What role do threads play in fasteners?

1. Core function: Connection and fastening

This is the most basic and well-known function of threads. By screwing the bolt (external thread) into the nut or threaded hole (internal thread), two or more components can be tightly connected together.

Generate significant axial force (clamping force): When tightening a nut or bolt, the thread converts the rotating torque into a huge tensile force along the bolt axis direction. This tension causes the bolt itself to elongate, pressing the connected components tightly together like a very hard spring. This clamping force is the key to preventing loose connections and separation.

2. Core Function: Transmitting Load

Once fastened, the connection point needs to withstand various external forces. Threaded connections can effectively transmit these loads:

Shear force: resisted by the tight contact between the bolt shaft and the hole wall of the connected component.

Vibration and impact: The frictional force generated by strong clamping force can effectively resist and attenuate vibration and impact energy, preventing relative displacement between components.

3. Core function: Sealing

In some applications, threads can serve as a seal to prevent liquid or gas leakage.

Pipeline sealing: In pipe threads (such as NPT, BSPT), the shape of the thread is conical. When tightened, the thread profile will deform and squeeze against each other, filling the small gaps between the threads and forming an effective seal. Usually, it is also combined with raw material tape or sealant to enhance the sealing effect.

4. Core role: regulation and measurement

The spiral structure of the thread makes it very suitable for precise linear movement and position control.

Adjustment: For example, micrometers, focusing knobs for microscopes, C-clamps, etc., all precisely control the displacement of moving parts by rotating threads.

Measurement: A micrometer is used to convert angular displacement into linear displacement using high-precision threads (with known pitch), thereby achieving high-precision length measurement.

5. Core function: Power transmission

Threads can convert rotational motion into linear motion and be used as a transmission mechanism.

Examples: Jack, feed screw of machine tool, Z-axis lifting of 3D printer, etc. The threads in these applications are commonly referred to as "lead screws" and have high requirements for strength, accuracy, and wear resistance.

The physical principles behind the implementation of these functions by threads:

A thread is essentially an angled plane wrapped around a cylinder.

Effort saving principle: Just like pushing a heavy object up a slope is easier than lifting it vertically, the force generated by rotating the thread along the slope direction is decomposed into a huge axial force (clamping force) and a smaller circumferential force. This allows us to generate significant clamping force with relatively small torque (the force of a wrench).

Self locking principle: When the slope angle (thread lift angle) is small enough and the friction between threads is large enough, the system will be in a "self-locking" state. This means that the force applied in the axial direction (the force attempting to pull off the connector) cannot be converted back into a rotational force that causes the nut to loosen. This is a prerequisite for fasteners to remain tight even in vibration environments.

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