Static balancing finds and removes the heavy side of a rotor by resting its shaft on two parallel, level steel bars so that it rolls until the heavy side hangs at the bottom, then adding a small weight on the light side or removing metal from the heavy side until it stays at rest in any position. The lightest weight that makes a balanced rotor turn on the stand sets how fine the balance can be, so test the stand with a known trial weight before trusting it.
Part of Centrifuge Building
Static balancing corrects the offset of a rotor’s center of mass from its axis, measured at rest by letting gravity find the heavy side.
Gravity Does The Measuring, So The Stand Must Be Level And Hard
The carrier article gives the method: mount the rotor on knife edges (two parallel, level steel bars), the heavy side rolls to the bottom, add a small weight (tape, putty or a setscrew) to the light side, and repeat until the rotor sits still in any position. The method works because a rotor whose center of mass is off the axis has a torque from its own weight whenever the offset is not directly below the axis.
What You Need
- Two hardened steel bars with smooth, straight, level top edges, set parallel, with a gap that the rotor’s journals can span; hardened and polished edges roll with less friction
- A spirit level or a plumb line to level both bars in both directions
- The rotor on its own shaft, with the journals clean
- Putty, small weights, or a setscrew, and a scale that reads to 0.1 g
- A felt-tip marker or chalk
Method
Step 1: Level the bars, along their length and across, with the level on both bars.
Step 2: Place the shaft journals on the bars and let the rotor roll freely to rest. Mark the lowest point on the rim; that is the heavy side.
Step 3: Add weight to the opposite point, or remove metal at the marked point (drilling a shallow hole in non-critical material, filing), in small steps.
Step 4: Repeat, rotating the rotor to four starting positions 90° apart each time. If it comes to rest at the same point of the rotor every time, it is still heavy there; if it rests at different points, it is nearly balanced.
Step 5: Finish when it stays put in every position.
Worked Example: How Good Is The Stand?
A balanced rotor on knife edges will still stop in random places if friction holds it, so friction sets a floor on the balance you can reach. Measure it.
- With the rotor at rest on the bars, stick a small known weight (for example 0.1 g) at the rim and release. If the rotor turns to bring the weight down, the stand responds to 0.1 g at that radius. Reduce the weight until the rotor stops responding; the smallest weight that makes it turn is the stand’s limit.
- Convert to residual unbalance: weight × radius. If a 0.2 g trial weight at 75 mm still turns it and 0.1 g does not, the stand resolves unbalance to about 0.2 g × 75 mm = 15 g·mm.
- Compare with what the rotor needs. For the 3 kg, 8,000 RPM bowl in Bowl Construction, the balance grade used there allows about 22.6 g·mm of residual unbalance. A stand that resolves 15 g·mm is only just good enough; one that resolves 2 g·mm leaves a comfortable margin.
The grade is a general-knowledge standard and the 3 kg mass an assumption, so use this as a guide. If your stand is coarser than the rotor needs, improve the edges (harder, smoother, truer) or accept a lower speed.
Limits Of Static Balancing
It corrects the offset of the center of mass, not the couple. A rotor with a heavy spot at the top at one end and the bottom at the other balances statically and still wobbles at speed. For anything longer than a disc, follow with Dynamic Balancing.
Common Mistakes
- Unlevel bars. The rotor rolls downhill and you add weight for no reason.
- Dirty or dented journals. The rotor sticks.
- Balancing without the parts that rotate with it. A bowl balanced without its disc stack is not balanced with it; see Disc-Stack Design.
- Correcting with tape. It peels at speed.
Safety
A weight added to balance is a loose mass at speed
The putty or setscrew you add to correct the rotor becomes a projectile if it comes off at 62.8 m/s. Use a clamp, a screw or a weld, and check every correction weight before a run. Never spin the rotor on the knife edges. They are a measuring stand, not a bearing, and the rotor will leave them.
What’s Next
- Learn next: Dynamic Balancing — balancing at speed
- Combine with: Blade Balancing — the same stand on a wind rotor