Abstract: In numerous fields such as machinery manufacturing, pipeline engineering, and more, different types of threads play a pivotal role. NPT, PT, G, TR, S, ZG, and RC threads are several common thread types, and they have significant differences in application scenarios, standard sources, and tooth pattern characteristics. This article elaborates on the definitions and characteristics of these threads, and helps readers clearly distinguish them through comparative analysis, providing reference for practical engineering applications.

Key words: thread type; NPT thread; PT thread; G thread; Application differences

1. Introduction

Threads, as indispensable elements in mechanical connections and transmissions, are widely used in various equipment and piping systems. Different types of threads have unique structural and performance characteristics, making them suitable for different working environments and requirements. NPT, PT, G, TR, S, ZG, and RC threads are common, and accurately understanding and distinguishing them is crucial for proper selection, ensuring connection quality, and system reliability.

2. Definition and characteristics of various types of threads

(1) NPT thread

• Definition: NPT is the abbreviation of National Pipe Taper, which means American standard cone pipe thread, which belongs to the 60-degree cone pipe thread.

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• Tooth angle: The tooth angle is 60 degrees, which is one of its distinctive features.

• Taper: Has a 1:16 taper, which is a 1-inch diameter change per foot of length. This taper design allows the thread to be sealed through its own elastic deformation when connected, and with tightening, the thread fit becomes tighter and tighter, so as to achieve a good sealing effect.

• Application scenarios: Mainly used in pipeline system connections in North America, such as oil, gas, water and other media transmission pipelines, and also in pipeline connections of some industrial equipment and instrumentation.

(2) PT thread

• Definition: PT is the abbreviation of Pipe Thread, which is an imperial tapered pipe thread with a tooth angle of 55 degrees and a taper of 1:16.

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• Tooth Angle: Unlike NPT threads, PT threads have a tooth angle of 55 degrees.

• Sealing method: It also relies on the sealing effect produced by the taper, but due to the difference in tooth angle, its sealing performance and applicable pressure range are different from NPT threads.

• Application scenarios: Commonly used in some countries in Europe and Asia, widely used in the pipeline connection of low-pressure fluids such as water and gas, and more common in some old industrial equipment and building water supply and drainage systems.

(3) G thread

• Definition: G stands for cylindrical pipe thread, which is a 55-degree non-sealed pipe thread.

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• Tooth angle: The tooth angle is 55 degrees.

• Sealing: It is mainly used for the connection and guidance of pipelines, and needs to be combined with sealing materials (such as raw material belts, sealing rings, etc.) to achieve sealing.

• Application scenarios: Widely used in the connection of various piping systems, such as building water supply and drainage, HVAC, fire protection, etc., for connecting pipes of different diameters or transitional connections between pipes and equipment.

(4) TR thread

• Definition: TR usually refers to a trapezoidal thread with an isosceles trapezoidal tooth shape.

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• Tooth angle: The tooth angle is generally 30 degrees (sometimes 29 degrees).

• Transmission efficiency: The tooth profile design of trapezoidal threads allows them to have high transmission efficiency and load-bearing capacity, and can withstand large axial forces and torques.

• Application scenarios: Mainly used in transmission occasions, such as the lead screw of machine tools, the spiral transmission of jacks, etc., to convert rotational motion into linear motion or transmit power.

(5) S thread

• Definition: S thread generally refers to the zigzag thread in ordinary threads, with a straight line on one side and a zigzag diagonal line on the other side.

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• Tooth profile characteristics: The tooth profile of zigzag threads is asymmetrical, which makes it have high load-bearing capacity and transmission efficiency when stressed in one direction.

• Application scenarios: It is often used in spiral transmission that bears unidirectional axial force, such as presses, jacks, and other equipment.

(6) ZG thread

• Definition: ZG is an old name for cone pipe thread, which is now generally expressed by RC, RP, etc., and it belongs to the 55-degree cone pipe thread.

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• Tooth angle: The tooth angle is 55 degrees.

• Historical applications: It is more common in some standards and engineering applications in the past, and with the development of standards, it has gradually been replaced by more explicit thread representation methods such as RC and RP.

(7) RC thread

• Definition: RC stands for Imbrak Sealed Cone Female Thread, which is a type of 55 degree sealed cone pipe thread.

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• Tooth angle: The tooth angle is 55 degrees.

• Sealing: When used with the corresponding tapered male thread, it can achieve a good sealing effect, suitable for sealing connections of medium and low pressure fluid pipelines.

• Application scenarios: Commonly used in pipeline systems in construction, chemical, machinery, and other fields to connect pipes and fittings that need to be sealed.

3. Comparative analysis of various threads

(1) Difference in tooth shape angle

The NPT thread profile angle is 60 degrees, while the PT, G, RC, ZZG thread profile angle is 55 degrees, the TR thread tooth angle is about 30 degrees, and the S thread is zigzag asymmetrical tooth shape. Different tooth angles directly affect the strength, sealing performance, and transmission efficiency of threads. For example, NPT threads with a 60-degree thread produce different friction when sealing than PT threads with a 55-degree thread, making them suitable for different pressures and sealing requirements.

(2) Comparison of sealing performance

NPT, PT, RC threads have self-sealing characteristics, relying on taper and thread fit to achieve sealing; The G thread itself is not sealing and needs to be sealed with the help of sealing materials; TR and S threads are mainly used for transmission, and sealing issues are generally not considered. When selecting threads for sealing occasions, it is necessary to choose appropriate self-sealing threads or threads with sealing materials according to factors such as working pressure and medium properties.

(3) Distinction between application fields

NPT threads are mainly used in North America; PT threads are used in some countries in Europe and Asia; G thread is widely used in various pipe connections; TR thread is used in transmission occasions; S thread for unidirectional force transmission; ZG threads are gradually replaced by RC and so on. RC threads are used for medium and low pressure sealed pipe connections. In practical applications, it is necessary to select the corresponding thread type according to the region, engineering standards and specific work requirements.

4. Practical selection precautions

(1) Consider regional standards

Different regions have different preferences and regulations regarding thread standards. NPT threads should be given priority when engineering design and equipment selection in North America; In some regions of Europe and Asia that follow imperial standards, threads such as PT, G, RC, etc. are more common.

(2) Choose according to the working pressure and medium

For high-pressure, flammable, explosive, or corrosive media, threads with good sealing performance and high strength, such as NPT or RC threads, should be selected, and the quality of the threaded connection and the reliability of the sealing material should be ensured. For low-pressure, ordinary media pipe connections, the G thread can also meet the requirements with the appropriate sealing material.

(3) Combined transmission requirements

In the transmission occasion, such as the lead screw transmission of the machine tool, the TR thread or S thread with high transmission efficiency and strong bearing capacity should be selected, and the type should be reasonably selected according to the direction and force characteristics of the transmission.

5. Conclusion

There are obvious differences in NPT, PT, G, TR, S, ZG, and RC threads in terms of tooth angle, sealing performance, and application fields. Accurately understanding and differentiating between these thread types is crucial for engineering activities such as mechanical design, pipe installation, and equipment maintenance. In practical applications, the appropriate thread type should be comprehensively considered according to regional standards, working pressure, media properties, transmission requirements and other factors to ensure the reliability, tightness and transmission efficiency of the connection, and ensure the safe and stable operation of the engineering system.