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Calibrating
without Blowing a Gasket
Bellows finger makes adjustments through
sealed avionics gauge
While
designing their 9A1990 liquid oxygen level gauges for
high-altitude military transports, the design engineers at
Transicoil Inc, Norristown, PA, were challenged to find a
method for mechanically adjusting a hermetically sealed
instrument from the outside — without breaching the seal.
Specs for the oxygen gauges required two field-adjustment
capabilities for potentiometers inside the seal: one to vary
the measurement range and another for periodic calibration.
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These tiny nickel bellows-based fingers adjust
rheostats inside the LOX gauge while preserving the
gauge’s hermetic seal.
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Mounted on the instrument panels of C130s
and C141s, the gauges tell the pilot how much oxygen is left
for their critical breathing apparatus. Each plane model has
different sized liquid oxygen storage tanks, necessitating the
gauges’ range adjustment. Each aircraft also has its own
power supply requirements, thus demanding the field
calibration spec.
The hermetic seal prevents fogging of the
display glass by condensation. To this end, the gauges are
purged to 10-8 atm, then partially refilled with dry nitrogen.
This standard practice in avionics instrumentation counters
the atmospheric air "pumping" that drives water
vapor into the internals of "open" instruments.
Water vapor is the leading cause of damage to the instruments’
viewing glass, besides making readings difficult at
potentially critical moments.
To provide the field adjustment capability,
Transicoil engineers designed rotary adjustable potentiometers
into the instrument internals within the hermetic barrier.
While the easy solution would have been to extend the
adjustment knobs through the instrument shell (with O-rings
providing the seal), the engineers decided that the seal
integrity was too important to risk failure.
The team started looking for different
methods of transmitting a mechanical movement from one side of
the seal to the other. They had been using electrodeposited
nickel bellows as pressure compensation and pressure actuation
devices in other instruments, and were therefore familiar with
the bellows’ absolute impermeability, ductility and
suppleness, reliability and extremely low weight. The question
was how to put these properties to use in the new function.
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A twist of the screwdriver adjusts a calibrating
rheostat inside the LOX gauge without breaking the
instrument’s hermetic seal.
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Their solution was to create a mechanical
"finger" based on a custom bellows, much like gloves
in gloveboxes. Servometer, Cedar Grove, NJ, customized their
bellows with a stiff conical point at one end. Thus, the
bellows’ main body is as supple as a Slinky, yet the conical
end is stiff enough to turn a knob or linkage inside a gauge.
A nib on the outside of the bellows, in turn, engages the
potentiometer to make the adjustment. "You might think of
the nib as a ‘fingernail,’" says Chet Wasniewski of
Transicoil. "Who among us hasn’t turned a loose screw
with a fingernail?"
To give motion to that fingernail,
Transicoil used a slender 0.187-in. dia. shaft with a
45-degree bend near its end, running down the center of the
bellows. The top end is exposed at the rear of the gauge and
has a slotted head for easy turning. As the installer or
maintenance technician turns the head, its opposite end
traverses a 0.3750-in. dia. orbit. "It’s like ‘runout’
in a bent drillbit, but very slo-mo and controlled,"
explains Wasnieski. "Here, runout is a good thing!"
In fact, it is all that is needed to articulate the end of the
bellows and adjust the potentiometers on the other side of the
seal.
The finger assembly is soldered to the rear
of the instrument to keep the hermetic seal intact. The
conical tip is EB welded to one end of a conventional bellows,
and an annular flange EB welded to the other end. The
0.125-in. hole in the center of the flange accommodates the
offset shaft. The bellows unit measures 0.75-in. long by
0.25-in. dia., with 0.0007-in. walls. To ensure an airtight
assembly, Servometer checks each one on a Helium Mass
Spectrometer leak tester, rejecting any leakage beyond 10-9
std cc/sec.
Various materials, including copper, were
considered for the bellows, but both companies returned to
nickel. "Electrodeposited nickel is ideal here for its
ductility, high strength-to-weight, corrosion resistance and
fabricability," says Paul Hazlitt, chief engineer at
Servometer. Nickel’s higher elastic limit meant that the
device would retain its original shape better than other
materials, despite repeated deformations. The nearest
competing material weighed 20% more than the nickel, which is
significant in aerospace applications.
The finger solution also reduced part
count. "We basically have a one-piece assembly —
bellows and built-in offset screw — that engages the
potentiometers directly, with no other linkages
necessary," maintains Wasniewski. Hazlitt adds that this
application is rather unique for bellows, "We’ve seen
bellows used for aneroid functions, pressure compensation,
seals, flexible couplings and self-locating electronic
connectors, but this is one of the few as a mechanical
manipulator."
—SG
For more information:
Transicoil Inc,
www.rsleads.com/402df-261
Servometer,
www.rsleads.com/402df-262
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