Results of Vibration Assessment for Pneumatic Hammers Taking into Account Higher Harmics

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

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

Many works by various authors have been devoted to studies of hand-held pneumatic hammers, but the question of vibration isolation has still not been completely resolved. The need for the most rapid solution of this question has been substantiated by sanitary-hygienic workers on the basis of an analysis of pathological processes in the bodies of workers using hand-held mechanized tools [1—3].

This article presents the results of studies and assessment of chipping and riveting hammers according to sanitary vibration standards, taking into account its spectral composition.

In studying the occurrence and development of vibration disease, hygienists note the important significance of vibration frequency. The most characteristic symptom of vibration disease is spasm of the blood vessels. Examination of workers using pneumatic tools showed that the first signs of vascular spasm appeared at a vibration frequency of 35 cps, while the greatest number of illnesses occurs at a frequency of 100 cps [1].

In order to know the amplitude and frequency of vibration at 60—100 cps, it is necessary either to test the hammers with vibration measurements by sensors tuned to particular frequencies, or to use the method of harmonic analysis. The authors obtained the results described here by means of harmonic analysis using Taylor’s grouping method [4], the simplest for a small order of harmonics.

In studying chipping and riveting hammers of the MR and KE types at the Tomsk Polytechnic Institute, vibrograms of the hammers were obtained at various air pressures and pressing forces, and it was established that the dependence of the vibration amplitude of the fundamental frequency on the pressing force has a complex extremal character (Fig. 1). As the pressing force increases, the vibration of the hammer body is first minimal in amplitude and then maximal. The amplitude minimum corresponds to the “floating” mode of oscillation of the hammer body, and the maximum to the limiting fundamental mode. From the hygienic point of view, operation in the “floating” mode is the most favorable, but it is very difficult to maintain this mode. For the most part, the worker creates a pressing force corresponding to the limiting fundamental mode of oscillation of the hammer, i.e., under the most unfavorable conditions; therefore, the assessment of the mo-

hammers according to sanitary standards is carried out by us precisely in this most dangerous mode.

Evaluation of KE- and MR-type hammers by the vibration amplitude of the fundamental frequency showed that all hammers, except MR-4 and MR-5, meet the requirements of the sanitary standards. If, however, one takes into account the necessity

Fig. 1. Amplitude–force dependences of six vibration harmonics of the KE-16 hammer:
— 1 (fundamental) harmonic, — — — 2nd harmonic, — ● — 3rd harmonic, — △ — 4th harmonic, — □ — 5th harmonic, — × — 6th harmonic

for reducing the permissible vibration amplitude by 2 or 3 times in the presence of subharmonics [3], then not a single hammer will pass the sanitary standards.

In the harmonic analysis we limited ourselves to six vibration harmonics, since for different hammers they cover a frequency spectrum up to 360 Hz, that is, the entire frequency range causing vibration disease [1].

On the basis of the analysis results, spectrograms were constructed (Fig. 2), in which the amplitude of the fundamental frequency was taken as unity, and the remaining harmonics were evaluated by the ratio of the amplitude of the higher harmonic to the fundamental. As the pressing force increases, the amplitudes of the higher harmonics increase. With an increase in the weight of riveting hammers, the amplitude maximum in the “floating” mode shifts from 60 Hz to a frequency of 80 Hz, and in the limiting fundamental mode—from 100 to 60 Hz. In chipping hammers, in both modes, as the hammer weight increases, the amplitude maximum shifts toward harmonics of higher orders.

Amplitude–force relationships constructed for each higher harmonic (Fig. 1) have the same character of variation as for the fundamental harmonic, i.e., in the “floating” regime the amplitude is minimal, and in the limiting fundamental regime it is maximal.

Fig. 2. Spectrograms of oscillations of the body of the KE-16 hammer at an air pressure in the network of 5 ati and various pressing forces.

For assessing the vibration of hammers according to sanitary standards, taking higher harmonics into account, safety factors were determined as the ratio of the permissible vibration amplitude to the actual one. The safety factor of riveting hammers increases with an increase in the weight of the hammer (Table 1). For example, the KE-16 and KE-19 hammers satisfy the standards for the first and second harmonics (frequency 20–60 Hz), the KE-22 and KE-28 for the first three (frequency 16–50 Hz), while the KE-32 satisfies the sanitary standards for all subharmonics practically in all possible regimes.

Let us show, using the KE-28 hammer as an example, the need to assess hammers separately for each harmonic. In the limiting fundamental regime the actual amplitude and frequency of vibration of the fundamental harmonic of the KE-28 hammer are respectively equal to 1.35 mm and 14 Hz. Taking into account the presence of subharmonics with a frequency of 50–80 Hz, we must reduce the permissible amplitude of 2.10 mm by a factor of 2, i.e., it must be taken as equal to 1.05 mm. In this case the KE-28 hammer does not satisfy the sanitary standards, since its safety factor is less than unity

\[ \left(\frac{1.05}{1.35}=0.78\right). \]

When assessing according to the standards the amplitude of each of the higher harmonics in the “floating” regime (Table 1), only the amplitude of the fifth harmonic is less than the permissible one, while in the limiting fundamental regime the KE-28 hammer fully satisfies the requirements of the sanitary standards. Consequently, the assessment of hammers must be carried out taking into account the amplitude of each harmonic.

Proceeding from an analysis of the operating regimes and of the vibration-frequency spectrum of pneumatic hammers, the basic requirements for vibration-damping devices can be formulated:

1) the vibration-damping device must be designed to transmit to the hammer the possible pressing forces exerted by the worker (up to 40 kg);

2) the vibration-damping element must provide effective filtration of vibration with a frequency of up to 360 Hz and an amplitude of up to 3–4 mm.

One way of sharply reducing vibration, meeting the stated requirements, is to create hammers with resilient (spring-

Table 1

Values of safety coefficients for the six harmonics of the hammer

loaded) handles and elastic couplings to isolate the worker’s left hand from the vibration of the tool.

REFERENCES

1. E. Ts. Andreeva-Galanina, V. G. Artamonova. Assessment of Work Capacity in Vibration Disease. Medgiz, Leningrad, 1963.

2. N. N. Malinskaya. Hygienic assessment of a hand-held mechanized tool. In: Proceedings of the Seminar “Experience in Combating Noise and Vibration in Industry.” Moscow, House of Technology, 1963.

3. Collection of the Most Important Official Materials on Questions of Occupational Hygiene and Industrial Sanitation. Issue II. Medgiz, 1962.

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