Fastener Standards: Types, Grades, and Applications

Fasteners, the fundamental hardware components holding our engineered world together, come in an immense variety of standardized types, grades, and materials. Understanding these classifications is crucial for selecting the right fastener for strength, safety, corrosion resistance, and cost-effectiveness. Here's an overview of major categories:

1. Threaded Fasteners (Bolts, Screws, Studs)

1. Hex Head Bolts/Screws (ISO 4014/4017, ASME B18.2.1, DIN 931/933):

       Grades/Classes: Metric (ISO 898-1): 4.6, 5.6, 5.8, 6.8, 8.8, 9.8, 10.9, 12.9. Imperial (SAE J429 / ASTM A307, A325, A490): Grade 2, 5, 8, A325 (Structural), A490 (Structural High-Strength).

       Key Features: External hex drive, used with nuts or tapped holes. Full or partial thread.

       Applications: General assembly (machinery, frames), structural steel connections (buildings, bridges - A325/A490), automotive chassis, high-stress applications (8.8+/Grade 5+). Hex head offers high torque capability.

2. Socket Head Cap Screws (SHCS) (ISO 4762, ASME B18.3, DIN 912）

       Grades/Classes: Metric: Primarily 12.9, also 8.8, 10.9. Imperial: Alloy Steel (high strength). 

       Key Features: Internal hex (Allen) drive. Cylindrical head with flat bearing surface. High strength-to-size ratio.

       Applications: Machine tools, precision equipment, aerospace, applications requiring high clamp force in confined spaces, assemblies needing a flush or low-profile head. Common where high strength and a clean look are needed.

3. Socket Button Head Screws (ISO 7380, ASME B18.3, DIN 7380):

       Grades/Classes: Similar to SHCS (8.8, 10.9, 12.9).

       Key Features: Internal hex drive, low-profile domed ("button") head.

       Applications: Applications requiring a very low-profile, rounded head (fixtures, jigs, consumer products like electronics, furniture), where aesthetics or clearance are critical.

4. Set Screws (ISO 4026-4036, ASME B18.3, DIN 916/913):

       Grades/Classes: Metric: 8.8, 10.9, 12.9, 45H (Hardened). Imperial: Hardened.

       Key Features: Headless, various point types (Cup, Flat, Cone, Dog). Internal drive (Hex, Torx, Slotted).

       Applications: Securing components onto shafts (gears, pulleys, collars), preventing relative rotation or axial movement. Often used with a drilled detent (flat point) or directly against the shaft (cup point).

5. Machine Screws (ISO 1580, ASME B18.6.3, DIN 84/85):

       Grades/Classes: Lower strength than hex bolts/SHCS. Metric: 4.8, 5.8. Imperial: Various (often comparable to Grade 2).

       Key Features: Smaller diameter, finer threads than bolts. Pan head, flat head, round head, truss head. Driven by screwdriver (slotted, Phillips, Pozidriv, Torx) or hex socket.

       Applications: Securing panels, covers, light fixtures, electrical components, appliances, joining thinner materials to tapped holes or nuts. Common in electronics and light assemblies.

6. Self-Tapping Screws (ISO 1478/1481, DIN 7504/7981):

       Types: Thread Forming (plastic, soft metals), Thread Cutting (metal, wood), Self-Drilling (with drill point - Tek Screws).

       Key Features: Creates its own mating thread in the substrate material. Various head/drive styles.

       Applications: Fastening sheet metal, plastic, wood, composite panels (construction, HVAC, automotive trim, electronics enclosures). Self-drilling types eliminate need for pre-drilling in steel.

7. Studs (ISO 4016, ASME B16.5/B16.47, DIN 975/976):

       Grades/Classes: Match requirements of mating bolts/nuts (A193 B7 common for flanges).

       Key Features: Threaded on both ends or along entire length. No head.

       Applications: Flanged connections (piping, pressure vessels), components requiring frequent disassembly (cylinder heads), mounting parts where a through-bolt isn't feasible.

1. Non-Threaded Fasteners

1. Pins:

       Dowels (ISO 2338/8734, ASME B18.8.2, DIN 7): Precision cylindrical or tapered pins. Used for alignment and shear load transfer in assemblies (gearboxes, fixtures).

       Clevis Pins & Cotter Pins (ISO 2340/1234, ASME B18.8.1, DIN 11024/94): Used with clevises for linkages (control rods, hydraulics). Cotter pins secure castle nuts or clevis pins.

       Roll Pins / Spring Pins (ISO 8752, DIN 1481): Elastic deformation provides radial pressure. Used for shaft/hub location, shear transfer, hinge pins.

2. Retaining Rings / Circlips (ISO 8750-8753, DIN 471/472, ASME B18.27):

       Types: External (shaft groove), Internal (bore groove). Axial retention.

       Applications: Securing bearings, gears, pulleys on shafts or in housings. Low-cost, space-saving axial retention.

3. Rivets:

       Solid Rivets (ASME B18.1.1, DIN 660/124): Permanent, installed by upsetting (hammering/bucking). High strength/reliability.

       Blind Rivets (Pop Rivets) (ISO 15977-15984, DIN 7337): Installed from one side. Tubular body with mandrel.

       Applications: Solid: Aerospace structures (critical joints), boilers. Blind: Sheet metal assembly (electronics, appliances, ducting, vehicles), applications where only one side is accessible.

   III：Nuts

1.  Hex Nuts (ISO 4032/4034, ASME B18.2.2, DIN 934):

       Grades/Classes: Must match bolt grade (e.g., 8 nut for 8.8 bolt - ISO 898-2, ASME/SAE J995 Grade 2H, 5, 8).

       Applications: General use with hex bolts/screws. Full height (ISO 4032) or reduced height (ISO 4034).

2. Locknuts:

       Nylon Insert Locknuts (ISO 7040/7042, DIN 985/982): Prevailing torque from nylon insert. Vibration resistance.

       All-Metal Locknuts (ISO 7043, DIN 980/6927): Distorted threads or top/bottom features (flange, serrations). Higher temperature capability than nylon.

       Applications: Critical joints subject to vibration (engines, machinery, transportation).

3. Flange Nuts (ISO 4161, DIN 6923):

       Key Features: Integrated washer-like flange distributes load, serrations often resist loosening.

       Applications: Automotive, sheet metal assemblies, where load distribution or vibration resistance is needed without a separate washer.

1. Washers

1. Flat Washers (ISO 7089/7090, ASME B18.22.1, DIN 125):

       Types: Standard, Wide, Extra Wide.

       Applications: Distributing load under bolt head/nut, protecting surface, compensating for oversize holes.

2. Spring Washers (ISO 10683, DIN 127/128):

       Types: Helical Spring (most common), Belleville (Disc Spring).

       Applications: Providing axial spring force to maintain tension and resist loosening (though effectiveness is debated vs. other locking methods). Belleville washers handle high loads/deflections.

3. Lock Washers:

       Tooth Lock Washers (Internal/External Teeth) (ISO 7091, DIN 6797): Teeth bite into nut/bolt head and surface.

       Applications: Resisting rotation/loosening under vibration (less reliable than locknuts for critical joints).

Key Standards Bodies & Selection Considerations

   ISO: International Standards (Widely adopted globally).

   ASME/SAE/ASTM: Primarily North American standards.

   DIN: German standards (still widely referenced in Europe/globally).

   JIS: Japanese standards.

Selection Factors:

1. Load: Tensile, Shear, Fatigue, Vibration? (Dictates Material/Grade).
2. Environment: Corrosion (Stainless - A2/A4, Coated Steel, Alloy), Temperature (Alloy Steel, Nickel Alloys), Chemicals?
3. Assembly/Disassembly: Permanent (Rivets, Adhesives)? Frequent access? Tool access (Drive type)?
4. Materials Being Joined: Metal, Plastic, Wood, Composite? (Affects thread type, clamping pressure).
5. Space Constraints: Head height, clearance for tools?
6. Cost & Availability: Standard vs. specialized.

Understanding fastener standards, grades, and their intended applications is fundamental to safe, reliable, and efficient design and assembly across countless industries. Always consult the specific standard and relevant engineering guidelines for critical applications.