7 Reasons Centrifugal Pump Shafts Break

What is a centrifugal pump?

A centrifugal pump is a pump that transports different types of fluids by use of centrifugal force. The centrifugal force adds pressure and velocity to the pumped liquid to ensure it reaches the intended destination. The source of power for this pump is either a diesel engine or an electric motor. The pump has a component known as an impeller, which has curved vanes that help generate the centrifugal force. Depending on the type of pump, the impellers could be one or more. Also, the impeller type is crucial as it dictates the fluid the pump can transport.

How does the centrifugal pump shaft enhance fluid pumping?

The shaft is a critical component as it transmits power from the prime mover to the impeller. Without the shaft, the pump cannot work since no power will be delivered to the fluid. Also, if the shaft is broken, pump may not work or it will be inefficient. When the operator turns the pump on, power from the prime mover is transmitted to the pump via the shaft. Due to the available power, the shaft rotates at the same speed (RPM) as the prime mover shaft. The impeller is mounted on the shaft; thus, as the shaft rotates, the impeller also rotates at the same RPM. Impeller rotation provides pressure and velocity energies responsible for moving the pumped fluid.

Figure: Components of centrifugal pump.

Reasons Centrifugal Pump Shaft Break

Manufacturing or metallurgical process flaws

Pump shafts are made of metallic materials. If the material is not processed correctly, it could have metallurgical flaws. Also, such defects could be due to faults during manufacturing of the material used to make the shaft. Flaws cause premature failure. Hardly would a shaft fail due to manufacturing or metallurgical defects. In most cases, shaft failure occurs due to the pump’s operating conditions, as discussed below.


Vibration is one major cause of shaft failure. It mainly occurs due to passing vane frequency, critical speed, operating the pump outside the best efficiency point, and cavitation. Due to vibrations, bearings start wearing out, causing lateral movement on the shaft, and thus, the shaft flexes and finally fails. Also, there are extra stresses on the pump shaft due to vibration harmonics, thus increasing the chances of failure.


Centrifugal pump shaft imbalance causes problems when the pump is in use. Imbalance causes vibration and reduces bearing and mechanical seal life. Common causes of imbalance are:

  • The buildup of foreign materials on the impeller.
  • Trimmed and unbalanced impeller.
  • New and never-balanced impeller.
  • Plugged balance holes.
  • Foreign materials stuck in the vanes.
  • Vanes out of plane or bent.


Pipe strain, poor installation, sheave misalignment, and severe belt tension cause undue stress on the shaft and bearings. If there is misalignment, the bearings will likely fail before the shaft. So misalignment is one of the causes of shaft failure in centrifugal pumps as it causes bending moments on the shaft, making it weak over time and thus causing failure.

Fluid properties

If your pump is designed to transport low-viscosity fluids but you use it for high-viscosity liquids, the shaft will have a high chance of breakage. It is recommended to check on the fluid’s viscosity with temperature changes. Check fluid specifications for the pump, as large solids/slurries increase stresses and thus damage the shaft.

Hydraulic shock

Hydraulic forces cause severe damage. For instance, when a foot valve slams shut, it interrupts fluid flow, causing substantial shock waves. The shock waves result to reverse flow, causing fluid to move downstream. When the pump and shock waves collide, especially when the shock is powerful enough, the centrifugal pump shaft breaks or bends instantly or with time.

Poor assembly

The shaft will likely fail if the coupling and impeller are incorrectly assembled. Incorrect fit also causes wear. Slight wear leads to fatigue damage. Also, the same problem may result from incorrectly assembled keyways and or keys.

Incorrect speed

Depending on the impeller inertia, each pump is designed to operate at a certain speed limit. The pump speed range depends on the mass of the fluid pumped.

Types of shaft misalignment

Parallel misalignment

This misalignment is either vertical or horizontal. Parallel horizontal misalignment occurs when the prime mover shaft is moved away from the pump shaft, which causes the shafts to be parallel to each other on a horizontal plane. On the other hand, vertical shaft misalignment occurs due to the movement of the prime mover away from the pump shaft, making the two shafts parallel to each other on the vertical plane.

Figure: Parallel shaft misalignment.

Angular misalignment

This misalignment is either vertical or horizontal. It occurs when the motor/engine shaft is oriented at an angle to the centrifugal pump shaft.

Figure: Angular shaft misalignment.