Hi, After a long absense I found this board again. Great to see it still going strong and many of the same names of 10? yrs ago. I left the VA dept at our mill about then but am now back as reliability engineer looking after that dept which has re-kindled interest in VA. (not that I ever lost it!)
Has anyone used Dynamic absorbers on paper machine calender stacks? I am thinking of giving it a go. Our stack barrs at 90-100hz or 10x our intermediate CC roll (beloit double shoe type) after about 2 months with a newly ground roll. We are exploring why this is happening i.e. paper corregations from earlier in process, hard/soft spots in the CC roll chill, not grinding deep enough etc. We have played with offsets to avoid common path lengths etc. Barring has started since speeding up machine from 1050-1150m/min so could be resonence related hense q about absorbers. Also if anyone has any other solutions to calender stack barring that would be great!
Thanks in advance.

I put 1T on top of a stack that actually was running above and close to structural resonance, the same as the old with cast iron frame did but closer. Shape was such that there was a maximum half way up the stack and on the top so placing as much weight on top as could be put there did give some more space speedwise from the peak and lowered vibration levels. Problems was with the rollers in the middle in this case. Make run up/down test and see if you get resonance indications, knock test with empty tambour with some paper left and the crane and 2 guys with ropes is also a lot of fun. Heaviest thing I ever hit anything with. Also used small VFD controlled motor with unbalance bolted to the frame to try to find where the peak was and some crude ODS using tacho from motor (this when machine is not operating). Not exactly dynamic absorber but rather de tuning. Good luck. Olov

I have done some ODS tests on this type of structure before also. Normally found the structual weakness straight away.

Best Regards,
Tom Murphy

We are using absorbers. They worked very well for about 5 years and now they don't work so well. We are trying a new procedure from Kuester's that shows promise.

Your roll people may ask them about it. I would give details but apparently it is proprietary information. I will say it did drop the barring amplitudes about 75%. Ours were in the same range as yours.

Aubrey

boilrmakri(at)gmail(dot)com

This message has been edited. Last edited by: Aubrey,

Planty,
We also have barring on our calendar stack. We run telephone directory paper. Our stack seems to bar in the 86-90 hz range. We had a firm come in to analyze and as close as they could figure out, the fan pump was also pulsing in that range. They seem to think it was coming from their or the head box. We still haven't solved the problem. just another thought.

We had a barring problem last year. There were bars visible in the sheet roll paper at the drum reel. The bars were spaced at every 4 inches. There was a frequency of 92 hz. which came from the second press roll in the wet end of the machine.
This press roll turned at 1840 fpm.

1840 fpm = 368 inches per second.

368 ips/92 Hz = 4 inches.

We concluded that the cause of the problem was that the granite roll had remained loaded in contact on the press roll a long time without having been retracted (operation error) crushing the felt between the 2 rolls during a stop of operation of the paper machine. There was this first fold on the felt which created other folds all along the felt at every 4 inches with each time the granite roll bounced on the press roll after the machine was restart. They changed this felt and all returned to normal condition.

This message has been edited. Last edited by: Alec,

Thanks for all your help. After reading Jims reply I just had another look at some recent headbox pressure testing we do where we hook up a pressure transmitter to an FFT analyser. The fmax was set to 50Hz but the waveworm data was in the report and contained a higher freq. When I counted the peaks what do you know, 100Hz the same as our barring freq. Not 100% convinced yet but its got to be a contender. Next step is to find the source of the 100Hz, possibly fan pump same as Jim.
Alec, We had an issue with second press roll wear last yr. Ours were beloit CC rolls with a Metso Gstrip cover (Continious wire wound) These were also barring at about 90-100Hz or 14x around the circumference. (I now realise it was probably headbox pulsations again!)We had a lot of pitch around at the time and a new clay filler and thought it may be forming an abbraisive mix and wearing away the roll. Have since gone to tungsten carbide coated rolls and no problems.
Cheers and thanks again,
Mike Plant.

Do you have a 50Hz grid? 100Hz = 2xLF?
Or is it not 100.0Hz? Olov

Planty,
Whenever we have had barring problems, its been traced to the fan pump and head box. Our conjecture after talking to the roll manufacturers has been that the barring is in the paper from the wire to the calendar stack. As the paper goes through the stack, the barring is transferred to the rolls.
Just one of the opinions we've gotten, but the one that seems to follow the closest as to whats going on.

Oli, We have 50Hz grid. Are you suggesting pressure transitter could be affected by stray currents? Good Thought. Will get more accurate measurement of freq. Also I will get ops to vary fan pump speed and see if pulsations follow.