Threaded fittings occupy a large portion of the world of fittings as they have features that make them ideal for rapid assembly and disassembly, allowing them to cope with failures, malfunctions, maintenance, replacements and changes in projects.
In particular, within the most commonly used threads, especially in hydraulics, there are
GAS threads and NPT threads. At the base of their differences there is, as often happens within the world of mechanics and current regulations, a use based on the geography of the manufacturer and user. In fact, GAS threads are more widely used worldwide while, as regards the United States and Canada, NPT threads are more widespread and used to produce threaded fittings. This geographical location should not be surprising: just think of all the regulations that differ between Europe (and in general the other continents) and North America.
A fundamental aspect of these types of threads is the unit of measurement: in fact, the classic metric system is not used but the designation is made in inches.
The GAS thread
The GAS thread is the most widely used for joining pipes and hydraulic fittings; it can be either parallel (cylindrical) or tapered.
- Cylindrical GAS thread: also known as “not pressure-tight on the thread,” it is regulated worldwide by ISO 228-1. Sealing is not generated by the shape of the male and female threads (both cylindrical and therefore not suitable for creating interference), but by the insertion of an appropriate sealing material that ensures the conveyed fluid does not leak, preventing losses, inconvenience, and malfunctions.
- Tapered GAS thread: also known as “pressure-tight on the thread,” it is regulated by EN 10226-1. In this case, the male thread is tapered while the female thread is cylindrical: the interference between the two creates the seal, and no additional sealing materials are required, although in practice gaskets are always added to increase safety and minimize potential issues.
Regardless of the type, the GAS thread is a Whitworth thread with a very fine pitch and, as such, is characterized by a thread angle of 55° (not 60° like the classic metric thread).
NPT Thread
The NPT thread (National Pipe Thread) represents the American standard in accordance with AISI B1.20.1 and is the second most widely used in the field of hydraulic fittings. Compared to the GAS thread, it has a 60° thread angle, which results in a coarser pitch (i.e., fewer threads per inch). Another important feature is that this type of thread is tapered, with no parallel version; therefore, it generally ensures better sealing, although the use of gaskets is still recommended.
Characteristic Parameters of Threads
Threads, as seen, can be of various types based on thread angles and structural characteristics. Whether they are GAS, NPT, or any other type (such as metric threads, the most widespread worldwide in industrial applications), there are important differences based on the parameters that define angles, dimensions, and helices. In particular, the fundamental variables are:
- Thread angle: characterizes the thread and its shape, significantly influencing mechanical and mating characteristics.
- Height H: the height of the generating triangle of the thread.
- Major diameter: corresponds to the diameter of the thread crests in the case of a screw and is equal to the diameter measured at the thread roots in the nut.
- Nominal diameter: the diameter used to designate threads.
- Minor (core) diameter: the opposite of the previously mentioned major diameter.
- Pitch diameter: as the name suggests, it represents the average diameter of the screw or nut.
- Pitch: the distance between two consecutive crests or roots of a thread. In particular, a distinction is made between fine pitch and coarse pitch to specify differences in this characteristic.
- Number of turns: the number of revolutions of the screw.
- Number of starts: represents the number of different thread starts on a screw, used to increase pitch while maintaining an adequate load-bearing core section.
- Direction of rotation: the screw may be right-hand or left-hand depending on the tightening direction during assembly.
During the design and selection phase of joints, it is essential to evaluate all these parameters to make an informed choice that minimizes the risk of failure during operation and ensures a long service life of the joint, avoiding risks and failures that could damage structures and systems. As with all thread types, GAS and NPT threads are also subjected to this decision-making process in conjunction with structural calculations, which are always decisive in a project.
Manufacturing Processes for Thread Production
To comply with applicable standards, threads must be produced using manufacturing technologies that ensure conformity to quality requirements. Among the main processes are:
- Turning: a machining process involving material removal in which the component being formed—here, the screw—rotates while the tool moves linearly and removes material, creating chips. Rotational speed, tool cutting angles, and feed rate are among the key parameters.
- Milling: whether tapered or cylindrical, this is a chip-removal process in which the tool undergoes both rotational and translational motion to shape the material. Once again, rotational and feed speeds are important, as well as tool geometry and material.
- Tapping: a chip-removal process carried out using a tool called a “tap,” which is responsible for creating the thread. The shape and size of the tap are crucial to achieving the required thread morphology.
- Thread rolling: unlike the previous processes, this is a plastic deformation process. No material is removed, and the resulting surface is more resistant to stress due to the work hardening generated in the material.
Threading and Stainless Steel: Considerations and Precautions
Stainless steel components, such as fittings, are often characterized by threads—both GAS and NPT—necessary for connection with other components in complex assemblies. However, producing a thread requires certain precautions. To perform it correctly, it is necessary to consider some phenomena that occur during the operations described above.
Among these, the main phenomenon to consider is work hardening. The use of unsuitable parameters, such as excessive cutting speed, can cause sliding on the surface rather than effective cutting. This leads to heat generation and surface hardening of the steel, resulting in difficulties during subsequent passes.
The main effects are:
- Increased machining times: surface hardening requires more time to remove material, extending overall processing times.
- Greater tool wear: as steel hardness increases, tribological effects between the tool and metal also increase, leading to higher tool consumption and reduced service life.
- Greater economic impact: increased time and consumption require higher costs for component production, especially in large quantities and series production.
Advantages and Benefits of Using GAS Threads
GAS threads are among the most common—along with metric threads—for construction applications, but they are certainly preferred for connecting pipes and hydraulic components. This preference is due to the characteristics of this thread, which make it particularly suitable for applications where fluid sealing is essential and where leakage risks can lead to significant service disruptions or safety hazards for people and surrounding environments.
The main characteristics include:
- Hermetic sealing: very tight couplings that minimize fluid leakage.
- Safety: this type of fitting is used in safety-critical applications, performing reliably even in pressurized systems where the fluid generates forces that heavily load the threads.
- Strength: with the addition of gaskets, GAS threads can withstand even considerable loads without losing functionality.
- Versatility: available in both tapered and cylindrical configurations, making them suitable for various conditions and applications.
For all these reasons, as mentioned earlier, GAS threads are widely used in hydraulic applications, where sealing failures can lead to flow losses and malfunctions of entire systems.
