Nominal values do not control the extremes
An O-ring groove can look ideal at nominal dimensions and fail at the tolerance limits. Minimum squeeze usually combines the smallest effective cord with the largest gland height. Maximum squeeze combines the largest effective cord with the smallest gland height. Maximum fill combines maximum O-ring volume with minimum available groove volume.
Those governing combinations are not necessarily the same for every result, so a single “worst case” geometry is insufficient.
Start from signed dimensions
Store every major dimension as nominal, positive tolerance and negative tolerance. Convert it into explicit minimum and maximum values before calculating. Avoid an ambiguous single “± tolerance” field when the drawing is unilateral.
For a simple axial face seal:
squeeze = (cord diameter − gland height) / cord diameter
The minimum squeeze uses the combination that minimizes the ratio; the maximum uses the combination that maximizes it. For radial seals, calculate the effective diametral or radial gland height from the actual bore, rod, piston and groove dimensions at each limit.
Fill requires a separate volume stack
O-ring volume depends on installed circumference and effective cross-sectional area. Groove volume depends on width, depth and path length. Therefore, maximum squeeze is not a substitute for maximum fill. Expected media swell increases the maximum O-ring-volume case, while temperature can change both elastomer and hardware.
Include functional contributors
The drawing tolerance stack may also need:
- bore-to-shaft eccentricity and runout;
- coating or plating thickness;
- pressure-induced hardware expansion;
- thermal expansion of each material;
- parting-line mismatch;
- ring stretch and corresponding cross-section change;
- wear allowance in dynamic service.
Do not hide these contributors inside an undocumented “safety factor.” Model them as named inputs so a reviewer can see the basis.
Worst-case and statistical methods
Arithmetic worst-case limits are appropriate when every allowed part must meet the design without selection. Statistical methods can describe production yield when dimensions are controlled distributions, but they do not turn an out-of-limit seal into an acceptable one. Use a statistical stack only with manufacturing capability data, correlation assumptions and an agreed acceptance policy.
Review sequence
- Verify units and dimension direction.
- Build explicit minimum and maximum ring and gland geometry.
- Calculate squeeze, stretch and fill independently.
- Apply the active engineering criterion to each governing result.
- Record which input extremes controlled each verdict.
- Confirm the result against actual measured parts and service tests.
The site calculators perform nominal/minimum/maximum geometry on the server and retain failed criteria in the report. Use them to expose the stack, then make the production drawing and inspection plan control the same dimensions.