Why Multiple Hydraulic Cylinders May Not Move Together: Diagnosing an Unbalanced Hydraulic Circuit

When several hydraulic cylinders are used to move the same mold plate, designers naturally expect them to move simultaneously and at the same speed.

In practice, this is not always the case.

A technical case analyzed by the Vega Team involved a mold equipped with four hydraulic cylinders. The Customer reported that the cylinders could not move together correctly, preventing the mold from performing the required movement smoothly.

The case is particularly interesting because the investigation showed that replacing the cylinders was not necessarily the correct solution. The problem could originate in the hydraulic circuit, mechanical resistance, cylinder tolerances, internal leakage or the way the hydraulic flow was distributed.

The key lesson is:

When several hydraulic cylinders do not move together, the cylinders themselves should not automatically be considered the cause. The complete hydraulic and mechanical system must be analyzed.

A Real Application with Four Hydraulic Cylinders

The mold in this case had dimensions of approximately 1020 × 900 × 1850 mm and used four hydraulic cylinders.

The Customer reported that the cylinders were unable to move simultaneously, causing the mold movement to become incorrect.

The Customer initially asked Vega to help identify a cylinder configuration that could improve the synchronized movement of the four cylinders. Several cylinder configurations were subsequently evaluated during the technical discussion.

However, the Vega Team immediately recognized an important point:

Changing the cylinders alone might not solve the problem.

The hydraulic circuit itself could be responsible for the unequal movement.

Why Identical Hydraulic Cylinders Do Not Always Move at the Same Speed

It is tempting to think that four identical cylinders connected to the same hydraulic system should automatically move at the same speed.

Hydraulic systems are more complicated.

Even cylinders manufactured according to the same design can have small differences in:

  • seal friction;
  • guide friction;
  • machining tolerances;
  • seal compression;
  • internal resistance;
  • hydraulic efficiency.

The Vega Team specifically noted that differences in the tolerances of seal seats and guide-bushing seats can create different levels of interference between otherwise identical cylinders.

These differences are normally small.

However, when several cylinders are mechanically connected to the same mold plate, even relatively small differences can become significant.

Current Vega technical documentation confirms the same principle: identical cylinders can experience differences caused by friction, manufacturing tolerances, hydraulic resistance, pipe dimensions, load distribution and guide friction.

The Hydraulic Circuit May Be the Real Problem

One of the most important observations in the case was that part of the hydraulic circuit passed through the mold plate.

The Vega Team could not clearly identify all the oil inlet and outlet passages inside the mold.

This is important because hydraulic flow does not automatically divide equally simply because several cylinders are connected to the same source.

The actual flow reaching each cylinder depends on the resistance of its hydraulic path.

Differences can be caused by:

  • pipe length;
  • pipe diameter;
  • internal passages;
  • fittings;
  • connectors;
  • manifolds;
  • pressure losses.

If one cylinder receives a slightly different flow from another, its piston speed will also be different.

Vega’s current technical documentation reaches the same conclusion: significant synchronization problems are often more closely related to an unbalanced hydraulic circuit than to a defective cylinder.

Hydraulic Pipe Length Matters

One of the practical recommendations made by the Vega Team was to use the same tube length between each cylinder and the flow distributor installed on the machine.

Why?

Because hydraulic resistance increases with the characteristics of the flow path.

If one cylinder is connected through a substantially different hydraulic path, the pressure loss can differ from that of the other cylinders.

This can result in unequal flow distribution and therefore unequal piston speeds.

For applications where several cylinders move the same mold plate, it is therefore preferable to design the hydraulic circuit as symmetrically as possible.

Equal Pipe Length Is Helpful—but It Is Not Enough

Making all hoses the same length is a useful engineering measure, but it does not guarantee perfect synchronization.

The hydraulic passages inside the mold can still be different.

The Vega Team specifically considered the possibility of different hydraulic passages inside the mold plate and the resulting imbalance in oil distribution.

Current Vega documentation similarly identifies differences in:

  • passage length;
  • passage diameter;
  • pressure losses;
  • manifold configuration

as possible causes of unequal cylinder speed.

Therefore, the complete hydraulic path must be considered—not only the external hoses.

Manufacturing Tolerances Can Also Contribute

Even when the hydraulic circuit is properly balanced, cylinders are not mathematically identical.

The Vega Team explained that differences in the tolerances of seal seats and guide-bushing seats can produce different interference levels between components.

This can create a difference in the resistance required to move one piston compared with another.

Under normal circumstances, these differences should remain small.

The problem becomes significant when the mechanical system is extremely sensitive to synchronization.

For this reason, a noticeable difference in cylinder speed should not automatically be interpreted as evidence that one cylinder is defective.

A Damaged Piston Seal Can Create a Different Problem

The Vega Team also considered another possibility: one cylinder could have a damaged piston seal, creating an internal pressure loss.

Internal leakage can affect cylinder performance because part of the hydraulic energy is lost inside the cylinder rather than being converted into useful piston movement.

If one cylinder behaves differently from the others, it is therefore important to investigate whether there is:

  • internal leakage;
  • damaged sealing;
  • excessive friction;
  • guide problems;
  • abnormal mechanical resistance.

This is another reason why replacing all cylinders without diagnosis may be unnecessary.

The Mechanical System Must Also Be Checked

Synchronization is not purely a hydraulic problem.

Imagine four cylinders connected to a large mold plate.

If one section of the plate encounters greater mechanical resistance, the cylinder connected to that section may slow down even if the hydraulic supply is correct.

Possible causes include:

  • guide friction;
  • misalignment;
  • uneven mechanical loading;
  • structural deformation;
  • guide-bushing resistance;
  • lateral forces.

Current Vega technical documentation emphasizes that cylinder synchronization must be considered together with the mechanical guiding system and mold structure.

This is particularly important for large plates.

Air in the Hydraulic Circuit

The Vega Team also specifically recommended bleeding the hydraulic circuit before starting and moving the cylinders.

Air in a hydraulic system can make cylinder movement less predictable because air is compressible while hydraulic oil is effectively incompressible under normal operating conditions.

For this reason, after installing or modifying a hydraulic circuit, proper bleeding should be part of the commissioning procedure.

Why the Problem Continued Even with Fewer Cylinders

An interesting part of the case is that the Customer had tried using only two cylinders, but the problem still occurred.

This was a valuable diagnostic observation.

If the problem were caused only by the presence of four cylinders, reducing the system to two might have been expected to eliminate it.

Since the problem remained, the investigation needed to consider other factors, including:

  • hydraulic circuit design;
  • flow distribution;
  • mechanical resistance;
  • individual cylinder behavior.

This is a good example of why controlled testing can be more useful than simply replacing components.

Replacing the Cylinders May Not Solve the Problem

The Vega Team explicitly warned that replacing the existing cylinders with other cylinders—whether Vega or another brand—might not solve the problem if the actual cause was in the hydraulic circuit.

This is a very important principle for maintenance departments.

If the hydraulic circuit is unbalanced, installing four new cylinders can simply reproduce the same problem with new components.

Before replacing the cylinders, it is therefore worth asking:

Is each cylinder actually receiving the same hydraulic conditions?

Flow-Control Valves for Individual Cylinders

One possible solution identified by the Vega Team was the use of individual flow-control valves for each cylinder.

The idea is to regulate the flow supplied to each actuator independently.

This can be useful when small differences between cylinders need to be compensated.

However, there is an important limitation.

The Vega Team noted that achieving a perfectly balanced movement using individual flow regulators is not easy.

Each adjustment affects the behavior of one cylinder, while the cylinders remain mechanically connected to the same moving structure.

The final adjustment can therefore become a complex process.

Hydraulic Flow Dividers

When synchronization is more critical, a hydraulic flow divider can provide a more controlled method of distributing hydraulic flow between several cylinders.

A flow divider is designed to split the incoming hydraulic flow between separate circuits.

This makes it particularly useful when several hydraulic cylinders must move together.

Vega’s current technical documentation specifically discusses flow dividers as a solution for synchronizing multiple hydraulic cylinders and explains why their selection requires information about the complete hydraulic system.

A Flow Divider Cannot Be Selected by Cylinder Quantity Alone

One of the most important engineering points is that saying:

“We have four cylinders, therefore we need a four-way flow divider.”

is not enough.

Before selecting a flow divider, the hydraulic system needs to be understood.

Relevant information includes:

  • operating pressure;
  • pump flow rate;
  • number of cylinders;
  • cylinder bore;
  • cylinder stroke;
  • required movement time.

Vega’s current technical article explicitly highlights these parameters as necessary for correctly selecting a flow divider.

Without these data, selecting a divider would essentially be guesswork.

Flow Rate Determines Cylinder Speed

Cylinder speed is directly related to hydraulic flow.

A greater flow produces greater piston speed, while a lower flow produces lower speed.

When several cylinders operate simultaneously, the required total flow also increases.

This affects:

  • pump capacity;
  • valve sizing;
  • pipe dimensions;
  • fittings;
  • pressure losses;
  • flow-divider selection.

Vega’s technical documentation emphasizes that hydraulic circuit sizing must therefore be considered together with cylinder sizing when multiple cylinders operate simultaneously.

Phase-Correction Systems

Some flow-divider systems can also incorporate phase-correction valves.

Their purpose is to correct small differences in cylinder position that may accumulate during repeated cycles.

This can be particularly useful in injection molding applications where several cylinders must remain closely synchronized over thousands or millions of cycles.

The Vega technical documentation describes phase correction as an additional method of maintaining synchronization between multiple hydraulic cylinders.

However, this should not be interpreted as a way to compensate for fundamentally incorrect mechanical design.

A flow divider controls hydraulic flow.

It does not eliminate:

  • guide friction;
  • mechanical misalignment;
  • structural deformation;
  • lateral loads;
  • incorrect mold design.

Synchronization Is Both a Hydraulic and Mechanical Problem

This case demonstrates why cylinder synchronization should be considered as a complete engineering problem.

The system consists of:

Hydraulic pump

↓

Valves and distributor

↓

Hydraulic pipes and internal passages

↓

Cylinders

↓

Mold plate

↓

Guides and mechanical structure

Every element can influence the final movement.

If one part of the system introduces additional resistance or a different pressure loss, the cylinders may no longer move together.

A Practical Diagnostic Procedure

When multiple cylinders are not synchronized, the following sequence can help identify the real cause.

1. Observe the movement

Determine:

  • which cylinder moves first;
  • which cylinder lags behind;
  • whether the problem occurs during extension, retraction or both;
  • whether the behavior is repeatable.

2. Inspect the hydraulic circuit

Check:

  • hose lengths;
  • hose diameters;
  • fittings;
  • manifolds;
  • internal mold passages;
  • distributors.

3. Compare the hydraulic paths

Ideally, the hydraulic paths from the distributor to each cylinder should be as similar as possible.

The Vega Team specifically recommended using equal-length tubes between the cylinders and the machine’s flow distributor.

4. Bleed the circuit

Remove trapped air before repeating the synchronization test.

5. Check the mechanical system

Inspect:

  • guides;
  • bushings;
  • alignment;
  • friction;
  • lateral forces;
  • plate deformation.

6. Test the individual cylinders

If one cylinder behaves differently, investigate:

  • seals;
  • internal leakage;
  • guide resistance;
  • piston condition.

7. Consider individual flow controls

These can be used when individual adjustment is required, although achieving perfect synchronization through separate regulators can be difficult.

8. Evaluate a flow divider

If synchronization is critical and circuit balancing alone is insufficient, a correctly sized flow divider can provide more controlled flow distribution.

The Most Important Lesson

The technical case shows why it is dangerous to diagnose a synchronization problem simply by saying:

“One cylinder is slower, so that cylinder must be defective.”

The slower cylinder may actually be responding correctly to the hydraulic conditions it receives.

The real cause could be:

  • an unbalanced circuit;
  • different pipe lengths;
  • different hydraulic passage diameters;
  • pressure losses;
  • air in the circuit;
  • mechanical friction;
  • manufacturing tolerances;
  • damaged sealing;
  • unequal mechanical loading.

The Vega Team therefore recommended investigating the hydraulic circuit before assuming that replacing the cylinders would solve the problem.

Conclusion

When multiple hydraulic cylinders are required to move the same mold plate, synchronization cannot be taken for granted.

The case analyzed by the Vega Team involved four cylinders that could not move simultaneously on an injection mold. The investigation showed that the cause could be related to the hydraulic circuit rather than simply to the cylinders themselves.

The recommended approach was to:

  • investigate the hydraulic circuit;
  • use hydraulic paths with similar characteristics;
  • use equal-length tubes where possible;
  • bleed the system correctly;
  • consider mechanical resistance;
  • check individual cylinders for internal leakage or damaged seals;
  • use individual flow controls where appropriate;
  • consider a flow divider when more controlled synchronization is required.

The fundamental engineering principle is:

Hydraulic cylinder synchronization is a system problem, not simply a cylinder problem.

Before replacing a cylinder, the hydraulic circuit, flow distribution and mechanical conditions should be analyzed.

In many cases, solving the real cause of the imbalance is more effective—and considerably less expensive—than replacing components that were not actually defective.

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All the following links are on the official Vega Cylinders domains.

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