Industrial Fasteners Guide: Types, Threads, and Assembly

In the global machinery and equipment industry, fasteners are fundamental components that connect individual parts. Their design, selection, and installation directly affect the reliability and service life of mechanical systems.Industry data shows that failures caused by improper design, incorrect selection, or installation errors are among the main reasons for mechanical malfunctions. From general machinery assembly to high-precision systems such as aerospace and automotive components, the proper use of fasteners is essential.

1.0Definition and Core Types of Fasteners

1.1Definition

A fastener is a standardized hardware component used to join two or more parts together. It allows for quick installation and removal through manual or powered tools (such as torque wrenches) or automated equipment (like fastener insertion machines). The key features of fasteners are removability and connection stability, distinguishing them from semi-permanent joining methods such as welding or riveting. Fasteners are widely used across industries including automotive, aerospace, and electronics.

1.2Core Types

Based on thread characteristics and application methods, fasteners are categorized into three main types, often paired with automated equipment (e.g., press-in machines) for mass production efficiency:

2.0Key Terminology Explained

Understanding fastener terminology is critical for accurate design and selection. Below are internationally recognized definitions (based on ISO standards):

3.0Thread Type Selection: Application Logic of Coarse and Fine Threads

Fastener threads follow ISO standards (metric: ISO 724; imperial: ANSI/ASME B1.1) and are selected based on material strength matching:

3.1Common Misconceptions

• Myth: Coarse threads are always stronger.
• Fact: Strength depends on material pairing:

• Fine threads in weak materials (e.g., aluminum) cause thread stripping.
• Coarse threads in strong materials (e.g., steel) waste strength and reduce preload accuracy.
  • Example: Automotive engine assemblies (cast iron blocks + aluminum heads) use coarse-thread bolts, tightened by automated insertion systems to balance strength and efficiency.

4.0Thread Manufacturing Processes: Rolled vs. Cut Threads (Key Equipment: Thread Rolling Machine)

Thread processing directly impacts fastener performance. High-quality fasteners globally use rolled threads (ISO 898-6) over traditional cut threads, with the thread rolling machine as core equipment:

5.0Core Function and Application Restrictions of Fasteners

5.1Sole Core Function: Clamping Connection

Fasteners exist to clamp components via preload (not for positioning, load-bearing, or transmission):

5.2Critical Application Restriction: No Shear Load on Threads

Threads are the weakest fastener part—never subject them to shear. Key reasons:

1. Thread minor diameter < shank diameter (only 60–80% of shank shear strength).
2. Limited thread-hole contact area (reduces stability; causes loosening).
3. Loosened threads abrade hole walls (enlarges clearance; accelerates failure).

Correct Practice:

• Use dowel pins to bear shear loads.
• Position the shank (unthreaded portion) in the shear plane (e.g., double-shear joints).
• Sheet metal applications: Press-in bolts keep threads outside shear zones; insertion machines control tightening depth for optimal shank placement.

6.0Preload Control: Risks of Over-Tightening and Under-Tightening

Preload is critical for joint stability. Over-tightening (within yield limit) is safer than under-tightening (based on ISO 6892-1 stress-strain behavior):

6.1Key Insights from the Stress-Strain Curve

Fastener materials (e.g., alloy steel, ISO 898-1) have three stages:

1. Elastic Stage: Material returns to original shape; insufficient preload causes fatigue failure under cyclic loads (e.g., engine vibrations).
2. Yield Stage: Permanent deformation begins; avoid exceeding yield strength.
3. Strain-Hardening Stage: Preload near (but not above) yield improves stability and resists external loads.

6.2Common Misconceptions

• Myth: Lock washers, anaerobic adhesives, or safety wires replace preload.
• Fact: These only delay loosening (do not prevent it). Proper preload is the only guarantee of stability.
• Automation Solution: Insertion machines with torque sensors monitor preload in real time; manual assembly uses ISO 6789 torque wrenches (no “feel-based” tightening).

7.0Fastener Preload and Torque Calculation (with Practical Verification)

Preload is controlled via torque. Below is a globally accepted simplified method (±20% accuracy, ISO-derived):

7.1Core Formulas

• Tensile Stress for Preload: Avoid permanent deformation; use 90% of yield strength:\(\sigma_t \approx 0.9 \times \sigma_y\)(Example: ISO 898-1 Grade 8.8 bolt → \(\sigma_y = 640\) MPa)
• Preload Calculation:\(F_i = \sigma_t \times A_t\)(\(A_t\) = thread tensile stress area; e.g., M10 threads → \(A_t = 58.0\) mm², ISO 6560)
• Torque Calculation:\(T \approx 0.2 \times F_i \times d\)(d = nominal diameter; 0.2 = coefficient for \(\mu = 0.15\); adjust for lubrication/coatings per ISO 16047.)

7.2Example Calculation (M10 × 1.5, Grade 8.8 Bolt)

1. From ISO 898-1:\(\sigma_y = 640\) MPa
2. Tensile stress:\(\sigma_t = 0.9 \times 640 = 576\) MPa
3. Tensile stress area:\(A_t = 58.0\) mm²
4. Preload:\(F_i = 576 \times 58.0 = 33,408\) N ≈ 33.4 kN
5. Torque:\(T = 0.2 \times 33,408 \times 10 = 66,816\) N·mm ≈ 66.8 N·m

7.3Experimental Verification (ISO 16047 Fatigue Test)

For a 3/8-inch bolt (180,000 psi tensile strength, ANSI/ASME B18.2.1):

• Industry Application: Boeing’s fastener manuals require 70–80% yield preload, achieved via torque-feedback insertion machines.

7.4Torque Verification Methods

1. Manual Assembly: Use ISO 6789-compliant torque wrenches.
2. Automated Assembly:

• Insertion machines integrate laser elongation sensors (preload accuracy ±5%).
• Press-in machines use pressure feedback for self-clinching fasteners (verifies preload indirectly).

8.0Fastener Selection and Identification Standards

8.1Selection Principle: Use Only Standard, Off-the-Shelf (OTS) Fasteners

Global Standards: OTS fasteners conform to ISO, ANSI, or DIN (avoid custom fasteners: 12–24 week lead time, high cost).

Extreme Conditions: Custom fasteners (e.g., Inconel alloys) only for high-temperature/corrosion environments.

Automation Compatibility Checks:

1. Self-clinching fasteners match press-in machine head types (e.g., ISO 14587 nuts).
2. Bolt heads fit insertion machine gripping mechanisms.
3. Thread parameters fall within thread rolling machine capabilities.

8.2Identification Rules (Global Formats)

9.0Fastener Hole Design: Tap Drill Holes and Clearance Holes

Hole accuracy directly impacts joint strength. Design follows ISO standards and aligns with automated equipment (insertion/press-in machines):

9.1Tap Drill Holes (for Internal Threads)

Purpose: Pre-drilled holes for tapping (ISO 239).

Critical Sizing:

Design Notes:

1. Adjust size for material hardness (e.g., +0.1–0.2 mm for aluminum vs. steel).
2. Reference ISO 239 tap drill tables.
3. CNC-machined holes work directly with insertion machines (no secondary alignment).

9.2Clearance Holes (for Through Bolts)

Purpose: Allow bolt shank passage (ISO 273-1).

Design Rules:

1. Strictly follow ISO 273-1 (e.g., M10 hole at 9.5 mm blocks bolt passage).
2. Hole perpendicularity ≤0.1 mm/m (prevents preload transfer issues).

10.0Fastener Joint Design: Enhancing Stability and Strength

Joint design maximizes load capacity (ISO 16047, ISO 26262) and accommodates automated equipment:

10.1Prefer Double-Shear Joints

Automation Workflow:

1. CNC batch-drills double-shear holes.
2. Press-in machines install fasteners in the middle plate.
3. Insertion machines tighten bolts for final assembly.
   • Industry Example: Wind turbine blade joints use double-shear designs, with thread rolling machines ensuring high-precision threads for uniform stress distribution.

10.2Edge Distance Requirements

Rule: Distance from bolt hole center to workpiece edge ≥ 1× major thread diameter (ISO 14587).

Example: M10 bolt → ≥10 mm edge distance.

Consequences of Poor Sizing:

Design Note for Sheet Metal:Add ≥3 mm clearance for press-in machine heads (avoids edge interference during installation). Align edge distance consistency with thread rolling machine-produced fastener lengths (prevents excessive protrusion or insufficient engagement).

11.0Conclusion

In global manufacturing, fasteners are the critical link between components and system reliability. Their design and application must adhere to ISO standards and prioritize automation compatibility—from thread rolling machine processes to insertion/press-in machine integration. Every step, from thread selection to joint design, relies on engineering principles and proven industrial practices to minimize failure risks and enhance global market competitiveness.