Threaded Fastener Basics螺纹紧固件基础

Bolts, screws and self-tapping screws螺栓、螺钉与自攻螺钉

Threaded fasteners are usually grouped by how they are installed:螺纹紧固件通常按安装方式来区分：

• Bolt — passes through clearance holes in the parts and is tightened with a nut.螺栓：穿过被连接件上的通孔，用螺母拧紧。
• Screw — threads directly into a tapped (internally threaded) hole in one of the parts.螺钉：直接拧入被连接件上已攻好内螺纹的孔中。
• Self-tapping / thread-forming screw — creates its own mating thread in a plain hole in sheet metal, cast metal or plastic, so no separate tapping operation is needed.自攻 / 自挤螺钉：在薄板、铸件或塑料的光孔中自行形成内螺纹，省去单独的攻丝工序。

Head styles常见头型

The head transmits the tightening force and determines how the fastener sits on the part.头部承受拧紧工具的作用力，并决定紧固件在零件上的安装形态。

Drive types驱动方式

The recess or external shape that the tool engages affects how much torque can be applied, how well the tool stays engaged and how easy automated assembly is.驱动形式决定了能施加多大的扭矩、工具是否容易打滑，以及是否适合自动化装配。

• Slotted — simple, but the tool slips out easily; low torque.一字槽：结构简单，但工具容易滑出，只能承受较小扭矩。
• Cross recess (type H / type Z) — self-centring and fast; the tool tends to “cam out” at high torque.十字槽（H 型 / Z 型）：自动对中、装配快；扭矩较大时工具容易“爬出”。
• Hexagon socket — good torque transfer in a compact head.内六角：头部紧凑，扭矩传递好。
• Hexalobular socket — six-lobe profile; higher torque transfer and very little cam-out, well suited to automated assembly.内六角花形（梅花）：六瓣形结构，扭矩传递更大，几乎不打滑，适合自动化装配。
• External hexagon — highest torque capacity; tightened with wrenches or sockets.外六角：可承受的扭矩最大，用扳手或套筒拧紧。

Thread types螺纹类型

• Machine thread (e.g. M8 coarse, M8×1 fine) — mates with a nut or tapped hole. Coarse thread is the default; fine thread gives finer adjustment and slightly higher strength in thin-walled parts.普通螺纹（如 M8 粗牙、M8×1 细牙）：与螺母或内螺纹孔配合。粗牙最常用；细牙便于微调，在薄壁件中强度略高。
• Tapping thread (e.g. ST4.2) — wider pitch and pointed end for sheet metal and plastics.自攻螺纹（如 ST4.2）：螺距较大、尾部尖，用于薄板和塑料。
• Thread-forming thread — forms the internal thread by displacing material rather than cutting it, giving no chips and good resistance to loosening.自挤（三角牙）螺纹：靠挤压而非切削形成内螺纹，不产生切屑，防松性能好。

Property classes (metric steel)性能等级（公制碳钢）

Steel bolts and screws are marked with a property class such as 8.8 (ISO 898-1). The first number × 100 is the nominal tensile strength in MPa; the second number × 10 is the ratio of yield strength to tensile strength in percent.碳钢螺栓、螺钉按 ISO 898-1 标注性能等级，如 8.8。第一个数字 × 100 为公称抗拉强度（MPa），第二个数字 × 10 为屈服强度与抗拉强度之比（%）。

Stainless steel grades不锈钢等级

Stainless fasteners are marked per ISO 3506, e.g. A2-70. The letter and digit give the steel group (A2 is broadly equivalent to 304, A4 to 316), and the number after the dash × 10 is the minimum tensile strength in MPa.不锈钢紧固件按 ISO 3506 标注，如 A2-70。字母和数字表示钢种（A2 大致对应 304，A4 大致对应 316），短横后的数字 × 10 为最低抗拉强度（MPa）。

• A2-70 — general corrosion resistance, indoor and general outdoor use.A2-70：一般耐腐蚀，适用于室内及一般户外。
• A4-70 / A4-80 — better resistance to chlorides; marine, chemical and food applications.A4-70 / A4-80：耐氯离子腐蚀更好，适用于海洋、化工、食品等环境。

Inch (imperial) fasteners英制紧固件

Inch bolts are graded under SAE J429 — Grade 2, Grade 5 and Grade 8 are the most common, marked on the head with radial lines (none, 3 and 6 lines respectively). Grade 5 is roughly comparable to metric 8.8 and Grade 8 to 10.9. Structural bolting in North America commonly uses ASTM F3125 (formerly A325 / A490).英制螺栓按 SAE J429 分级，最常用的是 2 级、5 级和 8 级，头部分别以无线、3 条、6 条放射线标识。5 级大致相当于公制 8.8 级，8 级大致相当于 10.9 级。北美钢结构螺栓常用 ASTM F3125（原 A325 / A490）。