A Real Engineering Case on Breakaway Force, Wedge Mechanisms and Hydraulic Cylinder Selection
Self-locking hydraulic cylinders are widely used in injection molds and die casting tools because they eliminate the need for continuous hydraulic pressure to maintain a locked position.
However, many designers assume that a self-locking cylinder is suitable for every application.
In reality, the effectiveness of a self-locking cylinder depends not only on its locking mechanism but also on the forces that must be overcome to release the lock.
When external loads become too high, the cylinder may no longer be capable of generating sufficient breakaway force, making normal operation impossible.
This real engineering case explains how the Vega Technical Department analyzed the design of a new mold after the customer experienced failures on an existing tool. The engineering investigation demonstrated that the originally selected self-locking cylinder was unsuitable for the application and recommended both hydraulic and mechanical design improvements.
The Customer’s Existing Mold
The customer was already producing the component using a mold equipped with a DEMA SPI tie-rod hydraulic cylinder.
The existing cylinder featured:
- 80 mm bore
- 36 mm rod diameter
- thrust force of approximately 98.5 kN
- pulling force of approximately 78.5 kN
- operating pressure of 20 MPa (200 bar).
Despite these specifications, significant mechanical problems had developed during production.
The Real Problem Was Not the Hydraulic Cylinder
The customer initially requested an evaluation of the hydraulic cylinder for a new mold.
However, after discussing the application, the Vega Technical Department concluded that the main problem did not originate from the cylinder itself.
Instead, the failures appeared to be caused by excessive clearance within the wedge locking system and by the extremely small cross-section of the front engagement area connecting the moving slide to the central insert.
The cracks observed on the lateral wedges strongly supported this conclusion.
This illustrates an important engineering principle:
Hydraulic cylinders are often blamed for failures that actually originate in the mechanical design of the mold.
Understanding Breakaway Force
One of the most overlooked aspects of hydraulic cylinder selection is the distinction between working force and breakaway force.
A cylinder may easily generate enough force to move a mechanism during normal operation while still being unable to initiate movement from a locked position.
During the first millimeters of travel, the cylinder must simultaneously overcome:
- static friction;
- wedge friction;
- seal friction;
- guide friction;
- the resistance of the self-locking mechanism;
- the external mechanical load generated by the mold.
If the available cylinder force is lower than the total breakaway force, the mechanism will remain locked even though the hydraulic system is operating correctly.
Why Wedge Geometry Is Critical
Wedge mechanisms transform the linear movement of a hydraulic cylinder into high locking forces.
This mechanical advantage is essential for mold rigidity, but it also increases the force required to release the mechanism.
Poorly designed wedges may generate:
- excessive friction;
- localized stresses;
- cracks;
- permanent deformation;
- increasing clearances after repeated production cycles.
Simply increasing hydraulic pressure rarely solves these problems.
Improving the geometry of the locking system is usually a far more effective engineering solution.
Why the Self-Locking Cylinder Was Not Suitable
After evaluating the application, the Vega Technical Department confirmed that the CF084 self-locking hydraulic cylinder was not suitable.
The reason was simple:
the force required to release the locking mechanism exceeded the useful force that the cylinder was capable of producing.
This is a common misconception.
A self-locking cylinder must not only generate the force required to move the load.
It must first unlock its own internal locking mechanism while simultaneously overcoming every external resistance acting on the mold.
When these combined forces exceed the cylinder capacity, reliable operation becomes impossible.
The Alternative Hydraulic Solutions
Rather than selecting a larger self-locking cylinder, the Vega Technical Department proposed two different hydraulic solutions.
The first consisted of:
- V450CM compact hydraulic cylinder
- 100 mm bore
- pilot-operated check valve
- maximum operating pressure of 300 bar
The second consisted of:
- V215CR tie-rod hydraulic cylinder
- 125 mm bore
- pilot-operated check valve
- maximum operating pressure of 190 bar.
Both solutions replaced the internal locking mechanism with external hydraulic load holding.
This configuration offers greater available operating force while maintaining safe load retention.
Why a Pilot-Operated Check Valve Can Be Superior
A pilot-operated check valve locks hydraulic oil inside the cylinder instead of relying on an internal mechanical locking device.
This approach offers several engineering advantages:
- lower breakaway force;
- higher usable cylinder force;
- improved reliability;
- simpler maintenance;
- reduced mechanical complexity;
- better long-term durability.
For heavily loaded mold mechanisms, this solution is frequently more reliable than using a self-locking cylinder.
Improving the Mold Design
The hydraulic system alone could not solve the customer’s problem.
The Vega Technical Department therefore recommended improving the mold mechanics as well.
The proposed modifications included:
- increasing the resistant cross-section of the two lateral wedges;
- adding a third hydraulically actuated radial wedge near the rear of the central slide;
- verifying whether the molding machine could independently control two radial cylinders equipped with dedicated position sensors.
These recommendations addressed the true root cause of the failure instead of simply increasing hydraulic force.
Common Engineering Mistakes
Technical support engineers frequently encounter the same design errors:
- selecting a self-locking cylinder without evaluating breakaway force;
- increasing hydraulic pressure instead of improving the mechanical design;
- underestimating wedge geometry;
- ignoring structural stiffness;
- neglecting fatigue loading;
- assuming that a larger cylinder automatically solves every problem.
Successful hydraulic cylinder selection always requires evaluating the complete mechanical system.
Engineering Lessons Learned
This case demonstrates that hydraulic cylinder selection cannot be based solely on catalog specifications.
A complete engineering analysis must consider:
- breakaway force;
- wedge geometry;
- friction;
- structural rigidity;
- load path;
- hydraulic locking strategy.
Only after understanding the interaction between these factors can the most reliable hydraulic solution be selected.
Conclusions
The investigation carried out by the Vega Technical Department demonstrated that the original self-locking cylinder was not unsuitable because of a manufacturing defect.
Instead, the hydraulic architecture and the mechanical design of the mold generated a breakaway force greater than the cylinder could overcome.
Replacing the self-locking cylinder with a conventional hydraulic cylinder assisted by a pilot-operated check valve, together with redesigning the wedge system, provided a far more robust engineering solution.
The most important lesson is simple:
When a hydraulic cylinder appears undersized, the real problem may not be the cylinder itself. Very often, the answer lies in understanding the mechanics of the entire mold.
Further Technical Reading
Understanding the engineering principles discussed in this case requires a solid knowledge of hydraulic force calculations, cylinder sizing and mold mechanics.
For a deeper understanding of these topics, we recommend the following technical articles:
- How to Calculate the Correct Hydraulic Cylinder Size for Injection Molds
https://www.icvega.com/support/how-to-calculate-the-correct-hydraulic-cylinder-size-for-injection-molds - Choosing the Right Cylinder for Mold Core: Pushing Force
https://www.icvega.com/choosing/choosing-the-right-cylinder-for-mold-core-pushing-force
These articles explain the engineering calculations behind hydraulic cylinder sizing, operating pressure, safety factors and force transmission in injection mold applications.
