Thesis (PhD (Industrial Engineering))--University of Stellenbosch, 2007. / Off-road working vehicles are subjected to high levels of vibration input on the rough
terrain and irregular roads they work. The human operators are therefore exposed to high
levels of whole body vibration (WBV) and at risk of developing health problems. A
number of international standards address the matter of whole body vibration, and the
European Union issued a directive which limits the exposure of workers in the EU to
WBV. Unfortunately, to date there is no law in South Africa requiring compliance with
any of these EU standards nor guidelines.
There are vehicles which are not fitted with suspension and/or suspension seats. The three
wheeled logger used in forestry is a highly manoeuvrable and effective bulk handler, but
without any form of suspension and no space under the operator’s seat to install a
suspension seat. However, a suspension cushion can be retrofitted to existing vehicles
largely alleviating the problem.
To isolate low frequency vibration large suspension travel is required which makes an air
suspension cushion attractive, as it can fully collapse. Additionally, a Helmholtz resonator
if added to the cushion in the form of a pipe and tank, provides anti-resonance at a specific
frequency. The resonator can be tuned by adjusting the pipe’s length and diameter as well
as the volume of the tank. Larger diameter pipes have less friction and give better
reduction of the transmissibility curve at the anti-resonance frequency.
The SEAT value is a single number used to compare suspension seats for a specific input
vibration. It is calculated from the weighted input acceleration power spectral density
curve and the suspension seat transmissibility curve. The former is obtained from the
vehicle and is vehicle, path and speed dependent. The latter is the only variable that can be
improved by using a better suspension seat/cushion. The input power spectral density
often contains significant energy at frequencies where the human operator is most
sensitive. The cushion resonator could be tuned to position the anti-resonance in the
transmissibility curve at these frequencies. The resultant output vibration would thus be
lower than the input vibration at that frequency.
In this dissertation an analytical model describes the state variables in the cushion, pipe
and tank. A Simulink model predicts the transmissibility curve with a solid mass as well
as with a two degree of freedom seated human model. Initially the prototype was tested
with a solid mass to compare the transmissibility curve produced by the simulation with
the experimental results. It was required to evaluate the contribution of the resonator
without the complexity of the human impedance. Subsequent tests were carried out with
human subjects. Test results showed high inter subject similarity at the anti-resonance
frequencies.
Design guidelines are formulated that can be used by the suspension cushion designer to
specify the pipe diameter and length and the volume of the tank to determine the optimal
transmissibility. Input psd from ISO7096 class EM3 vehicles is used as an example during
the design process.
A prototype air suspension cushion was designed to reduce output vibration on the three
wheeled logger. Laboratory tests with human subjects showed a significant improvement
at the problematic frequencies through the tuning of the resonator. Using a Helmholtz
resonator with the air suspension cushion the overall SEAT value improved by 25%
compared with a 100mm foam cushion. However, the current tank and pipe need to be
reduced in size for practical implementation to the vehicle.
Future work would include finding an alternative mass to replace the air in the pipe. This
should reduce the size of the tank and the pipe required. Additionally the simultaneous
effect of multiple resonators at different frequencies should be investigated. This is
required for vehicles having an input psd with significant energy at more than one
frequency band.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/1209 |
| Date | 03 1900 |
| Creators | Van der Merwe, Andre Francois |
| Contributors | Van Niekerk, J. L., University of Stellenbosch. Faculty of Engineering. Dept. of Industrial Engineering. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 3832205 bytes, application/pdf |
| Rights | University of Stellenbosch |
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