AXILINEOn-site balancing of a forest mulcher rotor, without removing it from the host machine
A forest mulcher rotor has it harder than any other drum: rocks, stumps, lost teeth, and hardfacing of uneven mass. Every tooth knocked out turns into an impact load on the bearing housings at 1500–2500 rpm. We balance rotors like this right on the mulcher head or the tractor-mounted mulcher, on your own site, running off the machine's own hydraulic drive or the tractor's PTO (power take-off) shaft. You get a before-and-after measurement, welded-on weights in two planes, and a report.
Symptoms: how unbalance shows up specifically on a mulcher
On a forest mulcher, unbalance is almost always tied to an event. Yesterday you were working a rocky patch; today the head is hammering into the excavator's boom. You changed the tooth set, and the tractor-mounted mulcher started shaking the tractor through the linkage. If you remember what that event was, tell us when you book: it speeds up the diagnosis.
Telling unbalance apart from other problems by feel alone is hard, but there are characteristic signs.
- Vibration appeared suddenly, after hitting a rock or a stump, and hasn't changed since.
- Shaking started after replacing part of the tooth set, or after hardfacing the carbide teeth.
- Noise and shaking build with rotor speed and drop when hydraulic-motor flow or PTO speed is reduced.
- The head shakes at idle rotor speed, even before it touches any wood.
- The bearing housings at the ends of the housing run hot, and fresh grease doesn't help.
- Cracks have appeared in the housing's weld seams, and the guard-panel fasteners have worked loose.
- The operator feels the shaking through the boom, the arm, or the tractor seat.
Vibration that only appears under load and disappears at idle usually isn't coming from unbalance — it's coming from cutting and feed. Balancing won't fix that, and we check for it with the measurement too.
Rotor construction, and where its unbalance comes from
A forest mulcher rotor is a steel tube-drum with welded-on tooth holders carrying either fixed carbide teeth or swinging hammers and knives. The drum sits on two bearing housings at the ends of the housing and is enclosed by a cover with a flap guard made of chains or rubber. The drum's length is many times its diameter, so its unbalance is almost always dynamic, with a couple component: a mass skew along the length doesn't just pull the drum to one side, it rocks it from both ends.
The factory balances the drum once, before its first shift in the forest. After that, the work itself is what wrecks the balance.
Lost and chipped teeth
A tooth knocked out at the drum's radius shifts the centre of mass immediately and noticeably. Chipped carbide does the same, just less. Worst of all is a holder torn off along with its tooth: that's both a mass loss and a damaged mounting point.
Uneven-mass replacement and hardfacing
A new tooth is heavier than a worn one. Replace three teeth out of forty, and the rotor gets three local heavy spots. Hardfacing the holders with carbide by eye produces a mass scatter around the circumference that nobody's controlling.
Soil and wood-pulp buildup
Working wet ground packs the holder pockets and the inside of the cover with a mix of soil and wood chips. The buildup isn't distributed evenly and partly breaks off during operation, so the vibration drifts. Balancing a dirty drum makes no sense — we require a wash first.
Bending from an impact
Hitting a boulder or hidden metal bends the drum tube or the shaft. That's no longer unbalance, it's a geometry problem: weights can't compensate for it across the whole speed range at once. We check shaft runout before balancing.
Drive: hydraulic motor or PTO, and what that changes about the measurement
Excavator and loader mulcher heads are turned by a hydraulic motor; tractor-mounted mulchers run off the PTO through a cardan shaft, a gearbox, and a belt drive. This isn't a small detail for balancing: the drive determines how stable the speed is, and where we mount the phase mark.
A hydraulic drive holds speed only as steady as the pump's flow and the oil temperature are. Cold oil is thicker, and the rotor turns at a different speed. So we warm the hydraulic system up to operating temperature first, lock in the host machine's engine speed and pump setting, and only then take measurements. Every run — initial, trial, and verification — happens at the same speed, otherwise the influence coefficients won't reconcile — the machine's measured response to a trial weight, which the instrument uses to calculate the correction.
A cardan drive has a different subtlety. When the joint angle is noticeable, the cardan's joints turn the shaft unevenly within each revolution, and worn universal joints add their own components to the spectrum. That's why we stick the reflective mark for the laser phase sensor on the rotor itself: on the drum shaft's end or its pulley, never on the cardan shaft or the PTO stub. A mark on the drive side, past a belt transmission, is simply meaningless — that's a different rotating speed entirely.
- Hydraulic drive: warm up the oil, fix the regime, and check speed with the instrument on every run.
- PTO and cardan: minimum joint angle, the linkage set to the correct height, no play in the universal joints or splines.
- The phase mark always goes on the rotor or its shaft, never on the cardan shaft.
- Speed mustn't drift between runs: a drift of more than a few percent means repeating the run.
What we check before fitting weights
Balancing removes only the once-per-revolution component of vibration. So first comes a measurement at both bearing housings: overall vibration, the share coming from the 1x component (vibration at rotor speed), phase, spectrum. If something other than rotating frequency dominates the spectrum, we deal with the mechanics, not the weights. How to read these numbers is covered in our article on overall vibration, 1x, and phase.
- The tooth set: we count against the manufacturer's layout, check positions, and look for chips and empty holders.
- Consistency of the teeth: a mixed set of new and worn ones gets spot-checked by mass.
- Drum and shaft runout: a rotor bent from an impact goes for straightening, not balancing.
- The bearing housings at the ends of the housing: play, heat, seal condition, and housing tightness.
- Cleanliness: the holder pockets, the inside of the cover, and the drum cavity, all free of soil and wood chips.
- The belt drive on a tractor-mounted mulcher: tension, belt condition, and pulley runout.
- The head's mounting to the boom, or the mulcher's to the linkage: loose fasteners mimic unbalance.
- Resonance (speed coinciding with the natural frequency of the cover or the boom): we change speed and watch how the amplitude and phase behave, and we don't balance inside a resonance zone.
Bearings worn out from a long run on an unbalanced rotor produce vibration of their own. We assess their condition from the spectrum and temperature before balancing, following standard vibration-diagnostics methods, and we tell you honestly if a replacement is needed first.
Sources: ISO 13373-3:2015 · ISO 281:2007 · ISO 20816-1:2016
How the work goes on your machine
- 01
Preparation and safety
The machine sits on level ground, the drum washed clean. We lower the head onto the ground or onto stands, shut the engine off, and bleed off any residual pressure in the hydraulic lines per the machine's manual. We never work under raised equipment: any work near the rotor only happens with the head lowered and secured.
- 02
Access to the rotor
We remove or tie back the flap guard, open the access ports, and take off cover panels if needed. This is the only way to reach the drum ends, where the correction planes will be (the spots for correction weights), and the sensor mounting points.
- 03
Sensors and mark
We fit two magnet-mounted accelerometers on both bearing housings, radially, on spots cleaned down to bare metal. We stick the reflective mark on the drum shaft's end. We don't change the measurement direction until the job is finished.
- 04
Initial run
The machine's own drive spins the rotor up. Everyone stays clear of the drum's plane of rotation: with the flap guard off, the throw-off zone is open. Only the machine's operator starts and stops it, on our instruction. We record overall vibration, 1x, phase, speed, and the spectrum.
- 05
Trial runs
With the rotor stopped and secured, we fit the trial weight in the first plane, run, and measure. We move the weight to the second plane and run again. The readings need to change noticeably, by twenty to thirty percent in amplitude or phase, otherwise we increase the weight. More detail in our article on trial weights.
- 06
Fitting weights and verification
The software calculates the mass and angle for each plane. We weld the weights to the drum, clean the scale off the weld, clear anything loose out of the cover, then run a verification pass. If needed, the software suggests a small addition, and we fine-tune the rotor down to the agreed value.
- 07
Reassembly and report
We put the flap guard and panels back on and hand over the report: before-and-after figures, weight masses and locations, speed, and any mechanical findings. We keep the influence coefficients on file: after the next tooth-set change, fine-tuning will take fewer runs.
We work with the hydraulics according to your machine's own rules: pressure bled off before any access to the rotor, supports under any raised assemblies, and the ignition key out whenever we're working by hand inside the cover. We don't bypass lockouts, and we don't ask the operator to bypass them either.
One plane or two: a mulcher drum needs two
A forest mulcher drum is elongated: its length is noticeably greater than its diameter. A rotor like this has couple unbalance on top of the static kind, and no single weight will remove it. Put a mass in the middle, and vibration drops at one bearing housing while rising at the other. That's why we work in two planes, one trial run per plane, with the instrument measuring both housings at once. Why that's the case is covered in detail in our article on choosing the number of planes.
We take the correction planes at the drum ends, as close to the bearing housings as possible: end discs, the outer bands of the tube, or clear spots between the outermost tooth holders. The further apart the planes sit, the smaller the masses needed to offset the couple component.
We agree the target value before starting work: residual unbalance by G class from ISO 21940-11 (the lower the number after the G, the stricter the tolerance) and vibration level at the bearing housings, referenced against the ISO 20816 zones. We record the applicable part and edition of the standard for the specific machine in the report, because these documents have formal scope limits, and for attached forestry equipment those need to be spelled out separately.
Sources: ISO 21940-11:2016 · ISO 20816-1:2016
Fitting weights: welding onto the drum, and the requirements for it
A mulcher rotor usually has no removable balancing positions, so we weld the correction weights on. That's more reliable than a bolted solution in an environment where the rotor is pounding through rocks, but it comes with requirements that can't be cut corners on.
Consider the load: a 100 g weight at a 250 mm radius at 2000 rpm pulls outward with a force of about 1100 N — over 100 kgf. Only the weld holds it in place, so we run a continuous seam around the plate's perimeter, not a quick tack job.
Weight material: ordinary mild steel, a plate or strip, weighed before it goes on. No cast iron, no random offcuts. Welding location: an end disc or the tube band near the end, well clear of the tooth holders and their welds, and never over hardfaced wear-resistant material. On drums made of high-strength wear-resistant steels, we weld with the material's weldability in mind: edge preparation, suitable electrodes or wire, and preheating the weld zone if needed. We clean and inspect the weld before the verification run.
- The weight is weighed, and its mass and radius are recorded in the report before welding.
- The weld is continuous, around the perimeter, sized to handle the centrifugal load with a safety margin.
- We never weld over holders, carbide, or hardfacing, and we don't leave any undercuts on the drum tube.
- Before welding, we clear wood dust and chips out of the cover: it's combustible material, and we keep a fire extinguisher close by.
- If the software calls for removing mass rather than adding it, we use the fixed-position drilling calculation, but on a thin-walled drum tube we drill sparingly and more often choose to weld a weight on the opposite side instead.
After sharpening or replacing the tooth set, a repeat balancing job goes faster: the saved influence coefficients let us fine-tune without new trial runs, often in just one or two passes.
Sources: Balanset-1A operation manual
When on-site balancing won't help
We tell you this based on the first measurement, not after the weights have already gone on.
- The drum or shaft is bent from an impact. Runout shows up on a dial indicator and in the measurement: straightening or replacing the tube comes first, balancing after.
- The tooth set is incomplete or mismatched. Fit the rotor with a matched set of teeth per the manufacturer's layout first, otherwise we'd be balancing a random state that disappears with the next replacement.
- Worn-out bearing housings, play in the drum's fit, or cracks in the housing are driving most of the level. That's a repair job, not weights.
- The hydraulic drive won't hold speed: it sags under load and jumps around while warming up. Measurements won't repeat, and influence coefficients can't be calculated.
- Operating speed falls inside a resonance of the cover, the boom, or the linkage: the amplitude is inflated and the phase drifts. You need to get out of resonance first — change the structure's stiffness or the operating speed; we have a separate article on this.
- No access to the drum ends: the flap guard won't come off, there are no access ports, the cover is sealed. Then the rotor gets removed and balanced off the machine.
Vibration that only appears under load, with a clean idle, means the source is in the cutting process or the drive. That's a diagnostics question, not a balancing one, and we'll honestly tell these cases apart with the measurement.
What you get, and how to book a visit
The result: a rotor that holds speed without shaking, and a report with figures. The report includes initial and final vibration at both bearing housings in mm/s, spectra, the mass, radius, and location of every weld-on weight, speed, mechanical findings, and recommendations. This document lets you compare the same head's condition a season later.
We're the engineers who design and manufacture the Balanset instruments and use them ourselves out in the field. We're based in Vila Nova de Gaia, near Porto, and travel across all of Portugal. Vibration diagnostics with a report is 300 EUR per unit, balancing adds from 250 EUR, the minimum invoice per visit is 500 EUR, and the calculator on the site gives you the exact figure for your machine.
For one mulcher head, we usually fit within a working day, provided the drum is washed and the ends are accessible. Several machines from the same fleet in one visit work out cheaper per rotor.
- Tell us the mulcher's model and the host machine: excavator, loader, or a tractor with a mounted mulcher.
- Tell us the drive: hydraulic motor, or PTO with a cardan shaft and belt drive.
- Describe the event: an impact, lost teeth, a set change, or hardfacing.
- Send photos of the rotor from the ends, the bearing housings, and the cover with the flap guard off, if you can: they let us pick the correction planes in advance.
- Have the drum washed and a full tooth set ready before we arrive.
Sources: Balanset-1A manufacturer specification
Frequently asked questions
Do you need to remove the mulcher head from the excavator, or the mulcher from the tractor?
No. We balance the rotor right on the machine, in its own bearing housings, and it's the machine's own drive that spins it. This is more accurate than a bench: the rotor is being tested with its actual supports, cover, and mounting. The rotor only comes off if the shaft is bent or there's simply no way to reach the drum ends.
Our hydraulic motor doesn't hold perfectly steady speed. Is balancing possible?
Yes, as long as speed is stable within a few percent and repeats from run to run. The instrument picks out the once-per-revolution component using the phase mark, so small fluctuations don't get in the way. We warm the hydraulic system up to operating oil temperature, lock in the engine and pump settings, and check speed on every run. If the drive sags and speed drifts between runs, we stop the work and get to the bottom of the hydraulics first.
Can weights be welded onto a drum made of wear-resistant steel at all?
Yes, as long as it's welded correctly and in the right place. We weld weighed mild-steel plates onto the end discs or the tube bands, well clear of the tooth holders and any hardfacing, with a continuous perimeter weld. For high-strength steels, we account for weldability: edge preparation, suitable welding consumables, and preheating if needed. We never weld over carbide or wear-resistant hardfacing.
Where do you put the phase mark if the rotor is driven by a cardan shaft off the PTO?
Only on the rotor itself: on the drum shaft's end or its pulley. A cardan shaft with joints turns unevenly within each revolution, and past a belt transmission the rotating speed is different again — so a mark on the drive side would give a wrong phase and break the correction calculation.
We replaced the whole tooth set with new ones. Is balancing needed?
Take a measurement. A full, matched set fitted per the layout often leaves the rotor within tolerance. But even teeth from the same batch have some mass scatter, and holders wear unevenly, so there's no guarantee. The measurement takes under an hour and answers the question with numbers. If we've already balanced this rotor before, the saved influence coefficients let us fine-tune it in just one or two passes.
What should we prepare before you arrive?
A drum washed clean of soil and chips, a full tooth set, level ground, a machine in good working order with a warmed-up hydraulic system or a properly functioning PTO, access to the drum ends, and an operator to run the machine for us. If the flap guard and panels are hard to remove, it's best to take them off in advance. The less preparation needed on site, the more time is left for measuring and fine-tuning.
Related content
On-site balancing of mulcher, mower, and agricultural-machinery drum rotors
Yes, we balance the rotors of mulchers, mowers, shredders, and the drums of harvesting machinery on site, in the machine's own bearing supports. Three conditions are needed: the rotor holds a stable speed, there's access to the correction plane — the spot on the rotor where the correction weight goes — at least through a hatch or a removed panel, and the vibration is genuinely driven by the running-speed component rather than a worn support, loose fasteners, or a worn driveshaft. We spend the first hours of a visit checking these conditions. If balancing won't help, we say so right away and explain what to do instead.
On-Site Rotor Balancing for Agricultural Mulchers and Flail Mowers
Yes, we balance the rotors of agricultural mulchers, flail mowers, residue shredders and haulm toppers on-site, in the machine's own bearings, without removing the drum. The conditions are simple. The flail or blade set is complete and matched by mass, the drum is washed clean of soil and plant matter, the tractor holds a steady PTO speed with no drifting, and there's access to the drum ends through a folded-back apron, a removed guard, or an inspection hatch. One reservation specific to flails: measurements only make sense at operating speed, where centrifugal force has swung the hinged working elements out to their radius. If the set is mismatched, the pins and bracket eyes are worn, or the drum tube is bent after a stone strike, weights won't fix that, and we'll say so before taking the job on.
Savings from balancing without removal: how to calculate the full cost and avoid overpaying in downtime
On-site balancing wins not on the price of the work itself, but on what it doesn't include: disassembly, rigging, transport, reassembly, shaft alignment after reassembly, and calendar days of downtime. Calculate both options against the same boundary: from the moment the machine stops to the moment it's back within tolerance and running again. In the overwhelming majority of cases, the outcome comes down to two line items: your hourly downtime rate, and whether you have a standby unit. Removal stays cheaper wherever a weight physically can't be fitted on site, where the rotor is flexible, where the geometry is damaged, or where acceptance requires a report against a balance quality grade G.
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.