I don't think that well considered posts such as yours have ever been aggressively shot down so I wouldn't worry too much about that.Richard Casto wrote: ↑07 Mar 2018, 18:32
In one of Wazari’s posts, he talks about the bearing in question being a sealed bearing. Maybe there was no expectation that this bearing would ever come into contact with engine oil (contaminated or otherwise). And that the migration of engine oil back through a scavenge path may have introduced oil into an area in should never have been, but making contact with the sealed bearing, breaking down the seal on the bearing, washing away the desired lubricant and ultimately triggering its failure.
Lastly… given the significant expertise here (of which I am not) and the ease in which theories are aggressively shot down, I am hesitant to post on a regular basis. I feel it may result in less people trying to contribute to the discussion (i.e. don't post unless you know what you are talking about). But I am willing to take the risk as I think common sense and a general engineering knowledge might occasionally provide something of interest (I can only hope).
Richard
So back to bussiness. I think that cavitation has been properly addressed in the previous posts so there's nothing to add to that.
Turbos have traditionally used either hydrodynamic bearings or angular contact bearings.
Hydrodynamic bearings have the advantage of providing very good radial damping at the expense of (arguably) higher friction, higher oil flow requirement, susceptibility to hydrodynamic-induced rotor instability (oil whip) and the inability to acommodate thrust loads.
Angular contact bearings offer next to no damping however they can operate with very little oil supply and take both radial and thrust loads. At high shaft speeds, the balls start climbing out of the inner bearing race under the action of centripetal loads leading to a decrease in bearing radial stiffness (which can lead to the excitation of a rigid rotor mode) and an increase in Herz stresses between the ball and the race. To counteract this effect, the bearings are pre-loaded axially - effectively squeezing the balls between the inner and out races to prevent radial movement at high speeds. In turn this leads to increased friction, increased heat generation and increased oil flow requirement (still significantly lower than that of a hydrodynamic bearing) in order to remove the heat.
A sealed rolling element bearing has no mechanism for removing heat at high shaft speeds and loads. Such a bearing would last very little in a turbo.
The lack of damping in angular contact bearings poses significant problems when the rotor is expected to operate super-critically (above its first natural frequency). This is usually mitigated by squeeze film dampers - effectively a cartridge that holds the bearings and is fixed from rotating but allowed to move radially in an oil fed cavity. The ends of this cavity are normally blocked (sealed) in order to prevent poiseuille flow and provide a higher damping per unit area. I am under the impression that something got lost in translation and Wazari meant sealer dampers rather than sealed bearings. I am waiting for him to clarify.
Of course it is quite easy to see how any problem with the oil supply to the oil cavity would result in a loss of damping and an increase in bearing forces leading to failure.
