Help in jet engine

[arslanturbojet]

I have made a Turbojet Engine but on its first start bearing burnt out please guide 

[racket]

How much oil pressure were you running through the bearings ??

[turbotom]

A full ceramic bearing isn't the best choice for applications where the shaft get very hot. The thermal expansion of the ceramics is lower than the one of the shaft inside (though ZrO2 isn't the worst choice), also since the heat transfer between the shaft and the ceramics is low.
An intermediate fit of the bearing bore and the shaft, as required for balance reasons, results in cracking of the inner race. Witout a squeeze film around the outer race, the engine will probably also suffer from shaft bending oscillation at the critical speed that almost certainly lies within the operational range.

Recommendation: Use hybrid ball bearings (preferably cageless or with a riveted, machined or single-piece metal cage) and a shaft reinforcement tube of sufficient thickness between the bearings to increase shaft critical speed.

[jetkart]

Great info Tom, what would you say the benefit is of a single piece machined cage hybrid bearing vs a full complement cageless hybrid bearing?

[turbotom]

My experience with full complement hybrid bearings is that they are a little less troublesome to operate, i.e. they have less strict demands on the lubrication system. On the downside, they tend to wear more quickly since the relative velocity of the contact area between two balls is twice that of caged bearings. If there is the slightest tolerance in the diameters of the individual balls, the balls tend to rub permanently against each other, so very high precision of ball selection is required. Nevertheless, some contacts between balls cannot be avoided and usually lead to microscopic damage of the ball surface. This causes the bearings to run noisier and rougher over time. The bearings failure is a slow process over several tens of hours, so condition monitoring is very well possible.

The load carrying capability of hybrid bearings is typically lower than of steel bearings due to the much higher Young's modulus of the ceramics vs. the steel races. Hence, most of the elastic deformation takes place in the surface of the races, leading to earlier fatigue at the same load compared to steel bearings, where the deformation is approximately evenly shared between balls / rollers and races.

A combination of ceramics balls, machined phenolic or other plastic cages and steel races has been proven very good for ultra high speed bearings. The cage material may limit operating temperature, though.

For higher load applications, steel bearings may be equipped with machined steel cages that are silver plated for better tolerance to operation in marginal lubrication conditions. The typical cause of wear in those bearings is micro-weldings between balls and races which cannot happen in hybrid bearings. For this reason, experiments have been performed with steel bearings where one or two steel balls had been replaced with just a few micrometers smaller ceramics balls. They work like "smootheners", closing micropittings in the race surfaces. Surprisingly, this approach works very well to prolong service life of such bearings.

In general, many bearing configurations will work in high-speed applications, even some that aren't even running "correctly" from a kinematic point of view. So, for example, most of the roller bearings used in small turbine engines / apus don't properly roll but rather perform a mix of sliding /rolling motion, supported by the lubrication oil film. The relatively high clearance in these bearings permit the build-up of an oil film, effectively separating the metal surfaces from each other. The higher the radial load, the more this oil film gets compressed / squeezed out and the faster the roller assembly revolves, moving closer to the "proper" kinematic operation. Even though this sounds very unreliable, this arrangement performs surprisingly well.

The most important this to keep in mind: Bearings made of standard 100Cr6 steel have to be operated at as low temperatures as possible. 160C can be considered the limit for them. If a turbine bearing is supposed to run at higher temperatures than that, special high temperature version have to be chosen. Stainless steels like X30CrMoN15-1 can be used at more than 400C but those bearings usually are speciality items and have their price.

[jugge]

Try make them with twin bearings at back and front if you can, with cage less ceramic design. Ensure lubrication and not overheating the turbine.