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
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superseded by more modern advances, especially in high temperature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.
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
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.
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.
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.
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.
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
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.
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