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Filed under: Rollers, Traction --- Tim Walker @ 08:00 AM

Contributed by Tim Walker -

Q: I have tried to add a nip roller to my process, but used a nip roller narrower than the fixed roller and also narrower than the web. The result is WRINKLES. Should I use a nip roller wider than the web?

A: Yes! Unless you have a thick and stiff web, nip rollers should always be wider than the web. Usually the two nip rollers are the same width and wider than the web. In some limited cases, you might be able to have a nipping roller just less than the web width, but I wouldn't leave more than 0.25-0.5 inches unnipped on either edge.

Why is a partial nip a problem? Nips will tend to speed up the web underneath them. If you speed up one section, say the middle, but not the edge, the result will be a severe localized speed variations that induces shear stress into the web. If the web is thin, it will wrinkle in response to this shearing.

Filed under: Rollers, Traction --- Tim Walker @ 11:23 AM

Question:
I have a S-wrap configuration that is causing poor film quality because the delta T across the rolls is very high. The temperature varies because the chilled water is too cold. The water flow through the rolls is at a dribble to help the problem but the film heats the water causing the temp across the rolls to increase. Do you recommend chilled water, at what temp? or process water, normally 70 degF?
Michelle Perry
michelle.perry@amcor-Flexibles.com

Reply:
I'm note sure I understand the problem here.
Am I correct to assume the temperature variation is too high across the width of the rollers?
You state the water flow is at a dribble. Wouldn't a high flow rate reduce the temperature variation? At a dribble, it seems you are asking the web to control the roller temperature.

Regarding chilled or 70F water, the lower the temp, but more ability you have to fight the web heat transfer. Too low leads to the risk of dropping below the vapor temperature and forming condensation or sweating, but if you have a lot of heat from the web, this shouldn't be a problem until you stop the line.

tjw

Filed under: Rollers, Traction --- Tim Walker @ 09:01 AM

Motor-shaft-roller-web.
Motor-shaft-gearbox-shaft-shell-roll-web

These are a couple examples of how torque gets out of the motor and to the web, creating tension. The first case is the simple direct driven roller. The second case is driving a winder or unwinder. (I always use the term 'roller' for what transports the web and 'roll' for a wound roll of material.)

At the AWEB conference, Bill Fields of American Roller Co. presented several interesting calculations that go into a good roller. The one that I found interesting, was the effect of the small machined radius at the transition from shaft diameter changes. I'm sure I learned this somewhere in my mechanical engineering training, but it must have been pushed out of my brain.

Transmitting torque through a uniform diameter shaft is a simple analysis. But most shafts have diameter step changes, often at the ends or journals to fit into a small diameter bearing or coupling. This step change, if made in a sharp square profile transition, is a great stress concentrator. If you don't make a smooth transition here, you're just asking for a crack.

It's a relatively easy problem to solve once you watch out for it. Simply machine the transition with a radius or weld a fillet and the stresses are greatly reduced.

I'm glad someone is thinking about this. Thanks Bill Fields from American Roller for participating in the first Applied Web Handling Conference by AIMCAL.

Filed under: Rollers, Traction --- Tim Walker @ 11:14 AM

Practical, applied web handling knowledge...

This is the intended product of the AWEB conference next month in Charlotte.

I'm busy getting my slides ready. I've got two presentations and a half-day class going on. Here's some background on pulling together the plan for the first presentation I'm working on. It's outcome of a nice three-way collaboration.

Presentation #1 - Measuring pressure in rubbber-steel nip roller systems.

What do you get when you combine precision diameter measurement with a thin film pressure sensing system?

I've know Steve Huff at Abba for many years through his alway informative presentations at AIMCAL's Fall Technical Conferences. Steve gave me a tour of the Abba facility, including their LMS 3000 precision diameter measurement system.

I met Vin Carrara of Tekscan in November when he was on a sales trip to the Minnesota area. I've know about the Tekscan system for many years and new it can be an effective tool for measuring nip pressure variations.

Sometime in December, we were planning the AWEB conference and I thought..."wouldn't it be great to see these two devices, the diameter and pressure measurements, put together in an AWEB paper." Well, a couple emails and a few phone calls later and we had a plan.

Each team member brought something to the table:

Abba Rubber, design and built the nip test stand and rollers and volunteered Steve's time and the LMS 3000 use.

Tekscan, provided the Tekscan sensors, software, and computer, plus Vin Carrara time and travel to Ontario, CA

TJWalker + Asso., mostly, I played Hannibal Smith to this A-Team, but also donated my time and travel to Ontario, and completed the data crunching and slides with my XLS and PPT skills.

The result: I'll post more hear after the AWEB conference.

Filed under: Rollers, Traction --- Tim Walker @ 08:30 AM

One blog visitor recently asked:
Is there a way how we can convert the pressure on the dancer (or nip roller) to PLI?

How to convert dancer pressure (psi) to PLI (lbs/inch of width)?

I often find myself at the side of a converting process, looking at PSI supplied to a dancer roller or nip roller wondering what the created load is. The answer to both of these scenarios is the same.

Pressure x area x number of cylinders = Fc (force out of the cylinders)

Fc (Total cyl. force) x Lcyl / Lroller = Fc,eff (Effective cyl force at the roller)

For pivoting dancers and nips, Lcyl is the length from the pivot point to the cylinder load point, Lroller is the length from the pivot point to the roller's center. For linear dancers and nips, Lcyl/Lroller may be equal to 1, so the Fc = Fc, eff.

For a horizontal nip or dancer, the roller weight is perpendicular to the load on the web, so it is ignored.
N(in PLI) = Fc,eff / width

For vertical nips or dancers, the roller weight needs to be added or subtracted from the Fc,eff.
N(in PLI) = (Fc, eff +/- Wroller)/ width

What is the width?
For dancers, width is web width.

For thick webs in nips, where the web prevents roller-roller contact outside the web width, the width is web width.

For thinner webs and conforming nips, where the roller contact each other outside the web width, the width is the roller contact width. Also, depending on the ratio of web thickness to engagement, you may have a significantly higher pressure at the web due to the engagement difference.

Estimating on and off web nip variations.

Continue reading"Converting PSI to PLI, Nips and Dancers Rollers"

Filed under: Rollers, Traction --- Tim Walker @ 06:49 AM

A roller = cylindrical shell + shaft + bearing + mount to equipment.

End of story.....................it would seem.

What a simple device? If you buy an idler roller for $300 you may wonder why this sophisticated rolling pin cost the same as an iPod. Many new-to-converting design and build shops think the same thing and say 'Why don't we just build our own roller?' If your new equipment supplier tells you they plan to do this, what to do? Very quickly, write a roller specification (or talk them out of it).

Webs, especially high modulus, thin materials are sensitive animals. When your product only stretches 0.1 percent, it's is sensitive to errors in your rollers (or relative alignment of rollers) in this same order of magnitude.

If I wrote a list of 'rollers gone bad', it would go one for some time. Maybe today I'll start that list. Want to help?

Filed under: Rollers, Traction --- Tim Walker @ 07:15 AM

I've had a few different client consults lately that all center around nip rollers, usually for laminating, but also embossing or printing.

What are nips for? Or what are the important properties to understand webs in nips.

1. Nip are for high pressure.

If you don't need pressure, why nip? Nip pressure can easily be 10-100x higher than the pressure of a tensioned web over a roller.

Many processes use nip pressure, usually to move something. Pressure moves a coating or extrusion fluid into the valleys of web surface roughness and exclude excess liquid (control coat weight). High enough pressure moves solid material around, such as in embossing, die cutting, crush or shear slitting. Low pressure moves air or liquid around, excluing bubbles in a laminate or squeegeeing liquid off a web as it leaves a submerged process.

2. Air elimination

This is part of high pressure, but in some cases, this is the primary function of the nip. Air is never completely eliminated (unless you have a vacuum process), but it can be reduced 15x or more with nip pressure. How much needs to be eliminated to stop bubbles? It depends on how much air your product can hide. Air bubbles at some size are usually invisible to the naked eye. Many solids and liquids can absorbs a limited amount of air. Reducing air is used to keep wound rolls tight and at the entrance of heating rollers to improve conduction.

Other nip effects:
3. Web elongation and speed control (web metering)
4. Web-roller slip (burnishing or calendaring)
5. Web compression
6. High friction

I'll have to write more thoughts on these last 4 later.

Filed under: Rollers, Traction --- Tim Walker @ 10:38 AM

How hard, that is, resistant to wear, is an anodized alumninum surface?

Quite good, actually.

Here's an interesting link to a Products Finishing magazine article on anodization.

"Anodized Bright; Anodized Tough"

Scroll to the bottom of this page to see an interesting graph of wear testing (using a Taber abraser testor) for anodized aluminum, stainless steel, chrome, regular aluminum and some other surfaces.

Result: Steel is initially more wear resistant, but anodized aluminum wins in the long run (per this test).

Filed under: Rollers, Traction --- Tim Walker @ 10:29 AM

More web resources useful to understand roller surfaces:

What surface modification processes produce what range of roughnesses?

One answer, courtesy of Precision Devices, Inc web page (a maker of surface roughness measurement equipment).

1. Surface Profile Parameters
Defines Ra and other roughness parameters and show how they are calculated.
2. Drawing Indication of Surface Texture
A table showing the range of roughnesses typically produced by differ processes and how to indicate surfaced on engineering drawings.

Filed under: Rollers, Traction --- Tim Walker @ 12:09 PM

While doing some surfing on roller surfaces this week, I can across a nice roller specifying checklist at the Webex, Inc. website.

Here's their list of the 'ten most critical roller specifications will assist the converter in matching optimum roller performance to system fit and function'

1. Application
2. Quantity
3. Roller material
4. Loading
5. Speed
6. Surface finishing
7. Coatings & coverings
8. Driven or idler roller
9. Elevated temperatures
10. Tolerances

www.webexinc.com/tech1.html

I'm a frequent web surfer. I'll try to keep posting useful sites here at convertingblog.com under the title 'Surf's up!'

tjw

Filed under: Rollers, Traction --- Tim Walker @ 08:32 AM

In a reply post to my question on 'What's your favorite rough roller?', the following questions were brought up:

'We have issues regarding keeping rollers clean, and grooved or knurled rolls present a challenge. Any thoughts on this?'

'What are your thoughts around elastomeric surfaces (can probably get higher coef. of friction, but more difficult to control roughness, except through grooving)?'

In my experience, a shallow knurled roller, where the roller is knurled, re-ground to remove burrs and the tops of the knurled pyramids, then anodized (talking aluminum roller here), leaving a groove that is 0.010-0.020" deep and wide on a 10-20 pitch is fairly easy to clean. Lots of small grooves are better to reduce air lubrication than a few big grooves, plus shallower grooves (like from the knurling process described above) are easier to clean. (continued...)

Continue reading"More on Rough Rollers"

Filed under: Rollers, Traction --- Tim Walker @ 07:03 AM

I always tell people that "My dog knows the belt equation". When he's out for a walk on his leash and wants to sniff the local news posted by a previous dog at a telephone pole, he doesn't sniff on the near side of the pole. He always walks around to the other side, wrapping his leash around the pole with about a 180 degree wrap. Why? He knows that if I want him to stop, the friction that develops between the leash and the pole will make it harder for me to pull him.

The belt equation is used every day on every converting line. This simple exponential equation says the friction force required to slide a tensioned web relative to a cylinder will be related to tension, wrap angle, and friction coefficient. (continued...)

Continue reading"Favorite Analogies #3: The Belt Equation and Dog Leashes"

Filed under: Rollers, Traction --- Tim Walker @ 09:07 AM

Continuing my on-going themed entries on favorite analogies of web handling. Here's #2

The traction between a web and roller is like the grip of a car tire and the road.

More car weight creates more tire/road friction.
More tension creates more web/roller friction.

The ability to exchange force withoug slipping will be a function of the coefficient of friction of the tire/road (and web/roller).
(continued...)

Continue reading"Favorite Analogies #2: Traction on rollers and cars"

Filed under: Rollers, Traction --- Tim Walker @ 03:16 PM

I continue to run into applications where one side of a web can't be touched. Sometimes one side is freshly coated, another time the web is easily scratched, and recently, the web had a big part attached to it sticking out one side. It's hard to go over a roller with a big bump on your web.

The first choice in one-side contact is to design your line like a scroll. Place the unwind in the center of one side of the scroll and your winder in the other side of the scroll. If your roll loading is cantilevered (rolls go in and out sideways), you can form a pretty tight sprially path around the unwind, as many times as are needed for your tension, guiding, and processing plan. When your done with your unwind spiral, move over to the winder side and spiral inward as needed until you hit the windup (or sheeting, the more likely finale to the bumpy product. All rollers will rotate the same clockwise or counterclockwise direction.

For tension control, you can still have 2 or more zones, but driven rollers will likely have 90 degrees of wrap (and of course no nips). A rubber roller can support a good tension change with 90 degrees, but a vacuum suction roller can do even better (again use rubber for high friction, if possible). Tension feedback will likely be with a transducer roller, since most people prefer 180 degrees on dancer rollers.

Other tricks?

Continue reading"One-Sided Contact Web Handling"

Filed under: Rollers, Traction --- Tim Walker @ 07:23 PM

I've never seen where a groove is a problem because it is too small.

I have seen grooves that are too big, usually big meaning too wide, sometime meaning they are too deep.

I've never seen the math on this, but at some point a web will begin to fall into a groove. For films, this usually happens when the web thicknes is less than 0.002" for stiffer polymers or less than 0.004" for softer polymers. For these materials, I think 0.25" is definitely too wide. As you go thinner, less than 0.001" thick films, even 0.1" wide grooves may be too much. The groove sensitivity drops with lower tension and when the wrap angle is near zero.

How about really wide grooves? (continued)

Continue reading"When do roller grooves get to be a problem?"

Filed under: Rollers, Traction --- Tim Walker @ 09:14 AM

Here's a Top 10 reasons to align your rollers.

#1 keeping the web on centerline.

The parallel entry rule makes the web want to track parallel to the directing a roller is spinning (which is perpendicular to its axis of rotation). Parallel rollers, with good traction, keep a web tracking parallel to the machine's centerline.

#2 Reduce wrinkles

Roller misalignment is a big cause of wrinkles, especially in thin webs. Wrinkle causes are additive. So if you have misalignment, it may not create a wrinkle by itself, but it will lower your threshold to wrinkling from cause X.

#3 Reduce breaks

Misaligned rollers create web path variations between lanes of the web, shifting the tension from average across the width to high on one or both sides. The reason I advise running web handling tension at 10-20% of bad tension (the load to break or yield the web) is to provide a safety factor against misalignment-induced tension variations.

More...

Continue reading"Why Align Your Rollers?"

Filed under: Rollers, Traction --- Tim Walker @ 10:45 AM

As an engineer, sometimes you trip across something that gives you that "best invention since slice bread" feeling. Well, there is one at CMM that your should take a peak at.

Laser gyro roller alignment.

The Paralign(tm) from Pruftechnik looks like it could fly a helicopter. You "sweep" one roller as your reference, send the date wirelessly to a laptop, then continue "sweeping" other rollers in your web line to get tram and level errors from the reference roller. Their literature says it is accurate to 0.4 mils per foot, which should exceed most converters needs. The system is fast, so you can do many, many rollers with minimum downtime. Whether the cost of this service can be justified for you operation is based on what downtime costs you.

The folks at Pruftechnik has brought this new device to the US. They are back in a corner booth at the show, so you may not have notice them, but please do. Also, get info at their website:
http://www.pruftechnik.com/alignment/products/paralign/paralign.htm

Filed under: Rollers, Traction --- Tim Walker @ 07:35 AM

If you come from the world of paper products and are moving into films, one of the first things you'll fight is scratches. You relatively rough, high tension paper could drive you old overly-greased rollers, but the smooth, low tension film - no way. The result - scratches. When I run in this, I recommend a dual action plan. 1) Improve you idlers to be easier to turn (a whole topic in itself), and 2) switch to rough rollers. Many people balk the idea of rough rollers, since they assume a rougher surface will create more scratches. Are they right? Sure, if you put in a rough sand-blasted roller and keep the old high-drag bearings, you may increase your scratches, but that not the goal. If the rough roller prevents air lubrication and keeps a roller speed-matched to the web, then you will be scratch-free. I can rest my eyeglasses on a piece of sandpaper without scratching them. Abrasion requires two things: normal force and relative motion. It's the relative motion that underlies scratching.

Continue reading"Should you be afraid of rough rollers?"

Filed under: Rollers, Traction --- Tim Walker @ 12:55 PM

Handling smooth films or papers on smooth rollers is like driving through puddles of water with bald tires. You can do it slowly, but its hazardous at even moderate speeds.

Many converter never run into this problem. If you product is porous (like many nonwovens) or rough (like abrasive-coated papers), you have a way to prevent air lubrications. But if you run smooth, non-porous webs, you have to manage the lubricating air layer with roller roughness.

What is your favorite rough roller?

Continue reading"What's your favorite rough roller?"