Understanding one-way bearing failure

[tsoebirk]

Two months ago, I crashed my eXo. In the circumstances it seemed to me like an abrupt loss of power after about 3 minutes of flight. I managed to get it right-side up, but lost all headspeed resulting in a drop from about 10-15 feet straight onto the skids. Apart from some canopy breakage, it looked like the only damage was that the screws holding the skids were bent and pulled out of the cross member threads. Not bad, considering.

I haven't had time to repair it until now, but have been wondering on and off what could have caused the power loss. It wasn't a complete power loss, since I retained cyclic and collective control, and the ESC should have only given me a slowdown in case of LVC. During rebuilding today, I noticed that the main pulley was rotating the wrong way on the one way bearing. Very strange since the only thing I had done when I disassembled the heli was to pull the head and main shaft out before removing the pulleys and motor, after which I immediately put the pulleys back on the shaft in order to keep the parts in one place. In other words, I have not removed the one-way bearing or turned the pulleys relative to one another. It was as if the one-way bearing had simply reversed itself. I then took the pulleys off the main shaft and pulled out the autorotation sleeve. Promptly one of the rollers fell out of the one-way bearing.

So the one-way bearing is obviously broken, and I am now thinking that perhaps this was actually the cause of the power loss during flight. The heli was a good distance away at the time, so I am not sure if I would be able to hear if the motor was still running - I simply assumed that it was not, and I reflexively hit TH before it hit the ground.

I would be quite happy if the crash was caused by the failure of the one-way, because then I can simply replace it and get back to flying (once the weather improves) without worrying about other potential causes, such as electronics, etc.

What do you think? Is it plausible that the one-way could have failed in flight and then afterwards have run in the wrong direction?

[Dogdipstick]

Short answer yea. Ive destroyed one ways switching from 2200 - 24-500. They sometimes work for a little after a mild inc. but if there was power on it when it went it is probally a goner, I have seen them change from one way to the other way for a little they just fall apart after any kind of shock or impact alot of the time.

I have also had ones that went through one or two "incidents" and never broke the rollers, thanks to TH.

I had it happen on a Trex 450 needle one way more than once. Turning the rotor the other way but locking on the normal rotational way.

[tsoebirk]

Originally Posted by Dogdipstick

Short answer yea.

Thanks! Good to know that I have likely identified the root cause

[Dylan Hudock]

Yes a failed one way bearing can definitely have symptoms like you described. I have seen them fail as you described, where it is a loss of power, along with times where it no longer allows rotation and appears to be locked to the one way shaft inside the bearing.

Definitely replace the one way. I would keep the first flight or two on it after repair pretty mild, just to get an idea if it was actually the one way that failed or was an electronics issue.

[dunkonu23]

Were there any strange noises such as an abrupt increase in motor noise when you noticed the failure? I'm asking because loss of the OWB would likely cause a sudden increase of motor noise until the governor (if used) caught up.

Scott

[tsoebirk]

Originally Posted by dunkonu23

Were there any strange noises such as an abrupt increase in motor noise when you noticed the failure? I'm asking because loss of the OWB would likely cause a sudden increase of motor noise until the governor (if used) caught up.

Scott

Not that I can recall. But the heli was around 25 meters away when it happened, so I don’t know how much I could have heard.
I use the Brain gov, and I expect it would quite quickly correct for the motor overspeeding.

[Ah Clem]

Over the years I have seen quite a few ramp/roller clutch failures.

Usually, the failures are caused by some of the rollers not fully engaging when the clutch is locked. This can be due to grit or the use of sticky grease in the clutch (causing some of the rollers to remain and the bottoms of their ramps and failing to take any of the load).

When the clutch is stressed with only some rollers engaged, it can force the engaged rollers over the tops of their rams,either jamming the clutch (to where it won't unlock), or causing the rollers to round off the tops of the ramps, go past the ramps, and break the cage. This usually results in failure of the clutch to lock/endless free-wheeling in both directions (not good if it happens in flight).

Sometimes the clutches are bad. Sometimes damage from previous crashes goes undetected. Sometimes the sleeve that goes through the center of the clutch is machined slightly too small. Sometimes the press-fit of the clutch is not tight enough, allowing the outside of the clutch to expand and freeing the rollers). In a few cases, the clutch used is not large enough for the load (have not seen this in a long time).

On some older designs there are no radial bearings above and below the clutch, allowing the main gear to tilt, and the clutch to take side loads-also not good from a longevity standpoint.

The usual culprit, from what I have seen in working with these things since around 1980 is grease, causing the rollers to stick.

Triflow oil or ATF works very well. ATF is, IMHO, the best.

[tsoebirk]

Thanks for elaborating. I have used grease up to now, but will try using ATF next time. Looking forward to receiving some new one-ways in the mail within a few days, so I can finish rebuilding. When the weather will be good enough to fly in is another story
How do you guys apply ATF to the OWB?

[Ah Clem]

"How do you guys apply ATF to the OWB?"

I have a small amount of ATF in a syringe (not the kind with a needle).

I put a few drops into the clutch, put the sleeve in it and roll it, then take the sleeve out and repeat the process.

It doesn't take much.

[cyclo]

To apply ATF on the OWB, here are the steps I follow:

1. Soak the OWB and sleeve in isopropyl alcohol using a small bowl then carefully rub the bearings with a foam tipped bud soaked in isopropyl to remove traces of the factory applied grease.



2. Soak the OWB and sleeve in a small bowl filled with ATF.



3. Rotate the sleeve a couple of times against the OWB to make sure the ATF penetrates the roller/pin bearings.

4. Repeat steps 2 and 3 a few times.

I have been doing the above steps for all my helis except the X3. I have had no problems with the respective OWBs throughout the years... Knock on wood. On the X3, the needle bearings in the OWB are too delicate for the process outlined above so I just install them with the factory grease.

[Ah Clem]

Great advice, Cyclo!

[GimpyGolden]

I've had a One Way reverse on me too. Actually also o a Compass.
Zapped all head speed and Whap!

I found I could 'pop' it back to normal but it wasn't usable. Just toss and get another.

[fluxcapacitor121]

Do you think one could submerge the main pulley/one way bearing/radial bearing assembly in alcohol and then atf?

It would take about 16 oz of each I suppose.

[tsoebirk]

Thanks for all the replies!

I have now rebuilt and successfully test-hovered my eXo

I ended up soaking the OWB in ethanol to clean out the factory grease, and I then used a syringe to apply ATF after pressing the bearing into the hub. It seems to be running smoother than ever.

[Ah Clem]

OUTSTANDING tsoebirk!

[Dylan Hudock]

Happy to hear that!!

[fluxcapacitor121]

I pressed mine out last night and am going to give her a long bath in some valvoline synthetic dexron.

[fluxcapacitor121]

So how long does the atf last in there? I would think it would weep out slowly, but maybe not.

Do you guys re-lube every 25 or 50 flights?

[Ah Clem]

I re-lube about once a year.

If I have to tear a ship down, I also do it then.

[tsoebirk]

I got 3 flights on my eXo yesterday. All were uneventful - so much so that I forgot to fly carefully in anticipation of a problem

I am ready to conclude that the previous failure was, in fact, due to the OWB. No more worries about other possible causes!


On a side note, the head seems to spin much easier than ever. Thanks for the tips on lubing the OWB.