Why Hydraulic Cylinder Selection for Unscrewing Mechanisms Starts with Force Calculation

Engineering Begins with the Load, Not with the Cylinder

Selecting a hydraulic cylinder for an injection mold unscrewing mechanism is often more complex than selecting a standard linear actuator.

Many engineers begin by looking at cylinder bore, stroke or available installation space.

However, none of these parameters should be the starting point.

The engineering process begins with understanding the mechanical load that the cylinder must overcome.

Only after calculating the required force and analysing the mechanical transmission can the correct hydraulic cylinder be selected.

A real engineering case handled by the Vega Technical Department clearly illustrates this methodology.

When the Guangzhou office requested assistance in selecting a hydraulic cylinder for an unscrewing application, the engineering team first analysed the available technical information before recommending a suitable solution.

The Customer’s Request

The Guangzhou Vega office received a request from a customer who needed assistance in selecting the correct hydraulic cylinder for an unscrewing mechanism.

The available drawings provided some information about the application but did not include complete details regarding the gear transmission.

Because of these missing data, the Vega Technical Department explained that it was only possible to calculate:

  • the required unscrewing thrust and traction force;
  • the appropriate rack module.

The final cylinder stroke and rack length would only be defined after the customer had completed the design of the gear system.

This approach demonstrates an important engineering principle.

A hydraulic cylinder should never be selected before the mechanical transmission has been properly evaluated.

Force Calculation Comes First

One of the most valuable lessons from this case is that hydraulic cylinder selection does not begin with a product catalogue.

It begins with engineering calculations.

The first objective is to determine the useful force required to perform the unscrewing movement.

Only after this value is known can engineers evaluate:

  • the available hydraulic pressure;
  • the mechanical transmission ratio;
  • the required cylinder size;
  • the appropriate operating conditions.

Without these calculations, cylinder selection becomes little more than guesswork.

The Gear System Influences Cylinder Selection

An unscrewing mechanism is not driven directly by the hydraulic cylinder.

The cylinder generates linear motion.

That motion is then transmitted through:

  • a rack;
  • one or more gears;
  • the rotating threaded core inside the mold.

For this reason, the mechanical characteristics of the gear system directly influence the hydraulic cylinder that will ultimately be required.

The Vega Technical Department specifically noted that the cylinder stroke and the rack length could not be determined until the customer had finalized the gear system design.

This highlights an essential engineering concept.

The mechanical transmission defines the cylinder—not the other way around.

Engineering Means Working with Available Information

Another interesting aspect of this case is the engineering methodology itself.

Instead of making assumptions about missing data, the Vega Technical Department clearly explained which calculations could be completed with the available information and which parameters still depended on future design decisions.

This approach reduced the risk of selecting an unsuitable hydraulic cylinder.

Professional engineering does not attempt to fill missing information with assumptions.

It identifies what is known, what remains unknown and what decisions must be made before the project can move forward.

Hydraulic Cylinder Selection Is a System Design Process

Many engineers naturally think of a hydraulic cylinder as an independent component.

In reality, an unscrewing system is composed of several interacting elements.

These include:

  • the hydraulic cylinder;
  • the rack;
  • the gears;
  • the threaded core;
  • the available hydraulic pressure.

Each element influences the others.

Changing the gear ratio may require a different cylinder stroke.

Changing the available pressure may require a different cylinder bore.

Changing the rack module may affect the entire transmission.

For this reason, hydraulic cylinder selection should always be considered part of the complete mechanical system rather than an isolated purchasing decision.

Engineering Support Adds Value Beyond Product Selection

One of the strengths demonstrated in this case is that the Vega Technical Department did far more than recommend a product.

The engineers first analysed the application, identified the calculations that could already be performed and explained which design parameters still depended on the customer’s mechanical choices.

This collaborative engineering approach allows mold designers to make informed decisions before finalising the hydraulic system.

Rather than simply supplying components, technical support becomes part of the engineering design process itself.

Engineering Starts with the Application

This case reminds us that selecting the correct hydraulic cylinder is never simply a matter of choosing a bore size from a catalogue.

The correct cylinder is the result of understanding:

  • the required force;
  • the transmission system;
  • the available hydraulic pressure;
  • the mechanical layout of the mold.

Only after these parameters have been analysed can the hydraulic cylinder be selected with confidence.

This systematic methodology is one of the key principles of professional hydraulic engineering.

Engineering Calculations Lead to the Correct Cylinder Selection

In Part 1, we saw that selecting a hydraulic cylinder for an unscrewing mechanism should never begin with the cylinder itself.

Instead, the engineering process starts by calculating the required force and analysing the mechanical transmission.

The engineering case handled by the Vega Technical Department demonstrates how this methodology allows the correct hydraulic cylinder to be selected only after evaluating the available technical information.

Once the force calculation had been completed, the engineers were able to identify the most suitable hydraulic cylinder for each application.

Two Different Applications, Two Different Force Requirements

The technical evaluation included two different mold designs.

For Drawing 4888, the Vega Technical Department calculated:

  • required unscrewing thrust/traction force: approximately 512 kgf;
  • rack module: Module 2.

Based on these values, the recommended solution was:

  • CS040 hydraulic cylinder operating at a minimum working pressure of 80 bar.

As an alternative, the engineers also proposed:

  • CR040018 hydraulic cylinder, also operating at 80 bar.

This illustrates how force calculations directly determine cylinder selection.

A Larger Force Requires Different Operating Conditions

A second application required significantly higher performance.

For Drawing 4955, the engineering calculations produced:

  • required unscrewing thrust/traction force: approximately 872 kgf;
  • rack module: Module 2.5.

To achieve this higher force, the recommended cylinders remained in the same product families but required a minimum operating pressure of 100 bar.

This comparison highlights an important engineering concept.

As the required mechanical load increases, cylinder operating conditions must also be reviewed.

The hydraulic cylinder cannot be evaluated independently from the mechanical transmission.

Hydraulic Pressure Is Part of the Design

Many engineers focus primarily on cylinder dimensions.

The calculations performed by the Vega Technical Department demonstrate that available hydraulic pressure is equally important.

For the first application:

  • minimum pressure: 80 bar.

For the second application:

  • minimum pressure: 100 bar.

This difference shows that cylinder selection depends not only on force but also on the pressure available within the customer’s hydraulic system.

Increasing hydraulic pressure may allow the same cylinder family to generate higher output force without increasing its overall dimensions.

The Rack Module Cannot Be Ignored

Another important aspect of the calculations concerns the rack module.

The two applications required:

  • Module 2;
  • Module 2.5.

Although the hydraulic cylinder produces linear motion, the rack module determines how that motion is transmitted through the gear system.

A different rack module changes the characteristics of the mechanical transmission and therefore becomes part of the cylinder selection process.

This demonstrates once again that unscrewing mechanisms should always be designed as complete mechanical systems rather than as independent components.

Engineering Offers Alternatives, Not Just One Solution

Another interesting aspect of this case is that the Vega Technical Department did not recommend only one hydraulic cylinder.

For both applications, the engineers proposed:

  • a CS series cylinder as the primary recommendation;
  • a CR series cylinder as an alternative.

Providing alternative solutions gives mold designers greater flexibility.

The final choice may depend on installation space, mold design, customer preferences or other project-specific requirements.

Engineering support is therefore not limited to selecting a product.

It provides multiple technically valid solutions.

Collaboration Produces Better Engineering

One of the strongest messages contained in this engineering correspondence is that hydraulic cylinder selection is a collaborative process.

The Vega Technical Department clearly explained that the available information allowed certain calculations to be completed immediately, while other parameters—such as cylinder stroke and rack length—would only be finalized after the customer completed the gear system design.

This collaborative approach prevents costly design changes later in the project.

Instead of forcing the customer to adapt the mold to an already selected cylinder, the cylinder is selected to match the final mechanical design.

Engineering Means Understanding the Entire System

This case demonstrates that a hydraulic cylinder is only one element of an unscrewing mechanism.

Reliable performance depends on the correct interaction between:

  • hydraulic force;
  • rack module;
  • gear ratio;
  • cylinder stroke;
  • available hydraulic pressure;
  • mold geometry.

Considering these elements together allows engineers to develop systems that are reliable, efficient and correctly matched to the application.

Conclusion

This engineering case demonstrates that selecting a hydraulic cylinder for an unscrewing mechanism is fundamentally an engineering calculation rather than a catalogue selection.

The Vega Technical Department first calculated the required unscrewing force and rack module using the information available, clearly explaining that the cylinder stroke and rack length would only be defined after the customer finalized the gear system. Based on these calculations, the engineers recommended appropriate CS040 and CR040018 hydraulic cylinders operating at 80 bar or 100 bar, depending on the application requirements.

This case reinforces one of the most important principles of hydraulic engineering:

The correct hydraulic cylinder is not selected first. It is the result of understanding the complete mechanical transmission, calculating the required force and integrating the hydraulic system into the overall mold design.

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