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Question
I just had an air pump failure on my aircraft
in less than 30 hours even after a cooling shroud was installed.
After seeing your website I thought I would contact you
for a possible solution. Have you heard of this type of quick
time failure in the past??
Bob G.
Columbus, OH
Answer
Mr. G.
 After
our phone conversation and receipt of your failed air pump,
photos were taken that indicated the cause of your failures.
With your permission, we have included some photos in this
article so other pilots or owners might be aware of the situation.
For those that are not familiar with the air pump manufactured
by Sigma-Tek (see adjacent photo) we insert the following.
| 1. |
Designed as a bi-directional air pump
to replace both the 211CW (Clockwise rotation) and 211CC
(counter-clockwise rotation) air pump manufactured by
Airborne, Rapco, Aero Accessories and various other manufactures. |
| |
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Advantage – No need to worry about
proper engine/pump rotation match. |
| |
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Advantage – Dealers stock ½ the
inventory |
| 2. |
Provided with carbon composite vanes |
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Advantage – Longer lasting than
commonly used carbon vanes. |
| 3. |
Provided with Aluminum rotor. |
| |
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Advantage – Will not self-destruct
in the event vanes break do to foreign material ingestion. |
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Disadvantage – Aluminum
rotor will expand with the internal heat created within
the air pump chamber. |
Clearance between the rotor and inner surface of the pump
bore is measured in thousandths of an inch. Air pumps incorporating
carbon rotors can maintain this clearance relatively close
because during operation even though pump interior temperatures
increase, the coefficient of expansion for carbon is fairly
low. When internal temperatures increase the rotor stays
the same size (diameter) and the housing increases (being
aluminum) with the clearance between rotor and housing
increasing. Worst-case scenario, when the pump overheats
less pressure (or vacuum) is produced. See Q & A
#3 for carbon rotor and carbon vane pictures.
 Design
of the Sigma-Tek aluminum rotor pump however created a
new problem. When internal temperatures increase the diameter
of the aluminum rotor also increases. As long as the air
pump housing increases it’s internal bore size with
this rotor size increase, there is no problem. However, with
high flying aircraft like the Beech F33, Cessna 210, Mooney
231 or 252, the engine compartment can operate at a relatively
cooler temperature, which means the aluminum housing will
not experience the same expansion rate as the internal rotor.
This problem was first identified in the field with the introduction
of Mooney 252 aircraft in the late 80’s, their owners
usually flying at the higher altitudes.
 Examination of your air pump, even though not installed
on a high-flying aircraft, resulted in the same failure.
Installation of a cooling shroud on the Sigma-Tek air pump
reduced pump bore expansion while at the same time internal
pump temperatures caused the rotor to increase in size.
You can see the results in the adjacent photo, abrasion
of the rotor and in the close up picture of the pump interior,
bore damage from the fast spinning, expanding rotor.
Rubbing
of the rotor with the internal bore causes a continued
increase in rotor temperature, the rotor expands even more
until binding occurs and the rotor shaft (or coupling)
is sheared due to the force of engine rpm. (You are not
going to slow the rotation of a 200 hp aircraft engine!)
 During our conversation, you did mention that the first
sign of trouble was a bouncing movement of the vacuum gage
needle, that gradually got more erratic until pump failure
occurred and the needle fell to ‘0.” That erratic
bounce was the first sign the aluminum rotor was scuffing
the inside of the pump bore. However, at that point, I
don’t believe there was anything you could do except
realize you were about to have an “in-flight air
pump failure”!
My Advice!
- If you are going to keep installing the Sigma-Tek air
pump, remove the cooling shroud.
- Stay out of high altitudes and cold weather, a
suggestion that is probably not practical since you’re
located in the Columbus, OH area.
If your air pump is failing do to an overheating condition
caused by faulty system components or air leaks, check
the system for proper operation as outlined in the “DATA
SHEETS“ section of this website.
If you decide to go back to the carbon rotor, carbon vane
type air pump I would still suggest you check your system
for proper operation. If you have read any of the Q & A
articles posted on this web site you know by now that an
engine run-up and visual check of the vacuum or pressure
gage does not indicate what pressures
your air pump is generating.
Regardless of which air pump you
purchase,
Check the pneumatic system with
shop air
and the proper
test equipment,
Before you install the next pump!
Fly “Heads-Up”,
R. Heysek
Aerotech Components, Inc.
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