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.
On Thu, 11 Sep 2014 14:47:15 -0700, Jeff Liebermann wrote as underneath :
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 years, perhaps 20-30, most have been sintered oilite bearings, these are the ones that tend to go sticky after about 5 years use depending, slow down and quietly stop eventually, usually they dont burn out windings but they do overheat. They can be relubricated to use but the lube will need replacing after only a year or so depending. RPM measured for interest! I took a fan out of the spares box yesterday: Bog standard 12v 0.25A
80mm case fan, definitely not ball bearing!
12v normal speed 2520 rpm air hose "by mistake 1s breeze" 4200 rpm (turbine noise definite) airhose straight on blast 7900 rpm airhose 80 psi directed single jet 9750 rpm. Still worked OK afterwards! IMHO its always safer to stop the fan(s) turning when you use an airhose. C+
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...
** 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.
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.
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Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
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