Static and dynamic balancing: understanding the difference
The two are often confused, yet they correct different physical effects and call for distinct methods. Here is why a machine tool spindle almost always needs two-plane dynamic balancing.
Static unbalance: a fault detectable even at rest
TLDR: It exists when the rotor's centre of gravity is offset from its axis of rotation. It shows without spinning the part — the heaviest point swings downward — and is corrected in a single plane.
Static unbalance is defined by an offset between the rotor's centre of gravity and its geometric axis of rotation. Its distinctive feature, and the origin of its name, is that it can be found without setting the part in rotation: placed on very low friction supports — knife-edge ways, for instance — it lets its heaviest point swing downward, revealing where the fault sits.
In rotation, this unbalance generates a constant centrifugal force perpendicular to the axis, whose magnitude depends on the mass involved, its distance from the axis and the square of speed. Its practical advantage: it is corrected in a single plane. Wherever material is added or removed along the axis, what matters is that quantity and radial position offset the centre of gravity shift.
Dynamic unbalance: a phenomenon that only appears in rotation
TLDR: An unbalance moment that survives a perfect static correction, when the rotor's axis of inertia is neither coincident with nor parallel to its axis of rotation. Very common on elongated parts, it demands correction in two planes.
Dynamic unbalance is subtler. It can survive a perfect static correction, when the rotor's principal axis of inertia — the one about which mass is theoretically distributed symmetrically — is neither coincident with nor parallel to the real geometric axis of rotation.
This is the most common configuration in the field, particularly on an elongated part such as a spindle: two unbalance masses can sit at different positions along the axis, oriented so as to cancel out statically — the rotor sits perfectly still on knife-edge ways — while generating in rotation a force couple that makes the real axis wobble.
Correcting it means measuring the centrifugal forces in two distinct planes along the axis, on a balancing machine where the part actually turns, then applying an independent correction in each.
Why spindles almost always need two-plane balancing
In practice, nearly every rotor met in the field, and machine tool spindles first among them because of their elongated geometry, carries an unbalance that is dynamic rather than purely static. Balancing a spindle therefore never stops at a static check: it goes through a measurement on a balancing machine, in real rotation, with detection in two separate planes, to pin down the nature and position of the unbalance before correcting anything.
How a dynamic balancing measurement actually runs
The measurement is taken in two planes, with the spindle mounted on a dedicated balancing machine, generally suspended or on precision bearings. As the force paths turn with the part, the signal is sinusoidal: its amplitude gives the scale of the unbalance, its phase — angular position against a fixed reference on the spindle — says where to correct.
The permissible residual unbalance is then set by the target quality grade, factoring in rotor mass and service speed. Those two parameters read straight off a standardised chart to give the maximum tolerated value for compliance. The ISO 21940 grades in detail.
Frequently asked questions
Can a part balanced perfectly statically still vibrate in rotation?
Yes, and it is very common. A purely dynamic unbalance is not detectable by a static test: it only shows in real rotation, on a balancing machine.
Do all spindles need two-plane balancing?
In practice, the great majority do. Their elongated geometry means the unbalance is almost always dynamic in nature, which requires correction in two planes.
How long does dynamic balancing of a spindle take?
It depends on the scale of the initial unbalance and the grade targeted. A routine balance takes a few tens of minutes on a dedicated machine, excluding preparation and disassembly.
Can a part be balanced without a dedicated balancing machine?
A purely static unbalance can be detected roughly without specialised equipment. Reliable, measurable dynamic balancing cannot: it requires a balancing machine.
Do you have the equipment needed for dynamic balancing?
Yes. Dynamic balancing is built into our overhaul process for spindles and motor spindles.
Professional dynamic balancing of your spindles
Our workshop measures and corrects unbalance on a dedicated machine, to the grade your application actually calls for — not to a generic catalogue value.
Our teams are available from our sites in Beaurepaire (France) and Lussery-Villars (Switzerland).
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