Many pump users mistakenly blame the choice of shaft material when a shaft breaks, thinking they need a stronger shaft. But choosing this “stronger, the better” path is often a symptom-based solution. Shaft failure issues may occur less frequently, but the root cause still exists.
A small percentage of pump shafts fail due to metallurgical and manufacturing process issues, such as undetected porosity in the base material, improper annealing and/or other processing. Some failures are due to improper shaft machining, and a smaller number fail due to insufficient design margins to withstand torque, fatigue and corrosion.
Another factor for manufacturers or users is the shaft flexibility factor ISF=L3/D4 in cantilever pumps
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 equals the shaft diameter at the mechanical seal sleeve (mm), and L is the span between the impeller outlet centerline and the radial bearing (mm).
1. Working away from the BEP: Running away from the allowable area of the pump BEP may be the most common cause of shaft failure. Working away from the BEP creates unbalanced radial forces. Shaft deflection due to radial forces creates bending forces twice per revolution. For example, a shaft rotating at 3550 rpm will bend 7100 times/minute. This bending dynamic creates shaft tensile bending fatigue. Most shafts can handle multiple cycles if the amplitude (strain) of the deflection is low enough.
2. Shaft Bend: Shaft bend issues follow the same logic as shaft deflection above. Purchase pumps and spare shafts from manufacturers with high standards/specs for shaft straightness. Due diligence is prudent. Most tolerances for pump shafts are in the 0.0254mm to 0.0508mm range, measured as the Total Indicator Reading (TIR).
3. Impeller or Rotor Imbalance: If the impeller is unbalanced, the pump will have "shaft play" when it is running. The effect is the same as the result of shaft bend and/or deflection, even though the pump shaft will be straight when you stop the pump and inspect it. It can be said that impeller balancing is just as important for low speed pumps as it is for high speed pumps. The number of bending cycles in a given time frame is reduced, but the amplitude of displacement (strain) (due to unbalance) remains within the same range as with the higher speed factor.
4. Fluid properties: Often, issues related to fluid properties involve pumps designed for one (lower) viscosity but subjected to a higher viscosity fluid. An example might be as simple as a pump selected and designed to pump No. 4 fuel at 95 F and then later used to pump fuel at 35 F (a difference of about 235 centipoise). An increase in specific gravity will cause similar problems. Also note that corrosion will greatly 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, shaft torque increases. For example, a 100 hp pump running at 875 rpm requires twice as much torque as a 100 hp pump running at 1,750 rpm. In addition to the maximum brake horsepower (BHP) limit for the entire shaft, the user must also check the BHP allowed for each 100 rpm limit in the pump application.
6. Misuse: Ignoring manufacturer guidelines will lead to shaft problems. If the pump is driven by an engine, rather than a motor or turbine, the power factor of many pump shafts will be reduced because of intermittent torque 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 significantly reduced. Many self-priming garbage and slurry pumps are designed to be belt driven, so there are few problems. Pumps built to ANSI B73.1 specifications are not designed to be belt driven (unless a jackshaft 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 optional accessories that can correct the symptom when the root cause cannot be corrected.
7. Misalignment: Misalignment between the pump and the drive, even the slightest misalignment, will cause bending moments. Often this problem manifests as bearing failure before the shaft breaks.
8. Vibration: In addition to misalignment and imbalance, vibration caused by other problems (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 fit and clearance, either too tight or too loose). Improper fit can lead to wear. Slight wear leads to fatigue failure. Improperly installed keys and/or keyways can also cause this problem.
10. Improper speed: Based on the impeller inertia and the (circumferential) speed limit of the belt drive, there is a maximum pump speed (for example, the maximum belt speed for ANSI pumps is generally agreed to be 6,500 feet per minute). In addition, in addition to the increase in torque problems, low-speed operation should also be noted, such as the loss of the Lomakin effect.
