AXILINEHow to tell whether you need balancing: a decision checklist
A fan starts humming after cleaning, and someone on the shop floor has already said it needs balancing. Sometimes that is the right call, and sometimes a weight on the impeller just masks a worn bearing or a loosened frame, and you are back at the same machine a month later. Below is a practical checklist: which observations point toward unbalance, which point away from it, what you can check yourself in twenty minutes, and when balancing is done regardless of the symptoms.
What balancing actually fixes: one force, once per turn
Unbalance is a mismatch between the rotor's principal central axis of inertia and the axis of rotation. An unbalanced mass m at a radius r creates a centrifugal force F = m·r·ω², which rotates along with the rotor and pushes the bearing housing in one direction relative to the shaft, once every turn.
Every sign you use to make the decision follows from this one formula. The force acts radially, so unbalance shows up mainly in the horizontal and vertical directions, while the axial component stays small. The force arrives exactly once per turn, so a peak rises at the running speed, and unbalance on its own does not create harmonics or half-order harmonics. The heavy point does not move around the rotor between runs, so the phase of the running-speed component — the angle showing at what point in the turn the push arrives — is stable and repeatable. The force is proportional to the square of the angular speed, so the level depends sharply on speed.
From there the logic is simple: if the vibration pattern on your machine does not look like this, a correction weight will not remove it. We cover reading the numbers on the screen themselves in a separate article on overall vibration, 1x and phase; here we just use them as a ready-made tool.
Balancing only reduces the running-speed component. That is not a limitation of the instrument — it is physics: a weight changes the rotor's mass distribution and nothing else. It does not touch a knocking bearing, play in a fit, misaligned coupling halves, or the frame's natural frequency.
Five signs balancing will probably help
You check the first two signs by ear and by changing the speed through a VFD. The third comes from the machine's history, and you read the last two off a two-channel instrument.
- The level rose specifically at the operating speed, and the sound is a steady hum, with no knocking, rustling or metallic grinding.
- You halved the speed, and the vibration dropped by roughly a factor of four. That is how centrifugal force behaves when the operating frequency is well clear of resonance.
- The rise began after a clear event: cleaning the blades, replacing a blade or knife, weld build-up, repairing the impeller, removing and refitting the wheel, product build-up, or the loss of a piece of patch or a balancing weight.
- The spectrum is dominated by a peak at the running speed, and overall vibration is only a little higher than 1x.
- The 1x phase repeats from run to run within a few degrees, the shift between horizontal and vertical at one support is around 90°, and axial vibration is noticeably lower than radial.
One sign is a hypothesis; three or more is already a decision. If all five line up, the only things left to discuss are the number of correction planes and access to where the weights will be fitted.
Six signs balancing will almost certainly not help
Each of these signs means something other than the rotor's mass is shaping the level, and a weight will only muddy the picture.
Knocking, rustling and impulses at any speed
Unbalance sounds like a steady hum that weakens along with the speed. Knocking, crackling and high-frequency noise that you can still hear at half speed point to a bearing, play, or rubbing. The time waveform shows individual impacts, and the spectrum shows lines that are not multiples of running speed. Check the bearing first, everything else after.
The level does not depend on speed
You change the speed across a wide range, and the number on the instrument barely moves. The source is outside the rotor: a neighbouring machine through a shared frame, flow pulsation, an electromagnetic cause. The check for electrical causes is simple: cut the power and watch the first second. Mechanical vibration decays along with the coast-down; the electromagnetic component disappears instantly.
A comb of harmonics and half-order harmonics
The spectrum shows not one peak but a series at frequencies that are multiples of running speed: 1x, 2x, 3x and on up, sometimes 0.5x and 1.5x too. This is mechanical looseness: foundation bolts that are not fully torqued, a crack in the frame, a worn fit, a worn-out bearing housing bore. A torque wrench does more good here than weights, and 1x often drops by itself once everything is tightened.
The 1x phase jumps around from run to run
Two runs at the same operating point gave phases that differ by tens of degrees. This is how a rotor behaves with a loosened wheel or pulley fit, a worn key, or a machine sitting close to resonance. The influence coefficient — the calculated link between a fitted weight and the change in vibration — comes out unstable from a phase like this, and the result will not repeat.
Large axial vibration and 2x across the coupling
Axial 1x is comparable to radial or larger, the 2x-to-1x ratio exceeds roughly 0.5, and the axial phase on the two sides of the coupling differs by around 180°. This is shaft misalignment, often together with a soft foot — a support that does not sit flush against the frame. Fix the feet first, then align the shafts. Balancing here would simply push the unbalance from one plane to another.
Everything changed after work on the foundation
Vibration rose by a large factor after a foundation repair, a frame replacement, relocating the machine, or fitting new vibration mounts — while the rotor itself was never touched. The system's stiffness changed, and the operating speed may now sit in the zone of a natural frequency of the structure. In resonance, amplitude climbs by a large factor for the same residual unbalance, and the fix is stiffness, mass, or speed.
Sources: ISO 13373-3:2015 · ISO 281:2007
Decision table: observation, likelihood of unbalance, first step
Use the table as an initial filter: it does not make a diagnosis, it shows you where to spend the first hour of work.
| What you observe | Likelihood it is unbalance | What to do first |
|---|---|---|
| The hum got louder at operating speed, the tone is steady, no knocking | High | Measure overall and 1x at both bearing housings, work out the 1x share |
| The level rose right after cleaning the blades, replacing a blade, or repairing the impeller | Very high | Check the wheel's fit and run-out, then balance on site |
| Speed was halved, vibration dropped by roughly a factor of four | High | Balance at operating speed, in the machine's own bearings |
| Speed was halved, the level barely changed | Low | Look for an external source, structural resonance, or an electromagnetic cause |
| 1x dominates, the phase repeats, the shift between horizontal and vertical is around 90° | Very high | Fit a trial weight and calculate the correction |
| 1x is large, but the 2x-to-1x ratio is above 0.5, axial vibration is noticeable | Medium, shaft misalignment is likely | Check for a soft foot, perform shaft alignment, measure again |
| A comb of harmonics 1x, 2x, 3x, with 0.5x present, phase jumping around | Low | Torque the fasteners to spec, check play, fits and the grout |
| Knocking or high-frequency noise at any speed, impulses in the time waveform | Almost zero | Look at the bearing: clearance, lubrication, quality of the installation |
| A sharp peak over a narrow speed range, followed by a drop, phase shifting by 180° | Low, this is resonance | Change the stiffness of the supports, the mass, or the operating speed |
| Vibration changed by a large factor after a foundation repair or relocating the machine | Low | Check that all feet bear evenly and check the frame's natural frequencies |
| 1x changes from run to run with no obvious cause | Medium, but too early to balance | Check the wheel and pulley fits, the key, thermal bow in the rotor |
A single observation often has several causes at once: a soft foot produces induced misalignment, false signs of looseness, and a rise in 1x, all together. That is why the table picks a first step, and the hypothesis gets confirmed by a measurement after that step, not by reasoning alone.
Sources: ISO 13373-3:2015
What you can check yourself in twenty minutes
The first five minutes need nothing at all, the next seven need a single vibration meter, and the last eight need a two-channel system with a phase sensor.
- 00–05
By hand, wrench and eye
Stop the machine and lock out the start. Inspect the impeller: built-up product, dust on the blades, ice, edge erosion, a lost balancing weight, signs of rubbing against the housing. Rock the wheel on the shaft: check whether it has turned relative to the key. Rock the shaft radially — noticeable play means a bearing issue. Go over the foundation bolts and the bearing-housing bolts with a wrench, and flag any that turn without resistance. Check the belt tension and pulley run-out. Some machines get ruled out right here.
- 05–12
Vibration meter: one number at each support
Fit the sensor on a magnet, on a clean, flat pad on the bearing housing, as close to the bearing itself as you can get. Take the RMS (root mean square) vibration velocity in mm/s in three directions at each support: horizontal, vertical, axial. Record the speed and the operating condition, including the valve position. Compare the numbers against the approximate zones A, B, C, D for your group of machines — condition zones running from "good" to "unacceptable." Then run the main low-cost test: if there is a VFD, take the same readings at 70% and 50% of the speed. Unbalance produces a sharp drop in the level; an external source or resonance does not behave that way. With no speed control, look at the level during coast-down — the machine's free deceleration after the drive is switched off.
- 12–20
Two-channel system: 1x, phase, spectrum
Two sensors on the two supports, a laser phase sensor aimed at a reflective marker on the shaft. Take overall vibration and 1x with phase at the same time on both channels, plus the spectrum. Work out the 1x share of the overall level, and check for 2x, a comb of harmonics, and lines that are not multiples of running speed. Then stop the machine, start it again at the same operating point, and repeat the measurement. If the phase matches within a few degrees, balancing will go predictably. If it differs by tens of degrees, the mechanics come first.
All three levels of the check require the same conditions between readings: the same point, the same direction, the same sensor mounting method, the same speed, the same flow and temperature conditions. Otherwise you are comparing your own technique, not the state of the machine.
Sources: ISO 20816-1:2016
Three outcomes of these twenty minutes
What makes the check useful is that it ends in a decision. There are only three.
Balance now
The mechanics are sound, the fasteners are torqued, there is no play, 1x dominates, the phase repeats, and the level depends sharply on speed. From here it is routine work: choose the number of planes from the length-to-diameter ratio, get the trial weight to change amplitude by 20–30% or phase by 20–30°, fit the correction, and run a check.
Mechanics first, balancing after
There is a comb of harmonics, a soft foot, loose fasteners, a worn fit, or shaft misalignment. The order is strict: get the feet sitting flush, torque everything to spec, sort out fits and play, align the shafts, and only then balance. Measure again after the mechanical work — some machines are already within tolerance by that point.
Do not balance at all
A bearing on its way out, resonance in the frame or pipework, cavitation at a pump, an electromagnetic cause, a flexible rotor running above its first critical speed — the speed at which the shaft starts to bend noticeably. Here, balancing either has no effect, or the effect lasts a week. A different fix is needed, and it is better to find that out before sending an instrument out to site.
When balancing is mandatory, even if the machine is not noisy
There are situations where you do not wait for symptoms. A rotor almost always picks up a new unbalance after any intervention, and the only question is how much. A measurement before start-up is cheaper than working it out after a month of running with extra load on the bearings.
- After repairing or replacing the impeller, blades, vanes, crusher knives, or a screw conveyor, and after weld build-up on edges.
- After rewinding a motor, if the rotor was removed, the cooling fan was replaced, or the fitting surfaces were touched.
- After replacing a pulley, a coupling half, a brake disc, or any other substantial part on the shaft.
- After fitting a new grinding wheel on a spindle, and after dressing it, because the wheel wears unevenly.
- At commissioning of a new or reinstalled machine: the measurement gives you a baseline you will compare against for years to come.
- When a rotor comes back from a contractor with no residual-unbalance report, and you do not know what grade it was actually balanced to.
- Periodically, on machines prone to build-up and erosion: extractor fans, mulchers, crushers, centrifuges, screw conveyors.
- After balancing on a balancing machine, if the picture in the machine's own bearings differs from what the rig showed.
For acceptance, record the conditions, not just the figure: the measurement points and directions, the frequency band, the operating condition, the type of supports and foundation. The required precision is set by the G grades — balance quality grades: the lower the number, the finer the tolerance — and the permissible residual unbalance is calculated from the rotor's mass and operating speed. Treat the G-grade numbers and the vibration-zone boundaries as working guidance: check which part and current edition of the standard applies to your specific machine, since there are exceptions by power, speed and machine type.
Sources: ISO 21940-11:2016 · ISO 20816-1:2016
Five traps that lead to the wrong decision
- Comparing against someone else's machine instead of your own trend. A level of 4 mm/s can be normal for a large machine on a flexible foundation and a red flag for a small pump. A doubling from your own baseline matters more than the absolute figure.
- Measuring in the wrong place. A sensor on the housing, guard or ductwork gives big, impressive-looking numbers that do not describe the rotor's condition. Measure on the bearing housing, where the load actually passes through.
- A hand-held probe instead of a proper mount. A probe is good up to roughly 300–500 Hz and gives almost no repeatability. A magnet on a clean, flat pad covers the 10–1000 Hz band, which is enough for the overall level, while the bearing-frequency bands need adhesive mounting or a stud.
- Comparing numbers from different bands and different instruments. A balancing instrument's RMS vibration-velocity measurement band is 5–200 Hz per its manual, while the approximate zones are written for 10–1000 Hz. These are different quantities, and on a machine with a bearing defect they will diverge noticeably.
- One measurement instead of two. A single run will not tell you whether the phase is stable, and it is exactly that phase stability that separates unbalance from looseness and resonance. A second run costs five minutes and clears up half your doubts.
The sixth trap is haste. Balancing looks like the fast fix, so people try it before torquing the fasteners and checking for resonance. The weight goes on the wheel, and the level comes back within a week.
Sources: Balanset-1A operation manual · ISO 20816-1:2016
What to send AXILINE to get an assessment before a visit
We are engineers who design and manufacture the Balanset instruments and use them for on-site balancing ourselves. So the conversation is short and number-driven: send us what you already have, and we will tell you whether it looks like unbalance or not.
If your numbers point away from unbalance, we will say so plainly and explain what to check first: a site visit that ends with "balancing will not help here" is no use to you or to us. If the picture fits unbalance, we come and balance on site, in the machine's own bearings, with no disassembly and no sending the wheel off to a balancing machine, in one or two planes.
- Machine type: fan, extractor fan, screw conveyor, crusher, mulcher, centrifuge, spindle, motor rotor.
- Power, operating speed, whether there is a VFD.
- Type of supports and foundation: a rigid bed, a welded frame, or vibration mounts; whether the rotor is overhung or sits between the supports.
- Overall vibration and 1x at each bearing housing, with the point and direction noted, if measurements already exist.
- 1x phase, spectrum, or screenshots, if you are working with a two-channel instrument.
- Photos of the machine and of the spots where mass could be added or removed.
- What changed over the last month: cleaning, repairs, part replacements, foundation work, operating conditions.
- How the level behaves when the speed changes, and during coast-down.
Want to do this yourself? The Balanset-1A is two vibration sensors, a laser phase sensor, a two-channel USB module with preamplifiers, integrators and an ADC, and Windows software. It measures speed, and the amplitude and phase of vibration velocity separately for overall and for 1x, shows the time waveform and the spectrum, acquires both channels at once, tells you whether the trial weight was a good size, splits the weight across fixed positions, calculates a drilling correction and the tolerance against the G grades, stores influence coefficients, and keeps an archive for reports. The same system works both on site visits and as the measurement side of a soft-bearing balancing machine.
Sources: Balanset-1A manufacturer specification · Balanset-1A operation manual
Frequently asked questions
Can you tell whether balancing is needed with no instrument at all?
Partly. Without an instrument, you can rule out the obvious: build-up on the blades, a lost weight, loose fasteners, play in a bearing, a wheel that has slipped on its shaft. You can also hear the difference between a steady hum and knocking. But you cannot confirm that the running-speed component is what is actually shaping the level without measuring 1x and phase.
I have 7 mm/s at a fan bearing. Is that a lot?
Depends on the machine. For a medium machine of 15–75 kW, that is zone C: operation is allowed for a limited time, and the cause needs fixing. For a large machine on a flexible foundation, the same figure falls in zone B. What matters more is what the level used to be. A rise from 2 to 7 mm/s deserves investigation even if you are formally within tolerance.
Vibration tripled after cleaning a fan. Should I balance it right away?
First check whether a balancing weight came off along with the dirt, and whether build-up is still left on some of the blades. Then check the wheel's fit and run-out. If the mechanics are sound and 1x dominates, this is a classic case for on-site balancing: two or three runs in a single visit.
How do you tell unbalance from shaft misalignment in a single measurement?
Look at three things at once. The 2x-to-1x ratio: above roughly 0.5, suspect shaft misalignment. The axial direction: with unbalance, axial vibration is small; with angular misalignment, it is pronounced. The axial phase on the two sides of the coupling: a difference of around 180° points to shaft misalignment. With unbalance, the phase is stable, and the shift between horizontal and vertical at one support is around 90°.
Balancing helped, but the vibration came back a month later. What does that mean?
Almost always one of three things. The unbalance physically came back: product build-up, blade erosion, worn knives. The weight's fixing or the wheel's fit loosened, and the mass shifted. Or the cause was never the rotor's mass at all, but looseness or resonance, and balancing only compensated part of the level temporarily. Start with a fresh measurement of 1x, phase and the spectrum, not with new weights.
Is it mandatory to balance the rotor after a motor rewind?
If the rotor was removed, the fitting surfaces were touched, or the cooling fan was replaced, then yes: the mass symmetry changes, and a new unbalance appears even with careful reassembly. At the very least, take a measurement at operating speed after reassembly and record it as a baseline. Balancing is done on site, in the machine's own bearings, without dismantling the machine again.
Related content
How to tell unbalance from shaft misalignment: signs, phase and the right order of work
Look at four things at once: the ratio of 1x to 2x (vibration at the running speed and at twice that frequency), axial vibration, phase, and how the reading behaves during warm-up. Unbalance produces a dominant 1x, a clean spectrum, a small axial component, and a difference of around 90° between horizontal and vertical at one support. Shaft misalignment produces a noticeable 2x, large axial vibration, a phase shift of around 180° across the coupling in the axial direction, and a level that drifts as the machine warms up. If the signs point to shaft misalignment, you must not balance: you would be introducing real unbalance to compensate for a force that was never the rotor's, and after the shafts are aligned, the vibration will end up higher than it was.
The phase sensor and reflective marker: how the instrument measures phase
The phase sensor produces one short pulse for every rotor revolution. From the interval between pulses, the instrument calculates rotation speed, and from the timing of the pulse it measures the phase of the vibration's running-speed component: the angular delay between the marker and the peak of the 1x sine wave. Without that reference point, there's nothing to extract 1x from and nothing to measure degrees from, so all you're left with is overall vibration as a single number, and no correction weight can be calculated. The marker sets the zero point for phase, but not the zero point for the correction angle: the weight's mounting angle is measured from the trial weight's location instead.
On-site balancing of crushers and shredders: rotors, hammers, knives
Yes, we balance the rotors of crushers, hammer mills, shredders, pelletizers, and disintegrators on site, in their own bearing supports, with no dismantling and no balancing machine. Three conditions apply: the set of hammers or knives is matched by mass, the machine holds a stable idle speed, and there's access to at least two correction planes with the rotor stopped. If a hammer is missing, broken, or the set is mismatched by mass, balancing is pointless: the set comes first, weights come second. If overall vibration (the combined level from all causes) is many times higher than the running-speed component — the vibration at the rotor's rotational frequency, which is what imbalance produces — the cause isn't imbalance, and we'll say so plainly rather than fit weights just to close out a report.
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.