The use of low-vibration seats on trucks and mobile machinery is an important technical measure for the protection of vehicle drivers against vibration at their workplaces. The vibration characteristics of the seats must be matched to the magnitude and spectrum of the vehicle's vibrations. Laboratory testing of the vibration characteristics is essential for design and optimization of the seats, and for selection of the correct seat for a given vehicle group. In the past, test persons have been employed for laboratory testing of the seats. In order to avoid vibration exposure and to increase the accuracy of measurement, mechanical substitute systems, "dynamic dummies", were developed whose vibration characteristics simulate those of the human body in the frequency range up to 20 Hz.
A more highly developed mechanical dynamic dummy, developed by the Federal Institute for Occupational Safety and Health (BAuA) for test persons of differing weight, was subjected, initiated and promoted by the BAuA, to a round-robin test to ascertain its suitability for laboratory use. The vibratory excitation was generated in the laboratory of the BG Institute for Occupational Safety and Health (BIA) by a "Hydropulse" simulator. The excitation corresponds to four different classes of earth-moving machinery according to DIN EN ISO 7096:2000, "Laboratory evaluation of operator seat vibration". Of these classes, one also applies to the testing of fork-lift truck seats in accordance with prEN 13490, "Mechanical vibration - Industrial trucks - Laboratory evaluation of operator seat vibration", one to seats for articulated trucks to DIN 45678, "Articulated trucks - Laboratory method for evaluating vehicle seat vibration", and one to tractor seats to ISO 5007, "Agricultural wheeled tractors - operator's seat - Laboratory measurement of transmitted vibration". One seat currently available on the market was provided for each of these four vehicle groups. In BIA a fork-lift truck seat with a pitching characteristic was also tested. Seven test bodies took part in the laboratory tests. With the exception of two test bodies which studied only one test subject per weight class, three test subjects per weight class (55 kg, 75 kg and 98 kg) and three dynamic dummies of the same weight classes were studied. Each measurement was performed three times. The test criterion was the vibration transmission factor SEAT, the quotient of the frequency-weighted vibration acceleration on the seat and at the seat mounting point.
On all seats, the seat transmission factor SEAT, which serves as a dimension for attenuation of vibration, improved, i.e. dropped, as the weight of the test subjects increased. Comparison of the results, obtained on the one hand by means of the dynamic dummies and on the other by human test subjects, also showed the seat transmission factor measured with the dynamic dummy to be on average 11% lower than that obtained with human test subjects. In other words, results of tests performed with the dynamic dummy suggested better vibration attenuation by the seat than that attained in tests involving actual human test subjects. The dynamic dummy with a weight of 75 kg delivered the closest approximation, that with a weight of 55 kg the worst. As anticipated, the repetitive accuracy with the dummy was better than that with the test subjects. In order to achieve closer correspondence between the results for the test subjects and those for the dynamic dummy, the dummy was modified again: the mass distribution between vibrating mass and additional mass was modified, as was the damper fluid. Three test laboratories took part in the subsequent test. In comparison with the first series of tests, the seat transmission factor with the modified dynamic dummy on the earthmoving machine seat was higher, i.e. the dynamic dummy provided better reproduction of the actual attenuation. The relative deviations for this seat from those for human test subjects were only 4%; testing by means of the dynamic dummy continued to yield better results than testing with human test subjects, however. For the semitrailer machine seat, modification of the dynamic dummy produced, on average, no improvement: in fact, the relative deviation of the seat transmission factor worsened in tests performed by some laboratories. Overall, the seat transmission factor (SEAT) obtained with the dynamic dummy was around 6% below that obtained with human test subjects, i.e. the seat returned a result excessively favourable by this percentage when the dynamic dummy was used. Whereas the results varied by 20% or more between human test subjects, the variance was below 10% when the dynamic dummy was employed. The dynamic dummy has thus demonstrated its suitability in principle as a substitute for human test subjects. At the same time, correction factors for the dummy must clearly be determined for the various standard excitation spectra. The effect of the dynamic dummy is shown to vary according to the frequency characteristic of the excitation spectrum.
-cross sectoral-Type of hazard:
Prüfverfahren, Prävention, VibrationDescription, key words:
whole-body vibration, drivers' seats, seat test, dynamic dummies, test persons, earth moving machinery, articulated trucks, wheeled agricultural tractors