3A screws with 2A threads? 1

[Koether]

For some drawings that I have been revising I have been trying to get away from re-inventing the screw and am just referring to ASME B18.3 (SHCS) for screw dimensions and tolerances when I can.

It was recently pointed out to me that ASME B18.3 requires the use of class 3A threads. This is a very common screw specification but it not common to see 2B threads called out for tapped holes. I have to think it is commonplace for 3A screws to be used in 2B tapped holes. Is this combination something that anyone has thought about? Is there any literature that discusses this?

The difference between 2B and 3B is only on the max side so I think that means the allowance should be the same between a 3A/3B and 3A/2B interface (potential for no allowance).

 

[KENAT]

If I remember correctly if you use 3A & 3B together you get a close fit up to a line fit.

If you switch one part to class 2 then you'll get a fit between 2 & 3.

I suspect the only time you might have issues are if you are very highly torquing/loading the threads.

We use off the shelf socket cap screws in 2B holes frequently with no problem - but we don't highly load them usually.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[bcd]

This is normal. Has been this way for as long as I have been in Engineering (30 years). Forgot why, but I think it has to do with manufacturing tolerances. Commercial screws typically have rolled threads and can achieve more accurate dimensions. Tapped holes on the other hand are cut and will tend to be less consistent in sizing. End result is 3 and 2 tolerance scales, but the parts fit together and work perfectly.

 

[kmenke]

Outside of some of the aerospace standards, most all standard inch series nuts are 2B thread.

 

[Koether]

Thanks for the input. That makes sense, it is odd that none of literature seems to acknowledge that the classes are or can be mixed.

 

[MintJulep]

The requirements for screw thread fits for specific application are predicated on end use and can be met by specifying the proper combinations of thread classes for the components. For example, a Class 2A external thread may be used with a Class 1B, 2B or 3B internal thread.