Think of a dryer section no4, 16 cylinders 100RPM cast iron frame babbitts underhalf bearings. When refurbed last they were mounted as they liked and that was heavy side turning down and vibration was impressive. Fast fix was to turn all upper cylinders 180 deg and they now compensate so the vibration at operation at 1xRPM is low. Still there is a problem with sheet breaking. How do this situation affect dynamic loading in the frame? Does it work like balancing shafts in a car engine? I find a 12Hz possible resonance in the frame when knocking bearings in horiz, that is also approx the dominating freq. when running and may be 2 time the ingoing drive shaft speed but the levels are not that high. It drops drastically when running half speed. So the problem is the breaking sheet but what made me curious was the unbalance distribution. Any comments or ideas? Olov

Oli,

As I understand it, they assembled the cans wo that sequential cans are 180 deg out of phase.

One dryer that I am familiar with (no idler rolls, idler gears or felt) has alternating short and tall stands with identical gears between the cans. The tall stands have are prone to resonating at certain speeds. I would think that in that event, and with all cans 180 degrees out of phase could cause an exaggerated movement where you get some slack in the sheet and then a snap back to the opposite position creating a sheet break.

Given those circumstances, I would expect fairly smooth operation with no sheet and the possibility of frequent breaks with one.

Possible fixes would be balancing of the cans in the area(s) of the break or stiffening of the stands. That could lead to chasing breaks all over the dryer and balancing lots of cans. Depending on the stand configuration it may be possible to link them together structurally changing the stiffness and thus the natural frequency.

Good Luck,

Danny

quote:

Fast fix was to turn all upper cylinders 180 deg and they now compensate so the vibration at operation at 1xRPM is low.

Hmmm! This seems strange to me.

Were all 16 dryers running in phase with the same basic amplitude of unbalance? What holds the upper dryers in place, are they geared to the lower dryers or are all pulled by the felt or the sheet or by the felt and the sheet? When the sheet breaks and if there is no felt and if they are not geared together doesn't the top ones roll heavy side down again? Surely they are felted if not geared, huh?
Or am I visualizing the setup all wrong?

Oli,

Sounds like a little residual unbalance there. By setting the heavy spots opposite for the upper and lower dryers, you may have phased the balance effects out, but there still is the issue of them lagging on the uphill side and falling on the downhill side of the heavy spot (loping). Depending on how much backlash you have in the gears, the dryers could rock quite a bit. I think this will be a major issue with the draw on the sheet leading to web breaks. Have you considered balancing the dryers? We now use a consultant to come in and perform an ODS then calculate the dryers that need balancing and how much weight to add, etc. In my older days as a millwright, we used the poor man's balancer. We would pull the gear and let the dryer roll down to the heavy spot. From there a 55 gallon drum was suspended to a head bolt 90 degrees around. We would fill the drum with water until it rotated the dryer. This became our corrective weight amount and 1/2 that amount was attached 180 degrees from the heavy spot on each side. Not too scientific, but allowed us to speed our machine up.

Ahh, the good(???) old days,
Gary B

Danny, as usual there are no space left to do any stiffening...

Ralph, yes they are all geared together and it was kind of a surprise when I got to know.

Gary, nice suggestion with the drum one problem is that it takes like 2 days to pull off 1 gear to be able to do that and that´s one of the problems. I can see the ODS idea but how that enable even a consultant to calculate the weight and position from the ODS info beats me. All cylinders are geared so you can only see the combination, summed result. Any idea how that magic is done? I know of cylinder balancing on site but that has been done by driving the cylinder using either hydraulic or motor and belt and that goes back to the time it takes to pull it apart that makes problems. Also the current compensation is too good so I get very small levels of 1xRPM so I have hard to tell if they all are equally bad that is what everybody think. So I guess you are correct, it all ends up in balancing one way or another. Thank you for the input, it was good help to my thinking. Olov

Olov,
I had an associate many years ago who balanced a large roll with a torsional sensor. The same balancing rules apply, except you are measuring either angular velocity or displacement. You may find that this method allows satisfactory balance results without any disassembly of roll drives. I have the torsional measurement capability, but I just haven't had an opportunity to try this method myself.

Walt

Walt, correct me if I am wrong, this seems interesting. You apply an encoder that give you enough pulses per rev and "demod" this to a analog signal and use this together with the traditional tacho pulse and go balancing and only to the torsion that is found on that shaft w/o influence from the other geared shafts that have exactly the same speed. Seems possible, once again I am happy tp learn new things every day and I am glad I keep asking about funny things. Thanks! Olov

Olov,

The "demod" is realy a F/V converter to convert time-varying encoder pulses into an analog signal. The method should be OK for a group of rolls with different diameter/speed. It could be worth a try, depending on the situation. I plan to try this method on my rotor kit (bench grinder) to see how it compares to conventional in-place balancing. You might consider this before sweating on a paper machine!
I am planning on making torsional measurements on a diesel (V-8 2-cycle) generator next week that has high 4xSS vibrations. I hope my encoder schemes work.

Walt

Walt, yes I can see that, problem here, they are all the same and geared together but the play in the gear gallery may anyway give a individual speed variation during 1 rev. So it could be interesting to look at. Good luck with the diesel those I worked on had a dominating 1xRPM and was possible to balance with about 0.5-1Kg in the flywheel. Olov