Vibration of rollers with air flow through the gaps

[Ianmitchel]

I have a bed of rollers ( diameter 70mm) close together, 1 mm gap between each. Over this bed of rollers there is a running web of thin paper material. To keep the material in place and stop it from creasing a vacuum of 200mm H2O is generated. I have experienced severe vibrations of the web and when I have changed the natural frequency of the rollers the vibrations are still there.
Can you tell me if the vibrations are caused by the rollers singing or is it down to air flow around the moving web?

 

[ronfrend]

Ian,
The vibration associated with air movement through the gap between the rollers is called a Karman vortex and it is calculated quite easily.

fD/V=0.22

where
f is the frequency of the vibration in Hz
D is the diameter of the roller
V is the velocity of the air (same units per second as in D)

0.22 is the Strouhal number and the whole thing is described well in Hartogs "Mechanical Vibrations" page 305 (Dover Press).

You will need to measure or find some way of estimating the actual air flow past the roller which could be the difficult bit.

Good luck


Ron Frend
tel: 011 44 1253 400541
ron.frend@predicon.net

http://www.predicon.net

 

[Ianmitchel]

Ron,
Thank you for your suggestion.
I now have a copy of the den Hartog book and have taken some measurements of the air flow between the rollers.
I have varied the fan speed and am now in a postion to begin predicting the range of air velocities that will cause the problem.
Should be well on the way to solving this problem early in the new year.
Ian Mitchell