Problems with Rapco dry vacuum pumps.

Google for what? You must have diligently spent time looking for the
first link (or are you from Australia). Not easy to find.

Here is an interesting extract from it (the second link is not all that
useful)
"...dry vacuum pumps .... have been a source of continual premature
failure...."

Better fix that oil leak.
Who said I didn't? But you still refuse to acknowledge the point that
an oil leak could develop that you or your mechanic DO NOT KNOW ABOUT
and you are taking a risk. And as you said, Rapco (and others) already
know how to fix the problem.

Who said I didn't? But you still refuse to acknowledge the point that
an oil leak could develop that you or your mechanic DO NOT KNOW ABOUT
and you are taking a risk. And as you said, Rapco (and others) already
know how to fix the problem.

With enough money, MANY (but not all) problems can be solved. The
amount of money needed may exceed the utility gained by doing so This
is going to get a little long-winded, so bear with me.

Dry vacuum pumps are mounted either vertically, as in the O-200, or
horizontally, as in most every other engine generally encountered. The
oil that gums up the vanes and causes them to snap probably leaks from
the accessory case shaft. If the vacuum pump is mounted vertically,
you've got a problem - seals drip, even good ones, so oil WILL migrate
towards your pump shaft, maybe a little, maybe a lot, but it happens
ALL THE TIME. But the key to a solution lies within the problem itself
- you have a RELIABLE source of oil to keep the seal on the vacuum pump
happy. Your goal is to essentially regulate the amount of oil that
gets INTO the vacuum pump to a slight sheen on the input shaft and no
more. Between the flinger and the seal, you can do that. (Even so,
I'll bet the life of vacuum pump on a C-150 is, on average, less than
that on a C-172 - those of you with maintenance experience in training
fleets can shout me down on this if you know better)

Mount the accessory drive and the vacuuum pump horizontally, and you no
longer have access to this solution. You no longer have a reliable,
everyday source of oil to lubricate the seal that "abripl" thinks
should be there on the input shaft of the pump (some oil may creep
along the shaft, but gravity is taking most of it down toward the
ramp). If you put one there anyway, it would burn up in a few hours
running dry on the steel shaft, and wouldn't be there when you REALLY
needed it, for the non-standard rate of leakage out of the accesssory
case or wherever.

There are seals that will run dry. Some exotic elastomers claim they
can, as lip or face seals, but life is short even then. A labyrinth
seal, as used in steam turbines, might do it, but they require
exquisite precision not just in the seals themselves, but also in the
bearings that support the shaft.

The engineers at Rapco, and Edo designed their pumps for the
application at hand with a compromise between cost and risk, and given
that Nature Abhors a Vacuum, it's a friggin' marvel they work as well
as they do, as long as they do, for as cheap as they are.

Who said I didn't? But you still refuse to acknowledge the point that
an oil leak could develop that you or your mechanic DO NOT KNOW ABOUT
and you are taking a risk. And as you said, Rapco (and others) already
know how to fix the problem.

Listen: the vacuum pump has an open frame between the drive
input and the pump itself. Oil has to get into this space to get past
the pump's seal. Oil getting into this space is also going to be
apparent as a mess on the engine, inside the cowling, on the belly, and
on the ground. I work on these things every day (I suspect you don't)
and I won't tolerate leaks that present any hazard to any component.
The oil coming off the engine crankcase breather is bad enough for me.
Oil ruins wiring harnesses, air hoses, steering rod boots, ignition
leads, magnetos, alternators, and vacuum pumps. Get ready to spend more
money.
Some people won't take advice and suffer the consequences,
wondering why the bad stuff always happens to them. I spent 12 years
remanufacturing air brake components for heavy trucks and industrial
machinery, and most of the warranty comebacks followed the same
pattern: control valves full of dirt, compressors destroyed by dirt
getting past old filters or through a lack of oil pressure when the
mechanic used a bunch of silicone to seal a fitting, thereby plugging a
gallery, or chunks of rubber from rotten hoses fouling things up. The
same things happen to aircraft vacuum pumps: dirt, carbon in the hoses
from the last blow-up pump, bits of rotten hose liners, ancient and
crumbling relief valve filter media, or oil from unrepaired leaks; all
stuff allowed into them that has no place there. We used to put
instructions in every box, with every air brake component, telling the
mechanic what to watch for if he wanted the thing to work right the
first time and for a long time. It was a free education that most
refused, and we were forever trying to convince them to clean up the
factors that caused the failure of the original component and
necessitated the replacement in the first place. They just kept
replacing stuff, usually at our expense. They didn't gain much respect
as mechanics.
What killed your original vacuum pump? You seem to be missing
the point that a successful seal that would keep oil out of a pump is
the sort of seal that would need oil to keep it alive. The vacuum pump
seals are designed to run dry and are not the type of seal that can
keep significant quantities of oils out. Seal technology is a whole
science of its own, and there is no single seal that can cover all
possible applications.
Aircraft maintenance includes making sure there's a good air
cleaner to keep dirt out of the engine. It should also include making
sure oil leaks aren't bothering your pump.
Dan

Google for what? You must have diligently spent time looking for the
first link (or are you from Australia). Not easy to find.

Took two minutes. I Googled "Airborne Vacuum pumps."
Edo/Sigma-tech and Airborne built vacuum pumps for aircraft. You'd have
to be in the industry to know that, I suppose.

Dan

I agree with you. I am in the vacuum pump business, and I don't
understand why they can't make the pump a little more reliable. I can't
change the pump design myself, because it is certified the way it is.
It would be to costly to redesign it. If you need any vacuum pump
assistance, you can call me at 800-621-2531. Good luck to you. Ron

rons321 wrote:
I agree with you. I am in the vacuum pump business, and I don't
understand why they can't make the pump a little more reliable. I can't
change the pump design myself, because it is certified the way it is.
It would be to costly to redesign it. If you need any vacuum pump
assistance, you can call me at 800-621-2531. Good luck to you. Ron

I would bet that any seal material or design that might work,
if it exists, would be so expensive that the pump price would rise just
enough that it would lose its competitive place in the market. Or if
there WAS a suitable seal, someone would have incorporated it by now
and marketed their pump as a superior, new and improved, longer lasting
oil- and solvent-proof design. Rapco HAS made small changes to their
pumps, things that weren't part of the original certification, such as
cooling fins, the inclusion of a small amount of salt in the vanes to
absorb a small amount of moisture and increase the vane life (low
humidity increases vane wear), and the flinger/oil drain idea for the
Continental vertical mount. Such changes lead me to believe that the
ideal shaft seal isn't available or affordable.

Dan

What I have learned from these posts is that dry vacuum pumps are a lot
more problematic than just the oil leak - as per
http://www.casa.gov.au/airworth/aac/PART-1/1-098.HTM link. Its too bad
that there is not more warnings before purchase instead of "wonderful"
features. The one hard to find link is hardly a help for a homebuilder.
I have perused these newsgroups frequently during building process and
there is hardly anything mentioned about inherent dry vacuum pump
problems. EAA has homebuilder vacuum systems help article
http://members.eaa.org/home/homebuilders/building/instruments/A%20Gyro%20Instrument%20System_%20(Your%20Options) .html#TopOfPage
with no operational warnings mentioned as in first article. I suspect,
Dan, that you are not a builder but either a certified plane pilot or
A&P making a living from repairs.

I have eventually replaced the vacuum system with electric for about
the same price and saved weight and expect a lot less problems.

How about a slinger followed by a felt seal? The slinger can be made
from an o-ring. Felt will catch any tramp oil, is cheap, & can run dry.

If you have a homebuilt aircraft, consider this:

Pull the plugs from the engine. Put your ear on the pump. Have someone
slowly turn the prop. Listen to the vanes inside the pump fall from
the inside to the outside of the rotor slot. There should be a regular
"click" as each or the six, ( I think that there are six) vanes fall.

If you do not hear the vanes fall, they most likely are "hung up" due
to some contamination. Open the pump, and clean everything. Put it
back together. I use a little silicon seal to act as a gasket on the
back joint of the pump.
I think that the pumps fail when the vane gets sticky and refuses to
slide in and out of the slot in the rotor. I know that the slots in
the rotor will wear larger eventually, but I listen to my pump every
25 hours at the oil change and have had to clean the pump twice in the
last 250 hours. The pump now has about 800 hours on it.

Good luck.










On 21 Sep 2005 14:03:08 -0700, "nrp" wrote:

How about a slinger followed by a felt seal? The slinger can be made
from an o-ring. Felt will catch any tramp oil, is cheap, & can run dry.


Zenith CH-300 Driver.

I suspect,
Dan, that you are not a builder but either a certified plane pilot or
A&P making a living from repairs.

Close, but not quite:
-Canadian Aircraft Maintenance Engineer (AME) M1
-Commercial Pilot, Instructor rating
-Homebuilder: building or restoration projects have included
Taylor Monoplane, AMF-S14, Jodel D-11. Auster VI restoration project.
-Since 1972 in homebuilts, flying since '73

I currently spend most of my time in the shop maintaining
aircraft, but make no extra money from doing more work. I work in a
College-based flight training program and teach a class on Aircraft
Systems; between the maintenance and the teaching I have plenty of
reason for research into aircraft problems. When I'm not doing that I'm
flying, either to figure out some problem, or with a student, which
isn't too often anymore, or in the Jodel.

What's your experience?

Dan