Home > News > Content

Standard Of Thread

Apr 27, 2020

         Threads, do mechanical daily dealings, especially hydraulic and pneumatic, a long time domestic, metric, imperial, straight cone, sealed non-sealed, inside and outside, 55 degrees and 60 degrees . In short, I am often confused. I use it once to check it from start to finish. I am here to summarize it and hope to help.

        My approach is to print it out and put it on the table. When you use it, you can check it at any time. After a long time, you will naturally remember it.

NPT is a general-purpose American standard taper pipe thread with a tooth angle of 60°
The PT tooth is an inch taper thread with a tooth angle of 55°, which is most commonly used in sealing. The inch pipe thread is a fine thread, because the tooth depth of the coarse thread will seriously reduce the strength of the pipe with the outer diameter of the thread.
The PF tooth is a parallel thread for the tube.
G is a 55 degree non-threaded sealed pipe thread, belonging to the Wyeth thread family. Marked as G for cylindrical thread, G is the general name for the pipe thread (Guan), 55, 60 degree division is functional
ZG is commonly called tube cone, that is, the thread is made of a conical surface. The general water pipe joints are like this. The old national standard is labeled as Rc.
The metric thread is expressed by the pitch of the thread. The US and British thread is expressed by the number of threads per inch. This is the biggest difference. The metric thread is a 60-degree equilateral tooth type, and the inch thread is an isosceles 55-degree tooth type. 60 degrees. Metric threads are used in metric units, and American-English threads are used in imperial units.
        The pipe thread is mainly used for the connection of the pipe, and the inner and outer threads are closely matched, and there are two kinds of straight pipe and tapered pipe. The nominal diameter refers to the diameter of the pipe to which it is connected. It is obvious that the diameter of the thread is larger than the nominal diameter.

1/4, 1/2, 1/8 are the nominal diameter of the inch thread in inches.

1, inch uniform thread
Widely used in inch countries, this type of thread is divided into three series: coarse tooth series UNC, fine tooth series UNF, extra fine tooth series UNFF, plus a fixed pitch series UN.
Marking method: Thread diameter - number of teeth per inch series code - accuracy level


  • Coarse tooth series 3/8-16UNC-2A

  • Fine tooth series 3/8—24UNF—2A

  • Extra fine tooth series 3/8—32UNFF—2A

  • Fixed pitch series 3/8-20UN-2A

The first digit 3/8 indicates the outer diameter of the thread in inches. The conversion to the metric unit mm is multiplied by 25.4, ie 3/8×25.4=9.525mm; the second and third digits 16, 24, 32, 20 are The number of teeth per inch (the number of teeth in the length of 25.4mm); the character code UNC, UNF, UNFF, UN after the third digit is the serial number, and the last two digits 2A are the precision grades.
Conversion of 2, 55° cylindrical pipe threads
55° cylindrical pipe thread, which is derived from the inch series, is widely used in metric and inch countries. It is used to connect liquid pipe, gas and wire fittings to pipes. However, the codes of different countries should be pressed. The foreign code in the table (control table) is converted into the code name of our country. The 55° cylindrical pipe thread code of each country is listed in the following table.
Country code

  • China G

  • Japan G, PF


  • France G

  • Germany R (internal thread), K (external thread)

  • Former Soviet Union G, TPУБ

  • ISO Rp

3, 55 ° conical pipe thread conversion
The 55° conical pipe thread means that the thread has a profile angle of 55° and the thread has a taper of 1:16. This series of threads is widely used in the world, its code name, different national regulations, see the table below. According to the foreign code in the table below, it is converted into our country code.
Country code

  • China ZG, R (external thread)

  • UK BSPT, R (external thread), Rc (internal thread)

  • France G (external thread), R (external thread)

  • Germany R (external thread)

  • Japan PT, R

  • ISO R (external thread), Rc (internal thread)

4, 60 ° conical pipe thread conversion
60° conical pipe thread refers to pipe thread with a tooth angle of 60° and a taper of 1:16. This series of threads is used in China's machine tool industry and the United States and the former Soviet Union. Its code name, China's past regulations for K, and later for Z, is now changed to NPT. The thread code comparison table is shown in the table below.
Country code

  • China Z (old) NPT (new)

  • United States NPT

  • Soviet B

5, 55 ° trapezoidal thread conversion
The trapezoidal thread is a metric trapezoidal thread with a tooth angle of 30°. This series of threads is relatively uniform at home and abroad, and its code name is also quite consistent. See the thread code below.
Country code

  • China T (old) Tr (new)

  • ISO Tr

  • Germany Tr

  • Former Soviet Union Tr

(Third) the thread classification
According to the use of thread, it can be divided into:
1. International Metric Thread System
China's national standard CNS uses the thread. The crest is flat, easy to turn, and the bottom of the tooth is rounded to increase thread strength. The thread angle is 60 degrees and the specification is indicated by M. Metric threads can be divided into coarse and fine teeth. The representation is as M8x1.25. (M: code, 8: nominal diameter, 1.25: pitch).
2, the US standard thread (AmericanStandardThread)
Both the top and the root of the thread are flat and have good strength. The thread angle is also 60 degrees and the specification is expressed in a few teeth per inch. Such threads can be divided into coarse teeth (NC); fine teeth (NF); special fine teeth (NEF) three. The representation is 1/2-10NC. (1/2: outer diameter; 10: number of teeth per inch; NC code).
3, unified standard thread (UnifiedThread)
It is jointly developed by the United States, the United Kingdom, and Canada, and is the commonly used inch thread.
The thread angle is also 60 degrees and the specification is expressed in a few teeth per inch. Such threads can be divided into coarse teeth (UNC); fine teeth (UNF); extra fine teeth (UNEF). The representation is 1/2-10UNC. (1/2: outer diameter; 10: number of teeth per inch; UNC code)
4, V-shaped thread (Sharp VThread)
The top and the root are pointed, the strength is weak, and it is not often used. The thread angle is 60 degrees.
5, Hui style thread (WhitworthThread)
Threads used in British national standards. The thread angle is 55 degrees, indicating that the symbol is "W". Suitable for rolling method manufacturing. The representation is as W1/2-10. (1/2: outer diameter; 10: number of teeth per inch; W code).
6, round thread (KnuckleThread)
Standard thread for German DIN. Suitable for connection of bulbs and rubber hoses. The symbol is "Rd".
7, pipe thread (PipeThread)
To prevent leakage of threads, it is often used for gas or liquid pipe joints. The thread angle is 55 degrees, which can be divided into straight pipe thread code "P.S., N.P.S." and inclined pipe thread code "N.P.T.", with a taper of 1:16, which is 3/4 inch per foot.
8, square thread (SquareThread)
The transmission efficiency is large, second only to the ball thread, and it cannot be adjusted with the nut after wear, which is a disadvantage. Generally used for the screw of the vise and the screw of the crane.
9, trapezoidal thread (TrapezoidalThread)
Also known as the Aikem thread. The transmission efficiency is slightly smaller than the square thread, but it can be adjusted with a nut after wear. The metric thread angle is 30 degrees and the inch thread angle is 29 degrees. It is generally used as a lead screw for lathes. The symbol is "Tr".
10, zigzag thread (ButtressThread)
Also known as the oblique thread, it is only suitable for single direction transmission. Such as screw jacks, presses, etc. The symbol is "Bu".
11, ball thread
It is the most efficient thread for transmission, and it is difficult to manufacture and costly. It is used in precision machinery. Such as the lead screw of CNC machine tools.
Representation of inch bolts:
LH 2N 5/8 × 3 - 13UNC-2A
(1) LH is a left thread (RH is a right thread and can be omitted).
(2) 2N double thread.
(3) 5/8 inch thread, outer diameter 5/8”.
(4) 3 bolt length 3".
(5) 13 Threads 13 teeth per inch.
(6) UNC unified standard thread coarse teeth.
(7) 2nd level fit, external thread (3: tight fit; 2: medium fit; 1: loose fit) A: external thread (may be omitted) B: internal thread
Inch thread:
The size of the inch thread is usually expressed in terms of the number of threads per inch of thread, referred to as "number of teeth per inch", which is exactly the inverse of the pitch. For example, a thread of 8 teeth per inch has a pitch of 1/8 inch.