Shaft breakage


Most shaft breakage is the result of fatigue. High stress starts a small crack that worsens as the shaft flexes. Here are a few causes of the original crack:

  • Stress concentration at a keyway. The sharp corners in the keyway cause a stress riser.
  • A fretted shaft.
  • The shaft diameter reduced under a grease or lip seal installed to protect the bearing.
  • Chloride Stress Corrosion if you are using stainless steel shafts.

The high radial load on the shaft can be caused by

  • Too high an L3/D4 shaft number
  • Operating too close to shut off
  • Starting most centrifugal pumps with the discharge valve all the way open or closed.
  • Shifting to a specific gravity fluid like sulfuric acid can cause an excessive load on the impeller.
  • The shaft may have been purchased from someone other than the original manufacturer (OEM) and does not meet specifications.

You can calculate the hydraulic force on your shaft if you refer to: Shaft bending, S040


  • On February 17, 2018