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I would like to know which types of bearings are commonly used to support the shafts in jet engines and why. I also read something about hydrodynamic bearings, how common are they?

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    $\begingroup$ I edited the title because it was incredibly broad and didn't jive with the question body. The question body is much more focused, though the hydrodynamic part would probably best be served as a stand alone question. $\endgroup$
    – FreeMan
    Oct 26 at 16:02
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Rolling bearings

I would like to know which types of bearings are commonly used to support the shafts in jet engines and why

Rolling bearings are mostly of two types, depending on what's rolling: Cylinders or spheres:

  • Roller bearings to counter friction. They are used mainly for centering shafts within another component (e.g. a frame).

    enter image description here
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    We want to center the shaft but we don't want the shaft or the other part to heat, nor the rotation to be slowed down by friction. So we replace as much friction/sliding as we can by a part rolling between journals. It's like replacing sleigh skates by wheels between the ground and the sleigh, when the ground is not snowy.

  • Ball bearings. In addition of acting against friction like a roller bearing, an angular contact ball bearing also prevents any longitudinal movement between the shaft and the other part. Thrust for example pushes longitudinally on shafts which would slide out of the engine if not blocked. The roller bearings act as blocking elements, shafts remain at their position withing the engine, and thrust is transmitted to the wing via the bearings, the frame and the pylon.

    enter image description here
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You can see roller (magenta squares) and ball (circles) bearings in this engine schematics taken from In a turbofan what holds the spinning axis:

enter image description here

Frames, shafts, bearings. Adapted from CFM56-7B Familiarization Manual

What we see on this design:

  • Two ball bearings are used to give N1 and N2 shafts their longitudinal position relatively to the fan frame.

  • These ball bearings also center the shafts within the frames, allowing rotors to be centered in the engine walls.

  • Three roller bearings are used to center the shaft at other selected locations, due to the shaft length and loads.

Other bearings are used for the gearboxes which connect the N2 shaft to the accessory gearbox, visible here:

enter image description here

Source: Where is the generator in a large turbofan of a commercial airliner?

(There are many other types of rolling bearings with needles, tapered rollers, double row, ... each having some advantages in a particular use.)

Hydrodynamic bearings

I also read something about hydrodynamic bearings, how common are they?

Hydrodynamic bearings reduce friction, not by rolling parts like balls or a rollers, but by injecting oil between the rotating elements, friction is decreased, only oil viscosity tends to slow down the rotation.

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It's the traditional bearing, with a lubricant, like waxed skates.

The "dynamic" part is because this is the motion of the rotating part which creates the oil film, using the same principle than for accelerating air over a wing: Shaft velocity creates a boundary layer in oil. After the film is created, the motion is no more between metal and metal, but between metal, oil and metal.

Their main advantage is they can bear large loads and still be accurate in positioning. You can read more this technology here. They are commonly found in rocket turbo-pumps. Here's one which is able to bear a load along the longitudinal axis, borne by the flat pads.

enter image description here
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Actually roller and ball bearings also use oil to minimize rolling resistance. The difference is rolling versus sliding. Rolling is usually more efficient to decrease friction effects than sliding since the area at the interface (metal-metal or metal-oil-metal) is reduced.

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    $\begingroup$ And what is the purpose of the final figure of a thrust bearing ( tilting pad ?) ? $\endgroup$ Oct 26 at 0:17
  • $\begingroup$ I agree with @blacksmith37, the tilting pad thrust bearing seems out of place here. They are rarely used in jet engines (partly because there is not that much thrust on the shaft: most of it is cancelled out between the turbine and compressor). It's just an opportunity to repeat common misconceptions: for example, these bearings can be more efficient than roller bearings, particularly under high load. $\endgroup$
    – Zeus
    Oct 26 at 1:04
  • $\begingroup$ @blacksmith37: The OP asked about what is a hydrodynamic bearing. This answers this point, even if they are not used in the engine shown. $\endgroup$
    – mins
    Oct 26 at 10:24
  • $\begingroup$ It's much better now, although the last sentence is still misleading: the area has almost nothing to do with it. And in the explanation of hydrodynamic bearings, I would add one perhaps most important clue: they don't just form a "boundary" of fluid between the surfaces; it is hydrostatic bearings that rely on a plain boundary. Hydrodynamic bearings form a wedge of fluid; this is why the shaft is shown off-centre on the picture and why you need tilting pads on the thrust bearing shown. They indeed behave a bit like a wing. It is none other than Osborne Reynolds who created this theory. $\endgroup$
    – Zeus
    Oct 27 at 1:14
  • $\begingroup$ @Zeus: You would save time by writing your own answer, free of misconceptions. $\endgroup$
    – mins
    Oct 27 at 7:57
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You don't see hydrodynamic bearings in aviation very much. Jet engines are generally ball and/or roller, and piston engines with machined one-piece crankshafts use plain babbit type bearings that depend on pumped oil pressure to separate the metal of the bearing and journal (piston engines with multi-element cranks may use ball and roller bearings).

Another bearing you see in components like Air Cycle Machines (used in air conditioning packs) are "air bearings", where a gas is injected into the journal to create sufficient pressure to separate the metal parts; more or less a plain bearing but with air instead of oil. Air bearings require a conventional low friction bearing that supports the shaft during startup until the gas pressure becomes sufficient. Once the pressure is sufficient to lift the shaft clear, these bearings are near frictionless.

The CRJ700 and 900 Air Conditioning packs use Liebherr Aerospace air bearing ACMs that pressurize their bearings using main engine or APU bleed.

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  • $\begingroup$ Why wouldn't a Babbitt bearing on a connecting rod, main, or cam bearing not be a hydrodynamic bearing? $\endgroup$
    – RetiredATC
    Oct 27 at 14:57
  • $\begingroup$ A hydrodynamic bearing works passively, using the motion between the surfaces and the ramp effect to build up a pressure wave of oil. Babbit bearings are helped by ramp effect, but under high stress they are dependent on oil pressure forced in from the pump, and a larger inlet than exit area, to keep the surfaces apart. A plain babbit bearing once pressurized has an infinite life in theory, as long as oil pressure is adequate. So oil pressure below the green arc on takeoff on a piston engine means your bearings are being destroyed by metal contact, but at or near idle, not such a big deal. $\endgroup$
    – John K
    Oct 27 at 15:45
  • $\begingroup$ oil is injected between the bearing and shaft. The oil wedge created (the oil between the bearing and shaft $\endgroup$
    – RetiredATC
    Oct 28 at 18:35
  • $\begingroup$ From sciencedirect.com/topics/materials-science/hydrodynamic-bearing $\endgroup$
    – RetiredATC
    Oct 28 at 18:36
  • $\begingroup$ oil is injected between the bearing and shaft. The oil wedge created (the oil between the bearing and shaft at the load point) is only about 20–25 μm tick, about the thickness of a human hair. $\endgroup$
    – RetiredATC
    Oct 28 at 18:37

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