Balancing the shaft


Not having access to a dynamic balancer, I used the tried and true method described by Wren in the instruction manual. Both the compressor (not shown) and the turbine wheel were balanced in this fashion.

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.

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.

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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