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Metallurgical/Manufacturing Process Flaws

Occasionally there are flaws in the metal or manufacturing process that can
cause shafts to fail prematurely.
When a pump shaft breaks, the natural reaction is to blame the manufacturer.
But in reality, shaft breakage is more often a direct result of the pump's
operating conditions. Only a very small percentage of shaft failures are due to
manufacturing flaws.

Vibration is most commonly caused by cavitation, critical speed, passing vane
frequency, and operating outside the pump's best efficiency point. Pump
bearings begin wear, allowing the shaft to move laterally, causing the shaft to
flex and eventually fail. The vibration harmonics also puts extra stress on the
pump shaft.

Imbalance creates problems while the pump is running, though the shaft will
measure straight if stopped. It's a source of vibration and will reduce machine,
bearing, and mechanical seal life.
Impeller imbalance is caused by:

 New, never balanced impeller

 Trimmed and not balanced impeller
 Foreign object stuck in vanes
 Vanes bent or out of plane
 Balance holes plugged
 Product build up on impeller

Poor installation, pipe strain, extreme belt tension and sheave misalignment
on direct drive pumps all put undue stress on a pump's bearings and shaft.
Note, in a misalignment scenario, the pump's bearings are more likely to fail
before the shaft does. Nonetheless, because misalignment creates bending
moments for the shaft, weakening it over time, we've included it here.
Fluid Properties
When a pump is selected to move lower viscosity fluids, but subjected to
higher viscosity fluids, likelihood of shaft breakage is higher. Be mindful of
fluids that experience changes in viscosity due to temperature, or being at
rest. Fluids/slurries that dewater as they sit, such as paper stock, can also be

Hydraulic Shock
Serious damage can be caused by hydraulic forces. For example, when a
check valve slams shut, interrupting the flow of fluid, a massive shock wave
results. This shock wave reverses flow and travels back downstream. When
the shock wave collides with a pump, assuming the shock is strong enough,
the shaft could bend or break instantly or over time. Read more about water
hammer in our post, "What Is Water Hammer?"

Reverse Flow
What happens when reverse flow causes a pump's impeller and shaft to turn
backwards and the pump suddenly kicks on? You have a stressed or broken
shaft. This scenario occurs when check valves upstream are partially or
completely plugged, or otherwise not functioning properly.
Is there a pump in your facility that frequently breaks shafts? Something
sinister may be at play. Consult an engineer in your area well versed in pumps
and fluid hydraulics.