Design of Fasteners

1. Introduction

Fasteners are mechanical elements used to join or hold two or more components together. They are widely used in machines, structures, and assemblies.

Common Fasteners

• Bolts and nuts
• Screws
• Rivets
• Studs

2. Types of Fasteners

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(A) Temporary Fasteners

• Can be removed without damage
• Examples: Bolts, screws, nuts

(B) Permanent Fasteners

• Cannot be removed easily
• Examples: Rivets, welded joints

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3. Modes of Loading on Fasteners

(A) Tensile Load

• Force acts along the axis

(B) Shear Load

• Force acts perpendicular to axis

(C) Combined Loading

• Both tensile and shear forces act simultaneously

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4. Stress in Fasteners

(A) Tensile Stress

$\sigma_t = \frac{F}{A_t}$σt​=At​F​

Where:

• $F$F = Axial load
• $A_t$At​ = Tensile stress area

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(B) Shear Stress

$$\tau = \frac{F}{A_s}$$τ=As​F​

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(C) Bearing Stress

$$\sigma_b = \frac{F}{d \cdot t}$$σb​=d⋅tF​

Where:

• $d$d = Diameter
• $t$t = Thickness of plate

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5. Design of Bolted Joints

Types of Bolted Joints

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(A) Lap Joint

• Plates overlap
• Bolts pass through both plates

(B) Butt Joint

• Plates aligned end-to-end
• Connected using cover plates

Failure Modes of Bolted Joints

1. Tensile failure of bolt
2. Shear failure of bolt
3. Crushing (bearing) failure
4. Tearing of plate

6. Design of Riveted Joints

Types of Riveted Joints

• Lap joint
• Butt joint

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Failure Modes

• Shear failure of rivet
• Crushing failure
• Plate tearing

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7. Eccentric Loading on Fasteners

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• Occurs when load does not pass through centroid
• Causes:
  • Direct shear
  • Torsional moment

Analysis

• Load distributed based on distance from centroid
• Maximum stress occurs at farthest fastener

8. Preloading of Bolts

• Initial tightening force applied
• Improves joint strength
• Prevents loosening

9. Design Considerations

(A) Factor of Safety

• Depends on:
  • Material
  • Type of loading
  • Application

(B) Thread Design

• Metric threads commonly used
• Stress concentration occurs at thread roots

(C) Fatigue Loading

• Important in dynamic conditions
• Avoid fluctuating stresses

10. Materials for Fasteners

• Mild steel
• Alloy steel
• Stainless steel
• Non-ferrous materials (Aluminum, brass)

11. Advantages of Fasteners

• Easy assembly and disassembly
• Reusable (for temporary fasteners)
• Cost-effective

12. Disadvantages

• Stress concentration
• Possibility of loosening
• Requires maintenance

13. Applications

• Machine assemblies
• Structural connections
• Automotive and aerospace industries
• Pressure vessels

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