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.

 

[ThomasH]

Many times "rules of thumb" works, especially for floors. But I have done some larger pedestrian bridges and that requires analysis. But that is not the current subject .

You say that startup ususlly isn't a problem so I leave that for now.

A few thoughts:
I gree that reducing stiffness can cause seviceability problems or other problems so that is probably not a good idea. At least not without more information. Increasing the stiffness is also questionable because it will move the natural frequency (8 Hz) closer to the machines frequency (16 Hz). I would not start with that approach.
Reducing the mass would also not be something i prefer, but increasing can be worth to check.

I don't have a lot of experience with dampers. But the damping effect from a "real" damper compared to the effect from natural damping, in my experience the real damper is much better. So that would definetly be an option. But I am curious, why du you use 4 Hz?

One concern is that I have so far only seem one natural frequency, 8 Hz. There has to be more but I don't know it they are any reason for concern.

But it seems like the OP has left the discussion.

 

[human909]

Many times "rules of thumb" works, especially for floors. But I have done some larger pedestrian bridges and that requires analysis.

I don't doubt it. Mu comment about rules of thumb was mean to be self deprecating regarding my own expertise in that area (aka, none). I presumed you were talking about large pedestrian bridges and I certainly recognise that something more than a rule of thumb is necessary.

A few thoughts:...

Those are the exact thought I had here. I linked this earlier but I presume you may not have seen it. I thought things would need stiffening but the calcs suggested otherwise.

I don't have a lot of experience with dampers.

I hadn't either until dealing with the problem in the last thread

But I am curious, why du you use 4 Hz?

Because the product was what was available, economic and would attenuate the effects by ~93%. I could have gone an even lower frequency but that would be a more expensive product.

I did do a deep dive into plenty of the theory. But this software and product supplier does make things awfully easy to buy their stuff with their helpful calculator.

https://www.mecanocaucho.com/en-GB/vibration-isolator-pro/

 

[ThomasH]

I don't doubt it. Mu comment about rules of thumb was mean to be self deprecating regarding my own expertise in that area (aka, none). I presumed you were talking about large pedestrian bridges and I certainly recognise that something more than a rule of thumb is necessary. thumbsup2

I think you probably have more experience regaring the topic for this thread than I have.
But regarding pedestrian bridges, since I mentioned "larger", the largest span I have worked with so far was 120 m, cable stayed bridge. It was a challenge from several perspectives.

Those are the exact thought I had here. I linked this earlier but I presume you may not have seen it. I thought things would need stiffening but the calcs suggested otherwise.

I had missed that link. But I think my thoughts are fairly general regarding what usually happens when you play with mass and stiffness.
But I can also give a simple and slightly fun example of how things can go wrong. I was involved in a project many years ago when somebody had designed a small dance floor. The approach was that if you get a hign enough natural frequency all will be good, "rule of thumb". So the floor was designed as lightweight as possible to maximize the frequenzy. That did not work out, but since the floor was small it was fixable with a reasonable approach.

When i comes to dampers I can say that for a few of the larger pedestrian bridges we have in the design stage sometimes calculated vibrations that were "on the limit". For a dynamic analysis the dynamic load is usually based on persons/area. For a large bridge that can mean a unreasonable number of persons on the bridge. In that case we can prepare the bridge for a TMD damper but we test the real bridge before we install the damper.

What I mean by that is that for dynamics there are a lot of parameters. Some we can control and to a large extent also analyze. Others are more difficult to control. In those cases it can be nice to have a damper as a method the modify the system.

https://www.mecanocaucho.com/en-GB/vibration-isola...

That was an interesting link. Thanks.

 

[canwesteng]

Good link - minor nitpick is that you are actually isolating vibration rather than damping it in that case. But dampers are better for that application because they will transmit much less load during ramp-up/down and also should increase the numbers of cycles before you get to steady state vibrations. I've generally seen springs on equipment that is quickly turned on/off (like screens) vs dampers on centrifuges or other equipment that has a more pronounced ramp up.

 

[human909]

What I mean by that is that for dynamics there are a lot of parameters. Some we can control and to a large extent also analyze. Others are more difficult to control.

I have a dead blow sledge hammer for this reason. So I can measure the fundamental frequencies of a structure rather than calculate it.

Good link - minor nitpick is that you are actually isolating vibration rather than damping it in that case.

True, true. I've been a bit slack in my terminology here. I agree it is important to recognise the different behaviours and functions of a damper vs an isolator.

About half a decade ago this equipment manufacturer used to offer an add on kit that that was springs/weights and shock absorbers. (SHOWN BELOW) But now I believe their current kit offering is just weights and spring isolators. For the reasons you mention it was likely recognised that shock absorbers weren't really needed.

Anyway.... That is probably me done here. I hope the OP's installation goes well.

 

[ThomasH]

I have a dead blow sledge hammer for this reason. So I can measure the fundamental frequencies of a structure rather than calculate it.

When working with modifications of existing structures measured data is always valable. But most of the work I do is not with existing structures.

It has been an interesting discussion. But I think that to have any more ideas regarding the original question the OP must provide some input.

 

[bootlegend]

Those of you that are measuring displacements and accelerations, what software/hardware are you using? Or are you using 3rd party to gather data?

 

[WARose]

Those of you that are measuring displacements and accelerations, what software/hardware are you using? Or are you using 3rd party to gather data?

Measure, as in the field? I have a device (made by LANDTEK) that is pretty good at measuring vibrations (as long as the forcing frequency is 10 Hz or more). The info it is going to get is kind of limited (without more equipment). If i need a lot of (highly detailed) info.....I'll hire a third party.

 

[bootlegend]

Measure, as in the field?

Yes, field measurements. The LANDTEK device sounds useful. I'll check them out.

I've got a couple accelerometers and a Dewesoft data collector that I've been trying to learn but the user interface and the online help is not great.

 

[WARose]

The LANDTEK device sounds useful. I'll check them out.

You can get some good bang for your buck with them. (I.e. a device for around $300.) Vibration equipment can get pretty outrageous in terms of cost.

 

[bootlegend]

So does the unit you have measure acceleration, velocity, and displacement? If you have a model number that would be great. I see several versions online. And I often deal with crushers and feeders that operate below 10Hz but this tool sounds useful even if only for the cases with machines operating above that.

 

[WARose]

So does the unit you have measure acceleration, velocity, and displacement?

Yes. (All three.)

If you have a model number that would be great.

VM-6370

 

[bootlegend]

VM-6370

Thanks!

 

[human909]

A standard mobile phone is also quite effective. Though generally they have a sampling rate of 100-200hz so you are effectively limited to 100hz or less. Though if you are doing this sort of measurement frequently or are really after the best quality data then a dedicated device is a no brainer.

 

[bootlegend]

Interesting. Stumbled on this when looking at some of the vibration meter offerings. This is a USB accelerometer that you plug into your phone or laptop. About $800 but you can use a lot of apps and software which might make it more versatile. And can go down to 2 Hz.

https://www.modalshop.com/digital-sensing/products/digital-usb-accelerometer

https://www.modalshop.com/digital-sensing/products/software