What is bolt tightening torque?

Bolt tightening torque is the rotational force applied to a fastener to generate a controlled clamping preload in the joint. It is calculated using T = K × F_i × d, where K is the nut factor, F_i is the bolt preload, and d is the nominal diameter.

What is a nut factor (K-factor)?

The nut factor (K) is a dimensionless coefficient that accounts for thread friction, under-head friction, and thread lead angle. Typical values are K ≈ 0.20 for dry steel, K ≈ 0.15 for lubricated, and K ≈ 0.13 for MoS₂-coated fasteners, per VDI 2230.

What is bolt preload and why does it matter?

Bolt preload is the tensile force generated in a bolt when it is tightened. It creates the clamping force that holds the joint together. Standard practice is to target 75–90% of proof load to ensure reliable joint performance without yielding the bolt.

What is the difference between ISO Class 8.8 and Class 10.9?

ISO Class 8.8 has a yield strength of 640 MPa and is the most common structural bolt. ISO Class 10.9 has a higher yield strength of 940 MPa and is used in high-strength joints. Class 12.9 is the strongest at 1080 MPa yield, used in critical applications.

Engineering DisclaimerEngineering calculations are provided for preliminary design and educational/reference purposes only. Users must verify all results according to applicable engineering standards, supplier data, manufacturing requirements, and professional engineering judgment.

ISO 898-1 / VDI 2230

Fastener Design

Bolt Torque Calculator

Calculate the recommended tightening torque, bolt preload clamp force, and tensile stress for metric (ISO) and imperial (UNC/UNF) fasteners using the verified nut-factor method per VDI 2230.

BOLT TORQUE & PRELOAD WORKBENCH

VDI 2230 / ISO 898-1

Deterministic 3-term friction torque reaction engine & von Mises yield stress audit

Workspace Mode:

Unsaved Configuration

Fastener Spec & Method

Calculation EngineMethod

Fastener Size

Property Class

S_y: 240 MPaA_t: 57.99 mm²

Surface & LubricationFriction μ

μ_G = 0.18μ_K = 0.18K_eff = 0.230

Tightening Tool (Scatter α_A)

Target Preload (% S_y)90%

Wrench Torque (M_A)

19.32N·m

14.2 ft·lb

Clamp Load (F_M,zul)

8.4kN

Scatter: 5.6 - 8.4 kN

VDI 2230 Stress Verdict

PASS100%

σ_red,M = 216 MPa

VDI 2230 Assembly Stress & Preload Scatter Audit

Evaluates combined von Mises stress (σ_red,M = √(σ_z² + 3τ_k²)) and tool clamping dispersion (α_A).

PASS — Elastic Range

Yield Strength Utilization (σ_red,M / S_y)100.0%

Axial Stress (σ_z)

144.7 MPa

From bolt stretch elongation

Torsional Stress (τ_k)

92.6 MPa

From thread flank friction

von Mises (σ_red,M)

216.0 MPa

Combined tightening stress

Scatter Band (F_M,min - F_M,max)

5.6 - 8.4 kN

Tool scatter α_A = 1.50

Verification Summary

Equivalent von Mises tightening stress is within allowable yield utilization limit.

Torque Reaction Component Breakdown (VDI 2230 / Motosh)

Over 85% of applied wrench torque is consumed by friction rather than stretching the fastener.

19.3 N·mTotal Torque

Pitch Stretch (T_p)10.4%

2.00 N·m

Useful elastic axial stretching work

Thread Flank (T_t)40.8%

7.87 N·m

Sliding friction on 60° thread flanks

Head Bearing (T_b)48.9%

9.44 N·m

Sliding friction on clamped bearing face

MechToolsHub

VDI 2230 Bolt Tightening Torque & Preload Report — Calculation Report

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Date: —

Standard: ISO 898-1 / VDI 2230 / DIN 946 / ISO 16047

VDI 2230 Bolt Tightening Torque & Preload Report

Design Inputs

Calculation Engine: VDI 2230 / DIN 946 (Full 3-Term Engine)
Bolt Size Selection: M10 × 1.5
Property Class / Fastener Grade: ISO Class 4.6
Lubrication / Thread Condition: Dry (Unlubricated Steel)
Tightening Tool (Scatter α_A): Calibrated Torque Wrench (Manual)
Target Preload Utilization Ratio: 90 % S_y
Tensile Stress Area (A_t): 57.99 mm²
Effective Nut Factor (K_eff): 0.230

Calculated Results

Permissible Preload Force (F_M,zul):	8.4 kN
Min Assembly Preload (F_M,min):	5.6 kN
Target Wrench Torque (M_A):	19.32 N·m
Pitch Stretch Torque (T_p):	2.00 (10.4%)
Thread Flank Friction (T_t):	7.87 (40.8%)
Underhead Bearing Friction (T_b):	9.44 (48.9%)
von Mises Tightening Stress:	216.0 MPa
Yield Safety Audit Verdict:	PASS

Verification Steps

1. Thread Profile & Fastener Geometry (m10):
- •Major diameter (d) = 10 mm
- •Thread pitch (P) = 1.5 mm
- •Pitch diameter (d₂) = 9.026 mm [d - 0.649519 P]
- •Core minor diameter (d₃) = 8.160 mm [d - 1.226869 P]
- •Tensile stress area (A_s) = 57.99 mm² [ISO 898-1 / ASME B1.1]
- •Mean bearing diameter (D_Km)= 12.50 mm [(d_w + d_h) / 2]
2. Material Strength Thresholds (class-4.6):
- •Minimum yield strength (R_p0.2 / S_y) = 240 MPa
- •Target utilization limit (ν) = 90% of yield
3. Friction Coefficients & Tool Scatter (dry):
- •Thread friction coefficient (μ_G) = 0.18
- •Bearing friction coefficient (μ_K) = 0.18
- •Effective Nut Factor (K_eff) = 0.230
- •Tightening scatter factor (α_A) = 1.50
4. VDI 2230 / Motosh 3-Term Torque Reaction Decomposition:
- •Pitch stretch torque (T_p) = 2.00 N·m (10.4%) [Elastic stretching]
- •Thread flank friction (T_t) = 7.87 N·m (40.8%) [Sliding flank resistance]
- •Head bearing friction (T_b) = 9.44 N·m (48.9%) [Clamped surface resistance]
- •Total tightening torque (M_A)= 19.32 N·m (100.0%)
5. Permissible Assembly Preload Scatter Envelope (VDI 2230):
- •Maximum assembly preload (F_M,max) = 8392.4 N
- •Minimum assembly preload (F_M,min) = 5594.9 N [F_M,max / α_A]
6. VDI 2230 Combined von Mises Stress Safety Audit:
- •Axial tensile stress (σ_z) = 144.7 MPa
- •Thread core torsional stress (τ_k) = 92.6 MPa
- •Combined von Mises stress (σ_red,M)= 216.0 MPa [√(σ_z² + 3 τ_k²)]
- •Assembly Verification Verdict = PASS

Engineering Disclaimer: Calculations comply strictly with VDI 2230 Blatt 1:2015, ISO 898-1, and DIN 946. Tightening tool scatter factor α_A defines the achievable minimum clamp load range. Verify against specific gasket seating or joint member compliance requirements for safety-critical flange assemblies.