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?
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).
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.
You can see roller (magenta squares) and ball (circles) bearings in this engine schematics taken from In a turbofan what holds the spinning axis:
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:
(There are many other types of rolling bearings with needles, tapered rollers, double row, ... each having some advantages in a particular use.)
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.
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.
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.
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.