Fitting Information For The Compressed Air UserFittings information.
On this page I'll take a look at all kinds of air fittings in an effort to make fitting identification and how to select the right fitting for a specific application easier for you.
The NPT Standard
NPT stands for National Pipe Taper. If an air fitting is manufactured to that standard, it will have dimensional compatibility with other NPT threaded fittings. One manufacturer's fitting can be replaced with another's, 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.
NPT Size
Fitting sizes can be a bit confusing! Please don't make the assumption – as I used to - that a 1/4 NPT fitting has a thread diameter that's actually 1/4 inch in diameter!
Visit your favorite hardware or plumbing store and ask for a 1/4” fitting. The north-American standard fitting they will likely show you will be a 1/4” NPT. You might be surprised to see that the actual diameter of the 1/4” NPT thread will be just over 1/2”.
To help you “get a grip” on the actual sizes of NPT fittings, see the chart below. It shows the comparison between NPT fitting sizes and their actual inch equivalents.
On this chart I have listed the NPT thread size first, and then what that NPT thread size is in actual (decimal) inches, and then the closest common size fraction.
If you've not used NPT fittings before, the size 'discrepancy' will surprise you.
Sizing The Hole
To determine the size of a threaded air port in an air tool handle, air cylinder, an valve, or the female thread in any compressed air component measure the diameter of the hole in inches.
Compare that result to the fractional sizes on the chart to help guide you to the right male NPT fitting size to use.
As mentioned, NPT is an acronym for National Pipe Taper, an industry dimensional standard. Recognize that the threads will connect, but just that they will connect together doesn’t mean that all of the fittings will necessarily look the same.
What You Need To Know
What do you need to know when you are trying to determine what fitting to use and where to use it?
You need to identify the...
- Port size / fitting thread size into which the male thread is screwed
- Size of tube or hose that you want to attach to the fitting; and note, tube and hose do NOT connect the same way
- Style of fitting; whether it is a straight, elbow, Tee , run Tee etc.)
- Way the hose or tube is to be connected to the fitting
The NPT Port Sizes
In NPT sizes, you will commonly see the following;
- 10-32
- 1/8 NPT
- 1/4 NPT
- 3/8 NPT
- 1/2 NPT
There are certainly much larger NPT sizes. Those shown above are the commonest fittings in industrial compressed air applications.
How Fittings Seal
The NPT type thread seals by penetrating the thread boss until tight and sufficient threads on the fitting are interlaced with sufficient threads in the boss to generate a seal.
To aid in the 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 allowing those male threads to turn deeper into the female boss improving thread-to-thread penetration distance and sealing.
Teflon Tape Problem
Some industrial compressed air professionals prefer not to use this tape as it is easy for pieces of the tape to overlap the end of the fitting and impede the air passageways. Or, sometimes pieces break off in the air turbulence inside the fitting and create problems downstream in other 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 easy to use so just be careful to keep the tape wound well back from the end of the fitting thread, and don’t let it 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, rather than loosen.
Already 'Doped' Fittings
The norm of late seems to see standard NPT fittings coming complete with a Teflon paste or other similar type of sealing compound already coating the threads.
Like the Teflon tape, this sealing compound allows the NPT threads to seat further into the boss allowing greater thread-to-thread contact, and improve sealing by filling micro-voids and unevenness in the threads. This is a nice feature of modern air fittings, and is both readily available and well accepted.
Liquid Thread Sealant
Alternatively, you can purchase liquid thread sealant to add to the threads of fittings. Using the liquid on smaller fittings is problematical if care isn't taken to keep the sealant from overlapping and blocking the actual air passage.
The Straight Fitting
Just as it's name suggests, the straight fitting accepts an air line (tube or hose) on one end, and on the other there is an NPT male or female thread that screws into or onto a boss.
A boss is the device into which you insert or thread a fitting. The boss could be an air tool, a valve, an air cylinder, an air manifold, or any number of other compressed air components.
The Elbow Fitting
Elbow fittings are those that allow the supplying line to connect to the fitting tangentially, out to one side rather than straight in.
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.
Careful! A swivel elbow is not a rotary device, and it's purpose is NOT to allow continuous movement of the swivel as the equipment to which the air line is connected moves. In this instance, the swivel portion of the elbow fitting will fail.
If the elbow fitting is to be swiveling continuously, then you need to install a rotary union.
Rotary Union
There are some applications that require a 90 degree elbow that swivels continuously.
That might be one that's attached to a moving component that oscillates continuously. That would quickly wear out a typical swivel elbow.
Another area where rotary unions are used is 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 that is designed to allow continuous movement of the swivel part of the fitting.
Rotary Unions will likely contain ball bearings in their construction and the proper seals necessary to ensure reliable, often high speed, continuous rotation without fitting failure or air leaks