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.

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.

As a rule of thumb the width is roughly 1.3 to 1.5 times the cord diameter, chosen so that fill lands in the 60–85 % band across the tolerance stack. Wider grooves (nearer the low fill limit) suit fluids that cause high swell; narrower grooves suit stable media. Because depth and width interact through both squeeze and fill, they are best solved 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.