Fans are the most commonly balanced machines in the field, and for good reason: an impeller picks up imbalance constantly — dust and process residue build up unevenly on the blades, blades erode or corrode, and repairs (a welded patch, a replaced blade) shift mass. The good news is that a fan is also one of the most rewarding rotors to balance in place: access is usually reasonable, the rotor is rigid, and a session from first measurement to verified result typically fits in an hour.
This guide walks through the in-situ procedure: instrument setup, the measurement runs, weight placement, and acceptance. It applies to radial and axial industrial fans, exhausters, blowers, and cooling-tower fans alike.
Before you balance: rule out everything else
Imbalance is only one of several reasons a fan vibrates. Balancing a fan with a failing bearing or loose anchor bolts wastes a shift and helps nothing. Spend ten minutes first:
- Clean the impeller. Caked dust is removable imbalance — wash or scrape it off before adding permanent weights, or the next cleaning will throw the rotor out of balance again.
- Check the blades for cracks, erosion, and loose patches.
- Rock the shaft to feel for bearing play; listen to the bearings at speed.
- Check anchoring — frame bolts, vibration isolators, duct connections.
- Measure first, conclude second. Take a vibration reading in vibrometer mode: dominant vibration at 1× shaft speed points to imbalance; strong 2× suggests misalignment; a raised broadband floor suggests bearings. If the picture is not clearly 1×, see When Balancing Doesn’t Help before going further.
Setting up
Sensors. Mount the two vibration sensors on the bearing housings — as close to the bearings as you can get, on clean, flat metal, axis perpendicular to the shaft. One sensor per bearing support: sensor 1 near the impeller-side bearing (plane 1), sensor 2 near the drive-side bearing (plane 2). A magnetic base needs a snug, paint-free spot; the sensor must not touch anything that moves.
Tachometer. Stick a piece of reflective tape on the shaft or pulley, set the laser tachometer on its magnetic stand 50–300 mm away, and aim the beam at the tape. Two things matter: the stand must not move between runs, and direct sunlight or a bright lamp shining into the optics can corrupt the pulse — shade it if needed.
Planes. For a narrow, disk-like impeller (width much smaller than diameter), single-plane balancing is usually enough. For wide impellers, squirrel-cage wheels, and rotors with two bearing spans, balance in two planes. When in doubt, start with one plane — if residual vibration stays high with a clean 1× signature, switch to two.
The measurement runs
The influence-coefficient method needs two or three short runs at a steady operating speed:
- Run 0 — baseline. Start the fan, let the reading stabilise, record amplitude and phase.
- Run 1 — trial weight in plane 1. Stop, fix a trial weight of known mass at any convenient blade in plane 1, note its position. Run again. The reading must change noticeably — at least 20% in amplitude or phase. If it barely moves, the trial weight is too light: double it rather than continue with weak data.
- Run 2 — trial weight in plane 2 (two-plane balancing only). Move the same weight to plane 2 and repeat.
A reasonable starting point for the trial mass:
mtrial ≈ Mrotor / ( R × (N/100)² )
where M is rotor mass in grams, R the mounting radius in centimetres, and N the speed in rpm. Heavier rotor → more mass; faster rotor → much less.
The software then computes the correction: a mass and an angle for each plane.
Placing the correction weights
The angle convention causes more failed balances than any other single mistake: the angle is counted from the trial-weight position, in the direction of rotation. Counting it against rotation puts the weight in a mirror-image position and the vibration goes up, not down.
Ways to attach the correction mass on fans: welded plates (most permanent), bolted balance weights on a flange, or clip-on weights on blade edges where the design allows. If adding mass at the computed spot is impossible, you can instead remove material 180° opposite — grinding is sometimes the only option on a closed wheel.
Remove the trial weight before the verification run unless the software was told it stays.
Verifying and accepting the result
Run the fan once more. Two acceptance views:
- Vibration severity — compare the bearing-housing reading against ISO 20816 zones for your machine class. Zone A/B is the normal target for a fan returned to service.
- Residual unbalance — the built-in ISO 21940-11 tolerance calculator converts rotor mass, service speed, and the chosen G-grade (G6.3 covers most industrial fans; G2.5 for more demanding duty) into permissible residual gram-millimetres.
If the first correction lands close but not inside the target, run a trim: the software refines the correction from the latest data without starting over. One trim run is routine; needing three or more usually means something else is wrong — loose weights, a moved tachometer, or a fault that is not imbalance at all.
Save the PDF report while the kit is still connected: before/after amplitudes, weights, and angles. The next time the same fan needs attention, those influence coefficients make the job a single-run exercise.
Worked examples
Real jobs with numbers, photos, and timing: a cooling-tower fan stabilised in 25 minutes and a generator cooling fan balanced without dismantling. For choosing the instrument itself, see Choosing a Field Balancer.