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Old 01-07-2013, 08:13 PM   #1 (permalink)
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Question one way bearing complaint

Guys
Ive had a couple of crashes with the 700. This is really an awesome machine but I cant figure out why the one way is shot after a crash. I tic tocked into the ground by accident. Destroyed the gear and the blades.
that was it, so I thought. After I repaired the damage and replaced the blades I found the one way was slipping, this is the upgraded system with the 2 bearings and the one way.
Have any of you experienced this. With my other machines, except the avant, I can say I've never had to change a one way.
Is there a better quality one way available?
Kindly let me know
thanks
d
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Old 01-07-2013, 08:44 PM   #2 (permalink)
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It has been mentioned elsewhere to check the one-way after a crash.
What is different compared to other helis, is the 2 stage drive system. The one way bearing is on the first stage of the gear train on the Goblin. So only about half the gear reduction is done at this point. This is double the amount of torque the bearing would be subject to seeing in a crash than it would in another machine.

Possible solutions would be an additional oneway bearing in line, or a larger diameter first stage drive shaft and bearing that would be able to handle a higher shock load.
I do not believe it is the quality of the bearing at all, I believe it is the design limit of the bearing at this size.
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Old 01-08-2013, 02:24 AM   #3 (permalink)
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My OWB also stopped working after my 700 crash. completely spins freely both ways.
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Old 01-08-2013, 03:31 AM   #4 (permalink)
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Quote:
Originally Posted by snemi00 View Post
What is different compared to other helis, is the 2 stage drive system. The one way bearing is on the first stage of the gear train on the Goblin. So only about half the gear reduction is done at this point. This is double the amount of torque the bearing would be subject to seeing in a crash than it would in another machine.
Actually the one way bearing sees less torque in the middle shaft then on the mainshaft. And most helis have their owb on the mainshaft as far as i know.
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Old 01-08-2013, 04:41 AM   #5 (permalink)
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Originally Posted by djamgils View Post
Actually the one way bearing sees less torque in the middle shaft then on the mainshaft. And most helis have their owb on the mainshaft as far as i know.
Correct. Less torque, greater rpm than main shaft mounted owb.
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Old 01-08-2013, 06:14 AM   #6 (permalink)
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Djamgils and seefest you are both absolutely correct I'm not sure what the heck I was thinking last night.
Absolutely, the torque on the intermediate shaft is less and the rpm is about 3x more relative to the mainshaft. Power remains constant throughout the drivetrain. Basic mechanics, my apologies.

I still believe the issue is the shock load that the one way is able to handle with respect to it failing in crashes.
It's a 10mm ID bearing, whereas most helis with the oneway on the mainshaft use a larger one. Also, most other designs have the oneway riding on a sleeve on the mainshaft, making it an even larger ID.
The Logo XXtreme has a 14mm ID oneway riding directly on the mainshaft
The Trex 700 has a 15mm ID oneway on a sleeve over the mainshaft
7HV uses a sleeve over the mainshaft as well, I'm not sure of the diameter.

I know I've had a few failures on the tiny oneway on my mini Protos design that places the oneway right on the main motor shaft for super high rpm exposure. I wonder how that will work out in their new 700. I believe Miniature Aircraft did the same thing with the oneway up at the motor.
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Old 01-08-2013, 06:39 AM   #7 (permalink)
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Thanks Guys
So are we saying that there is no better owb available?
D
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Old 01-08-2013, 07:11 AM   #8 (permalink)
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Quote:
Originally Posted by snemi00 View Post
I still believe the issue is the shock load that the one way is able to handle with respect to it failing in crashes.
It's a 10mm ID bearing, whereas most helis with the oneway on the mainshaft use a larger one. Also, most other designs have the oneway riding on a sleeve on the mainshaft, making it an even larger ID.
The strange thing is that a 12mm owb can handle ~2.3 times the force of a 10mm owb and a 14mm owb can handle ~3.25 times the force of a 10mm owb.
So the 10mm of the goblin is roughly equivalent to the strength of a 14mm mainshaft owb.
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Old 01-08-2013, 09:14 AM   #9 (permalink)
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I did some basic calcs to see what was happening here.

Standard spec for a 1012 oneway bearing lists a torque rating of 5.3Nm

If we assume a powerful motor like the Scorpion 4525 Ultimate/Limited the max continuous power is about 4500W. After losses if we assume an electrical and mechanical efficiency of 85% we get about 3800W at the bearing.

With a 2100rpm headspeed after reduction, the secondary shaft with the bearing is actually spinning at 2100x68/20=7140rpm

Torque(Nm)=Power(W)/2πxSpeed(rev/sec)
T=3800/2πx7140/60
T=5Nm

So we are just under the maximum torque handling capability of the bearing, 5 vs 5.3Nm

This is of course assuming a very loaded condition where the motor is putting out its maximum continuous rated output. With a 4 minute flight using up 4Ah of battery at an average voltage of 3.7v/cell you get 178Wh/(4min/60min/h)=2670W average power consumption. Again with 85% efficiency this is about 2270W average power transmitted through the bearing.
So average load on the bearing is more in tune with 3Nm in an aggressive flight

In a crash or in transients the peak loading of the motor can spike to 10,000W
Again with 85% efficiency this is 8500W and this can translate to 11.4Nm, far exceeding the spec of the bearing.

Important: Just because the spec of the bearing can be exceeded does not mean a catastrophic failure of the bearing. It just means the bearing can not hold the torque.
When this happens, the shaft slips momentarilty until it catches again. The more times the shaft slips on the bearing, the more wear and tear and eventual failure.
This is why you will see scoring on an unhardened shaft or marring on a hardened one.
The outer diameter tolerance of the shaft is also super critical to proper torque holding.
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Old 01-08-2013, 09:18 AM   #10 (permalink)
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Someone has broke out the force calculations from their Engineering schooling days!
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Old 01-08-2013, 09:59 AM   #11 (permalink)
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Quote:
Originally Posted by OICU812 View Post
Someone has broke out the force calculations from their Engineering schooling days!
Sorry...Class of 95
This was bugging me all morning, mostly that I screwed up last night in my mind and I had to clear it up. Had a few moments of clarity and this is what came out.
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Old 01-08-2013, 02:56 PM   #12 (permalink)
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Geek.
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Old 01-08-2013, 06:40 PM   #13 (permalink)
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lol
I ordered one from boca today. Same dimensions but stainless steel.
I'll let you know how it works out.
D
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Old 01-08-2013, 09:57 PM   #14 (permalink)
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Quote:
Originally Posted by deepak View Post
lol
I ordered one from boca today. Same dimensions but stainless steel.
I'll let you know how it works out.
D
More importantly... let us know your replacement procedure if you are successful getting the old one out and new one in without damaging the gear itself :-) I have three gears hanging on the wall with bad OWBs in them just waiting for a chance at a second life!

-Adam
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Old 01-08-2013, 10:17 PM   #15 (permalink)
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the easiest method is to heat the gear up and the bearings come out easily clean it up and then heat the gear again and the new bearings should slide in using some bearing retainer
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Old 01-08-2013, 10:17 PM   #16 (permalink)
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i say from experience having my OWB fail as well
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Old 01-08-2013, 10:35 PM   #17 (permalink)
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sweet! I'll try that and have me three spares :-)
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Old 01-20-2013, 08:53 AM   #18 (permalink)
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Hi, I had to take out the one way bearing (with heat gun) but now I am not sure the direction I have to put the new one
Can someone take a photo where we see the arrow ?
Thanks
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