- April 01, 2011, By Timothy J. WalkerContributing Editor
“No nips, please.” I need a button that says this. Every day, dumpsters and truckloads around the converting world are filled with waste generated due to unnecessary nipped rollers. Nipped rollers are a double-edged sword, creating high pressure for value-adding processing that quickly turn into value-subtracting wrinkling machines.
Nip-related waste may be your web supplier's fault (excessive bagginess), or your equipment supplier's fault (undersized rollers or poor choices in entering span length and pre-nip wrap angle), or your own fault (uneven nipping or deflection-excessive loads). However, the worst sin of nipping is the unnecessary nip. The nip that isn't there doesn't create waste.
You may not be able to get rid of your need for nip pressure in your value-adding processes, but the nips I'd like to see avoided are the ones used to create friction on driven pull rollers. To avoid these nips, we have to understand why they are used and how they can be bypassed or removed without upsetting our tension control.
To wean you of “tension isolation” nips, we have to answer four questions:
- Why do we have driven rollers in our process?
- How much friction or traction is required at each roller to avoid slip and maintain tension control?
- Why are nip rollers a popular choice to create drive roller friction?
- What are the alternatives to meet driven roller traction needs without nipping?
I touched on some of these answers in my January 2008 column, “Get a Grip: Driving Your Web” (pffc-online.com/web_handling/grip_driving_web_0101). However, in that column I don't think I went far enough to persuade you to give up your nipped drive rollers or help you to understand how to do it.
Why do we have driven rollers in our process? Driven rollers control process speed and local tension or draw ratio.
At least one of your driven rollers is the lead section of your process with the important job of controlling line speed. Beside the lead section, all the other driven points of your process are follower pull rollers in charge of local process tension or draw ratio.
How many driven rollers your process has may be due to the need for local process-specific tensions or to make up for tension losses of undriven roller drag and inertia.
How much traction does a drive roller need? Drive roller traction needs to be greater than the tension differential between the incoming and outgoing webs. If not, the drive roller will slip, losing control of tension or speed.
Figuring your tension differentials may be more difficult than you would think. The true tensions in the spans just before and after the driven roller may differ significantly from the values displayed on the operator control panel.
Getting to the real input and output tensions requires some work. For load cell feedback, we need to confirm their calibration. For dancer rollers, we need to confirm the tension required to counter the dancer's force.
For draw control processes, it can be quite challenging but not impossible to estimate the process tension from upstream tension, draw ratio, and web mechanical properties. Most importantly, if there are a number of rollers or a process between the feedback roller and the drive roller, you have to consider the losses or gains in tension from drag and inertia.
The goal of all this work is to understand and estimate the real tension differential needed at every drive roller. In next month's column, we continue on the nip-free path and discuss why nips are so popular and how to reduce our dependence on them.
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; email@example.com; www.webhandling.com.