انتخاب اندازه اورینگ

Follow these steps:

1. Identify application: piston seal, rod seal, or face seal
2. Determine diameter: bore diameter (piston) or shaft diameter (rod)
3. Choose cross-section: larger CS for higher pressures and larger diameters. CS 3.53 mm is the most common general-purpose size
4. Find standard ID: for piston seals, ID = bore − 2×CS. For rod seals, ID ≈ rod diameter. Select nearest standard size from ISO 3601
5. Verify squeeze: aim for 15–25% (static) or 10–20% (dynamic)
6. Check stretch: should be ≤5% of ID

Squeeze is the percentage compression of the O-ring cross-section in the groove:

Squeeze % = (CS − compressed height) / CS × 100

It creates the initial contact stress needed for sealing. Guidelines:

• 10–20% for dynamic seals (piston/rod) — balances sealing and friction
• 15–30% for static seals (face/flange) — higher squeeze for reliability
• Too little squeeze → leakage, poor seal
• Too much squeeze → excessive friction, heat, spiral failure, short life

Stretch occurs when the O-ring is installed on a groove diameter larger than its free-state ID. Effects:

• Reduces cross-section — the CS becomes thinner as the O-ring stretches, reducing effective squeeze
• Increases stress — higher internal stress accelerates aging
• Max 5% for most elastomers (NBR, FKM)
• Max 3% for hard/brittle materials (PTFE-encapsulated, FFKM)

Conversely, compression (negative stretch, ID larger than groove) can cause the O-ring to buckle or twist in the groove.

Common O-ring materials and their applications:

• NBR (Nitrile) — −40 to +120°C, excellent oil resistance, most common
• FKM/Viton — −20 to +200°C, excellent chemical and heat resistance
• EPDM — −50 to +150°C, water/steam/brake fluid, NOT for oils
• Silicone (VMQ) — −60 to +200°C, food-grade, medical
• FFKM (Kalrez) — −25 to +325°C, universal chemical resistance, expensive
• PTFE — −200 to +260°C, chemically inert, used as encapsulation or backup rings

Backup rings prevent O-ring extrusion into the gap between mating surfaces under pressure. Use backup rings when:

• Static seal: pressure > 3.5 MPa (500 psi)
• Dynamic seal: pressure > 7 MPa (1000 psi) with 70 Shore A O-ring
• Large gaps: diametral clearance > 0.1 mm at low pressure, less at high pressure

Place one backup ring on the low-pressure side. For bi-directional pressure, use two backup rings (one on each side). PTFE is the standard backup ring material.