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July 24, 2006
Proof of Nip-Induced Tension in Winding
Filed under: Winding, Unwinding, Roll Defects --- Tim Walker @ 03:40 PM
I was recently asked:
"Are there articles where nip-induced tension equation is developed and verified by experiment?
The simple equation is:
T(WOT) = T(WH) + COF*N
Where:
T(WOT) is the Wound-On Tension, in force per width (PLI or N/m) the sum of the web handling tension and nip-induced tension effect.
T(WH) is the web handling tension, also in force per width. It is the tension upstream of the nip roller or the winding tension for gap winding.
COF is the coefficient of friction between side A and side B of the web.
N is the nip force per width.
The best paper showing this theory and proof is:
Author: Dr. J. Keith Good
Title: “Modeling Nip Induced Tension in Wound Rolls”
Source: Proceedings of the Fourth International Conference on Web Handling, 1997.
There are several articles other papers, most published in past proceedings from IWEB (The International Conference on Web Handling), held in odd-numbered years at Oklahoma State University (OSU). These proceedings are available for purchase from OSU’s Web Handling Research Center.
In more advanced windig nip theory, they have found the COF*N over-estimates the nip-induced tension as the nip loads go above 10 PLI, but it is a good simple model to understand that nip create winding tension and increase roll tightness.
July 17, 2006
Torque Control is the Best Tension Control?
Filed under: Tensioning, Web Mechanics --- Tim Walker @ 02:06 PM
A few months back, I made post asking 'Is motor torque control EVER the right?' This was a post based on my experience with several converting controls engineers.
Pete Werner, formerly of Rockwell Automation, posted his comments on this and made a strong case of torque-based motor control should be the first choice. This is quite a contrast from what I've seen over the last 20 years.
Since then, Pete has continued to educate me on this topic. Pete made an interesting presentation at the AWEB conference last month. He makes some good points about how speed regulation and dancer rollers the popular choice, but these are devices invented to solve problem many years ago.
Why does speed-based tension control continue to dominate converting applications?
Probably several reasons. First, controls engineers have learn this and know it works. Speed is easy to understand.
Why use speed reducers (gearboxes)?
Supplier like to boast speed accuracy. Controls engineers like using speed reducers to reduce motor size and increase efficiency. But you shouldn't be thinking about energy or motor costs when your goal is good tension control. Yes, eliminating the speed reduce increase motor size and cost (both torque related), but you save some of that back by not buy a gearbox. The most important motive should be good (or great) tension control.
I always had it drilled into me that you want a motor to run using its full operating speed range. If the motor has 0.01 percent speed accuracy, this is 0.2 rpm variation over 2000 rpms. If you run the motor at 100 rpm with no speed reducer, you're asking for 0.2 percent speed variations. Why would you do that?
This is the wrong question. Motor are not inherently speed devices. They are torque generators. Torque output divided by the mechanical leverage of radius creates tension. Why care about speed accuracy at all?
These are great questions.
I like Pete's question where he says 'You don't usually consider putting a speed reducer on a braked or clutched application, why do it on a motor?'
Today's drives call for new answers. I get the feeling Pete would like to eliminate 90 percent of the speed reducer (gearboxes) and dancer roller in the converting industry.
Let me know your thoughts or experiences regarding speed or torque motor control and when dancer rollers can be eliminated.
tjw