Gas Turbine oil seal question

Studying jet engines recently, it crossed my mind how is the oil "sealed" in the mainshaft. At the high RPM, I cant imagine it being a common oil seal
like used on car engine crankshafts, and a total loss system sounds dubious to me for a modern engine. Anyone know ? Bob
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Emimec wrote:

Not very knowing here, but afaik it isn't? - there is no oil sump to seal?
Now jet engines do have oil, and oil seals, but they are only used to lubricate the bearings, which is a continuous flow process. There is no big wet sump, or the like, just a return tank which has no rotating parts or seals.
or so I imagine, don't really know.
I do know that small turbines as in model turbines and Predator-type drones and cruise missiles do use total loss, and actually maybe big ones might too - why not, the fuel is oil anyway?
At the high RPM, I cant imagine it being a common oil seal

I know quite a lot about high temp and high pressure differential seals, but those are used in rocket engines, not jets.
-- Peter fairbrother

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Peter Fairbrother wrote:

:)
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On Thu, 9 Dec 2010 22:21:28 -0000, "Emimec"

I did some test & research work on an old Rover GT (30 years ago) and later some studies on helicopter enigines & drives. The main shaft oil is generally only encouraged not to leave too quickly rather than 'sealed'. Some systems use fine wire brushes, but more advanced designs have pumping labyrinths eg a coarse 'thread' on the rotating part what runs with a small clearance within the housing. It works similar to an Archemedian screw. Some systems use a pressure bleed from the compressor to 'blow' lubricant through the bearings.
My (very limited) experience was 25 years ago so it may well have been superseded by more modern advances, especially in high temperature materials for fully closed seals.
Richard
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Probably a myth, but:
In the very early days of the jet engine Pratt and Whitney made an engine under licence from RR. Everything went well until they tested it and had to put a large container underneath to catch the oil which was being expelled at a high rate. They also had to feed it in very quickly to make it up. The oil seal on the main shaft had a thread on it so that it "screwed" the oil back in. The Americans had made the thread the wrong way round so that it expelled the oil rather than retaining it.
John
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Sounds plausible given that NSAS couldn't convert Imperial to metric for the shuttle, but on the other hand British bikes of that era were notorious for leaking oil so British engineering skills were equally poor.
Cliff Coggin.
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Cliff Coggin wrote:

I spent a couple of years living in Wichita Kansas and a guy I knew there had dealt with British bikes in the past, he said the problem wasn't with the engineering but with the assembly. He said if you took the bike engine apart and put it back together properly then in his experience they didn't leak.
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Thanks to everyone's input. I figured the oil must be under pressure for the "main shaft bearings", as in a car engines main bearings, but couldn't imagine a car type oil seal holding it all in, especially at the RPM a turbine runs at. I'll settle for the screw idea, seems the most likely answer, along with it being forced back into the engine by air/wind. Bob
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AFAIK they all run on ball bearings, so no need for "oil pressure". They are supplied with oil mist (not liquid oil). I suspect they rely on "centrifugal" catchers, possibly wind-back seals, also clever control of gas pressures. There won't be anything deliberately rubbing for oil sealing because of the speeds. To get the gas leakage down at blade tips they use "abradable" seals which wear to a small clearance, but will cope with transients without wrecking the blades.
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Bob,
I have no personal expertise in this field, but I do have a book on turbine engine design. It's from 1997, so reasonably current.
Apparently, most such engines use a dry sump system. Oil is filtered and pumped under pressure to the bearings and gearboxes and other parts requiring lubrication. A lube scavenge system draws oil from the sump, passed through a detector to look for metal particles in the oil, and then to a filter, heat exchanger and back to a reservoir tank. Pressurised air from the compressor stage is bled into the appropriate parts to prevent oil from leaking through seals, and also to cool the sump to minimise degradation of the oil.
The oils used are totally synthetic (i.e. not simply derived from crude oil) and have near-constant viscosity over the range of temperatures encountered.
Not sue if this is a complete explanation, but it's pretty well all I can add from a casual reading of the book's section.
David
--
David Littlewood

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http://www.exxonmobil.com/lubes/exxonmobil/emal/files/TTopic14_JetEng2.pdf
jsw
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Nice link, thanks!
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wrote:

http://www.exxonmobil.com/lubes/exxonmobil/emal/files/TTopic14_JetEng2.pdf
jsw
Excellent, many thanks, answers everything Bob
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