What do these two terms in a structural dynamic problem mean? 1

[bojoka4052]

[ul]
[li]Rotating mass. The structure will have rotating masses attached to it.[/li]
[li]Rounds per minute (RPM). They say critical RPM is 6000 for object attached to a structure, when the machine starts there are some critical oscillations (?) and we need to accelerate past this RPM to avoid damage.[/li]
[/ul]

 

[le99]

The two terms combined seem to describe a rotating mass with a speed of 6000 RPM (revolution per minute). The "rotating mass" could be the blades/tire that rotate about the axle.

 

[DayRooster]

Are you a mechanical engineer reading a note off a structural drawing? Or is this an actual test question or problem?

Context is important but it may be trying to tell you the natural resonance frequency of the structure so the machine frequency doesn’t align with it. Again I’m not sure because I’m missing some overall context of the situation…

 

[AskTooMuch]

It means to me the dude is saying you need to avoid resonance frequency.

 

[WARose]

From that statement it appears there is a transient phase of the vibration that you can't stay in too long.

 

[JoshPlumSE]

When they say "critical oscillations" I suspect that they mean the natural frequency of the structure or some equipment attached to the rotating mass. If you run the equipment at that frequency you get something called resonance which means that the deflections and stress increase rapidly because the frequency of the rotating mass and the frequency of the supports coincide.

If you can "throttle through" that frequency reasonably quickly, then you can avoid damage.

 

[TigerGuy]

It's part of the dynamic foundation analysis. The unit has an overall weight, of which a lesser weight is in rotational motion. The goal is to design the foundation such that the rotational mass accelerating and decelerating to operational RPM's does not pass through a resonant frequency for the structure.

 

[HTURKAK]

MR. bojoka4052 (Mechanical),

Is this thread for Rotordynamic and lateral instability due to asymmetric shaft?

If so, keep far from rpm ranges of instability say 9000 RPM, minimize eccentricity ..

 

[centondollar]

TigerGuy,

There is no way to design a foundation in such a way that no critical speeds exist. During start-up, there will be a transient period which causes slight vibrations, and the goal is (usually) to ensure that steady-state operating frequency far away from resonant frequencies.

Bojoka,
You should ask your supervisor about this. Eng-tips is not going to teach you how to solve the design problem of a rotating unbalanced mass, or how to solve it with dampers or by adjusting foundation stiffness, or how to convert RPM into angular speed of rotation, or what natural frequencies are - those things are best explained in textbooks, in University and by your colleagues if you are unsure about some of the details.

 

[DayRooster]

Centondollar,

I understand what you’re saying and that would make since if your machine frequency was higher than the natural frequency (mode shape/s with about 95%+ participation) of the foundation. Let’s say your machine runs at 30 HZ and your foundation is a block foundation on sand that has a natural frequency of 12 HZ. Then it will pass by 12 HZ at start up. This is the under-tuned scenario.

But, if I’m not mistaken, if your foundation is over-tuned then it should not pass by the reasonable frequency. Let’s say your machine frequency is 30 HZ but you have a block foundation sitting on rock that has a natural frequency of 45 HZ. During startup, it should never cycle faster than 30 HZ and you should stay away from natural frequency.

Or am I missing something. I understand there are multiple directions to consider too. Just pointing out that a stiff enough foundation has the potential to be overturned in vertical, lateral,etc.

 

[centondollar]

DayRooster,

It is difficult to make the foundation very stiff. For practical purposes, it is almost never rock, and even if it is, there will exist a concrete slab and possibly extra steel structures between the machine and the rock. Such slabs and steel structures have a non-negligible bending stiffness, which is far smaller than the vertical stiffness of solid rock. Furthermore, the bending stiffness of concrete slabs is difficult to model (creep and cracking cause a very big variation in stiffness), and the modelling of the foundation-soil or foundation-rock response is even more tricky - the Winkler model is often used, but it is simplistic and does not necessarily give a very accurate estimate of the natural frequencies of the coupled footing-soil system.

Technically, you are correct in your analysis, but actually producing an over-tuned (i.e., very stiff) foundation upon which machinery is installed is very difficult. It is my understanding that in real engineering applications, the under-tuned scenario is expected, and the final tuning for preventing steady-state vibrations is done on-site by adding mass dampers (with or without viscously damping hydraulic pistons) or adjusting the foundation stiffness in some other manner.

 

[TLHS]

There's a lot of range of practice for dynamic foundations. For slow cycling items, it's totally reasonable to keep your foundation natural frequency out of the operating range and the whole range of startup frequencies.

For high speed systems, this isn't really feasible. At best, you can keep the operating frequencies outside of the foundation natural frequencies. The real joy comes from variable speed high frequency equipment that can operate in a whole range of frequencies. It becomes impossible to keep foundation resonance outside of the operating frequencies using reasonable designs. Then you get to do dynamic analyses that account for damping and resonance so that you can keep the resulting vibration amplitude/velocity during resonance below limiting values, or you lock out certain operating ranges on the equipment.

The fun part is when you get measurements of foundation vibrations and realize how hand-wavey any math you're doing is. Stuff that shouldn't work by orders of magnitude ends up working in the field because of additional damping you can't account for, or because the machinists just balanced it until vibration hit limits. I'm definitely on board with using math to get in the range but when it's critical having a plan to tune the system somehow if need be.

 

[JoshPlumSE]

I used to work for a company that did HUGE compressor foundations. One of the old grey haired guys related an awesome story to me. We did the engineering on this compression and found that one of the natural frequency of the foundation was significantly less than the operating frequency (probably about 70% of the normal operating frequency).

As part of our deliverables, our company put together an "operations manual" for this piece of equipment. In that manual, it was clearly spelled out that in a frequency range near the foundation natural frequency, they would begin to experience some vibration and that it was important that the operators "throttle" past that frequency or they would risk damaging the the structure or equipment.

Of course, the operator himself never read the operating manual. So, when he gets to the natural frequency of the foundation he gets nervous and decides it would be 'unsafe' to increase the speed until this is all figured out. Ergo, he leaves it there and they operate it at that frequency for who knows how long before the engineers are called into to investigate why they equipment isn't operating properly.

Cue engineer telling "Joe Redneck, (i.e. the operator)" to find the operations manual and flip to the page which tells him that under no circumstances should the equipment be run at the frequency at which he decided to run it. face palm!

I believe the owner of the equipment had to do a lot of testing to make sure the equipment wasn't damaged at all. And, they may have voided their warranty for a multi-millions dollar compressor.

 

[WARose]

As part of our deliverables, our company put together an "operations manual" for this piece of equipment. In that manual, it was clearly spelled out that in a frequency range near the foundation natural frequency, they would begin to experience some vibration and that it was important that the operators "throttle" past that frequency or they would risk damaging the the structure or equipment.

I've included something like that in the drawings myself. And it's smart to include a range: one of the biggest mistakes I see engineers make in this is they think the spring constants they come up with (to represent the foundations) will be exactly the values they come up with.....not so.

 

[ThomasH]

@bojoka4052

It is difficult to give a specific answer since you don't provide a lot of information.

As I understand it, you have a structure that will experience forces due to rotating masses. There are a number of machines that can cause this. In my experience there is often a recommendation to avoid the working frequency with a certain margin, say +/- 20%. You have 6000 rpm which equals 100 Hz, so you structure should not have any natural frequencies between 80 and 120 Hz, based on the mentioned criteria. I don't know what type of structure you have but that may prove difficult.

These are not the typical low frequencies that we deal with for wind etc, these are higher. And once you have calculated them you need to check the mode shapes against the loading. Is it reasonable that the specifick loading can excite that specific mode shape? And if it can, you need some criteria from the manufacturer.

But if you want more specific answers I think you need to provide more information. For example, what type of structure are you designing?

Thomas

 

[bojoka4052]

You guys were right, its meant to say to avoid resonance frequency.

 

[TigerGuy]

Centondollar, thank you for articulating what I could not.

Our last design did not pass through the resonant frequency. That takes a lot of concern out of the design.