A bolt’s strength is described by three related numbers, and knowing the difference helps you choose and tighten fasteners correctly.
- Proof load — the maximum the bolt can take without permanent stretch; the safe working ceiling.
- Yield strength — where the bolt begins to deform permanently.
- Tensile (ultimate) strength — where it finally breaks.
Grades encode strength
SAE Grade 5/8 and metric 8.8/10.9/12.9 markings tell you these values — higher grade, higher strength (metric 10.9 ≈ SAE Grade 8). The numbers in metric grades are even meaningful: in 8.8, the 8 relates to tensile strength (~800 MPa) and the .8 to the yield-to-tensile ratio.
Why preload matters
A bolted joint is strongest when the bolt is tightened (preloaded) enough to clamp the parts firmly — this keeps the joint from loosening and helps the bolt resist fatigue. That’s why torque specs exist: they target a preload safely below proof load. Higher-grade bolts are stronger but more brittle, so they aren’t always best under shock or fatigue loads.
Frequently asked questions
What is proof load? The max load a bolt takes without permanent stretch — the safe ceiling.
What does grade 8.8 mean? Roughly 800 MPa tensile strength, with a defined yield ratio.
Why does torque matter? It sets preload — the clamping force that makes the joint strong and stable.