Vibration Dynamic Absorber

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This is one I recently finished and thought it would be a great one to share so people know what can be achieved.

 

Background:

We had three pump sets suffering from elevated vibration levels when operated in different combinations. Conventional vibration analysis was performed and this indicated a structural resonant condition.

The pump motors are mounted on a false floor:

and the pump barrels are below the floor:

The pump with the worst motion was on pump 3, the one far away from the edge of the drop. Also this pump has the least structural support under the floor. When ran in certain combinations pump 3 would be excited very badly.

 

The cost effective solution.

I designed a vibration dynamic absorber.

Dynamic Absorbers are often overlooked and not used, they can be seen as a band aid or a last option for some vibration problems. Whereas in some cases they can be the only cost efficient option, and they are very effective.

The Vibration Dynamic Absorber is a unique bespoke item, maintenance free, that is designed to absorb unwanted energy. It is tuned to have the same resonant frequency as the structure to set up an out of phase signal reducing the signal generated by the structure.

 

How did I design these?

For this one it was more of a ‘gut feel’. I looked at the motor and then drew out a design that wouldn’t look out of place when mounted, and that had some adjustment to it when fitted as theory doesn’t always pan out in real life. Then from this I worked backwards to get the correct material dimensions/configuration so it was resonant at the target frequency. I also made some weight configurations so I could cover my target range.

I will be going back in 6 months to see how it fairs. I did consider a round bar and weight but thought that with the rectangular bar you have more control on what way it will be resonant. As once you have performed phase analysis on the motor you then know what way it is moving and can mount the absorber accordingly.

Image of Pump 2 Vibration Dynamic Absorber:

Image of Pump 3 Vibration Dynamic Absorber:

Pump 2 Live Motion Video

 

Pump 3 Live Motion Video

 

Pump 3 Slow Motion Video

 

What am I covering?

On pump 3 I am covering the one problem frequency, 1 Order, but the two arms are of different lengths in terms of the length from the point of pivot (clamping) to the mass. Also the arms are of different dimensions with different mass at the end so they could be tuned to the same frequency.

I also did find that the sweet spot was not necessary the point of higher deflection of the absorber and that the three motors all reacted differently.

 

Final Review of actual vs theory:

I have had time to review the final theoretical tuning of the three pumps to actual results. They are all different and no one motor is the same, they all have their own personalities dynamically wise.

Pump 3 had the highest overall vibration, one dominant frequency at 1 Order on pump 3 and this was successfully reduced.

Pump 1 and pump 2 had two frequencies in the data. And both of the vibration dynamic absorbers were tuned to the lower frequency not the one order.

 

Table of final overall levels:

Before After % reduction
Pump 1 Motor NDE (Top) 4.314 2.854 33.84%
Motor DE (Coupling end) 2.092 1.617 22.71%
Pump 2 Motor NDE (Top) 9.95 6.959 30.06%
Motor DE (Coupling end) 4.05 3.012 25.63%
Pump 3 Motor NDE (Top) 27.02 7.59 71.91%
Motor DE (Coupling end) 10.73 5.113 52.35%

 

Pump 1:

Pump 1 actually showed the text book results. The theoretical calculations for the tuned damper was for the lower frequency not the running speed (1520 CPM yes they are on soft start VFD). It actually split the frequency – text book……….beauty!!

 

I have more questions and theories now, this is pretty exciting stuff. Hopefully I can keep this going on other pumps.

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