Slitters: Increased speed: Myth. Chapter 35
Increasing the slitter rewinder's speed:...We will continue our discussion in this post. Plant managers and converting plant owners make the common mistake of equating speed with increased production. In the last example we had a roll with 500 lineal feet of paper. If your slitter rewinder has a top speed of 500 feet a minute, that doesn't mean you can wind this roll in one minute.
Imagine yourself sitting in a sports car with a top speed of 200 m.p.h. You are at a stop light waiting for the light to turn green. The next stop light is 500 feet away.
The sensible thing to do would be to start out real slow and then speed up. Once you are at a safe distance from the next stop light you start slowing down and then come to a full stop at the stop light.
Slitter rewinders have to be operated in the same way. The machine starts out real slow, picks up speed and then about a 100 feet from the end it ( the winder) starts to slow down. It comes to a dead stop at precicely 500 feet, because that is what the customer wants...a roll with 500 feet of paper on it.
In technical terms here is how it goes..... Machine has a "soft start" function built into the motor controls. The soft start assures that the machine cycle starts out smoothly without any sudden jerks. Any sudden jerking might break the web or introduce wrinkles or soft spots in the web. In addition this allows the operator to make sure that everthing is starting out as planned. In case of a problem, the machine can be shut down without too much waste.
Once the machine starts in the soft start mode, it takes a couple of seconds and then goes into the "acceleration" mode. Speed starts to pick up and then reaches the "ramp speed". Ramp speed is the optimum speed at which the winder can run, based on the stock being run and the actual lineal footage in question. Please note, "ramp speed" is not the "top speed". Ramp speed is simply the best speed for the job at hand.
Once the ramp speed is achieved, the "deacceleration mode" kicks in. This is the point at which the motor starts to slow down, smoothly. Just like the car driver starts to "slow down" before the next stop light. This slowing down is the "deacceleration mode". A good driver will do this without any jerking or "hitches". Controlled deacceleration plays a vital role in the quality of the finished product.
Let us look at this in slow motion.
1) Machine starts out real slow ( soft start). Paper starts to wind around the core. This cycle takes about 5-8 seconds. This can always vary depending on operator expertise.
2) Machine starts to accelerate slowly and then reaches ramp speed. The acceleration cycle may take another 3-8 seconds.
3) Machine reaches ramp speed. It continues to run at this speed till the signal to slow down is given.
4) Machine goes into the deacceleration mode. This takes about 5-10 seconds, depending on the ramp speed.
5)Machine comes to a full stop. The brake module had been activated at the deacceleration mode and now it applies the final brake.
We will examine each aspect of this cycle as a function of time and actual lineal footage wound.
This is Dr. Winder saying bye bye. Live long and prosper.
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