-- Specifying a Servo for Motor for Pinch rollers --

Gary
Read post 1 again
The way I understand ...
He is unwinding a roll and pulling the material with the pinch rolls

Stopping and starting
Post 8

I just threw a couple of concept sketches up because that is how I generally see these in the field
 
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Rapid change is speed is not consistent with tension / torque control
so either the pinch rolls run at a fairly constant speed with slower speed change or they have a rapid speed change and not using them for tension control in that case there need to be another drive of some kind both can't be true
as I said we are not getting the full picture I have done web controls with both speed the tension control but there is more needed then what we are seeing
 
The dancer acts more like a small accumulator.
Both are speed controlled. The pull roll speed actual is sent to the unwinder as a speed reference. The dancer just trims the unwinder.

This isn’t tension control

Think of it as an unwind on a form fill seal machine or similar

A pic from the OP would help a lot !
 
This is an accumulator
Actually a slow speed one
I have worked on machines with accumulators with rolls that are 72” wide and weight about 10 tons
https://www.youtube.com/watch?v=9rErS94Tl8M
https://www.youtube.com/watch?v=KJJ8LUKl5zs
there are many other examples of accumulators out there

on some of the high speed web presses the accumulator holds about 500 ft or more of paper to allow for roll change with shutting down the press run over 500 ft / min

I have never run into a dancer that acts like an accumulator. Dancer rolls are never powered they are a feedback for tension on the web. Dancer position or the newer one are mounted on load cells.
The example shown is a drop loop and yes some consider it an accumulator but it only works at low speed with a small loop of the web. A drop loop can never be used for tension it should be obvious here is no tension at all
If you look closely you will see most of the unwinders are using a brake on the unwind roll to maintain tension on the web newer machines use a VFD running in regeneration mode to control tension.
To control you need something to pull the web, something to either hold back the web(Break, Drive running in Regen) and some mechanism to measure and feed back the tension

I will say it again we are not getting the full picture the OP may not know the full picture. But without the information nobody can point you in the right direction.
 
Keith
those examples are just babies compared to what's out there
a double web unit is huge 2 to 3 block long 3 stories high the have their own substation to supply power just to the press
I wish I could have takes pictures but camera's are not allowed (proprietary processes)
they run 24/7 over 2,000 impressions per min
but now with the digital age printing is almost at an end
 
Learn summat new every day!

I had only ever heard the phrase dancer before for a device that looked like this: my brother designed and maintained web handling and laminating system; he referred to dancers, and those were continuous processes (1500-2500'/min, 96" wide, 6' diameter rolls).


So, even though they appear similar, there is a more-than-semantic difference between a dancer and an accumulator?

  • dancer maintains tension for a process that is continuous on both sides
  • accumulator is the interface between two dissimilar sections of a process: one section is continuous and the other is batch (discontinuous).
Am I close?
 
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I looked at you video what you are talking about is just a simple unwinder not tension control at all.
The 3 rolls form a dancer control this dancer just controls the speed of the rubber pinch rolls most likely a simple POT mounted on the pivot arm of the dancer 0-100% speed. Gravity pulls the dancer arm down the web pulled into the wrapping machine pulls the dancer up. When the dancer is at the bottom the pinch rolls speed is 0 I would guess that the pinch rolls is driven by a simple DC universal motor and drive. No speed reference from anything other than the dancer POT. The rubber pinch rolls only pull the web off the source so it can be pulled into the forming filling sections without any tension at all.
If toy look closely at the source roll you will see there is no drive at all just a very light mechanical brake to keep the web from unwinding when the pull rolls stop.
None of the dancer rolls are powered they are just idlers.
 
Drbit

An accumulator is used for continuous web lines so you can do a roll change without stopping the line. (And when you don’t want wrinkles in the material unlike a j-scray)

I’m assuming the OP is just unwinding a roll for a stop/start process and just need to keep his roll from unwinding on the floor. Plus he is unwinding plastic so you don’t want wrinkles on it either
 
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Drbit

An accumulator is used for continuous web lines so you can do a roll change without stopping the line.

I’m assuming the OP is just unwinding a roll for a stop/start process and just need to keep his roll from unwinding on the floor




Ah, so it's like the capacitor or battery on my nightstand clock, or my MicroLogix 1100, so I can unplug the device to move it somewhere else, or last through a power outage, and when power is restored to the device it has not lost the time.
 
Look at Gary’s video of an accumulator
Post 20

I’ve seen those fall and crash too :)

The high end ones have composite rolls to keep them light weight etc
 
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does the splice always happen upstream of the accumulator, with the accumulator feeding the process, or could it be on either side and still be an accumulator e.g. downstream to change out a full winding roll?
 
They can be at exit end or entry
Sometimes both

They can be counter weighted with a clutch and a motor/or just a motor... all different ways

You clamp the material and it goes up or down to keep feeding the material
Then you change the roll

I see them a lot in textile lines

And get off your pc and go outside
I hear it’s snowing there
 
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