Internal Hydraulic Cylinder Leakage: Causes, Diagnosis and Solutions

A Real Engineering Case on Piston Seal Failure, Internal Bypass and Hydraulic Cylinder Troubleshooting

Hydraulic cylinder leakage is one of the most common problems encountered in industrial hydraulic systems. However, not all leaks are visible.

While external leakage is usually easy to identify because hydraulic oil escapes from seals or ports, internal leakage is often much more difficult to diagnose.

A hydraulic cylinder affected by internal leakage may appear perfectly dry from the outside while gradually losing force, speed and positioning accuracy.

In many cases, the cylinder simply stops performing as expected because hydraulic oil bypasses the piston instead of generating useful force.

This real engineering case demonstrates how the Vega Technical Department investigated a customer’s report of internal leakage affecting several hydraulic cylinders and identified the most probable causes before recommending a complete technical inspection.

The Customer’s Problem

A customer operating more than 20 Vega hydraulic cylinders reported that three or four cylinders had developed internal leakage.

The reported symptom was very specific.

Whenever hydraulic oil was supplied to one port of the cylinder, oil simultaneously appeared at the opposite port, indicating that hydraulic fluid was passing internally from one chamber to the other.

Unlike external leakage, no oil was escaping outside the cylinder.

The problem occurred entirely inside the hydraulic actuator.

What Is Internal Hydraulic Cylinder Leakage?

Internal leakage occurs when hydraulic oil flows from the pressurized chamber of the cylinder to the opposite chamber without producing useful mechanical force.

Instead of remaining separated by the piston sealing system, the oil bypasses the piston.

The consequences include:

  • reduced pushing force;
  • lower pulling force;
  • slower cylinder movement;
  • inability to maintain position;
  • loss of holding force;
  • unstable machine operation.

Because the leakage remains inside the cylinder, the fault is often overlooked until machine performance deteriorates significantly.

Recognising Internal Bypass

One of the easiest ways to identify internal leakage is observing the hydraulic ports.

If pressure is applied to one side of the cylinder and oil immediately exits through the opposite port without piston movement, an internal bypass is likely present.

This was exactly the condition described by the customer in this engineering case.

Although additional testing is always recommended, this behaviour is a strong indication that the sealing system is no longer separating the two pressure chambers correctly.

The Most Common Causes

After reviewing the information provided by the customer, the Vega Technical Department identified two possible causes.

1. Damaged Piston Seals

The piston seals are responsible for maintaining complete hydraulic separation between the two chambers.

If these seals become worn, damaged or chemically degraded, hydraulic oil can bypass the piston.

Even relatively small seal damage may produce a noticeable reduction in cylinder performance.

2. Scratches Inside the Cylinder Bore

The second possible cause was the presence of scratches or scoring inside the cylinder body or tube.

Surface damage inside the bore can prevent the piston seals from maintaining proper contact with the cylinder wall, creating an internal leakage path.

Why Internal Leakage Cannot Be Diagnosed Remotely

One important aspect of this case is that the Vega Technical Department did not immediately identify a definitive cause.

Instead, the customer was asked to return the affected cylinders for detailed inspection and analysis.

This reflects good engineering practice.

Several different failure mechanisms may produce similar symptoms, including:

  • damaged piston seals;
  • worn wear rings;
  • scored cylinder bores;
  • contaminated hydraulic oil;
  • incorrect assembly after maintenance.

A proper technical inspection is therefore essential before replacing components.

The Importance of Correct Serial Numbers

During the investigation another practical issue emerged.

One of the serial numbers supplied by the customer was incomplete, making it impossible to identify the exact cylinder configuration.

The Vega Technical Department therefore requested the correct identification before proceeding with the analysis.

This highlights the importance of accurate product identification when diagnosing hydraulic problems.

Different cylinder models may use different sealing systems, tolerances and spare parts.

Preventing Internal Leakage

Although seal wear cannot be eliminated completely, several preventive measures significantly increase cylinder life:

  • maintain clean hydraulic oil;
  • replace filters regularly;
  • avoid contaminated hydraulic circuits;
  • prevent rod damage;
  • avoid excessive side loads;
  • operate within specified pressure limits;
  • perform regular preventive maintenance.

Early detection of internal leakage often prevents more extensive cylinder damage.

Engineering Lessons Learned

This case demonstrates several important engineering principles.

Whenever internal leakage is suspected:

  • distinguish between internal and external leakage;
  • verify the hydraulic ports for internal bypass;
  • inspect piston seals;
  • inspect the cylinder bore for scoring;
  • confirm the cylinder identification;
  • perform a complete technical inspection before replacing components.

Correct diagnosis almost always reduces maintenance costs and avoids unnecessary replacement of otherwise serviceable cylinders.

Conclusions

Internal hydraulic cylinder leakage is one of the most misunderstood hydraulic failures because no external oil leakage may be visible.

In this real engineering case, the Vega Technical Department identified damaged piston seals and scratches inside the cylinder bore as the most probable causes of internal bypass, recommending detailed inspection before determining the final corrective action.

The key lesson is clear:

When oil flows internally between the two cylinder chambers, the problem is rarely visible from the outside. Accurate diagnosis requires careful inspection of the sealing system and the internal cylinder surfaces.

Further Technical Reading

To learn more about hydraulic cylinder reliability and troubleshooting, we recommend these technical resources from the Vega Technical Blog:

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