Gaps Closed by Bolts

[drawoh]

I am working on a structure that is going into an aircraft. The aircraft has a hole in the floor. There are aluminium angles bolted to the floor on either side of the holes. The structures mounted on the angles consist of a plate with two rails screwed to the bottom. Each rail picks up an opposing face of each angle. Bolts are put through the rails and angles to secure everything. On top of everything will be a surveying instrument.

Obviously, there has to be a gap between the rails and angles. This gap will be reduced to zero or to some small clearance when the bolts are tightened.

Structurally, this makes me nervous. I don't think the angles on the floor or the rails and plates are very flexible. If the bolts are tightened down hard, the structure will see a strain equal to the gap between the rails and angles. The act of tightening bolts could place the structure under high stress, before any heavy loads are imposed on the system in flight.

Has anyone any experience with mounting schemes like this? It is hard to analyze because I don't know the exact spacing of the angles in the aircraft. I need to know the gap accurately. Our structures are fairly rigid. Will the aircraft floor bend into shape? Is this even a good idea?

--
JHG

 

[rb1957]

yes, we don't like closing gaps with fasteners, basically you're enforcing a displacement on the structure being closed up.

either
1) shim (even liquid skim would be better than nothing) the gap;
2) build up the structure on the plane to mimimise the gap, ie if the rails are mounted on the floor then install only one angle on the plate untill it's installed, and then the 2nd angle is installed snug to the rail; or
3) live with closing the gap ("we do it all the time") and do some sort of calc on the structure so you can sleep at night ... measure (or estimate the gaps) consider the effect on the standing legs of the angles/rails on an enforced displacement.

i suspect that the working stress in these pieces is low enough that this isn't a Problem (though it isn't Best practice).

another day in paradise, or is paradise one day closer ?

 

[3DDave]

Shims are good; there used to be a material referred to as "Ren" that is a gap filling epoxy; applied wet and then bed the part, wait for cure before tightening. It was used behind armor plate over aluminum to prevent high spots becoming stress concentrations. The shop also abused it when angle grinding left notches, but as filler it was good.

I think it must be a product -like- Ren Patch 5008 Repair Paste but I'm sure that wasn't it exactly. Maybe it was this

http://www.freemansupply.com/RenPaste1250HighDe.htm

 

[Sparweb]

Can you adjust the steps taken during the assembly?

If the floor reinforcing rails were installed in place, but only loosely, then they could shift their positions as the bolts to secure the instrument are tightened.

Another way to go about it is to install the instrument as you described, then loosen the floor-to-rail fasteners to relax them. Parts will shift until the streses are relieved, or until they run out of clearance in their bolt holes. Then re-tighten the fasteners in the floor rails. You might have to support the instrument from dropping down while you do it, but as you say, there's a convenient hole in the belly for a cradle.

In both cases, can the rail-to-floor fasteners be in loose clearance holes, or would that hamper the effectiveness of the floor reinforcements? Perhaps you can reason that those fasteners closest to the instrument mounts only need to have wide clearance holes, and the rest can be tight.

Hope this instrument doesn't have to come out of the aircraft on a regular basis!

STF

 

[drawoh]

rb1957,

The shims sound like a good idea. I may be able to attach an angle or a rail flexibly, so the gap is eliminated by assembly procedure. This thing comes apart regularly, so the shimming paste is not an option.

I don't know how they have been installing stuff until now. They may not be inspecting down where the gap is. The position is hard to see. This is the sort of thing that works fine until someone crash lands the 'plane. So far, we have not done this.

--
JHG

 

[rb1957]

shims better build sequence ? i'd've thought it would be better to eliminate the gapping as much as possible by installing one piece at the last minute (as opposed to building two pairs of rails and hoping they'll match up).

if the gap is hard to see, how about using liquid shim to get an idea of the gap ?

how critical is the design ? can you live with a closed up gap ?

another day in paradise, or is paradise one day closer ?