General Question about Vibrations

In summary, the conversation revolved around a conducted experiment with a metal beam and a motor attached to it, measuring data for free/forced and damped/undamped vibrations. The individual was unsure why the resonance frequency was higher than the natural frequency and questioned if the rotating system played a role. The conversation also touched on calculating the natural frequency using the half power method and the possibility of the mass and position of the motor affecting the results. Ultimately, it was concluded that the system has infinitely many degrees of freedom and the expression relating the resonant frequency to the natural frequency will always give a higher value.
  • #1
tomadevil
11
1
Hello Everyone,

I conducted an experiment with a metal beam which had a motor attached to it in with an eccentric mass on it. The two ends of the beams were fixed with a roller and a hinge(as I remember). This was a one degree of freedom experiment.
vib.png

I had to collect data during free/forced and damped/undamped vibrations. My data clearly shows that the resonance frequency is higher than the natural frequency but I don't really know why. I believe they should be the same.
I was thinking it might be related to the fact that this is a rotating system. Am I on the right track?
Can someone explain to me what causes this difference?

Thank you for the answer.
Thomas
 

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  • #2
That sounds like an interesting experiment.

Can you show us how you calculated the natural frequency?
 
  • #3

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  • #4
What about the mass, shape and position of the motor, brackets and the mass? Did you include those in the natural frequency calculation?
 
  • #5
anorlunda said:
What about the mass, shape and position of the motor, brackets and the mass? Did you include those in the natural frequency calculation?
No, I didn't. We have to assume that the motor is exactly in the middle of the beam and I think that would be beyond the scope of this exercise.
 
  • #6
I asked that because the mass of the motor would probably change the natural frequency. Attach a large mass to a tuning fork and the pitch of the sound changes. The natural frequency of the shaft and the natural frequency of the system are not necessarily the same.

Your original question was why experiment does not agree with calculation. So you are seeking something wrong with your calculation.
 
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  • #7
The test frequency being higher than the calculated would indicate that possibly either the mass of the system is lighter than that used in your calculation or that the actual damping factor is less than your calculated one.
 
  • #8
If the actual system looks like your sketch above, then the motor is almost certainly the most significant mass in the system.

In truth, this is a system with infinitely many degrees of freedom. To say that it is 1 DOF involves a significant approximation, but it may be useful.

The expression you have relating the resonant frequency to the natural frequency must always give the resonant frequency higher than the natural frequency. You are calculating the resonance frequency by dividing the natural frequency by a number less than 1.0. I think you have some factors mis-arranged.
 

1. What are vibrations?

Vibrations are rapid, repetitive movements that occur in an object or system. They can be caused by external forces or internal energy sources.

2. What causes vibrations?

Vibrations can be caused by a variety of factors, including mechanical forces, sound waves, and electrical signals. They can also be a result of natural phenomena such as earthquakes and ocean waves.

3. How are vibrations measured?

Vibrations can be measured using instruments such as accelerometers and seismographs. These devices detect the movement of objects and convert it into data that can be analyzed and interpreted.

4. What is the relationship between vibrations and sound?

Sound is a type of vibration that travels through a medium, such as air or water. When an object vibrates, it creates waves of pressure that propagate through the medium, causing sound to be heard.

5. How do vibrations affect our daily lives?

Vibrations play a crucial role in many aspects of our daily lives, from the functioning of machines and vehicles, to the production of sound and music, to the behavior of natural systems such as earthquakes and weather patterns.

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