View Full Version : Why fan bearings get noisy....

Terry Porritt
21-08-2002, 09:23 AM
A tribological explanation... tribos, from the Greek for rubbing, friction, ie the study of friction, wear, and lubrication.

Fan bearings are always getting noisy and attract quite a few postings, why do they get noisy?

Many fans are fitted with porous bronze, oil filled plain bearings.

The oil is vacuum impregnated into the bearing during manufacture.

The lubrication mechanism in porous bearings is "marginal lubrication", where the surfaces are separated by a thin oil film most of the time but also get metal to metal contact, particularly at start up and stop when most wear takes place.

When the surfaces are separated by a hydrodynamic oil film the fan will run silently. As the bearings age, the oil leaks out of the porous bronze, dries out, and more and more metal to metal contact takes place.

Thus the bearings will get noisier and noisier, dirt getting into the fan will exacerbate the problem.

Lubricating the bearings with a drop of watch oil can only give a temporary solution as surface tension prevents the oil getting back into the porous bronze.

21-08-2002, 09:57 AM
To be honest, I've never had a problem with fan noise associated with bearings... <I guess I've now jinxed myself and am expecting problems within the next week>


Graham L
21-08-2002, 03:51 PM
My experience with sintered bronze bearings has been that the fans stop. With ball bearings, especially "cheap" ones (and bearings in computer fans will be cheap) the "lifetime lubrication" will dry out. Hard steel to hard steel is not an ideal antifriction design. They will rattle as clearances appear.

I have relubricated sintered bronze bearings. I just heated them and dropped them in Molyslip instrument lube. The replacement ones which arrived some months later were sealed in plastic with lots of a simimlar lubricant.

Terry Porritt
21-08-2002, 04:04 PM
That's a good way of re-lubricating a sintered bush, Graham, not only does the moly. provide a good boundary lubricant, but heating the bush allows oil to be drawn back in as it cools in the fluid and the air in the voids contracts.
I was really talking about adding a drop of oil to an installed bearing, it just tends to sit on the surface without penetrating.
It is usual practice to have an oil filled wick to continue to provide oil after that in the bush begins to disappear, but they dont do that in fans.

Graham L
21-08-2002, 04:17 PM
Textronix (rackmountable) scope maintenance manuals told you where on the label on the back of the fan hub to poke your hypodermic syringe needle to relube the bearing.

Chris Wilson
21-08-2002, 05:13 PM
>>but also get metal to metal contact, particularly at start up and stop
>>when most wear takes place.

Another good reason never to turn your computer off! ;)

Graham L
21-08-2002, 05:20 PM
You can get infinite fan life if you never turn the thing on. ]:)

Terry Porritt
21-08-2002, 05:32 PM
Yes, Tektronics would be a bit Rolls Royce for a PC, but their fans would probably last the lifetime of the PC and more. Motors and good fans usually have wick reservoirs.

Fine dust is the killer, that's why the fan stops eventually, efficient sealing is difficult because the power is so low, that a rubbing seal would have too much friction. A proper labyrinth seal would just cost too much for an el cheapo PC fan and be difficult to assemble. A wind-back seal wouldnt cost much more, just the extra die costs.

Rolling bearings dont offer any advantage over plain bearings for this very reason, once dirt get in, thats it. At least plain bearings can be cleaned out and and made to go again.

Graham L
21-08-2002, 06:37 PM
The RM545s would have been about 25 or more years old before I looked in the maintenance manuals for information about fans. They were getting about 6-12 hours a day of use. But that's professional gear.