bolt-torque

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):

1. Hand-tighten all bolts (snug condition)
2. Cross-pattern (star) at 30% target torque
3. Cross-pattern at 60% target torque
4. Cross-pattern at 100% target torque
5. 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).