Mechanical Resonance

[gsalih]

In a mechanical system of say 5 elements of different stiffness and masses are mechanically assembled together.
One of the 5 elements start to resonate by an external force.
The amplitude of the vibration is measured in each of the elements during resonance. Each element shows different amplitude of vibration at resonance frequency.
The task is to determine which element is actually resonating.
Is the element with the highest amplitude of vibration is the one that is resonating?

[zeke]

What task? If this is homework, we have some rules here about helping students. But..

If you have 5 discrete masses interconnected elastically , you have 5 degrees of freedom and therefore 5 natural modes of vibration.
Typically at each of the frequencies, the amplitudes are different, so you can't say one is resonating. In fact, they are ALL resonating at each frequency.

If you can write the 5 dynamic equations, you may be able to infer some of this.

If you are not familiar with this, then you will need a refresher in vibrations. There are many elementary books on this.

[gsalih]

I wish i was still a student. I was merely oversimplifiyng a practical situation.
The real situation is as follows: there is motor which is bolted on top of an oil tank, tank itself is bolted onto the baseframe. There is also another element bolted onto the baseframe which supports another motor when it is connected.
The baseframe itself is bolted onto a seismic bolt.
When a sweep test is carried out to 16000rpm which is the operating range of the motor, there is a resonance frequency at around 13,600rpm.
Vibration tests carried out show that the horizontal vibration reduces in a vertical line from the motor to the baseframe. Maximum amplitudes at resonance motor 6.7, oil tank 2.5, other motor element 1.9, baseframe 0.92 and seismic block 0.92 - all mm/s rms.
There are 5 sensors placed on motor and the tank each horizontally in line and they all seem to move almost in parallel at resonance.
It is also found that horizontal vibration on baseframe is less at resonance further the sensor from the motor.
The task is to determine what is causing the resonance.

[PinkertonD]

Am I over-simplifying it? But, if there are two (motors) sources inducing vibrations, would it not be prudent to run one motor only and test, then run the other then test again? See where (or if) the resonance moves to?

[gsalih]

For the vibration tests we are only running one motor. Eventually this motor will drive an electric vehicle motor.

[zeke]

A sketch of the setup would help us evaluate this.
When you say a sweep, where is the excitation?
Is it not at the structure beneath the seismic block?
I guess that the isolators are between the block and some understructure?
A lot of guessing .
Also, I'm having trouble with your units mm/srms. Why the seconds?Why not g's ?

What was the sweep rms or g level, and what direction was the sweep ?
I assume you are evaluating a design isolation system so tell us what you really have for isolation.
and what you are trying to accomplish.

[maximillian]

Hello All,
Does anyone know how to calculate the mechanical resonance of a 3D micro coil?
Thanks.