Analyse van de blootstelling aan trillingen bij gebruik van heftrucks

Analysis of the exposure to whole-body and hand-arm vibrations using forklift trucks

Huub H.E. Oude Vrielink

 

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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, in order 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 forklift trucks is considered to be one of the working activities that need advice on 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 forklift trucks during a normal working action. In addition, it was investigated to which extent exposure can be influenced actively by the driver and the management by flattening the driving surface, reducing the driving speed, or the choice of tyre type, type of chair, or truck type.

Exposure measurements have been performed with 2 experienced professional forklift drivers on 5 different truck–chair combinations during straight drive over different surfaces and during a simulated loading-unloading task performed at normal and hasty speed on flat cement flooring. 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 truck chassis at the chair base were measured to determine the effectivity of damping of the chair implemented (SEAT-value). Vibrations for each measurement site were sampled 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 outcome variables were performed off-line. The effect of different drivers (n=2), type of tyre (massive; rigid or soft rubber), truck type (3 modern forklift trucks in the medium power range), load (unloaded or carrying approximately 1100 kg), chair type (mechanically or pneumatically damped), and fork damping (on or off) on vibration exposure was determined during straight, constant velocity drive at 3 speeds (4, 9 and 14 km/h) over 3 different surface types (flat asphalt, cement flooring and Stelcon plates). For the interpretation of the vibration values measured into daily exposure values, it was assumed that the activity tested is performed during a full 8 h working day.

Performance of the loading-unloading task at normal speed on the cement flooring demonstrated the X-direction to be dominant and exposure to vibrations (whole body and hand-arm) to be below the action value (median range over the trucks: awx=0.23-0.29 m/s2, ahv=0.75-0.82 m/s2). Hasty performance of the task resulted in a considerably increased exposure: median values of awx and ahv over trucks were 0.35-0.46 m/s2 en 0.98-1.26 m/s2, respectively. Exposure appeared to be determined particularly by the degree of (un)evenness of the surface and the driving speed. When driving straight over flat surface (asphalt or cement flooring), exposure remained below the action value for the whole range of velocities investigated. The vertical axis appeared to be the dominant vibration direction. Straight drive over more uneven surface (Stelcon flooring) resulted in an exceeding of the action value at driving speeds of 9 and 14 km/h (median awz over all trucks 0.36 – 0.71 m/s2 and 0.43 – 0.84 m/s2, for both speeds, respectively; median ahv 1.36 – 2.57 m/s2 and 2.48 – 5.58 m/s2, respectively).

If exposure is to be reduced, the most effective measures appear to be flattening of the driving surface and/or reducing the driving speed. The effects of the use of soft-rubber massive tyres, the carrying of load or the installation of a damper in the lifting mechanism of the fork were only limited. An advanced damping technique of the front axle, as being applied into one of the forklift trucks, seems to have prospects, since both vibration emission of the truck and vibration exposure on the seat were reduced, compared to both other rigid front axle forklift trucks. The question whether this finding also holds for a broader range of conditions, as found in practice, needs further research. It must be noted that on this truck a different chair was mounted, which could be responsible for part of the differences in exposure measured on the seat.
 

Figure: effectivity of vertical damping, expressed in terms of SEAT value, of the chair of five different forklift truck - chair combinations as a function of traveling speed

Finally, the measurements showed that damping of the chair mounted should fit the vibration emission of the truck. Although for all chairs tested the SEAT value was below 100%, both the pneumatically damped chairs demonstrated a remarkably elevated exposure to vertical whole body vibrations compared to the standard mechanically damped chairs. It is advised, in order to facilitate the choice of a correct chair, that producers of forklift trucks provide data on the dominant vibration emission frequency of a truck, as determined under a broad range of conditions and while having different types of tyres mounted. In addition, producers of chairs should provide data on the cut-off frequency at different body masses and on the working range of a chair. The cut-off frequency of a chair should be below the dominant emission frequency of the truck.
    

Source: www.ergolabresearch.eu.