| name | o-ring-design |
| description | O-ring seals — groove design, squeeze, stretch, compression set, material selection, static/dynamic applications, Parker handbook, pressure limits, AS568 sizes. |
| metadata | {"priority":7,"promptSignals":{"phrases":["O-ring","o ring","groove design","seal groove","compression set","Parker seal"],"minScore":3}} |
O-Ring Seals — Complete Skill
O-Ring Dimensions (AS568 Standard)
AS568 dash numbers: -0xx (small) through -4xx (large)
Cross-section diameters: 0.070" (1.78mm) to 0.275" (6.99mm)
ID from 0.029" to 25.940"
Parker Size (ISO 3601): metric equivalent for international use
Groove Design
Static Face Seal (Axial Compression)
Groove depth (gland depth): d_g = d_w × (1 - squeeze%)
Recommended squeeze: 15-30% for static; 10-20% for dynamic
squeeze = (d_w - d_g) / d_w × 100%
Groove width: W_g = 1.25-1.5 × d_w (cross-section diameter)
Allows radial expansion when compressed; room for thermal expansion
Standard face seal groove (Parker):
d_w = 2.62mm (–112): depth = 2.06mm (21.4% squeeze), width = 3.43mm
Finish: Ra ≤ 0.8 μm (32 μin) for static, ≤ 0.4 μm (16 μin) for dynamic
Groove surfaces: ≤ 1.6 μm (63 μin); no spiral tool marks (use plunge grinding)
Dynamic Piston Seal (Radial)
Squeeze: 8-15% (less squeeze reduces friction and wear)
Groove in piston; bore is running surface
Backup rings: required for pressure > 1500 psi (10 MPa) to prevent extrusion
Dynamic Rod Seal
O-ring in groove in housing; rod reciprocates
Squeeze: 8-15% on rod OD
Surface finish on rod: Ra ≤ 0.2-0.4 μm (8-16 μin)
Hard chrome or ceramic rod surface for durability
Stretch / Compression
Stretch on shaft: ID of O-ring stretched over shaft
Allowable stretch: 1-5% (max) — do not exceed 6%
Compression from above: use only recommended squeeze
Volume fill of groove:
At assembly: V_oring / V_groove = 75-90% (must fill > 70%; prevent rolling)
Material Selection
| Material | Temp Range | Fluid Compatibility |
|---|
| Nitrile (NBR) | -40 to 120°C | Oils, fuels, water — standard |
| EPDM | -55 to 150°C | Steam, hot water, brake fluid; NOT petroleum oil |
| Silicone (VMQ) | -65 to 200°C | Extreme temperature; poor abrasion |
| Fluorosilicone | -65 to 175°C | Fuels + temp extremes |
| Viton (FKM) | -20 to 200°C | Chemicals, solvents, high temp |
| PTFE (encapsulated) | -200 to 260°C | Aggressive chemicals; minimal elasticity |
| FFKM (Kalrez) | -10 to 320°C | Any chemical; expensive |
Hardness: 70 Shore A (standard); 90 Shore A (high pressure, extrusion resistance)
Pressure Limits
Without backup ring: max pressure per cross-section and hardness
70 Shore A, d_w = 5.33mm: max P ≈ 6 MPa (900 psi) static; 1.5 MPa dynamic
With single backup ring: 3× pressure rating
With double backup ring: ~20 MPa static
Extrusion gap limit: max diametral clearance
At 6 MPa, 70 Shore A: gap ≤ 0.04 mm; at 20 MPa: gap ≤ 0.01 mm
Compression Set
O-ring permanent deformation after time under compression:
CS% = (d_initial - d_recovered) / (d_initial - d_groove) × 100%
CS < 25%: acceptable for static; CS < 15% for dynamic
Measures rubber aging / chemical attack; high CS → seal leaks on decompression
Accelerated testing: 70 hrs at 100°C (per ASTM D395 Method B)
Output
Provide: groove depth [mm], groove width [mm], % squeeze, % fill, material recommendation, backup ring requirement, max pressure P_max [MPa], surface finish requirement, standard AS568 dash number.
