My talk today is about the time I spent a day on top of a full-web press.
When I owned an R&D and manufacturing company, I was asked to build a tension control for chill-rolls on Harris web presses. My solution was radical.
Transcripts
# Chill Roll Control
Thank you for selecting my podcast today. My talk today is about the time I spent a day on top of a full-web press.
When I owned an R&D and manufacturing company, I was asked to build a tension control for chill-rolls on Harris web presses.
Here’s some background: A web press uses large rolls of blank paper, each weighing perhaps as much as a ton. As the paper is unspooled, it travels across many rollers. These rollers are cylinders of either steel or synthetic rubber, and they are as wide as the printing press and are used to keep the paper accurately tracking through the entire process of printing, drying, chilling, and sheeting.
The fundamental challenge for a high-speed web press is to take low moisture paper, lightly stretch it as it comes off the roll of paper, and guide it through a series of individual presses. Each press applies a single color, and the ink causes the paper to deform slightly. Too much tension or too little, and you end up with an expensive mess.
One of the challenges is that the paper, itself, does not belong to the printing company. The customer supplies an exact amount. If too much paper is wasted, then the printing company is in a bind.
Most web presses are four-color presses. The four colors are Cyan, Magenta, Yellow, and Black, commonly known as CMYK. One-color is used for each press. This approach enables the web press to print “full color” images. The press I’m talking about today printed Reader’s Digest, Sports Illustrated, and other full-color magazines.
Some web presses have additional presses to do such things as making the paper glossy by applying a coating.
As the paper passes through each press, the paper gains moisture and chemicals, which make the paper slightly deform. Air turbulence is also a problem since the presses run at approximately 400 feet per minute to 500 feet per minute (which is about fifty thousand 8.5 by 11-inch sheets per hour).
When the paper passes through the last press, it goes into a drier unit. The drier looks and works like a pizza oven. The drier dries the ink and evaporates the moister. However, the ink still isn’t stable and is easily smeared. So, when the paper leaves the drier, it goes through a set of chill rolls, and this is where I come in.
Chill rolls are large cylinders with water pumped through them to keep them ‘cold.’ The intent is to remove as much elasticity from the paper as possible and permanently ‘set’ the ink.
The paper is usually fed to a circular ‘sheeter,’ which is a rotating cylinder with metal shear protruding from it. There are different designs for sheeters. Some use a guillotine approach which requires the metal shears to be set at an angle since the paper is moving as it is cut. It’s mesmerizing to watch the rotating sheeter cut the wide paper and carefully land it on a stacker that gathers the sheeting together for the magazine.
So, the tension of the paper between the chill rolls and the sheeter is critical to producing sheets that are cut on the trim lines and will be cut so the sheets can go through the binders and be formed into a magazine. That tension is difficult to manage because the paper constantly varies in weight – how much ink – and elasticity because of moister and skew because of different tensions on the paper from one edge to the other edge.
The traditional way to control chill roll was based on analog signals. The approach was to install a roller from one side of the press to the other side. The roller was fitted with tension sensors attached to each end of the roll. These sensors would send an analog signal to an A/D converter and then a PID controller, directly controlling the chill roll motor.
This approach seems reasonable, but tension gauges are affected by temperature and constant stress. The analog signals are very low voltages, and temperature variations affect the accuracy of the tension signals. And, of course, the momentum of large chill rolls filled with water cause a lag time between when the speed control received a correction signal and when the chill roll slowed or sped up. These tension adjustments were always small, so the tension sensors operated within a tiny segment of its tension sensor’s designed range. These control challenges were exacerbated because the temperature of the drier is regularly changed.
Because of these problems, I took a very different approach. The way I solved this problem was to install a harmonic gearbox on the chill roll motor. Instead of using tension sensors, I installed optical rotary encoders on the chill and pincher rolls.
I calibrated the chill roll’s rotation angle to the pincher rolls’ rotation angle when the paper was at zero tension – not sagging or tight. Then when the web press ran, my single-board computer read both encoders, calculated the angular difference, which was due to too much or too little tension. That differential controlled a stepper motor connected to the harmonic gearbox, thereby adjusting the angular ratio between the chill roll and pincher roll to achieve zero tension. Greater or lesser tension could be dialed in.
The solution worked like a dream. The encoders didn’t suffer from low voltage signals or the need for regularly calibrating the tension sensors.
The voltage signal variations due to temperature changes did not affect the encoders since the encoders were digital. Also, the chill roll speed didn’t suffer from over-shoot or lag in speed adjustments. The speed inaccuracy was eliminated because the harmonic gearbox adjusted the gear ratio while isolating the ‘strength’ of the electric motor from speed changes.
The downside was that the chill roll control was installed at the highest point of the web press and perpendicular to the drier’s end. So, while I was perched on the small platform, it felt like being inside of a pizza oven. During the chill roll control installation, I spent about 12 hours up there baking like a thick crust, Chicago-style pizza.
Due to the contortion of my body to gain access to the electrical enclosure, when I finished, I couldn’t walk down the ladder. It took about ten minutes to move my body correctly, but it was sweet when it all came together.
By the way, I only did the electronics and programming. A separate company did all of the mechanical engineerings. They also built and installed the sheeter. The system worked so well that a business sprung up replacing the OEM Harris chill roll controls with mine.
It was a fun project and injected some needed funds into my company.
Anyway, thanks for listening. Bye
Photo by Bank Phrom on Unsplash
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