Air Shaft Sizes Explained: 3-Inch and 6-Inch Cores

One of the most common mistakes when ordering an air shaft is assuming that “a 3-inch shaft” or “a 6-inch shaft” is enough of a description. Those terms define the inside diameter of the core, but a shaft needs many more dimensions to fit the machine correctly and carry the roll safely. This article covers the common core sizes, the key dimensions of an air shaft and what to watch for when taking measurements.

What does “standard” actually cover?

It is worth being clear from the start: there is no single, industry-wide formal standard that defines every dimension of an air shaft. In roll-based industries such as packaging, paper, film and labels, core inside diameters have largely settled on sizes expressed in inches. This is an industry convention. Shaft length, journal (end) dimensions and connection details vary with the design of the machine.

So when people talk about “standard sizes”, they usually mean the core inside diameter; the rest of the shaft is specific to the machine.

Common core sizes

Nominal size	Metric equivalent (approx.)	Typical use
3 inch	76.2 mm	Common in flexible packaging, film, labels and narrower rolls
6 inch	152.4 mm	Common for heavier, larger-diameter rolls and parent rolls

Other inch sizes such as 1, 1.5, 2 and 4 inch are also used, as are metric cores in some applications. Which size is used depends on the material, roll weight and the habits of the sector.

Nominal versus actual size

Cores may be made of cardboard, plastic or metal, and they come with manufacturing tolerances. The actual inside diameter of a core sold as “3 inch” can differ slightly from the nominal value. Cardboard cores can also become oval over time due to moisture, impact and repeated use. An air shaft is designed to slide into the core easily when deflated and grip it firmly when inflated, which allows for these variations.

Key dimensions of an air shaft

To select a shaft correctly or order a replacement, you need to know:

• Nominal core size: the core the shaft is designed for (e.g. 3 inch).
• Deflated outside diameter: slightly smaller than the core inside diameter so the core slides on and off easily.
• Expanded outside diameter: the diameter the lugs or leaves reach when extended; slightly larger than the core inside diameter so they can grip it.
• Working (effective) width: the length of the lugged section that carries the cores.
• Overall length: end-to-end length of the shaft; must match the distance between the bearings on the machine.
• Journal (end) dimensions: diameter, length and profile of the ends that sit in the safety chucks or bearings. Square, flatted or special profiles may be used.
• Air inlet position: which end the air valve is on and how it is positioned.

How to measure correctly

Core inside diameter

Measure the core inside diameter with callipers at several angles, at both ends of the core and in the middle if possible. A single reading can be misleading on an oval core. It also helps to measure new and used cores separately.

Existing shaft

If you are replacing an existing shaft, take the overall length, working width and journal dimensions from it. Photograph the journal profile as well as measuring it, especially the section that sits in the safety chuck. Bear in mind that measurements taken from a worn journal may not reflect the original dimensions exactly.

Machine side

Check the seat dimensions of the bearings or the inside-frame safety chuck that the shaft sits in. A mismatch between safety chuck and shaft end can cause both safety and runout problems.

If the shaft has to be removed for measuring, stop the machine, lock out energy sources and release the air from the shaft first.

Working with different core sizes

If the same machine runs both 3-inch and 6-inch cores, there are two main options: keep a separate shaft for each core size, or, where suitable, use an adapter that lets a smaller shaft carry a larger core. Provided roll weight and speed allow it, an adapter can reduce the number of shafts you need to hold. You can check suitability on the mechanical aluminium adapter product page.

Pre-order checklist

Information	Note
Core inside diameter	Nominal and measured value
Core material	Cardboard, plastic, metal
Overall length and working width	In mm
Journal dimensions and profile	Measurements plus photo
Maximum roll weight	In kg
Maximum running speed	Line speed or rpm
Number of cores	Single or multiple

This information also makes it easier to choose the right shaft material and type. An aluminium air shaft suits light and medium loads, while a steel air shaft can be considered for heavy rolls. Once your measurements are ready, contact AYS Makina through our contact page.