10G vibration test w/ the shaker table excited at 24 - 1000 Hz

[bernardg]

In a nutshell...
We manufacture permanant magnet dc motors... and our motor need to pass the 10G vibration test w/ the shaker table excited at 24 - 1000 Hz.

We have the magnets, attached to the shell using adhesive tape and a clip (clip, as a secondary retention)... which is our only area of concern.

The weight of the magnet is 390 grams (0.86 lbs). And at 10 G vibration, it would encounter a force of 8.6 lbs force ~ calculated using Newton's second law of motion (F=ma)

question 1:
The shell - magnet assembly moves with the shaker table in the +ve X/Y/Z- direction, then at the end of each cycle itz deccelerated and starts to accelerate back in the -ve X/Y/Z-direction. Kindly clarify to me as to whatz the implication of the frequency (24 - 1000 Hz) in the calculation of the force.

question 2:
Along with the vibration testing,
we want to conduct the axial push-out test on the shell - magnet assembly.... during which, we'll clamp the shell - magnet assembly vertically in a fixture; and apply the push-load on the magnets in an increment of 2 lbs-force, to find at what load the magnets starts to move.

For that reason, we want to determine the peak value of lbs-force that a shaker table at 10 G would apply on the 390 gram magnet. Could any of you help me with a way to calculate?.... or if itz a standard value, would you kindly give me that value?

Thanks in advance for your help!!

Bernie

 

[pjhype]

mpoc,

You will need to be a little more specific regarding the 10g load. Is this a random vibration load where the overall level in the 24-1000 Hz range is 10g-RMS? Or is this a sine sweep with a peak value of 10g over the frequency range?

Regarding question #1: Structures will tend to respond at a higher vibration level to an input vibration at the structural vibration modes. This amplification (sometimes called the Q-factor) can be as high as 20-50 times the input vibration level at certain frequencies, depending on the damping in the structure. You would need to design your stucture to have a resonance much higher than the highest excitation frequency to avoid the amplification factor. Alternatively, you could greatly increase the damping in your system to reduce the amplification. Before you test your structure, you should perform a low level sine sweep and measure the response of your structure with an accelerometer to identify potential vibration modes.

Regarding question #2: You will need to know the natural frequency of your system to determine the equivalent static load that is applied to your magnet. You might want to ask the question backward... that is, given an equivalent static load to push out the magnet, what is the maximum allowable dynamic load (as a function of frequency) that can be withstood?

pj

 

[bernardg]

Thanks PJ

Well, I feel that I haven't defined my question properly. We manufacture 3 inch permanant magnet dc motors, that goes into the blower/defroster/windshield wiper units of various cabs.

Since our motors are mounted on to the cabs that operate in different rough terrains, our customer performs a 10G random vibration testing w/ the shaker table excited at 24-1000 Hz, to qualify our motors.

Now that we are in the initial stages of fixing the specifications,
our only area of concern is the magnet-shell bonding ~ we attach the magnets to the shell using a pressure sensitive adhesive tape & a clip. (If you could send me your email id to bgruban@yahoo.com, i'll send you the picture of the magnet-shell assembly, which would give you a better understanding of what i am talking about).

Our customer wants us to determine the specifications for the push-out force that our magnet bonding should withstand - to a level compared to the amount of force that a shaker table, excited at 24-1000 Hz frequency @ 10G would exert on to the magnet bonding. (FYI, we send our motors to an outside facility that performs the vibration testing for us. The motor is tested in a "No powering up" condition... so there would be no vibration as such caused by the motor... and all the vibration our motor faces is strictly due to the vibration exerted by the shaker table).

Knowing that the weight of the magnet is 390 grams (0.86 lbs), we want to determine the peak value of lbs-force that a shaker table at 10 G would apply on the magnet. Is there any equations to come with that value? Kindly help me with this.

Thanks a bunch,
bernie

 

[pjhype]

Bernie,

Send the e-mail to pjhype@mail.com and I will take a look at it.

There is an equation to calculate the equivalent static load on a structure when it is subjected to broad band random excitation. Essentially, it assumes that the structure behaves as a narrowband filter at the resonant frequency of the structure. Therefore, the peak acceleration is about equal to 3*sqrt(f*W/Q), where f is the resonant frequency, W is the power spectral density of the input at the resonant frequency, and Q is the structure amplification factor.

You can then take the peak acceleration and multiply it by the mass of the magnet to calculate the static equivalent force at the resonant frequeny of your structure. Knowing (through test) what the push-out force of the magnet is, you can then establish a margin of safety for the magnet when subjected to the random vibration load.

pj