NBR O-Rings for Hydraulic Seals
Cost-effective nitrile seals for mineral oil, hydraulic fluid and lubricating oil — the default choice for industrial cylinders, pumps and mobile equipment.
NBR (nitrile / Buna-N) accounts for the majority of hydraulic O-ring installations worldwide because it balances petroleum oil resistance, mechanical strength and cost. For standard mineral-based hydraulic fluid service from -40°C to +120°C, NBR 70 Shore A is the correct default unless temperature, fluid type or pressure demands a step-up to HNBR, FKM or PU.
Typical hydraulic sealing points
- Cylinder rod and piston glands — dynamic reciprocating seals (often with backup rings above 140 bar)
- Pump casings and valve bodies — static face seals on SAE and ISO flange patterns
- Manifold interfaces and hose couplings — crush seals on metric and inch port threads
- Filter bowls and reservoir caps — low-pressure static seals in maintenance-heavy locations
NBR performance depends on ACN (acrylonitrile) content: standard 33–36% ACN grades offer the best balance of oil resistance and low-temperature flexibility. High-nitrile (39–40% ACN) compounds improve fuel and aromatic resistance but stiffen below -30°C.
For high-temperature hydraulic oil above +120°C, phosphate-ester fluids, or bio-based hydraulic fluids, review HNBR hydraulic options or FKM for elevated temperature. See also our hydraulic cylinder application guide and oil-resistant O-rings overview. Full material data: NBR product page.
Why NBR for This Application
Proven oil compatibility
Excellent resistance to petroleum-based hydraulic oils, lubricating oils and most mineral hydraulic fluids in standard industrial temperature ranges.
Wide size availability
AS568, ISO 3601 and metric sizes stocked in 70 and 90 Shore A — covers the vast majority of cylinder, pump and valve grooves.
Cost-effective maintenance
Lowest cost per seal among performance elastomers — ideal for MRO kits and high-volume replacement programs.
Hardness selection
70 Shore A for general static/dynamic service; 90 Shore A for high-pressure pump heads and extrusion-prone grooves.
Common Sizes
| Size | Typical Use |
|---|---|
| AS568 -010 to -050 (1.78 mm CS) | Small valve and pilot-line static seals |
| AS568 -110 to -178 (2.62 mm CS) | Standard cylinder piston and rod seals (most common hydraulic range) |
| AS568 -210 to -284 (3.53 mm CS) | Large-bore cylinders, pump flanges and SAE port seals |
| ISO 3601 P/G series | Metric hydraulic equipment from European and Asian OEMs |
| Backup ring pairs | Required with NBR above ~140 bar dynamic or tight extrusion gaps — see [backup rings](/products/backup-rings/) |
Related Application Guides
FAQ
Is NBR suitable for all hydraulic fluids?
NBR suits petroleum-based mineral hydraulic oils and most lubricating oils. It is not recommended for phosphate esters (use EPDM or specialty compounds), water-glycol without compound verification, or synthetic esters — confirm compatibility before specifying.
NBR 70 or 90 Shore A for hydraulic pumps?
Use 70 Shore A for general static and moderate dynamic seals. Use 90 Shore A for high-pressure pump heads, plunger seals and grooves with large extrusion gaps where harder compound resists blow-out.
What temperature limit for NBR hydraulic O-rings?
Standard NBR: -40°C to +120°C continuous. Above +120°C, oil oxidation accelerates NBR degradation — specify HNBR or FKM for high-temperature hydraulic circuits.
When do I need backup rings with NBR?
Install PTFE or polyamide backup rings on the low-pressure side when dynamic pressure exceeds ~140 bar or diametral extrusion gap exceeds 0.15 mm for 2.62 mm cross-section seals.
Can NBR seal water-glycol hydraulic fluid?
Some NBR compounds tolerate water-glycol, but EPDM is generally preferred for water-based hydraulic fluids. Always verify compound compatibility with your specific fluid supplier data sheet.
Do you supply hydraulic O-ring kits in NBR?
Yes. Standard AS568 assortment kits in NBR 70 and 90 Shore A are available — see [O-ring kits](/products/kits/) or request a custom kit matched to your equipment BOM.
Quote NBR hydraulic O-rings
Provide bore ID, groove dimensions or AS568 dash number, operating pressure and fluid type. We ship from MOQ 1 with 7–15 day lead time on custom compounds.