- December 01, 2008, By Edward Boyle, Contributing Editor
For most printers and converters, tension controls are like the utilities coming into your home or office — the only time you give them any thought is when they're not working. Yet, controlling tension is perhaps the most critical factor in attaining faster production speeds and delivering a higher quality end product.
“What makes tension controls so important is that they remove any type of deviation from the [production] process so you can better control the output,” says Mark Breen, marketing manager for Dover Flexo Electronics. “It's all about getting better output off the end of your machine at the end of the run.
“The faster you're running, the more important it is to control tension in each of your zones to maintain quality,” he says. “It's all about quality output, and you start adding a lot of variation to the process when you're running at higher speeds. There are all kinds of things that can happen to a web, so having tension controls at more points throughout the process makes sense to help maintain quality.”
Conversely, if the tension is not properly controlled, wrinkles in the material can occur and cause product defects. If the roll of material is improperly rewound, the outer layers can crush the inner layers of material or the rolls can telescope, resulting in damage when the rolls are handled. When printing, improper tension control results in smearing of the ink and distorted images due to poor registration.
“Good tension control also results in the ability to run the process at higher speeds without sacrificing product quality,” explains Don Strenio, product manager/web tension transducers and controls for Cleveland Motion Controls. “And when it comes to tension transducers, converters want it as simple as possible to integrate them with the rest of the machine.”
Substrates Getting Lighter
Vincent Genovese, president of Nireco America, says the broad range of materials being converted on equipment that originally may have been designed to handle just one substrate also has offered special tension challenges for converters and suppliers alike. For example, controls that utilize brakes to maintain tension on paper and board substrates aren't ideal for lighter materials, such as thin gauge films, because they can stretch the film. Consequently, many tension controls now use four quadrant regenerative drives in which the motor acts as a brake and regulates tension when the web is accelerating and decelerating both forward and reverse.
“In the industry we're seeing a greater decrease in the number of brakes and the number of stand-alone tension controllers being required,” says Genovese, citing the lighter materials being converted at higher speeds. “When you're using any sort of brake, the web has to provide the energy to accelerate the rolls. But with lighter materials, you cannot sustain the tension requirements to accelerate and decelerate the machine; the tension required to accelerate the rolls exceeds the nominal tension of the materials, and you get undue stretching. So you need to provide the energy, and for the most part, the way to do that is to put a drive in place of the brake.”
Genovese says the best way to solve the problem is to use regenerative four-quadrant drives, which are able to accelerate and decelerate the web while running both forward and reverse. “When the motor is moving forward, you need to slow down to keep the tension, and unless the motor is physically capable of absorbing the energy back out of the motor and putting it back into the AC line, it will just idle there,” he explains. “So you need to operate in both a drive and a regenerative mode to maintain tension, and a normal servo drive doesn't have regenerative capabilities.”
Doug Brockelbank, national sales manager of Montalvo Corp., agrees that many OEMs are installing regenerative motors on the direct drive machine rewinds to assure that the rolls “hold up well” for further processing. “We find that pneumatic controls provide for what I call infinite resolutions, which can give a much finer level of control than digital drives.”
Rating the Units
One of the recent developments in transducers has been the broader use of an IP (Ingress Protection) rating system that indicates how impervious the units are to contamination and water. The standard ratings are IP 65, which indicates the unit is impervious to airborne contaminants such as dust, and 67, which indicates the unit still can function after being submerged for up to 15 minutes in three feet of water. This is most commonly found in papermaking operations.
“The other area in which it's beneficial is in wash down, because you don't have to be careful if you spray them down since they're rated to be submerged,” notes Darren Irons, product manager for Magpowr. Adds Brockelbank, “IP-67 is pretty much the highest degree of controlled environment.”
Transducers or “load cells” measure actual web tension in almost any moving web or filament, then output an accurate, reliable signal to an indicator or controller. The controller then automatically compensates for variations in roll diameter, speed, and web material characteristics to maintain desired tension.
“It is extremely critical these load cells work in all types of atmospheres and climates,” says Scott Durfee, district sales manager for Maxcess Intl. “A carpenter expects his measuring tape to function whether it's 45 degrees [Fahrenheit] out or 90 degrees, whether it's a dry or wet climate. Basically, converters should expect the same [performance] from their load cells. The IP-67 rating means more than [being able to] just stick it in the middle of a pond; it means that none of this is going to affect the ruggedness and reliability of the load cells.”
Finally, Durfee says more converters will be taking advantage of Ethernet connections as they become more standard on tension controls. They allow converters to update the software on their control systems or allow the manufacturer to troubleshoot drive problems directly over the Internet without a service call.
“It's available now, and a lot of people are using it,” says Durfee. “Most of the larger OEMs are building it into their equipment, and a lot of them are actually requiring an Ethernet connection from their machine. It's part of their warranty. This is absolutely the future of where our industry is going, remotely being able to diagnose and assess a situation or a job.”
Flexo Electronics | www.dfe.com
Cleveland Motion Controls | www.cmccontrols.com
Nireco America | www.nirecoam.com
Montalvo Corp | www.montalvo.com
Magpowr | www.magpowr.com
Contributing editor Edward Boyle, based in Reading, PA, has covered the converting industry for more than 24 years. Contact him at EJB Communications; 610-670-4680; firstname.lastname@example.org.
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