Dynamic Absorber
Dynamic Absorber
(OP)
We have a 20,000lb vertical pump/motor with vibes over 0.38 ips in the horizontal direction from running at the natural frequency of the system. We tried adding mass and were successful to a point, but needed more then we thought prudent. We are discussing dynamic absorbers and I was wondering how to guesstimate the weight I need on the absorber which will have minimal "whip". I know how to calculate the support length and I realize I can split the weight into a couple of Absorbers to reduce the weight on a single support. Also...any good references I should look at to design a dynamic absorber? Thanks for the help.
stay safe;
paul
stay safe;
paul





RE: Dynamic Absorber
http
Rockwell has taken over Entek and I can't find the "Enteract" conference articles anymore.
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RE: Dynamic Absorber
I designed various dynamic absorbers for cases up to 25000 lbs and as a rule of thumb you could use an absorber mass of 5 to 10% total mass. With a 3% added mass, it starts to be a bit difficult to tune the absorber, but it's still possible to get good results.
Good luck!
RE: Dynamic Absorber
I did find the Entek article at ht
but there appears to be mistakes in the formula (unit analysis shows a discrepency).
The rule of thumb was just what I was looking for, thats going to be alot of weight. I will have to see if I can find the Hartog text.
stay safe;
paul
RE: Dynamic Absorber
On page 7 there is a formula
W2 = stuff - otherstuff
The otherstuff has a factor of L in the numerator. If you change that L to L^4, the formula should be correct.
I am impressed that you found the file. How did you find it? Is there an address where all the Enteract files are available?
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RE: Dynamic Absorber
I found the Entek article using a Google search for "Dynamic Absorbers". I am sorry to say I do not know where the rest of the articles went.
stay safe;
paul
RE: Dynamic Absorber
less added mass means a more difficult tuning... you could also use 1% mass, but that would mean you should devise a means of fine tuning the mass position (i.e. a screw to precisely position the resonant masses). Remember that, from the tuning position, you will have two resonances really near: one to the far side of the rod and the other to the near... a difference in postion of just 1/32" could change the state from total vibration absorption to resonance!
The difficulty increases when using more than one added mass. In this case you should tune the first one, then fasten it (prevent the mass from oscillating) then proceed with the next one, fasten it and so on.
On the basis of experience I strongly encourage you not to go under the 3% ratio if you do not want to have seriuos headaches during tuning... 5% if you do not have precision vibration measurement equipment.