Nuts and bolts
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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

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Nuts and Bolts

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Nuts and bolts

Nuts and Bolts

Threaded Fastener Issues:

  • Types

  • Materials/Grades

  • Tightening Torque


Threaded fasteners

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

Why are fasteners used?

Advantages

  • Removable

  • Easy to install

  • Wide variety of standard parts

    Disadvantages

  • loosening

  • failure

  • cost


Types

Types

  • Machine screws

  • Wood screws

  • Tapping screws


Standard thread systems

Standard Thread Systems

Unified or American ACME

SI (ISO)

Pipe Whitworth (UK)


Typical designation

Typical Designation

1/2” - 13 UNC - 2A

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

Bolt Grades

  • Grade indicates the tensile strength of the bolt

  • Determined by bolt material and heat treating


Tightening torque

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

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

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

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.


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