What is a tension zone?

Definition:tension zone is a section of the web line, between two torque or speed controlling devices (a.k.a., drive points), where the tension is set by design or by control.

Tension set by control implies an operator can easily adjust the value up or down by changing a set point or adjusting air pressure.

Tension set by design implies the tension control or change in tension is fixed into the machine design.

For example, if two rollers are driven at a slight speed differential, creating or changing tension in draw control, this could be tension set by control of the speed ratio can be adjusted on a control panel or it could be tension set by design if the speed ratio fixed by the roller size and gearing of the equipment.

Single Tension Zone Unwinds:

Example 1: Toilet Paper

When you pull toilet paper from a roll you have a single tension zone web line. There are two devices working together: 1) your hand is the speed control, and 2) the resisting torque of the roll in its holder controls the tension. If you maintain constant speed, the toilet paper tension is fairly constant. The weight of the roll on its spindle creates a frictional force. The leverage you have against this friction (the roll diameter divided by the core’s inner diameter) will determine how much tension is created. As the roll diameter decreases, you may of may not see a tension increase. Since the back tension is a function of roll weight, the loss of leverage will be offset by a loss of friction at the core. If you accelerate or decelerate, there will be a tension increase or decrease due to inertial effects. Accelerate too fast, especially those jumbo, high density toilet paper rolls popular at airports and Fortune 500 companies, and the inertial torque may create enough tension to break the web. Decelerate too quickly and the jumbo TP rolls may not stop and dump TP on the floor as it coasts to a stop.

Example 2: Tape Dispenser

When you pull tape from a tape dispenser, you have a single tension zone web line. Like the TP example, your hand controls the line speed. The unwinding resistance (friction and inertia) combines with the adhesive peel force to set the tension.

Single Tension Zone Rewinder

Example 3: Simple Rewinder

Mount an input roll on a shaft attached to a brake. Thread the web to a core mounted on a shaft attached to a motor. Turn on the motor and run. The rewind motor controls the line speed. The unwind brake controls the tension. Is the speed constant? Probably not. As the building roll diameter increases, the line speed will increase proportionally. Is the tension constant? Probably not. Assuming a constant brake torque, the tension will increase inversely as the unwinding roll diameter decreases. Will varying speed and tension cause problems? Maybe. Since this is a simple rewinder, the speed increase may not be a problem, but if this was a coating line or other speed-sensitive process, the speed increase could lead to out-of-spec product. Winding may not be overly sensitive to the speed increase, but the increasing tension as the roll builds, what is often considered reverse taper tension, will lead to poor roll quality, especially defects like starring or spoking.

Improved Single Zone Rewinder

How do you avoid the changing speed and reverse taper tension of the simple rewinder?

Option 1: Add closed-loop speed control using feedback from an encoder monitoring a non-slipping idler roller. Add diameter compensation to the unwind torque control by adding some form of diameter measurement.

Option 2: Control speed at the unwind, adjusting RPMs proportional to the decreasing unwinding roll diameter. Drive the winding roll in torque control, allowing the tension to decrease as the winding roll diameter increases.

Option 3: Add a pacer section and upgrade to a two-tension zone process, using a single motor to drive the pacer section and also the input side of a clutch to control winding torque.

Though Option 2 is a clever, simple system that compensates for Option 1 drawbacks, my preference is for option 3. I think the two-zone web line with a pacer should be the foundation for all web lines.