AXILINEOn-Site Balancing of Blowers and Gas Blowers: Roots, Centrifugal, Drive
A blower on wastewater-treatment aeration or pneumatic conveying runs around the clock, and nobody's stopping it for long. At the same time, half the complaints about its vibration aren't about imbalance at all: positive-displacement machines pulsate by their very nature, and the piping and silencer can be the ones humming. We come out, measure, and balance what can actually be balanced on site, in the machine's own bearing supports. We're based in Vila Nova de Gaia, near Porto, and travel throughout Portugal.
Symptoms: how a blower complains
Low-pressure blowers live on a punishing schedule: wastewater-treatment aeration, pneumatic conveying, purging, dust extraction. The machine doesn't stop for months, its speed is higher than an ordinary fan's, and a rise in vibration is often noticed late, once the bearing housings are already running hot or the gearbox casing has started leaking. The earlier the measurement is taken, the cheaper everything else works out.
A blower has a feature a fan doesn't: a noticeable part of the noise and shaking comes not from the rotor but from the pulsating flow on the discharge side. So the first question isn't "how much is shaking" but "what exactly is shaking": a bearing support at the rotational frequency, or the piping at the blade-pass frequency (the rate blades pass a fixed point, several times higher than the rotational frequency). The answers to these two questions lead to different work.
- Vibration at the bearing supports has grown to one and a half times the usual level, even if the absolute figure is still in the acceptable zone
- Noise intensified after cleaning the flow path, replacing the filter, or repairing the drive
- Bearing housings run hot, grease darkens before its replacement interval, oil mist has appeared at the gearbox casing
- On a belt-driven machine, the belt squeals, runs hot and comes off the pulley, which runs out when turned slowly
- The gearbox has started howling, or the meshing tone has changed: more often that's gear wear, not imbalance
- The discharge piping and silencer walls shake and hum, the check valve bangs: this looks like pulsations, not imbalance
- Vibration changes together with discharge pressure, or when the suction filter is clogged
- The drive has already been balanced, but vibration came back within a few weeks
A trend is more informative than any table: overall vibration in mm/s RMS (root-mean-square value) at the bearing supports rising to one and a half times the baseline level is already reason enough for a measurement. Check the absolute limits against the applicable part and edition of ISO 20816, accounting for your machine's power and support type.
Sources: ISO 20816-1:2016 · ISO 13373-5:2020
How blowers are built, and where their imbalance comes from
The word "blower" covers machines with very different mechanics, and it's the type that determines what we can do on site. What they have in common is this: speeds are higher and clearances smaller than on fans, so the requirements on residual unbalance are tighter too.
Roots-type positive-displacement machine
Two profiled rotors with two or three lobes turn toward each other, synchronised by a pair of timing gears. Clearances between the rotors and the housing run to tenths of a millimetre, and contact isn't acceptable. The rotors are balanced by the manufacturer, and on site nothing is done to them: any weight on a lobe eats into the clearance. Imbalance here comes from dust and oil-film buildup on the rotors, from erosion, and from bearing wear shifting the axes. This is fixed by cleaning and repair during disassembly, where the block is also balanced on a machine.
Single-stage centrifugal
The impeller sits overhung on the shaft, often right on the motor shaft or on the gearbox's high-speed shaft. Imbalance builds up the classic way: buildup on the blades, erosion, corrosion, traces of previous repairs. This is the prime candidate for on-site balancing: access opens up through the suction nozzle or a removed inlet cone, with a single correction plane at the hub.
Multistage centrifugal
A stack of impellers on a common shaft in a sectioned housing. The correction planes are hidden inside the sections, and can't be reached on site. If the rotor really is what's causing the vibration, it's pulled out and balanced assembled on a machine. On site we're useful for diagnostics: separating imbalance from bearings, misalignment, and pulsations, so an expensive teardown doesn't turn out to be for nothing.
Belt drive and step-up gearbox
The impeller's speed isn't the same as the motor's, so we put the reflective tape and aim the laser phase sensor at the shaft we're actually balancing. A step-up belt drive adds its own vibration sources: pulley runout, a loosened taper bushing, an uneven belt. A worn gearbox produces gear-mesh frequency (the frequency at which gear teeth re-engage) with sidebands. All of this lives in the spectrum right next to 1x and readily masquerades as imbalance.
Oil system and seals
Bearings and gears run in end-mounted oil casings, separated from the flow path by seals. A worn seal lets oil mist into the chamber, dust sticks to the oil film, and the rotors or impeller pick up mass unevenly. Oil in the flow path isn't just an air-cleanliness issue — it's also an early warning of a floating imbalance that will come back after any balancing job.
Pulsations and noise: what balancing doesn't fix
A positive-displacement blower delivers air in discrete pulses. Two three-lobe rotors produce six discharges per revolution, so pulsations at the lobe-pass frequency and its harmonics are always present on the discharge side. That's a feature of the design, not a defect. Pulsations are damped by silencers and properly designed piping — weights on the rotor have nothing to do with them.
In the spectrum these are distinguished right away. Imbalance sits exactly at the rotor's rotational frequency. Pulsations sit at a frequency that's a multiple of the number of lobes, usually four or six times the rotational frequency. If the lobe-pass component dominates and the piping is humming in resonance, we say so after the first reading and don't start any trial runs.
- A broken-down silencer packing: the fill and baffles wear out, noise rises, and pieces of packing travel downstream and stick further along
- Piping or support resonance: the wall hums at the lobe-pass frequency, fixed by stiffness and additional supports, not weights
- A clogged suction filter: the machine drifts off its operating point, and centrifugal units start experiencing flow separation and surge
- A rattling check valve: the vibration is impulsive, showing a broad comb of peaks in the spectrum rather than a clean 1x peak
- Wear in the timing-gear mesh: howling and gear-mesh frequency with sidebands
How to separate these components from the running-speed one is covered in our articles on reading a vibration spectrum and on identifying the cause of vibration. The practical takeaway is short: balancing reduces only 1x, and on a blower this rule has to be checked more often than on any other machine.
Sources: ISO 13373-3:2015
What we check before the first weight
The walk-around takes up to half an hour and often saves an entire visit. Everything done by hand is done on a stopped machine with the start-up locked out and the pressure in the tract relieved.
- Frame and bearing-support fasteners, soft foot (a support that doesn't sit flush against the frame), the condition of the anti-vibration mounts: looseness produces a comb of harmonics (a series of multiple peaks in the spectrum) and an unstable phase
- Axial and radial play in the rotors of a Roots-type machine: noticeable play means bearing wear, and there's nothing to balance in that state
- The impeller's fit on the shaft, and the condition of the hub, the keyed joint, and the taper bushing
- Belt drive: pulley runout with a dial indicator, groove wear, alignment, tension per the manufacturer's instructions
- Gearbox: oil level and condition, metal shavings, meshing noise both by ear and in the spectrum
- Shaft seals and traces of oil in the flow path, buildup on the impeller blades or on the rotors
- Suction filter and silencers: tract resistance, integrity of the packing
- Operating point: discharge pressure, valve positions, VFD setpoint. We record it and don't change it until the work is finished
- Access and safety: how the suction nozzle or hatch opens, who's responsible for start-up lockout
Next comes the measurement. Two accelerometers (vibration sensors) go on the bearing supports of the shaft being balanced, with the laser phase sensor aimed at the reflective tape. We compare overall vibration against the running-speed component 1x at each support and take two runs in a row: with a securely fastened mass, the phase repeats within a few degrees. If it wanders by tens of degrees, something isn't fastened securely, and we look for where first.
How the on-site work proceeds
- Request
Reviewing the machine from photos and data
You send us the blower type, speed, power, drive type, photos of the machine, the drive, and the nameplate, and for a centrifugal unit, photos of the impeller if access has already been opened up. We tell you what can be balanced on site here, which correction planes are accessible, and what to prepare before our visit.
- Run 0
Baseline measurement at the operating point
The machine runs at its working discharge pressure. We record speed, overall vibration, the 1x amplitude and phase at each bearing support, the spectrum, and the time waveform. Pressure and operating point stay unchanged from this point until the work is finished.
- Diagnosis
Separating causes
We compare 1x against overall vibration, check the lobe-pass frequency, the gearbox's gear-mesh frequency, the high-frequency bearing zone, and the second harmonic associated with misalignment. If imbalance isn't confirmed, we stop right here and hand over a finding instead of running useless trial runs.
- Trial
Trial weight and calibration
We fit a weighed trial weight in the correction plane with a bolt in a factory hole, or a clamp around the hub: a magnet won't hold on an aluminium impeller. A valid trial run changes the 1x amplitude by at least 20-30 percent, or the phase by 20-30 degrees. This way the instrument obtains influence coefficients for your specific system: the relationship "weight fitted, vibration changed by this much" that the correction is calculated from.
- Correction
Weights or metal removal
The software gives mass and a position number. On an impeller we more often remove metal by drilling in the hub or the load-bearing disc, per the software's calculation: nothing sticks out, and the clearance with the housing isn't affected. On pulleys and coupling halves we work with bolts and washers in factory holes.
- Confirmation
Post-work measurement and report
A confirmation run at the same speed and the same discharge pressure. If needed, a trim balancing pass using the saved coefficients, usually one short iteration. Then a report with before-and-after figures for each support.
Every stop of the blower means relieving the pressure and opening up access, so it's your operating schedule that sets the real timeline, not the measurements. A single correction plane needs a baseline and one trial run; two planes need one more trial run. The criterion for a valid trial run is covered in more detail in our article on the trial weight.
Sources: Balanset-1A operation manual
One plane or two, and where they are on a blower
The general rule on the rotor's length-to-diameter ratio is covered in a separate article on choosing the number of planes. For blowers the practical picture is short: accessible planes are few, and the choice is dictated by the construction of the specific machine, not by theory.
| Rotor | Planes | Where and why |
|---|---|---|
| Impeller of a single-stage centrifugal | One | A narrow overhung wheel, with imbalance that's practically static. Correction plane at the hub, access through the suction nozzle or a removed inlet cone |
| Impeller plus pulley or coupling half on one shaft | Two | Planes on both sides of the bearing supports: the impeller and the pulley. This layout addresses the couple component without disassembly |
| Drive motor's rotor | Two | A classic between-bearings rotor. Correction on the cooling fan and the accessible ends of the rotor — more detail on our page about electric-motor rotors |
| Pulley of a step-up belt drive | One | A short disc. A bolted weight in the spokes or rim, or metal removal by drilling |
| Rotors of a Roots-type machine | Two, shop only | The manufacturer balances the rotors on a machine. On-site intervention is ruled out: any weight eats into the clearance with the housing and the other rotor |
| Rotor of a multistage centrifugal | Shop, on a machine | Correction planes inside the sectioned housing are inaccessible, the rotor is pulled out and balanced assembled |
Higher speeds tighten the tolerance: the faster a rotor turns, the smaller the allowable residual unbalance in grams at the same grade G to ISO 21940-11. The high-speed shaft downstream of the gearbox may run close to its first critical frequency (the speed at which the shaft itself enters resonance); in that case we separately check whether on-site balancing applies, against the part of ISO 21940 covering rotors in a flexible state.
Sources: ISO 21940-11:2016 · ISO 21940-12:2016
Attaching weights: grams, clearances, and aluminium
Correction masses on a blower are small. Speeds are high, and the required accuracy grade is reached with grams, not tens of grams. A one-centimetre error in the mounting radius costs more here than on a slow-running fan, so we measure the actual radius, from the shaft axis to the centre of mass of the weight.
The second constraint comes from the clearances. The impeller runs within a few millimetres of the housing and the inlet cone, and a weight is not allowed to protrude into those clearances, either during a trial run or once permanently fitted.
- Metal removal by drilling in the hub or the load-bearing disc: the main method on impellers. The software calculates the hole diameter and depth for the needed mass, and nothing protrudes outward
- A bolt in a factory-provided balance hole, where the manufacturer included one. We lock the nut and check the bolt's length against the clearance to the housing
- Many impellers are aluminium: welding is ruled out, and a magnetic trial weight won't hold. We fasten the trial mass with a bolt or a clamp around the hub
- On pulleys and coupling halves we work with balance bolts and stacked washers in factory holes
- We fit nothing at all on Roots-type machine rotors: neither a weight nor on-site metal removal is acceptable
- After fitting any weight, we turn the rotor through a full revolution by hand and check the clearances before starting
Attachment methods and their material constraints are collected in our article on attaching correction weights. For a blower, remember the main point: nothing protruding, the actual radius, and a clearance check after every fitting.
When on-site work won't succeed or won't help
An honest answer before the visit is cheaper than a wasted one. Here are the blower cases where we either decline to balance on site, or suggest something else first.
- The vibration is coming from inside the Roots block: buildup on the rotors, erosion, wear on the bearings or gears. Balancing the block is done in the shop during disassembly; on site we confirm the diagnosis with a measurement and a finding
- A multistage rotor in a sectioned housing: there's no physical access to the correction planes
- Overall vibration is several times higher than 1x: bearings, gear mesh, looseness. We'll do the diagnostics and tell you what to fix, but weights won't change anything
- The lobe-pass frequency dominates: pulsations, a broken-down silencer, piping resonance. The work needs to be on the tract, not the rotor
- Surge (unstable operation with reverse flow pulses) or flow separation on a centrifugal unit with a clogged filter or a throttled network: restore the operating point first, then measure
- Drive misalignment on a coupling connection: fixed by shaft alignment — mass on the rotor doesn't compensate for it
- Play in the impeller's fit, a crack, critical blade erosion: repair first, balancing after
- The machine doesn't hold stable speed and pressure: the influence coefficients come out unreliable, and the calculation doesn't converge
- Frame or piping resonance: stiffness and supports first, otherwise the result won't even hold through the end of the shift
If none of these causes fits, and vibration still doesn't drop after balancing, take a look at our article on cases where balancing doesn't help, and our article on choosing between on-site and shop-lathe balancing.
What you get, and how to book a visit
As a result of the work you get a report: initial and residual vibration at each bearing support in mm/s RMS, with a separate line for the running-speed component 1x; speed and discharge pressure at the time of measurement; masses, radii and positions of all weights, or the metal-removal parameters; spectra before and after; an assessment against the zones of the applicable part of ISO 20816, and a tolerance calculation by grade G where the rotor's mass is known. Plus a mechanical finding: what we found on the walk-around and what's worth fixing. Your machine's influence coefficients are saved, so a repeat balancing job goes ahead without trial runs and is noticeably shorter.
The balancing is carried out by the engineers who design and manufacture Balanset instruments and do the on-site work themselves. We work with the Balanset-1A: two accelerometers on the bearing supports, a laser phase sensor reading the reflective tape, a two-channel USB module and software on a laptop, single- and two-plane calculation by the influence-coefficient method, fixed positions, drilling calculations, and a report archive. The same instrument can be bought, so you can service your own fleet independently.
Prices: vibration diagnostics with a report 300 EUR per unit, balancing from 250 EUR, minimum invoice for a visit 500 EUR. The total depends on the number of rotors, the number of correction planes, and how far the site is; the calculator on the website gives an exact figure. If several blowers stand near each other, or a blower and its motor both need attention at the same time, count them as one visit: a second machine works out significantly cheaper than a separate call-out.
- Machine type: Roots, single-stage or multistage centrifugal, make and model
- Speed and drive type: direct, belt drive, gearbox
- Discharge pressure, the media and its dust content, whether there's oil in the flow path
- Photos of the machine, the drive, the nameplate, and the impeller if access has been opened up
- Where it shakes more, and when: at the supports, on the piping, at which operating point
- Your shutdown window, and who on site is responsible for start-up lockout
Send us a request with answers to this list, and we'll tell you straight: whether your blower can be balanced on site, whether diagnostics are needed first, or whether the block will have to be disassembled. If the photos show that weights won't help, we'll write that before the visit, not after.
Sources: ISO 20816-1:2016 · Balanset-1A manufacturer specification
Frequently asked questions
Can the rotors of a Roots-type blower be balanced on site?
No, and we say so right away. Roots rotors run with clearances of tenths of a millimetre between each other and the housing, are synchronised by gears, and are balanced by the manufacturer on a machine. Any weight or metal removal on site changes the clearances and the profile geometry, and contact between the rotors at running speed destroys the machine. If the block is vibrating, the cause is usually buildup on the rotors, or wear on the bearings or the gear mesh. On site we take a measurement, separate these causes from each other, and balance whatever periphery is accessible: the pulley, the coupling half, the motor's rotor. The block itself is balanced in the shop during disassembly.
The piping hums and shakes, but the bearing supports are almost calm. Is that imbalance?
Most likely not. A positive-displacement blower delivers air in discrete pulses, and there are always pulsations on the discharge side at the lobe-pass frequency: for two three-lobe rotors that's six pulses per revolution. If the pipe or silencer has landed in resonance with that frequency, it's the piping that's humming. This shows up immediately in the spectrum: the peak sits not at the rotational frequency but several times higher. Balancing reduces only the running-speed component and has no effect on pulsations. What works here are silencers, the condition of their packing, and the piping's stiffness and supports. We measure this and show it in numbers, so you don't pay for weights where supports are what's needed.
The blower has a step-up belt drive. Where do you put the mark and the sensors?
We put the reflective tape and aim the laser phase sensor at the shaft we're actually balancing, usually the impeller shaft: its speed isn't the same as the motor's, and tape on the motor would give the instrument the wrong frequency. The accelerometers go on the bearing supports of this same shaft, in the same radial direction across all runs. We separately check the pulleys with a dial indicator: an eccentric pulley or a loosened taper bushing produces vibration exactly at the rotational frequency and masquerades very effectively as impeller imbalance. A pulley like that gets put right first, or is balanced as a separate rotor.
Why are the weights so small, and can't a plate just be welded on, like on a fan?
Because of speed and clearances. At the same grade G, the allowable residual unbalance is inversely proportional to rotational frequency, so a high-speed impeller is balanced with grams. At the same time the impeller runs within a few millimetres of the housing and the inlet cone, and a protruding weight doesn't fit into those clearances. On top of that, many blower impellers are aluminium: welding is ruled out by the material, and a magnetic weight won't stick to aluminium even as a trial. So the main correction method here is metal removal by drilling in the hub or the load-bearing disc, and we only fit weights with bolts into factory holes.
Oil has appeared in the flow path. Is that connected to the vibration?
Often yes, and with a delayed effect. A blower's bearings and gears run in oil casings separated from the flow path by seals. A worn seal lets oil mist into the chamber, dust from the air sticks to the oil film, and the rotors or impeller pick up mass unevenly. The result is a floating imbalance: vibration rises, drops after cleaning, then comes back. It's possible to balance in that state, but the result only lasts until the next layer of buildup. The right order: replace the seals, wash out the flow path, then measure and balance.
How much does balancing a blower cost?
vibration diagnostics with a report costs 300 EUR per unit, balancing adds from 250 EUR, minimum invoice for a visit 500 EUR. The total 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 the website gives an exact figure for your machine. One thing to keep in mind: on a blower, two rotors are often balanced in a single visit, for example the impeller and the pulley, or the motor's rotor, and that works out cheaper than two separate visits. If the photos and data show that on-site balancing won't help and a shop is needed, we'll write that before the visit.
Related content
Balancing compressors, blowers and gas blowers at the point of operation
Yes, but not every piece of compressor equipment. On site we balance whatever gives access to a correction plane: open impellers on blowers, air-blower units and gas blowers, overhung impellers on single-stage centrifugal compressors, pulleys, flywheels, half-couplings, compressor shafts and drive-motor rotors. Screw-compressor rotors, multistage rotors in a closed casing, and high-speed turbocompressor rotors don't get balanced on site — those need a workshop and a balancing rig. Before we reach for weights, we always prove with a measurement that unbalance is genuinely what's producing the vibration.
On-Site Balancing of Centrifuge Drums and Baskets
We balance the drum or basket on site, in the machine's own bearing housings. But first we answer a different question: is this imbalance at all. On a filtering or sedimentation centrifuge, the dominant vibration more often comes from unevenly settled product than from the rotor. We tell them apart by phase repeatability — the angle by which the vibration is tied to the rotor. We run several starts in a row on a clean, dry basket and check whether the phase of the 1x running-speed component — vibration at the rotation frequency — stays put. If it does, it's mechanical imbalance and weights will help. If it drifts from run to run, weights would only mask the problem for a single cycle. We fit masses at the standard spots on the rim or the reinforcing ring. We don't drill the perforated shell, we don't weld on stainless steel, and we don't put foreign parts in the product zone.
On-site balancing or shop balancing on a machine: how to choose without overpaying in downtime
Balance on site if the rotor can be safely run up to its operating speed, you have access to the correction plane (the location on the rotor where the correction weight goes), and the operating mode repeats from run to run. Take it to a balancing machine if the rotor is flexible or passes through critical speeds, its geometry is damaged, there is no access to the correction planes, or acceptance requires a report against a balance quality grade G. In every other case, start with an on-site measurement: the data from the first reading will show on its own whether removal is actually needed.
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.