1. Introduction: Symptoms of failure
A critical incident was recorded on the stamping line of metal parts: the main hydraulic press with a force of 2500 kN stopped due to a sharp drop in pressure in the system. The operator reported a significant hydraulic fluid leak due to a master cylinder rod seal. The duty cycle time increased by 15% during the last 48 hours before complete failure, indicating a gradual decrease in volumetric efficiency.
A visual inspection revealed a puddle of hydraulic oil (ISO VG 46) with a volume of about 15 liters on the bed of the press. The temperature of the cylinder body in the area of the front cover was 85°C (normally 55°C). The failure resulted in equipment downtime and the risk of disrupting the production schedule. The purpose of this analysis is to determine the exact technical causes of seal failure and to develop a strategy to prevent such incidents.
2. Overview of the component and operating conditions
The object of research is a double-acting hydraulic cylinder. Main technical characteristics:
- Piston diameter: 160 mm
- Rod diameter: 100 mm
- Rod stroke: 800 mm
- Nominal working pressure: 210 bar
- Maximum peak pressure: 250 bar
- Working fluid: mineral oil ISO VG 46
The stem seal system consists of a dirt trap (NBR), a main U-seal (polyurethane, PU) and a buffer seal (PTFE with bronze filler). Guide rings are made of fabric-reinforced phenolic resin.
Directional hydraulic distributors are controlled via relay logic, where the key time delay element for smooth valve switching is the Siemens 7PU44402AN20 time relay. This component is responsible for forming the acceleration and deceleration time intervals of the massive sliding press, preventing the valves from closing abruptly.
3. Signs of failure (Evidence base)
During the disassembly of the hydraulic cylinder in the conditions of the repair workshop, the following physical evidence was found:
3.1. Condition of seals
The main polyurethane seal has signs of extrusion (squeezing) from the low pressure side. The edge is torn, there are traces of thermal degradation (darkening of the material). The buffer seal made of PTFE is deformed, the gap in the lock has increased from 0.2 mm to 1.5 mm.
3.2. Condition of the rod and guides
Longitudinal scratches are recorded on the chrome surface of the rod. Measurement with a profilometer showed a roughness of Rz = 6.3 μm (the nominal value for stable operation of seals is Rz ≤ 0.4 μm). Guide rings have asymmetric wear: the thickness on one side is 2.1 mm, on the opposite side - 3.4 mm (initial thickness 3.5 mm).
3.3. Analysis of working fluid and control parameters
Laboratory analysis of an oil sample taken from the tank showed a ISO 4406 purity class of 21/19/16 (the norm for this type of system is 17/15/12). A high content of silicon and metal shavings was found.
When checking the electrical control circuit, it was found that the setting of the Siemens 7PU44402AN20 relay was wrong: the time delay for the main drain valve to close was 50ms instead of the estimated 250ms.
4. Investigation of root causes (Method 5 Why)
To systematize the evidence, the "5 Why" method was applied in three main areas: pollution, distortion, pressure jumps.
Direction 1: Contamination of the system
- Why did longitudinal scratches appear on the rod? Due to the penetration of abrasive particles between the rod and the seal.
- Why did the particles end up in this area? The dirt remover failed to clean the stem during the reverse stroke.
- Why did the dirt remover fail? Its working edge was worn due to the high concentration of dust in the workshop and the lack of a protective corrugation.
- Why did the oil contamination level (21/19/16) exceed the norm? The filter element of the drain filter was operating in bypass mode (bypass valve open).
- Why did the filter work in bypass? The filter contamination indicator was faulty, the regular replacement of the filter element (MTBF 500 hours) was ignored.
Direction 2: Mechanical skew
- Why do the guide rings have asymmetric wear?A significant radial (lateral) load was applied to the rod.
- Why did the lateral load occur? The axis of the hydraulic cylinder did not coincide with the axis of movement of the sliding press.
- Why did the discrepancy occur? Loosening of the fastening bolts of the hydraulic cylinder flange.
- Why did the bolts loosen? Lack of regular torque control and high level of vibration.
Direction 3: Pressure jumps (Hydroshock)
- Why did the polyurethane seal extrusion occur? The pressure in the cylinder cavity exceeded the strength limit of the seal material for the available mounting clearance.
- Why did the pressure exceed the permissible limit?Hydraulic shock occurred during slider stop.
- Why did the water hammer occur? The main drain valve closed too quickly, instantly stopping the flow of oil.
- Why did the valve close quickly? Time relay Siemens 7PU44402AN20 tripped in 50ms instead of 250ms.
- Why did the relay setting change? Unauthorized intervention of the operator in the control cabinet in order to speed up the working cycle of the press.
5. Identified root causes
Based on the collected data, a matrix of causes was formed with an assessment of the probability and degree of influence on the destruction of the seal.
| The root cause | Probability / Impact | Mechanism of destruction | The evidence |
|---|---|---|---|
| 1. Hydraulic shock (Pressure surge) | High / Critical | The rapid closing of the valve due to a failure of the Siemens relay settings led to a pressure jump (estimated to 380 bar). This caused the polyurethane to be squeezed (extruded) into the gap between the piston and the cylinder. | Seal extrusion, 50ms relay setting, oil overheating due to throttling. |
| 2. Abrasive wear (Contamination) | High / Significant | The particles of silicon dioxide and metal acted as an abrasive, destroying the surface of the rod and cutting the edges of the seals. | Code ISO 4406 21/19/16, scratches on rod Rz 6.3 microns, wear of dirt remover. |
| 3. Radial load (Skewing) | Average / Moderate | The misalignment resulted in metal-on-metal friction after wear of the guide rings, which increased the local temperature and deformed the sealing assembly. | Asymmetric wear of guides (1.3 mm difference), loosened flange bolts. |
6. Corrective actions
The following measures have been implemented to restore the equipment's operability and eliminate the causes of failure.
Immediate actions (Short-term)
- Component replacement: New seal kit (PU + PTFE) and new guide rings installed. The hydraulic cylinder rod was replaced with a new one coated with hard chrome (thickness 30 μm, Rz 0.2 μm).
- System Cleaning: Performed a complete flushing of the hydraulic system using a mobile filter station to a cleanliness class of ISO 4406 16/14/11.
- Control calibration: The Siemens 7PU44402AN20 relay has been recalibrated for a 250ms delay. A seal is installed on the potentiometer to prevent unauthorized tampering.
Preventive actions (Long-term)
- Filtration modernization: Drain filters with absolute filtration of 10 μm (β10 ≥ 200) and electronic pressure drop sensors integrated in the ACS TP were installed.
- Geometry alignment: Laser centering of the hydraulic cylinder relative to the sliding press was carried out. The misalignment tolerance is set at the level of 0.05 mm/m. The use of torque wrenches for tightening flange connections has been implemented.
- Water hammer protection: A membrane hydraulic accumulator with a volume of 4 liters is integrated into the hydraulic line to dampen peak pressure surges in case of electronic failures.
7. Quick diagnostic checklist for technicians
This checklist is designed to be used on tablets during daily equipment rounds (Go/No-Go format).
- [ ] Visual control of the rod: Absence of an oil film in the form of drops. The stock should be slightly moist, but without dripping.
- [ ] Rod surface condition: No visible longitudinal scratches, gouges or chrome discoloration (blueing indicates overheating).
- [ ] Temperature regime: The temperature of the cylinder body (measured by a pyrometer) does not exceed 60°C. Temperature difference between cover and sleeve ≤ 5°C.
- [ ] Fixing: Checking the presence of paint marks on the fastening bolts (indicates the absence of loosening).
- [ ] Filtration: Filter contamination indicators are in the green zone.
- [ ] Valve operation: The sound of switching hydraulic distributors is smooth, without sharp metal impacts.
- [ ] Relay parameters: Visual check of integrity of seals on timers and relays (including Siemens 7PU44402AN20) in the control cabinet.
- [ ] Noise level: No cavitation noise or high-frequency squeak from the pumping station.
8. Prevention strategy and condition monitoring
To increase the mean time between failures (MTBF) from the current 2000 hours to a target of 8000 hours, a predictive maintenance strategy is implemented.
Oil analysis: Sampling every 500 working hours. Three parameters are controlled: purity class (ISO 4406), water content (ppm) and kinematic viscosity at 40°C. Exceeding the purity code above 18/16/13 requires offline filtration to be connected immediately.
Vibration Control: Installation of accelerometers on the main pump housing to detect pressure pulsations that may indicate valve problems or relay logic failures.
Planned replacements: Hydraulic cylinder seals are transferred to the category of critical components. Regular replacement is performed every 8,000 operating hours or every 2 years, whichever comes first, regardless of visible leaks.
9. Summary
Failure analysis has shown that failure of hydraulic cylinder seals rarely has a single cause. In this case, a combination of contaminated working fluid, mechanical misalignment, and a critical pressure spike due to incorrect control relay settings resulted in rapid degradation of the polyurethane and PTFE. A systematic approach to maintenance, which includes control of oil purity, precise centering of mechanics and strict control of electro-automatic parameters, is the only reliable method of ensuring uninterrupted operation of hydraulic systems.
For the selection of certified seals, filter elements, automation components (including time relays) and diagnostic equipment, use the UNITEC-D E-Catalog. Our catalog contains a complete technical specification for the correct selection of spare parts according to ISO and DIN standards.
10. Normative references
- ISO 4406:2017 — Volumetric hydraulic drive. Working fluids. The method of coding the level of contamination by solid particles.
- ISO 5598:2020 — Hydraulic and pneumatic systems. Dictionary of terms.
- DSTU EN ISO 4413:2014 — Volumetric hydraulic drive. General rules and safety requirements for systems and their components.
- ISO 10771-1:2015 — Volumetric hydraulic drive. Fatigue tests of metal shells operating under pressure.
