Understand a dynamic load representation 2

[nivoo_boss]

Hey everyone!

So I'm designing a factory that has a vibrating equipment on one floor. The load is given like this:

Do you have any thoughts what that "6 pol" might mean?

I'm not too familiar with dynamic analysis but am trying to educate myself a little in Robot Structural Analysis.

 

[Denial]

I tried to enlarge the image a bit to make your queried text slightly more readable.[ ] It didn't help very much, but got me wondering whether what you read as "pol[ ]50Hz" might be "p[sub][ ][/sub]at[ ]50Hz".[ ] But then that would merely move the problem along to what the "p" is.

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[sup]Engineering mathematician / analyst.[ ] See my profile for more details.[/sup]

 

[bootlegend]

I would guess it means a 6 pole 50 Hz motor on the machine. Then there is a load cycling at 16 Hz. What is the machine?

 

[nivoo_boss]

It's some sort of a grain grinding mill I think.

 

[ThomasH]

I agree that it probably is a 6 pole 50 Hz motor. The load is +/- 2445 N at 16 Hz, but what is the direction of the load. For a rotation machine I would assume a rotation load but here it might be vertical +/-. And how does it vary in time, sinus-function at 16 Hz or something else?

 

[canwesteng]

That looks like a layout drawing or something. You should ask for the vendor drawing, it should usually be clearer.

 

[WARose]

The load is +/- 2445 N at 16 Hz, but what is the direction of the load. For a rotation machine I would assume a rotation load but here it might be vertical +/-. And how does it vary in time, sinus-function at 16 Hz or something else?

For a electric motor (at least the kind I am use to dealing with) there would be both vertical and horizontal unbalanced force (one of them having a different phase angle than the other). Of course, we don't know what the machine actually is.

 

[human909]

Of course, we don't know what the machine actually is.

I'd guess with a 90% certainty that it is a vibrating sieve for sorting out suitable sized output from unsuitable sized output. The loads size, the loads frequency and the floor penetrations are very good clues. The fact that it is a grain mill is a big give away. (The hexagonal columns of bulk storage bins on the top of the drawing are also a good give away.)

I've dealt extensively with these machines and the problems their vibrations cause. I've seen them vibrate entire building and their foundations. With vibrations being observable over 50m away through the earth.

It's some sort of a grain grinding mill I think.

You really should clarify this. Is it an animal feed mill my guess. A flour mill? Or something else.

The load is +/- 2445 N at 16 Hz, but what is the direction of the load. For a rotation machine I would assume a rotation load but here it might be vertical +/-. And how does it vary in time, sinus-function at 16 Hz or something else?

The vibration is mostly up and down with a 25% front to back longitudinally. The screens run lengthwise so you want the vibration motion in that direction.

I'm not too familiar with dynamic analysis but am trying to educate myself a little in Robot Structural Analysis.

Be extremely careful especially if the structure is steel rather than concrete. 16hz is a horrible frequency for floors to deal with. If you are inexperienced then I'd be asking for assistance.

Also I'd be chasing the supplier or other suppliers for methods of dampening. That is my general approach. Why spend a fortune on the tuning the structure when you can dampen the vibration my ~97%?

 

[ThomasH]

For a electric motor (at least the kind I am use to dealing with) there would be both vertical and horizontal unbalanced force (one of them having a different phase angle than the other).

That's what I meant with the "rotating" load .

 

[canwesteng]

There is a difference between a rotating dynamic load and a combination of horizontal and vertical dynamic loads with respect to the steady state response of the structure. It generally isn't important some distance away from resonance and seems to be captured in FEA properly by using sin loads with phase angles, but just something to keep in mind.

 

[human909]

The machine has a vibrator or several on it. So it will be a heavy unbalanced load mounted on a frame that is made to vibrate at the preferred frequency. As mentioned it will be orientated with the axis of up-down the page. So the vibration will be left&right as well as in and out of the page.

There is a difference between a rotating dynamic load and a combination of horizontal and vertical dynamic loads with respect to the steady state response of the structure. It generally isn't important some distance away from resonance

Well in this case we have machinery that has a combination of horizontal and vertical dynamic loads. It is heavy and vibrating. And I don't know where you get the notion that it isn't important some distance away from the resonance. As I've said this type of machine can shake buildings, foundations and the earth around them if you aren't careful.

 

[Tomfh]

What is the difference between a rotating unbalanced load and horizontal and vertical dynamic loads? Isn’t that what a rotating unbalanced mass does? Induce horizontal/vertical dynamic loads perpendicular to the axis?

 

[canwesteng]

Where did I say vibration was unimportant away from resonance? The fact is that the steady state response of the structure is different when the forcing function is a rotating mass vs an externally forced system.

Tomfh - an externally forced system will have force amplification go below 1 and trend to zero as the ratio of forcing frequency to natural frequency goes above 1, and a rotating mass excited system will do the opposite. Here is a pdf I found from Brown explaining it better

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

Good to understand the fundamentals if you are going to be dealing with these types of problems, though I find the equipment manufacturers must be putting some ample safety factors on their dynamic loads.

 

[human909]

Canwesteng it seems you have been talking at cross purposes with both Tomfh and me.

My quote was quoting you directly "It generally isn't important some distance away from resonance" (albeit with the word the inserted), though your meaning in that context may have been misinterpreted.

I'm not sure where this externally forced distinction comes into relevance here. We were always talking about a piece of equipment that was vibrating and the fact that it was rotationally induced was pretty damn implicit.

 

[canwesteng]

The difference between rotating mass and external forcing isn't as important away from resonance. The distinction between rotating mass and a force acting up and down is fairly important for how you model the loading, something the OP should take into account when modelling the load.

 

[human909]

The distinction between rotating mass and a force acting up and down is fairly important for how you model the loading.

You are continuing to muddy the waters. The FORCE IS a force that is mostly acting up and down. (unless te equipment is something different from what I've been lead to believe.)

And designing for the up and down motion is absolutely what is required.

Here is a video for you:

https://www.youtube.com/watch?v=6F8hV8Yjtb8

While the true severity of the vibration isn't completely evident, having a 3.5T machine vibrating 16 times a second with a decent and measurable amplitude (on a ruler) is no joke when it comes to structural design.

 

[canwesteng]

I don't think I'm the one muddying the water, you've switched from saying the force comes from a rotating mass, to now saying the force is a vertical only forcing function. In any case, the OP should get the actual info from the vendor, and make the sure the behaviour observed in their model matches theoretical for either external forcing or a rotating mass.

 

[nivoo_boss]

OP here. Received a little more info on the machine. The "6 pol 50 Hz" does indeed mean that it is a motor with 6 poles operating at 50 Hz as many of you thought as well. It is a screening machine/sieve for separating grain, I think. It has four supports with some dampers as well (in red on the drawing). But other than that the supplier could not answer what the function for the dynamic load might be or what would be the allowable deflections for the support structure.

This is a schematic for the machine:

One more thing - the supplier wants the openings between the steel beams cast in concrete under this thing, something like this is done in other factories - there is a 200 mm concrete slab between the I-beams:

I did run a time history analysis in Robot Structural Analysis for the beams that support the thing - but I really don't know how to adequately take into account the concrete between the beams so I just added it as mass to the beams and then ran the calculation - even though the concrete in reality probably adds quite a bit to the stiffness of the beams. The natural frequency of the system modelled this way is around 8 Hz. I then ran the analysis with the given dynamic load at the given frequency with the load pulsing to a sine function. The maximum displacement of the beams under this dynamic load is 0,4 mm according to the analysis. But this is achieved with some quite mean beams - IPE400 sections while the span is only about 5 m

 

[human909]

And I'll confirm that I was bang on to the exact machine and brand. All my above comments apply. This is not the sort of machine you want to mess around with.

I had to deal with several of these which were shaking the building to its foundations and beyond (quite literally).

https://www.eng-tips.com/viewthread.cfm?qid=498714

 

[nivoo_boss]

Thanks! I read from the link you posted that elastomer dampers where eventually used to reduce the vibrations - does this mean the support beams where not stiffened at all then? And can you please specify how the dampers where detailed?

Also in the thread you posted someone's advice was to get the natural frequency of the floor to basically double to that of 16 Hz - seems pretty insane to achieve that with steel beams.