Common Causes of Shaft Failure Andrew Ouwejan METLAB LIMITED · 2018. 11. 26. · Location of...
Transcript of Common Causes of Shaft Failure Andrew Ouwejan METLAB LIMITED · 2018. 11. 26. · Location of...
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Principal Sponsor 13-15 November 2018Rotorua, NZ
Common Causes of Shaft FailureAndrew OuwejanMETLAB LIMITED
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Material – Proven material, new, defective, heat treatment, correct material?
Operation – Overloaded, History, maintenance
Design – Sharp corners, small cross-section?
Principal Sponsor 13-15 November 2018Rotorua, NZ
Design – Sharp corners, small cross-section?
Environment – Corrosive, abrasive?
Somewhere, someone knows something…….
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Failure mechanisms of ShaftsTensionCompression (eg buckling)Torsion (twisting)BendingFatigue
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Fatigue - Bending fatigue (reverse bending, rotating)- Torsional fatigue- Contact fatigue (wear)- Corrosion
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Examples of shaft failuresOverloadSharp keywaysDamaged keyways/keysPoor material/heat treatment
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Poor material/heat treatmentSharp cornersStraightening shafts
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OVERLOAD
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Torsional shear – caused by jamming of driven gear
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Sharp Keyway and the result of overload
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Principal Sponsor 13-15 November 2018Rotorua, NZ
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Milled keyway – very common manufacturing method Risk area - Sharp corners in keyway causing stress raiserPump received a shock load and no one thought to check the shaft.8 months later shaft fracture
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8 months later shaft fracture
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Damaged key / keyway
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Principal Sponsor 13-15 November 2018Rotorua, NZ
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Cause of fatigue
Old key not dressed upon installation. When tapped into place caused a stress-raiser / notch in the shaft.
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Poor Heat treatment
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Induction hardening did not surround circlip groove
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Result – Tensile overload through stress-raiser. Unhardened circlip groove unable to sustain load
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Gearbox shaft – fracture after 3 months
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Ratchet marks show multiple fatigue initiation sites and reverse torque – i.e. shaft rotating both directions
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Shaft drawing
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Location of fracture
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Cause of failure:Sharp corner in shaft caused stress raiser which resulted in reverse torque fatigue failure.
Cost of failure:Instant stoppage in the production lineBack-log of product
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Back-log of productDamaged gearbox Lost productivityDown time to replaceClaim on manufacturer$ estimated to be 300-400K
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Stress raisers in shafts 1) SHAPEStep changes, Holes, Corners and keyways
2) SURFACE DISCONTINUITIES
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Damage, nicks, notches, machining marks, ID Stamps
3) INTERNAL DISCONTINUITIESPorosity, inclusions, cracks
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Shaft Repairs
Its cheaper…….
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Its cheaper…….
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Straightening shafts
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Bent Duplex Stainless steel shaft
Shaft was bent in an accidentShaft heated and straightenedCrack tested – no cracks presentPut back into service
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Put back into service
The result 1 year later….
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WHY?Austenite phase in stainless steel locally work hardening during straightening operation.During service this harder zone gradually got harder until a crack developed.
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Questions: Should you straighten a shaft?Should the shaft be regularly crack tested after the straightening operation?
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Weld Repair
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Sometimes we cant tell why a shaft failed…
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Material – Proven material, age, defective, heat treatment, correct material?
Operation – Overloaded, History, maintenance – Repaired?
Design – Sharp corners, small cross-sections?
Environment –
Principal Sponsor 13-15 November 2018Rotorua, NZ
Environment – Corrosive, abrasive?
Consider crack testing your shafts!!!
Remember that somewhere, someone knows something…….