Hole edge clearance question

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BJC

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Reference: Lycoming IO-360-B1E engine and a custom air induction elbow on a Glasair.

I need to attach a 4130 steel brace that runs between between the 1/4" accessory case bolt that is just to the left and below the left magneto, to the 5/16" bolt that attaches the throttle body to a 90 degree elbow between the intake and the throttled body. The brace provides support for the throttle body.

I know that the recommended edge clearance, measured from the center of the hole is twice the diameter of the hole. The strap that I have will not fit if I follow that. The tabs on the strap are 1/8" thick, which is relatively thick for the bolt sizes used. If the in-service throttle body weighed 5 pounds (it is less, but I haven't taken it off and weighed it) then the max load at 6 g would be around 30 pounds. Factor in the mounting angle. a safety factor, some robustness for engine vibration and thermal expansion, call the max load 400 pounds.

Using a yield strength of 60,000 PSI for the 4160 would indicate the need for less than 1/8" of edge metal, or an edge distance of 3/8" and 9/32".

What issues would you anticipate with the reduced edge distance in this application?

Thanks,


BJC
 

wsimpso1

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It sounds like it ought to be fairly robust to begin with. I will let folks with direct experience on this mount talk to us.

The favorite approach is the empirical one. What have other folks done, and how many hours have their brackets run?

The usual issue is crack generation starting from flaws both inside the hole and on the outside radius. I will describe the downtown job to distance yourself from this sort of failure. Might be huge overkill. Might never be enough.

Drill undersize, ream to size, deburr edges, polish the hole/chamfer bright;
Profile the outside shape oversize, then file to size, deburr, sand out all scratches, polish bright;
Shotpeen to an appropriate spec for the material;
Paint with corrosion resistant high temp paints. I like KG GunKote, there are others. All really wear resistant and tough to penetrate. Sandblasting is the only reliable removal method,

Other precautions are washers under all fasteners and torquing to spec all fasteners in the system.

Would I do this a lot of places? Nope. Would I do this for a known problem child application? Sure. Would I model the parts in SolidWorks, then apply the vibe input and run the FEA? Maybe. The results will all be related to the severity of the vibratory environment. Have you got the results of a 3D vibe survey to base things on? If yes, this can be a great tool. If you are guessing as to the 3D vibe picture, who knows what it needs.

Billski
 
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BJC

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The favorite approach is the empirical one. What have other folks done, and how many hours have their brackets run?
Most seem to use a longer bolt into the accessory case, with a spacer between the case and the tab to move the tab away from the case and make it fit. I'm not aware of any strap failures, but there have been elbows that failed at the weld on the engine end. They looked like poor welds, but I am not aware of any actual analysis. There is no longer any factory support for the Glasair.
Have you got the results of a 3D vibe survey to base things on?
All I have is experience flying behind four cylinder Lycomings: I know that it will shake.

Thanks Bill.


BJC
 

TFF

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My answer is going to be more “ beat it to fit, paint it to match”.

If the arm is made ready to go, make it work.

If you are making the arm, I would go with the spacer.

Maybe a small section of tubing matching the bolt size as a boss and weld that to the rod instead of a flattened end.

I bet it’s fine in the original way. Those case bolts are not generous with room.
 

Victor Bravo

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Where you cannot add edge distance, you can often add thickness, resulting in the desired volume of metal around the hole.

If you look at the type of "welded tube hinges" used on the elevators and rudders of almost all tube and fabric airplanes (I believe the Pitts is this way), what is the "edge distance" of the hinge pins? Far less then half a bolt diameter. But an engineer will say "but that's different, you have a whole tube there which is plenty strong to keep the bolt from tearing out".

So to me, this means there is a balance of material thickness versus resistance to fastener tear-out. A thin strap with a hole in it has to have more edge distance than a thick piece of material, and if you go to a really thick material (like a tube) then you need very little edge distance.

As I said, I'm guessing that this is related to the volume of metal around the hole, regardless of shape.

Sooo.... IMHO weld a washer onto the end of your strap to double the thickness, and you can have the hole closer to the edge.
 
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Toobuilder

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Ran across a similar problem mounting my oil cooler.

Here's what I came up with - A few pieces of tube and one AN 970 washer:

Oil Cooler Brace.jpg
 
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Dan Thomas

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The longer bolt and spacer is what I would use, but I'd also look at making a three-holed bracket that would fit under two adjacent accessory case bolts, with spacers, and the brace to the third hole. Bend the bracket so most of it lines up with the brace tube so the tube's thrust-and-pull doesn't try to bend it and crack it around the case bolts, or even start levering the bolts out.
 

Mad MAC

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Since no one has mention it the analysis method for the flattened end is lug analysis (its pretty simple, a bit of geometry and a couple of graphs).

I would use a thicker than normal washer under the bolt head to ensure the strap is well clamped to make sure any bending in the strap finished well before the bolt hole in the strap.

For something similar to tool builders bracket but shorter, I have riveted a couple of pieces of the same profile of channel together.
 

BBerson

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Get a long nut and bolt on the outside end of the long nut. No clearance issue.
About the same as a thick spacer, I suppose.
 
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BJC

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Just finished bending and drilling the tabs on the support brace. I will use a longer 1/4” accessory case bolt, with a stack up of case, washer, spacer, about 1/4” long, washer, lock washer, and bolt head.

Thanks for all the suggestions.

Can anyone confirm that the above bolt is a standard grade 5 coarse thread bolt?

If not, what is special about the Lycoming bolt?

Anyone have good experience with Aircraft Specialties in Tulsa as a source of Lycoming parts?

Thanks again.


BJC
 

TFF

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Aircraft Specialties is good. They deal with homebuilts and our issues. Nice to talk to at Oshkosh.
I forget the bolt spec, but it is not as strong as a grade 5.
 
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Dan Thomas

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If not, what is special about the Lycoming bolt?
Its price.

I have seen many bolts on Lycomings with the three radial dashes indicating grade 5. The weaker part of the whole affair is the thread in the aluminum case.
 
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