Answer
Fatigue failure in a centrifugal pump occurs due to cyclic, repeated stress concentrations operating below the ultimate tensile strength of the metal over an extended period.
- Primary Causes / Mechanisms:
- Shaft Deflection / Cyclic Bending: Running the pump far off its Best Efficiency Point (BEP) creates high radial thrust forces on the impeller, bending the shaft once per revolution.
- High Vibration & Unbalance: Dynamic unbalance of the impeller (due to uneven wear, cavitation erosion, or partial blockage) induces cyclical stresses on shafts and bearing housings.
- Misalignment: Angular or parallel misalignment between pump shaft and motor shaft subjects the pump shaft to continuous reversed bending.
- Severe Cavitation & Vane Pass Shock: Pressure pulsations from collapsing vapor bubbles or hydraulic shocks as impeller vanes pass the volute cutwater.
- Stress Concentration Points: Sharp keyways, improper shaft radii, or corrosion pitting acting as stress risers where fatigue cracks initiate.