On Methods for Obtaining Reciprocating Motion of the Striker in Electromagnetic Impact Machines
Abstract Generated abstract
The paper examines three schemes for producing reciprocating striker motion in electromagnetic impact machines, including spring return, two electromagnets, and a single electromagnet with free striker runout. Using simplifying assumptions about losses and impact behavior, it derives conditions for unbraked operation and compares blow frequency, impact energy, specific power, machine weight, cooling area, and steady-state heating for the schemes when comparable electromagnets are used. The analysis suggests that single-coil machines with free runout can reduce weight and, under ideal timing, raise specific power, but this advantage is accompanied by increased heating and is diminished in existing machines where blow frequency is reduced more strongly. The authors conclude that a decisive quantitative comparison requires experimental data on braking and current overlap effects in the working cycle.
Full Text
N. P. RYASHENTSEV, A. P. TRONOV
METHODS FOR OBTAINING RECIPROCATING MOTION OF THE STRIKER IN ELECTROMAGNETIC IMPACT MACHINES
(Presented by the Department of Mining Machines and Mine Transport)
In electromagnetic impact machines, the following methods for obtaining reciprocating motion of the striker have found the widest application [1].
Method A. The electromagnet serves to draw the striker in one of the directions (for example, during the return stroke). At the end of the stroke the striker compresses a spring and, under the action of its elastic forces, returns to its initial position, striking the working tool (the forward stroke of the striker). The electromagnetic hammers of Bevy and Simbi operate according to this scheme [2].
Method B. Two electromagnets are included in the scheme. One of them draws in the striker during the return stroke; the forward stroke is performed under the action of the elastic forces of the spring and of the forces developed by the second electromagnet. The AEG, Cintron, Miller-Falls, and EK-1M hammers are based on this principle [2, 3].
Method C. One and the same electromagnet is switched on during the working cycle both for the forward and for the return stroke of the striker. In this case, at the moments when there is no current in the coil, the striker makes free runs from the middle position relative to the electromagnet coil [4]. This method is used in the MS-15, MS-18/36, and Dyatel-2 hammers [5, 6].
It has been established that the most expedient method for obtaining reciprocating motion of the striker in electromagnetic hammers is method C [1], since “...the use of the same coils for both the forward and the return stroke doubles the power of the hammer...” [7] in comparison with machines using method B.
However, experimental studies have shown that, in hammers based on methods B and C, the ratio of useful power to weight is practically the same [3].
This required additional consideration of the question of the effectiveness of methods for obtaining reciprocating motion of the striker in electromagnetic impact machines.
Conclusions regarding the influence of the method used to obtain reciprocating motion of the striker on the frequency of blows, specific power, and heating of the machine can be drawn if, in different schemes, the same electromagnets are used, whose parameters for a single attraction of the striker are close to optimal.
In works [7, 3] it is shown that an essential drawback of hammers with rectifiers in the power-supply circuit is electromagnetic braking—
motion of the striker. Let us consider what conditions must be satisfied in order to carry out unbraked working processes of machines based on various methods of obtaining reciprocating motion of the striker.
Assume the following:
- There are no friction losses.
- The coefficient of restitution of velocity when the striker impacts the buffer spring is equal to unity.
- The time of collision of the striker with the spring is small in comparison with the time of current flow during the return and forward strokes of the striker.
In this case, in order to achieve unbraked operation of hammers using method A, it is necessary to satisfy the condition
\[ V_{\mathrm{avg.\,rs}}\, t_{\mathrm{rs}} = V_1 t_{\mathrm{n}}, \tag{1} \]
where \(V_{\mathrm{avg.\,rs}}\) is the average velocity of the striker during the return stroke;
\(t_{\mathrm{rs}}\) is the time of current flow during the return stroke of the striker;
\(V_1\) is the velocity of the striker at the moment it is under the poles during the return stroke;
\(t_{\mathrm{n}}\) is the time during which current must not flow in the coil (Fig. 1, a).
In hammers based on method “B,” electromagnetic braking of the striker will be absent if the equality
\[ V_{\mathrm{avg.\,rs}}\, t_{\mathrm{rs}} = V_{\mathrm{avg.\,fs}}\, t_{\mathrm{fs}}, \tag{2} \]
is satisfied, where \(V_{\mathrm{avg.\,fs}}\) is the average velocity of the striker during the forward stroke;
\(t_{\mathrm{fs}}\) is the time of current flow during the forward stroke.
It is obvious, for \(t_{\mathrm{fs}} = t_{\mathrm{rs}}\), which occurs in real machines of this type, that electromagnetic braking of the striker is inevitable, since \(V_{\mathrm{avg.\,rs}} < V_{\mathrm{avg.\,fs}}\). To eliminate braking of the striker it is necessary to reduce the time of current flow during the forward stroke of the striker to the value (Fig. 1, b)
\[ t_{\mathrm{fs}} = \frac{V_{\mathrm{avg.\,rs}}}{V_{\mathrm{avg.\,fs}}}\, t_{\mathrm{rs}}. \tag{3} \]
Suppose that the heating of this machine, at the specified duration of energization, has proved to be optimal.
In order to reduce the weight of the hammer, we shall attempt to transfer the functions of the two coils of hammers using method B to one coil of a hammer with free run-out of the striker, while preserving the quantitative and qualitative aspects of the process of converting electrical energy into mechanical work per single attraction of the striker.
Fig. 1. Curves of the working processes of electromagnetic hammers.
\(\Delta S\) is the magnitude of the striker stroke; \(i_{\mathrm{rs}}\), \(i_{\mathrm{fs}}\) are the currents in the coils of the hammers during, respectively, the return and forward strokes of the striker; \(S_{\mathrm{rs}}\), \(S_{\mathrm{fs}}\) are the curves of, respectively, the return and forward strokes of the striker; \(S_{\mathrm{fr,rs}}\), \(S_{\mathrm{fr,fs}}\) are the curves of the free run-out of the striker during, respectively, the return and forward strokes.
The working process of hammers with free striker runout, satisfying the stated requirements, is illustrated by the curves in Fig. 1, a, while the condition for their unbraked operation is described by the expression
\[ V_1 t_{\mathrm{vo}} = V_{\mathrm{av.f}} t_{\mathrm{f}} \tag{4} \]
which is identical to equalities (1) and (2), since the striker runout time during the return stroke is \(t_{\mathrm{vo}} = t_{\mathrm{r}}\).
When the same electromagnets are used in different circuits, the process of accumulating kinetic energy during the return stroke of the striker in machines of all types proceeds according to the same laws (Fig. 1). The machines under consideration differ only in the method by which the forward stroke of the striker is effected.
From this point of view, the kinematics of the working processes of machines using methods A and B is different; the forward stroke of the striker in machines based on method B includes the corresponding stages of the working cycle of hammers of the first two types (Fig. 1).
For convenience in further considering the influence of circuits for obtaining reciprocating motion of the striker on the parameters of the machines, let us introduce the following notation:
\(W_{\mathrm{rs}}, W_{\mathrm{fs}}\) — the amount of mechanical energy stored by the striker during the return and forward strokes, respectively.
\(t'_{\mathrm{p}} = t_{\mathrm{rs}} + t_{\mathrm{fs}}, \quad t_{\mathrm{p}} = t_{\mathrm{vo}} + t_{\mathrm{vp}}\) — respectively, the total time of current flow in the electromagnets switched according to circuits B and V, and the time during which no current flows in the coils of hammers with free striker runout, where \(t_{\mathrm{vp}}\) is the time of free striker runout during the forward stroke.
The steady-state overheating value of the machine is \(\Theta = K_\theta \dfrac{Q_2}{S}\), where the useful power is \(Q_2 = k A n\).
In the equations given, \(S\) is the cooling surface; \(K_\theta\) is a proportionality coefficient depending on the efficiency of the electromagnet and on the heat-transfer coefficient, taken as constant; \(k\) is a proportionality coefficient; \(n\) is the number of blows; \(A\) is the energy of a single blow. The weight of a single-coil hammer is
\[ G = G_{\mathrm{A}}(1 + K_G), \]
where \(G_{\mathrm{A}}\) is the weight of active materials,
\(G_{\mathrm{n}}\) is the weight of inactive materials.
\[ K_G = \frac{G_{\mathrm{n}}}{G_{\mathrm{p}}}. \]
The specific power of the machine is
\[ q = \frac{Q_2}{G}. \]
The parameters of electromagnetic machines using the same electromagnets but different methods of obtaining reciprocating motion of the striker are described by the expressions given in Table 1. To compare the first principle for obtaining reciprocating motion of the striker with the second and third (Table 1), it is necessary to have data on the ratios
\[ \frac{W_{\mathrm{rs}}}{W_{\mathrm{fs}}} \quad \text{and} \quad \frac{t_{\mathrm{rs}}}{t_{\mathrm{fs}}}, \]
whose optimal values can be obtained experimentally.
Therefore, it appears possible to compare only machines based on the second and third methods of obtaining reciprocating motion of the striker. These hammers have the same blow energy, but different blow frequency and weight (Table 1). In passing from the second circuit to the third, the weight of the machine is reduced by half, while the blow frequency...
Table 1
| Machine parameters | Method of obtaining reciprocating motion of the striker: A | Method of obtaining reciprocating motion of the striker: B | Method of obtaining reciprocating motion of the striker: C |
|---|---|---|---|
| Number of blows per minute, $n$ | $\dfrac{1}{t_{\mathrm{ox}}+t_{\mathrm{vo}}}$ | $\dfrac{1}{t_{\mathrm{p}}}$ | $\dfrac{1}{t_{\mathrm{p}}+t'_{\mathrm{n}}}$ |
| Impact energy, $A$ | $W_{\mathrm{ox}}$ | $W_{\mathrm{ox}}+W_{\mathrm{px}}$ | $W_{\mathrm{ox}}+W_{\mathrm{px}}$ |
| Useful power, $Q_2$ | $\dfrac{kW_{\mathrm{ox}}}{t_{\mathrm{ox}}+t_{\mathrm{vo}}}$ | $\dfrac{k\left(W_{\mathrm{ox}}+W_{\mathrm{px}}\right)}{t_{\mathrm{p}}}$ | $\dfrac{k\left(W_{\mathrm{ox}}+W_{\mathrm{px}}\right)}{t_{\mathrm{p}}+t'_{\mathrm{n}}}$ |
| Weight of the machine | $G$ | $2G$ | $G$ |
| Specific power, $q$ | $\dfrac{kW_{\mathrm{ox}}}{t_{\mathrm{ox}}+t_{\mathrm{vo}}}\cdot\dfrac{1}{G}$ | $\dfrac{k\left(W_{\mathrm{ox}}+W_{\mathrm{px}}\right)}{t_{\mathrm{p}}}\cdot\dfrac{1}{2G}$ | $\dfrac{k\left(W_{\mathrm{ox}}+W_{\mathrm{px}}\right)}{t_{\mathrm{p}}+t'_{\mathrm{n}}}\cdot\dfrac{1}{G}$ |
| Cooling area | $S$ | $2S$ | $S$ |
| Steady-state overheating of the machine, $\Theta$ | $\dfrac{k\Theta}{S}\cdot\dfrac{kW_{\mathrm{ox}}}{t_{\mathrm{ox}}+t_{\mathrm{vo}}}$ | $\dfrac{k\Theta}{2S}\cdot\dfrac{k\left(W_{\mathrm{ox}}+W_{\mathrm{px}}\right)}{t_{\mathrm{p}}}$ | $\dfrac{k\Theta}{S}\cdot\dfrac{k\left(W_{\mathrm{ox}}+W_{\mathrm{px}}\right)}{t_{\mathrm{p}}+t'_{\mathrm{n}}}$ |
is reduced by a factor of $(1+K_t)$, which is less than two, since in actual machines
\[ 0<K_t=\frac{t'_{\mathrm{n}}}{t_{\mathrm{p}}}<1. \]
Consequently, in hammers with free run-out of the striker the electromagnet is used more intensively over time, which makes it possible to increase the specific power of the machine by a factor of
\[ \frac{2}{1+K_t}. \]
However, the heating of the hammer increases by the same factor.
The machines provide the same number of blows at $K_t=0$ (Fig. 2). In this case, the specific power and heating of hammers with free run-out of the striker are twice as great in comparison with hammers based on method B (the case for which the conclusions set forth in the literature [7] are valid).
At $K_t=1$, the electromagnets of hammers operating according to schemes B and C are loaded identically, since the blow frequency with free run-out of the striker is reduced by half in comparison with hammers using method B. Therefore the machines prove equivalent both in specific power and in heating.
In the real case, as was noted above, it is necessary to take as a guide the values of specific power and heating corresponding to the condition $0<K_t<1$.
To reduce the heating of a machine based on method C, it is necessary to increase the cooling surface. In this case the specific power will decrease somewhat, since the weight of inactive materials increases. Let us assume that, with an increase in the cooling surface proportional to the increase in heating, the weight of inactive materials increases...
increased by \(C\) times. Then the ratio of the specific powers of machines using methods В and Б is determined by the expression
\[ \varphi_{\mathrm{В,Б}}=\frac{2(1+K_\sigma)}{(1+K_t)(1+cK_\sigma)} . \tag{5} \]
Substituting into formula (5) the real values of the coefficients \(k_\sigma=0.3-0.5\), \(K_t=0.3-0.5\), \(c=\dfrac{2}{1+K_t}=1.54-1.33\), we obtain \(\varphi_{\mathrm{В,Б}}=1.2-1.37\).
Thus, when working processes are carried out according to the schemes in Fig. 1, б, в, the weight of hammers with free run-out of the striker, in comparison with hammers of type Б, is reduced by half, and the impact frequency is reduced by \((1+K_t)=1.3-1.5\) times at unchanged energy of a single blow. Owing to this, the specific power of hammers with free run-out of the striker, at the same heating, proves to be greater by a factor of \(1.2-1.37\).
Fig. 2. Parameters of electromagnetic hammers as a function of \(K_t\)
1, 2 — dependence of the number of blows on \(K_t\), respectively for machines using methods В and Б.
2, 3 — dependence of the specific power and heating on \(K_t\) for machines operating respectively according to schemes Б and В.
In existing machines with free run-out of the striker, the impact frequency, in comparison with machines operating according to scheme Б, is reduced not by a factor of \(1.3-1.5\), but by a factor of two [3]. Therefore, from the standpoint of specific power, none of the real schemes has an advantage.
Real machines with free run-out of the striker can provide greater specific power provided that, for equal weight and energy expenditures, the impact energy of these machines is greater than the impact energy of machines of type Б. This is possible in the case when electromagnetic braking in machines with free run-out of the striker affects the process of converting electrical energy into mechanical energy to a lesser extent than the overlapping of currents in hammers of the second type [3].
A quantitative estimate of the influence of the features of the working processes of the indicated hammers on the energetics of the working cycle can be made on the basis of the corresponding experimental data.
LITERATURE
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A. I. Moskvitin. Electrical Machines of Reciprocating Motion. Publishing House of the Academy of Sciences of the USSR, Moscow–Leningrad, 1950.
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N. P. Ryashentsev, A. P. Tronov. Some results of an experimental study of electromagnetic hammers. The article is included in the present collection.
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Inventor’s certificates No. 42962, 1933; No. 132578, 1960; No. 139622, 1960.
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N. P. Ryashentsev, V. V. Ivanov. Solenoid hammers for installation work. Technical and Economic Bulletin of the Kemerovo Council of the National Economy, No. 10, 1960.
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N. P. Ryashentsev, A. P. Tronov. A solenoid crushes granite. Inventor and Innovator, No. 9, 1963.
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A. I. Moskvitin. Electric, or solenoid, hammer. Electricity, Nos. 5, 6, 1934.