Air Shaft Balance Problems: Symptoms and Likely Causes

An air shaft is a rotor: it carries the roll and turns with it. A small mass imbalance that goes unnoticed at low speed can turn into vibration, noise and poorer winding quality as speed rises. Because balance problems usually develop gradually, operators may accept the symptoms as “just how the machine runs” for some time. This article covers how to recognise a shaft balance problem, how to tell it apart from runout, and how to carry out initial checks.

What is a balance problem?

A shaft is out of balance when its mass is not evenly distributed around the axis of rotation. As it turns, the unbalanced mass creates a centrifugal force that changes direction on every revolution. This force increases with the square of the speed: doubling the speed roughly quadruples the force caused by the imbalance. That is why an imbalance that causes no trouble on a slow machine can become obvious when the same shaft is used on a faster line.

Balance and runout are not the same

Runout is a geometric measure of how far the shaft surface deviates from the axis as it rotates, and is measured with a dial indicator. Balance is about mass distribution. The two often appear together, since a bent shaft produces both runout and imbalance. However, a geometrically true shaft can still be out of balance because of mass differences in its internal construction. Keeping this distinction in mind helps when tracing the source of a problem.

Symptoms

• Vibration that rises with speed: if vibration becomes noticeable above a certain speed and eases when the speed is reduced, balance is one of the first suspects.
• Rhythmic hum or noise: a sound repeating at the same rhythm as the rotation may be heard.
• Heat in safety chucks and bearings: constant dynamic load can cause faster-than-expected wear and heating in the bearings and safety chucks.
• Poorer winding quality: rippling on the roll surface, uneven hardness or irregular edges may appear.
• Tension fluctuation: imbalance can introduce periodic fluctuations into the tension that the tension control system is trying to hold steady.
• Vibration in the frame and surroundings: on lighter machine structures in particular, vibration can spread to the frame and connected equipment.

Some of these symptoms can also come from bearing faults, loose connections, core problems or the drive system. A symptom on its own is not a diagnosis.

Likely causes

Cause	Explanation
Impact or dropping	A dropped shaft can bend the body or the journal (end) area.
Journal wear	Wear on the ends that sit in the safety chucks can push the shaft off centre.
Lug or leaf damage	Missing, damaged or unevenly expanding lugs can hold the core eccentrically.
Later modifications	Welding, drilling or adding parts on site can upset the mass distribution.
Build-up	Adhesive, ink or dust deposits can create imbalance over time.
Roll-related imbalance	An eccentric core or an unevenly wound roll creates imbalance even when the shaft itself is sound.

Initial checks

Before any of the checks below, stop the machine completely, secure all energy sources according to the manufacturer’s lockout procedure and make sure all rotating parts have come to rest. Never inspect a rotating shaft on a running machine at close range.

1. Observe the problem against speed. Note the speed range at which vibration starts. Vibration directly linked to speed strengthens the case for imbalance or runout.
2. Take the roll out of the equation. If possible, try the same shaft with an empty core or a different roll. If the problem changes with the roll, the source may be the roll or the core.
3. Swap shafts. Run another shaft in the same position. If the problem moves with the shaft, suspicion falls on the shaft; if it stays in the same position, look at the bearings or the machine side.
4. Measure runout. With the shaft supported appropriately and turned slowly, runout on the body and journals can be checked with a dial indicator.
5. Inspect visually. Look for wear marks on the journals, damaged lugs and dents in the body.

How is a shaft balanced?

Dynamic balancing is carried out by spinning the shaft on a dedicated balancing machine to measure the size and position of the imbalance, then correcting it by adding or removing mass. The required balance quality is typically expressed using the “G” grades of ISO 1940-1 (now ISO 21940-11), and the appropriate grade depends on the shaft’s operating speed and application. This is not a job to attempt in a workshop without the proper equipment.

A bent shaft cannot be fixed by balancing alone; its geometry has to be assessed first. Depending on the extent of the damage, the decision is then whether to repair or replace it.

A preventive approach

• Store shafts on racks that protect them from drops and knocks.
• Do not weld, drill or add parts to a shaft on site.
• Before increasing line speed, confirm that the existing shaft is suitable for that speed.
• Add runout measurement and journal inspection to the periodic maintenance checklist.

On high-speed and wide machines, shaft material matters too. A steel air shaft suits heavy loads, while a carbon fibre air shaft is worth considering for wide applications that need both low weight and stiffness. To discuss your shaft’s condition or suitable options for your application, contact AYS Makina through our contact page.