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

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

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

Types Machine screws Wood screws Tapping screws

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

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 #)

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

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

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

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.

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.