I ran into an interesting problem today and I’m wondering if anyone else has had a similar experience.

I was called to investigate a loud "hi pitched squeal" coming from a dryer section (Pulp & Paper) at one of our facilities.

The squeal pitch and duration could be affected by changing the tension of the woven fabric (felt) that carried the paper. It was also difficult to pinpoint the source as it seemed to be resonating off of every thing in the immediate area. We were certain that it coming from this section (dryer) because it would stop if the section stopped. The squeal would also stop every time the fabric seam passed over the tension roll.

After checking all of the bearings and other likely possible sources without success, we started looking elsewhere. We were finally able to isolate it to one particular area across the machine (between sections). After some searching we identified the source as the juncture of the fabric and the felt tensioning roll.

Listening across the machine (front to back) it became obvious that one area of about 12-18” sounded much different than the rest. Utilizing a flashlight we inspected the porosity of the fabric and found one section that had become completely blocked (filled) with contamination.

It appears that the air is being “captured” between the contaminated fabric and the roll surface, and emanating the “squeal” while escaping. Unfortunately this theory can’t be confirmed until the next down. Until then……..

Anyone else had a similar experience?

Denny,

What sound detection/localization technique did you use?

1) Ears (observation)
2) Audible Sound (microphone)
3) Ultrasound (airborne sensor)

Walt

Walt,
We used ultrasound to localize the sound (UE 2000 w/Rubber Focusing Probe) and then we could observe the sound change (with our ears)once we were in close proximity to its apparent source. We did have to have our ears within a couple of inches of the apparent source to observe the difference.

Prior to localizing it, it seemed to reflect off off everything in the general area. it was deinitely "surround sound". You couldn't identify where it was coming from.

Being somewhat frustrated, I just started scanning the entire area using the Focusing Probe and, quite frankly, got lucky. In the process I came across the general area where there was a definitive change in amplitude. Again, once you got your ears close enough you identify the source.

We also identified that the bearing supporting the tension roll (general area where the sound was coming from) are undersized for the application. Being undersized when they were put under high tension they could be causing the roll to vibrate (oscillate)creating another source of the sound generation.

Interesting to say the least.....

Denny,

Your theory about "trapped air release" being the cause of the unusual sound is plausible. Another possibility is that sound is caused by friction with a "stick-slip" phenomena. The contaminated fabric could have different friction on roll, and the relative movement creates vibrations that radiate sound. Perhaps IR thermography could confirm that friction is present by showing a hot spot?
No matter what the sound generation method is; you were still able to identify abnormal fabric condition, and that is good.

Walt

Denny,
When you used the ultraprobe did you try changing the frequency on the UP2000 to a lower frequency? Remember the squeal if heard by human ears is low frequency, but, by gradually lowering the frequency from 40kHz towards 20kHz and the meter set to "Log" of the UP2000 you may had been able to isolate the sound (even though it may had muffled the squeal). The sound may not of had enough high frequency properties to hear it in the "Fixed Band" which represents "40kHz plus or minus 10kHz".
Also, when using the rubber focusing probe, keep it off of the unit until you are close to the area, unlike other sensors the "trisonic scanning module" will cover a wider field-of-view when trying to locate.
Last thing... was there an air source near the roller or felt (air knife, etc..?) anything that would of allowed air to get under the felt? If so, yes, I could understand the contaminated area making the noise.
When I go into a plant to give on-site training, I have the students/attendees take the ultrasound with a splitter for the headphones and a second set of headphones (for me) and and we walk and scan the area, production line, and/or other parts of the plant compressor rooms, switchgear, etc... then I have the students tell me what they heard and we try and identify them. Good practice for any plant or manufacturer to periodically listen for "anomalies".

Walt,
Good idea on the IR. We'll take a look at the possibility that there's some "stick-slip" happening.

Jim,
I had the UE 2000 set to 25khz - Linear which seemed to work best att he time. I didn't try the Log setting although that's a good thought and will certainly experiment more.

There's no air supply source near the unit so that doesn't appear to be a contributing factor.

I do like your ideas about the dual headphones and will aquire a set along with the splitter.