Analyse van de blootstelling aan trillingen bij gebruik van motorkettingzagen

Analysis of the exposure to hand-arm vibrations using petrol-engine chainsaws

Huub H.E. Oude Vrielink

 

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Summary

European and Dutch legislation define maximum values, i.e. action value and limit value, for hand-arm vibration exposure to which employees may be exposed on a working day. A transition period is defined until July 2014 for agriculture and forestry, but this holds only for machinery and equipment available for the workers before July 2007. The legislation is meant to protect the workers from health damage due to exposure to vibrations. If the action value of 2.5 m/s2 is exceeded, organisational and/or technical measures are to be taken and health surveillance is to be organised, to limit or reduce the exposure. In the case of exceeding the limit value (5 m/s2), exposure should be brought back immediately to below this limit value. The use of hand-held, petrol-engine chainsaws in professional forestry and maintenance of the green area is generally considered to be one of the working activities that need advice how exposure to hand-arm vibrations can be reduced. The present research aims to evaluate the consequences for exposure to hand-arm vibration when using different types and brands of modern petrol-engine chainsaws. In addition, the influence of working technique, chain and wood was investigated.

Measurements have been performed with two experienced workers on two categories of professional chainsaws: low power (around 50 cc, n=4) and medium power (approximately 75 cc, n=3).

Hand-arm vibrations have been measured according to ISO-5349-1 (2001) at both handles, during idling mode, racing (unloaded, maximum speed) and during cutting of slices of trunks of oak and birch. During cutting, three different working techniques have been measured: (A) cutting in up-to-down direction while keeping the comb of the chainsaw in close contact with the trunk, (B) cutting downwards while having contact with the trunk only via the chain en saw blade, and (C) cutting upwards, again without contact between trunk and engine block. All measurements have been performed with new, unused chains; in the trunk of birch, the three working techniques were also measured with a chain made blunt. Measured data were processed online on a portable computer, displayed and stored. Stable data segments were selected off-line and processed to yield frequency-weighted acceleration data. In addition, data on exposure times for the activities of forestry and green area workers during a normal working day were derived from time study of a group of six workers, and from interviews.


Table: exposure of the right ('handvat achter') and left hand ('handvat voor'), expressed as frequency-weighted rms-acceleration (ahv, in m/s2), as median values over two persons, for the seven types of chain saws ('Zaag') and the three types of load: idling ('stationair'), racing ('vol gas onbelast') and during cutting ('vol gas zagen'). Calculation of the latter is done only with the values obtained from American oak, using a new chain and applying all of the three working techniques investigated. The values between brackets are the emission values as given by the manufacturers (according to ISO 7505 of 22867). The saw-numbers in the dark areas are the median power saw types.


 

 

Time study and interviews showed that during a normal working day 3.5 – 4.7 h is spent on cutting. Operating the chainsaw in idling mode varied between 1.2 and 2.3 h/day. Racing could hardly be distinguished as a separate activity: < 0.1 h/day. Between 2 and 3 hours per day were spent on activities without exposure: consultation, fuel replenishment, sharpening the chain, and walking with a chainsaw switched-off.

A large difference in exposure was observed between the chainsaws investigated. Exposure values for the rear handle for the low power saws category were 3.4 – 5.1 m/s2, 1.6 – 2.8 m/s2 and 2.9 – 5.6 m/s2 for idling, racing, and full load cutting of oak, respectively (values indicate minimum and maximum value for the saws investigated). For the front handle, these values were (in the same order): 3.8 – 5.3 m/s2, 2.8 – 7.4 m/s2 en 3.3 – 4.7 m/s2. The medium power category of chainsaws showed higher exposure values: 5.5 – 7.0 m/s2, 3.2 – 4.0 m/s2 and 3.8 – 9.3 m/s2 (rear handle; same order as indicated above) and 5.3 – 8.3 m/s2, 3.5 – 6.3 m/s2 and 4.4 – 6.4 m/s2 (front handle). Generally, the emission values indicated by the manufacturers appeared to be lower than the exposure values measured. However, deviations in the opposite direction were also observed.

Exposure values and normal workday time data were combined to indicate maximal working times per day for each chainsaw. When using the low power category of chainsaws, the action value is exceeded after 2.7 – 5.1 hours, depending on the type and brand of the chainsaw. This value is inclusive all activities without exposure. The limit value is exceeded after 10.9 – 20.4 hours. When using the medium power category chainsaws, action value and limit value are exceeded after 1.1 – 3.2 h, and 4.4 – 12.4 h, respectively.

Exposure values differed largely between both workers. This difference was statistically significant and was observed for most of the chainsaws and for both handles. Working technique A (sawing downwards, keeping contact between comb and trunk) resulted in a lower exposure for the rear handle, compared to both other working techniques. Working technique C (upwards) demonstrated a higher median exposure compared to sawing in opposite direction. A sharp chain and soft wood tended to result in a lower exposure.

It is concluded that exposure and, hence, maximal daily working times, heavily depend on the choice of the chainsaw. In general, lower exposure can be achieved by using a lower power chainsaw. However, none of the saws tested may be used for a full working day without additional measures. Effective measures to restrict exposure in practice are a working technique during which the engine block is kept in contact with the trunk, working with a sharp chain, and restricting the time that the equipment is held while running in idling mode. According to the present results, a worker in practice aiming to select a chainsaw that results in the lowest possible vibration exposure, cannot rely on the manufacturer’s emission data, although determination of the latter is described in detail in ISO-regulations. Therefore, exposure measurements on a larger scale are advised. In addition, it is advised to further investigate the relatively large spread of the exposure data between the workers, as seen in this investigation. To which extent a subject can moderate exposure by increasing or reducing the activity of the muscles of hands and arms is yet unclear.


    

Source: www.ergolabresearch.eu.