What is a trial weight in rotor balancing?

A trial weight (also called a test weight or calibration weight) is a known mass temporarily attached to a rotor at a known radius during single-plane balancing. By measuring the vibration change caused by the trial weight, the balancing instrument calculates the correct permanent correction mass and angle. The trial weight is removed after the measurement run.

How do I choose the right trial weight size?

The ideal trial weight should produce a measurable vibration change without overloading the rotor or bearings. A common engineering guideline is to use 5% to 10% of the permissible residual unbalance (per ISO 21940) divided by the correction radius. Too small a trial weight may not produce a reliable reading; too large may cause excessive vibration or safety hazards.

What happens if the trial weight is too heavy?

An excessively heavy trial weight can cause dangerously high vibration during the trial run, potentially damaging bearings, seals, or the rotor itself. It may also exceed the linear range of the vibration sensors, leading to inaccurate balancing results. Always start with the calculated minimum (5% of U_per / r) and increase only if the vibration change is insufficient.

Where should I place the trial weight on the rotor?

Place the trial weight at the correction plane — the axial location where permanent balance weights will be added. For the radial position, attach it at the maximum available radius to minimize the required mass. Common attachment methods include bolting to existing holes, using magnetic weights, or taping weights to the rotor surface. Ensure the trial weight is securely attached and cannot detach during rotation.

Why use 5–10% of permissible unbalance for the trial weight?

The 5–10% range is a practical engineering guideline that balances two requirements: the trial weight must be heavy enough to produce a clearly measurable vibration change (signal-to-noise ratio), but light enough to avoid excessive vibration. Using permissible unbalance as the reference ensures the trial weight is scaled appropriately for the rotor's mass, speed, and balance grade.

How much centrifugal force can a trial weight safely create?

A common field-balancing guideline is that the centrifugal force of the trial weight, F = m·r·ω² (mass in kg, radius in m, ω = 2πN/60 in rad/s), should not exceed about 10% of the rotor weight (W = Mr·9.81 N). Higher forces risk bearing damage, sensor overload, or the weight detaching. This calculator caps its recommendation at that 10% force level automatically. The goal of the trial run is a measurable but safe response — typically a 20–30% change in vibration amplitude or a 20–30 degree phase shift.

Free Engineering Tool

Trial Weight Calculator for Rotor Balancing

Calculate the recommended trial weight mass for single-plane rotor balancing using an empirical field formula. Accounts for rotor mass, speed, correction radius, support stiffness, and vibration severity — and automatically caps the result so the trial-weight centrifugal force stays below 10% of the rotor weight.

Empirical Vibromera Method Support Stiffness Vibration Level 10% Force Cap

Rotor Mass (Mr) (kg)

Rotor Speed (N) (RPM)

Installation Radius (Rt) (mm)

Support Stiffness (Ksupp) (0.5–5.0)

Vibration Level (mm/s RMS)

Quick presets

Results

Recommended Trial Weight (Mt)

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Trial Weight Centrifugal Force (F)

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Rotor Mass (Mr)

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Trial Radius (Rt)

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Support Stiffness (Ksupp)

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Vibration Coefficient (Kvib)

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Radius in cm (Rt)

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Speed Factor (N/100)²

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Trial Weight Formula

The trial weight mass is estimated using an empirical field formula (based on Vibromera balancing experience, not derived from ISO 21940) that accounts for support conditions and vibration severity:

Mt — trial weight mass (g)
Mr — rotor mass (g) — enter in kg, converted to grams internally
Ksupp — support stiffness coefficient (0.5–5.0)
Kvib — vibration level coefficient (0.5–3.0) — derived from measured vibration in mm/s
Rt — trial weight installation radius (cm) — enter in mm, converted to cm internally
N — rotor speed (RPM)

Support Stiffness Coefficient (Ksupp)

This coefficient accounts for how the machine support structure affects the vibration response to unbalance:

Ksupp	Support Type	Description
5.0	Very rigid	Massive concrete block, stiff steel structure. Vibration barely changes with unbalance — need heavier trial weight (high Ksupp).
4.0	Rigid	Concrete foundation, stiff pedestal. Typical for large pumps and compressors.
2.0–3.0	Medium	Standard industrial mount, baseplate on concrete. Most common situation for fans, motors, and general machinery.
1.0	Flexible	Spring mounts, rubber isolators. Machine vibrates freely — lighter trial weight sufficient (low Ksupp).
0.5	Very flexible	Suspended mount, soft isolators, balancing jig/cradle. Maximum vibration response — lightest trial weight.

Rule of thumb: Rigid supports (Ksupp = 4–5) “absorb” vibration, so you need a heavier trial weight to produce a measurable change. Flexible supports (Ksupp = 0.5–1) amplify the response, so a lighter trial weight works.

Vibration Level Coefficient (Kvib)

This coefficient reflects the current vibration severity of the machine before balancing:

Kvib	Vibration Level	Condition
0.5	Good (≤ 1 mm/s)	Very smooth running. Use a light trial weight so the already-low vibration signal is not overpowered.
0.8	Good (1–2 mm/s)	Smooth running. Fine-tuning only. Light trial weight.
1.0	Acceptable (2–3 mm/s)	Noticeable but acceptable vibration. Standard balancing job.
1.2	Acceptable (3–4.5 mm/s)	Moderate unbalance. Typical field scenario.
1.5	Elevated / High (4.5–11 mm/s)	Clear, significant unbalance. The most common field-balancing case — the default range.
2.0	Dangerous (11–18 mm/s)	Large unbalance, urgent balancing. Heavier trial weight OK — vibration is already high.
2.5	Dangerous (18–28 mm/s)	Severe unbalance. Heavier trial weight acceptable to ensure a measurable vector change.
3.0	Extreme (> 28 mm/s)	Extreme vibration. Inspect the machine before balancing; heaviest trial-weight band.

Why This Formula Works

The formula Mt = Mr × Ksupp × Kvib / (Rt × (N/100)²) captures the key physics:

Heavier rotors need heavier trial weights (linear with Mr)
Higher speeds generate more centrifugal force per gram, so less trial weight is needed (inverse square of N)
Larger radius means more moment per gram, so less weight needed (inverse of Rt)
Stiffer supports need more weight to produce detectable vibration change (higher Ksupp = 4–5)
Flexible supports amplify the response, so less weight is needed (lower Ksupp = 0.5–1)
Higher existing vibration means larger existing unbalance — proportionally larger trial weight (higher Kvib)

Centrifugal Force Safety Cap

The empirical formula alone can suggest a mass that is unsafe at speed — especially with high Ksupp and Kvib values. That is why the calculator always checks the centrifugal force the trial weight would generate:

F = m × r × ω² , ω = 2πN / 60

F — centrifugal force of the trial weight (N)
m — trial weight mass (kg)
r — installation radius (m)
ω — angular speed (rad/s), N in RPM

A widely used field-balancing guideline is that this force should not exceed about 10% of the rotor weight (W = Mr × 9.81 N). If the empirical formula suggests a heavier mass, the calculator automatically limits the recommended trial weight to the 10%-of-rotor-weight force level and shows a warning. The centrifugal force of the recommended weight (in newtons and as a percentage of rotor weight) is always displayed in the results.

Practical Example

Example — Centrifugal Fan

Given: Mr = 111 kg = 111,000 g, N = 1111 RPM, Rt = 111 mm = 11.1 cm, Ksupp = 1.0, Vibration = 11 mm/s → Kvib = 1.5

Step 1: Speed factor: (N/100)² = (1111/100)² = 11.11² = 123.43

Step 2: Denominator: Rt(cm) × (N/100)² = 11.1 × 123.43 = 1,370.1

Step 3: Numerator: Mr(g) × Ksupp × Kvib = 111,000 × 1.0 × 1.5 = 166,500

Step 4: Empirical estimate: Mt = 166,500 / 1,370.1 = 121.5 g

Step 5 — force check: ω = 2π × 1111 / 60 ≈ 116.34 rad/s. For 121.5 g at 0.111 m: F = 0.1215 × 0.111 × 116.34² ≈ 182.6 N — that is ≈ 16.8% of the rotor weight (111 × 9.81 ≈ 1,089 N), above the 10% guideline.

Step 6 — safety cap: Mt(max) = 0.10 × 1,089 / (0.111 × 116.34²) ≈ 0.0725 kg = 72.5 g

Result: Use approximately 72 g trial weight at 111 mm radius (capped by the 10% force limit; the raw empirical estimate of 121.5 g would create excessive centrifugal force).

⚠️ Safety Note: An excessively heavy trial weight can cause dangerously high vibration. The goal of the trial run is a measurable but safe response — typically a 20–30% change in vibration amplitude or a 20–30° phase shift. Keep the trial-weight centrifugal force below about 10% of the rotor weight (this calculator enforces that limit automatically). If in doubt, start with half the calculated weight and increase gradually. Always ensure the trial weight is securely attached and cannot detach during rotation.

Comparison with ISO 21940 Method

The classic ISO approach uses balance grade G to calculate permissible unbalance, then takes 5–10% of it (divided by the correction radius) as trial weight. This Vibromera formula is an empirical field shortcut, not an ISO-derived equation; it gives comparable results while directly accounting for real-world conditions (support stiffness and current vibration level) that the ISO method assumes are ideal. The added centrifugal-force cap keeps its recommendations within safe limits even when the machine is already vibrating heavily.