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August 31, 2005
Two Pacers is One Too Many
Filed under: Tensioning, Web Mechanics --- Tim Walker @ 01:11 PM
All web processes should have a pacer. A pacer is a speed controlled section that drives a roller (and hopefully the web) to the process speed set point. The pacer doesn't care about web tension. In a slitter-rewinder, it's quite common to have a pacing section of driven rollers in the slitting and transport section and torque controlled unwind and rewinds.
If a process has two driven sections, say a two nipped pull roller stations, then one is the pacer and the other should be a follower. The follower pull roller drive can be controlled in draw control (sometimes called ratio control), in torque control (like a clutched or braked roller or a torque controlled motor), or in tension feedback / closed-loop speed control.
Occasionally, I run across a new web process that has two pacers - two speed driven sections ignoring web tension and just working to satisfy the speed set point. I'm not sure this should be thought of as two pacers, but a pacer and follower in 1:1 draw or ratio control. Is this a good idea? Not usually.
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August 23, 2005
Is This Roll of Polyester Properly Wound?
Filed under: Winding, Unwinding, Roll Defects --- Tim Walker @ 11:54 AM
I rec'd a questions that asked "How can I determine if a roll of polyester if properly wound?"
Upon a little follow-up Q&A, this question expanded with: 1) In unwinding the polyester rolls, they are sometimes loose on one side or the other.
2) Incoming polyester rolls sometimes haved ridges or ribs.
Films like Mylar (the Dupont brand name for their biaxially oriented polyester), often develop "bagginess" or permanent crossweb length variations from the combination of crossweb caliper variations, internal wound roll stresses, and storage time/temperature.
For small amounts of bagginess, increasing the tension will "pull out" the bagginess, so the running web doesn't have any zero tension lanes. If the bagginess is too severe, it isn't possible to pull it out with reasonable tensions.
Ribs and ridges form for a couple of reasons...
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August 09, 2005
Winding Tension Answers? Not 'Exactly'.
Filed under: Winding, Unwinding, Roll Defects --- Tim Walker @ 02:29 PM
In response to my August 5 on differential winding, a couple of blog readers commented with specific questions. Case 1: A reader asked for the 'exact data of rewind tension' required for a series of products and listed 5 different webs (plus two mysterious laminates called 3 Ply and 2 Ply), listing only the thickness of one web, one maximum width, and one maximum speed. Case 2: A second reader asks if there is a system to differentially wind to 60-in. OD for a specified range of steel and aluminum webs.
How do I reply to these well-intentioned inquiries? I have problems on three levels.
1. There is a great deal of missing information?
2. Even with near perfect information, is it possible to provide 'exact data on rewind tension'?
3. Is answering this type of specific question the purpose of convertingblog.com?
Lets go through each one of these a little bit...
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August 04, 2005
Differential Bars, Part 1 - What Torque? Tension?
Filed under: Winding, Unwinding, Roll Defects --- Tim Walker @ 10:55 AM
When people look at differential bars, they often focus on their main function and benefit - the ability to allow many individual rolls on a common shaft to turn independent, finding their appropriate rpms to compensation for variations in roll-to-roll diameter differences and lane-to-lane length variations.
OK, this is great, but let's move from this function onto other specification that will make or break your winding yield.
#1 The right winding tension
What is the right winding tension for your product, including the initial tension, the final tension, and any wacky tapering or profile you may want to run.
Winding and handling tension are usually in the same ballpark, but unusual wound roll characteristics many mean winding tension needs special considerations.
Slippery or thick products may need more tension (and pack). Thin and pressure sensitive (thick adhesives and other special coatings) product many need quite low tension.
#2 Torque creates tension
More...
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