Air Tube

Air Tube;

On earlier pages I've talked about the various types of fittings for plumbing your compressed air to your tools and cylinders.

Now, let's talk a bit about the air tube used for those fittings.

What's the difference between air tube and air-hose?

One difference is the size; how it's measured.


Tube Size

The ‘T’ in TOD stands for TUBE.

A 1/4" poly tube (the compressed air industry workhorse air line) will be exactly 1/4" on the O.D.

The T.O.D. (tube O.D) versus H.I.D. (hose I.D.) concept is pretty easy to demonstrate. If you take a 3/8” hose and then you take a piece of 3/8” P.E. (polyethylene) tube, the 3/8” P.E. tube will slide inside the 3/8” hose perfectly.

The same for 1/4” tube into 1/4” hose, 1/2” tube into 1/2” hose, etc.


Applications

The vast majority of air lines used in industry are polyethylene or polyurethane tube.

Polyethylene is the most common as it is least expensive, easily obtained, comes in 100 foot rolls (and up), is easily cut, and quickly attached to an instant type fitting.

By the way, you would not normally use air tube to supply an air tool as the fittings used for poly tube do not lend themselves to any kind of sideloading. And, of course, when moving an air tool to do work, you would be sideloading the tube-to-fitting connection all the time.

Polyurethane is sometimes used to plumb air to valves and air cylinders too, as it is softer, more malleable and able to be bent in tighter circles without kinking.

It is also more expensive than polyethylene tube.

Polyurethane also may have some leakage or “blow off” issues when used with some types of instant fittings that aren't designed for a soft-surfaced air line.

The ‘natural’ polyethylene air tube is rated to handle industrial air pressure in the 120+ PSI range in normal ambient temperature environments.


Colored Poly Tube

Colored forms of P.E. tube get more expensive and are sometimes harder to find.

Yet plumbing a machine with dozens of air valves and air cylinders with colored tubing makes problem diagnosis easier on a larger machine. For example, you might consider making the cylinder extend air lines one colour, the retract another. Or, if you have pneumatic logic on a machine, you might make all of the control air lines one color, and the power air lines another.

In this manner, it's easy to trace a specific air line from the source to it's air component, and thus make it easier to determine if there is a problem with air flow to that unit.

For instant fittings, remember that the fitting, into which you insert a square-cut end of poly tube, has an or some "O" rings inside it, and the "O" ring seals on the outside of the tube inserted into the fitting.

This is why an accurate O.D. is critical for tube. Once I blamed a batch of fittings for being out of spec, and it turned out that the poly tube was undersized by a couple of thousandths, enough, as it turned out, to allow a continuous leak and "blow off" of the tube from time to time.

Inside the tube orifice of the fitting there will also be a ring with teeth, a collet. When the tube is inserted, these teeth penetrate the surface of the tube llightly to hold the tube in the fitting. When compressed air is introduced to the poly line, it expands the poly tube slightly, and the teeth grip even harder preventing air line blow out.

The use of an instant fitting then requires a certain surface hardness of the tube to work properly.

Air line tubing that is quite soft or stretches easily may be better installed with a different style of fitting as the tube O.D. will lessen as the tube stretches, and it might pop out of the fitting. This sometimes happens with polyurethane tube.

Since you measure the tube on the O.D., (TOD) then the mating connecting hole on the fitting must be the same size as the tube. (1/4” tube will have an actual 1/4” mating size on the fitting).

Over recent years, the “instant” style fitting has become the predominant style of fitting to connect air lines in the 10-32 through the 1/2” tube size.