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Case Studies -
Improving the Safety of Detaching Hooks
 Following
an accident in the Vaal Reefs mine in the Republic of South Africa,
HSL was commissioned by SIMRAC (the South African Safety in Mines
Research Advisory Committee) and HSE to investigate the possible
failure mechanisms of detaching hooks used on drum winding installations.
Detaching hooks are designed to detach a conveyance from the winding
rope in the event of an overwind by the action of two actuating
plates entering a special housing simultaneously. However, it was
thought that in the incident, falling debris had caused the hook
to release by striking only one of the actuating plates.
A vertical drop rig was manufactured to test this hypothesis, to
compare the resistance to accidental opening of a range of hook
designs and measure the energy required to open these hooks.
Work was carried out on the three main existing designs together with
some modified designs to establish both partial and full release.
The conclusion reached by HSL was that a 750kg mass falling from 22m
could fully open some hook designs while a 250kg mass falling from
12m could cause a partial release. However, a much smaller mass (say
50kg) could also cause a full release if it fell from several hundred
metres. HSL's work showed that some hook designs can open if struck
on one side and that certain changes in hook design can reduce the
likelihood of release.
This work has alerted the industry to the potential dangers of falling
debris.
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