| name | bolt-torque |
| description | Bolt torque and preload — torque-tension relationship, K-factor (nut factor), friction coefficients, torque specification, preload scatter, torquing sequences, ASME PCC-1, VDI 2230 basics. |
| metadata | {"priority":7,"promptSignals":{"phrases":["bolt torque","bolt preload","torque tension","nut factor","K factor bolt","torque tightening","bolt clamping force"],"minScore":3}} |
Bolt Torque and Preload — Complete Skill
Torque-Tension Relationship
Basic torque equation:
T = K × F_b × d [N·m; K = nut factor; F_b = bolt preload (clamping force) [N]; d = nominal bolt diameter [m]]
Nut factor K:
K includes all friction effects (thread + bearing surface)
K = 0.10–0.15 (lubricated, calibrated)
K = 0.16–0.20 (as-received, clean, no lube)
K = 0.20–0.25 (dry, rusty, rough)
K = 0.10–0.12 (cadmium plated)
K = 0.08–0.12 (PTFE coated or waxed)
Torque split (approximate):
~50% bearing face friction (under head or nut)
~40% thread friction
~10% bolt helix (useful tightening)
Full expression (detailed):
T = F_b × [d₂/2 × tan(λ + φ) + μ_b × d_b/2]
d₂ = thread pitch diameter; λ = helix angle; φ = thread friction angle; μ_b = bearing face friction; d_b = bearing diameter
Helix angle:
tan λ = p / (π d₂) [p = thread pitch; p ≈ d/6 for coarse thread; p ≈ d/10 for fine thread]
Minimum Bolt Preload
Minimum preload for joint not to separate:
F_b_min = F_ext / (1 - Φ) + F_seating [N; Φ = compliance ratio]
Φ = δ_joint / (δ_bolt + δ_joint) [typically 0.1–0.3 for stiff joints]
Typical target preload for structural bolts:
F_b_target = 0.70–0.80 × F_proof [80% of proof load is common; proof = 0.85–0.92 × σ_y × A_stress]
Proof strength (ASTM A307 to A490):
A307 (Grade A): F_proof = 0.85 × 250 MPa × A_s
A325 (Grade 5): F_proof = 0.85 × 635 MPa × A_s; standard structural high-strength
A490 (Grade 8): F_proof = 0.85 × 900 MPa × A_s
Stress area A_s (ISO metric):
A_s = π/4 × (d - 0.9743/n)² [n = threads per mm; A_s ≈ 0.78 × d²/4 for approximate]
Exact: from ISO 898-1 tables
Preload Scatter
Sources of scatter:
- Thread friction variation: ±25–30% of mean F_b
- Bearing face friction: ±15%
- Operator torque accuracy: ±10% (calibrated wrench); ±20% (uncalibrated)
Total torque scatter (1σ):
σ_F/F_mean ≈ 25–35% (rough estimate for bolted joints)
Minimum preload for design:
F_b_min = F_b_target × (1 - 3σ/F_mean) [3σ lower bound; use in joint separation check]
F_b_max = F_b_target × (1 + 3σ/F_mean) [upper bound; check bolt yield]
Torque-angle method: reduces scatter to ±5–10%; apply initial seating torque, then add defined angle (120° = 1/3 turn, etc.)
Standard Torque Values
Metric bolt torques (approximate, dry, K = 0.20):
| Bolt size | Grade 8.8 [N·m] | Grade 10.9 [N·m] | Grade 12.9 [N·m] |
|---|
| M8 | 25 | 35 | 42 |
| M10 | 50 | 70 | 84 |
| M12 | 87 | 122 | 146 |
| M16 | 212 | 298 | 357 |
| M20 | 425 | 600 | 720 |
| M24 | 735 | 1035 | 1240 |
Adjust by K_actual/K_assumed if lubricant used
Imperial bolt torques (approximate, SAE Grade 5, dry):
1/4-20: 7.5 ft·lb; 3/8-16: 31 ft·lb; 1/2-13: 75 ft·lb; 3/4-10: 265 ft·lb
Torquing Sequences (Bolted Flanges)
Multi-bolt pattern (ASME PCC-1):
- Hand-tighten all bolts (snug condition)
- Cross-pattern (star) at 30% target torque
- Cross-pattern at 60% target torque
- Cross-pattern at 100% target torque
- Final sequential pass (clockwise) at 100% to verify no movement
Minimum number of passes: 3 (30%, 60%, 100%)
Purpose: ensures uniform gasket compression; prevents high-spot leaks
Hydraulic torque/tensioning:
Bolt tensioners: pull bolt directly to target stress; more accurate (K = 1.0 conceptually)
Torque multipliers: planetary gear amplification; up to 20,000 N·m output
Locking Methods
Prevention of vibration loosening:
Split lock washer: minimal effect against sustained vibration (Junker test → fails)
Nord-Lock washer: wedge locking; maintains preload under vibration
Thread-locking adhesive (Loctite): chemical bond; effective; need heat or special tool to remove
Self-locking nut (nylon insert): one-time use (nylon deforms); adequate for low vibration
Prevailing torque nut (all-metal deformed thread): reusable; higher torque; aerospace use
Nord-Lock evaluation:
Junker vibration test (DIN 65151): cross-slot vibration; Nord-Lock retains > 90% preload; standard spring washer retains < 50%
Flange Sealing with Torque
Gasket seating stress:
σ_g = F_b_total / A_gasket [MPa; F_b_total = Σ F_b over all bolts; A_gasket = compressed gasket area]
Minimum σ_g ≥ 2 × operating pressure × m [m = gasket seating factor from ASME VIII Appendix 2]
Maximum σ_g ≤ gasket crush limit:
Soft gaskets (spiral wound): crush limit 100–140 MPa
Solid RTJ (ring type joint): 240–690 MPa
Check both minimum and maximum
Standards
| Standard | Scope |
|---|
| ASME PCC-1 | Bolted flanged joint assembly procedures |
| ISO 898-1 | Mechanical properties of fasteners |
| ASTM A325 / A490 | High-strength structural bolts |
| VDI 2230 | Systematic calculation of high-duty bolted joints |
| EN 1591-1 | Gasketed circular flange connections |
Output
Provide: bolt grade and size, nut factor K, target preload F_b [kN], required torque T [N·m], preload scatter (±%) at 3σ, minimum and maximum preload [kN] for joint design, torquing sequence (cross-pattern, pass percentages), locking method if vibration present, gasket seating stress σ_g [MPa] (if flange), comparison to minimum and crush limit, and applicable standard (ASME PCC-1, ISO 898-1, VDI 2230).
