O-Ring Groove Design: How a Gland Is Sized

An O-ring groove is sized from three numbers: the cross-section of the ring, the squeeze the seal type needs, and the free volume the ring must have to swell and expand into. Parker's handbook holds gland fill between 60 and 85 percent of groove volume, with 75 percent as the optimum. This guide explains the rules that sit behind the numbers for an AS568 ring, from the published guidelines of Parker, Trelleborg, Precision Associates and Apple Rubber; the groove dimensions themselves come from the standard or the maker's design chart your drawing names.

Which gland type, and which convention?

Three gland types cover most O-ring seals: the static face (axial) seal, where the ring is squeezed between two flat flanges; the static radial seal, where a piston or rod carries the ring into a bore or tube without motion; and the dynamic reciprocating seal, where the rod or piston moves. Squeeze falls from face to radial to dynamic, because motion adds friction and wear. Two conventions publish dimensions: the AS568 industrial tables (Parker and MIL-G-5514 lineage, carried by SAE AS4716 and AS5857) and ISO 3601-2 for metric hardware. They disagree slightly on static gland depth for the large sections, so a calculation should use one convention and say which.

Static face seals

Internal and external pressure share the same groove; only the stretch rule differs.

Which groove diameter is sized to the ring depends on the pressure direction: Precision Associates sizes the groove outside diameter to the ring outside diameter plus the ring's tolerance, and Trelleborg keeps the ring outside diameter equal to, or at most 1 to 2 percent larger than, the outer groove diameter.

Static radial, dynamic and rotary glands

The depth and the squeeze of a radial gland change with the duty. Trelleborg allows piston seals 2 to 8 percent stretch (static) or 2 to 5 percent (dynamic); Parker does not recommend more than 5 percent, and above 2 to 3 percent it calls for a gland-depth correction. Rod seals sit at zero stretch.

Rotary service is its own design case. For the -0xx, -1xx and -2xx sections Precision Associates' rotary design chart (Design Chart 5) gives groove widths of 0.080, 0.117 and 0.157 in, and the groove depths worked out from its shaft and groove diameters come to 0.065, 0.098 and 0.133 in. Trelleborg sizes the ring 2 to 5 percent larger than the shaft, and Parker's Design Chart 5-4 puts the squeeze at 0 to 11, 1 to 8.5 and 0 to 7 percent on those sections. The ring is sized larger than the shaft, not smaller, so that it does not shrink onto the shaft and overheat, and the Parker handbook discourages plain O-rings as rotary seals beyond low speeds. The squeeze guide explains what temperature and swell do to the squeeze.

Gland fill: the rule that catches the tolerance stack

Fill is the ring's cross-sectional area divided by the groove's width times depth, where the ring area is pi over four times the cross-section squared. Parker's handbook band is 60 to 85 percent with 75 percent optimum; Trelleborg's ceiling is 85 percent; Apple Rubber allows the ring volume up to 90 percent of the minimum gland void, and 95 percent for a static crush seal only. The void exists because thermal expansion, fluid swell and the tolerance stack all need somewhere to go. Evaluate fill at the worst case, maximum ring section against minimum groove width and depth: a gland that reads 80 percent at nominal can read 90 percent at the extremes.

Surface finish and edges

Two cautions. RMS and Ra are not the same measure, but the seal makers use them interchangeably at these magnitudes; treat a 32 RMS and a 32 microinch Ra callout as the same requirement and never convert between them. And the number alone is incomplete: the lay of the finish should run with the ring, not across it. The gland entry gets a chamfer so the ring is not cut on installation.

Dovetail and backup-ring grooves

A dovetail groove holds the ring in a face seal for maintenance (vacuum chambers, lids that open). Trelleborg recommends it for the 0.139 in section and larger and calls it expensive; because its section is a trapezoid, the rectangular fill formula overstates the free volume, so fill is checked against the real shape. When pressure exceeds what the plain groove can hold, a backup ring goes on the low-pressure side (Parker's handbook, section VI) and the groove is widened to take it; the ring maker's sheet sets the width. The durometer guide carries the pressure-versus-clearance figures that decide when a backup ring is needed.

The ranges in this guide are the published design guidelines of the manufacturers named beside them (Parker, Trelleborg, Precision Associates, Apple Rubber), re-arranged by seal type and AS568 cross-section family. They are not Conquest Seal engineering recommendations: Conquest distributes seals and cuts gaskets, and does not design parts. If a drawing calls out SAE AS4716, SAE AS5857, ISO 3601-2 or MIL-G-5514, the standard governs and these ranges are context.

Sources

1. Parker O-Ring Handbook ORD 5700 (2021 printing), sections 3.5 to 3.7 (stretch, squeeze and gland fill), read 2026-09-24; section 4.1 (surface finish); section 5.27 and Design Chart 5-4 (rotary O-ring seal glands, page 5-39), read 2026-10-13; section VI (back-up rings, page 6-2), read 2026-09-25
2. Trelleborg Sealing Solutions, O-Rings and Back-up Rings catalog (June 2024 edition), read 2026-09-06; Table 20 (surface finish), read 2026-09-24
3. Precision Associates, Application Data: Seal Gland Design (2011), including Design Chart 5 (rotary seals), read 2026-09-06
4. Apple Rubber Products, Seal Design Guide (Rules of Thumb; O-Ring Installation), read 2026-09-06
5. SAE AS4716 and AS5857, ISO 3601-2 and MIL-G-5514: titles and scope only, from the SAE, ISO and ASSIST records read 2026-09-06