dynamic frequency assessment

[Ericson66]

In determining the frequency modes for a structural frame supporting vibrating equipment, in addition to the equipment mass do we also include operating material load in the analysis?

thanks in advance

 

[r13]

If the operating material is rigidly attached to the equipment, then I think its mass should be included. Otherwise, no.

 

[Agent666]

I'd say you'd need to look at several scenarios and see if the material mass affected the dynamic response. There is no point in looking at the equipment empty for example if the addition of the material causes a different and possible resonant response.

 

[ldeem]

In my world material mass is usually small relative to mass of everything else so I generally ignore it. However if I had a hopper or something that would generally hold material I would include that mass.

I recently learned the hard way to also look at local flooring and handrail dynamic properties as well.

 

[Ericson66]

I have a structure supporting a vibrating screen, the vendor specifies the primary supporting members to have a natural frequency of 2.5 x operating frequency (16.3Hz), OVERTUNED - I am getting a natural frequency of the structure will all the dead masses = 8Hz. How does one overtune a structure by a factor of 5??

 

[ldeem]

Is your screen running 16.3 Hz or is that 2.5x target?

I do quite a few screens for aggregates and they typically run in the 14 Hz range and I target modal response at least 30% off so typically 14*0.7=9.8 Hz. I have only designed steel support structures and response frequency is always below the screen. During startup the screen goes through natural frequency (I use the term loosely) but that hasn't been an issue.

I can't imagine how you could get a natural frequency to 16.3 Hz in a steel structural frame.

The screen vendors I have worked with will provide dynamic properties of their equipment but provide very little or nothing regarding design parameters for the supporting structure. The +/- 30% is what I use but others on this form use +/- 50%, I generally end up somewhere closer to 50% ; 30% is a bare minimum.

 

[Ericson66]

Ideem,

here is the notes on the drawing, the operating frequency = 16.3Hz, I am reading this as the supporting structure is to be overtuned x2.5, however this is near impossible. As you pointed out, the structure is undertuned by 50% so it should be OK, however they are experiencing issues onsite.

 

[r13]

Suggest to discuss this matter with the supplier to have a clear understanding. From the note, the supplier seems very thorough and have some experience in mind.

 

[ldeem]

I have not seen a note like this before from a screen vendor. I understand why they would think the support recommendation would work - it is very conservative. However I have no idea how you could achieve the recommendation with steel framed structure.

I would agree with retired13 to ask the vendor and see if they can provide some direction on how other installations have achieved the specified requirement. 16.3*2.5 is 40.75 Hz - maybe a giant chunk of concrete on grade?

It sounds like you have an existing structure and the client is saying there is a problem. You really have to go onsite and see these things. It may be the client perceives some problem with the structure but it is really a secondary structure (hand rail, floor grating, etc.) that is the problem. Chute work can pick up vibrations as well. I had a structure recently where the screen was upgraded to a unit with much larger dynamic forces but the frequency was the same as the previous screen. The main structure was fine (you could perceive some vibration on primary framing) but some handrails and larger deck plate spans had to be reinforced. Floor grating or deck plate spans can be controlled by dynamic responses in high dynamic load situations.

I am not sure your exact situation but I had success adding concrete blocks to a pump support platform that was experiencing dynamic's problems.