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Calculate recommended torque values for bolts based on size, grade, and lubrication conditions. Get clamp load, tensile stress, and torque ranges for proper fastening.
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Bolt torque is the rotational force applied when tightening a bolt or fastener, typically measured in foot-pounds (ft-lbs) or Newton-meters (Nm). Proper torque is essential for ensuring:
The recommended torque depends on the bolt size, grade (strength), material, thread condition, and lubrication.
Reference torque values for standard bolt sizes - Dry condition, 75% yield method:
| Size | Grade 2 | Grade 5 | Grade 8 |
|---|---|---|---|
| 1/4" | 5 | 11 | 20 |
| 3/8" | 18 | 40 | 75 |
| 1/2" | 39 | 85 | 155 |
| 5/8" | 65 | 145 | 260 |
| 3/4" | 105 | 237 | 430 |
| 1" | 230 | 520 | 950 |
| Size | 8.8 | 10.9 | 12.9 |
|---|---|---|---|
| M6 | 7.5 | 10 | 12 |
| M8 | 18 | 25 | 30 |
| M10 | 37 | 53 | 65 |
| M12 | 67 | 95 | 120 |
| M16 | 160 | 225 | 275 |
| M20 | 290 | 410 | 505 |
Common bolt grades and their strength properties:
| Standard | Grade/Class | Yield Strength | Tensile Strength | Common Use |
|---|---|---|---|---|
| SAE | Grade 2 | 45,000 psi | 60,000 psi | General purpose, low stress |
| SAE | Grade 5 | 90,000 psi | 120,000 psi | Industrial, automotive fasteners |
| SAE | Grade 8 | 140,000 psi | 170,000 psi | High-stress, safety-critical |
| Metric | Class 8.8 | 640 MPa | 800 MPa | Industrial general purpose |
| Metric | Class 10.9 | 900 MPa | 1000 MPa | High-strength applications |
| Metric | Class 12.9 | 1100 MPa | 1200 MPa | Ultra-high strength, critical |
Where:
Clamp Load represents the tension force created in the bolt when tightened to the specified torque.
Lubrication reduces friction, allowing lower torque values to achieve the same clamp load.
Torque is the rotational force applied to tighten a bolt (measured in ft-lbs or Nm). Clamp load is the tension force created in the bolt as a result of tightening. Torque is the input; clamp load is the output.
Lubrication reduces friction between the bolt threads and the fastened surface. This means the same amount of applied torque creates more clamp load. Conversely, you need less torque to achieve the same clamp load with lubrication, reducing the risk of over-tightening.
Use a calibrated torque wrench set to the recommended value. For critical applications, apply torque in a cross-pattern (if multiple bolts) to ensure even pressure. Some applications require "calibration" bolts or a specific sequence.
Over-tightening can stretch the bolt beyond its yield strength, causing permanent deformation or breakage. It can also damage threads on the fastened materials and create uneven stress distribution that can lead to premature failure.
No. Higher grade bolts have greater yield strength and can safely handle higher torque values. Always use the torque specification for the specific bolt grade you're using.
SAE uses Grade numbers (2, 5, 8) while Metric uses Class designations (8.8, 10.9, 12.9). The numbers indicate different strength levels. Metric Class 8.8 is roughly equivalent to SAE Grade 5, and Metric 10.9 is similar to SAE Grade 8.
Critical bolts should be re-torqued after the first use (after equipment is first run or loaded). Vibration and settling can cause bolts to loosen. For safety-critical applications, periodic inspection and re-torquing may be required per maintenance schedules.
No. Clean bolts with a dry surface before applying torque. Rust and moisture affect the friction coefficient and can lead to inconsistent clamp loads. Use a wire brush to clean the bolt and threads before tightening.
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