V. F. GORBUNOV, V. I. BABUROV, Yu. A. OPARIN, L. S. REDUTINSKY

ASSESSMENT OF THE VIBRATION CHARACTERISTICS OF CHIPPING AND RIVETING HAMMERS ACCORDING TO SANITARY STANDARDS

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

The vibration of hand-held pneumatic hammers has a harmful effect on the worker’s organism. The vibration effect is transmitted to the right hand from the oscillations of the hammer handle and to the left hand from the oscillations of the tool. The action of vibration on the human organism causes various painful consequences and ultimately leads to disorders of the central nervous system [1]. In this connection, much attention is being devoted to questions of combating vibration and to studies of the influence of various factors on its magnitude.

In the present work an assessment is given of the vibration of chipping and riveting hammers of the Tomsk Electromechanical Plant according to the current sanitary standards [2], and comments are set forth on the content of the latter that arose as a result.

Chipping hammers of the MR type (MR-4, MR-5, MR-6) and riveting hammers of the KE type (KE-16, KE-19, and KE-22), manufactured by the Tomsk Electromechanical Plant, are used for dressing steel, cast-iron, and nonferrous castings, as well as for trimming and eliminating various defects on rolled products and forgings.

The vibration amplitude of chipping and riveting hammers (Fig. 1) depends to a significant degree on the magnitude of the forces pressing on the hammer handle. Experimental studies show that this dependence (we shall call it the amplitude–force characteristic) is very complex. Under certain operating conditions it has bends, minima, and maxima. Consequently, the establishment of the actual vibration amplitude must be carried out at specific pressing forces. At the same time, in our opinion, it is necessary to take into account not only the magnitude of the pressing force, which determines one or another mode of oscillation of the hammer, but also the operating conditions under which the internal processes proceed more stably. This follows from the following considerations. In Fig. 1, a, the amplitude–force characteristic of KE-16 hammers at 5 atm is divided into separate characteristic modes of oscillation.

In mode 1 (Fig. 1, a), which we shall call the first transitional mode, the operation of the hammers is very unstable. The pressing forces are small in comparison with the magnitude of the internal disturbing force created by the air pressure on the walls of the housing. The hammer housing moves away from the tool so far that the striker delivers blows on the bridge of the barrel and can destroy it (the bridge in chipping and riveting hammers is provided structurally to prevent the striker from falling out of the barrel).

In the first transient mode the oscillations of the housing have the greatest amplitude; however, this mode occurs very rarely in hammer operation (in the dressing of small castings and in riveting work with small-diameter rivets).

Fig. 1. Amplitude-force relationships for hammers KE-16 (a) and MR-6 (b). Operating mode of the hammer: 1 — first transient; P — “floating”; 2 — second transient; PO — limiting basic; 3 — third transient; U — reverse impacts of the tool against the hammer housing. — — — 4 atm; ——— 5 atm; — · — · — 6 atm.

Of interest is the so-called “floating” mode of hammer oscillation [3] (Fig. 1, a), in which the oscillation amplitude of the housing is significantly reduced and there are no abrupt changes in the velocity of its motion. This mode is the most favorable from the sanitary-hygienic point of view. However, it is practically impossible to maintain the manual pressing force corresponding to the “floating” mode.

The most characteristic modes in practice are: mode 2 (we shall call it the second transient), in which the hammer housing, after several cycles, seats itself on the shoulder of the tool; the limiting basic mode [4] (PO, Fig. 1, a), in which seating occurs in every cycle; mode 3 (we shall call it the third transient); and the “U” mode, characterized by reverse impacts of the tool against the hammer housing. From the standpoint of the intensity of the vibration effect on the worker’s body, the worst conditions are created in the limiting basic mode, since the vibration amplitude is greatest here and the velocity

the motion of the housing changes sharply in magnitude and direction at the moment of collision with the limiting shoulder of the tool.

In an experimental investigation of the nature of the internal processes of hammers as a function of the pressing force, it was established that, as the pressing force increases, the blow frequency of the hammers at first increases, and then, at a certain pressing force, becomes constant [5]. As a rule, stabilization of the internal processes occurs in the limiting fundamental mode. Consequently, the limiting fundamental mode is characterized by the highest vibration frequency.

When the pressing force on the hammer is increased above 30–35 kg (at an air pressure of 5–6 atm), a mode of oscillations with small amplitudes is created, arising mainly as a result of return impacts of the tool against the hammer housing.

Taking into account the above characteristics of the fundamental oscillatory modes, it appears advisable to assess the vibration of hammers while they are operating in the limiting fundamental mode.

The pressing forces that ensure this mode can be determined from the formula

\[ R_{\text{lim. f.}} = \frac{n}{60} \left[ \sqrt{2Am}\,(\varepsilon+1) + \frac{f_{\text{sh}}\cdot S\cdot p_0}{K} + M_k V_k(\varepsilon_k+1) \right] \pm M_k g \sin \alpha, \tag{1} \]

where \(n\) is the blow frequency, blows/min;

\(A\) is the blow energy, kgm;

\(m\) and \(M_k\) are the masses of the striker and the housing, \(\dfrac{\text{kg}\cdot \text{s}^2}{\text{m}}\);

\(\varepsilon\) and \(\varepsilon_k\) are the rebound coefficients of the striker upon impact on the tool shank, and of the housing when seating on the tool shoulder;

\(V_k\) is the housing velocity during seating, m/s;

\(f_{\text{sh}}\) is the area of the end face of the tool shank, cm\(^2\);

\(S\) is the distance from the end face of the tool shank to the center of the exhaust openings of the hammer cylinder, cm;

\(p_0\) is the air pressure in the line, kg/cm\(^2\);

\(K\) is an empirical coefficient accounting for the influence of the factors \(S\) and \(p_0\) on the magnitude of the air-pressure impulse in the front cavity.

For hammers of type MP, \(K=7000\), and of type KE, \(K=870p_0+3600\);

\(\alpha\) is the angle of inclination of the hammer to the horizon, degrees;

\(g\) is the acceleration due to gravity, m/s\(^2\).

If the weight of the hammers is taken into account, then, in order to ensure the limiting fundamental mode, a worker operating hammers in different positions must apply different forces (Table 1).

Table 1

Pressing forces corresponding to the limiting fundamental mode of oscillation of the hammer housing
(according to experimental data, \(p_0=5\) atm)

When testing hammers under laboratory conditions (factory or research), it is possible to fix the pressing force on the hammer. The magnitude of the pressing force necessary to ensure the limiting basic operating mode of particular hammers can be determined by formula (1) or experimentally.

Let us now consider the recommendations of the temporary sanitary standards concerning pressing force. The standards [2] recommend determining the amplitude and frequency of vibration at a pressing force of 15 kg. At the same time, they do not indicate at all what material the blows should be delivered to, or in what position the hammer should be tested. If it is assumed that the hammer is to be tested while working vertically downward and its weight is taken into account, then the reduced pressing force will be equal to

\[ R_{\mathrm{n}} = Q_{\mathrm{m}} + 15\ \text{kg}, \tag{2} \]

where \(Q_{\mathrm{m}}\) is the weight of the hammer, kg.

Figure text:
Vertical axis: double amplitude of vibration, mm.
Horizontal axis: pressing force, kg.
Annotation: \(R_{\mathrm{n.min}} = 18\ \text{kg}\).

Fig. 2. Effect of pressing force on the vibration amplitude of hammer KE-22 when working on various materials — ×—×— steel; ·—·— cast iron; — — — aluminum; —·—·— granite; —— wood.

That is, for hammer KE-32 \(R_{\mathrm{n}} = 26.3\) kg; KE-28—25.3 kg, etc. These forces are close to the limiting basic operating mode of the hammers (Table 1).

Testing KE-type hammers in the horizontal position at a pressing force of 15 kg is generally inadvisable, since under these conditions impacts of the striker against the bridge of the barrel occur.

On the basis of the foregoing, we believe that, with respect to the pressing forces recommended when testing hammers for vibration, the sanitary standards require clarification.

Fig. 3. Influence of the elastic properties (coefficient of rebound) of the material on the amplitude of oscillations of the tool (a), and on the frequency and energy of hammer impacts (b).

Experimental studies undertaken to establish the influence of the properties of the materials being worked on the vibration characteristics of the KE-22 hammer showed that the worst conditions, from the point of view of vibrational effect on the worker’s organism, are created when impacts are delivered on steel (Fig. 2). With an increase in the modulus of elasticity of the medium being worked, the fundamental frequency increases (Fig. 3, b)

and the vibration amplitude (Fig. 2). In this connection, determination of the actual vibration characteristics of hammers (amplitude and frequency) must be carried out while they are operating on media corresponding to the actual operating conditions of the hammers. It should be noted that such recommendations are absent from the temporary sanitary rules [2].

The temporary sanitary rules for limiting the effect of vibration on workers using hand-held pneumatic tools state that, if vibrations of higher frequencies are present along with the fundamental frequency, the requirements for the vibration amplitude of the fundamental frequency must be changed in the direction of reducing it by a factor of 2–3.

In practice, when working with a pneumatic hammer in any regime except the “floating” one, there are subharmonic vibration components of higher frequencies than the fundamental.

Fig. 4. Amplitude-frequency characteristic of a pneumatic tool (according to sanitary vibration standards).

These subharmonics are caused by the presence of impacts between the striker, the housing, and the tool, and appear to varying degrees depending on the pressing force. And even in the “floating” regime of oscillations of the hammer housing, the worker’s left hand, with which he controls the tool, is subjected to vibration of the tool, in whose spectrum high frequencies predominate. The amplitude of the tool oscillations also depends on the force

pressure on the hammer and on the elasticity of the material being worked (Fig. 3).

In connection with the above, we consider it advisable to introduce into the sanitary rules and standards the corresponding corrections, taking into account the presence of transient processes in the vibration spectrum of pneumatic hammers.

Table 2

Vibration characteristics of chipping and riveting hammers

Note: 1. The amplitudes indicated in parentheses were obtained from Fig. 4.
2. The table gives “double” amplitudes.

Let us dwell further on certain shortcomings of the provisional sanitary standards for vibration. The permissible amplitudes of oscillation of hand tools are given in a table in which the lower frequency limit is restricted to 20 Hz. It is therefore difficult to determine the permissible amplitudes for hammers with a lower impact frequency. For example, the KE-22, KE-28, and KE-32 hammers have impact frequencies of 1100, 950, and 800 impacts/min, respectively.

In assessing the vibration of these hammers the authors used the following procedure. The tabulated values of the permissible amplitude-frequency characteristics were represented in the form of a graph, which was extended in the low-frequency region to the vertical axis of the graph (segment \(AB\), Fig. 4). Along the straight line \(AB\), the values of the permissible amplitudes for the KE-22, KE-28, and KE-32 hammers were established as a function of frequency.

On the basis of experience in using the sanitary rules, we believe that it would be preferable, in the vibration standards, to present the permissible amplitude-frequency characteristic of hammers in the form of a graph.

The sanitary rules also do not specify the operating conditions of the hammers. In particular, the vibration of the hammer body changes depending on whether the chipping-hammer tool is moving, removing chips from the workpiece, or is motionless. With a motionless tool, the vibration is more intense.

Table 2 gives comparative data on the vibration of hammers without taking into account additional oscillations or vibration subharmonics caused by reverse impacts of the tool against the hammer body. That is, the vibration of the hammers is assessed only by the fundamental frequency and amplitude. Comparing the actual amplitudes with the data of the sanitary standards, it should be noted that all the tested hammers, except the KE-32 and MR-6, have vibration amplitudes at the fundamental frequency, at an air pressure of 6 atm, above the permissible values. At an air pressure of 4 and 5 atm, only the MR-4 and MR-5 hammers, which have an increased impact frequency, do not meet the vibration standards.

Conclusions

1. Experimental studies of pneumatic hammers have shown that the relationship between vibration amplitude and pressing force is very complex, reflecting the action on the body of forces that differ in magnitude and duration. The principal among these forces are: variable air pressure on the walls of the hammer body, impact of the body when it is seated on the collar of the tool, the worker’s pressing force, reverse impacts of the tool against the body, and impacts of the striker on the bridge of the barrel.

2. The pressing force determines various forms of motion of the pneumatic-hammer body. Depending on the form of motion of the body, a number of the most characteristic operating regimes of the hammer can be distinguished: the first transient regime, the “floating” regime, the second transient regime, the limiting principal regime, the third transient regime, and the regime in which oscillations of the body occur only from the action on it of reverse impacts of the tool.

3. The vibration of pneumatic hammers should be assessed at pressing forces that ensure their operation in the limiting principal regime, since this regime is the most hazardous from the standpoint of the vibrational effect of the hammer’s oscillations on the worker’s organism. In addition, it is also the most common regime in hammer operation.

4. The provisional sanitary rules for limiting the effect of vibration of pneumatic hammers on the worker’s organism have a number of substantial-

significant shortcomings that must be eliminated in the near future. In our opinion, it is first of all necessary to amend and supplement those provisions of the rules that concern the conditions for testing hammers (pressing force, air pressure, type of material being worked, etc.), to refine the amplitude–frequency standards for low-impact hammers (less than 20 cps), and also to take into account the subharmonic components of vibration of the hammer body and tool that arise as a result of their impacts.

1. Tests showed that the chipping hammers MR-5 and MR-4 do not meet the sanitary standards with respect to vibration amplitude and fundamental frequency. The remaining hand-held pneumatic hammers manufactured by the Tomsk Electromechanical Plant, according to these indicators, satisfy the standards when used at an air pressure in the network of no more than 5 ati.

REFERENCES

1. E. Ts. Andreeva-Galanina, V. G. Artamonova, E. A. Drogichina. Vibration disease. Medgiz, 1961.

2. Collection of the most important official materials on questions of occupational hygiene and industrial sanitation. Issue 11, Medgiz, 1962.

3. B. V. Sudnishnikov. On the theory of recoil of hand-held impact machines. In: Impact Machines, Novosibirsk, 1953.

4. A. M. Petreev. On reducing the recoil of hand-held pneumatic impact machines by improving the working cycle. Izvestiya SO AN SSSR, Technical Sciences Series, No. 6, issue 2, 1963.

5. V. I. Baburov, V. F. Gorbunov. Investigation of internal processes and parameters of chipping-and-riveting hammers. Izvestiya TPI, vol. 123, Publishing House of Tomsk State University, 1963.