Abstract
M.Ing.
In the mining industry safety is a primary concern. Especially so when it comes to the large conveyances transporting people, equipment and ore up and down the shaft.
Even though it has occurred seldomly, a so-called "slack rope event" - when the conveyance gets stuck in its guides - is extremely dangerous. If the conveyance should become freed it will fall freely until all the slack rope has been taken up and then whiplash. The immense strain could lead to complete failure of the rope or suspension mechanisms. The large deceleration rates experienced could in itself be sufficient to cause serious injury or even death to passengers. A strategy to alleviate this danger, originated by Greenway and Hymers (41), is to have passive energy absorber units in
parallel to the suspension mechanism of the conveyance. In the event of an emergency the suspension mechanism will be detached and the absorbers will then be activated, safely absorbing the kinetic energy the conveyance has gained.
The dynamic behaviour of the system with and without the energy absorbers in a free fall situation was studied and it was clear that the hazards associated with a slack rope event could be successfully alleviated using the energy absorbers.
The question that now presented itself was, which energy absorbing mechanism should be used in this application. An encompassing set of qualifying criteria was subsequently set and after extensive research and evaluation the cyclic plastic bending energy absorber was deemed to be most suitable for this application, as also suggested by Greenway and Hymers (41).
The device uses a metal element being bent and unbent while being pulled through a set of rollers. The kinetic energy is transformed to plastic metal deformation and dissipated as low grade heat.
Further research was then done on this energy absorbtion
device. It revealed a substantial amount of background
information and two approximate equations for prediction of the
resistive force delivered by the device.
It was subsequently attempted to find an analytical solution
from first principles, to predict the resistive force and
characteristic behaviour of the device. Two solutions were
obtained, using different approaches to the problem.
A number of experiments were then conducted to study the
actual characteristics and behaviour of the device. After
manipulation of the results it was found that non-dimensional
parameters could be formulated which would make it possible to
predict the behaviour of full-scale prototypes using small-scale
models.
Upon comparing the experimental results with the analytical
solutions it was found that the two analytical solutions provide
an upper and lower bound to the experimental results.
Finite element analysis was also utilized to characterize
the behaviour of the device and to attempt to predict the
resistive force the device delivers. The finite element models
revealed some interesting characteristics of the device and
mimicked the behaviour of the actual device. However, the
resistive force values obtained, deviated slightly more from the
experimental values than the scaling method or the analytical
solutions.
It is thus possible to predict the behaviour of the cyclicplastic-
bending energy absorber by utilizing scaling techniques,
analytical solutions or the finite element method.
The cyclic-plastic-bending energy absorber has a wide
spectrum of applications as it is a very versatile and reliable
energy absorber.