Rotor and drum balancing: when and why

A rotating conveyor drum must be balanced — otherwise every revolution pushes the unit sideways. Imbalance is invisible to the eye, but it destroys bearings and loosens the frame. In this article we explain how to recognise imbalance, how static balancing differs from dynamic, and why this operation should not be skipped.

What imbalance is and where it comes from

Imbalance is an uneven distribution of mass relative to the axis of rotation. The drum’s centre of mass shifts from the geometric axis, and during rotation a centrifugal force appears that changes direction every revolution. This force is the source of vibration.

The causes of imbalance vary: uneven wall thickness during manufacture, product build-up on the drum, corrosion, deformation after an impact, uneven wear of the lagging. Even a new part without balancing almost always has residual imbalance.

The force of imbalance grows in proportion to the square of the rotation speed. This means a unit that worked “tolerably” at low speed may start destroying itself after switching to higher rpm. That is why with any change of operating mode — a throughput increase, a drive replacement — the balancing is worth rechecking.

Signs of imbalance

Imbalance is rarely diagnosed straight away — more often its consequences are noticed. Characteristic signs:

• Frame vibration that intensifies as speed rises.
• A steady-frequency hum, synchronous with drum rotation.
• Accelerated bearing wear — repeated failures of the same unit.
• Loosening of the frame’s bolted joints under alternating load.
• The belt drifting to one side with no visible mistracking cause.

Static and dynamic balancing

There are two levels of balancing, and they are not interchangeable. The choice depends on the part’s geometry and operating speed.

Parameter	Static	Dynamic
What it eliminates	Centre-of-mass shift	Shift + axis skew
For which parts	Short (disc, pulley)	Long (drum, shaft, rotor)
Equipment	Knife-edges or scales	Balancing machine
Operating speed	Up to 1000 rpm	Any, especially high
Correction planes	One	Two or more

For conveyor drums, where the length is noticeably greater than the diameter, static balancing is not enough — a part may be statically balanced but have a mass “skew” along its length. That is why we balance drums dynamically in two planes.

Engineer’s tip. Do not balance a drum with product build-up or old lagging — you will “balance” the dirt, and after cleaning the imbalance will return. First a full clean and replacement of worn lagging, and only then balancing. Otherwise the operation will have to be repeated.

How balancing is performed

On a balancing machine the part is spun up, sensors capture the amplitude and phase of vibration at the supports. The instrument calculates where and how much mass to add or remove. Correction is carried out by drilling metal in the “heavy” zone or welding balancing weights in the “light” one. The procedure is repeated until the residual imbalance drops to the permissible accuracy class.

The sequence of work on our projects is as follows:

1. Cleaning and defect inspection — we remove product build-up, check the drum for cracks and journal ovality.
2. Mounting on the machine — the part is placed on roller supports that minimise their own friction.
3. Measuring the initial imbalance — a trial run shows the magnitude and phase of the unbalance in both planes.
4. Calculating the correction — the system determines the mass and angle of the correction weights separately for each plane.
5. Correction and a control run — we add or remove metal and repeat the measurement; 2–3 iterations are usually enough.

For conveyor drums we aim for accuracy class G 6.3 to ISO 1940 — it covers their operating rpm with a margin and does not require expensive “turbine” balancing.

Balancing is logical to combine with bearing replacement — if the drum is already removed, checking its balance is inexpensive, while a second dismantling costs a whole downtime.

The quality of balancing is assessed by an accuracy class — the permissible residual imbalance for a specific type of part and its operating speed. For conveyor drums a moderate class is usually enough: they rotate slowly, and there is no point chasing “turbine balancing”. The main thing is that the residual imbalance is guaranteed not to give vibration at working rpm.

Consequences of skipping balancing

The temptation of “it works anyway” is understandable, but an unbalanced unit pays for itself later. The alternating load from vibration cuts bearing service life by 2–3 times, loosens the frame’s welds, accelerates fatigue failure of the shaft. Vibration transmits to neighbouring units and to the foundation, worsens the accuracy of scales and sensors. In the end, a small saving on balancing turns into an unplanned repair of the whole drive unit. This is a classic example of a hidden TCO item.

Balancing when manufacturing a new drum

It is a mistake to think balancing is only a repair operation. A new drum almost always has residual imbalance due to tolerances on the tube stock, unevenness of the weld and the welding-on of journals. That is why in our own production we balance the drum as a finishing operation — after welding, machining of the seats and applying lagging, if it is provided.

Balancing at the manufacturing stage is cheaper than “repair” balancing: the drum is on the machine anyway, correction is done before painting, and the line is not yet stopped. We hand the customer a report with the recorded residual imbalance — this is part of the product’s technical passport and a reference point for future maintenance.

Conclusion

Balancing a rotor or drum is not an unnecessary operation but insurance for the service life of bearings, frame and shaft. For long conveyor drums, dynamic balancing in two planes is required, carried out after cleaning the part. Neglecting it means paying with vibration and unplanned repairs. Need drum balancing or vibration diagnostics? Get in touch — we will perform it on our own equipment.