AXILINEOn-site balancing of crushers and shredders: rotors, hammers, knives
You fit a new set of hammers, start up the crusher, and the frame starts humming hard enough to shake the walkways around the machine. Nobody wants to pull a one-tonne rotor and haul it to a balancing machine — and there's no need to: rotors like this get brought back into spec right in their own bearing supports, in two or three runs. We come out with the instrument, sort out the causes of the vibration, and give you a straight answer on whether balancing will help. Below is exactly what we do on crushers, mills, shredders, and pelletizers.
Symptoms: what to look for before calling
A crusher rarely works its way up to shaking gradually. It's usually like this: you change the hammers, start it up, and the machine starts humming in a new way. Or it ran quietly for a week, then an oversized piece came through the feed, and after that run the level never went back to what it was.
Imbalance on rotors like this is almost always man-made. It shows up the moment working elements are replaced, something breaks, or wear becomes uneven. That means you have a link to a specific event, and it speeds up diagnosis a lot. Think back to the last thing done to the machine.
- Vibration increased right after replacing hammers, beaters, or knives, or after hardfacing the working elements.
- A dull, rhythmic knock is audible once per revolution, and the frame and service platform rock noticeably.
- It shakes just as much at idle as it does under load. So it's not the feed.
- The level rises as the hammer set wears in and drops after the hammers are flipped or replaced.
- Bearing housings run hot, grease gets pushed out, and bearings no longer last until the scheduled overhaul.
- During coast-down — a long one on a heavy rotor — vibration spikes sharply in some narrow speed band.
- Mounting seats are worn out, a key has sheared, and cracks are appearing along the weld seams of the rotor discs.
One number worth taking before you call: overall vibration in mm/s RMS at each bearing housing, horizontal direction. With that in hand the conversation gets concrete right away, and we know in advance what to bring.
What machines in this family we balance on site
What all these rotors have in common is that they break material apart by impact or cutting, so the mass of the working elements keeps changing throughout operation. Imbalance here isn't a manufacturing defect but a normal consequence of use, and it has to be removed again and again as the set wears.
The method is the same across all these subgroups. What differs is access to the correction planes, the number of ready-made weight positions, and how the weight is fixed.
Rotor, hammer, and impact crushers
Imbalance is produced by hammers and beaters of unmatched mass after replacement, one-sided wear, broken or lost elements, and product buildup in the rotor pockets.
Centrifugal crushers and disintegrators
High speed and abrasive wear on the accelerating vanes and pin discs. A small loss of metal on one side produces a noticeable centrifugal force here.
Mills: hammer and rotor type
A long shaft with several rows of hammers. Mass spread within a row and uneven wear along the length produce both static and couple imbalance.
Shredders, choppers, and pelletizers
Knife rotors: after regrinding, knives lose mass unevenly, the fasteners and shims differ, and the drum picks up buildup from soft material.
Wood and plastic crushers
Resinous and fibrous material settles unevenly and breaks off in chunks. On top of that, combustible dust in the shop often puts welding weights off-limits.
Mineral feedstock, construction materials, pigments, and paints
Crushers for mineral feedstock and construction materials wear from abrasion. Pigment grinders, rotors in paint-manufacturing equipment, and rotors in process mixing-and-grinding units suffer more often from buildup and from careless reassembly after a repair.
We always put the laser phase sensor's mark on the rotor shaft itself, not on the motor. With a belt drive the rotor speed differs, and a mark on the motor would have the instrument picking out the wrong frequency.
What we check before fitting the first weight
Some crusher requests get closed out without any weights at all. Vibration on these machines comes from several causes at once, and balancing only fixes one of them: unbalanced rotor mass.
So the first hour of a visit goes on measurement and inspection. We compare overall vibration against the 1x running-speed component (1x is the standard notation for vibration at the rotational frequency), look at the spectrum — vibration broken down by frequency — and listen to the machine as it coasts down.
- The hammer and beater set: is it matched by mass. Hammers are weighed and arranged so the sums in diametrically opposite groups match. A spread of a few hundred grams at a 300 mm radius can't be cleanly removed with a weight on the hub.
- Condition of the working elements: a missing, cracked, or broken hammer, chipped-out hardfacing, a bent knife.
- Material buildup on the discs and between hammers. Damp clay-like or resinous product changes the picture from one run to the next.
- Rotor-to-shaft fit, keys, disc fasteners, locking elements, condition of the weld seams.
- Bearing housings: play, clearances, noise, temperature, condition of the lubricant.
- Foundation mounting: anchor bolt tightness, soft foot (a support that doesn't sit flush against the frame and skews the housing when tightened), cracks in the concrete, condition of the vibration isolators.
- Drive: belt tension and condition, pulleys' own imbalance, and for coupling drives, shaft alignment.
- Resonance: we watch how amplitude and phase behave during coast-down. A heavy rotor gives us a conveniently long, smooth coast-down for this.
Imbalance and shaft misalignment produce similar numbers on a single channel but are fixed in opposite ways. Balancing a misaligned machine is harmful: the weight compensates for a force that belongs to something else, and once the shafts are aligned, vibration ends up higher than it started. The order is: foot contact, shaft alignment, then weights. More detail is in our article on telling imbalance apart from shaft misalignment.
Sources: ISO 13373-3:2015 · ISO 281:2007
How the work proceeds on site
A visit to one machine takes a full shift. Most of the time doesn't go on measurements but on stoppages: opening the guard, waiting for a heavy rotor to come to a complete stop, locking out the drive, fitting the weight, closing up, and restarting.
- Step 1
Agreeing on safety and mode
Before we arrive, we agree on who stops and locks out the machine, who opens the guards and hatches, and whether a hot-work permit is needed. On crushers this isn't a formality: the correction planes are almost always behind a guard, and every iteration costs you a full stop-and-start cycle.
- Step 2
Fitting sensors and the mark
Two accelerometers on the rotor's bearing housings, as close to the actual bearings as possible, magnet-mounted on a cleaned spot or on a stud. Direction is usually horizontal-radial, and we keep it the same from run to run. We aim the laser phase sensor at a reflective tape mark on the rotor shaft.
- Step 3
Pre-work measurement
A run at operating speed, usually at idle. We record overall vibration in mm/s RMS, 1x amplitude and phase, speed, spectrum, and time-domain signal on both channels. This is your baseline, and it goes into the report. This is also where we decide whether balancing is worth doing.
- Step 4
Trial weight
We fit a temporary mass of known size at a known radius in the first plane, run it, then move it to the second. The instrument calculates the influence coefficients — exactly how your rotor-supports-foundation system responds to the added weight. A valid trial run changes 1x amplitude by at least 20–30% or phase by 20–30°. On a heavy rotor the trial mass ends up substantial, and we fix it just as securely as a permanent weight.
- Step 5
Correction
The software outputs mass and angle for each plane, or a position number if we're working in fixed-position mode: by hammer-pin rows, disc holes, or knife lines. The weight is welded on, bolted, or metal is drilled out at the heavy spot.
- Step 6
Verification run and report
Same speed, same points, same sensors. If we don't hit tolerance right away, we add small masses to what the software already calculated. Next comes a measurement under load, the report, and a list of mechanical findings.
We save your machine's influence coefficients. Next time the hammer set is replaced, the trim balancing runs off those, with no trial runs, which means it's faster and needs fewer shutdowns.
Sources: Balanset-1A operation manual · Balanset-1A manufacturer specification
One plane or two on rotors like these
Geometry sets the number of planes. The rule of thumb is simple: the ratio of rotor length L to diameter D at the weight-mounting zone. A short disc with L/D under roughly 0.5 is usually brought into spec with one mass. An elongated rotor needs two planes, otherwise couple imbalance remains — a pair of forces rocking the rotor from both ends — and you get the familiar picture: one support is fine, the other hasn't changed at all.
Most rotors in crushing equipment are elongated, so we almost always work in two planes. There are exceptions, and there aren't many of them.
| Rotor type | Typical geometry | Correction planes |
|---|---|---|
| Rotor, hammer, and impact crushers, hammer and rotor mills | Elongated shaft with several rows of hammers or beaters, L/D of 1 or higher | Two planes: the end discs or the end rows of positions |
| Knife rotors of shredders, pelletizers, plastic and wood crushers | Long drum with knife lines running the full length | Two planes, the drum's end discs |
| Centrifugal crushers with an accelerator rotor | Short disc-type rotor, elevated speed | Usually one plane, a second one if a noticeable residual remains at the second support |
| Disintegrators with pin discs | One or two discs on an overhung shaft | Two planes if both discs are accessible, otherwise one with both supports monitored |
| Rotors of mixing-grinding units and paint-manufacturing equipment | Overhung shaft with a cutter head or disc on the end | One plane at the working element, two for a long shaft |
| Pigment grinders and fine-grinding rotors | Small disc, speed from 3000 rpm and up | One plane, but a tighter tolerance: we select the balance quality grade G to match the speed |
A second plane costs one extra trial run, meaning one extra stop with a full coast-down. On a heavy rotor that's a noticeable chunk of the shift, and we flag it in advance. We have a separate article on the L/D rule and choosing the number of planes.
Sources: ISO 21940-11:2016 · ISO 21940-12:2016
What gets in the way, and when on-site won't work
Let's be honest: on crushing equipment, the share of cases where weights don't solve the problem is higher than on fans and pumps. The reason is impact loading. It breaks down not just mass symmetry but also fits, welds, bearings, and the frame.
- A mismatched set of hammers. Until the masses in opposite positions are brought together, you're compensating with a weight for something that's cheaper to fix by matching. We'll ask you to weigh the hammers and show you how to arrange them by position.
- A missing or broken hammer, a broken beater, a chipped knife. That's a failure, not imbalance. Replacement first, measurement second.
- Material buildup. Balance it to the current surface condition, and within a shift the buildup will break off in a chunk and everything will be back where it started. Rotors like this get cleaned first.
- Cracks in the discs, a worn shaft fit, a sheared key. The 1x phase stops repeating from run to run, and the correction calculation becomes unreliable.
- Worn bearings and damaged housing bores. Components not a multiple of running speed appear in the spectrum, and overall vibration is many times higher than 1x. Balancing won't save a machine like that.
- Resonance in the frame or the service platform. A heavy rotor on a flexible steel structure falls into it easily, especially after the VFD setpoint changes.
- Combustible and explosion-hazard dust. Where wood, plastic, or pigments are being ground, welding weights is often simply banned. In that case we work with mechanical fixing or metal removal, but that needs to be agreed before the visit.
- No access to the correction planes, or the machine won't hold a stable speed. In the first case it's more sensible to pull the rotor for workshop balancing; in the second we discuss the operating mode in advance.
The worst-case scenario for us sounds like this: the vibration comes from the machine's overall condition, not the rotor. In that case you get measurements, spectra, and a prioritized diagnosis, not weights. We've written a separate article on cases where balancing doesn't help.
Sources: ISO 20816-1:2016 · ISO 13373-3:2015
How we fix the correction weights
This deserves its own section, because on a rotor that works by impact, a weight only stays put with the right fixing. A few hundred grams of mass breaking loose at operating speed is dangerous both for the machine and for people.
- Welding. The primary method on cast and welded rotors of crushers and hammer mills: a plate of matching steel, cleaned down to bare metal, welded all around the perimeter, not tacked. We check the rotor's steel grade and whether welding is allowed against the manufacturer's documentation.
- Bolted fixing. Where welding is banned: factory holes in the discs, weight washers under the hammer-pin bolts, welded bosses fitted at manufacture. We lock the threads.
- Metal removal. On a thick-walled disc it's simpler to drill metal out at the heavy spot; the software calculates the diameter and depth for the required mass. We don't do this on a thin wall: holes reduce strength.
- Fixed positions. On most crusher rotors a position grid is already there: hammer-pin rows, disc holes, knife lines. We enter their count into the software and get back not an angle in degrees but a position number and a mass, or a split between two adjacent positions. That rules out a mirror-image error in reading the angle.
- Rearranging the working elements. Sometimes correction doesn't need any weights at all: hammers are swapped between positions to balance the set. We check this option first, and it's free.
We record the weight's mounting radius in the report. At the next service you'll fit the mass at the same radius without having to recalculate: an error in radius is a direct error in mass.
What you get after the visit
The result of the work isn't the phrase "it got better" — it's numbers you can check again six months from now.
- Pre-work measurement: overall vibration in mm/s RMS and the 1x running-speed component at each bearing housing, speed, points, and directions.
- Post-work measurement: same points, same sensors, same mode. The drop in 1x is visible directly.
- Masses, radii, and positions of all fitted weights, and the fixing method.
- Spectra and time-domain signal before and after: they show what's left in the vibration besides imbalance.
- An assessment of the machine's condition by overall-vibration zone, citing the applicable part and edition of the standard.
- A residual-imbalance and G-class tolerance calculation, if needed for acceptance.
- A list of mechanical findings and recommendations for matching the set at the next hammer or knife replacement.
"Within tolerance" in the software means one thing: residual 1x is below the target value entered. It's not an assessment of the machine's overall condition by overall vibration, and it's not confirmation of G-class accuracy. We keep these three tolerances separate in the report so you're not comparing things that aren't comparable.
Sources: ISO 20816-1:2016 · ISO 21940-11:2016
Price and how to book a visit
We're the engineers who design and manufacture the Balanset instruments and use them on-site ourselves. We're based in Vila Nova de Gaia near Porto, and we cover all of Portugal.
vibration diagnostics with a report costs EUR 300 per unit, balancing adds from EUR 250, and the minimum invoice for a visit is EUR 500. The calculator on our website gives an exact figure for your machine. If there are several machines on site, count them all together: it's one visit.
- Type and model: rotor, hammer, impact, or centrifugal crusher, mill, shredder, pelletizer, disintegrator, grinder.
- Drive power, rotor operating speed, drive type: belt or coupling.
- Approximate rotor mass, working-section length, and diameter.
- What was measured and with what, if anything was measured: figures in mm/s and the measurement points.
- What was done to the machine before the vibration appeared: set replacement, hardfacing, support repair, a change in speed setpoint.
- Access to the rotor when stopped: hatches, removable guards, service platform.
- Whether hot work and weld-on weights are permitted on site.
- Photos of the rotor, the supports, and the machine's nameplate: they let us see the correction planes and the weight-fixing method in advance.
If you'd rather do this in-house, we sell the same instrument we use ourselves. The Balanset-1A: two accelerometers, a laser phase sensor via reflective tape mark, a two-channel USB module, software on a laptop. One- and two-plane balancing by the influence coefficient method, overall vibration and 1x, phase, speed, FFT spectrum and time-domain signal, fixed positions and drilling calculation, saved influence coefficients, trim balancing, G-class tolerance calculation, archive and reports. We advise on fitting sensors to your specific machine.
Sources: Balanset-1A manufacturer specification · Balanset-1A operation manual
Frequently asked questions
Can a crusher rotor be balanced without removing it?
Yes, that's the primary method. The rotor is balanced in its own bearing supports at operating speed: sensors on the supports, a laser phase sensor on the shaft mark, weights fitted through a hatch or removed guard. For a heavy rotor it's also cheaper: removal, transport, and reassembly usually cost more than the balancing itself. The rotor has to come off if there's no access to the correction planes, the shaft fit is worn, or G-class acceptance on a machine is required.
We fitted a new set of hammers from the manufacturer. Why weigh them?
Because casting or forging mass tolerance hasn't been abolished, and there's always some spread within a set. At a 300 mm radius, a 200-gram difference between opposite positions produces a distinctly noticeable centrifugal force. Weighing the set and arranging the hammers so the sums in diametrically opposite groups match takes half an hour. It's the cheapest way to cut vibration, and it's done before any instrument comes into play.
One hammer broke off. Is balancing enough?
No. A weight will remove the vibration — the one outward sign the fault is visible by — but it won't restore the machine. The broken piece may have damaged the housing liner and the grate, and the load on the discs and bearings will keep climbing. The order is: replace the element, inspect the rotor and housing, check the set by mass, then measure. Balancing comes last, and only if the running-speed component is still dominant.
How long does on-site crusher balancing take?
Usually a full shift for one machine. The measurements themselves are fast; time is eaten up by the stoppages: a heavy rotor coasts down slowly, the guard has to be opened and closed, the drive locked out, a welded weight left to cool. Two-plane balancing adds one full stop-and-start cycle. If the rotor also needs cleaning, add time for that and measure again afterward.
Can weights be welded onto a crusher rotor?
On most cast and welded rotors, yes, subject to agreement with the manufacturer and a hot-work permit. We check the steel grade, clean the spot down to bare metal, and weld the plate all around its perimeter. Where welding is banned — a common situation in shops with combustible wood, plastic, or pigment dust — we work with factory holes and bolted weights, or remove metal by drilling at the heavy spot.
How long does the result hold on a crusher?
Until the next noticeable change in rotor mass. On abrasive feedstock, wear is continuous, so vibration creeps up as the set wears in, and that's normal. A sensible practice: a check measurement a few weeks after balancing, then periodic measurement at the same point and the same mode to track the trend. A repeat balancing after the set is replaced is done off the saved influence coefficients, with no trial runs.
Related content
On-Site Shredder and Granulator Balancing: Knife Rotor, Regrinding, Set Mass
Yes. We balance the rotors of granulators, plastic knife crushers, disintegrators and high-speed shredders on-site, in their own bearing housings, usually in two planes at the rotor's end discs. Conditions: a full knife set matched by mass after regrinding, a rotor clear of wrap-up and caked-on melt, steady speed, and access to the rotor ends on a stopped, locked-out machine. An important reservation on speed: below roughly 150–200 rpm the centrifugal force from imbalance is small, and the running frequency drops to the lower edge of the measurement band, so we generally do not balance slow-speed twin-shaft shredders. Their vibration comes from the gearbox, cutting jerks and loose fits, and that is a matter for diagnostics, not weights. We only balance a large hydraulically driven shredder when the drive can hold constant speed for the duration of the measurements.
On-Site Wood Grinder and Hog Balancing: Disc, Drum, and Knife Overhang
Yes, we balance the rotors of wood chippers and chipping machines, both drum and disc types, on-site, in their own bearing housings, with no dismantling. Conditions: the knife set has been reground and weighed, every knife's overhang is set equal, the wedges are torqued to spec, the rotor is clear of resin and bark, the machine holds a steady speed, and the feed conveyor is stopped for the measurement. If a knife has been chipped by a nail or a stone, replacement and a counter-knife inspection come first, measurement after. In plants with combustible wood dust, we work without welding: bolted weights in factory holes, or metal removal by drilling. If the vibration is coming from something other than the rotor — clearance, resonance, or bearings — we will say so at the first measurement and will not fit weights.
On-site balancing or shop balancing on a machine: how to choose without overpaying in downtime
Balance on site if the rotor can be safely run up to its operating speed, you have access to the correction plane (the location on the rotor where the correction weight goes), and the operating mode repeats from run to run. Take it to a balancing machine if the rotor is flexible or passes through critical speeds, its geometry is damaged, there is no access to the correction planes, or acceptance requires a report against a balance quality grade G. In every other case, start with an on-site measurement: the data from the first reading will show on its own whether removal is actually needed.
Describe your equipment and the problem
We'll answer your questions, clarify the details, and let you know what's needed for an estimate and a visit.