Trial (calibration) weight: why it's needed and how to choose it

A fan starts humming after the impeller is cleaned, you arrive with the instrument, fit sensors to the bearing housings, and take the baseline vibration reading. The software shows 1x — the vibration at rotation frequency — its phase, and the spectrum (vibration broken down by frequency), but it doesn't tell you how many grams to fit or where. It's missing one run — the one with the trial weight. Below: why the calculation is impossible without it, how to choose the mass, how to tell whether the trial run worked, and where people go wrong most often.

Updated 27 August 2026 · by AXILINE · Vila Nova de Gaia

What a trial weight is

A trial weight (also called a calibration weight) is a temporary mass you fit to a correction plane for just one run. It doesn't fix the vibration. Its job is to make the machine show how it responds to an added imbalance.

The word “known” is the key here. The weight only does its job when you know three things about it precisely.

• Mass. Weighed, in grams, not “about twenty.” Scales are included with the Balanset-1A.
• Mounting radius. The distance from the rotation axis to the weight, in millimeters. The instrument uses it to convert vibration into imbalance (g·mm) and to check tolerance.
• Angular position. The point the software will later measure the correction weight's angle from. Mark it with chalk or a marker right away.

If you plan to balance this same rotor later using saved coefficients, fit the trial weight at the same angular position as the tachometer's reflective mark. That way the influence coefficients stay valid.

Why the correction can't be calculated without a trial run

After the first run, the instrument knows one thing: the vibration vector at rotation frequency — the 1x amplitude and phase. That's not enough. The software doesn't know the rotor's mass, the stiffness of the bearing housings, how compliant the frame is, the effect of a belt drive, or how much the foundation gives under load. The same 50 g·mm imbalance on a rigid steel base will produce one vibration reading, and on a welded frame with soft vibration mounts, a completely different one.

The trial weight closes that gap. You introduce an imbalance whose magnitude you know exactly (mass times radius) and watch how far and in what direction the vibration vector moves. The ratio of one to the other is the influence coefficient — the specific rotor-supports-foundation system's sensitivity, capturing both the magnitude and the direction of the response.

Formally, it looks like this: α = (V1 − V0) / Ut, where V0 is the 1x vector before the weight, V1 is the vector after, and Ut is the trial weight's imbalance. The software then solves the inverse problem and outputs the correction imbalance Uk = −V0 / α — that is, the mass and angle. The subtraction here is vector subtraction, not arithmetic: 5.4 mm/s after the weight instead of 7.2 mm/s before it doesn't mean things got “25% better.” The phase could have swung by 135°, in which case the change in the vector is larger than either measurement on its own.

A worked arithmetic example, not a site report. Starting point V0 = 7.2 mm/s ∠ 75°. Fit 30 g at a 350 mm radius, get V1 = 5.4 mm/s ∠ 210°. The vector difference ΔV ≈ 11.7 mm/s ∠ 236°, influence coefficient α ≈ 0.39 mm/s per gram, correction mass around 18.5 g at the same radius. The balancing software does this calculation itself and draws a polar diagram, but it's worth understanding the logic — you'll immediately notice when a result looks suspicious.

The three-run method for two planes, and the two-run method for one, only works on a linear system. If the speed, temperature, belt tension, or sensor position changes between runs, the influence coefficient is now describing a different machine.

The 30/30 rule: the criterion for a valid trial run

The trial weight has done its job if the machine responded noticeably. The practical threshold is this: the 1x amplitude changed by at least 20-30% (in either direction, up or down), or the phase shifted by at least 20-30°. Balanset's documentation calls this the 30/30 rule.

The logic is simple. Any vibration measurement has spread: the speed drifts, the flow in a fan varies, the bearing temperature rises. If the weight changes the reading by 5%, you can't tell its effect apart from that noise, and the influence coefficient comes out as essentially a random number. The software will dutifully calculate a correction, you'll fit 40 g instead of the 12 you actually needed, and the vibration will go up.

While you're at it, check the stability of the readings themselves. The 1x amplitude and phase shouldn't change by more than 10-15% over the course of a reading. If they drift more than that, the machine is running close to resonance, and balancing it in that state is pointless.

The Balanset-1A tells you itself whether the trial weight was adequate. That prompt doesn't replace your own read of the numbers: the instrument evaluates the change in the vector, not whether the weight is actually held on securely.

How to choose the mass and radius

1. 1

   Assess where you're starting from

   Look at the baseline 1x and the speed. The heavier the rotor and the lower the speed, the larger the mass you'll need. The higher the speed, the more careful you need to be: centrifugal force grows with the square of the rotation frequency, and a weight that's harmless at 500 rpm creates 36 times the load at 3000 rpm.

2. 2

   Start small

   Take a mass that clearly won't drive the vibration up to a dangerous level. One wasted run is cheaper than a weight flying off. You can always build the mass up later.

3. 3

   Weigh it and enter the actual figures

   Enter the weighed mass in grams and the measured radius in millimeters into the software. Percent mode is convenient for a quick pass, but it costs you the residual-imbalance calculation in g·mm and the ability to save the influence coefficients.

4. 4

   Fasten it at a strong, marked point

   Choose a spot where the weight holds securely and where you can actually fit the correction mass afterward. Mark the point: the software will measure the angle from it.

5. 5

   Make the run at the same speed

   The trial run's speed has to match the baseline run's speed. Wait for a steady operating condition before starting the measurement.

6. 6

   Check the change against the 30/30 rule

   Change is enough — move on to calculating the correction. Not enough — remove the weight, fit a heavier one, enter the new mass, and repeat. Don't try to squeeze an influence coefficient out of readings that barely moved.

Fit the correction mass at the same radius as the trial weight. If that's not possible, recalculate it inversely proportional to the radius, or use the software's function for recalculating weights onto other planes and radii. The correction weight's angle is measured from the trial-weight location, in the direction of the rotor's rotation.

Mounting and safety

A trial weight is only temporary in purpose. In terms of mounting requirements, it's no different from a permanent one: it spins at the same speed and the same radius. A 50 g plate that comes loose on a fan impeller will punch straight through the guard.

• Fasten the trial weight as securely as a permanent one: a bolt, a clamp, a tack weld; a magnet only at low speed and on a clean surface.
• Check clearances: the weight must not touch the guard, the diffuser, guide vanes, or any other stationary part.
• Don't fit the weight to thin sheet metal, a blade of doubtful strength, or a location with a crack.
• Account for the load on the bearing housings: the trial weight adds a dynamic force, and on worn bearings this is noticeable.
• Stay clear of the plane of rotation during the run, and don't leave anyone standing near the guard.
• Stop the machine and wait for it to come to a complete standstill before removing or repositioning the weight. Never reposition it during coast-down.
• Decide in advance at what vibration level you'll abort the run, and don't chase a nice-looking number if the machine is clearly overloaded.

A weight that's too heavy is more dangerous than an extra run. If you're unsure about the mass, fit less and do one more run instead.

Common mistakes

When a trial weight doesn't help

The influence-coefficient method assumes the system is linear and that imbalance is what's causing the vibration. Where that assumption doesn't hold, the trial weight will show you honestly: there's a response, but balancing isn't improving the picture. Building up the mass any further from there is pointless.

You get the first sign even before the trial run. Compare the overall vibration (the total level across all frequencies) with the 1x running-speed component. If the overall level is several times higher than 1x, something else is producing most of the vibration, and balancing won't remove it.

• Resonance. Amplitude jumps around in a narrow speed range, the 1x phase swings by almost 180°, readings drift from measurement to measurement. Change the speed or the foundation-mounting conditions, then balance.
• Loose fasteners and “soft foot.” The system is nonlinear, the influence coefficient changes from run to run. Torque the fasteners and check the supports first.
• Shaft misalignment. 2x is strong in the spectrum, axial vibration is often high. It's fixed with shaft alignment, not weights.
• Bearing defects. Peaks at high frequencies not related to running speed, a raised noise floor. Balancing won't help here.
• Cracks, rubbing, build-up, and uneven wear. Imbalance comes and goes on its own, the result is unstable. Look for a mechanical cause.
• The rotor is flexible at operating speed. A single pair of correction planes doesn't describe its behavior; different methods and a different part of the standard are needed.

A trial run is useful even in these cases. It shows the system's response, and that response often makes it clear that the vibration isn't coming from imbalance. That saves you half a day of trying to chase a number down with weights.

Sources: ISO 20816-1:2016 · ISO 13373-3:2015 · ISO 13373-5:2020 · ISO 21940-12:2016

Who does this, and with what

Everything described here is done by one person with a two-channel instrument. The Balanset-1A gives you two accelerometers for the bearing housings, a laser phase sensor with a tachometer, a USB module, and Windows software. The instrument measures speed, the amplitude and phase of vibration velocity, overall vibration and 1x, and builds the FFT spectrum. The software tells you whether the trial weight was adequate, calculates and stores influence coefficients, splits the weight across fixed positions, proposes drilling, checks tolerance against grade G, and stores the results in an archive for the report. Balancing is done in one or two planes, directly in the machine's own bearings.

Treat the grade-G numbers and the vibration zones in mm/s as a working guide. Before contractual acceptance, check against the applicable part and current edition of the standard, and record the measurement points, the frequency band, the operating mode, and the support type.

If there's no time to work through all this on your own fan or crusher, AXILINE's engineers come to the site and balance it themselves. We design and manufacture the Balanset instruments and use them ourselves on site visits, so the question “what trial weight should I use on this wheel” is routine for us. We start with vibration diagnostics: we check 1x against the overall level and the spectrum, so we're not balancing something balancing can't fix. The instrument can also be bought and learned independently — consulting support is there for that case.

Sources: Balanset-1A manufacturer specification · Balanset-1A operation manual · ISO 21940-11:2016 · ISO 20816-1:2016

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