ISO or FED-STD-H28/2B engagement length 1

[loiner]

Engineersedge(

http://www.engineersedge.com/thread_strength/thread_minimum_length_engagement.htm

&

http://www.engineersedge.com/thread_strength/thread_minimum_length_engagement_fed-std-h28.htm)

has two equation sets for required thread engagement length. One is "ISO" and the other is FED-STD-H28/2B. I am looking at a M56 (metric) bolt with nut tensile strength 430MPa and bolt tensile strength 850MPa and the two sets of equations when modified by the J factor give big differences. The FED-STD-H28 gives a much lower/better value. Which equation set should I use for a M56 bolt ? Where do the "ISO" equations come from (they are not in ISO898)?

 

[loafer09]

The difference appears to be in how the thread shear area is calculated.

I always use the FED standard, and have never had any problems. I’ve also done my own tests on some materials to verify that the bolt will fail before the threads strip using the FED standard.