PFFC - June 2008 - (Page 12)

WEB LINES The Spin on Idler Roller Testing I By Timothy J. Walker Contributing Editor t’s an election year, so I’d like to join the poSpin-down testing is more complicated, but it litical pundits and offer some spin of my own, will give you a sense of your safety factor for slipbut my spin will be free of political opinions. ping idlers. The spin-down test tools are simple: a Instead, I’ll be giving you my spin on freestopwatch, a tachometer, and some way to accelerspinning rollers. ate the roller (I recommend a rubber wheel on a Not every converter needs to consider improving cordless screwdriver). idler roller performance. If you have a high-tension process, giving up a few pounds to your idler rollers is okay. If you have a scratch-insensitive product, If the roller a few rollers that fail to spin may not affect your yields. But not all converters are so lucky. If you have a low-tension process, you can’t afford to give up much tension to your idler rollers. If you have rollers where the available traction is marginal, such as extra-low tension, low wrap That’s the same angle, low web-to-roller COF, or a reduced traction coefficient due to air lubrication, then thinking the captain keep reading. Before we can talk about improving idler rollers, of the we need to measure their performance. There are three tests I recommend: break-away drag, had just before that at-speed, and spin-down testing. Break-away testing is the simplest test and someunsuspected iceberg. thing that comes naturally, like kicking the tires of a new car. With the web out of the machine, reach out and spin an idler roller, noting the ease or difficulty in getting the roller started. If you want to The spin-down test is a balance of the roller’s quantify this value, use a small spring scale and inertia, wanting to keep the roller spinning and the measure the surface force required to move the bearing drag trying to decelerate the roller. The roller at creep speed. spin-down test itself is simple: 1) Drive the roller to At-speed testing answers the question, “Are a given speed; 2) measure the roller speed; 3) note your idler rollers turning at line speed?” You can the speed and start the stop watch; and 4) measure use a contact or noncontact tachometer. Using a the time until the roller stops. Long times indicate noncontact tachometer eliminates the concern the free-turning rollers (or high inertia). measurement load will slow the roller down, but it You can use spin-down times to compare a set will take more work on your part, requiring you to of nearly identical rollers, but the real value of this attach a small piece of reflective tape to the side of test is a few calculations away. Calculate the roller’s your roller. You also will need to know the roller’s rotational inertia from function of diameter, wall diameter, so you can convert rpms to speed in feet thickness, width, and density. Calculate deceleraor meters per minute. tion in radians per second-squared from the spinMany people feel at-speed testing—compardown speed and time. Multiply inertia by deceleraing the speed of your idler rollers to speed tion, and you get bearing torque. Divide by of your web or to a non-slipping roller radius to get surface force of the bearing such as a nipped or vacuum-assisted drag. Compare this surface force to driven roller —is the acid test. If the the available traction (COF, tension roller isn’t slipping, what could be force, and wrap angle), and you will wrong? That’s the same thinking know quickly if there are any idler the captain of the Titanic had just bearing icebergs in your process. IDLER ROLLER before that unsuspected iceberg. In July we will cover the key facBEARINGS At-speed tests will measure failure tors in idler roller bearings that will but won’t tell you if you are living on reduce bearing drag without sacrificthe edge. ing bearing life. isn’t slipping, what could be wrong? Titanic COMING NEXT MONTH Web handling expert Tim Walker, president of TJWalker+Assoc., has 20+ years of experience in web processes, education, development, and production problem solving. Contact him at 651-686-5400; tjwalker@tjwa.com; www.webhandling.com. 12 | JUNE 2008 WWW.PFFC-ONLINE.COM http://www.webhandling.com http://WWW.PFFC-ONLINE.COM

Table of Contents Feed for the Digital Edition of PFFC - June 2008

PFFC - June 2008 Contents First Glance Calendar of Events From the Editor Web Lines Process Management News Clips Narrow Web & Label Reporter Narrow Web & Label What’s New Products Carton & Box Reporter Carton & Box What’s New Products Paperboard Packaging Cartons/Boxes Cover: New Materials Offset Printing TLMI 75th Diamond Anniversary What’s New Products Services Directory Classified Marketplace Equipment Buyers & Sellers Advertisers Index Experience Speaks

PFFC - June 2008

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.