Balancing and vibration glossary: 57 terms an engineer actually uses

Unbalance

Unbalance is the condition in which a rotor's mass is not distributed symmetrically about its rotation axis, so rotation produces a centrifugal force the bearings have to carry. It is measured in g·mm — mass times radius — and defined in the ISO 21940 series. The force grows with the square of speed: 10 g at a radius of 200 mm pulls about 49 N at 1500 rpm and about 197 N at 3000 rpm.

Static unbalance

Static unbalance is the case where the rotor's principal inertia axis is displaced parallel to the rotation axis, so a single mass in one plane corrects it. It shows as in-phase vibration at both bearings, with a phase difference between the supports of under roughly 30°. ISO 21940-11 treats a disc-shaped rotor with a length-to-diameter ratio below about 0.5, running under about 1000 rpm, as a single-plane job.

Couple unbalance

Couple unbalance is two equal unbalance masses in different planes, 180° apart, which leave the centre of mass on the rotation axis but tilt the inertia axis away from it. A static check on knife edges will not find it — the rotor sits balanced at rest and shakes the moment it turns. Its signature is roughly 180° of phase difference between the two supports, and it always needs two correction planes.

Dynamic unbalance

Dynamic unbalance is the general case, a combination of static and couple unbalance, in which the inertia axis neither coincides with nor runs parallel to the rotation axis. Nearly every real rotor has it. It is corrected in two planes at once, which is why a long rotor normally needs three to four runs on site instead of two.

Correction plane

A correction plane is the plane perpendicular to the rotation axis in which the balancing weight is actually fitted. It has to be physically reachable with the machine assembled — a shroud disc, a fan hub, a coupling rim. The further apart the two planes are, the smaller the weights: moving them from 150 mm to 400 mm apart cuts the couple correction masses by roughly a factor of 2.5.

Residual unbalance

Residual unbalance is what remains in the rotor after the correction weights are fitted, stated in g·mm for each correction plane. It is judged against the permissible value derived from the balance quality grade, the speed and the rotor mass. A report line reading «residual unbalance 1200 g·mm» means nothing until you also know the rotor mass, the plane and the grade it was worked to.

Specific unbalance

Specific unbalance is residual unbalance divided by rotor mass, in g·mm/kg, and it is numerically equal to the offset of the centre of mass from the rotation axis in micrometres. This is the quantity the ISO grades are written in, so it lets you compare a 12 kg impeller with a 3-tonne roll. A 300 kg rotor carrying 12000 g·mm has e = 40 g·mm/kg, meaning its centre of mass sits 40 µm off the axis.

Balancing

Balancing is adding or removing mass on a rotor so that the residual unbalance, and the vibration it causes, fall within a stated tolerance. It reduces the 1x running-speed component and nothing else. If 1x is not the dominant part of the overall level, weights will change nothing — that is a repair problem, not a balancing problem.

Field balancing

Field balancing is balancing the rotor in the machine's own bearing housings, at working speed, without removing it. Two to four runs are normally enough, and no dismantling, transport or re-alignment follows. It is the right choice when a correction plane can be reached and the rotor behaves as a rigid one at working speed.

Trial mass

A trial mass, also called a calibration weight, is a known weight fitted temporarily at a known angle so the instrument can learn how the rotor responds to mass in that plane. It is sized to change the picture measurably without endangering anything: aim for at least a 20% change in amplitude or a 20–30° shift in phase. If neither moves, the trial mass was too small — or the machine is sitting at resonance.

Influence coefficient

The influence coefficient is the measured ratio between the change in vibration at a support and the trial mass that caused it, in both amplitude and angle. It is what the instrument computes after the trial run and then inverts to give the correction mass and its angle. Two-plane work produces four of them, because each plane influences both supports.

Correction mass

The correction mass is the final weight, of calculated size and angle, permanently fitted in the correction plane. It can be welded, bolted or riveted on, or replaced by drilling metal away at the opposite angle. AXILINE charges +150 EUR per rotor for fitting masses without welding and +400 EUR with welding, plus VAT.

Trim balancing

Trim balancing is a follow-up correction computed from influence coefficients already stored from an earlier job, with no new trial run. It turns a repeat visit into one run instead of three, which matters on rotors that are rebuilt, recoated or re-bladed routinely. The stored coefficients hold only while speed, sensor points and correction planes stay unchanged.

Single-plane and two-plane balancing

Single-plane balancing corrects unbalance with one weight; two-plane balancing corrects it with weights in two planes at once. The choice follows rotor geometry and speed, not preference: a narrow disc under about 1000 rpm usually takes one plane, a long rotor or anything faster normally takes two. Choosing one plane on a rotor with couple unbalance costs a shift, because the solution never converges.

Angle reference

The angle reference is the agreed zero point on the rotor and the agreed direction of counting, against which every weight angle in the report is stated. The instrument counts from the reflective mark, normally against the direction of rotation. On a 12-blade impeller the fixed positions are 30° apart, and counting the wrong way puts the mass at the mirror angle, so vibration rises instead of falling.

Balance quality grade G

The balance quality grade G is the ISO 21940-11 class that sets the permissible specific unbalance; the number is the product of specific unbalance and angular velocity, in mm/s. G6.3 covers most fans, pumps and standard electric motors, G2.5 covers turbines, compressors and machine-tool drives, G1 and G0.4 belong to grinding spindles. Each step down roughly halves the permissible mass and multiplies what it costs to get there.

ISO 21940-11

ISO 21940-11 is the standard that gives balance quality grades for rigid rotors and the method for turning a grade into a permissible residual unbalance. It superseded the older ISO 1940-1 and is the reference to quote in a balancing report. It also sets out how the permissible unbalance is divided between two correction planes according to their position relative to the bearings.

Permissible residual unbalance

Permissible residual unbalance, U_per, is the largest unbalance a rotor may retain after balancing, derived from the grade, the speed and the rotor mass. The chain is e_per = 9549·G/n in µm, then U_per = e_per·M in g·mm, then divide by the correction radius to get grams. A 300 kg fan at 1500 rpm to G6.3 gives e_per = 40 µm, U_per = 12000 g·mm, or 30 g at a radius of 400 mm.

ISO 20816

ISO 20816 is the standard for measuring and evaluating machine vibration on non-rotating parts, and it defines the zone limits that decide whether a machine may run. It superseded ISO 10816 and is split into parts by machine type and size. Measurements are taken on the bearing housings, in mm/s RMS, over the 10–1000 Hz band.

Evaluation zones A, B, C, D

Zones A, B, C and D are the ISO 20816 bands that classify a measured level as new-machine condition, unrestricted long-term operation, restricted operation, or damaging. For medium machines of 15–75 kW on rigid supports the boundaries are 1.12, 2.8 and 7.1 mm/s RMS. Above 7.1 mm/s the machine is in zone D and operation is treated as inadmissible — check the part and edition that apply to your machine, because the numbers move with power, speed and mounting.

Baseline level

The baseline is the vibration level recorded on a machine known to be in good condition, against which every later measurement is compared. A trend against a baseline finds a developing fault far earlier than an absolute limit does: a rise from 1.0 to 2.0 mm/s is still inside zone B, but a doubling is a real signal. Record it after commissioning or after repair, at the same points, direction and load.

ISO 13373

ISO 13373 is the standard for condition monitoring and diagnostics of machines by vibration, covering measurement procedures, signal processing and diagnostic interpretation. It describes how a route, a measurement chain and a diagnosis should be built, while ISO 20816 supplies the numeric limits. Part 1 deals with general measurement procedures; later parts cover processing and diagnostic techniques.

ISO 281

ISO 281 is the standard for the basic rating life of rolling bearings — the L10 life, at which 10% of a population is expected to have failed. For ball bearings life varies with the cube of the ratio of dynamic capacity to load, so halving the load from unbalance multiplies calculated life by about eight. That relationship is the whole financial argument for balancing before a bearing starts making noise.

RMS vibration velocity

RMS vibration velocity is the root-mean-square of the velocity signal, in mm/s, and it is the quantity the ISO 20816 limits are written in. It is measured on the bearing housing, radially and axially, over the 10–1000 Hz band. RMS is used because it tracks the energy in the signal; for a pure sinusoid it equals 0.707 of the peak value.

Vibration acceleration

Vibration acceleration, in m/s² or g, is what a piezoelectric sensor actually measures and the quantity that makes high-frequency faults visible. Bearing and gear defects live above 1 kHz, where integration to velocity has already suppressed them. One g equals 9.81 m/s².

Vibration displacement

Vibration displacement, in micrometres, is the physical amplitude of the movement, and it dominates the picture at low frequencies. It is the natural unit for shaft-relative measurements on sleeve bearings, taken with proximity probes. Below about 10 Hz it shows what velocity hides: 100 µm peak-to-peak at 5 Hz is only about 1.1 mm/s RMS.

1x component

The 1x component is the part of the vibration at the rotation frequency itself, and it is the only part that balancing can reduce. At 1500 rpm it sits at 25 Hz, at 3000 rpm at 50 Hz. If 1x accounts for most of the overall level, balancing will work; if it is a minority of it, look at mounting, alignment, bearings or resonance first.

Phase

Phase is the angular delay between the once-per-revolution reference pulse and the peak of the 1x vibration, expressed in degrees. It tells you where the heavy spot sits, and comparing it between the two supports separates static unbalance from couple unbalance. Phase that will not repeat from run to run points at resonance, looseness, or a rotor whose geometry is changing.

Spectrum

A spectrum is the vibration signal decomposed by frequency, showing where the energy actually sits. It is what separates unbalance from misalignment, looseness and bearing damage, all of which look identical in the overall level. A dominant 1x means unbalance; a strong 2x with high axial vibration means misalignment.

Time waveform

The time waveform is the raw vibration signal plotted against time, before any frequency analysis. It shows what a spectrum averages away: impacts, clipped peaks, modulation, rubbing. A repetitive spike train in the waveform usually means the machine is hitting something, and no correction weight will change that.

Envelope spectrum

The envelope spectrum is an analysis that isolates the repetition rate of impacts by filtering a high-frequency band and then analysing how its amplitude is modulated. It is the standard way to find rolling-element bearing damage, normally over a band of about 2–10 kHz. It reveals a defect long before the fault shows in the ordinary velocity spectrum or in the overall level.

Crest factor

The crest factor is the ratio of the peak value of the time signal to its RMS value, and it tells you whether the vibration is smooth or made of impacts. A pure sine wave gives 1.41 and healthy machinery runs at about 3 to 4. Values above roughly 5 or 6 mean impacts, which points at bearings, looseness or rubbing rather than at unbalance.

Fmax and number of lines

Fmax is the upper frequency limit of a spectrum and the number of lines is how many frequency bins it is divided into; the ratio between them is the resolution. For balancing, 200–500 Hz with 1600 lines is enough. For bearing diagnostics Fmax has to reach roughly 10 kHz, and 1600 lines then give 6.25 Hz per line — too coarse to separate closely spaced bearing frequencies.

Accelerometer

An accelerometer is a piezoelectric sensor that converts the acceleration of a surface into an electrical signal, and it is the standard sensor for field balancing and route measurements. The instrument displays mm/s because the acceleration signal is integrated inside it, not because the sensor measures velocity. Choose it for wide bandwidth, low mass and tolerance of a hot bearing housing.

Sensor mounting

Sensor mounting is how the accelerometer is attached to the machine, and it sets the highest frequency at which the measurement can be trusted. A stud on a machined pad is good to about 10 kHz, a flat magnet on clean bare metal to roughly 2 kHz, a hand-held probe to no more than about 1 kHz. A magnet on paint or on a curved surface loses exactly the high frequencies where bearing faults live.

Phase sensor (laser tachometer)

The phase sensor is an optical sensor that produces one short pulse per revolution, giving the instrument both the running speed and the reference point for phase. Speed comes from the interval between pulses, angle from the instant of the pulse. A laser type holds a stable reading at a working distance of up to roughly half a metre, which lets it stand clear of the guard.

Reflective mark

The reflective mark is a strip of retroreflective tape stuck to the rotating shaft or hub, which the phase sensor uses as its once-per-revolution target. One mark, and only one — a second reflector makes the instrument read a multiple of the true speed. A strip about 10–20 mm wide is enough, and it must be the only bright reflector the beam can see.

Measurement point

The measurement point is the exact place, direction and mounting used for a reading, and it is part of the measurement itself, not a detail around it. Put the sensor on the bearing housing, as close to the bearing as possible, on clean flat metal. A reading taken 200 mm away on sheet steel cannot be compared with the machine's own baseline and cannot be judged against ISO 20816.

Balanset-1A

Balanset-1A is a two-channel field balancing instrument and vibration analyser: two accelerometers, a laser phase sensor and software that computes correction masses in one or two planes. It stores influence coefficients, so a repeat job can be done as a trim without a new trial run. AXILINE supplies it at 1975 EUR plus VAT, and the Balanset-1A OEM measuring system for balancing machines at 1735 EUR plus VAT.

Rotor

A rotor is any body that turns in bearings — an impeller, an armature, a drum, a roll, a shaft assembly — and whose mass distribution about the rotation axis determines its unbalance. For balancing purposes it is the whole assembly as it runs, coupling and pulley included, not the bare shaft. Change anything on it — a new pulley, a rewound armature, one replaced blade — and the unbalance changes with it.

Rigid rotor

A rigid rotor is one that does not deform appreciably at working speed, so a correction made in two planes holds across the whole speed range. ISO 21940-11 is written for rigid rotors, and in practice a rotor is treated as rigid while it runs below roughly 70% of its first critical speed. Almost all field balancing is rigid-rotor work.

Flexible rotor

A flexible rotor is one that bends measurably at working speed, so its unbalance distribution changes with speed and a two-plane correction no longer holds everywhere. It appears where the machine runs at or above its first critical speed — long turbine shafts, multistage pump rotors. It needs modal or multi-plane methods, and a correction that works at 3000 rpm can be wrong at 1500.

Critical speed

Critical speed is the speed at which the rotation frequency coincides with a bending natural frequency of the rotor itself, so the shaft bows and the 1x vibration climbs sharply. The phase turns by about 180° as the machine passes through it. Keep the working speed at least 15% clear of a critical speed; balancing inside that band produces numbers that will not repeat.

Overhung rotor

An overhung rotor sits outside its bearings, cantilevered off the end of the shaft, as on most single-inlet fans and pumps. Both correction planes are on the same side of the supports, so cross-effect is strong: a weight in one plane visibly moves the other support. It also produces high axial 1x vibration, which on a between-bearings machine would have suggested misalignment instead.

Between-bearings rotor

A between-bearings rotor carries its mass between the two supports, as on most electric motors, blowers and double-inlet fans. Cross-effect between the correction planes is weaker and the calculation converges faster — two to three runs is normal. Static and couple unbalance separate cleanly here, which is why comparing phase between the supports works so well on this layout.

Bearing pedestal

The bearing pedestal, or bearing housing, is the fixed structure that carries the bearing and transfers the rotor's forces into the frame and the foundation. It is where the sensor goes and where the ISO 20816 limits apply, because it is the non-rotating part that responds to the rotor. A cracked or soft pedestal changes the machine's response, and a balance done on it will not repeat.

Shaft misalignment

Shaft misalignment is a cause of vibration: the axes of two shafts joined by a coupling do not coincide when the machine is running. It can be parallel, angular or — almost always in practice — both at once. The signature is a strong 2x component together with high axial vibration, and no correction weight will remove it.

Shaft alignment

Shaft alignment is the corrective action: adjusting shims and machine position until the two shaft axes coincide within tolerance in the running condition. It is the fix for misalignment, and the two terms are not interchangeable — one names the fault, the other the work. Do it after soft foot is cured and before balancing, because alignment work moves the machine and invalidates the balance.

Soft foot

Soft foot is the condition where one machine foot does not sit flat on its base, so tightening the bolts distorts the frame. Loosen one bolt at a time with a dial gauge on the foot: movement above about 0.05 mm is treated as soft foot and shimmed out. It is the cheapest fault to find on site, and left in place it corrupts both alignment and balancing results.

Mechanical looseness

Mechanical looseness is a loss of stiffness in a joint — a bolt that no longer holds, a worn fit, a crushed shim, a cracked weld. The spectrum answers with harmonics of running speed, often 2x, 3x and beyond, and sometimes with half-order components. Balancing a loose machine wastes a shift, because the response changes between runs and the coefficients never settle.

Resonance

Resonance is the amplification of vibration that occurs when an exciting frequency coincides with a natural frequency of the rotor, bearings, frame or foundation. Three signs together identify it: a sharp narrow amplitude peak over a speed range, a phase turn of about 180° through it, and readings that will not repeat between runs. Balancing does not cure it — the structure or the speed has to change.

Rub

Rub is intermittent contact between the rotor and a stationary part — a seal, a shroud, a labyrinth — during rotation. It makes the system non-linear, because stiffness only appears at the instant of contact, so influence coefficients stop being valid and the balancing solution never converges. Look for half-order components in the spectrum and truncated peaks in the time waveform.

Thermal bow

Thermal bow is a bend in the rotor caused by uneven heating, which changes its unbalance as the machine warms up. It makes the machine non-stationary: 1x amplitude and phase drift for the first 20 to 60 minutes after start. Balance only once the machine has reached a steady thermal state, or the correction will be wrong at working temperature.

BPFO

BPFO, the ball pass frequency of the outer race, is the rate at which rolling elements pass a defect on the stationary outer ring of a bearing. It is a non-integer multiple of running speed — roughly 0.4 times the number of rolling elements times the running speed. It is looked for in the envelope spectrum, where it appears with harmonics long before the overall velocity level moves.

BPFI

BPFI, the ball pass frequency of the inner race, is the rate at which rolling elements pass a defect on the rotating inner ring. It runs at roughly 0.6 times the number of rolling elements times the running speed, higher than BPFO on the same bearing. Because the defect turns through the load zone, BPFI usually appears modulated by running speed, with 1x sidebands around it.

Cavitation

Cavitation is the formation and collapse of vapour bubbles in a pump when local pressure falls below the vapour pressure of the liquid. It sounds like gravel passing through the pump and gives a broadband rise of roughly 1 to 10 kHz with no clear line in the spectrum. It is a hydraulic problem — correct the suction conditions or the operating point, because balance weights do nothing for it.

Blade pass frequency

Blade pass frequency is the running speed multiplied by the number of blades or vanes — the rate at which blades pass a fixed point such as the volute tongue. A 12-blade impeller at 1500 rpm gives 300 Hz. A rise there points at the gap between impeller and casing, at flow conditions or at a damaged blade, not at unbalance.

Rotor unbalance: what it is and why it is dangerous

Rotor unbalance is a mismatch between the rotor's principal central axis of inertia and the axis of rotation set by the bearings. In plain terms: the mass is distributed asymmetrically, and rotation produces an uncompensated centrifugal force that sweeps around the support once every turn. The force equals the unbalance multiplied by the square of the angular speed, so doubling the speed means quadrupling the force. Unbalance is dangerous not because it breaks the machine outright, but because of the cyclic loading: bearing life falls by a large factor, fatigue cracks grow in welds and the frame, fasteners work loose, and machining accuracy drifts.

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Vibration Limits per ISO 20816: Zones A, B, C, D and How to Use Them

ISO 20816 (ISO 10816 used to play this role before it) assesses a machine's broadband vibration, measured on non-rotating parts: bearing housings and pedestals. There's one parameter: the root mean square (RMS) of vibration velocity in mm/s, in the 10–1000 Hz frequency band. The result is compared against four zones: A is a new machine or one after a quality repair, B is a level at which extended operation is acceptable, C means limited operation until the cause is fixed, D means an unacceptable level. Zone boundaries depend on the machine's power and on the type of support, and a flexible (resilient) foundation allows higher vibration than a rigid one.

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On-site balancing of fans and smoke exhausters: service visits across Portugal

Yes, we balance ventilation equipment right at the site where it operates: from roof fans and blowers to boiler-house smoke exhausters and cooling-tower fans. The impeller turns in its own bearing supports, so there's no need to remove the rotor or dismantle the ductwork. Three conditions apply. The machine has to reach stable operating speed. The correction plane — the spot on the impeller where the correction weight goes — needs to be reachable through an inspection hatch, a removable cover, or an opening in the housing. And the vibration has to be dominated by the 1x running-speed component — vibration at the impeller's rotational frequency, which is what imbalance produces — rather than by bearings, shaft misalignment, or housing resonance. We check the first two conditions from your photos when you submit a request; we measure the third ourselves in the first half hour on site, and we'll tell you honestly if weights won't help here.

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