Groove depth
1.97 mm
Groove width 3.66 mm, anywhere between 3.01 and 3.80 mm works. Gland fill 75 percent, inside the normal 70 to 85 percent band.
First pass for a static face seal in a machined enclosure. Always check against the seal manufacturer table for your compound, particularly for dynamic seals, radial seals, high pressure, vacuum or anything that has to hold a rating. Temperature, media swell and compression set all move these numbers.
What the tool is doing
Three relationships, all of which have to be true at once for a groove to seal.
| Quantity | How it is worked out | Normal range |
|---|---|---|
| Squeeze | Cord diameter minus groove depth, as a percentage of cord diameter | 15 to 30 percent, static face seal |
| Groove depth | Cord diameter times one minus the squeeze | Follows from the squeeze |
| Groove width | Set so the compressed seal fills 70 to 85 percent of the groove | Usually 1.15 to 1.45 times the cord |
| Gland fill | Seal cross section area divided by groove area | 70 to 85 percent |
| Corner radius | Several times the cord diameter on internal corners | Keeps the seal from bunching |
The four things that leak
- A groove modelled on the cord. Zero squeeze, zero seal.
- A sealing face machined before the pockets. The part relaxes and the face is no longer flat. Finish it last.
- Tight corners. The seal stretches on the outside of the corner and bunches on the inside.
- A box that cannot breathe. Temperature cycles pump air past the seal and pull moisture back in. Add a protective vent.
On the drawing
- O-ring part number, cord diameter and compound
- Groove width and depth with tolerances
- The squeeze those came from
- Corner radii on the groove path
- Surface finish on groove and face
- Flatness on the sealing face, and masking during anodise
Questions
What squeeze does a static face seal need?
Typically 15 to 30 percent of the cord diameter, with 20 to 25 percent a common target on a machined enclosure. Less and the seal depends on surface finish, more and the rubber has nowhere to go and takes a permanent set.
Why does groove width matter as much as depth?
Because the compressed seal has to fit in the groove. Gland fill, the proportion of the groove the seal occupies once squeezed, should sit around 70 to 85 percent. Over that and the groove is overfilled, which lifts the lid and stops it sealing.
Does the groove need a special surface finish?
Around Ra 0.8 to 1.6 on the groove walls and the mating face, with machining marks running along the seal path rather than across it. A polished groove is not better and costs more.
Longer version: machining a drone enclosure that actually keeps water out. Related: sealed enclosures and housings.
Sealed housing to quote?
Send the model and the o-ring part number. We check the groove against the seal table before anyone cuts metal, and the first article is measured on the groove dimensions as well as the outside. Reply within 24 business hours.