Web Lines | The Case for Shorter (and Longer) Acceleration Times
- Published: February 05, 2014, By Timothy J. Walker
I recently visited a converting operation where the slitter/rewinder was set with an acceleration time of 180 sec (3 min) to reach 1,000 fpm (300 mpm). I recommended changing to a faster acceleration time for several reasons.
The case for faster acceleration
- Many thin products, including the one that was running on this slitter, wrinkle much more at low speed. At higher speeds, air lubrication reduces the web-to-roller traction, reducing the ability of a roller to shift or gather the web or hold the web in a buckled, wrinkled shape.
- I was surprised to see the acceleration set in time (seconds). In my experience, most converting machines have acceleration set in units of velocity per time (mpm/s or fpm/s), not simply time. For example, limiting acceleration to 10 mpm/s would reach 100 mpm in 10 sec, 300 mpm in 30 sec, and 800 mpm in 80 sec. Setting an acceleration time creates a slower acceleration rate for slowing speed set points. I would prefer to see a single acceleration speed independent of target speed.
- This particular slitter also was set up to have a two stage acceleration, first accelerating to and running at a low speed, then when the slitting winding process looks good, accelerating to the higher target speed. I can understand this approach for developing a set of winding conditions, when you might see, in the first few wraps, that wrinkling or shifting layers is a problem and want to stop, but the ultimate goal for slitter winding should be a rapid acceleration to line speed without an intermediate speed. The extra time at low speed and inertia upsets of speed changes will create undesired tension variations and roll defects.
- The target accelerations rate should be much faster than 300 mpm/180 sec. Many slitter/rewinders are designed to accelerate at 100 fpm/s (30 mpm/s)—18x faster than this slitter was set to run reaching 1,000 fpm in 180 sec. I recommend at least 30 fpm/s (10 mpm/s) or reducing acceleration time to 30 sec to reach 1,000 fpm (300 mpm). Ultimately, you may be able to reach 1,000 fpm (300 mpm) in 10 sec, greatly reducing the time and waste at low speeds.
The case against rapid acceleration
- Many advocates of rapid acceleration do so in the name of machine output. Obviously, any time spent at lower speed does not process as much material as higher speeds. In the case of extremely short slit rolls on fully automated slitter/rewinders, then there is a strong correlation between accelerations times and output. However, for slitters running long roll lengths, there is extremely small output benefit of reducing the acceleration times.
- A machine that runs larger diameter heavy rolls and does not have inertia compensation (such as many torque controlled unwinds and rewinds) will see dramatic tension variations during rapid acceleration.
- The added torque of rapid acceleration can put unnecessary loads on coupling, driven shaft, belts, chucks, shafts, and other components, leading to early failure and associated downtime and costs. Also, idling and driven rollers are more likely to slip during rapid acceleration, leading to roller wear or product abrasion (generating surface contamination and scratching).
- Rapid acceleration will always be a challenge to closed-loop tension control systems, creating tension transients high or low, especially without optimizing the PID or other control algorithm.
- Tension and speed transients during acceleration are commonly associated with registration losses in printing and die-cutting and coating variations. Wrinkling also can occur in the changes from high to low tension as the Poisson’s effect alters the web width from narrow to wide.
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.
