Tube connections are not threaded but have other means of creating a tight seal and strong grip on the tube.  There are multiple types of tubing and tube connections may be applicable to only one tube type.  Tube connections include barbed, compression, push to connect, and quick disconnect.

Barbed:  Barbed fittings are most often used with soft tubing and hoses.  Barbed fittings are easy to install and are ideal for strong mechanical seals.  However, barbed fittings offer a variety of options as to the number of barbs which determines the sealing properties.  To create the tightest seal of tubing connection the use of a single barbed fitting is ideal.  Multi-barbed fittings offer a tighter grip that might not provide as tight of seal.  For an additional grip the use of hose clamps is ideal. 

Compression: Compression fittings are used with hard or medium tubing.  Compression fittings have two parts a sleeve and a nut. The connection is a simple attachment.  Slide nut over the tubing, and insert the tubing into the compression fitting sleeve, tighten the nut over tubing to form a tight seal around.  When selecting a compression fitting consider temperature, pressure, and use. 

Push to Connect: Push to connect is an instant connection and is a quick connection for hard and medium fittings.  To attach insert tubing into fitting until collet (or collar) snaps into place, there is no need to tighten. To remove push the collet in and tubing will be released. 

Quick Disconnect: Quick Disconnect fitting also offer an instant connection without any need for tightening.  The quick disconnect fittings are a two part component with a socket and a plug.  The plug is inserted into the socket.  The plug has ridges or grooves that latch in the socket lock into to create a tight, leak-free seal.  The groove on the plug allows for plug to rotate 360 degrees without getting stuck on the socket’s latches, preventing the tubing from kinking.  To remove press the sockets latch and remove the plug.