● System Active

BOLTCALC TORQUE & CLAMP FORCE CALCULATOR

Tightening Torque

—

N·m

🔩 TORQUE

Clamp Force

—

⇧ CLAMP

Stress Area

—

mm²

Bolt Stress

—

MPa

K Factor

—

nut factor

Bolt Cross-Section

Bolt Size — Metric (M)

📊

Grade 8.8 · High Strength

Min tensile 800 MPa · Proof 640 MPa · Common structural fastener

Parameters

Bolt Grade8.8

Friction ConditionLightly Oiled

Preload (% of Proof)75%

50%70%90%

Safety Factor1.0×

1.0×2.0×3.0×

Formula Reference

TRQT = K × F_i × d · K = nut factor (surface friction)

CLFF_i = σ_p × A_s × η · η = preload ratio

AREAA_s = π/4 × (d − 0.9382p)² · p = thread pitch

Bolt Torque Calculator: Fastener Tightening Guide

Correct bolt tightening torque is critical for joint integrity. Under-tightening causes loosening and leakage; over-tightening risks bolt breakage or thread stripping. This calculator accurately computes tightening torque, clamp force and stress for metric (M-series) and UNC/UNF imperial fasteners.

📋 How to Use This Calculator

Select bolt size and grade/property class (e.g. M12, Grade 8.8).
Enter the nut factor (K), lubricated: 0.15, dry steel: 0.20, zinc-plated: 0.17.
Select desired clamp load percentage (typically 70–80% of proof load).
Note the calculated tightening torque, clamp force and tensile stress results.

📐 Formula & Working Principle

Tightening Torque T = K × d × F. Where K = nut factor, d = bolt nominal diameter (m), F = desired clamp force (N). Clamp Force F = (% preload × Proof Load Area × Proof Stress).

💡 Worked Example

M16 bolt, Grade 8.8, K=0.20, 75% preload. Proof stress = 600 MPa, Stress area = 157 mm². Clamp Force = 0.75 × 157 × 600 = 70,650 N. Torque = 0.20 × 0.016 × 70,650 = 226 N·m.

❓ Frequently Asked Questions (FAQ)

Q: What torque wrench accuracy is required?

Use a torque wrench with ±4% accuracy for industrial applications. For critical joints (engine, structural) use ±2% accuracy.

Q: Why does lubrication change the torque value?

Lubrication reduces friction (lower K value), less torque is needed for the same clamp load. Dry bolts: K=0.20, molybdenum-greased: K=0.13.

Q: Is re-tightening safe?

Used bolts may have fatigue damage. In critical structural applications, bolts should be replaced after a single use.

Q: What special precautions apply to high-strength bolts (10.9, 12.9)?

These bolts have a hydrogen embrittlement risk. Follow the correct torque sequence and take lubrication specifications from the manufacturer's datasheet.

Q: What is torque sequence?

In a multi-bolt pattern, tighten first to 50%, then to 100% of the torque value in a cross-pattern (star sequence) for even load distribution.