PU O-Rings (Polyurethane)
The ultimate material for high-pressure hydraulic seals, rod wipers and dynamic reciprocating applications.
Overview
Polyurethane (PU) O-rings, also designated AU (polyester urethane) and EU (polyether urethane), deliver the highest abrasion resistance, tensile strength and tear resistance of any elastomeric seal material. These properties make PU the default choice for hydraulic rod seals, piston seals, pneumatic cylinders and any application involving reciprocating motion under pressure.
With hardness options from 60 to 95 Shore A, PU O-rings resist extrusion into clearance gaps far better than softer elastomers. This allows PU seals to operate reliably in hydraulic systems at pressures exceeding 40 MPa without backup rings. The material also exhibits very low compression set and excellent rebound characteristics, contributing to long service intervals in dynamic equipment.
Standard polyester PU operates from -35C to +80C. For wet or humid hydraulic environments, polyether-based PU grades offer improved hydrolysis resistance. PU is not suitable for hot water, steam, glycol-based fluids or polar solvents, where NBR, EPDM or FKM should be used instead.
Material Properties
| Temperature Range | -35C to +80C (-31F to +176F) |
| Hardness Range | 60-95 Shore A |
| Tensile Strength | 25-55 MPa |
| Elongation at Break | 350-650% |
| Abrasion Resistance | Excellent (best in class) |
| Tear Strength | 40-100 kN/m |
| Color (standard) | Amber / Translucent yellow |
| Standards | DIN ISO 1629, ASTM D2000 BG |
Typical Applications
Hydraulics
Rod seals, piston seals, wipers and cushion seals in high-pressure hydraulic cylinders
Pneumatics
Cylinder seals, valve spool seals and pneumatic actuator dynamic seals
Construction Machinery
Excavator, loader and crane hydraulic seal systems exposed to dirt and vibration
Material Handling
Forklift mast seals, conveyor tensioners and lifting equipment hydraulics
Automotive
Shock absorber seals, steering rack seals and suspension dampers
Chemical Compatibility Summary
Compatible With
- - Mineral oils and greases
- - Hydraulic fluids (petroleum-based)
- - Silicone oils and greases
- - Aliphatic hydrocarbons
- - Fuels (diesel and gasoline)
Incompatible With
- - Hot water and steam
- - Glycol-based fluids
- - Strong acids and alkalis
- - Polar solvents (ketones, esters)
- - Chlorinated hydrocarbons
Compare Nearby Materials
Further Reading
X-Rings vs O-Rings
A dynamic sealing comparison for engineers working on cylinders, rods, and moving glands.
Read articleBackup Rings vs O-Rings
Know when profile choice is not enough and anti-extrusion support is the real fix.
Read articleKey Advantages
Highest Abrasion and Wear Resistance
PU outperforms all other elastomers in abrasive and dynamic conditions. This translates to longer seal life in reciprocating hydraulic and pneumatic equipment.
Superior Mechanical Strength
With tensile strengths of 25-55 MPa and tear resistance 3-5x higher than NBR, PU seals resist damage during assembly and high-pressure extrusion.
Excellent Extrusion Resistance
The high modulus and hardness range of PU (up to 95 Shore A) means it can withstand pressure gaps that would cause NBR or EPDM to extrude and fail.
Low Compression Set
PU maintains elastic recovery over long service intervals, reducing leakage and maintenance downtime in continuous-duty hydraulic systems.
Frequently Asked Questions - PU
What makes PU O-rings ideal for hydraulics?
Polyurethane offers the best abrasion, tear, and extrusion resistance of any elastomer, making it the top choice for hydraulic rod and piston seals in high-pressure systems.
What is the temperature limit for PU O-rings?
Standard PU operates from -35C to +80C. Specialty grades extend this slightly, but PU is not suitable for high-temperature applications.
Can PU O-rings be used with water?
Ester-based PU has poor hydrolysis resistance and should not be used with hot water or steam. Ether-based PU offers better water resistance for wet hydraulic systems.
What hardness is typical for hydraulic PU seals?
Hydraulic seals typically use 85-95 Shore A PU for rod wipers and piston seals, while lower hardness (70 Shore A) is used for low-pressure pneumatic applications.
Is PU compatible with glycol brake fluid?
No. PU is attacked by glycol-based fluids, brake fluids, and polar solvents. NBR or EPDM are better choices for these media.