Reflections on Deflection
- Published: December 01, 2010, By By Timothy J. Walker Contributing Editor
After reading last month's column, you should have resolved all of your deflecting roller problems. However, the alert web handler is not done with deflection diagnosis yet.
A winding roll is much like a roller: cylindrical and rotating. It's amazing that 3-in. dia rollers are rare in processes wider than 60 in., but the 3-in. (75-mm) core remains quite popular by customers and hence must be accommodated by winders. These winding shafts with a length-to-diameter ratio above 20:1 are deflecting, whipping noodles ready to destroy your profits.
Any spinning shaft has a critical velocity. Most converters won't run into these conditions, but combining deflection with high rpms will wobble a core or shaft into its own destruction.
Not all 3-in. shafts are prone to the same problem. The core, the journal supports, the winding layers of the roll, and well-placed nip rollers or support arms can alleviate the woes of deflection.
A shaft will have less deflection when winding a full width product. In these cases, the shaft often will be reinforced by the pressure and structure of the winding layers. Slit rolls will not create this same benefit since their independent stucture will have a negligible reinforcing effect.
How a shaft is captured or mounted into the winder will affect deflection. If the shaft is simply supported (held loosely from each end), the mounting will not help to reduce deflection. If the shaft is rigidly mounted (holding the ends level), deflection will be cut in half. Shafts that extend beyond the supporting mounts have surprising benefits from the extra length, even more so if the extended lengths have dual support points at each end.
Differential winding shafts have the most deflection per load and will see the most problems when winding narrow and large diameter rolls. As narrow slit rolls grow, the combination of roll weight, tension, and nip loads will tilt the out cuts inward, leading to weave or full roll collapse.
A solid steel shaft will have the lowest deflection, but it also will weigh you down (2 lb/in.). Many shafts are hollow, using air pressure and moving elements to grab the core. Obviously, reducing the cross sectional area will lead to more deflection.
Core-independent differential winding shafts will be worse yet, having a series slipping core-grabbing element mounted on something less than a 3-in. dia (often 2.5 in.). Advanced shaft designs replace missing steel or aluminum structures with tensioned carbon fibers, making a fine (but more expensive) substitute for steel.
In slitting, shear knife systems are the most sensitive to deflection. Shear slitting is best with minimal overlap between top and bottom knives; however, the force of slitting fracture will deflect an undersized knife shaft, leaving the poor options of jumping knives or excessive overlap.
Both differential and shear knife shaft deflection problems often are resolved easily with a single central support arm that can reduce deflection by eight times. In winding, a strategically placed winding nip roller will serve the same anti-deflection function.
Nipping from above or horizontally will only aggravate or rotate deflections; however, nipping from below with a large diameter roller is a great preventer of winding deflections. The paper industry has done this for years.
Anti-deflection nips are much less common in film winding, relying on their higher internal wound roll pressure to stiffen the winding roll, but I think even film roll quality improves with anti-deflection nips.
As with rollers, once deflection is diagnosed, either structural or strategic solutions will do the trick. If you haven't tired of deflection problems at this point, look next to the rigidity of how your equipment is connected to the Earth.
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.
