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Turbo shaft failure - what do you think?


Zeus of the Sluice
May 20, 2020
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Click image for larger version Name:	26F76B88-A9B1-4C06-9A01-FA9213FF97C4.jpeg Views:	25 Size:	78.2 KB ID:	351164 Click image for larger version Name:	182D9ECC-8193-4886-B258-DA2D438D9726.jpeg Views:	25 Size:	107.2 KB ID:	351163 7.3 powerstroke with T4 S366 w/ Turbonator VGT/VNT On friends truck posted on fte.

whats yalls opinions of failure mode?
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Sticking VNT leading to turbo chatter (or whatever the correct term is) and eventual failure as a result.

Edit: compressor surge.
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Looks, like a big compressor surge when he steps off the gas at around the 35 sec mark. I'd like to see graphs of the MAP vs the exhaust pressure. On the other pulls he seems to ease off the gas rather than step off which gives the VNT more of a chance to respond. Also if he'd been making a lot of smoke previously that can clog up the VNT again making it slower to respond.
Fatigue fracture. Stress reversals and thermal cycling created micro cracks that slowly propagated over time until strength was reduced enough to cause failure.

Boots were on too tight.
Obvious signs of fatigue cracking, which looks to have initiated at about 7 o'clock in the picture on the outside of the shaft. The progression and origin is consistent with primarily bending load, as is the final failure plane (perpendicular to shaft).

What I'm less sure of is what caused that loading - imbalance is the most likely source of bending load to me, but the only cyclic change is in magnitude and that only with change in compressor shaft speed, which means relatively few cycles of fairly low magnitude - for that to cause failure it would probably have to be significant imbalance and/or significantly higher than design speed. That being said, I am not sure what compressor surging looks like in term of shaft load - I would expect vibration and torsional loading, but I'm not sure if you get enough pressure imbalance across the wheel to cause significant bending load, and with rapid enough change to cause cyclic loading. Also, I would be surprised to see enough load to cause shaft fatigue without also having visible compressor blade damage.
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