What is a screw thread？

A screw thread is a helical structure used to convert rotational motion into linear motion and to fasten objects together. It is the essential feature of screws, bolts, and other fasteners, and serves two main purposes: fastening materials and transferring motion.

Components of a Screw Thread

・Crest: The topmost point of the thread.

・Root: The lowest point of the thread, between two adjacent threads.

・Pitch: The distance between corresponding points on adjacent threads. It is the distance the screw advances in one full rotation.

・Thread Angle: The angle formed between the flanks (the sloping sides of the thread).

・Major Diameter: The largest diameter of the thread, measured from crest to crest (external diameter for screws, internal diameter for nuts).

・Minor Diameter: The smallest diameter of the thread, measured from root to root.

・Flank: The angled surface that connects the crest to the root of the thread.

Types of Threads

・External Threads: Found on the outer surface of screws and bolts.

・Internal Threads: Found on the inside surface of nuts or tapped holes.

Thread Classifications

・Coarse Threads:

Have larger pitch, with fewer threads per unit of length.

Better for quick assembly and are more tolerant to dirt or damage.

Commonly used in wood and soft materials.

・Fine Threads:

Have smaller pitch, with more threads per unit of length.

Provide greater holding strength and are often used in metal applications.

More resistant to loosening due to vibration.

・Left-Hand vs. Right-Hand Threads:

    Right-Hand Threads: Tighten when turned clockwise (the most common type).

    Left-Hand Threads: Tighten when turned counterclockwise. These are used in specific applications, such as machinery, where rotation may loosen right-hand threads.

Functions of Screw Threads

・Fastening:

Screws and bolts use threads to clamp materials together tightly. The rotational force applied to the screw is converted into linear force, creating friction between the screw threads and the material, which secures the connection.

・Motion Transfer:

Threads are also used in mechanisms like lead screws or jackscrews to convert rotational motion into precise linear movement, such as in vises, clamps, or CNC machines.

Thread Standards

・Metric Threads:

Defined by ISO standards, typically labeled as "M" followed by the diameter and pitch in millimeters (e.g., M8 × 1.25).

・Unified National Thread (Imperial):

Used in the United States, labeled by thread size and threads per inch (TPI), such as 1/4"-20, which has a 1/4-inch diameter and 20 threads per inch.

・Pipe Threads:

Tapered threads used in plumbing to create a seal, such as NPT (National Pipe Thread) in the U.S.

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