Structural Shim

gutboy,

What structure is being repaired?

Please attach a sketch of the repair with some details.

Keep em' Flying
//Fight Corrosion!

I don't have a picture of the repair yet, but the repair is to a web of a frame. A doubler is required to repair damage to the web. The web has a .04" step in the area of the damage, so I need to have a .04" shim added to the stepped region to make a flat region for the doubler. The repair doubler is fastened to the web using #5 D solid rivets (some of these go through the shim). According to the SRM 51-30-01 a structural shim is defined when shim thickness / fastener dia. * 100 is greater than 21% (.04/.156 * 100 = 25.6%). So, my shim is classified as structural. The requirements for a structural shim is it be of a solid material equivalent to the repaired structure and it has to be fastened to either the aircraft structure or the added repair part to prevent movement that could allow bending loads in the repair fasteners that aren't evaluated for that loading.

I have not found guidance in the rest of the SRM on how the shim is to be fastened to the aircraft structure or the repair part. How many fasteners are needed to secure the shim in place? What size and type of fastener is needed to secure the shim? Adhesive bonding isn't allowed.

Once I figure out the shim fastener rules, I then know I'll be pulling out my hair on how to place the fasteners in the shim's limited real estate.

Thank you.

Hope this is clearer than used motor oil.

From your description I'm not 100% sure if what you are describing is a shim or simply a filler. There is a difference. You say you've added a "shim" to make a flat region for the doubler. The typical function of a shim (normally tapered in thickness) is to add a transition between thicknesses for an overlaying layer of material. Does your repair part transition up/down the step, or does your "shim" span the length of the doubler so there is effectively no step? Think of a lap filler for a repair at a lap joint.

Also, an 0.040" step? The 737 classics are all formed sheet metal or built-up frames, so what is this step? 0.040 seems thin for a chord flange or fail-safe chord/angle. Are you running into a shear tie or something? Which stringers are you at?



Keep em' Flying
//Fight Corrosion!

Its in Bruhn, under the title "filler" page D3.5

Thank you guys. My vocabulary needed some work. My shimming issue was actually a filler problem. Thank you for all of the references. I can say that I'm always learning. Thank you again.

1) Shims in aerospace applications don't have to be tapered.
2) The term filler and shim are more or less interchangeable in most cases, especially with repairs. So the terminology can get a bit confusing.
3) A structural shim, in this case would have to extend one row of fasteners further than the doubler for a typical application.

The main purpose of considering a shim structural is that by having it independently secured to the structure, it is not free to 'float' between the doubler and the repaired structure. That float is what causes fastener bending stresses that are difficult to account for (there's analysis for it, but it's not really well understood/researched so the analysis is very conservative).

I don't have access to any sketches off hand, or I would attach one, but hopefully you've sorted the repair by now?