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I've read phrases like "All gas turbine engines use pressurized seals for their shaft bearing compartments to keep [...]" but I do not understand what "seals" means. In spanish we haven't a word for it.

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    $\begingroup$ I think this is better on-topic at ell.stackexchange.com $\endgroup$ – Sanchises Mar 23 '16 at 10:07
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    $\begingroup$ I'm voting to close this question as off-topic because it is a request for a dictionary definition which applies to almost any mechanical system so isn't really a question about aviation. $\endgroup$ – David Richerby Mar 23 '16 at 15:54
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    $\begingroup$ Although it request dictionary definition, it can be answered in aviation sense given OP gave enough context. $\endgroup$ – vasin1987 Mar 23 '16 at 16:27
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    $\begingroup$ Of course there is a spanish word for it: junta. Seventh definition: dle.rae.es/?id=Md1HbkD $\endgroup$ – Gypaets Mar 23 '16 at 18:29
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    $\begingroup$ @DavidRicherby But you only know it isn't related specifically to aviation if you know about aviation. It's entirely possible that aviation uses a different (or more likely, a specialized) definition of a seal that carries with it implicit knowledge of avionics. I'd be worried if the site was nothing but terminology questions, but I don't see anything wrong with the question itself. $\endgroup$ – corsiKa Mar 23 '16 at 19:21
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Translation

Seal means sello mecánico.

While the answer could stop here, we wouldn't know what seal types are used in turbine engines, and how some are pressurized to prevent contaminants entering the aircraft pressure vessel.


Need for bearing compartments and seals

I've read phrases like "All gas turbine engines use pressurized seals for their shaft bearing compartments..."

In turbine engines, there are one or more shafts holding the rotating parts of the compressors and turbines (rotors). Bearings are inserted between a shaft and its stationary supports to limit destructive and heat-generating friction. Bearings also bear the large axial load transferred by compressor and turbine rotors to the shafts.

In turbine engine bearings, solid friction is replaced by fluid friction using an oil film between balls or rollers and races. Bearings are enclosed in a cavity and oil is injected into the cavity. This cavity is known as a sump.

The following drawing is taken from an answer to In a turbofan what holds the spinning axis?. You can see the five bearings within their sumps:

enter image description here
(Source)

Oil must be circulated from the oil tank to the sumps and back, and cooled before being reused. As oil quantity is limited, oil must be prevented from leaving the oil system. This is where seals enter into action. In particular, seals are used to prevent leakage between a rotating shaft and stationary sump apertures.

High speed seals

Turbo engines' oil seals are specially selected to bear the high rotation speed of the shafts (wear), and the high operating temperature of the engine core (adjustment and bending). They are of two main types: Carbon or labyrinth. Both reduce oil pressure while it travels in the interstice between the shaft and the sump wall.

Carbon and labyrinth seals
Carbon (source) and labyrinth (source) seals

Air sealing

All gas turbine engines use pressurized seals

As one would think, these seals are not perfect: a small quantity of oil will be able to make its way across the seal, and this quantity will increase with seal wear. Unfortunately, burnt oil releases hazardous fumes which could enter the aircraft air conditioning system as cabin air is tapped most of the time into the engine compressor.

To limit the leakage further and to recover leaked oil, a second cavity is built around the first seal and pressurized air is injected into it. This has two effects:

  • Air pushes oil back into the sump.
  • The second cavity being also sealed, any leaked oil can be drained and dumped overboard.

For example, the forward sump (on the left of the first picture) encloses 3 bearings (from left to right: #1, #2 and #3). Oil must be contained within the sump near bearings #1 and #3. Let's look at a CFM56-7B engine mounted on Boeing 737 NG. This is the lower half of the forward sump:

CFM56-7B forward sump sealing
CFM56-7B forward sump, based on drawing from CFM56-7B Familiarization Manual

  • Oil enters the sump and is directed to bearing races using tubes and oil nozzles.
  • An oil seal prevents oil from escaping the sump in large quantity.
  • As some oil would nevertheless cross the seal, compressed air is injected on the other side of the seal, to counter oil leakage. Air entering the sump is then dumped overboard by the centrifugal air-oil separators.
  • The space where air is injected is closed by a cap on the sump with a air seal on the shaft.
  • Air and a small quantity of oil able to cross the oil seal are eventually drained and released overboard.
  • Oil crossing the bearing is scavenged and returns to the oil tank after air has been removed.

Metal particles in scavenged oil are captured by magnets to detect abnormal wearing of bearings. From time to time oil is added to the oil tank to compensate for small leakages in the bearing sumps (and in the gearboxes).


See on Youtube: Carbon Seals - Turbine Engines: A Closer Look.

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    $\begingroup$ I find searching for "juntas de estanqueidad" on google gives me gaskets, not seals. Is there some other translation? $\endgroup$ – Andy Mar 23 '16 at 13:34
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    $\begingroup$ @Andy: Thanks for pointing out. I updated with sello mecánico (Google search). $\endgroup$ – mins Mar 23 '16 at 13:44
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"Seal" is used here to mean "closure". Specifically, a seal is a mechanism which prevents gases (or liquids) from crossing a barrier.

For instance, a rotating shaft must be connected to a fixed structure by means of a bearing (which allows it to rotate). If dirt or liquid were to enter the bearings, it would damage the mechanisms inside. Therefore, seals are used to keep dirt and liquids out of the bearings. Here is an example from general machinery: http://www.machinerylubrication.com/Read/838/protect-bearings

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Seals are things like this:

Pressurised shaft seal

they prevent oils and other fluid from contaminating areas where they do not belong (and they prevent impurities from contaminating the oils).

You can see one of these used in this assembly diagram (the "oil seal") to protect the ball bearing assembly:

Shaft assembly diagram

(Image source)

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