10 Common Reasons of Pump Shaft Break

Many pump users mistakenly blame the choice of shaft material when the shaft is broken, believing that they need a stronger shaft. But choosing this "stronger, better" shaft is often a symptom rather than a permanent cure. The shaft failure problem may occur less frequently, but the root cause still exists.

A small number of pump shafts will fail due to metallurgical and manufacturing process issues, such as no pores detected in the matrix material, improper annealing and/or other unsuitable processing. Some failures are due to improper shaft machining, and other failures are due to insufficient design margins to withstand torque, fatigue, and corrosion.

For manufacturers or users, another reason is the shaft flexibility of the cantilever pump ISF=L3/D4. It indicates how much the shaft will deflect (bend) due to radial force when the pump deviates from the design point ( best efficiency point or BEP).

  • Work away from BEP: Deviating from the permitted area of the pump BEP may be the most common cause of shaft failure. Working far from the BEP will produce unbalanced radial forces. The deflection of the shaft due to radial forces generates bending forces, twice per revolution. For example, a shaft rotating at 3550 rpm will bend 7100 times per minute. This kind of bending dynamics will produce axial tensile bending fatigue. If the amplitude of deflection (strain) is low enough, most shafts can cope with multiple cycles.

  • Shaft bending: The shaft bending problem follows the same logic as the above-mentioned shaft deflection. Purchasing pumps and spare shafts from manufacturers with high standard/specification shaft straightness,due diligence is prudent. Most tolerances of the pump shaft are in the range of 0.0254mm to 0.0508mm, and the measured value is the total indicator reading (TIR).

  • The impeller or rotor is unbalanced: If the impeller is unbalanced, the pump will produce "shaft movement" during operation. The effect 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 low-speed pumps and high-speed pumps. The number of bending cycles within a given time frame is reduced, but the amplitude (strain) of the displacement (due to imbalance) remains in the same range as the higher velocity coefficient.

  • Fluid characteristics: Generally, problems related to fluid characteristics involve pumps designed for a (lower) viscosity fluid but withstand a higher viscosity. An example may be simple. The pump selected and designed can be used to pump No. 4 fuel at 95 F, and then used to pump fuel at 35 F (a difference of about 235 centipoise). An increase in the proportion will cause similar problems. Please also note that corrosion will greatly reduce the fatigue strength of the shaft material. In these environments, shafts with higher corrosion resistance are a good choice.

  • Variable speed: Torque is inversely proportional to speed. As the pump decelerates, the shaft torque increases. For example, a 100 hp pump at 875 rpm requires twice the torque of a 100 hp pump at 1,750 rpm. In addition to the maximum brake horsepower (BHP) limit for the entire shaft, the user must also check the allowable BHP per 100 rpm limit in the pump application.

  • Misuse: Ignoring the manufacturer's guidelines will cause shaft problems. If the pump is driven by an engine instead of an electric motor or a steam turbine, many pump shafts power factor will decrease because of intermittent torque and continuous torque. If the pump is not directly driven (via coupling), such as belt/pulley or chain/sprocket drive, the shaft may be significantly lowered. Many self-priming waste pumps and slurry pumps are designed to be belt driven, so there are almost no problems. Pumps manufactured in accordance with ANSI B73.1 specifications are not designed to be belt driven (unless a jack shaft is used). ANSI pumps can be belt or engine driven, but the maximum allowable horsepower is greatly reduced. Many pump manufacturers provide heavy-duty shafts as an optional accessory to resolve the symptoms when the root cause cannot be corrected.

  • Misalignment: Misalignment between the pump and driver, even the slightest misalignment can cause bending moments. Usually this problem manifests as bearing failure before the shaft breaks.

  • Vibration: In addition to misalignment and imbalance, vibration caused by other problems (such as cavitation, passing blade frequency, critical velocity and harmonics) can also cause stress on the shaft.

  • Incorrect assembly: Another reason is that the impeller and coupling are incorrectly installed (incorrect assembly and clearance, regardless of whether it is too tight or too loose). Incorrect fit may cause wear. Slight wear causes fatigue damage. Incorrectly installed keys and/or key ways can also cause this problem.

  • Incorrect speed: Based on the impeller inertia 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 6,500 feet per minute). In addition to the problem of increased torque, attention should also be paid to low-speed operation, such as the loss of the Lomakin effect.

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