Rotary Cutter application & PLC Law 18.1

[ssommers]

I've got a problem which directly applies PLC Law 18.1. PLC Law #18.1: When the machine is down, the blame will be assigned to whichever component the person assigning the blame understands the least. I have a rotary cutting motor that is controlled by a Reliance GV3000 drive. It's setup in V/Hz mode just like all the other rotary cutters in the plant. In fact, they all had the same size motor & parameters in them until yesterday. Late last week, the #7 tube mill started making the thickest wall tube for the first time since the mill was renovated. The tube needs to be automatically cut into about 1 ft pieces when the tube coiler changes from one basket to another. The operators claimed that the cutter motor was slowing down & not cutting the tube completely causing the mill to jam. The mill was shutdown yesterday on day shift so maintenance could "fix it". I'm the Controls Engineer, but they didn't call me (grrr...) until the electrician had changed out the drive and then the motor and then put in a larger HP motor - went from a 3 HP to 5 HP. Nothing changed except the cutter could go a few more pieces before jamming. I was called in at that point so it could be officially declared a "mechanical problem". The Mech Eng went through a series of adjustments and replaced each mechanical part - cutting blade, gear box, bearings, etc. - and it still jammed after about 3-5 cuts. Then I went into the drive & played with parameters yesterday to get more torque during cutting. It gave me about 2 more cuts for a total of 5-7 cuts max. Production "claims" that the lower HP motor used to cut the thicker wall tube "all the time with no problems" before the upgrade. The cutter motor & drive were not changed as part of the upgrade. I know I've got enough HP to do the job, because the drive isn't faulting out. It keeps going until the motor physically stalls. The operator & production supervisor are still blaming the drive & motor. What else can I look at? Thanks in advance, Susan ps. They changed back to a thinner wall tube (which the cutter cuts fine) so I no longer have a chance to try anything until the mill changes to a different size tube.

[forqnc]

Do you know if productions "claim" is correct, if so what was changed in the upgrade/renovation including anything mechanical. If it worked before I would be looking at the changed items.

[TimWilborne]

Are you sure the motor is stalling? Did you physically see the MOTOR not turning?

[ssommers]

Yep, I'm sure the motor is stalling. The cutter stalls, the speed goes to 0, the amps shown on the drive go up almost to max. The motor pushes so hard that it will sometimes slip the crimped tube past the cutting point and bring the cutting blade around for another try. The problem is that by the time that happens, the operator has to e-stop the machine to keep the rest of the machine from crashing. Oh, and I can now play with the cutter all I want. They messed up a gear box in another part of the mill due to someone installing it without putting oil in first... At least they didn't blame it on an electrical problem this time!

[TimWilborne]

Well I'm not trying to play the blame game but if something works for a while then quits it is usually a good sign something is heating up and beginning to seize. Let it cool off for a while and the expansion from heat goes away, then it will work again until it heats back up. Now I haven't seen what you got so I don't know if it is possible, but can you feel of everything and see if something is heating up more than the rest? Well besides the cutter itself. A temperature gun may help.

[ssommers]

Found the problem... They've never actually automatically cut tube with walls this thick before! This info came from the old operator who was moved to another part of the plant. They used 2 operators to do this - one to cut the tube with a set of bolt cutters & then move swiftly out of the way and the other to guide the new tube end into the empty basket. The Mechanical Engineer calculated the required forces and it will take a lot heavier motor and/or a new cutter design to automatically cut the tube. We're lucky that it cuts 1 or 2 times with the current setup! It will have to be re-engineered because management wants to eliminate the second person requirement. I believe that Law #18.2 applies here... PLC Law #18.2: American Money says "In God we Trust" - with PLC's all others bring relevant data. Thanks, Susan