Results of the Development and Hygienic Study of Pneumatic Hammers with Vibration Isolation

V. F. Gorbunov, Z. M. Butkovskaya, V. I. Baburov, Yu. A. Oparin,
E. I. Volodina, A. N. Shchipunov, S. S. Karpushin, Yu. S. Shevchenko

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

Studies of the working process and vibration of pneumatic hammers have established the main directions for limiting the harmful effect of their vibration on the worker.

Among the known methods of damping and isolating vibration, an important place is occupied by:

a) changing the working process of the hammer (the nature of the action of the compressed-air pressure on the cylinder walls);

b) introducing vibration-isolating devices into the design of the hammer;

c) using protective vibration-isolating couplings for the tool, protective sleeves, etc.

At the Tomsk Polytechnic Institute (TPI), the second of the indicated directions is being developed. In collaboration with, and with the participation of, employees of the Electromechanical Plant (TEZ), several models of pneumatic pick and chipping hammers with vibration-isolating devices have been developed and studied. This article presents the latest results of comparative technical and medical-biological studies of the pneumatic chipping hammers MR-6 (production model of TEZ) and MRV-16 (experimental model of TPI).

On the basis of an analysis of the designs of hammers with elastic handles and of their individual assemblies, we developed a basic schematic of a hammer (Fig. 1) with vibration-isolating devices, in the calculation of which it was assumed that:

a) the energy of the return impact of the tool is absorbed by the rear buffer of the bushing and is not transmitted to the hammer body;

b) the blows of the striker against the crosspiece are absorbed by the front buffer of the bushing and likewise are not transmitted to the hammer body;

c) the vibration-isolating element of the elastic handle is a conical spring, which is calculated for the action of the force from the compressed air in the rear cavity of the hammer,

\[ Q = p \cdot S,\ \text{kg}, \]

where \(p\) is the working pressure of the compressed air, atm; \(S\) is the cross-section of the barrel cylinder, \(\text{cm}^2\).

The working deformation of the spring is 30 mm. The diameter of the largest working coil is determined structurally.

In calculating the front and rear chuck shock absorbers, the compression was determined. The amount of compression must not interfere with the normal operation of the hammer. For rubber shock absorbers it is determined by the formula

Fig. 1. Design of the MRV-16 pneumatic hammer.
1 — handle, 2 — conical spring, 3 — intermediate link, 4 — air-distribution device, 5 — rubber shock absorber, 6 — “false” barrel, 7 — barrel, 8 — chuck, 9 — rear shock absorber of the chuck, 10 — front shock absorber of the chuck, 11 — nut, 12 — connecting hose, 13 — tool

\[ \lambda = \frac{F \cdot H_1}{S_1 \cdot E_{\mathrm{dyn}}},\ \text{cm}, \]

where \(F\) is the force acting on the shock absorber, kg;

\[ H_1 = H - \frac{A}{8} \]

is the working height of the shock absorber, cm;

$H_1$ — total height of the shock absorber, cm;
$A$ — transverse dimension of the shock absorber, cm;
$S_1$ — cross-sectional area of the shock absorber, cm²;
$E_{\text{dyn}}$ — modulus of elasticity of rubber in compression under dynamic loading, kg/cm².

The magnitude of the force acting on the rear shock absorber was determined experimentally and amounted to 150–200 kg.

The magnitude of the force acting on the front shock absorber was determined by the formula

\[ F=\frac{G\cdot V}{q\cdot t},\ \text{kg}, \]

where $G$ — weight of the striker, kg;
$V$ — velocity of the striker at the moment of impact, m/sec;
$q$ — acceleration of free fall, m/sec²;
$t$ — contact time of the striker with the bushing during impact, sec.

The distinctive features of this basic hammer scheme are:

a) the use of a special vibration-damping bushing;
b) the use of a conical spring installed as an elastic element between the handle and the barrel;
c) the supply of compressed air from the handle to the air-distribution device is effected through a flexible hose;
d) the presence of a casing or “false barrel.”

The design of the vibration-damping bushing provides not only a reduction in vibration of the body due to collision forces with the tool, but also protection of the barrel from destruction under the action of blows of the striker against the crosspiece. In the middle, the bushing has an annular projection that rests on the rear and front shock absorbers (rubber sleeves), which are placed in an annular cavity in the front part of the barrel. Centering of the bushing is accomplished by means of the annular projection and the cylindrical part facing the handle. This prevents misalignment of the bushing relative to the longitudinal axis of the machine.

The inner surface of the bushing, which comes into contact with the shank of the working tool, is made in any shape depending on the nature of the work.

The proposed vibration-damping bushing has two disadvantages:

1) it can rotate about its axis, which is highly undesirable when performing certain operations;
2) it does not damp the transverse vibrations of the hammer.

However, despite the noted shortcomings, we believe that the positive qualities of the bushing make it possible to recommend it for use.

The larger-diameter coils of the handle’s conical spring rest on the bottom of the cavity in the handle, and the smaller-diameter coils rest on the intermediate link. Thus, the spring does not hinder the sliding of the handle along the intermediate link, which serves to fasten the air-distribution device to the barrel. The locking of the intermediate link is carried out in the same way as that of the handle in hammers of the MR and KE types.

A flexible hose connects the air duct at the inlet valve with the pre-valve chamber.

The “false” barrel in the front part slides along a support ring fitted onto the barrel, and in the rear part it is fastened to the handle.

To lighten the hammer, its barrel was milled on two sides, in the places where there are no channels for supplying air to the front cavity, and its diameter was reduced compared with the barrel of the MR-6 hammer. The exhaust holes were drilled along the radius of the barrel.

The indicated changes made it possible to manufacture a hammer with vibration-isolating devices weighing 5.7 kg. The weight of the production model MP-6 hammer is 6 kg.

Technical studies of the MP-6 and MPV-16 hammers for vibration were carried out in the laboratories of TPI and of the Pneumatika plant (Leningrad). In the course of the studies, diagrams of the vibration displacement of the hammer handles were recorded by means of electric sensors and an MPO-2 oscillograph. When the hammers were studied at TPI, the steel-cutting conditions were varied over a wide range (air pressure from 4 to 6 ati, handle pressing force from 10 to 40 kg), while at the Pneumatika plant the operating mode of the hammers was set in accordance with the requirements of the current sanitary standards (air pressure 5 ati, pressing force 15 kg), and the hammers operated on the ERD-3 stand*).

The main results of the tests of the hammers in terms of technical characteristics are given in Table 1.

It is seen from Table 1 that the frequency and amplitude of vibration displacement of the MPV-16 hammer do not exceed the values permitted by the temporary sanitary

Table 1

Technical characteristics of pneumatic hammers

*) According to data from the Pneumatika plant.

standards (No. 191—55 of August 11, 1955), whereas these characteristics for the MP-6 hammer are 1.6–1.9 times higher than the sanitary standards.

It should be noted that, when comparing the vibration-displacement amplitude with the standards, the complex nature of the vibration of pneumatic hammers was taken into account and, in accordance with paragraph 8 of the “Temporary Sanitary Standards,” the permissible amplitude was calculated from the expression

\[ a_{\mathrm{d}}=\frac{a_{\mathrm{n}}}{k}, \]

where \(a_{\mathrm{n}}\) is the permissible vibration amplitude in harmonic oscillation;

\(k\) is a coefficient taking into account the presence of high harmonics (overtones).

*) Employees of the Pneumatika plant and of the vibration-and-noise laboratory of the All-Union Central Council of Trade Unions Scientific Research Institute of Labor Protection took part in the hammer tests.

The spectral composition of the vibration of both hammers studied is very complex (Fig. 2); however, the acceleration values in all frequency ranges are considerably lower for the hammer with vibration isolation, MPV-16, than for the standard MP-6 hammer.

From the oscillograms of the vibration displacement of the hammer handles (Fig. 3) it is also evident that, in addition to the difference in the magnitude of the displacement amplitude,

Fig. 2. Spectral composition of vibration of the MP-6 and MPV-16 hammers (according to data from the All-Union Scientific Research Institute of Labor Protection of the All-Union Central Council of Trade Unions, Moscow)

Fig. 3. Oscillograms of vibration of the handle of the MP-6 hammer (a) and MPV-16 (b)

there is a difference in the shape of the curve. Thus, the rise time of the maximum displacement amplitude in the MP-6 hammer is considerably less than half the period, whereas in the MPV-16 hammer, with a smaller amplitude, it is approximately equal to half the period. It is known that the steeper the rise of the maximum amplitude, the more pronounced the effect of recoil on the organism.

Physiological studies were carried out on four practically healthy subjects. In them, the state of the peripher-

peripheral circulation, the neuromuscular apparatus, and vibration sensitivity. To judge changes in these systems, the following were determined: the time of appearance of spreading reactive hyperemia (after anemia induced by a tourniquet) on the right and left hands and chest; the tone of the muscles of the thenar group on the right hand; muscular strength in the hands and endurance at 80% of maximum effort; the magnitude of biopotentials on the right hand (muscles of the thenar group); vibration sensitivity on the index fingers of both hands; and pulse rate.

The data obtained from all subjects were summed and statistically processed with calculation of Student’s \(t\) criterion. Differences in the indices were considered reliable when the mean-square error did not exceed 5%.

All studies of physiological functions were carried out before, after, and 20 minutes after completion of work with the hammer. Experimental work with the hammer was performed for fifteen minutes. The pace of work was somewhat more strenuous than under production conditions: after each 30 seconds of continuous work with the hammer there followed a 15-second rest. Thus, the ratio of work to rest was 2:1, and the net time of experimental work was 10 minutes.

The experimental chipping consisted of removing chips from a steel hardened blank weighing 50 kg. The air pressure in the line during work was 5 atmospheres. The work was performed in canvas mittens. For protection against noise during work, the subjects worked in TsNIIOT-2 helmets. In all, two variants of investigations were carried out, comprising 70 experiments and about 3000 determinations of various functional indices.

If one considers the nature of the shifts in the body and compares the changes obtained after work with the MPV-16 and MR-6 hammers, the following features are noteworthy.

No reliable changes in pulse rate after work with either hammer were found. The nature of shifts in peripheral circulation can be judged from changes in the time of appearance of reactive hyperemia and in skin temperature. Lengthening of the time of appearance of reactive hyperemia and a decrease in skin temperature after work indicate narrowing or a tendency toward spasm of the peripheral vessels, which is characteristic of the negative effect of vibration. After work with the MPV-16 hammer, no lengthening of the time of appearance of spreading hyperemia was noted. On the left hand, an acceleration of the time was noted after work and during recovery, which is evidently connected with dilation of the vessels due to the inflow of blood to the working hand and heating of the hand by the warmed chisel. After work with the MR-6 hammer, a lengthening of the time of appearance of reactive hyperemia on the right hand was noted, which recovered within twenty minutes. On the left hand, no changes in the time of appearance of reactive hyperemia were noted. After work with the MPV-16 and MR-6 hammers, a reliable increase in skin temperature on the hands and chest was noted. After 20 minutes the temperature on the chest was restored, while on the hands it remained elevated. However, comparison of the magnitude of the shifts shows that both immediately after work with the MPV-16 hammer and 20 minutes after its completion, the temperature shifts on the hands were greater than after work with the MR-6 hammer. Thus, immediately after work with the MPV-16 hammer the temperature increased by 4.7° on the right and by 5.8° on the left hand, and after 20 minutes by 4.2° on the right hand and by 4.8° on the left; after work with the MR-6 hammer the temperature increas—

in the right hand by 3.2°, in the left by 4.0°, and after 20 minutes in the right hand by 2.2°, in the left by 2.7°.

No differences in chest temperature were observed when working with the different hammers.

The rise in temperature in the right hand is explained by dilation of the vessels and increased blood flow in the arterioles as a result of the influx of blood to the working hand; in the left hand, in addition, it is also explained by heating of the chisel. However, the smaller rise in temperature after work with the hammer without vibration damping than with the hammer with vibration damping is explained, in our opinion, by the fact that in the former case vasoconstriction occurs as a result of the action of vibration. This vasoconstriction was revealed in the study of the time of onset of reactive hyperemia.

A study of the biopotentials of the muscles of the thenar group in the right hand showed a tendency toward an increase in electrical activity after work with the MP-6 hammer and a decrease in electrical activity after work with the MPV-16 hammer (change in the amplitude of biopotentials).

Our previous studies showed that an increase in electrical activity is usually observed with a rapid rise in the maximum displacement value, as a reaction to impact. It was already indicated above that a rapid rise in displacement amplitude is characteristic of the recoil of the MP-6 hammer.

Muscle strength after work with the hammers changed practically not at all. Muscular endurance decreased somewhat in the left hand after work with the MPV-16 hammer. The reason for this decline in muscular endurance after work with the hammer with vibration damping is not entirely clear. It is unlikely that it can be explained by greater fatigue after work with the MPV-16 hammer, since shifts in the other indicators do not confirm this (changes in bioelectrical activity, muscle strength, muscle tone).

No significant changes in muscle tone after work with the hammers were found.

Changes in vibration sensitivity after work with the hammers were directed toward its decrease. A decrease was noted in sensitivity to the perception of frequencies of 100 and 50 Hz (the shifts lie within 28–15% of the initial level). During 20 minutes these shifts recovered.

It is noteworthy that a decrease in vibration sensitivity after work with the MPV-16 hammer was observed only in the left hand, whereas after work with the MP-6 hammer it was observed in both the right and left hands. This can be explained by the fact that the vibration protection of the MPV-16 hammer is more effective at the handle than at the chisel.

On the basis of the results of the technical and medico-biological studies, the following may be concluded:

a. The vibration characteristic of the MPV-16 hammer corresponds to hygienic standards with respect to the magnitude of the displacement amplitude, whereas no such correspondence was noted for the MP-6 hammer. In terms of the time of rise to the maximum displacement amplitude, the shape of the recoil curve of the MPV-16 pneumatic hammer is significantly safer than that of the MP-6 hammer (the rise time corresponds to 1/2 and 1/4 of the oscillation period). The acceleration values of each individual component in the recoil spectrum of the MPV-16 hammer are considerably smaller than the values of these components in the recoil of the MP-6 hammer.

b. Physiological studies established that experimental work with the MPV-16 hammer does not cause substantial shifts in the investigated functions. Work with the MP-6 hammer causes

some undesirable shifts in the functions under study, the most significant of which is the tendency toward spasm of the peripheral vessels.

Thus, the study of the physiological and hygienic characteristics of pneumatic riveting hammers MR-6 (serially manufactured) and MRV-16 (experimental model) showed certain advantages of the MRV-16 hammer over the MR-6 hammer.

On the basis of the studies carried out, it is possible to recommend manufacture of a production batch of the MRV-16 hammer. For the final physiological and hygienic evaluation, it is necessary to test the production batch under industrial conditions.