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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: 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: Tensioning, Web Mechanics --- Tim Walker @ 08:26 AM

How do you measure a baggy web?

A common, practical, but tough question. I suggest several options, some off-line, others on-line.

1. Measure skew from a straight line in sweeping out a web (measure asymmetry only).
2. Measure crossweb droop variations in a lightly tensioned horizontal span.
3. Qualitatively judge a sheet of film as you pull it from a roll, grading it a 1, 2,..5 as you pull a web to tautness with finger, wrist, elbow, shoulder, or full body strength.
4. Combine 2 and 3 above, measuring tension to pull horizontal span up to less than X (say 1") droop.
5. Mark crossweb lines in a wide web, then cut it into machine direction strips and measure the length differential of the strips.
6. Lay flat test, place sheet on flat plate and measure ripple or curl deviations from planar.

On-line tests (measuring the moving web):
1. Droop test with scanning distance measurement, such as laser triangulation micrometer or ultrasonic sensor.
2. Low angle laser - Visualize droop with a low-angle linear laser or combine this with a camera and vision system to quantify bagginess (I don't know of any commercially available system to do this.)
3. Multi-segment tension roller - Dover flexo makes this. Segmented tension roller.
4. Multi-segment tension bar - Same idea, but for scratch-insensitive webs, using a segmented bar instead of rolling elements. (I don't know of a commercially available design, but Paprican will talk about this at the AWEB next week.)
5. Air turn back pressure - Float the web over an air bar and measure the back pressure in discrete positions across the web width. (I haven't seen this commercially available in the US, but I expect any air turn manufacturer could put one together, such as MEGTEC. I have seen this marketed in Japan by Bellmatic Ltd.
6. Speed of sound - Measure the speed of a sound wave in the web. First demonstrated by Dr. Richard Lowery at the OSU WHRC. Limited marketing of this device by Ron Markum of Advanced Web Systems.

More. I don't think any of these have completely satisfied the need. Let me know if you want to add to the list.

tjw

Filed under: Tensioning, Web Mechanics --- Tim Walker @ 07:24 AM

This is a question I'm often asked by prospective clients for my services as a web handling specialist.

Have you ever worked with thin webitronium? (Yes, this is a ficticious material.) Have you ever handled nylon film? Have you ever worked with nickel-steel foil? Can you handle my complex laminate?

My answer is "I don't care what you call it, let's talk about the material properties:
elastic modulus (in x, y, and z directions),
thickness,
width,
speed,
roll sizes (core diameter, final diameter)
surface friction or adhesion,
expansion coefficients (thermal or moisture), roughness,
break strength,
yield strain-strength (in x, y, z directions),
conductivity,
etc.

Once these are defined, it doesn't matter what name you want to give a product. Call it Fred if you want to. Once I know this mechanical data, we can apply engineering and physics to solve your problem.

I hope this doesn't sound too theoretical. I'm also quite practical. I like to apply what works. One thing you get from bringing in someone from outside your industry is a solution that may already be proven elsewhere that can solve your problem. In that regard, I'm a honeybee that cross-pollenates technology from one industry to another (of course, the non-proprietary knowledge, only).

I look forward to handling Fred and preventing his wrinkles.

tjw

Filed under: General Business Topics --- Tim Walker @ 07:30 AM

Two news items on the AWEB front.

1. AWEB is drawing a crowd.

A lot of late registrations are rolling in and the attendance for AWEB is now set for over 100. A great draw for our first conference.

2. AWEB-IWEB make peace

I wrote last month how it seems there was some minor contraversy about the IWEB folks at Oklahoma State University's Web Handling Research Center being a little miffed at our plans for another web handling conference (bothered by our similar name or playing a little too close to their sandbox).

I'm glad to say that it looks like any conflict is now well on the way to being resolved.

Dr. Keith Good from OSU called and said he'd like to take us up on the offer to advertise next year's IWEB at AWEB next week and that they would be happy to reciprocate.

This is great. I've been making the case for how AWEB-IWEB is a great 1-2 punch to help the how web handling community. I think both events will prosper by working together.

tjw