On these pages I'll take a look at all kinds of compressed air fittings in an effort to make fitting identification, and how to select the right fitting for a specific application, easier for you.
Once you have reviewed the information here, if you have not found the information on air fittings you would like, visit here and Ask A Compressed Fitting Question.
If a specific air fitting is manufactured to the NPT standard, it will have dimensional compatibility with other NPT threaded fittings of other manufacturers.
In other words, one manufacturers fitting can be replaced with the same NPT size fitting of another manufacturer, and both - assuming they are the same size - will fit the same threaded hole in your air cylinder, actuator, air tool handle, valve...or whatever the application is.
Fitting sizes can be a bit confusing! Do not make the assumption - as I used to - that a 1/4" NPT fitting thread is anywhere near 1/4 inch in diameter! Not even close.
Visit your favorite hardware or plumbing store and ask for a 1/4" fitting. The fitting they will show you will likely be a 1/4" NPT. The actual diameter of the 1/4" NPT thread will be just over 1/2".
On this chart I have listed the NPT thread size first, and then what that thread size is in actual (decimal) inches, and then the closest common size fraction to that NPT size.
Compare that result to the fractional sizes on the chart to help guide you to the right male NPT fitting size to use.
NPT, as an acronym for National Pipe Taper, is an industry dimensional standard. The threads of different fittings with the same NPT size will connect, but that does not mean that all of the fittings will necessarily look the same.
You need to identify the...
There are certainly much larger NPT sizes, though in my experience the above are the commonest fittings in industrial compressed air applications.
To aid in the thread sealing process, and to render the thread leak proof, it is common for folks to select Teflon tape as a sealing aid. They coat the male thread with the tape, thus allowing those male threads to turn deeper into the female boss improving thread-to-thread penetration distance, and allowing tighter sealing.
Or, as compressed air flows very rapidly, sometimes pieces of the Teflon tape can break off in the air turbulence inside the fitting and create problems downstream by lodging in other air components. For example, pieces of tape can lodge in smaller air orifices and block them, or prevent another sealing surface from closing properly, actually generating a leak.
Yet Teflon tape is convenient, inexpensive and relatively easy to use. If you do use it, be careful to keep the tape wound well back from the end of the fitting thread, and do not let the Teflon tape overlap air passages when the fitting is threaded into the hole.
Wind the Teflon tape onto the male threads against the thread, so when the fitting is threaded into the port, the Teflon tape would tend to tighten on the threads rather than loosen.
Like the Teflon tape, this sealing compound allows the NPT threads to seat further into the boss allowing greater thread-to-thread contact, filling micro-voids and unevenness in the threads and improving sealing. This is a nice feature of modern air fittings, and is both readily available and well accepted.
A boss is the device into which you insert or thread a fitting. The boss could be an air tool, an air valve port, an air cylinder port, an air manifold, or any number of other compressed air components.
To make orientation of the air line easier, modern elbows are swivel type. After the thread is screwed tight into the port, the air line connection can be rotated to allow plumbing the air line in the most convenient direction.
If the elbow fitting is to be swiveling continuously, by a reciprocating part or some such on the machine, then you need to install a rotary union, NOT a swivel fitting.
A moving component that oscillates continuously would quickly wear out a typical swivel elbow either breaking it outright, or causing an air leak.
Rotary unions are commonly used on index tables, where there is continuous rotary movement of the fitting elbow.If you need to connect an air line to or from a moving component, make sure you select a Rotary Union of quality, on with ball bearings in their construction and the proper seals necessary to ensure reliable, often high speed, continuous rotation without fitting failure or air leaks.