Figure 1 (top left).
A drive chain being
extracted from the top of
the arch girders. At this
point, the tip of the crane
is about 200 feet above
ground level.
Figure 2 (top right).
Handling drive chains at
the very top of the dome.
Figure 3 (below left).
Extracting a drive box
from the dome. At
this point the shutter
structure, to the left in
this photo, has been
immobilized so that
it will not move once
disconnected from the
drive chain.
Gemini North Shutdown Ends
On August 25th, Gemini North returned to
regular night-time operations after an ex-
tensive seven-week shutdown. As quoted,
the project’s definition was to “place the
shutter mechanism in an as-new state or
better, and ensure that no major failure oc-
curs in the coming 15 years or longer.”
You may recall the failure of shutter drive
boxes, which cost significant observing time
in late 2013/early 2014, and again in in mid-
2014. Although we fixed those failures by in-
stalling spares, a deep analysis of root causes
revealed two fundamental issues: (1) the
drive chains suffered differential stretching
over time, and (2) the drive assemblies were
not mechanically free enough within their
drive boxes. These issues contributed to the
further failure of a lower-shutter drive box in
August 2016. At that time, we responded by
locking the lower shutter in place until the
work required to fix it could be planned and
budgeted. As we said in the October 2016 is-
sue of GeminiFocus, only complete replace-
ment of the chains and refurbishment of the
drive boxes would fully mitigate the chance
of future failures.
The drive chains are essentially stationary
relative to the dome arch girders; the drive
boxes are attached to the dome shutters,
and crawl along these chains carrying the
shutters with them. Extracting and replacing
the drive boxes require pinning the dome
shutters at various locations; the chains can
only be replaced by extracting them from
the very top of the dome and inserting new
chains in the same way. The top of the Gem-
ini dome is the highest point in the Pacific,
thus we needed the largest crane on the is-
land of Hawai’i to do the chain replacement.
Figure 4 (right).
The extracted drive
being lowered to
ground level for
refurbishment.
19
GeminiFocus
October 2017