What is a fastener? History, manufacturing and types of industrial fasteners and fastening elements
The probably best known types of fasteners and fastening elements are screws and bolts made from a cylindric piece of wire. They consist of a head at one end and a shaft with a thread which is helically formed around its center line. The counterpart is characterized by a cylindric hole with a corresponding internal thread.
The difference between a bolt and a screw is that a bolt is designed to be inserted through holes in assembled parts and secured by torquing a nut. In this way, two components can be assembled together. A fastener with the ability to form its own thread within the component being fastened is called a screw.
The head of such fasteners is (usually) wider than the shaft and is the bearing surface for driving the screw and holding together the assembled components. These assemblies can be released again. In many cases washers are used in bolt-nut joints. Washers, which are available in different shapes and sizes, have the main purpose to prevent nuts and bolts from coming loose because of vibration and hold them in place.
Rivets are industrial fasteners for a permanent connection. They consist of a cylindrical shaft with a head on one end. The other end is called tail. A rivet is inserted through holes of parts which are assembled together. After installation, the protruding tail is expanded by deforming and holds the rivet in place. Typical application examples are railway bridges. The Eiffel Tower in Paris contains more than 1 million steel rivets which connect its components. Today, the variety of fasteners is virtually unlimited, and there are many types of thread sizes, pitches and tolerance classes. When you ask “What is a fastener?” you can include also nails and pins – there are fasteners with almost endless application possibilities.
History of fasteners
Fastening elements had been in use since early civilizations, being employed in carts, agricultural equipment, buildings, military devices and ships. One decisive step was made with the invention of the screw thread by the ancient Greek scientist Archimedes in the 3rd century BC. The “Archimedes Screw” consisted of a tube with a giant screw inside and was actually a water pump: When the lower end was placed into water and the cylinder turned, water was drawn up to a higher level. In this way, it was possible to remove water from the hold of a large ship and to irrigate countryside fields with the water from streams or rivers. The same principle could be applied for handling other liquid and semi-liquid substances and bulk material such as grain, sand and ashes.
From a physical point of view, the thread of a screw is an inclined plane or a wedge which is helically formed around the center line of the screw.
Today, screws used as fastening elements are used to transmit forces and hold objects together. For hundreds of years the threads were inserted manually by cutting. There was nearly no technological improvement until the Industrial Revolution in the 19th century. Hand in hand with a multitude of technological innovations large numbers of screws, bolts, nuts, rivets and other fastening elements were needed. The age of industrialization brought the opportunity for mass production and for standardization. In 1841, Sir Joseph Whitworth, a British mechanical engineer, proposed the introduction of standard fastener sizes. These comprised specifications for the angle and pitch of screw threads.
When choosing and designing a fastener for a certain application, several factors should be considered such as the materials to be joined, the installation process and the later environment conditions. Common materials for metal fasteners are different kinds of steel and non-ferrous materials such as copper, copper alloys, aluminum alloys, titanium alloys, and special materials such as Inconel.
Most fasteners are manufactured by cold forging (or cold forming). The raw material, a wire or a rod, is fed into a cold forming press, straightened and separated into required lengths. These “cut off” blanks are transferred through a series of dies where they get their shape step by step. The more complex the fastener, the greater the number of dies. The result of this process is a “headed blank”.
Bolts and screws need to be threaded for many applications. So the next process is called "thread rolling". By rolling through two dies, a thread is inserted into the shaft of the headed blank. This process generates a thread that is far stronger and has a better grain flow than a cut thread. Moreover, the thread rolling enables higher production rates, and there is virtually no waste material produced.
Optional steps in fastener manufacturing comprise specialized processes to achieve specific results. A heat treatment is performed to increase the overall durability of the screw or bolt, while surface finishing processes are performed to improve the surface quality by, for example, enhancing corrosion resistance.
After all these steps have been completed, the fasteners are tested: They are checked for their hardness levels, threading accuracy, impact and tensile strength, torque levels, and coating thickness. When they have passed all these tests, they are packed and delivered.
Applications of fasteners
The biggest customer of the fastener manufacturing industry is the vehicle industry with their suppliers. Today’s passenger cars, for example, contain more than 1,000 screw connections. Apart from the automotive sector, significant buyers of fasteners are also the producers of household appliances, electrical equipment and furniture, the construction industry as well as machinery, apparatus and steel engineering, medical technology, the furniture industry, and the fittings industry.
Basic Fastener Handbook. Published by the Fastener Training Institute, Santa Clarita, CA, USA.
A selection of specialized literature in English language is listed on:
The information has been compiled by Dipl.-Ing. Konrad Dengler, technical journalist and translator specialised in industrial topics.
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