SENSOR OF THE RESULTANT FORCES ACTING ON THE PISTON OF A PNEUMATIC IMPACT MACHINE

V. F. GORBUNOV, V. I. BABUROV, L. S. REDUTINSKY, Yu. A. OPARIN

(Submitted by the Department of Mining Machines, Mine Transport, and Mining Mechanics)

The study of the internal processes of a pneumatic impact machine is of great importance for developing a theory of their operation and for creating new high-performance and economical designs. The source material for such studies is provided by diagrams of air pressure versus time in both chambers of the machine, recorded on photographic paper or photographic film. At present, the recording of air-pressure diagrams is carried out mainly by two methods: electrical—using strain-gauge transducers and electronic equipment [1, 2]—and optomechanical [3, 4]. The first method has become more widespread [7, 8, 9].

On the basis of pressure diagrams, all the principal parameters of a pneumatic machine are calculated. The method of this calculation was developed by B. V. Sudnishnikov [4—6]. Its essence is as follows.

To calculate the parameters of the machine’s operating cycle, it is necessary, on the basis of diagrams of the pressures above and below the striker, to construct the curve of the resultant force acting on the striker during the time of the operating cycle [4]. The diagram obtained is divided vertically into two parts in such a way that the sums of the static moments of the areas of the indicator diagrams during the working and return strokes of the striker are equal to each other. In this case, the vertical line will correspond to the instant at which the working stroke of the striker begins.

After the diagram of the resultant force has been divided, it is easy to find the velocity of the striker at the end of the working stroke, as well as the frequency and energy of a single blow. B. V. Sudnishnikov proposes determining the parameters without constructing the diagram of the resultant force [4]. In this case, it is necessary to take separately the static moments of the impulses acting on the striker from the rear and front chambers of the cylinder and to sum them for the working and return strokes of the striker. However, this in no way simplifies the calculations.

It should be noted that the above-mentioned method for calculating the energy parameters and displacement characteristics of the striker is generally recognized and has found wide application. However, in our opinion, it has one substantial drawback, namely the cumbersome nature of the constructions and calculations performed. Thus, for example, an experienced experimenter can, during one working day, carry out calculations for at most 5–8 cycles of machine operation.

The authors of the present work, while studying the working processes of hand-held pneumatic hammers, began searching for measures and ways to improve and accelerate the calculations being performed.

One such measure was the creation of a new sensor that makes it possible to record directly a diagram of the resultant force acting on the piston of a pneumatic machine. The sensor was named by the authors a differential strain-gauge manometer of the MDT-1 type.

The differential strain-gauge manometer (Fig. 1) is arranged as follows. Compressed air from both cavities of the machine passes through pipelines 9 into the internal cavity of the sensor body 10, which is divided into two equal parts by membrane 11. As a result, membrane 11 is subjected to air pressure from both sides and will deform in proportion to the pressure difference.

A working strain gauge 13 is bonded to membrane 11. A compensating temperature strain gauge 12 is bonded to the bottom of the body in the same half of the sensor cavity. The leads from the strain gauges are brought out through sealing gaskets 14 and then, through a shielded cable, are connected to the input of a PET-3VM amplifier.

Both halves of the body of the MDT-1 differential strain-gauge manometer are joined by bolts 15. The tubes supplying compressed air to the sensor are connected by means of fittings 7 and nuts 8. To eliminate the influence of losses of air pressure on the path from the cavity to the strain gauge on the quality of the recording, the connecting tubes 9 are made of equal length. The internal diameter of the tube is 4 mm, and the diameter of the internal cavity of the sensor is 30 mm. The diameter of the cavity was selected with allowance for the possibility of placing strain gauges with a base of 15, 20, and 25 mm. For the KE-22 hammer, when this sensor is used, the volume of the front cavity increases by 4.3 cm³, or by 6%, at the extreme rear position of the striker, and that of the rear cavity by 4.3 cm³, or by 4.4%, at the extreme forward position of the striker.

It follows from the figures given that the volume of the machine cavities on both sides of the striker increases only slightly. However, with a decrease in the volume of the machine cavities, the indicated ratios will change upward, which is highly undesirable. Apparently, for impact-action pneumatic machines with small cavity volumes, the use of the described sensor design is impossible. For such machines, it is possible, on the same principle, to create other designs of strain-gauge manometers that do not introduce changes into the working process.

The main advantage of the developed differential strain-gauge manometer is that it accelerates the processes of recording and processing pressure diagrams when calculating the parameters of impact-action pneumatic machines in which the useful areas of the striker piston are the same during its working and return motions. There is no need to record two pressure diagrams separately in the rear and front cavities of the machine. The use of the MDT-1 sensor sharply reduces the calculation of the characteristics of the machine being tested. With a linear characteristic of the wire strain gauge, it is possible, immediately after magnifying the resultant oscillograms, to carry out the corresponding calculations without reconstructing them.

It is only necessary to determine the force scale. Calibration of the device is carried out in the usual manner. First the sensor is calibrated with compressed air supplied to one side of the membrane, and then to the other. The sensor membrane was calculated according to formulas known in the literature [7—9]. The described sensor was tested in the laboratory of pneumatic machines of the Department of Mining Machines and Mine Transport. A well-studied riveting-

Fig. 1. Differential strain-gauge manometer MDT-1: 1 — hammer handle; 2 — valve box; 3 — valve; 4 — barrel; 5 — stop; 6 — striker; 7 — union; 8 — union nut; 9 — pipeline; 10 — sensor housing; 11 — steel diaphragm; 12 — compensating strain gauge; 13 — working strain gauge; 14 — sealing gaskets; 15 — bolt.

impact hammer of type KE-22, which, according to the manufacturer's technical specifications, has the parameters given in Table 1.

Table 1

At first the hammer was tested by the usual method, i.e., with two pressure sensors [1], and then with the new MDT-1 sensor. The test conditions were completely identical. The impact power of the hammer was transmitted to the working tool, a chisel weighing 0.9 kg, bearing against a cast-iron block. The position of the hammer was vertical. The pressing force was 25 kg; the air pressure in the supply network was 5 atm. A pressing force of 25 kg ensures stable operation of the hammer, since together with the weight of the machine, 9.5 kg, it is equal to the maximum value of the disturbing force. Under these conditions the influence of oscillations of the hammer body on the internal processes will be reduced to a minimum. The air consumption was measured with a float-type flowmeter, type V-1, made by the Pneumatika plant (Leningrad).

Figure 2 presents the results of testing the hammer by the usual method (with two sensors), i.e., with the recording of two oscillograms of air pressures: separately for the rear ($P_3$) and for the front ($P_{\text{п}}$) cavities, with subsequent construction of the graph of the resultant force ($R$) acting on the striker during one cycle (Fig. 2, b).

Fig. 2. Diagrams of compressed-air pressures on the striker in the rear—$P_3$ and front—$P_{\text{п}}$ cavities of a riveting hammer of type KE-22, and of the resultant force—$R$, acting on the striker during one cycle

The procedure for constructing the diagram $R$ (Fig. 2, b) is as follows. The air-pressure diagrams (Fig. 2, a), recorded on an MPO-2 oscillograph, were enlarged 10 times by an optical method using the PO-1 instrument.

Then each of them was replotted again to bring them to a single scale, since the calibration graphs for the two sensors are, as a rule, different. The replotted diagrams were combined into a common resultant one, taking into account the direction of action of the forces and the area of the striker. After the indicated constructions, the calculation of the parameters is carried out by the method of B. V. Sudnishnikov [4, 5]. The results of processing the diagrams for the KE-22 hammer are given in Table 1.

When testing the hammer with the new sensor, the latter was calibrated, as usual, before and after the experiments. The calibration graphs are presented in Fig. 3, b. Also shown here is the diagram of the resultant force—or, more precisely, of the air pressure on the striker—recorded with the aid of the aforementioned sensor (Fig. 3, a). It differs almost not at all from the diagram shown in Fig. 2, b. Since the dependence of the deflections of the loop beam on the oscilloscope screen on the air pressure on both sides of the membrane proved to be linear, processing of the diagram (Fig. 3, a) is reduced only to its preliminary enlargement, calculation of the scales, and determination of the impulse areas.

The force scale is equal to

\[ m_p = \frac{P_0 \cdot F}{L},\ \text{kg}\cdot\text{cm}, \]

where \(L\) is the deflection of the loop beam on the oscilloscope screen when the air pressure is changed by one atmosphere, cm;

\(P_0\) is the air pressure equal to one atmosphere, kg/cm\(^2\);

\(F\) is the area of the striker, cm\(^2\).

Fig. 3. Diagram of the resultant force acting on the striker during one cycle, obtained by means of an MDT-1 differential strain-gauge manometer (a), and calibration graph of the MDT-1 sensor (b)

The time scale is equal to

\[ m_t = \frac{n \cdot t_0}{l},\ \text{sec}/\text{cm}, \]

where \(l\) is the length of the cycle, cm;

\(t_0\) is the value of one division of the time marker, equal to 0.002 sec;

\(n\) is the number of time marks in one cycle, pcs.

The difference in the values of the principal parameters of the KE-22 riveting hammer, determined by the one and the other method (see table), is 1.5–3.0%. In both cases the deviations in the parameter values lie within the limits of the counting error. It follows from this that the selected dimensions of the MDT-1 sensor have practically no appreciable effect on the working

process of the hammer, and it can be recommended for use in determining parameters by the method of B. V. Sudnishnikov.

Conclusions

1. The paper describes the design of a differential strain-gauge manometer for pneumatic machines of impact action in which the working area of the striker piston is the same on both sides. The sensor makes it possible to oscillograph the resultant air pressure on the striker piston, which considerably simplifies and accelerates the processing and obtaining of experimental results.

2. Tests have shown that the proposed sensor can be used for pneumatic machines with cavity volumes of not less than 70–90 cm³. Otherwise, the presence of additional volume due to the strain-gauge manometer pipelines qualitatively distorts the internal process of the machine; the oscillogram records the natural oscillations of the air column located in the pipelines, and the accuracy of the results of the experiments being conducted is reduced.

3. In practice, the sensor is recommended for use in investigations of such machines as riveting hammers of types KE-19, KE-22, KE-28, KE-32, pneumatic impact tools of all types, impact units, etc.

Literature

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6. B. V. Sudnishnikov, L. I. Semenov. Calculation of a pneumatic recoil hammer. In: Machines of Impact Action. Novosibirsk, 1953.

7. V. V. Yushin. Determination of the parameters of the internal process of mine pneumatic machines of impact action by the method of electrical measurements. Abstract of a Candidate’s dissertation. Dnepropetrovsk, 1960.

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9. N. P. Raevsky. Methods of Experimental Investigation of Mechanical Parameters of Machines. Publishing House of the Academy of Sciences of the USSR, 1952.

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