Re: [CAD_CAM_EDM_DRO] magnetic spindle bearings

Elliot Burke wrote:

>Has anyone looked at magnetic spindle bearings?
>IBAG ( http://www.ibag.ch/e/index.shtml makes these things, they claim
>stiffness similar to ball bearings, 200,000 rpm, up to 125 HP (maybe not all
>at the same time).
>Evidently there is servo action to adjust the magnetic field and keep the
>spindle centered. One could imagine capacitive sensors, servo circuitry,
>and coils to push the spindle back to center made as part of the motor.
>Capacitive sensors have been made good to nm, and 200,000 rpm is only 3000
>Hz, not a very high speed for analog circuitry.
>
>

This is really complicated stuff. Especially when adapted to high speed
rotating machinery.
Air bearings would be a LOT easier to make. Using barrel laps and ring
laps to bring the
bearing and the shaft, respectively, to correct size is sufficient to
get VERY accurate fits.
If you can get clearance down to about .0002" per side, you get an
amazingly stiff bearing.
Air bearings in the 1 -2" diameter range routinely get stiffness of 1
million Lbs/inch
deflection. That means a 1 Lb load deflects the bearing only one
micro-inch. This is
MUCH stiffer than ball or typical oil-journal bearings. Needless to
say, an air
bearing has VERY low friction, and it doesn't need micro-inch sensors,
power amplifiers
servo loops, etc.

>air bearing-> high accuracy & speed, limited stiffness
>

If you check the literature, when designed for good radial stiffness,
air bearings do VERY well!

>magnetic-> high accuracy, speed, and stiffness. Cost is way up there!
>
>

Yes, I'm sure the last sentence is right!

Jon