- May 01, 2011, By Timothy J. Walker Contributing Editor
“No nips, please.” To qualify this statement, I just want to get rid of unnecessary nips, especially for preventing slip on tension controlling drive rollers.
“But,” you say, “if I don't have nips, my drive rollers slip, and I can't control tension.” Okay, I believe you, but let me ask these questions:
- Do you understand the tension differential across your drive rollers?
- Are you sure you can't support this differential without a nip roller?
- If you know you can't with your process as is, are you sure you can't change either your tension set points or your idler roller-related tension losses to reduce your need for drive roller differential and traction? You may find that you have two or three scenarios, some with high tension differentials, such as during rapid accelerations. You may find your nips are needed during acceleration, but they can be opened once steady line speed is reached.
Why are nip rollers a popular choice to create drive roller friction?
A Mainly, nips are simple to understand — pinch it, pull it. We can relate to nipped rollers — they create friction just like tires on the road. Nipped rollers are even able to pull when web tension on one side is zero, such as during machine threading.
Nipping two rollers together with 400 lb of load combined with a 0.25 web-to-roller coefficient of friction (COF) creates 100 lb of traction. The high pressure between nipped rollers will reject and compress air entrained by the moving web and roller, maintaining traction without worry of lubrication, even on fairly smooth web-roller combinations. It's only when wrinkling at your nipped rollers — from your baggy webs or roller deflection — that you wonder about the alternative to nipped rollers.
What are the alternatives to meet driven roller traction needs without nipping?
A The first choice for driven roller traction should always be an unnipped, high friction, grooved or textured, high wrap angle roller. To make an unnipped roller meet your needs, you may have to do three things you don't want to do:
- You need to restrain your tension differential.
- You have to think exponentially.
- You have to design to avoid air lubrication.
An unnipped roller creates tension by the capstan. The capstan equation calculates the maximum tension ratio of high-to-low tension across a wrapped cylinder.
To calculate the maximum ratio of any roller, raise Euler's number, e (2.7183), to the power of COF times the wrap angle in radians. For example, if your COF is 0.25 and your wrap angle is 3.14 radians (180 deg), the maximum high-to-low tension ratio without slipping is more than 2:1.
A ratio of 2:1 is sufficient for many applications, but why stop there? If you have a rubber-covered roller and a higher wrap angle, for example COF of 0.6 and a wrap of 200 deg, the non-slipping tension ratio is more than 8:1.
The Achilles' heel of the unnipped roller is air lubrication. You may measure web-to-roller COF as 0.3 or 0.6, but the true coefficient of traction (COT) may drop when entrained air exceeds the combined surface roughness of web and roller.
However, don't let fear of air lubrication keep you hip-deep in nip-related waste. Even subtle roughness or grooving easily keeps your web-to-roller at high traction levels.
It's easy to see why nipped rollers are naturally more popular than unnipped rollers. Nipping is simple. Who wants to worry about estimating true tensions, calculating exponentials, and avoiding air lubrication? Answer: Anyone who wants to enjoy the profits of wrinkle-free processes. Maybe that's you.
Unnipped rollers made easy: more wrap, more friction, more roughness, less wrinkles.
“Do you want nips (and wrinkle waste) with that?” No, thank you.
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.