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: S. Jana, V. Arun Kumar and M. Ananda. 5-axes levitation of a rotor towards indigenization of the magnetic bearing technology. In Air breathing engines and aerospace propulsion: proceedings of NCABE 2004, november 2004.

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.