Why Two Hydraulic Cylinders Never Move at Exactly the Same Speed

Understanding the Real Causes of Uneven Hydraulic Cylinder Motion

In injection molds, it is often assumed that two identical hydraulic cylinders supplied with the same oil pressure will always move at exactly the same speed.

From a theoretical point of view, this assumption appears reasonable.

If both cylinders have identical dimensions, receive the same hydraulic pressure and are installed on the same mold, many engineers expect perfectly synchronized movement.

Reality is considerably more complex.

Even small differences in hydraulic flow, internal friction, seal compression, oil contamination or installation conditions can produce noticeable differences in cylinder speed.

A real engineering case handled by the Vega Technical Department demonstrates why hydraulic cylinder synchronization should never be evaluated by considering only the cylinder itself.

Instead, the complete hydraulic system must be analysed before reaching any conclusions.

The Customer’s Observation

The customer was testing four V260CF self-locking hydraulic cylinders installed in an injection mold.

During the mold trials, another set of hydraulic cylinders manufactured by a different supplier was also tested under the same operating conditions.

According to the customer, both cylinder sets were tested:

  • on the same injection molding machine;
  • using the same hydraulic pump;
  • with the same hydraulic hoses;
  • under the same operating pressure.

Despite these apparently identical conditions, the customer observed that the two cylinder types did not behave in exactly the same way.

The cylinders supplied by the competitor appeared to move more smoothly and reached the expected operating speed, while the Vega cylinders initially appeared slower.

At first glance, this might suggest that the hydraulic cylinders themselves were responsible for the difference.

The Vega Technical Department, however, adopted a completely different engineering approach.

Never Blame the Hydraulic Cylinder First

One of the most important principles of hydraulic troubleshooting is to avoid assuming that the visible symptom immediately identifies the defective component.

When two hydraulic cylinders behave differently, many possible factors must be investigated before comparing the cylinders themselves.

Among them are:

  • hydraulic pump capacity;
  • oil flow rate;
  • oil viscosity;
  • hydraulic hose dimensions;
  • pressure losses;
  • seal friction;
  • internal leakage;
  • oil contamination;
  • mechanical alignment.

Only after analysing these variables can engineers determine whether the cylinder itself is responsible for the observed behaviour.

Pressure Does Not Determine Cylinder Speed

One of the most common misconceptions in hydraulics is confusing pressure with flow rate.

Pressure determines the force generated by a hydraulic cylinder.

Flow rate determines how fast that cylinder moves.

These two parameters are closely related but perform completely different functions.

Increasing hydraulic pressure does not automatically increase cylinder speed.

If the pump cannot supply sufficient oil volume, the cylinder will move slowly regardless of the pressure available.

Understanding this distinction is fundamental when analysing synchronization problems.

The Importance of Pump Flow Rate

After reviewing the customer’s videos, the Vega Technical Department immediately focused on the hydraulic pump used during the tests.

The engineering evaluation concluded that the pump originally used for testing did not provide sufficient nominal oil flow.

Subsequent testing performed with a hydraulic pump delivering approximately 35 litres per minute produced significantly better synchronization between the cylinders.

This observation demonstrates an important engineering principle.

Hydraulic cylinders cannot move faster than the oil supplied to them.

If the available flow is insufficient, even perfectly manufactured cylinders will not operate as expected.

Why Identical Cylinders May Still Behave Differently

Even when two hydraulic cylinders have exactly the same dimensions, perfect synchronization cannot always be expected.

Small manufacturing tolerances are normal in every mechanical system.

In hydraulic cylinders these tolerances may influence:

  • seal interference;
  • guide bushing friction;
  • piston friction;
  • oil film formation;
  • breakaway force.

Individually, these differences are usually very small.

Combined with variations in hydraulic flow, however, they may become visible during operation.

Seal Friction Also Influences Motion

After additional testing, the customer reported that the competitor’s cylinders continued to move slightly faster under identical operating conditions.

The Vega Technical Department proposed a possible engineering explanation.

According to the analysis, the competitor’s cylinders probably used piston seals with lower interference (lower seal compression) than the Vega cylinders.

This reduced seal interference produces lower friction during movement, allowing the cylinders to move more freely under the same hydraulic conditions.

This observation is particularly interesting because it highlights an engineering trade-off.

Reducing seal interference may improve movement speed.

However, higher seal interference often provides better sealing performance, reduced internal leakage and improved long-term reliability.

Hydraulic cylinder design therefore requires balancing speed, sealing efficiency and durability rather than optimizing only one parameter.

Testing Conditions Must Always Be Identical

Another valuable lesson from this case concerns comparative testing.

When engineers compare hydraulic cylinders from different manufacturers, every operating parameter should be carefully controlled.

Meaningful comparisons require:

  • identical hydraulic pumps;
  • identical oil temperature;
  • identical oil viscosity;
  • identical flow rate;
  • identical pressure;
  • identical hydraulic hoses;
  • identical loads;
  • identical mold conditions.

Even small differences in one of these variables may produce misleading conclusions.

For this reason, the Vega Technical Department based its evaluation on controlled testing rather than assumptions.

Looking Beyond the Visible Symptom

One of the strengths of this engineering case is the systematic diagnostic methodology.

Instead of concluding that the hydraulic cylinders were defective, the Vega Technical Department first analysed:

  • hydraulic pump performance;
  • available oil flow;
  • testing conditions;
  • seal friction;
  • possible mechanical influences.

Only after eliminating these variables did the engineers begin comparing the internal characteristics of the hydraulic cylinders themselves.

This structured engineering approach prevents incorrect conclusions and helps identify the true source of performance differences.

Looking Beyond the Cylinder to Find the Real Cause

In Part 1, we explained why hydraulic cylinder speed depends on much more than operating pressure.

The Vega Technical Department demonstrated that hydraulic flow rate, seal friction, oil delivery and testing conditions all influence cylinder performance.

However, the investigation did not stop there.

During maintenance, another important discovery completely changed the engineering analysis.

Internal Scratches Revealed Another Possible Cause

While inspecting the hydraulic cylinders, the Vega Technical Department discovered that two of the cylinders showed scratches inside the rear body.

Although the cylinders had been operating, these internal marks could increase friction and negatively affect movement smoothness.

Rather than attempting a repair, the engineering team decided to replace both cylinders free of charge, ensuring that subsequent testing would be carried out using components in perfect condition.

This decision demonstrates an important engineering principle.

Reliable troubleshooting requires eliminating every possible source of uncertainty before drawing conclusions.

Hydraulic Oil Cleanliness Is More Important Than Many Engineers Realize

The presence of scratches immediately suggested another possible explanation.

The Vega Technical Department considered that metallic particles circulating in the hydraulic oil could have caused the damage.

For this reason, the customer was advised to inspect the hydraulic pump and verify the cleanliness of the hydraulic circuit.

Hydraulic oil performs far more functions than simply transmitting pressure.

It also:

  • lubricates moving components;
  • protects sealing surfaces;
  • removes heat;
  • prevents wear.

When contamination enters the hydraulic system, even microscopic metallic particles can damage precision-machined surfaces.

Over time, this may increase internal friction, accelerate seal wear and reduce overall cylinder performance.

Friction Is Not Always a Defect

After the mold trials, the customer continued comparing the Vega cylinders with those supplied by another manufacturer.

Although the competitor’s cylinders appeared to move faster, the Vega Technical Department explained that this difference was most likely related to lower piston seal interference, resulting in reduced friction.

From an engineering perspective, this does not automatically mean one cylinder is better than another.

Lower friction may improve movement speed.

However, it may also reduce sealing effectiveness under certain operating conditions.

Conversely, a seal with slightly higher interference generally provides:

  • improved sealing stability;
  • reduced internal leakage;
  • better performance over long operating periods;
  • greater reliability in demanding applications.

Hydraulic cylinder design is therefore always a compromise between speed, sealing performance and service life.

Engineering Requires Understanding the Entire Hydraulic System

One of the most valuable lessons from this case is that the hydraulic cylinder was never analysed in isolation.

The Vega Technical Department considered the complete hydraulic system, including:

  • hydraulic pump capacity;
  • oil flow rate;
  • oil cleanliness;
  • internal component condition;
  • seal characteristics;
  • operating pressure;
  • testing procedures.

Only after evaluating all these variables could the engineers understand why apparently identical hydraulic cylinders produced different results.

This systematic methodology prevents incorrect conclusions and unnecessary component replacements.

Comparing Hydraulic Cylinders Requires More Than Watching Their Speed

Many engineers instinctively judge hydraulic cylinders by observing how quickly they move.

While movement speed is certainly important, it represents only one aspect of hydraulic performance.

A complete engineering evaluation should also consider:

  • sealing reliability;
  • resistance to contamination;
  • internal leakage;
  • durability;
  • maintenance requirements;
  • long-term operating stability.

A cylinder that moves slightly faster during a short demonstration is not necessarily the most reliable solution over millions of production cycles.

This distinction is particularly important in injection molds, where reliability is often more valuable than achieving the highest possible speed.

Root Cause Analysis Always Comes Before Conclusions

Perhaps the greatest engineering lesson from this case is the diagnostic philosophy adopted by the Vega Technical Department.

Rather than accepting the first explanation offered by the customer, the engineering team systematically investigated:

  • hydraulic flow rate;
  • hydraulic pump performance;
  • cylinder condition;
  • oil contamination;
  • seal interference;
  • comparative testing conditions.

Only after analysing the complete hydraulic system did the engineers provide a technical explanation for the observed differences.

This approach illustrates the essence of Root Cause Analysis (RCA).

Engineering is not about identifying the first visible difference.

It is about understanding every factor that influences the final result.

Conclusion

This real engineering case demonstrates that two hydraulic cylinders operating under apparently identical conditions may still exhibit different movement characteristics.

The investigation carried out by the Vega Technical Department showed that hydraulic flow rate, seal interference, oil cleanliness, internal component condition and testing methodology all contribute to overall cylinder performance.

Rather than attributing the different movement immediately to the hydraulic cylinders themselves, the engineering team systematically analysed the entire hydraulic system, replaced damaged cylinders found during inspection and considered every possible influencing factor before reaching its conclusions.

Ultimately, this case reinforces one of the most important principles of hydraulic engineering:

Perfect synchronization is achieved not by focusing on the hydraulic cylinder alone, but by optimising the complete hydraulic system—including flow rate, oil quality, sealing characteristics, maintenance and testing conditions.

Related Articles (Verified on icvega.com)

The following official Vega articles provide additional information related to hydraulic cylinder performance and application engineering:

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