AXILINEHow to reduce equipment vibration: a sequence that saves money
The machine is shaking, and everyone expects a fix by morning. The temptation is strong: pull the impeller and send it off for balancing. Sometimes that's the right call, but more often it's a wasted shift, because the vibration was actually coming from a soft foot (a support that doesn't sit flush on the foundation), a worn bearing, or frame resonance. Below is a six-step route that cuts out unnecessary work: cheap checks first, expensive ones later.
The route: six steps from “shaking” to normal
- 01
Make sure the reading is real
Repeat the measurement at the same operating mode, at the same point, in the same direction. A number you haven't reproduced twice isn't good enough to base a decision on.
- 02
Split the vibration into 1x and everything else
Compare the overall level with the running-speed component and look at the spectrum — the breakdown of vibration by frequency. This step determines whether you need balancing or a repair.
- 03
Check the mechanics
Fasteners, soft foot, play in the joints, the fit of the wheel on the shaft, the frame and foundation. The cheapest place to find the problem, and the one most often skipped.
- 04
Rule out resonance
Vary the speed and watch the amplitude and phase of 1x. In a resonance zone, balancing won't hold.
- 05
Balance if 1x dominates
Two or three runs on site, in the machine's own bearings, without disassembly. The weight goes on an accessible correction plane — a location on the rotor where it can be fastened.
- 06
Confirm the result
Take a reading after the work: same mode, same points, same frequency band. Record the before-and-after figures in the report.
Order matters more than speed. Balancing a machine with loose feet or shaft misalignment eats up a whole day and gets you nowhere, because you're fighting the wrong cause. Each step in the route is cheaper than the next, so it pays to go through it top to bottom.
Step 1. Take a reading you can trust
A fan starts humming after the impeller is cleaned. The first instinct is to strip it down and take the wheel off for balancing. Start cheaper. Put a sensor on the bearing housing and get a number that repeats.
Hold the sensor with a magnet, on a clean flat spot, as close to the bearing as possible. Use the horizontal-radial direction if you don't know for certain where the vibration is higher. From there on, keep that direction and that point fixed. Warm the machine up, set the same load and the same speed, or you're comparing two different machines.
Implausibly large numbers almost always mean a measurement problem, not a machine problem. The instrument shows 250 mm/s on the pump housing instead of the usual 3 mm/s? Check the cable, the connector, and how flush the magnet sits, then take the reading again.
- Measurement point: the bearing housing, not the guard or the protective screen.
- Sensor mounting is rigid: magnet on bare metal, no paint, no dirt, no gaskets.
- Same operating mode every time: speed, load, temperature, damper or valve position.
- The reading is repeated at least twice, with the spread within 10-15%.
- Speed is recorded. Without it you won't find 1x in the spectrum.
- Note what's running nearby: a neighboring unit can carry vibration in through the foundation.
Separately, record the baseline level for this machine in good condition. A rise from the baseline tells you more than the absolute number: 2.5 mm/s on a machine that always used to read 0.8 mm/s deserves attention, even though technically it's still zone B.
Step 2. Compare the overall vibration with the running-speed component
Overall vibration is everything at once, as one number in mm/s RMS (root mean square): imbalance, shaft misalignment, looseness, bearings, resonance, vibration picked up from neighboring machines.
The 1x running-speed component is the part of the vibration exactly at the rotation frequency, once per revolution. That's exactly where imbalance lives. The instrument extracts 1x using the tachometer mark and also gives its phase — the angle that shows at what point in the revolution the vibration reaches its peak.
Look at the proportion. 8 mm/s overall with 7 mm/s on 1x: balancing will remove almost all of it. 8 mm/s overall with 2 mm/s on 1x: balancing will remove a quarter of the level, the rest will stay, and you'll have to explain what was paid for.
Phase separates two defects that produce equally high 1x. With imbalance the phase is stable, and on one bearing the difference between the horizontal and vertical direction is around 90°. With shaft misalignment across the coupling in the axial direction, the shift is around 180°. The fixes here are opposite: balancing or shaft alignment.
- The 1x peak dominates, phase is stable: most likely imbalance, balancing is appropriate.
- Noticeable 2x plus axial vibration: shaft misalignment or a soft foot.
- A comb of harmonics — 1x, 2x, 3x and higher — a raised noise floor, phase jumping around: mechanical looseness, a crack, rubbing.
- Peaks at high frequencies not related to running speed, rising crest factor (the ratio of signal peaks to the average level): a rolling-bearing defect.
- Peaks below running speed (sub-synchronous) on a belt drive: the belt, its tension, pulley runout.
Balancing only reduces the running-speed component. No weight on the impeller will remove bearing knock, a harmonic comb from looseness, or vibration from shaft misalignment. That's the main rule that saves you site visits.
What you observe, the likely cause, the first step
| What you observe | Likely cause | First step |
|---|---|---|
| High 1x, phase stable, horizontal-to-vertical difference at the bearing around 90° | Rotor imbalance | On-site balancing, in the machine's own bearings |
| 1x plus noticeable 2x, strong axial vibration, phase shift around 180° across the coupling | Shaft misalignment, soft foot | Check how the feet sit, then perform shaft alignment |
| Harmonics at 1x, 2x, 3x, sub-harmonics, unstable phase | Loose fasteners, a cracked frame, rubbing | Torque the fasteners to spec, check for play and check the grouting |
| Sharp peak in a narrow speed range, 1x phase shifts by 180° during run-up | Resonance of the rotor or the structure | Shift the operating speed or change the stiffness of the supports |
| High-frequency noise, lines not related to running speed, impulses in the time waveform | Rolling-bearing defect | Replace the bearing, look into the mounting and lubrication |
| Level rose after cleaning, a blade replacement, or impeller repair | New impeller imbalance | On-site balancing after checking the fit |
| 1x changes from run to run with no obvious cause | Loose fit of the wheel, pulley, or screw on the shaft, or the key | Check the fit and runout, restore the mating surfaces |
| The component disappears instantly when power is cut | An electromagnetic cause, not a mechanical one | Coast-down test, check twice line frequency |
| Vibration rises as the valve closes, noise like gravel inside the pump | Cavitation, operating away from the design point | Bring the operating point back to spec, check the suction head |
The table helps you choose a first step, not settle on a final diagnosis. One symptom often has several causes: a soft foot can induce apparent misalignment, produce false looseness, and raise 1x all at once. Test the hypothesis with a measurement, not a guess.
Sources: ISO 13373-3:2015 · ISO 281:2007
Step 3. Mechanics: fasteners, soft foot, play, foundation
This is where a wrench, a dial indicator, and half an hour do the work. Sometimes that's enough to cut the level in half, and balancing turns out not to be needed at all.
Soft foot deserves special attention. One of the supports doesn't sit flush on the foundation: a gap, a tilt, a warped frame, dirt under the foot. Tightening the bolt distorts the housing, pulls the bearing seats out of position, and drives up 1x and 2x. Shaft alignment is pointless with a soft foot present — the result will drift as soon as you retorque the bolts.
- Torque the foundation bolts and bearing-housing bolts to the documented values, not by feel.
- Check for soft foot: loosen the bolt on one foot, watch for a change in vibration and the gap, retorque, move to the next foot. Shim the gap with calibrated shims.
- Inspect the frame and the grouting: cracks, separation from the foundation, stacks of random washers used instead of proper shims.
- Check the radial play in the bearing housings and the shaft runout with a dial indicator.
- Check the fit of the impeller, pulley, or screw on the shaft, and the condition of the key and keyway.
- On a belt drive, check the tension, belt wear, and pulley runout.
- Clear away any build-up: dust on the blades, product on the screw, ice, scale. Otherwise the imbalance will come back.
- Perform shaft alignment only after the soft foot is fixed, and only in that order.
Log the vibration level after every action. That way you'll see exactly what made the difference, instead of guessing later whether it was the retorquing or the alignment.
Sources: ISO 13373-5:2020
Step 4. Check for resonance, or the result won't hold
Resonance amplifies whatever is already there. A small residual imbalance at the resonant frequency produces a large amplitude. Balancing in this zone goes badly: the phase jumps around from run to run, the influence coefficient (the machine's response to a known trial weight) comes out unstable, and the result doesn't repeat.
Have a variable-frequency drive? Vary the speed smoothly and watch 1x. Resonance gives itself away with a narrow peak: moderate amplitude before it, a sharp rise at it, a drop after it. The 1x phase swings by about 180° as it passes through the peak. On a machine without speed control, look at the coast-down — how the vibration behaves while the machine freely runs down after the power is cut.
It's not only the rotor that can resonate. Often it's the frame, a bracket, piping, a service platform, or the housing itself. Apply the sensor to the frame and then to the support in turn: if the frame moves more than the machine does, you've found where the problem lives.
- Amplitude rises sharply in a narrow speed range and drops off beyond it.
- 1x phase shifts quickly near the peak and stays stable away from it.
- The support, frame, or piping moves more noticeably than the machine itself.
- Vibration changed by a large factor after foundation repair, a frame replacement, or relocating the unit.
Resonance isn't fixed with weights but by changing stiffness, mass, or operating speed: shift the working frequency, brace the frame with a diagonal strut, unload the piping, review the vibration mounts. A separate case is a flexible rotor running above its first critical speed (the speed at which the shaft itself goes into resonance). It balances differently from a rigid rotor and calls for a special method and a specialist.
Sources: ISO 21940-12:2016
Step 5. Balancing: how it's done, and what numbers to aim for
Reached this step and you're seeing dominant 1x with the mechanics in good order? Balance on site, in the machine's own bearings, with no disassembly and no sending the wheel out to a balancing machine. Two sensors on the bearing housings, a laser phase sensor aimed at a reflective mark, and you work at the actual operating speed under real conditions.
- Zone A: the condition of a new machine or one after a repair.
- Zone B: long-term operation is acceptable.
- Zone C: operation is acceptable only for a limited time; the cause needs to be fixed.
- Zone D: operation is not acceptable, the vibration is damaging the machine.
- The goal of balancing: bring the overall vibration into zone A or B and keep it there.
- 01
Run 0, baseline vibration
The machine at operating speed, the instrument records the amplitude and phase of 1x at both bearings. That's what you're going to reduce. It also gives you another look at 1x against the overall level.
- 02
Trial weight
You fit a known mass at a known radius on the first correction plane. The instrument observes how the system responded and calculates the influence coefficient. A valid trial run changes the 1x amplitude by 20-30% or the phase by 20-30°. A smaller change means the weight is too light: stop the machine, increase the weight, enter the actual mass, and repeat.
- 03
Calculating and fitting the correction
The software outputs a mass and an angle for each plane. In fixed-position mode it names the blade or hole number directly, so no protractor is needed. The angle is measured from the spot where the trial weight was fitted.
- 04
Verification run
A reading at the same operating mode. Didn't hit the target value? The software calculates an add-on to the weights already fitted, and you reach tolerance without a fresh calibration.
| Machine group | Zone A | Zone B | Zone C | Zone D |
|---|---|---|---|---|
| Class I, small machines up to 15 kW | ≤ 0.71 | 0.71-1.80 | 1.80-4.50 | > 4.50 |
| Class II, medium machines 15-75 kW | ≤ 1.12 | 1.12-2.80 | 2.80-7.10 | > 7.10 |
| Class III, large machines, rigid foundation | ≤ 1.80 | 1.80-4.50 | 4.50-11.20 | > 11.20 |
| Class IV, large machines, flexible foundation | ≤ 2.80 | 2.80-7.10 | 7.10-18.00 | > 18.00 |
The numbers in the table are given in mm/s RMS over the 10-1000 Hz band as a working guide, based on the classification previously used in ISO 10816-3. Check the applicable part and current edition of ISO 20816 for your specific machine: it has exceptions by power, speed, and unit type, and applicability needs to be checked separately for small fans and belt-driven units. An “in tolerance” reading in the instrument's software means exactly one thing: the residual 1x is below the target value you entered. It does not confirm an ISO 20816 rating or balance quality grade G under ISO 21940-11. A typical order of magnitude for fans: a unit at around 12 mm/s comes down to 1.5-2 mm/s, if imbalance really was the main source of the level. On a machine with shaft misalignment or a worn bearing, it won't.
Sources: ISO 20816-1:2016 · ISO 21940-11:2016 · Balanset-1A operation manual
What you can do yourself, and when to call in an engineer
We're engineers who design and manufacture the Balanset instruments and use them ourselves on site visits. So the conversation is usually short: send us the machine type, speed, power, and the current vibration level with the measurement point noted, and we'll tell you whether it looks like imbalance or not. If it doesn't, you don't need a site visit, and we'll say so directly. Consulting support on the instrument and the method is included.
On your own
A repeatable reading at the bearing housings. Torquing the fasteners to spec. Checking for soft foot by loosening the bolts one at a time. Inspecting the frame, the grouting, the belts and pulleys. Cleaning the blades and removing build-up. A trend: three or four readings a month at the same operating mode. This set of actions is enough to resolve a significant share of elevated-vibration cases without a site visit.
When to call in a specialist
The spectrum is ambiguous and looks equally like two different defects. The level stays high after retorquing and alignment. The 1x phase doesn't repeat from run to run. Resonance or a flexible rotor is suspected. The machine is critical, and you need a report with before-and-after figures. The rotor is large, and the trial weight needs to be calculated rather than guessed.
What we do on a site visit
We come to the machine, take vibration readings at the bearings, separate 1x from the overall level, and look at the spectrum and phase. If the cause is imbalance, we balance on site, in the machine's own bearings, in one or two planes. If the cause is something else, we say so plainly and explain what to do next. We work across Portugal: fans, exhaust fans, screw conveyors, crushers, mulchers, centrifuges, pulleys, and electric-motor rotors.
If you want to do this yourself
The Balanset-1A is two accelerometers, a laser phase sensor, a two-channel USB module, and Windows software. It measures speed, the amplitude and phase of vibration velocity, both overall and 1x, and displays the spectrum. It calculates the trial and correction weights, splits the weight across fixed positions, works out drilling, stores influence coefficients, and keeps an archive of results for reports. The same system works both on site visits and as the measuring part of a soft-bearing balancing machine.
Before you call, gather the minimum data: speed, power, type of supports, the overall level and 1x at each bearing, and what changed on the machine over the past month. With these figures, working it out takes minutes instead of another site visit.
Sources: Balanset-1A manufacturer specification
Frequently asked questions
Will balancing reduce vibration if the cause is shaft misalignment?
No. Balancing only reduces the 1x running-speed component. Shaft misalignment produces a noticeable 2x and strong axial vibration, and a weight on the wheel won't remove that. You'll spend a shift on trial runs and end up with the same level. First fix the soft foot, then perform shaft alignment, then take a fresh reading and decide whether balancing is still needed.
What mm/s level counts as normal?
As a working guide: for medium machines of 15-75 kW, zone A is up to 1.12 mm/s RMS, zone B up to 2.8 mm/s, zone C up to 7.1 mm/s, and above 7.1 mm/s operation is not acceptable. Measure on the non-rotating parts over the 10-1000 Hz band. Check the applicable part and edition of ISO 20816 for your specific machine — it has exceptions by power, speed, and machine type.
Can vibration be reduced without disassembly or removing the impeller?
In most cases, yes. Balancing is done on site, in the machine's own bearings: two or three runs at operating speed, with the weight fitted to an accessible correction plane. You only need to remove the wheel and take it to a balancing machine when there's no access to a correction plane, the rotor is flexible, or its geometry is already damaged.
Why did the vibration come back a month after balancing?
Three common causes. First: build-up, wear, or erosion — the imbalance simply came back, and the source of contamination was never removed. Second: the weight was held by a tack weld or adhesive and shifted. Third: the real cause was never imbalance but resonance, loose fasteners, or shaft misalignment, and balancing only masked the level for a while.
1x dominates, but balancing isn't reducing the vibration. What now?
Check four things. Speed close to resonance: the 1x phase will be unstable from run to run. A loose fit of the wheel, pulley, or screw on the shaft: 1x changes after every run. Thermal bow on a hot machine. An error in the angle reference direction: the weight ended up mirrored, and the level went up. If the case stays unclear, move to full vibration diagnostics instead of more trial runs.
Is an ordinary vibration meter enough to decide what to do?
A vibration meter gives you one overall-level number. It tells you the machine is unwell, but not why. To decide, you need 1x, phase, and the spectrum — which means a sensor on each bearing and a tachometer mark. A two-channel system with a laser phase sensor gives you all of that at once and lets you balance right away if 1x is confirmed as the cause.
Related content
How to Measure Vibration Correctly: Point, Mounting, Condition
Mount the sensor on the bearing housing, rigidly, on a spot cleaned down to bare metal, with its sensitivity axis exactly along the measurement direction. For an overall assessment, take vibration velocity in mm/s RMS (root mean square); for high frequencies and impacts, take vibration acceleration; for slow-turning shafts, take vibration displacement. Record the speed, load, and operating condition alongside the number, or there will be nothing to compare this reading against. Repeat the measurement three times: a spread greater than 10–15% means it's the measurement that needs fixing, not the machine.
How to find the cause of equipment vibration: a methodology, not a list of causes
The cause is found not from a single spectrum (the breakdown of vibration by frequency) but from the whole set of signs together: context (what changed and when it started), how reliable the reading is, the ratio of vibration across all bearing supports in three directions, how the frequencies relate to running speed, the shape of the time waveform, repeatability, and phase — the angle showing at what point in the turn the vibration reaches its peak. You gather this data first, then formulate a single hypothesis, and test it with one controlled action. The diagnosis is not made by the instrument — it comes from the combination of the signs, the machine's behaviour, and a confirming check.
Balancing production-line equipment at the point of operation
Yes, we balance production-equipment rotors on site. Three conditions are needed: the rotor starts up at running speed and holds it steadily, there's access to the bearing supports for sensors and to a correction plane for weights, and the once-per-turn component 1x — vibration at the rotor's rotation frequency, which is exactly what unbalance creates — accounts for most of the vibration. If 1x is small and the machine still shakes, the cause is in the fixings, misalignment, the bearings or resonance, and we'll say so before any work starts. In food, pharmaceutical and chemical production we work without hot work: bolted stainless-steel weights and studs, or removing metal by drilling in allowed locations.
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.