Classification & Detailing of Grades, Specifications & Sizes of Fasteners

Fasteners such as bolts, nuts, screws, and washers are manufactured in various grades, materials, sizes, and specifications to suit different applications. Selecting the right fastener requires understanding these technical classifications.

1. Classification of Fasteners

Fasteners can be classified based on:

A. By Type

1. Bolts – require a nut; provide strong clamping force.
2. Screws – self-threading or used with pre-tapped holes.
3. Nuts – mate with bolts to fasten components.
4. Washers – distribute load and prevent loosening.

B. By Head Type

1. Hex head
2. Socket head
3. Pan head
4. Flat head
5. Round head
6. Button head
7. Countersunk head

C. By Thread Type

1. Coarse Thread (UNC / M coarse)
2. Fine Thread (UNF / M fine)
3. Extra-fine Thread
4. Self-tapping thread
5. Wood thread
6. Metric / Unified / BSP / NPT (for pipe fittings)

D. By Material

1. Mild Steel (MS)
2. Carbon Steel
3. Alloy Steel
4. Stainless Steel (SS 304, SS 316, SS 202, SS 410)
5. Brass
6. Aluminum
7. Titanium

2. Fastener Grades & Their Meaning

Grades define the mechanical strength, hardness, and load-carrying capacity.

A. Metric Bolt Grades (ISO Standard)

B. Imperial Bolt Grades (SAE Standard)

C. Stainless Steel Grades

3. Standard Specifications for Fasteners

Fasteners follow recognized global standards:

ISO (International Organization for Standardization)

1. ISO 4014 / 4017 – Hex bolts
2. ISO 4032 – Hex nuts
3. ISO 4762 – Socket head cap screws

DIN (German Standard)

1. DIN 933 – Full-thread hex bolts
2. DIN 931 – Partial thread hex bolts
3. DIN 912 – Allen bolts

ASTM (American Standard)

1. ASTM A193 – High-strength bolts
2. ASTM A194 – Nuts for high-pressure service
3. ASTM A307 – Carbon steel bolts

ASME & ANSI Standards

Used widely in machinery, oil & gas, pipelines.

4. Fastener Sizes

A. Metric Sizes (mm)

1. Common diameters: M3, M4, M5, M6, M8, M10, M12, M16, M20, M24, M30
2. Lengths: 10mm to 300mm depending on type
3. Thread pitch:
4. Coarse example: M10 × 1.5
5. Fine example: M10 × 1.25

B. Imperial Sizes (inch)

1. 1/4", 5/16", 3/8", 1/2", 5/8", 3/4"
2. Thread pitch examples:
3. UNC – 1/4"-20
4. UNF – 1/4"-28

5. Thread Types and Selection

Metric vs. Imperial Threads

1. Metric (M) – globally used
2. Imperial (UNC/UNF) – common in US machinery

Coarse Thread

1. Stronger in stripping resistance
2. Best for softer materials

Fine Thread

1. Higher tensile strength
2. Used for precision assemblies

Self-Tapping Thread

1. For sheet metal
2. Creates its own threads

Machine Thread

1. For nuts or tapped holes

6. How to Choose the Right Fastener (Quick Checklist)

1. Application Type → structural, automotive, machinery, electrical
2. Load Requirement → static or dynamic load
3. Material Compatibility → avoid galvanic corrosion
4. Environmental Conditions → humidity, chemicals, heat
5. Grade Selection → strength requirement (8.8, 10.9, etc.)
6. Thread Type → coarse or fine
7. Coatings → zinc, hot-dip galvanized, black oxide
8. Size & Fitment → diameter, length, pitch

7. Coatings & Finishes