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.

Free Engineering Tool

Trial Weight Calculator for Rotor Balancing

Calculate the recommended trial weight mass for single-plane rotor balancing. Accounts for rotor mass, speed, correction radius, support stiffness, and vibration severity.

Vibromera Method Support Stiffness Vibration Level

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)

—

Rotor Mass (Mr)

—

Trial Radius (Rt)

—

Support Stiffness (Ksupp)

—

Vibration Coefficient (Kvib)

—

Radius in cm (Rt)

—

Speed Factor (N/100)²

—

Trial Weight Formula

The trial weight mass is calculated using a practical engineering formula 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
1	Low (< 2 mm/s)	Machine runs smoothly. Fine-tuning only. Lighter trial weight — otherwise may overpower existing unbalance signal.
2	Moderate (2–4.5 mm/s)	Noticeable vibration. Standard balancing job.
3	Elevated (4.5–7.1 mm/s)	Clear unbalance problem. Typical field balancing scenario. Default choice.
5	High (7.1–11 mm/s)	Significant unbalance. Urgent balancing needed. Larger trial weight OK — vibration is already high.
8	Very high (> 11 mm/s)	Dangerous level. Large unbalance. Heavier trial weight acceptable to ensure measurable vector change.

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)

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: Mt = 166,500 / 1,370.1 = 121.5 g

Result: Use approximately 122 g trial weight at 111 mm radius.

⚠️ Safety Note: An excessively heavy trial weight can cause dangerously high vibration. If the calculated weight seems too large, start with half 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% as trial weight. This Vibromera formula is a practical field shortcut that gives similar results while directly accounting for real-world conditions (support stiffness and current vibration level) that the ISO method assumes are ideal.