A comparison between assessment methods for whole-body vibration: preliminary study

Status:

completed 07/2006

Aims:

Energy-equivalent assessment methods are employed at present for prevention and for ergonomic assessment in relation to occupational disease BK 2110 (discogenic disorders of the lumbar spine caused by long-term exposure in a seated position to whole-body vibration, primarily in the vertical axis). These methods, which are based upon the root mean square of the frequency-weighted vibration acceleration, have been the subject of criticism, since they are believed to underestimate brief, intense vibration exposure. In international standards, a number of exponentiated energy-related methods employing powers higher than two are proposed for formation of the mean of the frequency-weighted acceleration, as are methods which consider the peak value of the acceleration characteristic during a defined duration. Experience continues to be lacking of the deviation between results obtained under different vibration excitation conditions, such as those arising on vehicles or mobile machines, on different roadway surfaces and under different operating conditions.

Activities/Methods:

With the aim of supporting the BGs in their task of hazard assessment in the course of routine measurements and analysis, a preliminary study was conducted in which the results of exponentiated energy-related and peak-value assessment methods were applied in addition to those from energy-equivalent assessments. The results were compared. One aim of the preliminary study was to develop a system permitting classification by vehicle/machine type, by roadway surface and by operating conditions.

Results:

A study of some 200 vehicles and mobile machines revealed no systematic correspondence for any machine group between energy-equivalent, exponentiated energy-related and peak-value assessment. A relationship typical of vibration characteristics which do not contain recurring high peaks was observed between the different analysis results for mean but largely even exposure, as for example during travel on the public highway. By contrast, relatively low mean exposure (a bus with hydraulic suspension on a roadway in good condition with traffic-calming ramps) and high mean exposure incorporating clearly prominent shock components (off-road terrain) yielded results in exponentiated energy-related and peak-value assessments which were significantly higher than those returned by energy-equivalent assessment. A systematic classification of vehicle and machine types was not possible, however. On the contrary: the roadway and operating conditions were the factors determining the differences between results. The parameters (ratios) stated in the standards, which are intended to indicate whether the energy-equivalent assessment is sufficient or should be supplemented by exponentiated energy-related and peak-value assessments, yield a range of variation which is approximately the same for all vehicle and machine types. Whereas the results for the exposure in absolute terms in the form of the energy-equivalent frequency-weighted acceleration, the exponentiated energy-related vibration dose or the peak moving RMS value vary in magnitude as a function of the exposure, the spread between energy-equivalent and exponentiated energy-related or peak-value assessment therefore corresponds to the parameters, and thus varies to approximately the same degree between approximately the same minimum and maximum values for all vehicle and machine types. The vehicle and machine types cannot therefore be characterized and classified generally in terms of the scale of brief, intense vibration exposure.

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