Electronics > Beginners
Question about DC motor failures
(1/2) > >>
dkuhn:
I work in Enterprise IT and we have recently been hit with a rash of component failures on a specific line of network switches.  The manufacturer is proactively sending out replacements for them, which is swell, but I'd love to figure out the failure mode as I find this stuff interesting.

The failure is specifically related to the cooling fan in the back of the switch failing over time.  The manufacturer claimed the issue was fixed about a year ago, and how have sent out another batch of proactive replacements that also covers the devices that were already replaced.  They claim that increasing the minimum fan speed prevents the issue from presenting itself.  This seems odd to me.  If it's a bearing failure, wouldn't spinning faster reduce lifetime?  Trying to make some sense of this with my colleagues and we haven't really gotten anywhere.  I'd potentially be willing to do a teardown of one of these devices if that helps.

https://documentation.meraki.com/zGeneral_Administration/Support/MS225%2F%2FMS210-48LP%2F%2FFP_Proactive_Replacement

Thanks in advance to anyone that can hopefully lend some light on this! (Total newbie when it comes to electrical things, apologies if I didn't include enough info)
edy:
I am assuming that if it is mounted sideways (vertically), not having a fast enough rotation may allow gravity to pull down and minimize the space between the center coil and magnets in the fan, causing wear and tear on the top part of the central hub. Increasing rotation creates an "air cushion" that maintains the space better and reduces wear?

Perhaps in a related issue, I've had a bunch of fans for my Dell CPU cases in my entire office (several machines) go after only a few years. They would start making these funny noises (like chirping sounds) and then eventually seize up altogether. And these fans were in some ridiculously customized "blower" type cases that clipped exactly into the chassis so you could not find an easy replacement. They looked kind of like this:



Anyways, when I opened them up, the central hub was full of rusty metallic dust and bone dry. The coil  was in the central hub, and surrounding it (part of the rotating plastic fan) were the permanent magnets. The plastic "blower case" also seemed slightly warped and because of the tight tolerances it seemed that the fan was scraping the inside edge of the opening.

Perhaps the plastic drying out and warping also caused premature failure, maybe due to heat in the case?
mzzj:
Many fan bearings nowadays have "hydrodynamic bearings" which means that the bearing surfaces are separated by a thin layer of grease/oil. This needs certain speed range to work (comparable to car hydroplaning/skidding on water)
If the speed is too slow there is going to be direct metal-to-metal contact and associated wear.
Ball bearings also have recommended speed range that would depend on lubricant viscocity etc.
dkuhn:

--- Quote from: edy on August 27, 2018, 05:10:06 pm ---I am assuming that if it is mounted sideways (vertically), not having a fast enough rotation may allow gravity to pull down and minimize the space between the center coil and magnets in the fan, causing wear and tear on the top part of the central hub. Increasing rotation creates an "air cushion" that maintains the space better and reduces wear?

Perhaps in a related issue, I've had a bunch of fans for my Dell CPU cases in my entire office (several machines) go after only a few years. They would start making these funny noises (like chirping sounds) and then eventually seize up altogether. And these fans were in some ridiculously customized "blower" type cases that clipped exactly into the chassis so you could not find an easy replacement. They looked kind of like this:



Anyways, when I opened them up, the central hub was full of rusty metallic dust and bone dry. The coil  was in the central hub, and surrounding it (part of the rotating plastic fan) were the permanent magnets. The plastic "blower case" also seemed slightly warped and because of the tight tolerances it seemed that the fan was scraping the inside edge of the opening.

Perhaps the plastic drying out and warping also caused premature failure, maybe due to heat in the case?

--- End quote ---

Oh, now this definitely seems like a potential winner here.  The switches have the same style of fan blower if I remember correctly.  I'm going to go find one and tear it apart a little later to see if that is indeed the case.


--- Quote from: mzzj on August 27, 2018, 05:19:06 pm ---Many fan bearings nowadays have "hydrodynamic bearings" which means that the bearing surfaces are separated by a thin layer of grease/oil. This needs certain speed range to work (comparable to car hydroplaning/skidding on water)
If the speed is too slow there is going to be direct metal-to-metal contact and associated wear.
Ball bearings also have recommended speed range that would depend on lubricant viscocity etc.

--- End quote ---

This also makes a lot of sense.  I also reached out to a contact I have at their engineering department to get their input.
ogden:

--- Quote from: dkuhn on August 27, 2018, 04:50:56 pm ---Thanks in advance to anyone that can hopefully lend some light on this! (Total newbie when it comes to electrical things, apologies if I didn't include enough info)

--- End quote ---

Rule #1: do not 100% trust manufacturer excuses explaining failures. They will try to save their reputation by all means, putting blame on anything but themselves. Provided reason of failure can be complete BS or maybe not. Only manufacturer knows, but they will not tell you anyway :)

They say:

We have redesigned and tested the switch with new fan hardware that is both more powerful and quieter than the previous component that caused issues.

It is obvious that fan they uses fails prematurely and they had to pick another one. End of story.
Navigation
Message Index
Next page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod