O-Ring Gland Design: Squeeze, Stretch & Fill to AS568 / ISO 3601

What AS568 and ISO 3601 are

AS568 (the US aerospace size standard) and ISO 3601 (its international counterpart) define the standard O-ring sizes — the inside diameters and cross-sections you can actually buy off the shelf — along with their dimensional tolerances. When someone specifies a "-214 O-ring," that dash number is an AS568 size. Designing a seal is mostly about choosing one of these standard rings and then cutting a groove (the "gland") around it that squeezes it the right amount.

The three numbers that make a seal work

• Squeeze — how much the groove compresses the ring's cross-section. Too little and it won't seal or will leak as pressure cycles; too much and the rubber over-stresses, takes a permanent set, and fails early. Design guides give recommended squeeze bands per cross-section and application (a static seal tolerates more squeeze than a dynamic one).
• Stretch — how much the ring is stretched over its groove diameter on installation. A little stretch keeps the ring seated, but too much thins the cross-section (reducing squeeze) and accelerates aging. Common limits are about 8% for small rings (d1 < 50 mm) and 6% for large ones, with roughly 0.5% cross-section reduction per 1% of stretch.
• Gland fill — how much of the groove volume the ring occupies. Elastomers are nearly incompressible, so the groove must have room for the squeezed ring to flow into, plus margin for thermal expansion and fluid swell. Industry practice keeps fill at ≤75% nominal (≤85% worst case). Over-fill it and the ring has nowhere to go — it jams and extrudes.

The failure mode the standards guard against: extrusion

Under pressure, an O-ring is pushed against the low-pressure side of the groove and tries to squeeze out through the clearance gap between the mating parts. Whether it survives depends on the clearance gap, the pressure, and the rubber hardness — a harder compound (higher Shore A) resists extrusion better. Design guides tabulate permissible extrusion clearances against those variables; exceed them and the ring shaves itself away.

How the geometry is actually computed

A gland calculation stacks these effects: the standard AS568 / ISO 3601 size and its tolerance, the stretch from the groove diameter (which thins the effective cross-section), the resulting squeeze against the groove depth, and the gland fill against the groove width. A rigorous check runs the worst case — every tolerance stacked at its unfavourable limit simultaneously — because that's the combination that actually shows up on a bad-but-in-spec part. Tolerances come from the AS568 / ISO 3601-1 Class A tables; ISO 286 fits on the bore and groove add their own contribution.