Unbalance is the most common source of vibration in rotating machinery, and it is almost entirely preventable. The physics is a single equation — F = m·r·ω² — but the square on the speed is what makes it matter so much.
Thirty grams is not a small number
Put 30 g at 100 mm radius on a rotor turning at 3000 rpm and it generates about 296 N of rotating force, every revolution, straight into the bearings. Take the same rotor to 6000 rpm and it is nearly 1.2 kN. Nothing about the rotor changed. That is why balancing tolerances tighten so sharply with speed, and why a machine that ran happily for years can shake itself apart after somebody fits a bigger motor.
What a balance grade actually specifies
ISO 21940-11 (formerly ISO 1940-1) defines grade G as the product of permissible eccentricity and angular velocity: G = e × ω, in mm/s. G6.3 covers general machinery, fans and pump impellers; G2.5 covers turbines and machine tool drives; G1 and G0.4 are precision spindle territory. Because G is a product, the permissible eccentricity halves every time the speed doubles — a rotor comfortably inside tolerance at 750 rpm is four times out at 3000 rpm without anything physically changing about it.
Eccentricity, unbalance and why the units look odd
Permissible eccentricity comes out in micrometres, and permissible unbalance in gram-millimetres. They are related by the rotor mass: U = e × m. Conveniently, e in µm is numerically identical to g·mm per kilogram of rotor, which is why the two units get used interchangeably and why it is worth checking which one a specification means.
One plane or two
A thin disc — a fan wheel, a grinding wheel — can be corrected in a single plane. Anything long enough for the unbalance to vary along its length needs two-plane balancing, because two equal masses at opposite ends and opposite sides produce zero net force but a very real rocking couple. As a rule of thumb, if the rotor is longer than about half its diameter, use two planes.
Rigid or flexible
All of the above assumes a rigid rotor, and that assumption holds only below roughly 70% of the first critical speed. Above that the shaft itself bends, the unbalance distribution changes with speed, and a correction made at one speed will not hold at another. Check where you are on the Critical Speed tool before trusting a single-speed balance.
What balancing will not fix
If vibration persists after balancing, it was not unbalance. Misalignment usually shows at twice running speed; looseness produces harmonics; bent shafts and cracked rotors have their own signatures; and a machine run near a resonance will amplify even a well-balanced rotor. Balancing corrects one specific fault, and correcting it well is cheap — work out the allowance on the Rotor Balance tool and the grades on the grade chart.