O-ring gland

The O-ring calculator starts from a ring size and gets you to the groove dimensions: depth, width, bottom diameter and the corner radii. This page explains what each number means and what to check before sending it to be machined.

The path

1. Pick the ring. The AS568 tab has the full series; the Free metric tab accepts any inside diameter and cross-section you type, for rings outside the standard.
2. Pick the type of mounting. It's the decision that changes the result the most — see the section below.
3. Enter the mating face, which is the diameter of the bore or the shaft the ring will work against.
4. Read the dimensions and check the three indicators: squeeze, fill and stretch.
5. Download the profile as .dxf to take straight into CAD.

The type of mounting is not a detail

A radial ring works compressed between the bottom of the groove and the wall of the mating face. A face-seal ring works squeezed between two flat surfaces, and tolerates more squeeze without suffering, because it isn't being sheared against a wall at the same time.

That's why the tool asks for the type before calculating: choosing "face" for a gland that is really radial gives you a groove that is too shallow, and the ring leaks.

The three indicators

Squeeze — how much of the ring's cross-section is compressed, as a percentage. Too little squeeze leaks; too much kills the ring through permanent deformation. The tool already aims at the right range for the chosen cross-section, and warns you when the mating diameter you typed pushes the value outside it.

Fill — how much of the groove volume the ring occupies. The groove has to have room left over, because elastomer is practically incompressible and it also expands with temperature and swells in some fluids. A groove that is too full blows out the edge.

Stretch — how much the ring is opened up to fit the working diameter. Stretching thins the cross-section, and a thinner section changes the actual squeeze. The tool takes that into account instead of calculating with the nominal section.

Watch out. High stretch is the most common origin of a leak that "shouldn't be happening": the groove dimensions match the table, but the installed ring has a smaller section than the table says.

The dimensions agree with each other

A detail that matters when machining: depth and bottom diameter are calculated in a chain, not rounded independently.

The bottom diameter comes from the already rounded depth — the one that goes on the drawing. Round the two separately and mating face − 2 × depth no longer matches the bottom diameter: half a hundredth of disagreement between two dimensions on the same drawing, and someone will spend time working out which of the two is right.

The DXF

The button generates the cross-section profile of the groove, not the revolved groove: a closed polyline in DXF R12, in millimetres, with the bottom corners already rounded.

It's the profile you use as a revolve or sweep sketch in CAD. The corners come rounded because a sharp corner at the bottom of the groove concentrates stress in the ring, and that's where it starts to tear.

Before machining

• Check the surface finish of the sealing faces. The calculator gives geometry; roughness is your call and depends on static or dynamic service.
• Check the chemical compatibility of the elastomer with the fluid and the working temperature. No dimension saves the wrong compound.
• At high pressure, consider a back-up ring. The tool doesn't size the back-up ring yet.