Class 3A thread... Why

[supergee]

Hi,

I love when students challenge me. I was teaching student about threading, and how Class 3 has a tighter tolerance than class 2 and even more than class 1.

I told them that class 2 is generally what is used but aerospace and other industries need tighter toleances, hence class 3.

A student asked me why is it better? If the screw is correctly tighten (right elongation) since the thread is acting like a cone so it should not move radially. why do we need Class 3??

I can't answer. at least, not with some reliable source. Can you help pretty please with sugar on top.

Gee

 

[3DDave]

Slightly more radial engagement and slightly more uniform loading down the spiral. No one wants to say "We saved $1 on the fastener that failed and led to a crash by using a less precise thread."

 

[SWComposites]

Huh? "Class" typically refers to the hole tolerance, not the threads. Class 1 is the tightest tolerance and typically used for composites. (well, actually interference fit is tighter than Class 1, and is used for fatigue critical metal joints). Class 3 is a sloppy hole for removable panels.

This question would be better asked in the Aircraft Engineering forum (there are mostly Civil Structures types here).

 

[3DDave]

Odds of accidentally referring to threads multiple times when meaning clearance holes is zero.

https://www.mwcomponents.com/fastener-thread-classes-explained

 

[supergee]

Thanks for taking the time to answer.

Slightly more radial engagement and slightly more uniform loading down the spiral. No one wants to say "We saved $1 on the fastener that failed and led to a crash by using a less precise thread."

When you say "slightly" do you mean the difference is slight yet significant or is it negligeable?

 

[dik]

The class 3 may give you a little more clearance for HDG, etc.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[3DDave]

If it was negligible there would not be a separate class.

 

[supergee]

If it was negligible there would not be a separate class.

I really feel stupid asking this question... I'm going to put it on the account of me teaching 7 hours of lecture in one day.