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Gear Tower Modification - How to Attach?

vomatic

Active Member
For those of you who have done the gear tower modification, is there any consensus or recommendation on how to attach the cover plate? Dimple? Countersink? Just pan screws? Thanks.
 
Whatever detailed way you install the screws, please realize that the cover plate and screws are re-establishing the shear strength of that panel, and the screws must transmit significant shear load. Make sure the holes in both the cover plate and the tower shear panel are good close (tight) fit to the screws, and there should be lots of screws --2" or less spacing. The plate-nuts and screws hold the pieces together, but it is the screw bearing on the sides of the holes that transmits the shear force.


The cover plate should be at least the same thickness as the tower shear panel. Probably too thick to dimple.
 
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I'm working on this now. Initially, I wanted to dimple, but didn't want to stretch the aluminum on the gear tower and cause rivet spacing problems with the pre-punched holes. Countersinking the top sheet would work, but it would end up being a very thick plate to accept a #8 screw head dimple. Ended up using 24 #8 structural pan-head screws on each gear tower. The important part being the smooth grip length where the shear loads are being carried. Don't want threaded portion to be exposed to loads. These were AN525 screws, and I just ordered the length that had the appropriate grip length for the pieces being held plus a washer..
 
Thanks guys. Good input. I'll get the tolerances as tight as possible. I have 20 screws planned. Is there a minimum edge distance for a #8 screw? May not matter since we're limited by the understructure of the tower that we bump up to which on mine is 9/16" from the edge.
 
Tower material is 0.040". It will dimple just fine, as will a 0.040" cover plate. A matched dimple joint will be superior.

Many moons ago I too started down the wrong road, with dome heads.
 
Thanks Dan. I agree on the dome heads. I'd prefer to dimple it for a cleaner look for sure. I think that is the way I'll go.
 
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Attach method dictates edge distance requirement

I've spent WAAAAY too much time on this gear tower modification. I appreciate everyone's input because it got me asking questions.

For those who are making this modification it seems that how the cover plate is attached matters. The reason it matters is how the plate is attached dictates the necessary edge distance of the screw. There isn't a ton of room in the tower itself to manipulate edge distance because there is a angle underneath that is 9/16" from the edge of the opening. This has to be avoided which dictates where the hole can be drilled.

I see three ways to attach the cover plate: 1) Flat screws. 2) Dimple with flush screws. 3) Countersink with flush screws.

It seems that each of these has a different minimum edge distance requirement. Military specification document MIL-R-47196 lays out the edge distance requirements for overlapping materials. The plate is called the visible piece and the tower is called invisible piece as it sits underneath. Each has a different edge distance requirement depending on which one of the three methods is used to attach the plate. This speaks to rivet sizes and not screws/bolts, but the logic should hold (I think).

Based on my measurements the largest edge distance available in the tower is about 6/16" (.375). This provides maximum edge distance to the opening and allows the nut plate to clear the angle underneath. The plate edge distance can be more than that as long as it stays inside the external rivet heads on top. For a 5/32" rivet (.156) the document calls for .281 - .344 minimum edge distance - 9/32" to 11/32".

The net is, doing just a flat head screw requires the smallest edge distance. Dimpling or countersinking will require more than 11/32" (.344) of edge distance (for a 5/32 rivet). For reference 6/16" of edge distance is .375.

Since the document doesn't give an edge distance for a #8 screw, I found a forum that the aerospace engineers use. They don't definitively answer the question and they don't like "rules of thumb", but they offer up an edge distance formula of 2 times diameter + .05. That would be .378 for a #8 screw. If we have 6/16" to work with that is .375(close enough?). The forum link is below.

Hope this helps for anyone making the modification. None of this speaks to which method would best carry the loading forces. I'll leave that up to the engineers in the group.

https://www.eng-tips.com/viewthread.cfm?qid=153469
 
David, are you removing just the connecting web between the lowest two access holes, and installing an oval cover plate? Or are you making the entire inboard face of the tower removable?
 
I removed the web between the holes on my QB fuse, and installed an oval cover.

Dimpled both the tower and the cover (cover of same thickness) with no issues.
 
Dan,
I'm just removing the connecting web between the lowest two access holes, and installing an oval cover plate. Am I over thinking this?
 
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