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May 16, 2009
News - Enginursday: Troubleshoot…
about 8 months ago
I thought you were going to say that it faulted when someone walked across the metal box that did the flipping.
Once I was sent out on a service call for a large steel slitting machine, similar to the paper machine you described above. It would unroll thin gauge steel, slit it into strips of various widths, then roll it back up. So the layout of a typical machine is that there is a station that the roll is placed on to be unwound. It has a brake to put a little back pressure on the steel. Then there is the slitting arbors, they are driven with a large industrial motor. The ones I worked on were typically DC motors. The steel then went down into a looping pit, then through a tension station, then on the the rewind, (also DC).The looping pit is required because believe it or not, coils of thin steel that are rolled up do not have the same thickness all the way across the roll. Before it is slit into strips, this isn’t a problem. Once they are slit, the small differences in thickness cause the rolls that are thicker to make a larger diameter roll, causing those to get tight, and the thinner ones to get loose. The looping pit takes up the difference in the diameter of the rewound rolls. But, if the coil is very thin and long, the loops can get so different in lengths that the ones are tight, and the others are scraping the floor of the pit. If that happens, they have to stop, cut some off to even them up, and then go again. Google “Steel Slitter” for pictures.
This particular machine had an extra section. It was called an infeed. It was powered, and it fed the material into a loop ahead of the slitter head. I think this was for thin material. Since it is before the slitter head, the steel is all still in one piece. There was an ultrasonic sensor aimed down at this primary pit to adjust the speed of the infeed section to keep a stable loop depth. Ultrasonics don’t work on the loop after the slitter head, too many strips to watch.
I was told the Ultrasonic sensor was messed up. I was also told I was the third technician to come and try to fix it. While the line was running, this loop would randomly come up to tightline, which leaves a mark on the material when the slitter heads slip on the steel. So I set myself to watch the machine and see it happen.
I noticed that when the operator saw that the loop after the slitter head got too deep, he would adjust the speed of the slitter down so that the slit set of loops could get shallower. Slowing down the slitter makes the primary loop get deeper, so the ultrasonic sensor would see the deeper loop, and slow down the infeed to make that primary loop get a little shallower. At that point I noticed that an extra loop would form in the material between the unwinder and the infeed. Mind you, the unwinder has a brake to keep tension between it and the infeed. This caused the unwind roll to actually stop, because it was no longer being pulled. Then the primary loop would get too shallow, and the ultrasonic sensor would command the infeed to speed up. So it sped up, taking up the slack that it had created in the extra unintentional loop between the uncoiler and the infeed. When it got tight again, it would “snap”, as the tons of steel sitting on the uncoiler has a lot of inertia, and does not start rolling instantly. When it “snapped” however, the infeed would lose grip on the steel, (it’s oily). Since it lost grip, the loop primary loop would continue to get shallower, and the ultrasonic would command more speed from the infeed to try and get the loop deeper. But after traction was lost, it was all over. It was like car tires on ice. The rolls would just start slipping faster and faster, until the primary loop tightlined and marked that material.
So the ultrasonic sensor was fine, it was the fact that there was not enough brake tension on the unwinder to prevent it from coasting forward when the line slowed down. Easy fix. I showed the operator and the maintenance personnel the problem, and told them to crank up the brake on the unwinder, (a normal operator control). I was told they couldn’t do that because then the infeed would not be able to pull the material off the unwinder. It seemed that the rubber rolls on the infeed were worn out, and did not have enough traction.
I then said, “My work is done, you either need new rolls on the infeed, or get them resurfaced”.
Two other technicians had each spent an entire day there, on separate occasions, attempting to speed match the infeed and calibrate the ultrasonic sensor. They even turned up the amount of influence the ultrasonic sensor had on the infeed speed, effectively making the problem worse.
Moral of the story? Sometimes you just have to sit back and look at the problem from the wider perspective to see the real problem.
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