Design the groove (gland) that houses an O-ring seal. Adjust the cord diameter, groove depth, groove width and shaft diameter to see the squeeze (compression), gland fill ratio and inner-diameter stretch update in real time, and find leak-free groove dimensions for static and dynamic seals.
Parameters
O-ring cord diameter d_CS
mm
Cross-section thickness. JIS/AS568 typical value (e.g. 3.53mm)
O-ring inner diameter d_ID
mm
Shaft (seal) diameter d_shaft
mm
Diameter of the shaft / groove floor the O-ring fits onto
Groove depth h_groove
mm
Groove depth. Making it shallower than the cord squeezes the O-ring
Groove width w_groove
mm
Groove width. Too wide lowers the fill ratio and lets the ring move
Seal application
Switches the recommended squeeze range
Results
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Squeeze (compression) (%)
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Gland fill ratio (%)
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Inner-dia. stretch (%)
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O-ring section area (mm²)
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Groove section area (mm²)
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Design verdict
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O-ring groove cross-section — compressed seal animation
A magnified cross-section of the O-ring seated and squeezed inside its rectangular groove between two metal surfaces. It shows the cord diameter, groove depth, groove width, squeeze gap and the empty space in the groove (fill ratio). Colour shows the design verdict (green = OK / orange = marginal / red = NG).
Fill ratio is the O-ring section area divided by the groove section area; stretch is the inner-diameter increase when fitted on the shaft. w_groove: groove width, d_shaft: shaft diameter, d_ID: O-ring inner diameter.
Adequate squeeze ensures sealing, while the fill ratio must leave room for thermal expansion and the squeezed material to flow.
What is an O-Ring Gland?
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An O-ring is just a round rubber loop, right? You only drop it into a groove — so why does it need "design"?
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Good question. The real design challenge is not the O-ring itself but the groove that houses it. That groove is called the "gland". When the O-ring is set into the groove and squeezed by the mating part, its round cross-section flattens slightly. That flattened part presses tightly against the mating face, and only then do you get a seal. So if the groove is too deep or too shallow, it leaks. The groove dimensions are what decide the performance.
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I see! So "squeeze" is that amount of flattening? When I make the groove depth shallower with the slider on the left, the squeeze climbs fast.
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Exactly. Squeeze (compression) is calculated as (d_CS − h_groove) / d_CS. A 3.53mm cord O-ring in a 2.70mm-deep groove is flattened by 0.83mm, so the squeeze is about 23.5%. For a static seal on a fixed part, aim for 15-30%; for a dynamic seal like a moving piston, 10-20%. Too little and the contact is weak and it leaks. Too much and assembly takes force and the rubber is over-strained, shortening its life.
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So as long as the squeeze is right, it's fine? But there's a "fill ratio" too...
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That is the other pitfall. Rubber, like water, is almost "incompressible", so the volume you squeeze out has to escape sideways. The fill ratio is what percentage of the groove cross-section the O-ring section occupies, and the rule is to keep it at or below 85%. If you fill the groove completely, the squeezed material has nowhere to go. And when the oil or the machine gets hot, the rubber expands, so it needs escape room too. A full groove makes the ring extrude and get nipped off.
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I'm also curious about "inner-diameter stretch". The O-ring stretches when you fit it onto a shaft, doesn't it?
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Nice catch. Slipping an O-ring onto a shaft stretches its inner diameter — that is the stretch, calculated as (d_shaft − d_ID) / d_ID. Keep it below 5%, usually 1-3%. Did you know the ring thins when you stretch it? Just like a rubber band that gets thinner as you pull it, the cord slims down and even the squeeze drops. Conversely, with zero stretch the ring is loose in the groove and tends to get pinched or twisted during assembly.
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Satisfying all three — squeeze, fill ratio and stretch — at once sounds pretty hard...
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Yes, and that is exactly what makes O-ring gland design interesting. The three move together through the groove depth, groove width and shaft diameter. Make the groove shallower and the squeeze rises, but the squeezed volume has less escape room so the fill ratio rises too. With this tool you watch all three numbers at once while moving the sliders, and you quickly find groove dimensions where all of them land in the recommended range. In practice, this three-point check is the basic routine.
Frequently Asked Questions
Squeeze is found from the O-ring cord diameter (cross-section) d_CS and the groove depth h_groove: squeeze = (d_CS − h_groove) / d_CS × 100 [%]. When fitted into the groove, the round cord section is flattened down to the groove depth, and that flattening fraction is the squeeze. Recommended values are 15-30% for static seals (fixed parts) and 10-20% for dynamic seals (reciprocating or rotating parts). Too little squeeze gives no initial sealing; too much raises assembly force and strain and shortens life.
The fill ratio is the O-ring cross-section area A = πd_CS²/4 divided by the groove cross-section area (groove width × groove depth). Rubber is almost incompressible, so the squeezed volume has to flow somewhere. On top of that, hydraulic fluid and the surroundings heat up and the rubber thermally expands. If you fill the groove completely (100%) there is nowhere for the material to go, causing extrusion or excessive pressure on the groove walls. Keeping the fill ratio at or below 85% leaves room for the squeezed material and thermal expansion.
Stretch is found from the shaft (seal) diameter d_shaft and the O-ring inner diameter d_ID: stretch = (d_shaft − d_ID) / d_ID × 100 [%]. Fitting an O-ring onto a shaft stretches its inner diameter and slightly thins the cord. The recommended value is below 5%, typically 1-3%. Excessive stretch thins the cord, which reduces the squeeze and accelerates stress relaxation (set). Zero stretch, on the other hand, lets the ring shift in the groove and get pinched during assembly.
A static seal (a non-moving location such as a flange face) can use a higher squeeze of 15-30% because friction is not a concern and reliable sealing is the priority. A dynamic seal (a reciprocating or rotating piston or rod) uses a lower squeeze of 10-20%, because high squeeze increases friction, heat and wear and shortens life. Dynamic seals also need attention to groove surface finish and, on the high-pressure side, anti-extrusion back-up rings. This tool switches the recommended squeeze range according to the selected application.
Real-World Applications
Hydraulic and pneumatic equipment: Almost every fluid-handling device — hydraulic cylinder pistons and rods, pneumatic valves, pump casings — uses O-rings. Fixed flanges are designed as static seals with a higher squeeze, while reciprocating pistons are dynamic seals with a lower squeeze. At high pressure (generally above 7 MPa) a back-up ring is added to prevent extrusion through the clearance gap, and the groove shape is decided on that basis.
Joints in automobiles and industrial machinery: O-rings seal oil, water and fuel in engine and transmission oil passages, fuel-system fittings and coolant joints. Because temperature swings are large, designers allow for both reduced sealing at low temperature and thermal expansion of the rubber at high temperature, keeping the fill ratio with a margin (at or below 85%). On face seals (clamped on a flat surface), a rounded-over or scratched groove corner leads directly to a leak.
Vacuum and semiconductor equipment: On vacuum-chamber flanges and gate valves, the O-ring separates atmosphere from vacuum. Here the squeeze is firmly secured to minimise leak rate, and low-outgassing materials (such as fluoroelastomer) are chosen. Loose control of the groove dimensions means the target vacuum level is never reached, and that becomes the limiting factor for the whole tool.
Design verification and troubleshooting: Many sealing failures — "weeping right after assembly" or "starting to leak after some use" — originate in the groove dimensions. A quick check of squeeze, fill ratio and stretch with a tool like this distinguishes an O-ring selection mistake from a groove-machining mistake. For detail it is combined with FEM analysis that accounts for rubber stress relaxation and compression set (CS).
Common Misconceptions and Pitfalls
The most common pitfall is "as long as the squeeze is right, everything is fine". Squeeze is the most important parameter for initial sealing, but looking only at it and ignoring the fill ratio leads to trouble. Rubber is incompressible, so the squeezed volume always flows sideways. If the groove width is too narrow and the fill ratio exceeds 100%, the O-ring will not even fit into the groove. Even just exceeding 85% leaves the rubber with nowhere to go during thermal expansion, inviting extrusion (extrusion failure) through the clearance gap. Always check squeeze, fill ratio and stretch as a set of three.
Next, "treating the O-ring nominal size as identical to the groove dimensions". When fitted onto a shaft, the O-ring stretches and its inner diameter increases while the cord thins slightly at the same time. Using the catalogue nominal cord diameter directly in the squeeze calculation makes the actual squeeze come out a little lower than calculated. The rubber also develops compression set (taking a permanent set) over temperature, fluid and time, so the initial compression reaction force decays as time passes. Secure ample squeeze at the design start, and for long-term use or high temperature also check the material's compression-set characteristics.
Finally, the complacency of "groove corners and surface roughness are minor details that can be left for later". An O-ring seal depends strongly on the condition of the mating seal face and the groove walls. A large rounded-over edge or a scratch on a groove corner becomes a leak path. On dynamic seals the surface roughness of the groove floor and seal face directly governs friction and wear: too rough abrades the ring, while too smooth can break the lubricating oil film and lead to seizure. Even when the dimension calculation is OK, the "design" is only complete once chamfers, corner radii and surface-finish callouts are specified as well.
How to Use
Enter the O-ring cord diameter (dcs) in mm—typical range 1.6–25.4 mm for industrial seals.
Input the shaft diameter (shaft) and groove depth (depth) in mm; depth commonly ranges 80–105% of cord diameter per ISO 3384.
Set the inner diameter of the groove (ido) to achieve target squeeze, typically 10–25% compression for dynamic seals.
Review the gland fill ratio (target 75–90%), inner-diameter stretch, and design verdict to confirm compliance with AS568 or ISO 3384 standards.
Worked Example
For an AS568-373 O-ring (cord diameter 3.53 mm, inner diameter 41.63 mm) installed on a shaft diameter of 40 mm: set groove depth to 3.0 mm (85% of cord) and ido to 41.5 mm. The simulator returns squeeze 18%, gland fill ratio 82%, inner-diameter stretch 0.3%, section area 9.8 mm², groove area 12.0 mm², and verdict "Acceptable" for low-speed hydraulic applications.
Practical Notes
Higher squeeze (20–25%) improves static sealing but increases friction and heat in dynamic applications above 5 m/s.
Gland fill ratio below 70% risks O-ring extrusion; above 95% causes pinching and stress concentration cracks.
For reciprocating pumps, maintain squeeze 12–18% and verify groove width ≥1.5× cord diameter to prevent rolling.
Temperature compensation: ISO 3384 requires depth tolerance ±0.1 mm for nitrile seals in 100–150°C aerospace hydraulics.