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)