Magnetic bearings have probably amassed more hours of contemplation among engineers than they have actual hours of running time. Their ability to operate unlubed without wear has always been held back by their high costs. Limited application, in turn, halts them from drawing much, if any, cost reduction through high volume production.
But the niche product may be coming into its own, says SKF Magnetic Bearings business development manager Todd Reitsma. “Thousands of turbomolecular pumps in the semiconductor industry still use a mechanical bearing solution,” Reitsma says, “but magnetic bearings are taking a bigger market share.”
That market share presently hovers around 20 percent, he adds. What’s making them increasingly attractive to turbomolecular pump users, besides their cleanliness and lengthy replacement intervals, is their ability to
control vibration energy, Reitsma explains. Keeping vacuum pump vibration out of the manufacturing of silicon wafers leads to better production numbers.
SKF magnetic bearings use inductive sensing to monitor shaft displacement directly. Coils resembling motor stator windings generate magnetic forces which hold a rotor on center or nudge it back into position after an
excursion. Each quadrant is one coil, and they can be energized in pairs to counter the gyroscopic effect of pushing on a rotating body and having it move in a direction 90 degrees away from the applied force. The bearing control loop updates at a 10 kHz frequency.
Shafts in the bearing region wear silicon-iron-steel laminations that withstand the heat generated by ever changing flux better than a bare shaft alone would. The speed limit for a rotating shaft supported in magnetic
bearings is determined by the mechanical stress these laminations feel.
|Hyperspin spindles (left) incorporate radial and thrust bearings in a complete assembly that includes motor, shaft, and controller. Adaptive Vibration Control (right) improves semiconductor wafer making by damping rotor and housing vibrations.
Unique to the bearings is the way in which stiffness and damping can be varied to accommodate a shaft’s journey up to and through its critical speeds. A 10,000 rpm compressor with a 7,000 rpm first resonance, for example, can be made to operate through the bearing’s digital controls with compliant bearings at the critical speed and stiffer bearings at the running speed. “Stiffer bearings beyond the resonance frequency are more robust to gas process changes,” Reitsma explains.
Of course, gas bearings compete with magnetic bearings particularly when supporting light shafts at high speeds, Reitsma concedes. But, gas bearings can’t work in a vacuum environment, he adds. The future’s promise for magnetic bearings may come with the development of high speed direct drive motors for pipeline compressors. Eliminating any gearing could also eliminate any need for an oil system, giving magnetic bearings an edge over hydrodynamic bearings.
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