Q. 

We have a 100 hp end suction pump that has a broken shaft.  What are the causes of shaft breakage?  

A. 

Pump shafts usually break in the fatigue failure mode.  High stress starts a small crack which slowly progresses as each rotation of the shaft causes a fluctuation or reversal of the stress.  The crack becomes larger until the final failure occurs.

The cause of the original crack can be from unexpected bending of the shaft.  Coupling misalignment could be a contributor.  Large radial forces from the impeller when operated near shut off is another possible cause.

The original crack may start from a point of stress concentration such as a shaft keyway.  The sharp corner in the keyway causes a stress riser in the shaft which progresses to a small crack and ultimately shaft failure.

In a similar manner, stress corrosion may begin in grooves or crevices or where impellers are mounted on the shaft.  As corrosion proceeds, the useable area of the shaft is reduced and failure eventually results.

Excess power may also be a cause.  When pumping liquids with high specific gravity, such as sulphuric acid, the normal power required by the pump is increased proportional to the specific gravity.  The shaft may be designed to pump water but high specific gravity can cause an overload.

Finally, the shaft material may not meet the original specification.  Replacement shafts may be procured from non-OEM shops which may not know when special materials are needed.