High Pressure O-Rings for Hydraulic Systems
Hydraulic seals for mobile and industrial cylinders, valves, pumps and manifolds — with material and backup-ring guidance for pressures up to 40 MPa.
Overview
High pressure o rings require the right compound, hardness and backup-ring support to prevent extrusion in dynamic hydraulic service. NBR 70 Shore A is the correct material for 90% of mineral oil hydraulic systems below +100°C. O-rings remain the most widely used sealing element in hydraulic cylinders, valve bodies, pump housings, and manifold connections because they are simple, reliable, and cost-effective when specified correctly.
At O-Ring Supply Co., we supply hydraulic O-rings in NBR, HNBR, FKM, and polyurethane for applications ranging from general industrial machinery to high-pressure mobile equipment. Our standard hydraulic grade NBR is formulated for 34% ACN content and 70 Shore A hardness, delivering the oil resistance and mechanical strength required for mineral oil-based hydraulic fluids.
High-Pressure O-Ring Design: hydraulic O-rings above 10 MPa need more than the correct material — they need the right hardness, clearance gap and backup-ring support. As a rule of thumb, add PTFE backup rings above 10 MPa for dynamic seals and above 14 MPa for static seals; above 21 MPa, upgrade from 70 Shore A to 80 or 90 Shore A NBR, HNBR or FKM depending on fluid and temperature. The diametral clearance gap between the rod or piston and bore is the dominant extrusion variable: at 150 bar a 70 Shore A O-ring needs clearance below 0.20 mm, while 90 Shore A extends the limit to approximately 0.30 mm. For pressures above 40 MPa, dual PTFE backup rings are required and clearance should be held below 0.10 mm. Hardness selection balances extrusion resistance against friction: 90 Shore A provides the highest pressure capability but increases dynamic friction and wear, so it is normally reserved for high-pressure static or slow-reciprocating positions. Use our backup rings PTFE backup rings alongside nitrile O-rings nitrile or HNBR O-rings HNBR O-rings, and verify squeeze with the compression calculator. For a deeper design discussion see high-pressure O-ring design guide. For synthetic-ester hydraulic fluids or continuous temperatures above +120°C, specify fluorocarbon O-rings FKM O-rings instead of NBR.
For cylinder-specific gland design — bore sizes, rod diameters, stroke pressure classes and material pairing — see the hydraulic cylinder O-rings application guide. For non-O-ring hydraulic sealing profiles, see our dedicated hydraulic seals hub, including rod seals for cylinder gland applications and backup rings for high-pressure extrusion protection.
Typical Components
Hydraulic Cylinders
Piston seals, [rod seals](/products/hydraulic-seals/rod-seals/), and wiper seals in single-acting and double-acting cylinders
Directional Valves
Spool seals, port seals, and body seals in directional control and proportional valves
Hydraulic Pumps
Shaft seals, housing gaskets, and high-pressure port connections
Manifolds
Static face seals between machined blocks and cover plates
Mobile Equipment
Excavators, loaders, agricultural tractors, and forestry machinery exposed to dirt and vibration
Recommended Materials
Common Failure Modes & Prevention
Extrusion and Nibbling
Cause: High pressure forcing the seal into clearance gaps without backup ring support
Prevention: Install PTFE or PEEK backup rings above 10 MPa in dynamic service
Compression Set
Cause: Thermal overage or over-compression causing permanent loss of elastic recovery
Prevention: Verify squeeze with a compression calculator and upgrade material if temperature exceeds limits Use compression calculator to verify the design before production.
Abrasion Wear
Cause: Contaminated hydraulic fluid grinding away the dynamic sealing surface
Prevention: Improve filtration to Beta 5 >= 200 and specify harder compounds or polyurethane
Spiral Failure
Cause: Dry running or side loading causing the O-ring to twist in the groove
Prevention: Ensure lubrication, correct piston alignment, and reduce squeeze to 10-12%
Typical Applications
- Hydraulic cylinder seals
- Pump shaft seals
- Valve spool seals
- Accumulator seals
- Hose fittings
- Pressure relief valves
- Filter housing seals
- Manifold seals
Relevant Standards
Frequently Asked Questions — Hydraulics
What is the best O-ring material for hydraulic systems?
NBR 70 Shore A is the standard choice for mineral oil-based hydraulic systems. For temperatures above 100C, specify HNBR. For phosphate ester or synthetic ester fluids, specify FKM.
At what pressure do hydraulic O-rings need backup rings?
Backup rings are recommended above 10 MPa for dynamic seals and above 14 MPa for static seals. They prevent extrusion into the clearance gap under pressure cycling.
What causes O-ring failure in hydraulic systems?
The most common causes are extrusion due to high pressure and inadequate backup rings, compression set from thermal cycling, abrasion from contaminated fluid, and spiral failure from dry running or misalignment.
Are self-lubricating O-rings available for hydraulics?
Yes. NBR and polyurethane compounds can be formulated with internal lubricants such as MOS2 or PTFE to reduce friction and stick-slip in low-lubricity or pneumatic-hydraulic hybrid systems.
Do you supply O-ring seal kits for hydraulic maintenance?
Yes. We supply hydraulic O-ring kits containing the most common AS568 sizes used in cylinders and valves. Custom kits with specific materials and sizes are available for fleet maintenance programs.
What O-ring material works best for high pressure hydraulics?
For mineral oil hydraulics up to 21 MPa, NBR 70 Shore A with PTFE backup rings above 10 MPa is the standard. Above 21 MPa or in abrasive service, specify NBR 90 Shore A, HNBR, or polyurethane. For synthetic ester fluids or temperatures above 120°C, specify FKM. Use our compression calculator to verify groove squeeze before final material selection.
How do I size O-rings for high-pressure hydraulic grooves?
Start by selecting the AS568 or ISO 3601 size that matches the groove ID and cross-section. Then verify the squeeze percentage with the compression calculator at compression calculator — target 10–15% for dynamic reciprocating seals and 15–25% for static seals. Check the diametral clearance gap against the pressure class: below 0.20 mm at 150 bar for 70 Shore A, below 0.30 mm for 90 Shore A. Add PTFE backup rings from backup rings above 10 MPa dynamic or 14 MPa static, and use dual backup rings above 40 MPa. If the standard size produces too much or too little squeeze, redesign the groove rather than accept a marginal compression rate.
What is the difference between hydraulic O-rings and high-pressure O-rings?
All high-pressure O-rings are hydraulic O-rings, but not all hydraulic O-rings are rated for high pressure. A standard 70 Shore A NBR O-ring in a properly designed groove seals most mineral-oil hydraulic systems below 10 MPa. High-pressure O-rings are the same geometry but are specified with harder compounds (80–90 Shore A), tighter clearance gaps and PTFE backup rings to resist extrusion above 10–21 MPa. The term 'high pressure' therefore describes the system design response — material hardness, backup-ring configuration and groove tolerance — rather than a different O-ring family. See high-pressure O-ring design guide for pressure-class design tables.