Controlling Rewind Tension with Friction Rings

On a slitter rewinder, a wide parent roll is slit into several narrower strips, which are usually rewound onto separate cores sitting side by side on the same rewind shaft. Simple as that sounds, it creates a real tension problem. Friction (differential) air shafts fitted with friction rings were developed to solve it. This article explains how friction rings work, how tension is set and what to watch for in operation.

The problem: one shaft, different roll diameters

The thickness of a web is never perfectly uniform across its width. Small thickness variations add up as winding progresses, so the rolls on a single shaft gradually end up with different diameters.

On a standard air shaft, every core is locked to the shaft and turns at the same speed. A larger-diameter roll tries to take up more material per revolution, a smaller one less, yet the web arrives at the same speed across all strips. The result:

• Tension rises on the larger rolls, and the material may stretch or break,
• The smaller rolls are wound loose,
• Rolls show differences in hardness, telescoping and uneven edges.

The solution: friction rings

On a friction air shaft, the cores do not sit directly on the shaft but on friction rings mounted along it. Each ring is driven by the shaft through controlled friction.

In simple terms, the principle is:

1. The shaft is driven slightly faster than the web requires.
2. Compressed air inside the shaft sets the force pressing on the rings, and therefore the friction between ring and shaft.
3. Each ring turns at the speed the web allows; the speed difference with the shaft is taken up by controlled slip.
4. Because the torque transmitted while slipping is limited by friction, each roll receives a similar torque regardless of its diameter.

Each roll can therefore turn at its own speed while tension is spread much more evenly between rolls.

How tension is set

Winding tension depends on the relationship between the torque applied to the roll and the roll radius: at the same torque, web tension falls as the roll grows. If constant tension is required, the torque must increase as the roll diameter increases.

On friction shafts, torque is usually adjusted through the air pressure supplied to the shaft:

• Higher pressure: more force on the rings, more torque transmitted.
• Lower pressure: slip comes more easily, less torque transmitted.

In many applications, pressure is adjusted with roll diameter by a tension controller or by the machine’s own control system. For some materials, tension is deliberately reduced somewhat as the roll builds up (taper tension), which can help prevent crushing of the inner layers. The right tension profile depends on the material and the roll structure required.

Points to consider in selection and set-up

Topic	Why it matters
Ring width vs core width	Cores must sit properly on the rings and be positioned correctly between them
Core inner diameter	Rings must suit the inner diameter of the cores
Required torque range	Set by material type, strip width and tension requirement
Speed difference	An excessive overspeed means more heat and wear from slip
Number of strips and layout	Ring count and spacers are planned around the slitting pattern

Points to watch at roll change

On a friction shaft, winding quality also depends on how the cores are set up along the shaft:

• Core positions must line up with the slitting knife positions, otherwise the strip is wound off-centre on the core.
• Each core must sit fully and squarely on its rings. A partially seated core can give that roll a different tension.
• Spacers and locating elements must be complete and fitted in the right order.
• Before the shaft goes back into the machine, check that the air pressure is at the set value.

Heat and wear

By design, a friction ring works by slipping continuously. Slip produces heat and wear, so:

• Keep the speed difference between shaft and rings at the level needed for tension, not higher,
• Inspect ring contact surfaces and friction elements regularly,
• If some rings wear noticeably faster than others, review the settings or the layout,
• Replace worn rings in good time, as they upset the tension balance.

Common problems

• Hardness varies between rolls: possible ring wear, contamination or loss of air pressure.
• All rolls wound loose: air pressure may be too low, or there may be a leak.
• Excessive heat: overspeed may be too high, or pressure higher than necessary.
• Core slipping on the ring: check the fit between core and ring and the ring’s core-gripping elements.

In every case, isolate the machine, deflate the shaft and follow the machine manufacturer’s instructions before working on the shaft.

For available options, see the friction (differential) air shaft and friction rings pages, or request a quote via our contact page.