Shifty Answers to Nip-Induced Tracking

Nipping rollers are used in many value-adding processes in web converting — roll coating, laminating, embossing, calendering, high-speed winding, and more. Imperfect nips, where the load is uneven, can create process variations, but they also may induce a lateral shifting of your web, leading to misalignment or wrinkling.

Before I can tell you about how uneven left-right nip loads will induce lateral tracking of your web, I must get on my anti-nip soapbox. The nip that isn't there will not shift your web (or wrinkle it).

In my experience, nips and webs are a volatile combination (baggy webs, in particular, and all webs are baggy to some degree). Nips should be avoided whenever possible. Nips are required for many processes (listed above), but nips are the last choice for creating friction.

Please choose large wrap angles on high-friction rollers or vacuum-suction rollers over nips if you need simple high friction. (Okay, I'm getting off my anti-nip soapbox now.)

How does a nip roller induce lateral shifts in your web? If you have left-right nip load variations, higher on one side than the other, which way will the web go? Do we know the answer?

Some work completed by the Web Handling Research Center (WHRC) at Oklahoma State Univ. sheds some light on these questions. The nip-induced lateral tracking phenomenon needs to be assessed in several steps.

  • Q1 | How does the nip create cross-web tension variations?
  • Q2 | Which way will the torque, created by the tension variations, bend the web?
  • Q3 | Will the web bending create a curvature as it enters the nip roller, violating the parallel entry rule (see “Web Lines,” August 2003, p20)?
  • Q4 | Does the nip have sufficient traction to bend the web to obey the parallel entry rule?

In the most predictable setup, the WHRC experiments showed, for a rubber-rubber nip, the web will track to the high nip load side. In these tests, the answers to the four questions above are as follows:

  • A1 | The rubber in the nip speeds up proportional to the nip load. If one side of a nip roller set has more load than the other, that side pulls the web faster than the other side, creating more tension on the high nip load side.
  • A2 | The high tension side torques the entering web span and may cause it to bend away from the high tension side. The web's first response is to move away from the high nip load side.
  • A3 | The web bending from the tension variations presents a web at an angle that violates the parallel entry rule, so if there is good traction, the web will spiral back toward the high nip load side, similar to how webs with good tracking will track to the large diameter side of a tapered roller.
  • A4 | Traction? You bet. This is a nip. Nips usually have an abundance of traction. Putting it all together, in a controlled experiment, the web will move to the high load side of a set of nipping rollers.
  • Bonus question | Will the web always track to the high load side of a nipped roller system?
  • Answer | Not necessarily. A slipping nip, such as a nip grabbing an oiled metal web, may shift the web away from the high load side.

Real-world nips include many complications to the simplified WHRC experiments. One or both rollers may be steel. High nip loads may create web-rubber slippage when the nip's rubber cover stretches many times more than a stiff web can.

If the web wraps either roller prior to the nip point, what effects will friction and deflection have? What if there is a big tension change across the nip that induces slippage?

As usual, questions about web shifting may get answers that are equally shifty.

Web handling expert Tim Walker, president of TJWalker+Assoc., has 25 years of experience in web processes, education, development, and production problem solving. Contact him at 651-686-5400; tjwalker@tjwa.com; www.webhandling.com.


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