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#11
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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. |
#12
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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. |
#13
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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 |
#14
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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 |
#15
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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 |
#16
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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 |
#17
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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. |
#18
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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. |
#19
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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. |
#20
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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 |
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