What Is an Air Shaft and How Does It Work?

In converting lines that handle paper, film, foil, labels or textiles, loading a roll onto the machine is one of the most frequently repeated tasks. An air shaft, also known as an expanding shaft, grips the core from the inside and makes roll changes quicker and more consistent. This article explains what an air shaft is, how it works and which main designs are available.

What does an air shaft do?

Web materials are usually wound onto a cardboard, plastic or metal core. For the machine to drive the roll, or to unwind material from it, the core has to be held on the shaft without slipping. An air shaft uses elements that expand under compressed air to grip the core firmly across its inner diameter. When the air is released, the core is free and can be slid off the shaft.

The practical benefits are:

• Shorter loading and unloading times,
• The core is held centred on the shaft axis,
• Less slippage and less damage to the inside of the core,
• Several cores can be positioned along a single shaft.

How does it work?

The operating principle is straightforward. Inside the shaft body there are one or more bladders (air tubes) running along its length. On the outside of the body sit lugs or leaves that rest on top of the bladder.

1. Inflation: compressed air is fed in through the air valve at the shaft end, usually with an air gun.
2. Expansion: the bladder inflates and pushes the lugs or leaves radially outwards.
3. Gripping: the expanding elements press against the inside of the core and lock it to the shaft.
4. Deflation: when the air is released through the valve, the bladder collapses, the elements retract and the core is released.

The gripping force depends on the air pressure in the bladder and on the contact area between the expanding elements and the core. For that reason the shaft should always be used within the pressure range specified by its manufacturer. Over-inflation can damage both the bladder and the core, while under-inflation leads to slippage.

Main components

Component	Function
Body	Carries the load and connects to the machine
Bladder (air tube)	Holds the compressed air and generates the expanding force
Lug / leaf	Contacts the inside of the core and grips it
Air valve	Used to inflate and deflate the shaft
Journals (shaft ends)	Sit in bearings or safety chucks

The shape of the journals determines how the shaft is mounted. On many unwind and rewind stations, the shaft ends are square journals that locate in safety chucks.

Types of air shaft

By expanding element

• Lug type: short lugs are spaced along the body and contact the core at discrete points. This is a widely used, versatile design.
• Leaf (strip) type: leaves run along the length of the shaft and contact the core along a longer line. The larger contact area spreads the load more evenly over the core, which can help with softer cores.

By body material

• Aluminium: lightweight, which is an ergonomic advantage where operators lift shafts by hand. Suited to moderate loads.
• Steel: chosen for heavy rolls and applications that need high torque. It is noticeably heavier.
• Carbon fibre: light and stiff. Worth considering for wide, long shafts where both weight and deflection matter.

Special designs

On slitter rewinders, several narrow rolls are wound side by side on the same shaft, and the tension of each roll needs to be balanced individually. This is the job of friction (differential) air shafts fitted with friction rings, which we cover in a separate article.

Core sizes

An air shaft is selected according to the inner diameter of the core it will run. The most common sizes are quoted in inches: a 3” core corresponds to roughly 76.2 mm and a 6” core to roughly 152.4 mm, although other diameters are also in use. When deflated, the shaft must slide easily into the core; when inflated, it must grip the core securely.

Points to watch in daily use

• Do not inflate the shaft until the core is fully in position, otherwise expansion may be uneven.
• Always deflate the shaft completely before removing the core.
• Check the air valve and bladder regularly for leaks. Loss of pressure can cause slippage during operation.
• When working with rotating and pressurised equipment, follow the machine manufacturer’s manual and your site’s lockout/tagout procedures.

Before you choose

The right shaft depends on core inner diameter, roll weight, shaft length and the type of application, all considered together. For available options, see the aluminium air shaft and steel air shaft pages, or request a quote via our contact page.