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Using shims with the shoulder harness anchors?

N546RV

Well Known Member
OK, so I'm working on fitting my upper aft skin, which includes fitting the F-858-1 shoulder harness brackets. These rivet primarily to the skin, but the aftmost rivet holes go through an upper bulkhead flange as well. The plans call for filing down this area of overlap so the anchor can fit tight to both the flange and the skin.

Seeing as how these must be fit with the skin clecoed to the bulkheads, that means a lot of climbing in and out of the tail cone, which sounds particularly unpleasant in the current ~100° Houston heat.

I'm wondering if there's any reason not to instead fabricate shims that are the same thickness as the bulkhead, and place those between the anchor and the skin. To my mind this is 1) easier and 2) does not require the same precision that filing would, which in my mind seems like a win-win.

Any thoughts on this?
 
Just to clarify what I'm talking about, I made some diagrams. Top shows the anchors per plans, with a relief filed for the bulkhead flange. Bottom shows my alternative idea, leaving the anchor unrelieved and adding an .032" shim between the anchor and skin.
 

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I can't remember what I did but if I had thought about this idea, I'm sure I would have done it. Of course, you'll need a longer rivet, maybe
 
My recommendation is to follow the plans here. This is a seat belt attach point, which would have high loading in the event of an incident.

Installing a "free" shim, particularly one of sizable thickness, causes a change to the way rivets (or other types of fasteners) load up. In a typical two sheet riveted joint, the rivets are in essentially pure shear (until close to the yield strength of the materials, at least). When you install a shim between the two sheets, that shim can now shift slightly under high loading, causing the rivets to "bend" in the middle. Rivets aren't great in bending or tension, so they will fail at a lower load level than without the shim. .032 might not seem like much, but if there are #3 rivets going through it then .032 is a sizable portion of the rivet diameter.

The other solution is to install a so-called "structural shim". Here, you'd make the shim larger than the joint and install fasteners between the shim and one of the sides only in the extra area (the original joint fasteners would remain unchanged). This causes the shim to move with whatever sheet it is most riveted to and causes the plane between the shim and the piece it does not have additional fasteners into to become a shear plane, putting the fasteners back into their intended structural mode.

This is a little difficult to explain without a whiteboard. If it doesn't make sense I can draw a quick picture.
 
Makes perfect sense. I did have an unfocused feeling that the shim addition would change the nature of the joint and how the rivets would be stressed, but couldn't quite put it into words. I think you've nicely covered the changes to the failure mode.
 
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