Course Thread/Fine Thread

[Steve_P]

No argument that fine threaded bolts are stronger than coarse. But that does not address the problem of shear in the parent material.

I already addressed shear strength in the parent material with math and science in my post. Not sure how you ignored that but you did. Threads shear ~ at the pitch diameter and the strength is determined by that area. Experiments and commercially available thread strength calculators confirm this- and they agree with the calculations I have shared. Been there and done that. But you are obviously smarter than all the Toyota, etc, engineers that specify fine threads when necessary. Where did you get your BSME?

Yes, per thread, as I showed, the fine thread has a smaller shear area. No smarts to notice that. But they also have more threads per inch. So it mostly equals out. And the fine thread bolt strength wins out. Which is why they are used in high torque and critical applications. Fine threads are generally not used in industrial applications because there is more design flexibility. I design custom machinery for a living also. But I'm also aware of math and science.

Obviously you will never be convinced, and that's ok.

 

[seber]

Using the standard formula for shear area of an internal thread.

For 1”-8 NC

A = pi x 8 x 1 x .998 (1/16 +0.57735 (.998-9168)) = .8733” for 1” engagement

For 1”-12 NF

A = pi x 12 x 1 x .9982 ( 1/24 + 0.57735 (.9982-.9941)) = .5274 for 1” engagement

You can find the equations for thread shear area here.
https://www.engineersedge.com/thread_strength/thread_bolt_stress.htm

That's over 60% greater area for coarse thread. Remember, we are talking about thread area not bolt strength.

 

[blarf]

Another point not yet mentioned is that rolled threads are stronger than cut threads. It's likely that when you are comparing puller prices some of the more expensive ones have been made using rolled threads and then hardened. If you're planning to replace a damaged forcing screw diy style it would be wise to use a grade 8 bolt or similar high strength type bolt.

Disclaimer: I'm not a machinist, just have read about threads in past, mostly in the "Machinist's Handbook".

If that was directed at me, I bit the bullet and bought replacement screws from BMW.

I'm not set up to do any metal fabrication and the original pivot screw has a fairly tall head with a hole in it that I couldn't easily replicate. Even just finding a long enough M16-1.5 screw or threaded rod is pretty challenging (there's about 100mm of threads).

As for strength, my understanding is that higher strength steel is more brittle. If a lower strength metal (or cut threads) mean that it'll fail more gracefully I'll take it. Having this slip off a ball joint is annoying but having the tool fail explosively would be a pretty bad day.

Were the steering knuckle on this car steel I'd just whack it with a couple hammers (not that there's a ton of room to swing a hammer here).

On the one hand metal fabrication as a hobby is space consuming and expensive. On the other, BMW engineers really like their non ******* standard tools. Dunno if the tools or car are worth more right now.

I, for one, would be happy if manufacturers would permanently stop using vague **** like "course", "fine", and "extra-fine" and simply precisely designate the actual damn thread pitch. Takes less room, even.

If memory serves coarse, fine, and extra-fine have specific meaning for metric thread.

 

[Jeff95TA]

Using the standard formula for shear area of an internal thread.

For 1”-8 NC

A = pi x 8 x 1 x .998 (1/16 +0.57735 (.998-9168)) = .8733” for 1” engagement

For 1”-12 NF

A = pi x 12 x 1 x .9982 ( 1/24 + 0.57735 (.9982-.9941)) = .5274 for 1” engagement

You can find the equations for thread shear area here.
https://www.engineersedge.com/thread_strength/thread_bolt_stress.htm

That's over 60% greater area for coarse thread. Remember, we are talking about thread area not bolt strength.

You're supposed to use the minimum major diameter of the external thread and the maximum pitch diameter of the internal thread. You used the maximums of the external for both. And you didn't do the arithmetic correctly. Using your values, they would be 2.745 and 2.353.

It should be 2.334 in^2 for coarse and 2.265 in^2 for fine. That's 3% greater area.

I'm not trying to be a jerk, just making sure people don't have wrong info. 60% vs. 3% is a big difference. And again, if the external and internal threads are the same material, the external thread is the driver, so fine would be better.

In the case of the OP's puller, the bolt may be much higher strength, hence the coarse threads.

 

[seber]

You're supposed to use the minimum major diameter of the external thread and the maximum pitch diameter of the internal thread. You used the maximums of the external for both. And you didn't do the arithmetic correctly. Using your values, they would be 2.745 and 2.353.

It should be 2.334 in^2 for coarse and 2.265 in^2 for fine. That's 3% greater area.

I'm not trying to be a jerk, just making sure people don't have wrong info. 60% vs. 3% is a big difference. And again, if the external and internal threads are the same material, the external thread is the driver, so fine would be better.

In the case of the OP's puller, the bolt may be much higher strength, hence the coarse threads.

You are right, my math was wrong. I ran those equations three times and got three different answers. Finally threw that damn rpn calculator away and went out to the shop for one that works. Your numbers are not correct either. It should be 2.745 for coarse and 1.657 for fine. It is still a 60% difference.

 

[Jeff95TA]

You are right, my math was wrong. I ran those equations three times and got three different answers. Finally threw that damn rpn calculator away and went out to the shop for one that works. Your numbers are not correct either. It should be 2.745 for coarse and 1.657 for fine. It is still a 60% difference.

I'm still getting 2.265 for fine and 2.334 for coarse using Mathcad and an online calculator. What values are you using for En max and Ds min?

All the data is attached.

 

[Provincial]

Out here in the real world, those of us without advanced education use simple rules.

1) Coarse threads into soft materials
2) Fine threads, using strong fasteners/nuts if you want it tight/strong.
3) Use a fastener system stronger than normal if the application is critical.
4) Using a Helicoil in softer material increases the pull-out strength of the fastener.
5) If you take it apart often, use studs and nuts rather than bolts. Otherwise, you can wear out the threads in the base material.

These are just the most obvious ones. There are plenty of other simple "rules."

A couple of comments on earlier posts:
* Vises have coarse threads in part to increase the speed of adjustment. A compromise to make the tool more practical.

* Increasing the root diameter of the coarse end of a stud that engages in a weaker material is a viable way of protecting against failure of the base material. I have encountered "stepped" studs where the coarse threads that engaged aluminum castings were a size larger than the fine threads on the nut end.

 

[Jeff95TA]

Out here in the real world, those of us without advanced education use simple rules.

Rules of thumb are great for getting the design together and may be all that's needed. I'm not sure what makes that "real world". When you have a lifting fixture for tens of thousands of pounds and you have to stamp the calculations, it's just as much the real world.