In hardware stores, a machine screw or its larger cousin, a bolt, is described by length, the type of head and the thread. In the United States, the convention for describing threads is to give the number of threads per inch, preceded by a gauge number if the bolt is smaller than a quarter-inch in diameter, otherwise by the diameter in fractions of an inch. So, for example, in the United States one might ask for a “2-inch quarter-twenty bolt,” which would be
A typical smaller fastener could be “a 1-inch 8-32,” “8” being a gauge number and “32” the number of threads per inch.
A bolt's diameter isn't all you need to know to buy a nut to fit it. A bolt a quarter-inch in diameter might have 20, 28 or even 32 threads per inch, and a nut that fit one would not fit another.
The relationship between diameter and number of threads per inch is standardized in a number of series, the most common in the United States being UNC and UNF. A more complete sizing of a quarter-inch bolt's thread might be ¼-20 UNC or ¼-28 UNF. A few examples:
Here is a table of UNC and UNF thread sizes up to 1 inch in diameter.
Most stores now also sell machine screws and bolts in metric sizes. The metric sizes are described in a different way, for example “M3.5 × 1.2”. The number following “M” is the diameter in millimeters; the number following “×” is the pitch (also in millimeters), which is the distance from one thread to the corresponding point on the next thread. Metric sizes are described in this table.
The thread situation, though complicated now, was much worse in the past. If you deal with imported equipment or anything manufactured before the end of the Second World War, consult the index of entries on national series.
An engineer must provide a much more precise description of a fastener. A description of an inch-based fastener might look like this:
The “2B” is a tolerance class. The standards for a thread series include specifications of tolerances. Most specify several different classes, because for some uses a close fit is essential, while achieving it for other uses would be a waste of money. For example, the old American National series had four classes of tolerance: Loose-fit (class 1), Free-fit (class 2), Medium-Fit (class 3), and Close-fit (class 4). The names are self-explanatory. For a fuller and more current description of classes, go here for inch-based and here for metric.
The “21” is the gaging system number, as defined in ASME/ANSI B1.3M.
Additionally, the grade of bolt may be specified.
Almost all threaded fasteners tighten when the head or nut is rotated clockwise. That is, as a viewer turns a nut clockwise it moves away from her. Such a fastener is said to have a right-hand thread; all screw fasteners are assumed to be right-hand unless otherwise specified. Left-hand threads are usually found only on rotating machinery. For example, the axles of bicycle pedals screw into threaded holes in the cranks. In a pair of pedals one will have a right-hand thread and the other a left-hand thread. That way the rotation of the pedals doesn't tend to unscrew their axles.
To designate a left-hand thread, the letters “LH” are placed after the class of fit, like this:
3/8-16 UNC 2B LH (21)
No comment is necessary for a right-hand thread.
Fasteners can be made with two or three parallel threads instead of the usual one, although no standard series includes such a thread. The advantage of such threading is that the bolt will be stronger than a single thread bolt whose nut would advance the same distance in one turn–or conversely, that the nut will advance twice as far in a single turn on a double thread bolt than it would on a single thread bolt with the same root diameter.
To designate a multiple thread the word “DOUBLE” (or “TRIPLE”, and so on) is placed after the class of fit, like this:
3/8-16 UNC 2B DOUBLE (21)
It might be thought that after 150 years so seemingly simple a thing as the bolt could have been brought to perfection, but that is not the case. All the thread forms discussed so far share one characteristic: they are symmetrical. But when a bolt is tightened, the forces on the two sides of the thread are different. That alone suggests that an asymmetrical thread form might be better, and so it seems. The U.S. space shuttle, for example, uses bolts with an asymmetric thread form. In time all threaded fasteners may have forms subtly different from those we use today.
National Bureau of Standards Handbook H28 (1957). See Appendix A.
ASA B1.7 Standard on Nomenclature, Definitions and Letter Symbols for Screw Threads.
ANSI B1.1 Unified and American Screw Thread Standards.
Industrial Fasteners Institute,
Metric Fasteners Standards, 1976.
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Last revised: 5 June 2008.