Notes on Nuts and Bolts

            A screw thread is a helical groove on a shaft. When used for delivering power, it is called a drive screw. Drive screws aren't really all that efficient, as they loose a significant amount of power to friction. However, this friction can be put to use in the case of threaded fasteners. You might say that a drive screw is an inclined plane wrapped around a post, while a fastener is a wedge wrapped around a post.

Bolt Terms:

A 1/2-13UNC-2A-3 bolt, with a 2" thread and a 1" shank.

               As nuts and bolts are not perfectly rigid, but stretch slightly under load, the distribution of stress on the threads is not uniform. In fact, on a theoretically infinitely long bolt, the first thread takes a third of the load, the first three threads take three-quarters of the load, and the first six threads take essentially the whole load. Beyond the first six threads, the remaining threads are under essentially no load at all. Therefore, a nut or bolt with six threads acts very much like an infinitely long nut or bolt (and it's a lot cheaper).

Stress on bolt threads. Note how the majority of the stress is on the first thread to the left. Image from Spiralock.		Thread	%	%Sum
		1	34%	34%
		2	23%	55%
		3	16%	71%
		4	11%	82%
		5	9%	91%
		6	7%	98%

           There is little point in having more than six threads in anything. Nuts with National Coarse threads typically have 5 threads in them, whereas nuts with National Fine threads have about 8 threads. Nuts are usually stronger than the bolts they are on, which is to say that the bolt will usually break before the nut strips.
           It is often said that two threads must be exposed above a nut. The reason for this is that the first two threads of a bolt are often poorly formed, and may not engage the nut properly. If they're not doing their share, the other threads in the nut will be overloaded, and the nut may strip.

Source : Gizmology