2.2 Rivet Layout, Edge Distance, Spacing & Driving Standards

Rivet Layout, Edge Distance, Spacing & Driving Standards

FAA Airframe Exam Focus: Structural joint integrity depends upon precise layout calculations (Rivet Diameter, Edge Distance, Pitch, Transverse Pitch, and Grip Length) and adherence to strict driven shop head (bucktail) dimensional tolerances per FAA AC 43.13-1B and MIL-R-47196.

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1. Rivet Diameter Selection Rule ($D = 3T$)

Selecting the correct rivet shank diameter ($D$) balances the shear strength of the fastener against the bearing strength of the parent sheet metal:

$D_{\text{nominal}} = 3 \times T_{\text{thickest}}$

where $T_{\text{thickest}}$ is the thickness of the thickest individual sheet comprising the joint.

  SHEAR FAILURE vs. BEARING FAILURE:
  
  [ Rivet Too Small (D < 3T) ]           [ Rivet Too Large (D > 3T) ]
       ───►                                   ───►
  ═══════╤═══════                        ═══════│░░░░│═══════  ◄── Sheet Crushed/Buckled
  ───────┼─────── ◄── Rivet Shears       ───────│░░░░│───────      (Bearing Failure)
  ═══════╧═══════                        ═══════│░░░░│═══════
       ◄───                                   ◄───

• If the rivet diameter is too small ($D < 3T$): The fastener lacks sufficient cross-sectional area to carry structural shear loads; the joint fails in pure rivet shear.
• If the rivet diameter is too large ($D > 3T$): The high driving force required to upset a massive shank crushes the thin surrounding sheet metal, causing bearing failure, sheet puckering, and sheet elongation.

Rivet Sizing Nomenclature

• Diameter: Designated in 1/32-inch increments (e.g., -3 = $3/32"$, -4 = $4/32" = 1/8"$, -5 = $5/32"$, -6 = $3/16"$, -8 = $1/4"$).
• Length: Designated in 1/16-inch increments (e.g., -4 = $4/16" = 1/4"$, -6 = $6/16" = 3/8"$, -8 = $8/16" = 1/2"$).
• Example Part Number: MS20470AD4-6 = Universal head (MS20470), 2117-T4 alloy (AD), $4/32"$ ($1/8"$) shank diameter, $6/16"$ ($3/8"$) length.

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2. Rivet Layout Geometry: Edge Distance, Pitch & Transverse Pitch

To ensure even load distribution and prevent structural tearing or moisture ingress, rivet patterns must adhere to strict geometric spacing limits.

  RIVET PATTERN GEOMETRY (Staggered Pattern):
  
  ◄─── ED ───►
  ┌──────────┬──────────────┬──────────────┬──────────┐
  │          │              │              │          │
  │   (•)    │     (•)      │     (•)      │   (•)    │ ◄── Row 1
  │    │     │      │       │      │       │          │
  │    │     │◄──── Pitch ─►│      │       │          │
  │    │     │              │      │       │          │
  │    │◄─── Transverse ───►│      │       │          │
  │    │        Pitch       │      │       │          │
  │    └────────────(•)─────┴──────┴──────(•)─────────┤ ◄── Row 2
  │                  │                                │
  └──────────────────┴────────────────────────────────┘

Critical Spacing Rules (AC 43.13-1B, Chapter 4)

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3. Rivet Length Calculation ($L = G + 1.5D$)

Before driving, the rivet shank must extend through the sheets with sufficient material to form a standard bucktail:

$L = G + 1.5D$

where:

• $L$ = Total rivet shank length (inches)
• $G$ = Grip length (combined total thickness of all sheets being joined, $G = T_1 + T_2 + \dots$)
• $1.5D$ = Allowance required to form the upset shop head ($1\frac{1}{2} \times \text{rivet diameter}$)

  RIVET LENGTH & SHOP HEAD GEOMETRY:
  
  [ BEFORE DRIVING ]                          [ AFTER DRIVING (UPSET) ]
     ┌───────────┐ Manufactured                  ┌───────────┐ Manufactured
     │           │ Head                          │           │ Head
  ═══╧═══════════╧═══                         ═══╧═══════════╧═══
  ═══╤═══════════╤═══ ◄── Grip Length (G)     ═══╤═══════════╤═══ ◄── Grip (G)
     │           │                               │           │
     │           │ ◄── 1.5D Allowance            ┌───────────┐ ◄── Height = 0.5D
     └───────────┘                            └───┴───────┴───┘ ◄── Width = 1.5D
       Shank (D)                                Driven Shop Head (Bucktail)

Step-by-Step Calculation Example

Problem: Calculate the correct rivet size to join two $0.040"$ 2024-T3 aluminum sheets using MS20470AD rivets.

1. Calculate Diameter ($D$): $D = 3 \times T = 3 \times 0.040" = 0.120"$. Select standard size: $1/8"$ ($0.125"$ or -4 diameter).
2. Calculate Grip Length ($G$): $G = 0.040" + 0.040" = 0.080"$.
3. Calculate Shop Head Allowance ($1.5D$): $1.5 \times 0.125" = 0.1875"$.
4. Calculate Total Length ($L$): $L = 0.080" + 0.1875" = 0.2675"$.
5. Convert to 1/16th inch increments: $0.2675" \times 16 = 4.28 \to$ round to nearest standard 1/16th: $5/16"$ ($0.3125"$, -5 length).
6. Final Part Number: MS20470AD4-5.

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4. Driven Shop Head (Bucktail) Dimensions & Tolerances

A properly driven solid rivet must conform to precise dimensional standards (verified using go/no-go rivet gauges):

$\text{Shop Head Height} = 0.5D = \frac{1}{2} D$ $\text{Shop Head Diameter (Width)} = 1.5D = 1\frac{1}{2} D$

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5. Riveting Tooling, Driving Technique & Defect Analysis

Tooling & Assembly Procedures

• Cleco Fasteners: Spring-loaded temporary clamping fasteners used to hold sheets in rigid alignment during drilling and riveting. Color codes: Silver ($3/32"$), Copper ($1/8"$), Black ($5/32"$), Brass ($3/16"$).
• Pneumatic Rivet Guns: Rated by stroke capacity (2X for $3/32"$, 3X for $1/8"$ to $5/32"$, 4X for $3/16"$ to $1/4"$). Rivet gun must be held exactly perpendicular ($90^\circ$) to the skin.
• Bucking Bar Selection: Bucking bar mass must be matched to rivet diameter. The face must be held square against the shank end without bouncing.

Riveting Defect Identification & Causes

  COMMON RIVET DEFECTS:
  
    Underdriven       Overdriven       Offset/Clenched    Cracked Head      Loose / Fretting
      ┌───┐             ┌───┐               ┌───┐            ┌───┐               ┌───┐
   ═══╧═══╧═══       ═══╧═══╧═══         ═══╧═══╧═══      ═══╧═══╧═══         ═══╧═══╧═══
      │   │             │   │               │   │            │   │               ▒   ▒
     ┌┴───┴┐         ┌─────────┐          ┌─┴───┘           ┌┴──/─┴┐            ┌┴───┴┐ ◄── Black Oxide
     │     │         └─────────┘          └───┘             │  /   │            │     │     "Smoking"
   H > 0.5D          H < 0.5D            Bar Tilted        Aged Past           Hole Elongated
   W < 1.5D          W > 1.5D                             Time Window

Proper Rivet Removal Procedure

1. Center-punch the exact center of the manufactured head.
2. Drill through the head using a drill bit equal to or one size smaller than the rivet shank, stopping exactly at the base of the head.
3. Snap off the rivet head using a pin punch.
4. Drive out the remaining shank with a drift punch while supporting the back of the structure with a bucking bar to avoid enlarging or elongating the hole.