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.

Decomposition is needed when:

You have a single-plane balancing result but the rotor has two correction planes
The calculated total correction needs to be distributed physically
You’re converting a single-plane measurement to a two-plane correction

This is common for rotors where the unbalance is primarily static (disc-like rotors) but two planes are available for correction.

Static unbalance — a single unbalance vector that displaces the principal inertia axis parallel to the rotation axis. Both planes get corrections at the same angle.

Couple unbalance — two equal and opposite unbalance vectors that tilt the principal inertia axis. The planes get corrections at opposite angles (180° apart).

Most real rotors have a combination (dynamic unbalance) requiring full two-plane balancing.

The CG position determines the lever ratio:

CG centered (L₁ = L₂): 50/50 split
CG closer to Plane 1 (L₁ < L₂): Plane 1 gets less mass, Plane 2 gets more
CG at Plane 1 (L₁ = 0): All correction goes to Plane 1

The formula: m₁ = m_total × L₂/(L₁+L₂) — the plane farther from CG gets more of the share.

Measure axially along the shaft from the center of gravity to each correction plane:

Use rotor drawings for CG location, or find CG experimentally on knife edges
Correction plane = the axial location where you attach weights
L₁ = distance from CG to Plane 1
L₂ = distance from CG to Plane 2

For symmetrical rotors, the CG is at the geometric center.

Guidelines:

Single-plane: L/D ratio < 0.5, disc-like rotors, primarily static unbalance
Two-plane: L/D ratio > 0.5, long rotors, significant couple unbalance
Two-plane: When single-plane doesn’t achieve the required quality
Two-plane: Rotors near or above critical speed

When in doubt, two-plane balancing is always more thorough.

The procedures described on this page were developed and field-tested by the Vibromera team using the Balanset-1A, a portable dual-channel balancer and vibration analyzer for on-site rotor balancing.