Tuned Resonant Absorber - Practical Experience 1

[butelja]

We have a couple of 3600 RPM blowers that are installed on elevated structural steel platforms and naturally exhibit high vibration readings. The correct fix in my mind would be to install inertia bases with spring isolators and expansion joints in the piping, along with ensuring proper balancing and alignment of the blowers. The balancing and alignment are no problem, but the $$ are not available to install the inertia bases.

It has been suggested that we try installing a tuned resonant vibration absorber. This would be a dumbell shaped piece of steel rigidly clamped in the center and mounted to the blower. It would be tuned to have its natural frequency tuned to the blowers operating frequency. Since there is a 180° phase shift at resonance, it should in theory exactly cancel out the blower vibration due to imbalance. DOES ANYONE HAVE PRACTICAL EXPERIENCE WITH TRYING THIS APPROACH? Some of the potential pitfalls that might happen include fatigue failure of the absorber and difficulty in tuning it properly. It seems that they should be mounted symmetrically in pairs in the plane of the impeller to avoid inducing moments into the system. ANY THOUGHTS BASED UPON PRACTICAL EXPERIENCE?

 

[Hatch]

I was involved in a group discussion on tuned dampers with 20 vibration specialists last year. The concensus was that everyone believed it works since the theory is relatively simple to understand and more importantly makes sense.

When the discussion became more detailed, it was obvious that less than 1/4 of the specialists had even applied the concept, and then only a few had any great success.

I am one of those who has not applied the concept but based on my research, I can offer the following info/suggestions.

1. There is no harm in trying the tuned mass damper.
2. Calculate the size of the mass required. If it is large, then don't do it. My practical limit is 5 lbs per absorber.
3. If the mass gets too large, then you can split it into several tuned absorbers. Answering your question, if it is large, it is also best to split the mass for symetry. Others have reported that small massess and a single absorber work well.
4.Yes fatigue is an important consideration, choose your metal (this also affects the spring rate) appropriately.
5. Don't fix the mass, design it to allow adjustment (with bolts) and after it is set, spot weld it in place.
6. Depending on the frequency and length required, it is a mass that is swinging back and forth at a considerable rate. Therefore provide a guard if possible.

Most importantly, tuned mass dampers only work at a single frequency and only if that frequency is a result of a resonance problem. If you do no have a resonace, then the damper will NOT work. Based on your information, it is possible that you have a structural resoance that is causing the high vibration. If that is so, then I would try the absorber (its cheap). If there is no single structural resoance frequency, then I don't think it will work.

 

[WIM32]

Did you think of trying to create a passive damped system? Since the excitation frequency is known (3600 RPM = 60 Hz), you could support the structure, by adding isolating pads, that will bring down the natural frequency of the system (= blower+supportstructure) to around 10 Hz; in this way the frequency ratio f,exitation/f,natural = around 6, which will result in a displacement transmissibility that is nearing 0. Then you should also add a restraint of some kind, so that at start-up resonance will be constrainted (starting up from 0 rpm to 3600 rpm).
We have applied this concept in practice to various equipment, such as fans and blowers, but also non-rotating machinery such as large transformers. There are of course some practical problems, but it is surely a cheap solution.
Good luck

 

[butelja]

WIM32,

The blowers themselves have a very light weight construction and vibrate even more severely if only isolation is attempted without adding mass. That is where the intertia base concept comes in, because you both add mass and tune it to a low natural frequency. The high cost is because the existing structure is very marginal and cannot accomodate the additional mass of an inertia base without extensive modifications.

 

[IPG]

I've used this concept to balance rigid and flexible rotors. It works very well, the best description is at Harris and Crede "Shock and Vibration Handbook" and the best choice (price and manufacturing) is a toroidal rotor with steel balls. The system must to be mounted on springs only. The best characteristic is that it tunes authomaticaly at every speed over resonance and it works alone during changes of umbalance, it is always balanced till it's capacity. If you have a drawing of your fan I can to help with details. In order to size you need to know maximum umbalance g*mm) of rotor during operation.

Regards

Ivan Perez G.

 

[Franko]

A consultant that we contacted appears to have much practical experience with tuned absorbers - RL Smith Engineering (603) 659-6868. See their article in Sound and Vibration Magazine, Nov. '98, pp 22-27. Should first confirm there really is a resonance - for structural resonance tests of large structures, Southwest Research (210) 522-2539 has required equipment and methods.

 

[attenuate]

Butelja:

I recommend asking the blower manufacturer for their input. I suspect the vibration problem is common, and the manufacturer may have solutions and a FEA model identifying critical points on the structure.

Regarding tuned dampers, you may want to add mass to critical points on the structure. It could be as simple as hanging masses (lower the structures natural frequency below the operating frequencies) or as complicated as stiffening the structure based on experience and/or a FEA. Again, the blower manufacturer may be able to offer guidance.

Adding isolators to the base will only address the energy going into the foundation. The disturbance within the blower will remain.

Sorry if I repeated anything from the other posts.

Lars

 

[IPG]

SHOCK AND VIBRATION HANDBOOK, BELEAVE IT OR NOT
REGARDS

 

[VibrationSpecialist]

Dear butelja,
I have applied this method for a vertical motor and it worked fine. It has been running for more than three years then the plant was mothballed.
The method as you can see is simple. You need to experiment to get the best design that suits your application.
Contact me if you need any further details.
;-)