BACKGROUND

Seal extrusion is a major concern for hydraulic applications, especially where pressures are high or extrusion gaps are larger than normal.  In the past, engineers relied on tables or approximate formulas to estimate extrusion potential.  However, as a result of research, experimentation, and FEA simulation, System Seals has developed an extrusion calculation tool which our engineers use to predict the potential for thermoplastics to fail due to extrusion.  As a result, System Seals uses such tools to develop new sealing technologies.

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Figure1.  Cross section of extruded seal

EXPERIMENTAL PROCEDURE AND FEA MODEL DEVELOPMENT

To investigate the relationships between e-gap, material, and extrusion, PTFE seals in a basic square profile were installed into a pressure intensifier as well as seals with a POM backup.  All profiles were tested at 21, 34, 48 MPa under 3 radial e-gaps (.5, 1, 1.5mm).  After testing concluded, the seals were immediately removed, visually inspected, and measured for extrusion. After, seals were sectioned and imaged for visual confirmation.  Simultaneously, FEA models were constructed to duplicate the seal size, pressures, materials, and e-gaps.  After a relationship was extracted between the FEA and experimental results, additional FEA models were constructed for the current and new seal designs under various operating conditions.

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The FEA models confirmed the experimental results, which indicated the relationships between pressure, e-gap, and extrusion potential.  And by running simulations on various seal designs, we were able to develop a new seal design with an active back-up ring that is extrusion resistant.

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Typical FEA model and probes

 

Because the initial experimentation and FEA simulations were so successful, we conducted nearly 300 additional FEA simulations which lead to the development of mathematical models that we use to predict extrusion for a wide variety of materials.

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Figure 19.  Design Tree for FEA Backup Ring Simulations

 

CONCLUSIONS

Older seal designs cannot effectively resist extrusion onset, due to larger e-gaps, higher pressures, and larger loads and deflections.  As a result, new designs are required.  To reduce the time and costs required to go to market with a new design capable of functioning in the more extreme operating conditions, a combination of both FEA and empirical experimentation were used for this investigation.  As a result, a next generation rod seal was developed which can withstand the higher pressures and e-gaps without extruding.  In conjunction, consideration was given to increases in e-gaps due to guidance system deflections and rod-to-cylinder eccentricity.  Furthermore, our design and simulation procedure included the impact of temperature on extrusion resistance of materials, as determined by DMA testing data.  Therefore, cylinder manufactures and engineers can design for larger e-gaps and clearances without the concern over seal extrusion and the associated system failures.