Rotor-to-Stator Rub: Pump Problems and Remedies

January 9, 2019

For pump repair techs, rotor-to-stator rubbing causes big headaches. The stator is the outside tube, the part of a pump that contains the rotor. So, when there's little clearance between the two mechanical assemblies, the parts rub together. The friction generated by the scraping parts can't be tolerated for long, a catastrophic fault transpires, and an overhaul is imminent. For progressive cavity pumps, stator rub signifies a deeper underlying problem.

High-Quality Pumps Are Immune

At least until they begin their path towards old age, high-quality pump architectures are immune to rotor-to-stator rub. But, and this is an important point, that statement applies to standard rotating devices, the ones that use solid, coil-lined stators. For poorly manufactured equipment, those pumps still use small clearances, but their bearings are manufactured from low-quality materials. The subpar parts wobble and generate finite quantities of side-to-side motion. Ordinarily, this effect could be ignored, but the small space between the rotor and stator doesn't allow for much error. The parts rub, they create abrasive energies, and pump torque is attenuated.

A Unique Helical Pump Issue

On helical pumps, the stator is manufactured from elastomeric compounds. It creates an interference fit that ensures the prompt formation of dozens of discretely travelling packets of semisolid materials. That's one of the headlining features of this pump type, by the way. Unfortunately, flexible stators can experience uncontrolled swelling events. When this issue occurs, the rubberized stator swells inwards until it contacts the helical rotor. That dreaded rotor-to-stator rub hampers pump performance, and problems pile up until the gear can no longer perform its regular duties.

Remedying Rotor-to-Stator Rub

To some degree, quality issues contribute to the problem. Superior helical geometries reduce the rubbing effect. If the internal and external threads are continually rubbing, a clearance adjustment procedure will compensate somewhat. Should the fault linger, a time-consuming overhaul can strip the rotor surface. This action restores the interference fit between the two threaded pump sections. More effectively, though, the engineering team will likely look for the cause, the reason why the elastomeric stator is swelling. Heat effects are common culprits here, as are certain chemical agents. As the chemical or heat inducing factors are diagnosed, their elimination will result in less swelling.

Depending on a stator's material characteristics, its ability to "bounce back," the rubbing effect may disappear altogether. Some monitoring will be required to see whether this is the case. If it's economically feasible, the overstressed stator needs replacing. Also, since the rubbing effect has impacted the surface of the rotor, the protective metal coating will need checking to see if it has been worn away.

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