What gland fill measures
Gland fill (or groove fill) is the percentage of available groove volume occupied by the installed O-ring:
fill (%) = installed O-ring volume / available groove volume × 100
For a simple, constant-width rectangular groove, the cross-sectional-area ratio gives the same result only if the ring and groove are evaluated over the same installed centerline length. This shortcut is useful for understanding the calculation; a real radial or face-seal design must account for its actual circumference, ring stretch, groove geometry and tolerances.
Worked example: why the nominal value is not enough
Assume a 3.00 mm nominal cord diameter (±0.10 mm), a 4.50 mm groove width (±0.10 mm) and a 2.40 mm groove depth (±0.10 mm). For this illustration alone, assume a straight rectangular groove, equal ring and groove path lengths, no corner radii, no stretch, and no temperature or fluid swell. The per-unit-length ring area is π × cord² / 4; the available groove area is width × depth.
| Case | Cord | Width | Depth | Fill | Squeeze |
|---|---|---|---|---|---|
| Minimum fill | 2.90 mm | 4.60 mm | 2.50 mm | 57.44% | 13.79% |
| Nominal | 3.00 mm | 4.50 mm | 2.40 mm | 65.45% | 20.00% |
| Maximum fill | 3.10 mm | 4.40 mm | 2.30 mm | 74.58% | 25.81% |
For example, nominal fill is π × 3.00² / 4 ÷ (4.50 × 2.40) × 100 = 65.45%. The spread from 57.44% to 74.58% exists before any service swell. These numbers illustrate the method, not a qualified groove recommendation or a pass/fail verdict. A circular gland and a stretched O-ring require the full installed-volume model in the radial static calculator or face-seal calculator.
Leave room to expand
Rubber is essentially incompressible — it changes shape, not volume. When it heats up or swells in service fluid, it needs somewhere to go. If the groove is already full, the ring over-stresses the gland, can extrude, and loses sealing reliability.
Which result controls the decision?
The applicable window depends on the verified Parker module. Face seals use 60%–90% for liquid and 75%–98% for gas; vacuum is mapped to 75%–98% as an engineering interpretation of Parker’s shared Vacuum/Gas narrow-gland reference, not as a separately observed Ideal value. The five radial bands use 60%–90%. The calculator does not interpolate a criterion for a non-standard cross-section. It displays the complete tolerance stack and applies expected media swell only to the maximum O-ring volume case.
Compare the entire calculated fill range with the active criterion, not just the nominal fill. If the minimum or maximum crosses a bound, inspect the dimensions that caused it before changing the groove. Squeeze, stretch and extrusion are separate checks; acceptable fill alone does not qualify a seal. The tolerance stack-up guide explains why their governing dimension combinations differ. For service-fluid effects, review material selection before relying on a dry, room-temperature result.