Question: What is the difference between NPT threads and UN threads?

This post was last edited by Chinese cabbage 5 on 2010-7-3 at 10:42. Unified threads in the inch system are widely used in this measurement system; such threads are divided into three series: the coarse-thread series UNC, the fine-thread series UNF, and the ultra-fine-thread series UNFF, in addition to a constant-pitch series called UN. Notation method: Thread diameter—TPI Series code—Precision 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 outer diameter of the thread in inches; to convert it to millimeters, multiply by 25.4, i.e., 3/8×25.4=9.525mm ； The second and third digits, 16, 24, 32, 20, represent the number of teeth per inch (the number of teeth over a length of 25.4 mm) ； The code letters after the third one, UNC, UNF, UNFF, and UN, are series codes, while the last two digits, 2A, indicate the precision grade. Take a look at what the old moderator summarized: The poster should be good at searching. This post was last edited by flute6 on 2009-12-31 at 10:57. Pipe threads have a 55-degree angle; they are used primarily for connecting pipes, ensuring a tight fit between their internal and external threads. There are two types: straight pipes and tapered pipes. Common pipe threads mainly include the following types: NPT, PT, G, etc. 1) NPT is the abbreviation for National (American) Pipe Thread; it refers to the 60-degree tapered pipe thread standard used in North America. For the national standard, refer to GB/T12716-1991. 2) PT (BSPT) is an abbreviation for Pipe Thread; it refers to a 55-degree sealed tapered pipe thread that belongs to the Whitworth thread family. It is widely used in Europe and the Commonwealth**, especially in the water and gas piping industries. The taper ratio is 1:16, and the relevant national standard can be found in GB/T7306-2000. It is called ZG in China. 3) G is a 55-degree unthreaded sealing pipe thread, belonging to the Whitworth thread family. Marked as G represents cylindrical threads. Refer to GB/T7307-2001 for national standards. Difference between metric threads and imperial threads: Metric threads are expressed in terms of pitch, while imperial threads are expressed in terms of the number of threads per inch ； Metric threads have an equilateral profile at 60 degrees, British threads have an isosceles profile at 55 degrees, while American threads have an isosceles profile at 60 degrees ； Metric threads use metric units (such as mm), while imperial and British threads use imperial units (such as inches) ； “Those in the industry usually refer to thread sizes in \"points\"; 1 inch equals 8 points, 1/4 inch equals 2 points, and so on. Additionally: ISO—metric thread standard 60 degrees ； UN—Unified thread standard 60 degrees ； API—American Petroleum Pipe Thread Standard 60 degrees ； W—British Whipple thread standard 55 degrees ； NPT BSP thread technology – Differences between NPT, PT, and G threads. NPT stands for National (American) Pipe Thread; it is a 60-degree tapered pipe thread that meets American standards and is used in North America. **The relevant standard can be found in GB/T12716-1991.** PT is an abbreviation for Pipe Thread; it is a 55-degree sealed tapered pipe thread that belongs to the Whitworth thread family and is commonly used in Europe and the Commonwealth countries. It is often utilized in the water and gas piping industries, with a taper ratio of 1:16. **The relevant standard can be found in GB/T7306-2000.** G refers to a 55-degree non-threaded sealed pipe thread that also belongs to the Whitworth thread family; the designation “G” indicates a cylindrical thread. **The relevant standard can be found in GB/T7307-2001.** Additionally, the designations 1/4, 1/2, and 1/8 refer to the diameter of the thread, measured in inches. Those in the industry usually refer to thread sizes in fractions, where 1 inch equals 8 fractions; thus, 1/4 inch is equivalent to 2 fractions, and so on. “G” is simply a general term for such threads; the distinctions between 55-degree and 60-degree threads are based on functional considerations, and these threads are commonly known as round pipes. That is, the thread is formed by machining a cylindrical surface. ZG is commonly known as pipe taper, meaning that the threads are formed by a conical surface; most water pipe fittings use this type of thread. According to national standards, metric threads are denoted by pitch, while imperial and British threads are indicated by the number of threads per inch – this is their main difference. Metric threads have an equilateral tooth profile at 60 degrees, British threads have an isosceles tooth profile at 55 degrees, and American threads also have a 60-degree tooth profile. Metric threads use metric units, while imperial and American threads use imperial units. Pipe threads are primarily used for connecting pipes; their internal and external threads fit tightly together, and there are two types: straight pipes and tapered pipes. The nominal diameter refers to the diameter of the pipe being connected; obviously, the thread diameter is larger than the nominal diameter. 1/4, 1/2, 1/8 are the nominal diameters of imperial threads, measured in inches. British pipe threads originated from British Whitworth threads; the series of diameters associated with Whitworth threads, together with the thread profiles of Whitworth threads, determined the basic dimensions of British pipe threads. Based on a 1/16 taper ratio, the radial diameter tolerances of Whitworth threads were converted into axial pitch tolerances for British sealed pipe threads (with some rounding and adjustments made). By referring to the tolerances of British sealed pipe threads, tolerances for British unsealed pipe threads were established (the tolerance distribution changed from bilateral to unilateral, with looser tolerances for the major diameter and less strict tolerances for the minor diameter). The three types of threads were developed as follows: British Whitworth threads were introduced in 1841, and new standards for Whitworth threads were issued in 1905 (BS 84). In the same year, standards for British sealed pipe threads were established (BS 21). From 1905 to 1940, Whitworth threads served as the standard for British unsealed pipe threads. In 1940, a series of unsealed pipe threads based on Whitworth threads was introduced (the BSP series) ； In 1956, a standard for British-style unsealed pipe threads (BS 2779) was issued separately. Europe and the Commonwealth were the first to adopt these British-style pipe thread standards. The ISO/TC5/SC5 Technical Committee for pipe thread standardization and its secretariat are under the control of Europe, and the British-style pipe thread standards were incorporated into ISO standards. In 1955, ISO introduced a standard for British-style sealed pipe threads (ISO R 7) ； In 1961, ISO introduced a standard for imperial unscrewed pipe threads (ISO R 228). In 1978, ISO issued official standards for two types of imperial pipe threads (ISO 7-1 and ISO 228-1). Today, imperial pipe threads are widely accepted outside North America and are used extensively in international trade. The imperial pipe thread standards defined by ISO have been converted to the metric system. Converting imperial pipe thread sizes to metric is very simple: multiply the original inch dimensions by 25.4 to obtain the millimeter values. The use of imperial pipe thread sizes is gradually being phased out; it is unrealistic to seek true pipe thread standards. There is no distinction between genuine metric pipe threads and fake metric pipe threads. Imperial screwed pipe threads come in two types of fits: “straight/tapered” and “tapered/tapered.” These two types use different thread gauges (cylindrical thread gauges and tapered thread gauges) as well as thread plugs (with their reference planes positioned at different distances, half a thread pitch apart). European countries mainly use the “straight/tapered” type of fit for pipe threads ； Outside of Europe, “taper/taper” thread fits are primarily used. For the same type of sealed pipe thread fitting, those that pass the inspection standards in Europe may fail to meet the requirements in regions outside of Europe. It is essential to be aware of this difference in international trade; otherwise, defective products may result. Before 1994, ISO’s standards for British-style sealed pipe threads and their related gauges were designed based on the “taper/taper” fit system. Chinese products using British-style sealed pipe threads could enter the international market without any issues, while European products of this type were at a disadvantage. After 2000, ISO’s standards for British-style sealed pipe threads and their gauges were redesigned using the “straight/taper” fit system. As a result, Chinese products using the original British-style sealed pipe threads faced difficulties entering the international market. To address this issue, China revised its standards for British-style sealed pipe threads in 2000, changing one standard into two, thereby reminding designers to pay attention to the differences between the two types of thread fits and to choose the appropriate one. When Japan revised its standards for British-style sealed pipe threads in 1999, it continued to use the ISO standards from before 1994. As a result, the international market for British-style sealed pipe threads became even more complex after 2000, requiring domestic manufacturers to be particularly careful. British-style sealed pipe threads are intended for general-purpose applications, and sealing materials must be added to the thread pair during use. Their advantage lies in their cost-effectiveness, with moderate requirements regarding processing precision. Threads that do not use sealing materials can provide a dry seal, but such dry-sealed threads do not exist within the British-style thread system. Sealed pipe threads serve both mechanical connection and sealing functions ； In contrast, unsealed pipe threads have only a mechanical connection function. Therefore, the precision required for sealed pipe threads is higher than that for unsealed pipe threads. Some people, seeing that the tolerance on the pitch diameter of unsealed pipe threads is half that of sealed pipe threads, assume that the precision of unsealed pipe threads is higher; this view is incorrect. Sealed pipe threads require precise tooth profiles, and the tolerances for their outer diameter, pitch diameter, and inner diameter are all the same ； The errors in the tooth side angle and pitch have a significant impact on the sealing performance. In contrast, non-sealing pipe threads generally have no requirements regarding tooth profile accuracy. The tolerance for the major diameter is greater than that for the minor diameter ； There are no tolerance requirements for its diameter. ； Additionally, some people believe that a combination of non-sealed cylindrical internal threads and sealed cylindrical external threads can be used for mating purposes. This view is also incorrect; doing so means relaxing the precision requirements for the sealed internal threads, which can lead to problems with sealing. Due to the different applications of sealed pipe threads, as well as variations in machining precision, assembly, and testing techniques, current pipe thread standards cannot guarantee that all threads that meet the standard specifications will provide proper sealing. The British standards for sealed pipe threads do not specify uniform precision requirements for individual thread parameters, and these parameters have a direct impact on sealing performance. The fundamental solution to this problem is for each industry or company to establish its own internal control measures tailored to its specific products. These internal control criteria are usually kept confidential and are not applicable to companies in other industries; manufacturers must be aware of this. Sealed pipe thread standards are not universal, and manufacturers need to pay close attention to sealing issues on their own. Before 1987, China did not have standards for American or British pipe threads. Yet, it was impossible to avoid using these two internationally common types of pipe threads in manufacturing. As a result, the old mechanical drawing standards specified their own codes for American and British pipe threads, based on Chinese pinyin letters, without considering whether they matched the codes used in foreign standards. Since these standards only defined thread codes without specifying thread parameters, the same code could represent different parameter values in different companies or industries. This made it difficult to determine who was at fault when defective products were produced. From 1987 to 1991, China introduced standards for British pipe threads. Since then, thread codes and markings must comply with these standards; the thread codes specified in the old mechanical drawing standards should be abolished immediately. 1. Sealed Pipe Threads (R): Basic dimensions and tolerances of British sealed pipe threads. Mating methods: There are two mating methods for British sealed pipe threads: a “cylindrical/conical” mating using cylindrical internal threads and conical external threads ； Tapered internal threads and tapered external threads form a “taper/taper” fit. In Europe, “shank/taper” fit threads are primarily used ； Outside of Europe, **“posts/cones” in combination with threads are primarily used; the inspection specifications for these two types of threads differ to some extent** ； The current ISO standard for sealed pipe threads in imperial units (ISO 7-2:2000) is based on a \"cylindrical/conical\" fit system. The complete designation for sealed pipe threads in imperial units consists of a thread characteristic code, a thread size code, and a rotation direction code. The characteristic code for sealed cylindrical internal threads in imperial units is: RP ； The characteristic code for imperial sealed tapered internal threads is: RC ； The characteristic code for British standard sealed tapered external threads is: R1 (used in combination with British standard sealed cylindrical internal threads) ； R2 (used in conjunction with British standard sealed tapered internal threads) ； The rotation direction code for left-handed threads is LH ； The direction code for right-hand threads is omitted. For sealed pipe threads, RP/R1 and RC/R2 are used to denote \"cylindrical/tapered\" and \"tapered/tapered\" thread pairs, respectively. 2. Unsealed pipe threads: The basic dimensions and allowable tolerances for British standard unsealed pipe threads – Marking: The complete marking for British standard unsealed pipe threads consists of the thread characteristic code, the thread size code, the pitch diameter tolerance class code, and the direction code. The characteristic code for British standard unsealed cylindrical threads is G; for British standard unsealed internal cylindrical threads, the pitch diameter tolerance class code is omitted ； The metric grade codes for the pitch diameter of British standard unsealed external cylindrical threads are A and B, respectively. The direction code for left-hand threads is LH ； The direction code for right-hand threads is omitted. When indicating a set of threaded connections for imperial non-sealed pipe threads, only the code for the external thread is specified. Examples: Right-hand, non-sealed circular internal thread with size code 2: G2; Right-hand, non-sealed circular external thread of grade A with size code 3: G3A; Left-hand, non-sealed circular external thread of grade B with size code 4: G4B-LH; A set of threaded connections consisting of a right-hand, non-sealed circular internal thread with size code 2 and a circular external thread of grade A: G2A. Threads in internal holes are formed using **tap screws, while external threads are formed using dies. Others: Conversion of 55° tapered pipe threads. 55° tapered pipe threads refer to threads with a thread pitch angle of 55° and a taper ratio of 1:16. This series of threads is widely used around the world, and its designation varies depending on the country. China, United Kingdom, France, Japan; ISO codes: ZG, R (external), G, PT, RR (external threads); Rc (internal); RR, Rc. Conversion of 60° tapered pipe threads: 60° tapered pipe threads refer to pipe threads with a thread pitch angle of 60° and a thread taper of 1:16. This series of threads is used in China’s machine tool industry as well as in the United States and the former Soviet Union. Its code was designated as K in our country in the past, then as Z, and is now changed to NPT.