The Bolt That Snapped (Wrong Grade)
We bolted a bracket (a load) to a machine. We used a standard bolt (grade 4.6, low strength). On the floor, the bolt snapped (the load vibrated, the bolt failed). The problem: the bolt’s grade was too low (4.6, not strong enough). We either upsized the bolt (bigger diameter) or used a higher grade (8.8). We switched to grade 8.8 (high strength). The bolt held. The mistake was using a low-grade bolt (4.6) on a vibrating load.
Fastener grade (8.8, 10.9) — pick the strength. This article covers the basics.
The Grade Numbers
The bolt’s grade (e.g., 8.8) encodes the strength:
- 4.6 (low): Yield 240 MPa. For a light load (a cover, a non-critical). Standard hardware.
- 8.8 (medium): Yield 640 MPa. For a general load (a bracket, a machine frame). The workhorse.
- 10.9 (high): Yield 940 MPa. For a heavy/vibrating load (a press, a motor mount). Strong (and brittle).
- 12.9 (very high): Yield 1100 MPa. For a critical load (a structural, a high-torque). Expensive.
Step 1: The Load (Static? Vibrating?)
Is the load light/static (a cover)? Use 4.6. Is it general (a bracket)? Use 8.8. Is it heavy/vibrating (a press, a motor)? Use 10.9. The bolt that snapped was 4.6 on a vibrating load. Use 8.8 (or 10.9).
The fastener rule: For a general load, use 8.8. For a vibrating/heavy load, use 10.9. The bolt that snapped was 4.6 (too weak). Don’t use 4.6 on a load (vibration snaps it). Use 8.8 (the workhorse).
Step 2: The Diameter (Size)
The bolt’s strength depends on the grade AND the diameter. A small 8.8 bolt (M6) is weaker than a big 4.6 bolt (M12). For a heavy load, either a bigger diameter (M12) or a higher grade (8.8, M8). Pick the grade + the diameter (together). Don’t rely on the grade alone.
Step 3: The Torque (Preload)
A bolt is tightened (torqued) to preload (clamp the parts). The torque depends on the grade (and the diameter). An 8.8 bolt (M8) is torqued to ~25 Nm. A 10.9 bolt (M8) is torqued to ~35 Nm. Don’t over-torque (it yields the bolt). Follow the torque table (per grade and diameter).
Step 4: The Material (Stainless?)
A stainless bolt (A2-70) is corrosion-resistant (but weaker than a steel 8.8). For a corrosive environment (washdown), use stainless (A4-80). For a general load (dry), use steel 8.8 (stronger).
| Grade | Yield | Use |
|---|---|---|
| 4.6 | 240 MPa | Light (cover) |
| 8.8 | 640 MPa | General (bracket) — workhorse |
| 10.9 | 940 MPa | Heavy/vibrating (press) |
| Stainless A4-80 | 600 MPa | Corrosive (washdown) |
A Fastener Checklist
- What is the load? (Light? Vibrating?)
- Is it general? (Use 8.8?)
- Is it heavy/vibrating? (Use 10.9?)
- Is the diameter right? (M8? M12?)
- Is the torque right? (Preload?)
- Is the environment corrosive? (Stainless?)
- Does the bolt snap? (Test?)
- Is the thread right? (Coarse? Fine?)
- Is the washer right? (Spring? Flat?)
- Is the quality right? (Marked?)
The Bottom Line
Fastener grade matches the strength to the load. The bolt that snapped was 4.6 (too weak). Use 8.8 (general). For vibrating, 10.9. Torque (preload). The bolt that held wasn’t the biggest bolt — it was the right grade.
