Balance: A Tiny Eccentricity Becomes an Enormous Force

A smear of paint on one side of a fan, or a fastener replaced with a slightly heavier one, is enough to put a rotor out of balance. Not because the mass is large, but because centrifugal force rises with the SQUARE of the speed. Double the speed and an unbalance produces four times the force. Which means a rotor that is perfectly acceptable at 500 rpm can be destructive at 3000, from exactly the same small amount of extra mass in the wrong place. It also means balancing is not optional above a certain speed, and that the tolerance tightens as speed rises. A balance grade is always quoted WITH a speed, and a rotor balanced for one is not balanced for another.

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The square law, in numbers

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Static and dynamic unbalance are different faults

STATIC unbalance is a single heavy spot. A rotor on frictionless supports will roll until that spot is at the bottom, which is how it is found without spinning anything, and it is corrected in one plane. DYNAMIC unbalance is two heavy spots on opposite sides at opposite ends. The rotor balances perfectly at rest — the two cancel — and wobbles violently when it spins, because they form a couple that tries to tumble it. So a rotor that passes a static check can still be badly out. Anything long relative to its diameter, and anything fast, needs balancing in TWO planes, which means a machine that can measure phase as well as amplitude.

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Unbalance has a signature: once per revolution

Unbalance vibrates at exactly the rotating frequency — once per revolution, 1X — and it is mostly radial. That signature is what distinguishes it from everything else on a machine. MISALIGNMENT shows strongly at twice per revolution, 2X, and often has a large axial component. A LOOSE foot shows many harmonics. A failing bearing shows high frequencies unrelated to shaft speed. So the frequency tells you which fault you have before anything is dismantled. Even without an analyser: vibration that rises smoothly with speed squared is unbalance, and vibration that appears suddenly at one particular speed is a resonance and is a different problem entirely.

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Correct by removing metal, or by adding it deliberately

Drill or grind material away at the heavy spot, or add a weight opposite it. Removing is usually preferred where the rotor allows it, because an added weight is something that can come off at speed. Whatever is added must be POSITIVELY retained — bolted, welded or fitted in a designed slot. A balance weight held on with adhesive that departs at 3000 rpm does not merely restore the original unbalance; it doubles it and becomes a projectile. Then re-measure. Balancing is iterative: the first correction is an estimate, and two or three rounds are normal. Stopping after one because the vibration is better is how a rotor ends up permanently mediocre.

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