The Ultimate magnetic balancer


Wren includes with the mark 2 kit an excellent pamphlet which describes the tried-and-true method of balancing a turbine rotor at home. This has worked well for me... my thrust MW54 has never had any problems with vibration, or bearing damage due to an unbalanced rotor. I know that the use of a magnetic prop balancer in its stock form is NOT adequate, but I decided for fun to experiment and see what could be done to improve upon this technique.

Magnetic prop balancers (Top Flite, and similar) suspend a model airplane propellor between two powerful magnets. The shaft is very free to rotate, being sharpened at both ends, and with only ONE end actually contacting a magnet face, the other hovering magically in the magnetic field. I discovered for propellors it is fine, but for a turbine wheel, the sensitivity is poor. This is due partially to the inability to correctly mount a turbine wheel, and also excess friction, making the turbine wheel "hang up" rather than having the heavy portion fall to the bottom.

 

  To remedy this, I cannibalized the magnets from the Top-Flite balancer and carefully lapped the surface to perfection on a surface plate, using 2000 grit silicon carbide paper. I then turned a 1/4" shaft of O-1 steel to a perfect 60 degree taper on one end. The taper is the pivot point for the balancing action, and rests against the lapped magnet face. This was a bit better than stock, but still not sensitive enough.
  The real breakthrough came when I was adjusting the distance between the magnets. I found that as they got closer together, the forces on the rotating point decreased. I figured with a fine micrometer control, I could bring the magnets slowly closer, to the point where the shaft was just barely secured at the pivot. An additional .0005" would cause the shaft to "snap" to the far magnet. In other words, the shaft is on the ragged edge of transitioning from one magnet face to the opposite. In this state, frictional forces were extremely low, and I found I could easily balance just the shaft, with no turbine wheel!
Shown here is the 60 degree cone which forms the pivot, resting against the lapped magnet face.
I began to balance the shaft before attempting any balancing of the turbine wheel. Note the slight grind marks here on the shaft, courtesy of a Dremel tool. Once the shaft was balanced as well as could be expected...
...I "chucked" the wheel on the shaft. Heavy spots were very easy to find, without any of the stickiness and errors of the ball bearing/tube method. The sacrificial ring was ground away where appropriate.
In the end, I had a turbine wheel with no detectable imbalance!

Please note, I still recommend the Wren-approved balancing method. In fact, I consider this wheel only superficially balanced. It will be mounted on the rotor shaft, and fine-balanced using the proven method.

But what this did do was probably 98% of the balancing required, with the final 2% to be done only as a completed rotor unit.

Post Script: 23 April 2002 - The turbine has been tested, and the balance was excellent. Please let me stress that the balancing done here was only preliminary, and after the rotor was completely assembled, it was further balanced using the standard, Wren-described method. There is no doubt in my mind, though, that this balancing did the vast bulk of the work; all that remained, when assembled as a complete rotor, was some touch up.

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