Key Components of Tension Control in Web Handling
Recently, Flexible Packaging had the opportunity to interview Mark Breen of Dover Flexo Electronics New Hampshire. Sure Controls is happy to share this great information. If you are in Wisconsin, Sure Controls is your go to resource for web handling needs. We understand both the art and the science of it. Our team of engineers have industry experience as well as several years experience on the account management side.
Although tension measurement and control equipment plays a vital role in the production quality aspect of flexible packaging, converting and web printing, it’s a process variable that our customers want to work invisibly, without them having to worry about how or why it’s working. The greatest compliment we can hear from customers is that they’re not even aware that their tension control equipment is doing anything. –Mark Breen DFE
What is Tension Control?
The term “tension control” as applied to materials processing, refers to the dynamic control of tension on a web being pulled through a machine’s processing zone. Tension is created either by a tensioning device pulling (usually a driven nip, lay-on roll or winder) on the web at one end (as in a rewind or intermediate zone), or by increasing the drag at the other end (with a brake, clutch or regenerative drive, as in an unwind zone).
Open Loop Systems
In an open-loop control system, the web tension is measured by a device, like a tension transducer, and then made available for read out on a meter or other type of display. The machine operator looks at the measured tension on the meter and then makes manual adjustments to a tensioning device to bring the actual web tension close to the desired value.
Closed Loop Systems
In a closed-loop system, the process variable (web tension in our case) in a particular zone of the process is measured many times a second. The controller’s output to the tensioning device is adjusted at a constant loop rate in response to the error signal generated as the controller compares the measured tension value to the desired reference (set point).
What is the difference between a transducer/load cell-based system and a dancer control system?
In a dancer system, the controller signals the dancer roll (which has the moving web crossing it) to move in response to a change in position. Although the initial position change is the result of a change in web tension, the controller does not measure tension or react to force changes as directly as a transducer-based system does.
If tension increases, the dancer changes position, moving a sensor, which is usually a potentiometer, and signaling the controller to reduce or increase torque to allow the dancer to return to its original position. If tension decreases, the opposite sequence occurs. Dancers are actually position controllers, not tension controllers. The system’s principal advantage is its web accumulation ability, which tension transducer schemes lack. A dancer system may be more forgiving of speed variations as created by out-of-round rolls or frequent starts and stops, but dancers have certain drawbacks. Compared to tension transducer systems, they are expensive to design, build and install. They are difficult to properly engineer, and they are prone to errors caused by mass, damping and friction. They cannot read out tension or compensate for small but significant tension transients in the process.
In a tension transducer system, specially designed force transducers (tension sensors) measure actual web tension in the converting process. The transducers are typically used in pairs with one installed on each end of an ordinary idler roll shaft.
What are some of the common parts/components that fail in a tension control system? Should converters consider stocking these parts to prevent lost productivity from malfunction?
Because of the static nature of most of these products, given that they bolt to the machine frame and typically experience minimal physical trauma in their lifetimes, we see very few true product failures in the field. The components that have the shorter lifespans are the ones at friction and wear points. Bearings in the transducers, for instance, which rotate with the idler roll shaft in the case of live-shaft mounting transducers, may wear over time and need to be replaced after a few years. Also, the friction pads on the pneumatic brakes are consumable items that need replacement on a routine schedule depending on the demand levels of the application.
Other than replacement brake friction pads, there are no components that we recommend to our customers to stock, except for maybe a spare transducer or two in the case of the rare need for an emergency malfunction replacement.
Say you have old tension control equipment that is seemingly working fine. When should someone consider replacing it?
That is a perplexing question for us. While we like to sell new product, our mission to our customers is to supply them with equipment and a total solution that will last them for many years without the need for replacement. The only real driver for a converter or printer to upgrade an old tension control system that is working fine is their personal preference. Two reasons to upgrade would be: 1) The newer tension control and indication electronic devices have advanced feature sets that might be appealing, like easier-to-use graphic interfaces with touch screens; and 2) Any type of warranty that comes with the purchase of new equipment.
As with consumer electronic devices, as microprocessors shrink and become more powerful, the number of features and benefits that can be realized in a single industrial electronics device increases, too. When a new tension controller with more advanced technology, a larger feature set and a smaller footprint becomes available on the market, and at a reasonable price, users can often justify the retirement of an older controller on a cost/benefit basis.
The truth is that many of our customers are happy to stick with their old equipment if it’s working fine. We have thousands of tension controllers, indicators and transducers on machines across North America that have been working steadily in industrial production for two and even three decades.