Misconception: Normal oil level (60%+) ≠ effective lubrication. Simply replacing bearings cannot solve "critical friction".

Truth: Deep disassembly revealed that a certain series of heavy-duty models under maximum tolerance combination, the male rotor engages the female rotor by approximately 35μm. Combined with asymmetric oil supply at the suction end and thermal deformation, the sealing line collapses without an oil film.

Conclusion: True repair must address both "reconstructing the tolerance chain from the manufacturing side" and "adding critical protection on the operational side".

Background

A heavy-duty screw compressor in industrial refrigeration duty seized after prior “oil starvation” diagnoses and bearing replacements. Sight-glass oil level had repeatedly been reported above ~60%. The recurring pattern forced a deeper teardown rather than another parts swap.

Symptoms

Symptoms included rising vibration, metallic contact signatures near the sealing line, and eventual lock-up. Oil level looked acceptable; operators therefore discounted lubrication as a cause.

Initial hypothesis

Initial hypothesis: oil starvation from return path or pump issues. That hypothesis fits low oil level—but it conflicts with a persistently “healthy” sight glass.

Measurements

Deep disassembly and metrology on an anonymized heavy-duty series showed that under a maximum tolerance stack-up, male-to-female rotor engagement reached approximately 35μm. Asymmetric suction-end oil supply and thermal deformation were documented qualitatively from wear maps; plant-specific continuous oil-flow meters were not available for publication.

Root-cause chain

Tolerance stack-up + thermal deformation + asymmetric oil feed → local sealing-line contact without a stable oil film → critical friction → seizure. “Oil level OK” masked “oil film collapsed.”

Corrective actions

Actions split into manufacturing-side tolerance-chain reconstruction for the affected series and operational-side critical protection (monitoring / interlocks / oil-feed symmetry checks). Bearing replacement alone was rejected as a closed corrective action.

Verification

Verification required cleared contact signatures after rebuild, confirmed oil-feed symmetry checks, and a defined observation window without recurrence. Exact vibration spectra and serial metrology sheets remain with the OEM/owner and are not reproduced here.

Prevention checklist

  • Do not equate sight-glass level with oil-film integrity under heavy-duty screw contact risk.
  • When seizures recur after bearing swaps, demand a tolerance + thermal stack review.
  • Check suction-end oil-feed symmetry after major overhauls.
  • Keep manufacturing and operations corrective actions paired.

Engineering boundary

This is an engineering postmortem. It does not assign legal liability, does not publish OEM drawing numbers, and does not replace OEM service procedures or local codes.

Anonymization note

OEM brand, exact model string, plant name, and customer identity are withheld. The ~35μm engagement figure is retained because it is central to the lesson and was established in the teardown metrology for the anonymized series.