TWI's experimental stress analysis team has successfully completed a project with a difference for a company whose main products are rough-terrain vehicles, forklift trucks, telescopic boom materials handlers and dump trucks. Such equipment usually has an arduous service life and experimental measurement used to support the detailed theoretical stress analysis that goes into design, however, this project centered on hydraulic performance as much as structural behaviour.
The machine under investigation was a telescopic boom materials handler, or teleporter. Teleporters have very powerful hydraulic systems and can handle and manoeuvre loads of several tonnes. The hydraulic forces react against the structure of the machine and can induce high local stresses, even when the machine is stationary and unladen. TWI's objective was to investigate the effect of hydraulic accumulators on hydraulic and structural behaviour.
Hydraulic accumulators do not influence peak steady-state pressures, but control the rate of pressure rise and limit the dynamic response of the system. The accumulators were already installed and could be turned on and off as required. Strain gauges were bonded round the hydraulic cylinder mounting points and pressure transducers fitted in the hydraulic lines. Analogue chart recorders captured the data, synchronised so that pressure and strain could be directly related n a common time base. Immediate output of data enabled test results to be interpreted and assessed as the tests progressed.
The results proved to be very illuminating. The stresses induced in the machine at the locations monitored were largely unaffected by the load. There was direct correlation between stress and hydraulic pressure. The machine operations and part-operations that generated high or low stresses were also clearly identified. The effect of the accumulators was in some respects quite different from that which was expected - and it was apparent that they reduced peak stress levels considerably in certain conditions.
Overall, the exercise proved very worthwhile because it increased the understanding of the complex hydraulic behaviour of the machine - as well as making the important discovery that accumulators alone could reduce the stresses created in the structure.
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