Engineering the Hydraulic System for Flanging and Stretch Forming Operations

Hydraulic cylinders are often selected by calculating the force required for a single actuator. In many industrial applications, however, the real engineering challenge is not sizing an individual cylinder but designing an entire hydraulic system capable of driving several cylinders simultaneously while meeting strict force and timing requirements.

This is particularly true in sheet metal forming, where multiple hydraulic cylinders must work together with gas springs, mechanical presses and moving tooling.

If even one element of the hydraulic system is incorrectly sized, the consequences may include:

• incomplete forming;
• excessive tool wear;
• dimensional inaccuracies;
• increased cycle times;
• overheating of the hydraulic system;
• premature failure of hydraulic components.

For this reason, hydraulic system design should always begin with a complete analysis of the forming process rather than with the selection of individual components.

A real engineering project developed by the Vega Engineering Team illustrates this design philosophy perfectly.

Understanding the Manufacturing Process

Before calculating hydraulic forces, engineers must understand exactly how the tool operates.

In this project, the forming die performs two completely different manufacturing operations.

The first operation is flanging.

The second operation is stretch forming.

Although both operations are performed within the same die, they generate completely different loading conditions.

The hydraulic system must therefore be designed to operate correctly during both phases.

According to the project documentation, the die operates in two distinct stages that require different interactions between the hydraulic cylinders, the press and the gas springs.

This immediately highlights an important engineering principle:

A hydraulic system should always be designed around the manufacturing process—not around the hydraulic cylinders themselves.

Phase One – Flanging

During the first stage, the upper flange of the sheet metal component is formed using the hydraulic press.

The workpiece is positioned on a movable punch.

At the same time, an external blank holder clamps the sheet against an internal blank holder mounted on a plate driven by hydraulic cylinders.

The clamping force is generated primarily by a series of nitrogen gas springs.

During this phase, the hydraulic cylinders remain fully extended and act as structural reaction members supporting the internal blank holder while the hydraulic press performs the flanging operation.

This is an interesting aspect of the design.

Although the cylinders are hydraulic actuators, they are not producing motion during this stage.

Instead, they function as load-bearing structural components.

Many engineers overlook this dual function when selecting hydraulic cylinders.

Phase Two – Stretch Forming

Once the flanging operation has been completed, the manufacturing process changes completely.

The hydraulic press remains closed.

The workpiece is still clamped.

However, the hydraulic cylinders are now activated.

Instead of extending, they operate in tension, pulling the moving plate and stretching the formed component.

The available time for this operation is limited.

According to the project documentation, the hydraulic press can remain closed for only about 10 seconds before excessive heating of its hydraulic circuit becomes a concern.

This creates an additional engineering constraint.

The hydraulic power unit must not only generate sufficient force.

It must also complete the required movement within the available time.

Hydraulic system design therefore becomes both a force problem and a flow-rate problem.

The Forces Acting During Stretch Forming

One of the most interesting aspects of this project is that the hydraulic cylinders do not pull against a single load.

Instead, they must overcome two independent resisting forces.

The first is generated by the blank holder.

The second is generated by the sheet metal stretching operation itself.

These forces are additive.

The hydraulic cylinders must therefore overcome the total of both loads simultaneously.

The Blank Holder Force

Initially, the external blank holder applies approximately 50 tonnes of clamping force using nitrogen gas springs.

However, gas springs do not maintain a constant force.

As they compress, the internal gas pressure increases.

Consequently, the clamping force also increases.

In this application, the blank holder force rises from approximately:

• 50 tonnes
• to approximately 77 tonnes

during compression.

This behaviour is typical of nitrogen gas springs.

Unlike hydraulic cylinders operating at constant pressure, gas springs behave according to the compression of the gas volume.

As the stroke decreases, the internal pressure rises.

This characteristic must always be considered during hydraulic system design.

Ignoring it may result in a severely undersized system.

The Additional Stretching Force

The blank holder is only one part of the load.

The stretching operation itself requires additional force.

Based on the forming calculations, the stretching process requires approximately:

35 tonnes

of additional pulling force.

The total hydraulic load therefore becomes:

• Blank holder force after compression: 77 tonnes
• Stretch forming force: 35 tonnes

Total required force:

112 tonnes

This total represents the minimum pulling capacity that the hydraulic system must deliver during the stretching phase.

Why Seven Cylinders Instead of Six?

Originally, the system was designed using six hydraulic cylinders.

After completing the engineering calculations, however, the Vega Engineering Team decided to increase the number to seven.

Why?

Because the available pulling force had to exceed the required process load while maintaining an appropriate engineering safety margin.

Each V450CM hydraulic cylinder with a 100 mm bore, operating at 300 bar, develops approximately:

18,781 kg

of pulling force.

Using seven cylinders provides a total pulling capacity of approximately:

126 tonnes.

This exceeds the required 112 tonnes while maintaining a reasonable reserve capacity.

The decision demonstrates another important engineering principle:

The objective is not to match the theoretical load exactly, but to provide sufficient reserve capacity to accommodate real operating conditions.

Load Sharing Between Multiple Cylinders

Whenever several cylinders operate simultaneously, engineers usually assume that the total force is equally distributed.

In an ideal system:

Word Equation Format

F_total=n×F_cylinder

where:

• F_total = total available force
• n = number of cylinders
• F_cylinder = force produced by each cylinder

In practice, however, perfectly equal load sharing rarely occurs.

Small differences in:

• manufacturing tolerances;
• hydraulic pressure losses;
• structural stiffness;
• assembly accuracy;
• mechanical deflection;

may cause individual cylinders to carry slightly different loads.

For this reason, designers normally include additional safety margins whenever multiple actuators operate together.

Why Hydraulic Pressure Alone Does Not Size the System

Knowing that the cylinders operate at 300 bar is only one part of the calculation.

Pressure determines the force generated by each cylinder.

It does not determine whether the hydraulic power unit can complete the required movement within the available time.

To size the hydraulic power unit correctly, engineers must also calculate:

• total oil volume;
• required flow rate;
• cylinder stroke;
• simultaneous cylinder movement;
• cycle time;
• pump displacement;
• electric motor power.

These calculations will determine whether the hydraulic power unit is capable of supplying seven cylinders simultaneously during the limited forming window.

Hydraulic Power Unit Sizing, Flow Rate and System Reliability

In Part 1, we examined how the Vega Engineering Team analysed the complete forming process before selecting the hydraulic cylinders.

The calculations showed that the stretching operation required approximately 112 tonnes of pulling force. Instead of using the originally planned six cylinders, the engineering team selected seven V450CM hydraulic cylinders with a 100 mm bore operating at 300 bar, providing a total pulling capacity of approximately 126 tonnes.

Although this verifies that the cylinders can generate sufficient force, the hydraulic system is still not completely designed.

An equally important question remains:

Can the hydraulic power unit move all seven cylinders quickly enough?

The answer depends not on pressure, but on hydraulic flow.

Force Alone Does Not Move a Cylinder

One of the most common misconceptions in hydraulic engineering is that pressure determines cylinder speed.

It does not.

Pressure determines force.

Flow rate determines speed.

This distinction is fundamental.

A hydraulic cylinder supplied with high pressure but insufficient oil flow may generate enormous force while moving extremely slowly.

Conversely, a large flow rate with insufficient pressure may produce rapid movement but inadequate force.

A properly designed hydraulic system must provide both simultaneously.

Pressure and Flow Perform Different Functions

Hydraulic engineers generally separate the design into two independent calculations.

Pressure determines whether the cylinder can overcome the external load.

Flow determines how rapidly the cylinder completes its stroke.

This explains why hydraulic power unit sizing always includes both pressure calculations and flow calculations.

Ignoring either parameter usually results in poor machine performance.

Determining the Required Oil Flow

Every hydraulic cylinder requires a specific oil volume to complete its stroke.

The oil volume depends on:

• piston diameter;
• rod diameter;
• stroke length;
• number of cylinders moving simultaneously.

The hydraulic pump must deliver enough oil to fill all seven cylinders within the available operating time.

In this project, the documentation specifies that the cylinder movement should be completed in approximately 5 to 6 seconds, allowing the operation to finish before the hydraulic press reaches its maximum allowable closed time.

This timing requirement becomes one of the principal design constraints.

Calculating Hydraulic Flow Rate

The required pump flow can be estimated using the relationship:

Word Equation Format

Q=V/t

where:

• Q = hydraulic flow rate
• V = total oil volume
• t = required movement time

This simple equation demonstrates why increasing the number of cylinders immediately increases the flow requirement.

If the total oil volume doubles while the required movement time remains unchanged, the hydraulic pump must also deliver approximately twice the flow.

For systems with multiple cylinders, flow calculations often become more critical than pressure calculations.

Seven Cylinders Must Operate Together

In this application, the cylinders do not operate independently.

They move simultaneously.

This creates additional engineering challenges.

The hydraulic circuit must distribute oil evenly so that every cylinder moves at approximately the same speed.

Unequal oil distribution may cause:

• uneven stretching;
• distortion of the workpiece;
• excessive stresses within the tooling;
• increased wear of guide components;
• poor dimensional repeatability.

Achieving good flow distribution therefore becomes as important as generating sufficient force.

Synchronisation of Multiple Cylinders

Perfect synchronisation is rarely achieved automatically.

Small differences in:

• hose length;
• pipe diameter;
• valve characteristics;
• manufacturing tolerances;
• internal friction;
• cylinder seal friction;

may cause one cylinder to move slightly before another.

Although these differences are often only fractions of a millimetre, they may produce significant load redistribution in large forming tools.

Hydraulic engineers therefore pay careful attention to circuit layout, manifold design and piping geometry to minimise these differences.

Why the Hydraulic Power Unit Must Be Sized Correctly

The hydraulic cylinders are only the visible part of the hydraulic system.

Behind them, the hydraulic power unit must provide:

• sufficient pressure;
• sufficient flow;
• adequate oil capacity;
• acceptable operating temperature;
• reliable continuous operation.

If the hydraulic power unit is undersized, several problems may occur:

• slow cylinder movement;
• inability to complete the cycle within the available time;
• excessive oil temperature;
• pump overload;
• reduced service life of hydraulic components.

For this reason, the Vega Engineering Team requested the hydraulic power unit to be sized specifically for the operating conditions of this project rather than selecting a standard unit.

Why Oil Temperature Matters

Hydraulic oil performs several functions simultaneously.

It:

• transmits power;
• lubricates moving components;
• removes heat;
• protects internal surfaces against wear.

If the hydraulic system is forced to operate continuously beyond its design capacity, oil temperature increases.

As temperature rises:

• oil viscosity decreases;
• internal leakage increases;
• volumetric efficiency falls;
• seal life is reduced;
• oxidation accelerates.

This explains why the project documentation limits the maximum time during which the hydraulic press remains closed.

Longer operating times would increase thermal loading within the hydraulic system.

Thermal management is therefore an essential part of hydraulic system design.

Structural Considerations Beyond Hydraulics

Although the article focuses on hydraulic sizing, the attached drawings also illustrate another important engineering principle.

The seven cylinders are arranged symmetrically beneath the moving plate. This configuration helps distribute the pulling force uniformly across the entire forming area, reducing bending moments and improving dimensional accuracy.

Similarly, the sectional drawing shows the interaction between the hydraulic cylinders, nitrogen gas springs and the forming die during both manufacturing stages.

These mechanical details demonstrate that successful hydraulic design cannot be separated from structural design.

The hydraulic system and the mechanical structure must always be engineered together.

Engineering Is About System Optimisation

One of the most valuable lessons from this project is that no single component determines machine performance.

The final result depends on the interaction between:

• hydraulic cylinders;
• hydraulic power unit;
• nitrogen gas springs;
• hydraulic press;
• moving plate;
• forming tool;
• structural stiffness;
• hydraulic piping;
• control valves.

Optimising only one component rarely produces the best overall system.

Instead, engineers optimise the complete machine.

Why Real Engineering Begins with the Process

Perhaps the most significant aspect of this case is the engineering methodology itself.

The Vega Engineering Team did not begin by selecting a pump.

Nor did they begin by selecting hydraulic cylinders.

Instead, they first analysed:

1. the sequence of manufacturing operations;
2. the forces generated during each stage;
3. the interaction between gas springs and hydraulic cylinders;
4. the total force required;
5. the available process time;
6. the number of cylinders required;
7. the hydraulic power unit capable of supplying those cylinders.

This structured engineering approach significantly reduces design risks while improving machine reliability.

Conclusion

This project demonstrates that hydraulic power unit sizing is far more complex than selecting a pump capable of producing the required pressure.

The complete hydraulic system must satisfy force, flow, timing and thermal requirements simultaneously.

By analysing the manufacturing process, calculating the combined forces generated by the gas springs and the stretching operation, increasing the number of cylinders from six to seven and considering the available operating time, the Vega Engineering Team developed a hydraulic solution capable of meeting both the mechanical and production requirements of the forming die.

The attached drawings further demonstrate how the hydraulic cylinders were integrated into the mechanical structure to ensure balanced load distribution and reliable operation throughout the forming process.

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

A successful hydraulic system is not designed by selecting components individually. It is designed by understanding the complete manufacturing process and engineering every component to work together as one integrated system.

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