Principal Sponsor 13-15 November 2018Rotorua, NZ

Common Causes of Shaft FailureAndrew OuwejanMETLAB LIMITED

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…….

Failure mechanisms of ShaftsTensionCompression (eg buckling)Torsion (twisting)BendingFatigue

Principal Sponsor 13-15 November 2018Rotorua, NZ

Fatigue - Bending fatigue (reverse bending, rotating)- Torsional fatigue- Contact fatigue (wear)- Corrosion

Examples of shaft failuresOverloadSharp keywaysDamaged keyways/keysPoor material/heat treatment

Principal Sponsor 13-15 November 2018Rotorua, NZ

Poor material/heat treatmentSharp cornersStraightening shafts

OVERLOAD

Principal Sponsor 13-15 November 2018Rotorua, NZ

Torsional shear – caused by jamming of driven gear

Principal Sponsor 13-15 November 2018Rotorua, NZ

Sharp Keyway and the result of overload

Principal Sponsor 13-15 November 2018Rotorua, NZ

Principal Sponsor 13-15 November 2018Rotorua, NZ

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

Principal Sponsor 13-15 November 2018Rotorua, NZ

8 months later shaft fracture

Damaged key / keyway

Principal Sponsor 13-15 November 2018Rotorua, NZ

Principal Sponsor 13-15 November 2018Rotorua, NZ

Principal Sponsor 13-15 November 2018Rotorua, NZ

Principal Sponsor 13-15 November 2018Rotorua, NZ

Principal Sponsor 13-15 November 2018Rotorua, NZ

Principal Sponsor 13-15 November 2018Rotorua, NZ

Cause of fatigue

Old key not dressed upon installation. When tapped into place caused a stress-raiser / notch in the shaft.

Principal Sponsor 13-15 November 2018Rotorua, NZ

Poor Heat treatment

Principal Sponsor 13-15 November 2018Rotorua, NZ

Induction hardening did not surround circlip groove

Principal Sponsor 13-15 November 2018Rotorua, NZ

Result – Tensile overload through stress-raiser. Unhardened circlip groove unable to sustain load

Principal Sponsor 13-15 November 2018Rotorua, NZ

Gearbox shaft – fracture after 3 months

Principal Sponsor 13-15 November 2018Rotorua, NZ

Principal Sponsor 13-15 November 2018Rotorua, NZ

Ratchet marks show multiple fatigue initiation sites and reverse torque – i.e. shaft rotating both directions

Principal Sponsor 13-15 November 2018Rotorua, NZ

Shaft drawing

Principal Sponsor 13-15 November 2018Rotorua, NZ

Location of fracture

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

Principal Sponsor 13-15 November 2018Rotorua, NZ

Back-log of productDamaged gearbox Lost productivityDown time to replaceClaim on manufacturer$ estimated to be 300-400K

Stress raisers in shafts 1) SHAPEStep changes, Holes, Corners and keyways

2) SURFACE DISCONTINUITIES

Principal Sponsor 13-15 November 2018Rotorua, NZ

Damage, nicks, notches, machining marks, ID Stamps

3) INTERNAL DISCONTINUITIESPorosity, inclusions, cracks

Shaft Repairs

Its cheaper…….

Principal Sponsor 13-15 November 2018Rotorua, NZ

Its cheaper…….

Straightening shafts

Principal Sponsor 13-15 November 2018Rotorua, NZ

Bent Duplex Stainless steel shaft

Shaft was bent in an accidentShaft heated and straightenedCrack tested – no cracks presentPut back into service

Principal Sponsor 13-15 November 2018Rotorua, NZ

Put back into service

The result 1 year later….

Principal Sponsor 13-15 November 2018Rotorua, NZ

WHY?Austenite phase in stainless steel locally work hardening during straightening operation.During service this harder zone gradually got harder until a crack developed.

Principal Sponsor 13-15 November 2018Rotorua, NZ

Questions: Should you straighten a shaft?Should the shaft be regularly crack tested after the straightening operation?

Weld Repair

Principal Sponsor 13-15 November 2018Rotorua, NZ

Principal Sponsor 13-15 November 2018Rotorua, NZ

Principal Sponsor 13-15 November 2018Rotorua, NZ

Principal Sponsor 13-15 November 2018Rotorua, NZ

Sometimes we cant tell why a shaft failed…

Principal Sponsor 13-15 November 2018Rotorua, NZ

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…….