When should I decompose a correction mass into two planes?

Decomposition is needed when the balancing measurement gives a single correction (e.g., from a single-plane measurement or a calculated total unbalance) but the rotor has two correction planes. This is common when you know the total unbalance but need to distribute it physically between two available correction locations.

What is the difference between static and couple unbalance?

Static unbalance is a single unbalance force that can be detected without spinning the rotor (it causes the rotor to hang heavy-side down). Couple unbalance consists of two equal and opposite unbalance vectors in different planes, which cancel statically but create a rocking motion during rotation. Most real-world unbalance is a combination of both.

How does CG position affect the decomposition?

The CG position determines the lever ratio for splitting the correction. If the CG is centered between planes, each plane gets 50% of the unbalance. If the CG is offset (e.g., 60/40), the plane closer to the CG gets less correction mass and the farther plane gets more, following the inverse lever rule.

How do I measure the distances to each plane?

Measure the axial distance from the rotor's center of gravity to each correction plane. The CG can be estimated from the rotor geometry or found experimentally by balancing on knife edges. The distances L₁ and L₂ are measured along the shaft axis.

When should I use two-plane vs single-plane balancing?

Use single-plane balancing for disc-like rotors (L/D 0.5), rotors with significant couple unbalance, or any rotor where single-plane balancing doesn't achieve the required quality.

Free Engineering Tool — #011

Correction Mass Decomposition

Split a single correction mass into two balancing planes using the lever rule. Accounts for different plane radii and CG offset.

2-Plane Split Lever Rule Static Decomposition

Total Correction

Total Correction Mass (g)

Correction Angle (degrees)

Plane Distances (from CG)

Distance CG → Plane 1 (mm)

Distance CG → Plane 2 (mm)

Plane Radii

Radius Plane 1 (mm)

Radius Plane 2 (mm)

Quick presets

Results

Mass — Plane 1

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Angle — Plane 1

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Mass — Plane 2

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Angle — Plane 2

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Unbalance — Plane 1

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Unbalance — Plane 2

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Distribution

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Total Unbalance Check

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Static Decomposition (Lever Rule)

A total unbalance at the CG is decomposed into two correction planes using the inverse lever rule:

Then the correction masses are:

ℹ️ Note: In static decomposition, both planes receive corrections at the same angle as the total correction. This only addresses static unbalance. Couple unbalance requires opposite-angle corrections in the two planes. When R₁ ≠ R₂, the entered correction mass is interpreted at the reference radius R_ref = (R₁+R₂)/2, and the total unbalance U = m × R_ref is conserved when split between the planes.

Practical Example

Example — CG Centered, Equal Radii

Given: 20g at 90°, L₁ = L₂ = 150 mm, R₁ = R₂ = 200 mm

U_total = 20 × 200 = 4,000 g·mm at 90°

U₁ = 4,000 × 150 / (150+150) = 2,000 g·mm → m₁ = 2,000 / 200 = 10.0 g at 90°

U₂ = 4,000 × 150 / (150+150) = 2,000 g·mm → m₂ = 2,000 / 200 = 10.0 g at 90°

⚠️ Note: This calculator handles static decomposition only. If the rotor also has couple unbalance, use a full two-plane balancing procedure.