O-ring Groove - Dynamic

What is dynamic sealing?

In dynamic sealing, the machine parts to be sealed move relative to each other. The most common form of movement is reciprocating, as in hydraulic and pneumatic cylinders. Sometimes there is also a combined rotary and linear movement, as in certain hydraulic drives. The O-ring then moves along with every stroke: it is stretched, displaced and re-formed. Friction is therefore a design parameter, not a side effect.

The compression of a dynamic O-ring is deliberately lower than in static applications. Not because lower sealing performance is acceptable, but because a higher compression creates more contact force, more friction and more wear with every stroke. The balance between sufficient sealing and acceptable friction determines the design range for the groove.

Lower compression in dynamic applications means: better surface finish required, tighter tolerances, and in pneumatics: deliberate lubrication.

Static versus dynamic: the core in one table

The table below makes the difference visible. Compression in dynamic applications is roughly half that of static applications. The roughness requirement for the sealing surface is four times stricter. And in pneumatics, where the medium does not lubricate, additional measures are needed.

Dynamic radial compression: hydraulics

In hydraulic applications, O-rings are used for reciprocating movements of pistons and rods. They are especially suitable when the available installation space is limited and a simple, cost-effective seal is sufficient. The medium, the hydraulic fluid, forms a lubricating film on the sealing surface that limits friction and protects the ring.

Compression in hydraulics is between 9 and 16% of the cross-section diameter. By comparison: in static radial compression, that is 15 to 30%. That lower compression reduces the contact force and therefore the friction per stroke. The downside is that there is less margin for deviations in groove geometry or the sealing surface. Therefore, always check the groove depth, width and surface quality before assembly.

At higher pressures, high stroke speeds, or when the service life must exceed 1 million strokes, special piston and rod seals are technically the better choice. An O-ring can still be used as a secondary seal or in combination, but no longer as the primary seal.

Complete groove parameters and size table: O-ring groove dimensions for dynamic radial compression: hydraulics.

Dynamic radial compression: pneumatics

Pneumatic O-ring seals operate at lower working pressures than hydraulics, typically between 4 and 16 bar in industrial systems. But they place higher demands on friction. Air has no lubricating effect: no lubricating film forms on the sealing surface. Every additional friction force costs cylinder force or disrupts the positioning accuracy of the drive.

Compression in pneumatics is therefore the lowest of all installation types: 7 to 13% of the cross-section diameter. This is deliberately chosen so low in order to limit dry-running friction. However, a lower compression requires a more precise sealing surface: the ring has less contact force to bridge small surface irregularities. The roughness requirement for the sealing surface is Ra max. 0.4 µm, the same as for hydraulics. In dry-running systems, lubrication with silicone grease or an oil mist is strongly recommended.

Complete groove parameters and size table: O-ring groove dimensions for dynamic radial compression: pneumatics.

Twisting: the most common cause of premature failure

Twisting is the rotation of the O-ring in the groove during linear reciprocating motion. It occurs if the groove is too wide, the compression is too low, or the surface quality is uneven around the circumference. The ring then rolls slightly with every stroke instead of sliding. After a number of strokes, the ring has turned a quarter turn or half turn. Under asymmetrical loading, the rubber then stretches unevenly and tears.

Twisting is difficult to recognize during inspection, because a twisted ring looks intact on the outside. The crack is on the inside of the ring. Prevention is easier than detection: keep groove width b1 strictly within tolerances, never use an excessively wide groove, and avoid extremely low compression at high stroke frequencies.

Signs of twisting: spiral wear on the sealing surface, cracks on the inside of the ring, or leakage that starts after a fixed number of strokes.

Shared requirements for all dynamic applications

Both hydraulic and pneumatic O-ring seals share the same standards for surface finish and gap width. Together with groove geometry, these are the two factors that determine the service life of the ring.

Surface finish

The roughness requirement for the sealing surface in dynamic applications is four times stricter than in static applications. A surface that is too rough wears the ring on every stroke. In unlubricated pneumatics, wear already occurs at values that are still acceptable in hydraulics.

Gap width

With pulsating pressures in dynamic applications, the risk of extrusion is greater than in static applications. The maximum gap width is therefore lower. For silicone materials, you must halve the values in the table.

Chamfer: critical during every assembly and disassembly

In dynamic applications, the O-ring is mounted more often than in static applications: at every overhaul or maintenance interval, the ring is replaced and the groove is inspected again. The chamfer on the insertion side is important at every assembly. Without a chamfer, the ring is damaged while sliding over the edge, even if the force is low and the ring is soft. A chamfer of 15 to 20 degrees, with the z length from the size table, prevents this completely. Also check that the chamfer is free of burrs after machining.

Material selection for dynamic applications

Material selection for dynamic O-rings largely follows the same logic as for static applications, but with two additional considerations: wear resistance and friction behaviour. Both vary by material and directly affect service life and cylinder performance.

• NBR: mineral hydraulic oils, standard industrial use: NBR is the standard choice.
• FKM: synthetic oils, higher temperatures, chemically loaded hydraulics: better choice than NBR.
• EPDM: pneumatics with water or steam, or applications without oil lubrication: good dry-running behaviour.
• Silicone: extreme temperatures or food contact: wear resistance is lower, combine with PTFE coating.
• PTFE variants: special dynamic applications with low friction: PTFE-coated or PTFE-filled rings.

Consult the chemical resistance guide and the material temperature guide for your specific application.

When is an O-ring not sufficient for dynamic use?

An O-ring is a simple and cost-effective solution for reciprocating sealing at moderate pressures, speeds and service-life requirements. For more demanding conditions, special seal profiles are technically better suited. Assess this per application based on the three criteria below.

Stroke speed above 0.5 m/s: heat generation due to friction increases rapidly. Lip seals and PTFE ring seals have a lower coefficient of friction and are more stable at higher speeds.

Service life above 1 million strokes: O-rings are less suitable here because of fatigue cracking in the rubber. U-cups, packings or profiled seals are designed for long service life at high cycle counts.

Operating pressure above 250 bar: at such pressures, even 90 Shore A rings without back-up rings are at their limit. Special piston and rod seals are more resistant to extrusion at high pressure.