Threads, as important mechanical connection elements, are widely used in various fields. Different types of threads exhibit significant differences in structure, performance, and application. This article will provide a detailed introduction to the distinctions among the commonly used NPT, PT, G, ZG, RC, and M threads, helping readers better understand and select these threads for use.

NPT Thread

NPT, or National Pipe Thread, is a 60° tapered pipe thread widely used in North America. The characteristic of the NPT thread is a 1:16 taper, meaning the thread tapers 1/16 inch per inch of length. This design allows NPT threaded joints to fit tightly with the inner wall of the pipe, achieving reliable sealing. NPT thread specifications are measured in inches, with common markings such as 1/4, 1/2, 1/8, etc., referring to the diameter of the thread size.

NPT threaded joints offer several advantages:

  1. Excellent sealing: The tapered pipe thread design ensures a tight fit with the inner wall of the pipe, providing reliable sealing.
  2. Pressure and temperature resistance: Made of high-quality materials, it can withstand high pressures and temperatures.
  3. Reliable connection: The connection is secure and not prone to loosening or leakage through the rotation of the thread.
  4. Easy installation: Simply insert the pipe into the fitting and rotate it to a certain angle to complete the connection.
  5. Versatile applications: Suitable for various piping systems, including petroleum, chemicals, pharmaceuticals, food processing, and also pneumatic and hydraulic systems.

When using NPT threads, note the following:

  1. Ensure the inner wall of the pipe is clean and free of debris and grease to ensure sealing.
  2. The length of the pipe inserted into the fitting should be appropriate, neither too long nor too short, to avoid affecting sealing and service life.
  3. Use a suitable wrench or tool to rotate the thread, avoiding excessive or insufficient force that may damage the thread or lead to a loose connection.
  4. Regularly check the sealing and condition of NPT threaded joints, and replace damaged joints or repair leakage points promptly.

PT Thread

PT, or British Standard Taper Pipe Thread, is a 55° taper pipe thread with a 1:16 taper. The characteristics of PT threads are smaller pitch and smaller heights of thread peaks and valleys, making the thread structure more compact and effectively enhancing the sealing performance of the connection. The thread angle of PT threads is 60°, capable of withstanding significant axial pressure and exhibiting strong compressive strength.

The PT thread standard is primarily applicable to piping connections requiring high sealing performance, such as hydraulic systems, pneumatic systems, and liquefied gas transmission pipelines. PT threads offer several advantages:

  1. Excellent sealing performance: The compact thread structure effectively enhances the sealing performance of the connection.
  2. Convenient installation: The smaller pitch allows for connection with just a small rotation angle.
  3. Strong compressive strength: Capable of withstanding significant axial pressure with strong compressive strength.

PT threads are widely used, particularly in hydraulic systems, pneumatic systems, liquefied gas transmission pipelines, and other fields. Due to their excellent sealing performance and easy installation, PT threads are extensively applied.

G Thread

G, or 55° Non-Sealing Pipe Thread, belongs to the Whitworth thread family. The characteristic of G threads is their high degree of international standardization, with uniform parameters and symbol representations that facilitate communication and coordination, improving work efficiency. G thread sizes are typically marked in inches, such as 1/4, 1/2, 1/8, etc., referring to the diameter of the thread size. In practical applications, the industry often uses “points” to describe these sizes, for example, 1/4 inch is 2 points.

G threads offer several advantages:

  1. High standardization: Uniform parameters and symbol representations enhance work efficiency.
  2. Quick installation: The design allows for simple and quick installation and disassembly without requiring excessive tools or time.
  3. Strong connection stability: Offers good thread engagement performance, capable of withstanding significant tensile and shear forces, effectively preventing thread loosening.
  4. Versatile applications: Widely used in machinery, automotive parts, construction, and engineering fields.

G threads play a crucial role in factory production lines, automotive assembly and maintenance work, and the construction process of buildings and structures. Their quick installation and disassembly significantly improve the efficiency of machine maintenance and replacement, ensuring automotive performance and safety, and securing and connecting various components.

ZG Thread

ZG, or Taper Pipe Thread, is a connection standard for sealing pipe threads. The characteristic of ZG threads is their tapered thread shape, which forms a tighter seal during connection, ensuring no fluid leakage. In China’s old mechanical drawing standards, ZG was used, but it has now been replaced by R/R1/R2 (external thread) and RC (internal thread).

ZG threads offer several advantages:

  1. Excellent sealing: The taper design results in gradually increasing contact area between threads during tightening, achieving better sealing.
  2. High connection strength: The tapered thread structure provides greater tightening force, enhancing connection strength and stability.
  3. Versatile applications: Widely used in petrochemicals, water conservancy and hydropower, pharmaceuticals, food processing, and other fields.

Due to their excellent sealing and stability, ZG threads play an irreplaceable role in applications requiring fluid leakage prevention.

RC Thread

RC, or Cylindrical External Thread, is a widely used thread type for mechanical connections. The tooth profile of RC threads is smoother than triangular threads, resulting in more uniform load distribution and better self-locking performance. The dimensions and specifications of RC threads are designed and produced according to the GB/T 193-1981 “Cylindrical External Thread” standard.

RC threads have several characteristics:

  1. High precision: Offers high dimensional accuracy and geometric tolerances, ensuring the quality of the tested thread.
  2. Simple structure: Consists of a threaded shaft and threaded sleeve, easy to use.
  3. Versatile applications: Covers a wide range of thread specifications, meeting the needs of various applications.

RC threads are widely used in the machinery manufacturing industry, precision instrumentation, aerospace, and other fields. In the machinery manufacturing industry, RC threads are used to inspect the thread quality of mechanical parts, ensuring interchangeability and reliability. In precision instrumentation, RC threads are used to inspect the threads of precision instruments, ensuring accuracy and stability. In the aerospace field, RC threads are used to inspect high-precision threads, ensuring the safety and performance of aircraft and equipment.

M Thread

M, or Metric Thread, also known as the public system thread, is a commonly used thread in China. The characteristic of M threads is a 60° tooth profile, with pitch referring to the distance between two thread peaks. Standard pitch values are arranged according to a segmented arithmetic series. The specification parameters of M threads are determined by metric units (millimeters), denoted as M + nominal diameter of the major thread diameter × pitch.

M threads offer several advantages:

  1. Unified measurement units: Measured in millimeters, facilitating measurement and calculation.
  2. Standard pitch: Pitch values arranged according to a segmented arithmetic series, easy to select and use.
  3. Versatile applications: Widely used in machinery, electronics, instrumentation, automobiles, aerospace, and other fields.

The selection and use of M threads depend on specific application scenarios and requirements to determine the thread diameter and pitch. Generally, a larger thread diameter provides greater carrying capacity but may also increase weight and cost. Pitch affects the tightening force and rotation speed of the thread. Common applications of M threads include the assembly of electronic products and precision instruments, and the fastening of large mechanical equipment.