Nuts and Bolts

[TThurston]

As part of my education as I consider building an RV, I found the following interesting information regarding bolts. It’s from the perspective of motorcycles, but I still learned a few things that I didn’t know. Perhaps it might by useful for others as well.

http://home.jtan.com/~joe/KIAT/kiat_1.htm

http://www.nobugs.org/bike/bolts.html

I have a copy of the RV-9/9A preview plans, and in Section 3 under Torque wrench, it says, “Aircraft nuts and bolds have specific torque values ... that can only be set with this tool. It is very easy to over torque the small AN3 bolts without one.”

I think I understand the need to tighten bolts to at least the specified value. I’m not sure I understand what harm there is in tightening to more than than that value – not that I intend to do so, I just want to understand. Of course I realize that at some extreme tightness, either the bolt or the material it is clamping will fail. Is that the risk?

Part of the reason for my question is the recent issue regarding engine mount bolts on the RV-12. I may be wrong, but it seems like one of the questions was what torque value to use.

My primary ground instructor told us his experience of having his propellor fall off during a cross-country flight at 9,500 MSL, fortunately within gliding distance of an airport:

http://dms.ntsb.gov/aviation/Acciden...2011120000.pdf

The probable cause was “inadequate torquing procedures of the propeller attachment bolts”.

So you see why I might have an interest in bolts and “proper torquing procedures”.

[terrye]

The websites you included in you post have pretty good information about nuts and bolts, and I think should have answered your question.

But you asked: "Of course I realize that at some extreme tightness, either the bolt or the material it is clamping will fail. Is that the risk?"

Yes, that is the risk. Without a torque wrench and with smaller bolts, say up to 1/2" diameter, the risk is generally overtightening. With larger bolts above that diameter, the risk is generally undertightening.

Your websites explained that a bolt is much like a spring with a very high spring rate. You should understand that most bolts are designed to be installed at about 70% of their yield stress. Yield stress is the stress at which the material undergoes non-reversible stretch. So your margin between "tight enough" (or 70% of yield stress) and "too tight" (non-reversible stretch) is only 30%. Sounds like a lot, but in reality it could be as little as 1/2 turn of the bolt or nut. Too loose is just as bad as the bolt will generally fail from fatigue in very few load cycles.

You should consider doing some tests to confirm this. Get some hardware store nuts and bolts, say 1/4" (SAE Grade 5), install them in a test block and tighten with your torque wrench. Then increase the torque until the bolt fails and note the value.

[DesertFlier]

Correct torque is much more critical in aircraft applications than in most other vehicle applications. Aircraft are generally made of soft materials like aluminum and fiberglass, and yet use fasteners made of much harder steel than automobiles or motorcycles. Over-torque a bolt going through an aluminum part and you'll damage the aluminum long before you shear the bolt.