10 Probable Reasons That a Pump Shaft Breaks
1.Operating Away from BEP: Departure from operating in the allowable region of the pump’s BEP is likely the most common cause of shaft failure. Operation away from BEP creates unbalanced hydraulic radial forces. The deflection of the shaft due to the radial forces creates a bending force that will occur twice per shaft revolution. For example, a shaft rotating at 3,550 revolutions per minute (rpm) will bend 7,100 times per minute. This bending dynamic creates a shaft tensile bending fatigue. Most shafts can handle the high number of cycles if the amplitude (strain) of the deflection is low enough.
2.Bent Shaft: Bent shaft issues follow the same logic as a deflecting shaft referenced above. Purchase pumps and spare shafts from manufacturers that have high standards/specifications for shaft straightness. Due diligence would be prudent. Most tolerances for pump shafts are in the 0.001- to 0.002-inch range with the measurement as total indicator readout (TIR).
3.Unbalanced Impeller or Rotor: An unbalanced impeller will create “shaft whip” while in operation. The effect is the same as if the shaft was bent and/or deflected, even though the shaft would measure straight if you stopped the pump and checked the shaft. It could be argued that balancing the impeller is just as important for slower-speed pumps as faster-speed pumps. The number of bending cycles in a given time frame is reduced, but the amplitude (strain) of displacement (due to the imbalance) remains in the same range as the higher-speed factors.
4.Fluid Properties: Discussions on Newtonian versus non-Newtonian fluids will appear in a future article. Normally, the issue concerning the fluid properties involves a pump that was designed for a fluid of one (lower) viscosity but subjected to higher viscosities. An example could be as simple as the pump was selected and designed for pumping number 4 fuel oil at 95 F and later it is used to pump fuel oil at 35 F (approximated difference of 235 centipoise). Similar issues will result from an increase in specific gravity. Also note that corrosion will significantly reduce the fatigue strength of the shaft material. Shafts with higher corrosion resistance are a good choice in these environments.
5.Variable Speed: Torque and speed are inversely proportional. As the pump slows down, the shaft torque increases. For example, a 100-horsepower (hp) pump at 875 rpm requires twice as much torque as a 100-hp pump at 1,750 rpm. Besides the overall shaft maximum brake horsepower (BHP) limits, users must check the allowable BHP per 100 rpm limits for the pump application.
6.Misapplication: Ignoring manufacturer guidelines will lead to shaft issues. Many pump shafts have a derate factor if the pump is driven by an engine in lieu of an electric motor or steam turbine due to the intermittent versus continuous torque. If the pump is not direct drive (through a coupling) such as belt/sheave- or chain/sprocket-driven, there can be a significant shaft derating. Many self-priming trash and slurry pumps are designed to be belt-driven, so there is little issue. Pumps built for American National Standards Institute (ANSI) B73.1 specifications are not designed to be belt-driven (unless a jack-shaft is utilized). ANSI pumps may be belt- or engine-driven, but the maximum allowable horsepower is greatly reduced. Many pump manufacturers offer heavy-duty shafts as an optional extra, which can address the symptom when the root cause cannot be corrected.
7.Misalignment: Misalignment between the pump and the driver even in the slightest amount contributes to the bending moments. Usually this issue manifests as failed bearings before the shaft will break.
8.Vibration: Vibration from issues other than misalignment and imbalance —such as cavitation, passing vane frequency, critical speeds and harmonics — will cause stress on the shaft.
9.Incorrect Fitting of Components: Another reason is incorrect mounting of impellers and couplings to the shaft (incorrect fits and clearances whether too tight or too loose). The incorrect fits may result in fretting. Fretting wear contributes to fatigue failure. Keys and/or keyways that are not properly fitted also contribute to the issue. This author prefers to hand file the key to fit the key-seat.
10.Improper Speed: There are maximum pump speeds based on impeller inertia and on (peripheral) speed limits for belt drives (for example, the maximum belt speed for an ANSI pump is normally agreed to be 6,500 feet per minute). Additionally, there are cautions for low-speed operation beyond the increasing torque issue—such as the loss of hydraulic dampening effects (Lomakin Effect).
