Winders, Increased speed: Myth: Chapter 34
Does increasing speed on a slitter winder translate into increased production?Answer: Depends on the application and the product in question.
Billions of dollars are spent by the aeronautical industry on R&D, to increase the speed of its jets. As we approach the speed of sound, a different set of problems arise.
The same is true for the automobile industry. Billions are spent on car design and fuel efficiency. There are sports car on the road that boast 1,000 H.P engines and top speeds in excess of 200 m.p.h. If the legal speed limit on our highways is 55 to 65 m.p.h. all this extra horse power is a total waste. Yet people who have the means still buy these sports car.
Slitter rewinders are not in the same league as jets or cars. Yet increasing speed has become the holy grail of paper mills and converting plants. Let us look at this in some detail, with actual examples.
Counter rolls:
In the last post, I showed how the plant increased production by about 20% without retrofitting or touching the actual slitter rewinder. We will stick with counter rolls in this example.
We take a 12 inch wide counter roll, wound on an inch and half core, with 500 lineal feet per roll. On a 65 inch wide slitter rewinder you can get a maximum of 5 rolls per cycle (5 x 12= 60 inches). The top speed of this winder was about 250 feet per minute. To make calculations easy, it would take about 2 minutes per cycle to wind a 500 foot roll. Wrong!
Imagine that you are in your car waiting for the stop light to turn green. The distance to the next stop light is 500 feet. Assume there is no speed limit. But here is the catch. You must come to a full stop, without crossing the line at the next stop light.
If you start out by burning rubber and hit the pedal to the metal, you will need extra heavy duty brakes and a parachute to come to a full stop. This will be a short, violent
and brutal drive. The strain on the engine, burning tires and other stresses will take a heavy toll on the car.
In a slitting rewinding operation you can't start out like a bat out of hell. The web will break. Before the cycle begins, the web has to be threaded and wound on the core.
The operator has to check to make sure that everything is in perfect alignment. There are no wrinkles: the web path is true: The core is in perfect position: etc. This takes the operator anywhere from 10 -15 seconds or more. The operator also has to load the cores on the rewind shaft and set the shaft in position. This takes another 30-60 seconds. In this example we are using only one operator.
Once the cores are loaded and all inspection checks have been made, the process begins. If we add all the time so far, we have spent anywhere from a minute to two minutes and we haven't even started yet!!!
Soft Start:
The machine has to start out real slow. This slow start allows the operator time to see if everything is going as planned. In addition it allows the web to start in motion and overcome any inertia problems. If something goes wrong at this stage, the operator hits the stop button. This saves a lot of material from being wasted, if the cycle has to be restarted from scratch. Had you started full blast, you would have wasted several hundred feet of paper. The less waste you have the more profitable it is. Obvious.
The old time winders did not have this automatic slow start feature. Several different schemes were used for this "soft start" procedure. This was an art and it took the operators months of trial and error to learn. I am sure you can see the amount of waste during this training period not to mention the drop in production.
Learning this "soft start trick" actually took a lot longer than a few months. Each grade of paper had it's own unique properties. Hence, what worked on one grade of paper did not work on a different grade of paper.
So far we have seen that about 2 minutes have gone by and we haven't even started the operation. We shall continue to explore this in our next post.
Till then, live long and prosper.
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