PU Seals for Hydraulic Applications
Polyurethane rod and piston seals for high-pressure hydraulics where NBR extrudes — superior abrasion and extrusion resistance.
Polyurethane (PU, AU/PU elastomer) is not a drop-in replacement for every NBR O-ring — but for reciprocating hydraulic rod and piston seals in high-pressure, contaminated fluid service, PU outperforms NBR in abrasion resistance, extrusion resistance and rebound after high load.
Where PU wins in hydraulics
- Mobile equipment cylinders in dusty environments (construction, mining, agriculture)
- High-pressure reciprocating rods (>200 bar) with moderate extrusion gaps
- Systems with frequent cold-start cycles requiring rebound elasticity
- Water-based HFC fluids when paired with hydrolysis-resistant PU grades
Where to keep NBR or FKM
- Static port and flange seals (PU is overkill; NBR is cheaper)
- Phosphate ester aircraft fluids (use FKM, not PU)
- Continuous +80°C oil above PU hydrolysis limits in standard grades
- Systems with aromatic solvents or strong acids
PU is commonly supplied as U-cups, lip seals and custom profiles rather than standard AS568 O-rings — but molded PU O-rings are available for specific OEM grooves.
Related: hydraulic cylinders application, hydraulic fluid compatibility guide, PU product page.
Why PU for This Application
Extrusion resistance
PU withstands higher pressure-gap combinations than NBR 70A — critical for worn cylinder bores.
Abrasion resistance
3–5× better wear life than NBR in contaminated hydraulic oil with silica and metal particles.
High rebound
Maintains sealing contact after pressure release — reduces stick-slip on rapid cycle equipment.
Hydrolysis-resistant grades
Ether-based PU compounds for water-glycol HFC fluids — verify grade before specifying.
Common Sizes
| Size | Typical Use |
|---|---|
| Rod seals (custom U-cup) | Quote rod diameter, bore, stroke and pressure — standard profiles available |
| AS568 -111 to -120 (2.62 mm CS) | PU O-rings for static port seals in high-pressure manifolds |
| AS568 -214 to -226 (3.53 mm CS) | Large-bore cylinder piston seals — custom profile often required |
| Metric 20×3 through 80×5 mm | European mobile hydraulic OEM sizes |
| Wear ring / guide ring | PTFE or POM wear rings paired with PU rod seals — quote bore × length |
Related Application Guides
FAQ
PU or NBR for hydraulic rod seals?
NBR 70–80A is the default for clean, moderate-pressure hydraulics. Upgrade to PU when you see extrusion nibs on NBR seals, abrasive wear from contaminated fluid, or pressures above 250 bar with marginal clearance.
Can PU O-rings replace NBR in mineral hydraulic oil?
Yes, for dynamic reciprocating seals in mineral oil. Verify temperature — standard PU is limited to about +80°C continuous in oil. Above that, use HNBR or FKM.
Is PU compatible with water-glycol HFC fluid?
Only hydrolysis-resistant ether-based PU grades. Ester-based PU hydrolyzes in water-glycol. For HFC fluids, EPDM O-rings are the safer default unless PU grade is validated by immersion test.
Why use PU U-cups instead of O-rings on hydraulic rods?
U-cups provide a lip geometry that maintains contact stress as pressure increases — self-energizing. O-rings rely on groove compression only. PU U-cups are standard on mobile equipment where bore wear increases extrusion gap over time.
What hardness for PU hydraulic seals?
90–95 Shore A is standard for rod U-cups. 80 Shore A for low-pressure applications requiring more compliance. Harder grades resist extrusion but need better surface finish on the rod.
Do you supply PU hydraulic seal kits?
Yes — custom kits by cylinder model including PU rod seals, wear rings and static NBR port O-rings. Provide cylinder OEM and serial range for BOM matching.
Specify PU hydraulic seals
Send rod/bore dimensions, pressure, fluid type and existing seal profile. We quote PU U-cups, O-rings and paired wear rings. MOQ 1 piece on custom profiles.