7 Criteria for Choosing the Right Air Shaft

An air shaft can look like a simple component, yet it has a direct effect on line efficiency, roll quality and operator workload. The wrong shaft can lead to core slippage, vibration, premature wear or, simply through excess weight, poor ergonomics. The seven criteria below summarise what to review before ordering a new shaft or replacing an existing one.

1. Core inner diameter

The starting point is the inner diameter of the core the shaft will run. Common sizes are quoted in inches: a 3” core is roughly 76.2 mm and a 6” core roughly 152.4 mm. If the line runs more than one core size, decide early whether each size will have its own shaft or whether adapters will be used. It is worth measuring actual cores rather than relying on nominal values, as worn or crushed cores reduce grip.

2. Roll weight

The heaviest roll the shaft will carry determines the body material and wall section. Consider the worst case, not the average roll: the largest diameter the line can handle combined with the densest material it processes. As roll weight increases, so do shaft deflection and the load on the journals and bearings.

3. Shaft length and deflection

On long shafts, deflection matters as much as weight. If the shaft bends too much under load, you may see uneven roll edges, vibration and tension differences across the web. As a general rule, for the same cross-section, deflection rises sharply as the shaft gets longer. On wide lines, body diameter, material and bearing span should therefore be considered together. Where in doubt, ask the manufacturer for a deflection calculation.

4. Body material

There are three main options:

Material	Key characteristic	Typical use
Aluminium	Lightweight, easy to handle	Moderate loads, frequent roll changes
Steel	Robust, carries high loads and torque	Heavy rolls, demanding conditions
Carbon fibre	Light and stiff	Wide, long shafts where weight is critical

On a line where operators lift the shaft by hand at every roll change, even a few kilograms make a difference over a shift. Equally, strength should not be sacrificed for lightness on very heavy rolls.

5. Expanding element and core material

Lug-type shafts contact the core at discrete points, while leaf (strip) type shafts grip along a longer line down the shaft. With soft cores such as cardboard, a larger contact area can help reduce crushing of the inner surface. Plastic and metal cores behave differently in terms of surface hardness and slip risk. The right element type should be chosen with both the core material and the torque to be transmitted in mind.

6. Torque and application

Whether the shaft is used on the unwind or rewind side, and whether it is braked or motor-driven, defines the torque it must transmit. On lines running at high tension or with frequent emergency stops, there must be enough grip to stop the core slipping on the shaft. On slitter rewinders where several narrow rolls are wound on one shaft, a friction (differential) air shaft should be considered instead of a standard shaft, because the tension of each roll needs to be balanced individually.

7. Machine interface, speed and runout

How the shaft fits the machine is one of the most commonly overlooked points:

• Journal type and size: will the shaft ends run in bearings or locate in safety chucks? Square journal sizes must match the existing chucks.
• Overall length: total length including journals must suit the distance between the machine frames.
• Air valve position: the valve should be easy for the operator to reach and clear of machine parts.
• Line speed: at higher speeds, runout and balance become more critical. A shaft with excessive runout can cause vibration, poorer winding quality and shorter bearing life.

Checklist before you order

Having this information ready speeds up any discussion with a manufacturer or supplier:

• Core inner diameter and material
• Maximum roll diameter, width and weight
• Overall shaft length and journal dimensions (a drawing if available)
• Unwind or rewind; brake or motor drive
• Line speed and material processed
• Number of cores to be run on the same shaft

Common misconceptions

• “The heaviest shaft is the safest.” An unnecessarily heavy shaft harms operator ergonomics and puts avoidable load on bearings.
• “If the core size matches, any shaft will fit.” Differences in journal size, length or valve position can prevent the shaft from fitting the machine.
• “More pressure will cure slippage.” Slippage may come from worn lugs, a leaking bladder or a damaged core. Never exceed the pressure specified by the manufacturer.

Once these seven points are clear, you can compare the aluminium, steel and carbon fibre air shaft options, or request a quote via our contact page.