Take me to hardware school please 1

[danielthb]

I've been noticing on some outline and installation drawings for electronic equipment onboard a ship the specification of -2A bolts and -3B self locking nuts for hold-down. The originator is not available to consult.

Are class 3 nuts typically used with class 2 bolts. Why or why not?

Are grade 5 bolts usually class 3 or could they be class 2?

Thanks,
Dan

 

[KENAT]

Assuming you're talking UN threads...

Looked into this a while back up to a point, -2A bolts will work with -3B nuts. The fit will be a little 'tighter'/more precise than 2A & 2B but looser than 3A & 3B.

Doesn't really answer your question though. We use class 3 for a lot of fine adjustment screws, not sure this is applicable to your situation though.

KENAT, probably the least qualified checker you'll ever meet...

 

[danielthb]

These are bolts to hold equipment racks on their foundations. I knew the class 3 was a more precise thread but didn't know if there was an engineering reason that mixing them was somehow superior.

Meanwhile, I've found a Grade 8 class 2 bolt specified. Is that one available? I thought the stronger the bolt, the tighter you wanted the fit--thread bearing surface deformation and all that.

There's a lot of inconsistency in what I'm looking at because it came in parts from a dozen or so places. I'm beginning to think the designers were no more qualified to spec hardware than I am.

Thanks again,
Dan

 

[KENAT]

Generally, class 2 are more common, from what I've seen since being in the states. I don't know for Grade 8 specifically.

Class 3 allow a line fit at max tol which doesn't work well with any contamination etc.

KENAT, probably the least qualified checker you'll ever meet...

 

[dvd]

I ran into this same problem with prevailing-torque locknuts. The Industrial Fastener Institute has specifications for locknuts (

http://www.indfast.org/publications.php?cat=3)

that call for the same class of fit on both screw and nut. My conversations with application engineers at the prevailing-torque locknut manufacturers did not come up with an answer. My application was used in a high-heat environment and the suspicion was that the class 2 nut and class 3 screw threads had something to due with thermal cycles. The individuals who decided on the fit mis-match were very thorough, and just like you, they are not around to ask why. Please post any findings you come up with.

 

[danielthb]

I've found yet another scenario. This one shows grade 8 bolts securing an aluminum rack to a steel platform. Can you fully torque or pre-tension grade 8 hardware with aluminum on one side?

Dan

 

[KENAT]

What grade aluminium?

This site will allow you to get a ballpark stress figure if I recall correctly.

http://www.boltplanet.com/default.asp

KENAT, probably the least qualified checker you'll ever meet...

 

[btrueblood]

"Can you fully torque or pre-tension grade 8 hardware with aluminum on one side?"

Certainly, with a wide enough and thick enough washer under the bolt head.

Of more concern to me is the (apparent) contact between steel and aluminum in a marine environment with no (apparent) concern for galvanic corrosion.

 

[danielthb]

Kenat, 6061-T6.

btrueblood, we're inside in a well air conditioned space--very well air conditioned.

Dan

 

[dimjim]

With a grade 8 bolt and aluminum, you may need a larger
od washer to prevent embedding the washer into the aluminum if you are using the full clamping force of the grade 8 bolts.