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Bolting threads for flanges

Bolting threads for flanges

Bolting threads for flanges

(OP)
Hi All,
What type threads should I select for flange bolting (ASME B16.5)? I understand that for smaller size bolts (up to 1") UNC is the normal way to go. But what about the larger sizes? Do I stay with UNC or do I switch to constant pitch threads like 8UN or similar? Which is more common? Are there advantages/drawbacks with one or the other?

Thanks
Gunnar Breimer

PS
Being European I am not that familiar with American engineering practises.

RE: Bolting threads for flanges

(OP)
The problem is that niether ASME nor Texasflange give the thread, only the thread diameter.

If we take the example of a 2" bolt. A 2" UNC has 4,5 threads per inch, while an 8-UN has 8 threads per inch. That to me would suggest that it would be easier to achive the required clamping force in the flange with less torque if you use an 8-UN thread rather then a UNC.

In ASME B1.1 sect. 3.6.1 it says: "8-Thread Series. The 8-thread series (8-UN) is a uniform-pitch series for large diameters <snip>. Although originally intended for high-pressure-joint bolts and nuts, <snip> for diameters larger than 1 in."

RE: Bolting threads for flanges

(OP)
Thanks John Breen,

I found my answer in your first link, section 5.2. It clearly says that for flanged joints the bolting should be UNC up to and including 1" bolts and 8-UN for bolts larger than 1".

Again thanks John, I have made bookmarks of your links. Brilliant!

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