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Bolt Preload & Torque

⚙️ Mechanical · Preload force and tightening torque calculation for a metric bolt

🧮 Calculate∑ Formula✅ Verification📚 Sources

Bolt Size

Property Class

Preload Fraction (× proof load)

Torque Coefficient K

Tensile stress area As84.3 mm²

Proof load Fp = Sp·As69.97 kN

Preload Fi52.48 kN

Tightening torque T = K·Fi·d125.9 N·m

💡 K ≈ 0.20 is for dry/lightly oiled assembly; it drops to 0.12 with MoS₂ grease. For reusable connections a preload of 0.75·Fp is recommended, 0.90·Fp for permanent ones (Shigley).

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T = K F_i d

In bolted joints — the most common type of detachable connection — applying the correct preload is the most critical factor determining the joint’s fatigue life and sealing. An insufficiently tightened bolt will loosen; an overtightened one approaches its yield limit and carries a risk of fracture.

The relationship between tightening torque and preload force is expressed with an approximate empirical formula that accounts for friction effects (T = K·Fi·d). The torque coefficient K depends on friction at the bolt/nut surface: for dry or lightly oiled assembly K ≈ 0.20, while with MoS₂-based grease it can drop to K ≈ 0.12 — meaning the torque needed to achieve the same preload is nearly halved.

Reading the property class: a marking like “10.9” indicates the bolt’s nominal tensile strength (1000 MPa) and its yield-to-tensile ratio (0.9) — the higher the class number, the higher the bolt’s strength, but it may also behave more brittle.

📖 Sourced Formula · Single Verification

This tool's formula is based on a reliable source (see the Sources tab below) and has been checked against a hand-computed numerical example. A second, independent literature cross-check has not yet been added. As with any engineering calculation, we recommend independently confirming results before relying on them for critical decisions.

[1] Budynas, Richard G. and Nisbett, J. Keith. Shigley's Mechanical Engineering Design, 11 ed.. McGraw-Hill, 2020. ↗
[9] ISO. ISO} 898-1 — Cıvata, Vida ve Saplamaların Mekanik Özellikleri. International Organization for Standardization. (TODO(oz): güncel baskı yılı doğrulanmalı) ↗

Bolt Preload & Torque — EngineersLab calc card

Formula: T = K·Fi·d

Sources: [1] Shigley's Mechanical Engineering Design; [9] ISO} 898-1 — Cıvata, Vida ve Saplamaların Mekanik Özellikleri

Tool version: 1.0.0 · Date:

Source URL: https://engineerslab.com.tr/en/araclar/civata

Results are for educational and preliminary-sizing purposes; final engineering design decisions must reference the relevant standards and a licensed engineer's approval.