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Home✈️Aircraft🚁HelicopterHelicopter Main Discussion › Metal Failure Experts, Please...
09-05-2006 01:58 AM  14 years ago
JKos

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Redondo Beach, CA

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Rob,
I can't tell you a number for the radius, but it can be seen pretty clearly in the pic I just posted.

If I can get it done locally, I'm going to have a new shaft made which will be 6 mm for the full length as well as long enough to be supported above pinion.

- John
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09-05-2006 02:08 AM  14 years ago
rscamp

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Ontario, Canada

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Another thing you can do is reduce the overhang. The gear teeth and therefore, where the radial load is applied to the pinion are a long way from the drive end bearing. Shorten this distance if you can to reduce the bending stress.

Rob
Coincidence is the science of the true believer. - Chet Rhaymo
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09-05-2006 02:12 AM  14 years ago
JKos

rrProfessor

Redondo Beach, CA

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Rob,
> Shorten this distance if you can to reduce the bending stress.

That would be nice but the motor can't go up much more at all. The mount would hit the tail drive gear.

- John
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09-05-2006 10:35 AM  14 years ago
justme

rrApprentice

israel

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Yep, by top view i meant straight down the shaft.
The final fracture does look like a sudden fracture caused when the shaft couldnt resist the stress,and it does look typical to a fracture caused by fatigue, but the propagation part doesnt look like fatigue, but thats just me, a top view will be better to look at.
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09-05-2006 07:54 PM  14 years ago
CK_

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Redondo Beach, CA

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I think you need a bearing to support the shaft above the gear. Without support, the gear is cantillevered and the side load at the gear bends the shaft back and forth every revolution. It looks like high cycle bending fatigue right at the stress concentration where the shaft changes diameter.

Chris
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09-05-2006 08:04 PM  14 years ago
JKos

rrProfessor

Redondo Beach, CA

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Rob,
> Shorten this distance if you can to reduce the bending stress.

Looked at this again. The motor could be moved up maybe 1/4" max. That would be putting the top surface of the motor right at the tail drive gear.

Chris,
> I think you need a bearing to support the shaft above the gear.

I would now agree. I wish there was an easy way to accomplish that with the stock motor shaft lengths. Both the Actro 32-3H and the Hacker A50-12L shafts are too short. A longer pinion with a top area to go into a bearing would work too (like a nitro pinion).

- John
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09-05-2006 08:19 PM  14 years ago
rscamp

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Ontario, Canada

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A third bearing virtually eliminates the radial load but it has its own issues. I would suggest that localized stresses can be reduced greatly before going to that extreme. You don't have to eliminate stresses, you just need to reduce them to a level for which the steel would survive virtually an infinite number of stress cycles (the fatigue limit).

Ignoring material for a moment, you can reduce the stress riser with a taper or larger internal radius to the smaller shaft diameter and with a shorter moment arm.

Rob
Coincidence is the science of the true believer. - Chet Rhaymo
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09-05-2006 11:16 PM  14 years ago
misskimo

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Alaska 17 years, before mississippi for 31y

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heli this big and that much power , theres no way to eleminate the wobble affect that is causeing this problem . its already known at its near max rpms that it wobbles , shaft fat.
to fix this , you can do away with the shaft side , like a Ion , turn the motor upside down , make a mount kinda like the Ions , get the shaft adaptor with a 10t MA pinion and bearing block above the pinion , and then you dont have to worry about this happen again .
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09-06-2006 12:56 AM  14 years ago
JKos

rrProfessor

Redondo Beach, CA

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The MA setup would put the motor much too low in my heli and then the batterys and motor would be trying to occupy the same space. But, a similar setup with a much shorter adapter may work.

- John
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