2 Threaded Fasteners Did you know that? the Boeing 747 uses about 2.5 million fasteners70,000 titanium costing $150,000400,000 other fasteners costing about $250,00030,000 squeeze rivets, 50cents each installedIn 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 installWide variety of standard partsDisadvantageslooseningfailurecost
5 Standard Thread Systems Unified or American ACMESI (ISO)Pipe Whitworth (UK)
6 Typical Designation 1/2” - 13 UNC - 2A external thread Terminology of screw threadsSharp vee threads shown forclarity; the crests and roots areactually flattened or roundedduring the forming operation.external thread(B means internal)Class of fit(1 is loosest tolerance, 3 is tightest)Thread SeriesUNC (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 TorqueIt is typical on engines for bolts to have a specified tightening torque. Why?It results in a quantified preload on the boltsInsures that parts never separateMaintains friction (no sliding to shear forces)Insures even distribution of loadingprevent warpage of mating partsuniform pressure distribution over seal or gasketPrevents bolt from looseningReduces fatique effects
9 Bolt Manufacturing Processes Forging (upsetting)Rollinga)b)Thread-rolling processes: a) reciprocating flat dies; and b) two-roller dies.Threaded fasteners, such as bolts, are made economically by theseprocesses 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 ReferencesKalpakjian, 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.