Small motor frame frequency

[cec17]

I am putting together a small frame for a motor, which has sent me down a rabbit hole on resonance and frequency.

The frame is a platform 3' high x 2' square for a centrifugal motor. According to manufacturer info, the motor is approximately 18500 RPM (seems high but I am not a mechanical engineer). Typically when I do motor or pump bases, they are on a foundation and I can size the mass appropriately using the mass of the pump. In this instance, I am trying to design the frame so we don't have resonance or vibration issues and obviously I can't match the mass of a concrete foundation.

For something this small (with rubber vibration isolators) is this even something usually considered? Is the motor frequency high enough that it's not a concern? Thanks in advance for any input.

 

[GC_Hopi]

What is the criticality of the equipment? Generally the best resource is the manufacturer. If is a "widget" as opposed to a "super scanning microscope" most design is done by experience.

A few rules of thumb for starting are braced frames are the best and get an example of a successful past install (the more detail you can get the better).

 

[TLHS]

If it's really small equipment you'd be surprised how quickly your frame steel mass can become significant.

At that RPM you're not going to beat resonance by being higher frequency. You're going to be way softer, which is also fair. Honestly, softer may be best for a high frequency system with low mass.

How often is it running, how critical is it, how big is it. What's around it.

If it's small, not in constant operation and there aren't people or things that would be irritated nearby then put something reasonably stout looking in place with good detailing practices (stiffeners everywhere, direct load paths, don't put plates into bending in connections, avoid bending anywhere it's reasonable in members as well, weld things or use tensioned bolts, etc) and you're fine.

If that doesn't cut it, you can do a dynamic analysis or just screen for resonance frequencies. That being said, the math is really not that accurate and with isolators you're likely basically guessing unless they're a *very* engineered isolation product. If it's pretty small you're likely still okay doing best practice detailing, stout members and then also have an allowance in your head for what to do if there's a vibration issue. Like, where are you going to throw some mass or add a brace.

If it's a large piece of equipment with high frequency or it's a precision machine it becomes an adventure.

 

[GregLocock]

Assuming the excitation is first order (balance) that's 300 Hz. Even if you made a box that size out of cast iron it would be unlikely to be stiff enough not to resonate at that sort of frequency. So your idea of using isolators is good, they can be quite stiff.

Cheers

Greg Locock


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