Nuts are designed to develop the full strength of the bolt and are graded with strength ratings compatible with their respective bolts. SAE grade 5 nut can support the proof load of a SAE class 5 bolt. The aircraft AN bolt in the picture above broke from overload. The nut remains intact.
Bolts in tension place the nut threads in shear. Fully engaged nut threads have sufficient shear strength to hold the bolt in tension. The hex nut tensile strength is usually not a consideration as bolt tension puts the nut in compression and the threads in shear.
Shear strength between the bolt and nut is based upon engagement of threads at the pitch diameter. Since the nut has more shear area than the bolt, failure of the bolt occurs before the nut (assuming bolt and nut are of the same materials).
Shear strength for steel is approximately 60% of Ultimate Tensile Strength.
The finer the thread, the shallower the height. Less material is engaged at the pitch diameter with fine threads than with coarse threads. This is why in general, a coarse thread takes a higher load.
A fine thread bolt is stronger in tension as it has a slightly larger root diameter than a coarse thread bolt.
A fine thread bolt has slightly greater tensile strength, whereas the same bolt in coarse thread has slightly greater shear strength at the threads.
When screwing a bolt into a low-strength material, such as a steel bolt into a tapped aluminum hole, coarse threads are desired because of their greater shear strength.
A strong bolt or nut is wasted if the joint isn't assembled correctly.
Class 3 thread tolerance (aerospace) by providing more shear area in the threads, is stronger than Class 2 (commercial) threads.
Aerospace UNJ threadform has greater fatigue strength (endurance when placed under cyclic load) than the same size commercial thread UNR or UN thread.
When bolts are placed in tension (stretched), the threads are in shear. Increased engagement of Class 3 (aerospace) tolerance means more material to resist shear forces.
Built to be very strong - but forgot to follow-through during assembly.
