Root Cause Analysis of Cylinder Seal Failures: Contamination, Misalignment or Pressure Surges

1. Introduction

The unplanned stoppage of the pressing line at the metallurgical plant in Zaporizhzhia started with the appearance of an oil puddle under the main hydraulic cylinder with a force of 250 tons. The operator recorded a drop in working pressure in the system from 280 bar to 210 bar during the forming cycle. An external inspection showed an intensive leakage of working fluid through the rod seal. The mean time between failures (MTBF) was only 1 450 hours instead of the calculated 8 000 hours according to the schedule. This case initiated an in-depth engineering investigation to determine the true root cause of the failure of the polyurethane seals.

2. Component Overview

Double-acting hydraulic cylinder is used in heavy forging equipment. Technical specifications of the unit:

• Working pressure: nominal — 250 bar, peak — 380 bar.
• Piston rod diameter: 200/140 mm.
• Working fluid: mineral oil HLP 46 according to standard DIN 51524 part 2.
• Temperature range: from +15 °C to +65 °C (locally up to +80 °C).
• Stroke speed: up to 0.5 m/s.

Sealing unit includes a stepped rod seal made of polyurethane (TPU), a metal-plastic support ring, and a two-stage wiper made of nitrile rubber (NBR). Hydraulic valve control and rod position monitoring in this system are integrated via industrial controllers, specifically modules of type Schneider Electric TSXBLY01, which are responsible for processing signals from inductive feedback sensors and encoders.

3. Failure Evidence

Disassembly and inspection of a hydraulic cylinder in a repair shop revealed the following actual data:

• On the rod working surface (hard chromium plating thickness of 30 µm), longitudinal grooves up to a depth of 0.15 mm are fixed in the lower forming zone.
• Dynamic seal urethane lip has local wear over 360 degrees with asymmetric thinning on the guide bushing side (wear eccentricity 0.4 mm).
• Oil sample analysis (ISO 4406) showed a cleanliness class of 21/19/16, which exceeds the permissible level ISO 18/16/13 for this type of seals.
• The telemetry data recorded regular hydraulic impacts with amplitude up to 410 bar, duration of 15–25 milliseconds.

4. Root Cause Analysis

For determining the single root cause, the Ishikawa diagram method and the 5 Whys analysis were used. Three main hypotheses were considered:

1. Contamination of working fluid with solid abrasive particles (iron oxides, sand).
2. Mechanical stem tilt due to wear of guide rings or misalignment of installation.
3. Hydraulic shocks and pressure peaks exceeding the strength limit of polyurethane.

Laboratory spectral analysis of wear particles confirmed the presence of quartz dust in the oil. At the same time, microscopy of the seal cross-section showed plastic deformation of the five seals, characteristic for extrusion into the clearance under the action of excessive pressure. Measurement of geometric parameters of the bearings revealed wear of the bronze bushing on one side.

5. Defined Root Causes

6. Corrective Actions

To eliminate the detected defects, a complex of immediate and long-term measures has been carried out:

• Immediate restoration: The piston has been ground and rechromed. The sealing set has been replaced with high-temperature polyurethane with PTFE support rings to prevent extrusion. New bronze guide strips have been installed in accordance with ISO 3320.
• Long-term maintenance: An additional off-line filtration station has been installed with a filtration accuracy of 3 µm (beta coefficient $eta_3 \ge 200$). The mounting taper sleeves of the cylinder have been adjusted to eliminate misalignment. Safety valves of the hydraulic station have been set to dampen peak pressures.

7. Quick Diagnostic Checklist

This checklist is intended for technical personnel on the production floor to inspect the condition of hydraulic cylinders directly next to the equipment:

• [ ] Check for external oil leaks around the piston rod and at static connections.
• [ ] Measure the cylinder housing temperature with a pyrometer (norm up to +65 °C; exceeding indicates internal leakage).
• [ ] Carry out a visual inspection of the rod surface for absence of corrosion, nicks and weld spatter.
• [ ] Check the condition of the dirt trap for damage and presence of abrasive dirt under the edge.
• [ ] Measure the rod play radially in the extreme positions using a dial indicator (tolerance according to the node passport).
• [ ] Check pressure indicators from the pressure gauges and control system logs for spikes.
• [ ] Take a sample of working fluid for an express test for the presence of water and contaminants.
• [ ] Verify the integrity of the filter elements (check the pressure drop on the filter indicators).

8. Prevention Strategy

To ensure reliable operation of hydraulic equipment and achieve the projected MTBF value of more than 8 000 hours, the following rules must be followed:

• Implement continuous oil cleanliness monitoring according to standard ISO 4406 using particle counters.
• Replace the filter elements according to the pressure drop sensor signal, but no more frequently than every 1 000 hours of operation.
• Perform planned checking of the rod centering and clearances in the guides every 6 months.
• Use certified original sealing materials resistant to the working fluid and temperature ranges of the specific production. The needs of enterprises for quality components and seals can be met through UNITEC-D E-Catalog.

9. Conclusion

Premature failure of hydraulic cylinder seals is a result of the combined effect of mechanical misalignment and contamination of the working fluid. A systematic approach to diagnostics, compliance with the standard ISO 3320 tolerances and oil quality control allow to avoid unplanned downtime on industrial sites. Selection of reliable spare parts and components is available through UNITEC-D E-Catalog.

10. References

• ISO 4406: Hydraulic fluid power — Fluids — Method for coding the level of contamination by solid particles.
• ISO 3320: Fluid power systems and components — Cylinder bores and piston rod diameters — Metric series.
• DIN 51524-2: Pressure fluids — Hydraulic oils — Part 2: HLP oils, minimum requirements.
• DSTU GOST 17752: Hydraulic power transmission and pneumatic power transmission. Terms and definitions.
• Technical documentation of the hydraulic equipment manufacturer and operation instructions for the Schneider Electric TSXBLY01 series controllers.