- March 01, 2011, By Timothy J. Walker Contributing Editor
I love vacuum pull rollers (VPRs). When it comes to driving a web without slipping and scratching or to knowing you have control of the tension differential from one zone to another, a vacuum pull roller can't be beat. They are especially great when you can't or don't want to touch one side of the web.
I was going to write this month's column about my love for vacuum pull rollers (this could have been my Valentine's Day column to VPRs), but after a quick web search, I realized Pete Eggen of Webex Inc., Neenah, WI, wrote a wonderful pro-VPR column, “Get a Grip on Your Web,” in the November 2009 issue of PFFC.
Pete's column did a great job covering the what, how, and why of VPRs. To read the article, visit http://pffc-online.com/web_handling/tension/paper-grip-on-web-1109.
What is there left to say about VPRs? As with anything you love, if you remove your rose-colored spectacles, you are able to see even your true love has faults. Here's a list of some the limitations and disadvantages of VPRs.
- VPRs cost more | A VPR is a special roller, similar to a heated or chilled roller, but instead of pumping water or oil through the roller, a VPR has an advanced pneumatic system to create a desired internal negative pressure without excessive leakage or flow volume. It's not as sophisticated as a low vacuum processing chamber, but it has its own sealing and pressure control challenges. This adds up to a roller that is much more expensive than a nipped or unnipped pull roller, both in upfront and operating costs.
- VPRs don't like dirt and dust | The extra traction of a VPR comes from suctioning the web down to the roller, but like any vacuum cleaner, a VPR isn't smart enough to know what or what not to suck up. This means debris, dust, and anything airborne or on the web may be sucked onto or into the VPR, clogging up the system, reducing its effective traction, and fouling the internal seals and pneumatics.
- VPRs are noisy | Like any pneumatic system, air flow and fans are noisy. Good designs will reduce noise, but a VPR always will be noisier than a non-vacuum roller.
- VPRs should be a tailored fit | The problems with dust and noise can be reduced by adjusting the VPR's effective vacuum area to match both the web width and roller wrap angle. However, this will add to the complexity and cost of the system, especially if you need to run varied web widths.
- VPR holes may mark your web or coating | Some VPRs are simply perforated cylinders with large holes to expose the web to the negative internal pressure. However, large holes will cause dimples, impressions, or wrinkles in thinner webs or sensitive coatings.
- VPRs may promote wrinkles | First, since VPRs are intended to have high traction and most wrinkles are dependent on good traction, VPRs with even slight misalignment or diameter variations will create wrinkles quickly. Some VPRs avoid large holes instead by using a screen sleeve, exposing the vacuum pressure over a finer, less-likely-to-dimple pattern. However, screen sleeves that are unsecured, except at the roller's ends, are known to balloon outward slightly at high rotation speeds. This turns your VPR into a high traction crowned roller, which is an excellent web wrinkler.
- VPRs won't work in a vacuum | This one shouldn't be a surprise. VPRs develop force by the difference in atmospheric pressure from their fan-induced low internal pressure. If you take away atmospheric pressure, you can't have a pressure differential to hold the web down.
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; firstname.lastname@example.org; www.webhandling.com.