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 commonly used for joining pipes and hydraulic fittings: it can be both cylindrical and conical.
- Cylindrical GAS thread: it is also defined as “not tight on the thread” and is regulated worldwide by the ISO 228-1 standard. The seal, in fact, is not generated by the shape of the male and female threads (both cylindrical and not adequate to create interference), but by the interposition of a suitable sealing material that ensures that the fluid conveyed does not escape causing leaks, inconveniences and malfunctions.
- Conical GAS thread: also defined as “tight on the thread” and is regulated by the EN 10226-1 standard. In this case, the male thread is conical while the female thread is cylindrical: the interference between the two generates a seal and no additional materials are required even if, on a practical level, the seals are always added to increase the degree of safety and reduce minimal inconveniences.
Regardless of the thread, in any case, the GAS thread is a Whitworth thread with a very fine pitch and, as such, is characterized by an angle at the apex of the thread of 55 ° (not 60 ° like the classic metric thread).
The NPT thread
The NPT thread (Nationale Pipe Thread) represents the American standard compliant with the AISI B1.20.1 standard and is the second most used in the field of hydraulic fittings. Compared to the GAS thread, it has a 60 ° thread angle which results in a larger pitch (which means a smaller number of threads per inch). Another important feature is that this type of thread is conical and there is no cylindrical type: therefore, this generally ensures greater tightness, even if the use of seals is still recommended.
Characteristic parameters of threads
The threads, as seen, can be of various types based on the angles of the thread and the constitutive characteristics. Whether it is GAS, NPT or any other type of thread (such as, for example, metric threads, the most widespread worldwide in the industrial sector), there are important differences based on the parameters that define their angles, dimensions and helices. In particular, the fundamental variables are:
- Thread angle: characterizes the thread and its shape, also greatly influencing the mechanical and coupling characteristics.
- Height H: is the height of the triangle generating the thread;
- External diameter: this denomination corresponds to the diameter relative to the crests of the threads in the case of the screw but is also equal to the diameter obtained considering the bottom of the threads in the nut screw;
- Nominal diameter: is the diameter used for the designation of the threads;
- Core diameter: it is the opposite of the external diameter introduced previously;
- Average diameter: as introduced in the name itself, it represents the average diameter of the screw or nut;
- Pitch: represents the distance between two consecutive ridges or bottoms of a thread. In particular, there is the distinction between fine and coarse pitch to specify the difference of this characteristic that can be found in the threads;
- Number of windings: it is the number of turns of the screw;
- Number of starts: represents the number of different types that can be found on a screw in order to increase the pitch but still maintain a resistant section suitable for the core;
- Direction of rotation of the screw: in fact, the screw can be right-handed or left-handed depending on the direction of screwing that distinguishes it during assembly.
During the design and selection phase of the connections it is essential to evaluate all these parameters in such a way as to make a conscious choice that minimizes the risks of breakage in operation and allows a prolonged life of the connection avoiding risks and failures that are harmful to the structures and to the systems. As with all types of threads, GAS threads and NPT threads are also subjected to this decision-making process in conjunction with the structural calculations that are always decisive in a project.
The processes necessary for the realization of the threads
In order to comply with current regulations, the threads must be made using production technologies that guarantee their compliance with the quality requirements. In particular, among the main ones it is possible to mention:
- Turning: machining carried out by chip removal which consists in the rotation of the component to be formed, in this case the screw, while the tool moves with a rectilinear motion and removes material, precisely creating the chip. The rotation speed, the cutting angles of the tool and the feed rate are just some of the fundamental parameters;
- Milling: be it conical or cylindrical, it is a machining by chip removal in which the tool is subjected to a rotational and translational motion going to form the material being machined. Once again the rotation and feed speeds take on importance, but also the shape of the tool and the material with which it is built;
- Tapping: it is a machining by chip removal carried out using a tool that assumes the name of "male" and is responsible for creating the thread. The shape and size of the male are essential to obtain the morphological characteristics required of the threads;
- Rolling: compared to the previous processes, it is a process by plastic deformation of the material. There is no removal of the material and the result is a surface more resistant to stresses than those obtained previously due to the work hardening generated in the material.
Threading and stainless steel: tips and precautions
Stainless steel components, such as fittings, are often characterized by threads, both GAS and NPT, which are necessary to allow connection with other components in complex sets. Making a thread, however, requires some precautions. In fact, to be carried out in a workmanlike manner, it is necessary to take into account some phenomena that occur during the operations presented in the previous paragraphs.
Among these, the main phenomenon to take into consideration is work hardening. The use of unsuitable parameters, such as an excessive cutting speed, can cause slipping on the surface rather than an effective cut. This circumstance brings with it the development of heat and the hardening of the steel surface with consequent problems for subsequent passes.
The main effects are:
- increase in processing times: surface hardening requires more time to remove the material and this lengthens the general processing times of the component;
- greater consumption of tools: by increasing the hardness of the steel, the tribological phenomena between the tool and the metal consequently also increase with greater consumption of the former and reduction of its useful life.
- greater economic impact: the increase in time and consumption requires a greater economic outlay for the creation of the component, especially when it comes to high numbers and series production.
