O-Ring Sizing and Gland Design Basics
An O-ring is a precision-engineered compression seal, not just a rubber band. The rubber itself only provides the reactive force; the machined gland does the structural work of containing that force and preventing extrusion. When a seal fails, the root cause is more often found in the gland dimensions and surface finish than in the elastomer compound.
AS568 Dash Numbers and Metric Sizing
The AS568 standard defines standard inch-series O-ring sizes using a three-digit dash number. The first digit indicates the cross-section series (0 through 5, covering sizes from 0.070 inch up to 0.275 inch), while the last two or three digits define the inside diameter (ID). For example, an AS568-014 is a 0.070-inch cross-section O-ring with an ID of 0.089 inches. Metric sizes follow ISO 3601-1 and are specified directly by their cross-section and ID in millimeters. Sourcing replacements requires matching both dimensions precisely, as a metric O-ring will not reliably drop into an AS568 gland, and vice versa.
When sourcing O-rings and kits, you must understand the difference between cross-section and ID. The cross-section determines the squeeze capacity and load-bearing area. The ID determines the stretch fit. An O-ring with the correct ID but a smaller cross-section than specified will under-compress in the gland, leading to immediate low-pressure weeping. An oversized cross-section will overfill the gland, causing high friction in dynamic applications or material rupture in static face seals.
The Gland Does the Sealing: Squeeze and Fill
The O-ring works by being squeezed into a deformation between two mating surfaces. The compression percentage—calculated as the reduction in cross-section relative to the free state—must be controlled within specific targets.
- Static face seals: 20% to 30% squeeze. High compression is acceptable because there is no dynamic friction to generate heat and wear.
- Static radial seals: 18% to 25% squeeze. Slightly lower to account for assembly drag over the mating part.
- Dynamic piston and rod seals: 10% to 20% squeeze. Lower compression reduces running friction, heat generation, and spiral failure in reciprocating applications.
Gland fill is equally critical. The gland volume must be larger than the O-ring volume to accommodate thermal expansion and fluid swell. As a general rule, gland fill should not exceed 85% in static applications and should be closer to 80% in dynamic applications. If the gland fills completely, the elastomer has nowhere to expand. Pressure then acts on a rigidly constrained volume, causing the O-ring to extrude into clearances or physically rupture.
Gland Types: Face, Piston, and Rod
Gland geometry dictates how the seal is installed and how pressure acts on it. The three primary configurations handle load differently.
Face seals are static seals where the compression load is applied axially. The gland is machined as a rectangular groove in one flange face, and the O-ring is compressed by the clamping force of the opposing flange. Pressure acts parallel to the groove axis, pushing the O-ring radially outward against the outer gland wall. The inner diameter of the groove is typically open to the fluid.
Piston seals (internal pressure) are dynamic or static seals where pressure acts from the inside out. The gland is machined into the inside diameter of a cylinder or housing. The O-ring is stretched over a piston or plug during assembly. Pressure forces the O-ring outward against the outer wall of the gland, expanding the seal and increasing contact force.
Rod seals (external pressure) are dynamic or static seals where pressure acts from the outside in. The gland is machined into the outer diameter of a rod or shaft. The O-ring is installed with slight interference on the rod ID. Pressure forces the O-ring inward against the inner gland wall. Because the pressure pushes the seal into the groove rather than out of it, rod seals generally resist higher pressures than piston seals of the same geometry.
For pneumatic and low-pressure hydraulic applications, standard elastomer O-rings are often sufficient. For higher-pressure hydraulic systems, dedicated hydraulic and pneumatic seals with engineered geometries—such as lip seals, cap seals, and composite buffers—offer better extrusion resistance and lower friction than standard O-rings.
Stretch Limits and Over-Squeeze Failures
During installation, a piston seal must stretch over the piston diameter to seat in the groove. The stretch limit for standard elastomers is generally 5% to 8% of the ID. Exceeding this causes permanent deformation, loss of cross-section (necking), and rapid relaxation. A stretched O-ring loses its ability to maintain compression force. If the seal must stretch over a large shoulder, a installation tool or a heated assembly process is required to prevent damage.
Over-squeeze is a common failure mode when technicians attempt to upsize an O-ring to fix a leak. If the gland is machined for a 0.070-inch cross-section and a 0.103-inch O-ring is forced into it, the gland fill exceeds 100%. The elastomer cannot compress; it can only deform. The material will flow into the clearance gap between mating parts, shearing off as the assembly moves. Even in static applications, over-squeeze causes the O-ring to harden and crack prematurely as the cyclic pressure loads a fully constrained, non-resilient mass.
Extrusion and Back-Up Rings
At high pressures, the elastomer will extrude into the clearance gap between the mating metal parts. This is a physical flow of the rubber into the gap, and it occurs when the fluid pressure exceeds the mechanical resistance of the elastomer. The pressure at which extrusion begins depends on the clearance gap, the elastomer hardness, and the temperature. Softer 70-durometer compounds extrude at lower pressures than 90-durometer compounds.
Extrusion failure is progressive. Once the material enters the gap, cyclic pressure causes a sawing action that eventually shaves the O-ring apart. The solution is to close the gap or block the flow path. Back-up rings—typically made from PTFE, nylon, or hard elastomer—act as a mechanical bridge across the clearance gap. They are installed on the low-pressure side of the O-ring. When pressure forces the elastomer toward the gap, the back-up ring deforms slightly to close the gap, preventing extrusion.
For pneumatic systems, such as those using SMC cylinders and valves, standard O-rings are usually sufficient because operating pressures are low. For heavy-duty hydraulic and bearing applications, such as those using SKF components, back-up rings and harder compounds are often necessary to handle high cyclic pressures.
Surface Finish Requirements
The surface finish of the gland and mating parts directly affects sealability and wear. A rough surface acts as a file, abrading the O-ring in dynamic applications and providing leak paths in static applications. The finish must be smooth enough to seal but not so smooth that it causes dry running in dynamic applications.
- Gland walls and bottom: 16 to 32 microinches Ra (0.4 to 0.8 micrometers). The walls must be smooth to prevent abrasion during assembly and pressure cycling.
- Dynamic mating surfaces: 8 to 16 microinches Ra (0.2 to 0.4 micrometers). The running surface must be highly polished to minimize friction and wear.
- Static mating surfaces: 32 to 63 microinches Ra (0.8 to 1.6 micrometers). Rougher finishes are acceptable because the O-ring is not moving.
Edges of the gland must be broken with a chamfer or radius to prevent cutting the O-ring during installation. A sharp edge will slice the seal as it slides past. The chamfer should be 0.005 to 0.010 inches and free of burrs.
If you need help sourcing standard or custom O-rings, back-up rings, or cross-referencing a specific seal dimension, our team can help identify the correct compound and size for your application.
More in Seals & Sealing
Need a quote on a specific part?
Request a Quote