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Did you know that?
the Boeing 747 uses about 2.5 million fasteners
• 70,000 titanium costing $150,000
• 400,000 other fasteners costing about $250,000
• 30,000 squeeze rivets, 50cents each installed
In certain applications (such as an engine
head), you should tighten the bolt as much as
possible, if it does not fail by twisting during
tightening, there is a very good possibility that
the bolt will never fail
Why are fasteners used?
Easy to install
Wide variety of standard parts
Standard Thread Systems
Unified or American ACME
Pipe Whitworth (UK)
1/2” - 13 UNC - 2A
(B means internal)
Class of fit
(1 is loosest tolerance, 3 is tightest)
UNC (Unified Coarse)
UNF (Unified Fine)
(also shown as decimal or screw #)
Terminology of screw threads
Sharp vee threads shown for
clarity; the crests and roots are
actually flattened or rounded
during the forming operation.
Grade indicates the tensile
strength of the bolt
Determined by bolt
material and heat treating
It is typical on engines for bolts to have a
specified tightening torque. Why?
It results in a quantified preload on the bolts
Insures that parts never separate
Maintains friction (no sliding to shear forces)
Insures even distribution of loading
• prevent warpage of mating parts
• uniform pressure distribution over seal or gasket
Prevents bolt from loosening
Reduces fatique effects
Bolt Manufacturing Processes
Thread-rolling processes: a) reciprocating flat dies; and b) two-roller dies.
Threaded fasteners, such as bolts, are made economically by these
processes at high rates of production
Manufacturing Processes -
Turning on screw machines
(a) Differences in the diameters of machined and rolled threads. (b) Grain flow in machined
and rolled threads. Unlike machining, which cuts through the grains of the metal, rolled threads
have improved strength because of cold working and favorable grain flow.
Kalpakjian, S. Manufacturing Engineering
and Technology, 2nd Edition, Addison
Spotts, M.F., Design of Machine Elements.
Prentice-Hall, Inc., 1985.
Shigley, Joseph and Mitchell, Larry,
Mechanical Engineering Design. McGraw-
HillBook Company, 1983.