Finding The Right Fit For Tubing

There is more to finding the right barb style connector for tubing than engineers might currently consider. Taking the extra time to conduct the proper tests will go a long way in ensuring a reliable system.

As medical disposables become more prevalent, it is crucial that all elements of the disposable be properly designed. Barbed fittings for connectors such as luers and tube-to-tube fittings are often selected through a trial and error process. Unfortunately, this can lead to field failures should the barb separate from the tubing either under fluid pressure or through stress on the tubing.

In general, the best practice is to utilize a barb style with the best combination of sealing integrity, high pull-off strength, and low assembly force for the type and size of tubing that will mate with it. The cylindrical surface behind the barb should allow the tubing to relax against the fitting. In choosing a barb style, ensure that there is no parting line on the sealing surface and the barb is designed with a sharp peak, allowing it to “bite” into the tubing. To narrow down the choice of barb style, the most practical method is to test an application by first requesting samples. This is often a fundamental mistake that designers make. Virtually all fitting vendors have a liberal sampling policy. Make certain testing has been performed on a large enough sample to fully understand the potential failure modes of the disposable.

The second most frequent mistake that design engineers make is not asking the vendor for technical assistance. Most fitting suppliers have excellent technical support groups. Take advantage of their expertise. It is likely that another company has had a similar application and the vendor can save a customer time by leading them in the right direction.

If the application requires a unique solution that cannot be satisfied with an off-the-shelf product, custom parts are sometimes an option or the vendor can often direct the customer towards another alternative. Custom parts are often less expensive than one might expect and worth further examination if a standard component will not meet the necessary requirements.

Most vendors offer a variety of barb styles and offer guidance on which are generally most appropriate for the durometer of the tubing (Figure 1). However, it is important to realize that tests may show that a different barb style might work better in a specific application than what is indicated on the graph.

Figure 1. Guidelines for selecting a barb fitting based on the durometer of the tubing.
To view graph larger, click here.

The most fundamental test performed on a barbed fitting is a simple pull-off test. Table 1 presents the results for a test using a variety of 75 durometer PVC tubes and barb styles. The fitting material was polypropylene. Notice how important the selection of barb style is to the results. Additionally, results across samples are consistent demonstrating the tubing is relaxing against the fitting and the barb is properly biting into the tubing.
Cable ties or other retaining devices are often utilized as further insurance against the tubing separating from the barb. While this is an acceptable technique and one that adds a certain amount of confidence to the connection, ties are often unnecessary if the barb size and style is properly matched to the tubing ID and durometer. Additionally, improperly applying a tie can create the leak path that the tie was intended to guard against in the first place.

An insertion force test should also be conducted (Table 2). This is critical since insertion force directly affects the manufacturability of the disposable. Selecting the lowest insertion force that meets the required pull-off strength reduces the assembly time and ultimately the cost of the disposable.

Design engineers are sometimes tempted to use an oversized barb assuming that the harder it is to push the tubing onto the barb, the better the pull-off strength will be. This is, in fact, often not true and oversized barbs can result in lower pull-off strength due to the inability of the tubing to properly relax behind the barb. Oversized barbs often create a worse seal by stretching the tube beyond its design limits and compromising its mechanical integrity. You may seriously affect the pressure a joint will handle when you weaken the mechanical properties of the tubing in this way. For a material such as PVC, expansion of the tubing ID by 50% or so is usually pretty reasonable. In extreme cases, damage to the sharp edge on the barb can result particularly with higher durometer tubing.

If a disposable is operated under pressure, it will need to be determined whether the connection meets the required pressure, in addition to clearing an appropriate safety margin. Many factors reduce tubing’s ability to perform under pressure including temperature, chemical degradation, mechanical stress, fluid pulsation, as well as the particular fitting chosen. Be certain to properly test the tubing and fittings under realistic conditions before making final selections.

Additionally, a bubble leak test should be performed. Either place the connection in water or apply a thin film liquid to the connection. Using compressed air at the maximum designed pressure, observe any bubbles that appear. Typically, any observed bubbles are unacceptable. There are a variety of more sophisticated test methodologies but these are usually not required for simple medical disposables.

It is common to make the insertion of barbs into tubes easier with solvents or by heating the tube. It is critical to understand whether these techniques are likely to be used in manufacturing so that testing can be performed using these same assembly techniques. Note that if used improperly, solvents can seriously affect the characteristics of the tubing. Also, it is important to consider the type of solvent as well as the length of time the solvent is in contact with the tube. Issues with tube heating typically involve inconsistent application of heat both in intensity and time. Both solvents and heating can create training and quality issues that can result in increased product costs. When using solvents or heating, make sure to contact the tubing vendor for advice.

To view tables larger, click here.

Tubing connections with barbed fittings must be properly designed and not implemented simply through trial and error. Do not assume that the more difficult it is to push a barbed fitting onto a tube, the better the seal. If a cable tie is used, do the testing to ensure that the tie is actually required to meet the designed pull-off strength and does not create inadvertent leak paths. Using oversized barbs or unneeded ties can significantly and unnecessarily increase the expense of the disposable. Also, be sure
to conduct testing utilizing the same assembly techniques that will be used in production. Finally, utilize the technical resources of both the tubing and fitting vendor to ensure that the best practices are being employed, resulting in the lowest cost, highest quality disposable.

Frank Lombardi is an engineer at Value Plastics. He is responsible for new product developm