Wing strut attach fitting problem

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BillDavis

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Oct 19, 2014
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Hi everyone I have a beaver rx28 ultralight and I was assembling it to fly this past weekend and during assembly of my last wing strut to the fuselage bracket the fitting on the wing strut broke. The fitting appears to be 6061 aluminum and is 1"x3/4" bar stock with a grove cut in the end. The manufacturing process on the part looks very crude. Does this part appear to be a factory part or ...? I know it is acceptable procedure to make these strut fitting out of bar stock but not sure if it is acceptable to cut a groove in it as was done here. I believe this groove weakened the fitting greatly and really am trying to figure out how to proceed. Any help appreciated.

Thanks
Bill
 

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Matt G.

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If it were made with a radius between the two lugs instead of a sharp corner, that would help. From your pictures, the slot on the original part appears to have square corners.
 

Dana

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If it were made with a radius between the two lugs instead of a sharp corner, that would help. From your pictures, the slot on the original part appears to have square corners.
That's my first reaction too. A stress concentration and a crack just waiting to happen.

Dana
 

FritzW

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I agree with everything said about the need for a radius on the corners but I'm surprised 6061 T6 broke like that. I'm not a metallurgist but it seems like there's something else going on: work hardening, vibration, fatigue, ...something?
 

BBerson

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Breakage on assembly implies misalignment stress during assembly or perhaps the bolt was tightened and broke it off.
 

Turd Ferguson

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Breakage on assembly implies misalignment stress during assembly or perhaps the bolt was tightened and broke it off.
The black fitting between the aluminum forks looks quite a bit smaller than the slot in the forks.
I am in agreement with BBerson and Kent Ashton.

If the tab was too thin and bolt through the fork was torqued so the tangs clamped on the tab.....could have applied too much bending load on those short, fat tabs. That's a clean shear type break. The distance from the hole to the bottom of the slot in the fork is a clue. I'd call the manufacturer and get a reference for that bolt's torque value.
 

Autodidact

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The black fitting between the aluminum forks looks quite a bit smaller than the slot in the forks. That would be a problem too
Measure the fork space to see if the forks were bent inward.
I am in agreement with BBerson and Kent Ashton.

If the tab was too thin and bolt through the fork was torqued so the tangs clamped on the tab.....could have applied too much bending load on those short, fat tabs. That's a clean shear type break. The distance from the hole to the bottom of the slot in the fork is a clue. I'd call the manufacturer and get a reference for that bolt's torque value.
Before I read these posts, I was starting to think, "Ah, the bolt could be over-tightened, that could do it." I can't see another way for that to happen; there's only one bolt through the strut and the fitting can self adjust for alignment, so that shouldn't be causing the stress...
 

cheapracer

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The fork spacing (the "groove") is too wide for the bracket, needs shims added to zero clearance, ideally you would need a weight to very lightly tap the shim into place.
 

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kennyrayandersen

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So, this appears to be a perennial problem with clevice-type joints. If the two parts rotate relative to each other then you don't want to tighten them down with pre-load. If the clevice tangs are long and thin and the tolerence between the clevice and the lug small then you can get away with tightening the clevice down ( a calculated stress due to the clamp-up must be superimposed onto the flight stresses and onto the fatigue atresses as well. If it is low enough then it may be acceptable.

However, the more common configuration is to bush one of the lugs such that the bushing slides on one tang of the clevice and clamps up on the other. That way tou dont put any preload on the clevice. If the parts don't move relative to each other then you can shim as was suggested so that you don't put a bending stress on the clevice.

the clevice in the pic appears to have enormously thick tangs or as we call the lugs. So I can only imagine how much stress was caused by clamping that thing up. Has that been done all over the aircraft or only at this location?

Thougj it is certainly true that a radius beteen the clevice lugs wiuld lower the corner stresses for increased fatigue life, I believe the failure you are looking at here is a static ultimate failure, not a fatigue failure ( if the broken surface is free from shiny, then it is a static failure otherwise you will see a significant area of shiny, then it might be fatigue.
 
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