1. Run away from BEP:

Operating outside of the BEP zone is the most common cause of pump shaft failure. Operation away from the BEP can produce excessive radial forces. Shaft deflection due to radial forces creates bending forces, which will occur twice per pump shaft rotation. This bending can produce shaft tensile bending fatigue. Most pump shafts can handle a large number of cycles if the magnitude of deflection is low enough.

2. Bent pump shaft:

The bent axis problem follows the same logic as the deflected axis described above. Purchase pumps and spare shafts from manufacturers of high standards/specs. Most tolerances on pump shafts are in the 0.001 to 0.002 inch range.

3. Unbalanced impeller or rotor:

An unbalanced impeller will produce "shaft churning" when operating. The effect is the same as shaft bending and/or deflection, and the pump shaft of deep well vertical turbine pump will meet the requirements even if the pump is stopped for inspection. It can be said that balancing the impeller is as important for low-speed pumps as for high-speed pumps.

4. Fluid properties:

Often questions about fluid properties involve designing a pump for a lower viscosity fluid but to withstand a higher viscosity fluid. A simple example would be a pump selected to pump No. 4 fuel oil at 35°C and then used to pump fuel oil at 0°C (approximate difference is 235Cst). An increase in the specific gravity of the pumped liquid can cause similar problems.

Also note that corrosion can significantly reduce the fatigue strength of the pump shaft material.

5. Variable speed operation:

Torque and speed are inversely proportional. As the pump slows down, the pump shaft torque increases. For example, a 100 hp pump requires twice as much torque at 875 rpm as a 100 hp pump at 1,750 rpm. In addition to the maximum brake horsepower (BHP) limit for the entire shaft, the user must also check the allowable BHP limit per 100 rpm change in pump application.

6. Misuse: Ignoring the manufacturer's guidelines will lead to pump shaft problems.

Many pump shafts have derating factors if the pump is driven by an engine rather than an electric motor or steam turbine due to intermittent vs. continuous torque.

If the deep well vertical turbine pump is not driven directly via a coupling, e.g. belt/pulley, chain/sprocket drive, the pump shaft may be significantly derated.

Many self-priming pumps are designed to be belt driven and therefore have few of the above problems. However, deep well vertical turbine pump manufactured in accordance with ANSI B73.1 specifications are not designed to be belt driven. When belt driven is used, the maximum allowable horsepower will be greatly reduced.

7. Misalignment:

Even the slightest misalignment between pump and drive equipment can cause bending moments. Typically, this problem manifests itself as bearing failure before the pump shaft breaks.

8. Vibration:

Vibrations caused by problems other than misalignment and imbalance (e.g., cavitation, passing blade frequency, etc.) can cause stress on the pump shaft.

9. Incorrect installation of components:

For example, if the impeller and coupling are not installed correctly on the shaft, the incorrect fit may cause creep. Creeping wear can lead to fatigue failure.

10. Improper speed:

The maximum pump speed is based on the impeller inertia and the (peripheral) speed limit of the belt drive. Furthermore, in addition to the issue of increased torque, there are also considerations for low-speed operation, such as: loss of fluid damping effect (Lomakin effect).