Web Tension & Guiding Talking about inertia
compensated tension control By Craig Thomson, sales manager, Martin Automatic.
W
eb printers and converters know that controlling tension in a web is essential to the quality of the product. Web tension upsets between, for example, an unwinding roll and the process may have several causes, including braking of the running roll, splicing action, acceleration of the new roll after splicing, and out-of- roundness or imbalance in the roll. Machine designers must consider the characteristics of the web, the weight of the roll, the mass of the system, and the capability of downstream (press) equipment, as well as the set tension and tension control requirements of the process. A dancer system using the principle of inertia compensation effectively absorbs tension upsets and delivers constant web tension to the process. A typical load cell or transducer system relies on the principle of ‘sense and correct’. It uses a tension-sensing device (e.g. a transducer roller) and, through an appropriate control system, attempts to maintain the output of the sensor constant. This type of system has no ‘storage’, and its response-time limits its ability to adjust for rapid changes, so that tension transients are passed along to the process.
An inertia compensated dancer, however, relies on the principle of ‘establish and maintain’. Tension is established by the dancer, and strictly proportional feedback from its travel is used to maintain the dancer within its stroke. Inertia compensation is achieved by engineering the system components to balance the translational (up and down dancer motion) and rotational (rotating dancer motion) inertias. So, unlike a typical load cell or transducer system, it actually ‘absorbs’ tension disturbances rather than passing them along to the process. The result is consistent web tension. Martin Automatic’s introduction of inertia compensated tension control devices to the coating, printing and converting industries represented a major technical contribution, and they remain the simplest and most accurate means of delivering constant tension.
Martin Automatic MTL non-stop unwind system for fibreglass mat, with inertia compensated accumulators and constant tension infeed.
This principle is used in infeed and outfeed tension control systems, as well as Martin’s extensive line of unwinding and splicing systems.
Inertia compensation can be applied to any web and nearly any application. It is especially beneficial in processes involving lower tensile strength substrates--such as nonwovens and extensible films--where an inertia compensated dancer absorbs tension spikes that could otherwise lead to neck down or permanent deformation of the web. Customers are able to control tension to within +/- 0.25 pounds (1.1 Newtons) total variation without the need for complex control logic.
A Martin zero-speed splicer includes a festoon (accumulator) section which not
only supplies web to the process at full speed during the splice cycle, but which also utilises inertia compensation to absorb tension upsets. When putting together splicer specifications, Martin engineers can select from several standard festoon configurations with different web storage and acceleration profiles.
Depending on the application, some accumulators include a separate inertia compensated dancer to provide infeed-quality tension control to the process. Martin festoon and dancer systems are also retrofittable, both as a means of increasing the speed of existing processes and for improved web handling of lighter or thinner materials.
Xwww.martinautomatic.com
28
September 2021
www.convertermag.co.uk
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