O-ring Groove - Design - Gap Size

What is gap extrusion?

Gap extrusion occurs when the operating pressure forces the rubber material of the O-ring into the gap between the two components. The ring is thereby partially pushed through the gap, on the low-pressure side of the groove. The rubber that has been pressed into the gap cannot recover: it has been damaged by mechanical overstretching. During the next pressure pulse, the protruding part is damaged further, and even more during the one after that. Eventually, a piece of rubber tears off, or the ring loses so much material that it no longer has sufficient sealing force.

Extrusion can be recognized by the characteristic nibbling damage: small frayed pieces of rubber on the low-pressure side of the ring. In static applications, extrusion can also remain invisible until the ring is inspected during disassembly. In dynamic applications, extrusion is almost always visible as rubber fragments in the system or as suddenly accelerated leakage after a number of operating hours.

Gap extrusion is irreversible. The ring must be replaced. But the gap that caused the extrusion remains just as large as before: without modification of the design, extrusion will occur again with the next ring.

The influence of pressure and hardness

The maximum gap width depends on two variables: the operating pressure and the Shore A hardness of the O-ring. Higher pressure creates more force to push the rubber through the gap: this makes the maximum allowable gap smaller. Higher hardness makes the rubber stiffer and less deformable: this makes the maximum allowable gap larger.

This explains why higher hardness grades are chosen at higher operating pressures or in designs where the gap cannot be made smaller for technical reasons. A 90 Shore A ring will always tolerate a larger gap than a 70 Shore A ring at the same pressure. The compromise is that a harder ring follows small irregularities in the sealing surface less well and therefore requires a smoother surface for good sealing at low pressures.

Maximum gap width in static applications

The values in the table below are valid for temperatures up to a maximum of 70 °C. At higher temperatures, rubber becomes softer and extrusion resistance decreases. Above 70 °C, the values should be applied more conservatively or a harder ring should be selected.

For silicone materials, all values must be halved. Silicone has a lower tensile strength than NBR or FKM and therefore extrudes at wider gaps than the table indicates.

Maximum gap width in dynamic applications

In dynamic applications, the maximum gap values are lower than in static applications at the same pressure and hardness. The pulsating pressure during each stroke creates a cyclic load on the ring at the gap side. This dynamic loading accelerates extrusion compared with a constant static pressure. Above 63 bar, 70 Shore A rings are no longer permissible in dynamic use without back-up rings, regardless of the gap width.

Pressure from both sides gives a different gap requirement than pressure from one side. With pressure from both sides, the rings are loaded on both flanks of the groove and back-up rings on both sides should be considered.

Back-up rings: the solution for excessively large gaps

When the gap between the components cannot be made smaller for design reasons, back-up rings are the most commonly used solution. A back-up ring is a hard, non-elastic ring, usually made of PTFE, PE or NBR, that is placed next to the O-ring in the groove, on the low-pressure side. The back-up ring fills the gap and blocks the passage of the rubber.

Pressure from one side: one back-up ring on the low-pressure side is sufficient.

Pressure from both sides or pulsating pressure: back-up rings on both sides of the O-ring.

Back-up ring material: PTFE has the lowest friction and the best chemical resistance. PE is cheaper. NBR (concave shape) is used in applications where slight resilience is desired.

View the full range of back-up rings at o-ring-stocks.eu/back-up-rings/.

Measuring and controlling gap width

In practice, gap width is not always easy to measure, especially in cylindrical connections where the gap depends on the fit between bore and rod. The gap is the difference between the nominal inner diameter of the bore and the outer diameter of the rod, divided by two. With a bore of 50.10 mm and a rod of 49.90 mm, the one-sided gap is 0.10 mm.

When designing, take the maximum gap over the full tolerance range into account: it is not the nominal fit that is decisive, but the worst-case combination of maximum bore and minimum rod. During maintenance, check whether the gap has increased due to wear of the rod or bore to above the permissible value.

Temperature effect: above 70 degrees Celsius

The gap values in the tables are valid up to a maximum of 70 °C. Above that, the modulus of elasticity of elastomers decreases: the rubber becomes softer and extrusion resistance is reduced. This means that at higher temperatures, the same pressure exerts a greater extrusion force on the material. As a guideline: above 70 °C, choose one hardness class higher than the table indicates for your pressure range, or reduce the gap further. For applications above 100 °C, it is advisable to determine the gap values in consultation with a technical specialist.