How It Works

The Torque-Tension Relationship

When tightening a bolt, the applied torque creates tension (preload) in the bolt shank. This relationship is governed by the equation:

T = K x F x d

T = Applied torque (N-m or lb-ft)
K = Nut factor (friction coefficient, typically 0.10-0.20)
F = Target preload/clamp force (N or lbf)
d = Nominal bolt diameter

Why K-Factor Varies with Lubrication

Only 10-15% of applied torque actually creates tension - the rest is lost to friction in threads and under the bolt head. Lubrication reduces friction, meaning less torque is needed for the same preload. Using dry torque specs on lubricated bolts can cause dangerous over-tensioning.

Why Preload Matters

Proper preload (typically 70-80% of proof load) ensures:

Joint stays clamped under external loads
Prevents fatigue from cyclic loading
Maintains gasket seal in flanged connections
Resists vibration loosening

Torque Control vs Tension Control

Torque Control (This Calculator)

Most common method - uses torque wrench
Simple and inexpensive
Accuracy: +/-25% due to friction variation
Suitable for most applications

Tension Control (Direct)

Hydraulic tensioners, ultrasonics, DTIs
Accuracy: +/-5% achievable
Higher cost and complexity
Critical for structural/pressure applications

Quick Select - Common Applications

Bolt Torque Calculator

Calculate recommended tightening torque and clamp force for bolted joints based on size, grade, and lubrication.

Unit System

Bolt Size

Bolt Grade

Lubrication / K-Factor

Target Preload (% of Proof Load)

50% 75% 90%

Typical: 70-80% for reusable joints, 75-90% for permanent

✓

Good Joint Quality

Preload in recommended range

Torque Specification

Recommended Torque --

Torque Wrench Range

Min

Target

Max

-- -- --

Clamp Force (Preload) --

Proof Load (100%) --

Tensile Stress Area --

Proof Stress --

Tightening Sequence Patterns

4-Bolt Cross

6-Bolt Star

8-Bolt Star

Tightening Steps:

Hand-tight (snug) all bolts first
First pass: 50% of final torque in sequence
Second pass: 75% of final torque in sequence
Final pass: 100% torque in sequence
Optional: Final check pass in circular order

Torque Formula

T = K x F x d

T = torque (N-m or lb-ft)
K = nut factor (friction coefficient)
F = target preload (N or lbf)
d = nominal bolt diameter

Grade Reference

Metric	Proof (MPa)	SAE	Proof (ksi)
4.6	225	Grade 2	55
8.8	580	Grade 5	85
10.9	830	Grade 8	120
12.9	970	A574 (SHCS)	140

K-Factor Reference

Condition	K-Factor
Rusty, dirty threads	0.20-0.30
Dry, as-received	0.18-0.20
Zinc plated	0.17-0.19
Lightly oiled	0.14-0.16
Anti-seize	0.11-0.13
Moly paste	0.09-0.11