Root Cause Analysis Before Repair: The Engineering Approach

In Part 1, we examined a real engineering case involving a stack mold equipped with 24 Vega hydraulic cylinders that reportedly lost approximately 200 litres of hydraulic oil after around 70,000 production cycles. Although the customer immediately suspected worn seals and requested replacement sealing cartridges, the actual source of the leakage had not yet been identified.

Rather than accepting this initial diagnosis, the Vega Technical Department followed a completely different engineering approach.

Instead of asking “Which seal should we replace?”, the first question became:

“Where is the oil actually leaking?”

That simple change in perspective is the foundation of every successful root cause analysis.

Replacing Components Is Not the Same as Solving the Problem

One of the most common mistakes in industrial maintenance is confusing a corrective action with a diagnosis.

Replacing a seal kit may temporarily eliminate an oil leak.

However, if the leak originated elsewhere, the problem will eventually return.

This not only increases maintenance costs but also creates unnecessary machine downtime.

Professional troubleshooting therefore follows a different sequence:

1. Confirm that a leak actually exists.
2. Locate the exact leakage path.
3. Determine why the leakage occurred.
4. Only then decide which components require replacement.

Skipping any of these steps significantly increases the probability of an incorrect repair.

Dynamic and Static Seals Behave Differently

A hydraulic cylinder contains several different sealing systems, each performing a specific function.

From a maintenance perspective, it is essential to distinguish between:

• dynamic seals, which operate on moving components such as the piston rod;
• static seals, such as O-rings located between fixed components.

These sealing systems fail for different reasons.

Dynamic seals are generally affected by:

• rod wear;
• contamination;
• inadequate lubrication;
• excessive temperature;
• side loading.

Static O-rings are more frequently influenced by:

• incorrect groove dimensions;
• improper compression;
• damaged sealing surfaces;
• assembly errors;
• ageing after prolonged service.

Without identifying which sealing system is actually leaking, replacing every seal inside the cylinder is rarely the most efficient solution.

The O-Ring Groove May Be Part of the Problem

One particularly interesting observation made by the Vega Technical Department concerns the O-ring installation itself.

Although Stefano Rogora did not believe that the manifold machining represented the primary cause of the leakage, he nevertheless recommended inspecting the condition of the existing O-rings after approximately 70,000 operating cycles.

If necessary, Vega proposed modifying the existing cylinders by increasing both the O-ring diameter and the dimensions of the O-ring groove.

This recommendation highlights an important engineering principle.

A sealing problem is not always caused by the sealing material.

Sometimes the geometry supporting the seal must also be reconsidered.

Even the highest-quality O-ring cannot compensate for an unsuitable groove design.

Every Leak Has a Physical Origin

Hydraulic oil cannot simply disappear.

Every leak follows a physical path.

Identifying that path is often more valuable than immediately replacing components.

Possible leakage paths include:

• piston rod seals;
• piston seals;
• static O-rings;
• hydraulic ports;
• threaded fittings;
• adapters;
• manifold interfaces;
• damaged machined surfaces;
• improperly assembled connections.

The larger the hydraulic system becomes, the more possible leakage points must be considered.

In this case, the reported loss of approximately 200 litres made accurate diagnosis even more important because the leakage source remained uncertain.

Engineering Means Challenging Assumptions

Perhaps the most valuable aspect of this case is not the technical recommendation itself.

It is the reasoning behind it.

The customer assumed that softer seals might solve the problem because the cylinders were operating at approximately 40 bar.

The Vega Technical Department did not reject this idea outright.

Instead, Stefano Rogora questioned whether the seals were actually responsible.

He reminded the customer that an earlier discussion had focused on a completely different issue involving the guidance system of the mechanical control shaft rather than hydraulic sealing.

This illustrates one of the most important habits of experienced engineers.

Good engineers do not simply answer the customer’s question.

They verify whether the customer is asking the right question.

Small Design Improvements Can Prevent Major Repairs

Another interesting aspect of this case is that Vega proposed improving the existing cylinder rather than replacing it.

By supplying a longer control shaft, the guidance of the mechanical switching system could be increased without changing the entire hydraulic cylinder.

This demonstrates another important engineering principle.

Reliability improvements are not always achieved through major redesigns.

In many situations, relatively small modifications can eliminate recurring maintenance problems while preserving the existing equipment.

A Structured Diagnostic Procedure

Cases like this illustrate why professional maintenance departments follow structured diagnostic procedures before ordering replacement parts.

A systematic investigation should include:

• identifying the exact leakage location;
• cleaning the cylinder before inspection;
• checking all hydraulic connections;
• inspecting static sealing points;
• inspecting dynamic sealing surfaces;
• examining rod condition;
• verifying manifold machining;
• reviewing operating pressure;
• confirming previous maintenance history.

Only after completing these checks should replacement components be selected.

This method reduces unnecessary repairs while significantly improving long-term reliability.

The Difference Between Repair and Engineering

The most important lesson from this engineering case is that replacing seals and solving a hydraulic problem are not necessarily the same thing.

Repairs based on assumptions often become expensive cycles of repeated maintenance.

Engineering, on the other hand, begins by understanding the real failure mechanism.

The Vega Technical Department did not simply provide replacement parts.

Instead, it analysed previous discussions, questioned the original diagnosis, evaluated alternative failure mechanisms and proposed modifications only after considering the complete application.

This approach transforms maintenance from reactive troubleshooting into preventive engineering.

Conclusion

This real engineering case demonstrates that hydraulic oil leakage should never be treated as proof of seal failure.

An oil leak is simply evidence that hydraulic fluid is escaping from the system.

The engineer’s task is to determine where, why, and under which operating conditions the leakage occurs before selecting a corrective action.

By challenging assumptions and following a structured root cause analysis, the Vega Technical Department avoided recommending unnecessary component replacements and instead focused on identifying the actual source of the customer’s problem.

The lesson extends far beyond this particular case.

Whether working on injection molds, die casting dies or any hydraulic system, the most effective repairs begin with a correct diagnosis—not with replacement parts.