On-site rotor balancing, in the machine's own bearing housings

The machine shakes, bearings wear out before their time, fastenings loosen on their own. Removing the rotor, taking it to a machine and putting it back together is expensive and slow. We come to the machine and balance the rotor right where it stands: in its own bearing housings, at running speed, without disassembly. Based in Vila Nova de Gaia, near Porto, we travel across Portugal.

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

When to call: the signs that point to unbalance

Unbalance rarely shows up suddenly. Vibration usually climbs over weeks: at first only the operator notices, then bearings start running hot, then welds crack and anchor bolts work loose. The earlier you call, the cheaper the visit works out, and the less likely you'll need a repair before you can even think about balancing.

Don't watch the absolute figure, watch the rise relative to what's normal for that machine. A machine that always ran at 2 mm/s and has climbed to 6 mm/s needs attention sooner than one that's lived at 5 mm/s unchanged for years.

• Vibration rose after a repair: a new impeller, a rewind, hardfacing, new knives, hammers or blades.
• Hum and shaking get stronger as speed rises and drop noticeably on coast-down, that is, during free deceleration after the machine is switched off.
• The bearing housing visibly shakes under your hand, and the mounting bolts loosen on their own.
• Bearings and seals fail more often than scheduled, and the shaft runs hot for no visible reason.
• You can see product build-up on the rotor, blade erosion, or a missing plate or rivet.
• The instrument shows that most of the level comes from a peak right at running speed.
• Cracks have appeared in the frame's welds, anchor bolts have snapped, the foundation grout has loosened.

Balancing only reduces the running-speed component of vibration, the part that occurs right at the rotation frequency. If the overall level is several times higher than that component, something else is causing the vibration, and weights on the rotor won't remove it. We check this in the first hour of work, before you pay for a correction.

Sources: ISO 13373-3:2015 · ISO 20816-1:2016

What balancing in the machine's own bearings is, and how it differs from a workshop

On a balancing machine, the rotor sits on the machine's own soft bearings and is spun by the machine's own drive. The machine measures the part's inherent unbalance and brings it within a precision grade. This is the right method when the rotor is new, when it's coming out for repair anyway, or when there's no access to the host machine.

On site we measure something different. The rotor turns in its own bearings, on its own frame, with its own drive and its own load. The instrument sees the response of the whole "rotor-bearings-foundation" system, and the correction is calculated for exactly that. That's why an on-site result more often matches the vibration level you actually see on the machine afterwards.

The two methods don't compete with each other. A rotor balanced on a machine often still shows vibration once it's installed back in the host machine: the fit, the key, the half-coupling, the frame's stiffness all play a part. On-site trim balancing closes exactly that gap. Precision grades for residual unbalance and vibration assessment on stationary parts are covered by different standards; we record the applicable part and edition in the report.

Sources: ISO 21940-11:2016 · ISO 20816-1:2016

What rotors we take on

The name of the machine is secondary to us. We look at the rotor's shape and where its unbalance comes from: that's what determines the number of correction planes and how the mass is attached.

Separate pages for each equipment type are collected in the equipment section: they cover access, typical correction planes and limitations for fans and induced-draft fans, pumps, crushers and mulchers, compressors, centrifuges, spindles and lines. We also take on non-standard and home-built assemblies; the criterion is the five conditions in the section below.

How the work runs on site

1. 01

   Enquiry and starting data

   You describe the machine in your own words and send photos of the rotor and the bearing housings, the speed, the drive type, and the rotor's approximate mass. We reply on whether on-site balancing is feasible, how many correction planes we're planning for, and how long the visit will take. At this same step we agree the window for the runs and the site's access requirements.

2. 02

   Inspection and mechanical check

   On site we work through a condition checklist: fastening tightness, soft foot (a mounting point that doesn't sit flush on the frame), bearing play, how the wheel sits on the shaft, rubbing, the condition of the frame and grout. Balancing doesn't fix mechanical faults, and on a faulty machine readings stop being repeatable. This is where it's decided: we balance today, or you repair first.

3. 03

   Initial reading

   We fit two accelerometers on the bearing housings, radially, close to the bearing, on cleaned flat spots. We stick on a reflective marker and aim the laser phase sensor. We bring the machine up to running speed and record the overall level, the running-speed component and the phase (the angle showing where the heavy spot sits on the rotor) at each bearing. We log the operating mode: speed, load, temperature, damper position.

4. 04

   Assessing the cause

   We compare the overall level with the running-speed component, and check the spectrum (vibration broken down by frequency) and the time waveform. A dominant peak at running speed means balancing is appropriate. A strong second harmonic, a peak at twice running speed, points to misalignment or looseness. A comb of harmonics points to looseness and rubbing, high-frequency peaks outside multiples of running speed point to bearings. If resonance is suspected, we check how amplitude and phase behave during run-up and coast-down.

5. 05

   Trial runs

   We fit a trial weight of known mass at a known radius in the first correction plane (the rotor cross-section where the weight will sit) and run it. For two planes, we move the weight to the second one and repeat. We consider a run usable if the running-speed amplitude changed by at least 20-30% or the phase by at least 20-30 degrees. If the response is weaker, we increase the weight and repeat, rather than calculate from readings that barely moved.

6. 06

   Correction

   The software outputs a mass and an angle for each plane. Where the rotor has ready-made mounting points, such as blades or flange holes, it shows a position number straight away instead of an angle: no protractor needed, and no way to get the direction of measurement wrong. We add mass with washers and plates on a bolt or by welding, and where welding isn't allowed, we remove it by drilling to the instrument's calculation.

7. 07

   Check reading and report

   We repeat the measurement at the same points and the same mode. If the level has dropped but hasn't reached the target, we add a small correction to the weights already fitted. We write up a report: figures before and after, weight masses and mounting locations, the measurement mode, what we found mechanically, and what's worth doing before the next season.

We work with the Balanset-1A, our own instrument: two channels simultaneously, a laser phase sensor, running-speed component and phase, overall level, spectrum and time waveform, tolerance calculation by precision grade, fixed-position and drilling modes, and a measurement archive. Saved influence coefficients (the recorded response of the machine to a trial weight) mean a repeat visit can skip the trial runs.

Sources: Balanset-1A manufacturer specification · Balanset-1A operation manual

One correction plane or two

The number of planes is set by the rotor's shape, not by a wish to save a run. Compare the rotor's working length with its diameter in the zone where the weight goes. A short disc, where the length is less than roughly half the diameter, is usually corrected with one mass. Anything elongated along its axis needs two planes, or couple unbalance will remain: you'll reduce vibration at one bearing and raise it at the other.

Speed matters too. The higher the rotation frequency, the more often two planes are needed, even for comparatively short rotors.

• One plane: pulleys, flywheels, brake drums, half-couplings, narrow disc knives, grinding wheels, thin impellers.
• Two planes: wide fan and induced-draft fan wheels, mulcher and crusher rotors, drums and screw conveyors, shafts and cardan shafts, assembled pump rotors, electric motor rotors.
• Two planes regardless of the length-to-diameter ratio, if vibration at the second bearing stays high after a single-plane correction.
• Two planes if the rotor is overhung and both correction planes are physically accessible.

Single-plane balancing means two runs: the initial one and one trial. Two planes means three runs: the initial one and two trials. Then comes the check run and, if needed, one trim run. A second plane costs one extra run and 150 EUR added to the package, and it almost always saves a day of fruitless attempts to make do with a single mass.

Sources: ISO 21940-11:2016

Five conditions for feasibility, and what needs to be sound

Go through these five points looking at your own machine. Five "yes" answers mean we'll almost certainly be able to balance the rotor on site. One "no" means we'll look for a workaround, and if there isn't one, we'll say so before any visit.

• Bearings with no critical play: no knocking, the rotor doesn't drop, axial float is within normal limits.
• Frame, bed and mounts with no cracks, no snapped anchor bolts, no loose bolts and no soft foot.
• The rotor doesn't rub anywhere against the guard, a seal, a grille or neighbouring parts.
• The rotor isn't deformed: radial and face runout are within limits, blades and knives aren't bent from an impact.
• The fit is tight: the wheel, pulley or drum doesn't spin or shift on the shaft.
• Misalignment has been corrected: coaxiality between the unit and its driving machine has been checked, and shaft alignment has been carried out.
• The running speed doesn't coincide with the natural frequency of the frame or the mounts.

If we find a fault, we show it to you and explain the order of work. Misalignment is corrected by shaft alignment, not by weights. Loose fastenings are corrected by tightening. Balancing is done after the repair, otherwise it just masks the level for a few weeks.

Sources: ISO 281:2007 · ISO 20816-1:2016

Fitting and welding work, the report, and what you're left with

Calculating the correction is half the job. The other half is fitting the mass to the rotor securely, or removing it without weakening the part. Who does that work directly affects the price of the visit.

If you have your own welder or fitter on site, the work of fitting the weights is included in the package and adds nothing to the bill. We give the mass, radius and exact location, your specialist fits it. If you don't have your own people, we do the fitting: without welding, plus 150 EUR per rotor, with welding, plus 400 EUR per rotor (a second, welding, specialist).

• A report with figures before and after at each bearing housing, stating the points and direction of measurement.
• The measurement mode on record: speed, load, temperature, damper position.
• The masses and radii of the fitted weights, position numbers or angles, a layout diagram by plane.
• The spectrum and time waveform of the initial condition, if they were used to decide on the cause of vibration.
• A mechanical finding: what was found during the inspection and what's worth fixing before the next season.
• A recommendation on the interval for the next vibration check on this machine.
• Saved influence coefficients, so a repeat visit can skip the trial runs.

Writing up the report is included in the vibration diagnostics. If contractual acceptance requires a specific precision grade, tell us in advance: trimming to G2.5 adds 175 EUR, to G1.0 adds 350 EUR, because it takes extra trim runs and more precise mass fitting.

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

When balancing won't help

An honest refusal is cheaper than a wasted visit. Balancing only removes the part of vibration that occurs at running speed. It leaves everything else untouched, and sometimes masks it for a few weeks.

• Resonance in the structure, frame, platform or the bearing mount itself. This calls for changing stiffness, mass or speed, not the rotor's mass.
• Misalignment between the unit and its driving machine. Cured by shaft alignment; the characteristic sign is a noticeable second harmonic and axial vibration.
• A wrecked or worn bearing. Readings stop being repeatable, the influence coefficient drifts, and the result won't hold.
• Loose fastenings, soft foot, cracked foundation grout. Tightening and repairing the foundation come first.
• The rotor rubbing against the guard, a seal or a build-up of deposit, as well as thermal bowing of the shaft.
• Hydraulic causes in pumps: cavitation, flow separation, operating far from the design point.
• Electrical causes in motors: they disappear the instant power is cut, and this is checked in a single coast-down.
• The rotor is flexible and runs above its first critical speed, the speed at which the shaft starts to visibly bend. A different method is needed, and we'll point you to a specialist contractor.
• The rotor is worn, deformed, or coated in a thick layer of build-up. Cleaning, straightening or replacing elements comes first, otherwise the unbalance will return within weeks.

If your photos and data show the issue isn't unbalance, we'll say so before any visit. If the decision can only be made on site, you'll know it within the first hour of work, not at the end of the day. We do vibration diagnostics and a finding on the cause as a separate piece of work, and it counts toward the cost of the visit.

Sources: ISO 13373-3:2015 · ISO 13373-5:2020

Price and how to order

The package is chosen by the rotor's power or mass. The amounts shown apply to a single rotor and to balancing in one correction plane. All prices are + VAT (charged at the applicable rate).

• The type of machine and its purpose in your own words: what it is, what it does, where it stands.
• Rotation frequency and drive type: direct, belt, through a gearbox, with a frequency converter.
• The rotor's mass and dimensions: diameter, length, overhang beyond the bearing if the rotor is overhung.
• Photos: the whole rotor, each bearing housing, the possible location for fitting a weight.
• A description of the symptom: when it appeared, after what, how it depends on speed and load.
• Any measurements you already have: the level in mm/s, the point, direction, mode. A screenshot of the spectrum is especially useful.
• Access: hatches and guards, mounting height, whether you need a platform, a scaffold or a lift.
• The window for runs: how many times and at what times the machine can be stopped.
• The site address, your desired timeframe, and any access-permit or induction requirements.

The minimum invoice per visit is 500 EUR, minimum time on site is 4 hours, deposit 50%. Several rotors in one visit work out noticeably cheaper than separate trips, so group your machines into one window. Send the details from the list above to axilinegeral@gmail.com or on WhatsApp to +351 931 831 229, and we'll reply on whether on-site balancing is feasible, how many correction planes we're planning for, how many runs will be needed, and what the visit will cost. You'll be dealing with engineers who design and manufacture Balanset instruments themselves and use them for on-site balancing themselves.

Frequently asked questions

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