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

 

[ccw]

Hi Bernie,
In the structural dynamics world, the attachment of the magnets to the shell of your stator can be modeled by sets of springs, dampers, and distributed clumps of mass. Any one combination of these groups can have a resonant frequency in the 24-1000 hz range. When the shaker sweeps the exciting frequency of your test motor through this resonance, force amplification can occur in one or some of these spring/mass/damper sets, greater than F = Magnet mass x 10g.
Can this structural system be modeled and the resonant forces predicted? Yes, but it is probably more reliable, maybe even cheaper, to do the testing.
Because of the unknown dynamic structural model properties, the static push out test does not tell the complete story. It would be interesting to take a few ac accelerometers (light weight and cheap) for a ride in the motor magnet parts and frame parts at various points of the assembled motor on the shaker to see if there are any resonances detected during the 24-1000 Hz sweep.
If it can be demonstrated that the magnet-to-frame attachment is rigid (magnet tracks frame in phase and amplitude) throughout the sweep, then the push-out test alone at magnet mass x 10g could be further justified.

 

[bernardg]

Hello,

I'll rephrase my question so that you can understand better at what I am trying to find...

We manufacture permanant magnet dc motors... and our customer demands that our motors need to pass the 10G Random vibration test with the shaker table excited at 24 - 1000 Hz frequency. Though our motor consists of a couple dozen parts, my only area of concern is the magnet bonding. We have the magnets, attached to the shell using adhesive tape and a clip (clip, as a secondary retention).
question:
Even though our customer is going to do vibration testing only on a few of our motors, they still want us to perform a 100% inspection to verify the magnet bonding. So we have concluded to do an axial push-out test as our standard inspection method.
During the engineering meeting, our customer threw up a value of 500 lbs-force ~ that our magnet shell assembly should pass during the push-out test. Just looking at our motors, I could see that the force that the magnet is going to encounter in the worsest of vibration could be far less than 500 lbs-force... So instead of doing a push-out test with a load of 500 lbs-force, if we prove theoritically that the amount of force the shaker table exerts on the magnet during the vibration test is lesser than 500 lbs-force, we can fix that value as the push-out load.
Though itz a crude way to determine one specific parameter of a test (which is not an end entity, and which is dependent on so many other factors)... and using that parameter to duplicate the test conditions in an entirely different test, I still cannot think of anything else....
Initially I started to calculate the force using Newton's law... 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). But looking at the value, I realize that there should be factors other than the mass and acceleration, that we need to take into consideration.
For the ease of calculation, let us assume that the magnet-shell assembly is held in a vertical position on the vibration/shaker table and is excited only along the Y-axis. On excitation of the shaker table, the shell - magnet assembly moves with it in the +ve Y- direction, then at the end of the cycle itz deccelerated and starts to accelerate back in the -ve Y-direction.
(1) Kindly clarify to me as to whatz the implication of the frequency (24 - 1000 Hz) in the calculation of the force.
(2) What would be the peak value of lbs-force that a shaker table at 10 G would apply on the 390 gram magnet. Could you help us with a way to calculate?
Thanks in advance for your help!!

Regards,
bernie

 

[ccw]

Hi Bernie,

I am sorry that I misinterpreted your first posting. I took it to mean that you had the shaker to do the testing, not the customer.

The resonant frequency of your magnet support system to the frame can, in an oversimplified way, be expressed as
rez freq. (Hz) = (1/2pi)SQRT(K/M). K would be the spring rate of the adhesive and spring retention structure. M would be the mass of the perm. mag. assembly. If you can show that this function is greater than 1000 Hz, then there should be no problem with your justification of a push out load less than 500 lbs. Determining the spring rate of your magnet retention system may be difficult. Perhaps you could perform a force/deflection test on the magnet frame assembly (carefully measuring and plotting force vs. deflection), and use the rough slope of the curve to obtain K.

If you cannot show this function is greater than 1000 Hz, then you go to the next step which is to determine the damped force/displacement amplification factor of the magnet support system. For this step you need the damping value of the adhesive and spring retention system. I guess that the damping value (force/velocity function) in shear of the adhesive would dominate, but you may have trouble obtaining the value. Perhaps your adhesive vendor publishes damping values in shear for that product. Whatever you determine for the amplication factor can be multiplied by the 10G and by the magnet mass to determine the peak maximum force on the magnet-frame-joint system, that it would see when swept through the resonant frequency.

Curious, why is the main concern just the magnet -frame-joint? What about the effect of the armature banging against the end bells and bearings during the 24-1000 Hz 10G sweep?

Why is the customer's request for 500 lbs on the push-out test considered too much? What is the shear stress on the joint when the magnet-frame assembly is loaded to 500 lbs.?

Send me an e-mail and I will send you the steady state solution of a simple damped spring mass structure, so you can compute the force amplification factor, once you know or guess at K, M, and C.

 

[bernardg]

Hello,

Thanks a bunch for your reply.... I very much appreciate it.
I am so very sorry for getting back with you this late... (I was involved in a different project which demanded all my time and attention).

To answer your questions....
(1) We do not have any concerns on the armature banging against the end bells and bearings... 'coz we addressed it by changing from burnished bearing bore to machined bearing bore where we could maintain tighter tolerances. On top of that, we use a heavy pre-load spring (wavy washer). We have vibration tested dozens of our motors... and we had no problem with that.
(2) Initally instead of using the adhesive tape to attach the magnet to the shell... we used a varnish dip process, where we attached the magnets to the shell using the clips... and dipped the magnet shell assembly in the varnish... and baked it. The curing/setting time on this process is almost half a day.... and that's one of the reasons why we went with the adhesive tapes. (Now we are into a single-piece flow).

Using the clip & dip process, our magnet-shell assembly easily passed a 500 lbs push-out test. This is the number our customer always hits us with. Though we know that our magnets are not going to experience this amount of force in the real-time application nor are they going to experience during the 10G 1000Hz frequency, we are forced to continue the push-out test at 500lbs force until we prove to them theoritically that the amount of force the shaker table exerts on the magnet during the vibration test is lesser than 500 lbs-force. (around 350 lbs-force or so).

In your message you said that you'll send me the steady state solution of a simple damped spring mass structure.Using that I guess I would be able to compute the force amplification factor). Kindly send that to me in this forum or at bgruban@yahoo.com

Thanks in advance your time and expertise.

Regards,
bernie

 

[drkumars]

We are facing a diffrent problem.I want some diagnostics.
Like the magnet when mounted with an epoxy resin and when subsequently cured is cracking during the push force. What shall we conclude? The properties of the magnets are superb from all stand point. Do we blame it on the epoxy? or should I go for the varnish like the other guy in the thread? Or is it I should go for double (like clip and glue)
What do You suggest?

 

[IRstuff]

Magnet material is generally quite brittle and cracking is generally due to excessive stress, which, from your description indeed seems to be correlated with the epoxy.

It may be that your epoxy undergoes excessive (for the magnet) shrinkage after cure.

One obvious avenue would be go back to the epoxy supplier to explore alternate formulations.

Likewise, a change in thickness of the epoxy might change the distribution of stress.

TTFN

 

[drkumars]

thanks for the suggestion.What I feel is I should calculate the force and see if the compression strength due to dynamic loading is exceeded. Also I should probably do some damping so that resonance and the associated amplicaion factor at resonance is avoided. how to calculate the natural freq of the magnet plus glue plus etcc set up? Here I forgot to tell one thing. There is an induction heating plus curing of the epoxy.Is it causing the danger? It was alternatively tried through slow means also but still the carcking continues. rate is slightly beyond acceptable levels. Should I go for some thing like micro wave curing?
can you help me?

 

[IRstuff]

It's also possible that the substrate to which you bond the magnet might be causing stress.

One way to check this might be to change the amount of surface area bonded and to leave gaps in the bondline. This would potentially separate the effects of the epoxy from the substrate.

TTFN