Anyone here know the differences in strength? I have been emailing a friend and he is arguing that course thread and fine thread bolts have the same strength. How come on airplanes they always use fine thread?

As a machanical design engineer I can assure you that fine thread are a bit stronger for one reason: The root of the thread is not as deep therefore the root dia of the fastener is larger. Larger dia = more cross-sectional area. Greater area means more stress can be applied.

They also have greater pull-out strength because more threads are engaged over a given distance than on a coarse threaded fastener.

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1986 SE 350 V8

I am no expert but have noticed fine threaded bolts used in more applications where vibration was a factor and I understand this is because it is less likely to loosen because there is more area of threads in contact and thus stronger for this reason.

quote Originally posted by tesmith66: As a machanical design engineer I can assure you that fine thread are a bit stronger for one reason: The root of the thread is not as deep therefore the root dia of the fastener is larger. Larger dia = more cross-sectional area. Greater area means more stress can be applied. They also have greater pull-out strength because more threads are engaged over a given distance than on a coarse threaded fastener.

Here we go everybody bored. Its ok, these questions are actually fun to us engineers. The rest of you take a nap. The first statement is true. The finer thread is stronger because there is more material because the root diameter is larger. The second part is irrelevant because of the first. Normally the weakest part of any bolted assembly is the cross section at the root diameter. That is where the bolt will break. Normally it won't matter where fine is stronger, because that is not where failure will occur. The statement that "more threads are engaged over a given distance" is true but the threads are smaller and thus irrelevant. Fine threads produce more clamping force per unit of torque because the pitch is lower. They are also capable of sustaining more clamping force because they are stronger.

I would like to clarify what I said about the thread engagement: Yes, there are more threads engaged, and yes, they are smaller than the threads on a coarse bolt, therefore the net gain in strength is insignificant.

I specify UNF when I am going in to thin material to get more threads, therefore I get more pull out resistance.

Well obviously the grade of the bolt determines the strength.
Theoretically yes, fine grade bolts are stronger, because of the amount of material engaged. You guys covered it above.
In the real world though you have to deal with defects at the threads, or the material not being as hard as it should. It's pretty irrelevant to what we are talking about though.
Rodney- I've also heard that airplanes use fine thread to dial in the torque better.

Jason

The main difference of usage would be , in my experience, into what material the screw is being fitted.If the receiving material is weak(aluminium for example,) the threads need to be thicker, because of the inherent weakness of such a material.If the thread were to be too fine, it would strip out easily under torque.Just my .02c...this is just an opinion, I have no evidence it is true!!!

fierofetish

Larry,

You mentioned that the failure area is "the cross section at the root diameter" which I understand to be the narrowest part on the inside of the threads at the point the threading stops. I think that makes sense and it also jibes with what I've seen in real life, that's where bolts fail.

I think, if you can find it on their site, the SAE has torque standards for various grades and threads of bolts. IIRC, fine thread "normal" torque values are about 10% higher than coarse threads. I'll see if I can find a chart. I have one in the shop from ARP.

John Stricker

Edited, the chart I have is available online at ARP's Website.

quote Originally posted by larryemory: Here we go everybody bored. Its ok, these questions are actually fun to us engineers. The rest of you take a nap. The first statement is true. The finer thread is stronger because there is more material because the root diameter is larger. The second part is irrelevant because of the first. Normally the weakest part of any bolted assembly is the cross section at the root diameter. That is where the bolt will break. Normally it won't matter where fine is stronger, because that is not where failure will occur. The statement that "more threads are engaged over a given distance" is true but the threads are smaller and thus irrelevant. Fine threads produce more clamping force per unit of torque because the pitch is lower. They are also capable of sustaining more clamping force because they are stronger.

[This message has been edited by jstricker (edited 12-22-2004).]

To clarify a trivial thing: For a given size of bolt, increasing the thread pitch, or threads per inch, produces a helix who's angle relative to the bolt centerline is shallower. The decrease in angle reduces the turning forces imparted to a bolt as a result of vibrations traveling along the bolt's axis. When the turning forces exceed the friction of the bolt head and threads the bolt will turn by itself, hence vibrate out.

JazzMan

Coarse...........Fine....................as long as the HOLE is the same size, I'm making my OWN threads!
"I left my parrrrt, in San Franciscooooo."

No, really.........Good stuff!