ON THE ISSUE OF DEVELOPING VIBRATION MEASUREMENT EQUIPMENT

V. F. GORBUNOV, G. S. ZHARTOVSKY, Yu. A. OPARIN

(Presented by the scientific seminar of the Department of Mining Machines, Mine Transport, and Mining Mechanics)

In the study of impact-action machines, with the aim of improving their designs, and in production control to determine whether the vibration level conforms to current sanitary-hygienic standards, vibration measurement equipment is used.

Various vibration measurement instruments are produced serially by industry. They make it possible to record the oscillation process (VR-1) or to observe the variation of some vibration parameter by means of a pointer instrument (V-2, VPU-2M, UBP, PIU-1M).

The specific features of the operation of impact-action machines, from the standpoint of the use of vibration measurement equipment, are considerable overloads reaching several tens of \(g\), a spectral composition of vibrations extending from 10 to 10000 Hz, vibration displacement magnitudes ranging within \(0.01\)—\(10\) mm, and the fact that the form of the vibrations may be random in character.

Depending on the purposes for which the equipment is used, different requirements are imposed on it. In laboratory investigations, a set of vibration measurement equipment must meet the requirements of ensuring the specified accuracy of the experiment under conditions of impacts and random shocks, with high reliability of the results. The equipment must have a frequency passband from 10 Hz to 10 kHz, a dynamic amplitude range for overloads of \(0.05\)—\(100\,g\), and for displacements of \(0.01\)—\(10.0\) mm. In production control, the equipment must be portable, highly reliable, permit periodic calibration by simple means, and be easy to service (not requiring highly qualified maintenance personnel).

It is difficult to find serially produced domestic equipment that could satisfy such requirements. Instruments with low-frequency sensors and pointer outputs of the V-2, VPU-2M, UBP, and other types are of little use: the sensors are designed for a narrow frequency range (up to 100—150 Hz); the small values of overloads (up to \(10\,g\)) and displacements (up to 2 mm), and the presence of amplifiers in the electronic units, do not allow operation with transient processes. Domestic instruments also do not cover simultaneously the frequency range 10—10000 Hz. Even the best serial instrument, the PIU-1M, with a PDU-1 piezoelectric sensor, permits measurement of vibration accelerations only at vibration frequencies from 40 to 5000 Hz. The sensor

under conditions of sharp impacts it quickly fails: the quartz crystal crumbles.

Since the vibration of hammers is not constant over time and the amplitude of displacement changes from cycle to cycle, continuous recording of the oscillation is necessary for analyzing the vibration of vibro-impact mechanisms.

The manual vibrograph VR-1 makes it possible to record oscillations on paper tape coated with wax; however, the speed of tape movement is low, and therefore analysis of the oscillations is difficult.

This shortcoming is eliminated in apparatus with a potentiometric-type sensor [1]. Vibrograms are recorded on motion-picture film by means of a magnetoelectric oscillograph. When the required film speed is set, vibrograms convenient for qualitative analysis are obtained. This method is good for laboratory studies.

In production inspection there is no need for qualitative analysis of vibration. It is necessary only to establish whether the displacement amplitude conforms to the requirements of sanitary standards. Moreover, the inspection time must be short. In accordance with these requirements, experiments were carried out with an instrument equipped with a magnetoelectric (seismic) sensor (Fig. 1). In the course of the tests it was found that the shocks arising during operation of the hammer bring the sensor system out of equilibrium. In this case the pointer indicator of the instrument either goes beyond the limits of the scale under random shocks, or oscillates at the beginning of the scale, where reading is difficult. The results obtained made it possible to establish that seismic sensors are unsuitable for monitoring the vibration of pneumatic hammers.

Fig. 1. Magnetoelectric (seismic) vibration sensor

Fig. 2. Inductive (a, b) and control (c) sensors for monitoring hammer vibration.
1 — contact leaf, 2 — rivet, 3 — measuring winding, 4 — generator winding, 5 — coil frame, 6 — plug, 7 — ferrite rod, 8 — mandrel body, 9 — cap

Subsequently, an instrument was developed with an inductive-type sensor (Fig. 2, a, b). The generator winding of the sensor is supplied with a frequency

10 kHz from an audio generator. The signal from the measuring winding is fed to an electronic oscilloscope. The coupling between the sensor windings is transformer coupling and is effected by means of a ferrite rod enclosed in a mandrel.

When testing hammers, the coil with the windings is fastened to a fixed support, and the ferrite rod—to the hammer. The scale divisions on the oscilloscope screen are evaluated using a reference sensor (Fig. 2, c). An instrument with an inductive-type sensor is complicated to operate and requires precise setting of the supply voltage and frequency. Taking into account that the new sanitary standards provide for evaluating impact-vibration tools by vibration velocity, we have carried out development of an instrument that would make it possible to measure velocity, displacement, and acceleration simultaneously or separately.

The instrument has an inductive-type sensor (Fig. 3). The sensor consists of a magnet 3, fastened to the object of measurement, and a coil 2 with a winding. The coil is fastened to a fixed support. The sensor signal is proportional to velocity, but by connecting circuits to the output it is possible to obtain displacement, velocity, and acceleration. Moreover, all parameters can be recorded either simultaneously or separately. Dynamic calibration of the sensor is performed on any setup with known amplitude and frequency. Connecting the sensor to a circuit with cathode followers makes it possible to record vibration on a loop oscillograph.

Fig. 3. Inductive sensor. 1—coil frame, 2—winding, 3—magnet, 4—mandrel

The most complete characterization of the vibration of impact machines can be obtained from the vibration spectrum. Spectral decomposition is performed by analytical, graphoanalytical, and instrumental methods. From our point of view, for impact machines the instrumental method is the most acceptable.

Commercially produced spectrum analyzers SICH, ASChKh, AS-3, and others do not fully satisfy the requirements for vibration analysis of impact machines, either in frequency range (for example, SICH are infralow-frequency instruments, whereas ASChKh are high-frequency instruments), or in the minimum recorded level of the input signal.

The analysis data are either read from the analyzer screen or continuously recorded from the screen onto motion-picture film. Both of these methods are acceptable for hammer studies in the laboratory and are unsuitable for production conditions, especially when there is a large number of hammers. For production, obtaining digital information is more convenient; therefore instruments of the digital-voltmeter type must be connected to the analyzers.

Thus, vibration-measuring equipment for pneumatic hammers must include an instrument with a massless indicator, and, for monitoring the vibration spectrum, analyzers with a frequency range of 10–10000 Hz and an output to a massless digital instrument must be developed.

REFERENCES

1. V. F. Gorbunov, V. I. Gusarov, L. S. Berezin. Electrical method for recording vibrograms of pneumatic hammers. Proceedings of Higher Educational Institutions, Mining Journal, No. 12, 1962.