430.000.000+ O-rings in stock
Fast delivery
Nederlands 
Deutsch 
Français 
Italiano 
Español 
A static O-ring seal has one task: to remain in place under pressure, without moving. Yet there is not one standard solution. The groove type, the direction of pressure, the design and sometimes the orientation of the component together determine which installation type is most suitable. This article provides an overview of all five static installation types for O-rings, including the considerations for choosing between the types and the shared requirements for surface finish and gap width.
Static sealing means that the machine parts being sealed do not move relative to each other. The O-ring is trapped in a groove and compressed by assembly. That compression, the squeeze, creates the contact pressure on the sealing surfaces. As long as that contact pressure is greater than the working pressure of the medium, the seal remains tight.
Static sealing places different demands than dynamic sealing. There is no wear caused by movement, no risk of twisting and no friction that needs to be controlled. But there are still requirements for groove geometry, surface finish and material selection that are just as decisive for the result as in dynamic applications. Most leakage problems in static O-ring seals are not related to the ring itself, but to the groove: milled too deep, no chamfer, or a sealing surface that is too rough.
Rule of thumb: if both parts remain stationary after assembly, it is a static application. If one of the parts moves, it is dynamic.
There are five standardized static installation types for O-rings. They differ in groove shape, compression direction, application area and the machining requirements they impose. The table below provides a direct comparison.
|
Groove type |
Compression |
Pressure direction |
Characteristic advantage |
Typical application |
|
Radial compression |
15 to 30% |
Omni-directional |
Simple and universal |
Plugs, rods, bores |
|
Axial compression |
1 to 3% (compressed) max. 6% (stretch) |
Internal or external |
Flat connections |
Lids, flanges |
|
Trapezoidal groove |
Equal to radial |
Omni-directional |
Ring does not fall out of the groove |
Overhead assembly |
|
Triangular groove |
Depends on angle |
Omni-directional |
Integrated into the design |
Insufficient depth |
|
Vacuum sealing |
Higher than standard |
Outward (vacuum) |
Lowest leakage rate |
Lab, analysis, process installations |
Radial compression is the most commonly used static installation type and the standard choice for cylindrical connections. The O-ring is compressed in the radial direction between two cylindrical surfaces. The groove is located in the inner or outer component. The recommended compression is between 15 and 30% of the cross-section diameter. This is a wide range that allows room for design tolerances without putting the seal at risk.
Radial compression is suitable for plugs, rods, bores and all constructions in which two cylindrical surfaces are slid concentrically over one another. The groove has six parameters: cross-section diameter (d2), groove depth (t), groove width (b1), chamfer length (z) and two corner radii (r1 and r2). All are derived directly from d2.
More information and the full dimension table: see the article O-ring groove dimensions for static radial compression in our knowledge base.
In axial compression, the O-ring is compressed in the direction of the axis, between two flat mating surfaces. The classic example is a lid tightened onto a housing, or two flanges bolted together. The groove is located in one of the flat mating surfaces.
The critical difference from radial compression lies in the pressure direction. With internal overpressure, the ring must lie against the outer wall of the groove and is compressed by 1 to 3%. With external pressure, the ring lies against the inner wall and may stretch by a maximum of 6%. If you place the ring on the wrong side, the pressure pushes it away from the sealing surface instead of against it. That is the most common mistake in axial applications.
Complete groove parameters and dimension table: O-ring groove dimensions for static axial compression.
The trapezoidal groove solves a practical problem: after installation, the O-ring can no longer fall out of the groove. The groove is not rectangular but trapezoidal in cross-section, with a narrower base than opening. As a result, the ring is clamped in place as soon as it is fully inserted. In overhead assemblies or with machine parts that are opened regularly, this saves rework and lost rings.
Manufacturing is more complex than for a rectangular groove: a special profile cutter is required. The tolerances on depth and width are symmetrical and tighter (±0,05 mm). The sealing performance itself is comparable to that of a rectangular groove at the same compression. The trapezoidal groove is not a universal improvement, only a targeted solution to a specific assembly issue.
Dimension table and application advice: O-ring groove dimensions for the static trapezoidal groove.
For sealing lids or flanges, we normally recommend a rectangular groove. The triangular groove is an exception: it is used only when a rectangular groove is not structurally feasible, for example when there is insufficient material depth or in an existing component where milling would cause structural objections.
The triangular groove has only three parameters: cross-section diameter (d2), groove width (b) and the bottom radius (r3). The tolerances are tighter than for other groove types. The contact zone of the ring is narrower, which creates higher local stresses in the rubber at higher pressures. Never use the triangular groove by preference, only when the design truly requires it.
Dimension table and conditions: O-ring groove dimensions for the static triangular groove.
Vacuum sealing with O-rings places different demands on the groove, the surface and the material selection. Under overpressure, the operating pressure helps: it presses the ring harder into place. In vacuum, that help is completely absent. The ring must seal based on its own elastic preload, while the diffusion of gas molecules through the rubber must also be limited.
The latter requires a full groove: the ring must fill the groove approximately 100%. For that reason, the tolerance on groove depth t is negative (the groove may be shallower, not deeper), the tolerance on width b is symmetrical and tighter, and the surface requirement is Ra max. 0,8 µm, stricter than the standard 1,6 µm. FKM (fluoroelastomer) delivers the best performance in practice for vacuum applications because of its low gas permeability.
Complete vacuum specifications: O-ring groove dimensions for static vacuum sealing.
Regardless of groove type, the same basic standards for surface finish and gap width apply to all static applications. These are the two factors that are most often overlooked: the groove geometry is correct, but the system still leaks. In almost all cases, the cause lies here.
The surface quality of the sealing face directly determines leak tightness, especially at low operating pressures where the ring’s contact pressure is limited. One machining groove or scratch is enough to create a continuous leak path along the ring. Vacuum applications require stricter values.
|
Surface |
Ra max. (µm) |
Rz max. (µm) |
Rmax (µm) |
|
Sealing face |
1,6 |
6,3 |
10 |
|
Groove base |
3,2 |
10 |
12,5 |
|
Groove flanks |
6,3 |
12,5 |
16 |
|
Vacuum: sealing face |
0,8 |
1,6 |
3,2 |
A gap that is too wide between the components to be sealed causes extrusion at higher pressures: the rubber is forced into the opening and damaged irreversibly. The maximum gap width depends on the operating pressure and the hardness of the O-ring. For silicone materials, the values must be halved.
|
Pressure (bar) |
70 Shore A (mm) |
80 Shore A (mm) |
90 Shore A (mm) |
|
up to 63 |
0,20 |
0,25 |
0,30 |
|
63 to 100 |
0,10 |
0,20 |
0,25 |
|
100 to 160 |
0,05 |
0,10 |
0,20 |
|
160 to 250 |
not applicable |
0,05 |
0,10 |
|
250 to 350 |
not applicable |
not applicable |
0,05 |
Above 100 bar, back-up rings are recommended on the low-pressure side of the groove, regardless of hardness.
The groove geometry is the same for all materials. What differs by material is the chemical resistance, the temperature limits and, in vacuum applications, the gas permeability. The choices below serve as a guideline for the most common applications. Always check compatibility with your specific medium.
Check your medium via the chemical resistance guide on o-ring-stocks.eu (1.500+ media included).
An O-ring is highly suitable for static sealing in most industrial applications. There are, however, situations in which other sealing methods perform better. If the conditions match one of the points below, it is worth considering alternatives.
Extreme operating pressures above 400 bar: at such pressures, metal face seals or specially designed elastomer seals are more suitable. At very high pressures, O-rings require back-up rings and narrower gaps.
Strong thermal cycling: with repeated temperature cycles, elastomers relax and lose their preload. Metal springs or restoring seals may perform better here.
Extremely high vacuum requirements (below 10-10 mbar): for ultra-high vacuum, metal seals (such as aluminium or copper crush rings) are the standard. O-rings fall short under such requirements because of diffusion through the elastomer.
In static sealing, the components no longer move relative to each other after assembly. In dynamic sealing, there is relative movement, such as with a piston or rod. The compression percentages are higher in static sealing than in dynamic sealing, because there is no wear caused by movement.
For cylindrical connections: radial compression. For flat connections (lids, flanges): axial compression. If the ring must not fall out during assembly: trapezoidal groove. If a rectangular groove does not fit the design: triangular groove. For vacuum applications: a vacuum-specific groove with adjusted tolerances.
The three most common causes are: the groove has been milled too deep (too little compression), the sealing surface is too rough (leak path along the ring), or the gap between the components is too large (extrusion at higher pressures). Check the groove depth with a depth gauge and the surface quality with a roughness meter.
Use the formula: compression (%) = (d2 − t) / d2 × 100. The recommended range for static radial compression is 15 to 30%. For axial compression: 1 to 3% compression (internal pressure) or max. 6% expansion (external pressure). Use the O-ring calculation tool for your specific combination.
Yes, for all groove types in which the O-ring is pushed over an edge during assembly. That applies to radial compression. In axial compression, the ring is placed in a flat groove and a chamfer is not required. The same applies to the trapezoidal groove and the triangular groove as to axial compression: the ring is not pulled over an edge.
NBR covers most applications: mineral oils, water and greases at temperatures up to +100 °C. For applications in which NBR falls short, FKM is the next step because of its broad chemical resistance and higher temperature tolerance.
Back-up rings are required when the gap between the components is too large at the operating pressure for the given hardness of the ring. As a rule of thumb: above 100 bar in static applications, back-up rings are recommended regardless of hardness. Consult the gap width table for the exact limits per pressure range.
