Critical Speed below natural torsional frequency? 4

[vc66]

Hi All-

I have a simple shaft, at the end of which is a disk that rotates about the same center as the shaft. Simple problem to find the natural frequency, I know. I have the natural torsional frequency solved as 1050 rpm; however, when we run the assembly (ramping up speed slowly), it starts to shake violently as we approach 350 rpm. If we ramp it up quickly, and pass 350 rpm, there is no problem.

I'm not too familiar with resonance and vibration, so can someone school me as to possible reasons why this would vibrate near 350 rpm if this is only 1/3 the natural frequency?

Any and all help, is appreciated.

V

 

[BobM3]

So the inner shaft spins and the hub is "grounded"? Take a look at the springiness of the hub "grounding". Also, could the spinning plate be vibrating?

Certainly the bearing spacing will affect your calculations.

 

[vc66]

Thanks for all the replies.

The motor has a gear box, which I'm currently getting the specs for. I'll let you all know how it goes.

Greg-

Can you give me a definition of "coning" stiffness. I can't seem to find it anywhere.

Thanks again for all the helpful replies. You're all making me a better engineer.

V

 

[vc66]

The vendor really needs to get their act together. There is no gear box... It's a direct-drive torque motor. The rotor inertia is almost zero.

V

 

[GregLocock]

A rigid shaft has 6 degrees of freedom. Coning is the rotational degree of freedom about the two axes perpendicular to the main axis of the shaft.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Rob45]

This is an interesting academic discussion, but inasmuch as vc has determined the actual resonance to be around 350 rpm, what is his real question?
Does this vibrational mode create a problem?

 

[vc66]

The problem is that they want to run the machine at 350 rpm's.

V

 

[electricpete]

I have kind of lost track where we were headed as well. It might perhaps be helpful if you could start from the beginning with a a focused description of the machine, the symptoms, and what you are trying to accomplish.

Did you ever do a bump test?

If we are dealing with simple lateral resonance at such a low frequency, then likely the support stiffness is playing a big role. This may have several implications for your problem solving process:

1 - If the support will be different at the customer site than the test site, the resonant frequency may be much different.

2 - Try adding temporary bracing to see if you can reduce the magnitude of the vibration. If bracing substantially improves the vibration, it is a strong signal that you are dealing with simple lateral resonance.

3 - Permanent changes to stiffness of support (such as adding a permanent brace) may be part of the solution.

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[vc66]

Right now the company is not running any testing on the machine (as we're not selling anything, big surprise). As soon as we start up testing again, I'll have more info, but from what I've calculated, and witnessed, it looks as if stiffening the spindle will be adequate.

V