Tension calculation formula

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Tension calculation formula

Postby M. Scott » Wed Feb 15, 2012 8:26 pm

Does anyone happen to know the standard tension calculation formula for lbs vs web width and film gauge?

We print on poly, but at this time I cannot locate a guide to the exact tension (lbs, once we have converted over on our German press) for our initial settings for each tension zone.

We don't have a problem attaining our repeats or holding tension and we adjust as needed, but an industry-standard formula would be nice, if available.

Thanks,
M. Scott
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Re: Tension calculation formula

Postby Randyskalba » Thu Feb 16, 2012 9:02 am

We print on Poly and i try to keep it as simple and standard as possible.
Tension settings always between 14-18 lbs.

The narrowest web we print is 20 inch and widest is 60 so my formula would consist of this.

20-30 14 lbs
30-40 Inch - 15-16 lbs
40-50 16-17 lbs
50+ 18 lbs

Thats just what i have found the most success with on our 8 colour F&K
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Re: Tension calculation formula

Postby SknyRussianHillbilly » Thu Feb 16, 2012 11:24 am

Good rule of thumb: .5-1.0 lbs per linear inch of web width for each mil of thickness of material run.
Above is a good starting point
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Re: Tension calculation formula

Postby Frank Burgos » Thu Feb 16, 2012 11:40 am

Hey guys,

With SknyRussianHillbilly's input, we may be able to create a calculator:

if I understand correctly, using 1.0 lbs per linear inch, 1.0 mils thickness, and 30" web width, the value for tension is 30 lbs. for half-mil thick film, it would be half that, or 15 lbs.

Is that right SknyRussianHillbilly?

Frank
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Re: Tension calculation formula

Postby M. Scott » Thu Feb 16, 2012 11:53 pm

Well, using SknyRussianHillbilly's formula I would conclude the following:

Required LBS of Tension= .75 (between .5-1.0) x web width x mil thickness.

However, Randyskalba formula does not fall in line with this formula.

With this formula Randy should be running at least 22.5 lbs of tension for a 30 inch web providing his shop is running at leat 1.0 mil poly.

.75x 30+ 22.5 x 1.0 = 22.5 lbs of tension

Unless he is running .5 mil film. (?)

Sound about right?

We run anything from 1.5-3.0 mil film.

Thanks,
M. Scott
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Re: Tension calculation formula

Postby Frank Burgos » Fri Feb 17, 2012 3:09 am

OK, Guys,

I sent each of you a calculator. If you see anything wrong with it, let us all know.

Regards,
Frank
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Re: Tension calculation formula

Postby SknyRussianHillbilly » Fri Feb 17, 2012 8:06 am

Thanks for the calculator Frank.

If you really want to crunch numbers, here is another guide:
10-25% of the breaking strength on a tensile strength guage.

In the reality of printing variables though.....sometimes you have to just figure out what works on A MACHINE for A JOB.
And it is amazing to watch "The Old Guy" who, had to figure things out before readily available measuring devices in "The Old Days" just go up and kind of bang on the web with his educated open hand, and then walk over and just turn the tension control knob with his other educated hand.

The best to all.
Bill
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Re: Tension calculation formula

Postby Frank Burgos » Fri Feb 17, 2012 8:40 am

You're welcome, Bill.

".....sometimes you have to just figure out what works on A MACHINE for A JOB."

I agree 100%! and the rest of it, too.

Have a great one, folks.
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Re: Tension calculation formula

Postby SknyRussianHillbilly » Fri Feb 17, 2012 11:41 am

M. Scott@....got a 4 mil poly job on today .45 lbs PLI (per linear inch)

Sometimes we have run down as low as 30 ga. / highly extensable films that we specificly have to guess ahead of time at what print tooling to use and how to plate so we can hit the targeted image repeats for the packaging machine feeds.

The levels of "snap back" we see are wild, even subsequent rolls. Like printing on a mixed lot of rubber bands sometimes.

Somebody told me once that "The only rule is: That there are no rules, just guidelines" and "when you have a problem, YOU have a problem and you do not swear by ANYTHING".

I think that somewhere along the line you will be able to find a set of nice starting numbers, but you have to remember to handle the web correctly first, and compensate on the other processes. I made the mistake once of showing someone that my gearless press can do a 'print length correction'. He failed to understand that I am actually skidding my plates along on the substrate and almost began to assume that I can just use any size print sleeve and punch in the correction. Maybe I should have shown him a halfone dot first and let him do a little homework on the value of the dot.

Also, each tension zone should have a progressive increase until you get to winding. We find inital winding tensions to be the lowest of the set and apply a taper on top of that. I really hated having to "slab off" that core I firmly crushed on that winding shaft. That is a mistake I would have loved to have just had the opportunity to see someone ELSE make just once, so I did not have to learn on my own.

Got a humorous litte scenerio to share sometime, but I am not supposed to be here today and would like to enjoy my "day off". Maybe I will share it sometime later.

Outta here for now. Everyone have a nice weekend.

Try a Diet Mt. Dew with a nice splash of vodka...The Skinny Russian Hillbilly...a premium medicinal, recreational and theraputic beverage when consumed with wisdom.

Bill
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Re: Tension calculation formula

Postby Randyskalba » Fri Feb 17, 2012 4:55 pm

We run between .75 and 1.50 Gauge Film.
I could not imagine running 30 foot lbs of tension as we would have all sorts of web stretch issues and our repeat would be much larger then specifed.
Also at high tensions you are much more prone to web breaks.
Its possible that this is also do to our tooling and press.

Previous press's i operated ran at around 20-22 foot lbs of tensions, So it would depend on the type of press, tooling , etc
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Re: Tension calculation formula

Postby Randyskalba » Fri Feb 17, 2012 4:57 pm

We run what is called a "Taper Tension" That controls our Rewind tension controls which increases/decreases it throughout the roll size.
I find that 2 wide/across jobs require a much hire Taper Tension to keep the lanes from overlapping.
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Re: Tension calculation formula

Postby dkelley » Mon Feb 20, 2012 12:10 pm

Frank can you send me this calculator
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Re: Tension calculation formula

Postby Frank Burgos » Mon Feb 20, 2012 12:20 pm

Hi dkelly,

I just sent it.

A note to all:

All the calculator does is allow you to enter variables for:

Lbs per linear inch
Web thickness, in mils
Web width, in inches

From there, it calculates tension, in lbs., by multiplying all values linearly.

As suggested through this thread, tension is a nebulous monster with more to it than meets the eye. However, the calculator can help provide a starting point.

Frank
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Re: Tension calculation formula

Postby M. Scott » Thu Feb 23, 2012 5:37 pm

Thanks, Frank!

Trying to figure out how to convert this into a decimal to input into the German W&H.
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Re: Tension calculation formula

Postby Frank Burgos » Thu Feb 23, 2012 6:00 pm

Do you know the metric units? What inputs and outputs would you like?
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