We have, in past articles, discussed the importance of maintaining proper web tension within your converting process. Oftentimes, the question arises as to what web tension levels you are actually running. This information is sometimes hard to attain. When you want to know what your roll diameter is you simply pull out a tape measure and get an accurate measurement. If you want to know what your roll weighs you simply place your roll on a scale and acquire an accurate measurement. Unfortunately, accurately measuring web tension is not so simple. However, there are resources and methods to estimating and measuring actual web tension:
TAPPI (Technical Association of the Pulp and Paper Industry), as well as many other industry organizations, publish estimated tension levels for several different types of webs and laminations. However, keep in mind; these values are only guidelines and "best estimates" based on many years of combined experience. The best tension for your application will, most likely, vary from the guideline.
- A very general estimate of web tension is 10%-25% of your webs tensile strength. 10%-25% is a very broad range of estimate and your process may be running outside of these extremes, so attaining an accurate number based on this general estimate is impossible.
- If you have a load cell tension control and / or read-out, it may display total tension across your web in pounds or metric equivalent.
- If your machine has dancer roll assemblies, you can calculate the amount of force the dancer is applying to the web if you know the web geometry and loading force. Another method would be to simply measure the force or "weight" the dancer is placing on the web. Remember, as long as the dancer remains somewhere within its travel (between its physical limits of completely full or completely empty) the tension on the web is equal to the loading in the dancer.
- Another crude but very effective way to measure tension in the unwind zone is the "fish scale method". This method works for the unwind tension zone only. It can only be utilized if there is a brake on the unwind station. Unwrap a small amount of material off the unwind roll. Set the unwind brake to the torque output normally set for a machine run. Wrap the leading edge of the web around a bar and hook the "fish scale" through the bar. Pull on the fish scale until the unwind starts to turn. Record the weight reading on the fish scale. This is the actual tension on the web at this point. Do not attempt this method of measuring tension if there is a drive motor on the unwind or for a rewind with a drive motor. Serious injury could result if this method of measuring tension is attempted when there is a drive motor at the unwind or rewind zone.
- Tension can be calculated if you know the model of brake or clutch, the output level to that brake or clutch at a given roll diameter. By knowing the torque output of the brake or clutch we can plug the values into the equation; torque = roll radius X tension and extrapolate the actual tension the brake or clutch is delivering to the web.
These methods of estimating and measuring web tension should assist your web tension must be specified.
Jeffrey Damour, Sales Engineer, Converter Accessory Corporation, Wind Gap, PA,
Phone - 800-433-2413