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A simple aluminum tube
was turned for this exercise. Be sure the bearing outer
diameter in the tube is relatively loose... you don't
want the races to pop apart upon disassembly. The bearings and appropriate
spacer are mounted on the shaft, along with either the
turbine or compressor wheel and nut. I found that by
popping loose the seal from a set of cheap 688 BB and
washing out the grease with mineral spirits, that I had a
shaft which free-wheeled better than the ceramic
bearings. This has the added benefit of keeping the
ceramic bearings free of any contamination prior to use.
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The shaft must be
absolutely free-wheeling, with almost zero drag, for this
to work properly. I occasionally added drops of mineral
spirits to help things along. I also found that spinning
up the shaft to perhaps 5,000 RPM with some shop air, and
holding it there for a minute or so, helped the bearings
seat a bit and allowed even less drag. The tube is placed on a surface
plate and with fingertip pressure gently rocked back and
forth through perhaps 10mm of distance. The heavy portion
will work its way to the bottom. This is marked with a
felt pen, and repeated to be sure it is an actual heavy
spot.
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The turbine wheel has a
balance ring cast into the root of the wheel. This is
slowly ground away, and the process repeated until no
discernable imbalance remains. Be sure to polish the area
up a bit when you get close to minimize any chance of a
stress crack or fracture developing. For the compressor wheel, the back
face is gently filed or sanded, again being careful not
to create any weak spots!
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