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Mechanically retain O-rings in vertical and overhead face seals where gravity would drop the seal during assembly.
Quick answer
Use a dovetail groove when the O-ring must stay in place during vertical or overhead assembly — valve covers, horizontal flanges, and large face seals. Typical geometry: 15–20° wall angle per side (30–40° included), groove opening width narrower than the groove midpoint so the O-ring snaps in and is retained. Machine to tighter tolerances than standard rectangular grooves; compression rate remains 15–25% for static face seals.
Standard rectangular O-ring grooves rely on compression and friction to hold the seal during bolt-up. In vertical or overhead face seal applications, the O-ring falls out before the mating flange is positioned — causing assembly delays, contaminated seals, and misaligned installations. A dovetail groove solves retention by angling the groove walls inward so the opening is narrower than the seal cross section at the groove root. The O-ring is stretched slightly to enter the groove and then relaxes behind the undercut lips. Dovetail grooves cost more to machine and inspect than standard grooves. Specify them only when retention during assembly is a real constraint, not as a default for every flange.
Vertical flange faces where the cover is lifted into place from below or the side.
Overhead pipe flanges and vessel covers where the seal cannot be held manually during bolt insertion.
Large-diameter face seals where seal weight causes sagging before compression.
Automated assembly lines where robotic placement requires positive retention without adhesive.
Starting dimensions for static face seals. Adjust for specific cross section and flange load.
Wall angle: 15°–20° from vertical on each side (30°–40° total included angle). Steeper angles increase retention but make installation harder and reduce effective groove volume.
Opening width (groove mouth): approximately 0.85–0.92 × O-ring cross section for standard elastomers; use the lower end for encapsulated or stiff compounds.
Groove root width: approximately 1.25–1.35 × cross section — similar to standard rectangular grooves.
Groove depth: set for 15–25% compression when the flange is fully bolted, same as static rectangular design.
| Cross Section (mm) | Typical Opening Width (mm) | Typical Root Width (mm) | Wall Angle (per side) |
|---|---|---|---|
| 1.78 | 1.50–1.64 | 2.05–2.25 | 15°–20° |
| 2.62 | 2.20–2.41 | 3.00–3.30 | 15°–20° |
| 3.53 | 3.00–3.25 | 4.05–4.40 | 15°–20° |
| 5.33 | 4.50–4.90 | 6.10–6.65 | 15°–20° |
| 6.99 | 5.90–6.45 | 8.00–8.75 | 15°–20° |
Lubricate the O-ring lightly with compatible fluid. Press the seal into the groove at an angle, stretching slightly until it snaps behind the dovetail lips. Verify the seal is fully seated at the groove root — partial seating shows as a high spot when the flange is closed.
Do not use sharp tools to force the seal. A plastic cone or split installation sleeve reduces jacket damage on encapsulated O-rings.
Tighten flange bolts in a star pattern to compress the seal evenly. Uneven bolt-up can eject one side of the seal from a dovetail groove if retention lips are shallow.
Dovetail grooves require CNC machining or precision milling — manual layout is insufficient for consistent retention. Target groove width tolerance ±0.05 mm on opening and root dimensions.
Break all sharp edges at the groove mouth with 0.10–0.25 mm radius to prevent cutting the O-ring during snap-in.
Surface finish at the groove mouth: Ra 0.8–1.6 µm. Rougher surfaces increase friction during installation and may damage soft compounds.
Document the groove on the drawing with a dedicated section view — do not rely on a note referencing standard rectangular dimensions.
Temporary adhesive (cyanoacrylate or compatible RTV): acceptable for field maintenance but not for regulated processes or repeated disassembly.
Retainer ring or cover plate that captures the seal before flange closure — common on large vessel manways.
Square-cut or X-ring profiles with higher friction — sometimes sufficient for moderate vertical assemblies without dovetail machining.
Vulcanized large-diameter rings with tighter tolerance cord for static flanges where molded sizes are unavailable — see vulcanized cord options for ID above 300 mm.
Generally no. The undercut lips create stress concentrations and prevent uniform O-ring movement in reciprocating or rotary glands. Dovetail geometry is intended for static face seals only.
No. Standard tables specify rectangular groove dimensions for static and dynamic glands. Dovetail geometry is a custom retention feature defined on the equipment drawing.
Yes for static service, but installation requires extra care. Limit stretch, use lubrication, and verify the opening width does not force excessive deformation of the FEP jacket. Prefer shallower wall angles (15°) and the upper end of opening width tolerance.
After snap-in, attempt to roll the seal out with finger pressure. It should resist removal without tools. Rotate large seals to confirm uniform seating — high spots indicate incomplete snap-in or wrong cross section.
No. Use standard rectangular grooves when the seal can be held during assembly or when the flange closes horizontally. Reserve dovetail for vertical/overhead orientation or automated lines where retention is proven necessary during pilot assembly.
Submit your flange drawing or photos. We review retention geometry, compression, and material choice at no charge.
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