Notes on Nuts and Bolts (June 10, 2010)

Notes on Nuts and Bolts,drive screw,Bolt Terms,nuts and bolts,Thread Terms,Metric and American threads,screw thread,threaded fasteners,Nuts,nut or bolt

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

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).

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.

Thread Terms

Metric and American threads both conform to the same profile, a series of equilateral triangles with the crests chopped off and the roots rounded.

The depth of the threads is 54.127% of the distance between threads, and the radius of the rounded root is 14.434% of the distance between threads. Another way of looking at it would be to say that ¹/₈ of the height of each equilateral triangle is chopped off the top, and ¹/₄ of the height off the bottom, leaving only ⁵/₈ of the height available. (The height of an equilateral triangle is equal to the width times half of the square root of three; ⁵/₈ of this is 0.54127.)

The root diameter of the thread is the nominal diameter minus 108.3% of the pitch of the thread. This means that fine threads have larger root diameters than coarse threads, and thus larger tap drill sizes. For threading using a tap or die, most threads are not cut to full depth, but to 75% or so. The resulting threads are not quite as strong, but full depth threading is very hard on the tap or die. Threading on a lathe presents no difficulty cutting to full depth.

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