thread behavior mystery

[cervantes]

hi all,

i am wondering about physics of this situation

we have 3 screws: DIN 912 M6x30, 8.8 zinc, 10.9 zinc and 12.9 black
we are fastening them to steel high nut (around 15 mm height) thru a steel plate

now the results are:
1) 8.8 zinc screw - bolt is broken around 18 Nm, thread in nut and screw thread are ok
2) 10.9 zinc screw - nut thread is damaged @ around 21 Nm, screw thread is ok
3) 12.9 black screw - everything is ok at 30 Nm

different results at different classes, however why 12.9 didn't destroyed nut thread?.... the only reason I find here is an external diameter of screw which was around 0.1 higher than other classes (5,93 comparing to 5,83)

thanks for useful tips

 

[cervantes]

one more thing: consider experiment environment at the same level (same fricition etc.)

 

[CoryPad]

You can't assume friction is the same. Perhaps the 10.9 screw had lower friction and developed higher force which damaged the internal thread.

 

[ornerynorsk]

Frictional properties between zinc electroplate and black oxide are very different.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.