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Mayall 4-m Back On-line After Shutter Failure (1Mar96)
(from KPNO, NOAO Newsletter No. 45, March 1996)

In mid-October 1995 the gearbox that drives the dome shutter of the
Mayall telescope was found to have damaged gears, and we were forced to
suspend observing at the 4-m while we repaired it. Nightly operations
resumed on 28 December 1995. We would like to describe the nature of
the failure, the repair itself, and how the loss of observing
opportunity was addressed.

On 18 October, while searching for a noise that the dome shutter was
making as it closed, the shutter gearbox was found to have damaged
gears. The gears were damaged to such an extent that we judged it
unsafe to continue to operate the shutter on a daily basis, as further
use could have led to complete breakage of the gearbox and significant
damage to the shutter. We decided to keep the 4-m dome open until the
manufacturer was ready to repair the gearbox, or until we were forced
to close the shutter because of threatening weather. After twelve
nights--ten clear--our luck ran out. The night of 29/30 October saw the
first storm of fall arrive on the tropical jet stream. After riding out
a few drizzles during the night and morning--the Cassegrain cage was
wrapped in a tarp and the top-end sheltered under the dome--the storm
intensified, with embedded thunderstorms showing up on weather radar.
We decided just before noon on 30 October to close. That afternoon the
mountain received a strong rainstorm with the promised thunder and
lightning; significant water damage would have resulted had the 4-m
dome not been closed when it was.

	         	  [Photo not included]

While we were open, arrangements for the repair were made. As soon as
we closed the shutter, removal of the gearbox began. Not suprisingly,
the gearbox itself defined the critical path of the repair schedule.
The manufacturer estimated eight weeks to fabricate new gears, which
seemed like a long time until we learned that a complete gearbox would
have taken five months to be manufactured. While the gearbox was off
for repair, the motors, clutch and brake assemblies in the shutter
mechanism were refurbished, and the shutter drive was carefully
inspected and lubricated. Despite the usual surprises that come up in
undertaking a project of this magnitude--e.g., the gearbox went out
weighing 1100 lbs and came back weighing 650 lbs. more, requiring that
we re-do our rigging to lift the gearbox into place--we met our goal of
opening by year's end.

The root cause of the failure appears simply to be due to wear over the
nearly thirty years of use. The original gears were cast-iron with a
hardened surface. When the hardening wears through, rapid wearing of
the gears occurs with the teeth then easily breaking (see figure). All
of the gears showed pitting from wear, an indication of surface fatigue
failure, but without signs of scoring, which would indicate oil film
failure due to excessive loads or aging lubricant. All of the bearings
in the gearbox were worn out and were replaced at this time as well.
The new gears are made of solid steel with superior wear properties. In
addition, the two old motors--one for slow speed and one for fast--have
been replaced with a single DC motor that ramps up to the set speed,
thus placing less load on the gearbox when starting and stopping. The
entire shutter system is under review to ensure that any potential
failure modes that we have been unknowingly living with for decades
will be corrected.

Astronomers scheduled during the time that we expected to be closed
were informed of the extraordinary circumstances. All were offered
reimbursement of any direct expenses (such as non-refundable air-fare)
that they had made in advance of their runs. They were asked if they
would be available should we be finished before our projected opening
date, and if they would like to be re-scheduled in spring 1996.
Ultimately, by shortening the block assigned for DLIRIM proposals to
five nights, we were able to fit in two programs from earlier in the
semester. Five proposals canceled at the 4-m in fall 1995 have been
scheduled in early spring 1996 after due consideration by the TAC when
they met in November.

Ironically, the noise that was heard that led us to take a careful look
at the gearbox was still there when we were able to move the shutter
again. The noise turned out to come from a piece of sheet-metal from
one of the seals that had started to rub against the dome skin. That
has been taken care of as well.

A large number of personnel--both Kitt Peak and Tucson based--were
involved in this repair. Thanks to their hard work and teamwork the
telescope was ready for use by visiting astronomers essentially on the
day projected back in October when we shipped the gearbox off for
repair. They were also responsible for a significant amount of
additional work done to the dome shutter while the gearbox was away to
ensure reliable operation for years to come. We sincerely thank them
for all of their efforts.

Bruce Bohannan, Tony Abraham

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