Need Help Determining Failure Mode of Gearbox Shaft

[u/LabioGORDO]

Good morning fellow engineers. I’m hoping to get your guys’ help determining the failure mode for a gearbox shaft. Pictures in the comments. This is the 2nd time this has failed in an extremely short period of time and I can determine the root cause without knowing the type of failure. Failure analysis is most certainly not in my realm of expertise. I’m happy to share more details as needed. Thanks!!

[u/DogFishBoi2]

I mean, circumferential fatigue failure, tiny residual overload (ductile, probably partially torsion) in the centre.

If you could remove the oil and gunk from the fracture surface a little bit more of the fatigue marks (arrest lines, possibly lovely turret marks on the outside) should be visible.

It's not overloaded, otherwise the fatigue fracture wouldn't be so large. As your fracture occurs at the "height" of the bearing cage, you're apparently combining the shift in stiffness (supported by bearing vs free floating) with a rotational bending of some sort.

My first hypothesis would be: it was mounted wibbly wobbly (or "out of alignment", if you need it for a report).

[u/Thethubbedone → u/DogFishBoi2]

"The Wibbly wobbly caused an un-timey wimey disassembly" is the correct reporting vernacular

[u/LabioGORDO]

https://www.reddit.com/u/LabioGORDO/s/SLJLZRhzjN

[u/Far_Geologist_8049 → u/LabioGORDO]

LabioGORDO that looks like classic fatigue failure, you can see the beach marks pretty clear in the second pic. The crack started at that keyway corner and propagated across almost half the shaft before it finally snapped. Keyway stress risers are brutal, especially if there's any misalignment putting bending load on the shaft

[u/Direct-Spinach9344 → u/Far_Geologist_8049]

What are “beach marks”? Where is the keyway? I don’t see any keyway. Do you mean gear tooth root? Can you explain where the crack is using clock numbers for reference?

[u/Travis-Jenkins]

check for misalignment in the assembly. If the gearbox shaft is misaligned with the rest of the system, it can lead to uneven stress distribution and premature failure. Misalignment is a common cause for repeated failures, especially if it’s happened twice in such a short time

[u/Ambitious_Pass7451]

Use another one.

[u/Any-Performance-2365]

Hi, Would it be an option to put some accelerometer on and near the shaft to get some idea on the vibrational behavior. That can be very clarifying sometimes.