Sunday, October 1, 2023

Thread Types

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A thread is a continuous helical groove on the external or internal surface of a cylinder or cone. The function of the thread is to insert an element (screw, bolt, tube) inside a hole with a similar shape and with an equivalent thread, through a circular movement, in order to integrate both pieces. Depending on the type of thread, the assembly can have the function of securing a part or, in higher precision threads, that of preventing all kinds of fluid leaks. The latter happens specifically in the case of threaded tubes used for the passage of water, oil, gas and others.

Uses of Threads

Depending on their characteristics, threads are used for different purposes. Cylindrical threads are the most common and are used in screws, bolts and their corresponding nuts, with the aim of firmly joining two elements that are initially separate. Socket head cap screws are also used to gauge certain precision or measuring parts. Tapered threads are used for a hermetic union, as in the case of tubes through which fluids must circulate.

Thread Types

In addition to the broad division between tapered and cylindrical threads, there is a more detailed classification of threads based on their shape, the width of their flutes (pitch), and other characteristics.

Coarse Pitch Threads: As its name indicates, the pitch, that is, the width of each groove, is wide. Therefore, this type of thread does not have great precision in terms of the union of the element that is inserted (the male) and the hollow part where it is installed (the female). They are used for normal jobs that require firmness but not such a tight joint.

Fine Pitch Threads: they generate greater firmness in the joint, and are used above all in mechanics, in the automotive and vehicle industry in general.

Extra Fine Pitch Threads: they are used when greater precision is required, as in the case of elements that must be joined to thin walls.

Eight Thread Threads: they are so named because their pitch consists of eight grooves per inch; These threads are indicated for water pipes and other fluids. The characteristics of its surface allow greater resistance to pressure and prevent gas and liquid leaks.

Depending on the shape of each thread or groove, the threads can be: V, rounded, square, Witworth (whose shape is trapezoidal) and trapezoidal.

Names of the Most Common Threads

There is a wide variety of threads. Among the most common, the following can be highlighted:

  • BSP: Witworth Gas thread.
  • BSF: Witworth fine pitch thread.
  • BSPP: Parallel Witworth thread, used for tubes.
  • BSPT: Tapered Witworth Pipe Thread.
  • NPS: American cylindrical thread; It is used for joining pipes.
  • NPT: tapered American pipe thread.


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