Torque and Preload扭矩与预紧力

Three terms to know三个基本概念

• Torque — the turning force applied to the fastener with a tool (N·m).扭矩：用工具施加在紧固件上的旋转力（N·m）。
• Preload — the axial tension created in the bolt when it is tightened, stretching it slightly like a spring.预紧力：拧紧时在螺栓中产生的轴向拉力，使螺栓像弹簧一样被轻微拉长。
• Clamp load — the force pressing the joined parts together. It is what actually keeps the joint tight.夹紧力：把被连接件压紧在一起的力，真正让连接保持紧固的就是它。

What the design needs is the right clamp load. Torque is only the means we use to get there — and it is an indirect one.设计真正需要的是合适的夹紧力。扭矩只是达到这个目标的手段，而且是间接的手段。

Where the torque goes扭矩都去哪了？

When a typical dry steel bolt is tightened, most of the applied torque is used up overcoming friction. As a rule of thumb:拧紧一颗普通的干燥钢制螺栓时，大部分扭矩都消耗在摩擦上。经验上大致是：

only about 10% becomes useful preload只有约 10% 转化为有效预紧力

This is why small changes in friction — a different coating, oil on the threads, a rougher bearing surface — can change the preload considerably at the same torque.这就是为什么摩擦条件稍有变化（换了镀层、螺纹上沾了油、支承面更粗糙），同样的扭矩下预紧力就可能相差很大。

The torque–preload relationship扭矩与预紧力的关系

A commonly used simplified formula is:常用的简化公式是：

• T — tightening torque (N·m)T：拧紧扭矩（N·m）
• F — preload (kN)F：预紧力（kN）
• d — nominal diameter (mm)d：螺纹公称直径（mm）
• K — the “nut factor”, which lumps together all friction effects. About 0.2 for plain dry steel, lower when lubricated or with some coatings, and higher on rough or dirty surfaces.K：扭矩系数，综合了所有摩擦影响。普通干燥钢件约为 0.2，润滑或某些涂层下更低，表面粗糙或有污物时更高。

Example: for an M10 bolt with a target preload of 25 kN and K = 0.2, T ≈ 0.2 × 25 × 10 = 50 N·m.举例：M10 螺栓，目标预紧力 25 kN，K = 0.2，则 T ≈ 0.2 × 25 × 10 = 50 N·m。

What affects the result影响结果的主要因素

• Surface finish and coating (zinc, zinc-flake, black oxide, etc.)表面处理与镀层（镀锌、达克罗 / 锌铝涂层、发黑等）
• Lubrication or thread-locking adhesive润滑剂或螺纹锁固胶
• Hardness and roughness of the bearing surface; use of washers支承面的硬度和粗糙度，是否使用垫圈
• Thread tolerance and condition of the tapped hole螺纹公差及内螺纹孔的状态
• Tightening speed and tool type拧紧速度和工具类型
• Joint stiffness: a “hard” metal-to-metal joint reaches full torque in a small angle; a “soft” joint with gaskets or plastics needs much more rotation连接刚度：金属对金属的“硬连接”转很小角度就达到扭矩；带垫片或塑料件的“软连接”需要转更多圈

Choosing the tightening method拧紧方法的选择

How to set the right torque如何确定合适的扭矩

For important joints the reliable approach is a torque–tension test using the actual parts, coating, lubrication and tightening tool. The test produces a curve of preload against torque, from which a seating torque can be chosen that gives the target preload with a safe margin below yield. A common target is around 75% of the bolt’s proof load, but the right value depends on the application.对于重要连接，可靠的做法是用实际零件、实际表面处理、润滑条件和拧紧工具做扭矩–预紧力测试。测试得到预紧力随扭矩变化的曲线，据此选定拧紧扭矩，使预紧力达到目标值，并与屈服点保持安全余量。常见目标约为螺栓保证载荷的 75%，具体数值需按应用确定。