Q&A: Collecting Process Tension Data Over Time

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TENSION CONTROL IN CONVERTING MACHINES     

     In the converting industry we have all kinds of webs to control tension on, from the bonding of a zip lock strip to a plastic web for making zip lock bags to folding carton stock. The correct tension for these various webs varies greatly. Virtually any material processed in a strip form has tension. The amount of pull or torque applied to the strip creates the tension. This now must be controlled for printing, registration, coating, laminating, guiding and to prevent wrinkles during the process.

     Each process can be broken down into three types of tension zone. You have one or more unwind tension zones, you have multiple Intermediate tension zones and you can have a rewind tension zone. To control the tension in each of these zones, it must be measured by some means or approximated.

     Radius measurement by photo cell or sonic sensor can be used on an unwind or rewind to change an output to a controlling device as the roll diameter changes. This is less accurate than the two following measurement devices.

     A dancer roll in the web path can measure tension changes and control the tension in all three types of tension zones. It allows some forgiveness because it creates web storage. The load applied to the dancer can be representative of the actual tension except friction and inertia enter into the unknown picture. On an unwind control, a dancer does help to compensate for the out of round material roll by supplying storage.

     The transducer (or load cell) can also be used in all three tension zones. These are by far the most accurate for tension measurement because they are measuring the actual force the web is applying on the tension roll. They can be calibrated by a known weight or spring scale at the end of a rope passed throught the machine in the same fashion as the web path.

There are three (3) types of transducers used in the converting industry:
Strain gauge type transducers are used most often and are usually the least expensive. The total movement of the measuring beam is 0.004" - 0.008" for full load. Transducers are produced in a variety of mounting configurations with the roll supported on both ends or as a cantilevered idler roll or pulley.

The second type is the LVDT (Linear Variable Displacement Transformer) which has a movement of 0.020" - 0.030" for full load rating. These units are usually made to mount a pillow block bearing on the top and should be mounted so the direction of tension force is 90° to the top plate.

The third type uses an AC voltage field set up in a machined block of metal. The tension force causes a strain in the metal, which causes a change in the electromagnetic field. The movement is zero and this type of transducer can handle a wide tension range.

     Now that we have used one of these types of transducers to accurately measure the web tension in each of the tension zones, we want to automatically control the tension.

UNWIND ZONE:
The unwind tension can be controlled by a pneumatic brake, an electric brake or a regenerative drive. The tension measurement signal from the transducer would be the input to a tension controller. The controller would compare this signal to the desired tension set point and give an appropriate output to the control device. This output signal would decrease as the material roll decreases in diameter, thus maintaining a constant tension going into the machine. In the case of a laminator, the desired tension must be governed by the substrate to be laminated. If you are using two different materials, then you should be controlling tension at two different levels. The level of tension in each material must be such that after the bond has cured, the laminated substrate does not separate or curl. If one material has been stretched with excessive tension, it will cause either of these problems.

INTERMEDIATE ZONE:
There can be multiple of these zones, depending on the machine and process. An intermediate zone can be prior to the main drive station or after or both. In this zone the transducer signal is used only to trim the line speed signal as a follower of the main drive speed signal. This is usually 5% - 10% maximum. When controlling a nip prior to the main drive, the controller output would be slowing the nip down to create the correct tension with respect to the main drive section. If the tension controlled nip is after the main drive then the output would speed up the nip to create tension. Each section must have a positive grip on the web. These nip sections can be driven by clutches from a line shaft, a DC Regenerative drive, an AC Vector drive or Servo drive. The driven device must be able to provide torque in the reverse direction. This will allow you to have various tension levels throughout the machine. This is especially true with the last nip section prior to the rewind.

REWIND ZONE:
The rewind tension controller is very critical to winding a salable roll without starred ends or telescoping. The drive to be controlled is usually an electric clutch or a variable speed drive.

Taper tension is used depending on the maximum diameter roll of material to be wound and the surface of the substrate. This is done in the rewind tension controller by comparing the line speed signal and the rewind shaft RPM and calculating the roll diameter as it increases. If you start out with 100 lbs tension at the core and want 30% taper, then the controller would automatically reduce the tension as the roll builds to achieve 70 lbs tension at the full roll. This reduces telescoping and starred rolls. When using taper tension, this is why you have an outfeed nip so that the changing rewind tension does not effect the process tension.

When using a splicing turret winder the controls must include the speed matching circuitry to bring the empty core up to line surface speed, activate the lay-on roll, fire the knife, switch the tension control signal to the new core and reset the diameter calculator. The full roll would then come to a stop.

STAN AUSTIN

• Mechanical Design Engineering Degree - Wentworth Institute - Boston, Massachusetts
• 10 Years - Research & Design Engineer - S.D. Warren Paper Company
• 6 Years - Application Engineer - Kidder Tension System
• 23 Years - CMC Tension Control Sales Manager and Regional Sales Manager
• 6 Years - Dover Flexo Electronics - OEM Sales Manager