PFFC - November 2007 - (Page 12) WEB LINES T By Timothy J. Walker Contributing Editor Differential Winding Limits: Part I he purpose of differential winding is Most differential shafts have their torque to apply a desired torque to two or regulated by air pressure, either the air pressure more rolls winding on a single shaft. that pushes laterally on a stack of slipping cores Differential winding allows multiple and locked spacers or that pushes radially from rolls to turn at differing speeds, with each roll an internal bladder out against slipping elements free to slip at the speed required to compensate or the core’s inner diameter. for roll-to-roll diameter variations and strandAs you apply more pressure, the clutching to-strand length variations. mechanism will slip at a higher frictional force For more on why differential winding is and a winding roll will receive more torque. The needed, how it compares to locked-shaft torque applied at the core then is transmitted out winding, and some of their limitations, check through the radius of the winding roll to create out my three columns starting in November tension at the roll’s outer diameter. 2002 at www.pffc-online.com/web_lines. Unfortunately, this is not the only torqueDifferential shafts in all their varied creating mechanism of a differential shaft. designs are a great invention. They are the Gravity creates a force on the differential shaft “sliced bread” of slitter/rewinding operations. from the roll’s weight, creating an increased However, what I’d like to cover in this and torque component as the roll grows. For larger next month’s columns rolls, this may be all is the idea that even a the torque you need, great invention has its and the applied load correct application and from air pressure limitations. Let’s start should be turned off. with what is at the heart An advanced of differential shafts— differential slitter torque. How do they has a roll weight create torque and are you compensation control, getting what you need? but if you don’t tell Most webs run at 0.3– your machine the roll 3.0 lbf/in. of width (a.k.a. diameter, material PLI). On a typical 3-in. density, and inner diameter core, this is nominally a starting roll width, you probably don’t have this torque of 0.6–6.0 in.-lbs of torque. Many small important capability. rolls will wind great at constant torque, allowing Two last factors that add to torque are nips the web tension to drop off inversely with the and inertias. If you use a winding nip roller to roll diameter. With large roll buildups (final prevent air lubrication at high-speed winding, diameter/core diameter > 4), the tension of the nip load also loads the core against the constant torque winding may drop too much, differential shaft, creating an additional torque. making the roll’s outer layers too loose to hold Inertia isn’t much at roll starts, but as a large the roll together. winding roll decelerates, it doesn’t want to slow The primary differential winding torque is down (it’s a flywheel) and creates an additional created by the shaft’s frictional slip clutching torque proportional to your deceleration rate. mechanism. A clutch is any device that Differential torque is the sum of torques engages or disengages a rotating shaft created by: and a driving mechanism. A slip u applied load and slip clutch friction clutch is a clutch in that when u roll weight you engage it, instead of locking u nip load gears together like a car’s clutch, u roll inertia. loads two non-locking surfaces DIFFERENTIAL together. By controlling the load WINDING LIMITS It’s great, just like sliced bread, but between the two slipping surfaces, Part 2 it’s quite confusing to figure out how you control the friction-limited force wide our slices are going to be. that develops when the clutch slips. Even a great invention has its correct application and limitations. 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 | NOVEMBER 2007 WWW.PFFC-ONLINE.COM http://www.pffc-online.com/web_lines http://www.webhandling.com http://WWW.PFFC-ONLINE.COM
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