What do I need to know about web accumulators?
David Roisum, Ph.D., Consulting Technical Editor -- Converting Magazine, 10/1/2008 2:00:00 AM
Accumulators are used to temporarily store web material on medium-speed, continuous-processing lines. This storage allows a manual roll change to be made on an unwind or winder while stopped and yet still keep the process running by drawing upon that stored material. At very low speeds, say less than 100 fpm, manual roll changes can sometimes be made at speed without accumulators. At very high speeds, such as more than 1,000 fpm, accumulators become too large so that turret unwinds and winders are the only practical option. Though there are plenty of accumulators around, they can be challenging to design and maintain.
The first thing to do: Properly size the accumulator. You want it no bigger or smaller than needed for an efficient unwind or winder roll change. Once you know the length of storage (roll change time), you need to decide whether it will be tall (few rollers) or long (many rollers). Usually tall is used for sturdy materials, but thin or wrinkle-prone materials can't tolerate long spans even more than they dislike many rollers.
Pneumatic cylinders usually do the raising and lowering of the accumulator. However, the sides must be timed precisely by well-maintained precision chains or gear rack to preserve alignment. Sloppiness here causes the accumulator to be out-of-level on at least part of its stroke. Roller alignment must be considered here just as in any other machine; fortunately, accumulator alignment is simpler than with most machines. You need only allow for precision level of each individual roller because that is the tender in-plane bending direction. The square is not so important because that is the tolerant out-of-plane skew direction.
The festoons of an accumulator are wrinkle-prone components because there are many rollers, they are highly wrapped, and the spans may be long. We can also help the situation by operating procedure. Run the accumulator nearly empty until the very last minute before filling and changing the roll. This allows less time for large spans to let the web misbehave.
More demanding concerns are usually with tension control. All those rollers have inertia and bearing drag. You don't want combined tension upsets to exceed more than 10 percent of the nominal tension at any point. Another concern is the friction of the cylinders and timing mechanism. When this device is used like a dancer, tension quality is probably poor. Thus, the use of an accumulator as feedback to a drive should be limited to the fill and empty phases only. You should use a separate dancer or load cell for tension control during run because they are usually much more precise.
Finally, the accumulator will have an infeed or outfeed nip. This device is quite tricky both mechanically and control-wise. Because all nips are wrinkle-prone, the rollers must be precisely designed and maintained for a dead-level nip. Control is also tricky because the nip is in speed-control during fill and empty with s-curve ramps. However, the nip should be taken out of the system during run so that a precise dancer or load cell commands the unwind or winder directly without the now redundant nip.
920/312-8466, drroisum@aol.com, www.roisum.com
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