NUT             HEAD
          |      SHANK     |
  THREAD  |        |       |
    |    __        |      ___
    |   |__|     ________|__ |
  //////|  |/////        |   |
  //////|__|/////________|__ |
        |__|             |___|
         |      |________|
                 GRIP LENGTH


Bolts and screws fall under the umbrella term "fasteners." The two are often confused because a fastener can be both a bolt and screw at the same time.

Fasteners are broadly identified by function, shape, and relationship to fastened material. Machinery's Handbook defines a bolt as a threaded fastener that holds together unthreaded components using a nut; a screw, by contrast, either is applied to a threaded opening or creates its own threads during application (like a wood screw or self-tapping screw).

A bolt can also be a screw if it is inserted into a threaded opening and secured with a nut.

Un-threaded bolts acted as door hinges and locking mechanisms in Ancient Rome. They were notched to accept wedges, which were hammered into place.

Screw threads probably originated around 400 BC, when Archytas of Tarentum, a contemporary of Plato, used screw threads suspended in an unthreaded shaft to raise water. This device--the water screw--is also known as Archimedes' screw, because Archimedes more famously used it for the same purpose a century and a half later. Water screws also worked in Ancient Egypt to pump bilge from ships and allegedly in Assyria's Hanging Gardens of Babylon. Threaded wood shafts aided in the pressing of grapes in all three locations.

Metal bolts appeared in fifteenth century Europe. Their threads were cut by hand in a process that was most assuredly grueling; an early application was in Gutenberg's printing presses. Luc Besson devised thread-cutting machines in France in 1568 before developing ridged plates for use with lathes--a development currently in use. As was his way, da Vinci conceptualized a number of screw-cutting machines. J and W Wyatt achieved the mass manufacture of bolts in Britain in 1760.

Early bolts featured non-standard, square heads; hex-shaped bolt heads, smaller in area and approachable from more angles, evolved as a direct consequence of shrinking machinery. James Nasmyth introduced the mass-manufacture of hex heads in 1830; hex nuts would appear fifty years later, owing to a greater ability to produce and accurately machine mild steel.

The non-standardization of bolt threads occupies a shocking span of human history. It was in 1841 that Joseph Whitworth proposed a standard 55-degree thread pitch and standard number of threads-per-inch correspondent with shaft diameter (he had analyzed screws from all over Britain). Two decades later, William Sellers would engender the American Coarse and Fine Series; American threads (pitched 60 degrees) feature more pointed ridges and valleys than Whitworth's threads, which makes them easier to manufacture.

Between nations, non-standardization of bolt threads persisted through World War II (two). It was a particularly nasty specter for the Allies. Today's ISO thread metrics are prefaced by standards agreed upon by Canada, the US, and Britain in 1948, for all nations using Imperial measurements.

The last eight decades have witnessed a number of small changes to worldwide bolt and nut metrics, none of which are detailed here.



Types & Applications


The types of bolt in the world are myriad. This list is abbreviated to include the most common/important.

Hex bolts are standard in most applications, particularly structural. Variations on the hex bolt are also myriad, ranging from slotted heads to built-in washers to coarse threads for tapping into wood (as lag bolts).
Carriage bolts present a pleasing, featureless, domed head. The squared section just underneath digs into wood (often framing lumber), securing the bolt for the application of the nut.
U-bolts feature two threaded ends and are often paired with a bar, for securing pipe. The similar (though inferior) J-bolt is often used for the same purpose.
Shoulder bolts feature a free-spinning sleeve before the threaded section, allowing for the creation of a pivot joint.
Sex bolts are hollow, with a female (or internal) thread. They are paired with mating screws and are used wherever it is advisable to have a bolt head at both ends.
Eye bolts feature a looped head, to attach a chain or hook. The loop can be open or closed.

Note that "type" and "drive"--the means by which a fastener may be tightened--represent separate worlds unto themselves. Different types of drive are more applicable to screws; bolts are generally not driven, for reasons outlined at the beginning of this write-up.



Grades & Materials


As of October 2014, there exist 37 grades of bolt, divided between standards devised by SAE, ASTM, and ISO. All three organizations use the same metrics: proof load, yield strength, and tensile strength--respectively, the load the bolt can bear without deforming, the load at which the bolt deforms, and the load the bolt can bear while stretched.

Chances are very good that you have observed SAE markings on a bolt head:


     |              \|/ 
/ \ /|\
SAE Grade 5     SAE Grade 7


More "legs" denote a stronger bolt, typically of alloy steel. Regardless of classification system, bolts with no markings can be presumed to be of low or median steel.

Bolts manufactured to ISO and ASTM standards bear those organizations' alphanumeric codes. Stainless bolts display specific grades, usually (again) SAE markings such as 304 or 316.

Bolts made of non-metals like nylon are common in applications requiring electrical isolation.





The goal, if such a word be used, is to pre-load the bolt evenly. 

"Clamp force" refers to the amount of force exerted by the bolt head and nut, as the bolt stretches. It is directly proportional to the amount of applied torque. Tightening is usually limited by friction.

Whether one tightens the head or nut matters only if the materials bolted together are dissimilar, in which case tightening whichever side faces the softer material will allow for more load. It is important (and safe) to use an appropriately-sized wrench; adjustable and open wrenches should be turned in the direction the handle is cocked, so as to (in the case of the adjustable wrench) place the load on the solid jaw rather than the adjustable one. Teflon tape, grease, etc. reduce friction. Washers distribute force on the faces of bolted materials.

"Galling" is the seizing and tearing-free of threads inside the nut, usually a consequence of overtightening or unevenly pre-loading (a good way to do this is to put the bolts on your tire in the wrong order). Removing a galled nut is sometimes possible with the application of penetrating oil and much tapping/suffering; frequently it is impossible (or impractical, if the bolt is small enough to be twisted apart by hand). It is important to tighten bolts smoothly.

Combining a nut and bolt of dissimilar metals results in corrosion and seizing.





Nord-Lock Bolt Securing Systems. "The history of the bolt.", 9/27/14

Santa Rosa College. "The Nuts and Bolts Behind Engineering." (.pdf), 9/27/14

eHow. "The History of Hex Head Nuts and Bolts.", 9/27/14

ThomasNet. "The Evolution of Bolt Manufacturing.", 9/27/14

Bolt Depot. "Fastener Type Chart" and "How Fasteners are Identified.", 10/4/14

Engineer's Edge. "Hex Bolt Identification Guide.", 10/4/14

Wikipedia. "Bolt.", 10/4/14

Bolt Science. "Tightening the Nut or the Bolt?", 10/9/14