Digital Magazine

Force Is Needed to Shift a Web

When your web isn't where you want it to be, you have two choices: You can be happy where it is, or you can try to move it to where you want it. If you decide you want to move it, then you should understand that this doesn't happen for free.

Why do we want to bend a web? The most obvious need is in web guiding. Though you may be surprised to find most web guides don't need to bend a web, steering guides do need to bend the web reliably to put it in the right position.

Why would you not want to bend a web? In most cases, we want our web running on machine centerline, so we mostly don't want to bend a web. We also don't like one of the web's responses to bending — wrinkling. Since webs don't like to bend, when they do, the applied bending forces generate internal shear and compressive stresses that may lead to wrinkling.

Do we want sufficient forces applied to the web to bend them? It seems that we do and we don't. We like good steering, but we don't want wrinkling. My philosophy is mostly that good traction — the source of the force for web bending — is a good thing.

What you don't want is a web that is unpredictable. Insufficient web-roller traction leads to a web that may be in transition from stick to slip, creating a wandering web.

If you want to bend a beam, you have to push on it. If you aren't strong enough or your beam is stiff, you won't be able to bend it much.

If you want to bend a web, your rollers will need to push on it. That push is delivered by web-to-roller traction. If you don't have enough traction or your web and geometry are stiff, you won't be able to bend your web as far as you would like to bend it.

If you try to bend a beam with a rectangular cross-section, the physics of bending are straightforward. The lateral force required to bend a beam goes up with the stiffness, which is a function of the elastic modulus and cross-sectional geometry and how much you want to bend it.

The force to bend a beam also will go up inversely with how long the beam is, making longer beams much easier to bend. For a rectangular cross-section, it is also much easier to bend something along the smaller dimension (webs are much easier to bend around a roller than laterally or widthwise).

In webs, since webs are beams, all this is still true. A higher modulus web takes more force to move. A thicker web takes more force to move. Moving a web more takes more force. But the biggest effect in how hard a web is to move is the width-to-length ratio. The force to move a web increases by the width-to-length ratio to the third power.

If you double a web's width, the force to move it increases eight times. If you double the length of web you are trying to bend, the force to bend it drops eight times.

  • Example

    Take a 12-in. (150-mm)-wide web of paper or polyester (modulus of 500 kpsi/3.4 GPa) 2 mils (50 microns) thick at 1 lb/in. (175 N/m) tension. Use a 90-deg wrapped roller with a friction coefficient of 0.25.

    If you try to bend this web just 40 mils (1 mm) in a 20-in. (0.5-m) span, no problem. But try to bend this same web ⅛ in. (3 mm), and you won't have enough web-roller friction to do it. The web will bend as far as the friction available allows (just shy of 0.1 in. or 2.5 mm).

    If you increase the width fivefold (to 60 in. or 1.5 m), the force to bend will go up 125 times, but web-roller friction will go up only five times. You won't get the web to move very far (about 4 mils or 0.1 mm). If you want to bend a wide web, either go long (spans) or forget stable steering.

Web handling expert Tim Walker, president of TJWalker+Assoc., has 25 years of experience in web processes, education, development, and production problem solving. Contact him at 651-686-5400; This email address is being protected from spambots. You need JavaScript enabled to view it.; www.webhandling.com.

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