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Dynamic O-ring sealing in pneumatics works with lower operating pressures than hydraulics, but places higher demands on friction. Air does not lubricate. In hydraulics, the fluid forms a lubricating film on the sealing surface that protects the ring during every stroke. In dynamic O-ring sealing for pneumatics, that film is completely absent, unless the system is deliberately lubricated. Every additional amount of friction costs stroke force or disrupts the positioning accuracy of the actuator. The compression is therefore the lowest of all installation types: 7 to 13% of the cord diameter.
Dynamic radial sealing for pneumatics is very similar to hydraulics, but there are two essential differences. First, the working pressure in pneumatics is generally lower, rarely above 16 bar in industrial applications. This somewhat reduces the requirement for gap width and material. Second, the medium is air, and air does not lubricate. In hydraulic O-rings, the hydraulic fluid provides a lubricating film on the sealing surface. In pneumatics, that film is completely absent unless the system is deliberately lubricated. The compression is therefore set slightly lower in pneumatics than in hydraulics in order to limit dry-running friction. Nevertheless, the ring must always make sufficient contact to block the airflow. That balance determines the design range for the groove.
The six groove parameters are the same as in hydraulics: d2, t, b1, z, r1 and r2. The values of t are slightly higher than in hydraulics for the same cross-section diameter, resulting in even lower compression. The rest of the geometry is comparable. The result is a groove that presses the ring less firmly, which reduces friction at the cost of a slightly increased leakage risk under fluctuating pressures.
The cross-section diameter is the starting point for the complete groove calculation, also in pneumatic use. In pneumatic cylinders, smaller cross-section diameters are generally used than in hydraulics, because the working pressures are lower and the design is more compact. Common sizes are 1.78 mm, 2.62 mm and 3.53 mm for standard industrial pneumatics.
In pneumatics, the groove depth is even closer to d2 than in hydraulics. At d2 = 5.00 mm, t = 4.60 mm: a compression of only 8%. In hydraulics, this is 12% for the same cross-section diameter. That extra 4% lower compression noticeably reduces friction, but requires a tighter sealing surface. The tolerance on t is also positive here: the groove may be slightly deeper, but never shallower.
The groove width is comparable to hydraulics for the same cross-section diameter. In pneumatics, twisting plays an even greater role than in hydraulics, because the lower compression holds the ring less tightly in the groove. Keep b1 within the tolerances and never use a groove that is wider than the nominal value plus the tolerance.
The chamfer length on the insertion side is identical to hydraulics. Pneumatic systems are more often operated with less lubrication, which increases the chance of damage during assembly. Always make the chamfer exactly as indicated in the table: 15 to 20 degrees, length z depending on d2.
Compression in pneumatics: 7 to 13% of d2. Lower than hydraulics (9 to 16%). Lower compression requires even tighter surface finish.
Air does not lubricate, but many pneumatic systems use an oil mist or silicone grease as lubrication. This lowers friction and significantly extends the service life of the ring. Without lubrication, the ring wears faster and friction is higher on every stroke. The roughness requirement for the sealing surface in dynamic pneumatic use is Ra max. 0.4 µm (Rz max. 1.2 µm), the same as in hydraulics. With insufficient surface quality, wear already occurs at low pressures, especially without lubrication.
NBR is the standard choice for pneumatics at room temperature with or without oil mist. EPDM is used in systems with steam or water vapor, or in applications without oil lubrication where NBR suffers from drying out. FKM is the preferred option for aggressive gases, high temperatures or use with chemical cleaning agents. Silicone O-rings are used at extreme temperatures (below -50 °C or above +150 °C) but are less wear-resistant and elastic than NBR or FKM.
Check the compatibility of your medium via the chemical resistance guide.
| d2 | t +0,05 | b1 +0,25 | r1 |
| 1,50 | 1,30 | 1,80 | 0,3 |
| 1,52 | 1,30 | 1,80 | 0,3 |
| 1,60 | 1,40 | 1,90 | 0,3 |
| 1,63 | 1,40 | 2,00 | 0,3 |
| 1,78 | 1,55 | 2,10 | 0,3 |
| 1,80 | 1,60 | 2,10 | 0,3 |
| 1,83 | 1,60 | 2,20 | 0,3 |
| 1,90 | 1,65 | 2,30 | 0,3 |
| 1,98 | 1,75 | 2,30 | 0,3 |
| 2,00 | 1,75 | 2,40 | 0,3 |
| 2,08 | 1,85 | 2,40 | 0,3 |
| 2,10 | 1,85 | 2,50 | 0,3 |
| 2,20 | 1,95 | 2,60 | 0,3 |
| 2,26 | 2,00 | 2,60 | 0,3 |
| 2,30 | 2,05 | 2,70 | 0,3 |
| 2,34 | 2,10 | 2,70 | 0,3 |
| 2,40 | 2,15 | 2,80 | 0,3 |
| 2,46 | 2,20 | 2,90 | 0,3 |
| 2,50 | 2,25 | 2,90 | 0,3 |
| 2,60 | 2,35 | 3,00 | 0,3 |
| 2,62 | 2,35 | 3,00 | 0,3 |
| 2,65 | 2,40 | 3,10 | 0,3 |
| 2,70 | 2,40 | 3,10 | 0,3 |
| 2,80 | 2,50 | 3,30 | 0,3 |
| 2,92 | 2,65 | 3,40 | 0,3 |
| 2,95 | 2,65 | 3,40 | 0,3 |
| 3,00 | 2,70 | 3,50 | 0,3 |
| 3,10 | 2,80 | 3,70 | 0,6 |
| 3,50 | 3,15 | 4,20 | 0,6 |
| 3,53 | 3,20 | 4,20 | 0,6 |
| 3,55 | 3,20 | 4,20 | 0,6 |
| 3,60 | 3,25 | 4,30 | 0,6 |
| 3,70 | 3,35 | 4,40 | 0,6 |
| 4,00 | 3,65 | 4,70 | 0,6 |
| 4,30 | 3,90 | 5,20 | 0,6 |
| 4,50 | 4,10 | 5,50 | 0,6 |
| 5,00 | 4,60 | 6,10 | 0,6 |
| 5,30 | 4,90 | 6,50 | 0,6 |
| 5,33 | 4,90 | 6,50 | 0,6 |
| 5,50 | 5,05 | 6,70 | 0,6 |
| 5,70 | 5,25 | 6,90 | 0,6 |
| 6,00 | 5,50 | 7,30 | 0,6 |
| 6,50 | 6,00 | 7,90 | 1,0 |
| 6,99 | 6,45 | 8,50 | 1,0 |
| 7,00 | 6,45 | 8,50 | 1,0 |
| 7,50 | 6,95 | 9,10 | 1,0 |
| 8,00 | 7,40 | 9,70 | 1,0 |
| 8,40 | 7,80 | 10,20 | 1,0 |
| 8,50 | 7,85 | 10,30 | 1,0 |
| 9,00 | 8,35 | 10,90 | 1,0 |
| 9,50 | 8,80 | 11,50 | 1,0 |
| 10,00 | 9,30 | 12,10 | 1,0 |
| 10,50 | 9,75 | 12,70 | 1,0 |
| 11,00 | 10,25 | 13,30 | 1,0 |
| 11,50 | 10,70 | 13,90 | 1,0 |
| 12,00 | 11,15 | 14,50 | 1,0 |
Air does not lubricate, which means that higher contact pressure immediately causes higher friction. To maintain the stroke and positioning accuracy of the cylinder, the compression is deliberately kept low. This does, however, require a tighter sealing surface.
Not mandatory, but strongly recommended. Silicone grease or an oil mist reduces friction, extends service life and lowers the risk of twisting. Use grease that is compatible with the rubber and with the medium.
The ring itself can be the same, but the groove is dimensioned differently. The groove depth t is greater in pneumatics (lower compression) than in hydraulics for the same cross-section diameter. Always use the table that belongs to the installation type.
With a gap of 0.15 mm and 80 Shore A, the maximum pressure in dynamic use is approximately 30 to 63 bar. Most pneumatic systems operate at 6 to 16 bar, which is well within that range.
