After reading this article you will learn about Vibrations:- 1. Introduction to Vibrations 2. Sources of Vibrations 3. Measurement 4. Effects on Human Body 5. Diseases 6. Safety Measures.
Contents:
- Introduction to Vibrations
- Sources of Vibrations
- Vibration Measurement
- Effects of Vibrations on Human Body
- Vibration Diseases
- Safety Measures against Vibration
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1. Introduction to Vibrations:
Machinery, equipment and power-driven tools, which are encountered in all sectors of modern industry, generate intense vibration that may be transmitted to the workers who operate them. Vibration may affect comfort, reduce work output, and cause disorders of physiological functions in man, giving rise to the development of disease in the event of intense exposure.
Vibration is a physical factor which acts on man by transmission of mechanical energy from sources of oscillation.
Some of the sources of vibration are:
(a) Knocks and frictions of machine mechanisms;
(b) Inaccurately centred or loudly balanced rotating masses; and
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(c) Pressure pulses of compressed air.
Even a single machine or tool can include various sources of vibration with different spectral compositions, and different amplitudes, for the various frequencies, changing randomly with time.
Vibration may be sub-divided into the following two categories:
(a) Whole-body vibration (acts on the body of sitting or standing persons); and
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(b) Local vibration (mainly transmitted to the hands and arms).
2. Sources of Vibration:
Sources of whole-body vibration are encountered in the manufacturing and construction industries, agriculture, and transport (e.g. lorries, weaving looms, machines for making pre-fabricated concrete elements, tractors, harvesters, threshers, and self-propelled or tractor-towed agricultural or construction equipment).
Whole-body vibration may be subdivided, according to its source, into the following three categories:
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(a) Transport vibration (due to locomotion);
(b) Mixed technical and transport vibration (produced by machines operating both in a fixed position and when moving along an especially prepared path); and
(c) Technical vibration (generated by stationary machines, or transmitted to workplaces without sources of vibration).
Sources of vibration transmitted to workers hands are mainly power-driven tools.
Some of these tools in wide-spread use in various sectors of industry are as follows:
(a) Rock drills and pneumatic picks (used in mining and quarrying);
(b) Chain saws and branch cutters (used in logging);
(c) Fettling and riveting hammers;
(d) Portable grinders and polishers;
(e) Nut runners (used in mechanical engineering);
(f) Demolishing hammers and concrete breakers (used in building construction).
The tools mentioned above may be subdivided by type into the following three classes:
(a) Pneumatic tools;
(b) Engine-powered tools; and
(c) Electrical tools.
On the other hand, such tools may also be subdivided, according to their operating principle, into the following four classes:
(a) Rotary tools;
(b) Percussion tools;
(c) Combined rotary and percussion tools; and
(d) Pressing tools.
The vibration characteristics which are of interest to the occupational hygienist and safety engineer are those which affect man, i.e:
(a) The root-mean-square (RMS) velocities (in m/s); and
(b) Their levels (in dB) in the octave frequency bands.
3. Vibration Measurement:
In order to obtain a full idea of vibration, it is necessary to know its amplitude variation in time, and its frequency distribution. If the vibration is steady, it is sufficient to measure the RMS velocity (in m/s) or its logarithmic levels (in dB) in the octave bands.
Vibration is measured at the locations where the human body is in contact with the vibrating surfaces (separately in each direction along x, y and z axes), on the handles or working elements of the tools for local vibration, and on the seat or usual standing point (e.g., working platform) for whole-body vibration.
These measurements are carried out on a vibration measuring bench comprising a transducer (to ensure reliable results in the direction of vibration), a preamplifier and amplifier (of the levels to be measured), and a level recorder (or measuring magneto-phone). Under industrial conditions, it is preferable to use magnetically recorded signals, which can be analysed subsequently under laboratory conditions.
4. Effects of Vibrations on Human Body:
The harmful effects of vibration on the human body arise from its local irritant and damaging action on the tissues, and on the extero- and interoceptors embedded in them. Since these receptors form part of the central nervous system, a refectory action is exerted on various systems of the subject concerned.
The effect of vibration depends on the physical characteristics of the oscillating process, and on the duration of contact between the body and vibrating surfaces. When vibration propagates in the human body, the oscillations are damped.
The higher the frequency of vibration, the stronger is the damping; but the damping factor does not depend on the intensity of oscillations in the area of excitation. This explains also the differences in response to the action of low-frequency and of high- frequency vibration.
At low frequencies (up to 10 Hz), the oscillations propagate through the entire body (regardless of the location of input). Such low-frequency oscillations are hardly damped, and they confer an oscillating motion to the trunk and head. The behaviour of various parts of the body under the action of vibration is characterised by the mechanical input impedance, which reflects the resistance offered by the body structure to the oscillating motion.
In the case of low-frequency vibration, the mechanical impedance of the vibration-exposed hand is basically determined by the stiffness of the system. Any increase in muscular effort of the hand results in an increased mechanical impedance.
This implies that the vibration conductivity augments as a function of increased stiffness of the hand. That is why the muscles are generally involved in cases of systematic exposure to low-frequency vibration.
If there is an exposure to high-frequency vibration, the zone of propagation is limited by the area of contact. In this case, the vibration power transmitted to the hand produces an elevated energy density in the soft tissues, thus causing changes in the walls of the blood vessels.
The magnitude of these changes increases as a function of the frequency, and is inversely proportional to the diameter of the vessels. Thus the vascular disorders, therefore, are generally observed in cases of prolonged exposure to high-frequency vibration.
Exposure to whole-body vibration of certain frequency ranges (4-5 Hz and 8-12 Hz) is associated with resonance phenomena (i.e. large increase in oscillation amplitude of the anatomic organ and system structures). As a result, the vibration of these frequencies has the most adverse effects. In particular, whole body vibration affects the central nervous system.
Inhibition processes start being preponderant in the cerebral cortex, the normal cortical-subcortical inter-relations are impaired, and autonomic dysfunctions may be observed. The stress produced by whole-body vibration exposure gives rise to disturbances of the neuro-humoral regulation and metabolic processes, to dysfunctions of various organs and systems, and to increased energy losses of the organism.
Exposure to high levels of whole-body vibration may also result in damage to internal organs. Long-term vibration exposure is associated with various types of histological, histochemical and biochemical alterations leading to dystrophyic changes.
Moreover, vibration also affects the sensory systems. Exposure to whole- body vibration impairs visual acuity, narrows the field of vision, diminishes the light sensitivity of the eye, and disturbs the vestibular function. Similarly, exposure to local vibration reduces the tactile, pain, temperature, vibration, and proprioceptive sensitivities.
5. Vibration Diseases:
Prolonged exposure to vibration (especially in combination with other harmful factors such as cold, noise, static loads, etc.) may lead to the development of vibration disease. In this disease is due to local vibration, its most prominent feature is a vascular syndrome, accompanied by spells of “white finger” after general or local body cooling, and also by impaired sensitivity to vibration, pain, and temperature.
Poly-neuritic and angio-dystonic syndromes with symptoms related to spasms of the peripheral vessels are characteristic in this case.
If the disease has been caused by whole-body vibration, it is characterised by considerable changes of the central nervous system. It is also associated with general angio-dystonia and a poly-neuritic syndrome, which is more pronounced in the “lower extremities.
Medical examinations of a large number of workers exposed to vibration have distinguished several stages in the clinical picture of vibration disease. There are not many symptoms in the initial stage. Pain may be felt occasionally, with periodic spells of paraesthesia in the hands. Objective examination reveals a slight loss of vibration sensitivity of the finger tips. There is a propensity to arteriolar spasm.
In the second stage, there are moderately pronounced changes, the pain and feeling of numbness are more persistent. The loss of sensitivity spreads to all fingers and to the forearm. The skin temperature of the fingers diminishes. There are cyanosis and hyperhidrosis of the hands, and discreet functional deviations in the central nervous system. However, these changes are reversible.
The third stage of vibration disease is characterised by pronounced vasomotor and trophic disorders. The fingers become white. The hands are generally cold and moist, and all types of sensitivity of the hands are diminished. The muscular changes are more pronounced. The functional changes in the central nervous system become noticeable. All these changes, moreover, are persistent.
The changes become general in the fourth stage of vibration disease. Vascular disorders can be observed in the arms and legs. The spasms may involve the cardiac and cerebral vessels. There may be fits of dizziness and semisyncopic states.
The sensitivity is even more diminished. This state is persistent and not much reversible. The diagnosis of vibration disease is supplemented by the designation of the character of the basic syndrome, and an indication of the dominating frequency in the vibration exposure.
The various designations of the character of the basic syndrome are as follows:
(a) Angio-dystonic;
(b) Angio-spastic;
(c) Neuromuscular impairment;
(d) Vestibular nerve impairment; and
(e) Di-encephalic syndrome.
The above stages of the vibration disease do, however, not reflect all the clinical details. In a number of cases, various syndromes may be combined.
For example, the condition caused by exposure to medium-frequency whole-body vibration is characterised by symptoms denoting functional disorders of the central nervous system in combination with peripheral neurovascular disorders of the upper and lower extremities.
On the other hand, the condition due to vibration of motor vehicles with low-frequency spectra is characterised by lumbalgia and chronic lumbosacral radiculitis. As the vibration disease evolves against the background of adaptation responses of the organism, the degree of compensation is expressed by distinguishing between compensated, sub-compensated, and decompensated forms.
6. Safety Measures against Vibration:
In view of the increasing mechanisation of labour-intensive processes, the protection of the worker against vibration is one of the important problems for medical men and safety engineers.
The successful solution of this problem is the pre-requisite for greater productivity, and the preservation of the working capacity and health of the workers. The prevention of the harmful effects of industrial vibration includes technical, organisational, hygienic, prophylactic and therapeutic measures.
Technical measures comprise the following:
(a) Automation and remote control;
(b) The design of safe machines and tools (lowering the vibration parameters of the vibrating sources by damping the vibration); and
(c) Use of personal protective equipment.
Organisational measures should be directed at maintaining tools and machines in good working condition by applying the standards of their preventive maintenance. The organisation should also try to introduce work-rest schedules to reduce the duration of vibration exposure.
Since it is not possible or practicable to replace all vibrating tools and machines, maximum permissible vibration parameters have been se< up. In addition, a number of other associated factors (e.g., weight of hand-held machines, physical effort during work, etc.) are subject to restrictions.
When it comes to prophylactic and therapeutic measures, pre-employment and periodical medical examinations play an important part. These examinations must be conducted by specialised physicians (Otolaryngologist, neuro-pathologist and therapeutist).
They must go hand in hand with the following laboratory tests:
(a) General blood analysis;
(b) Capillaroscopy; and
(c) Radiography (if indicated) of the hand bones (for those exposed to local vibration), and of the spinal column (for those exposed to whole-body vibration.
Early diagnosis of the disease adds to the probability of a favourable outcome. In order to prevent the development of the vibration disease, and to preserve the working capacity, persons exposed to industrial vibration should be advised:
(i) To do special gymnastics;
(ii) To undergo hydrotherapy, massage and ultraviolet radiation; and
(iii) To take vitamins.
If certain symptoms become evident, prophylactic treatment under ambulatory conditions or in sanatoria is recommended. Thorough expertise of the working capacity with a view to the timely removal from vibration exposure is of great importance.