Sudden Drop in Roll Force by One Load Cell in Two Cell Operation

Contact the suppliers of your equipment, NOW! That's as early as it gets.

An anonymous forum has no help for you.

Again, get technical help from the people who supplied your load cells, and data acquisition and control equipment.

This is a waste of your time. And ours.

CALL NOW!

Thanks for your reply sir.

We have checked the mill housing by removing all the rolls. No abnormality was found. The back up rolls which were in mill during roll spall also replaced.

We could roll with hydraulic pressure. During that time also, the DS roll force from load cell is fluctuating abnormally where as OS is steady . When we stop the roll rotation, the fluctuation is not there and the reading shows very small negative value (-0.06 MN) and steady. In the following photo, see the sudden drop of ABB DS value where as OS remains mostly steady. The behaviour of Oil roll force is steady and the diffrence between OS & DS are normal.

The dropout looks to be highly periodic. Is related to roller rotation speed? Towards the right side of the chart is the machine slowing down? If it is related to speed, is the dropout in-sync with the rate of (a) particular component(s)?

What do you see if you run the mill without the rollers in contact with stock, ie, just running the rollers without their applying force? Does the signal (albeit now at a small value) still fluctuate?

Last time i saw this in a roll mill was when the backup rolls were out of round on one end by .003". Same pattern.

Ron

And I'm betting the signal fluctuated in sync with the backup-roll rotation rate. That's one way to tell which roller is at fault, with the affected cell telling at which end.

What's odd about the OP's situation is that the problem showed up three days after the spalling incident. I'm assuming they replaced the rollers the day of the incident, though the OP did not actually say so. If they had, and replaced with a roller that is out-of-round, why would this problem show up three days later?

"In last month, suddenly during rolling we observed a different type of behavior. The roll force from one side (Drive side) load cell was suddenly dropping by about 1 MN compared to that of other side (Operator side)."

This is what's puzzling. They'd been rolling and suddenly the problem appeared. An out-of-round would be out-of-round the whole time.

Thats how we detected the problem. A dig storage o scope and calculating the circumference of the roll and the rotation rate it came up to the same spot on the backup roll. Nobody was checking the run out on the backup roll in the machining center along the roll length. Re ground the roll with a dial indicator along the length and it was out .003" at one end, after grind run out was <.0005". Re installed the roll and problem solved.

What we saw was the run out was binding the rolls and loading the thrust bearings which took some time to warp and eventually caused failure.

That would certainly explain the delay. Was the failure sudden in your case, or did it gradually appear as the bearing warped? In the OP's case it looks to be a sudden mechanical failure, a crack suddenly developing in the bearing perhaps? Something gave way in the OP's machine. I don't think it's a load-cell problem at all; the load cell is just the messenger. The problem is mechanical.

No, it was not sudden. Took about 5 days to start up. Hydraulic loading would shut off .

I started seeing the hoses doing a dance and then the unit would shut down. Later investigation showed the wrong material being used for the thrust bearings as well as excessive runout on the backup rolls. As usual it was a compound error and not single.

They had one job.

interesting: I was gonna say it's time to replace the bearings in the roller.

I'd say so too, as well as find out why they failed.

Bad bearings and out of round rolls have caused me grief with load cells before.

You may want to check your cable runs also to make sure they aren't damaged. Chafed cables can cause peculiar problems also.

Get your vibration folk involved - they may be able to identify the problem area.

My experience with ABB load cells has been excellent. Kelk load cells are also excellent - they use a different technology but can be form and fit to mount in the same place.

Hello

The cable was physically checked from panel to load cell. It looks fine. Also after powering up the load cell electronics, a system test consists of 64 steps have to do before starting the rolling.During this test it checks the healthiness of both cable,load cell and matching units. This test is getting cleared every time.

also we could not observe any abnormal vibration in the mill housing during full speed rolling.

kRajesh, have you found any of this useful? Especially what ronc said about the problems they had with run-out and bearing failure? Have your guys checked along similar lines?

Thanks for your suggestions.

We got 3 pairs of back up rolls with different chokes and bearings. All the sets shows similar behaviour. The run-out of back up rolls after last grinding measured in our CNC machine was very low for these rolls.

We couldn't notice any abnormal vibration in the mill during rolling. The bearings of the removed rolls are yet to open and check.

I am adding the enlarged version of the graph where the sudden drop in RF is happening. this time the variation in Load cell at OS and the force from Oil are not fluctuating as in the case of DS Load Cell. This behaviour disturbs the rolling. This is not happening in every coil and comes randomly at different instances (passes) of rolling. This behaviour is similar even after changing load cells, back up rolls and pressure transducers.

Are you sure the "screw downs" or actuators are not to blame? Perhaps the load cells are only reporting what is actually happening? Try to sort out the cause and effect.

Just on the face of things, there is something else causing the issue, if I understand correctly that you have replaced the load cells and ABB electronics on driver and driven sides of the mill.

Without seeing a strip showing the harmonics going on, plus the fluctuation being seen, to give a pat answer would not serve you well.

There are many issues outside the load cell that could make it appear to be something it is not.

I would highly recommend you call in a competent Engineer from ABB and get his diagnosis. You are using his equipment, have him give you a reason why you are observing this being outside controlled limits. It could be a floating ground issue, it could be the mill itself moving due to load, many other things.

I take it you are in India, and I know ABB, and they are an honorable company. Let them resolve this issue, and take notes.