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What is tension?

Typically, tension is measured in PLI (Pounds per Linear Inch) in the US. If you know PLI and you want to know total tension applied to the web, multiply PLI times the width of the material in inches. If you know total pounds of tension applied to the web and you want to know PLI, divide the total pounds of tension across the web by the width of the web in inches.

• PLI (Pounds per Linear Inch) = total pounds of tension / web width in inches

• Total pounds of tension = PLI X web width in inches

The tension applied to a web can be described as the webs tautness as if you hung a weight off the edge of the web. The tension on the web would be equal to the weight in pounds. PLI would be equal to the weight in pounds divided by the web width in inches.

Why is tension important to me?

• It is impossible to control your web without proper tension being applied. The web must be “in traction” with all machine idler rolls and driven rolls to ensure proper web handling and control.

• The web handling principal states a web will seek to align itself perpendicular to an idler or driven roll. This web handling principal is applied to route webs through processes with parallel idler rolls and driven rolls. It is applied when web guides are used to steer webs. And it is also applied in many wrinkle removal devices. However, the web handling principal does not apply to webs that are not in traction with idler or driven rolls. In other words if the web slips over the face of an idler or driven roll it can wander from side to side or if the web slips on web guiding idler rolls it will not move where the web guide attempts to steer it. Tension must be applied to webs to keep them in traction with idler and driven rolls.

• All webs stretch in the machine direction as tension is applied to them. It is important to apply proper tension to a web so that it can be handled through the machine and processes without over-stretching

• It is impossible to unwind rolls into a process without proper tension control at the unwind station. Telescoping, “dished” rolls, wrinkles and even web breaks will occur when tension is not controlled at the unwind station.

• It is impossible to rewind rolls from a process without proper tension control at the rewind station. Telescoping, “dished” rolls, wrinkles and even web breaks will occur when tension is not controlled at the rewind station. 

• Many converting operations require proper registration to print or die cut stations. Proper tension control is essential to controlling print to print or die cut registration.

• Laminating operations require layers to be laminated with proper tensions to avoid web curl.

• Slitting operations require proper slit position, which is a function of tension and web guiding.

• When tension is too high webs will stretch in the machine direction and compress in the cross machine direction. This narrowing of the web width can cause wrinkles to occur.

• When tension is too low webs will shrink in the machine direction and web width will widen in the cross machine direction. This widening of the web width can cause wrinkles to occur. 

Understanding what web tension is, its function in your converting process and why it is important are the first steps to properly controlling web tension to benefit your operation.