Web Tension: A Pop Quiz

Today's column is a pop quiz on web tensioning. Feel free to work as a team on the answers. If you score poorly, don't worry, but it's worth your while to do your homework and work toward getting the answer right.

Step 1: What is your web's tension? (5 points and possible 5 points extra credit)

What is your web's tension? Hands down, this is the most important web tensioning question, but surprisingly few will have the answer. If you have a calibrated tension sensing roller in your process, congratulations, give yourself 5 points. However, if you control your process with torque from a brake or clutch, if you control tension with a dancer roller, if you control tension with a speed ratio, if you set tension in percent of anything, or you set it by air pressure, mark down 0 points, but I'll give you a chance for partial or extra credit.

Options for extra credit: Have you calibrated your clutch or brake, or used a force gauge to measure the performance of your clutch, brake, differential shaft, or dancer roller? (3 points) Have you calculated the likely change in web tension from your speed ratio or draw setting from web thickness, width, and modulus? (2 points)

Step 2: How does your product react to tension? (5 points)

Do you know your product's break or yield point in terms of tension and elongation? (2 points) Do you use this data to set your product's web handling tension, running your products with tension proportional to their width, thickness, and modulus, web's spring constant, yield point, or break point? (2 points)

Super special extra credit: Have you measured your product's modulus as a function of temperature, moisture, or strain rate, or have you measured the effective modulus of your coated or laminated product? (1 point)

When anyone asks me what the right tension for their product is, I send them off looking for modulus, yield, and break data. My advice is always to start with a tension of 10%-20% of the product's trouble point, which is usually yield or break. Think of tension in terms of stress and strain. In general, tension will double each time you double the width, thickness, or modulus. There is a diminishing need to increase tension proportionally as the product has natural stiffness, so a tenfold thickness increase likely won't need a tenfold tension increase.

Step 3: Bonus round (possible 10 points)

Do you prefer unnipped or vacuum drive rollers over nipped drive rollers? (1 point) Have you used the belt equation to figure the tension ratio of an unnipped roller? (1 point) Have you measured friction of your various webs to the drive roller surface to estimate the tension change that would cause slip? (1 point)

Does your winder and unwind tension control include inertia compensation (where the drive tuning is adapted to changes in width, diameter, and density)? (1 point)

Do you minimize the use of dancer rollers, only using them between drive points with anticipated relative speed variations? (1 point) Have you designed your dancers to have minimum friction, inertia, and pneumatic hysteresis? (1 point)

For speed-controlled tension loops, do all your drives share a line speed reference from the pacer or lead section? (1 point)

When writing specs for new equipment, especially winders, do you take time to calculate your minimum and maximum torque requirements? (1 point) Do you include torque to accelerate or decelerate large rollers or wound rolls? (1 point) If your torque range (max/min) is more than 30, do you figure how to make concessions to reach a reasonable torque range, including taper tension? (1 point)

Timothy J. Walker has 20+ years of experience in web handling processes, education, development, and production problem solving. Contact him at 651/686-5400; tjwalker@tjwa.com; www.tjwa.com.

Report Card

0-5 Summer school anyone?
6-10 Better bring an apple for the teacher!
11-15 Most excellent.
15-20 Your web likes you.
>20 Certified web tensioning guru.

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