1. Nuts and Bolts Threaded Fastener Issues: Types Materials/Grades
Tightening Torque

2. Threaded Fasteners 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

3. Why are fasteners used? Advantages Disadvantages Removable
Easy to install
Wide variety of standard parts
Disadvantages
loosening
failure
cost

4. Types
Machine screws
Wood screws
Tapping screws

5. Standard Thread Systems
Unified or American ACME
SI (ISO)
Pipe Whitworth (UK)

6. Typical Designation 1/2” - 13 UNC - 2A external thread
Terminology of screw threads
Sharp vee threads shown for
clarity; the crests and roots are
actually flattened or rounded
during the forming operation.
external thread
(B means internal)
Class of fit
(1 is loosest tolerance, 3 is tightest)
Thread Series
UNC (Unified Coarse)
UNF (Unified Fine)
Pitch (threads/inch)
Nominal Diameter
(also shown as decimal or screw #)

7. Bolt Grades Grade indicates the tensile strength of the bolt
Determined by bolt material and heat treating

8. Tightening Torque
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

9. Bolt Manufacturing Processes
Forging (upsetting)
Rolling a) b)
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

10. Manufacturing Processes - continued
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.

11. References
Kalpakjian, S. Manufacturing Engineering and Technology, 2nd Edition, Addison Wesley, 1992.
Spotts, M.F., Design of Machine Elements. Prentice-Hall, Inc., 1985.
Shigley, Joseph and Mitchell, Larry, Mechanical Engineering Design. McGraw-HillBook Company, 1983.