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August 31, 2005
Two Pacers is One Too Many
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.
The first problem with draw control is the system time constant (equal to the draw zone web length divided by web speed). It takes at least three time constants or three flushes of the system for a draw zone to get to 95 percent of the steady state conditions. If you have a tension sensitive process, this is an unwelcome pile of process waste. Torque control and closed-loop tension control react much quicker based on the response of the mechanical system, independent of line speed and web length.
For low modulus (stretchy) products, draw control is an easy way to develop web tension and avoid excessive stretching or necking. However, for high modulus products (many papers, polyester, nylon, and most foils) speed control variations, as small as 0.01-0.1 percent, can cause 10, if not 100 percent, tension swings.
If a process has two pacers there are two choices on how to improve the processes. 1) Change one of the two driven section to closed-loop tension control using a dancer or transducer roller for feedback. 2) Add a clutch to one of the driven sections, putting the web between the sections in torque control.
Posted by Tim Walker at August 31, 2005 01:11 PM
Comments
If the surface speeds of the two pacers are very close, then there should be no problem. In fact, this is sometimes used as a tension control element "Driven S-Wrap", as in an IWEB paper by Al Forrest of DuPont. If the speeds differ by more than a fraction of 1%, then friction on the roller surface is insufficient to give the tension change which the speed change would impose. the web must start to slip somewhere. I wrote about this in a couple of IWEB papers.
In particular, roller speeds are often stepped up going through a machine. This is only correct if the web is not elastic, but creeps under tension. An elastic web will probably start slipping over some of the rolls in the sequence of rollers.
Posted by: Dilwyn Jones at September 1, 2005 01:59 AM
In a lamination process that has a reverse roll coater - drying oven - lamination nip rolls. Both the reverse roll coater and the laminating nip are driven with their own drives. We have been using a ratio control between these drives to maintain the web tension in the oven. The ratio control is being replaced with a closed loop automatic tension control system. Based on your experience should the reverse roll coater or the laminating nip be the pacer for this process?
Posted by: Rich Abella at September 12, 2005 07:27 AM
This may not be directly applicable to the web industry but many manufacturers of dual layer wire & cable use 2 capstan units (effectively 2 drive rollers) with accumulation in between. The accumulator provides tension control & feedback to the second capstan to ensure that it is at the proper speed.
Posted by: John Wysk at September 12, 2005 07:52 AM
I agree with both Rich's and Dilwyn's posts. The most common 'two pacer' system is the 'S' wrap pull roller. Most common, these are installed with 1-to-1 ratio and act like a single large-wrap driven roller.
I like S wrap drive rollers because they can often be used to replace nipped drive rollers. Why don't I like nipped drive rollers? Because they will always increase wrinkling of baggy or loose webs. Unnipped rollers are more forgiving to web bagginess, and that's a good thing.
tjw
Posted by: tim walker at October 10, 2005 01:09 PM
Just wanted to say Hi. Looks like you are busy again. I'm looking for a paper by Ron Swanson, do you know how I can get a copy of the one he did on the spreader rollers. It was a land mark paper presented at a Web handling Conferance some years ago.
Posted by: Jim at October 18, 2005 06:23 PM
