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Countersink hole for AN bolt?

spaceflightmeow

Active Member
Say I have an AN3 bolt. The grip measures 0.189" with a caliper. I install it into a 0.189" reamed hole. However, there's a radius between the grip and the bearing surface of the bolt head, so the head bearing surface (dimension H) will not sit flat on the sheet. Instead, the bolt will rest on the radiused portion:
fx6zdC0.png


That seems undesirable. Is this a practical concern? Would the hole need to be slightly countersunk to accept the radius portion of the bolt?

Thanks in advance.
 
It's our old friend the Z-bracket.
Here's an example:
3ah1zDQ.jpg


EDIT: Answering my own question I guess.

In the case of the Z-brackets, their IS a washer under the bolt. So that solves this issue in that case.

In cases where there is a nut on the other side, the washer would go under the nut. Would there need to be another washer under the bolt head (or a countersink)?

Also note the above picture is from scrap material - the bolt is upside down relative to how it would actually be installed.
 
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Something is not right if the bearing surface of the bolt is not meeting the surface of the material after tightening. IMO countersinking would be treating the symptom and not the root problem. For a bolt that goes into a nutplate vice a nut, I would add a washer under the bolt and never CSK the hole. FWIW, I've never heard of anyone CSKing for a bolt.
 
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The pictures above are before tightening. If I were to add a nut and washer (under the nut), then tighten the nut, would the radius just dig into the parent material? Is that acceptable?
 
The radius won't dig in--the proper torque for an AN bolt just isn't that great. More than likely you'd snap the head off before you did that. In any event, where you are tightening the bolt by turning the bolt instead of the nut as in the case where there's a nutplate, there should always be a washer under the bolt.
 
OK, sanity check found here:
http://www.experimentalaircraft.info/articles/aircraft-building-3.php
(and also in the van's build manual):

Washers must go under the bolt head to act as the bearing surface. This seems contrary to the plan drawings.

On the RV-7 plans, the aileron brackets are installed with bolts and nuts. There is only one washer, and it is drawn as going under the nut. That leaves the bolt head without a washer.

That's because you are turning the nut not the bolt. Just follow the plans--they are correct .
 
If the radius doesn't dig in (take the aileron bracket example), then isn't the bolt bearing surface unsupported?

DnUQie6.jpg

(Photos borrowed from an online build log, not mine)


EDIT: Here's what my aileron bracket bolts look like:
dcyNqXS.png


It does appear that the bolts are sitting just fine after torqueing. But I'm still wondering how it's OK (it must be, because that's what the plans say...)
 
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Hole accomodates radius?

If the hole has to be large enough to accommodate the radius, and the radius is 0.01" a side, doesn't that mean the hole diameter has to be 0.02" larger than the bolt diameter? That seems excessive. A 0.189" hole would now be a 0.209" hole.

(thanks for your patience in continuing to field my questions)
 
No worries. You're over thinking this, I believe your math is wrong because the arc segment defined by that radius doesn't measure out to anywhere near the size of the radius itself. I can't describe it in math terms as I'm not that smart. All I can tell you is use the appropriate drill bit for the size fastener you're installing, washers where applicable and you won't need to do anything else.
 
I just ran out to the shop, took a #12 reamed hole with no countersink on it, and installed an AN3 bolt. It sat on the radius. I then torqued a nut and washer on the opposite side, then untorqued it. The AN3 head now sits flush to the material. I removed the bolt, and found that the hole is now radiused (crushed).
 
Typically a zero clearance hole is reamed and is only used for critical sheer applications. A close clearance hole for a 3/16 bolt is .201. A loose fit would be .209.

If using a zero or close clearance hole and a washer is not used under the head, a relief chamfer is required for the radius. These are standard design criteria for aircraft.

Your observatio was good. Hope that helps.
 
If the radius doesn't dig in (take the aileron bracket example), then isn't the bolt bearing surface unsupported?

DnUQie6.jpg

(Photos borrowed from an online build log, not mine)


EDIT: Here's what my aileron bracket bolts look like:
dcyNqXS.png


It does appear that the bolts are sitting just fine after torqueing. But I'm still wondering how it's OK (it must be, because that's what the plans say...)

In these situations, you can radius the bottom of the washer by hand on a grinder so that it conorms to the radius of the bracket. You don't want a sharp edge digging into the radius of the bracket.
 
Perhaps the term you're looking for is 'chamfer' instead of countersink. Basically 'breaking' the edge of the hole; maybe analogous to taking the sharp edge off a fresh cut.

Charlie
 
the arc segment defined by that radius doesn't measure out to anywhere near the size of the radius itself.
Doing the math, that 0.01" radius results in a 0.00414" deep fillet, which is very shallow indeed. Deburring the hole with a single twist of a drill bit would create the needed chamfer to care of this. Whew! (perhaps I've had too much coffee today).

Typically a zero clearance hole is reamed and is only used for critical sheer applications. A close clearance hole for a 3/16 bolt is .201. A loose fit would be .209.

If using a zero or close clearance hole and a washer is not used under the head, a relief chamfer is required for the radius. These are standard design criteria for aircraft.

Your observatio was good. Hope that helps.

Yes, that helps a lot!

Interesting that you say 0.201" is considered close clearance (what's your source for the info?). 3/16 is 0.1875". Elsewhere on this forum people recommend a #12 reamer (0.189") for AN3 bolts. At this point I've assumed that this needs to be done everywhere, with even greater care for the wing attach bolts.

Are the aileron brackets in "critical shear"? How about the fuel tank Z-brackets bolts? Can I use 0.201" holes in those areas?
 
More digging, I found this interesting discussion on AN3 bolts

AN3 bolts are loaded in TENSION. As such, where the sides of the hole are doesn't matter as much - there is intended to be a little gap there. The way a non-precision-fit bolted joint works is the preload on the bolt (generated via the torque applied), puts the items attached together into compression - yes, they actually deflect a little into the direction the bolt force is pressing on them. Failure of this joint is defined as applied load exceeding the preload. There is separation of the materials at this point.

So how are there so many joints in aircraft, made with AN3 bolts that are loaded in tension, but it looks like the force on the parts is sideways through the bolts? How does that work? Well, the friction generated by the preload keeps the parts from sliding relative to each other. The bolts aren't loaded in shear.

So what I take from that is, AN3 bolts are not used for "critical shear" applications, even in places like the fuel tank Z-brackets. Nonetheless, use a #11 drill (0.191"). Nowhere near the 0.201" number. I found some sources online for #10 "screws" - NOT bolts.
 
The aileron hinge bolts are loaded in both tension and shear. The fuel tank z-brackets are loaded primarily in shear.

On my RV-3B project, I ream holes for AN3 bolts to 3/16.

Dave
 
Doing the math, that 0.01" radius results in a 0.00414" deep fillet, which is very shallow indeed. Deburring the hole with a single twist of a drill bit would create the needed chamfer to care of this. Whew! (perhaps I've had too much coffee today).



Yes, that helps a lot!

Interesting that you say 0.201" is considered close clearance (what's your source for the info?). 3/16 is 0.1875". Elsewhere on this forum people recommend a #12 reamer (0.189") for AN3 bolts. At this point I've assumed that this needs to be done everywhere, with even greater care for the wing attach bolts.

Are the aileron brackets in "critical shear"? How about the fuel tank Z-brackets bolts? Can I use 0.201" holes in those areas?

I have worked for 3 major aerospace firms that use those numbers for general applications. Consider this. If a bolt is to act in tension, the hole/ bolt fit is not so important within reason. The hole/bolt fit doesn't significantly alter the bearing area of the bolt head to the surface of the sheet. If the bolt is to act in sheer for ligher loads, then the bearing of the bolt on the edge of the hole is not so important. But if the bolt is highly loaded in sheer, the bearing on the edge of the hole needs to pretty much be distributed over as much of the circumference of the edge as possible to reduce the point loads. Therefore the bolt and hole diameters need to closely match.
 
A couple more pieces of info for you....

More digging, I found this interesting discussion on AN3 bolts



So what I take from that is, AN3 bolts are not used for "critical shear" applications, even in places like the fuel tank Z-brackets. Nonetheless, use a #11 drill (0.191"). Nowhere near the 0.201" number. I found some sources online for #10 "screws" - NOT bolts.

Contrary to this info you found in the EAA Forums?.... RV's (and most light aircraft for that matter) are designed with (other than a few exceptions) most all fasteners primarily loaded in shear.

All of the RV prototypes and engineering static test articles were built using the standard in the plans of sizing holes for AN3 bolts to #12. If you choose to go bigger, you will be deviating from Van's recommendation and standard aircraft construction practices, and you will have an RV that may not age the same as most of the fleet does (I.E., there is value in making a copy of an airplane that is way ahead of you in flight hrs for discovering potential problems as it ages, etc.)
 
Z-bracket bolt loading

I remember reading your article in Kitplanes a few months ago (which is great, thanks for writing it!) You showed an example free body diagram analysis on the Z bracket. Here's the example again:

V is the applied shear load in one Z-bracket:

V = F *a/L ; V = 123.7 lbf
b = 1.125 in Depth of a Z-bracket (see Fig. 3)
c = 2.66 in Distance between bolt holes (see Fig. 3) on the aft flange
n = 5 There are 5 rivet holes in the forward flange.

We want to know what the tension and shear forces are on one bolt:

Pc = F * b / c ; "Limit" tension load on one of the mounting bolts. ; Pc = 228.4 lbf
Vb = V / 2 ; V "Limit" shear load on one of the mounting bolts. Note that there is both shear and and tension on these bolts. ; Vb = 61.9

And the load on one rivet:

Vrivet = V / n "Limit" shear load on a single rivet between the baffle and the Z-bracket. ; Vrivet = 24.7 lbf

I see that the limit tension load on the bolts is more than 3 times higher than limit shear load. Is the AN3 bolt meant to be used as a shear pin here, or is meant to be used primarily in tension, with the shear shear force reacted by the friction between the bracket and spar?
 
I remember reading your article in Kitplanes a few months ago (which is great, thanks for writing it!) You showed an example free body diagram analysis on the Z bracket. Here's the example again:



I see that the limit tension load on the bolts is more than 3 times higher than limit shear load. Is the AN3 bolt meant to be used as a shear pin here, or is meant to be used primarily in tension, with the shear shear force reacted by the friction between the bracket and spar?

Isn't the more important question, what were the holes like in the wing that was static tested to limit and ultimate load values?

As I attempted to say in my previous post.... I happen to know that they were # 12 holes.
 
Doing the math, that 0.01" radius results in a 0.00414" deep fillet, which is very shallow indeed. Deburring the hole with a single twist of a drill bit would create the needed chamfer to care of this. Whew! (perhaps I've had too much coffee today).

.....

I think that is the reason it's simply not a problem.

The act of deburring a bolt hole per the general Vans instructions takes care of any effects of the very small bolt radius. :)
 
I remember reading your article in Kitplanes a few months ago (which is great, thanks for writing it!) You showed an example free body diagram analysis on the Z bracket. Here's the example again:
------
I see that the limit tension load on the bolts is more than 3 times higher than limit shear load. Is the AN3 bolt meant to be used as a shear pin here, or is meant to be used primarily in tension, with the shear shear force reacted by the friction between the bracket and spar?

Oops, I should have reread my article. Still, that article was a) for education only, b) for just fuel loads (not aero loads), and c) sized for an RV-3B, since I had one in my shop.

Still - it does suggest that there are substantial tension loads on one of those bolts, doesn't it? Probably best to assume that every bolt carries both tension and shear.

Dave
 
Oops, I should have reread my article. Still, that article was a) for education only, b) for just fuel loads (not aero loads), and c) sized for an RV-3B, since I had one in my shop.

Still - it does suggest that there are substantial tension loads on one of those bolts, doesn't it? Probably best to assume that every bolt carries both tension and shear.

Dave

It looks like the analysis is correct, I was trying to draw some conclusions. In this case the bolts are loaded in tension and shear due to the bending moment, but nonetheless it seems that the choice of bolt (non-precision) suggests that it's intended to be used mainly in tension. Otherwise Van's would have specified a reamed hole and a close-tolerance bolt.

I guess I'm just to build some understanding for why Van's would suggest it's OK to elongate or enlarge those holes to #10.
 
Deburring

A slight countersink of a hole is properly called a chamfer. .010 would be considered a chamfer.
Deburring can be a chamfer or simply the removal of excess material caused by making the hole. IE deburring CAN be simply removing the excess material with fine emery or a pad.
 
Hole

Common protocol fro a hole where bolt is going to be rotated to torque the assembly(anchor nut) is to make the hole slightly oversize. Too tight a fit often results in galling the bolt.
#10 hole for an AN3 bolt is pretty common in this case.
 
A slight countersink of a hole is properly called a chamfer. .010 would be considered a chamfer.
Deburring can be a chamfer or simply the removal of excess material caused by making the hole. IE deburring CAN be simply removing the excess material with fine emery or a pad.
Thanks for the clarification.

Common protocol fro a hole where bolt is going to be rotated to torque the assembly(anchor nut) is to make the hole slightly oversize. Too tight a fit often results in galling the bolt.
#10 hole for an AN3 bolt is pretty common in this case.

What you're saying makes sense. If you don't mind my asking, where does this common protocol come from?
 
It's worth adding that my article shows one way to analyze the Z-bracket bolts, showing a tension load on them. But there's another way that's equally valid, perhaps more so -

That tension load came from a moment on the Z-bracket due to the shear forces (down on one flange and up on the other, tending to rotate the Z-bracket). This moment can also be reacted by heel-toe pressure on the faces of the flanges from the tank baffle and the spar web. The actual distribution of the load between the bolts and the pressure on the flanges would be determined by the relative stiffnesses of the various parts, and that's more complicated than I can describe here or in the article.

Dave
 
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