On-site balancing of fans and smoke exhausters: service visits across Portugal

The smoke exhauster shakes so hard the whole flue hums, and the boiler can only be stopped on the night shift. The dust-extraction fan started hitting harder after the impeller was cleaned than it did before. We come to your site and balance the impeller in its own bearing supports, with no rotor removal and no dismantling the ductwork, measuring before and after. We're based in Vila Nova de Gaia near Porto, with service visits across Portugal.

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

Symptoms: what usually brings people to us

Fans generate more balancing requests than any other type of equipment. The reason is simple: a large impeller, an open flow path, and dirty air. Imbalance here is almost never a built-in defect — it builds up during operation.

Residual imbalance isn't just shop-floor noise. It's a rotating force that loads the bearings, the impeller-to-shaft fit, and the weld seams around the clock, right alongside belt tension and rotor weight. The bearing service life calculated to ISO 281 doesn't account for it unless someone built it in. So a machine that starts out just humming usually ends up needing new bearing units within six months.

• Humming and shaking increased after cleaning the impeller, replacing blades, or repairing the housing
• Vibration is noticeable by hand on the bearing housing; the duct, ductwork, or roof under a roof fan hums
• Bearings have started running hot, grease darkens faster than usual, seals leak
• A rhythmic knock is audible once per revolution, amplitude jumped without any change in operating mode: looks like a chunk of buildup broke off
• Bearing housing and foot bolts keep loosening, frame gussets crack, the foundation grouting is breaking up
• Route vibration readings show the level climbing from one round to the next, even if the absolute value is still within limits
• The machine fails acceptance after a repair, and a report with numbers is needed
• Balancing was already done, but the result didn't hold for even a month

A straightforward trigger for calling us: overall vibration (the combined level from all causes at once) at the bearing housings, in mm/s RMS, has risen by half again relative to the level at which the machine ran quietly. The absolute limits depend on the applicable part and edition of ISO 20816 and on whether the machine sits on a rigid or a flexible foundation, but the trend tells you more than any table.

Sources: ISO 13373-5:2020 · ISO 281:2007

What ventilation equipment we balance on site

All these machines share one thing: a rigid rotor, moderate speed, and an impeller that in most cases can be reached through a hatch. What differs is where the imbalance comes from, and that's what determines the order of work.

If your machine isn't on this list but it has an impeller, two bearings, and an operating speed, chances are we can balance it. Send us photos of the rotor and the supports and we'll give you a straight answer.

What we check before fitting the first weight

The walk-round takes fifteen to thirty minutes and decides whether it's even worth starting the procedure. Half the machines people call "out of balance" need a wrench, not weights. Anything handled by hand is done on stopped, locked-out equipment.

• Fasteners on the bearing housings, casing feet, and frame. Looseness produces a whole comb of harmonics in the spectrum (2x, 3x, and above) and unstable phase — there's nothing to balance in this state
• Impeller-to-shaft fit: play at the key, a worn hub, a loosened taper-lock bushing. Any play means a repair, not balancing
• Blades and impeller: cracks at the root and in weld seams, bent edges, thinning from erosion, torn-off wear plates
• Cleanliness of the impeller on both sides of the blades and inside the shroud. A two- or three-millimetre layer on just one sector already gives a noticeable imbalance and is barely visible from outside
• Belt drive: pulley runout checked with a dial indicator, groove wear, parallelism, tension per the manufacturer's instructions
• Impeller clearances in the volute or housing, signs of rubbing, condition of the inlet guide vanes
• Support structure: cracks in the frame and gussets, condition of the vibration isolators, foundation settling, a flexible platform or roof curb
• Process mode: operating speed, damper position, VFD setpoint. Measuring and balancing has to happen at the mode the machine actually runs in
• Access and safety: how the inspection hatch opens, how start-up is locked out, who holds the key

Then comes the measurement. We fit two accelerometers on the bearing housings and a laser phase sensor on a reflective tape mark on the fan shaft. Then we compare overall vibration against the 1x running-speed component, look at the spectrum (vibration broken down by frequency, which shows how much each cause contributes) and at phase repeatability — the angle that shows where the heavy spot sits on the impeller. Balancing only reduces 1x. Nine millimetres per second of overall vibration with two on 1x isn't a job for weights, and we'll say so up front, not after three trial runs.

Sources: ISO 13373-3:2015

Why balancing a dirty or eroded impeller is pointless

This is the main reason fan balancing doesn't hold. The correction calculation assumes the rotor's mass is fixed and constant. A smoke-exhauster impeller with ash buildup, or an aspiration impeller that keeps wearing away, doesn't meet that assumption.

The rule sounds dull but saves money: wash first, measure second. And clean the whole impeller, not just where buildup is visible. Partial cleaning often makes things worse, because you strip deposit off one sector and end up with more imbalance than you started with.

Checking for a shifting imbalance takes five minutes: two or three consecutive runs at the same speed, comparing 1x amplitude and phase. On a clean, rigid impeller, phase repeats within a few degrees. If it drifts by tens of degrees, the mass isn't fixed, and the cause needs to be found first.

How the on-site work proceeds

1. Request

   Assessing the machine from photos

   You send us the machine type, speed, drive power, impeller diameter, drive type, duct temperature, and photos of the impeller, supports, and inspection hatch. We tell you whether it can be balanced on site, how many correction planes to expect, and what to prepare before we arrive.

2. Walk-round

   Checking the mechanics and the operating mode

   On site we start with the fasteners, the impeller fit, the condition of the blades, the belt drive, and the support structure. This is also where we agree on the operating mode we'll measure at and the lockout procedure.

3. Run 0

   Baseline measurement

   Two sensors on the bearing housings, a laser phase sensor on the fan-shaft mark. We record speed, overall vibration, 1x amplitude and phase at each support, and the spectrum. This step decides whether it's imbalance or something else.

4. Marking out

   Blades as fixed positions

   The blades, the factory holes in the back disc, and the gussets form a ready-made position grid. We number them in the direction of rotation, measure the actual mounting radius, and enter it into the software. From then on the instrument gives back not an angle but a position number and a mass, sometimes splitting the mass between two adjacent blades.

5. Trial weight

   Calibrating to your rotor

   We fit a weighed trial mass just as securely as we'll fit the permanent one. A run counts as valid if the 1x amplitude changes by at least 20 to 30 percent, or the phase by at least 20 to 30 degrees. That's how the instrument gets the influence coefficient for your specific rotor-supports-foundation system.

6. Correction

   Fitting the correction weights

   The software outputs masses and positions for each plane. We fix them with a bolt through a factory hole, weld on a plate where the manufacturer allows welding and the zone permits it, or remove metal by drilling in a heavy part of the hub or disc. Magnetic weights stay trial-only.

7. Verification

   Post-balancing measurement and report

   Same mode and same speed as the baseline run. If we don't hit the target on the first pass, trim balancing follows — fine-tuning with small additional weights, usually one or two short iterations. We then record the before-and-after numbers for each support and hand over the report.

The procedure and common mistakes on fans are covered in more detail in our article on how to balance a fan, and the criterion for a valid trial run is covered in our article on trial weights. What matters more here is that all measurements are taken at your actual operating mode, because measuring with the damper partly closed and running with it open produce different vibration.

Sources: Balanset-1A operation manual

One correction plane or two

The number of planes is chosen from the ratio of L to D, where L is the impeller width between possible correction planes and D is the diameter at the weight-mounting zone. Fans have an extra factor: on most machines the impeller is overhung, meaning it sits outside both bearing supports. A weight at the shroud disc affects both supports at once, sometimes the far one more strongly. Hence the typical picture: after correcting in one plane, a noticeable residual remains at the second support.

The L/D rule and how to choose the number of planes are covered in a separate article. The practical takeaway for ventilation machines: anything that isn't a thin disc gets balanced in two planes with a two-channel instrument, as long as both planes are physically accessible. A machine running close to its first critical speed (the speed at which the rotor hits its own resonance) behaves like a flexible rotor, and two planes don't always handle that. Check applicability against the current edition of the relevant standard part.

Sources: ISO 21940-12:2016 · ISO 21940-11:2016

When on-site balancing won't work or won't help

An honest conversation is cheaper than a wasted site visit. Here are the cases where we either decline to balance or suggest something else first.

• No access to the correction plane. The impeller sits inside a sealed, fully welded housing, there's no hatch, and dismantling the ductwork costs more than pulling the rotor. In that case the impeller gets removed and balanced on a machine
• The machine won't hold a stable speed. The VFD drifts, the drive follows the load, the damper gets moved mid-process. The influence coefficient comes out unreliable
• There's a crack, critical blade thinning, or play in the impeller-to-shaft fit. That's a repair job; balancing comes after it
• Overall vibration is many times higher than 1x. That means bearings, looseness, or shaft misalignment are driving the vibration. We'll run the diagnostics and tell you exactly what to fix, but weights on the impeller won't change anything here
• Shaft misalignment at the drive. It's fixed by shaft alignment, and no mass on the impeller will compensate for it
• Resonance in the frame, housing, roof curb, or ductwork. Balancing doesn't fix resonance. What's needed is a change in structural stiffness, speed, or mounting
• Aerodynamic vibration: flow stall at low flow rate, poor inlet conditions, spread in blade-setting angle. The symptoms look similar, but the fix is different
• The impeller keeps getting contaminated or worn down. We can balance it, but the result only lasts until the next cycle. Cleaning needs to be sorted out first
• An explosion-hazard zone in a paint booth, or a manufacturer ban on welding. In that case we work only with bolted fixing through factory holes and metal removal, which sometimes limits the achievable result

There's a separate topic: cases where balancing doesn't help even when none of the causes above apply. Seven such cases are covered in our article on why vibration doesn't drop after balancing, and in the article on choosing between on-site and workshop balancing.

What you get in the end

The result of the work isn't just reduced noise — it's numbers you can attach to a sign-off report, show to management, or compare against six months from now.

• Baseline and residual vibration for each bearing support and each measurement direction, in mm/s RMS, plus the 1x running-speed component separately
• Speed, damper mode, and measurement conditions, so the result can be reproduced on the next round
• Masses, radii, and position numbers of all fitted correction weights
• Spectra and time-domain signal before and after, noting any extraneous components if present
• An assessment of overall vibration by zone under the applicable part of ISO 20816, and a tolerance calculation under the G classes (balance quality grades) if you provide rotor mass and correction radius
• A mechanical findings summary: what we found during the walk-round and what needs fixing before the next shutdown
• Saved influence coefficients for your machine. On a repeat balancing of the same fan, trial runs are no longer needed, and the next visit is shorter

Balancing is carried out by the engineers who design and manufacture the Balanset instruments and use them on-site themselves. We work with the Balanset-1A: two accelerometers, a laser phase sensor via a reflective tape mark, a two-channel USB module, and software on a laptop. The instrument calculates the correction in one and two planes using the influence coefficient method, works in fixed-position mode by blade, calculates drilling-based metal removal, assesses tolerance by G class, and keeps an archive for reports. The same instrument can be bought and used to balance on your own.

Sources: ISO 20816-1:2016 · Balanset-1A manufacturer specification

Price and how to book a visit

vibration diagnostics with a report costs EUR 300 per unit, balancing adds from EUR 250, and the minimum invoice for a visit is EUR 500. The final amount depends on the number of rotors, the number of correction planes, how far the site is, and whether diagnostics are needed before balancing. The calculator on our website gives an exact figure for your machine.

A practical tip: if there are several fans, smoke exhausters, or aspiration impellers on site, group them into one visit. The sensors and the mark are already in place, the method is already dialled in, and the second machine costs substantially less than a separate visit.

• Machine type and purpose: smoke exhauster, centrifugal fan, blower, aspiration impeller, cooling-tower fan
• Impeller operating speed, drive power, drive type: direct, through a coupling, or belt
• Impeller diameter and width, approximate rotor mass if known
• Ambient temperature and whether the atmosphere is aggressive or explosion-hazardous
• How the inspection hatch opens and whether there are factory holes for weights on the back disc or housing
• Photos of the impeller from both sides, the bearing supports, the drive, and the frame
• Whether the impeller is clean or needs washing, and when it was last cleaned
• Your shutdown window and who on site is responsible for lockout

We're based in Vila Nova de Gaia near Porto, with service visits across Portugal. Send a request with answers to the list above, and we'll tell you straight: whether your machine can be balanced on site, whether diagnostics are needed first, or whether the impeller will have to come off. If your data shows weights won't help, we'll say so before the visit, not after.

Frequently asked questions

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