The relationship between the application pressure and the gland clearance gap size is important in order to design a sealing system that maintains a proper seal under various pressure conditions without the O-ring failing or extruding into the gap.

O-Ring Application Pressure

The extent of pressure against the o-ring varies depending on the application, ranging from low pressures in simple static seals to very high pressures in hydraulic systems. As pressure increases, the force on the o-ring also increases, pushing it against the gland walls more firmly.

Gland Clearance Gap Size

The gap where the o-ring is seated between the surfaces being sealed contributes to how much the o-ring is compressed and how well it seals.

Recommendations

• Excessive clearance and/or pressure can result in seal extrusion and failure
• A larger gap can be acceptable in low pressure applications as the o-ring is less likely to extrude into the gap
• For higher pressure applications, a smaller gap can prevent the o-ring from being forced into the gap
• Consider the use of back-up rings or harder materials for high pressure applications

Tolerances for both the o-ring and gland are important to ensure optimal seal performance and life.

Dimensional Tolerances

• The inner diameter (ID) of an o-ring must fit precisely around the component it is sealing
• The outer diameter (OD) ensures the o-ring fits snugly within the gland where it is seated.
• The cross-sectional (CS) diameter of the o-ring ensures the correct amount of compression when the o-ring is installed and can affect the seal's tightness and performance.
• O-rings are often manufactured to AS568 standards, which specify acceptable tolerances for different o-ring sizes.

Material Tolerances

• O-ring materials hardness level (Shore A durometer) can affect how well the o-ring conforms to the sealing surface and resists extrusion under pressure
• Material composition can influence the o-ring's performance including chemical resistance, temperature stability, and seal life

For more information about material read the table above

Surface Finish

The surfaces against the o-ring should be smooth and free of defects and the seal material must fill all voids in surfaces.

• Rough surface finish can result in abrasion and wear on the o-ring or spiraling even with a static seal
• Lower durometer materials can be used to seal rough surface finishes

Surface Roughness

Roughness Average (Ra), usually measured in micrometres (µm) or micro-inches (µ-in), measures the average height of a surface's texture, or how far each point on the surface deviates from the mean height.

• For static surfaces, Ra of 16 - 32 is recommended
  16 to seal gas
  32 to seal liquid
• For dynamic surfaces, 8 - 16 is recommended

Sharp or abrupt transitions at the edges or corners of the gland where the o-ring is seated can cause problems in many ways, resulting in reduction in the integrity and longevity of the seal.

Recommendations

• Corners should be chamfered during seal installation to limit damage and reduce wear and tear, stress, and maintenance issues
• In high-pressure applications, back-up rings can help prevent the o-ring from extruding into sharp corners
• In applications where sharp corners cannot be avoided, a harder material may reduce cutting or tearing

For more information about material read the table above

The gland or groove is designed with a specific volume to accommodate the o-ring. Gland fill is the ratio of gland volume vs. o-ring volume

• If the gland fill is too high and, the o-ring fills too much of the gland, it may become over-compressed - extruding into the clearance gap which can result in higher risk of seal failure
• If the gland fill is too low and the o-ring does not sufficiently fill the gland,, the o-ring may not compress adequately, resulting in an ineffective seal

Recommendations

• Space in needed in the groove to allow for volume swell, thermal expansion, and increasing width due to squeeze
  A narrower groove is required for sealing vacuum or gas
• 75% nominal gland fill / 25% void space for standard applications

Both of these factors can greatly impact the performance and lifespan of a seal. Proper alignment, careful installation, use of backup rings, appropriate material selection, and gland design can prevent issues related to o-ring eccentricity and side loading.

Eccentricity

• Occurs when the o-ring is not perfectly centered within its gland, resulting in uneven distribution of compression
• Too much squeeze on one side and not enough on the other or none at all
• Can result in leaks where the compression is insufficient and excessive wear or extrusion where the compression is too high

Side Loading

• An off-center or lateral force against an o-ring, causing it to be pushed against one side of its groove or gland
• Occurs in dynamic applications, where the o-ring encounters uneven forces during operation
• Can open too wide a clearance gap and result in extrusion of one portion of seal leading to leaks