SOME ISSUES IN THE METHODOLOGY OF MEASURING AND EVALUATING WORKPLACE VIBRATIONS IN PRODUCTION

L. V. NECHAEV

(Presented by Prof. Dr. V. K. Nechaev)

In a number of industries (foundry shops, plants for the manufacture of products made of vibrated concrete, etc.), vibrating machines of various types are becoming increasingly widespread. The operation of such machines is accompanied by vibration of the foundation, which is transmitted to workplaces (shop floors, platforms, etc.).

Considerable attention is paid to workplace vibrations by design and engineering organizations involved in the design, manufacture, and operation of vibrating machines. Thus, for example, the design and construction of foundations for vibrating machines must be carried out with allowance for the acting dynamic loads and with the vibration of the foundation limited by the relevant standards, bearing in mind that the intensity of workplace vibrations depends primarily on the vibrations of the foundation [3, 4].

However, very often, for a number of reasons (some conventional assumptions in such calculation data as the nature of the vibrations of the machine’s working members, the elastic characteristics of the foundation material, the appearance of rigid connections not provided for in the calculation), the vibration isolation of the working members of vibrating machines proves insufficient, as a result of which the actual vibrations of the foundation considerably exceed the calculated ones. Workplace vibration under these conditions, as a rule, proves excessive, sharply worsening the working conditions of the servicing personnel [2].

In the Soviet Union, the “Temporary Sanitary Rules and Standards for Limiting Workplace Vibration” [1] are in force. These rules and standards establish the maximum permissible vibrations in terms of the absolute amplitudes of workplace oscillations, as well as in terms of the amplitudes of the velocities and accelerations of these oscillations. These “Standards...” also give certain recommendations on the methodology of vibration measurement and on measuring equipment as applied to the measurement of vibrations at workplaces.

At the request of the Tomsk trust “Stroidetal,” staff members of the Department of DVS, E. N. Nevstruev and L. V. Nechaev, under the supervision of Prof. Dr. V. K. Nechaev, carried out measurements and a study of the nature of workplace vibrations at several reinforced-concrete-products plants in the city of Tomsk.

In carrying out this work and evaluating its results, as well as with respect to the measurement procedure, we were guided by the above-mentioned standards.

As a result of the work carried out on measurement and on the general study of the character of workplace vibrations, we noted certain shortcomings in the measurement procedure and permissible standards recommended by the “Standards…,” which are discussed in detail in the conclusions of this article.

Measurements were made at four plants where SM-476 vibrating machines are installed (with long frames—about 6 m) and vibrating tables, each with two S-413 vibrators (with short frames—about 2.5 m).

Vibrations were measured on the ground; on wooden and metal decks covering the pits in which the SM-476 vibrating machines are mounted; and on the footboards of concrete-placing-machine operators servicing these machines.

The vibration measurements were carried out with an electrodynamic transducer (of seismic type) and the corresponding electronic apparatus. In order to obtain the fullest and most reliable measurement results possible, we used, in parallel, an indicating instrument (an MVL-3 tube millivoltmeter) and, for recording vibrograms, an MPO-2 loop oscillograph.

The voltage from the electrodynamic transducer, before being fed to the indicating instrument and the oscillograph, was amplified by the corresponding tube amplifiers (2) and (5), which had a rectilinear frequency characteristic, and by a power amplifier (6) with a push-pull output using 6P6 tubes. The block diagram of the measuring channel is shown in Fig. 1¹).

Fig. 1. Block diagram of the measuring channel:
1—electrodynamic transducer; 2—tube amplifier; 3—integrating and differentiating devices; 4 and 5—MVL-3 millivoltmeter with amplifier; 6—power amplifier; 7—MPO-2 loop oscillograph.

In this form, the measuring apparatus made it possible to measure the amplitude of the velocity of oscillatory motion at the point where the transducer was installed, both from the readings of the indicating instrument and from vibrograms of vibration velocity recorded on photographic film by the oscillograph.

¹) The installation of the measuring channel, as well as the manufacture of the electrodynamic transducer and power amplifier (6), was carried out by E. N. Nevstruev.

Taking into account that the “Standards...” stipulate all three parameters—displacement, velocity, and acceleration amplitudes—integrating and differentiating devices (RC circuits) were introduced into the measuring path between amplifiers (2) and (5), making it possible to measure the amplitudes of the velocity and acceleration of vibrations from the readings of the pointer instrument and to record the corresponding curves on photographic film. The vibration frequency was determined with the aid of time marks placed on each vibrogram during the process of recording vibrations, from an internal time marker installed in the MPO-2 oscillograph and having its own frequency of 500 cps.

Before vibrations were measured and recorded at workplaces, in the laboratory of the Department of DVS the transducer was calibrated, and the frequency and amplitude characteristics of the transducer together with the entire measuring path were taken on a special vibration stand designed and built in that department.

From the structural standpoint, the vibration stand is an elastic beam, one end of which is clamped as a cantilever on two supports (one of the supports can be moved), while the other is connected to an electrodynamic exciter. Power for the field coil of the exciter is supplied from an AVS-5 full-wave mains rectifier and an automobile storage battery connected in parallel with the rectifier. The storage battery was used by us as a kind of damper, smoothing the pulsations of the magnetic field of the vibrator’s field coil. The moving part of the exciter is a thin-walled brass cup with a coil wound of copper wire (diameter 0.25 mm, 180 turns), fed from a ZG-2A audio generator through a KINAP amplifier (from the narrow-film projection cinema apparatus “Ukraina”). The layout of the vibration stand and of the apparatus used in calibration is shown in Fig. 2.

Control of the amplitudes of the end of the beam, on which the transducer was fastened, was carried out with the aid of the optical head of a Vickers microscope, rigidly fixed to the frame of the vibration stand. A piece of a safety-razor blade was glued to the end of the beam; its sharp edge was perpendicular to the axis of the microscope objective and lay in the plane of vibration of the end of the beam. In reality, during vibrations the free end of the beam describes a cylindrical surface. However, with a large beam length (about 1000 mm) and small vibration amplitudes (not more than 0.7 mm), it is quite permissible to assume that, during its vibrations, the end of the beam moves in a certain plane perpendicular to the longitudinal axis of the beam.

Before being glued on, the blade was dipped several times in paint, and on a fine whetstone the paint layer was ground off down to the metal. Only in this way were we able to obtain on the beam a reference mark (from which the amplitude is read) about 5 microns wide. This circumstance made it possible to measure small amplitudes with greater accuracy.

In the stand described, in order to obtain beam-vibration amplitudes sufficiently large for reading in the frequency range 30–500 cps, the resonance principle of operation was used, whereby the exciter coil is supplied with alternating current at a frequency close to, or equal to, the natural frequency of the elastic suspension. To make it possible to calibrate the transducer at different frequencies when the stand operates according to this principle, our stand provides for: a) a movable beam support, making it possible to change the natural frequency of the elastic suspension (beam); b) the use of beams of different stiffness.

In addition to the elements and assemblies listed, the vibration-stand set includes: an ICh-6 frequency meter (for measuring the frequency of the current, ...

…of the moving coil of the exciter), an EO-7 oscilloscope (for monitoring the shape of the emf generated by the sound generator and the transducer), and a stroboscope (for monitoring the form of the beam oscillations).

When calibrating the transducer together with the measuring channel, we noted certain positive qualities of the stand described: a) reliability and simplicity of construction; b) with cantilever mounting of the elastic suspension and operation of the vibration stand at resonant and near-resonant frequencies, the exciter consumes only a small amount of power in the range of 30–500 cps and at oscillation amplitudes of the end of the beam of 0.5–0.05 mm (in our case the power consumption did not exceed 5–7 watts).

Fig. 2. Diagram of the vibration stand with apparatus for calibrating vibration transducers:
1—ZG-2A sound generator, 2—electronic wide-band amplifier, 3—ICh-6 frequency meter, 4—EO-7 cathode-ray oscilloscope, 5—AVS-5 selenium full-wave rectifier, 6—electrodynamic vibrator, 7—elastic beam, 8—microscope, 9—vibration transducer, 10—storage battery, 11—massive cast-iron plate with supports.

Some characteristics of the measuring channel are given below:

natural frequency of the transducer—20–25 cps,

velocity sensitivity of the transducer

at a vibration frequency of 50 cps,

\[ \rho_v = 4.7 \frac{\text{mV sec}}{\text{cm}}, \]

the same at a vibration frequency of 500 cps,

\[ \rho_v = 2.0 \frac{\text{mV sec}}{\text{cm}}, \]

velocity sensitivity of the transducer, measured in mm of amplitude of deflection of the oscilloscope beam on the film at a vibration frequency of 50 cps,

\[ \rho_v^{0} = 0.89 \frac{\text{mV sec}}{\text{cm}}, \]

at the same vibration frequency of 500 cps, \(a_v^{0}=0.8 \dfrac{\text{m sec}}{\text{cm}}\).

Calibration showed that the transducer, together with the entire measuring channel, has good linearity of the amplitude characteristics.

Measurements and recording of vibrations at workplaces were carried out with only one vibration machine operating, near which the measurements were made.

Processing of the vibrograms and analysis of the readings of the pointer instrument made it possible to draw the following conclusions:

1. Vibrations of workplaces at different points near vibrating platforms in most cases have a complex, nonharmonic character, especially near vibrating platforms with long frames (Fig. 3 a and b).

2. The vertical vibrations of long frames are not the same in amplitude and character at different points along the length of the frame, Fig. 4 a and b; the vibrograms were taken with a VR-1 vibrograph. The nonconstancy of the character and amplitude of vibrations of long frames along their length is, evidently, a consequence of their insufficient rigidity, which leads to the appearance of considerable natural oscillations of the frame. This complexity of the frame vibrations—the forced vibrations of the frame as a whole (as a rigid body) and its natural oscillations (as an elastic body)—is one of the causes of the complex, nonharmonic character of vibrations of the floor and decking near vibrating machines with long frames. It is quite natural that the complex character of the vibrations of the long frames of vibration machines considerably complicates the calculation and design of effective vibration isolation of workplaces.

Fig. 3. Vibrations of workplaces at different points near vibrating platforms of type SM-476 with a long frame:
a) vibrations of the shop floor near the electric-motor vibrator at the edge of the vibrating platform;
b) vibrations of the shop floor near the middle of the frame of the vibrating platform;
c) vibrations of a thin metal sheet covering the end well of the vibrating platform.

1. Table 1 gives the magnitudes of workplace vibrations measured by us at the above-mentioned plants, and Table 2 gives the permissible vibration parameters taken from the “Temporary Sanitary Rules and Standards...” [1].

From a comparison of the corresponding parameters of the actual vibrations with the permissible ones, it may be concluded that a number of workplaces have impermissible vibrations in all three parameters (for example, metal decking—

Fig. 4. Vibrations along the length of the frame of a vibrating platform of type SM-476: a) at the edge of the frame near the support stand of the insert for forming cavities in the product; b) in the middle of the frame.

Table 1

decking); the vibrations of other places are impermissible only in one or two parameters.

1. The nature of the vibrations of a workplace and their magnitude in a number of cases (for example, the footrest of a concrete placer) depend on the load on the vibrating machine (in the case of the concrete placer, on the amount of concrete in the hopper, Fig. 5 a and b).

2. The vibrations of the soil and decking at different points near one vibrating machine have different character and amplitudes (Fig. 3 a and b).

Table B

Note: If the duration of exposure is 10–15% of working time, the amplitudes indicated in the table may be increased, but by no more than 3 times.

The nature of the vibrations of the soil and of the floors is, as a rule, different (Fig. 3 a and c).

The work carried out on measuring vibrations and studying their nature at workplaces in reinforced-concrete-products plants allows us to draw the following general conclusions regarding the methodology for measuring vibrations and the apparatus required for this as applied to measurement at workplaces, as well as certain comments directly on the “Temporary Sanitary Rules...” [1].

Fig. 5. Vibrations of the frame of a concrete placer:
a) hopper with concrete, b) hopper without concrete.

1. The nonharmonic, complex nature of the vibrations of the working members of vibrating machines that have insufficient rigidity (in our case, long frames) indicates that the usual calculations of foundations for vibrating machines, taking dynamic loads into account, are of a very approximate nature, since the natural oscillations of nonrigid machine elements are not taken into consideration. In connection with this, the actual amplitudes of displacement, velocities, and accelerations may differ significantly from the calculated ones, in most cases considerably exceeding them. This testifies to the absolute necessity of measuring workplace vibrations at all enterprises that have vibrating equipment.

2. The use, in the apparatus we employed, of a pointer instrument for measuring amplitudes and of an oscillograph for recording the curves of displacement, velocity, and acceleration of vibrations showed that, between the vibration parameters of one and the same point, determined from the readings—

...readings of the pointer instrument and those obtained by processing vibrograms, there is a certain difference, the greater the more the character of the vibrations at the measurement point differs from harmonic (Table 1). This indicates that the complex, non-harmonic character of the vibrations introduces distortions into the readings of the pointer instrument, which points to the need for cautious use of such instruments when measuring vibrations with electronic apparatus. When the character of vibrations is complex, the results obtained by processing vibrograms of displacement, velocity, and acceleration are more reliable.

1. Measurement of vibrations by means of vibrographs of the VR-1 type, whose use is permitted by the “Standards...” [1], is acceptable only for an approximate determination of vibrations with fairly large amplitudes (not less than 0.02 mm). The calculated determination of the amplitudes of velocity and acceleration from displacement curves recorded with a vibrograph must necessarily be carried out with allowance for the character of the vibrations; and, when necessary, when the vibrations of the measurement point have a non-harmonic character, a harmonic analysis of the displacement curve must be performed. Otherwise the calculated amplitudes of velocity and acceleration will be very far from the actual values and, as a rule, will always be considerably underestimated.

From this point of view, the measurement method and apparatus proposed by the “Standards...” [1] are very imperfect and do not provide practical accuracy of measurement, do not make it possible to evaluate actual vibrations correctly (one may even draw a false conclusion as to their permissibility) or the effect of vibration isolation, and, consequently, they limit the application of the standards themselves. In the investigation and measurement of vibrations at workplaces it is necessary to use more complex apparatus that ensures the recording of displacement, velocity, and acceleration curves (such as, for example, that which we used).

1. Taking into account item 2 of the conclusions concerning vibration measurements at reinforced-concrete plants, it may be concluded that the “Standards...” [1] should include a qualification regarding the permissibility or impermissibility of actual vibrations in cases where they exceed the standards in one or two parameters. It would also be highly desirable for the “Standards...” to indicate which parameter (when and where) should be regarded as the principal one in evaluating actual vibrations.

REFERENCES

1. Temporary sanitary rules and standards for limiting vibrations of the workplace (approved by the Main State Sanitary Inspectorate of the USSR, Feb. 17, 1959, No. 280—59). Handbook of Safety Engineering and Industrial Sanitation. Sudpromgiz, 1959.

2. E. Ts. Andreeva-Galanina. Vibration and Its Significance in Occupational Hygiene. Medgiz, 1956.

3. Technical specifications for the design of foundations for machines with dynamic loads. Instruction SN-18-58.

4. Instruction for the design and calculation of vibration isolation of machines with dynamic loads and of equipment sensitive to vibrations (I 204-55 MSPMKhP). Moscow, 1955.