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.