What Are The Meanings Of RC1/8, G3/4 And NPT1/4? What Is The ...

What are the meanings of RC1/8, G3/4 and NPT1/4? What is the difference between BSPT, RC, G and other threads?

2021-07-23

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Yesterday, I did a simple push z-axis, which is used to push a suction device. This device, after pumping out the waste gas produced by plasma cleaning, rises to the highest point to avoid the higher material on the conveyor belt below it. The driving stroke is 150 mm, because there is no precision requirement, ± 1 mm is OK. Therefore, the shaft is driven by double acting cylinder and guided by ordinary single slide linear guide (Use fork sensor to detect whether it has been pushed to the highest position). Since the cylinder is used, naturally, a simple gas path is involved. Because we want to keep the suction device at that position for a certain period of time in case of power failure at the highest point, we need a three position five way middle seal solenoid valve ( Plus double side speed control valve (speed control). Search the system and soon the solenoid valve will be selected. At first glance, the gas connection is RC1/8. I can’t help thinking of the types and parameters of various gas connections.

For example, in the supplier’s product catalog, there are often NPT1/8, RC3/4, G1/4 and other interfaces.

Table of Contents hide What do these pipe thread interfaces NPT, RC, G, etc. mean? What’s the difference between NPT, BSPP and BSPT? BSPP (British Standard Parallel Pipe) BSPT (British standard pipe tape) How to choose the thread? What is the global application of the two pipe thread systems?

What do these pipe thread interfaces NPT, RC, G, etc. mean? What’s the difference between NPT, BSPP and BSPT?

Speaking of these problems, we have to talk about Wyeth thread. As we know, the profile angle of ordinary connecting thread is 60 degrees. This Wyeth thread is different from ordinary thread in that it has a 55 degree profile angle. Why is it 55 degrees? Originally, there is an interesting thing. Joseph Whitworth, the inventor of Wyeth thread, established his own machine tool factories along with the wave of British Industrial Revolution. These different factories, because there is no unified thread standard, manufacture thread according to their own experience, resulting in different sizes, which leads to the interchangeability of thread parts become a problem. For example, the screw hole at the same position of the same part may not be locked after changing a screw. As the scale of production continues to expand, Whitworth is aware of this serious problem. Subsequently, he collected thread samples from different factories, measured size and shape and other data. Based on this, he proposed a unified standard for thread processing, which is the origin of Wyeth thread BSW (British Standard Whitworth) and the first thread standard in the world.

In this standard, the thread profile angle is 55 degrees (BSF = British Standard Whitworth Thread fine). Although Wyeth thread is not common in nuts and screws nowadays, it has been replaced by unified series and metric series. However, BSPP (British standard pipe parallel) and BSPT (British standard pipe tape) of 1:16 are developed on this basis, and become international standards (ISO), which have been used up to now.

BSPP (British Standard Parallel Pipe)

Online some literal translation into parallel pipe thread, bloggers feel a bit inappropriate. Another pipe thread corresponding to it is called “conical pipe thread”, which is not only a conventional name, but also a standard name in textbooks. Translating British standard pipe parallel into “parallel pipe thread” may make people feel a little confused and unprofessional. For historical reasons, BSPP also has other different names. Such as BSPF (British standard pipe fitting), BSPM (British standard pipe mechanical) and PS (British standard pipe straight), etc. However, with the development of ISO standardization, all of them were replaced by a simple and clear letter. This is the G-thread. Therefore:

• G = BSPP = BSPF = BSPM = PS = ……

The letter G is the abbreviation of gas (it comes from German Gewinde, meaning “screw thread”, bloggers prefer the former), because it was used in gas pipeline at first. It is a coincidence that we call G pipe thread because the initials of G and “Guan” are the same. G thread is divided into internal and external threads, which are used for non sealing connection. The executive standard is BS EN ISO 228-1, and the old standard is BS 2779, which is now invalid. In addition, the meanings of several common versions of relevant standards are as follows:

• ISO XXX = International Standard = International Standard;
• EN-ISO-xxx = European version of the international standard;
• BS-EN-ISO-xxx = British version of the European version of the international standard.

Common BSPP/BSPF thread size table
Sign	Size	Type	Major diameter mm d=D	Pitch mm p	Number of teeth TPI	Pitch diameter mm d2=D2	External thread Narrow path d3	Tooth height mm H1	Bottom hole mm
G1/8	1/8″	BSPP	9.728	0.907	28	9.147	8.566	0.581	8.70
G1/4	1/4″	BSPP	13.157	1.337	19	12.301	11.445	0.856	11.60
G3/8	3/8″	BSPP	16.662	1.337	19	15.806	14.950	0.856	15.00
G1/2	1/2″	BSPP	20.955	1.814	14	19.793	18.631	1.162	19.00
G5/8	5/8″	BSPP	22.911	1.814	14	21.749	20.587	1.162	20.75
G3/4	3/4″	BSPP	26.441	1.814	14	25.279	24.117	1.162	24.50
G7/8	7/8″	BSPP	30.201	1.814	14	29.039	27.877	1.162	28.00
G1	1″	BSPP	33.249	2.309	11	31.770	30.291	1.479	30.50
G1-1/8	1-1/8″	BSPP	37.897	2.309	11	36.418	34.939	1.479	35.00
G1-1/4	1-1/4″	BSPP	41.910	2.309	11	40.431	38.952	1.479	39.50
G1-3/8	1-3/8″	BSPP	44.323	2.309	11	42.844	41.365	1.479	41.50
G1-1/2	1-1/2″	BSPP	47.803	2.309	11	46.324	44.845	1.479	45.00
G1-3/4	1-3/4″	BSPP	53.746	2.309	11	52.267	50.788	1.479	51.00
G2	2″	BSPP	59.614	2.309	11	58.135	56.656	1.479	57.00
G2-1/4	2-1/4″	BSPP	65.71	2.309	11	64.231	62.752	1.479	63.00
G2-1/2	2-1/2″	BSPP	75.184	2.309	11	73.705	72.226	1.479	72.50
G2-3/4	2-3/4″	BSPP	81.534	2.309	11	80.055	78.576	1.479	79.00
3	3″	BSPP	87.884	2.309	11	86.405	84.926	1.479	85.50
G3-1/4	3-1/4″	BSPP	93.98	2.309	11	92.501	91.022	1.479	91.00
G3-1/2	3-1/2″	BSPP	100.33	2.309	11	98.351	97.372	1.479	97.75
G3-3/4	3-3/4″	BSPP	106.68	2.309	11	105.201	103.722	1.479	104.00
G4	4″	BSPP	113.03	2.309	11	111.55	110.072	1.479	110.50
G4-1/2	4-1/2″	BSPP	125.73	2.309	11	124.251	122.772	1.479	123.00
G5	5″	BSPP	138.43	2.309	11	136.951	135.472	1.479	136.00
G5-1/2	5-1/2″	BSPP	151.13	2.309	11	149.651	148.172	1.479	148.50
G6	6″	BSPP	163.83	2.309	11	162.351	160.872	1.479	161.50

What’s more interesting is that in Wyeth thread, it is also stipulated that the top and bottom of the tooth should be made into arc.

Why? This is actually related to Whitt’s keen market insight and technology accumulation. Because in the beginning, the purpose of building huite factory was to sell machine tools, that is, “hens”, rather than selling the parts processed by machine tools. It may be that he is optimistic about the prosperous industrial revolution driven market. Therefore, when he submitted the thread standard to the British Industrial Standards Association, he implanted the arc. It can be said that he was very wise and made his own thread become the industry standard. Of course, at that time, only his family could make this kind of circular tooth top and bottom, which was a good technical barrier. This is inseparable from his industry accumulation of more than 10 years. Even now, if we want to set up standards in an industry, it is not easy to have strong core competitiveness (not necessarily in terms of technology, but in terms of positioning, brand, etc.), because it is easy to be imitated, copied, and lost in the competition. From the point of view of use, the arc crown and the tooth base have its advantages. It can reduce the wear of the thread appliance on the crown, and its fatigue strength is better than that of the flat bottom thread. Because of the excessive arc, the stress distribution is optimized. Later, in order to reduce the difficulty of thread processing, the U.S. changed the profile angle to 60 degrees, and the top and bottom of the tooth to be flat according to this standard. Only then did the U.S. standard pipe threads NPT and npsc: National pipe tape thread and national pipe straight coupling thread come into being. So now, as far as pipe thread is concerned, two pipe thread standard systems with ISO standard as one side and American Standard as the other side have been formed all over the world.

BSPT (British standard pipe tape)

BSPT – inch tapered pipe thread, also known as Pt thread, is only used for general sealing connection. The old standard of BSPT is BS 21, but in the New Standard BS EN 10226-1, R thread is used instead. Therefore:

• R = BSPT = PT = ……

Having said the two systems of pipe thread, it is easy to understand the classification and difference of various pipe threads, because R series and G thread belong to English pipe thread.

• RP: Inch sealing straight pipe thread;
• RC: Imperial sealed internal cone pipe thread;
• R: British sealing external cone pipe thread, matching with RC, RP, G;
• R1: British sealed conical external thread, matching with British sealed straight internal pipe thread, RP/R1;
• R2: English sealing cone external thread, matching with English sealing cone internal thread RC/R2;
• G: Cylindrical / straight pipe thread.

The design profile of sealing cylindrical pipe thread (RP) is completely consistent with that of non sealing pipe thread g, and the difference lies in the thread precision. The taper pipe thread (R) for sealing is converted into technology based on the taper of 1:16 on the basis of cylindrical pipe thread. The design teeth are as follows:

Common BSPT/R thread size table
Size	Number of teeth TPI	pitch of screw mm P	Tooth height mm h	Datum diameter mm D=d	Pitch diameter mm D2=d2	Narrow path mm D1=d1	Reference distance mm	Limit deviation T1/2 mm	Effective thread length mm	Assembly allowance mm	Limit deviation of internal thread diameterT1/2 mm
1/16	28	0,907	0,581	7,723	7,142	6,561	4	+ /- 0,9	6,5	2,5	+ /-0,071
1/8	28	0,907	0,581	9,728	9,147	8,566	4	+ /- 0,9	6,5	2,5	+ /-0,071
1/4	19	1,337	0,856	13,157	12,301	11,445	6	+ /- 1,3	9,7	3,7	+ /-0,104
3/8	19	1,337	0,856	16,662	15,806	14,95	6,4	+ /- 1,3	10,1	3,7	+ /-0,104
1/2	14	1,814	1,162	20,955	19,793	18,631	8,2	+ /- 1,8	13,2	5,0	+ /-0,104
3/4	14	1,814	1,162	26,411	25,279	24,117	9,5	+ /- 1,8	14,5	5,0	+ /-0,104
1	11	2,309	1,479	33,249	31,77	30,291	10,4	+ /- 2,3	16,8	6,4	+ /-0,180
1 1/4	11	2,309	1,479	41,91	40,431	38,952	12,7	+ /- 2,3	19,1	6,4	+ /-0,180
1 1/2	11	2,309	1,479	47,803	46,324	44,845	12,7	+ /- 2,3	19,1	6,4	+ /-0,180
2	11	2,309	1,479	59,614	58,135	56,656	15,9	+ /- 2,3	23,4	7,5	+ /-0,180
2 1/2	11	2,309	1,479	75,184	73,705	72,226	17,5	+ /- 3,5	26,7	9,2	+ /-0,216
3	11	2,309	1,479	87,884	86,405	84,926	20,6	+ /- 3,5	29,8	9,2	+ /-0,216
4	11	2,309	1,479	113,03	111,551	110,072	25,4	+ /- 3,5	35,8	10,4	+ /-0,216
5	11	2,309	1,479	138,43	136,951	135,472	28,6	+ /- 3,5	40,1	11,5	+ /-0,216
6	11	2,309	1,479	163,83	162,351	160,872	28,6	+ /- 3,5	40,1	11,5	+ /-0,216

It is worth mentioning that many friends think that BSP thread refers to G thread and can be used in common. We have different views on this view. The sealing effect of general sealing cylindrical pipe thread (RP) is realized by adding sealing filler (e.g. PTFE sealing belt) to the whole thread when necessary, while the sealing effect of non sealing cylindrical pipe thread (g) is realized by using sealing equipment (e.g. O-ring) at both ends of the thread connection or around it. In addition, there are also differences in the usage and design tolerance between general sealing pipe thread and non sealing pipe thread. For details, please refer to the standard manual of metric, American and English screw threads. In fact, when machining BSP threads, many machining enterprises usually encounter that the thread ring plug gauge is qualified, but the combination is poor sealing. The main reason is that the RP internal thread for sealing is confused with the G internal thread for non sealing. Some thread codes and standards related to BSP thread:

• G – inch cylindrical pipe thread (standard: BS 2779, BS EN ISO 228-1, ISO 7-1);
• BSPP – Inch cylindrical pipe thread (standard: BS 2779, BS EN ISO 228-1, ISO 7-1);
• BSPT – Inch tapered pipe thread (standard: BS 21, BS EN 10226-1, ISO 7-1);
• R (RP, RS, RC, R1, R2) – Inch tapered pipe thread (standard: BS 21, BS EN 10226-1, ISO 7-1, DIN 2999);
• BSP thread classification diagram.

Use a diagram to show the relationship of inch pipe thread, as follows (from tap notes).

Above we understand the basic classification of pipe thread.

How to choose the thread?

Here also need to understand the matching way and sealing way of pipe thread. The first is the matching method: the taper external pipe thread can be matched with the taper internal pipe thread and the straight internal pipe thread. As shown in the figure above, it is written as RC/R2 and RP/R1. The left represents internal thread and the right represents external thread. However, straight external pipe thread cannot be matched with tapered internal pipe thread. In addition, because NPT thread and BSPT thread have different thread profile angle, tooth top, tooth bottom, and thread pitch, they are generally incompatible (except for some 1/2 and 3/4 threads, because their thread pitch is the same). For example, NPT male cannot be used to fit BSP female.

Intuitively, use the following figure to show the matching method.

For sealing: cylindrical pipe threads are generally sealed with O-ring or gasket with sealing ring, or with raw material belt, sealant, etc. The sealing of taper pipe thread depends on the deformation of taper thread, forming extrusion force to seal. But there is always the possibility of air leakage. Therefore, for places with strict requirements, it will be sealed with raw tape or sealant. Intuitively, the sealing method can be shown in the following figure:

Therefore, when we choose the connector, we must first see clearly the thread system, BSP with BSP, NPT with NPT, m with m, UNF with UNF, etc. Then it should be noted that straight pipe external thread can not be matched with tapered pipe internal thread, for example, G1/4 matched with RC1/4 is wrong, and only tapered external pipe thread can be matched with straight pipe internal thread, for example, RP/R1 or RC/R2. Finally, for sealing, straight pipe thread must be sealed, while taper pipe thread can be sealed by itself. For applications with high sealing requirements, sealant or raw tape should be added. However, suppliers usually bring their own sealing methods when they leave the factory.

What is the global application of the two pipe thread systems?

BSPP and BSPT are Wyeth pipe threads, which are mostly used in Europe, Commonwealth countries, Australia, New Zealand, etc. In Asia, such as China, South Korea, Japan and so on, BSPT thread is used (Japanese industry standard is called PT). Many solenoid valve products we use, and the gas interface belongs to this system. The United States, Canada and other regions are using NPT thread. Here’s a few more words about NPT threads. NPT thread does not need or cannot be sealed with O-ring, because it is a conical thread, which can only be screwed in, and it can be sealed usually by thread deformation. If you want to ensure the sealing, you can use PTFE tape or loctite to seal. It should be pointed out that NPTF = National pipe tape fuel refers to American dry sealing tapered pipe thread, including internal thread and external thread. It differs slightly from NPT only in the apex and the base of the tooth, and the rest is the same. It is easy to mistake NPTF for national pipe thread female. Finally, a table is used to represent the commonly used British and American threads, and another table is used to compare the commonly used threads, as follows.

Source:China Thread Fittings Manufacturer– Yaang Pipe Industry (www.steeljrv.com)