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  #111  
Old 09-27-2013, 11:19 AM
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bruceh bruceh is offline
 
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Maybe a stupid question, but why isn't it standard practice to put a star lock washer under the jam nuts on the rod end bearings to keep these from coming loose?
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  #112  
Old 09-27-2013, 11:34 AM
Bob Axsom Bob Axsom is offline
 
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Default Well...

Sharp edges dig into spar ...
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  #113  
Old 09-27-2013, 11:54 AM
Daniel S. Daniel S. is offline
 
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Default Lock Washer

This is a very interesting discussion! Reading the last couple posts here regarding the lock washer idea made me think about the Flap rod-end-bearing install (just because I'm there now)... It calls for loc-tite, flat wash, lock washer then jam-nut. Just food for thought. I wonder if that would be a good /bad idea. I'll let you engineer types fret that out. You know more than I. I just follow directions closely
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  #114  
Old 09-27-2013, 11:56 AM
Wayne Gillispie Wayne Gillispie is offline
 
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Quote:
Originally Posted by Bob Axsom View Post
Sharp edges dig into spar ...
causing stress concentrations leading to cracks. Also more susceptible to corrosion. A better choice would be Loc Tite or a self locking jam nut on top of these nuts.
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Last edited by Wayne Gillispie : 09-27-2013 at 12:09 PM. Reason: spl
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  #115  
Old 09-27-2013, 12:08 PM
Wayne Gillispie Wayne Gillispie is offline
 
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Quote:
Originally Posted by Daniel S. View Post
This is a very interesting discussion! Reading the last couple posts here regarding the lock washer idea made me think about the Flap rod-end-bearing install (just because I'm there now)... It calls for loc-tite, flat wash, lock washer then jam-nut. Just food for thought. I wonder if that would be a good /bad idea. I'll let you engineer types fret that out. You know more than I. I just follow directions closely
If using two non self locking jam nuts...getting the correct torque on each nut is difficult.

Here is a good two nut article...

http://www.boltscience.com/pages/twonuts.htm

Here is some good NASA Fastner Design information. See page 7 for the two nut paragraph...

http://ntrs.nasa.gov/archive/nasa/ca...1990009424.pdf
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  #116  
Old 09-27-2013, 12:20 PM
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rocketbob rocketbob is offline
 
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Quote:
Originally Posted by bruceh View Post
Maybe a stupid question, but why isn't it standard practice to put a star lock washer under the jam nuts on the rod end bearings to keep these from coming loose?
I always put a star washer and a thin AN960L under the rod end nuts. Why? Because I've found mine loose.
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  #117  
Old 09-27-2013, 12:28 PM
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Default Thanks Dan

Dan,
Thanks for your theory, drawing and explaination.
It makes perfect sense to me.
So we need to reduce the vibration or spread out the load.


Mark



Quote:
Originally Posted by DanH View Post
All true. However, Mark is sure his bearings were not misaligned, and reports no binding or odd feel. Right now I'm inclined to discount misalignment as a cause.



Bill, perhaps the cycles are vibratory, not aerodynamic. Explanation as we go along.

But first, here's a sketch with what I think is the bending mode when cycled:



In this illustration the tip of the elevator has been forced upward in relation to the fixed stabilizer pivot. The resulting load is as shown. The lower rivet is in tension. The spar material below the rivet is unsupported.
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  #118  
Old 09-27-2013, 12:40 PM
BillFear BillFear is offline
 
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Default another thought

When ours was pulled apart, we noticed that the hinge bearing rod end (on the damaged attach point) was adjusted well out of the nut plate to the point where it is pretty much flush with the back side of the nutplate. I would think having this lever longer would not be helping the situation.



Quote:
Originally Posted by Mark Burns View Post
Dan,
Thanks for your theory, drawing and explaination.
It makes perfect sense to me.
So we need to reduce the vibration or spread out the load.


Mark
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  #119  
Old 09-27-2013, 12:40 PM
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Low Pass Low Pass is offline
 
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Quote:
Originally Posted by Wayne Gillispie View Post
causing stress concentrations leading to cracks. Also more susceptible to corrosion. A better choice would be Loc Tite or a self locking jam nut on top of these nuts.
This engineer adds a vote for LocTite (thread lock compound).

Also, earlier this week (not waiting for an SB, AD, etc, etc.), I inspected my empennage and found loose elev jam nuts and differential play between control surfaces. I added added the second fastener with spacer (AN3), then inspected, lubed and replaced the main AN3 fastener connecting horn to pushrod. No more differential play, and my elevator assy is much more able to withstand asymmetrical loading of spiraling slipstream, and taxiing on rough surfaces, etc.

Last edited by Low Pass : 09-27-2013 at 12:47 PM.
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  #120  
Old 09-27-2013, 12:49 PM
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Bob Kuykendall Bob Kuykendall is offline
 
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Quote:
Originally Posted by bruceh View Post
Maybe a stupid question, but why isn't it standard practice to put a star lock washer under the jam nuts on the rod end bearings to keep these from coming loose?
As Bob A. points out, the sharp edges of the star washer would dig into the spar, creating a crack initiation risk.

Another consideration is that for this application to function correctly, the clamping force must be distributed evenly around the hole in the spar where the rod end inserts. The teeth of the star washer might create an uneven clamping force distribution, which would be magnified when up or down forces are applied by the control surface. And that would be a fatigue liability.

As I've asserted before, this is a sub-optimal application of male-shank rod ends, though one with a long and pretty much satisfactory service history. In order for it to function as desired, care must be used to stay within the envelope of established practice. That is, the rod end extension (number of exposed threads) must be minimized (or at least kept to what the plans suggest), and the nuts must kept tight.

That said, and with all due respect to the previous cautions about doing too much "beefing up" and just creating problems elsewhere, I suspect that overall strength and fatigue resistance of the structure would be enhanced with a modestly-sized doubler on the forward side of the spar that would help distribute stresses more evenly into the thinner parts of the surrounding structure.

Actually, and I know I'm not alone in this, what I think would be best would be to ditch the threaded rod ends altogether and go to a machined part with a sizable footing attached to the spar at multiple points so that it does not rely so much on clamping forces. But that cuts close to the kind of idle speculation that we're trying to avoid.

Regardless, I have full confidence that the engineering crew at Vans is taking this issue very seriously and proceeding with all haste to understand it in all its complexity and to address it. I've known Van for twenty-odd years, and I've met a lot of his engineers, and they are not the kind of people to take this stuff lightly.

Thanks, Bob K.
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