Leak repair part 4 - Repairing circumferential cracks in hydraulic tubing

The next step in our series on troubleshooting leaks in hydraulic systems is to understand what causes the cracks that appear around the circumference of hydraulic tubing. We are especially interested in those located close to a tube fitting in the hydraulic system, as shown in picture 1.

Picture 1

In the first blog post in our series on troubleshooting leaks in hydraulic systems, we highlighted the following issues in relation to circumferential cracks in hydraulic tubing.

• High vibration in the system
• Insufficient/incorrect clamping of the tube
• Incorrect/misaligned tube routing
• Incorrect tube routing

Before reworking or repositioning the cracked hydraulic tube, it is important to determine which of the likely causes listed above is the root of the leak. All of these causes should be investigated, as in some cases a leak may have several causes. By identifying the correct cause of the leak, you minimise the risk of the damage recurring in the same place.

To get the most accurate picture, it can make good sense to disassemble the system where the leak has occurred, since circumferential cracks are hard to spot and are often located underneath the tube support ring. As we have mentioned in all our articles in this series of blog posts, it is important always to take the proper safety precautions and to make sure the system has been shut down before taking it apart. If you are unsure which safety precautions are required, seek advice from someone who knows.

This blog post contains information and images featuring O-lok, Triple-Lok/JIC, but also applies to DIN fittings and mm tubing.

High vibration in the system

When designing a hydraulic system, vibration must be taken into account. Excessive vibration in a system containing tube fittings can be harmful. Tube fittings are rigid connections, so the vibration is absorbed into the connection. If too much vibration occurs, the connection will fail quickly and create a leak. This failure can be anything from a loosened nut to a broken fitting or a cracked hydraulic tube.

If you find that your hydraulic system has a circumferential crack in a tube, you should inspect the system carefully for other signs of excessive vibration, such as a loose tube nut. This type of failure is caused by metal fatigue and will not have a predictable time frame for when it breaks. Hydraulic systems with high vibration can cause other and more expensive component failures; a loose nut or a crack in a tube may just be the first symptoms to appear. To prevent vibration from affecting the tubing, the tubes must be clamped correctly to avoid unnecessary metal fatigue failures.

Insufficient or incorrect clamping of the tube

You can expect your hydraulic system to have a certain amount of movement. To protect your system, however, the tubes must be clamped to a rigid structure such as a wall wherever possible. Correct clamping helps dampen flexing and vibration in the tube routing, thereby preventing failures caused by metal fatigue. If you notice a circumferential crack in a hydraulic tube, the system may not have the correct tube clamping.

Below is an extract translated into Danish from Parker's Tube Fittings Division catalogue 4300 & 4100 on tube clamping.

Tube clamps serve two primary purposes in the hydraulic tube system: mounting and vibration damping. Metal fatigue failures due to mechanical vibration account for the majority of tube routing failures. Correct clamping of the tube also reduces system noise.
Use a clamping system such as Parker Ermeto Tube clamps together with the correct clamping spacing, as specified in the section on mounting and installation.
For tube clamps to dampen vibration effectively, they must be secured to a rigid structure, e.g. a wall.

Hydraulic tubing must be securely clamped at regular intervals, as shown in picture 2. If the clamping is not carried out correctly, it can affect the system adversely, which will impact the entire system and not just the individual connection.

When designing a hydraulic system, it is easiest and safest to design the tube clamps in from the outset.

Picture 2

Misalignment and poor fit between tube and nut

A critical step in the assembly process is aligning the tube with the fitting. If the tube and the sealing surface of the fitting are not aligned correctly before the nut is tightened, a leak will occur. One symptom of misalignment is a circumferential crack in the tube, caused by excessive stress on the tube assembly. The correct procedure is always to align the tube and the sealing surface of the fitting properly before tightening the nut. Take a look at your connection: if the tube looks like the one on the left in picture 3, the tube and fitting are misaligned. If it looks like the one on the right in picture 3, it is aligned correctly but has a poor fit. Both can result in excessive stress and ultimately a potential fatigue failure.

Picture 3

With a tube that is not aligned correctly, instead of forcing the gap shut by pressing the tube down until it fits, you should bend the tube properly so that it lines up with the fitting. If the tube does not fit the span because it is the wrong length or has been bent in the wrong place, you should not try to force it into place either. In most cases, you will have to remove the tube and make a new one so that it reaches the connection – see picture 4.

If there is a gap between the fitting and the tube due to misalignment, a leak can occur because the sealing surfaces do not meet correctly. Do not use the nut to close the gap. If the tube is not aligned correctly, you can put unnecessary stress on the system, which can lead to long-term problems such as cracks in the tubes.

Picture 4

Misalignment or poor fit can affect two things:

• The tightening torque
• The quality of your connection

If you use the nut to make up for the gap between tube and fitting, you put stress on your connection. If the connection is not aligned correctly before the nut is tightened, most of the torque will be used to align the tube and fitting, and there may not be enough clamping load, which is necessary to assemble the connection correctly.

When the tube is correctly aligned, the nut can be screwed all the way on by hand, until the final tightening with an open-end wrench.
We strongly advise against using a nut to pull a misaligned connection together!
It will place excessive stress on the assemblies, leading to leakage, circumferential cracks in the tube or, in extreme cases, deformation of the fitting.

Incorrect tube routing

How you plan to route the tubes should be a priority when designing a hydraulic system. Incorrect routing can result in excessive stress on the tube assembly, which can lead to a crack in the tube and connection problems in the form of misalignment and poor fit.

Tube routing is one of the most difficult yet most important considerations when designing a hydraulic system. Correct routing involves creating a connection from one point to another – along the most logical path, while considering the other factors described below. The most logical path is not always the most direct one, and it should have the following characteristics:

• Avoid excessive stress on the tube assembly – A stressed tube assembly will always start to leak eventually. A straight tube assembly (without bends) or a tube assembly forced into place are common examples of stressed tube assemblies.
• Allow for expansion and contraction – Use a “U” bend or a hose in long runs to allow for expansion and contraction caused by pressure and temperature fluctuations.
• Allow for movement under load – Even some apparently fixed systems move under load. Use an offset (“S”) bend.
• Route around obstacles without using too many 90 degree bends – There is a greater risk of pressure loss with a 90^o degree bend than with two 45^o bends.
• Keep the hydraulic tubes away from components that require regular maintenance.
• Keep the tube assemblies as accessible as possible – Inaccessible assemblies are harder to assemble and tighten correctly, and they are also more time-consuming to service.
• Make sure the hydraulic system looks tidy – that makes troubleshooting, maintenance and repair easy.
  
  
  

Summary

The cause of leaks in a hydraulic system is not always obvious, and if incorrect assumptions are made as well, the leak will soon reappear. This article should help you identify the cause of the leak or leaks, so you can have them repaired correctly and make sure they do not happen again.

If you are unsure about the cause of a leak in your hydraulic system, or what to do about the leak once you have found it, you are welcome to contact us. We are here to help.