Several Common Factors Causing Pump Shaft Breakage

Jun 01, 2025

Many pump users mistakenly blame the choice of shaft material when the shaft breaks, believing that they need a stronger shaft. But choosing this' stronger, better 'path often only treats the symptoms rather than the root cause. The frequency of shaft failure issues may be low, but the root cause still exists.
A small portion of pump shafts may fail due to metallurgical and manufacturing process issues, such as undetected pores in the matrix material, improper annealing and/or other processing. Some faults are caused by improper shaft machining, while smaller parts fail due to insufficient design margin to withstand torque, fatigue, and corrosion.
For manufacturers or users, another factor is the shaft flexibility system ISF=L3/D4 in cantilever pumps
It represents how much the shaft will deflect (bend) due to radial force when the pump deviates from the design point (optimal efficiency point or BEP). Among them, D is equal to the shaft diameter at the mechanical seal shaft sleeve (mm), and L is the span between the centerline of the impeller outlet and the radial bearing (mm).
1. Working away from BEP: Operating outside the allowable range of the pump BEP may be the most common cause of shaft failure. Working away from BEP will generate unbalanced radial forces. The deflection of the shaft due to radial force will generate bending force every two rotations. For example, a shaft rotating at 3550 rpm will bend 7100 times per minute. This type of bending dynamics can result in axial tensile bending fatigue. If the amplitude (strain) of deflection is low enough, most shafts can handle multiple cycles.

2. Axis bending: The issue of axis bending follows the same logic as the aforementioned axis deflection. Purchase pumps and spare shafts from manufacturers with high standard/specification shaft straightness. Due diligence is prudent. Most of the tolerances for the pump shaft are within the range of 0.0254mm to 0.0508mm, and the measured value is the total indicator reading (TIR).
3. Unbalanced impeller or rotor: If the impeller is unbalanced, the pump will experience "shaft movement" during operation. Its impact is the same as the result of shaft bending and/or deflection, even when the pump is stopped and the pump shaft is checked, the pump shaft will still be straight. It can be said that the balance of the impeller is equally important for both low-speed and high-speed pumps. The number of bending cycles within a given time range decreases, but the amplitude of displacement (strain) (due to imbalance) remains within the same range as the higher velocity coefficient.
4. Fluid characteristics: Typically, issues related to fluid characteristics involve designing a pump for a fluid with (lower) viscosity but capable of withstanding higher viscosity. An example may be simple, selecting and designing a pump that can be used to pump No. 4 fuel at 95 F, and then used to pump fuel at 35 F (with a difference of approximately 235 centipoise). The increase in proportion will lead to similar problems. Please also note that corrosion will greatly reduce the fatigue strength of the shaft material. In these environments, shafts with high corrosion resistance are a good choice.
5. Transmission: Torque and speed are inversely proportional. As the pump decelerates, the shaft torque increases. For example, a 100hp pump with a speed of 875 rpm requires twice the torque as a 100hp pump with a speed of 1750 rpm. In addition to the maximum brake horsepower (BHP) limit for the entire shaft, users must also check the allowable BHP for every 100 rpm limit in the pump application.
6. Misuse: Ignoring the manufacturer's guidelines will result in 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 decrease due to intermittent torque and 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 lowered. Many self-priming garbage pumps and slurry pumps are designed as belt driven, so there are almost no problems. Pumps manufactured according to ANSI B73.1 specifications are not designed to be belt driven (unless using a jack shaft). 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 address the symptom when the root cause cannot be corrected.
7. Misalignment: Misalignment between the pump and the driver, even the slightest misalignment, can cause bending moments. Usually, this problem manifests as bearing failure before the shaft fractures.
8. Vibration: In addition to misalignment and imbalance, vibrations caused by other issues (such as cavitation, blade frequency passing through, critical velocity, and harmonics) can also cause stress on the shaft.
9. Incorrect assembly: Another reason is incorrect installation of the impeller and coupling (incorrect assembly and clearance, whether too tight or too loose). Incorrect fit may lead to wear and tear. Slight wear leads to fatigue damage. Improper installation of keys and/or keyways can also cause this issue.
10. Incorrect speed: According to the inertia of the impeller and the (circumferential) speed limit of the belt drive, there is a maximum pump speed (for example, it is generally agreed that the maximum belt speed for ANSI pumps is 6500 feet per minute). In addition, in addition to increasing torque issues, attention should also be paid to low-speed operations, such as the loss of the Lomax effect.