Bearings/Splines
Gentlemen
I've yet to read the new links...... Will take some time later on. In the Navy, it's called an "Even Strain". Any rotating assembly can lose its stress/containment v/v a vibration. In other words, at particular frequencies, a Shaft can "Float" in its bearings. This is not good at all, for it allows chaotic and random (intermittent) 're-acquisition/loss' of constant bearing loads, and extreme heat and wear. It also beats the Oil out of its proper position at the Shaft/Bearing effacement.
Essentially then (as the result of Float), an independent Shaft (And its coupling) are "free" of containment in highly engineered clearances. Note for example the position of the Spline coupling (IP) short of the Plane of its thrust bearings. This allows out of 'bearing boundary' chatter, this geometry has puzzled before.
Think 'False Bearing', and a situation in which the Engine's shafts beat their mountings to a 'pulp'. Vibrations are thoroughly covered of course, but unusual "Novelties" ?? As in a transient Resonance at certain load, a relaxation of "Even Strain", a certain RPM, and Thrust value.
As to "Aft Migration" of the IP Stub shaft (aft shaft), this happens when Splines are worn beyond a certain value such that continued Resonance rapidly fails the Tooth/Trough pairs. The position of the Splines out of direct and balanced (Planar) position relative to the Bearing #2 can instantly aggravate the rigid couple into a disconnect (wear through) of the coupling.
The likely genesis of the vibration causing the Float would be the Fan.
Static balance of the Fan I'm sure is demanding. The Fan itself has issues (well addressed) in dynamic thrust that can unbalance the equation, and shift exterior and intense loads into the core.
Last edited by bearfoil; 2nd Feb 2011 at 16:18.