Class 3A screw in 2B nut 3

[tmech77]

Dimensionally should a class 3A screw always work with a class 2B nut if they are both made out of steel? If you use anti-seize, should it work if they are both stainless steel?

 

[JohnRBaker]

I think that there should be no problems.

John R. Baker, P.E. (ret)
EX-Product 'Evangelist'
Irvine, CA
Siemens PLM:
UG/NX Museum:

The secret of life is not finding someone to live with
It's finding someone you can't live without

 

[tmech77]

Thanks John.

 

[JohnRBaker]

The only consideration is that if you're required to meet a certain tolerance spec, the fit of a combination such as this is defined by the lower tolerance class used, in this case it would be considered a Class 2 fit.

John R. Baker, P.E. (ret)
EX-Product 'Evangelist'
Irvine, CA
Siemens PLM:
UG/NX Museum:

The secret of life is not finding someone to live with
It's finding someone you can't live without

 

[3DDave]

The limits of American Standard threads are absolute for individual profile - there's a boundary that each side is to keep away from. The lower the class, the larger the nominal gap.

However, there are practical limits to pitch error accumulation and, more minor, straightness.

Much beyond one diameter this error accumulation will see the fit become tight.

There is also a contribution of finish - in previous questions about stainless steel screw threads people have noted that galling happens if the threads are rough and that the galling stopped with only a small amount of material removal to polish the threads. Anti-galling material, of course, will also help.

 

[tmech77]

Thanks 3DDave.

 

[DAVIDSTECKER]

It sounds to me that you are concerned about how badly stainless steel fasteners gall and lock up.
We had that issue here on a regular basis until the guys finally realized that applying the anti-seize as instructed really did work.
If this is the case for you just use the anti-seize and don't get all wrapped up about class of the threads.
I've had screw pin SSTL shackles with really loose fits lock up with out anti-seize.

 

[tmech77]

Thanks David. Do you think a steel screw could possibly gall up with a stainless steel nut?

 

[JohnRBaker]

Generally galling only occurs when both of the threaded items are made of the same material. With dissimilar metals, there are different issues, such as galvanic corrosion.

John R. Baker, P.E. (ret)
EX-Product 'Evangelist'
Irvine, CA
Siemens PLM:
UG/NX Museum:

The secret of life is not finding someone to live with
It's finding someone you can't live without

 

[DAVIDSTECKER]

As JohnRBaker states it is similar metals that is the main issue.
I recently spoke with one of my suppliers that said that they use 304 (18-8) bolts with 316 nut to reduce galling along with anti-seize.
Belt and suspenders approach.