Thread pitch strength??

[chuck hahn]

Is a finer pitch stronger than a courser thread as far as holding more torque and not easily stripping??? Base metal would be aluminum.

[Cyrano]

Yes, a finer thread pitch means more metal to metal contact area. A finer pitch is often used when it is not practical to up a size in diameter.

Take things to an extreme and imagine a nut and bolt with a very coarse thread with only one turn available. This would easily strip out.

I've noticed that in many automotive applications there are many times when a fine pitch is used instead of a 'standard' pitch. e.g. wheel spindles and nuts.

[tkent02]

I think they are less likely to back out from vibration or whatever, because the smaller thread pitch puts it at more of a perpendicular angle to the bolt. I don't think fine threads are any stronger for ultimate strength to failure. Picture the total amount of aluminum that would have to be pulled out for a coarse thread to fail, it's a lot more metal than pulling a fine thread out. Also the coarse thread will take a lot more abuse than a fine thread, as far as sand or rust on the threads, loosening and tightening many times, cross threading, over torque, under torque and rattling around, any of these things and fine threads will get damaged easier.

We need a real live mechanical engineer who designs this crap to post the truth.

[Steve]

A quick Google search found THIS, which basically confirms what Cyrano and tkent are guessing.

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[Dogma]

BSME checking in...

TL;DR: Fine threads are for stronger screws, not stronger female threads. You can't make aluminum stronger by putting a stronger screw in it.

A screw is a wedge wrapped around a cylinder, so finer threads are a sharper wedge. That means less torque to deliver the same tension, so if you apply the same torque, it strips easier. With regards to stripping threads, the tension you can hold for the same length of engagement is not significantly different. This is easier to see in inch fastener designations: 1" of thread engagement of 1/4-20 threads divides that 1" into 20 threads. 1" of thread engagement for 1/4-28 divides the 1" into 28 threads. That's the same Ø0.25 x 1" cylinder of material carrying the load in the female thread (The male thread root diameter is larger and stronger in the fine thread). Now, that's a little bit of an oversimplification, since the stress distributions are a little different and I think the fine thread does carry a few % more tension before stripping. I'm not sure because it's close enough to never think about. As Tom pointed out, fine threads are easier to mess up. Engineers usually design everything with coarse threads, except the #10-32 inch size (easier tap breakage on the #10-24 for some reason).

Fine thread fasteners do have a larger root diameter (larger cross-section), so the screw itself is stronger and less likely to break, or strip the male threads. So in some circumstances where higher tension is required in a stronger fastener, but a larger nominal size is not desired, fine threads are used. Of course, the female threads need to be in a sufficiently strong material.

So, fine threads in aluminum are not a good idea. That would just be easier to strip out by over-torquing. Go up a nominal size, or go deeper, or add more fasteners, or use a torque wrench, or add a Helicoil. Adding thread inserts like a Helicoil not only repairs threads in aluminum, but reinforces them. The insert's threads are more durable, and where they engage the aluminum they are larger than what used to be there. So a M6 hole becomes almost M8 where it engages the aluminum. Helicoil probably has design reference materials that would give values for the increase in pull-out strength.