Hello,
I had rather bad experience with Cobra motors for stunt because of weak Chinese bearings.
Last year, one of my Cobra 2820/12 stopped mid air due to overheating caused by bearings and I barely saved my eParrot with moderate damage to the fuselage and wing. There exists a separate thread in which I described the entire affair.

This motor has been replaced with the brand new 2820/12, the damages have been repaired and I started flying the pattern with eParrot again.

Unfortunately, this particular motor started showing the bearings damage effects after only twelve patterns: two in November 2017 and ten in March 2018.

This time, the growing damage to the bearings manifested itself as strong vibrations in each corner, with clearly audible "bzzzz...."sound.
I could feel the vibrations on the handle.

To confirm my suspicion, I run the motor holding the plane in my hands and flipping it to simulate the corner. I could feel the vibrations and hear "bzzzz..." with each small fast pitching move.

I have checked the motor mounting bolts - they were tight. I have checked the balance of the prop and spinner - no problem here. I have checked the structure of the fuse front for cracks - there were none. I have checked the battery and ESC mounts - both were securely held down by Velcro tape.

Finally, I have tried to rotate the propeller very slowly by hand listening carefully and....bingo! There was a "click" sound associated with momentarily growing rotational resistance. There was only one such "click" for one prop. rotation. When I stopped the prop. in the "click" position and applied the moment perpendicularly to plane of the prop. rotation, the motor shaft rotated slightly in this plane. This means that one of the bearings lost its ability to support the shaft radially in this particular position.

Consequently, the gyroscopic moment acting on propeller in corners was not reacted properly, causing the shaft to momentarily deviate from the line of its required rotation. This is the cause of vibrations and "bzzz.." noise.

I will disassemble the motor today, remove both bearings and find the one causing the vibrations. I believe there is only one bad bearing at the moment but I will know soon.

General comment: Cobra motors are well made and, except of bearings, represent a decent quality. If the Made in China bearings are replaced by better bearings, perhaps Made in Japan ( I mean actually made in Japan not bought by some Japanese company in China and sold to the world as Made in Japan), it may be possible to substantially increase the number of patterns on one set of bearings.

Where can I find good quality metric ball micro bearings in America?

Regards,
MP Stunt Academy Toronto

Per the INNOV8TIVE WEB SIGHT (  http://innov8tivedesigns.com/parts/brushless-motors/cobra-c-2820-12-brushless-motor-kv-970  )  the Cobra 2820/12 has 3 BB. Two each 16mm x 5 mm x 5 mm.  One each 11mm x 5mm x 5mm.  I checked the Boka web sight and they do have this size.  I would call or live chat with them.  They should be able to tell you what quality bearings you need for a brushless aircraft motor.

Hi John,
Thank you for the link to Boca Bearings. I will contact them.

For your information: Cobra 2820/12 has two bearings: one (on the green, stator side) measures 16 mm. OD x 5 mm. ID x 5 mm. thickness.
The other bearing is: 11 mm. OD x 5 mm. ID x 5 mm. thickness.

Cobra 2826 and 3515, 3520 and 3525 have three bearings: one 16 mm. OD and two 11 mm. OD. with other dimensions the same.
Cobra 4120 and 4130 have also three bearings.

When cheap Chinese Bearings in Cobras are replaced by better quality bearings, I anticipate much higher number of patterns per bearings set.

I am flying a lot, will be flying more and if I cannot get, say, about 80 patterns on one set of bearings, I may stop using Cobras and switch to Plettenbergs.

Regards,
M

I've used Cobra motors for the last several years and have them front mounted and use an APC 13x5.5 cut down to 12.  No problems with the bearings what-so-ever.  I think Howard nailed it with your rear mounted Cobras.  I used 2826 and lately 3515/18.

Hi Motorman,
EParrot (attached) uses APC 11x5.5 MRP propeller, weighting 18 grams (0.63 oz.).
This propeller is lighter than APC 11x5.5 E that weights 25 grams (0.88 oz.).

APC 11x5.5 MRP has smaller hub and slightly wider and thinner blades. The bending stiffness of this prop. blades is about 75% of the bending stiffness of the APC 11x5.5 E blades. I know that the gyroscopic moment in corners can bend less stiff propeller blades out of the plane of the prop. rotation and that this creates a momentary dynamic imbalance with the buzzing sound.

I have tried flying eParrot with APC 11x5.5 E propeller and it flies a bit slower at 9,800 RPM. The difference in speed is especially visible in the second corner of the hourglass that becomes lazy with the tendency to stall the model.

If this abysmal weather that we have in Toronto ever improves, I will try flying eParrot again with APC 11x5.5 E, boosting the RPM to, say, 10,000.

Anticipating the question: EParrot is well trimmed now. It grooves well, maintains the level normal and inverted flight with no hunting and almost "by itself", and reacts predictably to the input. It flies on 60' long (eye-to-eye), 0.015" stranded lines and, in general, it is a competition quality airplane now.

Howard, Crist,
Like I wrote, I will be soon flying another, larger model using Cobra 3515/18 with Igor's 11x5 carbon composite 3 blade prop. and/or with 11.5x6 carbon composite 3 blade prop. "Pol-Prop". For your information: "Pol-Prop" is slightly heavier than Igor's 11x5 but also much stiffer. Cobra 3515/18 will be front mounted.

Happy Flying,
M

 

Howard, Crist,
Like I wrote, I will be soon flying another, larger model using Cobra 3515/18 with Igor's 11x5 carbon composite 3 blade prop. and/or with 11.5x6 carbon composite 3 blade prop. "Pol-Prop". For your information: "Pol-Prop" is slightly heavier than Igor's 11x5 but also much stiffer. Cobra 3515/18 will be front mounted.

Happy Flying,
M

Thumbs up!

Like I wrote, I will be soon flying another, larger model using Cobra 3515/18 with Igor's 11x5 carbon composite 3 blade prop. and/or with 11.5x6 carbon composite 3 blade prop. "Pol-Prop".

Matt,
Where do you get the "Pol-Prop"?  Price?

If bearing is of low quality, it will not survive in corners long, but there many other things related to short life time. For example biggest difference make 3 blade prop instead 2 blade. 2 bades do not make symmetrical mass inertia moment (mass concentrated in one line) and thus also gyro moment in corner "vibrates" twice per revolution, it is killing game for any bearing.

But there are also smaller effects, for example proper axial free play: Too much or too little is also not good, that is reason why some makers fit axial free play by shimming and does not allow user to accidentally destroy it.

Oiling: Pure bearing makes feel that it needs oiling, but oiling will dissolve and clean orriginal lubricant in bearing and in that case it need oiling on regular basis.

RPM: Every outrunner has rotor constructed as a bell which has some specific frequency of resonantion. That is usually on motors of our size somethere at 9000rpm. If you run at that rpm bering will get free play soon and that is way for problem, it will soon start make strange noices and soon fail. In that case any replacing does not help, it will happen again with any bearing. The solution is usage under or over that RPM, that is reason why we do not use 6" props which needs just those 9000 rpm. Going to 5" seems to solve those problems on AXI motors which we know from past ( and attempts for supporting by 3rd bearing on back of motor). And I know peaple also running 7" props also without such problems. Another safe solution is ring bearing on rotor like used on Plettenbergs, so it cannot vibrate at all.

Orientation: I personally use reverse mounting and I do not have any problems with life time. I use it especially for better ventilation. Moments from precession are same on both sides of shaft if you mount front or back. But only in case that rop is well balanced. If prop is not so well balanced, it is clearly better to have that large bearing close to prop.

Interesting video. I’ll have to stop banging on my motors with a mallet. 

Are Boca Bearings considered to be high quality? I’ve replaced a few bearings using their products, and thus far have had no problems. I’ve only bought their economy bearings, I don’t see the point in paying big bucks for ceramic bearings that offer marginally better performance.

Of course I’m doing everything wrong. Rear mount, 2 blade 6 pitch prop, RPM near 9000.