# Rotor Balancing After Rewinding and Repair: Assembled Trim Balancing On Site

> The motor is back from repair and humming louder than before. A familiar story: rewinding, a fresh varnish impregnation, new bandages, different bearings — and a rotor that ran quietly for years before the repair has become a source of vibration. We measure with a two-channel vibration analyser, separate assembly-related causes from imbalance, and bring the rotor down to tolerance right in the assembled machine, in its own bearing supports. If the cause is electrical — a rewinding defect — you'll get numbers to take to the repair shop, not weights that would change nothing.

**In short:** Yes, we balance rotors on site, assembled and at running speed, after rewinding, repair and bearing replacement. Two conditions apply: the running-speed component 1x — vibration at the rotor's rotational frequency, which is what imbalance produces — must dominate the spectrum, and the fits and bearings must be sound after the repair. If the rotor isn't assembled into the machine yet, it's more honest to balance it on a lathe in the shop first, then take a confirmation reading after assembly and do an assembled trim balance if needed. Lathe balancing of the bare rotor doesn't account for the cooling fan, the coupling half, the key, or the stiffness of the actual bearing supports, so "balanced in the shop" and "runs quietly in the machine" are not the same thing.

Source: https://axiline.pt/en/equipment/rotor-balancing-after-rewinding-repair/  
Publisher: AXILINE · Vila Nova de Gaia, Portugal · +351 931 831 229 · axilinegeral@gmail.com

## Symptoms: the motor runs worse after repair than before it

The complaint almost always sounds the same: it was quieter before the repair. That's a valuable observation. The vibration has a specific triggering event rather than accumulated wear, and the search narrows immediately to whatever was touched during the repair. Bring numbers, not impressions, to the conversation with the repair shop: a reading from before the repair, if you have one, and a reading from after.

- [x] Noise and vibration increased right after the motor came back from rewinding or repair
- [x] On the repair shop's test bench the motor ran quietly through break-in, but started humming once loaded as part of the unit
- [x] Vibration builds up as the motor warms and eases off as it cools: suspect uneven varnish impregnation or a turn-to-turn short
- [x] New bearings started humming or running hot a few weeks after replacement
- [x] Vibration on the drive side increased after a new coupling half was pressed on
- [x] The rear bearing started vibrating after a new cooling fan was fitted
- [x] The repair shop handed over the rotor with a lathe-balancing report, yet assembled the machine still sits in zone C or D of the applicable part of ISO 20816 (condition zones from A — good, to D — unacceptable)
- [x] Grease darkens and gets squeezed out, bearing housings run hotter than before the repair

> If the motor is only about to go in for repair, take a "before" reading at the working load: overall level in mm/s RMS (root-mean-square vibration velocity) and, if the instrument allows, the 1x amplitude at each bearing. This baseline turns post-repair acceptance from an argument into a comparison of two numbers. Residual unbalance hits new bearings hardest of all: the rotating radial force isn't factored into the life calculation under ISO 281, and a fresh bearing ends up lasting half as long as expected.

Sources: [ISO 281:2007](https://www.iso.org/standard/38102.html) · [ISO 20816-1:2016](https://www.iso.org/standard/63180.html)

## What exactly upsets the balance during a repair

A repair shifts the rotor's mass distribution around the circumference in several places at once. It's usually not one single operation to blame, but the sum of three or four small contributions, each of which looks harmless on its own.

### New winding and impregnation

A rewound wound rotor, armature or pole assembly gets a fresh copper layout: a different cross-section, a different winding density, different end windings. Impregnating varnish and compound run and set unevenly while curing, especially if the rotor was baked in one fixed position. Tens of grams of varnish at the radius of the end windings is already a noticeable imbalance.

### Bandages and wedges

The old bandage is cut off, and a new fiberglass bandage is wound on site with a tension that never exactly repeats the factory winding turn for turn. New slot wedges differ in mass from the old ones. Both changes sit at a large radius, where every gram produces the maximum imbalance.

### Restoring fit surfaces

Weld build-up, spray coating and repair sleeves add metal asymmetrically. Subsequent turning brings the surface to size, but the internal asymmetry of the built-up layer remains. If the turning setup was offset relative to the core's axis, fit eccentricity gets added on top.

### New cooling fan

A cast aluminium or plastic fan arrives with its own inherent imbalance: uneven blade thickness is normal for a casting. A clearance fit secured by a retaining ring adds eccentricity. The fan gets replaced often during repairs, because the old one breaks during removal.

### Pressing on the coupling half

The coupling half sits on the shaft overhang, beyond the bearing, and its unbalance acts on a long lever arm. Add the eccentricity of the press fit and the question of the key: it can be accounted for in balancing in two ways — full key or half key. If the repair shop and the coupling manufacturer used different conventions, the "balanced" pair ends up with an imbalance equal to exactly the mass of half the key.

### Bearing replacement

New bearings don't create imbalance themselves, but they change the rotor's position in the air gap and the stiffness of the supports. Vibration amplitude and phase are different after the change, and the old assumption that "this machine always hummed like that" no longer applies. If the fit or the bearing type was changed at the same time, the whole dynamics of the system changes too.

## Shop lathe or assembled trim balancing: why you need both

After rewinding, the rotor is balanced on a lathe in the shop, and that's the right approach: every surface is accessible, and residual unbalance is measured in g·mm and compared against the tolerance for balance quality grade G from ISO 21940-11. For electric-machine rotors, G2.5 is usually adopted. The lathe is the only way to get a documented record of the balance quality of the rotor itself.

But lathe balancing only accounts for the bare rotor. The fan and coupling half go on afterward, each with its own imbalance and fit eccentricity. Lathe speed is lower than running speed. The lathe's supports are stiffer than the actual bearing end-shields and behave more predictably. Transport, press-fitting and tightening add their own contribution. That's why the assembled machine on its foundation almost always shows a residual vibration that wasn't and couldn't have been present on the lathe.

Hence the workflow we consider correct: the lathe gives the rotor its grade G, a confirmation reading in the assembled machine shows what remains, and on-site trim balancing removes it in two to three starts. Assembled trim balancing accounts for everything bolted on and the actual stiffness of the supports, because the influence coefficients — the machine's response to a trial weight — are measured on the machine itself. A full comparison of the two approaches is covered in our article on on-site versus shop-lathe balancing.

| Criterion | Shop lathe | Assembled, on site |
| --- | --- | --- |
| What's balanced | The bare rotor, no bolt-on parts | The whole assembled shaft: rotor, fan, coupling half, keys |
| Speed | Lathe speed, usually lower than running speed | Running speed and the actual operating point |
| Supports | Lathe supports | The actual bearing supports and real foundation |
| Documentation | Residual unbalance in g·mm, grade G to ISO 21940-11 | Vibration before and after in mm/s RMS, zones per ISO 20816 |
| When to choose it | Rotor removed: rewinding, bandage replacement, fit-surface restoration | Machine is assembled, re-disassembly is costly, a result on the real supports is needed |

> If the repair shop hands over the rotor without a lathe-balancing report, insist on it: rotor mass, balancing speed, residual unbalance by plane, the grade G adopted, and how the key was accounted for. Without this report, any dispute about the cause of vibration once assembled turns into finger-pointing.

Sources: [ISO 21940-11:2016](https://www.iso.org/standard/54074.html)

## What we check before fitting weights

After a repair, the checklist is longer than usual, because the machine was reassembled and repositioned on its foundation from scratch. A third of what turns up at this stage has nothing to do with balancing at all.

- [x] Shaft and coupling-half runout with a dial indicator: straightening and press-fitting can leave residual bending
- [x] Bearing fits in the end shields and on the shaft: after a repair this is defect location number one
- [x] Foot bolt tightness, soft foot, shim stack: the motor was repositioned on the frame from scratch
- [x] Shaft alignment with the driven machine: misalignment after motor reinstallation shows up almost as often as imbalance — the distinguishing signs are covered in our article on imbalance versus misalignment
- [x] Both keys are in place, the coupling half and fan sit with no play
- [x] Checking for an electrical cause by cutting the power: on coast-down — while the de-energized rotor keeps turning under its own inertia — the electromagnetic component drops in a step together with the current, while imbalance fades off gradually with speed
- [x] A reading on the cold machine and a repeat after 30-60 minutes under load: thermal behaviour after rewinding must be checked
- [x] Comparison with the pre-repair reading, if one exists

> The coast-down check is especially important after rewinding: incorrect winding layout, a turn-to-turn short, and an uneven air gap are typical rewinding defects, and all of them live at line frequency and its second harmonic, not at 1x. We cover how to separate electrical causes from mechanical ones in detail in our article on electrical causes of electric-motor vibration; here we simply apply that check to every machine after a repair.

Sources: [ISO 13373-3:2015](https://www.iso.org/standard/40840.html)

## How assembled trim balancing proceeds

1. **Baseline reading and comparison** — The machine at running speed. We record overall vibration, the 1x amplitude and phase at both bearing supports, the spectrum, speed, and temperature. We compare against the pre-repair reading and the lathe-balancing report, if either exists.
2. **Separating causes** — Power-off on coast-down, checking 100 Hz and the sidebands, verifying phase repeatability over two starts, a reading on the warmed-up machine. Only after that do we decide: balance, or fix an assembly or electrical cause first.
3. **Sensors and the mark** — Two accelerometers on the bearing end shields, as close to the bearings as possible, radial and in the same position on every start. Reflective tape on the shaft extension or coupling half, with the laser phase sensor aimed at it.
4. **Trial weights** — A weighed trial weight, one at a time, in each correction plane — each rotor cross-section where weights will be fitted. A valid trial run changes the 1x amplitude by 20-30 percent or the phase by 20-30 degrees. This way the instrument obtains influence coefficients for this specific assembled machine, not the lathe.
5. **Calculation and fitting** — The software gives mass and position for each plane. On the fan blades and coupling-half bolts we work in fixed-position mode: the instrument states a position number and a mass, ruling out any error in the direction of angle measurement.
6. **Verification run and trim** — The same speed and operating point. If the residual is above target, the software calculates additional weights to add to what's already fitted. After rewinding, one iteration is usually enough.
7. **Verification on the warmed-up machine** — We repeat the reading after 30-60 minutes running under load. If 1x rises with temperature and the phase drifts, we note it separately in the report: thermal behaviour isn't fixed with correction weights.
8. **Figures for the record** — Before and after for each bearing, spectra, weight masses and positions, measurement conditions. Influence coefficients are saved in the instrument's archive: the next balancing job on this machine will go ahead without trial runs.

> We work with the Balanset-1A instrument: two accelerometers, a laser phase sensor, a two-channel USB module and software on a laptop. Single- and two-plane balancing by the influence-coefficient method, fixed-position mode, tolerance calculation by grade G, an archive and reports.

Sources: [Balanset-1A operation manual](https://vibromera.eu/balanset-1a-operation-manual/) · [Balanset-1A manufacturer specification](https://vibromera.eu/product/balanset-1/)

## Correction planes: where they are on an electric-machine rotor

An electric-motor rotor is almost always long: the length-to-diameter ratio exceeds 0.5, so the standard approach is two correction planes. We get by with a single plane only on short rotors of small machines; the selection rule itself is covered in our article on the number of correction planes.

Once assembled, the accessible planes are set by the design. On one side there's the cooling fan under a removable cover, on the other the coupling half or pulley. They're spread across the ends of the shaft — convenient geometry for a two-plane calculation. On the shop lathe, the choice is wider: the manufacturer builds in dedicated correction locations, and a competent repair makes use of them.

- Balance rings and grooves, where the manufacturer provided them: weights are secured in the slot without drilling
- Pressure washers on a DC-machine armature: solid metal that allows removal by drilling
- The armature's bandage rings as a mounting location for sector weights — we don't touch the bandage itself
- The end faces of the squirrel-cage short-circuiting rings: only factory bosses and holes, where the design provides them
- The cooling fan's blades and rim: bolted weights through factory holes
- Coupling-half bolts: calibrated washers under the bolt heads, the fastest method on the drive side

> A weight on the coupling half sits on the shaft overhang, beyond the bearing, so its effect on the far support isn't always obvious in sign. We don't estimate that by eye: a trial run gives the actual influence coefficient for each plane on this specific machine.

## Attaching weights and off-limit zones on a rewound rotor

On an electric-machine rotor there are zones where nothing may be fitted, let alone drilled. This is the main difference from a fan or pulley, where almost any solid point is fair game for correction.

- Allowed: bolted attachment through factory holes on the fan and coupling half, weights in balance grooves, washers under coupling-half bolts
- Allowed with a caveat: welding on a plate only to solid parts, well away from the winding, without heating the core, and with the bearings protected from welding current
- Allowed with a caveat: metal removal by drilling only in pressure washers, the fan rim, and other solid parts, with depth and diameter calculated by the software
- Forbidden: drilling the rotor core — it breaks magnetic symmetry and produces exactly the vibration at twice line frequency that can't be removed afterward
- Forbidden: drilling or heating the cage bars and short-circuiting rings — a notch in a bar turns into a crack, and a crack into a broken-bar fault with sidebands in the spectrum
- Forbidden: touching the fresh bandage, commutator, slip rings, and the end windings of the new winding — any intervention here voids the rewinding warranty
- Magnetic weights — only as trial weights, for the duration of the measurement

> Attachment methods and their reliability at run-up speeds are covered in our article on attaching correction weights. On a rewound rotor, one extra rule applies: agree any welding or drilling with the repair shop in writing while its warranty is in force.

## When on-site trim balancing won't give a result

### Rewinding defect: an electrical cause

A turn-to-turn short, an incorrect winding layout, an uneven air gap after assembly. Vibration sits at line frequency and its second harmonic and doesn't depend on weights. We'll show that with numbers from the coast-down test and the spectrum — with that report, the motor goes back for warranty repair, and you don't pay for a useless balancing job.

### Thermal bow after impregnation

Uneven impregnation and clogged cooling ducts heat the rotor asymmetrically. 1x grows as it warms up, and the phase shifts by tens of degrees. Balancing would give you quiet at one thermal state and vibration at every other. Why this happens is covered in our article on rubbing and thermal rotor bow.

### Fits not restored

A worn bearing end shield, a spun ring, play in the fan or coupling half. The correction calculation assumes the rotor's mass is fixed in place, and that's not the case here. Mechanical repair first, weights after.

### No access to the planes

A cast, non-removable cover, an enclosed coupling guard, a fan that can't be exposed without dismantling the unit. If not a single correction plane can be reached, on-site work is physically impossible — the rotor can only be balanced on a lathe.

### Flexible or high-speed rotor

Near its first critical speed a rotor bends, and two planes in the assembled state may not be enough. We check whether low-speed balancing applies to such rotors against ISO 21940-12, and we say plainly when a machine belongs on the lathe.

Sources: [ISO 21940-12:2016](https://www.iso.org/standard/50429.html)

## Post-repair acceptance: what to put in the record, and how to order

Vibration-based acceptance of a motor after repair is a comparison against a number, not a feeling. Write into the record: measurement points and directions, frequency band, operating point and load, overall vibration in mm/s RMS at each bearing, the applicable part and edition of ISO 20816 with the zone achieved. The target after a repair is zone A or B. As a separate line: residual unbalance by grade G from the lathe-balancing report, and the masses and positions of the assembled trim-balancing weights. A detailed structure for this document is in our articles on the balancing report and on post-repair acceptance testing.

Cost: vibration diagnostics with a report 300 EUR per unit, balancing from 250 EUR, minimum invoice for a visit 500 EUR. The calculator on the website gives an exact figure — it accounts for the number of rotors, planes, and how far the site is. We're based in Vila Nova de Gaia, near Porto, and travel throughout Portugal. Diagnostics without balancing is also a full result of the visit: sometimes the whole repair comes down to returning the motor under warranty, report in hand.

- Machine type, power, speed, number of poles
- What was done during the repair: rotor or stator rewinding, replacement of bearings, bandages, fan, coupling half
- Whether a lathe-balancing report for the rotor and a pre-repair vibration reading exist
- Photos from both sides: bearing end shields, fan cover, coupling half, frame
- Possible shutdown windows, and whether the repair shop's warranty is still in force

> AXILINE is engineers who design and manufacture Balanset instruments and do the on-site balancing themselves. If your repair shop rewinds motors regularly, assembled trim balancing is a procedure your maintenance team can master with its own instrument; we'll support you with advice on specific machines.

Sources: [ISO 20816-1:2016](https://www.iso.org/standard/63180.html) · [ISO 21940-11:2016](https://www.iso.org/standard/54074.html) · [Balanset-1A manufacturer specification](https://vibromera.eu/product/balanset-1/)

## Frequently asked questions

**The rotor was balanced on a shop lathe. Why does the assembled machine still vibrate?**

The lathe balances the bare rotor on its own supports and at its own speed. Afterward, the fan and coupling half go onto the shaft, each with its own imbalance and fit eccentricity, the key question comes into play, and the stiffness of the actual bearing supports differs from the lathe's. The sum of these contributions is exactly the residual you can hear once assembled. It's removed by on-site trim balancing in two to three starts, because the influence coefficients are measured on the machine itself.

**The stator was rewound, the rotor wasn't touched. Is balancing needed at all?**

Stator rewinding by itself doesn't change the rotor's balance. But the machine was fully disassembled for it: the fan was removed and refitted, bearings were replaced, the coupling half was pressed on, the motor was repositioned on the frame and the shafts realigned. Each of these operations is a candidate cause of vibration. We start with a reading and a power-off coast-down check: in a single start it separates an electrical cause from a mechanical one, and only then do we decide whether weights are needed.

**Can a rotor be trim-balanced after rewinding without removing the motor from its foundation?**

Yes, that's exactly the standard trim-balancing scenario. Sensors go on the bearing end shields, the phase mark on the shaft extension, weights on the cooling fan and coupling-half bolts. All you need is a removable fan cover or access to the coupling, and the possibility of two or three machine stops. Removing the motor is only necessary when not a single correction plane can be reached, or when the rotor is flexible.

**Vibration increased after bearing replacement. Can new bearings really be the cause?**

Rarely on their own. New bearings change the rotor's position in the air gap and the stiffness of the supports, so vibration amplitude and phase come out different: a previously hidden imbalance can surface. More often the accompanying assembly work is to blame: misalignment during press-fitting, a damaged fit in the end shield, a fan that was removed and refitted. We check the fits and the spectrum before proposing balancing.

**The motor is under the repair shop's warranty. Will fitting weights void it?**

It can, if you intervene without approval. Bolted weights at the factory locations on the fan and coupling half usually raise no questions, but agree drilling and welding on the rotor in writing while the warranty holds. If our reading shows an electrical cause or thermal bow, we don't fit weights at all: you get a report with numbers, and the motor goes back for warranty repair with it.

**What tolerance should be considered normal for a rotor after rewinding?**

There are two different assessments here, and they shouldn't be mixed up. The balance quality of the rotor itself on the lathe is set by grade G to ISO 21940-11: G2.5 is usually adopted for electric-machine rotors, and the exact grade should be agreed with the manufacturer. The condition of the assembled machine is assessed by overall vibration in mm/s RMS under the applicable part of ISO 20816: zone A or B is the target after a repair. Both figures, with the standard edition stated, belong in the acceptance record.