The straight side of a thread profile connecting crest and root. Unified and ISO metric threads have a 60 degree included angle, so each flank sits 30 degrees off the perpendicular, and under load the bolt thread bears on the nut thread along one flank, the pressure flank. Flank angle and straightness errors from worn dies or tools concentrate load on part of the thread and lower stripping strength even when the diameters check good.
Threads are polished on one flank only, so that bolt's been loaded hard, look for a crack at the first engaged thread.
Also heard as
- thread flank
Related terms
Pitch diameter
The diameter at which thread thickness equals the gap between threads, the dimension that controls fit.
Thread stripping
Failure where the threads of a nut, tapped hole or bolt shear off instead of the bolt breaking.
Galling
Adhesive wear where thread surfaces cold-weld and tear under load, seizing a nut partway down a bolt.
Fatigue
Cracking that grows a little with every load cycle until a bolt breaks far below its static strength.