Making reliable, clean connections for medical and laboratory applications

From Colder Products Company

A hose barb is defined as one or more continuous ridges or bumps on a connector that are used to grip the inside diameter of a tube and seal the connection.

Introduction
Designers spend a lot of time choosing the right tubing or hose for an application yet sometimes the decision on fittings or couplings receives less scrutiny. In fact, a secure and reliable connection is really determined by how well the connector and tube work together. This makes selecting the right connector just as important as tubing choice.

Connectors—which include both fittings and quick disconnect couplings—with hose barbs offer a simple, dependable and inexpensive means for termination of tubes and hoses.

Connectors may have one barb or several barbs. Barbs may be spaced evenly or with extra space behind, between or in front of each barb. The connector itself may be constructed of various plastic or metal materials and have barbs in different configurations. All of these subtle differences are meant to improve the seal and grip for various grades of tubing available on the market.

Mold parting lines and rounded barb edges, such as those shown here, on plastic connector barbs can present a significant weakness, causing fluid leaks, or tube blow-offs or pull-offs.

In any system, connections can be the weakest link. Without a proper match between tubes and fittings, connections can be pulled off from vibration or tension, blown off from a pressure spike, leak just enough to cause problems or even result in a difficult installation.

Finding the proper connection is essential for a safe and secure system for transferring everything from oxygen to viscous substances, such as blood and medicines.

When you think about it, tubing connections and hose barbs are everywhere. Respiratory therapy, dialysis, pumps, drains, blood pressure cuffs—even sterilization and ice making equipment; all of these depend upon secure tubing connections. No matter how the connection is made—with or without bonding or ties—it is the barb on the connector that grips the tube and seals the connection.

To avoid weak connections, look for connectors with parting-line-free hose barbs. Using advanced molding processes, the mold parting line should extend only from the body of the connector no further than the base of barb.

The Hose Barb Grips and Seals
A hose barb is defined as one or more continuous ridges or bumps on a connector that are used to grip the inside diameter of a tube and seal the connection.

As a tube is installed on the connector, it expands over the barb. Grip and seal occurs as the tube tries to relax to its original inside diameter behind the barb. For many applications, one hose barb on a connector provides 100 percent of the sealing and most of the holding for a tube termination.

When a single hose barb does the sealing, the quality of the hose barb is most important. Mold parting lines on plastic connector barbs can present a significant weakness.

Certain applications or tube quality require a tie or clamp to provide additional holding power. While compression and screw-on connectors have their place, the barbed connector remains the dominant system for connecting tubes.

Hose barbs on connectors come in many shapes and sizes. Slope and depth of the barb, sharpness of the gripping edge, number of barbs, and spacing of barbs are all factors that contribute to the gripping and sealing ability of hose barbs.

A smooth, uniform surface on the barb is necessary for tight tubing grip and seal.

The internal diameter of the tube, flexibility of the tube material, wall section, and the intended application all factor into the decision on which connector to choose for the job.

Rubber hoses and plastic tubing available today all exhibit different levels of flexibility, a factor that plays a large part in how well a connection performs. In general, the softer the tubing, the more likely that a hose barb type connector is appropriate for the application.

While it is the inside diameter of the tube that largely determines selection of the proper connector, there are other factors to consider when selecting a barbed connector.

• Pull-off resistance. When a tube is pulled, it tends to contract and grab the hose barb tighter. A particular connector will therefore exhibit different tensile strength characteristics for different sizes and grades of tubes. If the barb is too sharp and the tubing is very soft, enough pulling and vibration could cut the material and cause leaks. On the other hand, a shallow or rounded barb mated with very stiff tubing may enable a tube to disengage with minimal pulling force.
• Blow-off resistance. Spikes in pressure can make tubes expand, potentially loosening the grip of the barb. For high pressure applications, the shape and arrangement of barbs and the relative flexibility of the tubing determines how the connection will perform under expansion of the tubing material.
• Ease of installation. Shape and placement of barb(s) on connectors combined with tube flexibility determine the force required to install the barb in the tubing. Easing the burden of installation is one issue to consider. More important, if it is too difficult to push a tube over the connector, the tube may not be installed correctly and open the possibility for leaks.

Clearly, selection of tubes and connectors is best made in tandem. Securing samples of each is always a good idea. The performance of tube and connectors using the hose barb system can be traced to several factors—and several myths.

Other characteristics to keep in mind when selecting the right hose barb include the sharpness of the barb, surface finish and the barb angle all contribute to the overall quality of the connection. Also, the number of barbs should not be the focus, but rather the quality of the barbs that grip the tubing – this is what determines the barb’s performance.

Would You Like Metal or Plastic?
For high pressure applications, for handling certain chemicals and compounds, or where the environment can be rough, a metal connector is often the best choice.

Yet just because metal is an inherently stronger material doesn’t mean that metal connectors provide gripping force that is superior to plastic connectors. With high quality plastic connectors, the gripping edge of the barb can be molded precisely with no radius—sharp enough to grab, but not too sharp to cut the tube. This is not always the case with barbs on metal connectors.

Different manufacturing techniques, such as application of a plating layer to the metal connector can create a radius over the barb that dulls the gripping and sealing edge. In terms of gripping tubing and holding the pressure, plastic barbs can perform equally, if not better, than metal barbs. In addition, plastic connectors will generally cost less.

Of course, not all plastic connectors are created equal. In making a plastic part, a mold opens and closes as each piece is produced. In this process, a parting line is created, a seam that produces a slight imperfection on the plastic part where the two halves of the mold meet.

On the barb of a connector, a mold parting line can invite leaks. Significant parting lines are especially prevalent with a poorly molded part or where the manufacturer has skimped on maintenance and investment in manufacturing equipment.

Mold parting lines on barbs can present a significant frailty for plastic connectors with hose barbs. To remedy the situation, look for manufacturers that have been able to create parting-line-free hose barbs.

Using advanced molding processes, the mold parting line extends from the body of the connector no further than the base of barb. This more expensive manufacturing process ensures a smooth, uniform surface on the barb that is necessary for a tight grip and seal.

Leak paths can occur due to parting lines in connectors. It is therefore important to insist upon connectors manufactured under extremely high tolerances or without mold parting lines altogether.

The Right Number of Barbs
Because for many things in life more is often better, it is also tempting to conclude that more barbs on a connector provide better holding and sealing power.

After all, hose barbs are designed to grip the inside of a tube; it seems logical that more barbs will provide more gripping power. In reality, this is not always true.

There are many things that work together to determine the quality of the connection between a hose barb and the tubing it connects.  The sharpness of the barb, surface finish and the barb angle all contribute to the overall quality of the connection.  Failing to optimize these technical aspects will result in a poor connection, regardless of the number of hose barbs used. 

In some cases, a single hose barb may work well. For example, connectors used in applications like biopharmaceutical manufacturing or chest drains that use extremely flexible, silicone tubing may work best with a single hose barb design.

However, that same single hose barb design will not work well with a higher durometer tubing, or thicker wall section where the tubing does not relax as well around the barb. In this case, multiple, well-made, sharp hose barbs will provide more holding force and a more secure connection.

Some manufacturers claim that a single hose barb design is superior due to its ability to be parting line free and sharper than a multiple barb design. However, multiple barb designs can also be made with advanced engineering and manufacturing techniques to provide a barb that adapts to a wider range of tubing and provide similar sealing and blow-off resistance.

Again, the number of barbs should not be the focus, but rather the quality of the barbs that grip the tubing – this is what determines the barb’s performance.

To Tie or Not to Tie
While the hose barb effectively grips the inside of the tube, ties (or clamps) provide an extra measure of holding power from the outside. Use of ties depends upon the tubing material, the pressure of the system and environmental conditions.

Some users will use a tie no matter the situation; there is something to be said for peace of mind by adding a few extra cents per tubing termination. For tubes without memory, ties or clamps are often necessary.

Tubing that has been repeatedly installed and removed over one or more barbs and is no longer elastic enough to relax behind a barb will require a tie, too. If pressures and temperatures are unknown or vibration can be an issue, ties are an important element for connecting tubes to connectors.

On the other hand, with higher quality materials such as braided tubing, or in a lower range of pressure systems, ties may not be necessary. In these cases, the hose barb provides more than enough blow-off and pull-off resistance.

There is a downside to ties. If installed incorrectly, ties can actually induce leaks. If a tie-type clamp is cinched too tight on a soft tube, it can lift the tube away from the connector and open a leak path. Securing a tie over the portion of tubing that is stretched directly over a barb can have the same effect—degrade the seal enough to cause leaks of materials or reduce pressure.

Installed properly behind hose barbs, the tie is highly effective for providing additional grip on tubes. Yet ties can introduce a point of failure, so it is worth investigating the pull-off and blow-off resistance of barbed connectors before you commit to ties. That’s because in many cases, the proper combination of tube and connector alone will perform to specifications.

Go with the Flow
Plastic connectors look deceptively simple, but when done right, they are actually highly engineered components. It takes advanced manufacturing techniques to control or eliminate mold parting lines and sophisticated plating capabilities to ensure the crucial tube-to-fitting connection is reliable. This same attention to detail is a requirement for the inside of connectors, too.

Many applications require a consistent flow path not only for steady delivery of materials but also to avoid damaging media, such as blood or other active cell culture material as it is delivered through a tubing system.

The pathway along tubing is always smooth; yet connections can pose issues where material can eddy or meet obstacles that can actually damage sensitive material.

To achieve a consistent flow path, advanced manufacturing of connectors results in a leading edge inside the barb that is typically sharper and tapered. The sharper leading edge of a well-designed connector faces oncoming fluids or gases moving through the tubing system.

This design detail helps fluids move unencumbered through the connection, removing entrapment areas where material can coagulate, easing fluids through the connection area with the tapered inside wall. In addition, a sharper leading edge helps reduce damage to sensitive media that can occur with connectors machined or molded with more blunt leading edges.

Choosing the Right Connector
Throughout this paper, the term “connectors” has been used to refer to both simple fittings and couplings—both of which are common termination choices for tubing. Both can use one or more barbs to grip and seal tubing. Yet this is where the similarity ends.

Even though fittings and couplings are not the same, these tubing termination devices are often lumped together for evaluation. For example, when someone removes a tube from a simple fitting only to have blood splatter around the room, the problem is misdiagnosed as a problem fitting—even though a quick disconnect coupling would have prevented the incident. Perception is reality.

On a purchase order, fittings appear to be cheaper than couplings—and with good reason. The quick disconnect coupling is an intricate device with internal shutoff valves, latching mechanisms and other features that allow a secure connection on a repeatable basis without re-cutting tubing between uses. In general, quick disconnect couplings are specified for handling tubing that is connected and disconnected repeatedly.

Fittings with a hose barb are a means to connect tubing-not disconnect it. Couplings allow tubing to be safely disconnected. Couplings are not only more flexible than simple fittings, but also safer, with features such as non-spill shutoff valves.

By automatically stopping the flow, there is little risk of dangerous or messy spills of fluids or loss of pressure on critical equipment when tubing is disconnected.

With any component, there is a total cost of ownership. If you seek the least expensive product, you may be surprised to find many hidden costs beyond the purchase price. Here are some questions to ponder when evaluating whether to use a simple fitting or quick disconnect coupling for an application:

• What is the true cost of replacing leaky parts in the field?
• Is user safety a concern? What is the impact if blood or other hazardous media is splattered all over the room?
• What is the value of a quick disconnect that saves time for the user?
• What is the assembly cost for a fitting? Do you also include the cost of a cable tie, a pinch clamp or other components that may be eliminated with a proven, well-designed quick disconnect coupling?
• What is the end user’s perception of your device if the primary interface — a tubing connection — is cumbersome, time consuming and messy?

When working with a manufacturer it is important to consider the following:

• Does the vendor have a quality system that meets your requirements — for example ISO 9000?
• Is local inventory readily available?
• Does the vendor also invest in maintaining the tooling—no matter how many parts you order?
• Does the vendor have lot traceability?
• Does the vendor have detailed change control procedures?
• Does the vendor have the technical staff to support you both before and after the specification is complete?
• Does the vendor have multiple options available to allow you to pick the right hose barb system for your application?
• Does the vendor have a proven history in manufacturing quick disconnect couplings?

If any of these issues are in question, your total cost of owning a quick disconnect type of connector is much lower than the upfront price of the low cost fitting.

Conclusion
The numbers of barbs or “metal vs. plastic” are not necessarily the most important considerations for a more secure, reliable system for connecting tubes and hoses with fittings and couplings. Reliable connections for your system start with finding the proper combination of tubes and connectors.

One of the biggest issues surrounding tube terminations is the accuracy of inside diameters of tubes. If someone is unfamiliar with specifying tube and connector combinations, the tube selected for a job may be too big or too small.

Given the inherent variability in tubing and hose material, it is important to carefully select the fitting or coupling as a system rather than an individual component. Evaluating samples of tubes, hoses, fittings and couplings is a great way to start.

In the end, it is important to understand the humble hose barb. Matched properly with a tube or hose, the gripping and sealing power of connectors with a hose barb system offers a connection with pull-off and blow-off resistance for most applications. Hose barbs are an integral part of today’s connector solutions, providing a high quality, low cost system for connecting many different types of tubes and hoses.

Colder Products Company is a provider of fittings and quick disconnect couplings to industrial, life sciences, chemical and packaging markets. If you would like more information visit www.colder.com