Causes Why Pump Shafts Can Keep Breaking

December 7, 2018

Helical shaft pumps have gained something of a grand reputation. They just work, and they rarely experience fluid conveyance problems. If we were to dwell on a potential worst-case scenario, though, it would be pump shaft breakage. Granted, surface imperfections do cause trouble, but a break in the power transmission train is far worse. Answering this frustrating question, we look at metallurgical stress factors.

Helical Stress Factors

A plain cylinder is a geometrically stable construct. Loads propagate throughout the shaft, power flows smoothly, and the pumping equipment functions for days on end. Built as a helical rotor, there's a third dimension working along the shaft like a coiling thread. The screw-like shaft, perhaps carrying a concealed metallic flaw, fractures because material loads become trapped in the grooves of the pump shaft. Back with the plain cylindrical shaft, even if there is a material discontinuity concealed inside the alloy, the rod will continue to spin as long as it's not stopped suddenly by a clog or bearing failure.

Introducing Bearing Failures

Here's a shaft pumping error that troubles all powertrains. If the friction-mitigating bearings on a pump shaft dry out, then all of the moving parts are in trouble. The shaft will seize, a profiled shaft edge will suddenly crack, and the equipment will come to a grinding halt. For self-lubricating bearings, poorly applied lubricant films can cause massive problems in pumping equipment, and that's because their oily coatings only coming into existence as the pump shaft gathers speed. If the load and starting currents are excessive, the pump shaft will break before the self-lubricating bearings generate their oily films.

Recruit a Systematic Troubleshooting Approach

Breaking once, in all likelihood the pump shaft fractured because of a weakness in the metal grain. That's unfortunate, but it likely won't happen again. If, like being struck by lightning, the breakage occurs again, though, there's a second problem at work. The bearings are seizing up, the impeller is experiencing shaft-jamming clogs, or there are unusual radial and axial forces causing the malfunctions. Perhaps there's even contamination, a few grains of sand would be enough, getting into the rolling elements of the bearings. To solve that last glitching error, just fit a series of sealed bearings. That should do the trick.

Like a detective, the troubleshooting engineer moves through his checklist. The bearings are just fine, and so is the grain of the metal. Clogs seem to be causing the breaking problems, at least this time around. On conventional pumps, clog remediation may require a secondary mechanism. A material grinding stage will yield good results. For helical structures, swollen stators stop overheated rotors, which break pump shaft linkages. On the trail of the fault, the engineer looks for stator overheating causes.

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