Dynamic forces on platform supporting machinery 1

[nivoo_boss]

Hey everyone!

I have to design a support platform for some machinery. The manufacturer has given me dynamic loads that I should apply to the supporting structure. The machine is quite small and light and the forces are small as well. Below is a screenshot of the table with these dynamic forces.

The problem is I have never applied dynamic forces in my software (SCIA) and I'm not well versed with dynamic design. Since the loads are very small (maximum vertically around 800 N, that's 80 kgf), would it be acceptable if I just multiply these values with a factor of 5 or something and use static design ?

 

[ldeem]

If the vendor is giving detailed acceptance criteria then you need to make sure the platform passes. Even if loads are relatively small they could give larger defections if close to resonance.

I am not familiar with your program but a proper time-history analysis will give you the results you are looking for. Assumptions on dynamic masses, connection fixity and support conditions will all play a part. Often there is a range of possible assumptions so it may require several runs of different combination to verify none are exceeding the acceptance criteria.

I am not familiar with the program you are using but try to verify your setup with hand calcs or example problems.

 

[dik]

Loads seem quite small, but the 7.1Hz may be a little bit of a problem. Do they have a spectrum of all loads and frequencies? If your spans are short, it might be prudent just to make it 'really stiff' (not ignoring frequencies).

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[human909]

For a vibrating load I would not call those loads small. More information about the piece of equipment would be helpful. Eg size, weight and type of operation.

Those maximum allowable displacements are similarly quite harsh.

 

[dik]

In my neighbourhood, 200lbs is a small load. I just noticed, the displacements are 'really small'; what is the span?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[WinelandV]

0.21 mm of dynamic displacement - I have a feeling you'll need to design for that first. Maybe with that small of displacement, you'll end up with a stiff enough floor to have a natural frequency outside of the 7.1 Hz.

 

[nivoo_boss]

The displacement requirements are quite ridiculous, yes. I think it's nearly impossible to get this with a steel structure - which is what they want. Anyway, the machinery itself weighs around 15 kN or 1500 kg.

 

[canwesteng]

Hate to be the bearer of bad news - at 7.1 HZ a deflection of 0.45mm would be considered rough operation. Don't expect them to come back and move the decimal place for you if you push back on this. If you just add a multiplication factor 5, and the deflection meets their requirements, my quick mathcad sheet says to get the steady state amplification factor to match that your natural frequency needs to between ~8 to 6 Hz with 2% damping. If you're not comfortable with dynamic analysis in your software maybe you can make sure there are no natural frequencies of the structure around there and add the amplification factor.

There is a great free resource from brown here, get yourself that ivy league education

https://www.brown.edu/Departments/Engineering/Courses/En4/Notes/Vibrations/Vibrations.pdf

 

[dik]

almost... one is mass, the other if force.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

canwest... great explanation... I had to read it twice.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[nivoo_boss]

almost... one is mass, the other if force.

Yes, I know that - but does it really matter right now?

 

[dik]

Not really, but to be more correct; without my Scotch, I'm anal...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[human909]

Vibrating machinery on steel structures is no joke. Vibrating machinery with maximum deflection requirements would require even more stringent design.

I recently was on site with a small vibratory feeder that was resting on a medium sized hanging platform I designed. The entire platform was shaking. Needless to say I was not impressed. (The client just thought it was what you expect from having a vibrating machine on a platform.) Took me a bit of time to figure it out and it wasn't the unapproved site modifications.

It turns out the columns themselves were vibrating all the way down to the ground! There was an entire plane of the structure completely unbraced and I'm pretty sure that the embedded columns were not designed with moment resistant foundations! I was not impressed at all. But my objections to the responsible engineers have so far fallen on deaf ears.

 

[nivoo_boss]

Looked more into the machine. It's this seed grain cleaner:

https://static1.squarespace.com/sta...05936b40/1634644313376/Product+sheet_S_GB.pdf

Basically it has a sieve in it that vibrates constantly side to side and sorts the grain, shouldn't be too sophisticated machinery.

 

[human909]

That is a baby one. Only 2T/hr and 5kw. Those dynamic values likely include the unit weight as well. Unless you really skimp on steel then building a stiff enough frame for this should not be a challenge.

That said I've seen large units of these vibrate entire steel buildings.

 

[nivoo_boss]

It is small and light, yes. But I still find it unrealistic to limit the deflection to the specifications (with steel supports). Anyway, I contacted the contractor as well and he said that these machines are built on top of all sorts of bascially home made frames without any problems.

 

[ldeem]

The vendor says that now but if there is a problem down the road related to vibration they will be claiming the structure is at fault.

Better to do the analysis and show you considered dynamics and it may the vendor criteria.

 

[ThomasH]

Hi

Considering that the loads are dynamic I would not say that they are very small. Certainly not of you are driving the machine on the structures natural frequency.

As for the deflection limit (0.21 mm @7.1 Hz). If I compare to an acceleration 0.21 mm *(2*pi*7.1)^2 = 418 mm/s^2, if you stand on the structure at that vibration level you will clearly feel the vibrations. And since it is a steel structure you will probably have a very low damping, I would assume 1% of critical. The comfort criteria is not relevant for the design, it is just to show that the requirement is not very stringent.

It's a fairly small machine but it has nasty properties from a dynamic perspective. I would avoid 7.1 Hz with some margin, as a starting criteria, and since there is no data for any other frequency I would do the initial analysis based on that the given loads apply for all frequencies. If the structure meets the requirements with that approach I would probably consider it ok.

First I thought it was some type of badly balanced engine but then I realized of is a machine that uses vibrations to clean grain.

THomas

 

[human909]

I literally was examining one of these machines the other week.

A well braced structure should have you covered especially if you are close to the ground. If you are elevated and don't have appropriate bracing then all bets are off.

The grain sieve I saw didn't have big vibration issues. However in a different part of the plant the vibratory feeder (roughly similar frequency and power) was shaking a large area. The lack of bracing along one column line was the obvious culprit. (Neither I nor my company designed that steelwork!)