METHODOLOGY FOR OBTAINING VIBRATION SPECTRA OF PNEUMATIC HAMMERS

V. F. Gorbunov, V. I. Baburov, A. I. Pashnin, Yu. A. Oparin, A. F. Kozlov

The vibration of a pneumatic hammer is a complex oscillatory process caused by the action of several forces on the hammer body [8]. Until now, researchers have been interested mainly in the components of vibration—displacement, velocity, acceleration. The need to evaluate pneumatic hammers according to sanitary standards [5] raises the question of obtaining vibration spectra. This work considers issues of instrumental analysis of the oscillation spectrum of pneumatic hammers.

In the general case, a vibration-measuring instrument consists of a transducer, a functional circuit, and a recording instrument. Our studies have established that, among known transducer designs [3, 4], inductive and magnetoelectric sensors of the nonseismic type are most applicable for measuring the vibration of pneumatic hammers. They possess high sensitivity and are little affected by mechanical wear and temperature.

An inductive transducer is a coil (Fig. 1) with measuring winding 3 and generator winding 4. The coupling between the windings is transformer coupling, and the transformation coefficient changes with a change in the position of the core inside the coil. The generator winding is supplied with high-frequency current. Most often a supply frequency of 10–15 kHz is used [2, 4]. The advantage of the inductive transducer is that its sensitivity is in linear dependence on the supply voltage and, for a given sensor, can be regulated.

A magnetoelectric transducer (Fig. 2) consists of a solenoid 2 and a magnet 3 inside it. This transducer is of the generator type. The sensitivity of a specific sensor is constant and can be changed by means of the number of turns of the winding.

Since the inductive transducer makes it possible to measure displacement, and the magnetoelectric transducer—velocity, when it is necessary to obtain other vibration characteristics the indicated transducers should be connected to differentiating and integrating circuits.

According to the principle of decomposing oscillations, analyzers are divided into instruments with sequential and simultaneous analysis [9]. Since the oscillatory process of a pneumatic hammer is not strictly periodic—

Fig. 1. Inductive transducer: 1 — contact tab, 2 — rivet, 3 — measuring winding, 4 — generator winding, 5 — frame, 6 — plug, 7 — ferrite rod, 8 — mandrel

Fig. 2. Magnetoelectric transducer: 1 — housing, 2 — winding, 3 — magnet, 4 — mandrel

Since the vibrations are nonperiodic, spectrometers with simultaneous analysis must be used to decompose the vibrations. In this case the vibration spectrum is observed on the analyzer screen and photographed, with subsequent obtaining of spectrograms.

The spectrum of a pneumatic hammer for the most stable regimes [8] can also be obtained with the aid of an analyzer with sequential analysis. The transducer signal may be fed directly to the spectrometer or first recorded on magnetic tape, which is more convenient and universal. A signal recorded in this way is preserved for an indefinitely long time. This makes it possible to carry out coarse and fine analysis sequentially. An analyzer with octave filters is preferable. This requirement is met by spectrometers of the SICH, SZCh, and FS\(_p\) 10a types, manufactured by Brüel & Kjær. The FS\(_p\) 10a spectrometer has advantages over others, since it has two units that make it possible to analyze the process in frequency bands of 4–200 Hz and 40–20,000 Hz. In addition, this spectrometer is a quarter-octave type, i.e., fine analysis can be performed.

Fig. 3. Modulator circuit

When using a tape recorder as a means of preliminary recording, in order to obtain the spectrum of oscillations of pneumatic hammers it is necessary to take into account certain features of this method of obtaining the spectrum.

Mass-produced tape recorders have a frequency range of 40–10,000 Hz [7], which does not completely cover the dynamic range of hand-held pneumatic hammers [1]. It is necessary to extend the frequency range of the tape recorder into the low-frequency region. This is achieved by using special types of magnetic tape (for example, type 6), increasing the tape speed to 19 and 38 cm/s, and using a modulator, the circuit of which is shown in Fig. 3. Since the carrier frequency of the signal from the audio generator is recorded well on the tape, oscillations with a low modulating frequency, produced by the transducer, are also recorded well on the magnetic tape.

To correct the spectra, it is necessary to know the frequency characteristic of the tape recorder. For this purpose, a signal of a specified level (preferably as close as possible to the level of the signal being analyzed) is recorded on the magnetic tape over the entire frequency range. Then, during playback, the signals at the output of the tape recorder are measured with a vacuum-tube voltmeter. From the measured values a graph is constructed

\[ f(X)=\frac{U_{\mathrm{in}}}{U_{\mathrm{out}}}, \]

where \(U_{\mathrm{in}}\) is the voltage at the input of the tape recorder, V;
\(U_{\mathrm{out}}\) is the voltage at the output of the tape recorder, V;
\(X\) is the measured vibration characteristic.

The obtained spectrum is corrected with the aid of this graph.

Depending on the type of function being decomposed (displacement, velocity, acceleration), the corresponding spectra are obtained.

Recording a modulated signal on magnetic tape does not make it possible to judge the level of the useful signal; therefore, a large modulation depth must be achieved so that the useful signal does not fall below the sensitivity threshold of the analyzer. Tape recorders powered from the mains are characterized by interference at the industrial frequency, which makes analysis difficult. Installing a filter at a frequency of 50 Hz may also absorb the useful signal at this frequency; therefore, the recording of oscillations should be made on a tape recorder with autonomous power supply, or the tube-filament circuit of the mains-powered tape recorder (at least the first two amplifier stages) should be switched to direct-current supply.

Fig. 4. Structural diagram for analyzing vibrations of pneumatic hammers

Since the optimum supply frequency of the inductive transducer lies on the steeply falling portion of the tape-recorder characteristic, during recording one should switch to a lower supply frequency—6–8 kHz—and compensate for the loss of sensitivity by increasing the supply voltage of the converter. Both inductive and magnetoelectric converters must be calibrated dynamically. For this purpose, vibration stands of the ST-1000, GEMK, and other types are suitable.

Thus, the general scheme for obtaining the vibration spectrum (Fig. 4) includes a converter (transducer) 1, a tape recorder 2, a spectrum analyzer 3, and an oscilloscope 4 for intermediate monitoring of the quality of the magnetic recording.

Conclusions

1. The decomposition of the vibrations of a pneumatic hammer into a spectrum must be carried out on spectrometers with band-pass filters and simultaneous analysis.

2. The use of magnetic recording makes the method more universal. When recording vibrations of hand-held pneumatic hammers, the dynamic frequency range of the tape recorder should be expanded, especially in the low-frequency region.

3. To eliminate the influence of industrial-frequency interference, the recording of vibrations must be made on a tape recorder with autonomous power supply, or the filament circuits of the amplifier of a mains-powered tape recorder must be supplied with direct current.

LITERATURE

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1. G. S. Vilner. A stand and apparatus for vibration testing of chipping pneumatic hammers. In: Scientific Works of the Institutes of Labor Protection of the All-Union Central Council of Trade Unions, issue 1 (27). Profizdat, Moscow, 1964.

2. Yu. I. Iorish. Vibrometry. Mashgiz, Moscow, 1963.

3. L. F. Kulikovsky. Inductive Displacement Meters. Gosenergoizdat, Moscow–Leningrad, 1961.

4. Sanitary Norms and Rules for Work with Tools, Mechanisms, and Equipment that Generate Vibration Transmitted to Workers’ Hands, and for Limiting the General Vibration of Workplaces. Publishing House of the Ministry of Health of the USSR, Moscow, 1966.

5. M. G. Serebrennikov. Harmonic Analysis. Gostekhizdat, 1948.

6. Handbook for the Beginning Radio Amateur. Edited by R. N. Malinin. “Energiya,” Moscow–Leningrad, 1965.

7. B. V. Sudnishnikov, N. N. Esin. Elements of the Dynamics of Impact-Action Machines. Publishing House of the Siberian Branch of the Academy of Sciences of the USSR, Novosibirsk, 1965.

8. S. B. Stopsky. Spectrum Analyzers of Audio and Infrasonic Frequencies for Acoustic Spectrometry. Gosenergoizdat, Moscow–Leningrad, 1962.