- October 01, 2008, By Timothy J. Walker Contributing Editor
Faster, faster, faster. As you reach the designed capacity of your existing process, before buying new equipment, most converters will ask if they simply can increase speed to get more capacity and profitability. In an effort to increase profits everywhere, here is a checklist of what to consider as you think about moving to higher process speeds.
- How fast can you drive a roller or roll? Theoretically, there is no limit to how fast you can drive a roller or roll. (Don't you brace yourself anytime a point starts with the word “theoretically”?) Motor speeds typically are maxed out at 1,750 or 2,000 rpm, but line speed is rpm times roll or roller circumference and can be magnified with gearboxes or pulley-belt systems to get as much surface speed as you want.
- As you move to extreme rpms, you will need to bulk up your bearings, couplings, rollers, and other components to handle the speed increase. Just as an automobile can be designed to set land-speed records, a web line can be beefed up to meet just about any challenge. However, you likely already have a web line, one that was designed for a given maximum speed. Simply changing the gear ratio doesn't mean your line will function properly at high speeds any more than turning your '65 Chevy Impala into a hot rod will survive your drag racing dreams.
- In the paper industry, where speeds of 5,000, even 12,000 fpm are not unusual, critical speeds of all rotating elements must be addressed. Most web handling processes never will need to consider the critical speed of a roller, winding shaft, or core, but if you do, balance and deflection play strongly into what speed a rotating element will begin to whip and uncontrollably vibrate or buckle. You don't ever want to see your rollers or cores beyond their critical speed because they will actually explode.
So far, we've addressed equipment issues; what about the web?
- Air drag may create too much tension drop. Baggy webs may begin to flutter and tear apart like a flag in a hurricane. Entrained air pulled along by the moving web, roller, or winding roll will grow beyond the web or roller roughness. Too much air lubricates web-roller contact, reducing the traction to drive the web and idler rollers or hold the winding roll together. Too much air entrained in the winding roll will bleed out slowly over roll storage time and turn a tightly wound roll into a loose structure impossible to transport or unwind.
- Beyond the web-only issues, the more likely limit to increasing speed will be your processes. Your extrusion or coating system is limited in pound or volume per hour. Your drying or curing process requires time, so your process doesn't have enough length-to-speed ratio to get the job done. Heating, cooling, and moisturizing all may be time or length-to-speed limited.
- Web handling systems and components also may be speed averse. Web guides will have increasing following error as lateral offset occurs over a short time. Rubber-covered rollers generate heat due to hysteresis and need convective cooling, so high speed turns rubber coverings into goo. Clutches, brakes, and differential winding shafts generate heat proportional to the differential speed. At high speeds, these components won't have enough convective cooling, leading to thermal expansion and failure.
- The last consideration is operational issues. With increasing speed, unwinding rolls run out faster; winder roll transfers happen more frequently; start-up and winder indexing waste will increase; quality sampling inspection will see a smaller percent of your product; and 100% inspection systems will have to work harder.
Beware of the plan to increase speed, because as the old joke says: We're losing money, but we'll make up for in it volume.
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.