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Thread: Loose lift strut fork

  1. #1
    Marc Davis's Avatar
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    Default Loose lift strut fork

    One of our planes has a little play between the strut fork bolt an the strut tab hole. About 0.005" The bolt is the correct size so the issue is with the hole in the tab.
    I seem to remember that this needs to be a close fit that should have no play. Is that right? If so, what is the normal way to repair this?

    Ream the tab and install a bushing?
    Weld the hole and re-drill and ream?
    I can imagine reaming the hole and the fork for an oversized bolt, but modifying the fork seem like a very bad idea.

    Thanks for any input.

  2. #2
    Marc Davis's Avatar
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    Default Re: Loose lift strut fork

    Adding to this question. From the Piper print the hole size is 0.376 max. An AN6 bolt min diameter is 0.371 So perhaps 0.005 play of acceptable. I'm not sure if the clevis bolts have different specifications than an AN6.

    I'll have to do some more careful measurements.
    Last edited by Marc Davis; 05-13-2017 at 10:15 AM.

  3. #3
    Administrator Steve Pierce's Avatar
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    Default Re: Loose lift strut fork

    I have found two Cabon Cubs with a lot of slop in the strut attach fitting. Found them by grabbing the wing tip and pushing up and down. You coulod hear a pop and followed it down to the strut attach fitting. Took the Cub Crafters engineers many months to come up with an approved repair. They ended up reaming the forks oversize and then I had to ream the strut attach fittings by hand. Not an easy task since the fitting is made up of two pieces of .125" material. Then installed an NAS oversize bolt. The second one they sent me an oversized fork and an NAS bolt and had me knock it in the fitting. Here is a video of the slop, the hole was hourglass shaped.


    The J3 had bushings in that hole and a smaller strut fork and lower gross weight.

  4. #4

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    Default Re: Loose lift strut fork

    I would think the best solution would be to go oversize on the fuselage fitting, shrink fit a bushing, and ream to size after installation. The fitting has plenty of material, so edge distance isn't an issue.


    Sent from my iPad using Tapatalk

  5. #5

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    Default Re: Loose lift strut fork

    If all else fails JB Weld works pretty well

  6. #6
    Marc Davis's Avatar
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    Default Re: Loose lift strut fork

    Thanks for all the comments.

    I've taken the fitting apart and measured the parts. The hole in the fuselage is wallowed out, but only 0.002 (length vs width) that makes for 0.005 slop between the bolt and hole. Surprising how little you can feel. That's actually just within the specifications according to the print for an original fork, NAS high tolerance bolt, and the fuselage bracket. Still makes me worry that it will pound during flight and get worse quickly.
    If it was or gets worse the press in bushing idea was my first thought. I need to do a little analysis to be sure I feel good about removing material there. There is a lot of force on that joint and your life depends on it.
    The JB weld idea may have some merit. It might keep the pieces from moving in flight and pounding. And it can't hurt anything. I also thought about trying a piece of U shaped brass shim along one end of the hole. Perhaps glued in.

  7. #7
    andya's Avatar
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    Default Re: Loose lift strut fork

    Marc

    I think the pressed in bushing is the best option, JB Weld doesn't sound like a good solution plus it will likely show some and generate questions.
    the brass shim would wear quick in my opinion.
    "Progress is our most important problem"

  8. #8

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    Default Loose lift strut fork

    I would suggest starting with a piece of 7/16 x .035 wall 4130 tubing (theoretical id is .368. Measure the tubing, then open the hole in the fitting so you have a .0008 interference fit. Chill the bushing and heat the fitting, then assemble. After everything is back to room temperature ream the bushing to about .372/.374. That should give a nice fit. You will have to take out .004-.006 of material, so go slow. You don't want to spin the bushing.


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  9. #9
    Marc Davis's Avatar
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    Default Re: Loose lift strut fork

    Thanks for all the ideas everyone. The bushing is clearly the best option. I'm working the strength analysis to make myself feel OK with drilling out the hole.

  10. #10

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    Default Re: Loose lift strut fork

    Just kidding about the JB weld

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