A Real Engineering Case on Fatigue Failure, Pressure Hammering and Dynamic Loads
Hydraulic cylinders used in injection molds are designed to withstand extremely high pressures, millions of operating cycles and demanding industrial environments.
When a cylinder body suddenly cracks and begins leaking oil, the immediate assumption is often that the material was defective or that a manufacturing error occurred.
In reality, this conclusion is frequently incorrect.
Many structural failures originate from fatigue, a failure mechanism capable of breaking even perfectly manufactured components that fully comply with material specifications.
Unlike an overload failure, fatigue develops gradually.
A microscopic crack forms at a highly stressed location and grows with every operating cycle until the remaining material is no longer capable of supporting the applied load.
The final fracture often appears sudden, but the damage may have been accumulating for weeks, months or even years.
This real engineering case demonstrates how the Vega Technical Department investigated several hydraulic cylinders returned because of oil leakage and discovered that the aluminum body had cracked near the sensor fixing area. The investigation excluded both material defects and machining defects, identifying cyclic dynamic stresses as the most probable cause of failure.
The Customer’s Complaint
A customer returned several hydraulic cylinders after observing oil leakage during production.
The cylinders were subjected to hydraulic testing and every failed unit showed the same symptom:
Oil leakage directly from the cylinder body.
Oil leaking from the body is considerably more serious than leakage from a seal.
While a damaged seal can normally be replaced, a crack in the pressure-containing structure compromises the mechanical integrity of the entire cylinder.
The first objective of the engineering investigation was therefore to determine whether the failure originated from:
- defective material;
- machining errors;
- incorrect assembly;
- or excessive operating loads.
Only after identifying the true origin of the crack could an effective corrective action be proposed.
The First Inspection
The returned cylinders underwent a complete visual and hydraulic inspection.
The Vega Technical Department immediately identified a crack on the internal surface of the cylinder body, located close to the sensor fixing area.
This location was particularly significant.
Sensor grooves and threaded holes inevitably reduce the cross-sectional area of the cylinder body.
Every geometric discontinuity creates what mechanical engineers call a stress concentration.
Stress is rarely distributed uniformly throughout a mechanical component.
Instead, local geometric features cause stresses to become significantly higher than the average value acting on the entire structure.
These local stress peaks are often where fatigue cracks begin.
Material Quality Was Not the Problem
One of the most important findings of the investigation was what engineers did not find.
The inspection showed:
- no material defects;
- no casting imperfections;
- no machining defects;
- no manufacturing anomalies.
The report therefore excluded both material quality and machining as the origin of the failure.
This is a crucial engineering conclusion.
Many users assume that every crack automatically indicates poor manufacturing quality.
In reality, a perfectly manufactured component can still fail if the operating conditions generate cyclic stresses beyond the fatigue capability of the material.
Understanding Fatigue Failure
Unlike static overload, fatigue does not require extremely high loads.
Instead, it results from repeated loading cycles.
Even stresses well below the yield strength of the material can eventually produce failure if repeated millions of times.
The fatigue process generally develops in three stages:
- Crack initiation at a stress concentration.
- Progressive crack propagation during each operating cycle.
- Final sudden fracture when the remaining cross-section becomes too small to support the applied load.
This explains why a cylinder may appear to function perfectly for months before suddenly developing an oil leak.
The final failure occurs in seconds.
The damage that caused it may have accumulated over years.
Why the Crack Appeared Near the Sensor Area
The fracture did not occur randomly.
It appeared near the sensor fixing area, where the cylinder body contains additional machined features.
From an engineering perspective this is entirely predictable.
Every groove, threaded hole or machining pocket modifies the flow of mechanical stresses inside the material.
The sharper the geometric transition, the higher the local stress concentration factor.
For this reason, fatigue failures frequently originate near:
- oil ports;
- sensor grooves;
- threaded holes;
- sharp internal corners;
- sudden section changes.
The crack location therefore provided an important clue regarding the real failure mechanism.
Pressure Hammering: The Hidden Enemy of Hydraulic Cylinders
Once material defects and machining problems had been excluded, the Vega Technical Department focused on the operating conditions of the hydraulic cylinders.
The technical investigation identified several factors capable of generating extremely high dynamic stresses, even when the normal working pressure remained within the cylinder specifications.
Among these factors, one stood out above all others:
pressure hammering, also known as the hydraulic hammer effect.
Unlike static hydraulic pressure, pressure hammering is a transient phenomenon that occurs when the moving piston is forced to stop in an extremely short period of time.
The kinetic energy of the moving mass is converted almost instantaneously into pressure waves that propagate through the hydraulic circuit and the cylinder body.
Although these pressure peaks last only milliseconds, they may greatly exceed the normal operating pressure.
If repeated thousands or millions of times, these impacts become one of the most common causes of fatigue failure.
Dynamic Loads Are Often More Dangerous Than Static Loads
Many designers calculate hydraulic cylinders considering only the nominal working pressure.
However, cylinders operating in injection molds are subjected to continuous acceleration and deceleration.
Each movement generates inertia forces.
The kinetic energy of the moving system can be calculated using the classical engineering equation:
where:
- Ek = kinetic energy
- m = moving mass
- v = piston velocity
An important characteristic of this equation is that the energy increases with the square of the velocity.
Doubling the cylinder speed does not double the impact energy.
It increases it by a factor of four.
For example:
- Moving mass = 15 kg
- Cylinder speed = 0.25 m/s
Kinetic energy:
Ek = ½ × 15 × 0.25²
Ek = 0.47 J
If the speed increases to 0.50 m/s, the energy becomes:
Ek = ½ × 15 × 0.50²
Ek = 1.88 J
Although the velocity has only doubled, the impact energy has increased by 300%.
This simple calculation explains why apparently small increases in cylinder speed can dramatically increase fatigue loading.
Flow Rate Directly Influences Impact Forces
The technical report also identifies excessive oil flow as one possible cause of the observed failures.
Higher flow rates produce:
- higher piston velocity;
- greater moving momentum;
- higher deceleration forces;
- stronger pressure hammering.
This relationship explains why increasing production speed without reviewing cylinder sizing often reduces component life.
For this reason, the Vega Technical Department recommended installing flow control valves to reduce piston speed before the end of the stroke, thereby limiting dynamic loads.
Vertical Applications Increase Mechanical Stress
The investigation also highlighted that vertical installations can further increase the severity of dynamic loading.
In a vertical application, gravity continuously acts on the moving components.
Depending on the direction of movement, gravitational acceleration may:
- increase the effective moving mass;
- increase impact velocity;
- increase deceleration forces;
- amplify pressure hammer effects.
Consequently, cylinders installed vertically often require more careful hydraulic tuning than identical cylinders operating horizontally.
Temperature Also Influences Fatigue Resistance
Although the primary cause was identified as dynamic fatigue, the technical report also considered operating temperature.
The cylinder body was manufactured from Ergal 7075-T6, a high-strength aluminum alloy widely used in demanding mechanical applications.
However, like all heat-treated aluminum alloys, its mechanical properties gradually decrease at elevated temperatures.
The report specifically recommends avoiding continuous operation above 120°C, because prolonged exposure can reduce the fatigue strength of the material.
Temperature alone may therefore not generate the crack, but it can accelerate fatigue propagation when combined with repeated dynamic loading.
Tightening Torque Can Also Affect Fatigue Life
Another important observation concerns the tightening of the mounting screws.
Excessive tightening torque introduces additional stresses into the cylinder body before the machine even begins operating.
If these residual stresses combine with cyclic hydraulic loads, local stress concentrations become even higher.
The technical report therefore recommends avoiding excessive tightening of the mounting screws, since unnecessary preload may contribute to crack initiation near highly stressed areas.
This demonstrates that fatigue failures are rarely caused by a single factor.
Instead, they usually result from the combined effect of several seemingly minor conditions.
Engineering Recommendations
After completing the investigation, the Vega Technical Department proposed several corrective actions designed to eliminate the root causes rather than simply replacing the damaged cylinder.
These recommendations included:
- reducing cylinder speed using flow control valves;
- avoiding excessive hydraulic flow rates;
- verifying operating temperatures;
- preventing excessive tightening of mounting screws;
- replacing the damaged cylinder body;
- considering the use of V450 CBX hydraulic cylinders for particularly demanding high-speed applications.
Each recommendation directly addressed one or more factors contributing to fatigue loading.
Lessons Learned from This Real Engineering Case
One of the most valuable lessons from this investigation is that oil leakage is often only the final symptom of a much larger mechanical problem.
The crack itself did not originate because the aluminum alloy was defective.
Instead, it developed gradually through millions of loading cycles generated by hydraulic dynamics.
This case also illustrates why replacing a cracked cylinder without correcting the operating conditions may simply postpone the next failure.
True engineering solutions eliminate the cause—not just the consequence.
Engineering Conclusions
This real technical support case demonstrates that fatigue failure can occur even when the material, machining quality and dimensional inspections are all fully compliant with engineering specifications.
The investigation performed by the Vega Technical Department excluded manufacturing defects and identified cyclic dynamic loading, pressure hammering, excessive piston speed, elevated operating temperatures and excessive tightening torque as the most probable contributors to crack initiation near the sensor fixing area.
Rather than focusing solely on replacing the damaged cylinder, the engineering analysis proposed corrective actions capable of reducing the mechanical stresses responsible for fatigue, including flow control, thermal management and the use of cylinders specifically designed for high-speed applications.
The most important lesson is clear:
Hydraulic cylinders rarely fail because of a single overload. Most structural failures are the result of millions of small dynamic stresses that gradually accumulate until the material reaches its fatigue limit.
