Brief intro
We humans tend to take the marvel of Technology and Engineering around us for granted. Our
modern world apparently functions on machines ranging from our normal vehicles to complex
super computers. The structures which hold our modern world together too are nothing short
of impressive - from massive steel bridges to the our very own houses. Holding together a lot
of these wonders of science are seemly mundane yet incredibly useful – fasteners. In this report
we will focus on the most widely used class of the fastener family – the Nuts and bolts
A bolt can be defined as a cylindrical, externally threaded fastener with a head. If the bolt is not
threaded all the way from the head to the tip, the diameter of the unthreaded part is about
equal to the diameter of the thread crest (the maximum diameter of the thread). The shape of
the bolt's head can be square, hexagonal, round, countersunk, elliptical or oval. A bolt is
mounted into a nut which is what differentiates it from a screw.(1)
A nut can be defined as a block of metal, usually square or hexagonal, with a hole through its
center. The hole is internally threaded to mate with the external threads of a standard bolt.
Variations of design include locknuts, slotted or castle nuts and wing nuts. Most of the
variations seek to deal with the problem of preventing a nut from working loose in a vibrating
joint. (1)
Some idea of the structural importance of these fasteners can be conveyed in a list of familiar
objects with the number of fasteners (of all types) holding them together. A telephone has 73, a
dishwasher 115, a refrigerator 275, a fork-lift truck 940, a railroad boxcar 1,200. a turret lathe
1,650, an automobile 3,500 and a jet airplane 1.5 million. (1)Graves, Frederick E. “Nuts and
Bolts.” Scientific American, vol. 250, no. 6, 1984, pp. 136–145.
A sense of the economic importance of fasteners can be had from a recent estimate in the
United Kingdom that the joining of components accounts for between 20 and 40 percent of
total manufacturing costs. The same would be true for the rest of the world. Much of the
expenditure is for labor. Although the cost of fasteners for a mechanical assembly averages less
than 5 percent of the cost of the finished assembly, a little more than half of the total
production time is spent on fastening work. The result is that the cost of the installed fasteners
in an assembly ranges from three to 10 times the cost of the fasteners as parts. The fastener
industry is economically significant in its own right. For example in the U.S. some 625
companies employ between 50,000 and 60,000 workers to make approximately 250 billion
fasteners per year. The final users pay $10 billion just to buy these fasteners and much more to
install them. The final installed cost is some $50 billion.(1)
History
The idea that a threaded bolt and a matching nut could serve as a fastener dates back only to
the 15th century. Today nuts and bolts and ways of making them are still actively evolving. In
thinking about how machines and other manmade structures are fastened together one might
suppose the combination of nut and bolt is of ancient origin and is by now so well developed
that nothing new could be expected of it. In reality the threaded nut and bolt date only from
about the middle of the 15th century, when they were made by hand and a nut that fit one bolt
was highly unlikely to fit another. Moreover, even though the technology of the nut-and-bolt
fastening is well established, it has recently benefited considerably from several evolutionary
advances. They include electronic systems for tightening nuts, special coatings for nuts and
bolts and new tools for making nuts and bolts(1)
Although the threaded bolt dates from the 15th century, the unthreaded bolt, which is far more
limited in application, is much older. Records show that un-threaded bolts served in Roman
times for barring doors, as pivots for opening and closing doors and as wedge bolts: a bar or a
rod with a slot in which a wedge was inserted so that the bolt was secured in place and could
not be moved. The Romans also seem to have developed the first wood screw, which was made
out of bronze or even silver. Its threads were filed by hand or consisted of a wire wound around
a rod and soldered on. Apparently this invention disappeared with the Roman Empire, since the
first printed record of screws is in a book dating to early in the 15th century. Later in that
century Johann Gutenberg included screws among the fasteners in his printing press. Leonardo
da Vinci's books from the late 15th and early 16th century include several innovative designs for
screw cutting machines.(1)
It is widely agreed that it was the advent of the industrial revolution which accelerated the
research and development of the nut and bolt. The first application of machinery to make
screw-threads was made in France in 1569 by Jacques Besson, a French inventor who utilized a
screw cutting gauge in a lathe. In 1641, almost a century later an English firm, Hindley of York
greatly improved his design and put it into wider circulation.(2) W.R. Wilbur, “History of nuts
and bolt in America”, pp. 386, 1905.
In Britain in 1760, J and W Wyatt introduced a production process for the manufacturing screw
threads. However, this led to a different challenge: each company manufactured its own unique
threads, nuts, and bolts so there was an enormous range of various sized screw threads on the
market, causing problems for machine manufacturers. It wasn’t until 1841 that English engineer
Joseph Whitworth managed to find a solution to the problem. Whitworth after years of
research and collecting screw samples from various workshops, he suggested standardizing the
dimensions of the screw threads in Britain, for instance, someone could make a bolt in one part
of England and someone else in another part could make the nut and that they would both fit
together. His proposal was that the angle of the thread flanks be standardized at 55 degrees,
and the number of threads per inch should be defined for various diameters. While this issue
was being addressed in Britain, the Americans facing a similar situation initially resorted to
using the Whitworth standard thread.(3)
In 1864, William Sellers proposed a 60-degree thread form and various thread pitches for
various diameters. This developed into the American Standard Coarse Series and therefore the
Fine Series. One obvious advantage the Americans had over British system was that their
thread form had flat roots and crests. This made its production easier than the Whitworth
standard, which had rounded roots and crests. It had been found, however, that the Whitworth
thread gave better results in dynamic applications and that the rounded root of the Whitworth
thread improved fatigue performance. (3)
The Americans gradually moved to Sellers standard since it was easier, cheaper, and faster to
produce, and by the end of 18th century they had completely abandoned Whitworth standard in
favor of the Sellers standard which now became known as the American standard. In 1901
Europe too formally adopted the Seller thread at International Congress for Standards and
Gauges. The British however stuck to Whitworth standard. This didn’t have any major
consequences until the First world War, when the absence of consistency between screw
threads in several countries became an enormous obstacle to the war effort; during the Second
World War when the American supplies reached the devastated British forces in North Africa it
was found that American nuts did not fit British bolts, and vice versa. This incompatibility of
screw thread standards became a major hindrance in the Allied war effort. Hence the U.S had
to run two separate assembly lines for the remainder of the war, one to make British
equipment and weapons, and another to make American ones.(4) Anonymous, “Turn of the
Century”, Wired Magazine, #Magazine- 10.01, 2002.
After the war in 1948, Britain, the USA and Canada agreed on the Unified thread as the
standard for all countries that used imperial measurements. It uses a profile similar to the DIN
metric thread previously developed in Germany in 1919. This was a mixture of the simplest of
the Whitworth thread form (the rounded root to enhance fatigue performance) and of the now
popular Sellers thread (60-degree flank angle and flat crests). However, the larger root radius of
the Unified thread proved to perform much better than the DIN metric profile. This led to the
ISO metric thread which is employed by all industrialized countries today.(3)
Modern Development
In 1968 the International Organization for Standards (ISO) recommended the adoption of a set
of metric standards based on the basic thread profile. It appears likely that both types of
threads will continue to be used for some time to come. In both the Unified and ISO systems,
the crests of external threads may be flat or rounded. The root is usually made rounded to
minimize stress concentrations at this critical area. The internal thread has a flat crest in order
to mate with either a rounded or V-root of the external thread. A small round is used at the
root to provide clearance for the flat crest of the external thread. In the metric system, the
pitch is always expressed in millimeters, whereas in the American (Unified) system, it is a
fraction having as the numerator 1 and as the denominator the number of threads per inch.
Thus, 16 threads per inch, 1 /16, is a 16 pitch. Consequently, in the Unified system, threads are
more commonly described in terms of threads per inch rather than by the pitch.(5) Degarmo, E.
Paul; Black, J T.; Kohser, Ronald A., “Mannesmann Mill”, Materials and Processes in
Manufacturing, 9th ed., pp. 791, 2003
Types of Nuts and Bolts
The various classes of Nuts and Bolts are as diverse as their applications in modern world. Here
we will discuss only the more common types of bolt fasteners available in markets today.
ANCHOR BOLT
These are bolts which are designed to anchor a structural or non-structural components to
concrete. They may be set into place in wet concrete as it is being cast or they can be attached
to the concrete by drilling a hole in the concrete into which the anchor bolt is then inserted and
then fastened. Anchor bolts themselves come in a variety of different designs such as cast inplace anchor bolts, under-cut anchor bolts, bonded Anchor bolts. (6)
BLIND BOLT
Blind bolts are a type of incredibly versatile structural fasteners. It consists of a steel pin, collar,
and sleeve. When the blind bolt is inserted through the pre-drilled hole the expander opens on
the blind side of the hole and blocks the bolt through the hole as a nut is tightened on the
threaded side firmly securing the bolt its connection in place. No access to the blind side of the
hole is required, hence the moniker ‘blind’. The interesting thing about blind bolt is that it is
easier and faster to install than a rivet but still forms a stronger joint than it.(7) Mike Eichinger,
“Essential Definition: What is a Blind Bolt”, blog.baysupply.com, 2018.
CARRIAGE BOLTS
A Carriage bolt is a specialized bolt typically used for fastening wood to wood or metal to wood
but can also be used to fasten metal to metal. It consists of a shallow dome head, a square
section just below the head, and the normal threaded shaft. The Shallow circular head inhibits
tampering from the bolt head side of the fastener whereas a typical nut goes on the threaded
side. The square section slots into a square slot on the metal or a hole in the wood to hold the
bolt from turning while the nut is being fastened on it.(8) Anonymous, “What is the Difference
between Carriage bolts and Coach Bolts”, www.accu.co.uk, 2018
DOUBLE END BOLT
They are also called connecting rods or Double end studs. These bolts consist of a rod threaded
on its either side with a small unthreaded portion in the middle. The unthreaded part has
usually has a flat portion or some other form to allow the user to grip the bolt and prevent it
from turning when it is being fastened. It looks very similar to a threaded rod with the
exception that it not threaded in the middle and that the threads may be of dissimilar sizes and
even opposite in direction. It is commonly used to connect two female-threaded components
to one another. They are typically used in plumbing applications, metal fabrication and
construction works.(9) Anonymous, “Double-end threaded rods and studs”, grainger.com
EYE BOLT
Eye bolts are bolt having a circular ring attached to their head. They used to attach ropes and
cables to objects. Eye bolts typically don’t come with a shoulder but as it unnecessary as the
bolt can be tightened using the ring itself. Shouldered eye bolts are generally suitable for up to
45 angular loads while the eyebolts without one are generally not suitable for any significant
angular loads. Common applications include washing line support, shade support etc. For minor
loads they, may even be used as an anchored pulley.(6)
HEX BOLT
These are the most commonly used type of bolts available in the market. They come with a
hexagonal head which is used to grip or turn it with help of a wrench or socket. They came in
several different sizes typically in metric standard sizes but also in dimensional inch sizes. They
may be fully threaded or they may feature an unthreaded shoulder which may be used as a
pivot. They are used in almost every industry be it automotive industry, power generation
industry, water pumping plants, gadgets manufacturing industry, construction industry etc.
(6)Anonymous, “Types of bolts – A Thomas buying guide”, thomasnet.com
PENTA-HEAD BOLT
These are pretty similar to the hex bolt, with the exception that they sport a pentagonal head
instead of the typical hexagonal head. The purpose of manufacturing them with a pentagonal
head is to prohibit their access by the usage of commonly available standard tools since these
bolts require specialized tools to fasten/unfasten them hence they are mostly used to secure
places where non-authorized personnel entry is forbiddened, for example, manhole covers,
bank vaults etc. (6) Anonymous, “Types of bolts – A Thomas buying guide”, thomasnet.com
ROUND-HEAD BOLT
They are very similar to carriage bolts with the exception that they don’t have a square section
below their heads. Like carriage bolts, round-head bolts too find their use in woodworking. In
carriage bolts it is the square taper which locks the bolt and holds it from turning during nut
tightening, however in Round-head bolts it is the friction between the bolt and the wood
surface which hold the bolt from turning during said operation.(6) Anonymous, “Types of bolts
– A Thomas buying guide”, thomasnet.com
SHOULDER BOLT
Shoulder Bolts is typical machine screws having an unthreaded shoulder. Other types of bolts
having a shoulder on the bolt shaft are also a subclass of shoulder bolts. When the bolt is
installed into its application, the shoulder part sticks out of the application site’s surface. This
shoulder part can then act as a dowel or a pivot.(10) Anonymous, “FASTENERS 101™
Types of Fasteners”, albanycountyfasteners.com
SOCKET HEAD BOLT
The Socket head bolt features a hexagonal cavity in the center of its head, in which an L- key
(Allen wrench) or a hex socket screwdriver is inserted to screw/unscrew the bolt. These bolts
come in various headstyles such ranging from countersunk design to a flat domed head.(10)
Anonymous, “FASTENERS 101™ Types of Fasteners”, albanycountyfasteners.com
T-HEAD BOLT
The T-head bolt is called so because its vertical profile resembles a ‘T’. It typically consists of a
rectangular head, which slots into a slot or recess which secures it in place and prevents it from
turning while nut is tightened on the bolt. A Square head bolt is a close sibling of the T-head
bolt, differing only in the shape of the head as the Square head bolt has a square shaped head
instead of a rectangular one but functions the same way. A variety of the T-head bolt, the T-slot
bolt is an integral component of the T-slot track, a common workshop tool for securing a
workpiece in place.(11) Erik Oberg et al. "T-Bolts, slots, nuts and tongues", Machinery's
Handbook, 28th ed., pp. 1632–1634, 2008.
U-BOLT
U-Bolt is essentially a double end bolt, bent into a ‘U’ shape. The bent portion of the bolt lacks
threads while both of its ends are threaded. These bolts are typically used as supports for
pipings of various sizes. They are widely used in the automotive industry too to mount leaf
springs on the automobiles. A variation of the U-bent bolt design is the Square bend bolt which
has two right angled bend instead of a single continuous u-bend.(12) Anonymous, “U-bolt
Application and Design”, engineersedge.com, 2019
References
1. Graves, Frederick E. Nuts and Bolts., Scientific American, Vol. 250 no.1, 1984, pp. 136-145.
2. Wilbur, W.R. History of Nuts and Bolts in America. 1905. p. 386.
3. Anonymous. The History of the bolt. Nord-lock.com. [Online] 2017.
4. Anonymous. Turn of the Century., Wired Magazine, Vol. 10. 01, 2002
5. Degarmo, E. Paul, Black, J T. and Kohser, Ronald A. Materials and Processes in Manufacturing. 9th
ed. 2003. p. 791.
6. Anonymous. Types of bolts - A Thomas buying guide. thomasnet.com. [Online]
7. Eichinger, Mike. Essential Definition: What is a Blind Bolt. blog.baysupply.com. [Online] 2018. .
8. Anonymous. What is the Difference between Carriage bolts and Coach Bolts. accu.co.uk. [Online]
2018.
9. Anonymous. Double-end threaded rods and studs. grainger.com. [Online]
10. Anonymous. FASTENERS 101™ Types of Fasteners. albanycountyfasteners.com. [Online]
11. Oberg, Erik. T-bolts, slots, nuts and tongues. Machinery's Handbook. 28th ed. 2008. pp. 1632–1634.
12. Anonymous. U-bolt Application and Design. engineersedge.com. [Online] 2019.