Should Unwind Tension Be Lower Than Winding Tension?

Back on December 4, 2014, I posted a list of questions to consider when first looking to resolve a roll telescoping problem. Let’s take this a step further: You have identified that the telescoping occurs when your customer unwinds your thick film product. What do you do next?

This is most likely telescoping that is initiated by machine direction slippage (a.k.a. cinching) within the unwinding roll. Usually this is avoided with the first rule of winding and unwinding: “A roll’s unwinding tension should be lower than the tension it was wound with.” This advice is directly aimed to avoid applying excessive torque to an unwinding roll.

This simple rule is helpful but may not be enough. Why not? Does tension cause telescoping? If the winding roll used taper tension, changing tension with radius, which tension does unwinding need to be lower than?

Unwinding cinching—the machine direction slippage of layers within the roll—is a function of torque, not tension. Torques are a force at a distance; in this case, the force of tension (in lbs or N) at the distance of the roll’s radius (one half of roll diameter). It might be better to say “Don’t torque the unwinding roll more than it was torqued at winding.”

Torque can come from two sources—tension x radius and inertia x acceleration (or deceleration). If acceleration rates are low or diameters are small, there will not be much inertial torques. Then, if the starting unwind tension is lower than the final winding tension, the torque applied at unwinding will be less than winding. This should prevent cinching, but still not in all cases.

Some materials (especially films) may loosen up over time, losing some of their ability to transmit torque without cinching. In some films, slip additives or coatings change over time, dropping the product’s inside to outside coefficient of friction and again lowering the roll’s torque transmission capacity.

When a customer has trouble with telescoping while unwinding your roll, they will immediately assume you have changed something. As a supplier, if this is a new complaint for an old product or other customers with the same product not complaining, you will likely wonder if your customer has changed something at their end. If the customer has changed their unwinding process to unwind at higher tension (more braking torque), faster accel/decelerations (more inertial torque), or unwinding your rolls while they are cold, this may change their telescoping rate without any change in your process or product.

What to do?

1. Calibrate your winding process

Some winders are set up to run winding in engineering units (lbs or lbs/in.), but many operate in open loop control, setting air pressure or percent torque. To understand where your winding process is and where to go to improve it, a winder should be calibrated to know how it functions—how the controlled variables (such as current, air pressure, or percent settings) create torque and nip load. For differential or clutched winding, a handheld force gauge can measure the tension versus radius of rewind shafts and winding positions on the shaft to calibrate air pressure to winding torque and tension. The force gauge combined with roll density can measure and estimate nip load versus air pressure and roll weight.

2. Static torque capacity testing

There are a couple versions of this test, but both involve drawing a radial spoke line on a finished roll then applying a torque to see when there is machine direction slippage within the roll. The static torque can be applied on an unwind stand or with a roll strapped to a pallet and torqued from the core with a chuck and torque wrench. I'd recommend the unwind test. We could likely do this at your facility.

3. Contract converting

If the problem is due to equipment limitations, you may need to justify a new winder or winder upgrade. It might be worth running a trial at a contract converter to see if a machine with the feature your equipment lacks (e.g., automatic taper tension, higher torque range) can make rolls with higher torque capacity making rolls with a higher probability of surviving your customer's process. Besides sending rolls wound at a contract converter to your customers, you could also measure an improvement in torque capacity with the unwind torque slippage test.

4. Time effects

There may be time factors to consider. Your rolls may be loosening over time. It may be important to test some rolls for torque capacity after a storage time representative of what your customer sees. This could be done at the contract converter, too.

5. Roll hardness testing

You may have a trend of telescoping and thickness profile. The easiest way to detect thickness profile in winding is to measure the roll hardness profile. Do you test roll hardness? You may find rolls with uniform thickness profile and hardness are more prone to telescoping. You may also find rolls with tapered profiles are more prone to telescoping, as this is the usual driving force that turns machine direction slippage into lateral slippage. This is one defect where a couple subtle thick gauge bands can be helpful. Gauge bands locally increase interlayer pressures, making a roll have more total pressure, friction, and torque capacity.

6. Side flanges

When all else fails (or if you want a short term fix), use a couple of flange disks on each side of the unwinding roll, so when telescoping does happen, it is physically restrained.

As you can see, there is more to consider than simply having unwind tension lower than wind tension. Cinching and telescoping can usually be reduced or eliminated. Starting with lower unwind tension is the right direction, but the path to a solution may require a few more steps.

Tim Walker

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