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Filed under: Winding, Unwinding, Roll Defects --- Tim Walker @ 12:35 PM

Someone wrote me this week to get a sanity check on when center-surface winding makes sense.

Definition: Center winders drive a roll from the core or center. Surface winders drive a roll a nipping roller in contact with the roll's outer surface. Center-surface winders do both, driving the roll from a combination of center and surface driving.

The inquiring party makes several good points / questions:

> Center-surface winding makes some sense for very slippery (low web side A to side B coefficient of friction) webs.

Yes, this is to avoid how much torque needs to be transmitted through the roll.

> The bottom line on wound roll tightness is the wound-on-tension and the profiling of the WOT versus the roll diameter (a.k.a. WOT and taper tension).

FYI, WOT is the tension of each layer as it is added to the winding roll) and is a function of applied center torque and winding nip load.

> A center winder with nip pressure control can achieve a large enough range of WOT to wind most everything.

Yes, I agree again. The paper industry (and other LARGE roll winders) use surface winders since torque is independent of roll diameter. Center winders can be limited if the core to final roll ratio is too high (a.k.a. the buildup ratio).

> Why do people 'need' the extra complication and cost of a center-surface winder?

I think of most center-surface winders as surface winders with center assist. Surface winders can have trouble getting the initial layers near a core tight enough and the center assist can help with this. If you want to wind an ultra-soft roll, say a nonwoven or tissue product, your surface winding can only go as soft as the nip load required to turn the core, core shaft, and bearings. If you have a center assist, you can go to lower nip loads and softer rolls.

> What does the addition of the last 'T' knob (in TNT) give you besides more variables to mess-up?

Yes, may people that buy surf-center winders never figure out what to do with them.

There has been some interesting work out of Oklahoma State's WHRC how different nip rollers change the tension induced from the nip roller, but it's still all about modeling a roll from a given WOT vs. diameter. I think there is work yet to be done on how WOT from tension and WOT from nip may magnify or de-magnify crossweb roll variations from crossweb caliper and roll diameter variations, but I haven't seen any good work to point anyone in the right direction.

Conclusion: Use winding technology appropriate to your application. Don't get more knobs if you don't need them.

-tjw

Filed under: Winding, Unwinding, Roll Defects --- Tim Walker @ 09:00 AM

Where is the best position to nip a winding roll?
12:00 on top of the roll
3:00 or 9:00 on the side of the roll
6:00 on the bottom of the roll

12:00?
+ If you have a noodle-shaped nip roller (the L/D is over 10), the 12:00 position lets you use the roller's weight to create the nip load, avoiding the deflection of applying the nipping load from the roller's journals.
- This is a default closed system and needs to be counter balanced or other method to open the nip in case of a power loss or emergency.
- The nipping load is the combination of roller weight and external load. You may need to add or subtract from the roller weight to get the desired nip load.

3:00 or 9:00?
+ The nipping load is perpendicular to and independent of gravity, making it easy to convert pneumatic load into nip load.

6:00
+ If the nipping roller has enough stiffness, it can offset deflection in a noodle-shaped winding roll (again L/D>10) or deflection from heavy slit rolls winding on a lock-core or differential shaft.
+ In two drum surface winder, the rollers aren't at 6:00, but form a nest for the winding roll to sit and use the rolls weight to create much of the nip load.
- This position may be the most difficult to thread up.

I've seen all of these clock position used successfully. In most cases, the nip load and deflection issues can be managed with good engineering. I've lately grown fond of the bottom nip after realizing some of the best wound rolls I've seen have this orientation. Don't underestimate the value of a straight, rigid nip roller to impress its goodness into the winding roll.

-tjw

Filed under: Winding, Unwinding, Roll Defects --- Tim Walker @ 10:00 AM

My last column in Paper, Film, and Foil Converter magazine was on how to drive a winding roll, comparing and contrasting surface and center winding. One main point I make is surface winding tends to make a looser roll.

This prompted the following advice from one of my past clients.

He said that I should be careful when I say that a surface winder winds a looser roll. He has a surface winder that can't wind a roll loose enough. The rolls are so tight on his thin film product that it magnifies the gauge bands to point where the customer rejects the product. The winder is a true surface driven winder with no drive on the core and the roll weight creates the surface winding nip pressure. (So as the roll diameter and weight increases, the surface winding nip pressure goes up.)

My reply:

But I’m still going to stick to my ‘surface winding will be less tight’ statement. If you compare winding the same roll on a pure surface winder and a pure center winder using the same winding tension and winding nip force, the surface wound roll will be looser. In surface winding, the nip load is the #1 variable to determine roll tightness for a given material and roll geometry.

Why are your surface-wound rolls extra hard? I believe it is due to excessive nip load. It sounds like the roll’s weight controls your surface winding nip load (pounds per inch of width that the roll presses against the surface driving roll). I’m guessing your surface winder has the core positioned on the top or 12:00 position of the surface drive roll. Sometime the top load surface winders will have a ‘rider roller’ that allows the surface winding nip to be set above the roll’s weight, but don’t really have an option to relieve the gravity effect or run a nip load less than the roll’s weight.

Another surface winder design loads the core from the side (the 3:00 or 9:00) position and pneumatically controls the load to the optimum value, independent of roll weight. In this design, the surface winding nip load can be less, sometime much less, than the roll’s weight.

What is too high in surface winding nip load? The first estimate of surface winding tension is the nip load multiplied by the web-web friction coefficient. So if you roll weighs 10 pounds per inch of width (say winding polyester, density of 0.05 lbs/in^3, on a 4-inch OD core to a 9-in final diameter), then the effective winding tension is 10 x 0.25 (typical PET friction coefficient) or 2.5 PLI. If this was a 3-mil film, no problem, but if you are winding 0.5 or 1 mils film, the 2.5 PLI is way too high.

Filed under: Winding, Unwinding, Roll Defects --- Tim Walker @ 11:38 AM

Question:
When web (PET) is wound after slitting, the winding tension should be changed in case of paper core and plastic core?

When winding PET on a paper core you may need a high starting tension than for the plastic core. It depends on what defects you are seeing. A plastic core is usually stiffer than a paper core and will usually create higher pressure near the core for the same winding conditions. A paper core may show more spoking or starring defects. A plastic core may show more core impression or blocking defects.

Filed under: Winding, Unwinding, Roll Defects --- Tim Walker @ 08:00 AM

Q1: How can you minimize web sagging in a horizontal roller?
Q2: How can you eliminate sagging rolls?

A1: If the web is sagging in a web span between rollers, higher tension will reduce web sagging or shortening the web span by adding a roller. You can offset span sag with air nozzles also.

A2: A sagging roll was usually not initially that way. A sagging roll indicate either the web is getting longer over time (not very common) or the roll’s layers are getting thinner over time. Why would a layer get thinner? If the wound roll layer include air, then the air can bleed out over time and create roll looseness or sag. If the web has a thick coating, the coated layer may with ooze laterally (like an adhesive layer), or increase it’s density and get thinner, both loosening the roll and causing roll sag. Rolls don’t automatically get loose with a small amount of web thinning. Only after the thinning exceeds the strain of the roll’s layers will they loose all their tension and sag. Winding tighter will help, but only so much.

Filed under: Winding, Unwinding, Roll Defects --- Tim Walker @ 01:51 PM

Contributed by Tim Walker -

First rule of roll edge misalignment: Figure out if the web is shifting before it gets to the winding roll or if it enters the roll in the correct position and shifts later.

Q1: What causes one edge of a roll to be misaligned (but not the other edge)?
Q2: What causes roll edge misalignment only near core?
Q3: What causes misaligned roll edges as diameter increases?
Q4: What causes misaligned roll edges as speed increases?

A1: Anytime one side is aligned and the other side is not, it indicates the web width or tension is varying. When winding after slitting and one side if good, but one side is uneven, the top suspect is the slitting process. The web may be loose at the cutter causing the edge cut position to wander and width to vary or the slitting blade or knife assembly is wobbling.
If the winding alignment is controlled by a web guide, then one edge should always be well-aligned, but the other edge may wander due to width changes, tension and necking changes, or web wrinkling in the wound roll.

A2: I saw this recently on a differential winding shaft after slitting. This slitter used one of the newer differential winding shafts that have core stops to hold the core’s lateral position. However, due to core debris generation on these stops, the operators had moved them out a little, but this gave the core freedom to shift. At startup, the core shifted to one side over the first 100 feet of winding, causing a shifting of layers near the core. If you have severe shifting near the core, you likely have cinching (MD slippage) and telescoping.

A3: If you don’t use a winding nip (aka pack roller, lay-on roller, contact roller), then you may have slippage when the entrained air is greater than the product roughness. The lubricating air layer increases with roll diameter, so as some point, the air layer is too thick and the layers begin to shift.

A4: The same answer as #3, but now speed causes the increase in entrained air and lubrication.

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.

Filed under: Winding, Unwinding, Roll Defects --- Tim Walker @ 10:32 AM

Two questions from Greg Martz of Martz Technologies
engineering@martztechnologies.com

Q1: Center Winding:
Diameter Calculator, or Ultra-sonic. Which is better and why?

Reply1:
The best answer is BOTH. By a diameter calculator, I assume you are talking about a tachometer ratio calculation using the spindle rpm, and the diameter + rpm of a non-slipping roller. The tach ratio diameter calculator is the most reliable approach, but it doesn't give you any information when the system is stopped. Therefore, the need for the ultrasonic sensor.
I wouldn't rely on the u/s sensor for the entire winding or unwinding process. They are too easily affected by outside influences, such as rapid air temperature or humidity changes and blockage from a web scraps.

By diameter calculator, you may have been thinking of the web thickness, core OD, and revolutions methods. I don't see this used as often, but if you know thickness (which may be a good question), then this works well, too.

Winders may not need the u/s sensor if you always start with a give core OD or start where you stopped (you may need to add a operator 'new core' button).

Q2: Center Winding:
Drive in Speed mode with a Torque Trim, or Drive in Speed mode with a speed trim?

Reply2:
How about torque mode with torque trim? This is what Pete Werner, formerly of Rockwell Automation, has been trying to make the case for. The classic or traditional design would have a speed reducer, therefore, you would run in speed mode with speed trim, since the motor has a large torque noisy drive train.

See today's post on torque control for more on the debate of torque vs speed mode tension control.

tjw

Filed under: Winding, Unwinding, Roll Defects --- Tim Walker @ 09:21 AM

Question:
Has anyone modelled the effect of varying MD tension across the width on winding stresses? These are stresses created by previous spreading or steering devices, not a result of web thickness or bagginess variations. What do you think would happen?
Dilwyn Jones, Emral Ltd.
Dilwyn.Jones@physics.org

Reply:
My feeling is that the tensioning upstream of a winding roll, like any spreading device, is a temporary effect. The purpose, of course, is to eliminate wrinkling at winding. I expect once on the wound roll and away from the incoming span, the web would shift tension to conform to the shape of the roll. If the product was adhesive tape, then maybe the span tension would be locked into the layer.

Imagine a baggy edge web running into a nip. You can pull back on the baggy side with a misaligned roller or spreader and get it taut and into the nip wrinkle free, but the web comes out the other side of the nip still baggy. The same would happen in winding. You can get onto the wound roll, but a longer, low tension baggy lane will still have low tension.

Your thoughts?

tjw

Filed under: Winding, Unwinding, Roll Defects --- Tim Walker @ 09:21 AM

Question:
How good are winding models at predicting the diameter of wound rolls, for a given length, tension profile and lay-on force (The small variations from compression and air entrainment, of course!)?
Dilwyn Jones, Emral Ltd.
Dilwyn.Jones@physics.org

Reply (not exactly an answer):
I haven't seen this done.

Start with the easy case of a narrow, uniform thickness roll - I imagine that if you had the stack modulus data and modeled the roll pressure, you could map the web thickness on to the pressure model to get a thickness of each layer, total stack height, hence diameter.

The difficult case, as you've alluded to, is to incorporate thickness variations and entrained air. Some work along this line was presented at the last IWEB, Dr. Keith Good and xxx, but they were looking more at stresses, not stains and diameters.

What would you do with this info?
And what are your thought on this topic? Do tell.

tjw

Filed under: Winding, Unwinding, Roll Defects --- Tim Walker @ 08:10 AM

Question:
Does running a web through a 'z' pattern of rollers reduce tension variances? The 'z' pattern in question is made up of 4 rollers located immediately after the unwind.
Peter Krasucki
pkrasucki@rdspecialties.com

Answer:
Yes, to some degree, especially is the web had good friction on the roller (doesn't slip) and the rollers have modest to high inertia.

You can imagine a stopped web with stopped rollers. If you jerk on the web at the unwind and the web doesn't slip on the roller and the rollers don't turn, then NO tension shock would pass over the rollers.

A running web is different from this scenario. The tension or strain of the web travels with it. For a running web, if you change the unwind brake setting, the tension from the unwind to the next drive point will nominally go to the new level. So low frequency tesnion changes will pass through the system.

The benefit, if significant, will be in calming high frequency tension variations, where high frequency is relative to the span lengths (or the time to travel the span length). In some regards, longer spans will do more for tension noise attenuation than the rollers.

tjw

Filed under: Winding, Unwinding, Roll Defects --- Tim Walker @ 01:34 PM

What is K2? Besides one of the world's more difficult summits or a brand name in skis and inline skates, K2 is one of two parameters used to describe the compressibility of a stack of sheets of any web product. The compressibility of a stack is also known as the stack modulus or radial modulus, when considering the mechanical structure of a wound roll.

Why do we care about stack modulus? It is a critical material characteristic that defines whether a material is easy to wind. In general, the lower the stack modulus, especially the lower it is relative to tensile modulus, the easier a product will be to wind.

Stack modulus is a non-linear property. The more you push on a stack the harder it is to compress it more. Unlike tensile in-plane modulus, that is described as one value, stack modulus goes up with pressure and can only be described with a multi-variable equation.

K2 and K1 are two variables that form a exponential fit to a stack compression test, measuring the stress or pressure to compress the stack and the resuling stack compression or strain.

J. David Pfeiffer, formerly of Beloit Paper Machinery, developed the K1, K2 curve-fit to stack modulus in the mid-60s. After testing paper stack, he decided that an exponential equation was a reasonable fit. The equation takes the form:

Pressure = K1*(EXP(K2*Strain)-1)

There are more advanced stack modulus models. Zig Hakiel of Kodak developed polynomial equation, but

Stack modulus is not an easy property to test. Where many quality labs have tensile-elongation tester that can find a tensile modulus, few are set up for the high loads and fine strain measurements needed to find K2.

Thankfully, J. David Pfeiffer is sells a table top, stand-alone K2 tester. For information, visit his K2 Tester website.

Filed under: Winding, Unwinding, Roll Defects --- Tim Walker @ 10:22 AM

Inertia compensation is a big deal at unwinds and winders. The torque demand of a unwind or winder comes from three sources: tension x radius, inertia x accel/decel, and torque system losses (from nip, bearings, and couplings).

A question on roll inertia came in recently:

>I'm a novice to web tension control applications but I would like to learn what the proper calculations are that must be made in order to find out the inertia of a center driven unwinder and the torque required to maintain a certain tension.

>For example, let's assume a roll of plastic film weighing 1000 lb. and having a 24" diameter. The Desired tension is 1.0 PLI (Pounds per linear inch). I would like to learn how to make this calculation correctly.

Here's the calculation:
Torque from inertia is I x alpha
I = rotational inertia of the roll
alpha = accel / decel rate in radians per second^2

I = pi x rho x w x (Rout^4-Rin^4) / 2 / g
rho is density in lbs/in^3
g is gravity

[NOTE: Two sharp blog readers pointed out an error in the original post of this equation that was missing the width term. It is now correct, but I want to give out a big thanks for their help.]

I have a spreadsheet where I can calculate all this.
The 1000-lb roll of film at 24" is likely 50" wide on a 6" OD core. This crunches out to an inertia of 200 in-lbs-s^2.

What is alpha?

alpha can be found from line speed and accel time.
If you accelerate this roll to 1000 fpm in 15 seconds, the accel alpha is about 1 radians per second-squared.

So the inertial torque is I x alpha or about 200 in-lbs. At 12-in radius, this is 16 lbs.

If you have a braked unwind and the brake applies 800 in-lbs or 48-lbs of tension at 12-in radius. During the accel, the tension would increase to 60 lbs due to the added inertial torque.

Clear? Let me know.

Advanced control (what really should be standard) has inertia compensation built into the tension control plan to adjust automatically for this natural source of tension upsets.

Filed under: Winding, Unwinding, Roll Defects --- Tim Walker @ 03:34 AM

Unwinding rolls are seldom perfect. They are lumpy, eccentric, and out of alignment. If you handleds your web over rollers with these properties, you'd have wrinkles city.

What to do?

The simple solution is to have a relatively long span from the unwinding roll to the first roller. This demagnifies the problem. A 0.1" runout over 50 inches is a smaller upset than the same runout over a 10 inch span.

Filed under: Winding, Unwinding, Roll Defects --- Tim Walker @ 08:23 AM

Is your wound roll straight? Does the side look like a phonograph record or a neatly stacked deck of cards? Or do you use words like shifted, feathered, telescoped, dished, or offset to describe the side of your roll?

Let's talk about the top cause of lateral position error in wound rolls.

On the road to straight rolls, we divide the causes of lateral errors into four categories.
1) the web shifts prior to entering the winding roll
2) the web shifts after entering the winding roll
3) the web shifts in storage or handling
4) the web shifts upon unwinding

Write in with any area you'd like to know more about.

Filed under: Winding, Unwinding, Roll Defects --- Tim Walker @ 08:20 AM

Pro: Nips and the high pressure they create are required for many web processes. The high pressure or controlled gap of two nips rollers is used for coating, extrusion, calendaring, laminating, and embossing. Shear knives are a unique form of nips. Nip rollers at winding help controlled entrained air and increase roll tightness.

Con #1: Nips can create wrinkles from either uneven feeding of the web length or steering the web into shear or tracking wrinkles.

Con #2: Nips can't be perfect. The pressure within the footprint of the nip is not uniform and will vary with on web variations (crossweb thickness, wrinkles) and roller variations (diameter, hardness, alignment, deflection).

How to avoid these problems?

Nip roller systems are quite complex, but by following a list of nip system design 'rules of thumb' (ROT), many problems can be avoided.

Throughout today's live blog, I'll try to work up a long list of these ROTs, and get your comments.

Filed under: Winding, Unwinding, Roll Defects --- Tim Walker @ 08:58 AM

When I teach web handling classes or write article, I find it often helps to compare handling webs (a fairly abstract action) to something people are more familiar with.

So I'm going to start a series (maybe monthly) of my favorite analogies I find useful to put a human perspective on web handling. This is #1.

Winding rolls and creating wound rolls is like...
...constructing a building and making a building.
(Continued...)

Continue reading"Favorite Analogies #1: Wound rolls as buildings"

Filed under: Winding, Unwinding, Roll Defects --- Tim Walker @ 11:54 AM

I rec'd a questions that asked "How can I determine if a roll of polyester if properly wound?"
Upon a little follow-up Q&A, this question expanded with: 1) In unwinding the polyester rolls, they are sometimes loose on one side or the other.
2) Incoming polyester rolls sometimes haved ridges or ribs.

Films like Mylar (the Dupont brand name for their biaxially oriented polyester), often develop "bagginess" or permanent crossweb length variations from the combination of crossweb caliper variations, internal wound roll stresses, and storage time/temperature.

For small amounts of bagginess, increasing the tension will "pull out" the bagginess, so the running web doesn't have any zero tension lanes. If the bagginess is too severe, it isn't possible to pull it out with reasonable tensions.

Ribs and ridges form for a couple of reasons...

Continue reading"Is This Roll of Polyester Properly Wound?"

Filed under: Winding, Unwinding, Roll Defects --- Tim Walker @ 02:29 PM

In response to my August 5 on differential winding, a couple of blog readers commented with specific questions. Case 1: A reader asked for the 'exact data of rewind tension' required for a series of products and listed 5 different webs (plus two mysterious laminates called 3 Ply and 2 Ply), listing only the thickness of one web, one maximum width, and one maximum speed. Case 2: A second reader asks if there is a system to differentially wind to 60-in. OD for a specified range of steel and aluminum webs.

How do I reply to these well-intentioned inquiries? I have problems on three levels.

1. There is a great deal of missing information?
2. Even with near perfect information, is it possible to provide 'exact data on rewind tension'?
3. Is answering this type of specific question the purpose of convertingblog.com?

Lets go through each one of these a little bit...

Continue reading"Winding Tension Answers? Not 'Exactly'."

Filed under: Winding, Unwinding, Roll Defects --- Tim Walker @ 10:55 AM

When people look at differential bars, they often focus on their main function and benefit - the ability to allow many individual rolls on a common shaft to turn independent, finding their appropriate rpms to compensation for variations in roll-to-roll diameter differences and lane-to-lane length variations.

OK, this is great, but let's move from this function onto other specification that will make or break your winding yield.

#1 The right winding tension

What is the right winding tension for your product, including the initial tension, the final tension, and any wacky tapering or profile you may want to run.

Winding and handling tension are usually in the same ballpark, but unusual wound roll characteristics many mean winding tension needs special considerations.

Slippery or thick products may need more tension (and pack). Thin and pressure sensitive (thick adhesives and other special coatings) product many need quite low tension.

#2 Torque creates tension

More...

Continue reading"Differential Bars, Part 1 - What Torque? Tension?"

Filed under: Winding, Unwinding, Roll Defects --- Tim Walker @ 07:36 AM

I often advocate the benefits of controlling entry to a winding roll with a nip or gap roller. Both options will reduce wrinkles and improve the entering web's alignment. Nipping rollers provide three benefits beyond gap rollers. Winding nips will squeegee air entrained by moving web and roll. Winding nips will press down on large diameter lanes and let air in small diameter lanes, promoting a more cylindrical roll (if the winding nip is held parallel to the roll's axis of rotation). Winding nips will increase the tension of the winding roll's outer wrap above the upstream handling tension, increasing roll tightness. Definitions: Gap roller, gap winding - In center winding, a roller is held in close proximity, but not touching the winding roll. Winding nip roller, nipped winding - In center, surface, or combination winding, a roller is pressed with against the winding roll to squeegee entrained air, promote cylindricity, and increase roll tightness through nip-induced-tensioning. -tjw

Filed under: Winding, Unwinding, Roll Defects --- Tim Walker @ 10:05 AM

Rubber covered rollers are common in almost any nipped process. Without a compliant covering on one or both rollers, the nip pressure will vary greatly from roller misalignment, diameter variations, uneven loading, roller deflection, and web thickness variations. Steel just doesn't deform much, so even variations as small as 2-10 mils can create larger nip pressure variations.

The compliance of a rubber covered roller is forgiving. The rubber (or elastomer) will deform from the nip load, often as much as 30-100 mils. If the rubber covering is deflection 50 mils, a 5 mils error from product variation or deflection will create a 10% pressure variations. The same 5 mils error in a steel nip will likely cause a gap in the nip. In some cases, a 5 mil thick lane in the product could end up carrying 100% of the nip load, creating what could be a 100-1000% pressure change. Whew.

So what about winding nips. Should they be elastomer-covered or hard?

Continue reading"When should winding nips be rubber covered?"