What does nut strength class mean?

Nut strength class per ISO 898-2 indicates the proof load capability of the nut. It is designated by a single number (e.g., 8, 10, 12) that corresponds to 1/100th of the minimum proof load stress in MPa for the reference bolt-nut combination.

How do I match a nut to a bolt class?

The nut class number should be equal to or greater than the first digit of the bolt class. For example: bolt 8.8 → nut class 8; bolt 10.9 → nut class 10; bolt 12.9 → nut class 12. This ensures the nut threads are at least as strong as the bolt.

What is the proof load stress of a nut?

Proof load stress is the maximum axial stress a nut can sustain without permanent deformation, measured via a hardened mandrel test. For a class 8 nut (≤M16), this is 800 MPa. The actual proof load force depends on the bolt tensile stress area.

Why are nut proof loads size-dependent?

For nuts >M16, ISO 898-2 tabulates size- and style-dependent proof load stresses that differ from the small-size nominal values, reflecting changes in thread engagement and load distribution at larger diameters. Check the ISO 898-2 tables for exact values.

What hardness should a class 8 nut have?

A class 8 nut per ISO 898-2 should have Vickers hardness HV 200–302 (approximately Rockwell HRC 20–30). Nuts that are too hard may be brittle; too soft may strip threads under load.

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螺帽強度等級 — ISO 898-2

查詢 ISO 898-2 螺帽等級：保證負荷應力、硬度範圍及對應螺栓性能等級。選擇螺帽等級與尺寸。

ISO 898-2等級 5–12M5 – M36

結果

保證負荷力

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保證負荷應力 (Sp)

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硬度範圍 (HV)

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對應螺栓等級

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抗拉應力面積 (As)

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保證負荷

螺帽的保證負荷是指其在不發生永久變形的情況下所能承受的最大軸向力：

ISO 898-2 螺帽等級摘要

螺帽等級	Sp (MPa) ≤M16	HV 範圍	對應螺栓
5	500	130–302	4.6, 4.8, 5.6, 5.8
6	600	150–302	6.8
8	800	200–302	8.8
10	1040	260–353	10.9
12	1220	295–353	12.9

⚠️ 注意： 對於 >M16 尺寸，ISO 898-2 列出依尺寸與型式而異之保證負荷應力，可能與此處顯示之等級名義值不同（某些等級較高，而非較低）。請務必針對精確尺寸、螺距與螺母型式，依據實際 ISO 898-2 表格驗證保證負荷。

螺帽強度等級 ISO 898-2 查詢 | 免費線上工具

結果

保證負荷

ISO 898-2 螺帽等級摘要

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