On-Site Wood Grinder and Hog Balancing: Disc, Drum, and Knife Overhang

A chipper starts humming after a knife change, and the hum won't go away, not at idle and not under a load of chips. There's no need to remove a disc weighing a tonne and haul it to a shop: rotors like this get brought into spec in their own bearing housings. But on a knife machine, balancing does not start with weights — it starts with knife overhang and cleaning off the resin. Below is what we specifically do on wood chippers, both disc and drum types.

Updated 27 August 2026 · by AXILINE · Vila Nova de Gaia

Symptoms: how a chipper shakes

On a wood chipper, vibration is almost always tied to an event. Knives were reground yesterday, it's shaking today. Or a nail came through with the bark in the feed, the machine "kicked," and the level never went back down after that run. A gradual rise happens too, and it's usually resin: it builds up on the disc and in the knife pockets over weeks, then breaks off in a lump in a second.

Chipper speeds are low to moderate, typically 500–1500 rpm, so imbalance gets away with more here for longer than it would on a fan. The reckoning comes through the bearing housings: a heavy flywheel disc squeezes out the grease and wears the fits loose long before the vibration starts alarming anyone standing near the machine.

• Shaking starts from the very first run after regrinding or replacing the knife set.
• The level jumped in a step after metal or a stone came through in the feed.
• The hum is once per revolution, and it's audible at idle with no feed running too.
• Chips are coming out uneven: some thick, some torn. A sure sign of unequal knife overhang.
• During the long coast-down — minutes long on a flywheel disc — vibration spikes sharply in a narrow speed band.
• Bearing housings run hot, and grease is being forced out of the seals.
• The feed conveyor and the infeed chute are shaking, and the frame anchors have loosened.

A useful figure to have before you call: overall vibration — the total level of oscillation from all causes at once — in mm/s RMS (root-mean-square value, the standard measurement unit) at both rotor bearing housings, taken at idle with the conveyor stopped. That way you rule out feed vibration and give us a clean baseline.

Disc or drum: construction, and where the imbalance comes from

Industrial wood chippers are built around two layouts. On a disc chipper, the knives sit radially on a heavy steel disc that doubles as a flywheel. On a drum chipper, the knives run along the drum's length in pockets, clamped by wedges or bolts. In both layouts, a fixed counter-knife sits opposite the rotating knives, and the gap to it sets the quality of the cut.

The rotor itself is rigid and symmetric. Imbalance is brought in by the replaceable parts and by the environment.

Knives: why overhang matters more than weight mass

The main mistake on these machines is asking for balancing the moment vibration jumps. On a knife rotor, the order is different. Knives first, weights after, and never the other way around.

The reason is geometry. A knife sits at a radius of 300–700 mm and weighs from one to several kilograms. A one-millimetre overhang error shifts its centre of mass at that same radius, and in gram-millimetres that's comparable to a correction weight. But a weight will not fix it. Unequal overhang means unequal cutting: the protruding knife takes a thicker cut, and the impact repeats every revolution. So the vibration shows up not just at the running frequency but also at the knife-pass frequency — speed multiplied by the number of knives. A weight only removes the 1x running-speed component — the part of the vibration that repeats once per revolution. It does not touch the impact component from the cutting itself.

There's a third problem too: an undertorqued wedge. A knife that has shifted under load changes both its overhang and its mass at that radius. The 1x phase — the angle at which the instrument sees the rotor's heavy spot — stops repeating from run to run, and the correction calculation loses its meaning.

• Regrind knives as a set, with equal metal removal. Replace in pairs: a new knife opposite a new knife, not opposite a worn one.
• Weigh the knives after grinding. The difference between opposite positions should be minimal.
• Clean the knife pockets and mounting surfaces down to bare metal, removing resin and packed chip fragments. Resin under a knife means both an overhang error and a loosened torque.
• Set the overhang equal for all knives using a template or a dial indicator, to the precision given in the manufacturer's documentation. Usually that's tenths of a millimetre.
• Torque the wedges and bolts to the values in the manual, in the specified sequence. Check the torque again after the first hour of running.
• Set the gap to the counter-knife and inspect its edge: a chipped counter-knife tears the chips and shakes the machine just as badly as imbalance does.

Only after this does a measurement make sense. If the 1x running-speed component is still dominant on a clean, correctly assembled rotor, weights take over from there. A noticeable share of jobs on knife machines get closed out by overhang adjustment and cleaning alone, without a single gram of correction.

What we check before the first weight

Before weights, we separate out the sources of vibration. On a wood chipper there are at least three: the rotor itself, the drive, and the feed line. The conveyor and feed rollers have their own drives, and their vibration travels through the common frame into the sensors on the rotor bearings. So we take the baseline measurement at idle with the feed stopped, and we separate the conveyor's components out in the spectrum — the signal broken down by frequency: they are not multiples of rotor speed.

• Knife set: sharpening, masses, overhang, wedge torque. Without this item, the rest don't matter.
• Resin and bark buildup on the disc, in the pockets, and on the inside of the housing wall.
• Edge condition after metal has come through the feed: chipping, bending, cracks near the mounting.
• Counter-knife: gap, wear, mounting.
• Bearing housings: play, noise, temperature, and grease condition.
• Drive: belt tension and condition, plus the pulleys' own imbalance for a belt drive; shaft alignment for a direct coupled drive.
• Frame and foundation: anchors, cracks, foot flatness, and the condition of the vibration isolators.
• Resonance: the behavior of the 1x amplitude and phase during coast-down. A flywheel disc's long coast-down is convenient for this check.

Imbalance and misalignment look similar in the measurement but are fixed in opposite ways, so on a coupled drive we check shaft alignment before weights. How to tell these two defects apart is covered in our article on distinguishing imbalance from misalignment.

Sources: ISO 13373-3:2015 · ISO 281:2007

How the work proceeds on site

A visit to one machine takes a shift. On wood chippers, coast-down eats up the most time: the flywheel disc keeps turning for minutes after the drive is switched off, and many machines have no brake. So we plan the number of stops in advance and don't waste runs.

1. Step 1

   We agree on safety and preparation

   Before the visit, we agree who stops and locks out the drive and feed, who opens the hood or housing, and whether hot work is acceptable given your dust. We ask you to clean the rotor of resin and check the knife assembly beforehand: that saves you two or three runs.

2. Step 2

   We mount the sensors and the mark

   Two accelerometers go on the rotor's bearing housings, on magnets on cleaned-down pads, and we keep the measurement direction fixed from one run to the next. We stick the reflective mark on the rotor's own shaft. On a belt drive this matters: motor and rotor speeds differ, and a mark on the motor would give the instrument the wrong frequency.

3. Step 3

   Baseline measurement

   Run at operating speed, at idle, feed stopped. We record overall vibration in mm/s RMS, the 1x amplitude and phase, speed, spectrum and time waveform on both channels. This is where we decide whether it's imbalance or the knives, clearance, or mechanics.

4. Step 4

   Trial weight

   A temporary mass of known size at a known radius: on the back of the disc or on the rim for a disc chipper, on the end disc for a drum chipper. Run, then for a two-plane setup, move it to the second plane and run again. The instrument calculates the influence coefficients — exactly how your system of rotor, bearings and frame responds to the weight fitted.

5. Step 5

   Correction

   The software outputs a mass and an angle for each plane, or a factory hole number if we're working from fixed positions. We fasten with bolts, drill out metal at the heavy point, or weld if hot work has been agreed.

6. Step 6

   Check run and report

   Same speed, points and sensors. If the remainder is above target, the software calculates an addition to the weights already fitted. Then a measurement under a load of chips, a report with the before-and-after figures, and a list of mechanical findings.

We keep your machine's influence coefficients on file. The next balancing after a knife regrind runs as a trim adjustment: no trial runs, with a minimum of stops, which on a machine with a multi-minute coast-down saves a noticeable chunk of a shift.

Sources: Balanset-1A operation manual · Balanset-1A manufacturer specification

One plane or two: disc and drum behave differently

Disc and drum layouts part ways here. A disc is the classic short rotor: length-to-diameter ratio much less than 0.5, the moment component (mass skew along the rotor's length) is small, and one correction plane — one cross-section where we fit weights — is usually enough, with both bearings checked. A drum is elongated, with knives and buildup spread along its length, so we work in two planes at the drum's end discs.

A separate case is a massive pulley or flywheel on the shaft. It can itself be a source of imbalance, and at the same time a convenient second plane.

We have a separate article on the L/D rule and choosing the number of planes. A second plane on a machine with a long coast-down costs one extra stop, and we factor that into the time estimate.

Sources: ISO 21940-11:2016 · ISO 21940-12:2016

How we fasten weights when there's combustible dust around

Wood dust is combustible, and that is the main constraint. On many sites, a hot-work permit inside a chipper's housing simply won't be issued at all, so we discuss the fastening method before the visit, not standing at the open machine.

• Bolted weights in factory holes. Many discs and drum end discs have balance-weight holes built in by the manufacturer. We thread-lock everything: a machine with impact loads unscrews anything that isn't locked.
• Removing metal by drilling at the heavy point. A cold method, safe around combustible dust. The software calculates the drilling diameter and depth for the required mass. We do not drill on thin sections or near the knife pockets.
• Welding on a plate. Only with a hot-work permit and a full clean-down of the machine and area, clear of dust and chips. Welded around the perimeter, on the back of the disc or the drum's end disc, away from the knife pockets. We confirm whether welding is acceptable from the manufacturer's documentation.
• We do not touch the knife fasteners. Fitting washers under knife bolts or wedges is off-limits: it changes the torque and the overhang, creating a problem more dangerous than the original one.
• Repositioning knives. If the set has spread apart by mass, sometimes simply swapping knives so that opposite positions balance out is enough. We check this cost-free option first.

We record the mass, radius, position and fastening method of every weight in the report. At the next service, you can fit the correction at the same radius without recalculating: an error in the radius is a direct error in the mass.

When on-site work will not work

Let's be honest: some visits to chippers end not with weights but with a list of jobs for your maintenance team. Here are the typical cases.

• The knives aren't set to equal overhang, or the wedges aren't torqued. Balancing a rotor like this is pointless: everything changes the moment the knives are next drawn up.
• A chipped or bent knife after a nail or a stone. This is a replacement job, not balancing. We inspect the counter-knife and the inside of the housing while we're at it.
• The rotor is caked in resin. Cleaning down to bare metal comes first, measurement after: the buildup breaks off unpredictably, and yesterday's balancing job goes with it.
• Cracks in the disc or the drum's welds, a worn fit on the shaft. The 1x phase doesn't repeat from run to run, the calculation is unreliable, and the machine needs repair.
• Worn bearings and damaged bearing housings. Overall vibration is many times the 1x level, with spectrum components that aren't multiples of speed. Weights won't help.
• Resonance in the frame or the supporting steelwork, a common story on machines raised above a chip bin. Stiffening or a speed change comes first, balancing after.
• The machine won't hold a stable speed, or there's no access to the correction planes when stopped. In that case it's more honest to take the rotor to a shop, and we'll say so right away.

If the vibration isn't coming from the rotor, you get a measurement, spectra, time waveforms, and a prioritized diagnosis instead of weights. We have a separate article on cases where balancing doesn't help.

Sources: ISO 20816-1:2016 · ISO 13373-3:2015

What you get, and how to book a visit

We are the engineers who design and manufacture the Balanset instruments, and we do the on-site balancing ourselves. We are based in Vila Nova de Gaia, near Porto, and we travel throughout Portugal: sawmills, pellet plants, pulp sites, wood-waste processing.

After the work, you're left with a report: overall vibration and 1x at each bearing before and after, spectra and time waveforms, the masses, radii and fastening method of the weights, an overall-vibration zone assessment noting the applicable part and edition of ISO 20816, and, where needed, a residual-imbalance calculation to the G balance quality grades of the applicable part of ISO 21940. Vibration diagnostics with a report costs EUR 300 per unit, balancing adds from EUR 250, the minimum invoice for a visit is EUR 500, and the calculator on the website gives you an exact figure.

• Machine type: disc or drum, stationary or mobile, model.
• Rotor speed, drive power, belt or direct drive.
• Number of knives, disc or drum diameter, approximate rotor mass.
• When and what was last done to the knives: regrinding, replacement, overhang setting.
• Whether metal or a stone came through the feed before the vibration jumped.
• Measurement figures in mm/s and the points, if you have measured.
• Access to the rotor when stopped: hood, hatches, a removable section of the housing.
• Whether hot work is permitted. Photos of the rotor, the bearings, and the machine's nameplate.

If you want to track vibration yourselves, we sell the same instrument we use: the Balanset-1A. Two accelerometers, a laser phase sensor working off a reflective mark, a two-channel USB module, and software on a laptop. One- and two-plane balancing by the influence coefficient method, overall vibration and 1x, phase, speed, FFT spectrum and time waveform, fixed positions and drilling calculations, saved influence coefficients, trim balancing, G-grade tolerances, an archive and reports. With regular knife regrinding, this is a practical routine: measure after every reassembly and balance off the saved coefficients.

Sources: Balanset-1A manufacturer specification · Balanset-1A operation manual

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