What is bolt preload force?

Bolt preload (also called clamping force or initial tension) is the tensile force induced in a bolt during tightening. It creates the clamping force that holds the joint together and is essential for joint integrity, preventing loosening and fatigue failure.

What utilization factor should I use?

Typical utilization factors (η) are 0.7 to 0.9 of the yield strength. For controlled tightening (torque wrench), 0.7–0.75 is common. For precision methods (stretch measurement, hydraulic tensioner), 0.85–0.9 can be used. Never exceed 0.9 for static applications.

How is bolt preload related to torque?

The approximate relationship is T = K × d × F, where K is the nut factor (typically 0.2 for dry steel, 0.15 for lubricated), d is the nominal bolt diameter, and F is the preload force. This is an approximation with ±25% uncertainty.

Why is preload important in bolted joints?

Preload is critical because it: (1) prevents joint separation under external loads, (2) reduces bolt fatigue by minimizing load fluctuations, (3) creates friction that resists shear loads, (4) prevents bolt loosening from vibration, and (5) provides gasket seating force in sealed joints.

What happens if bolt preload is too high?

Excessive preload can cause bolt yield or fracture during tightening, thread stripping, gasket crushing, flange distortion, or joint member damage. This is why utilization factors are used to keep preload safely below the yield point.

Free Engineering Tool

Bolt Preload Force Calculator

Calculate the assembly preload force in a metric bolt from utilization factor, yield strength, and tensile stress area. F = η × Rp × As.

ISO 898-1 VDI 2230 M3 – M64

Bolt Size

Property Class

Utilization Factor η (%)

Nut Factor K (for torque est.)

Quick presets

Results

Assembly Preload Force (F)

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Estimated Tightening Torque

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Yield Load (100%)

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Tensile Stress Area

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Utilization

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Preload Force

The assembly preload force is the target bolt tension during tightening:

η — utilization factor (0.7–0.9 typical)
R_p — proof / yield strength (MPa)
A_s — tensile stress area (mm²)

Torque Estimation

Approximate tightening torque (short-form equation):

K — nut factor (friction-dependent, typically 0.15–0.20)
d — nominal bolt diameter (mm)

Practical Example

Example — M10 × 8.8 @ 70%

Given: M10, class 8.8 (Rp = 640 MPa), As = 58.0 mm², η = 70%

F = 0.70 × 640 × 58.0 = 25,984 N ≈ 26.0 kN

T = 0.20 × 10 × 25,984 / 1000 = 52.0 N·m

⚠️ Note: Torque-based tightening has ±25% uncertainty. For critical joints, use stretch measurement or hydraulic tensioning for greater accuracy.

Bolt Preload Force Calculator | ISO 898-1 | Free Online Tool

Published by admin on February 15, 2026 February 15, 2026

Results

Preload Force

Torque Estimation

Practical Example