Engine Dissection - Nuts & Bolts

7/29/2019 Engine Dissection - Nuts & Bolts

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product dissection 3/15/201

Engine Dissection - Nuts and

Bolts

Threaded Fastener Issues on Engine:

Types

Materials/Grades

Tightening Torque


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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 enginehead), you should tighten the bolt as much as

possible, if it does not fail by twisting duringtightening, there is a very good possibility thatthe bolt will never fail


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Why are fasteners used?

Advantages

Removable

Easy to install

Wide variety of standard parts

Disadvantages

loosening failure

cost


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Types

Machine screws

Wood screws

Tapping screws


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Standard Thread Systems

Unified or American ACME

SI (ISO)

Pipe Whitworth

(UK)


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Typical Designation

1/2 - 13 UNC - 2A

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 #)

Terminology of screw threads

Sharp vee threads shown for

clarity; the crests and roots areactually flattened or rounded

during the forming operation.


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Bolt Grades

Grade indicates the tensilestrength of the bolt

Determined by boltmaterial and heat treating




Materials

Lets look at the example of engine headbolts

The head is alumimum, so why are the headbolts made of steel ?

In what cases might you want to use analuminum bolt?




Tightening Torque

It is typical on engines for bolts to have aspecified tightening torque. Why?

It results in a quantified preload on the boltsInsures 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


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Bolt Manufacturing Processes

Forging (upsetting)

Rolling

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

a) b)


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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.


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References

Kalpakjian, S. Manufacturing Engineeringand 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.

Engine Dissection - Nuts & Bolts

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