10 common causes of broken shafts in pumps

Many pump users wrongly blame the choice of shaft material when the shaft breaks, thinking they need a stronger shaft. But choosing this "the stronger the better" is often a temporary solution rather than a permanent solution. Shaft failure issues can occur less frequently, but the root cause still exists.

A small percentage of pump shafts will fail due to metallurgical and manufacturing process issues, such as undetected porosity in the matrix material, improper annealing and/or other process handling. Some failures are due to improperly machined shafts, and smaller parts fail due to insufficient design margins to withstand torque, fatigue and corrosion.

Another factor for the manufacturer or user is the shaft flex system ISF=L3/D4 in the cantilever pump. It indicates how much the shaft will deflect (bend) due to radial forces when the pump deviates from the design point (Best Efficiency Point or BEP). Among them, D is equal to the shaft diameter (mm) at the shaft sleeve of the mechanical seal, and L is the span (mm) between the centerline of the impeller outlet and the radial bearing.

Figure Cantilever pump rotor

1. Work away from the BEP: Operation outside the allowable area of the pump BEP is probably the most common cause of shaft failure. Working away from the BEP creates unbalanced radial forces. The deflection of the shaft due to radial forces creates bending forces, twice per revolution. For example, a shaft rotating at 3550 rpm will bend 7100 times/min. This bending dynamic produces axial tensile bending fatigue. Most axes can handle multiple cycles if the amplitude (strain) of the deflection is low enough.

2. Shaft bending: The shaft bending problem follows the same logic as above for shaft deflection. Purchase pumps and spare shafts from manufacturers with high standard/spec shaft straightness. Due diligence is prudent. Most tolerances for pump shafts are in the 0.0254mm to 0.0508mm range, measured as total indicator reading (TIR).

3. Impeller or rotor unbalanced: If the impeller is unbalanced, the pump will produce "shaft play" when it is running. The effect is the same as the result of shaft bending and/or skewing, even when the pump is stopped and checked, the pump shaft will remain straight. It can be said that the balance of the impeller is equally important for low-speed pumps and high-speed pumps. The number of bending cycles in a given time frame is reduced, but the amplitude (strain) of the displacement (due to unbalance) remains within the same range as for higher velocity coefficients.

4. Fluid Properties: Generally, issues related to fluid properties involve pumps designed for one (lower) viscosity fluid but tolerating higher viscosity fluids. An example might be as simple as a pump chosen and designed to pump No. 4 fuel at 95 F and then again at 35 F (about a 235 centipoise difference). An increase in specific gravity will cause similar problems. Also, note that corrosion will greatly reduce the fatigue strength of the shaft material. In these environments, shafts with higher corrosion resistance are a good choice.

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

6. Misuse: Ignoring the manufacturer's guidelines will cause shaft problems. If the pump is driven by an engine rather than an electric motor or turbine, the power factor of many pump shafts will be reduced because of intermittent versus continuous torque. If the pump is not directly driven (through a coupling), such as belt/pulley or chain/sprocket drive, the shaft may be lowered significantly. Many self-priming waste and slurry pumps are designed to be belt driven, so there are few issues. Pumps built to ANSI B73.1 specifications are not designed to be belt driven (unless a jack shaft is used). ANSI pumps can be belt or engine driven, but the maximum allowable horsepower is greatly reduced. Many pump manufacturers offer heavy duty shafts as an optional accessory that can resolve the symptom when the root cause cannot be corrected.

7. Misalignment: Misalignment between pump and drive, even the slightest misalignment can cause bending moments. Usually, this problem manifests itself as bearing failure before the shaft breaks.

8. Vibration: In addition to misalignment and imbalance, vibration caused by other issues such as cavitation, passing blade frequencies, critical speeds and harmonics can also stress the shaft.

9. Improper assembly: Another cause is improper installation of the impeller and coupling (incorrect assembly and clearance, either too tight or too loose). Incorrect fit can cause wear. Minor wear leads to fatigue failure. Incorrectly installed keys and/or keyways can also cause this problem.

10. Incorrect speed: There is a maximum pump speed based on the impeller inertia and the (circumferential) speed limit of the belt drive (for example, it is generally agreed that the maximum belt speed for ANSI pumps is 6,500 feet per minute). Also, in addition to increased torque issues, low-speed operation, such as the loss of Lockheedin effect, should also be noted.