Evaluation of Vibration Characteristics and Human Response to Hand Transmitted Vibration of Single Axle Tractor with Varying Key Parameters
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Date
2021-07
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Addis Ababa University
Abstract
Hand-Arm vibration from a 15-horsepower single-axle tractor can be very powerful and can
induce operator fatigue plus various physiological disorders in the vascular, neurological and
musculoskeletal systems. The transmission of vibration is determined by the dynamic response
of the human body and depends on the physical characteristics of the individual hand, the contact
area, the grip force, the push force and the posture. The grip force is an integral part of the spatial
distribution of the touch pressure distributed over the entire hand and influences the surface of
the contact. This research aimed to test a single-axle tractor operator's vibration characteristics
and human response to hand-transmitted vibrations. The research was conducted at the Melkasa
Agricultural Research Center in Ethiopia in Oromia Regional State, East Shoa District.
Anthropometric data measurement, a physiological response and hand vibration of single-axle
tractor operators and single-axle tractor handle vibration were performed. For biodynamic
reaction, grip strength and vibration transmissibility measurements were assessed using 2, 3 and
4-DOF models. The demand for expenditure capacity, physical workload, and daily vibration
exposure of operators were calculated from measured data. To dampen the vibration of the single
axle tractor, mechanical dampers made of a rubber compound were fabricated and tested.
Without any damping mechanism, the mean measured resting heart rate, working heart rate and
heart rate estimated after operation of single axle tractor were 71±7, 162 ± 4 and 126 ± 24 beats
per minute, respectively; however, the maximum acceptable working heart rate for a work shift
of 8 h according to international standards is only 136 ± 8 bpm. The measured values of the
physiological workload or cardiovascular stress index and the ratio of work to resting heart rate
were 74 ± 10 per cent and 2.3 ± 0.4 (p < 0.0001) respectively. The calculated cardiovascular
strain and relative cardiovascular load was 131 ± 0.42 and 67 ± 17 % respectively.
Finally, the grip strength of single-axle tractor operators was measured and it was found that the
operator's grip strength decreased by 11.34 and 9.5% after vibration exposure of dominant and
non-dominant hands, respectively, relative to the grip strength before vibration exposure.
To improve the understanding of hand-arm vibration biodynamics, the two-, three-and fourdegrees-
freedom (DOF) models were used to simulate the dynamic reactions of the hand-arm system in three orthogonal directions (vertical, forward and lateral); results indicated the highest
magnitude of vibration transmission to be in the vertical direction.
Actual measurements of vibration transmitted to the tractor operators working in second and
third gear were contrasted with the vibration predicted by the 2, 3 and 4-DOF models. The 4-
DOF model showed the best agreement. It was observed that for all models the vibration
transmitted to the forearm of the palm wrist rose with frequency up to the resonance frequency,
and then decreased to zero at a frequency of 1250 Hz.
However, the 4-DOF model is the best model for the measurement of hand-arm transmitted
vibration in the hand-arm system, as it can be used to quantify the vibration transmitted to
fingers and upper arm-shoulder parts, unlike the 2-DOF model. The 4-DOF model also has the
advantage over the 3-DOF model in accounting for the vibration transmitted to the upper armshoulder
Following the installation of the rubber vibration damper, the vibration measurement was carried
out at the handle of the single axle tractor in the longitudinal, forward and lateral directions and
these were compared with the value before. There was a reduction in vibration magnitude of
42%, 38% and 46% following installation of the damper system in the vertical, forward and
lateral directions, respectively. Reductions in vibration magnitude in third gear of tractor
operation were 46%, 29% and 50% in the longitudinal, forward and lateral directions,
respectively following damper installation. The vibration total value after the vibration damper
implementation dropped by 42% and 46% in the second and third gears, respectively. The results
indicate a larger percentage decrease in vibration due to the addition of a damper while operating
in a third gear than the second gear.
The latency period is defined as the time elapsed before symptoms start to appear in the exposed
population. As the handle vibration total value magnitude decreases from 27 m/s2 to 16 m/s2 due
to damper installation, the latency period for hand-arm vibration syndrome increases from 2.55
to 4.76 years duration in second gear operation (89% increase). In third gear, the vibration total
value falls by 15 m/s2 upon installation of the vibration damper, while the latency time increases The findings suggest a substantial reduction in the handle vibration of the single-axle tractor in
all three directions due to the installation of the rubber vibration damper system. However, even
with the dampers installed, vibration transmission to the tractor operator remains at a hazardous
level; this can lead to adverse health impact on tractor operators according to international
guidelines and directives.
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Keywords
Varying Key Parameters, Vibration Characteristics, Hand Transmitted Vibration, Single Axle Tractor