Standard fasteners, particularly bolts, are graded for strength using different systems, with the higher the number indicating the stronger the bolt.
Fasteners are critical components in countless applications, from automotive to construction. Understanding their strength grading is essential for selecting the right fastener for a specific load requirement. The primary systems used for grading bolt strength are based on specifications from organizations like the Society of Automotive Engineers (SAE) in the United States and the International Organization for Standardization (ISO) for metric fasteners.
Common Fastener Strength Grading Systems
While many specific grades and classes exist, two systems are most commonly encountered for standard bolts: SAE (inch series) and Metric (ISO).
SAE Grades
The SAE system uses numerical grades, often marked on the bolt head. The reference highlights three common grades:
- Grade 2: Standard, low-strength bolt.
- Grade 5: Medium-strength bolt, heat-treated.
- Grade 8: High-strength bolt, heat-treated to a higher level than Grade 5.
Metric Classes
The metric system uses property classes, typically denoted by two numbers separated by a dot (e.g., 8.8). These numbers represent tensile strength and yield strength characteristics. The reference lists several common classes:
- Class 5.8: Medium-strength bolt.
- Class 8.8: High-strength bolt, common in many structural applications.
- Class 10.9: Very high-strength bolt.
- Class 12.9: Extremely high-strength bolt.
The Core Principle: Higher Number, Higher Strength
Regardless of whether the SAE or Metric system is used, the fundamental principle holds true: the higher number means the stronger the bolt. This applies when comparing grades within the same system (e.g., Grade 8 is stronger than Grade 5) or classes within the metric system (e.g., Class 10.9 is stronger than Class 8.8).
Visualizing Strength Grades
Bolt heads are often marked to indicate their grade or class. While Grade 2 bolts typically have no markings, higher grades have specific patterns or numbers.
| System | Common Grades/Classes | Strength (Relative) | Head Marking Example |
|---|---|---|---|
| SAE | Grade 2 | Standard | (No marks) |
| SAE | Grade 5 | Medium | Three radial lines |
| SAE | Grade 8 | High | Six radial lines |
| Metric | Class 5.8 | Medium | 5.8 |
| Metric | Class 8.8 | High | 8.8 |
| Metric | Class 10.9 | Very High | 10.9 |
| Metric | Class 12.9 | Extremely High | 12.9 |
Note: Specific markings can vary slightly by manufacturer or standard version.
Understanding these grades and the principle that a higher number signifies greater strength is crucial for selecting appropriate fasteners that can safely withstand the intended loads in an assembly. Using a fastener that is too weak can lead to failure, while using one that is excessively strong may be unnecessarily expensive. For more detailed information on fastener standards and strength properties, consult specific engineering handbooks or standards documents like those from SAE or ISO.
