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AN3 bolt torque

Wills_9A

Active Member
I was just wondering what most builders are doing out there as far as putting these bolts on properly. After shearing one off with relative ease...with a torque wrench mind you, I am a little concerned about the possible failure of the bolts I just used to attach the aileron brackets with. The torque wrench did not seem accurate when it would take nearly 20 in lbs. just to turn the bolt in a nutplate or nylock nut.

I had a tech counselor and A&P who told me that realistically all I needed to do was to tighten these up a little bit tighter than hand tight, which is what I did, but I am not sure how close I am to the failure point...I mean, what if I was only a quarter turn from the sheer point? I would think it might not be fun losing the aileron hinge brackets in flight!

Any comments?

Will
wills-rv9a.com
 
If you were able to shear one are you sure you weren't trying to get to 20 ft*lb not in*lb?

I have been torquing to 25 in*lb just fine. It is basically just past snug. The conversion is ~2 ft*lb which in simple terms is using a 12" wrench and hanging 2 pounds off the end of it. That's not much.

Scott
#90598 Fuselage (just about to transition to Finish Kit)
 
All I'd advise at this point is to get a better torque wrench. If you're snapping 'em with a torque wrench, then you're either not using it properly, or the torque wrench is "bad" (it doesn't click etc.. when you reach preset torque.. or even worse, the signal is so obscure that it's easy to miss it and end up overtorqueing).

I had that with my el-cheapo HF wrench. Then I got a good one, and realized what POS that HF thing was.. The new one clicks right on, and leaves no guessing whether you got the right torque or not. Basically, you can't miss it, and that's exactly what I need.

PS.. I don't remember any of the AN3 bolts needing 20 in.lbs. just to turn it (eg. before reaching the washer etc.).. but either way, set the wrench to 23-24 and you'll be all set.
 
Just the standard torque value plus whatever it takes to overcome the locking feature.
For example, an AN3 is 20-25 inch pounds. If it takes 3 inch lbs (Example only) to turn it in a nut plate before it bottoms out, then you would shoot for a torque range of 23-28 inch lbs. Same concept if you were torquing from the head side through a thick stackup that had lots of friction.

I've got a fancy torque wrench that captures this running torque digitally so you can see what to add, but with nutplates they are all a little different depending on how many times they've been used.

As far as "just a little past finger tight" Yeah, in concept I get it, but unless people have done it a bunch and have a finely calibrated elbow, they almost always over torque the fastener.
 
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As far as "just a little past finger tight" Yeah, in concept I get it, but unless people have done it a bunch and have a finely calibrated elbow, they almost always over torque the fastener.

I got bitten by this a while back. The RV-8 has 30 or so #8 flush screws around the base of the gear towers, and my SO and I were working on getting those installed; she was on the outside inserting the screws and I was sitting under the inverted canoe tightening the nuts.

Since it was pretty snug under there - especially for the screws inside the gear towers - I convinced myself that I didn't need a torque wrench, and that my finely calibrated gorilla elbow was good enough. So we went, and after about an hour we were almost done with the screws. Of course it was about the third-to last one where, as I torqued the nut, the screw broke.

Breaking the screw was annoying in its own right, but then there was the realization that if I snapped one, I almost surely overtorqued every single one of them. So I ended up ordering all new hardware, removing every single one of those screws, and doing the whole exercise again - but this time with the right tool.
 
Get a good torque wrench. One that come with a certificate stating it's been tested and what the values were.
 
Me too

I broke off a AN3 bolt using a torque wrench. In my case, all I can figure was the two pieces weren't aligned and when the bolt was torqued, it sheared the bolt. Ordered a new bolt, made sure everything was aligned and the torque took well.
 
Having purchased a couple of "high-end" torque wrenches early in the build, I have decided that before using these tools that I have over tightened every nut/bolt that was less than 1/4" in my entire life. Silly me!

As I recall, the first bolt that calls for a specific torque on the -14 build is the elevator bearing. I remember that after using the torque wrench to tighten to the proper spec I thought, "jeez, is that gonna hold?".

Today, and several hundred bolts and screws later; getting to 20-25 inch pounds (adding in the prevailing torque) on an AN3 seems like lots of torque.
 
I have a small CDI torque wrench like many builders here. I don't remember which model, but you want to have one that can go down to below 20 in-lbs and up to 60-70 in-lbs at a minimum. I think mine goes down to 10 in-lbs but I'm not 100% sure. I have the 1/4" drive wrench and then I have a 3/8" adapter so I can use any of my 1/4" or 3/8" sockets. These wrenches run a little over a $100.

Also remember for your torque wrench to always release the pressure on the spring after you use it. This will help extend the life of the tool so that the spring doesn't compress over time and mess up your torque.
 
I broke off a AN3 bolt using a torque wrench. In my case, all I can figure was the two pieces weren't aligned and when the bolt was torqued, it sheared the bolt. Ordered a new bolt, made sure everything was aligned and the torque took well.

I'm still trying to understand this. Are you saying the bolt broke before your wrench clicked over (or, if you had a beam type wrench, before the wrench indicated max torque)? If that's the case, the bolt must have been bad in order to shear before the torque limit.
 
I did this once by putting FT-lbs on a bolt instead of IN-lbs. Strangely enough, it was the 3rd one that broke. First two were significantly over torqued, but didn't break. Yes they were all replaced.
 
A torque screwdriver is plenty for AN3 and smaller.

I visited a -10 builder early on, and he had this big Craftsman digital wrench. Waaaay too big for our stuff. Not only that, I took it in to our cal lab and checked it, and it was off quite a bit. Recalibrated it, but at full scale I could hardly keep the calibrator and bench it was attached to in place!
 
It's interesting (for me anyway) to note that there's such a tool as a torque screwdriver that go as high as 40 inch-pounds, which is half again or more the normal torque for a -3 bolt. It's a bit of a mental reset to understand that the proper tightness doesn't even require wrench-type leverage, just a firm grip. I have a wrench, not a driver, but this idea has made me very careful about using it - at such low settings the click is soft and easy to miss.
 
I have a wrench, not a driver, but this idea has made me very careful about using it - at such low settings the click is soft and easy to miss.

And that folk is the big danger with the el cheapo click type torque wrenches. At very low torques (and an AN3 has VERY low torque) it is extremely easy to miss the very subtle click and keep on torquing. Newbies do it all the time.
 
None of you know this, but this conversation has answered a question that has been bugging me. "How on EARTH can the torque called for be SO LITTLE?" An AN-3 bolt is apparently a delicate little thing. I'm beginning to understand that the torque values specified are not to ensure you've tightened it enough, but to prevent you from torqueing it too much! I did not see that coming.
 
Regarding ScottK and the torque screwdriver. Ditto. I always had leaking valve cover gaskets. Would replace them and in short order the new ones would leak. Talked to a guy from Pinnacle Aircraft engines in Fairhope, Alabama... (they rebuild engines and do great work BTW)... He said I was over torquing. I bought a torque screwdriver a couple years ago and haven't had a leak since! I was WAY WAY over tightening them! I also agree with claycookiemonster... we are likely over torqueing bolts without the use of a good torque wrench/screwdriver.
 
It's a bit more complicated

None of you know this, but this conversation has answered a question that has been bugging me. "How on EARTH can the torque called for be SO LITTLE?" An AN-3 bolt is apparently a delicate little thing. I'm beginning to understand that the torque values specified are not to ensure you've tightened it enough, but to prevent you from torqueing it too much! I did not see that coming.

Torque values are not necessarily intuitive.

For a fastener in shear application (which is typically the preferred design approach), the torque value would yield a strain/stress at ~ 50% of the plastic limit of the parent material.

For a fastener in tension application, the torque should yield a stress/strain @ ~ 90% of the plastic limit of the parent material.

The design criteria here is not the about ultimate strength of the fastener. It is intended to provide a high enough stress/resultant force to prevent a reversing load; thus, limit the exposure to cyclic loads and resulting fatigue failures.

There are different classes of torque when moving beyond typical EAB projects. A value for shear apps (Class3). A value for tension apps (class1). A value for castellated apps (class 4, which happens to be the lowest value of the class 3/shear app range then tightened until next hole/castellation alignment) if I recall these correctly. Several more.

So what about reversing load/fatigue mitigation in shear apps? That is where close tolerance holes are used. Even better, an interference fit like a hi-lok or rivet.

There's also rules like torqueing to mid range (above prevailing torque) when torqueing from the nut side (preferred). Torqueing to the high range when torqueing from the bolt side, etc.

You can't learn it all at once. Keep asking questions. This place is a great resource. For immediate benefit, when talking to someone who knows the difference between shear and tension bet them a craft brew which application receives a higher torque value. You'll win more than you lose.
 
Anyone have some good recommendations for the a screwdriver torque wrench? I've seen a couple out there on the net and a youtube review, and would be interested in the view of the VAF brain trust.

Leaning towards the PB Swiss Tools DigiTorque V02 since it seems to be about as cool as a tool can get, and my other tools from them are very solid.

https://www.pbswisstools.com/en/videos/torque-handles
 
We use Utica and Sturtevant-Richmont torque screwdrivers on the production line at work and they are very good, but expensive.

I have a S-R myself, but I'm sure there are more affordable options out there that are fine for our occasional use. The bicycle racers and gunsmiths have some suitable tools that look good.
 
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