On-Site Balancing of Combine Threshing and Chopping Drums

The combine shakes with the threshing mechanism engaged, even on a dry run before any crop feeds through, and after a rasp-bar change the hum only got louder. We balance threshing and chopping drums right on the combine, in their own bearing housings, from the machine's own drive. The drum is long relative to its diameter, so we work in two planes almost every time and check both bearings at once. We are based in Vila Nova de Gaia, near Porto, and travel throughout Portugal.

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

Symptoms: how drum imbalance shows up on a combine

A combine has a convenient property: you can run the threshing mechanism dry, with no crop going through it. Drum imbalance shows up right there. If the machine shakes with the threshing mechanism engaged while parked, before any feed at all, the source is in the rotating rotors, not in the threshing process.

A second clue: the event. On combines, vibration almost always appears after a specific repair: rasp bars were replaced, chopper knives were sharpened, a wrap-up was pulled out, or the drum caught a stone or a bolt. Tell us about that event when you get in touch, and the diagnosis will go faster.

• The combine shakes with the threshing mechanism engaged at idle, before any crop feeds into the feederhouse.
• Vibration increased after replacing rasp bars, bar-carrier strips, or part of the chopper knife set.
• The straw chopper started humming after the knives were sharpened or flipped.
• The hood panels, mirrors and cab windows shake, and the threshing mechanism's paneling rattles.
• The drum's bearing housings on the sides of the threshing mechanism run hot, and fresh grease gets forced out of the seals.
• Bearing-housing bolts keep loosening, and cracks appear on the side panels and frame brackets.
• Vibration changes along with drum speed when the variator is adjusted.
• After a stone or metal object went through, the machine started shaking immediately and has shaken the same way ever since.

The threshing mechanism has several rotors: the threshing drum, the beater, the accelerator drum, the straw chopper, the cleaning fan. We separate them out by their rotational frequencies in the spectrum (vibration broken down by frequency) and by phase — the angular link between the oscillation and the shaft's rotation. There's also a simple field trick: change the threshing drum's speed with the variator. If the vibration follows it, that's the culprit.

Sources: ISO 13373-3:2015

Drum construction, and where the imbalance comes from

All three types of drum share the same geometry: length many times the diameter, removable working elements, a wet and abrasive environment. What differs is speed, how the working elements are mounted, and exactly what breaks the balance.

Why one new rasp bar without the full set creates imbalance

A rasp bar's ribs wear down over a season, and a worn bar is lighter than a new one. The difference reaches hundreds of grams. Fit one new bar among worn ones, and the drum picks up a local heavy spot running its full length. Two hundred extra grams at a 300 mm radius, at 1000 rpm, pulls outward with a force of around 650 N. That's close to seventy kilograms hammering the bearing housings every revolution — sixteen times a second.

What's worse is this. A rasp bar runs the full length of the drum, so the excess mass is spread out along the rotor's length. Imbalance like this isn't purely static: it has a moment component, and one weight in the middle won't remove it. That's exactly why, after replacing a single rasp bar, a combine shakes harder than a farm would expect from such a minor job.

On a straw chopper the arithmetic is even harsher. A 50-gram difference between knives at a 250 mm radius, at 3000 rpm, produces over 1200 N. That's why chopper knives are only ever replaced in weighed pairs, fitted at diametrically opposite positions.

• Replace rasp bars as a complete set. This is the only option that gives the drum a chance of staying within tolerance without balancing.
• If a complete set isn't possible, replace in diametrically opposite pairs, and match the pairs by mass on a scale, not by eye.
• Follow the rib-direction alternation pattern: a bar with left-hand ribs cannot go in a right-hand position.
• Fit straw-chopper knives in equal-mass pairs at opposite positions, along with new bushings and pins if the old ones are worn out.
• After any replacement, run the threshing mechanism dry and listen to the machine before heading out into the field.

Even a complete new set doesn't guarantee balance: parts from the same batch have a mass spread, and the bar carriers and discs wear unevenly. A matched set removes the gross imbalance; the measurement shows the remainder in figures.

Checking the drum before the season: an order of operations that saves your harvest

Balancing in the middle of harvest costs the most, because machine downtime during the most expensive days of the year gets added on top of the visit price. The right time for this work is the off-season and pre-season prep. The check procedure is simple and fits into half a day per machine.

• Clean last year's crop residue out of the threshing mechanism, wash the drum and the internal surfaces, and let it dry. Damp chaff weighs a noticeable amount and is spread unevenly.
• Inspect the rasp bars and knives: rib wear, cracks near the mounting holes, chips, and completeness. Check the positions against the manufacturer's layout.
• Torque the rasp-bar, bar-carrier, and knife fasteners to the specified value. A loose bolt changes both the balance and the rotor's stiffness.
• Check the bearing housings: play by rocking them, noise while turning by hand, and the condition of the grease and seals.
• Check the shaft for runout with a dial indicator, especially if the drum caught stones or metal last season.
• Inspect the variator belts and pulleys: cracks, oil contamination, pulley runout, and the condition of the variator's springs and hydraulic cylinder.
• Run it dry at operating speed and check the vibration by hand at the bearings. A tremor that wasn't there in the autumn is a reason for a measurement before harvest starts.

If you replaced the rasp-bar or knife set over the winter, build a pre-season vibration measurement into your plan. An hour's measurement in March is cheaper than a day of downtime in July.

Sensors, drive, and what we check before the first weight

We mount two accelerometers on magnets to the drum bearing housings, one on each side of the threshing mechanism, radially, on pads cleaned down to bare metal. We reach the bearings through the side hoods and removable guards. We stick the reflective mark for the laser phase sensor on the pulley or the end of the drum's own shaft. It cannot go on the engine shaft or an intermediate shaft: the variator changes the ratio, and the phase would lose its meaning.

The drive here is convenient. The combine's diesel engine with its governor holds a steady speed, and we set the drum's operating speed with the variator and leave it untouched for the rest of the job. Every run — baseline, both trials, and the check — is done at the same setting, otherwise the influence coefficients won't converge — the calculated relationship of "fit a weight, get a response" that the software uses to compute the correction.

Balancing only removes the 1x running-speed component — the part of the vibration that repeats once per rotor revolution. So before the first weight, we look at the spectrum and the mechanics: how much of the level is the imbalance itself, and how much comes from the bearings, the fasteners, and the neighboring rotors. How to read these numbers is covered in our article on overall vibration, 1x, and phase.

• Completeness and consistency of the rasp bars or knives, fastener torque, and the arrangement layout.
• Stalks and twine wrapped around the shaft at the discs: removed before the measurement, not compensated with a weight.
• Cleanliness of the drum and the internal guards: there's no point balancing caked-on chaff.
• Play and noise in the bearing housings, temperature after a run, and grease condition.
• Shaft runout after a foreign object has gone through.
• Drive belts and pulleys, how the variator performs, and speed stability from run to run.
• The contribution of neighboring rotors in the threshing mechanism: separated out by rotational frequency in the spectrum.
• Panel and frame resonance: we vary the speed with the variator and watch the amplitude and phase.

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

How balancing proceeds on your combine

1. 01

   Preparing the machine

   The combine sits on level ground, the threshing mechanism cleaned out, the drum washed and dried. The hoods are open, and whatever blocks access to the drum ends and bearings is removed or lowered. The engine is shut down, and the threshing mechanism's drive stays off for the whole of any hands-on work.

2. 02

   Sensors and mark

   We fit accelerometers on both bearing housings, stick the mark on the pulley or the end of the drum shaft, and check that the laser sensor can see it through a hatch or an opening. We fix the measurement direction and don't change it for the rest of the job.

3. 03

   Baseline run

   The operator engages the threshing mechanism from the cab and brings the drum up to operating speed. Everyone stays clear of the plane of rotation and the straw chopper's discharge zone. We record overall vibration (the total level of oscillation), 1x, phase, speed, and spectrum at both bearings.

4. 04

   Mechanical checks

   With the threshing mechanism stopped and de-energized, we work through the checklist from the previous section. We show you everything we find before fitting any weights: some problems remove vibration without any balancing at all.

5. 05

   Trial runs

   We fasten a weighed trial weight in the first correction plane, run, measure. We move it to the second plane, run again. The response has to be noticeable in amplitude or phase; the criterion is covered in our article on the trial weight.

6. 06

   Correction and check

   The software calculates the mass and location of the weight for each plane. We fit the weights and run a check at the same speed. If needed, a trim adjustment: a small addition based on the influence coefficients already calculated.

7. 07

   Reassembly and report

   We put the guards and hoods back. We hand over a report: before-and-after figures at both bearings, the masses and locations of the weights, speed, and mechanical findings. We keep the influence coefficients on file for your machine.

Starts and stops are made only by the operator, on our instruction. We do not bypass the combine's factory lockouts, and we don't ask you to either. What to prepare before we arrive is covered in detail in our article on preparing for a visit.

Sources: Balanset-1A operation manual

One plane or two: a combine drum needs two

A threshing drum runs roughly 1.2–1.7 m long at a diameter of 0.55–0.8 m. A straw chopper is even more elongated. Rotors like this always have a moment component alongside the static imbalance, and a weight in one plane won't remove it: one bearing settles down while the other shakes harder. So we work in two planes, and the two-channel instrument shows both bearings at once. Why this is the case is explained in our article on choosing the number of planes.

We take the correction planes at the drum's outer discs, as close to the bearings as possible. We reach the threshing drum's discs through a lowered or removed concave, the stone trap, and the side hatches. We reach the straw chopper's rotor through the rear hood and the removed spreaders. We stick to a single plane only on the short rotors in this family: pulleys, short beaters, and the cleaning fan's impeller.

We agree the target value before starting work: residual imbalance to the G balance quality grades of ISO 21940-11, and the vibration level at the bearings measured against the zones of ISO 20816. We record the applicable part and edition of the standard in the report: for a self-propelled agricultural machine, the scope limits of these documents need to be stated separately.

Sources: ISO 21940-11:2016 · ISO 20816-1:2016

Fastening weights on a combine drum

We never fit weights on the rasp bars or knives: these are replaceable working elements, and the correction would leave along with them. Weights go on the drum discs and the rotor's end sections.

If there are free holes in the discs, we work with bolted weights through them in fixed-position mode: the software outputs a position number and mass instead of an angle. This is convenient for the future too: after the next set replacement, the weights are easy to recalculate and reposition. If there are no holes, we weld weighed low-carbon steel plates to the discs with a continuous weld, well clear of the bearing fits and the bar-carrier fasteners. On the straw chopper, with its speed, we size the weld with margin: centrifugal force on the weight there is many times higher than on the threshing drum.

Welding inside the threshing mechanism demands discipline: dry chaff and dust are all around. Before welding, we clean the work area, clear combustible material out from under the drum, keep a fire extinguisher on hand, and inspect the area afterward. If the calculation calls for removing mass rather than adding it, drilling the discs is also an option, but on thin metal we more often choose welding on the opposite side.

• The weight is weighed before installation, with mass, radius and position recorded in the report.
• Bolted weights are torqued and secured against loosening.
• Welded weights: a continuous bead around the perimeter, cleanup, and an inspection before the check run.
• No weights on the rasp bars, knives, bar carriers, or knife pivot pins.
• Fire preparation of the welding area is mandatory: the threshing mechanism means dry, combustible dust.

When on-site balancing won't help

We say this based on the first measurement, not after fitting weights.

• The drum shaft is bent after a stone or metal object went through. Runout shows up on a dial indicator: straightening or shaft replacement comes first, balancing after.
• The rasp-bar or knife set is mismatched. A rotor like this can be balanced, but the result disappears with the next replacement. Matching the set comes first, weights after.
• Worn-out bearing housings or loosened fits are producing most of the level. This is a repair job, and we separate it from imbalance using the spectrum.
• The drum has caked up with damp crop residue, and the buildup changes from run to run. Washing and drying comes first, otherwise we'd be balancing dirt.
• Operating speed falls into a panel or frame resonance: amplitude is inflated, phase drifts, measurements don't repeat. Dealing with the resonance comes first.
• It only shakes under load, and a dry run is clean. Then the source is the feed and threshing process, not rotor balance, and that's a matter for diagnostics.
• No access to the drum ends and correction planes. This is rare on combines, but when it happens, the drum is removed and balanced off the machine.

How imbalance differs from other causes of vibration, and when balancing is fundamentally useless, is covered in separate articles. On combines, three cases come up most often: a bent shaft, a mismatched set of working elements, and buildup.

What you get, the price, and how to book a visit

The result: a drum that holds its operating speed without knocking, and a report with figures. It includes the initial and final vibration at both bearings in mm/s, spectra, the masses and positions of the weights by plane, speed, mechanical findings, and recommendations for next season. The saved influence coefficients stay with your machine: after the next knife sharpening or rasp-bar set replacement, fine-tuning will take one or two runs.

We are the engineers who design and manufacture the Balanset instruments, and we do the on-site balancing ourselves. Vibration diagnostics with a report cost 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 for your machine and address. One drum usually fits within a working day; several rotors on one threshing mechanism in a single visit come out noticeably cheaper per rotor, because the combine is already prepped and the sensors are already mounted.

The best window for this work is the off-season and pre-season prep. We do come out during harvest too, but the queue is longer, and machine downtime costs more than the balancing itself.

• Tell us the machine and drum type: grain combine, forage harvester, straw chopper.
• Describe the event: rasp-bar replacement, knife sharpening or replacement, a foreign object, a bearing repair.
• Photograph the drum from the ends, the bearing housings, and the speed nameplates, if you have access.
• Tell us which hoods and hatches open and whether the concave comes off: from the photos we can choose the correction planes in advance.
• Before we arrive, have the drum washed, a complete set of working elements, level ground, and an operator on hand for the runs.

Sources: Balanset-1A manufacturer specification

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