Turbofan:Bearings

Bearings and cooling

Rotational speed achieved by the engine will probably be around 40000rpm. At this speed, regular ball bearings may overheat or suffer from a too fast wear. In real engines, bearings are constantly lubricated by an oil bath, which poses problems with regards to oil pressurization and leaks in other parts of the engine. Seals are consequently placed close to bearings to prevent leaks, generally carbon leaks. Accessories like oil pumps, pipes, fixations, filters and so on, are also required.

Bearings

Ball bearings

Ball or roller bearings are the obvious way to guide rotating parts. They provide good mechanical constraints on the axis orthogonal to the rotation, they are inexpensive and their integration is reasonably simple.

Silicon nitride bearings have lots of improvements over regular metal ball bearings. Balls are more than 60% less heavy, thus having a lower inertia at high speeds, implying a more softer contact with the tracks, allowing longer lifetime or higher reachable speeds. They also require less lubrication. Fortunately, silicon nitride bearings have reached marked with a large production, and are not over-expensive.

Alternate bearings

Fluid or magnetic bearings should be considered. They allow much higher rotation speeds and lower friction, but have two main drawbacks. At standby state, they release the hard constrain on moving parts orthogonally to the rotating axis. In reduced-size turbomachinery, where rotor and stator have to be adjusted to tens of microns, it's quite complicated to use those bearings. The second drawback is that they require more external hardware, to pressurize the fluid or to provide magnetic energy.

However, magnetic bearings have been demonstrated in this paper , in which axial position accuracy is measured below 150µm for a 4kg rotor at around 2000rpm. The rotor position sensor has a resolution of 2µm per mV. However, no indication is given about the resting position of the rotor and how that impacts the clearance between rotor and stator.

Foil bearings are a particular type of fluid bearing, that "Unlike aero or hydrostatic bearings, foil bearings require no external pressurisation system for the working fluid, so the hydrodynamic bearing is self-starting". In this other paper , a small centrifugal turbojet is built to evaluate the ability of MiTi's product, a foil bearing, to sustain very high rotation speeds (120'000rpm) and high temperature (800°C). The bearing has a low spacing between the rotor's journal and the stator fixation, but it is secured, in this paper, using a ball bearing on the compressor side, where the temperature is low. They planned to make a dual-foil bearing, we'll need to check on that. MiTi also demonstrated a hybrid foil magnetic bearing, that has the advantages of magnetic bearings at low speeds and those of foil bearings at high speeds.

Oil displacement without external pumping

Screw pumping will be used as a way to move the oil through the engine, i.e. parts that have to be lubricated and cooled. the work of the turbine will thus directly drive the oil pumping without requiring external accessories. However, cooling the oil may require external hardware, and sealing is absolutely required.

External hardware required for lubrication

A cooling device will be required if the oil gets too hot, which is likely. A basic oil-to-air heat exchanger should be sufficient.

Sensors will be required too, at least for oil temperature and displacement. Oil temperature may inform about the status of the engine, and with sufficient experiments and modeling can be used to infer turbine temperature. Oil displacement sensor is required to ensure that there is no problem with the oil/cooling flow in the engine and that the measured temperature is not bogus.