- September 01, 2008, By Timothy J. Walker Contributing Editor
A good accumulator should be like money in the bank — easy to deposit and readily available to withdraw when you need it. A web accumulator is like a bank, a safe place to keep extra web in a buffer between two sections of a web line running temporarily at differing speeds.
Most accumulators are found at either end of a web line, buffering between the main constant speed process, such as coating or printing, and the stop-start processes of splicing at the winders. For intermediate stop-start processes, such as non-rotary stamping or heat sealing, you will need two accumulators — one each to buffer the input and output speed differentials. Accumulators also can be used just for threading up a long web path tank or oven process.
Accumulators store anywhere from a few inches to hundreds of feet of web by moving one or more rollers relative to other fixed position rollers, increasing or decreasing the length of the web path between points A and B. A dancer roller system is a small web accumulator designed to collect and dispense web tension control delays. While a dancer system usually moves just one roller, most accumulators will move two, five, or tens of rollers to create the required path length differential.
To calculate the required storage capacity of your accumulator, multiply line speed times the sum of deceleration (or acceleration) time and the required time at zero speed. If you need to stop an unwinder running at 100 fpm with a deceleration time of six seconds and require 30 seconds to make a splice, then you need an accumulator with 60 feet of storage (100 fpm × 0.6 minutes). This accumulation could be met with a six-roller accumulator in which each roll translates five feet, accumulating and dispensing ten feet of web each.
If you increase your line speed, your accumulator size must increase proportionally. When the calculated accumulator size becomes unreasonable, you have two options:
- Reduce the accumulation times further with faster accel/decel rates and shorter zero-speed processes (such as automatic splicing);
- Avoid accumulation altogether and upgrade to an at-speed splicing system or rotary process.
Regarding accumulator design options, most favor vertical over horizontal motion to avoid problems of catenary sag. Linear motion is more common than pivoting, though this leads to many problems of misalignment, wander, and wrinkling during their translation.
Try to include mechanical design features that will hold a tight alignment during translation. For dispensing buffers, wait until just prior to the dispensing need before filling the buffer to minimize time of running with the long accumulated web spans.
Some accumulators have an ease of threading feature in which the bottom rollers are able to rise above (and between) the top rollers. This allows a simple manual threading, using the accumulator's motion to create the complicated serpentine web path.
Controlwise, most accumulators are simply multiroller dancer systems, loading the web with the combined forces of gravity and pneumatic loads with position feedback to close the tension loop. More sophisticated systems drive the accumulator position, using feed-forward to position the accumulator relative to input and output process speeds, and using a dancer or tension roller to close the tension loop.
Accumulators with a large number of rollers may see scratching or width-variation-induced wrinkling during the tension swing from drag and inertia. Both low inertia rollers and reducing line speed just prior to splicing will help.
The correctly designed accumulator should be like a bank in another aspect: Both should give back your deposit with at least equal the value from when you put it in.
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; email@example.com; www.webhandling.com.