Pipe thread types, sizes and GOSTs

One of pipe connection methods is a thread, which is a spiral with constant pitch.

Pipe threads vary in the following parameters:

• Diameter measuring system: metric and inch thread
• Thread direction: right hand, left hand
• Thread location: external, internal
• Number of threadings: single, multiple
• Purpose: fastening, fastening and sealing, running, special, etc.

The main characteristics of pipe threads are:

• Outside diameter
• Inside diameter
• Pitch: the distance between nearby turns
• Run: the move distance of the fastener in the longitudinal direction in one full turn. With single knurling, the run is equal to the pitch, with multiple one the pitch is multiplied by the number of threadings.

Metric thread

Metric thread with the relevant designator “M” as per GOST is characterized by the measurement of the main parameters in millimeters. It is widely applicable in diameters ranged from 1 to 600 mm and pitch from 0.25 to 6 mm. The metric thread profile is equilateral triangle with apex angle 60° and theoretical height H-0.866025404.
General metric thread dimensions are currently determined according to the current GOST 24705-2004, adopted by the CIS Interstate Council for Standardization, Certification and Metrology, as well as by the national authorities of standardization of the Russian Federation, Azerbaijan, Armenia, Belarus, Georgia, Kazakhstan, Kyrgyzstan, Moldova, Tajikistan, Turkmenistan, Uzbekistan, Ukraine.

Metric thread, sizes and GOSTs (download file .pdf)

Inch thread

Inch thread express all parameters in inches, it is designated "Tr" as per GOST. The inch thread is based on the British thread standard BSW (British Standard Whitworth), patented by the British engineer Whitworth back in 1841, and complies with the British standard pipe thread (BSPT).
One inch is equal to 2.54 cm, and the graphic symbol for inch is a double stroke to the right and above the number (1 "= 1 inch). For inch pipe threads, the size indicates the inside diameter of the pipe, not the outside one. There are four pitches: 28, 19, 14 and 11 threads per inch.
Inch profiles are distinguished by sharper ridges and troughs with angles 55° and theoretical height H = 0.960491, with the rounded prong tops.
It is impossible to combine metric and inch threads in one connection; a special adapter is required for this.
The inch pipe thread size is set by the current GOST 6357-81 "Basic interchangeability standards".

Pipe Thread Size Table

Unified thread

The so-called unified thread, or UTS (Unified Thread Standard), is common In the USA and Canada. It is characterized by apex angle 60° and theoretical profile height H = 0.866025P.
UTS in turn is divided into the following standards depending on the pitch:
• UNC (Unified Coarse);
• UNF (Unified Fine);
• UNEF (Unified Extra Fine);
• 8UN;
• UNS (Unified Special).

Edison profile (round thread)

Round thread, or the so-called Edison profile, are sometimes applied for pipe connection. This pipe thread profile is characterized by interlacing hogbacked and dished arcs with the same radius. The round thread type is used in connections that are exposed to frequent loosening and screwing, as it provides high endurance.

Cylindrical thread

Cylindrical pipe threads are applied to connect pipes, pipework and fittings with diameter from 1/16 to 6 inches with the number of threads from 28 to 11 per inch.
The cylindrical pipe thread standard is GOST 6367-81, where letter G, thread size designation and the average diameter accuracy class must be included as marking symbols for the cylindrical pipe thread. For left-hand threads, designation LH is adopted.
The threaded connection is indicated by a fraction, with the internal thread accuracy class is indicated in the numerator, and the external one in the denominator.
The internal cylindrical pipe thread as per GOST 6357-81 is compatible with the external tapered thread according to GOST 6211-81. That complies with BSPP standard.

Tapered thread (trapezoidal as per BSPT)

The tapered pipe thread is applied to ensure tightness and locking without wrench. Thus, with this type of part connection, the desired effect is achieved due to the thread deformation.

Today, the metric standard for foreign technology is the most general one with ISO marking. Other standards are also fairly widespread in the world, that is associated with historical differences in measuring systems of the different countries. Furthermore, the use of inch or metric threads depends on the scope of application. For example, pipes, relevant to international standards for cylindrical inch threads in ¼, ½ and 1 inch, are generally used in housing construction.
Metric threads are used in industry. The tapered pipe thread is distinguished by exceptional pipe connection reliability, therefore it is applied for high pressure pipelines, i.e. in gas and oil production and on boring rigs.
The American standard for the unified pipe thread UNC 1/4 (1/4 "x1.25 mm) is accepted worldwide to serve photo and video equipment mountings.

Pipe thread dimensions

Pipe thread dimensions are determined according to a special table after measuring the main parameters:
• pitch (P) - the distance between the same name sides of the profile, measured in meter fractions, in inch fractions or in the number of threads per inch (the denominator of the simple fraction, where the numerator is an inch, expressed as natural number)
• outside diameter (D, d) - the diameter of the cylinder, circumscribed around the tops of the external thread (d) or troughs of the internal thread (D).
• average diameter (D2, d2) - the diameter of the cylinder, which generatrix intersects the thread profile in such a way that its segments formed at the intersection with the trough are equal to half of the nominal thread pitch.
• inner diameter (D1, d1) - the diameter of the cylinder inscribed in the troughs of the outer thread (d1) or tops of the inner thread (D1)
• run (Рh) - the amount of relative movement of the original midpoint along the thread helix at angle 360°.

How to cut threads: threading equipment and tools

The choice of threading equipment depends on the production type and scale, pipe material and dimensions, thread accuracy requirements, and other factors.

The thread can be obtained by knurling or cutting methods.

The knurled thread is produced using special threading rollers. On this purpose, the workpiece is installed between rollers with the required thread contour and the thread turns are produced on thread rolling machines and semiautomatic machines, and sometimes on lathes and revolving machines. Owing to the smooth spreading of tension waves between the turns because of the metal deformation, such thread is featured with high mechanical characteristics and performance.

However, there must be ensured a very high accuracy level of the applicable rollers, and the material must have improved mechanical properties; highly doped stamped steels are mostly used.

Cut threads are easier to manufacture, but inferior to rolled threads in mechanical properties and endurance limit. This is explained by the presence of the sharper profile edges and the higher tension factor.

According to the threading method, threads are divided into manual type and special machine threading.

Special devices are used for manual cutting, such as tap (for the internal pipe threading) and screw-thread dies (for the external pipe threading), or a special threading hand tool - a die-cutter, which performs cutting using movable adjustable ridge incisors. Manual cutting is recommended for pipe diameters up to 1 inch, as large diameters require essentially more physical strength.

Nowadays, there is a huge selection of reliable threading tools for large loads and volumes of work, from portable electric threading tools to special machines that can cope with any type threading on all diameter pipes made of any materials.