AXILINEConveyor Pulley Balancing: Drive, Take-Up, and Bend Pulleys
A belt conveyor pulley turns slowly, often below 100 rpm, and that changes the whole logic of the work. Centrifugal force grows with the square of the speed, so on a slow-turning pulley even gross imbalance loads the bearings less than a belt splice or belt mistracking does. We put sensors on the bearing units and first find out what's actually shaking the conveyor. We balance once the measurement confirms it.
Why imbalance is rarely the culprit on a slow-turning pulley
The force from imbalance is calculated as mass times radius times the square of the angular velocity. The square decides everything. A 1 kg chunk of rock stuck to a shell with a 400 mm radius, at 75 rpm, pulls on the bearings with a force of about 25 N. That's less than the chunk's own weight. The same kilogram at 1500 rpm would press with nearly a tonne of force — a difference of four hundred times.
So on a pulley running at 50-100 rpm, imbalance is almost never the dominant source of vibration. Other forces shake the bearings: the belt-splice impact, tension fluctuation from shell eccentricity, side thrust from belt mistracking, gearbox gear-mesh frequencies, bearing defects. None of these depend on the pulley's mass distribution, and weights don't touch them. Hence an order we don't break: before any talk of balancing, we check what breaks more often and shakes harder.
- Belt tension: over-tensioned overloads the bearings, under-tensioned slips on the drive pulley and jerks the whole conveyor structure.
- Belt tracking: drifting sideways, rubbing against the frame, misaligned pulley axes.
- Lagging: uneven wear changes the working radius and pulls at the tension once per revolution.
- The motor-gearbox unit: its input shaft turns tens of times faster than the pulley and often overshadows everything else in vibration level.
- The pulley's bearing units: play, a worn fit, a failed cage.
- The steel structure: loosened bearing-housing fasteners, cracked welds, a compliant gantry frame that amplifies any of the other sources.
How to tell imbalance apart from other causes by spectrum and phase is covered in separate articles. The takeaway for a conveyor: first rule out the belt, the lagging, and the gearbox, then bring out the trial weights.
When pulley balancing is still needed
Imbalance moves to the forefront where the pulley turns faster, or where mass has shifted grossly. A 400 mm pulley with a belt speed of 3 m/s already runs at about 143 rpm, and centrifugal forces are noticeable there. A second typical situation doesn't depend on speed at all: water or spillage has gotten inside the hollow body, and the mass wanders from run to run.
Check yourself against the list. If two or three items match, a measurement will almost certainly confirm imbalance.
- Vibration appeared after a repair: welding the shell, replacing an end disc, or hard-facing the shaft journals.
- The pulley was re-lagged, and the layer went on with an uneven thickness.
- There's a clean patch with weld-tack marks on the shell or end disc: a factory balancing weight used to sit there.
- Turning it by hand, you can hear water or spillage sloshing around inside the body.
- Material has built up on the inside through a leaking seam and won't wash out.
- The pulley is fast-turning: a small diameter, belt speed from 2.5-3 m/s, speed above 120-150 rpm.
- Vibration at the bearings is strictly at running speed: a peak exactly at the rotation frequency, with the phase stable from run to run.
- The bearing housings run hot and fail repeatedly, even with normal belt tension and tracking.
We check phase stability — the angle by which the vibration is tied to the marker on the pulley's end — with the laser sensor. If the phase drifts by tens of degrees between runs, the mass inside the body is mobile: drain the water first, then weights.
Design: where the vibration in each unit comes from
A conveyor pulley is a welded structure: a shell, two end discs, hubs and a shaft, with lagging on top. It turns in two split bearing units on the frame. Let's go through it unit by unit, since both the diagnosis and the fix depend on which one is involved.
Drive pulley and the motor-gearbox unit
The only pulley with its own drive, so it's the only one that can be turned with the belt slackened. The motor-gearbox unit hangs on the shaft or sits alongside it. Its input shaft and gear pairs produce vibration at frequencies tens of times higher than the pulley's running frequency, and the spectrum separates them out immediately.
Take-up pulley
Sits on a movable carriage or the take-up frame. A compliant mount means a low natural frequency, and the carriage readily rocks in response to any force. It has no drive of its own and turns only by the belt: it can be diagnosed on site, but balanced only with the conveyor running.
Bend and tail pulleys
A small wrap angle, with the same speed or higher due to a smaller diameter. Imbalance here most often comes from buildup: the pulley contacts the dirty, non-carrying side of the belt. Working scrapers and belt cleaners solve more than weights do.
Lagging
Rubber, either smooth or with a diamond-groove pattern, sometimes ceramic. Uneven wear shifts the center of mass and, at the same time, changes the working radius. The second effect is the trickier one: the belt tensions and relaxes once per revolution, which looks like imbalance in the spectrum but is only fixed by re-lagging.
Buildup and water inside
Wet concentrate, clay, and cement dust build up on the shell in an uneven layer that changes over a shift. A separate case: water inside the hollow body, through a defective seam. You can hear it on the coast-down — the free run-down — and on starts it makes the phase random.
Bearing units and the frame
Split housings with double-row spherical roller bearings on adapter sleeves. The sleeve loosens, the housing wears out its fit, and the frame fasteners stretch. The gantry frame is compliant, and at its own natural frequency it amplifies any source several times over.
The pulley is balanced by the manufacturer to begin with. If vibration has risen, look for a triggering event: a repair, a lost weight, buildup, water, or lagging wear. Mass distribution doesn't change on its own.
The lower speed limit: what can realistically be measured
Pulley speed is easy to work out: divide the belt speed by the circumference. A 1.6 m/s belt with an 800 mm pulley gives 38 rpm; a 2 m/s belt with a 630 mm pulley gives 61 rpm. The running frequency comes out to 0.6-1.7 Hz, and that's already a problem: at frequencies like these an accelerometer puts out only scraps of signal, and the vibration velocity in mm/s stays small even with a noticeable imbalance.
The practical limit for on-site balancing runs at around 120 rpm, meaning 2 Hz of running frequency. Above that we confidently record the 1x running-speed component's amplitude and phase and work by the influence-coefficient method: the instrument learns from trial runs exactly how your pulley responds to an added weight. Below it we say plainly: speed and spectrum can still be assessed, but phase accuracy drops, and, more importantly, there's usually nothing to fix there anyway.
There's a regulatory side to this too. ISO 20816-3, in its current edition, applies to machines running at 120 rpm and above; below that there simply are no formal assessment zones. We assess a slow-turning pulley by trend against its own baseline, not against a zone table.
If your pulley turns at 40-60 rpm and is vibrating, a balancing visit is probably not what you need. What you need is diagnostics: the belt, the lagging, the gearbox, the bearings, the frame. We do that too, but we call things by their name over the phone, before we ever come out.
Sources: ISO 20816-1:2016
What we check before trial weights
A measurement on a conveyor starts not with the instrument but with a walk-around. Balancing a pulley with a dirty shell or a wandering belt is pointless: the correction will leave with the next chunk of buildup that falls off, or get drowned out by vibration the pulley isn't even producing.
- Belt tension against the conveyor's data sheet, take-up carriage travel, and the condition of the gravity take-up unit.
- Belt centering, both empty and loaded, and signs of rubbing against the frame and skirts.
- Lagging: thickness at the edges and center, gouges, delamination, and the condition of the groove pattern.
- Shell: buildup on the outside, signs of prior welding, and runout by dial indicator while turning slowly.
- The body's cavity: water or spillage sloshing, heard while turning it by hand, and the condition of the drain plugs.
- Bearing units: play, temperature, and the condition of the adapter sleeves and housing fasteners.
- Motor-gearbox unit: vibration level and spectrum at its mounts, and the condition of the coupling.
- Frame and support beams: cracked welds, loosened fasteners, and a shaky platform.
- Belt scrapers and cleaners: whether they're fitted, whether they're pressed against the belt, and whether they're working.
Bearing defect frequencies at slow-turning housings sit in the single-digit hertz range and are hard to read, so we assess the units as a whole: spectrum, temperature, play, lubrication. There's a separate article with more detail on bearing diagnostics.
Sources: ISO 281:2007 · ISO 13373-3:2015
How balancing the drive pulley proceeds
- 01
Inspection and decision
We work through the checklist above and calculate the pulley's speed from the belt speed and diameter. If the speed is below the practical limit, or if there's no pulley running-speed component in the vibration, we stop at diagnostics and a report.
- 02
Sensors and the marker
We place two accelerometers on the pulley's bearing units, radially, on cleaned spots on the housings. We stick the reflective marker on the end disc or hub, and mount the laser phase sensor on a magnetic stand attached to the frame. Your staff removes the guards for the duration of the measurement.
- 03
Baseline measurement
We run the conveyor empty, with no material on the belt. We record the overall level, 1x amplitude and phase, speed, and spectrum. The belt splice hits with a period equal to one full belt loop — tens of seconds. Synchronous averaging keyed to the marker (the instrument averages the signal over many pulley revolutions) separates the splice impact from the pulley's own running-speed component.
- 04
Trial runs in two planes
The pulley has two correction planes: the end discs at the shell, on both sides. We fix the trial weight with a clamp or a magnet, run the pulley, and the instrument calculates the influence coefficients and outputs a mass and an angle for each plane. On narrow pulleys with the planes close together, one is sometimes enough.
- 05
Fitting the correction weights
The standard method: we weld steel plates to the outside of the end discs, close to the shell, where the radius is greatest. The rubber lagging sits nearby, so we shield and cool the weld zone and arrange the hot-work permit in advance. If welding is prohibited, we bolt the weights to existing holes in the disc.
- 06
Check measurement and the report
A repeat run confirms the result. In the report we record the before-and-after amplitude and phase, speed, the weights' masses and angles, the residual unbalance, and the grade achieved. We save the influence coefficients: the next balancing of this pulley will proceed without trial runs, in one or two starts.
Take-up and bend pulleys have no drive of their own: we balance them on the running conveyor by the same method, if that pulley's running-speed component stands out clearly at the bearings. With worn lagging, it's more honest to pull the pulley for repair.
Sources: Balanset-1A manufacturer specification · Balanset-1A operation manual
Tolerance: what level we balance to
For conveyor pulleys, manufacturers usually specify balance quality grade G16 under the current edition of ISO 21940-11, with G6.3 assigned to fast-turning, small-diameter pulleys. We calculate the allowable residual unbalance from the pulley's mass and operating speed, and the instrument compares the result against the tolerance automatically.
Grade G governs residual unbalance, not bearing vibration. On a compliant gantry frame, even a pulley balanced within tolerance can show a noticeable level from neighboring sources. That's why we record both quantities in the report: residual unbalance in g·mm, and bearing vibration before and after.
| Pulley | Typical speed | Grade G | Comment |
|---|---|---|---|
| Drive, 630-1250 mm diameter | 40-100 rpm | G16 | Imbalance is rarely the main issue; diagnostics first |
| Drive, fast-turning, up to 500 mm | 120-200 rpm | G16 / G6.3 | On-site balancing is fully justified |
| Take-up and bend | depends on diameter and belt speed | G16 | Balancing on the running conveyor, if 1x stands out |
| After re-lagging in the workshop | any | G6.3 | Balancing on a rig before installation; on site, check only |
How grade G is chosen and how the tolerance converts into grams at the weight's mounting radius is covered in separate articles on balance quality grade and residual unbalance.
Sources: ISO 21940-11:2016
Visits throughout Portugal, prices, preparation
We're engineers who design and manufacture the Balanset instruments and do the on-site balancing with them ourselves. We're based in Vila Nova de Gaia near Porto and work throughout Portugal: quarries, ports, cement and woodworking plants, grain terminals. The Balanset-1A packs into a single case, and running it needs a 230 V outlet or an inverter.
Packages start from 550 EUR per unit (diagnostics 300 EUR + balancing from 250 EUR), with a minimum invoice per visit of 500 EUR; the calculator on our website gives the exact amount. A diagnostic visit without balancing is also possible: at the end of it you get a report with the readings and a prioritized list of causes.
On your side you'll need to arrange a stop and trial runs of the conveyor, the pulley guards removed, a washed shell, the water drained from the body, and either a hot-work permit or a decision to fasten weights by bolt. Prepare several pulleys on the same line for a single visit — it works out cheaper per rotor that way.
The full preparation checklist is in the article on preparing equipment for on-site balancing. For a conveyor, two points matter most: a clean shell and agreed trial runs.
Frequently asked questions
Can a pulley be balanced without removing the belt?
Yes, that's the standard approach. The drive pulley turns under its own motor-gearbox unit, and we run the conveyor empty. The belt splice hits with a period equal to one full belt loop, while the instrument extracts the pulley's own running-speed component synchronously, keyed to the marker. The belt only needs to come off if the pulley itself is being repaired.
The pulley turns at 60 rpm and vibrates badly. Will balancing help?
Almost certainly not. At 60 rpm the force from imbalance is negligible: even a kilogram of buildup at a 400 mm radius pulls on the bearings less than its own weight. Look for the cause in belt tension and tracking, the lagging, the gearbox, the bearing units, and the frame. We can come out with diagnostics and show you the source by spectrum, but weights aren't the cure here.
The shell was cleaned of buildup, but the vibration stayed. Why?
That means buildup wasn't the only cause. Common continuations: material has built up on the inside of the body through a leaking seam, the lagging is worn unevenly, a bearing unit has worn out, or the source isn't the pulley at all but the motor-gearbox unit or the belt. A measurement at the bearings, broken down by frequency, shows where to look next.
Can a take-up or bend pulley be balanced on site?
Conditionally. They have no drive of their own and turn only by the belt, so we take the measurements and trial runs with the conveyor running. That's possible when this particular pulley's running-speed component reads clearly at its bearings. With worn lagging, it's more honest to balance it on a rig after repair.
What balance quality grade does a conveyor pulley need?
Usually G16 under ISO 21940-11, with G6.3 used for fast-turning, small-diameter pulleys. The allowable residual unbalance is calculated from mass and operating speed: the slower the machine turns, the more gram-millimeters it's allowed. A slow-turning pulley doesn't need a tight tolerance — it simply wouldn't feel it.
How long does it take, and what should be prepared?
One drive pulley with normal access takes a few hours, including the check measurement and the report. What eats up the most time is logistics: waiting for a stop, removing guards, arranging hot-work paperwork. Have the trial runs, a clean shell, and the water drained from the body ready, and we'll fit within a single shift, diagnostics of the line included.
Related content
On-Site Belt-Drive Pulley Balancing: Geometry and Belts First, Weights After
Yes, we balance a belt-drive pulley on site, without removing it from the shaft. Conditions: the fit on the key, taper bushing or clamping sleeve is tight and doesn't turn, the rim's radial and face runout are within the manufacturer's tolerance, and the belts are a matched set with tension set correctly. A thin pulley is closed out with a single correction plane (weights fitted at one rotor cross-section) in two to three runs; a wide multi-groove pulley, or a shaft assembled with a pulley and coupling half, sometimes needs two. If the rim runs out or the grooves are worn shiny at the bottom, weights won't help: refitting or replacing the pulley comes first, and we'll say so before starting work.
On-Site Balancing of Combine Threshing and Chopping Drums
Yes, we balance threshing drums, forage-harvester chopping drums, and straw choppers on-site, with no removal from the combine. Conditions: a complete, matched set of rasp bars or knives, the drum washed clean of crop residue and dried out, an unbent shaft, and access to the drum ends through the hoods, inspection hatches, or a lowered concave. The drive from the combine's engine holds a steady speed, which only helps the balancing. If the set of working elements is mismatched, or the shaft is knocking after a foreign object went through, we'll say so after the first measurement, and we won't use weights to compensate for something that needs matching or straightening instead.
How to build vibration monitoring and balancing into your maintenance program
Build vibration monitoring into maintenance with three moves. First, sort machines by the consequences of failure and give each group its own measurement interval instead of one interval for the whole fleet. Second, set a baseline level and three thresholds for each point, and next to each threshold define the required action, the deadline, and who is responsible. Third, schedule balancing in advance for jobs that predictably cause unbalance: rotor repair, blade or hammer replacement, motor rewinding, installing or relocating the unit, and impeller cleaning.
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.