Troubleshooting Leaks: Understanding Circumferential Tube Cracks

Understanding how circumferential tube cracks happen in a hydraulic system and how to fix working man Tube Fittings Division
Next up in our six-part troubleshooting fluid system connection leaks article series is understanding the causes of circumferential tube cracks. Specifically, we are investigating circumferential tube cracks that are close to tube fitting connections in your system.  

A circumferential tube crack is a crack that goes around the circumference of the tube. Figure 1 is a visual example. 

Tube Fittings Division Understand the causes of circumferential tube crack Tube Fittings Division Circumferential Tube Crack





Figure 1. Circumferential Tube Crack 


In the first post of our troubleshooting leaks series, we outlined the following probable causes for a leak coming from a circumferential tube crack as the following: 

  • Excessive vibration 
  • Inadequate or improper clamping of tube 
  • Tube line misalignment 
  • Improper routing of lines 

Before remaking and replacing the cracked tube connection it is important to determine which of the probable causes listed above is the root cause. You will want to investigate all these probable causes as, in some cases, there may be multiple causes. By identifying the root cause(s) the potential of the issue happening again is eliminated.  

To get the most accurate picture, you may need to take the connection apart where the leak has been identified since circumferential cracks are difficult to see and are often located under tube sleeves.  As we have said throughout this article series, always use proper safety measures and be sure to lockout and power down the equipment before taking the connection apart. If you are not sure what safety measures are needed, ask someone for guidance.  


Excessive vibration 

When designing a system, vibration needs to be considered. Too much vibration in a system containing tube fittings can be detrimental. Tube fittings have rigid connections, so vibration is absorbed into the connections. If too much vibration is encountered the connection may fail prematurely, causing a leak. This failure could range anywhere from a loosened nut to a broken fitting or a cracked tube. 

If you find that your system has a circumferential tube crack, examine your system closely for other signs of excessive vibration such as loose tube nuts. This type of failure is caused by fatigue and will not have a predictable failure time frame. Systems with excessive vibration can lead to other more costly component failures; loose nut or a tube crack may only be the first symptom. To prevent this vibration from affecting tubes, properly clamp tubes to prevent excessive and unnecessary vibration fatigue failures. Proper clamping is discussed below. 


Inadequate or improper clamping of tube 

Some movement in systems containing tube fittings is expected. However, to protect your system, tubes need to be clamped to a rigid structure whenever possible. Proper clamping helps to dampen tube line flexing and vibration, hence preventing failures due to fatigue. If you notice a circumferential tube crack, your system may not have proper tube clamping. 

Here is an excerpt from Parker Tube Fittings Division’s Catalog 4300 about tube clamping: 

Tube line supports (clamps) serve two primary purposes in tube line systems: mounting and vibration dampening. Fatigue failure due to mechanical vibration accounts for the majority of tube line failures. Proper clamping of the tube also reduces system noise. Use a clamping system such as Parker’s ParKlamp along with proper clamp spacing provided in the Assembly and Installation sectionFor tube clamps to dampen vibration effectively, they need to be anchored to a rigid structure. Clamping several tubes together, without rigid structural anchoring, does not provide effective dampening. 

Tubes need to be securely clamped and at regular intervals as shown in the following table. If clamping is not done properly, the entire system may be negatively affected by vibration or movement, rather than just a single connection. 

Pro Tip: Planning for clamping in the design stage is the most effective way to ensure a system is properly supported and damped. Resources to help with this, like the table below and other helpful recommendations can be found in the Assembly and Installation section of Catalog 4300.

Troubleshooting Leaks: Understanding Circumferential Tube Cracks Example of proper tube clamping in a system Tube Fittings Division Tube Fittings Division








Troubleshooting Leaks: Understanding Circumferential Tube Cracks Chart shows the recommended tube clamp spacing Tube Fittings Division









Figure 2. Tube Clamp Spacing 

Read our article Best Practices: Tube Line Clamping for Hydraulic, Pneumatic and Lubrication Systems for additional information.


Tube line misalignment 

A critical step in the assembly process is aligning the tube end with the connecting fitting face. If a tube is not properly aligned with the sealing surface of the fitting before tightening the nut, a leak path will be created. One symptom of tube misalignment is a circumferential tube crack which is caused by excessive strain on the joint. The proper procedure is always to align the tube and connecting sealing surface of the fittings properly before tightening the tube nuts.  Look at your connection, if the tube looks like tube on the left side of Figure 3, you have a misaligned tube.  If the tube looks like the figure on the right, you have an improper fit. Both can result in excessive strain and potential fatigue failure. 


Troubleshooting Leaks: Understanding Circumferential Tube Cracks Incorrect tube alignment or fit in a system can cause tube cracks and other system leaks.Tube Fittings Division









Figure 3. incorrect alignment and fit 


For a misaligned tube, the tube will need to be adjusted or re-bent so that it is correctly aligned with the fitting rather than making up the gap with force. For an issue caused by an improper fit tube, it is the same situation as a misaligned tube, you need to correct this. In most cases, this will involve re-making the tube because it is not long enough to make the connection.  

If there is a gap between the fitting face and the connecting tube (due to misalignment or improper fit), a leak path will be created, as this is your sealing surface. Do not rely on the torquing the tube nut to make up this gap; using the nut to close the gap is never recommended.  If the tube alignment or fit is not correct, you may be introducing unnecessary strain into the system which can lead to long term issues such as tube cracks. 

Misalignment and improper fit can affect two things:  

  • The assembly torque  
  • The integrity of your connection    

If you are using your tube nut to correct the gap between the tube and the fitting, you are putting stress on your connection. If the connection is not aligned or fitted properly prior to tightening, most of the torque will be used up in achieving alignment and will result in inadequate clamping load needed to properly assemble the connection for leak free service.    

When properly aligned, the tube nut should be able to thread on completely by hand before wrench tightening.  We cannot stress this enough, do not use tightening of a tube nut for pulling a misaligned joint together, this will put an undesirable strain on the joint, leading to leakage, circumferential tube cracks, or, in extreme cases, possible shearing of the fitting.  

Troubleshooting Leaks: Understanding Circumferential Tube Cracks Ensure proper tube and fitting alignment in your system to avoid system leaks and tube cracks. Tube Fittings Division









Figure 2. correct alignment 


Improper routing of lines 

How tube lines are routed should be a priority when designing a system. Incorrect tube routing can result in excessive strain on a joint which can result in a tube crack, as well as tube fitting connection issues. Below is an excerpt from the Assembly and Installation section of Catalog 4300 with regards to routing. 

Routing of lines is one of the most difficult, yet most significant of these system design considerations. Proper routing involves getting a connecting line from one point to another through the most logical path, while considering other factors as discussed below. The most logical path is not always the direct path and should have the following characteristics: 

  • Avoid excessive strain on joint — A strained joint will eventually leak. A straight-line tube assembly (with no bends) or a joint that is forced into position are common examples of strain applied to tube assemblies. 
  • Allow for expansion and contraction — Use a “U” bend or a hose in long lines to allow for expansion and contraction due to pressure or temperature fluctuations. 
  • Allow for motion under load — Even some apparently rigid systems do move under load. Use an offset (“S”) bend. 
  • Get around obstructions without using excessive amount of 90° bends — Pressure drop due to one 90° bend is greater than that due to two 45° bends. 
  • Keep tube lines away from components that require regular maintenance. 
  • Leave fitting joints as accessible as possible — Inaccessible joints are more difficult to assemble and tighten properly, and more time consuming to service. 
  • Have a neat appearance and allow for easy troubleshooting, maintenance, and repair

For more details on proper routing read our article: Tube Routing Tips for Hydraulic, Pneumatic and Lubrication Systems.  


Finding the root cause of leaks is not always obvious and if incorrect assumptions are made, the leak will appear again soon. The details included in this post should help to identify the root cause(s) of the leak and from there, you can fix it and ensure that you do not have the same issue again.  

As we mentioned above, consider every probable cause we have listed in this section as there could be more than one problem. Though taking the time to inspect your system as outlined above can take some time, it is a drop in the bucket when considering the repeated system leakage that will happen when the true cause was not found and fixed properly. If you are unsure about a leak cause or what you should do about a leak once you find the cause, feel free to reach out. We have often seen the issue before and can get you pointed in the right direction. We are here to help.  

The next post in our troubleshooting leaks series is Troubleshooting Leaks: Longitudinal Tube Cracks  

Share your stories

Have you had any tube cracks or tube connection leaks where the source was not what you anticipated? Share your story below with what happened and how you determined what was really going on. 

Refer to our Tube Fittings Division Frequently Asked Questions page,, for a wealth of information and resources. 

If you have any questions or comments, please post them, and we will respond if warranted. To talk to our techConnect engineer team directly, they can be reached at Parker Tube Fittings Division, 614.279.7070. See Parker Tube Fittings Division's full line of tube fittings and hose adapters

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Troubleshooting Leaks: Understanding Circumferential Tube Cracks Emily Alexander, senior design engineer at Parker Tube Fittings Division


Article contributed by Emily Alexander, senior design engineer, Parker Tube Fittings Division





Troubleshooting Leaks: Understanding Circumferential Tube Cracks Ted Amling, senior project engineer, Parker Tube Fittings Division


and Ted Amling, senior project engineer, Parker Tube Fittings Division






Related resource articles:

Troubleshooting Leaks: Fixing a Leak Coming From a Tube or Swivel Nut

Troubleshooting Leaks: Fixing a Port End Connection Issue

Troubleshooting Leaks: Understanding a Leak from a ORFS Braze Sleeve

Leaking Ports? Troubleshooting for SAE J1926 and ISO 6149 Ports

Proper Assembly Steps for 37° Flare Fittings Using the Flats Method

Metric Ports: Which Fitting Goes With Which Metric Port?

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