What is Vibration?
Vibrations can be defined as oscillations of mass about a fixed point. When the body comes in contact with a mechanical source of vibration the tissues of the body become displaced from their resting position. In the work setting there are basically three types of vibration that are significant to the worker. These include whole body vibration, segmental vibration and resonance.
The most common form of whole body vibration is vehicular vibration. In this case vibration enters the body through the buttocks and the feet and to a lesser extent the hands.
Segmental vibration is the result of mechanical vibration that comes in contact with body parts and the effect becomes localized. This form of vibration is transferred into the hands and arms when we use power tools.
All physical systems have their own natural frequency. When tissues of the body are exposed to sources of vibration corresponding to their natural frequency these tissues go into resonance. That is, the strength or amplitude of the vibrations exceeds that of the source.
Why is Vibration Important?
When the body comes is contact with mechanical vibration there is a direct adverse physiological effect on the body. The effect interferes with work efficiency and in some situations can put the worker at risk for injury. Any factor that has the potential to impair the workerís function is a significant issue in ergonomics and therefore must be eliminated or reduced to the greatest extent possible.
What are the Effects of Vibration on the Body?
The human body is made up of organ systems composed of different tissues. When it comes in contact with a source of vibration it reacts as a set of linked masses. Each tissue mass has its own natural frequency. Smaller structures tend to resonate at higher frequencies and larger masses tend to resonate at lower frequencies.
Hand power tools tend to vibrate at relatively high frequencies so they would set up resonance frequencies at the point of contact at the fingers and the vibrations would become dampened (by reducing the amplitude) as they travel up the arm. In fact, any frequencies over 30 Hz become significantly dampened as they travel from the fingers into the larger tissues of the body.
When the muscles of the body are exposed to vibration they react by exhibiting a protective reflex. The reflex causes the muscles to contract and shorten resulting in an increase in energy consumption. These sustained static muscle contractions result in a rapid fatiguing of the muscle. Fatigued muscles are much more susceptible to injury.
The most significant effect is the contraction of the muscles that line the walls of the small blood vessels. These muscles serve to regulate blood flow by constricting and dilating the vessel size. Blood flow is cut off to the affected body part resulting in numbness and tingling. The worker consequently looses sensation and motor control. This condition is known as vibration syndrome or white finger syndrome. Susceptibility is particularly increased when coupled with cold working conditions, eliciting a condition known as Raynaudís disease.
It has been known for some time that truck drivers and heavy equipment operators are at risk for developing Intervertebral Disc Disease. Aside from the damaging effects on the lumbar spine from sitting for prolonged periods of time it has been suspected that vertical whole body vibrations have a weakening effect directly on the disc. Constant vibration of the lumbar discs coupled with long periods of constant sitting make the worker vulnerable to a lumbar disc injury. Tractor drivers have been found to have an above average incidence of prostate problems and hemorrhoids.
It is known that there are measurable effects on visual acuity with vibrations of the eyes beyond 4 Hz and especially in the range of 10-30 Hz. Images in the visual field becomes blurred and visual acuity is sharply reduced.
We can see that the heavy equipment operators and drivers are particularly at risk for accidents and injury due to the above factors. In addition, studies involving strong vibrations such as those seen in resonance frequencies, the brainís ability to process information and the performance of skilled motor tasks become impaired.
In the case of jack-hammers and chain saws, where the vibrations are lower than 40 Hz there is the risk of losing bone mass in the arms along with developing osteoarthritis and tendonitis in the wrist and elbow.
How can Mechanical Vibration be Managed in the Workplace?
The goal for the worker should be to eliminate or minimize all sources of vibration as much as possible. In the case of whole body vibration the worker should avoid continuous contact with vibrating surfaces. If possible, machines should be designed in such a way that the machine controls do not come in contact with vibrating surfaces. The workstation should be isolated from the source of the vibration. Excessive vibration can be avoided by keeping machinery properly maintained. For drivers it is desirable to utilize a vertical seat suspension system in order to attenuate unwanted vibrations.
In the case of segmental vibration such as found with hand tools it is important to use tools that have a built-in dampening system. Warm anti-vibration gloves should be used and the worker should not be exposed to more than 10 minutes of continuous use per hour.
1) Kroemer, K. H. E. and E. Grandjean. ď Noise and Vibration.Ē Fitting The Task to The Human: A Textbook of Occupational Ergonomics. London: Taylor and Francis, 1997
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