Using an air compressor, I was cleaning out the dust from a
consignment of used ATX power supplies, when one of my friends pointed
out that I was killing the fan by spinning the fan backwards. What he
does it use a stick or toothpick to prevent the fan from rotating.
This seemed odd to me as I've been using compressed air to clean fans
by spinning the rotor in both directions and usually faster than rated
RPM for maybe 25+ years without incident. Google found some articles
proclaiming that spinning the rotor with compressed air was a bad
idea. However, I couldn't find anything describing any failure modes.
So, is using compressed air to clean fans really a bad idea?
Is it bad to spin a cooling fan backwards?
What breaks or fails?
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Jeff Liebermann has brought this to us :
Using an air compressor, I was cleaning out the dust from a
consignment of used ATX power supplies, when one of my friends pointed
out that I was killing the fan by spinning the fan backwards. What he
does it use a stick or toothpick to prevent the fan from rotating.
This seemed odd to me as I've been using compressed air to clean fans
by spinning the rotor in both directions and usually faster than rated
RPM for maybe 25+ years without incident. Google found some articles
proclaiming that spinning the rotor with compressed air was a bad
idea. However, I couldn't find anything describing any failure modes.
So, is using compressed air to clean fans really a bad idea?
Is it bad to spin a cooling fan backwards?
What breaks or fails?

Well I was told that so far back that I can't remember (1960s) I think,
Long before PCs, mainframes had lots of fans.
Although I never saw a failure I always thought it was because you
could grossly overspeed the fan if you were stupid enough to put too
much air in. B-)

--
John G Sydney.

On 11.09.14 1:26, Jeff Liebermann wrote:

Using an air compressor, I was cleaning out the dust from a
consignment of used ATX power supplies, when one of my friends pointed
out that I was killing the fan by spinning the fan backwards. What he
does it use a stick or toothpick to prevent the fan from rotating.
This seemed odd to me as I've been using compressed air to clean fans
by spinning the rotor in both directions and usually faster than rated
RPM for maybe 25+ years without incident. Google found some articles
proclaiming that spinning the rotor with compressed air was a bad
idea. However, I couldn't find anything describing any failure modes.
So, is using compressed air to clean fans really a bad idea?
Is it bad to spin a cooling fan backwards?
What breaks or fails?

Spinning the rotor causes induction voltage, which
can be the wrong polarity, or to high, or both,
so internal electronics may be killed.

I can see that spining the fan backwards could be bad if the fan had a brush type motor and the brushes hit the rotor segments going backwards.

What Sjouke said is probably true but I believe there is Zener diode protection all over the place. If so, would it be a good idea to unplug the motor for this ? Maybe not. Let the Zeners conduct for a half a minute, should do no harm. On the other hand, disconneecting it might allow a highr voltage to be developed which could cause a breakdown of insulation in the wiring.

the other concern is the bearings. They are not quite Timkens. they are probably just brass wjhich means there is a functional limit to RPMs. It's a chemical metalurgical thing and I would have to ask my Father. Well, to do that we need to get some Gypsies and have a seance. (Timken bearing are the type used on non- drive car wheel's shafts. They are tapered and tightenable down to zero play and thrust, they are also used on better lathes and other machines)

Anyway, the regular brass bearings almost cannot be called bearings, they are more apty called busings and they wear with every revolution. What's more, the even minor friction in that system exacerbates the wear at at least an exponential rate because of the heat produced. you won;t be aware of it because it is localized right there. what can happen is that the life of the fan is greatly shortened. not that it will fail. but it will become noisy..

I never worried too much about noise until the fan in my laptop got noisy. you know the microphone picks that up ? I do now.

I would say if you can use two hands, use one for the air chuck and the other to limit the speed of the fan.

I was going to mention the 327 the olman built a long time ago that always wiped number one bearing at exactly 9,200 RPM. But that is a differenrt problem, they couldn't get enough oil to it. they did almost everytihng, graduated bearing clearances, high volume, cut the flow to the lifters and cam, and still it wiped out at about the same RPM every time. the problem was the mass of the oil trying to make it through the crank with the centrifugal forces acton on it. The ONLY way they could have reallt fixed it was to use two oil pumps, or a dual feed system. That was too much work.

So they got the crank machined a few thousandths under and had it hard chromed back to size. this way, the crank ws safe and all they had to do was to replace the bearings, and in some cases the connecting rod. And no, that did not unbalance the engine in any signifacant way.

But back to the topic, I THINK the major danger is to the bearings. You're an experimenter, experiment. (and of course post up the results)

Find a way to measure the RPMs when you blow that air through there.

Oh **** finding a way, you already know. It has a tach pulse. Just get the rated RPMs for a given fan, find what it is putting out, it is most likely one pulse per revolution. dunno, never had to check. But then blow that air and see what you get on the freak counter.

Hell maybe I'll do that just for fun. Just to learn something that day.

You know what you do shooting that air in ther ? YOu make a turbine engine. It is not quite as good as the stor boughten ones, but that is what it is.

Thanks for raising a new, interesting point that nobody ever thought of.

wrote:

I can see that spining the fan backwards could be bad if the fan had a brush type motor and the brushes hit the rotor segments going backwards.


** Computer fans have no brushes.

Brush motors ( both universal and permanent magnet types ) are reversible.


.... Phil

"I can see that spining the fan backwards could be bad if the fan had a brush type motor and the brushes hit the rotor segments going backwards. "

HEY, how about just using a really powerful vacuum cleaner instead ?

However, if the RPMs are exceeded... But then, it is probably better for the bearings to keep always going the same way. I am SURE tha tis true of the brushes.

Maybe it doens't make that much difference. Another thing, compressed air has no limit on force, vacuum is automatically limited.

Maybe be better ?

On Thu, 11 Sep 2014 03:06:01 +0200, Sjouke Burry
wrote:

On 11.09.14 1:26, Jeff Liebermann wrote:

Using an air compressor, I was cleaning out the dust from a
consignment of used ATX power supplies, when one of my friends pointed
out that I was killing the fan by spinning the fan backwards. What he
does it use a stick or toothpick to prevent the fan from rotating.
This seemed odd to me as I've been using compressed air to clean fans
by spinning the rotor in both directions and usually faster than rated
RPM for maybe 25+ years without incident. Google found some articles
proclaiming that spinning the rotor with compressed air was a bad
idea. However, I couldn't find anything describing any failure modes.
So, is using compressed air to clean fans really a bad idea?
Is it bad to spin a cooling fan backwards?
What breaks or fails?

Spinning the rotor causes induction voltage, which
can be the wrong polarity, or to high, or both,
so internal electronics may be killed.

My thanks to everyone for considering this problem.

I just spun a common 80 mm fan for about 3 minutes backwards using 80
psi air pressure. I could only run it 3 minutes because I was making
enough noise to have the neighboring office complain. I have no idea
of what RPM it was spinning, but I'm certain it was well above rated
RPM. Applying 12V afterwards to the wires demonstrated that it spun
normally and that nothing had failed. Incidentally, putting a VOM
across the leads, and spinning the fan in either direction produced no
detectable voltage.

I would think that any manufacturer of brushless fans that uses
internal electronics as a driver, which blows up if the fan is run as
a generator either too fast or backwards, is just asking for problems.
I'm fairly sure that the driver IC's are all well protected. That may
not have always been the case, where such fans were driven by discrete
xsistor circuits or the fans had brushes, but todays fans seem
impervious to such abuse.

The view of the inside:
https://www.google.com/search?q=brushless+fan+motor+schematic&tbm=isch
There should be a schematic in there somewhere.... yep:
http://www.nidec.com/en-NA/technology/motor/basic/00005/
Well, it's 3 poles instead of the usual 4 poles, but the principles
are the same.

Note the totem pole output of each driver transistor pair. Without
any applied power, all the transistors are normally open no matter
what the motor is doing. The only way that any generated voltage from
the motor/gen is going to go anywhere past those transistors is if at
least two of them are turn on by some magical source of outside
voltage. So, maybe I can kill this fan by applying a low voltage to
the leads, making the fan spin slowly, and then using air pressure to
spin it in either direction. Back in a few minutes while I try
that...

No damage. I tried 3.3v, 5v, and 12v applied, while I did the air
hose trick. With 12v applied, it made some rather impressive noises
thanks to the overspeed, but no damage. I had a difficult time
stopping the fan with 12v applied, but managed to do it for a few
seconds. Lesser voltages produced similar overspeed or stall, but to
lesser degrees. The transistors were certainly turning on when the
power was applied and the fan was spinning by itself, so any voltage
produced by the spinning magnets should have made it past the totem
pole switches, and into the osc/driver circuitry. So much for that
theory.

The bearing/bushing burnout theory is also interesting. This
particular fan, Mechatronics Model F8025H12B2 12v 0.145A claims to
have ball bearings inside. I'm not going to take it apart because I
know that it doesn't have ball bearings. It uses bushings like all
such cheap PS fans. The only fans I've ever seen with real ball
bearing are the larger and more expensive fans by Rotron which burn
20-40 watts instead of 2 watts. At the higher power level, any
imbalance is going to put a nice gouge or dent into a bushing, while a
ball bearing arrangement will distribute the vibration loading and not
be affected.

About the best I can guess(tm) is that fan overspeed will cause the
lubricant to get hotter than usual, which might blow the seal, leak
out, and produce premature failure. I've never seen that since most
such fans don't need my help with drying out the lube or leaking oil.
Still, the few seconds it takes to blow the dust off the fan is not
going produce any significant overheating and certainly not long
enough to pump out the oil.

Summary: Spinning the cooling fan rotor may have been a bad idea in
the distant past, but todays fans can easily take such abuse.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On Thu, 11 Sep 2014 09:33:21 +1000, John G wrote:

Well I was told that so far back that I can't remember (1960s) I think,
Long before PCs, mainframes had lots of fans.
Although I never saw a failure I always thought it was because you
could grossly overspeed the fan if you were stupid enough to put too
much air in. B-)

Methinks you have hit upon the answer. Overspeed and reverse rotation
may have been a problem for the fans of the 1960's, but todays
brushless computah fans are largely immune to such abuse. Running a
commutator type motor backwards will probably do some damage to the
commutator because the brushes are inserted at a slight angle (to help
clear out debris between the brushes and commutator sections).
However, none of the computah fan motors I'm currently dealing with
have commutators or brushes.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Jeff Liebermann wrote:

Using an air compressor, I was cleaning out the dust from a
consignment of used ATX power supplies, when one of my friends pointed
out that I was killing the fan by spinning the fan backwards. What he
does it use a stick or toothpick to prevent the fan from rotating.
This seemed odd to me as I've been using compressed air to clean fans
by spinning the rotor in both directions and usually faster than rated
RPM for maybe 25+ years without incident. Google found some articles
proclaiming that spinning the rotor with compressed air was a bad
idea. However, I couldn't find anything describing any failure modes.
So, is using compressed air to clean fans really a bad idea?
Is it bad to spin a cooling fan backwards?
What breaks or fails?

For Pc fans, it's harmless to clean them with compressed air, unless
you're trying to overspeed them by angling the air stream just right,
turning the fans into turbines.

I did once basically explode a squirrel cage blower with the pressure side
of a vacuum cleaner. It ran so fast that the fins expanded and touched the
housing and became a bent up mess before flying out of the housing across
the room. I was not expecting that.

Jeff Liebermann wrote:
On Thu, 11 Sep 2014 09:33:21 +1000, John G wrote:

Well I was told that so far back that I can't remember (1960s) I think,
Long before PCs, mainframes had lots of fans.
Although I never saw a failure I always thought it was because you
could grossly overspeed the fan if you were stupid enough to put too
much air in. B-)

Methinks you have hit upon the answer. Overspeed and reverse rotation
may have been a problem for the fans of the 1960's, but todays
brushless computah fans are largely immune to such abuse. Running a
commutator type motor backwards will probably do some damage to the
commutator because the brushes are inserted at a slight angle (to help
clear out debris between the brushes and commutator sections).
However, none of the computah fan motors I'm currently dealing with
have commutators or brushes.



Most ollder larger computers used ac fans. I've blown out a few fans, no
problems seen.

Greg

On Wed, 10 Sep 2014 16:26:00 -0700, Jeff Liebermann
wrote:


Using an air compressor, I was cleaning out the dust from a
consignment of used ATX power supplies, when one of my friends pointed
out that I was killing the fan by spinning the fan backwards. What he
does it use a stick or toothpick to prevent the fan from rotating.
This seemed odd to me as I've been using compressed air to clean fans
by spinning the rotor in both directions and usually faster than rated
RPM for maybe 25+ years without incident. Google found some articles
proclaiming that spinning the rotor with compressed air was a bad
idea. However, I couldn't find anything describing any failure modes.
So, is using compressed air to clean fans really a bad idea?
Is it bad to spin a cooling fan backwards?
What breaks or fails?

I'll bet this is a carry over from the days when running a brush motor
backwards would degrade/ruin the brushes. I know; I ruined one that way.

Jeff Liebermann wrote:



The bearing/bushing burnout theory is also interesting. This

particular fan, Mechatronics Model F8025H12B2 12v 0.145A claims to

have ball bearings inside. I'm not going to take it apart because I

know that it doesn't have ball bearings. It uses bushings like all

such cheap PS fans.



** FFS - wot an utter ignoramous.

Peal of the paper label and take a look sometime.

Brushless DC fans mostly use ball bearings.



.... Phil

Mythbusters, anyone?

To the best of my knowledge, there is nothing in any motor that
//mechanically// favors rotation in one direction. "Forwards" and "backwards"
are determined electromagnetically, not mechanically.

The maximum speed before the bearings are damaged depends on the bearing
quality, I assume. The only damage I can image is knocking the shaft
off-kilter.

"Cydrome Leader" wrote in message ...

I did once basically explode a squirrel cage blower with the
pressure side of a vacuum cleaner. It ran so fast that the fins
expanded and touched the housing and became a bent-up
mess before flying out of the housing across the room.
I was not expecting that.

"NO ONE expects a squirrel-cage blower explosion..."

On Thu, 11 Sep 2014 06:11:07 -0700 (PDT), Phil Allison
wrote:

Jeff Liebermann wrote:
The bearing/bushing burnout theory is also interesting. This
particular fan, Mechatronics Model F8025H12B2 12v 0.145A claims to
have ball bearings inside. I'm not going to take it apart because I
know that it doesn't have ball bearings. It uses bushings like all
such cheap PS fans.

Peal of the paper label and take a look sometime.
Brushless DC fans mostly use ball bearings.
... Phil

I've taken quite a few power supply and CPU fans apart in order to
clean off the ring of tar (formerly oil) that accumulates on the rotor
shaft and re-lubricate the fan. Usually, the shaft fits into what
looks like a sintered bronze sleeve. I don't recall seeing ball
bearings on the smaller (80mm or less) but may have been looking at
fans with both a sleeve and a ball bearing (1B1S), as described below
at bottom of page:

Anatomy of Computer Fans
http://www.hardwaresecrets.com/printpage/Anatomy-of-Computer-Fans/1039

Ball vs. Sleeve: A Comparison In Bearing Performance
http://www.nmbtc.com/fans/white-papers/fans_ball_vs_sleeve/

Fan Design (review) including info on bearing types:
http://www.hardwarecanucks.com/forum/hardware-canucks-reviews/4534-glacialtech-igloo-5750-silent-cpu-cooler-review-5.html

Let me see if I can dream up some other ways to destroy this fan with
compressed air before I dissect it. Stay tuned.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On Thu, 11 Sep 2014 07:23:47 -0700, "William Sommerwerck"
wrote:

Mythbusters, anyone?

Sounds like fun. Let's run some fans at Mach 2 in a wind tunnel and
watch them fly apart. (Anything worth doing is worth overdoing).

To the best of my knowledge, there is nothing in any motor that
//mechanically// favors rotation in one direction. "Forwards" and "backwards"
are determined electromagnetically, not mechanically.

The maximum speed before the bearings are damaged depends on the bearing
quality, I assume. The only damage I can image is knocking the shaft
off-kilter.

Oh-oh. I think you may have hit on a potential problem. As long as
the pressure is equal on all the rotor blades, the rotor shaft remains
centered in the middle of the bushing. However, with an air hose, one
could apply all the pressure on one side of the frame, producing an
asymmetrical side load on the rotor shaft. In other words, I'm
producing a bearing load condition for which it was not designed. If
the rotor shaft were dry and the lube gone, the high speed rotation of
the rotor shaft might put a gouge into the edge of the bushing. A
ball bearing would be unaffected.

I've seen a variation of the above potential problem with bushing type
fans, as in ATX type power supplies. The components inside the power
supply are not uniformly distributed around the fan intake area. The
result is that the pressure drop at different points around the fan is
not uniform. This produces an asymmetrical fan load which results in
a side load on the bushing. If lube were present and functioning, no
damage would be done. However, if the rotor shaft were mounted
vertically, and all the oil drips to one end of the shaft, the
potential for damage is quite real.

I guess the proper cleaning technique with bushing type fans is to
move the air hose nozzle rapidly around the circumference of the duct,
thus preventing bushing wear at any one spot.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On Thursday, September 11, 2014 10:23:47 AM UTC-4, William Sommerwerck wrote:
Mythbusters, anyone?

To the best of my knowledge, there is nothing in any motor that
//mechanically// favors rotation in one direction. "Forwards" and "backwards"
are determined electromagnetically, not mechanically.

I've seen (very cheap) brush-type motors that had asymmetric brush
springs, that would possibly buckle if the motor was reversed.

On Thu, 11 Sep 2014 06:11:07 -0700 (PDT), Phil Allison
wrote:

Jeff Liebermann wrote:
The bearing/bushing burnout theory is also interesting. This
particular fan, Mechatronics Model F8025H12B2 12v 0.145A claims to
have ball bearings inside. I'm not going to take it apart because I
know that it doesn't have ball bearings. It uses bushings like all
such cheap PS fans.

Peal of the paper label and take a look sometime.
Brushless DC fans mostly use ball bearings.
... Phil

I took apart the fan and much to my amazement, found two ball
bearings.
http://802.11junk.com/jeffl/pics/megatronics-fan/
I just hate it when I'm wrong. I have some more junk fans somewhere.
I'll see what I can find. In the past, they were mostly bushings, but
I guess that's changing, especially since this fan was fairly new.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

William Sommerwerck wrote:
"Cydrome Leader" wrote in message ...

I did once basically explode a squirrel cage blower with the
pressure side of a vacuum cleaner. It ran so fast that the fins
expanded and touched the housing and became a bent-up
mess before flying out of the housing across the room.
I was not expecting that.

"NO ONE expects a squirrel-cage blower explosion..."

Just one of those things you have to learn the hard way.

William Sommerwerck wrote:
Mythbusters, anyone?

To the best of my knowledge, there is nothing in any motor that
//mechanically// favors rotation in one direction. "Forwards" and "backwards"
are determined electromagnetically, not mechanically.

The maximum speed before the bearings are damaged depends on the bearing
quality, I assume. The only damage I can image is knocking the shaft
off-kilter.

well for brushed motors, there can be mechanical differences. Brushed
motors can be "timed" to alter speed/torque as well as to support
bidirectional use when the brushes are set to a neutral position. This is
determined by the angles between the brushes and the magnetic poles in the
motor. The same applies to universal motors as in power tools and such. As
you don't run a circular saw or angle grinder backwards, you can bet that
motor was designed for maximum performance in the direction it does run.
Maximum performance can also just mean getting by with the smallest
cheapest motor possible to get the job done.

Charlie+ wrote:
On Thu, 11 Sep 2014 14:47:15 -0700, Jeff Liebermann
wrote as underneath :

On Thu, 11 Sep 2014 06:11:07 -0700 (PDT), Phil Allison
wrote:

Jeff Liebermann wrote:
The bearing/bushing burnout theory is also interesting. This
particular fan, Mechatronics Model F8025H12B2 12v 0.145A claims to
have ball bearings inside. I'm not going to take it apart because I
know that it doesn't have ball bearings. It uses bushings like all
such cheap PS fans.

Peal of the paper label and take a look sometime.
Brushless DC fans mostly use ball bearings.
... Phil

I took apart the fan and much to my amazement, found two ball
bearings.
http://802.11junk.com/jeffl/pics/megatronics-fan/
I just hate it when I'm wrong. I have some more junk fans somewhere.
I'll see what I can find. In the past, they were mostly bushings, but
I guess that's changing, especially since this fan was fairly new.

Like most of us your only wrong some of the time!
Iv done a no. of brushless PC fan "keep going" repairs over the PC

I'm going to have to steal that "keep going" repair term.

"well for brushed motors, there can be mechanical differences. Brushed
motors can be "timed" to alter speed/torque "

I saw that on a vacuum cleaner motor a long time ago. It had slotted mounting holes so you could adjust it.

Bottom line here though is that if it has ball bearings and no brushes all you have to worry about is the blades flying apart, or the squirrel escaping...

wrote:


Bottom line here though is that if it has ball bearings and no brushes all you have to worry about is the blades flying apart, or the squirrel escaping...


** Anyone familiar with RC electric vehicles and planes would be well aware that small DC motors with bronze bushings and brushes are fine at 20,000 rpm and those with ball bearings can be used safely up to 50,000 rpms.

DC fans are simply not in the race.


..... Phil

"Phil Allison" wrote in message
...

DC fans are simply not in the race.

But the ball bearings are. (ba-DUM!)

In article ,
says...

"Cydrome Leader" wrote in message ...

I did once basically explode a squirrel cage blower with the
pressure side of a vacuum cleaner. It ran so fast that the fins
expanded and touched the housing and became a bent-up
mess before flying out of the housing across the room.
I was not expecting that.

"NO ONE expects a squirrel-cage blower explosion..."

especially when the squirrel is still in it!


Jamie

William Sommerwerck wrote:
"Phil Allison" wrote in message
...

DC fans are simply not in the race.

But the ball bearings are. (ba-DUM!)

ha.

On Thu, 11 Sep 2014 07:25:36 -0700, "William Sommerwerck"
wrote:

"Cydrome Leader" wrote in message ...

I did once basically explode a squirrel cage blower with the
pressure side of a vacuum cleaner. It ran so fast that the fins
expanded and touched the housing and became a bent-up
mess before flying out of the housing across the room.
I was not expecting that.

"NO ONE expects a squirrel-cage blower explosion..."

Years ago I was cleaning some bearings and was blowing the solvent out
with compressed air. I knew this was bad news but it was interesting
to really spin up up bearings. Anyway, I put this bearing on my index
finger and spun it up real good. The pitch of the whine from the
bearing rose higher and higher as the speed increased. Just when the
pitch got so high I couldn't hear it the outer race of the bearing
exploded and the balls went flying. I thought my finger was broken
but it just hurt like hell. Needless to say I bought new bearings.
Eric

On Fri, 19 Sep 2014 17:30:41 -0700, wrote:

Years ago I was cleaning some bearings and was blowing the solvent out
with compressed air. I knew this was bad news but it was interesting
to really spin up up bearings. Anyway, I put this bearing on my index
finger and spun it up real good. The pitch of the whine from the
bearing rose higher and higher as the speed increased. Just when the
pitch got so high I couldn't hear it the outer race of the bearing
exploded and the balls went flying. I thought my finger was broken
but it just hurt like hell. Needless to say I bought new bearings.
Eric

I had something like that happen to me with ball bearing assembly. The
problem was that the balls were equally spaced around the bearing and
held in place by a plastic retainer affair. As long as the bearings
were equally spaced, everything worked just fine. However, if one
removes the plastic spacers, all the bearing fall to one end of the
bearing assembly, the whole thing falls apart. I managed to do that
with some solvent, which embrittled the plastic spacers when the
solvent dried out. When I spun the bearings at high speed using air
pressure, the plastic cracked and I had balls flying everywhere.

Blundering forward, I discussed the problem with a competitor (who is
conveniently located next to my office). He mentioned that he has had
problems using compressed air to clean CPU fans on MacBooks. When he
uses compressed air, and overspeeds the fans, they usually work when
he's done. A week or two later, the customer returns with complaints
about fan noise. I've seen this exactly once with a PC laptop CPU
fan. What seems to be happening is the high speed causes heating,
which then causes the dried out oil to reflow. Centrifugal force
redistributes the oil away from the main shaft bushing, where it again
dries out. That leaves the shaft dry and unlubricated. The reason I
haven't seen it much on PC laptops is that I usually lube the CPU fans
on laptops with "turbine oil" even if they seem to be working
normally. I can spin those all day long, and the oil will remain
fluid and functional. I learned long ago that this prevents return
visits and complaints.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558