Common Factors of Shaft Breakage in Pumps
Many pump users erroneously attribute shaft failures to the choice of shaft material, assuming that a stronger shaft is the solution. However, this "stronger-is-better" approach often addresses only the symptoms rather than the root cause. While shaft failures may become less frequent, the underlying issues persist.
A small percentage of pump shaft failures can be attributed to metallurgical and manufacturing defects, such as undetected porosity in the base material, improper annealing, or other processing errors. Some failures result from inadequate machining of the shaft, while others occur due to insufficient design margins for withstanding torque, fatigue, and corrosion.
Another critical factor for manufacturers and users is the Shaft Flexibility Factor (ISF = L³/D⁴) in cantilever pumps. This factor indicates the degree of shaft deflection (bend) caused by radial forces when the pump deviates from its Best Efficiency Point (BEP). Here, D represents the shaft diameter at the mechanical seal sleeve (mm), and L is the distance between the impeller outlet centerline and the radial bearing (mm).
1. Operating Away from BEP: Deviating from the allowable operating range of the pump's BEP is one of the most common causes of shaft failure. Working outside the BEP generates unbalanced radial forces, leading to shaft deflection and bending twice per revolution. For instance, a shaft rotating at 3550 rpm will experience 7100 bending cycles per minute. This dynamic creates tensile bending fatigue, which most shafts can withstand if the deflection amplitude (strain) is sufficiently low.
2. Shaft Deflection: Issues related to shaft deflection follow the same principles outlined above. It is advisable to purchase pumps and spare shafts from manufacturers with stringent standards for shaft straightness. Most tolerances for pump shafts are within the range of 0.0254 mm to 0.0508 mm, measured as Total Indicator Reading (TIR).
3. Impeller or Rotor Imbalance: An unbalanced impeller can cause "shaft play" during operation, resulting in effects similar to those caused by shaft deflection or bend. Even though the shaft appears straight when inspected after stopping the pump, the imbalance-induced displacement (strain) remains significant. Proper impeller balancing is crucial for both low-speed and high-speed pumps, as the number of bending cycles decreases but the strain amplitude remains comparable.
4. Fluid Properties: Problems related to fluid properties often arise when pumps designed for lower viscosity fluids are used with higher viscosity fluids. For example, a pump selected for No. 4 fuel at 95°F might later be used for fuel at 35°F, increasing viscosity by approximately 235 centipoise. Increased specific gravity also poses similar challenges. Additionally, corrosion significantly reduces the fatigue strength of shaft materials, making corrosion-resistant shafts preferable in such environments.
5. Variable Speed Operation: Torque and speed are inversely proportional. As the pump slows down, shaft torque increases. A 100 hp pump running at 875 rpm requires twice the torque compared to a 100 hp pump running at 1,750 rpm. Users must verify not only the maximum brake horsepower (BHP) limit for the entire shaft but also the BHP allowed for each 100 rpm increment in the pump application.
6. Misuse: Ignoring manufacturer guidelines can lead to shaft problems. If the pump is driven by an engine instead of a motor or turbine, the power factor of many pump shafts decreases due to intermittent versus continuous torque. Non-direct drive systems, such as belt/pulley or chain/sprocket drives, can significantly reduce shaft life. Many self-priming slurry pumps are designed for belt drives, minimizing issues. However, ANSI B73.1-compliant pumps are not intended for belt drives unless a jackshaft is used. While ANSI pumps can be belt or engine-driven, the maximum allowable horsepower is substantially reduced. Heavy-duty shafts offered as optional accessories can mitigate symptoms when addressing the root cause is impractical.
7. Misalignment: Even slight misalignment between the pump and drive can induce bending moments, often manifesting as bearing failure before shaft breakage.
8. Vibration: In addition to misalignment and imbalance, vibrations caused by cavitation, passing blade frequencies, critical speeds, and harmonics can stress the shaft.
9. Improper Assembly: Incorrect installation of the impeller and coupling (improper fit and clearance) can lead to wear and eventual fatigue failure. Improperly installed keys and keyways can also contribute to these issues.
10. Improper Speed: Based on impeller inertia and circumferential speed, operating the pump at inappropriate speeds can exacerbate shaft problems.
