Blog

Accuracy of Universal Rotor Cutting Machines for Pumps

February 28, 2018

A look at a universal rotor cutting machine takes us neatly into familiar territory. As mentioned previously, the machined helical geometry assigned to this rotating shaft isn't cut arbitrarily. Quite the opposite, the rotor cannot form a fluid-conveying relationship with its stator unless it's machined accurately. This interference fit facilitates the formation of discrete fluid packets. Cut poorly, those isolated fluid packets will leak. Let's talk about rotor-cutting accuracy.

Recutting the Helical Recesses

The rotor geometry of a progressive cavity pump is relatively complex. Back at the manufacturing plant, the newly cast shaft was cut into its final shape by CNC (Computer Numerical Control) machines. Annealing treatments and chrome plating techniques finished the drive shaft, and there was a final inspection to ensure dimensional integrity. Recutting tasks emulate the original production process. Taken back to the repair factory for reconditioning, the rotor is milled and polished. A fast-moving cutting head adapts to the shaft geometry. It skirts the contours, cuts away waste, and follows the pitched threads of the rotors' helical profile.

Reconditioning the Rotor

All kinds of waste find their way onto the helical screw. Solid particles "weld" themselves to the screw surfaces while abrasive cuts undermine the sharply cut screw edges. The rotor looks shabby, but the milling equipment soon corrects this messy image. Rotating axially, the cutting head tilts while it scours, then it progresses to the next thread, never pausing for long. The damage is erased until the screw geometry is restored. Cutting accuracy is paramount here, for a poorly addressed cut is hard to correct. And, if that cut is too deep, it'll be tough to regulate the interference fit, as imposed by the elastomeric stator. In other words, it's easier to slowly conduct a subtractive milling process than it is to implement a process-correcting additive phase.

Adding the Chromium Coating

The helical screw is accurately cut, and it's mating beautifully within the flexible stator. However, the cutting process is abrasive. With that in mind, the protective plating needs to be reapplied. Passed through a continuous quality assessment program, the meticulously cut rotor is now exposed to a rigorously controlled chrome coating operation. The finished product is now clean, polished, dimensionally accurate, and protected from abrasive wear.

Incidentally, it's not always enough, this precision-regulated cutting procedure. An intelligently managed reconditioning process must account for certain processing variables, after all. For instance, if the fluid stream is hot, the helical shaft will expand. The accuracy of that universal rotor cutting machine won't matter if we don't account for this thermal expansion effect.

Contact Alpha Pumps:

Phone:    (03) 9311 7188
Fax:          (03) 9364 9554
Mobile:    0403 030 830
Email:      alphapumps@optusnet.com.au

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