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Classification of Threads
Classification of threads are standardized helical structures used to fasten components, transmit motion, or create seals in mechanical systems. They are primarily categorized by profile type (such as metric, unified, or trapezoidal), function (fastening, power transmission, or sealing), and direction (right-hand or left-hand). Each thread type features unique characteristics—like the 60° angle of metric threads for general fastening or the 30° trapezoidal design for precision leadscrews.
Classification of Threads |
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Classification Basis |
Thread Type |
Standard/Characteristics |
Thread Angle |
Applications |
Relevant Standards |
| By Thread Direction | Right-Hand Thread | Tightens clockwise | / | Most standard fasteners | ISO, ANSI, DIN |
| Left-Hand Thread | Tightens counter-clockwise | / | Special anti-loosening cases | ISO 7041 | |
| By Profile Type | Metric Thread | 60° symmetric profile, pitch in mm (e.g., M8×1.25) | 60° | International machinery | ISO 68-1 |
| Unified Thread | 60° profile: Coarse (UNC)/Fine (UNF)/Extra Fine (UNEF) | 60° | US equipment/aerospace | ANSI B1.1 | |
| Whitworth Thread | 55° rounded crest profile | 55° | British piping/old machinery | BS 84 | |
| Trapezoidal Thread | 30° asymmetric profile, high power transmission efficiency | 30° | CNC leadscrews/jacks | DIN 103, ISO 2901 | |
| Buttress Thread | 45° asymmetric profile (load-bearing side vertical) | 45°/7° | Unidirectional loads | DIN 513 | |
| Round Thread | Circular profile, high fatigue resistance | Radius | Hooks/drilling equipment | API 7-1 | |
| By Function | Fastening Thread | Standard connections (bolts/nuts) | 60°/55° | General machinery | ISO, ANSI |
| Power Thread | High-precision force transmission (leadscrews) | 30°/45° | Machine tools/hydraulic press | DIN, JIS | |
| Sealing Thread | Tapered design + interference fit | 55°/60° | Piping/pressure vessels | NPT, BSPT | |
| By Unit System | Metric | Diameter and pitch in mm (e.g., M8×1.25) | 60° | International standard | ISO 261 |
| Imperial | Diameter in inches, threads per inch (e.g., 1/4″-20 UNC) | 55°/60° | North America | ANSI B1.20.1 | |
| Special Threads | Self-Tapping Thread | Sharp crest for self-forming | 60° | Sheet metal/plastic | ISO 1478 |
| Micro Thread | Ultra-small size (diameter <1mm) | 60° | Precision instruments | JIS B0207 | |
| Multi-Start Thread | Multiple helical paths for rapid advancement | Varies | Bottle caps/quick-release | Custom | |
Key Notes:
Metric vs Imperial:
- Metric threads use pitch (mm) (e.g., M6×1)
- Imperial threads use threads per inch (TPI) (e.g., 1/2″-13)
Sealing Thread Differences:
- NPT (US 60° tapered pipe): Requires sealant
- BSPT (British 55° tapered pipe): Metal-to-metal seal
Power Thread Selection:
- High precision: Trapezoidal (e.g., Tr20×4)
- Unidirectional load: Buttress (e.g., BU28×5)
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