Analyse van de blootstelling aan trillingen bij gebruik van landbouwtrekkers

Analysis of the exposure to whole-body and hand-arm vibrations using agricultural tractors

Huub H.E. Oude Vrielink

 

(click on the figure to download the entire report: 4.2 MB)

Summary

European and Dutch legislation define maximum values, i.e. action values and limit values, for whole-body vibration (WBV) and hand-arm vibration (HAV) to which employees may be exposed on a working day. A transition period is defined until July 2014 for agriculture and forestry. The legislation is meant to protect the workers from health damage due to exposure to vibrations. If the action value (0.5 and 2.5 m/s2 for WBV and HAV, respectively) 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 case of exceeding the limit value (1.15 and 5 m/s2, for WBV and HAV), exposure should be brought back immediately to below this limit value. The use of tractors in agriculture and forestry is generally considered to be a set of working activities that need advice how exposure to vibrations can be reduced. The present research aimed to make an estimation of the daily exposure to WBV and HAV when using agricultural tractors during their normal working activities. In addition, it was investigated to which extent exposure can be influenced actively by the driver by his/her choice of different unevenness of surface, driving speed, tyre inflation pressure, tyre type or tractor type.

Exposure measurements have been performed with 7 experienced professional tractor drivers on 11 different tractors and during 17 different activities in agriculture and forestry. Twelve of the activities were evaluated with 2 drivers and on 2 different tractors; the other five activities were performed by 1 or 2 drivers on only one tractor. Vibration evaluation was performed according to ISO (2631-1, 1997, 2631-5, 2001 and 5349-1, 2001) on the seat (WBV) and at the steering wheel (HAV). Besides, vibrations of the tractor cabin at the chair base were measured to determine the effectivity of damping of the chair implemented (SEAT-value). Vibrations for each measurement site were measured in the 3 standard directions: X (for-afterward; awx), Y (sideward; awy), and Z (vertical; awz). During the tests, vibration results were displayed on-line digitally on a laptop computer and were stored. Processing of the data and calculation of definitive outcomes were performed off-line. The effect of different drivers (n=6), type of tyre (multi-purpose, ploughing, or low tension), tractor type (3 pairs of modern tractors in the light, medium and heavy power range, distinguishing within each pair with respect to the measures implemented to reduce vibration exposure) and tyre inflation pressure (rear: 0.75-2.0 bar; front: 0.5-1.0 bar) on vibration was determined during straight, constant velocity drive (speed in 5 categories, varying from 3-4 to 40 km/h) over 8 different surface types (flat asphalt road, brick road, unpaved road, flat field, grassland, rough terrain, meadowland, rough farmland). For the interpretation of the vibration values measured into daily exposure values, it was assumed that all of the agricultural activities tested are normally performed during a full working day of 8 hours.

Vibrations transmitted via the steering wheel appeared to exceed the action value for HAV only incidentally. No special measures need to be taken. The application of damping of the front axle appeared to parallel a considerably lower HAV exposure, compared to no front axle damping, and is likely to contribute to an improved driving comfort. Median values of WBV exposure during most of the agricultural activities investigated remain below or around the action value. This does not hold for ensilaging (median awx=0.56 m/s2), tedding of grass (awx=0.60 m/s2), transport of a trailer or a cubic metre box over uneven surface and at driving speeds above 15 km/hour (awx=0.63-0.99 m/s2), and lugging of trees out of the wood using a tractor (awy=1.16 m/s2). For the transport of a trailer or cubic metre box over asphalt road, the WBV exposure was measured to remain well below the action value for the whole range of speeds tested.

Surface type and driving speed appeared to be the most important contributors to WBV exposure. A more uneven surface resulted into higher exposure values, when keeping the driving speed constant; an increasing driving speed on the same surface type resulted in a progressively increasing WBV exposure. Damping of the tractor cabin and of the axles contributed to a reduction of vibration exposure but, however, only partly compensated for the effects of surface type and driving speed. The mounting of different tyre types or lowering of the tyre inflation pressure did not have a systematic effect on WBV exposure during straight drive at any velocity or surface type. In addition, no effect was observed of tractor mass or power on WBV exposure.

For most of the tractors investigated, the seats showed SEAT values for X- and Y-axis vibrations >100% for the whole range of driving velocities. This implies that cabin vibrations in the horizontal plane are amplified by the seat. Because vibrations in this horizontal plane appeared to be dominant during all of the working activities tested, more attention is needed for damping of the tractor chair in this horizontal plane and better tuning of the damping characteristics of the chair and tractor.

Figure: illustration of enhancement in stead of damping of the vibration exposure as seen in tractor number 6 (light blue): compare the difference between the middle and right panel (weighted data). X-axis: traveling speed in km / h; y-axis: unweighted (left) and weighted (middle) acceleration level at the chair base, and weighted acceleration level on the chair (right).

  

The application of a chair with improved damping in this horizontal plane is advised, in order to lower WBV exposure during ensilaging and tedding of grass. Whether this measure is sufficient to reduce exposure below the action value is to be confirmed by additional measurements. The exceeding of the action value during transportation activities over uneven surfaces at velocities above 15 km/h is conquered most effectively by reducing driving speed or provision (or choice) of a more even surface (or route). In addition, implementation of more advanced damping techniques (i.e. of axles and cabin) will contribute to exposure reduction, but to a limited extent. Again, it is noted that damping characteristics of the chair should be tuned with those of the tractor. The lugging of trees out of the wood using a tractor results into very high exposures. Besides, the impact load, especially in sideward and vertical direction, is such that the activity implies a serious health risk. The main cause is the very rough surface of felled wood and forest trails, where strictly speaking can be driven only at very low driving speeds. The alternative for this type of activity is the use of a forwarder: WBV exposure remains below the action value.


    

Source: www.ergolabresearch.eu.