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Real Performance Machinery Screen Printing Article :
Good Connections Stop Costly Air Leaks

Good Connections Stop Costly Air Leaks
Understanding tubes and fittings to avoid energy-wasting air leaks.
By Rick Fuqua

Air leaks are annoying and costly. Yet, although most air leaks are easy to fix, they often go unattended until the sound becomes sufficiently annoying. In many cases, it seems the noise is more objectionable than the energy being wasted. But another reason these leaks don't get the attention they deserve is a lack of comprehension of the amount of energy being wasted and the possible negative side effects.

A single 1/2-inch leak can spill out 100 cubic feet per minute (cfm) of air while a number of smaller, less noisy or noticeable leaks in combination easily can spill out more. The reason these leaks are costly is they cause the compressor to kick on more frequently and stay on longer to make up for the air being lost. This results in higher energy and maintenance/repair bills. In addition, if leaks divert enough air, they can cause performance problems with your printing machine. For these and other reasons, you should check for air leaks regularly and repair them, however big or small.

With the exception of rigid pipe running from the compressor to the machine, most air connections on your press are likely to be connectors used to join tubing to pneumatic parts. Today these connections are made using what many people refer to generically as push-in fittings or connectors. Push-in fittings have replaced the compression-style fittings—the type commonly used in bathroom water fixtures—that were used previously.

The preference for push-in over compression fittings can be traced to the former's ease of use and quick assembly. Some manufacturers claim push-in connectors save as much as 75% in assembly time. Although this may not mean as much to the machine user as the reliability and leak-resistance of the fitting (which, it could be argued, are not as great as with compression fittings), push-in fittings are here to stay. Thus, it's important to understand how they work—and how and why they fail. You also should know the specifications needed to order replacements.

Here's how push-in fittings work: A good, straight, clean-cut piece of tubing is inserted into the fitting while the air is shut off and exhausted. When the tubing is pushed in, the wide flange-release collar collapses so it is flush with the fitting body. The tubing should be pushed in as far as it will go. When pressure is no longer being applied to the tube and air runs through it, the wide flange-locking collar will pull away from the body of the fitting. Activated by the collar, the fitting's teeth grip the tube on the inside so it can't be pulled or pushed out by internal air pressure.

To reverse this process and disconnect the tube from the fitting, you must first shut off the air supply and exhaust the pressure. Then you must push-in the wide flange-release collar (which often is easier to do if you push in on the tubing at the same time). When the collar is up against the body of the fitting again, pull out the tubing. In effect, you have to push in the collar to open the teeth and release the grip on the tubing so it can be pushed in or pulled out.

The gripping ability of these fittings depends on the type and size of the tubing. No fitting works with the wrong size outer-diameter (O.D.) tubing, and some fittings work only with tubing made of a specific material.

Tubing Specs
Common sizes of tubing measured by the O.D. are 1/8”, 5/32”, 3/16”, 1/2", 5/16", 3/8", 1/2", and 5/8". The material the tubing is made of varies according to the application and performance qualities desired. One primary difference among material types is the level of flexibility or rigidity.

In addition to O.D. and inner diameter (I.D.), manufacturers also specify recommended working pressure, burst pressure, and other tubing properties. The two most popular tubing materials used in T-shirt machines are nylon and polyethylene. Generally, polyethylene is lighter in weight and more flexible than nylon. Polyurethane, another popular tubing material, is more flexible than nylon and polyethylene, but it is not often used in T-shirt press applications. In many applications, more rigid tubing is preferred as it is less likely to get caught in or trapped by moving machine parts. In addition, some push-in style connectors do not hold softer tube material securely. The design of the teeth or gripping device inside the fitting also varies by manufacturer, and some brand/design fittings do not seem to hold more flexible tubing as well as more rigid nylon or polyethylene materials.

Typically, push-in style connectors have one pipe-threaded end (unless the fitting is a union of tubing to tubing) that threads into the air component (air valve, cylinder, regulator, etc.). The other end is designed to allow the tubing to be pushed into the body to complete the air connection. Connected tubing can then be removed from the air component without tools simply by pushing in the colored locking collar with the tube and, while holding the collar in, pulling out the tubing. All connections/disconnections should be done with the air shut off and exhausted from the line or component. This type of connector allows quick and easy assembly repairs.
The downside of these connectors is that over time, the device that holds the tubing in place can wear down the O.D. of the tube, particularly if the connection is made repeatedly or improperly. This can result in an air Ieak.

Often, such leaks can be fixed by trimming the tubing back to expose a portion that has not been abraded by the locking collar of the fitting. However, in some cases, the collar itself may have become bent or damaged in a way that does not allow sufficient holding power to keep the tube engaged in the fitting.

In such cases, the tube may not stay connected or pop out under pressure. When this occurs, it's best to replace the fitting with a new one.
Some manufacturers of these devices recommend performing no more than five connections/disconnections on any one piece of tubing due to the risk of leaks. Also, the tubing end must be cut square to seal properly within the fitting. A tubing cutter is a smart and convenient tool for ensuring a proper, clean, and straight cut in these applications.

To choose the correct replacement fitting, you need to know the type and size of the pipe thread, the geometry of the connector ("T”, elbow, etc.), and the size and type of tubing connected to it.

Pipe Sizes
Although many geometric styles and manufacturer variations are available, the most common are: straight, elbows, "T"s, and "Y"s. Each of these shapes can have variations, depending on whether they have male or female connections, (unions are for tube-to-tube connections), the tubing size is the same on both ends or tapers, or a swivel connection is provided so the fitting can be screwed securely into place without the tube having to rotate to an undesirable location/angle.

Pipe Threads
One of the most confusing things to specify is pipe threads. Unlike bolt threads, which are determined and referred to by the number of threads per inch on the bolt, pipe threads are designated in a much less straightforward way. The designated size of a pipe less than 12" has nothing to do with its I.D. or O.D. A 3/8" pipe does not have a 3/8" outside or inside diameter. It is, however, reassuring that although the actual size of a pipe has nothing to do with its size designation, the outside diameter is consistent; that is, all 3/8" pipe has the same 0.675" diameter.

I have found the chart on page 116 to be helpful in making sense of pipe sizes. And, by the way, "IPS," "pipe size," "size," and "nominal pipe size" all mean the same thing.

Pipe Terminology
The most common pipe threads are National Pipe Threads (NPT). These threads are tapered for a leak-resistant fit. FPT refers to female pipe threads and MPT, to male pipe threads; both are NPT threads.

Dry-seal or NPTF threads are modified NPT threads that are virtually leak-free without a sealant. Dry-seal or NPTF threads can be used with NPT threads, but you no longer have the virtual leak-free seal.

Straight pipe thread is referred to as NPS and is not tapered. These threads are intended only for mechanical joining and do not possess any leak-proof characteristics. Other thread styles such as GHT for garden hose and NST for fire hose, exist, but the one to watch out for in England is the British Standard Taper Pipe Thread BSTP, which should not be used with any of the threads mentioned above.

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