The threads of fasteners are defined by five geometric elements: thread form, diameter, pitch, number of starts, and thread direction (handedness). Only when these five elements match perfectly can internal and external threads be used together.
Among these, thread form, nominal diameter, and pitch are the three primary elements. National standards specify certain standard thread forms, nominal diameters, and pitches. Threads that conform to these standards are called standard threads.
Threads with a standard thread form but non-standard nominal diameter or pitch are called special threads, while those with a non-standard thread form are referred to as non-standard threads.
1.3.3 Thread Forms
The cross-sectional profile of a thread, when cut along its axis, is called the thread form. Common thread forms include triangular, trapezoidal, buttress (sawtooth), and square, each serving different purposes and represented by distinct codes.
1. General-Purpose Threads (M)
General-purpose threads are the most commonly used fastening threads, with a triangular thread form and a 60° thread angle, designated by the code M. They are further divided into coarse (standard) threads and fine threads, which share the same designation.
· Coarse threads are typically used for general fastening.
· Fine threads have a smaller pitch and thread height compared to coarse threads of the same major diameter, making them suitable for thin-walled components.
2. Imperial Pipe Threads (G, R, Rp, Rc)
Pipe threads are primarily used for tubing connections, featuring a 55° triangular thread form. They are categorized into two types:
1. Non-pressure-tight (Parallel) Pipe Threads (G)
Designated by G, these are cylindrical threads (parallel) and do not provide a pressure-tight seal.
Commonly used in electrical conduits and other non-sealed piping systems.
These threads can form a pressure-tight seal when mated (tapered-to-tapered or parallel-to-tapered).
Used in water pipes, gas lines, and lubricant pipelines.
3. Trapezoidal Threads (Tr)
Trapezoidal threads are widely used for power transmission, featuring an isosceles trapezoidal thread form with a 30° thread angle, designated by Tr.
4. Buttress (Sawtooth) Threads (B)
Buttress threads are designed for unidirectional load-bearing applications, with an asymmetrical trapezoidal thread form (30° on the load-bearing side and 3° on the opposite side), designated by B.
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