A Real Engineering Investigation into Seal Leakage in Hydraulic Cylinders
Oil leaking from the rod seal is one of the most common problems reported in hydraulic cylinders used in injection molds.
When oil appears around the piston rod, the immediate conclusion is often that the seals have failed or that the cylinder was manufactured with defective components.
In many situations, however, the seals are not the real cause of the leakage.
Seal leakage is frequently the visible consequence of another mechanical problem affecting the entire hydraulic system.
Understanding the difference between the cause of the leak and the point where the leak appears is essential for preventing repeated failures.
This real engineering case demonstrates how the Vega Technical Department investigated an apparent seal problem, considered several possible causes and proposed a systematic engineering approach instead of immediately replacing components.
The Customer’s Problem
A customer had purchased four V450CM hydraulic cylinders during the previous year.
After remaining in storage for several months, the cylinders were installed in an injection mold and production started.
After approximately two days of serial production, oil leakage appeared on the piston rods of three cylinders.
The fourth cylinder was installed in a position that prevented visual inspection, making it impossible to determine whether leakage was also present there.
Initially, the hydraulic system operated at approximately 45 bar, and no leakage was observed.
During production, however, the operating pressure was increased to approximately 60 bar, after which oil streaks became visible around the piston rods.
The hydraulic system was capable of reaching 175 bar, while the cylinders themselves were designed for a maximum operating pressure of 450 bar.
The customer therefore asked an obvious question:
Was the problem caused by defective cylinders or by the application itself?
The First Engineering Assessment
Rather than immediately assuming that the seals had failed, the Vega Technical Department carefully reviewed the available information.
One detail immediately appeared unusual.
All three visible cylinders showed the same symptom.
From a statistical perspective, it is highly unlikely that several independently tested hydraulic cylinders would all develop identical seal defects after only a few days of operation.
Every Vega hydraulic cylinder is pressure-tested before shipment.
If a manufacturing defect had been present in the sealing system, it would almost certainly have been detected during factory testing.
This observation suggested that another external factor might be responsible.
Could Radial Loads Be the Real Cause?
After reviewing the photographs supplied by the customer, the Vega Technical Manager identified one possible explanation.
The hydraulic cylinders appeared to be connected to the moving mechanism through a rigid linkage with a long lever arm.
This type of arrangement can generate radial loads on the piston rod.
Unlike axial forces, which hydraulic cylinders are specifically designed to withstand, radial loads introduce bending forces that may cause slight rod misalignment during operation.
Even relatively small misalignment can have significant consequences for the sealing system.
The rod seal is designed to operate with the piston rod moving concentrically through the guide system.
When lateral forces are introduced, contact pressure on the seal becomes uneven, accelerating wear and allowing hydraulic oil to escape.
Why Floating Joints Matter
For this reason, the Vega Technical Department has always recommended using floating joint connections whenever possible.
A floating joint allows the cylinder rod to compensate for small alignment errors between the cylinder and the driven mechanism.
Instead of transmitting bending forces directly into the rod, the joint absorbs minor angular deviations.
This significantly reduces:
- radial loads;
- seal wear;
- guide wear;
- rod bending stresses;
- long-term maintenance costs.
Many oil leakage problems can be prevented simply by improving the mechanical connection rather than replacing hydraulic components.
Are the Seals Really Defective?
The customer also questioned whether the seals themselves could have deteriorated while the cylinders remained in storage for approximately one year before installation.
Although elastomer seals can age under unsuitable storage conditions, simultaneous leakage on multiple cylinders after only a few days of operation suggested that another explanation was more likely.
Without inspecting the returned cylinders, however, no definitive conclusion could be reached.
Professional engineering requires objective evidence rather than assumptions.
Why Returning the Cylinders Was Important
The Vega Technical Department explained that determining the real cause of the leakage required detailed inspection of the cylinders at the factory.
Only a complete examination could verify:
- seal condition;
- guide wear;
- rod alignment;
- surface finish;
- contamination;
- assembly conditions.
Without this analysis, any conclusion would remain speculative.
Root cause analysis always depends on direct inspection whenever possible.
Supporting the Customer While Continuing the Investigation
The customer explained that removing even one cylinder from the mold would stop production completely.
Understanding the urgency of the situation, the Vega Technical Department proposed a practical solution.
Rather than waiting for the complete investigation, Vega offered to supply replacement seal cartridges under commercial warranty, allowing production to continue while maintaining the opportunity to investigate the returned cylinders later.
This approach balanced customer support with sound engineering practice.
An Unexpected Recommendation About Hydraulic Pressure
One recommendation in the technical correspondence may initially appear surprising.
After replacing the seal cartridges, the Vega Technical Manager recommended increasing the hydraulic operating pressure to approximately 80 bar.
Many people assume that reducing hydraulic pressure always decreases leakage.
In reality, sealing systems operate within an optimum pressure range.
According to the engineering evaluation, the standard sealing system achieved its best performance when operating above approximately 70 bar.
Insufficient pressure may prevent the seal from operating under its ideal conditions, while excessive pressure creates different challenges.
Proper seal performance therefore depends on the complete operating conditions rather than pressure alone.
Lessons Learned from This Real Engineering Case
This case illustrates several important engineering principles.
Oil leakage should never automatically be interpreted as seal failure.
The visible leak may simply indicate another mechanical problem elsewhere in the application.
Likewise, replacing seals without investigating the root cause often results in the same failure reappearing after a relatively short period of operation.
Professional technical support requires evaluating the complete mechanical system, including alignment, loading conditions, operating pressure and installation design.
Only after considering all these factors can the true cause of leakage be identified.
Engineering Conclusions
This real technical support case demonstrates that rod seal leakage is not always caused by defective seals.
The Vega Technical Department evaluated the installation, considered the possibility of radial loads caused by a rigid mechanical connection, recommended the use of floating joints and requested the return of the cylinders for detailed engineering analysis.
At the same time, Vega supported the customer’s production by supplying replacement seal cartridges under commercial warranty and recommending operating conditions that would optimize the performance of the standard sealing system.
The most important lesson is clear:
Replacing leaking seals may solve the immediate problem, but identifying and eliminating the true mechanical cause is the only way to prevent the leakage from returning.
