What do the two numbers in a bolt class like 8.8 mean?

The first number × 100 gives the nominal tensile strength Rm in MPa. The second number × first number × 10 gives the nominal yield strength Rp0.2 in MPa. For class 8.8: Rm = 8 × 100 = 800 MPa and Rp0.2 = 8 × 8 × 10 = 640 MPa.

What is the difference between Rp0.2 and Sp (proof stress)?

Rp0.2 is the 0.2% offset yield strength — the stress at which the bolt takes a 0.2% permanent set. Sp (proof stress) is a guaranteed minimum stress the bolt must withstand without permanent deformation. Sp is typically 90–95% of Rp0.2 and is used to calculate the proof load for quality testing.

When should I use 10.9 versus 8.8 bolts?

Use 8.8 for general-purpose applications — it offers good strength with ductility. Use 10.9 when higher clamping force is needed in a limited space, such as automotive engine assemblies, structural steel connections, or high-vibration environments. 12.9 is reserved for the most demanding applications where maximum preload per bolt is essential.

How are bolt grades marked on the bolt head?

Per ISO 898-1, hexagon bolts and screws must be permanently marked on the head (top or side) with the property class designation (e.g., 8.8, 10.9, 12.9) and the manufacturer's identification mark. Socket head cap screws may be marked on the side of the head or on the recess.

How is proof load calculated?

Proof load Fp = Sp × As, where Sp is the proof stress (MPa) for the property class and As is the tensile stress area (mm²) for the thread size. The result is in Newtons (N). Divide by 1000 for kN. For example, an M12 class 8.8 bolt: Fp = 580 × 84.3 = 48,894 N ≈ 48.9 kN.

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螺栓強度等級 (4.6–12.9)

依 ISO 898-1 查詢任何螺栓性能等級的機械性質——抗拉強度、屈服強度、保證應力、硬度——並計算任何公制螺紋尺寸的保證負荷。

ISO 898-1 等級 4.6 – 12.9 M3 – M48

結果

保證負荷 Fp

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抗拉強度 Rm

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Rp0.2 屈服強度

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保證應力 Sp

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應力截面積 As

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斷裂伸長率

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硬度 HRC

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極限抗拉負荷

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級別	Rm (MPa)	Rp0.2 (MPa)	Sp (MPa)	伸長率 %	HRC	材料

解讀性能等級標示

雙數字系統（例如 8.8）編碼了螺栓的關鍵機械性質：

對於等級 8.8：R_m = 8 × 100 = 800 MPa，R_p0.2 = 8 × 8 × 10 = 640 MPa。

保證負荷計算

保證負荷是螺栓在不產生永久變形的情況下必須承受的最大力：

其中 S_p 為保證應力，A_s 為依 ISO 898-1 的抗拉應力截面積。

極限抗拉負荷

範例 — M12 等級 8.8 螺栓

已知： 等級 8.8，M12 (A_s = 84.3 mm²)，d ≤ 16 mm → S_p = 580 MPa

保證負荷 F_p = 580 × 84.3 = 48,894 N ≈ 48.9 kN

抗拉負荷 F_t = 800 × 84.3 = 67,440 N ≈ 67.4 kN

⚠️ 注意： 對於等級 8.8，S_p = 580 MPa（當 d ≤ 16 mm）且 S_p = 600 MPa（當 d > 16 mm）。這是因為較大螺栓的可硬化截面與總截面之比更高。

ℹ️ 設計提示： 一般螺栓預壓目標為保證負荷的 75–90%。鎖緊時切勿超過保證負荷。關鍵接頭請使用經校準的扭矩扳手或角度控制鎖緊法。