When building or maintaining a pneumatic or hydraulic system, it is important to understand the fittings you are using. Threaded and compression fittings look nearly identical, but they connect in very different ways while functioning the same.

You can explore the differences here.

Similarities and differences

Similarities – Threaded and compression fittings both use threads to help make their connections. However, they use the threads differently.
Differences – Threaded fittings have a 1-piece connection end that is used to connect to port/block. These threads create the seal for the connection with the support of Teflon tape or pipe dope and are manufactured in a tapered, cone-shape. Whereas compression fittings have a 3-piece connection that is tasked with its connection to tubes. Compression ends typically are a parallel shape and instead of sealing on the threads, the seal point is created with the assistance of a ferrule (ring, olive or sleeve) and a compression nut. So in this case, the threads simply needed to hold the fitting together with the compression nut.

How a compression fitting works

As you can see, compression fittings form a seal by applying force. This is done with the three piece assembly; (comprised of a) compression nut, connector and an inner ring (the ferrule). When the compression nut is properly tightened, the ferrule is pressed against the tube with enough force to eliminate excess space in the joint, which prevents fluid or gas from leaking. The ferrule forms the seal, not the nut, the threads or any mechanical connection. This takes all the pressure off the mechanical connection.

Identifying a compression fitting

If you’re looking at a fitting and wondering if it’s a threaded or compression, take a look inside. The compression fitting will have a seat inside for the tube. This is the quickest way to identify a compression fitting.

If you need more proof, look for a nut and the aforementioned ferrule (ring). The threads alone won’t tell you what you need to know, as it’s difficult to look at threads quickly and determine if they are tapered or parallel, let alone if they are port or tube connectors.

The advantages of compression fittings

Compression fittings eliminate the need for soldering or welding a connection, which makes them quick and simple to use. Further, no special skill or tool is needed to assemble and install the fitting. If a fitting is needed in an area that may need to be disassembled for maintenance, the joints can be broken and reassembled without damage to the connection.

When assembling a compression fitting, avoid the temptation to over-tighten the nut. This could damage the ferrule inside or the tube itself. Once the nut stops turning with reasonable force, stop tightening.

Remember the mention of not needing soldering or welding? That makes a compression fitting a good idea in an area near a high heat or potentially flammable source. Wrenches and pliers don’t typically burst into flames.

Considerations

Compression fittings are not as solid as a soldered connection. Keep this in mind, and don’t install a compression fitting in an area that’s subjected to frequent flexing or sudden pressure changes. These fittings are more sensitive to those stressors.

For a fast, easy assembly, you can’t beat a compression fitting. And as with all industrial components, always verify your TAMPS (temperature, application, material, pressure and size) to ensure a reliable system.

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