ANALYSIS OF THE COMPONENTS OF THE VIBRATION SPECTRUM OF PNEUMATIC HAMMERS

V. F. GORBUNOV, V. I. BABUROV, A. I. PASHNIN

Obtaining and studying the spectra of pneumatic hammers as a function of operating conditions is necessary in order to establish the causes of vibration, especially high-frequency vibration. On this basis it becomes possible to develop scientifically grounded recommendations for vibration isolation of hand-held pneumatic hammers.

In accordance with the sanitary standards (1966, No. 626), the vibration of hand-held mechanisms and tools must be evaluated from the vibration-velocity spectrum, since the vibrational energy is proportional to it.

Of all possible methods for obtaining the spectra of pneumatic hammers, instrumental analysis is the most acceptable; for its purposes the authors developed a methodology [1].

We investigated chipping-riveting pneumatic hammers manufactured serially by the Tomsk Electromechanical Plant. In studying the vibration-velocity spectra of pneumatic hammers, general regularities were established. The form of the envelope of the spectrum and the levels of its components depend on the operating regime of the hammer (air pressure in the line and pressing force). Figure 1 shows the vibration-velocity spectra of the riveting hammer KE-22 at various pressing forces and at a constant air pressure equal to 5 ati. As can be seen from the figure, with increasing pressing force the levels of the spectral components tend to decrease, and this decrease is not monotonic. The general form of the spectral envelope is preserved. But at certain frequencies (frequency bands), depending on the pressing force, new “peaks” appear or a redistribution of the vibration-velocity levels occurs. Thus, the level of vibrational energy, proportional to the area of the spectrum and transmitted to the worker, decreases with increasing pressing force.

Figure 2 gives the velocity spectra of the riveting hammer KE-16 at a constant pressing force but at different air pressures. With an increase in the air pressure supplied to the hammer, the levels of the spectral components increase noticeably. At the same time, the general form of the envelope of the spectral components has the same character.

Figures 1 and 2 show that the spectrum of a pneumatic hammer depends substantially on its operating regime. Let us note certain general features of the vibration spectra of hammers of different designs when they operate in one and the same regime characteristic of production, namely—

but: at an air pressure of 5 atm; a pressing force of 20 kg with a spring stiffness of 5 kg/cm; the impact-energy absorber was a steel plate.

Under these operating conditions of the hammers, the vibration-velocity spectra have the following general regularities:

Fig. 1. Envelopes of the vibration-velocity spectra of the KE-22 hammer at pressing forces: △—△—△—29 kg, ○—○—○—25 kg, ×—×—×—21 kg, ●—●—●—17 kg

a) all spectra begin with the fundamental (first) harmonic, equal to the impact frequency of the hammer; this is characteristic of all operating modes, although in some of them components of lower frequency may appear; their presence is explained by the random character of the vibration and the complexity of the form of the oscillations;

b) a decrease in the magnitude of the vibration velocity from the beginning of the spectrum up to frequencies of 100–300 Hz;

c) the presence, as a rule, of several maxima in the spectrum at frequencies above 300 Hz.

The variable air pressure in the cavities of the hammer cannot create high-frequency oscillations; therefore, in the spectral zone up to 300 Hz a decrease in the vibration-velocity levels is observed. If the presence of the low-frequency part of the spectrum can be explained by the action of vibration produced by the changing air pressure in the hammer, then the nature of the high-frequency “peaks” cannot be satisfactorily explained without special experiments.

For these purposes, the natural frequencies of vibration of the parts and of the assembled hammer were found. Some of their values are given in the table. The natural frequencies of vibration were determined by the method of frequency comparison. Figure 3 shows the layout of the experimental setup—

Fig. 2. Envelopes of the vibration-velocity spectra of the KE-16 hammer at an air pressure in the network of ●—●—●— 6 atm, ×—×—×— 5 atm, ○—○—○— 4 atm.

setup. Part 2 was suspended on a flexible thread 1. Oscillations at the natural frequency were excited in the part by an impact; the signal was picked up by an acoustic probe (AZ-1) 3 and amplified by a special attachment 4 of the UPU-1 type. The amplified signal was fed to oscilloscope 5. Simultaneously, a signal of a standard calibrated frequency from sound generator 6 was fed to oscilloscope 5.

Table

Natural vibration frequencies of the pneumatic hammer and its parts, in Hz

When the frequencies of these signals coincide, or when their multiplicity coincides, the corresponding Lissajous figure is observed on the oscilloscope screen, from which the natural vibration frequency of the part is determined.

It was not possible to determine the natural frequency of the assembled hammer by this method because of the rapid attenuation of the signal. Therefore, for this purpose a wide-band electrodynamic vibration stand (up to 5000 Hz) was used. When the excitation frequency coincides (resonance) with the frequency

sound effects and a sharp increase in acceleration were observed. The frequency data are given in the table.

If we compare the frequencies at which “peaks” are observed in the vibration-velocity spectrum, we note that they coincide with the natural-vibration frequency of the assembled hammers. Thus, the source of high-frequency vibration is the natural vibrations of the parts and of the hammer as a whole. Given the impact nature of the hammer’s operation, vibrations at the natural frequency are excited in the parts, thereby increasing the levels of the high-frequency components.

Analysis of the vibration-velocity spectra of pneumatic hammers shows that there are two main sources of vibration: the time-varying air pressure in the front and rear cavities of the hammer, and the natural vibrations of the parts and of the hammer as a whole. In this case, the first is the source of low-frequency vibration, and the second of high-frequency vibration. Proceeding from this, methods for combating vibration can be recommended. Suppression of low-frequency vibration is possible by vibration-isolating the handle with elastic elements, using dynamic absorbers, changing the operating cycle, etc.

Fig. 3. Diagram of the experimental setup

To reduce high-frequency vibration, only vibration damping is possible, since it is caused by vibrations of parts at their natural frequency. Therefore it is necessary:

a) to strive to eliminate impacts when seating the hammer body on the shoulder of the tool, by using special bushings and vibration-protective couplings;

b) to shift the natural frequency of pneumatic hammers as far as possible into the high-frequency region;

c) to select the material of hammer parts with a view to reducing the frequency of natural vibrations.

The natural frequency

\[ f = k \sqrt{\frac{c}{m}} \]

depends on the ratio of the stiffness of a part to its mass and on the conditions of its fastening. Increasing this ratio by reducing the mass is inadvisable, since the stiffness of the part decreases along with it. It is rational to increase \(f\) by increasing the stiffness of the part. For example, replacing the round-section hammer-handle holder with a box-section one leads to a sharp increase in stiffness without a noticeable increase in weight. However, when increasing the stiffness of the hammer handle, it is also necessary to take into account the aesthetic and technological requirements for its design from the standpoint of ease of gripping by the worker’s hand and reduction of the hydraulic losses of compressed air. In this case, the most convenient cross-sectional shape should be considered triangular with rounded vertices, proposed by Tuchny [2]. To reduce hydraulic air losses, the channels through which the air passes must be straightened, but their cross-section must not be reduced.

Conclusions

1. Spectral analysis of the vibration of mass-produced pneumatic hammers has established that, as the pressing force increases, the levels of the components of the vibration-velocity spectrum decrease, though not monotonically. The envelope of the spectrum has maxima at certain frequencies. An increase in the air pressure in the network leads to an increase in the levels of the components. For all types of hammers, the envelope of the spectrum in this case has one and the same character.

2. General regularities in the variation of the vibration-velocity spectra of hammers when they operate under identical conditions have been shown.

3. The sources of low- and high-frequency vibrations have been established, and a method for limiting them by changing the natural frequencies of vibration of individual parts of the hammer has also been shown.

References

1. V. F. Gorbunov, V. I. Baburov, A. I. Pashnin, Yu. A. Oparin, A. F. Kozlov. Method for obtaining vibration spectra of pneumatic hammers. (This collection).

2. P. Tichý. K hygiene práce s ručním nářadím. Praha, 1965.