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If you have an intermittent feed application, one where the web is constantly cycled with a feed / stop operation, you may have trouble with momentum and inertia in your unwind, process and possibly rewind tension zones. When your machine process pulls material off of a stopped unwind roll, the tension will spike until that roll gets moving. The process and the web being pulled through the process must overcome inertial forces in order to start the stopped unwind roll turning (a body at rest tends to stay at rest).
When the process halts during its' stopped portion of the cycle, the unwind roll wants to keep rolling due the momentum (a body in motion tends to stay in motion). This continued rotation will cause the web tension to decrease and the web may even “throw a loop”, in essence removing all web tension and causing you to lose control of the web feeding into the converting process.
Accumulators and nip roll assemblies can be used in conjunction with feed roll assemblies, such as nip rolls, to eliminate the stopping and starting of the unwind roll. Combined with a properly designed drive system these components will greatly reduce the effects tension fluctuations resulting from your start / stop cycles.
Basically, the system works like this. The feed roll assembly pulls material off the unwind roll at a near constant rate. This constant feed rate is closely synchronized with the stop / start intermittent feed rate. The accumulator or dancer is designed with enough storage for your longest single pull cycle, plus some amount of web length to ensure the web accumulation never “bottoms out”. Your process machine pulls web from the accumulator and the accumulator empties out the web necessary for the process. The feed rolls continue to feed material into the accumulator, so that it fills back up and gets ready for the next pull cycle of the process.
With this system, the unwind roll no longer has to start and stop. The web accumulator or dancer roll assembly can be designed so momentum and inertia losses are very minimal, optimizing web tension entering your process.
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THE AUTHOR ASSUMES NO LIABILITY CONNECTED WITH THE USE OF THIS INFORMATION OR THE RESULTS OBTAINED FROM IT.
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What is Web Handling?
Web Handling is the science, engineering and equipment necessary to successfully transport a continuous flexible material through various processes without defect. Enhancing Web Handling with proper equipment will ensure materials are processed at their maximum efficiency, allowing the process to generate maximum profits.
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