig. 4. Diagrams of the working processes of the Sintron and MS-3 solenoid hammers.

The middle magnetic circuit is common to the two coils, and therefore part of the magnetic flux closes through the magnetic circuit of the other coil, thereby creating braking forces.

To determine the influence of the braking forces acting on the striker during its motion in the working and return directions, we construct the curve of acceleration and resultant forces over the entire hammer cycle (Fig. 4, g). For the Sintron hammer, forces coinciding with the direction of motion of the striker are plotted upward along the ordinate axis, and braking forces downward (by convention). The area bounded by this curve and the abscissa axis gives the sum of the impulses of the working forces (upward along the ordinate axis) and of the braking forces (downward along the ordinate axis). The curves in the portion to the left of line A depict reflection from the buffer spring, and to the right of B—impact on the peak. It is clear from the graph that during the forward stroke, over the path section from 0 to 2 mm, the striker enters the coil with deceleration; then, over the section from 2 to 6 mm, it acquires acceleration; and subsequently it moves without acceleration over a section equal to 10 mm. Then deceleration again occurs and persists to the end of the stroke. During the return stroke of the striker, braking is observed over the section 8.5–12 mm. The sum of the impulses of the braking forces is almost equal to the sum of the useful impulses of the forces of the working-stroke coil. As a result, the working-stroke coil performs almost no useful work.

A. I. Moskvitin notes [1] that, because of the presence of braking forces, the design of the Sintron hammer is still far from perfect. Moreover, such causes of braking as current overlap cannot be eliminated, since they follow from the very principle of operation of the hammer. After the braking forces are eliminated, the impact energy of the hammer increases by a factor of 2–3.

To investigate the operating mode of the MS-3 hammer, a record was made of the motion of the striker and of the current-measurement curves in the coils of the working stroke \(i_p\) and the return stroke \(i_x\) of the striker, which are shown in Fig. 4, a.

It is apparent from the diagrams presented that there is no simultaneous flow of current in the coils of the working and return strokes. Consequently, in the developed hammer designs with a synchronous sparkless switch, one of the main causes of striker braking is eliminated—the overlap of current in the coils of the working and return strokes.

In addition, in the MS-3 hammer an unequal duration of the striker motion in the working and return directions is automatically established. This is achieved by the presence of a time interval during which no current flows in the coils (Fig. 4, a). It is clear from the oscillogram that during the first 5 msec of the cycle and in the period between 19 and 24 msec, no current flows in the coils. Consequently, for this operating mode of the hammer, the time of the striker’s return stroke is 65–70% greater than the time of the striker’s working stroke. This is a very important condition for the operation of the hammer without braking forces.

To determine the impact energy, we plot the velocity-versus-time curves (Fig. 4, б), from which we determine the velocity of motion of the striker just before the moment of impact (equal to \(6.85\ \text{m/sec}\)). Knowing the weight of the striker (\(1.55\ \text{kg}\)), we determine the impact energy from the expression

\[ A=\frac{mv^2}{2}. \]

Let us now see how the tractive forces of the striker are distributed over the entire cycle. As is evident from the curves shown in Fig. 4, г (MS-3), from the moment the current begins to flow in coil K3 (Fig. 1), the striker begins to move with acceleration. However, the maximum value of the tractive force is obtained not at the moment when the current reaches its maximum value, but earlier. This indicates that coil K3 is not operating under optimum conditions. With further displacement of the striker to the right, its motion slows down. This slowing of the striker occurs owing to its braking by the spring damper (Fig. 4, г). The striker reaches its extreme position and, under the action of the elastic forces of the spring and the electromagnetic forces of the working-stroke coils, begins to move with acceleration in the direction of impact. The increase in force diminishes at a distance of \(12\ \text{mm}\) before impact, and just before the moment of impact the magnitude of the acting forces even decreases. The decrease in the forces applied to the striker is explained by the considerable drop in current in the working-stroke coils and by the induction of eddy currents in the striker and in the poles.

From the curves (Fig. 4) characterizing the working process of the MS-3 hammer, it has been established that in this hammer design there is no electrical braking due to superposition of currents in the working- and return-stroke coils. The developed coil-supply circuits automatically establish an unequal duration of striker motion in the working and return directions. Slight braking of the striker is observed, caused by eddy currents and by the common magnetic circuit of the coils for the forward and return strokes of the striker. Proceeding from this, it may be said that this design has a higher efficiency than the Cintron hammer.

In further work to improve the design of solenoid hammers, it is necessary to take into account that each coil should not operate for half the cycle time, but should be used for both the working and the return stroke of the striker; i.e., on the basis of multi-coil designs, a hammer operating on the resonance principle should be developed. In this case the weight of the hammer per unit power may be reduced. The solution of this problem is greatly facilitated by the developed supply circuits, which ensure hammer operation without electrical braking. The magnetic circuit and the striker should be made of powdered magnetically conductive material, and the magnetic circuits of the coils for the forward and return strokes of the striker must be insulated from one another. In addition, the efficiency of the hammer can be increased by using a material for the magnetic circuit and striker with high

magnetic permeability. A further reduction in overheating of the casings of solenoid hammers can be achieved by better filling the winding space with copper (replacing a round-section conductor with a rectangular one) and by using, for their cooling, the liquid intended for flushing the boreholes or for sprinkling.

REFERENCES

1. A. N. Moskvitin. Electromechanical or solenoid hammer? Elektrichestvo, No. 5–6, 1934.

2. P. M. Alabuzhev, N. P. Ryashentsev. On the question of creating solenoid hammers. Izvestiya TPI, vol. 106, Metallurgizdat, 1958.

3. O. D. Alimov, I. G. Basov, V. F. Gorbunov, Ya. A. Serov. Methodology for conducting experiments in the study of impact assemblies of pneumatic drilling hammers and for processing experimental data. Izvestiya TPI, vol. 88, Tomsk, 1958.

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

Noted Misprints

Order 29