What is the formula for calculating bolt tightening torque?

T = K × F × d, where K is the torque coefficient (0.10–0.25), F is target preload force (N), and d is nominal bolt diameter (m).

What is K-factor in bolt tightening?

K-factor combines thread and bearing surface friction. Typical values: dry 0.20-0.25, oiled 0.14-0.18, MoS₂ 0.10-0.12, PTFE 0.08-0.10.

What is the recommended preload for bolts?

Typically 65-75% of yield strength for standard applications. F = S × As × η.

How does lubrication affect bolt torque?

Lubrication reduces K-factor significantly. Using wrong K-factor causes over- or under-tightening.

What is the correct bolt tightening sequence?

Tighten in multiple passes using cross/star pattern: snug → 30% → 70% → 100%.

What is the Kellermann-Klein equation?

T = F × [P/(2π) + 1.154·μth·d₂ + μb·(Do+dh)/4]. Separates thread and bearing friction.

Free Engineering Tool

Bolt Tightening Torque Calculator

Calculate the required torque to achieve proper bolt preload force. Supports coarse & fine pitch metric threads, imperial UNC, and multiple lubrication conditions.

This calculator was developed and verified by the Vibromera engineering team, makers of the Balanset portable balancing instruments.

ISO 898ISO 16047VDI 2230ASME B1.1

Bolt Size

Property Class / Grade

Strength Basis (strict mode)

Lubrication (affects K-factor)

Target Preload (% of Rp)

Joint Type (informational)

Tightening Method (affects range)

Number of Bolts (for pattern)

Results

Recommended Torque

—

Preload Force

—

K-Factor

—

Stress Area

—

Torque Range (based on tightening method accuracy)

———

Tightening Sequence

1Hand-tighten all bolts until snug
2Tighten to — (30% torque) in pattern
3Tighten to — (70% torque) in pattern
4Tighten to — (100% torque) — smooth motion

Recommended Tightening Pattern

Torque Calculation Formula

The required tightening torque is calculated using the simplified VDI 2230 nut-factor method:

T = K × F × d

T — tightening torque (N·m)
K — torque coefficient / nut factor (0.10–0.25)
F — target preload force (N)
d — nominal bolt diameter (m)

Preload Force

F = S × A_s × η

S — strength basis: Rp (yield) or Sp (proof stress)
A_s — tensile stress area (mm²)
η — utilization factor (0.50–0.90)

Kellermann-Klein Equation

T = F × [ P/(2π) + 1.154·μ_th·d₂ + μ_b·(D_o+d_h)/4 ]

K-Factor Reference

Surface Condition	K-factor	Notes
Dry	0.20–0.25	High scatter
Light oil	0.14–0.18	Most common
MoS₂	0.10–0.12	High loads, stainless
PTFE	0.08–0.10	Caution: over-torque
Zinc plated	0.17–0.20	Depends on coating
Anti-seize	0.11–0.13	High-temperature
Wax patch	0.09–0.11	Automotive OEM

Bolt Property Classes (ISO 898-1)

Class	R_m	R_p	S_p	Application
4.6	400	240	225	Non-critical
5.6	500	300	280	General
8.8	800	640	580/600	Standard structural
10.9	1040	940	830	High-strength
12.9	1220	1100	970	Critical, max load

Practical Examples

Pump Flange — M12 × 1.75, class 8.8, oiled

K=0.16, F=640×84.3×0.75=40,464 N → T=0.16×40464×0.012=77.7 N·m

Gearbox — M20 × 2.5, class 10.9, anti-seize

K=0.12, F=900×245×0.75=165,375 N → T=0.12×165375×0.020=397 N·m

Over-torquing strips threads or fractures bolt
Under-torquing → loosening, leaks, fatigue failure
Calibrate torque wrenches annually
Clean threads before assembly
Do not reuse class 10.9+ bolts
This is an engineering estimate — test per ISO 16047 for critical joints