Radial Deflection Of The Pump Shaft

There are multiple causes of a radial deflection of the rotating shaft from the pump centerline. They include:

  • Operating off the best efficiency point (BEP) causes the shaft to deflect in a radial direction. The deflection is normally 60° or 240° from the pump cut water, measured in the direction of shaft rotation if you are using conventional Francis Vane impellers with a specific speed between 1500 and 4000. Other specific speed numbers deflect in the same manner, but in a different axial direction.
  • Dynamic unbalance of the rotating assembly is a continual problem. Impeller wear and damage from solids in the pumpage contribute to the unbalance.
  • A bent shaft. Many shafts are distorted during the bearing and seal removal process.
  • A non-concentric shaft sleeve.
  • A non-concentric mechanical seal attached to a sleeve or shaft.
  • Misalignment between the pump and its driver. The coupling installed between the pump and its driver cannot compensate for this misalignment.
  • Pipe strain. Either physical or thermal.
  • Thermal growth in a non-centerline pump design.
  • Vibration will cause radial deflection. Vibration includes cavitation and water hammer.



  • On February 16, 2018