What is an O-Ring Face Seal?

A face seal is a groove machined to the top or bottom of a flat plate where the O-Ring is sealed on the top and bottom surface not the inner or outer diameter. When the mating cover is bolted down, the O-ring is squeezed in the axial direction to create a static seal. Groove depth, width, and surface quality are the key levers for performance.

Step-by-Step Design Checklist

1. 
   

   Start with the environment (then choose the compound)

   Define media, temperature range, pressure/vacuum, exposure to ozone/UV, washdowns/solvents, and any cleanliness or low-outgassing needs. From there, select the elastomer family and durometer (e.g., NBR, FKM, EPDM, Fluorosilicone, FFKM, Silicone, Neoprene) that best matches your conditions.

2. 
   

   Pick the cross-section and size

   Face seals usually use standard AS568 O-ring sizes but Sealing Devices can offer other standard or customer sizes if needed. Larger cross-sections are more tolerant of flatness variation and can be easier to seal, but they require deeper/wider glands and higher clamp loads. Balance space, squeeze target, and your fastener strategy.

3. 
   

   Engineer the gland for controlled squeeze

   • Depth sets axial squeeze → sealing force.
   • Width provides space for the compressed O-ring plus volume changes from temperature or media swell.
   • Edges with corner breaks/fillets reduce cutting or nibbling during assembly.
   • Volume fill: leave expansion room at high temperature; avoid overfilling the groove.
4. 
   

   Manage plate gap & extrusion risk

   Any clearance between the faces becomes a potential extrusion gap under pressure. Reduce risk by:

   • Minimizing gap with a robust fastener pattern and stiff cover design,
   • Choosing higher durometer when pressures climb,
   • Adding anti-extrusion (backup) rings if needed,
   • Verifying against your peak pressure and temperature during testing.
5. 
   

   Specify surface finish and flatness

   • Target a smooth sealing land (commonly 32 µin Ra for liquid service, and 16 µin Ra or finer for gas and vacuum service).
   • Keep machining lay circular and avoid radial paths from the inner and outer edges that could leak.
   • Ensure flatness across the seal path so minimum squeeze is maintained.
6. 
   

   Control the tolerance stack

   Axial squeeze is sensitive to accumulated variation: groove depth, cover thickness, gasket shims, fastener stretch, and thermal growth. Use a worst-case analysis (not just nominal) to keep squeeze within spec across the full operating range.

7. 
   

   Assembly best practices

   • Lightly lubricate (use a compatible lube) to cut friction and installation damage.
   • Deburr and radius the groove entry.
   • Use a cross/star torque pattern to seat the seal evenly.
   • Consider alignment pins for maintenance lids to protect the sealing land.
8. 
   

   Validate early

   Prototype on representative hardware and test: pressure cycling, temperature cycling, and media soak. Early testing reveals extrusion, compression set, and leakage behaviors long before field use.

Common Mistakes to Avoid

• Oversqueezing to “be safe” (accelerates compression set and shortens life).
• Ignoring plate gap at high pressure.