The meanings and types of thread designations such as NPT, PT, PF, G, and ZG

NPT (American Standard Tapered Thread)

PT (Japanese Old Standard Tapered Thread) is equivalent to ISO R, Rc.

DIN 2999 is a European, primarily German, pipe thread.

NPT

NPT(American Standard Tapered Pipe Thread) is a general-purpose American standard tapered pipe thread.

N stands for National (American), P for Pipe, and T for Taper, with a 60° thread angle.

This type of pipe thread is commonly used in North America and frequently seen in ANSI specifications. National standard GB/T12716-1991 can be found here.

Note: NPT threads must be used when using ANSI specifications!

PT

PT(British Standard Tapered Thread) is an imperial tapered thread with a 55° thread angle, most commonly used for sealing. It is widely used in Europe and Commonwealth countries. In Taiwan and Japan, JIS specifications also follow imperial standards, and China also uses imperial threads. It belongs to the Whitworth thread family. National standard GB/T7306-2000 can be found here. British Standard pipe threads are fine-pitch threads because coarse threads have a greater thread depth, which would significantly reduce the strength of the pipe with the cut thread outer diameter.

Furthermore, in practical applications with small-diameter pipelines, nipples are often used. The purpose is that purchased nipples have relatively thick walls, ensuring the strength of the tapped portion, while the main pipeline does not require a thick wall, thus saving costs.

PF threads are parallel pipe threads.

Note: PT threads, etc., must be used in JIS specifications!

G

G is a 55-degree non-sealing pipe thread, belonging to the Whitworth thread family. The marking G represents a cylindrical thread. The national standard can be found in GB/T7307-2001. G is a general term for pipe threads (Guan). The 55 and 60 degree classifications are functional, commonly referred to as pipe round. That is, the thread is machined from a cylindrical surface.

ZG

ZG (Zero-Gap) is commonly known as a pipe tapered thread, meaning the thread is machined from a conical surface. This is common in water pipe fittings. The old national standard designated it as Rc. Metric threads are expressed using pitch, while British and American standard threads are expressed using the number of threads per inch. This is their biggest difference. Metric threads have a 60-degree equilateral profile, while British threads have a 55-degree isosceles profile, and American threads have a 60-degree profile.

Metric threads use metric units, while British and American standard threads use imperial units.

Pipe threads are mainly used for pipe connections. Their internal and external threads fit tightly, and they come in both straight and tapered pipe types. The nominal diameter refers to the diameter of the connected pipe; obviously, the major diameter of the thread is larger than the nominal diameter. 1/4, 1/2, and 1/8 are the nominal diameters of British standard threads, in inches.

Identification of Unified Inch Threads

Unified inch threads are widely used in countries using the inch system. These threads are divided into three series: coarse thread UNC, fine thread UNF, and extra-fine thread UNFF, plus a fixed pitch series UN.

Marking Method: Thread Diameter — Threads per Inch Series Code — Accuracy Grade

Example: Coarse Thread Series 3/8—16 UNC—2A

Fine Thread Series 3/8—24 UNF—2A

Extra Fine Thread Series 3/8—32 UNFF—2A

Constant Pitch Series 3/8—20 UN—2A

The first digit, 3/8, represents the thread outer diameter in inches. To convert to metric units (mm), multiply by 25.4, i.e., 3/8 × 25.4 = 9.525 mm. The second and third digits, 16, 24, 32, and 20, represent the threads per inch (number of threads over a 25.4 mm length). The subsequent digits, UNC, UNF, UNFF, and UN, are the series code. The last two digits, 2A, indicate the accuracy grade.

Conversion of 55° Cylindrical Pipe Threads

55° cylindrical pipe threads originate from the inch system, but are widely used in both metric and inch-based countries. They are used for connecting pipe fittings to pipes for transporting liquids, gases, and installing electrical wires. However, the designations differ from country to country. The foreign designations in the table below should be converted to the Chinese designations. The designations for 55° cylindrical pipe threads from various countries are listed in the table below.

Country Designation

China G

Japan G, PF

UK BSP, BSPP

France G

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

Former USSR G, TPУБ

ISO Rp

Conversion of 55° Tapered Pipe Threads

55° tapered pipe threads refer to threads with a thread angle of 55° and a taper of 1:16. This series of threads is widely used worldwide, and its designations vary from country to country, as shown in the table below. The foreign designations in the table below should be converted to the Chinese designations.

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)

Conversion of 60° Tapered Pipe Threads

60° tapered pipe threads refer to pipe threads with a tooth angle of 60° and a thread taper of 1:16. This series of threads is used in my country's machine tool industry and in the United States and the former Soviet Union. Its code was previously K in my country, then Z, and now NPT. See the table below for thread code comparison.

Country Code China Z (old) NPT (new) USA NPT USSR B

Conversion of 55° Trapezoidal Threads

Trapezoidal threads refer to metric trapezoidal threads with a tooth angle of 30°. This series of threads is relatively uniform domestically and internationally, and its codes are also quite consistent. See the table below for thread codes.

Country Code

China: T (Old) Tr (New)

ISO: Tr

Germany: Tr

Former USSR: Tr

Thread Types

Depending on their application, threads can be classified as follows:

1. International Metric Thread System (IMS): The thread adopted by my country's national standard CNS. The crest is flat for easy machining, while the root 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 threads. The notation is as follows: M8x1.25 (M: code, 8: nominal diameter, 1.25: pitch).

2. American Standard Thread: Both the crest and root of the thread are flat, resulting in better strength. The thread angle is also 60 degrees, and the specification is indicated by the number of threads per inch. This type of thread can be divided into three grades: coarse (NC), fine (NF), and extra-fine (NEF). The notation is as follows: 1/2-10NC. (1/2: Outer diameter; 10: Threads per inch; NC designation).

3. Unified Thread: Jointly developed by the United States, the United Kingdom, and Canada, it is the most commonly used British standard thread.

The thread angle is also 60 degrees, and the specification is expressed in the number of threads per inch. This type of thread can be divided into coarse thread (UNC); fine thread (UNF); and extra fine thread (UNEF). The notation is 1/2-10UNC. (1/2: Outer diameter; 10: Threads per inch; UNC designation)

4. Sharp V Thread: Both the tip and root are pointed, resulting in weaker strength, and it is rarely used. The thread angle is 60 degrees.

5. Whitworth Thread: The thread adopted by the British National Standard. The thread angle is 55 degrees, and the symbol is "W".

Suitable for roll forming. The notation is W1/2-10. (1/2: outer diameter; 10: number of threads per inch; W designation).

6. Knuckle Thread: A standard thread defined by the German DIN standard. Suitable for connecting light bulbs and rubber hoses. The symbol is "Rd".

7. Pipe Thread: A thread used to prevent leaks, often used for connecting gas or liquid pipes. The thread angle is 55 degrees. It can be divided into straight pipe threads (designated "P.S., N.P.S.") and tapered pipe threads (designated "N.P.T."), with a taper of 1:16 (3/4 inch per foot).

8. Square Thread: High transmission efficiency, second only to ball screws, but its disadvantage is that it cannot be adjusted with a nut after wear. Generally used in vise screws and crane threads.

9. Trapezoidal Thread: Also known as Eckham thread. The transmission efficiency is slightly lower than that of square threads, but it can be adjusted with a nut after wear. The thread angle is 30 degrees for metric and 29 degrees for imperial. It is generally used in the lead screw of lathes. The symbol is "Tr".

10. Buttress Thread: Also known as a beveled thread, it is only suitable for unidirectional transmission. Such as screw jacks, presses, etc. The symbol is "Bu".

11. Ball Thread: It is the thread with the best transmission efficiency. It is difficult to manufacture and has a very high cost, so it is used in precision machinery. Such as the lead screw of CNC machine tools.

Imperial Bolt Representation:

LH 2N 5/8 × 3 - 13 UNC-2A

(1) LH is left-hand thread (RH is right-hand thread, which can be omitted).

(2) 2N double-start thread. (3) 5/8 Imperial thread, outer diameter 5/8”.

(4) 3 Bolt length 3”.

(5) 13 Thread, 13 threads per inch.

(6) UNC Standard thread, coarse thread.

(7) Class 2 fit, external thread (3: tight fit; 2: medium fit; 1: loose fit) A: External thread (can be omitted)

B: Internal thread

Imperial Thread: The size of an imperial thread is usually expressed by the number of threads per inch of length, simply called "threads per inch," which is exactly the reciprocal of the pitch. For example, a thread with 8 threads per inch has a pitch of 1/8 inch.