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This post was last edited by Reichert-Li Zongsheng on 2022-11-3 08:41. Many pump users wrongly blame the choice of shaft material when a shaft breaks, thinking that they need stronger shafts. But choosing this path of \"the stronger, the better\" often only provides temporary solutions rather than addressing the root cause. Axis failure issues may occur less frequently, but the root causes still exist. A small portion of pump shafts fail due to metallurgical and manufacturing process issues, such as the absence of pores in the matrix material, or improper annealing and/or other processing. Some failures are due to improper shaft machining, while others occur because the design margin is insufficient to withstand torque, fatigue, and corrosion. Another factor for manufacturers or users is the shaft flexibility in cantilever pumps, denoted as ISF=L3/D4. It indicates how much the shaft will deflect (bend) due to radial forces when the pump is operating away from its design point, namely the point of optimal efficiency or BEP. Here, D equals the shaft diameter at the mechanical seal sleeve (in mm), and L is the distance between the centerline of the impeller outlet and the radial bearing (in mm). Figure: Cantilever pump rotor 1. Operating away from the BEP: Operating outside the allowable range of the pump’s BEP is often the most common cause of shaft failures. Working away from the BEP generates unbalanced radial forces. The deflection of the shaft caused by radial forces generates bending forces every two rotations. For example, a shaft rotating at 3550 rpm will bend 7100 times per minute. This bending motion induces axial tensile bending fatigue. If the amplitude of deflection (strain) is low enough, most shafts can withstand multiple cycles. 2. Shaft bending: The issue of shaft bending follows the same logic as shaft deflection mentioned above. Purchase pumps and spare shafts from manufacturers with high standards/specifications for shaft straightness. Due diligence is thorough. Most tolerances for pump shafts range from 0.0254 mm to 0.0508 mm, with the measured value being the Total Indicator Reading (TIR). 3. Imbalance of the impeller or rotor: If the impeller is unbalanced, the pump will experience \"shaft drift\" during operation. Its effect is the same as that of shaft bending and/or deflection; even when the pump is stopped and the pump shaft is inspected, it remains 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 within a given time range decreases, but the amplitude of displacement (strain) (due to imbalance) remains within the same range as that at higher speed coefficients. 4. Fluid properties: Typically, issues related to fluid properties involve pumps designed for a (lower) viscosity but having to handle fluids with higher viscosities. An example might be simple: a pump selected and designed to pump fuel No. 4 at 95°F can then be used to pump fuel at 35°F (a difference of about 235 centipoise). An increase in specific gravity will lead to similar problems. Please note also that corrosion will **reduce the fatigue strength of the shaft material**. In these environments, a shaft with high corrosion resistance is a good choice. 5. Gear shifting: Torque is inversely proportional to speed. As the pump slows down, the shaft torque increases. For example, the torque required for a 100 hp pump operating at 875 rpm is twice that of a 100 hp pump operating at 1,750 rpm. In addition to the maximum braking horsepower (BHP) limit for the entire shaft, users must also check the BHP permitted per 100 rpm limit in pump applications. 6. Misuse: Ignoring the manufacturer’s 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 decrease due to intermittent torque versus continuous torque. If the pump is not directly driven (via a coupling), such as by a belt/pulley or chain/sprocket drive, the shaft height may decrease significantly. Many self-priming trash pumps and slurry pumps are designed to be belt-driven, so there are hardly any problems. Pumps manufactured in accordance with ANSI B73.1 standards are not designed for belt drive (unless a jacking shaft is used). ANSI pumps can be driven by belt or engine, but the maximum allowable horsepower is **reduced**. Many pump manufacturers offer heavy-duty shafts as an optional accessory, which can resolve this issue when the root cause cannot be corrected. 7. Misalignment: Misalignment between the pump and the driver; even the slightest misalignment can result in bending moments. Typically, this problem manifests as bearing failure before the shaft breaks. 8. Vibration: In addition to misalignment and imbalance, vibrations caused by other issues (such as cavitation, the frequency of the passing blades, critical speed, and harmonics) can also exert stress on the shaft. 9. Incorrect assembly: Another reason is the improper installation of the impeller and coupling (incorrect assembly and clearance, whether too tight or too loose). Incorrect fitting may lead to wear. Minor wear leads to fatigue failure. Keys and/or keyways that are not properly installed can also cause this problem. 10. Incorrect speed: Due to the inertia of the impeller and the (circular) speed limitations 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 to the issue of increased torque, attention should also be paid to low-speed operation, such as the loss due to the Lomakin effect.