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AFA ILion DVD CHAPTER: Threaded Fasteners

Threaded Fasteners

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Forces

Clamping force between two surfaces

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Forces

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First exposed threadFirst thread root

Underhead filletShank

Bolt head

Nut

Washer face

High Stress Areas

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Manufacturing

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Manufacturing

Bolts are cold formed

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Manufacturing Cut Vs Rolled Thread

CUT THREAD

CUT THREAD

ROLLED THREAD

ROLLED THREAD

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Caterpillar Bolts

Standard Cat bolts have grade 8 Classification

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Marking Metric - Inch

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Torquing

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Nuts Lower Hardness than the Bolt

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Knurled Bolts Provide better fit in bolt hole

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Threads This bolt have been torqued

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Abnormal Stress Raisers

• Pits• Nicks• Gouges• CracksWhere ?• Threads• Shank• Under head Fillet• Washer Faces

• Pits• Nicks• Gouges• CracksWhere ?• Threads• Shank• Under head Fillet• Washer Faces

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Fatigue Fracture Abnormal Stress raiser

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Fatigue Fracture Abnormal Stress raiser

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Fatigue Fracture Fatigue fracture at high stress area with no abnormal stressraiser indicate cyclic overload

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Fatigue Fracture Bending

Bolts are not supposed to carry bending loads

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Loose Joint No Preload

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Fatigue Fracture Bending

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Fatigue Fracture Bending

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Fatigue Fracture Reverse Bending

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Fatigue Fracture Bending

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Fretting Cyclically overloaded or slightly loose joint

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Fretting Bolt vibrating

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Ductile Fracture Single Overload Bending load

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Ductile Fracture Over torque

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Ductile Fracture Over torque Dark rough fracture

Polished threads

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Ductile Fracture “Bulleting” Plastic deformation

Heat discoloration

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Impact Shear

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Fracture Comparison ImpactShear

FatigueFracture

DuctileBendingFracture

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Fastener Problems

• Material• Forming• Hardening• Threading

• Material• Forming• Hardening• Threading

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Pre-crack Rolling Seam

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Pre-crack Rolling Seam

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Pre-crack Rolling Seam

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Forming Problems Not hot enough during forming

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Stress Corrosion Cracking

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Stress Corrosion Cracking Bolt failed when machine was not in use

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Stress Corrosion Cracking

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Stress Corrosion Cracking

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Stress Corrosion Cracking

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Heat treating Problems

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Heat treating Problems

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Loose Nuts Last thread of 2 strippedThread damaged after nut come of