What is web tension?
What are the engineering units of tension?

Definitions: A force tending to stretch or elongate something.
Tension can be viewed as a force, a force per width, or as a tensile stress.

Definitions: A force tending to stretch or elongate something.
Tension can be viewed as a force, a force per width, or as a tensile stress.

  • Tension may be a FORCE in units of Newtons (N) or Pounds (lb), sometimes Kilograms-force (kgf).
    10 N = 1 kgf (actually 9.81 N, but 10 is close enough). 10 N = 1 kgf = 2.2 lbs

  • Tension may also be described as a FORCE per WIDTH in Newtons per Meter (N/m) or Pounds per Inch (PLI) [Pounds per Lineal Inch of width] Alternatives include: kgf/dm or strange mixed units of kgf/in. 175 N/m = 1 PLI

  • Tension may also be described as tensile stress in units of FORCE per AREA or Force per (Width x Thickness) in units of Newtons per square Meter (N/m2) or Pascals (Pa) or Pounds per Square Inch (psi). 8 MPa = 200 N/m/25 microns = 1200 psi = 1.2 pli/mil

In equations, FT is force of tension, T is tension force per width, σ is tensile stress.

What tension is the right tension for a web?
What tension will break or deform a web?

The answer to the second question is part of the answer to the first question.
Most experts will advise to run your web at 10% of the tension that would damage the web, a safety factor of 10:1.

As a starting point or rule of thumb, average web tension should be 10 percent of the web’s break or yield point. This 10:1 safety factor may seem high, but this tension setpoint is the average tension in the web. Web tension is often talked about as one value, but it should be viewed as a contour, varying vs web width and web path length. Crossweb tension can easily vary by 2:1 or more across the web’s width due to web bagginess or roller misalignment. Tension variations over time of 10% are fairly normal and can vary as much as 50 or 100 percent during acceleration or within tension zones.

The most common web tension is 200N/m (1 PLI). Ninety percent of webs are run between 60-600N/m (0.3-3.0 PLI) and 95 percent of webs are run between 20-2000 N/m (0.1-10 PLI).

When a web will break or deform is a material property. In all materials, breaking and deforming will occur in response to not just force, but stress (which is the force over a cross-sectional area). In tensioning a web, the cross-sectional area is the width times the thickness. Material, thickness, and width will all contribute to a web’s strength and resistance to breaking or deforming (a.k.a. yielding).

A 25mm (1-inch) wide web will break easier than a 250mm (10-inch) wide web. A 50 micron (2-mil) thick web will be harder to break or yield than a 12 micron (0.5-mil) thick web. A steel web will be harder to break than a polyethylene web. (Harder to break = higher critical tensile break stress.)

A tensile-elongation test is the best way to find your web’s break or yield point. A tensile-elongation tester pulls on a strip of the web precisely measuring the elongation distance and force. When the web breaks is clear. When it yields may not be as clear. Yield is the point where the web will no longer elastically recover. When tension is removed, it will not return to its original dimension.

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What are recommended tensions by product?

These tension guidelines have been handed down and shared across many companies.

To find the recommended tension for your product, multiple the value in the table by the thickness or weight of your product.

Example 1:

Recommended polyethylene tension is 2 N/m/micron.
If your product is 75 micron PE, tension target is (2 N/m/micron)(75 microns) = 150 N/m
If your polyethylene is 1.5m wide, your target tension is (150N/m)(1.5m) = 225 N = 22.5 kgf

Recommended polyethylene tension is 0.3 pli/mil.
If your product is 3 mil PE, tension target is (0.3 pli/mil)(3 mil) = 0.9 pli
If your polyethylene is 60 inches wide, your target tension is (0.9 pli)(60 in.) = 54 lbs

Example 2:

Recommended paperboard tension is 4 N/m/gsm.
If your product is 80 gms, tension target is (4 N/m/gsm)(80 gsm) = 320 N/m
If your paperboard 2m wide, your target tension is (320N/m)(2m) = 640 N = 64 kgf

Recommended paper tension is 0.04 pli/(lbs/ream).
If your product is 50 lbs/ream, tension target is (0.04 pli/lbs/ream)(50 mil) = 2.0 pli
If your polyethylene is 60 inches wide, your target tension is (2.0 pli)(60 in.) = 120 lbs

Material N/m/micron PLI/mil
Aluminum 7.0 1.0
Polystyrene 7.0 1.0
Cellophane 5.0 0.7
Polyester 5.0 0.7
Acetate 3.5 0.5
Polyethylene 2.0 0.3
Polypropylene 2.0 0.3
Vinyl 0.7 0.1
Saran 0.7 0.1
Material Tension(SI) Tension(American)
Paper 3-4 N/m/gsm 0.03-0.04 pli/(lbs/ream)
Paperboard 4-5 N/m/gsm 0.3-0.4 pli/mil

Ream = 3000 sq.ft.

Why does target tension have such a large safety factor (10:1)?

Understanding and visualizing the tension variations along a web line are critical tools of any web handling problem solver, especially for wrinkling and roll defects.

Tension is often displayed as a single value of force or force per unit width. However, a single value is only the beginning of understanding tension. Just as the number on a thermostat doesn’t represent the temperature in every location in a building, measured tension represents an average at a roller, but doesn’t show how tension varies in the MD, CD, and ZD from web, equipment, and control imperfections.

It is easy to imagine how too high or too low average tension will cause problems. Thinking about tension variations is more complicated but important for avoiding web handling problems. Tension (and strain) will vary as the web is transported in the machine direction (MD), from the forces of processes, rollers, and gravity. Web tensile stresses and strains can vary through the web thickness (ZD) from bending around rollers or the wound roll, and from residual stresses in laminated or coated layers. Lastly, maybe the most important point to understand and diagnose is that tension can vary in the cross-machine direction (CD) from web bagginess, equipment variations, and imperfect splices.

A large tension safety factor is intended to keep a web safe from damage or breaks even when it has large MD, CD, and ZD tension variations. Each source of variation may be well within the safety factor, but when the extreme values occur at the same MD and CD location and time, they can combine to produce a damagingly high value, or slackness.

(pp 204-206 and Table 5.3, The Web Handling Handbook)