AXILINEOn-site balancing of any rotor: the five conditions that make it work
You have a machine that appears in no balancing-machine catalogue. A homemade drum, a modified screw conveyor, a crusher rotor after a hammer change, a drive built for one specific line. We don't need the name of the unit to answer you: we need to know whether the bearing housings are accessible, whether speed can be read, and whether the machine can be started and stopped a few times. This page shows how you can check feasibility yourself in ten minutes.
What brings people to us: the symptoms worth calling about
We approach every machine the same way. It doesn't matter who built the unit or whether there's any documentation for it. What matters is whether we can reach the rotor with two sensors, pick up the once-per-revolution component off the mark — vibration exactly at rotating speed, which is what unbalance produces — and run the machine a few times. If yes, we remove the unbalance without taking the rotor out of the machine.
It's better to call not once the unit has already failed, but once vibration has risen above the level this machine normally runs at. A rise from the baseline tells you more than any absolute figure.
- Vibration increased after a repair: an impeller change, weld build-up, or replacing knives, hammers, or blades.
- Noise and shaking build with speed and fade during coast-down.
- The bearing housing shakes noticeably to the touch, and mounting bolts keep working loose on their own.
- Bearings and seals fail more often than the service schedule calls for, and the shaft runs hot for no obvious reason.
- The instrument shows that most of the vibration level comes from a peak exactly at rotating speed.
- The rotor is non-standard, there's no equivalent to compare it to, and removing it is expensive or not technically possible.
- Cracks have appeared in weld seams, anchor bolts have sheared, or the frame has cracked.
Balancing reduces only the once-per-revolution component. If the overall level — total vibration across all frequencies at once — is several times higher than the peak at rotating speed, something else is causing the vibration, and weights on the rotor won't remove it. We check this before you pay for the work, not after.
Sources: ISO 13373-3:2015
The five conditions under which on-site balancing is feasible
This is the main section of the page. Go through the five points while 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 honestly and decline.
1. There's somewhere to mount vibration sensors
We need access to the bearing housings on both sides of the rotor. The sensor sits on a magnet, on a clean flat spot, as close to the bearing as possible, facing radially — across the shaft axis. A guard, a protective mesh, or a cladding panel won't do: they vibrate on their own and give false readings. If the housing is sealed shut or the bearing is recessed into the machine casing, we look for the nearest rigid point in the load path and tell you plainly how much that will degrade the picture.
2. Speed can be read with an optical tachometer
We stick a reflective mark on the shaft, hub, or pulley, and a laser sensor gives us rotating speed and a phase reference — tying the readings to the rotor's angle of rotation. The mark needs a direct line of sight: a gap in the guard, an open hatch, a removed belt-guard cover. Without phase, unbalance can't be calculated; all that's left is a level measurement. The laser sensor itself can be mounted on a stationary part near the shaft if the geometry allows, but you still need a clear sightline to the mark.
3. The machine can be started safely several times
Single-plane balancing takes two runs, two-plane balancing takes three, plus a verification run and sometimes one fine-tuning run. Between runs the machine must come to a complete stop and be reliably isolated and locked out, because we work with our hands inside it. If your process only allows one start per day, or stopping the machine takes down the whole line, tell us up front — we'll plan the time window differently.
4. There's somewhere to add or remove mass
We need an accessible, solid correction plane — a spot on the rotor where a balancing weight can be added or removed. Existing flange bolts and studs, holes in a disc, a wheel rim, a hub, or a drum end-plate all work. We add mass with washers and plates on a bolt, by welding on a plate or tacking it, and on rotors where welding isn't allowed, we remove mass by drilling or grinding instead. The instrument calculates the metal-removal option too. If there's nowhere to put a weight and modifying the part is off-limits, the job stops making sense.
5. Rotation speed is stable
Speed has to hold steady during measurement. A drifting frequency smears the once-per-revolution component and shifts the phase; the influence coefficients — the machine's measured response to an installed weight, the basis of the whole calculation — come out unstable, and the result won't repeat. A slipping belt, a worn transmission, load that jumps around, a VFD in auto-adjust mode: all of that has to be locked down for the duration of the work. Keep the operating regime constant too — speed, load, temperature, damper position.
The five conditions aren't about the class of machine, they're about the physics of measurement. That's exactly why we take on one-off and homemade units that have neither a balancing machine nor a catalogue tolerance to their name.
What has to be in good order before balancing
Balancing doesn't fix mechanical faults. A weight on the rotor won't stop a bearing from knocking, won't tighten a loose bolt, and won't straighten a bent blade. Worse, on a faulty machine the measurements stop repeating: the influence coefficient drifts, and you end up paying for runs that produce nothing.
That's why we spend the first hour on site checking the machine's condition. You can catch some of these issues yourself before we arrive, with a wrench and a dial indicator.
- Bearings with no critical play: no knocking, the rotor doesn't drop, axial float within normal limits.
- Frame, bed, and supports free of cracks, sheared anchor bolts, loose bolts, and soft foot (a support that doesn't sit flush against the frame and gets pulled down at an angle by its bolt).
- The rotor doesn't rub anywhere against the guard, a seal, a grate, weld build-up, or neighbouring parts.
- The rotor isn't deformed: radial and face runout within limits, blades and knives not bent from an impact.
- The fit is tight: impeller, pulley, drum, or disc doesn't turn or shift on the shaft.
- Rotation is stable: the amplitude and phase of the once-per-revolution component repeat from run to run.
- Operating speed doesn't coincide with the natural frequency of the frame or supports. In resonance, the result won't hold.
If we find a fault, we show it to you and explain the order of work. Shaft misalignment is corrected by shaft alignment, not by weights. Loose fasteners are corrected by tightening them. Balancing comes after, otherwise it just masks the vibration level for a few weeks. More on this in our article on cases where balancing doesn't help.
Sources: ISO 20816-1:2016 · ISO 281:2007
What rotors and rotating assemblies we take on
The list below is organised by group, not by factory model names. Look at the shape of your rotor and the nature of its loading: those are what determine where the unbalance comes from and how many correction planes you'll need.
Long rotors, shafts, and drums
Dryer and separator drums, screw and transport conveyors, drive and intermediate shafts, cardan shafts, roller and winding assemblies, spindles on converted machine tools. Unbalance here most often comes from uneven product buildup along the length, localised wear, weld build-up in one section, or curvature left over from straightening. These rotors almost always need two planes.
Rotors with impellers and fan wheels
Industrial fans and induced-draft fans, pump impellers, blower and compressor unit rotors, dust-extraction and exhaust wheels, centrifuge and separator rotors. The main sources: blade erosion, dust and product buildup, corrosion, a lost rivet or plate, replacing a single blade without matching its mass. Access is usually through an inspection hatch.
Rotors with cutting and impact tooling
Mulchers and mowers, wood and biomass shredders, general shredders, hammer and rotary crushers, pelletiser rotors, milling drums. Here unbalance appears instantly: one chipped hammer, a lost knife, or an uneven tool-set replacement. Fit tooling in matched pairs and by mass, otherwise balancing turns into a recurring service you need every time.
Non-standard and one-off units
Homemade rotors, converted and modernised machines, retrofitted units, test and running-in rigs, mixers and agitators, vacuum-equipment rotors, lab drives. There's no factory tolerance here, and often no drawing either. We set the tolerance from the required vibration level at the bearing housings and from an accuracy class based on rotor mass and speed.
Drive and auxiliary components
Pulleys and V-belt sheaves, flywheels and inertia masses, brake drums and discs, coupling halves, clutches, gear and chain wheels. Many of these behave like a thin disc, and a single correction plane is enough. With a belt drive, rule out belt tension and pulley runout first — they add their own vibration components.
We don't take on aviation rotors, turbines, or high-speed units running above the first critical speed — the speed at which a shaft starts to bend noticeably. Those are balanced using different methods and different equipment.
One correction plane or two: how it's decided for your rotor
The number of planes is set by the shape of the rotor, not by a wish to save a run. The rule of thumb is simple: compare the rotor's working length to its diameter in the zone where the weight goes. A short disc, where the length is under roughly half the diameter, is usually corrected with a single mass. Anything elongated along its axis needs two planes, or you'll be left with couple unbalance: vibration drops on one bearing housing and rises on the other.
That's exactly why we almost always balance long rotors, shafts, drums, mulcher and crusher rotors, assembled pump rotors, and wide fan wheels in two planes. The second plane costs one extra trial run and saves you a day of fruitless attempts. The length-to-diameter rule is covered in more detail in our article on choosing the number of planes.
| Rotor or unit | Shape | Usually planes | Where we put the mass |
|---|---|---|---|
| Pulley, flywheel, brake drum, coupling half | Thin disc, length under half the diameter | One | Holes in the disc, the rim, flange bolts |
| Narrow impeller, disc knife, small-width fan wheel | Disc-shaped, symmetrical | One | Wheel rim, existing holes |
| Fan and induced-draft fan impeller | Wide wheel, often overhung mounting | Two | Front and back wheel discs, welded-on plates |
| Drum, screw conveyor, drive or cardan shaft | Elongated, length noticeably greater than diameter | Two | End discs and flanges at both ends |
| Mulcher, mower, or shredder rotor | Long shaft with mounted tooling | Two | Hubs or discs at the rotor ends, bolted-on mass |
| Crusher, shredder, or hammer mill rotor | Long stack of discs on a shaft | Two | Outer discs, welded or bolted weights |
| Multistage pump rotor | Elongated assembled shaft | Two | Hubs or nuts of the outer impellers |
Single-plane balancing is two runs: an initial run and one trial run. Two-plane balancing is three runs: an initial run and two trial runs, one per plane. After that comes a verification run and, if needed, a fine-tuning run.
Sources: ISO 21940-11:2016 · ISO 21940-12:2016
How the work goes on site, in the rotor's own bearings
- 01
Inspection and mechanical check
We go through the mechanical checklist: fasteners, play, fit, rubbing, frame condition. We look at the spectrum (vibration broken down by frequency) and compare the overall level with the once-per-revolution component. This is where we decide whether we're balancing today or repairing first.
- 02
Sensors and mark
We mount two accelerometers on the bearing housings, radial direction, kept the same from one reading to the next. We stick on a reflective mark and aim the laser phase sensor at it. We lock in the operating regime: speed, load, temperature.
- 03
Initial run
We bring the machine up to operating speed and record the initial amplitude and phase of the once-per-revolution component at each bearing housing. That's the figure you'll see in the report as "before."
- 04
Trial runs
We fit a trial weight of known mass at a known radius on the first correction plane, start the machine, and measure. With two planes, we move the weight to the second plane and repeat. We count a run as valid when the amplitude has 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.
- 05
Calculating and fitting the correction
The software outputs a mass and angle for each plane, and on rotors with ready-made mounting points it gives a position number instead of an angle: no protractor needed, and no chance of getting the reference direction wrong. We fix the mass as securely as the calculation requires, or remove it by drilling to the calculated amount.
- 06
Verification run and fine-tuning
We repeat the measurement at the same points and under the same regime. If the level has dropped but not reached the target, we add a small correction to the weights already fitted. Saved influence coefficients let us skip the trial runs next time.
- 07
Report and recommendations
We put together the before-and-after figures, the weight masses and locations, and the measurement regime. Separately, we note what we saw on the mechanical side and what's worth doing before next season.
We work with our own Balanset-1A instrument: two channels, a laser phase sensor, the once-per-revolution component and phase, overall level, spectrum and time-domain signal, tolerance calculation by accuracy class, a fixed-position and drilling mode, and a measurement archive.
Sources: Balanset-1A manufacturer specification · Balanset-1A operation manual
When we'll decline or suggest another path
An honest no costs you less than a useless site visit. Here are the cases where we either won't take the job or will suggest a different solution.
- No access to the bearing housings: the bearings sit inside a sealed casing, are potted in, or run inside a process medium. We'll suggest removing the rotor and balancing it on a bench.
- Nowhere to add or remove mass: a smooth part where welding and drilling are ruled out by the process or the warranty. Balancing loses its physical meaning.
- Speed won't hold steady: a worn, slipping belt drive, load that jumps around, a VFD that can't be locked to a fixed setting. Fix the drive first, then balance.
- The machine can't be stopped and started the number of times needed. We'll discuss a time window or postpone the work until a scheduled shutdown.
- The rotor is flexible and runs above its first critical speed. That needs a different method, and we'll point you to a specialist contractor.
- The rotor is worn or deformed, or shows erosion or heavy buildup. Clean, straighten, or replace parts first, otherwise the unbalance will come back within weeks.
- The vibration isn't coming from unbalance: shaft misalignment, a failed bearing, looseness, resonance, or rubbing. We run vibration diagnostics and tell you what to repair.
- Emergency condition: a crack in the rotor or a disc, a torn-off blade, a cracked shaft. Balancing a machine like that is dangerous — it needs to be shut down.
We give you this kind of conclusion from photos and data, before the visit, whenever there's enough information. If the decision can only be made on site, you'll hear it in the first hour of work, not at the end of the day.
What to send us so we can answer on feasibility
You don't need to send drawings. We just need the minimum that lets us understand the geometry, the access, and the operating regime. That's usually enough for us to tell you whether we'll take the job, how many correction planes we expect, and how long the visit will take.
- The type of machine and what it's for, in your own words: what it is, what it does, where it's installed.
- Rotation speed and the drive: direct, belt, through a gearbox, with a VFD.
- Rotor mass and dimensions: diameter, length, and overhang beyond the support if the rotor is overhung.
- Photos: the whole rotor, each bearing housing, and the possible weight-mounting location.
- A description of the symptom: when it appeared, after what, and how it depends on speed and load.
- Measurements, if you have them: level in mm/s, point, direction, regime. A screenshot of the spectrum is especially useful.
- Access: hatches and guards, installation height, whether a platform, staging, or a lift is needed.
- The run window: how many times, and at what times, the machine can be stopped.
- The site address and your preferred dates, plus any access-permit or induction requirements.
If something on this list is missing, send what you do have. One good photo of the bearing housing plus the rotation speed is already enough for a preliminary answer.
Price and how to order
Vibration diagnostics with a report costs 300 EUR per unit, balancing adds from 250 EUR. The minimum invoice per visit is 500 EUR: the trip, the diagnostics, and the on-site work are billed as one visit, even if there's only one small rotor. The calculator on the site gives you an exact figure for your machine — it accounts for the number of rotors, the number of correction planes, distance, and access conditions. Balancing a second and subsequent rotor on the same visit costs less, so it pays to line up several machines in one time window.
The work is done by engineers who design and manufacture the Balanset instruments and use them themselves out in the field. We're based in Vila Nova de Gaia, near Porto, and we travel across all of Portugal.
- A mechanical check and vibration diagnostics before balancing, with a conclusion on the cause of the vibration.
- On-site balancing, in the rotor's own bearings, in one or two planes.
- Fabricating and securely fitting correction weights, or removing mass to the calculated amount.
- A verification measurement and a report with before-and-after figures, the measurement regime, and the weight locations.
- Recommendations on mechanical condition and on the interval for the next check.
To get an answer, send the details from the previous section through the request form. We'll write back to tell you whether on-site balancing is feasible, how many planes we're planning for, how many runs it will take, and how long the visit will last.
Frequently asked questions
Do you balance equipment that isn't on your list?
Yes. The list is there to help you understand what we do, not to screen you out. What decides it is the five conditions: access to the bearing housings for sensors, the ability to read speed with an optical tachometer off a reflective mark, several safe runs, an accessible correction plane, and stable rotation speed. A homemade unit, a converted machine, a one-off test rig — if the five conditions are met, the rotor balances the same way a production fan does.
Do you need to remove the rotor from the machine?
In most cases, no. We balance on site, in the rotor's own bearings, at operating speed. That gives a more accurate result, because we're accounting for the real stiffness of the supports and frame, and it costs you less downtime. We only remove the rotor when the bearing housings can't be reached, there's nowhere to fit a weight, or the rotor is flexible and runs above its first critical speed.
How do I know in advance whether it's one correction plane or two?
Compare the rotor's working length to its diameter in the zone where the weight goes. A length under roughly half the diameter means it behaves like a disc, and one plane is usually enough. Anything elongated along its axis needs two: long shafts, drums, screw conveyors, mulcher and crusher rotors, assembled pump rotors, wide fan wheels. One plane is two runs; two planes is three.
What happens if the bearings are worn but balancing is needed urgently?
We'll measure and show you the condition of the bearings, but balancing is no substitute for replacing them. On a worn-out support, measurements don't repeat well, the influence coefficient drifts, and the result won't hold. We sometimes agree to bring the level down as a temporary measure until a scheduled repair, but we say so plainly and note in the report that the root cause hasn't been fixed.
How many runs and stops do you need?
For one plane, two runs plus a verification run; for two planes, three runs plus a verification run. Sometimes one fine-tuning run gets added if we don't hit the target the first time. Between runs the machine has to be fully stopped, isolated, and locked out, because we work with our hands near the rotor. If your process doesn't allow for that kind of window, tell us in advance and we'll plan the work around a scheduled shutdown.
How much does it cost, and what does the price depend on?
Vibration diagnostics with a report costs 300 EUR per unit, balancing adds from 250 EUR, and the minimum invoice per visit is 500 EUR. The final figure depends on the number of rotors in one visit, the number of correction planes, distance to the site, and how difficult access is. The calculator on the site works out the exact amount for your machine. Several rotors in one visit cost less than separate visits.
Related content
Multistage Pump Rotor Balancing: What On-Site Work Can and Can't Do
Sometimes, but less often than with any other pump, and we say so before the visit. We balance a multistage rotor on site if it behaves as rigid. That means: the operating speed is well clear of the first critical speed — the speed at which the shaft goes into resonance and starts to bow; the dominant vibration is the 1x running-speed component, meaning vibration exactly at the rotation frequency; its phase repeats from run to run; and two correction planes — spots where a weight can be fitted — are accessible, at minimum the coupling half and the free end of the shaft. If the rotor behaves as flexible, or the imbalance is spread across the stages after wear and reassembly, weights at the shaft ends won't solve the problem. Then the visit turns into diagnostics: we measure, find the cause, give an assessment, and write up a work order for the workshop to balance the assembled rotor on a balancing machine.
On-site balancing of electric motor and generator rotors
Yes, we balance electric motor and generator rotors on site, in their own supports, at operating speed. Three conditions apply. First: the vibration has to be dominated by the 1x running-speed component — vibration at the rotor's rotational frequency, the main sign of imbalance — rather than by line frequency and its second harmonic. Second: at least one correction plane needs to be accessible, meaning a spot where a weight can be fitted: usually the cooling fan, the coupling, a balancing ring, or the rotor face. Third: the fits, bearings, and fasteners have to be sound, because weights don't cure play. If the rotor is coming off anyway (rewinding, bearing replacement, restoring the mounting surfaces), it makes more sense to balance it on a machine in the workshop, and we'll say so plainly rather than fit weights through a hatch.
Multi-bearing rotor balancing: a long shaft on three or more bearings
A multi-bearing rotor is a distinct problem, not an extended two-plane one. For a rigid rotor on two bearings, you're solving a system of two equations with two unknowns, and the influence coefficient matrix is 2×2. Three bearings give a 3×3 system with nine coefficients and a minimum of four runs; four bearings give sixteen coefficients and five runs. You have to measure at every bearing and solve the system as a whole: if you work through the planes one at a time, the vibration will keep running from bearing to bearing, and the process won't converge.
Describe your equipment and the problem
We'll answer your questions, clarify the details, and let you know what's needed for an estimate and a visit.