What the groove has to do

An O-ring groove (or gland) has two jobs: compress the cord by the right amount to create sealing contact, and leave enough free volume for the rubber to spread into as it is squeezed and as it swells with heat and fluid. Get the depth wrong and the seal either leaks or over-stresses; get the width wrong and the groove either over-fills (risking damage) or under-fills (wasting compression). Three numbers define most rectangular grooves: depth, width and corner radii.

Groove depth sets the squeeze

Depth is derived from the cord diameter and the target squeeze:

Groove depth ≈ cord diameter × (1 − target squeeze)

For example, a 3.53 mm cord at 20 % squeeze needs a gland height of about 3.53 × 0.80 ≈ 2.82 mm. In practice the “depth” that matters is the assembled gland height — the radial gap between groove bottom and the mating sealing surface — so you must include diametral clearances and tolerances, not just the machined groove depth.

Typical squeeze targets, which set the depth, are:

  • Radial static seals: ~15–25 %.
  • Reciprocating dynamic seals: ~8–16 % (lower, to limit friction and wear).
  • Face (axial) seals: ~20–30 % of cord.

These are starting ranges, not universal acceptance limits. The active criterion, actual compound and operating conditions determine whether a particular design qualifies.

Worked example: size the assembled gland height, then stack tolerances

Suppose a 3.53 ± 0.08 mm cord is being considered for a radial static seal and the preliminary target squeeze is 20 %. The starting assembled gland height is 3.53 × (1 − 0.20) = 2.824 mm. Rounding this to 2.82 mm and assuming an illustrative ± 0.04 mm assembled-height tolerance gives:

Condition Cord diameter Assembled gland height Calculation Geometric squeeze
Minimum squeeze 3.45 mm 2.86 mm (3.45 − 2.86) / 3.45 17.10 %
Nominal 3.53 mm 2.82 mm (3.53 − 2.82) / 3.53 20.11 %
Maximum squeeze 3.61 mm 2.78 mm (3.61 − 2.78) / 3.61 22.99 %

The nominal value is not exactly 20 % after rounding, and the tolerance extremes span almost six percentage points. The assumed tolerances are for illustration, not an AS568 size tolerance or a drawing recommendation. For a radial gland, derive the assembled height from the groove-bottom and mating diameters, including both parts’ tolerances; it is not automatically the machined groove depth. Stretch may also change the effective cord section. Use the radial static calculator and the tolerance-stack guide to evaluate the real design.

Groove width sets the fill

Width is governed by gland fill — the ratio of O-ring volume to groove volume. The ring must never completely fill the groove, because rubber is nearly incompressible and expands with temperature and fluid swell. A groove that is too narrow can hydraulically lock or force the ring against the edges.

An initial width may be around 1.3 to 1.5 times the cord diameter, but that ratio is not a fill calculation or a design limit. For the example above, 1.3 × 3.53 mm suggests a starting width of about 4.59 mm. Calculate the actual ring and gland volumes—including diameter, stretch, width and depth tolerances, and expected swell—before accepting it. Wider grooves may provide more swell allowance; narrower grooves reduce that allowance. Because depth and width interact through both squeeze and fill, solve them together rather than one at a time.

Corner radii and edges

  • Groove bottom radii are small (often 0.1–0.4 mm) to avoid a sharp stress concentration and to let the ring seat.
  • Groove top edges are lightly broken or radiused so the ring is not sheared or nibbled during assembly, especially on dynamic seals.
  • Lead-in chamfers on the mating part (typically 15–20°) prevent the ring from being cut as it is installed over threads or ports.

Radial versus face glands

A radial groove seals on a diameter — a piston in a bore or a rod in a housing — so the critical dimensions are the groove-bottom diameter, the mating (bore or rod) diameter and the resulting diametral clearance. A face (axial) groove seals between two flat surfaces, so the critical dimensions are groove depth, groove inner and outer diameters, and whether pressure acts inward or outward. The pressure direction decides which groove wall the ring is forced against, which in turn affects the width you should choose.

Surface finish and extrusion gap

Two secondary numbers finish the design. The surface finish of the groove and the sealing face controls leakage and wear — smoother for dynamic seals, and a controlled roughness on the static faces. The diametral extrusion gap — the clearance the ring could be pushed into under pressure — must stay small enough that the ring does not extrude; higher pressure and softer material demand a tighter gap or a back-up ring. See the extrusion guide for the gap-versus-pressure relationship.

Let the calculator solve it

Depth, width, squeeze and fill are coupled, so evaluate the entered O-ring and groove dimensions together across nominal, minimum and maximum conditions. The radial static, radial dynamic and face seal calculators report squeeze and gland fill against the active, source-labelled criteria and mark each governed result qualified or unqualified. A criterion set does not silently replace your drawing dimensions with brand-recommended groove geometry.

Frequently asked questions

Should the O-ring fill the whole groove? No. Leave 15–40 % free volume so the incompressible rubber has room to deform and expand. A completely filled groove can hydraulically lock and damage the ring.

How deep should an O-ring groove be? Deep enough to give the target squeeze — roughly cord diameter × (1 − squeeze). For a typical static seal that is about 75–85 % of the cord diameter as an assembled gland height.

Do I need corner radii? Yes. A small bottom radius avoids stress concentration and a broken top edge protects the ring during assembly. Standard groove tables specify these.