Getaran Manusia

Mempelajari mengenai pengaruh getaran mekanis pada tubuh manusia Definisi getaran: Gerakan osilasi dari partikel di sekitar reference pointnya dari titik seimbangnya dari solid body, liquid, atau gas. Orang yang bermain ayunan merupakan contoh gerakan pada sekitar titik seimbangnya.

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Getaran dapat dibagi menjadi: a. Getaran harmonis dan periodis b. Getaran random c. Getaran transient a. Getaran harmonis dan periodis Getaran harmonis dan periodis adalah getaran yang terbentuk dari satu atau beberapa sinusoidal komponen, getaran ini mempunyai karakteristik yaitu polanya berulang dengan waktu. Contoh: Getaran yang disebabkan oleh putaran roda yang tidak balans pada roda kendaraan.

Getaran Harmonis

Getaran Periodis

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b. Getaran random (stochastic) Getaran yang tidak berulang dengan sendiri. Contoh: Getaran yang terjadi ketika kendaraan melaju melalui jalan yang tidak rata.

Getaran random c. Getaran transient Getaran dengan durasi yang pendek dan disebabkan karena mechanical shock. Contoh: Getaran yang terjadi ketika kendaraan mengenai lubang.

Getaran transient

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Pada kenyataannya, getaran yang terjadi adalah kombinasi dari ketiga jenis getaran tersebut.

Karakteristik Getaran

Getaran ditentukan karakteristiknya lewat magnitude, frekuensi, durasi dan arah getaran. MAGNITUDE The vibration magnitude tells how "powerful" the vibration is. Usually the magnitude of vibration is indicated by the acceleration 'a', measured in metres per second squared (m/s²). Because the magnitude of the acceleration is continually changing, a single overall value was introduced. In most cases, when vibration does not contain shocks, the acceleration magnitude is expressed by the root-mean-square (rms) value.

∫=T

0

2rms dt)t(a

T1a

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CREST FACTOR In the case when vibration is transient, ie contains shocks, the rms underestimates the vibration, therefore, a crest factor has been introduced.

rmspeakfactorCrest =

If the crest factor exceeds 9 then the vibration effects are underestimated and other evaluation methods need to be used. ISO 2631-1, 1997 describes two different methods: the running rms; and the fourth power vibration dose, to be used when the crest factor is above 9. FREQUENCY The number of complete cycles that occur per second is called the frequency and is measured in hertz (Hz). A complete cycle includes the movement from the equilibrium position upwards to its highest position,

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followed by the downwards movement to the lowest position, and finally a return to the starting position. One complete vibration per second corresponds to 1 Hz (hertz). DURATION Human responses to vibration increase with increases in the duration of exposure, so it is important to define a dose measure which incorporates the exposure time factor. To determine the effective daily exposure duration it is necessary to gather information on the number of minutes or hours per day when exposure to vibration occurs, then the number of weeks or months per year during which work involving vibration is carried out. DIRECTION The exposure of the human body to vibration is assessed by measuring vibration entering the body. The vibration is normally measured along three perpendicular directions. If there is more than one point at which vibration enters the body, there will be more than one co-ordinate system for obtaining measurements. In the case of human vibration, a biodynamic set of axes is used with the point of contact as the origin of the co-ordinate system. The directions are:

x, back-to-front; y, right-left; and z, foot-head (ISO 2631-1:1997, ISO ).

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Coordinate system for the measurement of whole-body vibration (ISO 2631-1:1997)

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Coordinate system for the measurement of hand-arm

vibration (ISO 5348; 1986) In the case of hand-arm vibration when the hand grasps a handle, a basic central co-ordinate system is used where the front of the handgrip is used as the origin of the system, in which the plane x, z lies vertical to the palm of the hand, and the plane y, z passes horizontally through the longitudinal axis of the third mid-hand bone. RESONANCE All objects tend to vibrate at one particular frequency that depends on the composition of the object, its size and shape. This frequency of natural vibration is called the resonant frequency. A vibrating machine transfers the maximum amount of energy to an object when the machine vibrates at the object's resonant frequency.

EACH BODY SUB-SYSTEM HAS A RESONANCE FREQUENCY BAND To understand why human beings are more sensitive to some frequencies than to others, it is useful to consider

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the human body as having sub-systems, where each sub-system has its own resonance frequency band and the interactions between sub-systems are influenced by the body's position, for example, standing or sitting.

Simplified human body sub-systems

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WHOLE-BODY AND HAND-ARM VIBRATION

The human responses to vibration depend also on which part of the body is affected.

There are two major types of human exposure to vibration:

a. Whole-body vibration (WBV) b. Hand-arm vibration (HAV)

a. Whole-body vibration Mechanical vibration transmitted to the whole-body through a supporting surface, for example, the feet of a standing person or the buttock of a seated person. Frequency range (0.5 – 100 Hz) b. Hand-arm vibration Mechanical vibration applied to a part of the body ie. segmental vibration. When vibration is applied to the hand, it is termed "hand-arm" vibration. for example, vibration transmitted to the hand while operating power tool Frequency range (0.5 – 1500 Hz)

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ADVERSE EFFECTS OF WHOLE-BODY VIBRATION

From the experimental data, the adverse effects are divided into the following:

• effects on health • effects on activities • effects on comfort • motion sickness

EFFECTS ON HEALTH Severe, prolonged exposure to vibration can cause injury to the body. The parts of the body most likely to be injured during exposure to whole-body vibration depend on the magnitude of vibration, distribution of the motion within the body, and the vibration frequency, direction and duration. From the epidemiological studies, subjective data, biodynamic models and a knowledge of the physical properties of the body it is possible to establish some of the health effects:

• Spinal column disease and complaints: These are perhaps the most common diseases associated with the long-term exposure to whole-body vibration, where the back is especially sensitive to the 4-12 Hz vibration range.

• Digestive system diseases: Often observed in persons exposed to whole-body vibration over a long period of time. Associated with the resonance movement of the stomach at frequencies between 4 and 5 Hz.

• Cardiovascular system effects: Prolonged exposure to whole-body vibration at frequencies below 20 Hz

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results in hyperventilation, increased heart rate, oxygen intake, pulmonary ventilation and respiratory rate.

EFFECTS ON TASK PERFORMANCE Some of the whole-body vibration may affect:

• the senses, and create problems with collecting information;

• the processing information; • the level of arousal, motivation or fatigue; and • intentional actions.

Some of the effects can be frequency related as certain parts of the body are in resonance with the vibration received. Vision, for instance, is mainly affected by vibration at frequencies between 20 to 90 Hz. Posture is another example of effects related to frequency, because people subjected to vibration in the frequency range from 1-30 Hz experience difficulties in maintaining a correct posture and experience an increased postural swing. In general they cause imbalance, disorientation and lack of co-ordination, which then leads to stress, fatigue, interference with instrument readings, operation of tools etc. They can also result in impaired reflex action, distraction and annoyance. Some adverse effects are due to the combination of vibration with other physical factors like noise, temperature, non-ergonomic design of equipment, protective clothing etc that are common at workplaces. This combination can also cause stress, fatigue and problems with employee task performance.

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EFFECTS ON COMFORT Some motion can be a source of pleasure but some may cause dissatisfaction, discomfort and displeasure. Many factors determine the sense of well-being in an individual; vibration is only one of them. The whole-body vibration impact on comfort was studied mainly by the transport industry to establish how to improve ride comfort in vehicles such as buses and trains. They were trying to determine at what level, frequency, direction and duration vibration causes difficulty in performing certain operations like eating, reading or writing. It was established that the change in discomfort is proportional to a change in magnitude, that is a doubling in vibration magnitude causes a doubling of discomfort. Further, the discomfort is closely related to the vibration frequency: at low frequencies, (1-2) Hz the same movement is transmitted throughout the whole-body; at slightly higher frequencies various body resonances tend to amplify the motion and overall discomfort. If the frequency is increased further, the body provides an increasing attenuation of vibration reducing the discomfort. The discomfort tends to increase with increasing duration of vibration. Random vibration and multiple axis vibration produce more discomfort, therefore a special frequency-weighting and a root-sums-of-squares of vibration on all axes were included in the assessment methods.

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There are also many other factors affecting comfort, including body posture, age, gender, and noise. MOTION SICKNESS Vibration at frequencies of about 1 Hz and below, which occur in many forms of transport, might induce motion sickness (kinetosis). It results in nausea, dizziness, vomiting and can affect the safe handling of vehicles or the performance of other tasks. The symptoms are worst between approximately 0.125 and 0.25 Hz, and only rarely occur due to frequencies above 0.5 Hz. Motion sickness in ships is believed to be caused by vertical oscillation. Some form of air sickness are also due to vertical oscillation of the body, but car sickness is believed to be caused mainly by horizontal motion and is associated with acceleration, braking and cornering manoeuvres.

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EVALUATION OF HUMAN EXPOSURE TO WHOLE-BODY VIBRATION

ISO 2631-1:1997 International Standard ISO 2631-1:1997, Mechanical Vibration and Shock - Evaluation of Human Exposure to Whole Body Vibration - Part 1: General Requirements, defines the means to evaluate periodic, random and transient vibration with respect to human responses: health, comfort, perception and motion sickness. The Standard specifies direction and location of measurements, equipment to be used, duration of measurements and frequency weighting, as well as methods of assessment of measurements and evaluation of weighted root-mean-square acceleration. The Standard uses acceleration as the primary quantity of vibration magnitude. In certain cases, when vibration is low and in a very low frequency range, velocity measurements can be made and then translated into acceleration. FREQUENCY WEIGHTINGS The human sensitivity to vibration is highly frequency-dependant. The effect of frequency is reflected in frequency weightings labelled Wk, Wd, Wf, Wc, We and Wj. Different frequency weightings are required for the different axes of the body.

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The frequency-weighted acceleration (in m/s²) is multiplied by the weighting factor before its effect is assessed.

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Table 1 - Guide for the Application of Frequency-Weighting Curves

Frequency Health Comfort Perception Motion Sickness

Wk z-axis, seat surface z-axis, seat surface z-axis, standing

vertical recumbent (except head)

x-, y-, z-axes, feet (sitting)

z-axis, seat surface z-axis, standing

vertical recumbent

-

Wd x-axis, seat surface y-axis, seat surface

x-axis, seat surface y-axis, seat surface x- y-axes, standing

horizontal recumbent y-, z-axes, seat back

x-axis, seat surface y-axis, seat surface x-, y-axes, standing

horizontal recumbent -

-

Wf - - - vertical

Wc x-axis, seat back x-axis, seat back x-axis, seat back -

We - r x-,r y-r z-axes, seat surface

<I r x-,r y-r z-axes, seat

surface

-

Wj - vertical recumbent (head) vertical recumbent -

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Evaluation method

Basic evaluation method The basic evaluation method is the weighted r.m.s acceleration method. This method accounts for the frequency content and is expressed as:

∫=T

0

2wrmsw dt)t(a

T1a

where; aw is weighted acceleration T is duration of measurement

For those types of vibrations containing shocks, when the crest factor is more than 9, it is recommended to use additional evaluation methods like the running r.m.s or the fourth power vibration dose method. Additional evaluation method There are two alternative methods. a. Maximum Transient Vibration Value (MTVV)

This method takes into account occasional shocks and transient vibration by using a short integration time constant.

where;

aw(t0) is running r.m.s. aw(t) is instantaneous weighted acceleration τ is integration time t is time (integration variable) t0 is observation time (instantaneous time)

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and MTVV is given as maximum value of running r.m.s. with integration time of 1 seconds

MTVV = max (aw(t0)) b. Vibration Dose Value

This method is more sensitive to peaks than the basic evaluation method because it uses a fourth power instead of a second power of the acceleration time history. The fourth-power vibration dose value is expressed in m/s 1.75 or rad/s 1.75 .

where;

aw(t) is instantaneous weighted acceleration T is duration of measurement

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PREVENTION A comprehensive management system to prevent whole body vibration should cover the following steps:

1. Elimination 2. Substitution 3. Isolation 4. Engineering control

a. at source b. in path transmission

5. Administrative a. duration b. training

6. Health surveillance 7. Personal protective equipment

1. ELIMINATION Elimination is the first step that should be considered when addressing exposure to vibration at a workplace. In some cases it can be possible to eliminate the exposure; sometimes it could mean changing the operation completely. This is the most effective way of controlling the risk and should always be considered. For example, sending goods by train instead of employing several truck drivers. 2. SUBSTITUTION A proper selection of vehicles or machines plays a very important part in prevention methods. Sometimes a vibration source can be replaced by another machine with lower vibration magnitude, for example, machines with rotating instead of reciprocating parts, or belts instead of chains.

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3. ISOLATION Isolation means separating the vibration source from people involved in the work or from others standing by. It could mean relocating operators or others to positions away from the vibrating source. Placing vibrating machines on separated slabs or spring mounts to isolate the source from the surroundings is another example of isolation.

Vibration can be minimised by using properly designed spring mounts on machinery.

4. ENGINEERING CONTROL Vibration magnitude, frequency, direction and duration as well as body posture are the main factors that determine the severity of vibration exposure. Vibration exposure control means addressing one or more of these factors to minimise the health risk. Engineering vibration controls can be divided into two categories:

a. controls at source, b. in path by controlling the transmission.

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a. Reduce Vibration at Source Reducing vibration at source is often very complicated. It may require detailed analysis of a machine to identify the source of vibration. In other cases, there may be a simple solution such as:

a. Ensuring machinery is balanced. b. Levelling a road or track. c. Ensuring factory floors are without pot-holes (for

forklifts to fall into). d. Ensuring the rail track of a bridge crane gives a

smooth ride. e. Changing machinery speed, so the resulting vibration

frequency is not in a range that affects the human body.

f. Using additional mass to shift the natural frequency of the machine.

b. Reduce Vibration Transmitted The other way of controlling vibration by engineering means is by reducing the vibration transmitted to the vehicle body or by reducing the vibration transmitted to the operator.

Possible Locations of Suspension Systems in a Vehicle.

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Importance of Good Seat Suspension The most commonly used way of controlling vibration transmission is through the seat suspension. There are many different types of suspension seats available. Basically they all consist of upper and lower frames guided by linkages, usually with a combined scissor and rolling action, springs and a damper or a shock absorber. Usually there are two types of spring used: a mechanical (metal) one or a pneumatic (air) one. The mass-spring-damper system of a seat has a natural resonance frequency at which vibration is magnified. For a common foam and spring type of seat it occurs at around 4 Hz, whereas a suspension seat with the additional spring and damper mechanism under the seat has a resonance frequency lower at about 2 Hz. At lower frequencies these seats sometimes exhibit slight amplification.

Typical Transmissibility of Foam and Metal Sprung Seat and a Suspension Seat

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5. ADMINISTRATIVE CONTROLS Another way of reducing vibration exposure is by reducing the duration of exposure. It can be arranged by rotating operators so each one would spend less time being exposed to vibration.

Job Rotation to Reduce Exposure to Vibration

Training is a very important aspect in the prevention approach. Operators should be informed about human vibration, health and other effects of exposure to whole-body vibration and different methods of prevention. Training should also cover the proper use and adjustment of seats. A poor body posture is often regarded as partially responsible for back problems associated with exposure to vibration. Operators need to be trained on how to adopt a posture which minimises the transmission of vibration to the body. A standing person may reduce the transmission by bending the knees. A seated person may benefit from avoiding a too erect posture. Maintenance of the seat is also essential, as most seats and suspensions have a

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working life span shorter than the vehicle to which they are fitted. Training should also encompass how to choose a speed that is appropriate for the ground to be driven over, as well as the importance of avoiding potholes, bumps and other obstacles.

PERSONAL PROTECTIVE EQUIPMENT Personal protective equipment offers the last possibility of preventing vibration exposure. It should reduce the vibration transmission to the feet. Shoes with absorbing soles have been tried, but the measurements made on existing shoes show only a very small reduction. Protective belts are the other kind of PPE that have been tried by drivers of off-road vehicles and motor cycles. These belts are said to help maintain proper posture by stiffening the abdomen. However, many doctors are opposed to the wearing of these belts because of possible long-term muscular disorders. HEALTH SURVEILLANCE Medical precautions will not replace technical measures but can help to identify the problem early enough to prevent it from developing further. It is recommended to introduce an initial examination to identify any existing disorders of the spinal column, spinal disc, any illnesses of the gastrointestinal tract or any cardiovascular problems which could be exacerbated by whole-body vibration. Follow-up examinations during employment will identify if any of the existing problems are exacerbated or any new ones developed.